WO2025214735A1

WO2025214735A1 - Measurement of an ac signal from a component

Info

Publication number: WO2025214735A1
Application number: PCT/EP2025/057538
Authority: WO
Inventors: Selwyn Yannick Frithjof CATS; Christian Gerardus Norbertus Hendricus Marie CLOIN; Zahra MARVI; Ronald Van Der Wilk; Lucas Christiaan Johan HEIJMANS; Paul Jansen
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-04-08
Filing date: 2025-03-19
Publication date: 2025-10-16
Anticipated expiration: 2026-10-08
Also published as: TW202603510A

Abstract

An electrostatic clamp system for a component in a lithographic apparatus, the electrostatic clamp system comprising: an electrostatic clamp; at least one electrode associated with the electrostatic clamp; and a measurement circuit configured to measure an AC signal from the at least one electrode indicative of a varying potential of the component.

Description

MEASUREMENT OF AN AC SIGNAL FROM A COMPONENT

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24168928.0 which was filed on 08 April 2024 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to measurement of an AC signal. More particularly, embodiments of the present invention relate to an electrostatic clamp system for a component in a lithographic apparatus, a lithographic apparatus comprising the electrostatic clamp system, and a method of operating an electrostatic clamp system.

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 X 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 X, 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] Once the EUV radiation has been generated, it is directed through the lithographic apparatus by a plurality of mirrors to a component such as a patterning surface of a patterning device, which imparts the desired pattern to the EUV radiation.

[0008] It is desirable to control the energy level of the EUV radiation as being generated. The energy can be measured for calibration purposes. The measurement of the energy takes time and it uses a proportion of the EUV radiation that is generated.

SUMMARY OF THE INVENTION

[0009] An aim of the present invention is to reduce the time and/or amount of energy that it costs to measure the energy level of the EUV radiation.

[0010] According to the present invention, there is provided an electrostatic clamp system for a component in a lithographic apparatus, the electrostatic clamp system comprising: an electrostatic clamp; at least one electrode associated with the electrostatic clamp; and a measurement circuit configured to measure an AC signal from the at least one electrode indicative of a varying potential of the component.

[0011] Also according to the present invention, there is provided a method of operating an electrostatic clamp system for a component in a lithographic apparatus, the method comprising: measuring, from at least one electrode associated with an electrostatic clamp of the electrostatic clamp system, an AC signal indicative of a varying potential of the component.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] 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.

[0013] Figure 1 schematically depicts a lithographic apparatus.

Figure 2 schematically depicts a more detailed view of the lithographic apparatus. Figure 3 schematically depicts an electrostatic clamp system.

Figure 4 schematically depicts an electrostatic clamp, which may be a part of a lithographic apparatus in accordance with the present invention, and a patterning device. Figure 5 depicts a plot of the potential (V) of a first surface of a patterning device during a time in which two pulses of EUV radiation are generated by the lithographic apparatus.

Figure 6 schematically depicts an electrostatic clamp, which may be a part of a lithographic apparatus in accordance with the present invention, and a patterning device covered by a pellicle.

Figure 7 schematically depicts a measurement circuit of an electrostatic clamp system.

[0014] 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

[0015] Figure 1 schematically depicts a lithographic apparatus 100 including a radiation source 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.

[0016] 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.

[0017] 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.

[0018] 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. [0019] 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.

[0020] 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.

[0021] As here depicted, the lithographic apparatus 100 is of a reflective type (e.g., employing a reflective mask).

[0022] 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.

[0023] Referring to Figure 1 , the illumination system IL receives an extreme ultraviolet radiation beam from the radiation source SO. The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation. 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 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 radiation source 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 radiation source SO. The laser and the radiation source SO may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0024] 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 radiation source 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 radiation source SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Figure 2 shows the lithographic apparatus 100 in more detail, including the radiation source SO, the illumination system IL, and the projection system PS. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W. [0029] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 80 and a facetted pupil mirror device 81. The faceted field mirror device 80 and faceted pupil mirror device 81 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 80 and faceted pupil mirror device 81.

[0030] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 83, 84 which are configured to project the patterned EUV radiation beam B ’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 83, 84 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0031] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0032] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[0033] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0034] Figure 3 schematically depicts an electrostatic clamp system 10. The electrostatic clamp system 10 is for a component 128 in a lithographic apparatus 100. The component 128 may be, for example, a patterning device MA. Embodiments are described primarily in the context of the component being a patterning device MA. However, the component may be a component different from a patterning device MA. For example, the component 128 may be a substrate W, a sensor, a pellicle frame 132 or a fiducial. In general, the component 128 may be a component on which EUV radiation is incident during use of the lithographic apparatus 100.

[0035] In an embodiment the electrostatic clamp system 10 is for clamping the component 128. However, it is not essential that the electrostatic clamp system 10 clamps the component 128. For example, in an alternative embodiment, the electrostatic clamp system 10 may be separate from another electrostatic clamp that clamps the component 128. [0036] As shown in Figure 3, in an embodiment the electrostatic clamp system 10 comprises an electrostatic clamp 101. The electrostatic clamp 101 comprises a main body of material. The main body may be formed integrally.

[0037] As shown in Figure 3, in an embodiment the electrostatic clamp system 10 comprises at least one electrode 104. As shown in Figure 3, in an embodiment the electrostatic clamp system 10 comprises a plurality of electrodes 104. For example, Figure 3 shows four electrodes 104. The electrostatic clamp 101 may be a multipolar electrostatic clamp. Alternatively, the electrostatic clamp 101 may be a monopolar electrostatic clamp.

[0038] The electrostatic clamp 101 and the at least one electrode 104 are described in more detail with reference to Figure 4 and Figure 6, for example.

[0039] As shown in Figure 3, in an embodiment the at least one electrode 104 is associated with the electrostatic clamp 101. For example, the electrode 104 may be located in the electrostatic clamp 101. The electrode 104 may be embedded in the main body of material that forms the electrostatic clamp 101. However, it is not essential for the electrode 104 to be located in the electrostatic clamp 101. In an alternative arrangement the electrode 104 is at the electrostatic clamp 101 or near the electrostatic clamp 104. For example, the electrode 104 may be provided adjacent to a surface of the electrostatic clamp 101.

[0040] In an embodiment the at least one electrode 104 is configured to exert an electrostatic clamping force on the component 128. However, it is not essential for the at least one electrode 104 to exert an electrostatic clamping force on the component 128. In an alternative embodiment, at least one electrode 104 may be provided for measurement of an AC signal. At least one further electrode in the same electrostatic clamp 101 may be provided for exerting an electrostatic clamping force on the component 128. In a further alternative embodiment, one electrostatic clamp may be provided for exerting an electrostatic clamping force on the component. A different electrostatic clamp 101 may be provided for the measurement of an AC signal.

[0041] When the electrostatic clamp system is used for measuring the AC signal but not from electrostatically clamping the component 128, the electrostatic clamp system 10 may be referred to as an AC signal measurement system. When the electrostatic clamp 101 is used for outputting the AC signal but is not used for exerting an electrostatic clamping force on the component 128, the electrostatic clamp 101 may be referred to as an AC signal transfer body. The AC signal is transferred through the electrostatic clamp 101.

[0042] As shown in Figure 3, in an embodiment the electrostatic clamp system 10 comprises a measurement circuit 12. The measurement circuit 12 may be configured to measure an AC signal. In an embodiment the AC signal is indicative of a varying potential of the component 128. By measuring the AC signal, the potential of the component 128 may be monitored. In an embodiment the potential of the component 128 varies over time. The potential may be a time-varying potential. By monitoring the time- varying potential of the component 128, information can be determined relating to one or more properties of one or more parts of the lithographic apparatus 100. An embodiment of the invention is expected to allow greater and/or more accurate control of the lithographic apparatus 100. An embodiment of the invention is expected to allow for improved decisions about replacing one or more parts of the lithographic apparatus 100.

[0043] In an embodiment the component 128 is configured to emit electrons. For example, during use of the lithographic apparatus 100, the component 128 may be clamped using a high-voltage electrostatic clamp. The environment surrounding the patterning device and the electrostatic clamp may be maintained at a low pressure. This environment may be non-conductive. As such, electric charge may be built up on dielectric or ungrounded surfaces, such as the patterning surface of the component.

[0044] As a result of the photoelectric effect, the EUV radiation incident on the component may cause electrons to be ejected from the component and into the environment surrounding the patterning device. This may cause the component to become positively charged. In other words, the emission may be caused by incident EUV radiation, thereby varying a potential of the component 128.

[0045] By measuring the AC signal that is indicative of the varying potential of the component 128, information relating to the EUV radiation may be obtained. For example, in an embodiment the controller 500 is configured to determine, based on the measured AC signal, the energy of an EUV radiation beam incident on the component 128. The measurement of the AC signal allows for a realtime measurement of EUV radiation reaching the component 128. In an embodiment, the controller 500 is configured to control the lithographic apparatus 100 based on measurement of the AC signal.

[0046] For example, in an embodiment the controller 500 is configured to control the EUV source SO based on the measured AC signal. The controller 500 may be configured to monitor one or more properties of the AC signal over time. For example, if the controller 500 determines that the magnitude of the AC signal is varying over time, then this may be an indication of EUV drift. EUV drift refers to the gradual change in energy level of the EUV radiation beam over time. In an embodiment the controller 500 is configured to control the EUV source SO to reduce the energy level of the EUV radiation beam when the magnitude of the AC signal is determined to have increased over time. In an embodiment the controller 500 is configured to control the EUV source SO to increase the energy level of the EUV radiation beam when the controller 500 determines that the magnitude of the AC signal has reduced over time.

[0047] By monitoring changes in the energy level of the EUV radiation beam over time using the AC signal, the AC signal may allow calibration of the EUV radiation beam during use of the lithographic apparatus 100. It may not be necessary to perform a separate measurement of the energy level of the EUV radiation beam after the exposure of one substrate W and before the exposure of a subsequent substrate W. An embodiment of the invention is expected to reduce the time required for calibrating the energy level of the EUV radiation beam. An embodiment of the invention is expected to increase throughput, i.e. the number of substrates W that can be processed by a lithographic apparatus 100 in a given amount of time. [0048] The AC signal may be measured during use of the lithographic apparatus 100. For example, the AC signal may be measured during an exposure process. It may not be necessary to use a proportion of the EUV radiation beam exclusively for measurement of the energy level of the EUV radiation beam. An embodiment of the invention is expected to increase the proportion of the EUV radiation beam that can be used for exposure processes. An embodiment of the invention is expected to increase the energy efficiency of use of the lithographic apparatus 100.

[0049] In an embodiment the measurement circuit 12 is configured to measure the AC signal from the electrostatic clamp 101. For example, the AC current may pass through the electrostatic clamp 101 capacitively. The AC signal can be measured by the measurement circuit 12.

[0050] As shown in Figure 3, in an embodiment the lithographic apparatus 100 comprises a vacuum chamber 11. The electrostatic clamp 101 and the component 128 may be located in the vacuum chamber 11. In an embodiment the vacuum chamber 11 is configured to maintain a low pressure. For example, a vacuum apparatus such as a pump may be provided to remove gas from the vacuum chamber 11.

[0051] As shown in Figure 3, in an embodiment the measurement circuit 12 is configured to measure the AC signal outside of the vacuum chamber 11. In an embodiment the electrostatic clamp system 10 comprises electrical connections through the vacuum chamber 11. The electrostatic clamp 101 inside the vacuum chamber 11 may be connected electrically to the measurement circuit 12 outside of the vacuum chamber 11.

[0052] However, it is not essential for the AC signal to be measured outside of the vacuum chamber 11. In an alternative embodiment the measurement circuit 12 is located in the vacuum chamber 11. By locating the measurement circuit 12 in the vacuum chamber 11, the measurement signal 12 may be located closer to the electrostatic clamp 101.

[0053] In an embodiment the controller 500 is configured to control at least one electrode 104 of the electrostatic clamp 101 based on the measured AC signal. By measuring the AC signal closer to the electrostatic clamp 101, control of the electrodes 104 based on the measured AC signal may be improved. An embodiment of the invention is expected to allow greater control of the electrostatic clamp 101.

[0054] In an embodiment, the varying potential of the component 128 is at a surface of the component 128 facing away from the electrostatic clamp 101. For example, the varying potential may be at the first surface 110 shown in Figure 3 or Figure 4. The first surface 110 is arranged such that the EUV radiation is incident on the first surface 110. The photoelectric effect may occur at the first surface 110. Additionally or alternatively, the varying potential of the component 128 may be at another surface of the component 128 on which EUV radiation is incident.

[0055] As shown in Figure 3, in an embodiment the AC signal is measured at the opposite side of the electrostatic clamp 101 from the component 128. The electrostatic clamp 101 may be located between the component 128 and the measurement circuit 12. The AC signal passes through the electrostatic clamp 101 such that the AC signal can be measured by the measurement circuit 12 on the opposite side of the electrostatic clamp 101.

[0056] In an embodiment the measurement circuit 12 comprises a high pass filter. The high pass filter is configured to filter out a DC signal for powering the at least one electrode 104. For example, the DC signal may be provided to the at least one electrode 104 to exert the electrostatic clamping force. By filtering out the DC signal, the AC signal may be measured more accurately.

[0057] In an embodiment the measurement circuit 12 comprises a capacitor 16. As shown in Figure 3, in an embodiment a capacitor 16 is provided for each electrode 104 from which the AC signal is to be measured.

[0058] As shown in Figure 3, in an embodiment the electrostatic clamp system 10 comprises at least one electrical connector 14 for the respective at least one electrode 104 from which the AC signal is to be measured. As shown in Figure 3, in an embodiment the electrostatic clamp system 10 comprises a power source 13. The power source 13 is configured to supply the at least one electrode 104 with a DC signal. The DC signal may be for pairing the at least one electrode 104. For example, the DC signal may be for pairing the at least one electrode 104 to exert the electrostatic clamping force. As shown in Figure 3, in an embodiment the power source 13 comprises a power supply for each of the electrodes 104. In an embodiment the controller 500 is configured to control each electrode independently of the other electrodes 104. The electrical connectors 14A, 14B, 14C, 14D may be configured to connect the power supplies of the power source 13 to the electrodes 104. The electrical connectors 14 may be configured to transfer the DC signal from the power source 13 to the electrodes 104.

[0059] In an embodiment, the electrical connectors 14 are configured to transfer the AC signal from the electrodes 104. For example, as shown in Figure 3 in an embodiment the measurement circuit 12 comprises a measurement connector 15 for each electrode 104 from which the AC signal is to be measured. The measurement connector 15 is an electrical connector. As shown in Figure 3, the measurement connectors 15A, 15B, 15C, 15D may branch off from respective electrical connectors 14 A, 14B, 14C, 14D that connect the power source 13 to the electrodes 104. The measurement connectors 15 may be configured to transfer the AC signal from the electrodes 104.

[0060] As shown in Figure 3, in an embodiment the measurement circuit 12 comprises a measurement device 17. In an embodiment a plurality of measurement devices 17A, 17B, 17C, 17D may be provided. In an embodiment each measurement device 17 corresponds to a respective electrode 104. The measurement device 17 is configured to measure the AC signal transferred from the electrodes 104. For example, the measurement device 17 may be configured to measure the varying electric potential. As shown in more detail in Figure 7, in an embodiment the measurement device 17 is configured to measure the AC signal across a measurement resistor 20. As shown in Figure 3, in an embodiment the capacitor 16 is located electrically between the measurement device 17 and the electrode 104.

[0061] In an embodiment the measurement circuit 12 comprises a tuner. The tuner is configured to tune the measurement circuit 12 to a predetermined frequency range. For example, the tuner may be configured to tune the measurement circuit 12 to the source frequency. The source frequency is the frequency of pulses of the EUV radiation beam. In an embodiment, the controller 500 is configured to control the EUV source SO and the measurement circuit 12 such that the measurement circuit 12 is tuned to the same frequency as the EUV source SO. Merely as an example, in an embodiment the measurement circuit 12 comprises a lock-in amplifier. The lock-in amplifier may function as the tuner. By providing the tuner, the AC signal may be measured more accurately. The tuner may be located electrically between the electrode 104 and the capacitor 16. Alternatively, the tuner may be located electrically between the capacitor 16 and the measurement device 17. The tuner is not essential. In an alternative arrangement, the tuner may be omitted.

[0062] In an embodiment the predetermined frequency range comprises the pulse frequency at which the EUV source SO is configured to generate the EUV radiation beam that comprises pulses of EUV radiation.

[0063] In an embodiment, the electrical connectors 14A, 14B, 14C, 14D are signal lines. In an embodiment the measurement circuit 12 is configured to measure the AC signal via at least one signal line from the electrostatic clamp 101. As shown in Figure 3, in an embodiment the signal lines extend through a wall of the vacuum chamber 11. In an alternative arrangement, the signal lines may be within the vacuum chamber 11, for example when the AC signal is measured within the vacuum chamber 11. As shown in Figure 3, in an embodiment each signal line corresponds to a respective electrode 104 of the electrostatic clamp 101.

[0064] As shown in Figure 3, in an embodiment the measurement circuit 12 is located electrically between the power source 13 and the electrostatic clamp 101. As shown in Figure 3, the measurement circuit 12 may interrupt the AC signal output from the electrodes 104 via the electrical connectors 14. The electrical connectors 14 may be configured to transfer power to the electrodes 104 and to transfer the AC signal from the electrodes 104. The measurement circuit 12 is configured to measure the AC signal transferred via the electrical connectors 14. In an alternative arrangement, dedicated electrical connectors may be provided to connect the electrodes 104 to the measurement circuit 12 for the measurement of the AC signal.

[0065] Figure 4 schematically depicts, in cross-section, a portion of a support structure MT, which may be a part of a lithographic apparatus in accordance with the present invention. The cross-section extends in a vertical plane (i.e., parallel to the z-direction). The portion of the support structure MT may be a radially outward portion of the support structure MT. That is, the support structure MT may continue radially inwardly, (in the rightward direction as depicted in Figure 4). The support structure MT clamps the patterning device MA during lithographic operations. In accordance with the present invention, the support structure MT is an electrostatic clamp 101. The electrostatic clamp 101 may be considered to be part of the lithographic apparatus or may be considered to be part of an apparatus that is separate to the lithographic apparatus. [0066] The patterning device MA to be clamped by the electrostatic clamp 101 may comprise a first surface 110 and a second surface 111. The first surface 110 and the second surface 111 may be substantially parallel. The first surface 110 may be a patterning surface and comprise a patterning region. The patterning region may be configured to impart a pattern that is to be projected onto a substrate W. The patterning region may be configured to reflect a beam of radiation such that the reflected beam of radiation is a patterned beam of radiation. The second surface 111 is opposite the first surface 110.

[0067] Both the electrostatic clamp 101 and the patterning device MA 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 and/or other components within the lithographic apparatus such that gases and contaminant particles are substantially prevented from entering the patterning device environment 90. 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.

[0068] The electrostatic clamp 101 may comprise a clamping surface 102. When the patterning device MA is clamped to the electrostatic clamp 101, the clamping surface 102 may face the second surface 111 of the patterning device MA. When the patterning device MA is clamped to the electrostatic clamp 101, the first surface 110 of the patterning device MA may face away from the clamping surface 102. The clamping surface 102 may be generally planar. The electrostatic clamp 101 may be configured such that, when the patterning device MA is clamped to the electrostatic clamp 101, the clamping surface 102 of the electrostatic clamp 101 is substantially parallel to the second surface 111 of the patterning device MA.

[0069] The electrostatic clamp 101 may comprise a plurality of electrodes 104A-104D. The plurality of electrodes 104A-104D may be buried within the body of the electrostatic clamp 101. For instance, the plurality of electrodes 104A-104D may be buried beneath the clamping surface 102 of the electrostatic clamp 101. Each of the plurality of electrodes 104A-104D may be connected to a power supply such that a potential can be applied to each of the plurality of electrodes 104A-104D.

[0070] The electrostatic clamp 101 may comprise a plurality of burls 106. The plurality of burls may protrude from the clamping surface 102 of the electrostatic clamp 101. The electrostatic clamp 101 may be configured such that distal ends (that is, ends furthest from the clamping surface 102 of the electrostatic clamp 101) of the plurality of burls 106 form a planar surface. When the patterning device MA is clamped to the electrostatic clamp 101 , the second surface 111 of the patterning device MA may be in contact with the distal ends of the plurality of burls 106. Consequently, the clamping surface 102 of the electrostatic clamp 101, and the electrodes 104A-104D that may be buried beneath the clamping surface 102 of the electrostatic clamp, may be separated from the second surface 111 of the patterning device MA.

[0071] The clamping surface 102 of the electrostatic clamp 101, and the plurality of electrodes 104A- 104D that may be buried beneath the clamping surface 102 of the electrostatic clamp 101, may also be separated from the second surface 111 of the patterning device MA by a dielectric coating 105.

[0072] The plurality of electrodes 104A-104D may each be rectangular in shape. However, this is not essential to the present invention, and the shape of the plurality of electrodes 104A-104D is not particularly limited. The plurality of electrodes 104A-104D may be distributed uniformly over the clamping surface 102 of the electrostatic clamp 101. However, the particular arrangement of the plurality of electrodes 104A-104D over the clamping surface 102 of the electrostatic clamp is not particularly limited. As depicted in Figure 4, 4 electrodes 104A-104D are present in the radially outward portion of the electrostatic clamp 101. However, the number of electrodes 104A-104D in the electrostatic clamp is not particularly limited, and could be any number such as 1, 2, 3, 4, 5, 10, 20 or more.

[0073] The patterning device MA to be clamped by the electrostatic clamp 101 may be provided with a first conductive coating 112 on the first surface 110 and a second conductive coating 113 on the second surface 111. The first and second conductive coatings 112, 113 may cover a majority of the first and second surfaces 110, 111, respectively. The first conductive coating 112 and the second conductive coating 113 may be substantially electrically isolated. That is, the first conductive coating 112 may be electrically isolated from the second conductive coating 113, and both the first conductive coating 112 and the second conductive coating 113 may each be substantially electrically isolated from other components within the lithographic apparatus. Consequently, electric charge may build up on the first surface 110 and the second surface 111.

[0074] Each of the plurality of electrodes 104A-104D may be connected to a power source (not shown), such that a potential can be applied to each of the plurality of electrodes 104A-104D. A controller 104A-104D may control the potential applied to each of the plurality of electrodes 104A- 104D.

[0075] The plurality of electrodes 104A-104D may comprise a first set of electrodes 104A, 104C and a second set of electrodes 104B, 104D. The first set of electrodes 104A, 104C may be connected to one or more first voltage source(s) and the second set of electrodes 104B, 104D may be connected to one or more second voltage source(s). The potential applied to the first set of electrodes 104A, 104C may be different to the potential applied to the second set of electrodes 104B, 104D. For example, the potential applied to the second set of electrodes 104B, 104D may be of a similar magnitude but opposite polarity to the potential applied to the first set of electrodes 104A, 104C.

[0076] The first set of electrodes 104A, 104C and the second set of electrodes 104B, 104D may each be arranged uniformly over the clamping surface 102 of the clamping device 100. For example, the first set of electrodes 104A, 104C and the second set of electrodes 104B, 104D may be in a 2- dimensional checkerboard arrangement. In the checkerboard arrangement, the electrodes 104A-104D may alternate between belonging to the first set of electrodes 104A, 104C and belonging to the second set of electrodes 104B, 104D in a first direction and a second direction, where the first direction and the second direction are perpendicular to one another and in the plane of the clamping surface 102.

[0077] When a potential is applied to the plurality of electrodes 104A-104D, a high electric field is established between the plurality of electrodes 104A-104D and the patterning device MA, causing the patterning device MA to be attracted to the plurality of electrodes 104A-104D . In particular, the potential applied to the plurality of electrodes 104A-104D causes charge separation to occur within the second conductive coating 113. The displacement of charges within the second conductive coating 113 means that an attractive force may be established between the plurality of electrodes 104A-104D and the second conductive coating 113. Consequently, the electrostatic clamp 101 may exert a clamping force on the patterning device MA.

[0078] The potential applied to the plurality of electrodes 104A-104D may be large enough for the clamping force exerted on the patterning device MA by the electrostatic clamp to overcome the gravitational force exerted on the patterning device MA. The required potential may be dependent on the mass of the patterning device MA and the quality of the first conductive coating 112 and the second conductive coating 113. The magnitude of the potential applied to the plurality of electrodes 104A- 104D may be in the order of several kV. For example, the magnitude of the potential applied to the plurality of electrodes 104A-104D may be greater than 100 V, preferably greater than 300 V, and further preferably greater than 1 kV. The magnitude of the potential applied to the plurality of electrodes 104A- 104D may be less than 100 kV, preferably less than 50 kV, and further preferably less than 25 kV.

[0079] Masking blades 120 may be provided within the lithographic apparatus adjacent to the first surface 110 of the patterning device MA. For example, the masking blades 120 may be provided such that they are displaced from the first surface 110 in the z-direction. In the case that a pellicle 131 is provided in front of the patterning device MA, the masking blades 120 may be provided such that they are displaced from the first surface 110 in the z-direction. The masking blades 120 may be configured to selectively mask the patterning device MA from the beam of radiation during exposure. The lithographic apparatus may be configured such that the masking blades 120 can be moved in the horizontal plane to provide different levels of masking for the patterning device MA.

[0080] The lithographic apparatus may be an EUV lithographic apparatus. That is, the lithographic apparatus may be configured to project a beam of EUV radiation onto the substrate W. During exposure, the beam of EUV radiation may be incident on the patterning region of the first surface 110 of the patterning device MA. This may cause the release of electrons from the first surface 110, as a result of the photoelectric effect. Consequently, the first surface 110 may become positively charged.

[0081] In an EUV lithographic system, EUV radiation may be generated in pulses. That is, there are periods when EUV radiation is generated, and periods when EUV radiation is not generated. 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. During a typical EUV pulse cycle (for example, a period starting when a pulse of EUV radiation is first generated and ending immediately before the next pulse of EUV radiation is generated), the pulse of EUV radiation may only be generated for a short amount of time. For example, the pulse of EUV radiation may be generated for approximately 1% of the time for a typical EUV pulse cycle.

[0082] Figure 5 depicts, on the same axes: (1) a plot of EUV intensity (I) against time (the solid line), and (2) a plot of the potential (V) of the first surface 110 of the patterning device MA against time (t) (the dashed line). Figure 5 depicts a time period in which two pulses of EUV radiation are generated by the lithographic apparatus. The situation depicted may be one in which a pellicle 131 is not provided in front of the first surface 110 of the patterning device MA. Upon the initiation of a pulse of EUV radiation, the EUV radiation that is incident on the first surface 110 may cause the emission of electrons from the first surface 110, causing the first surface 110 to become positively charged. This brings the first surface 110 to a positive potential. After the termination of the EUV pulse, the first surface 110 may be discharged. That is, the magnitude of the positive charge on the first surface 110 may decrease. This may be such that the first surface 110 becomes approximately neutral.

[0083] Figure 5 shows a pulsating voltage. The pulsating voltage induces an AC signal. The induced AC signal may be at the same pulse frequency as the pulses of the EUV radiation beam. As shown in Figure 5, each EUV pulse causes photoemission from the component 128 which changes the potential of the component 128. The component 128 is subsequently discharged between the EUV pulses.

[0084] The discharge may be caused by plasma. For example, in an embodiment the EUV radiation is ionising for gas in the volume 90 adjacent to the component 128. For example, the EUV radiation may excite hydrogen molecules within the environment surrounding the patterning device to form a plasma. The electrons ejected from the component 128 may also contribute to this plasma. Free charges in the plasma may cause the component 128 to be discharged.

[0085] The discharging of the patterning surface 40 may be caused by free negative charges in the plasma that is formed within the patterning device environment 90. This means that, during an EUV lithographic process, a first surface 110 may cycle between being positively charged and being approximately neutral at a high frequency.

[0086] However, it is not essential for there to be background gas in the volume 90. In an alternative arrangement, the pressure in the volume 90 is low such that there is no significant plasma in the volume 90. The component 128 may be discharged by means other than the plasma.

[0087] For example, in an embodiment the electrostatic clamp system 10 comprises an electrical connector configured to controllably connect the component 128 to a reference potential. The reference potential may be a ground potential, or a potential closer to ground than the potential that the component 128 reaches as a result of the pulses of EUV radiation. In an embodiment the controller 500 is configured to connect the component 128 to the reference potential between the pulses of EUV radiation. For example, the component 128 may be grounded so as to discharge the component 128 between the pulses of EUV radiation.

[0088] As another example, in an embodiment the electrostatic clamp system 10 comprises a charged particle source, for example an electron source. The charged particle source is configured to generate charged particles for charging the component 128. For example, an electron beam may be used to charge the component 128 negatively between pulses of EUV radiation. The electron beam may discharge the component 128. In an embodiment, a combination of any of the plasma, grounding or an ioniser may contribute to discharge of the component 128 between pulses of EUV radiation.

[0089] In an embodiment the controller 500 is configured to determine a condition of the component 128 based on the measured AC signal. For example, the AC signal may be dependent on the condition of the component 128. By measuring how the AC signal varies over time, a changing condition of the component 128 may be monitored by the controller 500. The controller 500 may be configured to determine when the component 128 should be replaced.

[0090] For example, in an embodiment the component 128 is a patterning device MA. The patterning device MA may comprise a plurality of layers. The layers may be provided at or near a surface of the patterning device MA. Measuring the photoelectric current over time of the patterning device MA may indicate changes in the surface composition of the patterning device MA. For example, the photoelectric effect may depend on the layers at the surface of the patterning device MA. When the composition of the layers changes, the extent of the photoelectric effect as a result of the incident EUV radiation may change.

[0091] In an embodiment the controller 500 is configured to detect a change in the composition of the patterning device MA when the magnitude of the AC signal that is measured increases or decreases over time. Experiments may be performed to determine how the AC signal varies dependent on the condition of the patterning device MA. By performing the experiments, the controller may be calibrated so as to determine when it is appropriate for the patterning device MA to be replaced. Additionally or alternatively, the controller 500 may be configured to determine when the dose of EUV radiation is to be recalibrated.

[0092] During exposure, the second surface 111 may not become positively charged, because it is not exposed to EUV radiation. Rather, the second surface 111 may become negatively charged as a result of the free negative charges of plasma within the patterning device environment 90.

[0093] Figure 6 schematically depicts an electrostatic clamp 101 , which may be a part of a lithographic apparatus in accordance with the present invention, and a patterning device covered by a pellicle 131. The pellicle 131 may be provided opposite the first surface 110. For example, the pellicle 131 may be provided such that it is displaced from the first surface 110 in the z-direction. The pellicle may be a membrane assembly configured to protect the patterning device MA from contaminant particles within the patterning device environment 90. In order to minimize the absorption of EUV radiation by the pellicle, the pellicle is very thin and consequently very fragile. The pellicle 131 may be supported by a pellicle frame 132. The pellicle 131 may be stretched across the pellicle frame 132. The pellicle 131 may be substantially electrically isolated. That is, the pellicle 131 may be substantially electrically isolated from the patterning device MA and substantially electrically isolated from other components within the lithographic apparatus. Consequently, electrostatic charge may be accumulated on the pellicle 131.

[0094] As shown in Figure 6, in an embodiment the lithographic apparatus 100 comprises a pellicle 131. The pellicle 131 is configured to protect the component, such as a patterning device MA, from particles.

[0095] In an embodiment the controller 500 is configured to detect a state of the pellicle 131 based on the measured AC signal. In an embodiment the controller 500 is configured to detect a presence of the pellicle 131 based on the measured AC signal.

[0096] The measurement of the photoelectric current allows for a real-time measurement of the deposited charge and neutralisation charge on the component 128. This will be different in a situation where a pellicle 131 is present compared to when the pellicle 131 is broken or absent. For example, it is possible that the pellicle 131 can become ruptured during use.

[0097] When the pellicle 131 is broken or absent, the AC signal may change. For example, the magnitude of the AC signal may suddenly increase. In an embodiment the controller 500 is configured to detect that the pellicle 131 is broken or absent based on a determination that the magnitude of the AC signal has suddenly increased. By measuring the AC signal, it may be determined when a pellicle 131 broke or became absent. This may help to determine that the pellicle 131 requires replacement and/or to determine the likely effect on the exposed substrate W due to the breaking or absence of the pellicle 131 at that point in time.

[0098] The photoelectric effect and subsequent discharging of the component 128 cause voltage peaks as shown in Figure 5 on the first surface 110 of the component 128. The pellicle 131, being further away from the electrodes 104 may have a smaller AC current when it is intact compared to when the pellicle 131 is broken. Experiments may be performed to determine the extent by which the magnitude of the AC signal may change as a result of the pellicle 131 rupturing or becoming absent. In an embodiment the controller 500 is configured to determine the breaking or absence of the pellicle 131 when the magnitude of the AC signal changes by at least a predetermined threshold. The predetermined threshold may be a percentage of the magnitude of the AC signal when the pellicle 131 is present and intact. For example, in an embodiment the controller 500 is configured to detect breaking or absence of the pellicle 131 when the magnitude of the AC signal increases by at least 10% and optionally by at least 20%.

[0099] Figure 7 shows an alternative arrangement of a measurement circuit 12 for an electrostatic clamp system 10. As shown in Figure 7, in an embodiment the measurement circuit 12 is configured to combine the signal lines into a combined signal line. This is shown by the measurement connectors 15A, 15B, 15C, 15D being combined into the combined signal line 19 in Figure 7. In an embodiment the measurement circuit 12 is configured to measure the AC signal from the combined signal line 19. For example, as shown in Figure 7, in an embodiment the measurement circuit 12 comprises the measurement device 17 configured to measure the AC signal across the measurement resistor 20. As shown in Figure 3 and Figure 7, in an embodiment the measurement circuit 12 comprises a measurement terminal 18. In an embodiment the measurement device 17 is configured to output the measurement of the AC signal. For example, the measurement device 17 may be configured to send the measurement result to the controller 500.

[0100] As shown in Figure 7, in an embodiment the measurement circuit 12 comprises a single measurement device 17. By combining the signal lines into a combined signal line 19, the magnitude of the AC signal may be increased. By increasing the magnitude of the AC signal, the AC signal may be measured more accurately.

[0101] However, it is not essential to combine the signal lines into a combined signal line. For example, as shown in Figure 3, the signal lines may be kept separate from each other. A separate measurement device 17 may be provided for each signal line. As shown in Figure 3, the signal lines may be combined to the measurement terminal 18 electrically downstream of the measurement devices 17. The measurement results may be combined together. Alternatively, each measurement device 17A, 17B, 17C, 17D may output a measurement result to the controller 500.

[0102] In an embodiment the controller 500 is configured to control the power supply to the electrodes 104 based on the measured AC signal. For example, in an embodiment the controller 500 is configured to capacitively induce an AC voltage to the component 128 based on an earlier measurement of the phase of the AC signal. In an embodiment the measurement circuit 12 is configured to measure the phase of the AC signal. For example, by providing a lock-in amplifier, the phase of the AC signal may be measured. The measurement of the phase of the AC signal may be used for controlling the potential applied to the electrodes 104.

[0103] It may be possible to implement the system and method of the present invention in current models of electrostatic clamp without requiring any changes to hardware. Consequently, the system and method may be quick and easy to implement.

[0104] A lithographic apparatus in accordance with the present invention may be used for the manufacture of ICs.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

Claims

1. An electrostatic clamp system for a component in a lithographic apparatus, the electrostatic clamp system comprising: an electrostatic clamp; at least one electrode associated with the electrostatic clamp; and a measurement circuit configured to measure an AC signal from the at least one electrode indicative of a varying potential of the component.

2. The electrostatic clamp system of any preceding claim, wherein the component is configured to emit electrons, the emission caused by incident EUV radiation, thereby varying a potential of the component.

3. The electrostatic clamp system of claim 1 or 2, wherein the measurement circuit is configured to measure the AC signal from the electrostatic clamp.

4. The electrostatic clamp system of any preceding claim, wherein the varying potential of the component is at a surface of the component facing away from the electrostatic clamp.

5. The electrostatic clamp system of any preceding claim, wherein the AC signal is induced by the varying potential.

6. The electrostatic clamp system of any preceding claim, wherein the measurement circuit comprises a high pass filter configured to filter out a DC signal for powering the at least one electrode to exert the electrostatic clamping force.

7. The electrostatic clamp system of any preceding claim, wherein the measurement circuit comprises a tuner configured to tune the measurement circuit to a predetermined frequency range.

8. The electrostatic clamp system of claim 7, wherein the predetermined frequency range comprises a pulse frequency at which an EUV source of a lithographic apparatus comprising the electrostatic clamp system is configured to generate an EUV radiation beam comprising pulses of EUV radiation.

9. The electrostatic clamp system of any preceding claim, wherein the measurement circuit is configured to measure the AC signal via at least one signal line from the electrostatic clamp.

10. The electrostatic clamp system of claim 9, wherein each signal line corresponds to a respective electrode.

11. The electrostatic clamp system of any preceding claim, wherein the measurement circuit is configured to combine the signal lines into a combined signal line and to measure the AC signal from the combined signal line.

12. The electrostatic clamp system of any preceding claim, comprising: a power source configured to supply the at least one electrode with a DC signal for powering the at least one electrode to exert the electrostatic clamping force.

13. The electrostatic clamp system of claim 12, wherein the measurement circuit is located electrically between the power source and the electrostatic clamp.

14. The electrostatic clamp system of any preceding claim, comprising: an electrical connector configured to controllably connect the component to a reference potential.

15. The electrostatic clamp system of any preceding claim, comprising: a charged particle source configured to generate charged particles for charging the component.

16. A lithographic apparatus comprising the electrostatic clamp system of any preceding claim.

17. The lithographic apparatus of claim 16, comprising: an EUV source configured to generate an EUV radiation beam comprising pulses of EUV radiation that are directed to the component.

18. The lithographic apparatus of claim 17, wherein the EUV radiation is ionising for gas in a volume adjacent to the component.

19. The lithographic apparatus of any of claims 16-18, comprising: a controller configured to control the lithographic apparatus.

20. The lithographic apparatus of claim 19, wherein the controller is configured to monitor a variation of the measured AC signal over time.

21. The lithographic apparatus of claim 19 or 20, wherein the controller is configured to determine, based on the measured AC signal, energy of an EUV radiation beam incident on the component.

22. The lithographic apparatus of any of claims 19-21, wherein the controller is configured to control an EUV source of the lithographic apparatus based on the measured AC signal, the EUV source configured to generate an EUV radiation beam.

23. The lithographic apparatus of any of claims 19-22, wherein the controller is configured to determine a condition of the component based on the measured AC signal.

24. The lithographic apparatus of any of claims 19-23, comprising: a pellicle configured to protect the component from particles; wherein the controller is configured to detect a state of the pellicle based on the measured AC signal.

25. The lithographic apparatus of any of claims 16-24, comprising: a vacuum chamber in which the electrostatic clamp and the component are located.

26. The lithographic apparatus of claim 25, wherein the measurement circuit is configured to measure the AC signal outside of the vacuum chamber.

27. A method of operating an electrostatic clamp system for a component in a lithographic apparatus, the method comprising: measuring, from at least one electrode associated with an electrostatic clamp of the electrostatic clamp system, an AC signal indicative of a varying potential of the component.

28. The method of claim 27, comprising: controlling an EUV source based on the measured AC signal.

29. The method of claim 27 or 28, comprising: detecting, based on the measured AC signal, a state of a pellicle configured to protect the component from particles.

30. The method of claim 29, wherein detecting the state of the pellicle comprises detecting a presence of the pellicle.

31. The method of any of claims 27-30, comprising: capacitively inducing an AC voltage to the component based on a measurement of a phase of the AC signal.

32. The method of any of claims 27-31, comprising: applying a clamping force to the component with the at least one electrode.