WO2025201867A1 - Patterning device voltage biasing system - Google Patents

Patterning device voltage biasing system

Info

Publication number
WO2025201867A1
WO2025201867A1 PCT/EP2025/056585 EP2025056585W WO2025201867A1 WO 2025201867 A1 WO2025201867 A1 WO 2025201867A1 EP 2025056585 W EP2025056585 W EP 2025056585W WO 2025201867 A1 WO2025201867 A1 WO 2025201867A1
Authority
WO
WIPO (PCT)
Prior art keywords
patterning device
voltage
clamping surface
patterning
device support
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
PCT/EP2025/056585
Other languages
French (fr)
Inventor
Selwyn Yannick Frithjof CATS
Ronald Van Der Wilk
Bo Liu
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
ASML Netherlands BV
Original Assignee
ASML Netherlands BV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by ASML Netherlands BV filed Critical ASML Netherlands BV
Publication of WO2025201867A1 publication Critical patent/WO2025201867A1/en
Pending legal-status Critical Current
Anticipated expiration legal-status Critical

Links

Classifications

    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70691—Handling of masks or workpieces
    • G03F7/707—Chucks, e.g. chucking or un-chucking operations or structural details
    • G03F7/70708—Chucks, e.g. chucking or un-chucking operations or structural details being electrostatic; Electrostatically deformable vacuum chucks
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
    • H10P72/00—Handling or holding of wafers, substrates or devices during manufacture or treatment thereof
    • H10P72/70—Handling or holding of wafers, substrates or devices during manufacture or treatment thereof for supporting or gripping
    • H10P72/72—Handling or holding of wafers, substrates or devices during manufacture or treatment thereof for supporting or gripping using electrostatic chucks
    • H10P72/722—Details of electrostatic chucks

Definitions

  • the present invention relates to a patterning device support system, a lithographic apparatus comprising a patterning device support system, a method of loading a patterning device onto a patterning device support, and a method of unloading a pattering device from a patterning device support.
  • a lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate.
  • a lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs).
  • a patterning device which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC.
  • This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
  • a single substrate will contain a network of adjacent target portions that are successively patterned.
  • the patterning device may be supported by a patterning device support. While the patterning device is supported by the patterning device support, conductive members may be connected electrically to the patterning device, e.g. to a clamping surface of the patterning device.
  • Figure 9 schematically depicts a patterning device support system, during a loading period.
  • patterning device should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section such as to create a pattern in a target portion C of the substrate W.
  • the pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.
  • Examples of patterning devices include masks, programmable mirror arrays, and programmable liquid-crystal display (LCD) panels.
  • Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types.
  • the resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module.
  • output radiation e.g., EUV radiation
  • the laser and the source collector module SO may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.
  • the laser is not considered to form part of the lithographic apparatus 100 and the radiation beam B is passed from the laser to the source collector module SO with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander.
  • the source may be an integral part of the source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.
  • the illumination system IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illumination system IL can be adjusted.
  • the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section.
  • the radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA.
  • the radiation beam B After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W.
  • the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B.
  • the first positioner PM and another position sensor PSI can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B.
  • the patterning device (e.g., mask) MA and the substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.
  • a controller 500 controls the overall operations of the lithographic apparatus 100 and in particular performs an operation process described further below.
  • Controller 500 can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and nonvolatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus 100. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus 100 is not necessary.
  • one computer can control multiple lithographic apparatuses 100.
  • multiple networked computers can be used to control one lithographic apparatus 100.
  • FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS.
  • An EUV radiation emitting plasma 210 may be formed by a plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.
  • the radiation emitted by the radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212.
  • the space intervening between the projection system PS and the substrate table WT can be at least partially evacuated.
  • the intervening space may be delimited at the location of the projection system PS by a solid surface from which the employed radiation is directed toward the substrate table WT.
  • Contaminant particles P may also be present in the patterning device environment 90. Despite the separation of the patterning device environment 90 from the external environment and/or other components within the lithographic apparatus, it is possible that some contaminant particles P may enter the patterning device environment 90 from these locations. Also, contaminant particles P may be generated within the patterning device environment 90 by mechanisms such as abrasive wear, which occurs when there is relative motion between contacting surfaces.
  • the present disclosure refers to three main periods during the operation of the lithographic apparatus: (1) loading of the patterning device MA onto the support structure MT (referred to as a “loading” period or a “load” period); (2) exposure of the patterning device MA to expose the substrate W (referred to as a “production” period); and (3) unloading of the patterning device from the support structure MT (referred to as an “unloading” period or an “unload” period).
  • the term “period” is used to define specific portions of time during the operation of the lithographic apparatus.
  • the term “period” may be used interchangeably with, for example, “stage” and “phase”.
  • the loading of the patterning device MA onto the support structure MT may be performed before a series of lithographic operations are performed by the lithographic apparatus.
  • the loading of the patterning device MA onto the support structure may be such that the patterning device MA is in a position within the lithographic apparatus where it is able to receive the beam of EUV radiation and impart a pattern thereto.
  • the unloading of the patterning device MA from the support structure MT may be performed after the series of lithographic operations have been completed. Additionally, unloading of a patterning device MA from the support structure MT and loading of another patterning device MA onto the support structure MT may be performed during the series of lithographic operations performed by the lithographic apparatus, e.g. to change the patterning device MA which receives the beam of EUV radiation.
  • the patterning device MA may be moved towards support structure MT, i.e. moved such that a vertical distance between the patterning device MA and the support structure MT decreases.
  • the unloading period may comprise a first portion in which the patterning device MA is in contact with (e.g. supported by, e.g. clamped to) the support structure MT, and a second portion in which the patterning device MA is not in contact with (e.g. is separated from or spaced from) the support structure MT.
  • the patterning device may break contact with the support structure MT. That is, the first portion of the unloading period, in which the patterning device MA is in contact with the support structure MT, may come before the second portion of the unloading period, in which the patterning device MA is not in contact with the support structure MT.
  • a vertical direction is a direction such that, when the patterning device MA is supported by the support structure MT, the patterning device MA is below the support structure MT in the vertical direction.
  • the terms “radially outwards” and “radially inwards” are used in relation to the center of the patterning device MA, with the radial direction being perpendicular to the vertical direction.
  • the patterning device support system may be configured to apply a bias voltage to the patterning device MA. More specifically, the patterning device support system may be configured to apply a bias voltage to the conductive coating which forms the patterning surface 40 of the patterning device MA and/or to the conductive coating which forms the clamping surface 41 of the patterning device MA. Additionally or alternatively, the patterning device support system may be configured to electrically connect the patterning device to the ground. More specifically, the patterning device support system may be configured to electrically connect the conductive coating which forms the patterning surface 40 of the patterning device MA to the ground and/or to connect the conductive coating which forms the clamping surface 41 of the patterning device MA to the ground. This will be described in more detail below.
  • Figure 3 shows that, when the patterning device MA is clamped to the support structure MT, the clamping surface 41 is in contact with distal ends of the plurality of burls 70. It is not necessary for each of the plurality of burls to be in contact with the clamping surface 41. In general, distal ends of one or more of the plurality of burls 70 may be in contact with the clamping surface 41 of the patterning device MA.
  • the electrical connection between the patterning surface 40 and the clamping surface 41 may be via a path integral to the patterning device MA itself.
  • the electrical connection between the patterning surface 40 and the clamping surface 41 may be via an external path, such as a wire, as is shown in Figure 3.
  • one or more of the plurality of burls 70 may be configured to electrically connect the clamping surface 41 to the ground. This may allow the clamping surface 41 and the patterning surface 40 to be discharged.
  • the one or more of the burls 70 which electrically connect the clamping surface 41 to the ground may be the same as the one or more of the burls 70 which electrically connect the clamping surface to the voltage source 61.
  • the voltage source 61 may provide the ground.
  • the patterning device support system may comprise a mode-changing switch 65.
  • the mode-changing switch 65 may be configured such that the one or more of the burls 70 can be selectively connected to the voltage source 61 or a ground 67 other than the voltage source 61 (e.g. the frame of the lithographic apparatus).
  • the mode-changing switch 65 may be any suitable switching device, e.g. a transistor or a switch implemented in software.
  • the one or more burls 70 which electrically connect the clamping surface 41 to the ground 67 may be different to the one or more of the burls 70 which electrically connect the clamping surface to the voltage source 61.
  • the patterning device support system is configured to capacitively induce a bias voltage in the patterning surface 40 using the clamping means in the support structure.
  • the support structure MT is an electrostatic clamp comprising a plurality of electrodes. When a voltage is applied to the electrodes in the clamp, an electrostatic clamping force is exerted on the clamping surface 41 of the patterning device.
  • the average voltage of the plurality of electrodes in the electrostatic clamp can be controlled. When the average potential of the plurality of electrodes is not 0 V, an electric field may be present between the electrostatic clamp and grounded components in the lithographic apparatus, e.g. patterning device masking blades.
  • the patterning device MA may be disposed between the electrostatic clamp and the grounded components, and so within this electric field. Consequently, a bias voltage may be capacitively induced in the patterning surface 40 and the clamping surface 41 (or, more specifically, in conductive coatings which form the patterning surface 40 and the clamping surface 41). Capacitive induction of a bias voltage in the patterning surface 40 and/or clamping surface 41 of a patterning device MA is described in EP 23168046.3 filed on 14 April 2023, the entirety of which is hereby incorporated by reference .
  • the support structure may still comprise one or more burls configured to electrically connect the clamping surface 41 to the ground 67.
  • the patterning device support system may comprise one or more current-limiting components 62, 66 between the patterning surface 40 and the voltage source 61 and/or between the patterning surface 40 and the ground 67.
  • the current-limiting components may be, or may perform the functionality of, one or more of a resistor, an inductor and a diode.
  • the current-limiting components may be provided to maintain a current within the patterning surface 40 within acceptable levels. Such current-limiting components are described in EP 23176443.2 filed on 31 May 2023, the entirety of which is hereby incorporated by reference.
  • the one or more burls which are configured to connect the clamping surface 41 to the voltage source 61 and/or the ground 67 may be referred to as conductive burls.
  • the conductive burls may be coated with a conductive material.
  • the conductive burls are examples of conductive members.
  • conductive burls may form only a small proportion of the total number burls 70 of the support structure MT.
  • conductive burls may make up less than 10%, preferably less than 5% and further preferably less than 1% of the total burls 70 on the support structure MT. This may allow the clamping surface to be effectively discharged and/or brought to a bias voltage, without compromising the clamping of the patterning device MA to the support structure MT.
  • Figure 5 depicts a support surface 42 of a support structure MT.
  • the support structure MT depicted in Figure 5 is rectangular, but a support structure MT in accordance with the present disclosure may be any suitable shape.
  • a portion of the plurality of burls 70 on the support structure MT are conductive burls (e.g. conductive burls 70a, 70b, 70c).
  • the support structure MT further comprises a conductive track 68 formed on the support surface 42.
  • the conductive track 68 may be formed around a perimeter of the support surface 42.
  • the conductive track 68 may be formed outward of the burls 70.
  • the conductive burls 70a, 70b, 70c may be electrically connected to the conductive track 68.
  • the conductive coating of the conductive burls 70a, 70b, 70c may be connected to the conductive track.
  • the conductive burls 70a, 70b, 70c may be located in a border region of the support surface 42 of the support structure. Conductive burls 70a, 70b, 70c may be in an outermost ring of burls 70. The conductive burls 70a, 70b, 70c may be located in one or more corners of the support surface 42 of the support structure MT. Disposing the conductive burls 70a, 70b, 70c in such locations may reduce the effect that the grounding of the burls 70 has on the clamping of the patterning device MA to the support structure MT, or limit the regions in which the clamping of the patterning device MA to the support structure MT is affected to regions which are not critical to the quality of the image projected from the patterning device MA.
  • the conductive track 68 may be connected to an interface 69.
  • the interface 69 may allow the conductive track 68 to be connected to external circuitry.
  • the external circuitry may connect the interface 69 to the ground 67 and/or the voltage source 61.
  • the conductive material which forms the conductive track may be the same material that coats the conductive burls 70a, 70b, 70c.
  • the conductive burls 70a, 70b, 70c may be electrically connected to the conductive track 68 via one or more extensions of the conductive track (e.g. extensions 68a, 68b, 68c).
  • each conductive burl 70a, 70b, 70c is provided with an extension 68a, 68b, 68c.
  • the extensions 68a, 68b, 68c may extend inward from the conductive track 68 to the conductive burls 70a, 70b, 70c.
  • one extension 68a, 68b, 68c may connect a plurality of conductive burls 70a, 70b, 70c to the conductive track 68.
  • Figure 6 depicts a patterning device MA in which the patterning surface 40 is substantially electrically isolated.
  • the patterning device depicted in Figure 6 is a comparative example. In cases where the patterning surface is substantially electrically isolated, the patterning surface 40 may become positively charged as a result of the release of electrons from the patterning surface 40.
  • the EUV radiation within the patterning device environment 90 may causes contaminant particles P to become negatively charged. This may occur as a result of at least two main mechanisms.
  • a first mechanism is a result of the formation of plasma from the gas molecules within the patterning device environment 90, which are excited by the EUV radiation. Free electrons within the plasma may be absorbed by the contaminant particles P, resulting in those particles becoming negatively charged.
  • a second mechanism is a consequence of the photoelectric effect which causes the patterning surface 40 to become positively charged. Specifically, electrons that have been ejected from the patterning surface 40 as a result of the photoelectric effect may be absorbed by the contaminant particles P, causing them to become negatively charged.
  • Figures 8A and 8B depict plots of the displacement of a contaminant particle P relative to the patterning surface 40 over time.
  • Figure 8A shows the displacement of a contaminant particle P relative to the patterning surface 40 where the patterning surface 40 is substantially electrically isolated.
  • Figure 8B shows the displacement of a contaminant particle P where voltage biasing of the patterning surface is applied.
  • the plot in Figure 8B does not relate directly to a specific method by which the bias voltage is applied to the patterning surface 40.
  • the circumstances underlying the plot depicted in Figure 7A correspond to the circumstances underlying the plot depicted in Figure 8A
  • the circumstances underlying the plot depicted in Figure 7B correspond to the circumstances underlying the plot depicted in Figure 8B.
  • the production-bias voltage applied to the patterning surface 40 is a constant bias voltage of approximately -I V.
  • Figure 7A shows that, without the application of a production-bias voltage, the voltage of the patterning surface 40 begins at approximately 0 V.
  • the contaminant particle P is initially at rest in the direction perpendicular to the surface 40.
  • the contaminant particle P begins to accelerate towards the patterning surface 40.
  • the patterning surface 40 discharges, the magnitude of this acceleration decreases.
  • the contaminant particle P no longer accelerates towards the patterning surface 4, but continues to travel towards the patterning surface 40 at a constant velocity.
  • the contaminant particle P again begins to accelerate towards the patterning surface 40.
  • the displacement between the contaminant particle P and the patterning surface 40 becomes zero, i.e., the contaminant particle 40 is deposited into the patterning surface 40.
  • the time in which the contaminant particle 40 accelerates towards the patterning surface 40 may be sufficiently small such that, over time, the distance between the contaminant particle P and the patterning surface 40 increases. Consequently, the contaminant particle P is not deposited onto the patterning surface 40.
  • the production-bias voltage may be varied over the course of the production period. For instance, the production-bias voltage may alternate between being positive during EUV pulses (i.e. while EUV radiation is generated) and negative between EUV pulses (i.e. while the EUV radiation is not generated.
  • EUV-induced emission of electrons through the photoelectric effect from the patterning surface 40 contributes to the deposition of contaminant particles P on the patterning surface (and therefore imaging errors). This is because: (i) the emission of electrons causes the patterning surface 40 to become positively charged (and be brought to a positive potential), thus attracting negatively charged contaminant particles; and (ii) the emission of electrons introduces additional electrons to the plasma within the patterning device environment 90, which may increase the number of contaminant particles that become negatively charged or the magnitudes of the negative charges on the contaminant particles P. Consequently, by reducing or preventing the emission of electrons whilst the patterning surface 40 is exposed to EUV radiation, fewer contaminant particles P may be deposited on the patterning surface 40.
  • the patterning surface 40 By inducing a positive production-bias voltage in the patterning surface 40 whilst the patterning surface 40 is exposed to EUV radiation, the emission of electrons from the patterning surface can be reduced. Consequently, the patterning surface 40 may become positively charged to a lesser extent, and may contribute less electrons to the plasma in the patterning device environment 90.
  • the magnitude of the positive production-bias potential applied to the patterning surface 40 may be sufficient to prevent the emission of electrons by the photoelectric effect. That is, the magnitude of the positive production-bias may be such that the positive potential induced at the patterning surface 40 is greater than a stopping potential (V sto p).
  • V sto p a stopping potential
  • the maximum kinetic energy of an electron emitted though photoemission is given by Equation (2), where h is the Planck constant (4.14 x 10 15 eVs),/is the frequency of the radiation, and cp is the work function of the material (i.e., the minimum energy required to cause emission of an electron from a surface).
  • the work function is a property of the material of the surface from which electrons are emitted.
  • the wavelength of radiation may be approximately 13.5 nm.
  • the photon energy of a photon within a beam of EUV radiation may be approximately 92 eV.
  • the work function of the patterning surface 40 may be dependent on the material from which the patterning surface 40 is formed. In general, the work function may be between 2 eV and 7 eV. If the work function is 7 eV or less, it may be preferable for the potential induced on the patterning surface 40 to be approximately 85 V or greater to substantially suppress photoemission. If the work function is 2 eV or less, it may be preferable for the potential induced on the patterning surface 40 to be approximately 90 V or greater to substantially suppress photoemission.
  • a majority of electons released from the patterning surface 40 upon irradiation with EUV radiation generally have a low energy, e.g., an energy that is less than 10 eV. This may be because EUV photons are absorbed at effective depth of approximately 10 to 100 nm. As electrons which have absorbed an EUV photon propagate to the vacuum interface from the absorption position to the surface, they may lose energy. Considering this, to significantly suppress the emission of electrons through the photoelectric effect, it may be sufficient to apply a positive production-bias voltage which is greater than + 50V. In this case, the positive production-bias voltage may be less than 100 V to reduce the risk of discharge.
  • the positive production-bias voltage may be less than 50 V to further reduce the extent to which the positive production-bias voltage applied to the patterning surface leads to physically sputtering of ions onto grounded surfaces, such as the masking blades. Considering this, it may be preferable for the positive production-bias voltage to be greater than 5 V and less than 50 V.
  • the patterning device voltage production-biasing system may be synchronized with the pulses of EUV radiation generated by the lithographic apparatus.
  • the means by which the polarity of the production-bias voltage is switched is not particularly limited.
  • a timing switch may be provided between the patterning surface 40 and the voltage source 61 and/or between the patterning surface 40 and the ground 67.
  • the timing switch may be configured such that a production-bias voltage is applied to the patterning surface 40 between pulses of EUV radiation, but not during pulses of EUV radiation.
  • the timing switch may prevent the current within the patterning surface exceeding an acceptable limit while the EUV radiation is incident on the patterning surface.
  • the timing switch may be switched on and off (i.e. opened and closed) at a frequency of greater than 49 kHz, preferably greater than 59 kHz, and further preferably greater than 99 kHz.
  • the frequency at which the switch is switched on and off may be synchronized with a frequency of generation of the pulses of EUV radiation.
  • the timing switch may be any suitable switching device, e.g. a transistor a switch implemented in software.
  • the timing switch is described in detail in EP 22195470.4 filed on 13 September 2022, the entirety of which is hereby incorporated by reference.
  • the patterning device MA is loaded onto the support structure MT. That is, the patterning device MA is taken from, for example, a storage area, and moved such that it is brought into contact with the support structure. Once the patterning device MA is in contact with the support structure, clamping means within the support structure MT may be activated, such that the patterning device MA becomes clamped to the support structure MT.
  • the support structure MT comprises an electrostatic clamp
  • the clamping means may comprise a plurality of electrodes, which are described in further detail below.
  • the clamping surface 41 of the patterning device comes into contact with one or more conductive members.
  • the one or more conductive members may comprise conductive burls.
  • the conductive burls may be configured to apply a bias voltage to the clamping surface 41 and/or the patterning surface 40, e.g. a production-bias voltage to the clamping surface 431 and/or the patterning surface 40 during the production period. Additionally or alternatively, the conductive burls may be configured to electrically connect the clamping surface 41 and/or the patterning surface 40 to the ground 67.
  • a patterning device support system may comprise conductive members in addition to, or instead of, the conductive burls.
  • the patterning device support system may comprise a voltage biasing member.
  • the voltage biasing member may be configured to apply a bias voltage to the clamping surface 41 and/or the patterning surface 40 by contacting the clamping surface 41 when the patterning device MA is clamped to the support structure MT.
  • the voltage biasing member may be configured to electrically connect the clamping surface 41 and/or the patterning surface 40 to the ground when the patterning device MA is clamped to the support structure MT.
  • the voltage biasing member may not be configured to support the patterning device MA.
  • the conductive members may comprise any conductive component or feature which comes into contact with the clamping surface 41 of the patterning device MA when the patterning device is clamped to the support structure MT.
  • the present disclosure focuses on a patterning device support system in which the conductive members comprise conductive burls.
  • Figure 9 depicts a patterning device support system during the loading period.
  • the arrow shown in Figure 9 demonstrate the direction in which the patterning device MA is moved during the loading period.
  • the patterning device MA comprises a conductive coating 41C which forms the clamping surface 41.
  • the patterning device MA comprises a conductive coating 40C which forms the patterning surface 40.
  • the conductive coatings 40C, 41C are substantially electrically isolated from one another. However, in some cases, the conductive coatings 40C, 41C may be electrically connected, as has been described above.
  • the support structure MT is an electrostatic clamp.
  • the electrostatic clamp comprises a plurality of electrodes 45a-d embedded therein.
  • Each electrode 45a-d may be electrically connected to a voltage source 46a-d.
  • each electrode 45a-d may be electrically connected to a common voltage source.
  • the patterning device support system may be configured such that a different voltage can be applied to each of the plurality of electrodes 45a-d.
  • the support structure MT may be configured such that, when a clamping voltage is applied to each of the plurality of electrodes 45a-d, an electrostatic clamping force is exerted on the conductive coating 41C which forms the clamping surface 41.
  • the electrostatic clamp may be configured such that a positive clamping voltage is applied to some of the electrodes 45a-d, while a negative clamping voltage is applied to other electrodes 45a-d.
  • a positive clamping voltage may be applied to the electrodes 45a, 45d
  • a negative clamping voltage may be applied to the electrodes 45b, 45c.
  • an average voltage of the plurality of electrodes may be approximately zero.
  • the average voltage of the plurality of electrodes can be changed (i.e. such that the average potential of the plurality of electrodes 45a-d is positive or negative).
  • the conductive burls 70a are electrically connected to ground.
  • a potential difference may exist between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a. Consequently, as the clamping surface 41 approaches the conductive burls 70a, electrostatic discharge may occur.
  • the load-conductive member voltage reduces a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a.
  • the reduction of the potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may be relative to the case in which the conductive burls 70a are grounded.
  • the load-conductive member voltage is predetermined. That is, the load- conductive member voltage may be controlled based on known aspects of the operation of patterning device support system, e.g. the average voltage of the plurality of electrodes 45a-d in the support structure MT when the average voltage of the plurality of electrodes 45a-d in the support structure MT is intended to be (i.e. controlled to be) zero. In such embodiments, the load-conductive member voltage may not be actively controlled, i.e. there may not be a closed feedback loop.
  • a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may be established where the average voltage of the plurality of electrodes 45a-d in the support structure MT is not equal to zero.
  • the voltage of the conductive coating 41C which forms the clamping surface 41 can be determined or otherwise estimated based on the average voltage of the plurality of electrodes 45a-d.
  • the voltage capacitively induced in the conductive coating 41C which forms the clamping surface 41 may have the same polarity as the non-zero average voltage of the plurality of electrodes 45a-d.
  • the magnitude of the voltage capacitively induced in the conductive coating 41C which forms the clamping surface 41 may be between 0 V (i.e. ground potential) and the average voltage of the plurality of electrodes 45a-d in the support structure MT.
  • the load-conductive member voltage may be determined based on measurements made by the one or more sensors.
  • the load-conductive member voltage may be actively controlled, e.g. controlled using a closed feedback loop.
  • Figure 10 depicts a number of such sensors, e.g. sensors 31 and 32a-d, which are described in further detail below.
  • the patterning device support system may comprise only one of the sensors, and the determination of the load-conductive member voltage may be based on measurements made by this one sensor.
  • the patterning device support system may comprise more than one of the sensors (e.g. more than one type/location of sensor), and the determination of the load-conductive member voltage may be based on measurements made by some or all of the sensors.
  • the patterning device support system comprises a current sensor 31) configured to measure the current between the conductive burls 70a and the conductive member voltage source 61.
  • the current sensor 31 is an example of a first sensor.
  • the load-conductive member voltage may be determined based on the current measured by the current sensor 31.
  • the patterning device support system may comprise a sensor disposed in the patterning device handler 80.
  • a sensor disposed in the patterning device handler 80 is an example of a third sensor.
  • the load-conductive member voltage may be determined based on measurements made by the third sensor.
  • the sensor disposed in the patterning device handler 80 may be a voltage sensor configured to measure a voltage of the conductive coating 41 C which forms the clamping surface 41.
  • the load- conductive member voltage may be controlled to be the same as the voltage of the conductive coating 41C as measured by the voltage sensor in the patterning device handler.
  • the sensor disposed in the patterning device handler 80 may be a strain gauge.
  • the strain gauge may be configured to measure a strain of the patterning device MA. If there is a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a, a resulting electrostatic force between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may cause the patterning device MA to be deformed. This deformation may be measured by the strain gauge. Thus, the load-conductive member voltage may be determined on the basis of the deformation of the patterning device MA as measured by the strain gauge.
  • the sensor deposed in the patterning device handler 80 may be a force sensor configured to measure a contact force between the patterning device MA and the patterning device handler 80. If there is a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a, a resulting electrostatic force between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may cause the contact force between the patterning device MA and the patterning device handler 80 to change (e.g. decrease). This change in the contact force between the patterning device MA and the patterning device handler 80 can be measured by the force sensor. Consequently, the load-conductive member voltage may be determined on the basis of the contact force between the patterning device MA and the patterning device handler 80 as measured by the strain gauge.
  • This description of the loading period has focused on the case where the potential difference between the conductive members (i.e. the conductive burls 70a) and the clamping surface 41 is reduced by applying a load-conductive member voltage to the conductive members (i.e. the conductive burls 70a).
  • the potential difference between the conductive members (i.e. the conductive burls 70a) and the clamping surface 41 may also be reduced by applying a load-bias voltage to the clamping surface 41 as the patterning device MA approaches the support structure MT.
  • a load-bias voltage may be applied by controlling the average voltage of the plurality of electrodes 45a-d in the support structure MT.
  • the unload-bias voltage may be adjusted (S108).
  • the adjusting of the unload-bias voltage may be based on the measurements made by the one or more sensors when the patterning device MA was moving away from the support structure.
  • the adjusted unload-bias voltage may be applied to the clamping surface (SI 03), as described above.
  • the method may be repeated as many times as necessary until the charge on the clamping surface 41 is determined to be less than the threshold charge in SI 05, and the patterning device can be fully unloaded, and e.g. returned to a storage area.
  • the step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining whether a current between the one or more conductive burls 70a and the conductive member voltage source 61 (e.g. as measured by the current sensor 31) exceeds a first predetermined threshold. If the current between the one or more conductive burls 70a and the conductive member voltage source 61 does exceed the first predetermined threshold, it is determined that the charge on the clamping surface 41 exceeds the threshold charge, and the patterning device MA is returned to the loaded position in which the clamping surface is electrically connected to the one or more conductive members.
  • the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more positive (e.g. increasing the magnitude of a positive unload-bias voltage or decreasing the magnitude of a negative unload-bias voltage).
  • the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more negative (e.g. increasing the magnitude of a negative unload-bias voltage or decreasing the magnitude of a positive unload-bias voltage).
  • the extent to which the unloadbias voltage is adjusted i.e. the amount by which the unload-bias voltage is changed) may be a function of the current measured by the current sensor 31 and the rate of increasing separation between the clamping surface 41 and the support structure MT.
  • the step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining whether a current between the electrodes 45a-d and the clamping voltage source(s) 46a-d (e.g. as measured by the one or more current sensors 32a-d) exceeds a second predetermined threshold. If the current between the electrodes 45a-d and the clamping voltage source(s) 46a-d does exceed the second predetermined threshold, it is determined that the charge on the clamping surface 41 exceeds the threshold charge, and the patterning device MA is returned to the loaded position in which the clamping surface 41 is electrically connected to the one or more conductive members 70a .
  • the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more positive (e.g. increasing the magnitude of a positive unload-bias voltage or decreasing the magnitude of a negative unload-bias voltage).
  • the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more negative (e.g. increasing the magnitude of a negative unload-bias voltage or decreasing the magnitude of a positive unload-bias voltage).
  • the extent to which the unload-bias voltage is adjusted i.e. the amount by which the unload-bias voltage is changed) may be a function of the current measured by the one or more current sensors 32a-d and the rate of increasing separation between the clamping surface 41 and the support structure MT.
  • the step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining the amount of charge on the clamping surface 41, or a variable related to (i.e. indicative of) the amount of charge on the sample, using a sensor in the patterning device handler 80.
  • the step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining the voltage of the clamping surface 41 using the voltage sensor of the patterning device handler 80.
  • the patterning device MA may be returned to the loaded position, in which the conductive burls 70a are in contact with the clamping surface 41.
  • the unload-bias voltage applied may be excessively negative (or insufficiently positive).
  • the step of adjusting the unload-bias voltage may comprise making the unload-bias voltage more positive (i.e. increasing the magnitude of a positive unload-bias voltage or decreasing the magnitude of a negative unload-bias voltage). If the voltage measured by the voltage sensor after the patterning device MA has separated from the support structure is positive, the unloadbias voltage applied may be excessively positive (or insufficiently negative). Thus, the step of adjusting the unload-bias voltage may comprise making the unload-bias voltage more negative (i.e.
  • the size of the adjustment to the unload-bias voltage may be a function of the voltage measured by the voltage sensor and the separation between the patterning device MA and the support structure MT.
  • the step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise a step of determining a strain (i.e. deformation) of the patterning device MA using a strain gauge in the patterning device handler 80. Strain of the patterning device MA may be indicative of charge on the clamping surface 41. Thus, if the strain measured by the strain gauge exceeds a strain threshold, the patterning device MA may be returned to the loaded position in which the clamping surface 41 is in contact with the conductive burls 70a, and the unload-bias voltage adjusted. The amount of change to the unload-bias voltage may be a function of the magnitude of the strain of the patterning device MA as measured by the strain gauge.
  • the step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise a step of measuring a contact force between the patterning device MA and the patterning device handler 80 using the force sensor in the patterning device handler 80. If the clamping surface 41 is charged, an electrostatic attractive force may be exerted on the clamping surface 41 in the direction of the support structure MT.
  • the patterning device support system may be configured to return the patterning device MA to the loaded position in the case that the force measured by the force sensor is less than a threshold force.
  • an unload-bias voltage may be applied to the clamping surface 41 through any suitable means, e.g. through a voltage biasing member (as described above) or by controlling the average voltage of the plurality of electrodes 45a-d in the support structure MT.
  • the patterning device support system described above may be incorporated into a lithographic apparatus.
  • the lithographic apparatus may be used for the manufacture of ICs.
  • a computer program comprising instructions which, when executed by a controller for a lithographic apparatus, cause the lithographic apparatus to carry out the method of any of clauses 44 to 77.

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Abstract

A patterning device support system for use in a lithographic apparatus comprising: a patterning device support configured to support a patterning device having a patterning surface and a clamping surface facing the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a conductive member voltage source configured to apply a voltage to the one or more conductive members; and a controller configured to control voltage applied to the one or more conductive members, wherein, during loading of the patterning device onto the patterning device support, the controller is configured to control the voltage applied to be a load-conductive member voltage, wherein the load-conductive member voltage reduces a potential difference between the clamping surface and one or more conductive members relative to a case where one or more conductive members are grounded.

Description

PATTERNING DEVICE VOLTAGE BIASING SYSTEM
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of EP application 24167535.4 which was filed on March 28, 2024 and which is incorporated herein in its entirety by reference.
FIELD
[0002] The present invention relates to a patterning device support system, a lithographic apparatus comprising a patterning device support system, a method of loading a patterning device onto a patterning device support, and a method of unloading a pattering device from a patterning device support.
BACKGROUND
[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
[0004] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
[0005] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1): where is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, kl is a process-dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from Equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength 7. by increasing the numerical aperture NA or by decreasing the value of kl . [0006] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser- produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
[0007] The patterning device may be supported by a patterning device support. While the patterning device is supported by the patterning device support, conductive members may be connected electrically to the patterning device, e.g. to a clamping surface of the patterning device.
[0008] In certain periods during the operation of a lithographic apparatus (e.g. while the patterning device is supported by the patterning device support and is receiving EUV radiation), a bias voltage may be applied to the patterning device via the conductive members. Applying a bias voltage to the patterning device during the certain periods may reduce contamination of the patterning device. Additionally or alternatively, in certain periods during the operation of a lithographic apparatus (e.g. while the patterning device is supported by the patterning device support and is receiving EUV radiation), the patterning device may be grounded via the conductive members. Grounding of the patterning device during the certain periods may reduce the accumulation of charge on the patterning device.
[0009] The presence of the conductive members may introduce problems during loading of the patterning device onto the patterning device support and unloading of the patterning device from the patterning device support.
[0010] During loading of the patterning device onto the patterning device support, if there is a potential difference between the patterning device and the conductive members, sudden electrostatic discharge between the patterning device and the conductive members may occur.
[0011] Before the patterning device is unloaded from the patterning device support, the patterning device may become charged. If the patterning device is charged as the patterning device is unloaded from the patterning device support, sudden discharge between the patterning device and the conductive members (or between the patterning device and other conductive components) may occur.
[0012] Sudden discharge between the patterning device and the conductive members (or other conductive components) may result in wear of the patterning device and/or the patterning device support. This may decrease (i.e. worsen) the flatness of the patterning device and/or the patterning device support, resulting in an increase (i.e. worsening) of overlay error, and a reduction in the lifetime of the patterning device support and/or the patterning device.
SUMMARY OF THE INVENTION [0013] An object of the present invention is to provide an improved patterning device support system. More particularly, an object of the present invention is to provide a patterning device support system which, when used in a lithographic apparatus, results in a reduced (i.e. improved) overlay error and which extends the lifetime of the patterning device and the patterning device support.
[0014] According to an aspect of the present invention, there is provided a patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support a patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a conductive member voltage source configured to apply a voltage to the one or more conductive members; and a controller configured to control the voltage applied to the one or more conductive members, wherein, during loading of the patterning device onto the patterning device support, the controller is configured to control the voltage applied to the one or more conductive members to be a load-conductive member voltage, wherein the load-conductive member voltage reduces a potential difference between the clamping surface and the one or more conductive members relative to a case in which the one or more conductive members are grounded.
[0015] According to another aspect of the present invention, there is provided a patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support the patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a conductive member voltage source configured to apply a bias voltage to clamping surface via the one or more conductive members; and a controller configured to control the bias voltage applied to the clamping surface via one or more conductive members, wherein, in a period before unloading of the patterning device from the patterning device support, the controller is configured to control the conductive member voltage source to apply an unload-bias voltage to the clamping surface via the one or more conductive members, and the unload-bias voltage reduces a charge on the clamping surface, relative to a case in which the clamping surface is grounded via the one or more conductive members.
[0016] According to another aspect of the present invention, there is provided a method of loading a patterning device onto a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: moving the patterning device towards the patterning device support; and applying a load-conductive member voltage to one or more conductive members, wherein the one or more conductive members are arranged to be electrically connected to the clamping surface when the patterning device is loaded onto the patterning device support, and the load-conductive member voltage applied to the one or more conductive members reduces a potential difference between the clamping surface and the one or more conductive members relative to a case in which the one or more conductive members are grounded.
[0017] According to another aspect of the present invention, there is provided a method of unloading a patterning device from a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: in a period before unloading the patterning device from the patterning device support, applying an unload-bias voltage to the clamping surface via one or more conductive members; and then moving the patterning device away from the patterning device support, wherein the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device, relative to a case in which the clamping surface is grounded via the one or more conductive members.
[0018] According to another aspect of the present invention, there is provided a patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support the patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a voltage source configured to apply a voltage to the clamping surface and/or the conductive members; and a controller, wherein, during loading of the patterning device onto the patterning device support, the controller is configured to control the voltage source to apply a load voltage to the clamping surface and/or the one or more conductive members to reduce a potential difference between the clamping surface and the one or more conductive members.
[0019] According to another aspect of the present invention, there is provided a patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support the patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a voltage source configured to apply a bias voltage to the clamping surface; and a controller, wherein, in a period before unloading of the patterning device from the patterning device support, the controller is configured to control the voltage source to apply an unload-bias voltage to the clamping surface, and the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device.
[0020] According to another aspect of the present invention, there is provided a method of loading a patterning device onto a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, the patterning device support comprising one or more conductive members, wherein the one or more conductive members are arranged to be electrically connected to the clamping surface when the patterning device is loaded onto the patterning device support, and the method comprises: moving the patterning device towards the patterning device support; and applying a load voltage to the clamping surface and/or the one or more conductive members, wherein the load voltage applied to the one or more conductive members and/or the clamping surface reduces a potential difference between the clamping surface and the one or more conductive members.
[0021] According to another aspect of the present invention, there is provided a method of unloading a patterning device from a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: in a period before unloading the patterning device from the patterning device support, applying an unload-bias voltage to the clamping surface; and then moving the patterning device away from the patterning device support, wherein the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference symbols indicate corresponding parts.
Figure 1 schematically depicts a lithographic apparatus.
Figure 2 schematically depicts a more detailed view of the lithographic apparatus.
Figure 3 schematically depicts a patterning device support system.
Figure 4 schematically depicts a patterning device support system.
Figure 5 schematically depicts a plan view of a support surface of a patterning device support in accordance with the present disclosure.
Figure 6 schematically depicts a patterning device while being exposed to EUV radiation.
Figure 7A depicts a plot of Voltage of a Patterning Surface against Time for the case that a production-bias voltage is not applied.
Figure 7B depicts a plot of Voltage of a Patterning Surface against Time for the case that a production-bias voltage is applied.
Figure 8A depicts a plot of Distance from a Patterning Surface against Time for the case that a production-bias voltage is not applied.
Figure 8B depicts a plot of Distance from a Patterning Surface against Time for the case that a bias voltage is applied.
Figure 9 schematically depicts a patterning device support system, during a loading period.
Figure 10 schematically depicts a patterning device support system in accordance with the present disclosure, during a loading period.
Figure 11 schematically depicts a patterning device support system in accordance with the present disclosure, during a production period or during a first portion of an unloading period. Figure 12 schematically depicts a patterning device support system in accordance with the present disclosure, during a second portion of an unloading period.
Figure 13 depicts a method of unloading a patterning device from a patterning device support, in accordance with the present disclosure.
The features shown in the Figures are not necessarily to scale, and the size and/or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature.
DETAILED DESCRIPTION
[0023] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module SO according to one embodiment of the invention. The apparatus 100 comprises: an illumination system (or illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation). a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.
[0024] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.
[0025] The support structure MT may also be referred to as a patterning device support. The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.
[0026] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section such as to create a pattern in a target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit. [0027] Examples of patterning devices include masks, programmable mirror arrays, and programmable liquid-crystal display (LCD) panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.
[0028] The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.
[0029] As here depicted, the lithographic apparatus 100 is of a reflective type (e.g., employing a reflective mask).
[0030] The lithographic apparatus 100 may be of a type having two (dual stage) or more substrate tables WT (and/or two or more support structures MT). In such a “multiple stage” lithographic apparatus the additional substrate tables WT (and/or the additional support structures MT) may be used in parallel, or preparatory steps may be carried out on one or more substrate tables WT (and/or one or more support structures MT) while one or more other substrate tables WT (and/or one or more other support structures MT) are being used for exposure.
[0031] Referring to Figure 1, the illumination system IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module SO may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.
[0032] In such cases, the laser is not considered to form part of the lithographic apparatus 100 and the radiation beam B is passed from the laser to the source collector module SO with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.
[0033] The illumination system IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illumination system IL can be adjusted. In addition, the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section.
[0034] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PSI can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.
[0035] A controller 500 controls the overall operations of the lithographic apparatus 100 and in particular performs an operation process described further below. Controller 500 can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and nonvolatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus 100. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus 100 is not necessary. In an embodiment of the invention one computer can control multiple lithographic apparatuses 100. In an embodiment of the invention, multiple networked computers can be used to control one lithographic apparatus 100. The controller 500 may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus 100 forms a part. The controller 500 can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and/or an overall control system of a fab.
[0036] Figure 2 shows the lithographic apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. An EUV radiation emitting plasma 210 may be formed by a plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation. [0037] The radiation emitted by the radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212.
[0038] The collector chamber 212 may include a radiation collector CO. Radiation that traverses the radiation collector CO can be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module SO is arranged such that the virtual source point IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.
[0039] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the unpatterned beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the unpatterned beam 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT.
[0040] More elements than shown may generally be present in the illumination system IL and the projection system PS. Further, there may be more mirrors present than those shown in the Figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in Figure 2.
[0041] Alternatively, the source collector module SO may be part of an LPP radiation system.
[0042] As depicted in Figure 1, in an embodiment the lithographic apparatus 100 comprises an illumination system IL and a projection system PS. The illumination system IL is configured to emit a radiation beam B. The projection system PS is separated from the substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto the substrate W. The pattern is for EUV radiation of the radiation beam B.
[0043] The space intervening between the projection system PS and the substrate table WT can be at least partially evacuated. The intervening space may be delimited at the location of the projection system PS by a solid surface from which the employed radiation is directed toward the substrate table WT.
The Patterning Device Support System
[0044] Figure 3 depicts a schematic representation of a patterning device MA clamped to a support structure MT. The support structure MT is an example of a patterning device support. The patterning device MA and the support structure MT may form part of a patterning device support system. The patterning device support system may comprise other components in addition to the patterning device MA and the support structure MT. In some cases, the patterning device support system may not comprise the patterning device MA itself. This applies for all of the patterning device support systems of the present disclosure. [0045] As described above, the support structure MT may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may comprise a plurality of burls 70 (cone-shaped protrusions) on a support surface 42 of the support structure MT that faces a clamping surface 41 of the patterning device MA. When the patterning device MA is clamped to the support structure MT, the clamping surface 41 is in contact with distal ends of the plurality of burls. It is not necessary for each of the plurality of burls to be in contact with the clamping surface 41.
[0046] The patterning device MA comprises a patterning surface 40 opposite the clamping surface 41. During operation of the lithographic apparatus (and, specifically, while a substrate W is being exposed with EUV radiation), the patterning surface 40 is configured to receive a beam of EUV radiation. The patterning surface 40 comprises a pattern formed thereon. The pattern may be confined to a patterning region of the patterning surface 40 (i.e. a sub-portion of the patterning surface 40). The patterning surface 40 may be configured such that the beam of EUV radiation received by the patterning surface 40 is reflected. When the beam of EUV radiation is reflected by the patterning surface 40, the pattern on the patterning surface (or a portion thereof) may be imparted to the beam of EUV radiation.
[0047] The clamping surface 41 may be conductive. For example, the patterning device MA may comprise a conductive (e.g. metallic) coating, which forms the clamping surface 41. The conductive coating which forms the clamping surface 41 may be provided to allow the patterning device MA to be clamped to the support structure MT, which may be an electrostatic clamp. The conductive coating which forms the clamping surface 41 may be provided to a portion (i.e. not the entirety of) the patterning device MA. In this case, the clamping surface 41 may correspond to the area over which the conductive coating is provided. The patterning surface 40 may be conductive. For example, the patterning device may comprise a conductive (e.g. metallic, e.g. Ruthenium) coating, which forms the patterning surface 40.
[0048] Both the patterning device MA and support structure MT may be contained within a patterning device environment 90. The patterning device environment 90 may be separated from an external environment surrounding the lithographic apparatus 100 and/or other components within the lithographic apparatus such that gases and contaminant particles P are substantially prevented from entering the patterning device environment 90.
[0049] The patterning device environment 90 may be partially evacuated of gas. That is, the pressure within the patterning device environment 90 may be less than ambient pressure. This is to limit the attenuation of EUV radiation as it travels through the patterning device environment 90. Even though the pressure within the patterning device 90 is less than ambient pressure, it is not a perfect vacuum, so gas particles are present in the patterning device environment 90.
[0050] Contaminant particles P may also be present in the patterning device environment 90. Despite the separation of the patterning device environment 90 from the external environment and/or other components within the lithographic apparatus, it is possible that some contaminant particles P may enter the patterning device environment 90 from these locations. Also, contaminant particles P may be generated within the patterning device environment 90 by mechanisms such as abrasive wear, which occurs when there is relative motion between contacting surfaces.
[0051] The present disclosure refers to three main periods during the operation of the lithographic apparatus: (1) loading of the patterning device MA onto the support structure MT (referred to as a “loading” period or a “load” period); (2) exposure of the patterning device MA to expose the substrate W (referred to as a “production” period); and (3) unloading of the patterning device from the support structure MT (referred to as an “unloading” period or an “unload” period). The term “period” is used to define specific portions of time during the operation of the lithographic apparatus. The term “period” may be used interchangeably with, for example, “stage” and “phase”.
[0052] The loading of the patterning device MA onto the support structure MT may be performed before a series of lithographic operations are performed by the lithographic apparatus. The loading of the patterning device MA onto the support structure may be such that the patterning device MA is in a position within the lithographic apparatus where it is able to receive the beam of EUV radiation and impart a pattern thereto. The unloading of the patterning device MA from the support structure MT may be performed after the series of lithographic operations have been completed. Additionally, unloading of a patterning device MA from the support structure MT and loading of another patterning device MA onto the support structure MT may be performed during the series of lithographic operations performed by the lithographic apparatus, e.g. to change the patterning device MA which receives the beam of EUV radiation.
[0053] Throughout the production period, the patterning device MA is in contact with (e.g. supported by or clamped to) the support structure MT.
[0054] The loading period may comprise a first portion in which the patterning device MA is not in contact with (e.g. is separated from or spaced from) the support structure MT, and a second portion in which the patterning device MA is in contact with (e.g. supported by, e.g. clamped to) the support structure MT. During the loading period, the patterning device MA may come into contact with the support structure MT. That is, the first portion of the loading period, in which the patterning device MA is not in contact with the support structure MT, may come before the second portion of the loading period, in which the patterning device MA is in contact with the support structure MT. During the portion of the loading period in which the patterning device MA is not in contact with the support structure MT, the patterning device MA may be moved towards support structure MT, i.e. moved such that a vertical distance between the patterning device MA and the support structure MT decreases.
[0055] The unloading period may comprise a first portion in which the patterning device MA is in contact with (e.g. supported by, e.g. clamped to) the support structure MT, and a second portion in which the patterning device MA is not in contact with (e.g. is separated from or spaced from) the support structure MT. During the unloading period, the patterning device may break contact with the support structure MT. That is, the first portion of the unloading period, in which the patterning device MA is in contact with the support structure MT, may come before the second portion of the unloading period, in which the patterning device MA is not in contact with the support structure MT. During the portion of the unloading period in which the patterning device MA is not in contact with the support structure MT, the patterning device MA may be moved away from the support structure, i.e. moved such that a vertical distance between the patterning device MA and the support structure MT increases.
[0056] In the present disclosure, a vertical direction is a direction such that, when the patterning device MA is supported by the support structure MT, the patterning device MA is below the support structure MT in the vertical direction. The terms “radially outwards” and “radially inwards” are used in relation to the center of the patterning device MA, with the radial direction being perpendicular to the vertical direction.
Bias Voltage and Grounding
[0057] The patterning device support system may be configured to apply a bias voltage to the patterning device MA. More specifically, the patterning device support system may be configured to apply a bias voltage to the conductive coating which forms the patterning surface 40 of the patterning device MA and/or to the conductive coating which forms the clamping surface 41 of the patterning device MA. Additionally or alternatively, the patterning device support system may be configured to electrically connect the patterning device to the ground. More specifically, the patterning device support system may be configured to electrically connect the conductive coating which forms the patterning surface 40 of the patterning device MA to the ground and/or to connect the conductive coating which forms the clamping surface 41 of the patterning device MA to the ground. This will be described in more detail below.
[0058] Figure 3 depicts a patterning device support system which is configured to apply a bias voltage to the clamping surface 41 and the patterning surface 40. The patterning device support system may be configured to apply the bias voltage to the clamping surface 41 and the patterning surface 40 from a voltage source 61. The voltage source 61 is an example of a conductive member voltage source.
[0059] Figure 3 shows that, when the patterning device MA is clamped to the support structure MT, the clamping surface 41 is in contact with distal ends of the plurality of burls 70. It is not necessary for each of the plurality of burls to be in contact with the clamping surface 41. In general, distal ends of one or more of the plurality of burls 70 may be in contact with the clamping surface 41 of the patterning device MA.
[0060] The patterning device support system depicted in Figure 3 may be configured to apply a bias voltage to the patterning surface 40 via one or more of the plurality of burls 70. The voltage source 61 (which may be referred to as a conductive member voltage source) may be configured to apply a voltage (which may be referred to as a conductive member voltage) to the one or more of the plurality of burls via the support surface 42. The one or more of the plurality of burls 70 may be electrically connected to the clamping surface 41 of the patterning device MA. Thus, the application of the conductive member voltage to the one or more burls 70 may cause a bias voltage to be applied to the clamping surface 41 of the patterning device. The clamping surface 41 may be electrically connected to the patterning surface 40. Thus, applying the bias voltage to the clamping surface 41 may mean that the bias voltage is also applied to the patterning surface 40.
[0061] In general, the electrical connection between the voltage source 61 and the patterning surface 40 may comprise the support surface 42 of the support structure MT being electrically connected to the voltage source 61, the one or more of the plurality of burls 70 being electrically connected to the support surface 42 of the support structure MT, the one or more of the plurality of burls 70 being electrically connected to the clamping surface 41 of the patterning device MA, and the patterning surface 40 being connected to the clamping surface 41. That is, the bias voltage can be applied to the patterning surface 40 via the support surface 42 of the support structure MT, the one or more of the plurality of burls 70, the clamping surface 41 of the patterning device MA and an electrical connection between the clamping surface 41 and the patterning surface 40.
[0062] The electrical connection between the patterning surface 40 and the clamping surface 41 may be via a path integral to the patterning device MA itself. Alternatively, the electrical connection between the patterning surface 40 and the clamping surface 41 may be via an external path, such as a wire, as is shown in Figure 3.
[0063] It is not necessary for each of the plurality of burls 70 to be electrically connected to the support surface 42. In general, one or more of the plurality of burls 70 may be involved in the electrical connection between the patterning surface 40 and the voltage source 61. The burls 70 that are involved in electrical connection between the patterning surface 40 and the voltage source 61 are examples of conductive members.
[0064] In some embodiments, one or more of the plurality of burls 70 may be configured to electrically connect the clamping surface 41 to the ground. This may allow the clamping surface 41 and the patterning surface 40 to be discharged.
[0065] In this context, “ground” refers to an electric charge sink which is able to absorb a very large amount of electric charge relative to the amount of charge that may be built up on the patterning device MA during operation of the lithographic apparatus. The ground may be provided by, for example, the voltage source 61 or the frame of the lithographic apparatus.
[0066] In some embodiments, the one or more of the burls 70 which electrically connect the clamping surface 41 to the ground may be the same as the one or more of the burls 70 which electrically connect the clamping surface to the voltage source 61. In this case, the voltage source 61 may provide the ground. Alternatively, as depicted in Figure 4, the patterning device support system may comprise a mode-changing switch 65. The mode-changing switch 65 may be configured such that the one or more of the burls 70 can be selectively connected to the voltage source 61 or a ground 67 other than the voltage source 61 (e.g. the frame of the lithographic apparatus). The mode-changing switch 65 may be any suitable switching device, e.g. a transistor or a switch implemented in software. [0067] In some embodiments, the one or more burls 70 which electrically connect the clamping surface 41 to the ground 67 may be different to the one or more of the burls 70 which electrically connect the clamping surface to the voltage source 61.
[0068] In some embodiments, a bias voltage may be applied to the clamping surface 41 and/or the patterning surface 40 through means other than via one or more of the plurality of burls 70. For example, in some embodiments, the patterning device support system comprises a conductive member which is electrically connected to a voltage source and configured to contact the patterning surface 40 of the patterning device MA so that a bias voltage can be applied thereto. Such a conductive member is described in EP 22195470.4 filed on 13 September 2022, the entirety of which is hereby incorporated by reference.
[0069] In other embodiments, the patterning device support system is configured to capacitively induce a bias voltage in the patterning surface 40 using the clamping means in the support structure. In such embodiments, the support structure MT is an electrostatic clamp comprising a plurality of electrodes. When a voltage is applied to the electrodes in the clamp, an electrostatic clamping force is exerted on the clamping surface 41 of the patterning device. The average voltage of the plurality of electrodes in the electrostatic clamp can be controlled. When the average potential of the plurality of electrodes is not 0 V, an electric field may be present between the electrostatic clamp and grounded components in the lithographic apparatus, e.g. patterning device masking blades. The patterning device MA may be disposed between the electrostatic clamp and the grounded components, and so within this electric field. Consequently, a bias voltage may be capacitively induced in the patterning surface 40 and the clamping surface 41 (or, more specifically, in conductive coatings which form the patterning surface 40 and the clamping surface 41). Capacitive induction of a bias voltage in the patterning surface 40 and/or clamping surface 41 of a patterning device MA is described in EP 23168046.3 filed on 14 April 2023, the entirety of which is hereby incorporated by reference .
[0070] In embodiments in which patterning device support system is configured to apply the bias voltage to the patterning device MA through means other than via one or more of the plurality of burls 70, the support structure may still comprise one or more burls configured to electrically connect the clamping surface 41 to the ground 67.
[0071] The patterning device support system may comprise one or more current-limiting components 62, 66 between the patterning surface 40 and the voltage source 61 and/or between the patterning surface 40 and the ground 67. The current-limiting components may be, or may perform the functionality of, one or more of a resistor, an inductor and a diode. The current-limiting components may be provided to maintain a current within the patterning surface 40 within acceptable levels. Such current-limiting components are described in EP 23176443.2 filed on 31 May 2023, the entirety of which is hereby incorporated by reference.
Configuration of the burls [0072] The one or more burls which are configured to connect the clamping surface 41 to the voltage source 61 and/or the ground 67 may be referred to as conductive burls. The conductive burls may be coated with a conductive material. The conductive burls are examples of conductive members.
[0073] It may be preferable for the conductive burls to form only a small proportion of the total number burls 70 of the support structure MT. For example, conductive burls may make up less than 10%, preferably less than 5% and further preferably less than 1% of the total burls 70 on the support structure MT. This may allow the clamping surface to be effectively discharged and/or brought to a bias voltage, without compromising the clamping of the patterning device MA to the support structure MT.
[0074] Figure 5 depicts a support surface 42 of a support structure MT. The support structure MT depicted in Figure 5 is rectangular, but a support structure MT in accordance with the present disclosure may be any suitable shape. A portion of the plurality of burls 70 on the support structure MT are conductive burls (e.g. conductive burls 70a, 70b, 70c).
[0075] The support structure MT further comprises a conductive track 68 formed on the support surface 42. The conductive track 68 may be formed around a perimeter of the support surface 42. The conductive track 68 may be formed outward of the burls 70. The conductive burls 70a, 70b, 70c may be electrically connected to the conductive track 68. Specifically, the conductive coating of the conductive burls 70a, 70b, 70c may be connected to the conductive track.
[0076] The conductive burls 70a, 70b, 70c may be located in a border region of the support surface 42 of the support structure. Conductive burls 70a, 70b, 70c may be in an outermost ring of burls 70. The conductive burls 70a, 70b, 70c may be located in one or more corners of the support surface 42 of the support structure MT. Disposing the conductive burls 70a, 70b, 70c in such locations may reduce the effect that the grounding of the burls 70 has on the clamping of the patterning device MA to the support structure MT, or limit the regions in which the clamping of the patterning device MA to the support structure MT is affected to regions which are not critical to the quality of the image projected from the patterning device MA.
[0077] The conductive track 68 may be connected to an interface 69. The interface 69 may allow the conductive track 68 to be connected to external circuitry. The external circuitry may connect the interface 69 to the ground 67 and/or the voltage source 61.
[0078] The conductive material which forms the conductive track may be the same material that coats the conductive burls 70a, 70b, 70c. The conductive burls 70a, 70b, 70c may be electrically connected to the conductive track 68 via one or more extensions of the conductive track (e.g. extensions 68a, 68b, 68c). As depicted in Figure 5, each conductive burl 70a, 70b, 70c is provided with an extension 68a, 68b, 68c. The extensions 68a, 68b, 68c may extend inward from the conductive track 68 to the conductive burls 70a, 70b, 70c. In some embodiments, one extension 68a, 68b, 68c may connect a plurality of conductive burls 70a, 70b, 70c to the conductive track 68.
[0079] The conductive track 68 and/or the coating on the conductive burls 70a, 70b, 70c may be formed of any suitable conductive material. For example, the conductive track 68 and/or the coating on the conductive burls 70a, 70b, 70c may be formed of titanium nitride (TiN). The conductive material may be deposited onto the support surface 42 using any suitable technique. After deposition of the conductive material, the conductive material may be patterned to form the shape of the conductive track 68, the extension 68a, 68b, 68c and the coatings for the burls 70.
The Production Period
[0080] During the production period, the unpatterned beam 21 is incident on 40 patterning surface 40 of the patterning device MA. This causes the release of electrons from the patterning surface 40 by the photoelectric effect.
[0081] Figure 6 depicts a patterning device MA in which the patterning surface 40 is substantially electrically isolated. The patterning device depicted in Figure 6 is a comparative example. In cases where the patterning surface is substantially electrically isolated, the patterning surface 40 may become positively charged as a result of the release of electrons from the patterning surface 40.
[0082] The EUV radiation within the patterning device environment 90 may causes contaminant particles P to become negatively charged. This may occur as a result of at least two main mechanisms. A first mechanism is a result of the formation of plasma from the gas molecules within the patterning device environment 90, which are excited by the EUV radiation. Free electrons within the plasma may be absorbed by the contaminant particles P, resulting in those particles becoming negatively charged. A second mechanism is a consequence of the photoelectric effect which causes the patterning surface 40 to become positively charged. Specifically, electrons that have been ejected from the patterning surface 40 as a result of the photoelectric effect may be absorbed by the contaminant particles P, causing them to become negatively charged.
[0083] As a result of the patterning surface 40 becoming positively charged and the contaminant particles P becoming negatively charged, an attractive electrostatic force is exerted between the patterning surface 40 and the contaminant particles P. This causes the contaminant particles P to accelerate towards the patterning surface 40. Consequently, it is likely that contaminant particles within the lithographic apparatus will be deposited onto the patterning surface 40.
[0084] During the production period, the EUV radiation may be typically generated in pulses. Thus, the patterning surface 40 may be cyclically exposed to EUV radiation. That is, during the production period, there are portions of time when EUV radiation is generated, and portions of time when EUV radiation is not generated. In the portions of time when the EUV radiation is not generated, the patterning surface 40 may be discharged, i.e., the magnitude of the positive charge on the patterning surface 40 may decrease. This may be such that the patterning surface 40 becomes approximately neutral. The discharging of the patterning surface 40 may be caused by the plasma that is formed within the patterning device environment 90 from the gas particles excited by the EUV radiation. Specifically, electrons within the plasma may be attracted to the patterning surface 40, where they are absorbed by positive ions on the patterning surface 40. [0085] Pulses of EUV radiation are typically generated at a rapid frequency. This frequency may be, for example, approximately 50 kHz, approximately 60 kHz, or approximately 100 kHz. This means that, during the production period, a patterning surface 40 may cycle between being positively charged and being approximately neutral at a high frequency.
[0086] To prevent contaminant particles P accelerating towards the patterning surface 40 as a result of electrostatic attraction, the patterning device support system may be configured to apply a productionbias voltage to the patterning surface 40 of the patterning device during the production period. The production-bias voltage may be applied to the patterning surface 40 as described above. For instance, the production-bias voltage may be applied to the patterning surface using the arrangement depicted in Figures 3 and 4. The production-bias voltage may also be applied to the clamping surface 41.
[0087] The production-bias voltage may be negative. By applying a negative production-bias voltage to the patterning surface 40 during the production period, negatively charged contaminant particles P within the patterning device environment 90 are repelled from the patterning surface 40. The magnitude of the production-bias voltage may be greater than 0.5 V and preferably greater than 1 V. A voltage of this magnitude may be necessary to ensure that the distance between the patterning surface 40 and contaminant particles P increases over time (i.e. to ensure that contaminant particles P are accelerated away from the patterning surface 40). The magnitude of the production-bias voltage may be less than 10 V, preferably less than 5 V and further preferably less than 3 V. Voltages in excess of these values may result in an excessively large current being drawn through the patterning surface 40. This can cause the patterning surface 40 to heat up and deform, which can reduce the quality of the pattern projected from the patterning surface 40 to the substrate W.
[0088] Figures 7A and 7B depict plots of voltage of the patterning surface 40 against time. Figure 7A shows voltage of the patterning surface 40 where the patterning surface 40 is substantially electrically isolated during the EUV lithography process. Figure 7B shows voltage of the patterning surface 40 where the production-bias voltage is applied to the patterning surface 40. The plot in Figure 7B does not relate directly to a specific method by which the production-bias voltage is applied.
[0089] Figures 8A and 8B depict plots of the displacement of a contaminant particle P relative to the patterning surface 40 over time. Figure 8A shows the displacement of a contaminant particle P relative to the patterning surface 40 where the patterning surface 40 is substantially electrically isolated. Figure 8B shows the displacement of a contaminant particle P where voltage biasing of the patterning surface is applied. The plot in Figure 8B does not relate directly to a specific method by which the bias voltage is applied to the patterning surface 40. The circumstances underlying the plot depicted in Figure 7A correspond to the circumstances underlying the plot depicted in Figure 8A, and the circumstances underlying the plot depicted in Figure 7B correspond to the circumstances underlying the plot depicted in Figure 8B. For Figures 7B and 8B, the production-bias voltage applied to the patterning surface 40 is a constant bias voltage of approximately -I V. [0090] Figure 7A shows that, without the application of a production-bias voltage, the voltage of the patterning surface 40 begins at approximately 0 V. At t=ti, a pulse of EUV radiation is generated by the lithographic apparatus, which causes the voltage of the patterning surface 40 to rapidly increase, reaching a maximum at the point that the pulse of EUV radiation terminates. After this point, the voltage of the patterning surface 40 decreases as the patterning surface discharges, reaching approximately 0 V by t=ts- This process is repeated when the next pulse of EUV radiation is generated at t=t3-
[0091] In Figure 8A, the contaminant particle P is initially at rest in the direction perpendicular to the surface 40. When the pulse of EUV radiation is initiated (t=ti), the contaminant particle P begins to accelerate towards the patterning surface 40. As the patterning surface 40 discharges, the magnitude of this acceleration decreases. Once the charge of the patterning surface 40 has returned to approximately 0V (t=t2), the contaminant particle P no longer accelerates towards the patterning surface 4, but continues to travel towards the patterning surface 40 at a constant velocity. As the second pulse of EUV radiation is initiated (at t=t?), the contaminant particle P again begins to accelerate towards the patterning surface 40. Before the next pulse of EUV radiation is generated, the displacement between the contaminant particle P and the patterning surface 40 becomes zero, i.e., the contaminant particle 40 is deposited into the patterning surface 40.
[0092] Figures 7A and 8A only depict two pulses of EUV radiation. After these two pulses, the contaminant particle P is deposited onto the patterning surface 40. In practice, many more pulses of EUV radiation may be required to sufficiently accelerate the contaminant particle P such that it travels the distance between its initial position and the patterning surface 40. However, because the frequency of the pulses of EUV radiation in a typical lithographic apparatus is high, the effect of the EUV radiation on the path of the particle over time is large, even if the magnitude of the acceleration of the contaminant particle P towards the patterning surface 40 is relatively low.
[0093] In Figure 7B, the production-bias voltage applied to the patterning surface 40 is -1 V. Therefore, before the initiation of the first pulse of EUV radiation, the voltage of the patterning surface 40 is approximately -1 V. When the pulse of EUV radiation is initiated (at t=ti), as in the case where there is no bias voltage applied, the voltage of the patterning surface 40 increases. Figure 7B shows that this is such that the voltage of the patterning surface 40 becomes greater than 0 V (i.e., the patterning surface 40 becomes positively charged). However, this is not necessarily the case, and the magnitude of the negative production-bias voltage applied to the patterning surface 40 may be sufficient for the voltage of the patterning surface 40 to remain below 0 V (i.e., for the patterning surface 40 to remain negatively charged) throughout the duration of each pulse of EUV radiation. The voltage of the patterning surface 40 reaches a maximum at the termination of the pulse of EUV radiation. After the pulse of EUV radiation, the patterning surface 40 is discharged, such that the voltage of the patterning surface 40 becomes the same as the production-bias voltage applied to the patterning surface (at t = tz) . As in the case where no production-bias voltage is applied to the patterning surface 40, discharging is caused by the plasma within the patterning device environment 90. However, discharging is also caused by the application of the negative production-bias voltage. As a result of this, the rate of discharge of the patterning surface 40 is faster. Consequently, the length of time ti to t2 (and t; to ) is smaller in the case that a negative production-bias voltage is applied to the patterning surface 40 than in the case where a negative production-bias voltage is not applied to the patterning surface.
[0094] In Figure 8B, which depicts the position of a contaminant particle P relative to the patterning surface 40 over time for the case where a production-bias voltage is applied to the patterning surface 40, the contaminant particle P is initially at rest in the direction perpendicular to the patterning surface 40. At this point, the voltage of the patterning surface 40 is approximately -I V, which means that the contaminant particle P is repelled by the patterning surface 40, and accelerates away from the patterning surface 40. As the pulse of EUV radiation is initiated (at t=t i) and the voltage of the patterning surface 40 becomes positive, the particle accelerates towards the patterning surface 40. In Figure 8B, this is such that the direction of travel of the contaminant particle P is reversed, and the particle briefly travels towards the patterning surface 40. However, this may not be the case, and the acceleration of the contaminant particle P towards the patterning surface 40 may only be such that the contaminant particle P continues to travel away from the patterning surface 40, but with a decreasing velocity. In a case where the pulse of EUV radiation does not cause the voltage of the patterning surface 40 to become positive, the contaminant particle will not accelerate towards the patterning surface 40 at all. In this case, the contaminant particle P would continue to accelerate away from the patterning surface 40, but the magnitude of this acceleration would be temporarily reduced.
[0095] Returning to the scenario shown in Figure 8B, as the pulse of EUV radiation terminates and the voltage of the patterning surface 40 becomes negative, the contaminant particle P once again accelerates away from the patterning surface 40. This process is repeated when the second pulse of EUV radiation is initiated.
[0096] The time in which the contaminant particle 40 accelerates towards the patterning surface 40 may be sufficiently small such that, over time, the distance between the contaminant particle P and the patterning surface 40 increases. Consequently, the contaminant particle P is not deposited onto the patterning surface 40.
[0097] The embodiments described above have referred to the application of a negative productionbias voltage to the patterning surface 40, so that negatively charged contaminant particles P are repelled from the patterning surface 40. However, there may be circumstances which cause contaminant particles within the patterning device environment to become positively charged. In this case, a positive production-bias voltage may be applied to the patterning surface 40, such that the positively charged contaminant particles P are repelled by the positively charged patterning surface 40.
[0098] In some embodiments, the production-bias voltage may be varied over the course of the production period. For instance, the production-bias voltage may alternate between being positive during EUV pulses (i.e. while EUV radiation is generated) and negative between EUV pulses (i.e. while the EUV radiation is not generated.
[0099] As explained above, EUV-induced emission of electrons through the photoelectric effect from the patterning surface 40 contributes to the deposition of contaminant particles P on the patterning surface (and therefore imaging errors). This is because: (i) the emission of electrons causes the patterning surface 40 to become positively charged (and be brought to a positive potential), thus attracting negatively charged contaminant particles; and (ii) the emission of electrons introduces additional electrons to the plasma within the patterning device environment 90, which may increase the number of contaminant particles that become negatively charged or the magnitudes of the negative charges on the contaminant particles P. Consequently, by reducing or preventing the emission of electrons whilst the patterning surface 40 is exposed to EUV radiation, fewer contaminant particles P may be deposited on the patterning surface 40.
[0100] By inducing a positive production-bias voltage in the patterning surface 40 whilst the patterning surface 40 is exposed to EUV radiation, the emission of electrons from the patterning surface can be reduced. Consequently, the patterning surface 40 may become positively charged to a lesser extent, and may contribute less electrons to the plasma in the patterning device environment 90.
[0101] The magnitude of the positive production-bias potential applied to the patterning surface 40 may be sufficient to prevent the emission of electrons by the photoelectric effect. That is, the magnitude of the positive production-bias may be such that the positive potential induced at the patterning surface 40 is greater than a stopping potential (Vstop). The maximum kinetic energy of an electron emitted though photoemission is given by Equation (2), where h is the Planck constant (4.14 x 10 15 eVs),/is the frequency of the radiation, and cp is the work function of the material (i.e., the minimum energy required to cause emission of an electron from a surface). The work function is a property of the material of the surface from which electrons are emitted. kmax f <p (2)
[0102] Photoemission cannot occur when the energy supplied to the electrons by an electric field arising from the positive potential induced at the patterning surface 40 is greater than the maximum possible kinetic energy of the emitted electrons. Thus, the stopping potential can be defined as in Equation (3). stOp f (p (3)
[0103] In EUV lithography, the wavelength of radiation may be approximately 13.5 nm. Thus, the photon energy of a photon within a beam of EUV radiation may be approximately 92 eV. The work function of the patterning surface 40 may be dependent on the material from which the patterning surface 40 is formed. In general, the work function may be between 2 eV and 7 eV. If the work function is 7 eV or less, it may be preferable for the potential induced on the patterning surface 40 to be approximately 85 V or greater to substantially suppress photoemission. If the work function is 2 eV or less, it may be preferable for the potential induced on the patterning surface 40 to be approximately 90 V or greater to substantially suppress photoemission. A majority of electons released from the patterning surface 40 upon irradiation with EUV radiation generally have a low energy, e.g., an energy that is less than 10 eV. This may be because EUV photons are absorbed at effective depth of approximately 10 to 100 nm. As electrons which have absorbed an EUV photon propagate to the vacuum interface from the absorption position to the surface, they may lose energy. Considering this, to significantly suppress the emission of electrons through the photoelectric effect, it may be sufficient to apply a positive production-bias voltage which is greater than + 50V. In this case, the positive production-bias voltage may be less than 100 V to reduce the risk of discharge. To moderately suppress the emission of electrons through the photoelectric effect, it may be sufficient to apply a positive production-bias that is greater than 5 V. In this case, the positive production-bias voltage may be less than 50 V to further reduce the extent to which the positive production-bias voltage applied to the patterning surface leads to physically sputtering of ions onto grounded surfaces, such as the masking blades. Considering this, it may be preferable for the positive production-bias voltage to be greater than 5 V and less than 50 V.
[0104] To apply a negative production-bias voltage to the patterning surface 40 when the EUV pulse is off, and a positive production-bias voltage may be provided to the patterning surface 40 when the EUV pulse is on, the patterning device voltage production-biasing system may be synchronized with the pulses of EUV radiation generated by the lithographic apparatus. The means by which the polarity of the production-bias voltage is switched is not particularly limited.
[0105] In some embodiments, a timing switch may be provided between the patterning surface 40 and the voltage source 61 and/or between the patterning surface 40 and the ground 67. The timing switch may be configured such that a production-bias voltage is applied to the patterning surface 40 between pulses of EUV radiation, but not during pulses of EUV radiation. The timing switch may prevent the current within the patterning surface exceeding an acceptable limit while the EUV radiation is incident on the patterning surface. The timing switch may be switched on and off (i.e. opened and closed) at a frequency of greater than 49 kHz, preferably greater than 59 kHz, and further preferably greater than 99 kHz. The frequency at which the switch is switched on and off may be synchronized with a frequency of generation of the pulses of EUV radiation. The timing switch may be any suitable switching device, e.g. a transistor a switch implemented in software. The timing switch is described in detail in EP 22195470.4 filed on 13 September 2022, the entirety of which is hereby incorporated by reference.
The Loading Period [0106] As has been described above, during the loading period, the patterning device MA is loaded onto the support structure MT. That is, the patterning device MA is taken from, for example, a storage area, and moved such that it is brought into contact with the support structure. Once the patterning device MA is in contact with the support structure, clamping means within the support structure MT may be activated, such that the patterning device MA becomes clamped to the support structure MT. In the case that the support structure MT comprises an electrostatic clamp, the clamping means may comprise a plurality of electrodes, which are described in further detail below.
[0107] During the loading period, the clamping surface 41 of the patterning device comes into contact with one or more conductive members. As has been described above, the one or more conductive members may comprise conductive burls. The conductive burls may be configured to apply a bias voltage to the clamping surface 41 and/or the patterning surface 40, e.g. a production-bias voltage to the clamping surface 431 and/or the patterning surface 40 during the production period. Additionally or alternatively, the conductive burls may be configured to electrically connect the clamping surface 41 and/or the patterning surface 40 to the ground 67.
[0108] In some cases, a patterning device support system may comprise conductive members in addition to, or instead of, the conductive burls. For instance, the patterning device support system may comprise a voltage biasing member. The voltage biasing member may be configured to apply a bias voltage to the clamping surface 41 and/or the patterning surface 40 by contacting the clamping surface 41 when the patterning device MA is clamped to the support structure MT. Additionally or alternatively, the voltage biasing member may be configured to electrically connect the clamping surface 41 and/or the patterning surface 40 to the ground when the patterning device MA is clamped to the support structure MT. Unlike the conductive burls, the voltage biasing member may not be configured to support the patterning device MA. In general, the conductive members may comprise any conductive component or feature which comes into contact with the clamping surface 41 of the patterning device MA when the patterning device is clamped to the support structure MT. By way of example only, the present disclosure focuses on a patterning device support system in which the conductive members comprise conductive burls.
[0109] Figure 9 depicts a patterning device support system during the loading period. The arrow shown in Figure 9 demonstrate the direction in which the patterning device MA is moved during the loading period. The patterning device MA comprises a conductive coating 41C which forms the clamping surface 41. The patterning device MA comprises a conductive coating 40C which forms the patterning surface 40. As depicted in Figure 9, the conductive coatings 40C, 41C are substantially electrically isolated from one another. However, in some cases, the conductive coatings 40C, 41C may be electrically connected, as has been described above.
[0110] In the patterning device support system depicted in Figure 9, the support structure MT is an electrostatic clamp. The electrostatic clamp comprises a plurality of electrodes 45a-d embedded therein. Each electrode 45a-d may be electrically connected to a voltage source 46a-d. Alternatively, each electrode 45a-d may be electrically connected to a common voltage source. The patterning device support system may be configured such that a different voltage can be applied to each of the plurality of electrodes 45a-d. The support structure MT may be configured such that, when a clamping voltage is applied to each of the plurality of electrodes 45a-d, an electrostatic clamping force is exerted on the conductive coating 41C which forms the clamping surface 41.
[0111] The electrostatic clamp may be configured such that a positive clamping voltage is applied to some of the electrodes 45a-d, while a negative clamping voltage is applied to other electrodes 45a-d. For example, a positive clamping voltage may be applied to the electrodes 45a, 45d, and a negative clamping voltage may be applied to the electrodes 45b, 45c. In this case, if the magnitude of the clamping voltages applied to each of the electrodes 45a-d is substantially the same, and each of the electrodes 45a-d are of substantially the same size, an average voltage of the plurality of electrodes may be approximately zero. By varying the magnitude of the clamping voltages applied to each of the plurality of electrodes 45a-d, the average voltage of the plurality of electrodes can be changed (i.e. such that the average potential of the plurality of electrodes 45a-d is positive or negative).
[0112] As shown in Figure 9, the support structure MT comprises a plurality of burls 70 on which the patterning device MA is supported when the clamping means of the support structure is activated (i.e. when clamping voltages are applied to the plurality of electrodes 45a-d). Of the plurality of burls 70, a portion are conductive burls 70a. the conductive burls 70a are conductive by virtue of being coated with a conductive coating 68. As depicted in Figure 9, during the loading period, the conductive burls 70a may be electrically connected to the ground (e.g. the frame of the lithographic apparatus, or a power supply set to 0 V).
[0113] During the loading period, before the clamping means of the support structure MT is activated (i.e. before the clamping voltages are applied to the electrodes 45a-d in the electrostatic clamp), the patterning device MA may be supported by a patterning device handler 80. The patterning device handler 80 may be configured to move the patterning device MA towards the support structure during the loading period, and to move the patterning device MA away from the support structure MT during the unloading period. The support structure 80 may comprise one or more supporting members 81a, 81b configured to contact the patterning surface 40 of the patterning device MA.
[0114] During the loading period, as the patterning device MA is moved towards the support structure MT, a distance between the clamping surface and the conductive burls 70a decreases. If there exists a potential difference between the conductive burls 70a and the conductive coating 41C which forms the clamping surface 41, there is a risk that electrostatic discharge may occur between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a. Electrostatic discharge may cause degradation and/or wear to the conductive burls 70a and/or to the clamping surface 41. This may lead to a deterioration in the flatness of a support plane formed by the burls 70 of the support structure MT, and/or a deterioration in the flatness of the clamping surface 41. This may lead to an increase (i.e. worsening) in defects in the pattern that is imparted to the beam of radiation by the patterning surface 40, and/or an increase (i.e. worsening) of overlay error. The degradation and/or wear to the conductive burls 70a and/or to the clamping surface 41 may also reduce the lifetime of the patterning device MA and/or the support structure MT.
[0115] A potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may arise as a result of a number of mechanisms, some of which are described below. As will be understood, an overall potential difference between conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may be the result of a combination of the mechanisms described below.
[0116] During the loading period, before the clamping surface 41 is in contact with the conductive burls 70a, the conductive coating 41C which forms the clamping surface 41 may be substantially electrically isolated. As described above, in some cases, the conductive coating 41C which forms the clamping surface may be electrically connected to the conductive coating 40C which forms the patterning surface 40. In this case, the conductive coating 41C which forms the clamping surface 41 and the conductive coating 40C which forms the patterning surface 40 may, collectively, be substantially electrically isolated. Consequently, the conductive coating which forms the clamping surface 41 may be charged during the loading period. The conductive coating 41C which forms the clamping surface 41 may be charged in the loading period as a result of becoming charged before the loading period (e.g. during transportation or storage). Additionally or alternatively, the conductive coating 41C which forms the clamping surface 41 may become charged during the loading period, e.g. by charged particles being deposited thereon.
[0117] As depicted in Figure 9, the conductive burls 70a are electrically connected to ground. In cases where the conductive coating 41C which forms the clamping surface 41 is charged, and the conductive burls 70a are electrically connected to ground, a potential difference may exist between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a. Consequently, as the clamping surface 41 approaches the conductive burls 70a, electrostatic discharge may occur.
[0118] In some examples, the conductive burls 70a may be substantially electrically isolated during the loading period. In this case, the conductive burls 70a may become charged during the loading period. If the conductive burls 70a and the conductive coating 41C which forms the clamping surface 41 are charged, there may exist a potential difference between the conductive burls 70a and the conductive coating which forms the clamping surface 41, and thus a risk of electrostatic discharge therebetween. If the conductive burls 70a is charged but the conductive coating 41C which forms the clamping surface 41 is not charged, there may be a potential difference between the conductive burls 70a and the conductive coating 41C which forms the clamping surface 41 , and thus a risk of electrostatic discharge therebetween.
[0119] In some cases, a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may be established without charging thereof. This may occur when the average potential of the plurality of electrodes 45a-d in the electrostatic clamp is not equal to zero. The average voltage of the plurality of electrodes 45a-d in the electrostatic clamp may not be equal to zero during the loading period as a result of errors and inaccuracies in the clamping voltage source(s) 46a-d. That is, during the loading period, the clamping voltage source(s) 46a-d may be controlled to apply voltages to the plurality of electrodes 45a-d such that the average voltage of the plurality of electrodes 45a-d is zero. However, the actual average voltage of the plurality of electrodes 45a-d may not be zero.
[0120] The patterning device handler 80 may be grounded (but electrically isolated from the patterning device MA). Consequently, when the average voltage of the plurality of electrodes 45a-d of the support structure MT is not equal to zero, there may be an electric field between the support structure MT and the patterning device handler 80. As a result of this electric field, a voltage may be capacitively induced in the conductive coating 41C which forms the clamping surface 41. Where a voltage has been capacitively induced in the conductive coating 41C which forms the clamping surface 41 and the conductive burls are grounded, there exists a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a. Thus, there is a risk of electrostatic discharge between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a.
[0121] One aspect of the present disclosure is directed to reducing the risk of electrostatic discharge between the conductive burls 70a and the conductive coating 41C which forms the clamping surface 41 during the loading period. This is achieved by reducing the potential difference between the conductive coating 41 C which forms the clamping surface 41 and the conductive burls 70a as the clamping surface 41 approaches the support structure MT. As will be appreciated, the techniques described in the present disclosure in relation to reducing the risk of electrostatic discharge between the conductive burls 70a and the conductive coating 41C may be used irrespective of the exact mechanism by which a potential difference between the conductive burls 70a and the clamping surface 41 is established.
[0122] An embodiment of a patterning device support system during the loading period, in accordance with the present disclosure, is depicted in Figure 10. The patterning device support system depicted in Figure 10 may be substantially the same as the patterning device support system depicted in Figure 9, except as described below.
[0123] During the loading period, the conductive burls 70a are electrically connected to a voltage source 61. The voltage source 61 may be referred to as a conductive member voltage source 61. This is because it is a voltage source which applies a voltage to the conductive members (i.e. the conductive burls 70a). In general, the voltage applied to the conductive burls 70a by the conductive member voltage source 61 may be referred to as a conductive member voltage. The voltage applied to the conductive burls 70a by the conductive member voltage source 61 during the loading period may be referred to as a load-conductive member voltage.
[0124] The conductive member voltage source 61 may be the same as the voltage source 61 which is configured to apply the production-bias voltage to the patterning surface 40 during the production period. Similarly, the conductive burls 70a may be the same conductive burls that are used to apply the production-bias voltage to the patterning surface 40 during the production period.
[0125] The load-conductive member voltage reduces a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a. The reduction of the potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may be relative to the case in which the conductive burls 70a are grounded.
[0126] To reduce the potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a, the load-conductive member voltage (relative to the ground) may be substantially the same as a voltage of the conductive coating 41C which forms the clamping surface 41. The voltage of the conductive coating 41C which forms the patterning surface may vary as the patterning device MA is moved towards the support structure MT. Thus, to effectively reduce the risk of electrostatic discharge, the load-conductive member voltage (relative to the ground) may be substantially the same as a voltage of the conductive coating 41C which forms the clamping surface 41 at the point at which the clamping surface 41 comes into contact with the conductive burls 70a.
[0127] It is not necessary for the load-conductive member voltage to be exactly the same as the voltage of the conductive coating 41C which forms the clamping surface 41. Any reduction in the potential difference between the conductive burls 70a and the conductive coating 41C which forms the clamping surface 41 may be beneficial in reducing the risk that electrostatic discharge occurs and/or reducing the damage caused by electrostatic discharge if it does occur. For example, the application of the load- conductive member voltage reduces the potential difference between the clamping surface and the one or more conductive members (e.g. at a time before the clamping surface 41 contacts the conductive burls 70a) to less than 50 V, preferably less than 10 V, preferably less than 1 V, and preferably less than 0.1 V
[0128] In some embodiments, the load-conductive member voltage is predetermined. That is, the load- conductive member voltage may be controlled based on known aspects of the operation of patterning device support system, e.g. the average voltage of the plurality of electrodes 45a-d in the support structure MT when the average voltage of the plurality of electrodes 45a-d in the support structure MT is intended to be (i.e. controlled to be) zero. In such embodiments, the load-conductive member voltage may not be actively controlled, i.e. there may not be a closed feedback loop.
[0129] As described above, in some cases, a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may be established where the average voltage of the plurality of electrodes 45a-d in the support structure MT is not equal to zero. This is because a voltage is capacitively induced in the conductive coating 41C which forms the clamping surface 41. In such cases, the voltage of the conductive coating 41C which forms the clamping surface 41 can be determined or otherwise estimated based on the average voltage of the plurality of electrodes 45a-d. Generally, the voltage capacitively induced in the conductive coating 41C which forms the clamping surface 41 may have the same polarity as the non-zero average voltage of the plurality of electrodes 45a-d. The magnitude of the voltage capacitively induced in the conductive coating 41C which forms the clamping surface 41 may be between 0 V (i.e. ground potential) and the average voltage of the plurality of electrodes 45a-d in the support structure MT.
[0130] In some embodiments, the load-conductive member voltage may be determined based on measurements made by the one or more sensors. In such embodiments, the load-conductive member voltage may be actively controlled, e.g. controlled using a closed feedback loop. Figure 10 depicts a number of such sensors, e.g. sensors 31 and 32a-d, which are described in further detail below. In some embodiments, the patterning device support system may comprise only one of the sensors, and the determination of the load-conductive member voltage may be based on measurements made by this one sensor. In other embodiments, the patterning device support system may comprise more than one of the sensors (e.g. more than one type/location of sensor), and the determination of the load-conductive member voltage may be based on measurements made by some or all of the sensors.
[0131] As depicted in Figure 10, the patterning device support system comprises a current sensor 31) configured to measure the current between the conductive burls 70a and the conductive member voltage source 61. The current sensor 31 is an example of a first sensor. The load-conductive member voltage may be determined based on the current measured by the current sensor 31.
[0132] In the absence of external electric fields (e.g. an electric field created by a non-zero average voltage of the plurality of electrodes 45a-d), if the clamping surface 41 is negatively charged, current may flow towards the conductive burls 70a from voltage source 61 as the patterning device is moved towards the support structure MT. Thus, if the current sensor 31 detects current flowing towards the conductive burls 70a, it may be determined that the voltage of the conductive coating 41C which forms the clamping surface 41 is negative. Consequently, a negative voltage may be applied to the conductive burls 70a (i.e. the load-conductive member voltage may be negative). This may decrease the potential difference between conductive coating 41C which forms the clamping surface and the conductive burls 70a, thus reducing the risk of electrostatic discharge.
[0133] As is also depicted in Figure 10, the patterning device support system may comprise one or more current sensors 32a-32d disposed between the electrodes 45a-d of the support structure MT and the clamping voltage source(s) 46a-d. The current sensors 32a-d are examples of second sensors. The load-conductive member voltage is determined based on the current measured by the second sensor.
[0134] In the absence of external electric fields (e.g. an electric field created by a non-zero average voltage of the plurality of electrodes 45a-d), if the clamping surface 41 is negatively charged, current may flow towards the electrodes 45a-d from voltage source 46a-d as the patterning device MA is moved towards the support structure MT. Thus, if the current sensors 32a-d detect current flowing towards the electrodes 45a-d, it may be determined that the voltage of the conductive coating 41 C which forms the clamping surface 41 is negative. Consequently, a negative voltage may be applied to the conductive burls 70a (i.e. the load-conductive member voltage may be negative). This may decrease the potential difference between conductive coating 41C which forms the clamping surface and the conductive burls 70a, thus reducing the risk of electrostatic discharge.
[0135] The patterning device support system may comprise a sensor disposed in the patterning device handler 80. A sensor disposed in the patterning device handler 80 is an example of a third sensor. The load-conductive member voltage may be determined based on measurements made by the third sensor. [0136] The sensor disposed in the patterning device handler 80 may be a voltage sensor configured to measure a voltage of the conductive coating 41 C which forms the clamping surface 41. The load- conductive member voltage may be controlled to be the same as the voltage of the conductive coating 41C as measured by the voltage sensor in the patterning device handler.
[0137] The sensor disposed in the patterning device handler 80 may be a strain gauge. The strain gauge may be configured to measure a strain of the patterning device MA. If there is a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a, a resulting electrostatic force between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may cause the patterning device MA to be deformed. This deformation may be measured by the strain gauge. Thus, the load-conductive member voltage may be determined on the basis of the deformation of the patterning device MA as measured by the strain gauge.
[0138] Additionally or alternatively, the sensor deposed in the patterning device handler 80 may be a force sensor configured to measure a contact force between the patterning device MA and the patterning device handler 80. If there is a potential difference between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a, a resulting electrostatic force between the conductive coating 41C which forms the clamping surface 41 and the conductive burls 70a may cause the contact force between the patterning device MA and the patterning device handler 80 to change (e.g. decrease). This change in the contact force between the patterning device MA and the patterning device handler 80 can be measured by the force sensor. Consequently, the load-conductive member voltage may be determined on the basis of the contact force between the patterning device MA and the patterning device handler 80 as measured by the strain gauge.
[0139] This description of the loading period has focused on the case where the potential difference between the conductive members (i.e. the conductive burls 70a) and the clamping surface 41 is reduced by applying a load-conductive member voltage to the conductive members (i.e. the conductive burls 70a). However, the potential difference between the conductive members (i.e. the conductive burls 70a) and the clamping surface 41 may also be reduced by applying a load-bias voltage to the clamping surface 41 as the patterning device MA approaches the support structure MT. Such a load-bias voltage may be applied by controlling the average voltage of the plurality of electrodes 45a-d in the support structure MT.
The Unloading Period
[0140] During the unloading period, the clamping means of the support structure MT is deactivated, and the patterning device MA is moved away from the support structure MT. While the patterning device MA is moved away from the support structure MT, the patterning device may be supported by the patterning device handler 80 in the manner described above.
[0141] As described above, charge can accumulate at the isolated surfaces of the patterning device MA, e.g., the patterning surface 40 and the clamping surface 41 during the loading period. Residual charge can remain on a clamped patterning device MA once it has been released from the support structure MT. The residual charge that is likely to be present on the patterning device MA before the patterning device MA is unclamped from the patterning device support MT may be a negative charge on the clamping surface 41.
[0142] As the unclamped patterning device MA is moved away from the support surface 42, the increasing separation between the support surface 42 and the clamping surface 41 can lead to a decrease in capacitance, and an amplification of the voltage. That is, given the proportional relationship between charge and potential (i.e. Q = C.V) in a closed system, when the capacitance changes (in inverse proportion to the separation between parallel plates), any reduction in capacitance will result in a proportional increase in voltage. Thus, as the patterning device MA and patterning device support MT are separated, it is possible that the potential of the patterning device MA will rise sufficiently to cause electrical breakdown of the hydrogen gas to occur, and so electrostatic discharge between, for example, the clamping surface 41 and the conductive burls 70a. Consequently, it is preferable that the residual charge on the patterning device MA is small or non-existent before the patterning device MA is unloaded from the patterning device support MT.
[0143] As explained above, the unloading period may comprise a first portion in which the patterning device MA is in contact with (e.g. supported by, e.g. clamped to) the support structure MT, and a second portion in which the patterning device MA is not in contact with (e.g. is separated from or spaced from) the support structure MT. To discharge the clamping surface, it has it has been proposed that the clamping surface 41 should be electrically connected to the ground before the patterning device MA is moved away from the patterning device MA (i.e. in the first portion of the unloading period). For example, the clamping surface 41 may be electrically connected to the ground via the conductive burls, as has been described above. However, such grounding of the clamping surface 41 may not effectively discharge the clamping surface 41. In some cases, grounding the clamping surface 41 in this way may actually result in the charge on the clamping surface 41 increasing.
[0144] For example, where the clamping voltage source(s) 46a-d are inaccurate, and the average voltage of the plurality of electrodes 45a-45d is not zero during the first portion of the unloading period, electrically connecting the clamping surface 41 to the ground may increase the amount of charge accumulated on the clamping surface 41. For instance, if the average voltage of the plurality of electrodes 45a-45d is positive, and the clamping surface 41 is electrically connected to the ground, the clamping surface may become negatively charged. The negative charge acquired by the camping surface may cancel out the positive potential capacitively induced in the clamping surface 41 by virtue of the clamping surface 41 being disposed within the electric field between the support structure MT and the patterning device handler 80. When the clamping surface 41 loses contact with the conductive burls 70a, the negative charge may remain on the clamping surface 41.
[0145] An aspect of the present disclosure is directed to reducing the magnitude of the charge on the clamping surface during the unloading period. In accordance with this aspect of the present disclosure, the patterning device support system is configured such that in a period before unloading of the patterning device MA from the support structure, an unload-bias voltage is applied to the clamping surface 41. The unload-bias voltage may reduce a charge on the clamping surface, relative to a case in which the clamping surface is grounded via the one or more conductive members.
[0146] Figure 11 depicts a patterning device support system in accordance with the present disclosure, during the first portion of the unloading period. Figure 12 depicts a patterning device support system in accordance with the present disclosure, during the second portion of the unloading period. In Figure 12, the arrow indicates the direction of the movement of the patterning device MA.
[0147] The patterning device support system depicted in Figures 11 and 12 is configured to apply the unload-bias voltage to the clamping surface 41 via the one or more conductive burls 70a. That is, the applying the unload-bias voltage to the clamping surface 41 may comprise applying an unload- conductive member voltage to the conductive burls 70a. The unload-bias voltage may be applied to the clamping surface 41 from a conductive member voltage source 61
[0148] The conductive member voltage source 61 which applies the unload-bias voltage to the clamping surface 41 may be the same conductive member voltage source 61 that applies the load- conductive member voltage to the conductive burls 70a, as described above. The conductive member voltage source 61 may be the same as the voltage source 61 which is configured to apply the productionbias voltage to the patterning surface 40 during the production period. Similarly, the conductive burls 70a may be the same conductive burls that are used to apply the production-bias voltage to the patterning surface 40 during the production period.
[0149] To eliminate or reduce the extent of charging of the clamping surface 41 in the first portion of the unload period, the unload-bias voltage applied to the clamping surface 41 may have the same polarity as an average voltage of the plurality of electrodes in the support structure MT.
[0150] To eliminate or reduce the extent of charging of the clamping surface 41 in the first portion of the unload period, the unload-bias voltage may have a magnitude that is similar to the magnitude of the voltage which would be capacitively induced in the clamping surface 41 by virtue of the position of the clamping surface 41 in the electric field between the support structure MT and the patterning device handler if the clamping surface 41 (or the patterning device MA in its entirety) were to be substantially electrically isolated.
[0151] The magnitude of the voltage which would be capacitively induced in the clamping surface 41 by virtue of the position of the clamping surface 41 in the electric field between the support structure MT and the patterning device handler, if the clamping surface 41 (or the patterning device MA in its entirety) were to be substantially electrically isolated may be between the average voltage of the plurality of electrodes 45a-d in the support structure and ground voltage (i.e. 0 V). Thus, the unloadbias voltage applied to the clamping surface 41 may be greater than 70% of the average voltage of the plurality of electrodes, preferably greater than 80% of the average voltage of the plurality of electrodes, further preferably greater than 90% of the average voltage of the plurality of electrodes, less than 110% of the average voltage of the plurality of electrodes, preferably less than 100% of the average voltage of the plurality of electrodes, and further preferably less than 95% of the average voltage of the plurality of electrodes.
[0152] As has been described above, in some cases, the clamping surface 41 and the patterning surface may be electrically connected. In such cases, the unload-bias voltage is applied to both the clamping surface 41 and the patterning surface 40. Consequently, the amount of charge on the patterning surface 40 prior to the unloading of the patterning device MA from the support structure MT can be reduced.
[0153] In some embodiments, the unload-bias voltage is predetermined. That is, the unload-bias voltage may be controlled based on known aspects of the operation of patterning device support system, e.g. the average voltage of the plurality of electrodes 45a-d in the support structure MT when the average voltage of the plurality of electrodes 45a-d in the support structure MT is intended to be (i.e. controlled to be) zero. In such embodiments, the load-conductive member voltage may not be active controlled, i.e. there may not be a closed feedback loop.
[0154] In other embodiments, the patterning device support system may comprise one or more sensors, and the unload-bias voltage may be determined based on measurements made by the one or more sensors. The one or more sensors used in the determination of the unload-bias voltage may include the sensors described in relation to the determination of the load-conductive member voltage in the loading period. That is, the one or more sensors may comprise an current sensor 31 configured to measure the current between the one or more conductive burls 70a and the conductive member voltage source 61 (i.e. a first sensor). Additionally or alternatively, the one or more sensors may comprise one or more current sensors 32a-d configured to measure the current between the plurality of electrodes 45a-d and the clamping voltage source(s) 46a-d (i.e. a second sensor). Additionally or alternatively, the one or more sensors may comprise a sensor in the patterning device handler 80 (i.e. a third sensor). The sensor disposed in the patterning device handler 80 may be a voltage sensor configured to measure a voltage of the conductive coating 41C which forms the clamping surface 41. The sensor disposed in the patterning device handler 80 may be a strain gauge configured to measure a strain of the patterning device MA. The sensor disposed in the patterning device handler 80 may be a force sensor configured to measure a contact force between the patterning device MA and the patterning device handler 80.
[0155] In some embodiments, the determination and application of the unload-bias voltage to the clamping surface may be an iterative process in which the polarity and/or the magnitude of the unloadbias voltage is iteratively adjusted based on measurements made by the one or more sensors. In some embodiments, the unload-bias voltage is iteratively adjusted based on measurements made by the one or more sensors after the patterning device MA has moved away from the support structure MT. In some embodiments, the unload-bias voltage is iteratively adjusted based on measurements made by the one or more sensors as the patterning device MA is moving away from the support structure MT. In this way, a suitable unload-bias voltage can be determined and applied to the clamping surface 41, i.e. an unload-bias voltage that effectively reduces the charge on the clamping surface 41 can be determined and appliqued to the clamping surface 41.
[0156] An iterative method for determining a suitable unload-bias voltage is depicted in Figure 13. The method starts S101 in the first portion of the unloading period, i.e. when the patterning device MA is supported by the support structure MT.
[0157] The method comprises a step SI 02 of determining an initial unload-bias voltage. The initial unload-bias voltage may be a predetermined estimate. The predetermined estimate may be based on known characteristics of the patterning device support system, e.g. the average voltage of the plurality of electrodes 45a-d in the support structure MT when the average voltage of the plurality of electrodes 45a-d in the support structure MT is intended to be (i.e. controlled to be) zero. Alternatively, the initial unload-bias voltage may be determined based on measurements made by the one or more sensors.
[0158] The method comprises, after determining the initial unload-bias voltage, applying the unloadbias voltage to the clamping surface 41 of the patterning device MA. The unload-bias voltage may be applied to the clamping surface for a predetermined amount of time. The predetermined amount of time may be configured to be sufficient for discharging of the clamping surface 41 to occur. The unloadbias voltage may be applied to the clamping surface 41 using any suitable method, such as via the conductive burls 70a, as described above.
[0159] The method further comprises, after the predetermined amount of time, a step S103 of moving the patterning device MA away from the patterning device support MT. This may comprise disabling the clamping means of the support structure MT and controlling the patterning device handler to move away from the support structure.
[0160] While the patterning device MA is moving away from the support structure MT, and/or after the patterning device MA has moved a predetermined distance away from the support structure MT, measurements may be made with the one or more sensors described above.
[0161] The method may further comprise a step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge. The threshold charge may be an amount of charge above which there is a risk of electrostatic discharge occurring (e.g. electrostatic discharge occurring between the clamping surface 41 and the conductive burls 70a). The determination of the whether the amount of the charge on the clamping surface exceeds a threshold charge may comprise a direct measurement of the amount of charge on the patterning surface 41. However, this is not essential, and the step SI 05 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise indirectly determining the amount of charge on the patterning device MA. That is, the step S105 may be a step of determining whether a parameter indicative of the amount of charge on the clamping surface 41 exceeds a predetermined threshold. This is discussed in further detail below.
[0162] In the case that the amount of charge on the clamping surface 41 is determined not to exceed the threshold charge (S105 = NO), the patterning device may continue to be moved away from the support structure, i.e. the unloading process may be continued as normal. The method may then end (S109).
[0163] In the case that the amount of charge on the clamping surface 41 is determined to exceed the threshold charge (S105 = YES), the patterning device MA may be moved back towards the support structure MT, such that the conductive burls 70a come back into contact with the clamping surface (S107).
[0164] Once the clamping surface 41 has re-made contact with the conductive burls 70a, or while the patterning device is moving towards the support structure, the unload-bias voltage may be adjusted (S108). The adjusting of the unload-bias voltage may be based on the measurements made by the one or more sensors when the patterning device MA was moving away from the support structure.
[0165] Once the clamping surface 41 has re-made contact with the conductive burls 70a, the adjusted unload-bias voltage may be applied to the clamping surface (SI 03), as described above.
[0166] The method may be repeated as many times as necessary until the charge on the clamping surface 41 is determined to be less than the threshold charge in SI 05, and the patterning device can be fully unloaded, and e.g. returned to a storage area.
[0167] The step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge will now be discussed in further detail.
[0168] The step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining whether a current between the one or more conductive burls 70a and the conductive member voltage source 61 (e.g. as measured by the current sensor 31) exceeds a first predetermined threshold. If the current between the one or more conductive burls 70a and the conductive member voltage source 61 does exceed the first predetermined threshold, it is determined that the charge on the clamping surface 41 exceeds the threshold charge, and the patterning device MA is returned to the loaded position in which the clamping surface is electrically connected to the one or more conductive members.
[0169] If the current between the one or more conductive burls 70a and the conductive member voltage source 61 is in the direction of (i.e. towards) the conductive burls 70a, it may be determined that the clamping surface 41 is negatively charged. This may occur in cases where the unload-bias voltage is not sufficiently positive (e.g. negative with an excessively large magnitude, or positive with an insufficiently large magnitude. Thus, the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more positive (e.g. increasing the magnitude of a positive unload-bias voltage or decreasing the magnitude of a negative unload-bias voltage). If the current between the one or more conductive burls 70a and the conductive member voltage source 61 is away from the conductive burls 70a (i.e. towards the conductive member voltage source 61) it may be determined that the clamping surface 41 is positively charged. This may occur in cases where the unload-bias voltage is not sufficiently negative (e.g. positive with an excessively large magnitude, or negative with an insufficiently large magnitude. Thus, the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more negative (e.g. increasing the magnitude of a negative unload-bias voltage or decreasing the magnitude of a positive unload-bias voltage). The extent to which the unloadbias voltage is adjusted (i.e. the amount by which the unload-bias voltage is changed) may be a function of the current measured by the current sensor 31 and the rate of increasing separation between the clamping surface 41 and the support structure MT.
[0170] The step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining whether a current between the electrodes 45a-d and the clamping voltage source(s) 46a-d (e.g. as measured by the one or more current sensors 32a-d) exceeds a second predetermined threshold. If the current between the electrodes 45a-d and the clamping voltage source(s) 46a-d does exceed the second predetermined threshold, it is determined that the charge on the clamping surface 41 exceeds the threshold charge, and the patterning device MA is returned to the loaded position in which the clamping surface 41 is electrically connected to the one or more conductive members 70a .
[0171] If the current between the electrodes 45a-d and the clamping voltage source(s) 46a-d is in the direction of (i.e. towards) the electrodes 45a-d, it may be determined that the clamping surface 41 is negatively charged. This may occur in cases where the unload-bias voltage is not sufficiently positive (e.g. negative with an excessively large magnitude, or positive with an insufficiently large magnitude). Thus, the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more positive (e.g. increasing the magnitude of a positive unload-bias voltage or decreasing the magnitude of a negative unload-bias voltage). If the current between the electrodes 45a-d and the clamping voltage source(s) 46a-d is away from the electrodes 45a-d (i.e. towards the clamping voltage source(s) 46a-d), it may be determined that the clamping surface 41 is positively charged. This may occur in cases where the unload-bias voltage is not sufficiently negative (e.g. positive with an excessively large magnitude, or negative with an insufficiently large magnitude. Thus, the step S108 of adjusting the unload-bias voltage may comprise making the unload-bias voltage more negative (e.g. increasing the magnitude of a negative unload-bias voltage or decreasing the magnitude of a positive unload-bias voltage). The extent to which the unload-bias voltage is adjusted (i.e. the amount by which the unload-bias voltage is changed) may be a function of the current measured by the one or more current sensors 32a-d and the rate of increasing separation between the clamping surface 41 and the support structure MT.
[0172] The step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining the amount of charge on the clamping surface 41, or a variable related to (i.e. indicative of) the amount of charge on the sample, using a sensor in the patterning device handler 80.
[0173] The step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise (or may be) a step of determining the voltage of the clamping surface 41 using the voltage sensor of the patterning device handler 80. When the voltage measured by the voltage sensor of the patterning device handler exceeds a threshold voltage, the patterning device MA may be returned to the loaded position, in which the conductive burls 70a are in contact with the clamping surface 41.
[0174] If the voltage measured by the voltage sensor after the patterning device MA has separated from the support structure is is negative, the unload-bias voltage applied may be excessively negative (or insufficiently positive). Thus, the step of adjusting the unload-bias voltage may comprise making the unload-bias voltage more positive (i.e. increasing the magnitude of a positive unload-bias voltage or decreasing the magnitude of a negative unload-bias voltage). If the voltage measured by the voltage sensor after the patterning device MA has separated from the support structure is positive, the unloadbias voltage applied may be excessively positive (or insufficiently negative). Thus, the step of adjusting the unload-bias voltage may comprise making the unload-bias voltage more negative (i.e. increasing the magnitude of a negative unload-bias voltage or decreasing the magnitude of a positive unload-bias voltage). The size of the adjustment to the unload-bias voltage may be a function of the voltage measured by the voltage sensor and the separation between the patterning device MA and the support structure MT.
[0175] The step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise a step of determining a strain (i.e. deformation) of the patterning device MA using a strain gauge in the patterning device handler 80. Strain of the patterning device MA may be indicative of charge on the clamping surface 41. Thus, if the strain measured by the strain gauge exceeds a strain threshold, the patterning device MA may be returned to the loaded position in which the clamping surface 41 is in contact with the conductive burls 70a, and the unload-bias voltage adjusted. The amount of change to the unload-bias voltage may be a function of the magnitude of the strain of the patterning device MA as measured by the strain gauge.
[0176] The step S 105 of determining whether an amount of the charge on the clamping surface exceeds a threshold charge may comprise a step of measuring a contact force between the patterning device MA and the patterning device handler 80 using the force sensor in the patterning device handler 80. If the clamping surface 41 is charged, an electrostatic attractive force may be exerted on the clamping surface 41 in the direction of the support structure MT. Thus, the patterning device support system may be configured to return the patterning device MA to the loaded position in the case that the force measured by the force sensor is less than a threshold force.
[0177] This description of the unloading period has focused on the case where the unload-bias voltage is applied to the clamping surface 41 via the conductive burls 70a. However, an unload-bias voltage may be applied to the clamping surface 41 through any suitable means, e.g. through a voltage biasing member (as described above) or by controlling the average voltage of the plurality of electrodes 45a-d in the support structure MT.
General
[0178] The patterning device support system described above may be incorporated into a lithographic apparatus. The lithographic apparatus may be used for the manufacture of ICs.
[0179] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.
[0180] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.
[0181] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools.
[0182] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography.
[0183] Aspects of the invention are described in the following numbered clauses.
1. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support a patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a conductive member voltage source configured to apply a voltage to the one or more conductive members; and a controller configured to control the voltage applied to the one or more conductive members, wherein, during loading of the patterning device onto the patterning device support, the controller is configured to control the voltage applied to the one or more conductive members to be a load- conductive member voltage, wherein the load-conductive member voltage reduces a potential difference between the clamping surface and the one or more conductive members relative to a case in which the one or more conductive members are grounded.
2. The patterning device support system of clause 1 , wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprise one or more of the plurality of burls.
3. The patterning device support system of clause 1 or 2, wherein the application of the load- conductive member voltage reduces the potential difference between the clamping surface and the one or more conductive members to less than 50 V, preferably less than 10 V and preferably less than 1 V, and further preferably less than 0.1 V.
4. The patterning device support system of any of the preceding clauses, wherein the load-conductive member voltage is substantially the same as the voltage of the clamping surface, optionally wherein the load-conductive member voltage is substantially the same as the voltage of the clamping surface when the clamping surface comes into contact with the patterning device support.
5. The patterning device support system of any of the preceding clauses, wherein the patterning device support further comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes.
6. The patterning device support system of any of the preceding clauses, wherein the load-conductive member voltage is predetermined.
7. The patterning device support system of any of clauses 1 to 5, further comprising one or more sensors, and wherein the load-conductive member voltage is determined based on measurements made by the one or more sensors.
8. The patterning device support system of clause 7, wherein the one or more sensors comprise a first sensor configured to measure the current between the one or more conductive members and the conductive member voltage source, and the load-conductive member voltage is determined based on the current measured by the first sensor. 9. The patterning device support system of clause 7 or 8, wherein the one or more sensors comprises a second sensor configured to measure the current between the plurality of electrodes and the clamping voltage source, and the load-conductive member voltage is determined based on the current measured by the second sensor.
10. The patterning device support system of any of clauses 7 to 9, further comprising a patterning device handler configured to support the patterning device while the patterning device is moved towards the patterning device support during loading of the patterning device onto the patterning device support, and wherein: the patterning device handler comprises a third sensor; and the load-conductive member voltage is determined based on measurements made by the third sensor, optionally wherein the third sensor comprises one or more of: a voltage sensor configured to measure a voltage of the clamping surface, a strain gauge configured to measure a strain of the patterning device and a force sensor configured to measure a contact force between the patterning device and the patterning device handler.
11. The patterning device support system of any of clauses 1 to 6, wherein the load-conductive member voltage is based on an average voltage of the electrodes in the patterning device support.
12. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support a patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a conductive member voltage source configured to apply a bias voltage to the clamping surface via the one or more conductive members; and a controller configured to control the bias voltage applied to the clamping surface via one or more conductive members, wherein, in a period before unloading of the patterning device from the patterning device support, the controller is configured to control the conductive member voltage source to apply an unload-bias voltage to the clamping surface via the one or more conductive members, and the unload-bias voltage reduces a charge on the clamping surface, relative to a case in which the clamping surface is grounded via the one or more conductive members.
13. The patterning device support system of clause 12, wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprises one or more of the plurality of burls. 14. The patterning device support system of clause 12 or 13, wherein the patterning device support further comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes.
15. The patterning device support system of any of clauses 12 to 14, wherein the unload-bias voltage applied to the clamping surface has the same polarity as an average voltage of the plurality of electrodes.
16. The patterning device support system of any of clauses 12 to 15, wherein the unload-bias voltage applied to the clamping surface is greater than 70% of the average voltage of the plurality of electrodes, preferably greater than 80% of the average voltage of the plurality of electrodes, further preferably greater than 90% of the average voltage of the plurality of electrodes, and less than 110% of the average voltage of the plurality of electrodes, preferably less than 100% of the average voltage of the plurality of electrodes, and further preferably less than 95% of the average voltage of the plurality of electrodes.
17. The patterning device support system of any of clauses 12 to 16, wherein the patterning device support system is configured such that the application of the unload-bias voltage reduces a charge on the clamping surface and a charge on the patterning surface.
18. The patterning device support system of clauses 12 to 17, further comprising one or more sensors, wherein the unload-bias voltage is determined based on measurements made by the one or more sensors.
19. The patterning device support system of any of clause 18, wherein the one or more sensors comprises a first sensor configured to measure the current between the one or more conductive members and the conductive member voltage source.
20. The patterning device support system of clauses 19, wherein, after the patterning device is unloaded from the patterning device support, if a current between the one or more conductive members and the conductive member voltage source exceeds a first predetermined threshold, the patterning device is returned to a loaded position in which the clamping surface is electrically connected to the one or more conductive members.
21. The patterning device support system of any of clauses 18 to 20, wherein the one or more sensors comprises a second sensor configured to measure the current between the plurality of electrodes and the clamping voltage source.
22. The patterning device support system of clause 21, wherein, after the patterning device is unloaded from the patterning device support, if a current between the plurality of electrodes and the clamping voltage source exceeds a second predetermined threshold, the patterning device is returned to a loaded position in which the clamping surface is electrically connected to the one or more conductive members.
23. The patterning device support system of any of clauses 12 to 22, further comprising a patterning device handler configured to support the patterning device while the patterning device is moved away from the patterning device support during unloading of the patterning device from the patterning device support.
24. The patterning device support system of clause 23, wherein the patterning device handler comprises a third sensor, and the patterning device support system is configured to determine whether to return the patterning device to a loaded position in which the clamping surface is electrically connected to the one or more conductive members based on measurements made by the third sensor.
25. The patterning device support system of clause 24, wherein the third sensor comprises a voltage sensor configured to measure the voltage of the patterning device, and the patterning device support system is configured to return the patterning device to the loaded position in the case that the voltage measured by the voltage sensor exceeds a threshold voltage, optionally wherein the voltage sensor is configured to determine the voltage of the clamping surface.
26. The patterning device support system of clause 24 or 25, wherein the third sensor comprises a strain gauge configured to measure a strain of the patterning device, and the patterning device support system is configured to return the patterning device to the loaded position in the case that the strain measured by the strain gauge exceeds a threshold strain.
27. The patterning device support system of any of clauses 24 to 26, wherein the third sensor comprises a force sensor configured to measure a contact force between the patterning device and the patterning device handler, and the patterning device support system is configured to return the patterning device to the loaded position in the case that the force measured by the force sensor is less than a threshold force.
28. The patterning device support system of any of clauses 20, 22 and 24 to 27, wherein the patterning device support system is configured such that, after the patterning device has been returned to the loaded position, the unload-bias voltage is adjusted based on the measurements made by the first sensor and/or the second sensor and/or the third sensor.
29. The patterning device support system of any of the preceding clauses, wherein a period in which the patterning surface is cyclically exposed to EUV radiation is referred to as a production period, and, during the production period, the controller is configured to control the patterning device support system to apply a production-bias voltage to the patterning surface and/or the clamping surface of the patterning device.
30. The patterning device support system of any of the preceding clauses, wherein the patterning surface is electrically connected to the clamping surface.
31. The patterning device support system of clause 29, wherein the patterning device support system is configured to capacitively induce the production-bias voltage in the clamping surface and/or the patterning surface, optionally wherein the patterning device support system is configured to capacitively induce the production-bias voltage in the clamping surface and/or the patterning surface by controlling an average voltage of a plurality of electrodes in the patterning device support.
32. The patterning device support system of clause 29, wherein the production-bias voltage is applied to the clamping surface from the conductive member voltage source via the one or more conductive members.
33. The patterning device support system of clause 32, wherein the one or more conductive members are connected to the conductive member voltage source via a resistor or an inductor. 34. The patterning device support system of clause 32 or 33, wherein the one or more conductive members are connected to the voltage source via a diode.
35. The patterning device support system of any of clauses 32 to 34, wherein the one or more conductive members are connected to the voltage source via a switching device.
36. The patterning device support system of clause 35, wherein the switching device can be switched on and off at a frequency that is greater than 49 kHz, preferably greater than 59 kHz, and further preferably greater than 99 kHz.
37. The patterning device support system according to clause 35 or 36, wherein the frequency at which the switching device is switched on and off is synchronised with a frequency of generation of a beam of radiation in the lithographic apparatus, such that the production-bias voltage is applied to the one or more conductive members between pulses of radiation.
38. The patterning device support system of any of clauses 29 to 37, wherein the production-bias voltage is negative.
39. The patterning device support system of any of clauses 29 to 38, wherein the production-bias voltage is positive during times when the lithographic apparatus generates pulses of EUV radiation and negative between times when the lithographic apparatus generates the pulses of EUV radiation.
40. The patterning device support system of any of clauses 29 to 39, wherein a magnitude of the negative production-bias voltage to the patterning surface is greater than 0.5 V, preferably greater than 1 V, less than 10 V, preferably less than 5 V and further preferably less than 3 V, and/or a magnitude of the positive production-bias voltage is greater 1 V and preferably greater than 5 V, less than 100 V and preferably less than 50 V.
41. The patterning device support system of any of the preceding clauses, wherein the patterning device further comprises a conductive coating which forms the clamping surface.
42. The patterning device support system of any of the preceding clauses, wherein the patterning device support system comprises the patterning device.
43. A lithographic apparatus comprising the patterning device support system according to any of the preceding clauses.
44. A method of loading a patterning device onto a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: moving the patterning device towards the patterning device support; and applying a load-conductive member voltage to one or more conductive members, wherein the one or more conductive members are arranged to be electrically connected to the clamping surface when the patterning device is loaded onto the patterning device support, and the load-conductive member voltage applied to the one or more conductive members reduces a potential difference between the clamping surface and the one or more conductive members relative to a case in which the one or more conductive members are grounded. 45. The method of clause 44, wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprise one or more of the plurality of burls.
46. The method of clause 44 or 45, wherein, the application of the load-conductive member voltage reduces the potential difference between the clamping surface and the one or more conductive members to less than 50 V, preferably less than 10 V, further preferably less than 1 V, and further preferably less than 0.1 V.
47. The method of any of clauses 44 to 46, wherein the load-conductive member voltage is substantially the same as the voltage of the clamping surface, optionally wherein the load-conductive member voltage is substantially the same as the voltage of the clamping surface when the clamping surface comes into contact with the patterning device support.
48. The method of any of clauses 44 to 47, further comprising determining the voltage of the clamping surface, and determining the load-conductive member voltage based on the voltage of the clamping surface.
49. The method of any of clauses 44 to 47, further comprising determining the load-conductive member voltage based on measurements made by one or more sensors.
50. The method of clause 49, wherein the load-conductive member voltage applied to the one or more conductive members is applied by a conductive member voltage source, and the method further comprises: measuring a current between the one or more conductive members and the conductive member voltage source; and determining the load-conductive member voltage based on the measured current between the one or more conductive members and the conductive member voltage source.
51. The method of any of clause 49, wherein the patterning device support comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes, and the method further comprises: measuring a current between the plurality of electrodes and the clamping voltage source; and determining the load-conductive member voltage based on the measured current between the plurality of electrodes and the clamping voltage source.
52. The method of clause 49, wherein, while the patterning device is moved towards the patterning device support, the patterning device is supported by a patterning device handler, and the method further comprises: determining the load-conductive member voltage based on measurements made by a sensor of the patterning device handler, optionally wherein the sensor of the patterning device handler comprises one or more of: a voltage sensor configured to measure a voltage of the clamping surface, a strain gauge configured to measure a strain of the patterning device and a force sensor configured to measure a contact force between the patterning device and the patterning device handler. 53. The method of any of clauses 44 to 47, further comprising determining the load-conductive member voltage based on an average voltage of the electrodes in the patterning device support.
54. A method of unloading a patterning device from a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: in a period before unloading the patterning device from the patterning device support, applying an unload-bias voltage to the clamping surface via one or more conductive members; and then moving the patterning device away from the patterning device support, wherein the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device, relative to a case in which the clamping surface is grounded via the one or more conductive members.
55. The method of clause 54, wherein the applying the unload-bias voltage to the clamping surface via the one or more conductive members comprises applying an unload-conductive member voltage to the one or more conductive members, and the unload-conductive member voltage is also applied to the one or more conductive members after the patterning device has moved away from the patterning device support and the one or more conductive members are no longer in contact with the clamping surface.
56. The method of clause 54 or 55, wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprises one or more of the plurality of burls.
57. The method of any of clauses 54 to 56, wherein the patterning device support further comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes, and the unload-bias voltage has the same polarity as an average voltage of the plurality of electrodes.
58. The method of any of clauses 54 to 57, wherein the unload-bias voltage applied to the one or more conductive members is greater than 70% of the average voltage of the plurality of electrodes, preferably greater than 80% of the average voltage of the plurality of electrodes, further preferably greater than 90% of the average voltage of the plurality of electrodes, and less than 110% of the average voltage of the plurality of electrodes, preferably less than 100% of the average voltage of the plurality of electrodes, and further preferably less than 95% of the average voltage of the plurality of electrodes.
59. The method of any of clauses 54 to 58, further comprising determining the unload-bias voltage based on measurements made by one or more sensors.
60. The method of any of clauses 54 to 59, wherein the bias voltage applied to the clamping surface via the one or more conductive members is applied by a conductive member voltage source, and the method further comprises measuring a current between the one or more conductive members and the conductive member voltage source. 61. The method of clause 60, further comprising, if, after the patterning device has been separated from the patterning device support, the current between the one or more conductive members and the conductive member voltage source exceeds a first predetermined threshold: moving the patterning device towards the patterning device support, such that the clamping surface becomes electrically connected to the one or more conductive members; and adjusting the unload-bias voltage based on the measured current between the one or more conductive members and the conductive member voltage source.
62. The method of any of clauses 54 to 61 , further comprising measuring a current between the plurality of electrodes and the clamping voltage source.
63. The method of clause 62, further comprising, if, after the patterning device has been separated from the patterning device support, the current between the plurality of electrodes and the clamping voltage source exceeds a second predetermined threshold: moving the patterning device towards the patterning device support, such that the clamping surface becomes electrically connected to the one or more conductive members; and adjusting the unload-bias voltage based on the measured current between the plurality of electrodes and the clamping voltage source.
64. The method of any of clauses 54 to 63, wherein, while the patterning device is moved away the patterning device support, the patterning device is supported by a patterning device handler, and the method further comprises determining whether to return the patterning device to towards the patterning device support such that the clamping surface is electrically connected to the one or more conductive members based on measurements made by a sensor of the patterning device handler.
65. The method of clause 64, further comprising: measuring a voltage of the patterning device with the patterning device handler; and if, after the patterning device has been separated from the patterning device support, the voltage of the patterning device exceeds a third predetermined threshold: moving the patterning device towards the patterning device support, such that the clamping surface becomes electrically connected to the one or more conductive members; and adjusting the unload-bias voltage based on the measured voltage of the patterning device.
66. A method of supporting a patterning device on a patterning device support, the method comprising the method of loading the patterning device onto the patterning device support of any of clauses 44 to 53 and/or the method of unloading the patterning device from the patterning device support according to any of clauses 54 to 65.
67. The method of clause 66, wherein a period in which the patterning surface is cyclically exposed to EUV radiation is referred to as a production period, and the method further comprises applying a production-bias voltage to the clamping surface and/or the patterning surface during the production period. 68. The method of clause 67, wherein the production-bias voltage is capacitively induced in the clamping surface and/or the patterning surface, optionally wherein the production-bias voltage is capacitively induced in the clamping surface and/or the patterning surface by controlling an average voltage of a plurality of electrodes in the patterning device support.
69. The method of clause 67, wherein the production-bias voltage is applied to the clamping surface from the conductive member voltage source via the one or more conductive members.
70. The method of clause 69, wherein the patterning surface is electrically connected to the clamping surface, such that applying a production-bias voltage to the clamping surface via the one or more conductive members results in the application of the production-bias voltage to the patterning surface.
71. The method of clause 67 to 70, wherein the production-bias voltage is negative.
72. The method of any of clauses 67 to 71, wherein, in the production period, the production-bias voltage is applied to the clamping surface between pulses of radiation, and the patterning device is electrically isolated during pulses of radiation.
73. The method of any of clauses 67 to 72 wherein the production-bias voltage is positive during times when the lithographic apparatus generates pulses of EUV radiation and negative between times when the lithographic apparatus generates the pulses of EUV radiation.
74. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support the patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a voltage source configured to apply a voltage to the clamping surface and/or the conductive members; and a controller, wherein, during loading of the patterning device onto the patterning device support, the controller is configured to control the voltage source to apply a load voltage to the clamping surface and/or the one or more conductive members to reduce a potential difference between the clamping surface and the one or more conductive members.
75. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support the patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a voltage source configured to apply a bias voltage to the clamping surface; and a controller, wherein, in a period before unloading of the patterning device from the patterning device support, the controller is configured to control the voltage source to apply an unload-bias voltage to the clamping surface, and the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device.
76. A method of loading a patterning device onto a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, the patterning device support comprising one or more conductive members, wherein the one or more conductive members are arranged to be electrically connected to the clamping surface when the patterning device is loaded onto the patterning device support, and the method comprises: moving the patterning device towards the patterning device support; and applying a load voltage to the clamping surface and/or the one or more conductive members, wherein the load voltage applied to the one or more conductive members and/or the clamping surface reduces a potential difference between the clamping surface and the one or more conductive members.
77. A method of unloading a patterning device from a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: in a period before unloading the patterning device from the patterning device support, applying an unload-bias voltage to the clamping surface; and then moving the patterning device away from the patterning device support, wherein the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device.
78. A method of manufacturing a device comprising the method of supporting a patterning device according to any of clauses 44 to 77.
79. A computer program comprising instructions which, when executed by a controller for a lithographic apparatus, cause the lithographic apparatus to carry out the method of any of clauses 44 to 77.
[0184] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support a patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a conductive member voltage source configured to apply a voltage to the one or more conductive members; and a controller configured to control the voltage applied to the one or more conductive members, wherein, during loading of the patterning device onto the patterning device support, the controller is configured to control the voltage applied to the one or more conductive members to be a load-conductive member voltage, wherein the load-conductive member voltage reduces a potential difference between the clamping surface and the one or more conductive members relative to a case in which the one or more conductive members are grounded.
2. The patterning device support system of claim 1 , wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprise one or more of the plurality of burls.
3. The patterning device support system of claim 1 or 2, wherein the application of the load-conductive member voltage reduces the potential difference between the clamping surface and the one or more conductive members to less than 50 V, preferably less than 10 V and preferably less than 1 V, and further preferably less than 0.1 V.
4. The patterning device support system of any of the preceding claims, wherein the load-conductive member voltage is substantially the same as the voltage of the clamping surface, optionally wherein the load-conductive member voltage is substantially the same as the voltage of the clamping surface when the clamping surface comes into contact with the patterning device support.
5. The patterning device support system of any of the preceding claims, wherein the patterning device support further comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes.
6. The patterning device support system of any of the preceding claims, wherein the load-conductive member voltage is predetermined.
7. The patterning device support system of any of claims 1 to 5, further comprising one or more sensors, and wherein the load-conductive member voltage is determined based on measurements made by the one or more sensors.
8. The patterning device support system of claim 7, wherein the one or more sensors comprise a first sensor configured to measure the current between the one or more conductive members and the conductive member voltage source, and the load-conductive member voltage is determined based on the current measured by the first sensor.
9. The patterning device support system of claim 7 or 8, wherein the one or more sensors comprises a second sensor configured to measure the current between the plurality of electrodes and the clamping voltage source, and the load-conductive member voltage is determined based on the current measured by the second sensor.
10. The patterning device support system of any of claims 7 to 9, further comprising a patterning device handler configured to support the patterning device while the patterning device is moved towards the patterning device support during loading of the patterning device onto the patterning device support, and wherein: the patterning device handler comprises a third sensor; and the load-conductive member voltage is determined based on measurements made by the third sensor, optionally wherein the third sensor comprises one or more of: a voltage sensor configured to measure a voltage of the clamping surface, a strain gauge configured to measure a strain of the patterning device and a force sensor configured to measure a contact force between the patterning device and the patterning device handler.
11. The patterning device support system of any of claims 1 to 6, wherein the load-conductive member voltage is based on an average voltage of the electrodes in the patterning device support.
12. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support a patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a conductive member voltage source configured to apply a bias voltage to the clamping surface via the one or more conductive members; and a controller configured to control the bias voltage applied to the clamping surface via one or more conductive members, wherein, in a period before unloading of the patterning device from the patterning device support, the controller is configured to control the conductive member voltage source to apply an unload-bias voltage to the clamping surface via the one or more conductive members, and the unloadbias voltage reduces a charge on the clamping surface, relative to a case in which the clamping surface is grounded via the one or more conductive members.
13. The patterning device support system of claim 12, wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprises one or more of the plurality of burls.
14. The patterning device support system of claim 12 or 13, wherein the patterning device support further comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes.
15. The patterning device support system of any of claims 12 to 14, wherein the unloadbias voltage applied to the clamping surface has the same polarity as an average voltage of the plurality of electrodes.
16. The patterning device support system of any of claims 12 to 15, wherein the unloadbias voltage applied to the clamping surface is greater than 70% of the average voltage of the plurality of electrodes, preferably greater than 80% of the average voltage of the plurality of electrodes, further preferably greater than 90% of the average voltage of the plurality of electrodes, and less than 110% of the average voltage of the plurality of electrodes, preferably less than 100% of the average voltage of the plurality of electrodes, and further preferably less than 95% of the average voltage of the plurality of electrodes.
17. The patterning device support system of any of claims 12 to 16, wherein the patterning device support system is configured such that the application of the unload-bias voltage reduces a charge on the clamping surface and a charge on the patterning surface.
18. The patterning device support system of claims 12 to 17, further comprising one or more sensors, wherein the unload-bias voltage is determined based on measurements made by the one or more sensors.
19. The patterning device support system of any of claim 18, wherein the one or more sensors comprises a first sensor configured to measure the current between the one or more conductive members and the conductive member voltage source.
20. The patterning device support system of claims 19, wherein, after the patterning device is unloaded from the patterning device support, if a current between the one or more conductive members and the conductive member voltage source exceeds a first predetermined threshold, the patterning device is returned to a loaded position in which the clamping surface is electrically connected to the one or more conductive members.
21. The patterning device support system of any of claims 18 to 20, wherein the one or more sensors comprises a second sensor configured to measure the current between the plurality of electrodes and the clamping voltage source.
22. The patterning device support system of claim 21, wherein, after the patterning device is unloaded from the patterning device support, if a current between the plurality of electrodes and the clamping voltage source exceeds a second predetermined threshold, the patterning device is returned to a loaded position in which the clamping surface is electrically connected to the one or more conductive members.
23. The patterning device support system of any of claims 12 to 22, further comprising a patterning device handler configured to support the patterning device while the patterning device is moved away from the patterning device support during unloading of the patterning device from the patterning device support.
24. The patterning device support system of claim 23, wherein the patterning device handler comprises a third sensor, and the patterning device support system is configured to determine whether to return the patterning device to a loaded position in which the clamping surface is electrically connected to the one or more conductive members based on measurements made by the third sensor.
25. The patterning device support system of claim 24, wherein the third sensor comprises a voltage sensor configured to measure the voltage of the patterning device, and the patterning device support system is configured to return the patterning device to the loaded position in the case that the voltage measured by the voltage sensor exceeds a threshold voltage, optionally wherein the voltage sensor is configured to determine the voltage of the clamping surface.
26. The patterning device support system of claim 24 or 25, wherein the third sensor comprises a strain gauge configured to measure a strain of the patterning device, and the patterning device support system is configured to return the patterning device to the loaded position in the case that the strain measured by the strain gauge exceeds a threshold strain.
27. The patterning device support system of any of claims 24 to 26, wherein the third sensor comprises a force sensor configured to measure a contact force between the patterning device and the patterning device handler, and the patterning device support system is configured to return the patterning device to the loaded position in the case that the force measured by the force sensor is less than a threshold force.
28. The patterning device support system of any of claims 20, 22 and 24 to 27, wherein the patterning device support system is configured such that, after the patterning device has been returned to the loaded position, the unload-bias voltage is adjusted based on the measurements made by the first sensor and/or the second sensor and/or the third sensor.
29. The patterning device support system of any of the preceding claims, wherein a period in which the patterning surface is cyclically exposed to EUV radiation is referred to as a production period, and, during the production period, the controller is configured to control the patterning device support system to apply a production-bias voltage to the patterning surface and/or the clamping surface of the patterning device.
30. The patterning device support system of any of the preceding claims, wherein the patterning surface is electrically connected to the clamping surface.
31. The patterning device support system of claim 29, wherein the patterning device support system is configured to capacitively induce the production-bias voltage in the clamping surface and/or the patterning surface, optionally wherein the patterning device support system is configured to capacitively induce the production-bias voltage in the clamping surface and/or the patterning surface by controlling an average voltage of a plurality of electrodes in the patterning device support.
32. The patterning device support system of claim 29, wherein the productionbias voltage is applied to the clamping surface from the conductive member voltage source via the one or more conductive members.
33. The patterning device support system of claim 32, wherein the one or more conductive members are connected to the conductive member voltage source via a resistor or an inductor.
34. The patterning device support system of claim 32 or 33, wherein the one or more conductive members are connected to the voltage source via a diode.
35. The patterning device support system of any of claims 32 to 34, wherein the one or more conductive members are connected to the voltage source via a switching device.
36. The patterning device support system of claim 35, wherein the switching device can be switched on and off at a frequency that is greater than 49 kHz, preferably greater than 59 kHz, and further preferably greater than 99 kHz.
37. The patterning device support system according to claim 35 or 36, wherein the frequency at which the switching device is switched on and off is synchronised with a frequency of generation of a beam of radiation in the lithographic apparatus, such that the production-bias voltage is applied to the one or more conductive members between pulses of radiation.
38. The patterning device support system of any of claims 29 to 37, wherein the production-bias voltage is negative.
39. The patterning device support system of any of claims 29 to 38, wherein the production-bias voltage is positive during times when the lithographic apparatus generates pulses of EUV radiation and negative between times when the lithographic apparatus generates the pulses of EUV radiation.
40. The patterning device support system of any of claims 29 to 39, wherein a magnitude of the negative production-bias voltage to the patterning surface is greater than 0.5 V, preferably greater than 1 V, less than 10 V, preferably less than 5 V and further preferably less than 3 V, and/or a magnitude of the positive production-bias voltage is greater 1 V and preferably greater than 5 V, less than 100 V and preferably less than 50 V.
41. The patterning device support system of any of the preceding claims, wherein the patterning device further comprises a conductive coating which forms the clamping surface.
42. The patterning device support system of any of the preceding claims, wherein the patterning device support system comprises the patterning device.
43. A lithographic apparatus comprising the patterning device support system according to any of the preceding claims.
44. A method of loading a patterning device onto a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: moving the patterning device towards the patterning device support; and applying a load-conductive member voltage to one or more conductive members, wherein the one or more conductive members are arranged to be electrically connected to the clamping surface when the patterning device is loaded onto the patterning device support, and the load-conductive member voltage applied to the one or more conductive members reduces a potential difference between the clamping surface and the one or more conductive members relative to a case in which the one or more conductive members are grounded.
45. The method of claim 44, wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprise one or more of the plurality of burls.
46. The method of claim 44 or 45, wherein, the application of the load-conductive member voltage reduces the potential difference between the clamping surface and the one or more conductive members to less than 50 V, preferably less than 10 V, further preferably less than 1 V, and further preferably less than 0.1 V.
47. The method of any of claims 44 to 46, wherein the load-conductive member voltage is substantially the same as the voltage of the clamping surface, optionally wherein the load- conductive member voltage is substantially the same as the voltage of the clamping surface when the clamping surface comes into contact with the patterning device support.
48. The method of any of claims 44 to 47, further comprising determining the voltage of the clamping surface, and determining the load-conductive member voltage based on the voltage of the clamping surface.
49. The method of any of claims 44 to 47, further comprising determining the load-conductive member voltage based on measurements made by one or more sensors.
50. The method of claim 49, wherein the load-conductive member voltage applied to the one or more conductive members is applied by a conductive member voltage source, and the method further comprises: measuring a current between the one or more conductive members and the conductive member voltage source; and determining the load-conductive member voltage based on the measured current between the one or more conductive members and the conductive member voltage source.
51. The method of any of claim 49, wherein the patterning device support comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes, and the method further comprises: measuring a current between the plurality of electrodes and the clamping voltage source; and determining the load-conductive member voltage based on the measured current between the plurality of electrodes and the clamping voltage source.
52. The method of claim 49, wherein, while the patterning device is moved towards the patterning device support, the patterning device is supported by a patterning device handler, and the method further comprises: determining the load-conductive member voltage based on measurements made by a sensor of the patterning device handler, optionally wherein the sensor of the patterning device handler comprises one or more of: a voltage sensor configured to measure a voltage of the clamping surface, a strain gauge configured to measure a strain of the patterning device and a force sensor configured to measure a contact force between the patterning device and the patterning device handler.
53. The method of any of claims 44 to 47, further comprising determining the load- conductive member voltage based on an average voltage of the electrodes in the patterning device support.
54. A method of unloading a patterning device from a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: in a period before unloading the patterning device from the patterning device support, applying an unload-bias voltage to the clamping surface via one or more conductive members; and then moving the patterning device away from the patterning device support, wherein the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device, relative to a case in which the clamping surface is grounded via the one or more conductive members.
55. The method of claim 54, wherein the applying the unload-bias voltage to the clamping surface via the one or more conductive members comprises applying an unload-conductive member voltage to the one or more conductive members, and the unload-conductive member voltage is also applied to the one or more conductive members after the patterning device has moved away from the patterning device support and the one or more conductive members are no longer in contact with the clamping surface.
56. The method of claim 54 or 55, wherein the patterning device support comprises a plurality of burls configured to contact the clamping surface of the patterning device, and the one or more conductive members comprises one or more of the plurality of burls.
57. The method of any of claims 54 to 56, wherein the patterning device support further comprises a plurality of electrodes and a clamping voltage source configured to apply a voltage to each of the plurality of electrodes, and the unload-bias voltage has the same polarity as an average voltage of the plurality of electrodes.
58. The method of any of claims 54 to 57, wherein the unload-bias voltage applied to the one or more conductive members is greater than 70% of the average voltage of the plurality of electrodes, preferably greater than 80% of the average voltage of the plurality of electrodes, further preferably greater than 90% of the average voltage of the plurality of electrodes, and less than 110% of the average voltage of the plurality of electrodes, preferably less than 100% of the average voltage of the plurality of electrodes, and further preferably less than 95% of the average voltage of the plurality of electrodes.
59. The method of any of claims 54 to 58, further comprising determining the unload-bias voltage based on measurements made by one or more sensors.
60. The method of any of claims 54 to 59, wherein the bias voltage applied to the clamping surface via the one or more conductive members is applied by a conductive member voltage source, and the method further comprises measuring a current between the one or more conductive members and the conductive member voltage source.
61. The method of claim 60, further comprising, if, after the patterning device has been separated from the patterning device support, the current between the one or more conductive members and the conductive member voltage source exceeds a first predetermined threshold: moving the patterning device towards the patterning device support, such that the clamping surface becomes electrically connected to the one or more conductive members; and adjusting the unload-bias voltage based on the measured current between the one or more conductive members and the conductive member voltage source.
62. The method of any of claims 54 to 61, further comprising measuring a current between the plurality of electrodes and the clamping voltage source.
63. The method of claim 62, further comprising, if, after the patterning device has been separated from the patterning device support, the current between the plurality of electrodes and the clamping voltage source exceeds a second predetermined threshold: moving the patterning device towards the patterning device support, such that the clamping surface becomes electrically connected to the one or more conductive members; and adjusting the unload-bias voltage based on the measured current between the plurality of electrodes and the clamping voltage source.
64. The method of any of claims 54 to 63, wherein, while the patterning device is moved away the patterning device support, the patterning device is supported by a patterning device handler, and the method further comprises determining whether to return the patterning device to towards the patterning device support such that the clamping surface is electrically connected to the one or more conductive members based on measurements made by a sensor of the patterning device handler.
65. The method of claim 64, further comprising: measuring a voltage of the patterning device with the patterning device handler; and if, after the patterning device has been separated from the patterning device support, the voltage of the patterning device exceeds a third predetermined threshold: moving the patterning device towards the patterning device support, such that the clamping surface becomes electrically connected to the one or more conductive members; and adjusting the unload-bias voltage based on the measured voltage of the patterning device.
66. A method of supporting a patterning device on a patterning device support, the method comprising the method of loading the patterning device onto the patterning device support of any of claims 44 to 53 and/or the method of unloading the patterning device from the patterning device support according to any of claims 54 to 65.
67. The method of claim 66, wherein a period in which the patterning surface is cyclically exposed to EUV radiation is referred to as a production period, and the method further comprises applying a production-bias voltage to the clamping surface and/or the patterning surface during the production period.
68. The method of claim 67, wherein the production-bias voltage is capacitively induced in the clamping surface and/or the patterning surface, optionally wherein the production-bias voltage is capacitively induced in the clamping surface and/or the patterning surface by controlling an average voltage of a plurality of electrodes in the patterning device support.
69. The method of claim 67, wherein the production-bias voltage is applied to the clamping surface from the conductive member voltage source via the one or more conductive members.
70. The method of claim 69, wherein the patterning surface is electrically connected to the clamping surface, such that applying a production-bias voltage to the clamping surface via the one or more conductive members results in the application of the production-bias voltage to the patterning surface.
71. The method of claim 67 to 70, wherein the production-bias voltage is negative.
72. The method of any of claims 67 to 71, wherein, in the production period, the production-bias voltage is applied to the clamping surface between pulses of radiation, and the patterning device is electrically isolated during pulses of radiation.
73. The method of any of claims 67 to 72 wherein the production-bias voltage is positive during times when the lithographic apparatus generates pulses of EUV radiation and negative between times when the lithographic apparatus generates the pulses of EUV radiation.
74. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support the patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a voltage source configured to apply a voltage to the clamping surface and/or the conductive members; and a controller, wherein, during loading of the patterning device onto the patterning device support, the controller is configured to control the voltage source to apply a load voltage to the clamping surface and/or the one or more conductive members to reduce a potential difference between the clamping surface and the one or more conductive members.
75. A patterning device support system for use in a lithographic apparatus, the patterning device support system comprising: a patterning device support configured to support the patterning device having a patterning surface and a clamping surface opposite the patterning surface, wherein the clamping surface faces the patterning device support; one or more conductive members configured to be electrically connected to the clamping surface while the patterning device is clamped to the patterning device support; a voltage source configured to apply a bias voltage to the clamping surface; and a controller, wherein, in a period before unloading of the patterning device from the patterning device support, the controller is configured to control the voltage source to apply an unloadbias voltage to the clamping surface, and the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device.
76. A method of loading a patterning device onto a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, the patterning device support comprising one or more conductive members, wherein the one or more conductive members are arranged to be electrically connected to the clamping surface when the patterning device is loaded onto the patterning device support, and the method comprises: moving the patterning device towards the patterning device support; and applying a load voltage to the clamping surface and/or the one or more conductive members, wherein the load voltage applied to the one or more conductive members and/or the clamping surface reduces a potential difference between the clamping surface and the one or more conductive members.
77. A method of unloading a patterning device from a patterning device support, the patterning device comprising a patterning surface and a clamping surface opposite the patterning surface, and the method comprising: in a period before unloading the patterning device from the patterning device support, applying an unload-bias voltage to the clamping surface; and then moving the patterning device away from the patterning device support, wherein the unload-bias voltage reduces a charge on the clamping surface before the patterning device is separated from the patterning device.
78. A method of manufacturing a device comprising the method of supporting a patterning device according to any of claims 44 to 77.
79. A computer program comprising instructions which, when executed by a controller for a lithographic apparatus, cause the lithographic apparatus to carry out the method of any of claims 44 to 77.
PCT/EP2025/056585 2024-03-28 2025-03-11 Patterning device voltage biasing system Pending WO2025201867A1 (en)

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