pollution control

Abstract

A lithographic patterning device contamination control system comprising: a support structure configured to support a patterning device; and an electron beam source configured to emit a beam of electrons such that at least a portion of the beam is incident on a patterned surface of the patterning device supported by the support structure during EUV exposure.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims priority to European / U.S. Application No. 22212483.6, filed December 9, 2022, which is incorporated herein by reference in its entirety.

[0002] [Technical field] The present invention relates to contamination control in a lithographic apparatus. [Background technology]

[0003] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern in a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

[0004] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Lithographic apparatuses that use extreme ultraviolet (EUV) radiation having a wavelength in the range of 4-20 nm (e.g., 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate than lithographic apparatuses that use radiation having a deep ultraviolet (DUV) wavelength of, for example, 193 nm.

[0005] In conventional (DUV) lithography systems, a pellicle is attached to the patterning device. The pellicle is a membrane that is spatially separated from the patterning device. Contamination particles incident on the pellicle are out of focus when projected onto a substrate by the lithography system. As a result, the contamination particles do not introduce defects into the pattern that is projected onto a substrate from the patterning device by the lithography system. Summary of the Invention [Problem to be solved by the invention]

[0006] Pellicles may be used in EUV lithography apparatus. However, EUV radiation is absorbed by the pellicle, which reduces the intensity of the EUV radiation that may be used to expose a substrate. This, in turn, reduces the throughput of the lithography apparatus. Maintaining a clean patterning device during EUV exposure can be achieved by suppressing contaminant particle emissions and / or transporting particles from contaminated surfaces near the patterning device. This is an alternative to using a pellicle. However, some contaminant particles may still be incident on the patterning device.

[0007] It would be desirable to provide an apparatus that overcomes or alleviates one or more problems associated with the prior art. [Means for solving the problem]

[0008] According to a first aspect of the present invention, there is provided a lithographic patterning device contamination control system comprising: a support structure configured to support a patterning device; and an electron beam source configured to emit a beam of electrons such that at least a portion of the beam is incident on a patterned surface of a patterning device supported by the support structure during EUV exposure.

[0009] Advantageously, the electron beam provides contactless control of the potential of the patterned surface of the patterning device (i.e., no physical wire contact is required, which could create contaminants or damage critical surfaces of the patterning device). The electron beam can keep the time-averaged potential of the patterning device MA at a negative voltage (e.g., -0.1V...-10V) or prevent the patterning device MA from reaching a positive time-averaged potential (e.g., +1V...+5V). The negative potential of the patterning device creates a favorable electric field that repels contaminant particles from the patterning device.

[0010] The line of sight may extend from the electron beam source to the patterning device.

[0011] A line of sight, which may extend from the electron beam source to the patterning device, may exist for all positions of the patterning device during EUV exposure.

[0012] The electron beam source may be configured to emit electrons having an energy of at least 1 eV.

[0013] The electron beam source may be configured to emit electrons having an energy of up to 13.5 eV.

[0014] The electron beam source may be configured to emit electrons having an energy of up to 100 eV.

[0015] The electron beam source may be positioned at a distance greater than 10 cm from the patterning device, the electron beam source being configured to emit electrons having an energy of at least 10 eV.

[0016] The electron beam source may be positioned at a distance of 10 cm or less from the patterning device.

[0017] The electron beam source may be located at the bottom of a housing that defines the patterning device environment.The electron beam source may be located near a wall of an opening provided in the housing.

[0018] The electron beam source may be configured to emit electrons with a current of at least 100 μA.

[0019] The electron beam source may be configured to emit electrons with a current of up to 100 mA.

[0020] The electron beam source may be configured to output an electron beam having an elliptical cross-sectional shape. The elliptical beam may have an aspect ratio greater than 1, preferably greater than 2, and most preferably greater than 3.

[0021] The plasma electrode of the electron beam source may be in contact with one of stainless steel, tungsten, molybdenum, tantalum metals or alloys that are resistant to sputtering by hydrogen or helium ions having energies up to 100 eV.

[0022] The electron beam source may include at least one permanent magnet and a magnetic shielding material such as mu metal.

[0023] The electron beam source may have a housing that is grounded and in electrical contact with the frame of the lithography tool.

[0024] The housing of the electron beam source may be made from one of stainless steel, tungsten, molybdenum, or tantalum metals or alloys that are resistant to sputtering by the hydrogen EUV plasma.

[0025] The electron beam source may be connected to an RF power supply configured to provide power having a frequency in the range of 0.1 GHz to 10 GHz. The frequency is preferably in the range of 1 GHz to 3 GHz. The electron beam source may be connected to a DC power supply configured to provide a maximum of 100 mA. The electron beam source may be connected to a DC power supply configured to provide a maximum of -1 kV (to support electron beam extraction) or a maximum of +1 kV (to support positive ion beam extraction).

[0026] The electron beam source may be connected to a supply of at least one of H2 and He.

[0027] The electron beam source may be configured to direct an electron beam towards an exposure zone in the patterning device environment.

[0028] The electron beam source may be configured to direct the electron beam to a location that is offset from the exposure zone.

[0029] An additional electron beam source may be configured to direct an additional electron beam at a location offset in the opposite direction relative to the exposure zone.

[0030] The electron beam source may be one of a plurality of electron beam sources.

[0031] The electron beam source may be configured to maintain the patterning device at a negative potential, at least on a time-averaged basis, during EUV exposure.

[0032] The electron beam source may further comprise a controller configured to switch a polarity applied to the electron beam source such that the electron beam source outputs positive ions.

[0033] According to a second aspect of the present invention, there is provided a lithographic apparatus comprising the patterning device contamination control system as described above, and further comprising a housing in which a masking blade, a patterning device exchange system, and a patterning device environment are arranged.

[0034] The electron beam source may be provided in a wall that defines an opening leading to the patterning device environment.

[0035] The electron beam source may be positioned at a height corresponding to the height of the masking blades.The electron beam source may be offset in a direction orthogonal to the scanning direction of the patterning device support structure.

[0036] The electron beam source may be located near the patterning device exchange system.

[0037] According to a third aspect of the present invention, there is provided a lithographic apparatus comprising a source of electrons and an ionizer arranged in a patterning device environment.

[0038] The electron source and ionizer may advantageously generate a plasma that neutralizes charge on the backside of the patterning device during operation of the patterning device.

[0039] The lithographic apparatus may further comprise a controller configured to activate the ionizer when the lithographic apparatus is not performing a lithographic exposure.

[0040] According to a fourth aspect of the present invention there is provided a method of controlling contamination of a lithographic patterning device comprising directing an electron beam towards a patterned surface of the patterning device.

[0041] The electron beam provides contactless control of the potential of the patterned surface of the patterning device (i.e., no physical wire contact is required, which can create contaminants or damage critical surfaces of the patterning device).

[0042] A gas may be supplied to the plasma electrode of the electron beam source, the gas comprising at least one of H2 and He.

[0043] The electron beam may maintain the patterning device at a negative potential on a time-averaged basis during EUV exposure.

[0044] According to a fifth aspect of the present invention, there is provided a method of controlling charge on a back surface of a lithographic patterning device, comprising directing a beam of electrons or a beam of positive ions towards the back surface of the lithographic patterning device when the lithographic patterning device is handled by a patterning device exchange system.

[0045] Advantageously, this may prevent high voltages and resulting discharges that occur when the patterning device is manipulated.

[0046] Features of different aspects of the present invention may be combined together. [Brief explanation of the drawings]

[0047] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: FIG. 1 illustrates a schematic diagram of a lithography system including a patterning device contamination control system according to one embodiment of the present invention. FIG. 2 shows a more detailed schematic of the pollution control system. FIG. 3 shows a schematic of an electron emitter that forms part of a pollution control system. FIG. 4 illustrates a schematic of a patterning device contamination control system according to an alternative embodiment. FIG. 5 illustrates schematically a patterning device contamination control system according to a further alternative embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0048] 1 shows a lithography system comprising a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate a beam of EUV radiation B and to provide the beam of EUV radiation B to the lithography apparatus LA. The lithography 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.

[0049] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. In addition, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. Together, the facetted field mirror device 10 and the facetted pupil mirror device 11 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 facetted field mirror device 10 and the facetted pupil mirror device 11.

[0050] After being conditioned in this manner, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is produced. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To that end, the projection system PS may comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by a substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B' to form an image having smaller features 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 in FIG. 1 as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0051] The support structure MT may comprise a clamp used to hold the patterning device MA. The clamp may be an electrically actuated electrostatic clamp. There may be a dielectric layer between the clamp and the patterning device MA. At least a portion of the support structure MT may be electrically grounded.

[0052] The patterning device MA and other elements may be provided within a housing 24. The interior defined by the housing may be referred to as a patterning device environment 25. The housing 24 may be substantially closed except for an opening at the bottom end of the housing. A set of masking blades 20 is provided within the patterning device environment 25. The masking blades 20 are used to selectively mask areas of the patterning device MA so that only desired portions of the patterning device receive EUV radiation at any given time. During a scanning exposure, the patterning device MA and support structure MT move in the y direction, while the substrate W and substrate table WT move in the opposite y direction (and vice versa). In this way, a band of EUV radiation passes over the patterning device MA and through an exposure field on the substrate W.

[0053] An electron beam source 100 is provided within the patterning device environment 25. In FIG. 1, the electron beam source 100 is below and to one side of the blade of the reticle masking blade system 20. However, the electron beam source 100 may be provided at a different location (e.g., elsewhere within the patterning device environment 25). The electron beam 101 provided by the electron beam source, in combination with the EUV (photo effect) and EUV plasma (charging due to currents of ions and electrons), together determine both the instantaneous and average potential of the patterned surface of the patterning device MA (the patterned surface is conductive). While the electron beam 101 is illustrated as a line, in reality it may diverge and not necessarily travel in a straight line. Electrons output from the electron source 100 are scattered by gas molecules, ions, and other electrons. Additionally, due to charges on surfaces within the patterning device environment 25, the electrons are subjected to electrostatic forces. These forces accelerate the electrons.

[0054] A relative vacuum may be provided in the radiation source SO, the illumination system IL, and / or the projection system PS, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure. The same is true for the patterning device environment 25, i.e. a gas at a pressure below atmospheric pressure is present in the patterning device environment 25. The gas may be, for example, hydrogen. The gas may be partially ionized by the EUV radiation B and / or the electron beam 101.

[0055] The radiation source SO shown in FIG. 1 is of a type that may be designated, for example, as a laser-produced plasma (LPP) source. A laser system 1, which may include, for example, a CO laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn), provided from a fuel emitter 3. While the following description refers to tin, any suitable fuel may be used. The fuel may be, for example, in liquid form or may be, for example, a metal or alloy. The fuel emitter 3 may comprise, for example, a nozzle configured to direct the tin, in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident on the tin in the plasma formation region 4. The deposit of laser energy into the tin generates a tin plasma 7 in the plasma formation region 4. During de-excitation and recombination of electrons and ions of the plasma, radiation including EUV radiation is emitted from the plasma 7.

[0056] EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 may, for example, comprise a near-normal incidence radiation collector 5 (often more commonly referred to as a normal incidence radiation collector). Collector 5 may have a multi-layer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength, such as 13.5 nm). Collector 5 may have an elliptical configuration with two foci. The first focus may be at the plasma formation region 4 and the second focus may be at an intermediate focus 6, as described below.

[0057] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO by a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or beam expanders and / or other optical elements. The laser system 1, the radiation source SO and the beam delivery system may together be referred to as a radiation system.

[0058] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focus 6 in order to form an image at the intermediate focus 6 in the plasma present in the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near an opening 8 in a closure structure 9 of the radiation source SO.

[0059] Although Figure 1 depicts the radiation source SO as a laser-produced plasma (LPP) source, any suitable source may be used to produce EUV radiation, such as a discharge-produced plasma (DPP) source or a free-electron laser (FEL).

[0060] FIG. 2 shows a schematic representation of a portion of the lithographic apparatus LA in more detail. In particular, FIG. 2 shows a patterning device MA, a support structure MT, a masking blade 20, an electron beam source 100, and other elements provided within a housing 24. An opening 26 is provided at the bottom end of the housing 24. A beam of EUV radiation enters the housing 24 through this opening and, after being reflected from the patterning device MA, exits through the same opening 26. The masking blade 20 defines an exposure zone 31 through which the EUV radiation passes before impinging on the patterning device MA. The exposure zone 31 may be interpreted as a volume having sides defined by the masking blade 20, a volume extending to the patterning device MA. The area of ​​the patterning device MA that receives EUV radiation may be referred to as the exposure area.

[0061] A gas supply system 27 is provided within the housing 24. The gas supply system 27 is on one side of the exposure zone 31 and provides a flow of gas across the exposure zone. A gas removal system (not shown) may be provided to remove gas from the opposite side of the exposure zone 31. A beam attenuator 54 movable in the Y direction (scanning direction of the lithographic apparatus) is provided within the housing 24. The beam attenuator 54 may, for example, comprise a series of fingers that may be driven so as to partially intersect the EUV radiation beam, thereby providing some attenuation of the radiation beam when required.

[0062] A patterning device exchange system 102 connects to the housing 24. Only a part of the patterning device exchange system 102 is visible in Figure 2. The patterning device exchange system 102 is configured to receive a patterning device MA from the support structure MT so that the patterning device may be removed from the patterning device environment 25. The patterning device exchange system 102 is further configured to deliver different patterning devices MA to the support structure MT to enable different patterns to be projected onto the substrate W by the lithographic apparatus LA (see Figure 1).

[0063] The electron beam source 100 is connected to a current and voltage source 106 via an electrical connection 104. The electron beam source 100 is connected to a gas source 110 via a conduit 108. The electrical connection 104 and the gas conduit 108 may pass through a wall of the housing 24. The electron beam source 100 may be fixed to the housing 24. The electron beam source 100 may be located near the patterning device exchange system 102 (as shown schematically) or at a different location. The current and voltage source 106 may be configured to provide DC and / or AC power. The electron beam source 100 may emit electrons via field emission or thermal emission, or may extract electrons from a dedicated plasma source. The current and voltage source 106 may power the electron emission or maintain a dedicated plasma. The current and voltage source 106 may accelerate the emitted or extracted electrons to a desired energy. The gas source 110 may be configured to maintain a desired pressure (e.g., higher than the nominal pressure in the environment 25) within the electron source 100. Providing a pressure within the electron source that is higher than the nominal pressure can help maintain a dedicated plasma.

[0064] The gas source 110 may provide, for example, hydrogen gas or helium gas, with hydrogen and helium being preferred over other gases because other gases can result in sputtering by heavy collateral ions, contaminating the optical elements of the lithographic apparatus and reducing the EUV transmission of the optical elements.

[0065] An electron beam 101 extends from an electron beam source 100 to the patterned surface of a patterning device MA. A line of sight extends from the electron beam source 100 to the patterning device MA. The electron beam 101 is illustrated schematically as diverging as it travels towards the patterning device MA. In reality, the electron beam may diverge more than is shown schematically. The divergence may be non-uniform.

[0066] An example of an electron beam source 100 is shown schematically in FIG. 3. This example shows an electron source in which electrons are extracted from a plasma, specifically an RF plasma with optional magnetic confinement (ECR plasma). Other plasma sources, as well as thermal and field emissions, may also serve as the electron source. The electron beam source includes a chamber 120 into which a gas 121 (e.g., hydrogen) is delivered via a conduit 108. An RF antenna 123 within the chamber 120 receives an RF electrical signal. The electric field established by the RF antenna 123 ionizes the gas 121, generating a plasma 121 (the gas and plasma are both in the same location, so the same reference numeral is used). An electrode 122 is positioned near a wall of the chamber. A negative DC voltage is applied to the electrode 122, which pushes electrons out of the chamber 120 through an opening 124. The walls of chamber 120, which may be referred to as the plasma chamber for ease of identification, have the same negative DC voltage as electrode 122 (due to conduction by the plasma between electrode 122 and the walls of plasma chamber 120). A permanent magnet (not shown) may be used to confine electrons near electrode 122. In some embodiments, electrode 122 may be omitted. In such embodiments, a negative voltage may be provided directly to the walls of plasma chamber 120.

[0067] As described above, the plasma chamber 120 is biased with a negative voltage. The plasma chamber 120 is provided within a housing 126 that is connected to ground. The ground may be the frame of the lithographic apparatus. The housing 126 comprises a first chamber 128 and a second chamber 130. The first and second chambers 128, 130 are separated by a partition wall 132 through which an opening 134 is provided. Electrons 101a exiting the first opening 124 are accelerated by the negative voltage on the wall of the plasma chamber 120 (and the electrode 122, if present) and pass through the opening 134 in the grounded partition wall 132, forming an electron beam.

[0068] An einzel lens 136 is provided within the second chamber 130. The optional einzel lens 136 refocuses the electron beam to form the electron beam 101. When the electron beam is positioned sufficiently close to the patterning device MA (e.g., closer than 10 cm), the focusing of the einzel lens is not necessary and may be omitted.

[0069] The housing 126 used to initiate and sustain the ECR discharge (not shown), the housing of the plasma chamber 120, the electrode 122, and the RF antenna 123 may be formed from the same material. These components, and generally any metal in contact with the plasma 121 or electron beam 101, may be formed from stainless steel, tungsten, molybdenum, or tantalum metals or alloys that are resistant to sputtering by hydrogen or helium plasmas with ion energies less than 100 eV, or EUV hydrogen plasmas.

[0070] The above features, such as the grounding of the plasma chamber 120 and the materials of the housing 126 or other components, are also applicable to other electron sources (eg, electron sources having different configurations than the electron source shown in FIG. 3).

[0071] The electron beam source 100 may be, for example, a mini-ECR based source from Polygon Physics (Meylan, France).

[0072] The ECR-based electron beam source 100 has a significant magnetic field. It is undesirable for this magnetic field to extend into the patterning device environment 25 because it could interfere with the scanning movement of the patterning device. For this reason, the electron beam source 100 may be surrounded by a mu-metal box 144 (or other magnetic shield). The mu-metal box 144 includes an opening through which the electron beam 101 may exit (mu-metal is a nickel-iron soft-heat magnetic alloy). Because mu-metal has high transparency, it is advantageous for shielding the patterning device environment 25 from magnetic fields. Other highly transparent materials may also be used. If mu-metal is used, a coating may be provided on the mu-metal. The coating may be configured to prevent interaction between the hydrogen plasma in the patterning device environment 25 and the mu-metal, which may result in undesirable fragmentation, cracking, or hydrogen embrittlement. Examples of suitable coatings are NiP (nickel phosphorus), Cr, Mo, or other refractory or noble metals. The coating may have a preferred thickness of 0.5-5 μm. Instead of a coating, an additional box (not shown) may be provided around the mu-metal box 144. The additional box may be made of a metal, such as steel, that does not react with the hydrogen plasma. Another additional box may be provided inside the mu-metal box 144. This additional box may be made of a metal, such as steel, that does not react with the hydrogen plasma.

[0073] As shown schematically in Figures 1 and 2, an electron beam 101 extends from an electron beam source 100 to a patterning device MA. The electron beam 101 provides contactless control of the potential of the patterned surface of the patterning device MA (i.e., it does not require physical wire contact, which could create contaminants or damage critical surfaces of the patterning device MA). The electron beam 101 can keep the time-averaged potential of the patterning device MA at a negative voltage (e.g., -0.1V to -10V) or prevent the patterning device MA from reaching a positive time-averaged potential (e.g., +1V to +5V). The negative potential of the patterning device MA, in combination with the grounded masking blade 20, the grounded gas delivery system 27, and the grounded beam attenuator 54, creates a favorable electric field that repels contaminant particles from the patterning device MA. Most contaminant particles are negatively charged due to the effect of the EUV-induced plasma in the low-pressure hydrogen in the environment 25 and / or the photoelectron output from the patterned device due to EUV photon irradiation, so the negatively charged patterning device MA repels the negatively charged contaminant particles.

[0074] The electron beam 101 is incident on one end of the patterning device MA, but because the patterned surface of the patterning device is conductive, a negative potential is distributed across the patterning device.

[0075] In the absence of the electron beam source 100, the patterned surface of the patterning device MA becomes positively charged and attracts negatively charged contaminant particles. The positive charge, which may be on the order of 3 V, results from a combination of the effects of EUV-induced photoemission of electrons from the patterning device MA and the interaction of EUV-induced plasma in the vicinity of the patterned surface of the patterning device MA. The positively charged patterning device surface, in combination with the grounded masking blade 20 and other grounded components, attracts negatively charged contaminant particles toward the patterning device. This undesirable situation is avoided by embodiments of the present invention.

[0076] The current of the emitted electron beam may be sufficient to provide a current of at least 100 μA at the patterned surface of the patterning device MA. To provide a current of at least 100 μA at the patterned surface of the patterning device MA, the electron beam source 100 may be configured to output an electron beam 101 having a current significantly higher than 100 μA. For example, the electron beam source 100 may output an electron beam having a current of at least 1 mA. For example, the electron beam source 100 may output an electron beam having a current of up to 10 mA. The current may be sufficient to keep the voltage at the patterning device surface at a negative potential (e.g., on the order of −0.1 V to −10 V instead of on the order of +1 to +5 V on a time-averaged basis). The current may be sufficient to remove the positive voltage at the patterning device more than 10 μs after each EUV pulse (thus more quickly than the 20 μs gap between EUV pulses provided by the source SO). This ensures that the time-averaged potential of the patterning device remains negative.

[0077] The current of the electron beam 101 and the voltage of the patterning device may be set so as not to generate significant heat. The power absorbed by the patterning device from the electron beam 101 may be, for example, less than 10% of the power absorbed from the EUV beam B. The power absorbed by the patterning device from the electron beam 101 may be, for example, 1 W or less.

[0078] As shown by arrow 40 in Figure 2, the patterning device MA moves back and forth in the Y direction during scanning exposure. This establishes a dynamic system in which the patterning device MA moves through the EUV beam B, which continuously charges the patterning device positively. The current of the electron beam 101 should be sufficient to counter the photo effect of the EUV beam B so that the patterning device surface remains at a negative potential at all positions within the scanning movement of the patterning device.

[0079] The energy of the electrons in the electron beam 101 may be, for example, at least 1 eV. Having electrons with an energy of at least 1 eV is desirable because it helps minimize electron attachment ionization.

[0080] The electron beam source 100 may be located, for example, within 10 cm of the patterning device MA (e.g., when scanning movement of the patterning device moves the patterning device over the electron beam source). The energy of the electrons in the electron beam may be up to 13.5 eV. This is desirable because this energy is lower than the hydrogen ionization potential (13.6 eV) and the electrons do not ionize hydrogen atoms via electron impact in the patterning device environment 25. Hydrogen ions are undesirable because they promote particle emission from contaminated surfaces near the patterning device MA. While it is preferred that the energy of the electrons in the electron beam be up to 13.5 eV, the electron energy may be greater than this (although some ionization of hydrogen atoms may result). The electrons may have, for example, an energy of up to 100 eV. Having electrons with energies greater than 100 eV is undesirable because at such high energies the secondary electron yield becomes greater than 1, reducing the negative charge provided to the patterning device by the electron beam.

[0081] If the electron beam source 100 is, for example, more than 10 cm from the patterning device, the electrons in the electron beam 101 may be provided with an energy of, for example, at least 10 eV so that they have sufficient energy to travel to the patterning device. If the electrons have a lower energy, they may be attenuated or dissipated in hydrogen in the patterning device environment 25 before reaching the patterning device. The electrons may have an energy of, for example, greater than 13.5 eV. The electrons may have an energy of, for example, up to 100 eV. As mentioned above, it is undesirable to provide electrons with an energy greater than 100 eV.

[0082] The electron beam source 100 may have a line of sight to the patterning device MA (e.g., as shown in FIG. 2). In this case, the electron beam 101 may extend in a straight line to the patterning device MA. In other embodiments, the electron beam 101 may not have a line of sight to the patterning device MA. In this case, some electrons travel in a curved line relative to the patterning device. The EUV beam creates a positive potential on the patterning device, and as a result, the electrons may be attracted to the patterning device (and thus may bend toward the patterning device). Alternatively, at least some of the electron beam may reach the patterning device due to scattering of gas molecules, ions, other electrons, and interactions with capacitance or surface charges.

[0083] One embodiment of the present invention is shown in Figure 4. In this embodiment, an electron beam source 200 is embedded in the wall 150 of the opening 26 leading to the patterning device environment 25. In the embodiment shown, an electron beam 101 is directed generally upwards towards the patterning device MA. The electron beam source 100 may have a line of sight to the patterning device MA. The electron beam may be incident on the patterning device MA in the exposure zone 31. An advantage of this arrangement is that the patterning device MA does not move towards and away from the electron beam source 200, and the current provided to the patterning device by the electron beam remains generally constant. Alternatively, the electron beam 101 may be directed at the EUV beam B rather than the EUV illumination region of the patterning device MA.

[0084] In the present embodiment shown in Figure 4, the electrons in the electron beam 101 may have an energy greater than 10 eV (e.g., up to 100 eV). If the electrons have a lower energy, they are more likely to be attenuated before reaching the patterning device (lower energy electrons are more likely to diverge from the electron beam than higher energy electrons, and therefore dissipate more quickly). The electrons lose energy as they travel towards the patterning device, and as a result, they may have an energy (on average) less than 10 eV when they are incident on the patterning device. The electrons may similarly have an energy less than 10 eV when they are incident on other surfaces in the vicinity of the patterning device.

[0085] It is advantageous to direct the electron beam 101 towards the exposure zone 31 because a plasma generated in the exposure zone by the EUV radiation attracts the electrons of the electron beam and guides them towards the patterning device MA. The electron beam 101 may have a cross-sectional size that generally corresponds to or is generally smaller than the aperture defined by the masking blades 20. This advantageously avoids or reduces losses of the electron beam due to electrons impinging on the masking blades.

[0086] In a further alternative embodiment, the electron beam source 300 may be positioned at approximately the same height (i.e., at a position in the z-direction) as the masking blade 20. The electron beam source 300 may also be positioned to one side of the masking blade 20 (i.e., offset in the x-direction). In such a configuration, the electron beam source 300 may not have a line of sight to the patterning device MA. The electron beam may be bent towards the patterning device MA. An advantage of this arrangement is that the patterning device MA does not move towards and away from the electron beam source 300, and the current provided to the patterning device by the electron beam remains approximately constant. When this arrangement is used, a single electron beam source may be provided.

[0087] A further alternative embodiment of the invention is shown in Figure 5. In this further embodiment, the patterning device MA, support structure NT, and masking blade 20 are as further described above in connection with the other embodiments. The gas supply system, beam attenuators, and patterning device exchange system have been omitted from Figure 5 for simplicity of illustration. The height of the patterning device environment 25 shown has been reduced compared to that shown in other figures of the present application so that the relative positions of the apparatus elements shown in Figure 5 more closely resemble a physical lithographic apparatus (although Figure 5 is still schematic).

[0088] Unlike the other embodiments, a beam of EUV radiation B is shown. An exposure zone 31 can be seen in Figure 5. The exposure zone 31 is where, in use, the EUV beam B intersects the patterning device MA.

[0089] The bottom of the housing 24 containing the patterning device environment 25 comprises a floor which may be formed by the upper surfaces of the different apparatus elements. This is shown schematically in Figure 5 as a stepped floor 402. An opening 26 is provided in the stepped floor 402 of the housing 24. The opening comprises a pair of walls 404. The walls 404 are sloped to form an inwardly tapered space through which the EUV radiation beam B may pass and through which reflected EUV radiation (not shown) may also pass.

[0090] An electron beam source 400 is provided on one side (scanning, y direction) of the opening 26. The electron beam source 400 may be located at the bottom of the housing 24. The electron beam source 400 may be located near one of the walls 404 of the opening 26. The electron beam source 400 may form part of the floor of the housing 24 (e.g., a stepped floor 402 is formed from the top surface of the device elements).

[0091] Electron beam source 400 emits electron beam 401. Electron beam 401 is tilted relative to the vertical (z-direction) so that the electron beam generally points toward exposure zone 31. This may be achieved by positioning electron beam source 400 at an angle relative to the vertical (as shown) or by configuring the electron beam source to emit an angled electron beam (as described further below). Electron beam 401 has some divergence such that the size of the cross-sectional area of ​​the electron beam increases as a function of distance from electron beam source 400. Electron beam 401 may extend across the entire exposure zone 31 in the scanning direction (y-direction) of the lithographic apparatus (as shown). The middle of electron beam 401, indicated schematically by dotted line 403, may overlap exposure zone 31.

[0092] The electron beam 401 may be incident directly on the patterning device MA in the exposure zone 31. This advantageously means that the patterning device MA does not move towards or away from the electron beam 401 during use. Instead, the electron beam 401 remains continuously incident on the patterning device MA during a lithographic exposure. The current emitted by the electron beam source 401 to the patterning device MA may remain roughly constant during a lithographic exposure, or may be modulated by bias or RF power modulation. A constant current emitted by the electron beam source is more robust and reliable, and is therefore the preferred embodiment.

[0093] The spacing between the electron beam source 400 and the patterning device MA may be less than 10 cm. The spacing may, for example, be less than 4 cm. The spacing may, for example, be about 3 cm. Providing the electron source 400 at such a spacing relatively close to the patterning device MA is advantageous because it allows the electrons to be provided with energies less than 13.6 eV. The spacing is small enough that energies less than 13.6 eV are sufficient for most electrons to reach the patterning device MA. Advantageously, the electrons do not have enough energy to ionize hydrogen atoms via electron impact within the patterning device environment 25.

[0094] Although the spacing is small enough that most electrons with energies less than 13.6 eV reach the patterning device, electrons with higher energies may also be used, for example, electrons with energies of 30 eV or more, and electrons with energies up to 100 eV may be used.

[0095] The electron beam source 400 may provide an electron beam current of at least 100 μA. The electron beam source 400 may provide an electron beam current of up to about 10 mA. This may apply to other embodiments. These electron beam current values ​​are time-averaged currents for embodiments in which the electron beam is modulated (the electron beam may be constant or modulated). A current of at least 100 μA may be sufficient to maintain a negative voltage at the patterning device MA. A current greater than 10 mA may also be provided. However, a current of up to 10 mA suffices to maintain a negative voltage at the patterning device MA, and increasing the current beyond 10 mA introduces unnecessary stress (e-beam generated plasma) to the environment.

[0096] Generally, providing the electron beam source 400 near the bottom of the housing 24 and the wall 404 of the opening 26 may advantageously provide a line of sight from the electron beam source to the exposure area 31, with a spacing that allows the electrons to have an energy of about (for example) 10 eV, e.g., up to 100 eV, with an electron beam current of (for example) up to 10 mA, while providing a sufficient negative potential with respect to the patterning device during exposure. Electron energies greater than 100 eV and / or electron beam currents greater than 10 mA may be used, but these introduce unnecessary stress (e-beam generated plasma) to the environment without providing any benefit.

[0097] Multiple electron beam sources may be provided (for this or other embodiments). The multiple electron beam sources (not shown) may be distributed in the X direction such that the electron beams 401 are incident on the patterning device MA at different X-direction locations. The electron beam sources 401 may, for example, be spaced apart sufficiently small that adjacent electron beams overlap each other when incident on the patterning device MA. The multiple electron beam sources 400 may be distributed to provide overlapping electron beams such that a band of electrons extends across the entire patterning device MA in the X direction.

[0098] The electron beam source (for this or other embodiments) may be configured to output an electron beam having an elliptical cross-sectional shape. For example, referring to FIG. 3 , the Einzel lens 136 may have a rectangular cross-section (extending in the X direction) to provide an electron beam with an elliptical cross-section. Other electrode configurations may be used. The elliptical beam may have an aspect ratio greater than 1, preferably greater than 2, and most preferably greater than 3. An aspect ratio greater than 3 is most preferred because it spreads the electron beam more across the patterning device MA in the X direction (compared to aspect ratios less than 3). This, in turn, may provide complete coverage of the patterning device MA in the X direction using fewer electron beam sources (for a given electron beam divergence and a given spacing from the patterning device).

[0099] Over time, electrodes of an electron beam source of one embodiment of the present invention (e.g., electrode 122 providing RF excitation) may be damaged due to sputtering of hydrogen or helium ions (which may have energies up to 100 eV). To reduce the occurrence of such damage, a gas may be supplied to the plasma electrode. The gas may be a single element or a mixture of elements. The gas may comprise at least one of H2 and He.

[0100] To reduce electrode damage, the electrode may be in contact with one of stainless steel, tungsten, molybdenum, or tantalum metals or alloys that are resistant to sputtering by hydrogen or helium ions with energies up to 100 eV.

[0101] The electron beam source may comprise at least one permanent magnet. A permanent magnet (not shown) may be provided around the plasma chamber 120 and configured to confine electrons of the plasma 121. A magnetic shielding material, such as a mu material, may be disposed around the electron beam source to shield other parts of the lithographic apparatus from the magnetic field provided by the permanent magnet.

[0102] In the lithographic patterning device contamination control system according to this embodiment, the outer wall of the electron beam source is made of one of stainless steel, tungsten, molybdenum, or tantalum metals or alloys that are resistant to sputtering by the hydrogen EUV plasma.

[0103] The RF antenna 123 of the electron beam source may be connected to an RF power supply configured to provide power having a frequency in the range of 0.1 GHz to 10 GHz. The frequency may preferably be in the range of 1 GHz to 3 GHz. The RF antenna 123 may be connected to a DC power supply configured to provide a maximum of 100 mA. The walls of the plasma chamber 120 (and the electrode 122 on which they reside) may be connected to a DC power supply configured to provide a maximum of -1 kV (to support extraction of an electron beam) or a maximum of +1 kV (to support extraction of a beam of positively charged ions).

[0104] In the present embodiment shown in FIG. 5, the electron beam 401 is generally symmetrical about an axis extending from the electron beam source 400 (i.e., the electron beam is not tilted relative to the electron beam source). However, in other embodiments (not shown), the electron beam may be tilted relative to the body of the electron beam source. For example, the electron beam may have a tilt of up to 20°. The electron beam tilt can be achieved by offsetting the opening 134 in the grounded partition 132 of the electron beam source relative to the central axis of the electron beam source (e.g., the axis defined by the RF antenna 123) (see FIG. 3). Tilting the electron beam can be assisted by tilting the grounded partition 132 and by tilting the plasma chamber wall in which the opening 124 is provided. Alternatively, an auxiliary electrode or magnet can be positioned near the extraction ground electrode opening 134 to tilt the electron beam (101).

[0105] In general, the electron beam source may be provided anywhere in or near the patterning device environment 25 such that the electron beam 101 can reach the patterning device MA. If the electron beam source is offset in the Y direction (scanning direction) from the exposure zone 31, the current provided by the electron beam to the patterning device MA will vary as the patterning device MA moves towards and away from the electron beam source. The effect of the electron beam current in maintaining a negative voltage at the patterning device plane may also vary correspondingly. To avoid this varying effect, a first electron beam source may be offset in the Y direction from the exposure zone 31, and a second electron beam source may be offset in the −Y direction from the exposure zone 31. Alternatively, a single electron beam source may be provided, but with a higher output electron beam current (the current may be selected so that the patterning device plane always has a negative voltage).

[0106] In one embodiment (not shown), an electron beam source may be provided on either side of the exposure zone 31. If this is done, the current provided to the patterning device may remain generally constant, although such an embodiment may be more complex and expensive to implement than a single electron beam source embodiment.

[0107] During unloading of the patterning device MA from the support structure MT, it is desirable to avoid a negative potential on the backside of the patterning device MA. The unloading (and loading) of the patterning device MA from (and onto) the support structure MT by the patterning device exchange system 102 may be referred to as patterning device manipulation. The patterning device MA may act as one plate of a capacitor, and the support structure MT as the other plate. The capacitance of this capacitor decreases rapidly as the patterning device moves away from the support structure. If the backside of the patterning device is negatively charged, the reduced capacitance results in an increase in voltage. There is a risk that the voltage at the backside of the patterning device could reach kV levels, resulting in an electrical discharge. This is undesirable as it could damage the lithographic apparatus. Embodiments of the present invention may neutralize or reduce the negative potential on the backside of the patterning device.

[0108] In one embodiment, a current of positive ions may be delivered to the backside of the patterning device MA. This current of positive ions may neutralize the charge on the patterning device MA. The current of positive ions may be obtained by reversing the polarity applied to the electron beam source 100 (so that positively charged ions are delivered to the vicinity of the reticle exchange device 102). The backside of the patterning device MA is negatively charged, and therefore attracts positive ions even when there is no line of sight to the backside of the patterning device. The controller may control the electron beam source to deliver positive ions when the lithographic apparatus is not performing a lithographic exposure, for example, when the patterning device MA is being unloaded from the support structure. Alternatively, the electron beam source 100 operated in normal electron emission mode during patterning device loading / unloading can ionize hydrogen in the environment 25 and provide the necessary positive ions to the vicinity of the reticle exchange device 102.

[0109] In one embodiment, an electron beam source (or a different electron source) may be used to assist the ionizer in generating the plasma in the patterning device environment. The ionizer may be, for example, a coil configured to induce ionization of hydrogen gas in the patterning device environment. Electrons provided by the electron source may be accelerated, promoting ionization of the hydrogen molecules and providing a “kickstart” for the ionization provided by the ionizer. Once the initial plasma is generated, the plasma may be self-sustaining (assuming the ionizer is continuously running). The plasma flows to the backside of the patterning device and neutralizes the charge on the backside of the patterning device MA. In this way, the plasma reduces the risk of electrical discharge when the patterning device MA separates from the support structure MT. An electrostatic mirror may be used to direct the plasma to the backside of the patterning device.

[0110] Techniques other than electron extraction from a micro-plasma source can provide an electron beam suitable for controlling the potential of a patterning device. The electron source can be a heated wire providing thermionic emission. The electron source can also be, for example, a cold emission source (e.g., a tip held in an electric field strong enough to induce electron emission from the tip). The electron source can also be a DUV LED (e.g., with a wavelength in the 200-300 nm range) configured to illuminate a surface where photoemission occurs (i.e., a surface with a photoemission energy lower than the energy of the incident photons). Any other technique can be assisted by a power source to form a beam of electrons of the required energy (e.g., 10 eV ... 100 eV).

[0111] The ionizer may be, for example, an induction coil. 3 of (e.g., 1 cm 3 The inductive coil may have a smaller (smaller) volume. The inductive coil may be located within the patterning device environment. Multiple inductive coils may be located within the patterning device environment.

[0112] The controller may control the ionizer such that the ionizer is switched on when the lithographic apparatus is not performing a lithographic exposure, for example when the patterning device MA is unloaded from the support structure. The same may apply to the electron source.

[0113] The use of the terms "electron beam" and "electron beam source" does not necessarily imply that the electrons are provided as a collimated beam. Moreover, as noted above, significant divergence of the electrons may occur.

[0114] The electron beam source may be configured such that when the lithographic apparatus is exposing a substrate, the majority of electrons generated by the beam source and remaining in the patterning device environment may have an energy less than 13.6 eV.

[0115] Multiple electron beam sources may be provided.

[0116] In this document, references to ground may be interpreted as referring to electrical earth (which may also be referred to as electrical earth or simply earth).

[0117] In this document, an electron beam source may be configured to direct an electron beam towards a patterning device supported by a support structure. At some times, there may not be a patterning device on the support structure. In this case, the electron beam source configuration remains unchanged (although the electron beam may be switched off). This state of affairs may be encompassed by referring to the area of ​​the support structure that holds the patterning device. Thus, for example, a line of sight may extend from the electron beam source to the area of ​​the support structure that holds the patterning device. As another example, the electron beam source may be positioned at a distance of up to 11 cm from the area of ​​the support structure that holds the patterning device (the patterning device being about 1 cm thick).

[0118] Methods according to an embodiment of the present invention may be performed by a computing device. The device may include a central processing unit ("CPU") coupled to a memory. The methods described herein may be implemented in code (software) stored on a memory comprising one or more storage media and arranged for execution on a processor comprising one or more processing units. The storage media may be integrated into the CPU and / or may be separate from the CPU. The code, which may be referred to as instructions, is configured to be fetched from the memory and executed on the processor to perform operations in accordance with the embodiments discussed herein. Alternatively, it is not excluded that some or all of the functionality of the CPU may be implemented in dedicated hardware circuitry or configurable hardware circuitry such as an FPGA.

[0119] The computing device may comprise an input configured to allow a user to input data into the software program running on the CPU. The input device may comprise a mouse, keyboard, touch screen, microphone, etc. The computing device may further comprise an output device configured to output the results of the measurements to the user.

[0120] 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, including 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.

[0121] Although specific reference may be made in this text to embodiments of the invention in the context of a lithography apparatus, embodiments of the invention may also 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 objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum or atmospheric (non-vacuum) conditions.

[0122] Although specific reference may be made to the use of embodiments of the invention in the context of optical lithography, it will be understood that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0123] 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 as 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 manner readable by an apparatus (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, acoustic, or other forms of transmission signals (e.g., carrier waves, infrared signals, digital signals, etc.), etc. Furthermore, firmware, software, routines, and instructions may be described herein as performing particular actions. However, it should be understood that such description is for convenience only, and that such actions may in fact be caused by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., causing actuators or other devices to interact with the physical world.

[0124] While specific embodiments of the invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set forth below.

Claims

1. a support structure configured to support a patterning device; an electron beam source configured to emit a beam of electrons such that at least a portion of the beam is incident on a patterned surface of a patterning device supported by the support structure during EUV exposure; 1. A lithographic patterning device contamination control system comprising:

2. The lithographic patterning device contamination control system of claim 1 , wherein a line of sight extends from the electron beam source to the patterning device.

3. 3. The lithographic patterning device contamination control system of claim 2, wherein the line of sight extending from the electron beam source to the patterning device exists for all positions of the patterning device during EUV exposure.

4. 4. A lithographic patterning device contamination control system according to claim 1, wherein the electron beam source is configured to emit electrons having an energy of at least 1 eV.

5. 5. A lithographic patterning device contamination control system according to claim 1, wherein the electron beam source is configured to emit electrons having an energy of up to 13.5 eV.

6. 5. A lithographic patterning device contamination control system according to claim 1, wherein the electron beam source is configured to emit electrons having an energy of up to 100 eV.

7. the electron beam source is positioned at a distance greater than 10 cm from the patterning device; The electron beam source is configured to emit electrons having an energy of at least 10 eV.

5. A lithographic patterning device contamination control system according to any one of claims 1 to 4.

8. 7. A lithographic patterning device contamination control system according to claim 1, wherein the electron beam source is positioned at a distance of 10 cm or less from the patterning device.

9. the electron beam source is disposed at the bottom of a housing that defines a patterning device environment; the electron beam source is positioned adjacent a wall of an opening provided in the housing; 9. The lithographic patterning device of claim 8.

10. 10. A lithographic patterning device contamination control system according to claim 1, wherein the electron beam source is configured to emit electrons with a current of at least 100 μA.

11. 11. A lithographic patterning device contamination control system according to claim 1, wherein the electron beam source is configured to emit electrons with a current of up to 10 mA.

12. 12. A lithographic patterning device contamination control system according to claim 1, wherein the electron beam source is configured to output an electron beam having an elliptical cross-sectional shape.

13. 13. A lithographic patterning device contamination control system according to claim 1, wherein the plasma electrode of the electron beam source is in contact with one of stainless steel, tungsten, molybdenum, tantalum metal or alloys that are resistant to sputtering by hydrogen or helium ions having energies up to 100 eV.

14. 14. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source comprises at least one permanent magnet and a magnetic shielding material, such as mu metal.

15. 15. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source has a housing that is grounded and is in electrical contact with a frame of a lithography tool.

16. 16. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source housing is made from one of stainless steel, tungsten, molybdenum, or tantalum metals or alloys that are resistant to sputtering by hydrogen EUV plasma.

17. 17. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source is connected to an RF power supply configured to provide power having a frequency in the range of 0.1 GHz to 10 GHz.

18. The electron beam source is H 2 18. A lithographic patterning device contamination control system according to claim 1, connected to at least one supply of:

19. 19. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source is configured to direct the electron beam towards an exposure zone in a patterning device environment.

20. 19. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source is configured to direct the electron beam to a location that is offset from an exposure zone.

21. 21. The lithographic patterning device contamination control system of claim 20, wherein an additional electron beam source is configured to direct an additional electron beam at a location that is offset in an opposite direction relative to the exposure zone.

22. 21. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source is one of a plurality of electron beam sources.

23. 23. A lithographic patterning device contamination control system according to any preceding claim, wherein the electron beam source is configured to maintain the patterning device at a negative potential, at least on a time-averaged basis, during EUV exposure.

24. 24. A lithographic patterning device contamination control system according to any preceding claim, further comprising a controller configured to switch a polarity applied to the electron beam source such that the electron beam source outputs positive ions.

25. 25. A patterning device contamination control system according to any one of claims 1 to 24, and further comprising a housing in which the masking blade, the patterning device exchange system, and the patterning device environment are disposed. Lithography equipment.

26. 26. A lithographic apparatus according to claim 25, wherein the electron beam source is provided in a wall that defines an opening leading to the patterning device environment.

27. the electron beam source is positioned at a height corresponding to the height of the masking blade; the electron beam source is offset in a direction orthogonal to a scanning direction of a patterning device support structure.

26. A lithographic apparatus according to claim 25.

28. the electron beam source is positioned near a patterning device exchange system.

26. A lithographic apparatus according to claim 25.

29. A lithographic apparatus comprising a source of electrons and an ionizer disposed in a patterning device environment.

30. 30. The lithographic apparatus of claim 29, further comprising a controller configured to activate the ionizer when the lithographic apparatus is not performing a lithographic exposure.

31. A method for controlling contamination of a lithographic patterning device, comprising directing an electron beam against a patterned surface of the patterning device.

32. a gas is supplied to a plasma electrode of the electron beam source; The gas is H 2 and He, 32. The method of claim 31 .

33. 33. The method of claim 31 or 32, wherein the electron beam maintains the patterned surface of the patterning device at a negative potential on a time-averaged basis during EUV exposure.

34. 1. A method for controlling charge on a back surface of a lithographic patterning device, comprising directing a beam of electrons or a beam of positive ions towards a back surface of the lithographic patterning device when the lithographic patterning device is handled by a patterning device exchange system.

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