NL2013676A

NL2013676A - A method of clamping articles for a lithographic apparatus, a controller for a lithographic apparatus, a chuck, a method of using a chuck and a device manufacturing method.

Info

Publication number: NL2013676A
Application number: NL2013676A
Authority: NL
Inventors: Ronald Wilk
Original assignee: Asml Netherlands Bv
Priority date: 2014-02-11
Filing date: 2014-10-24
Publication date: 2015-08-17
Also published as: WO2015120923A1

Description

A METHOD OF CLAMPING ARTICLES FOR A LITHOGRAPHIC APPARATUS. A CONTROLLER FOR A LITHOGRAPHIC APPARATUS. A CHUCK. A METHOD OF USING A CHUCK AND A DEVICE MANUFACTURING METHOD

FIELD

[0001] The present invention relates to a method of clamping articles for a lithographic apparatus, a controller for a lithographic apparatus, a chuck, a method of using a chuck and a device manufacturing method.

BACKGROUND

[0002] 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.

[0003] 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.

[0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

where λ 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 λ, by increasing the numerical aperture NA or by decreasing the value of kl.

[0005] 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.

[0006] EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

SUMMARY OF THE INVENTION

[0007] A chuck may be provided for holding an article such as a patterning device for imparting a radiation beam with a pattern in its cross section or a substrate for receiving the patterned radiation beam. The chuck may be configured to hold the article by electrostatic attraction. The chuck may provide lohnsen-Rahbek (JR) clamping, Coulomb clamping, or both, for example.

[0008] In use the chuck holds the article onto a supporting component by electrostatic force. When the article is held, the clamping force can increase in time. This is because of the complex impedance of the dielectric material of the chuck. According to an electrical model, the dielectric material may be considered as a combination of multiple capacitor and resistor components. This reduces the predictability and controllability of the clamping force of the chuck. Additionally, the increase in clamping force can lead to a residual clamping force even when the chuck is disconnected from the power supply. This can make it difficult to release the article from the chuck and/or make it difficult to release the article smoothly.

[0009] It is desirable to provide a method of clamping substrates with improved predictability and controllability of the clamping force. It is desirable to provide a controller for a lithographic apparatus, a method of using a chuck and a device manufacturing method corresponding to the improved method of clamping substrates. It is desirable to provide a chuck that can be used in the method.

[0010] According to an aspect of the invention, there is provided a method of clamping articles for a lithographic apparatus, the method comprising: providing a chuck comprising an electrode connected to a power supply; providing an article on the chuck; applying with the power supply to the electrode a voltage having a clamping polarity so as to provide a clamping force between the chuck and the article on the chuck; replacing the article on the chuck with a succeeding article on the chuck; and applying a voltage having the clamping polarity to the electrode so as to provide a clamping force between the clamp and the succeeding article on the chuck.

[0011] According to an aspect of the invention, there is provided a controller configured to: control a power supply to apply to an electrode of a chuck a voltage having a clamping polarity so as to provide a clamping force between the chuck and an article on the chuck; control the power supply to apply to the electrode a voltage having the clamping polarity so as to provide a clamping force between the chuck and a succeeding article on the chuck.

[0012] According to an aspect of the invention, there is provided a chuck for use in holding an article onto a supporting component of a lithography apparatus by electrostatic force, said chuck comprising: a dielectric member; and an electrode configured to impart a charge to a surface of the dielectric member; wherein the dielectric member is formed of a material selected from a group consisting of cordierite, and aluminium oxide, parylene® and cyclotene®.

[0013] According to an aspect of the invention, there is provided a method of using a chuck for clamping articles for a lithographic apparatus, the method comprising: providing the chuck comprising an electrode connected to a power supply; providing an article on the chuck; applying with the power supply to the electrode a voltage having a clamping polarity so as to provide a clamping force between the chuck and the article on the chuck; replacing the article on the chuck with a succeeding article on the chuck; and applying a voltage having the clamping polarity to the electrode so as to provide a clamping force between the clamp and the succeeding article on the chuck.

[0014] According to an aspect of the invention, there is provided a device manufacturing method comprising: clamping substrates for a lithographic apparatus by: providing a chuck comprising an electrode connected to a power supply; providing a substrate on the chuck; applying with the power supply to the electrode a voltage having a clamping polarity so as to provide a clamping force between the chuck and the substrate on the chuck; replacing the substrate on the chuck with a succeeding substrate on the chuck; and applying a voltage having the clamping polarity to the electrode so as to provide a clamping force between the clamp and the succeeding substrate on the chuck; and for each succeeding substrate, using the lithographic apparatus to transfer a pattern from a patterning device to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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, and in which: [0016] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention; [0017] Figure 2 is a more detailed view of the apparatus 100; [0018] Figure 3 is a more detailed view of the source collector module SO of the apparatus of Figures 1 and 2; [0019] Figures 4 and 5 to 13 each depict a chuck according to an embodiment of the present invention; and [0020] Figures 6 and 7 each depict a graph showing how a potential at a surface of the chuck varies over time; [0021] Figure 8 depicts schematically a chuck according to an embodiment of the present invention; and [0022] Figures 9 to 11 each depict a graph showing how a potential at a surface of the chuck varies over time according to an embodiment of the present invention.

[0023] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DF.TATT F,D DESCRIPTION

[0024] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module SO according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation). a support structure (e.g., a mask table) MT constmcted 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 constmcted 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.

[0025] The illumination system 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.

[0026] The support stmeture 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 stmeture can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support stmeture may be a frame or a table, for example, which may be fixed or movable as required. The support stmeture may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0027] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0028] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks arc 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.

[0029] The projection system, like the illumination system, 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.

[0030] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0031] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0032] Referring to Figure 1, the illuminator IL receives an extreme ultra violet 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 may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0033] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module 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, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0034] The illuminator 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 σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0035] 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. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam 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 PS1 can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0036] The depicted apparatus could be used in at least one of the following modes: [0037] 1. In step mode, the support structure (e.g., mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

[0038] 2. In scan mode, the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0039] 3. In another mode, the support structure (e.g., mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0040] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0041] Figure 2 shows the apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector module SO. An EUV radiation emitting plasma 210 may be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum.

The very hot plasma 210 is created by, for example, an electrical discharge causing an at least partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0042] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap) that is positioned in or behind an opening in source chamber 211. The contaminant trap 230 may include a channel structure. Contamination trap 230 may also include a gas barrier or a combination of a gas barrier and a channel stmeture. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure, as known in the art.

[0043] The collector chamber 211 may include a radiation collector CO, which may be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to 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 is arranged such that the intermediate focus 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.

[0044] 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 radiation 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 beam of radiation 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 wafer stage or substrate table WT.

[0045] More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 may optionally be present, depending upon the type of lithographic apparatus. 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.

[0046] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with grazing incidence reflectors 253, 254 and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254 and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0047] Alternatively, the source collector module SO may be part of an LPP radiation system as shown in Figure 3. A laser LA is arranged to deposit laser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma 210 with electron temperatures of several 10’s of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near normal incidence collector optic CO and focused onto the opening 221 in the enclosing structure 220.

[0048] In an embodiment, a chuck 50 is provided for holding via electrostatic force onto a supporting component of a lithographic apparatus an article. Such a chuck 50 may be referred to as an electrostatic chuck. The article may be, for example, a patterning device MA or a substrate W. The supporting component may be, for example, a substrate table WT of the lithography apparatus. In the description below, the invention is described with reference to the article being a substrate W and the supporting component being a substrate table WT. This is for convenience and brevity. The article may be an article other than a substrate W, for example a patterning device MA. The supporting component may be a supporting component other than a substrate table WT, for example a mask table MT.

[0049] In use the chuck 50 holds the substrate W onto the substrate table WT by electrostatic force. When the substrate W is removed, there may be residual charge at the surface of the substrate W and/or at the surface of the chuck. Such a residual charge can produce a holding force even when the electrostatic chuck is no longer powered electrically. This can make it difficult to release the substrate W from the chuck and make it difficult to release the substrate W smoothly. Additionally or alternatively, this can cause problem related to loading a succeeding substrate W. For example, any residual surface voltage can lead to an unwanted clamping force, which in turn can lead to higher friction. Higher friction can lead to stress on the substrate W and consequent deformation of the substrate W.

[0050] When the chuck holds the substrate W on the substrate table WT, the voltage drop over the dielectric material of the chuck 50 can vary in time. In particular the clamping force on the substrate W can increase in time as the substrate W remains on the chuck 50. This can reduce the predictability and/or controllability of the clamping force on the substrate W.

[0051] Figure 4 illustrates an electrostatic chuck 50 according to an embodiment of the invention. In the embodiment shown, the chuck 50 is mounted on a substrate table WT.

In an embodiment the chuck 50 comprises a dielectric member 45. The dielectric member 45 may have a thickness in the region of from a bout 0.5 cm to about 2 cm, and preferably about 1 cm. The thickness of the dielectric member 45 is measured from an upper base surface 47 of the dielectric member 45 to a lower base surface 49 of the dielectric member 45.

[0052] In the following description, the terms ground and zero voltage are used for convenience. It is not necessary for any part of the lithographic apparatus to be grounded or to have zero potential. The terms ground and zero voltage should be interpreted to mean a voltage approximately equal to the voltage of the substrate W that is held by the chuck 50.

[0053] In an embodiment the chuck 50 comprises an electrode 40 configured to impart a charge to a surface (e.g. the upper base surface 47) of the dielectric member 45. In an embodiment the chuck 50 further comprises a lower electrode 44 configured to impart a charge to a surface (e.g. the lower base surface 49) of the dielectric member 45. In an embodiment the chuck 50 comprises more than one of the electrode 40 and/or more than one of the lower electrode 44.

[0054] The electrode 40 and the lower electrode 44 are each for applying a potential difference between the electrode 40, 44 and the surface (e.g. the upper base surface 47 or the lower base surface 49) of the dielectric member 45. The potential difference may be such as to cause a charge to accumulate on the upper base surface 47, for example. The accumulated charge can electrostatically attract and hold a substrate W in contact with the chuck 50. In particular the substrate W may be in close contact with the chuck 50, providing contact area and no-contact areas depending on the roughness of the materials used. The clamping force is located in the small no-contact areas that are formed due to the roughness of the materials. This effect is known as the Johnsen-Rahbek effect. A chuck that operates based on this effect may be referred to as a Johnsen-Rahbek chuck, a J-R chuck, or a JR chuck. In an embodiment the chuck 50 of the present invention is arranged to clamp the substrate W electrostatically at least partly by Johnsen-Rahbek clamping. The J-R attraction force increases approximately linearly with the voltage applied to the electrode 40.

[0055] In an embodiment, the dielectric member 45 may have a resistivity that is so high that no significant current can pass between the electrode 40 and the upper base surface 47. In this case, the Johnsen-Rahbek effect may not occur to any significant extent.

However, an attractive force between the electrode 40 and the substrate W may still occur due to the potential difference between these elements. The electrode 40 and the substrate W may act as two plates of a capacitor and be attracted to each other in the same way as the two plates of a charge capacitor. Chucks/clamps that operate predominantly on this principle may be referred to as Coulombic clamps. J-R clamps will also involve some degree of attraction by the Coulombic mechanism, but the J-R effect will normally be dominant in such clamps.

In an embodiment the chuck 50 of the present invention is arranged to clamp the substrate W electrostatically by Coulombic clamping.

[0056] Figure 5 depicts a chuck 50 according to an embodiment of the invention. In an embodiment the chuck 50 comprises a plurality of upper burls 57 protruding above the upper base surface 47 of the chuck 50. Each of the plurality of upper burls 57 has a respective distal end 58. The plurality of upper burls 57 are arranged such that, when the substrate W is supported by the chuck 50, the substrate W is supported by the respective distal end 58 of each of the plurality of the upper burls 57.

[0057] An advantage of the upper burls 57 is that they reduce the impact of contaminant particles on substrate flatness. This is because a particle can only cause a deformation of the flatness when on N upper burls 57, unless the particle is larger than the gap between the chuck 50 and the substrate W. In such an embodiment the contact area between the substrate W and the chuck 50 is a percentage of the total area of the upper surface of the chuck 50. For example the percentage may be in the region of from about 0.5% to about 10%, and preferably about 1.5%. In an embodiment the upper burls 57 may each have a height within the region of from about 2 micrometres to about 200 micrometres and preferably about 10 micrometres. The height is how far the upper burl 57 protrudes from the upper base surface 47 between the upper burls 57.

[0058] Another advantage of the upper burls 57 is that they allow back fill gas to sit between the substrate W and the chuck 50, thereby providing better thermal conductivity between the substrate W and the chuck 50. In such an embodiment, the region between the substrate table WT and the substrate W may be maintained at partial vacuum. Such an arrangement will tend to facilitate removal of the substrate W, for example because Van Dcr Waals forces acting between a substrate table WT and the substrate W are less strong.

[0059] In an embodiment the chuck 50 comprises a plurality of lower burls 59 protruding below the lower base surface 49 of the chuck 50. Each of the plurality of lower burls 59 has a respective distal end 60. The plurality of lower burls 59 are arranged such that, when the chuck 50 is supported on the substrate table WT, the chuck 50 is supported by the respective distal end 60 of each of the plurality of the lower burls 59. In an embodiment the dimensions of the lower burls 59 is the same as the dimensions of the upper burls 57.

[0060] In an embodiment the lithographic apparatus comprises a controller 500. In an embodiment the lithographic apparatus comprises a power supply 80. In an embodiment the controller 500 is configured to control the power supply 80 to apply to the electrode 40 a voltage having a clamping polarity so as to provide a clamping force between the chuck 50 and the substrate W on the chuck 50.

[0061] Figure 6 depicts how the voltage of the electrode 40 and the voltage (or potential relative to the substrate W, which may be at ground potential) on the upper base surface 47 of the dielectric member 45 varies over time during clamping of a substrate W. In Figure 6, the X axis represents time. The Y axis represents voltage (or potential). In Figure 6, the dotted line represents the voltage applied to the electrode 40. The solid line represents the voltage of the upper base surface 47 of the dielectric member 45.

[0062] A substrate W is positioned on the chuck 50 so that the substrate W is in contact with the distal ends 58 of the upper burls 57. Initially (i.e. at time t = 0), the power supply 80 does not apply any voltage to the electrode 40. Subsequently, the controller 500 controls the power supply 80 to apply to the electrode 40 a voltage having a clamping polarity. This provides a clamping force between the chuck 50 and the first substrate W1 on the chuck 50.

[0063] When the voltage having the clamping polarity is applied to the electrode 40, the voltage of the upper base surface 47 increases to be voltage Vix. The voltage Vix of the upper base surface 47 is less than the voltage having the clamping polarity applied to the electrode 40. While the first substrate W1 is being clamped, the voltage at the upper base surface 47 increases from the voltage V ix to the voltage V\y, as shown in Figure 6. The clamping force applied by the chuck 50 on the substrate W1 increases when the voltage at the upper base surface 47 increases. As the voltage of the upper base surface 47 increases from Vix to Viy, the clamping force on the first substrate W1 increases.

[0064] When the first substrate W1 is to be replaced with the second substrate (not shown in Figure 6), the controller 500 controls the power supply 80 so as to reduce the clamping force. For example, the controller 500 may be configured to control the power supply 80 to stop applying the voltage having the clamping polarity so as to reduce the clamping force. In an embodiment the voltage applied to the electrode 40 may be reduced not quite to zero.

[0065] In Figure 6, it is depicted that the voltage applied to the electrode 40 is decreased to zero, as shown by the dotted line. When the voltage applied to the electrode 40 is reduced to zero or near zero, there remains a residual voltage at the upper base surface 47 of the dielectric member 45. The residual voltage provides a residual clamping force on the first substrate W1. The residual voltage decreases over time to zero.

[0066] In an embodiment the method of clamping substrates for a lithographic apparatus comprises replacing the substrate W on the chuck 50 with a succeeding substrate W on the chuck 50. For example, the first substrate W1 on the chuck 50 may be replaced with the succeeding second substrate W2 on the chuck 50.

[0067] Figure 7 depicts how the voltage at the upper base surface 47 may vary in time when a typical method of clamping is employed. In the method corresponding to the graph of Figure 7, the polarity of the voltage applied to the electrode 40 is switched between substrates W. Initially, the voltage at the upper base surface 47 during the first clamping period for clamping the first substrate W1 is the same as depicted in Figure 6, for example. However, during the second clamping period for clamping the second substrate W2, the voltage applied to the electrode 40 has opposite polarity, but the same magnitude. When the voltage having the opposite polarity is applied, the voltage at the upper base surface 47 decreases by the amount Vs. The potential difference Vs is the step in voltage at the upper base surface 47. The magnitude of the voltage step Vs is the same for both the first substrate W1 and the second substrate W2.

[0068] When the voltage having the opposite polarity is applied to the electrode 40, the voltage at the upper base surface initially has the value V2X. The magnitude of the voltage V2x is less than the magnitude of the voltage Vrf, (i.e. the voltage at the upper base surface 47 at the beginning of the first clamping period). This is because of the residual voltage at the end of the first clamping period.

[0069] The voltage at the upper base surface 47 decreases over time due to the dielectric relaxation of the dielectric member 45 and reaches the voltage V2y at the end of the second clamping period. The magnitude of the voltage V2y is less than the magnitude of the voltage Viy. Hcncc, the clamping force applied to the second substrate W2 is less than the clamping force applied to the first substrate W1. This causes a problem in controlling and predicting the clamping force applied to succeeding substrates W.

[0070] Figure 8 depicts schematically the clamping system of the present invention in terms of two capacitors. The clamping system can be modeled as a capacitor between the electrode 40 and the upper base surface 47 and another capacitor between the upper base surface 47 and the substrate W. In an embodiment the substrate W may be at substantially zero voltage, i.e. at ground. However, this is not necessarily the case. In an embodiment the substrate W has a predetermined voltage. In an embodiment the electrode 40 is connected electrically to the power supply 80.

[0071] The clamping force applied by the chuck 50 on the substrate W increases when the strength of the electric field between the upper base surface 47 and the substrate W increases. The clamping force is proportional with the square of the electrical field. This means that, for example, the undesirable effects due to a deviation at the upper base surface 47 of 10V from a targeted level of 1000V is more severe than a deviation of 10 V from a targeted level of around (i.e. zero). Hence when half of the maximum voltage range (due to relaxation) is compensated for by an offset, then there wold be a very low clamping force when targeting zero potential at the upper base surface 47. The clamping force is indirectly proportional to the distance dg between the upper base surface 47 and the lower surface of the substrate W. The distance dg is constant and is not varied during use of the chuck 50. Hence, in use the clamping force depends on the strength of the electric field.

[0072] The strength of the electric field is directly proportional to the potential difference between the upper base surface 47 and the substrate W. The potential of the substrate W is substantially constant in use of the chuck 50. Hence, the clamping force is directly proportional to the voltage at the upper base surface 47 of the dielectric member 45.

[0073] The capacitor between the upper base surface 47 and the substrate W may be termed the gap capacitor having a gap capacitance Cg. The capacitor between the electrode 40 and the upper base surface 47 may be termed the dielectric capacitor having a dielectric capacitance of Cd- The voltage at the upper base surface 47 is related to the gap capacitance Cg and the dielectric capacitance Cd by the following equation.

[0074] The gap capacitance Cg remains substantially constant during use of the lithographic apparatus. However, the dielectric capacitance Cd is not that of an ideal capacitor. This is due to dielectric relaxation of the material of the dielectric member 45.

The dielectric relaxation of the dielectric member 45 results in an apparent increase of the dielectric capacitance Cd- When the dielectric capacitance Cd increases, the voltage at the upper hase surface 47 increases, as shown in Figures 6 and 7, for example, and as can be derived from the above equation.

[0075] Dielectric relaxation refers to the relaxation response of the dielectric member 45 to the external electric field caused by the electrode 40. This relaxation may be described in terms of permittivity of the dielectric member 45. The dielectric relaxation causes the permittivity of the dielectric member to increase, thereby resulting in an apparent increase of the dielectric capacitance Cd, which is related to an increase of the voltage at the upper base surface 47. This dielectric relaxation can cause problems as explained above in relation to Figure 7.

[0076] In an embodiment the method comprises applying a voltage having the clamping polarity to the electrode 40 so as to provide a clamping force between the clamp 50 and the subsequent substrate W on the chuck 50. For example, in an embodiment a voltage having the same clamping polarity is applied to the electrode 40 when clamping both the first substrate W1 and when clamping the second substrate W2. By applying the voltage having the same clamping polarity to the electrode 40 when clamping the succeeding substrate W on the chuck 50, the problems described in relation to Figure 7 above are overcome.

[0077] It has previously been typical for the polarity of the voltage applied to the electrode 40 to be switched between succeeding substrates W. This is because of an issue that will be described below with reference to Figure 9 in particular.

[0078] Figure 9 depicts how the voltage at the upper base surface 47 varies over time when a series of substrates Wl, W2, W3 are held by the chuck 50. As shown in Figure 6, when the first substrate Wl is held by the chuck 50, the voltage at the upper base surface 47 increases to Viy. When the first substrate Wl is to be replaced by the second substrate W2, the voltage having the clamping polarity is no longer applied to the electrode 40. A residual voltage remains at the upper base surface 47. The residual voltage decreases over time. However, the residual voltage docs not decrease to zero before the voltage having the clamping polarity is again applied to the electrode 40 in order to provide a clamping force on the second substrate W2.

[0079] As shown in Figure 9, this means that the initial voltage at the upper base surface 47 for clamping the second substrate W2 is greater than the initial voltage at the upper base surface 47 for clamping the first substrate W1. Over time the voltage at the upper base surface 47 increases to reach the voltage V2y· The voltage V2y of the upper base surface 47 at the end of the second clamping period for clamping the second substrate W2 is greater than the voltage Viy of the upper base surface 47 at the end of the first clamping period for clamping the first substrate W1. Accordingly, the clamping force applied to the second substrate W2 is greater than the clamping force applied to the first substrate Wl.

[0080] Figure 9 depicts a third clamping period in which a third substrate W3 is clamped by the chuck 50. As shown in Figure 9, the voltage of the upper base surface 47 increases to the voltage V3y at the end of the third clamping period. The clamping force applied to the third substrate W3 is greater than the clamping force applied to the second substrate W2 or the first substrate Wl.

[0081] It can be difficult to control and/or predict the clamping force applied to each substrate W. In particular there is a possibility that the clamping force increases with each succeeding substrate W.

[0082] Depending on the resistance of the dielectric member 45, there is a possibility that the voltage at the upper base surface 47 could continue to increase for each succeeding substrate W. For example, the voltage at the upper base surface 47 could, in theory, eventually reach the voltage applied to the electrode 40. This would be undesirable. For example this would increase the possibility of an undesirable electrical breakdown between the upper base surface 47 and the substrate W. This electrical breakdown could lead to a reduction of clamping force, and/or a high initial force when loading a succeeding substrate W.

[0083] However, the inventors of the present invention have found that the dielectric relaxation of the dielectric member 45 can reach a saturation point. At the saturation point, the permittivity and apparent dielectric capacitance Cd reaches a stable value and no longer increases.

[0084] Figure 10 depicts how the voltage at the upper base surface 47 varies over time for a series of succeeding substrates W being clamping by the chuck 50. In the example shown I Figure 10, for the first three substrates Wl, W2, W3, the voltage at the upper base surface 47 varies in time as depicted in Figure 9. However, during the fourth clamping period for clamping the fourth substrate W4, the dielectric relaxation of the dielectric member 45 reaches saturation point. The saturation point is reached at time ti. At time h, the voltage at the upper base surface 47 reaches the limit voltage Vi. The relaxation has been saturated in one direction (i.e. by applying a clamping polarity voltage to the electrode 40). The relaxation could be saturated in a different direction, for example applying a voltage having the opposite polarity to the electrode 40, and keeping the same polarity for succeeding substrates W.

[0085] As depicted in Figure 10, for the fifth substrate W5, the sixth substrate W6 and the seventh substrate W7 (and beyond), the clamping force applied to each substrate W is consistent. The clamping force applied to the fifth substrate W5 is the same as the clamping force applied to the sixth substrate W6 and the seventh substrate W7 and so on. This is because relaxation of the dielectric member 45 has been saturated.

[0086] Experimental measurements have shown that the resistance of the dielectric member 45 is much higher than had typically been expected in the art. The significantly higher resistance means that the dielectric relaxation of the dielectric member 45 can reach the saturation point. Due to the saturation of the dielectric relaxation, the maximum residual voltage Vr at the upper base surface 47 is capped, as shown in Figure 10.

[0087] In an embodiment, the step of replacing the substrate W with a succeeding substrate W is repeated a plurality of times. In an embodiment for each succeeding substrate W, a voltage having the clamping polarity is applied to the electrode 40 so as to provide a clamping force between the chuck 50 and the succeeding substrate W. Hence, for each succeeding substrate W, the clamping force is predictable and can be controlled. In particular the clamping force can be accurately controlled by the controller 500 controlling the power supply 80 to apply the voltage having the same clamping polarity to the electrode 40.

[0088] In an embodiment the chuck 50 comprises a dielectric member 45 having a capacitance Cd- The electrode 40 is configured to impart a charge to a surface (e.g. the upper base surface 47) of the dielectric member 45. In an embodiment the voltage having the clamping polarity is applied to the electrode 40 for a period of time long enough for the imparted charge to be substantially stable following dielectric relaxation. This is depicted in Figure 11.

[0089] As depicted in Figure 11, the first clamping period is longer than the subsequent clamping periods. The first clamping period is sufficiently long for the charge imparted to the upper base surface 47 to be substantially stable following dielectric relaxation. The charge imparted to the upper base surface 47 becomes substantially stable at the time q. The time q is the point at which the dielectric relaxation of the dielectric member 45 has reached the saturation point. Following the first clamping period, the clamping force applied to each subsequent substrate W is predictable and can be controlled accurately.

[0090] In an embodiment the dielectric member 45 is formed from a material selected from a group consisting of cordierite, an alkali-free aluminoborosilicate glass, aluminium oxide, silicone oxide, ULE®, and other isolators such as parylene® or cyclotene®.

[0091] The present invention reduces the electrostatic demands on the material used for the dielectric member 45. In particular, some materials had not previously been considered because of the large change in capacitance of the dielectric member 45 due to dielectric relaxation that occurs during use of the lithographic apparatus. For example, for an alkali-free aluminoborosilicate glass (e.g. AF32), the change in capacitance of the dielectric member 45 may be less than 5%. However, for some other materials such as cordierite and aluminium oxide, the change in apparent capacitance of the dielectric material due to dielectric relaxation could be much greater, for example an order of magnitude greater.

Hence materials such as cordierite and aluminium oxide had not previously been considered as a suitable material for the dielectric member 45 because the large capacitance change exacerbates the issue identified above and described in relation to Figure 9. According to the invention these materials such as cordierite and aluminium oxide can be used to form the dielectric member 45 of the chuck 50.

[0092] However, employing the present invention, a problem of an increase in capacitance of the dielectric material due to dielectric relaxation is reduced. This is because even when the capacitance can change a large amount, once the saturation point has been reached, the clamping force applied to each succeeding substrate W is consistent as shown in Figures 10 and 11, for example.

[0093] In an embodiment the voltage having the clamping polarity is lower than a voltage of the substrate W. For example, if the substrate W has a ground potential, then the voltage having the clamping polarity may be negative. In other words the clamping polarity may be negative. A negative clamping polarity has the advantage that it reduces the possibility of electrons travelling from the dielectric member 45 to a surface of the substrate W resulting in a reduction of clamping force.

[0094] In an embodiment between the two steps of applying the voltage having the clamping polarity (i.e. to succeeding substrates W), a surface (e.g. the upper base surface 47) of the chuck 50 is discharged so as to reduce any residual clamping force on the succeeding substrate W. There are various methods for discharging the upper base surface 47 of the chuck 50. For example, UV light may be used to make ions and free electrons from hydrogen. For example, the free electrons may be produced and attracted to the upper base surface 47 so as to discharge the upper base surface 47. Alternative methods such as using a conductive brush, using a plasma or using a conductive fluid may be employed.

[0095] In an embodiment between the two steps of applying the voltage having the clamping polarity (i.e. to succeeding substrates W) a voltage having a polarity opposite to the clamping polarity is applied by the power supply 80 to the electrode 40 so as to reduce any residual clamping force on the succeeding substrate W. For example, instead of applying zero voltage to the electrode 40 between clamping periods, a small opposite polarity voltage may be applied to the electrode 40 between clamping periods. For example, if the voltage having the clamping polarity is negative, then between clamping periods there may be a small residual clamping voltage (e.g. residual voltage Vr) on the upper base surface 47. By applying a small positive voltage to the electrode 40 between clamping periods, the residual voltage on the upper base surface 47 may be decreased, thereby reducing any residual clamping force on the substrate W. This makes it easier and smoother to replace the substrate on the chuck 50. This also reduces the friction when placing the succeeding substrate W.

[0096] In an embodiment the voltage having the polarity opposite to the clamping polarity is selected so as to counteract an increase in a charge imparted on the surface (e.g. the upper base surface 47) of the chuck 50 due to dielectric relaxation of the dielectric member 45.

[0097] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0098] 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 may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0099] The term “lens,” where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[00100] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, instead of a single electrode 40 there may be a plurality of electrodes 40 that effect clamping of the substrate W. Instead of a single lower electrode 44 there may be a plurality of lower electrodes 44 that effect clamping to the substrate table WT. In particular, there may be two electrodes 40 (and/or two lower electrodes 44). This is called bipolar clamping. For bipolar clamping the substrate W being clamped may not be at ground potential.

[00101] The invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein. 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 clauses set out below. Other aspects of the invention are set-out as in the following numbered clauses.

CLAUSES 1. A method of clamping articles for a lithographic apparatus, the method comprising: providing a chuck comprising an electrode connected to a power supply; providing an article on the chuck; applying with the power supply to the electrode a voltage having a clamping polarity so as to provide a clamping force between the chuck and the article on the chuck; replacing the article on the chuck with a succeeding article on the chuck; and applying a voltage having the clamping polarity to the electrode so as to provide a clamping force between the clamp and the succeeding article on the chuck. 2. The method of clause 1, wherein the step of replacing the article with a succeeding article is repeated a plurality of times, wherein for each succeeding article, a voltage having the clamping polarity is applied to the electrode so as to provide a clamping force between the chuck and the succeeding article. 3. The method of any preceding clause, wherein: the chuck comprises a dielectric member having a capacitance, the electrode being configured to impart a charge to a surface of the dielectric member; and the voltage having the clamping polarity is applied to the electrode for a period of time long enough for the imparted charge to be substantially stable following dielectric relaxation. 4. The method of any preceding clause, wherein the dielectric member is formed from a material selected from a group consisting of cordierite, an alkali-free aluminoborosilicate glass, aluminium oxide, silicon oxide, ULE®, parylene® and cyclotene®. 5. The method of any preceding clause, wherein the voltage having the clamping polarity is lower than a voltage of the article. 6. The method of any preceding clause, wherein between the steps of applying the voltage having the clamping polarity for succeeding articles, a surface of the chuck is discharged so as to reduce any residual clamping force on the article on the chuck. 7. The method of any preceding clause, wherein between the steps of applying the voltage having the clamping polarity for succeeding articles, a superimposed correcting voltage having a polarity opposite to the clamping polarity is applied by the power supply to the electrode so as to reduce any residual clamping force on the article on the chuck. 8. The method of clause 7, wherein the superimposed correcting voltage having the polarity opposite to the clamping polarity is selected so as to counteract an increase in a charge imparted on a surface of the chuck due to dielectric relaxation of the dielectric member. 9. The method of any preceding clause, wherein the article is a substrate for receiving a patterned radiation beam. 10. A controller configured to: control a power supply to apply to an electrode of a chuck a voltage having a clamping polarity so as to provide a clamping force between the chuck and an article on the chuck; control the power supply to apply to the electrode a voltage having the clamping polarity so as to provide a clamping force between the chuck and a succeeding article on the chuck. 11. The controller of clause 10, wherein the controller is configured to repeat, for each succeeding article, controlling the power supply to apply to the electrode a voltage having the clamping polarity so as to provide a clamping force between the chuck and the succeeding article on the chuck. 12. The controller of any of clauses 10 to 11, wherein: the chuck comprises a dielectric member having a capacitance, the electrode being configured to impart a charge to a surface of the dielectric member; and the controller is configured such that the voltage having the clamping polarity is applied to the electrode for a period of time long enough for the imparted charge to be substantially stable following dielectric relaxation. 13. The controller of any of clauses 10 to 12, wherein the controller is configured such that the voltage having the clamping polarity is lower than a voltage of the article. 14. The controller of any of clauses 10 to 13, wherein the controller is configured such that between applying the voltage having the clamping polarity for succeeding articles, the controller is configured to control a surface of the chuck to be discharged so as to reduce any residual clamping force on the article on the chuck. 15. The controller of any of clauses 10 to 14, wherein the controller is configured such that between applying the voltage having the clamping polarity for succeeding articles, the controller is configured to control the power supply to apply a superimposed correcting voltage having a polarity opposite to the clamping polarity to the electrode so as to reduce any residual clamping force on the article on the chuck. 16. The controller of clause 15, wherein the controller is configured such that the superimposed correcting voltage having the polarity opposite to the clamping polarity is selected so as to counteract an increase in a charge imparted on a surface of the chuck due to dielectric relaxation of the dielectric member. 17. A lithographic apparatus comprising: a chuck comprising an electrode; a power supply connected to the electrode; and the controller of any of clauses 10 to 16. 18. A chuck for use in holding an article onto a supporting component of a lithography apparatus by electrostatic force, said chuck comprising: a dielectric member; and an electrode configured to impart a charge to a surface of the dielectric member; wherein the dielectric member is formed of a material selected from a group consisting of cordierite, aluminium oxide, parylene® and cyclotene®. 19. A lithographic apparatus comprising: the chuck of clause 18. 20. A method of using a chuck for clamping articles for a lithographic apparatus, the method comprising: providing the chuck comprising an electrode connected to a power supply; providing an article on the chuck; applying with the power supply to the electrode a voltage having a clamping polarity so as to provide a clamping force between the chuck and the article on the chuck; replacing the article on the chuck with a succeeding article on the chuck; and applying a voltage having the clamping polarity to the electrode so as to provide a clamping force between the clamp and the succeeding article on the chuck. 21. A device manufacturing method comprising: clamping substrates for a lithographic apparatus by: providing a chuck comprising an electrode connected to a power supply; providing a substrate on the chuck; applying with the power supply to the electrode a voltage having a clamping polarity so as to provide a clamping force between the chuck and the substrate on the chuck; replacing the substrate on the chuck with a succeeding substrate on the chuck; and applying a voltage having the clamping polarity to the electrode so as to provide a clamping force between the clamp and the succeeding substrate on the chuck; and for each succeeding substrate, using the lithographic apparatus to transfer a pattern from a patterning device to the substrate.

Claims

A lithography device comprising: an exposure device adapted to supply a radiation bundle; a carrier constructed to support a patterning device, the patterning device being able to apply a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.