Pollution control

Abstract

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

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to European / US application 22212483.6 filed on December 9, 2022, and incorporates the entire content of that application herein by reference. Technical field

[0003] The present invention relates to contamination control in a lithographic apparatus. Background art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a lithographic apparatus can be used in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus can project a pattern present on a patterning device (e.g., a mask) onto a layer of radiation - sensitive material (resist) provided on a substrate.

[0005] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum feature size that can be formed on the substrate. Compared to a lithographic apparatus using radiation having a deep - ultraviolet (DUV) wavelength of, for example, 193 nm, a lithographic apparatus using extreme - ultraviolet (EUV) radiation (having a wavelength in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0006] In a conventional (DUV) lithographic apparatus, a pellicle is attached to the patterning device. A pellicle is a diaphragm that is spatially separated from the patterning device. When a lithographic apparatus projects contaminant particles onto a substrate, the contaminant particles incident on the pellicle will be defocused. Thus, the contaminant particles do not introduce defects into the pattern projected from the patterning device onto the substrate by the lithographic apparatus.

[0007] A pellicle can also be used in an EUV lithographic apparatus. However, EUV radiation is absorbed by the pellicle, and this reduces the intensity of the EUV radiation that can be used to expose the substrate. This in turn reduces the throughput of the lithographic apparatus. Maintaining the cleanliness of the patterning device during EUV exposure can be achieved by suppressing the release and / or transport of contaminant 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.

[0008] It may be desirable to provide an apparatus that overcomes or alleviates one or more problems associated with the prior art. Summary of the invention

[0009] According to a first aspect of the present invention, there is provided a contamination control system for a lithographic patterning apparatus, comprising: a support structure configured to support the patterning apparatus; and an electron beam source configured to emit an electron beam such that at least a portion of the electron beam is incident on a patterned surface of the patterning apparatus supported by the support structure during EUV exposure.

[0010] Advantageously, the electron beam provides non-contact control of the potential of the patterned surface of the patterning apparatus (i.e., no physical line contact that may generate contaminants or damage the critical surface of the patterning apparatus is required). The electron beam can maintain the time-averaged potential of the patterning apparatus MA at a negative voltage (e.g., -0.1 V,..., -10 V), or can prevent a positive time-averaged potential of the patterning apparatus MA (e.g., a potential reaching +1 V,..., +5 V). The negative potential of the patterning apparatus will form a favorable electric field that repels contaminant particles from the patterning apparatus.

[0011] A line of sight can extend from the electron beam source to the patterning apparatus.

[0012] A line of sight can extend from the electron beam source to the patterning apparatus, and this line of sight can exist for all positions of the patterning apparatus during EUV exposure.

[0013] The electron beam source can be configured to emit electrons having an energy of at least 1 eV.

[0014] The electron beam source can be configured to emit electrons having an energy of up to 13.5 eV.

[0015] The electron beam source can be configured to emit electrons having an energy of up to 100 eV.

[0016] The electron beam source can be positioned at a distance greater than 10 cm from the patterning apparatus, and wherein the electron beam source is configured to emit electrons having an energy of at least 10 eV.

[0017] The electron beam source can be positioned at a distance of 10 cm or less from the patterning apparatus.

[0018] The electron beam source can be positioned at the bottom of a housing that defines the environment of the patterning apparatus. The electron beam source can be positioned adjacent to a wall of an opening provided in the housing.

[0019] The electron beam source can be configured to emit electrons having a current of at least 100 μA.

[0020] The electron beam source can be configured to emit electrons having a current of up to 100 mA.

[0021] The electron beam source can be configured to output an electron beam having an elliptical cross-sectional shape. The elliptical beam can have an aspect ratio greater than 1, preferably greater than 2, and optimally greater than 3.

[0022] The plasma electrode of the electron beam source can be in contact with one of the following: stainless steel, tungsten, molybdenum, tantalum metal or alloy, which can resist sputtering by hydrogen or helium ions having an energy of up to 100 eV.

[0023] The electron beam source can include at least one permanent magnet and a magnetic shielding material, such as mu-metal.

[0024] The electron beam source can have a housing that is grounded and in electrical contact with the frame of the lithography tool.

[0025] The electron beam source housing can be made of one of the following: stainless steel, tungsten, molybdenum, tantalum metal or alloy, which can resist sputtering by hydrogen EUV plasma.

[0026] The electron beam source can 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 can preferably be in the range of 1 GHz to 3 GHz. The electron beam source can be connected to a DC power supply configured to provide up to 100 mA. The electron beam source can be connected to a DC power supply configured to provide up to -1 kV (to support electron beam extraction) or up to +1 kV (to support extraction of positively charged ion beams).

[0027] The electron beam source can be connected to a supply source of at least one of H2 and He.

[0028] The electron beam source can be configured to direct the electron beam to an exposure zone within the patterning device environment.

[0029] The electron beam source can be configured to direct the electron beam to a position offset from the exposure zone.

[0030] An additional electron beam source can be configured to direct an additional electron beam to a position offset in the opposite direction relative to the exposure zone.

[0031] The electron beam source can be one of a plurality of electron beam sources.

[0032] The electron beam source can be configured to maintain the patterning device at a negative potential based at least on time-average during EUV exposure.

[0033] The electron beam source can further include a controller configured to switch the polarity applied to the electron beam source such that the electron beam source outputs positive ions.

[0034] According to a second aspect of the invention, there is provided a lithographic apparatus comprising a contamination control system for a patterning device according to any of the preceding claims, and further comprising a shadow blade, a patterning device exchange system and a housing, wherein a patterning device environment is located within the housing.

[0035] An electron beam source may be provided in a wall defining an opening leading to the patterning device environment.

[0036] The electron beam source may be positioned at a height corresponding to the height of the shadow blade. The electron beam source may be offset in a direction orthogonal to the scanning direction of the patterning device support structure.

[0037] The electron beam source may be positioned adjacent to the patterning device exchange system.

[0038] According to a third aspect of the invention, there is provided a lithographic apparatus comprising an electron source and an ionizer positioned within a patterning device environment.

[0039] The electron source and the ionizer may advantageously generate a plasma which neutralizes charge on the rear side of the patterning device during manipulation of the patterning device.

[0040] The lithographic apparatus may further comprise a controller configured to activate the ionizer when the lithographic apparatus is not performing a lithographic exposure.

[0041] According to a fourth aspect of the invention, there is provided a method of controlling contamination of a lithographic patterning device, the method comprising directing an electron beam onto a patterned surface of the patterning device.

[0042] The electron beam provides non-contact control of the potential of the patterned surface of the patterning device (i.e., no physical line contact which may generate contaminants or damage critical surfaces of the patterning device is required).

[0043] A gas may be supplied to a plasma electrode of the electron beam source, the gas comprising at least one of H2 and He.

[0044] The electron beam source may maintain the patterning device at a time-averaged negative potential during EUV exposure.

[0045] According to a fifth aspect of the invention, there is provided a method of controlling charge on a rear surface of a lithographic patterning device, the method comprising: directing an electron beam or a positive ion beam towards the rear surface of the lithographic patterning device when the lithographic patterning device is being manipulated by the patterning device exchange system.

[0046] Advantageously, this may prevent high voltages and subsequent discharges from occurring when manipulating the patterning device.

[0047] Features of different aspects of the present invention may be combined together. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0049] - Figure 1 A lithography system including a contamination control system for a patterning device according to an embodiment of the present invention is schematically depicted;

[0050] - Figure 2 The contamination control system is more detailedly schematically depicted;

[0051] - Figure 3 An electron emitter forming part of the contamination control system is schematically depicted;

[0052] - Figure 4 A contamination control system for a patterning device according to an alternative embodiment is schematically depicted; and

[0053] - Figure 5 A contamination control system for a patterning device according to yet another alternative embodiment is schematically depicted. DETAILED DESCRIPTION

[0054] Figure 1 A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes 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.

[0055] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. In addition, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide an EUV radiation beam B having 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 10 and the faceted pupil mirror device 11.

[0056] After being so adjusted, the EUV radiation beam B interacts with the patterning device MA. Due to 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 this purpose, the projection system PS may include a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W, which is held by a substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image having features smaller than the 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 Figure 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).

[0057] The support structure MT may include a clamp for holding the patterning device MA. The clamp may be an electrically driven electrostatic clamp. There may be a dielectric layer between the clamp and the patterning device MA. At least a part of the support structure MT may be electrically grounded.

[0058] 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 the patterning device environment 25. The housing 24 may be substantially enclosed except for an opening at the bottom end of the housing. A set of masking blades 20 is provided in the patterning device environment 25. The masking blades 20 are used to selectively mask regions of the patterning device MA such that only the desired part of the patterning device receives EUV radiation at any given time. During the scanning exposure, the patterning device MA and the support structure MT are moved in the y direction, and the substrate W and the substrate table WT are moved in the opposite y direction (and vice versa). In this way, the EUV radiation band passes through the patterning device MA and through the exposure field on the substrate W.

[0059] An electron beam source 100 is provided in the patterning device environment 25. Figure 1The electron beam source 100 in [description] is located below and to one side of the blade of the mask blank shielding blade system 20. However, the electron beam source 100 can be arranged at different positions, such as other positions within the patterning device environment 25. The electron beam 101 provided by the electron beam source combines with EUV (photo effect) and EUV plasma (charged by ion and electron flows) to together determine both the instantaneous potential and the average potential of the patterning surface of the patterning device MA (the patterning surface is conductive). The electron beam 101 is illustrated as a line, but in reality may diverge and does not necessarily travel in a straight line. The electrons output from the electron source 100 are scattered by gas molecules, ions, and other electrons. Additionally, the electrons are subject to electrostatic forces due to the charge on the surfaces within the patterning device environment 25. The electrons are accelerated by those forces.

[0060] A relative vacuum, i.e., a small amount of gas (e.g., hydrogen gas) at a pressure far lower than atmospheric pressure, can be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS. The same applies to the patterning device environment 25. That is, there is gas in the patterning device environment 25 at a pressure lower than atmospheric pressure. The gas can be, for example, hydrogen gas. The gas can be partially ionized by EUV radiation B and / or by the electron beam 101.

[0061] Figure 1 The radiation source SO shown in [description] is of a type that can be referred to as a laser-produced plasma (LPP) source, for example. A laser system 1, which can include, for example, a CO2 laser, is arranged to deposit energy into a fuel, such as tin (Sn), via a laser beam 2. The fuel is provided by, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be in liquid form, for example, and can be a metal or an alloy. The fuel emitter 3 can include a nozzle configured to direct the tin, for example in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The deposition of laser energy in the tin creates a tin plasma 7 at the plasma formation region 4. During the de-excitation and recombination of electrons with the ions of the plasma, radiation is emitted from the plasma 7, including EUV radiation.

[0062] The EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 includes, for example, a near-normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 can have a multilayer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 can have an ellipsoidal configuration with two foci. As discussed below, the first of the foci can be at the plasma formation region 4, and the second of the foci can be at an intermediate focus 6.

[0063] The laser system 1 can be spatially separated from the radiation source SO. In such a case, the laser beam 2 can be transmitted from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown), which includes, for example, suitable steering mirrors and / or beam expanders, and / or other optical devices. The laser system 1, the radiation source SO, and the beam delivery system can be considered together as a radiation system.

[0064] 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 to form an image of the plasma existing at the plasma formation region 4 at the intermediate focus 6. The image at the intermediate focus 6 serves 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 the opening 8 in the enclosure structure 9 of the radiation source SO.

[0065] Although Figure 1 the radiation source SO is depicted as a laser-produced plasma (LPP) source, any suitable source such as a discharge-produced plasma (DPP) source or a free electron laser (FEL) can be used to generate EUV radiation.

[0066] Figure 2 A part of the lithographic apparatus LA is depicted schematically in more detail. Specifically, Figure 2 The patterning device MA, the support structure MT, the masking blade 20, the electron beam source 100, and other elements provided in the housing 24 are depicted schematically. An opening 26 is provided at the lowermost end of the housing 24. The EUV radiation beam enters the housing 24 through such an opening, is reflected from the patterning device MA, and then exits through the same opening 26. The masking blade 20 defines an exposure section 31 through which the EUV radiation passes before being incident on the patterning device MA. The exposure section 31 can be considered as a space having a plurality of sides defined by the masking blade 20, which extends up to the patterning device MA. The area of the patterning device MA that receives the EU radiation can be referred to as the exposure area.

[0067] A gas supply system 27 is provided within the housing 24. The gas supply system 27 is located on one side of the exposure section 31 and provides an air flow across the exposure section. A gas removal system (not depicted) can be provided to remove gas from the opposite side of the exposure section 31. A beam attenuator 54 that can be moved in the Y direction (the scanning direction of the lithographic apparatus) is provided in the housing 24. The beam attenuator 54 can include, for example, a series of fingers that can be moved to intersect the EUV radiation beam partially, thereby providing some attenuation of the radiation beam when desired.

[0068] A patterning device exchange system 102 is connected to the housing 24. In Figure 2Only a part of the patterning device exchange system 102 is visible. The patterning device exchange system 102 is configured to receive a patterning device MA from a support structure MT such that the patterning device can be removed from the patterning device environment 25. The patterning device exchange system 102 is also configured to transfer a different patterning device MA to the support structure MT in order to allow the lithographic apparatus LA to project a different pattern onto a substrate W (see Figure 1 ).

[0069] 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 the wall of the housing 24. The electron beam source 100 may be fixed to the housing 24. The electron beam source 100 may be positioned adjacent to the patterning device exchange system 102 (as schematically depicted) 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 thermionic emission, or may extract electrons from a dedicated plasma source. The current and voltage source 106 may provide power for electron emission or maintain a dedicated plasma. The current and voltage source 106 may also accelerate the emitted or extracted electrons to a desired energy. The gas source 110 may be configured to maintain a desired pressure within the electron source 100, which pressure may be, for example, higher than the nominal pressure in the environment 25. Providing a pressure higher than the nominal pressure within the electron source may help to maintain a dedicated plasma.

[0070] The gas source 110 may provide, for example, hydrogen or helium. Hydrogen and helium are preferred over other gases because other gases may cause sputtering of associated heavy ions, which would contaminate the optics of the lithographic apparatus and reduce the EUV transmittance of the optics.

[0071] The electron beam 101 extends from the electron beam source 100 to the patterning face of the patterning device MA. The line of sight extends from the electron beam source 100 to the patterning device MA. The electron beam 101 is schematically illustrated as diverging as it travels towards the patterning device MA. In practice, the electron beam may diverge more than schematically depicted. The divergence may be non-uniform.

[0072] In Figure 3FIG. 0 schematically depicts an example of an electron beam source 100. Such an example depicts an electron source in which electrons are extracted from a plasma, specifically from an RF plasma (ECR plasma) with optional magnetic confinement. Other plasma sources as well as thermionic and field emission can be used as electron sources. The electron beam source includes a chamber 120 into which a gas 121 (such as 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, thereby generating a plasma 121 (the same reference numeral is used for the gas and the plasma because both are in the same location). An electrode 122 is positioned adjacent to the wall of the chamber. A negative DC voltage is applied to the electrode 122. This pushes electrons out of the chamber 120 through an opening 124. For ease of identification, the wall of the chamber 120, which may be referred to as the plasma chamber, has the same negative DC voltage as the electrode 122 (this is due to the conduction of the plasma between the electrode 122 and the wall of the plasma chamber 120). Permanent magnets (not depicted) can be used to confine the electrons close to the electrode 122. In some embodiments, the electrode 122 can be omitted. In such embodiments, the negative voltage can be applied directly to the wall of the plasma chamber 120.

[0073] As described above, the plasma chamber 120 is biased with a negative voltage. The plasma chamber 120 is disposed within a housing 126 that is connected to ground. The ground can be the frame of a lithographic apparatus. The housing 126 includes a first chamber 128 and a second chamber 130. The first chamber 128 and the second chamber 130 are separated by a partition wall 132 provided with an opening 134. Electrons 101a exiting from the first opening 124 are accelerated by the negative voltage at the wall of the plasma chamber 120 (and the electrode 122, when present) and pass through the opening 134 in the grounded partition wall 132 and form an electron beam.

[0074] An Einzel lens 136 is disposed in the second chamber 130. The optional Einzel lens 136 refocuses the electron beam such that it forms an electron beam 101. When the electron beam is positioned close enough to the patterning device MA, such as less than 10 cm, Einzel lens focusing is not necessary and it can be omitted.

[0075] The housing 126 for igniting and maintaining the ECR discharge (not depicted), the housing of the plasma chamber 120, the electrode 122, and the RF antenna 123 can be formed of the same material. These components, as well as any metal that typically comes into contact with the plasma 121 or the electron beam 101, can be formed of stainless steel, tungsten, molybdenum, tantalum metal, or alloys that are resistant to sputtering by hydrogen or helium plasmas or EUV hydrogen plasmas having an ion energy of less than 100 eV.

[0076] The above features, such as the grounding of the plasma chamber 120, or the material of the housing 126 or other components, can be applied to other electron sources (e.g., electron sources having a configuration different from that of the electron source depicted in Figure 3 the electron source).

[0077] The electron beam source 100 can be, for example, a micro ECR-based source from Polygon Physics of Meyran, France.

[0078] The ECR-based electron beam source 100 has a significant magnetic field. It may not be desirable for such a magnetic field to extend within the patterning device environment 25, as this would interfere with the scanning movement of the patterning device. For this reason, the electron beam source 100 can be enclosed in a high-permeability alloy (i.e., permalloy) frame 144 (or other magnetic shield). The high-permeability alloy frame 144 includes an opening through which the electron beam 101 can pass. (High-permeability alloy is a nickel-iron soft thermomagnetic alloy). The high-permeability alloy is beneficial for shielding the patterning device environment 25 from the magnetic field because of its high magnetic permeability. Other high-magnetic-permeability materials can be used. If a high-permeability alloy is used, a coating can be provided on the high-permeability alloy. The coating can be configured to prevent the interaction between the hydrogen plasma in the patterning device environment 25 and the high-permeability alloy (which may lead to undesirable fragmentation or cracking or hydrogen embrittlement). Examples of suitable coatings are NiP (nickel-phosphorus), Cr, Mo, other refractory metals or noble metals. The coating can have a preferred thickness of 0.5 um to 5 um. Instead of a coating, an additional frame (not depicted) can be provided around the high-permeability alloy frame 144. The additional frame can be formed of a metal (such as steel) that does not react with the hydrogen plasma. Another additional frame can be provided inside the high-permeability alloy frame 144. Such an additional frame can also be formed of a metal (such as steel) that does not react with the hydrogen plasma.

[0079] As Figure 1 and Figure 2As schematically depicted, an electron beam 101 extends from an electron beam source 100 to a patterning device MA. The electron beam 101 provides non-contact control of the potential of the patterning surface of the patterning device MA (i.e., no physical line contact that can generate contaminants or damage the critical surface of the patterning device MA is required). The electron beam 101 can hold the time-averaged potential of the patterning device MA at a negative voltage (e.g., -0.1 V, …, -10 V), or can prevent a positive time-averaged potential of the patterning device MA (e.g., a potential reaching +1 V, …, +5 V). The negative potential of the patterning device MA, in combination with the grounded shielding blade 20, the grounded gas delivery system 27, and the grounded beam attenuator 54, will generate a favorable electric field that repels contaminant particles from the patterning device MA. Due to the influence of EUV-induced plasma in the low-pressure hydrogen gas in the environment 25 and / or due to the photoelectrons output from the patterning device due to EUV photon irradiation, most contaminant particles are negatively charged. The negatively charged patterning device MA thus repels the negatively charged contaminant particles.

[0080] Although the electron beam 101 is incident at one end of the patterning device MA, the patterning surface of the patterning device is conductive and thus a negative potential is distributed over the patterning device.

[0081] If there is no electron beam source 100, the patterning surface of the patterning device MA will be positively charged and will attract negatively charged contaminant particles. A positive charge of about 3 V is generated by a combination of the following effects: photoemission of electrons from the patterning device MA caused by EUV, and the interaction of EUV-induced plasma in the vicinity of the patterning surface of the patterning device MA. The positively charged patterning device surface, in combination with the grounded shielding blade 20 and other grounded components, attracts negatively charged contaminant particles towards the patterning device. Embodiments of the present invention avoid this undesirable situation.

[0082] The current of the emitted electron beam can be sufficient to provide a current of at least 100 μA at the patterning surface of the patterning device MA. To provide a current of at least 100 μA at the patterning surface of the patterning device MA, the electron beam source 100 can be configured to output an electron beam 101 having a current significantly higher than 100 μA. For example, the electron beam source 100 can output an electron beam having a current of at least 1 mA. The electron beam source 100 can output an electron beam having a current of, for example, up to 10 mA. This current can be sufficient to keep the voltage at the patterning device surface at a negative potential (e.g., about -0.1 V to -10 V on a time-averaged basis, rather than about +1 to +5 V). The current can be sufficient to remove the positive voltage of the patterning device within a time of less than 10 μs after each EUV pulse, and thus faster than the 20 μs gap between EUV pulses provided by the source SO. This ensures that the time-averaged potential of the patterning device is kept negative.

[0083] The current of the electron beam 101 and the voltage of the patterning device can be such that they do not generate significant heat. The power absorbed by the patterning device from the electron beam 101 can 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 can be, for example, 1 W or less.

[0084] As Figure 2 depicted by arrow 40 in

[0085] The energy of the electrons in the electron beam 101 can be, for example, at least 1 eV. It may be desirable for the electrons to have an energy of at least 1 eV because this will help to minimize electron attachment ionization.

[0086] The electron beam source 100 can be positioned, for example, within 10 cm of the patterning device MA (e.g., when the scanning movement of the patterning device has moved the patterning device such that it is located above the electron beam source). The energy of the electrons in the electron beam can be up to 13.5 eV. This is desirable because this energy is below the hydrogen ionization potential (13.6 eV), and thus the electrons will not ionize hydrogen atoms via electron collisions in the patterning device environment 25. Hydrogen ions are undesirable because they can promote the release of particles from contaminated surfaces near the patterning device MA. Although it is preferred that the energy of the electrons in the electron beam is up to 13.5 eV, the energy of the electrons can be greater than this energy (although some ionization of hydrogen atoms may then occur). For example, the electrons can have an energy of up to 100 eV. Electrons having an energy greater than 100 eV may be undesirable because at such high energies, the secondary electron yield will increase to greater than 1, and the secondary electron yield will reduce the negative charge provided to the patterning device by the electron beam.

[0087] If the distance of the electron beam source 100 from the patterning device is greater than, for example, 10 cm, the electrons in the electron beam 101 can 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 were to have a lower energy, they could be attenuated or dissipated in the hydrogen gas in the patterning device environment 25 before they reach the patterning device. The electrons can, for example, have an energy greater than 13.5 eV. For example, the electrons can have an energy of up to 100 eV. As explained above, providing the electrons with an energy greater than 100 eV may be undesirable.

[0088] The electron beam source 100 can have a line of sight to the patterning device MA (e.g., as shown in Figure 2 ). In such a case, the electron beam 101 can extend in a straight line to the patterning device MA. In other embodiments, the electron beam 101 can not have a line of sight to the patterning device MA. In such a case, some of the electrons will travel to the patterning device in a curved path. The EUV beam induces a positive potential on the patterning device, and thus the electrons can be attracted to the patterning device (and thus can be bent towards the patterning device). Alternatively, at least a portion of the electron beam can reach the patterning device due to scattering by gas molecules, ions, other electrons, and due to interactions with space or surface charges.

[0089] In Figure 4Embodiments of the present invention are depicted. In this embodiment, the electron beam source 200 is recessed into the wall 150 of the opening 26 leading to the patterning device environment 25. In the depicted embodiment, the electron beam 101 is generally directed upward and toward 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 section 31. The advantage of this arrangement is that the patterning device MA does not move toward and away from the electron beam source 200, and thus the current provided to the patterning device by the electron beam can remain generally constant. Alternatively, the electron beam 101 may be directed toward the EUV beam B, but not toward the EUV irradiated area of the patterning device MA.

[0090] In Figure 4 the depicted embodiment, the electrons of the electron beam 101 may have an energy greater than 10 eV (e.g., up to 100 eV). If the electrons were to have a lower energy, the electrons may be attenuated before reaching the patterning device (lower energy electrons tend to diverge more from the electron beam than higher energy electrons and thus diverge faster). The electrons lose energy as they travel to the patterning device and may thus have an energy of less than 10 eV (on average) when the electrons are incident on the patterning device. When the electrons are incident on other surfaces near the patterning device, the electrons may similarly have an energy of less than 10 eV.

[0091] Directing the electron beam 101 toward the exposure section 31 is advantageous because the plasma generated by EUV radiation in the exposure section will attract the electrons of the electron beam and direct the electrons toward the patterning device MA. The electron beam 101 may have a cross-sectional size generally corresponding to or generally less than the opening defined by the shielding blade 20. This advantageously avoids or reduces electron beam losses due to electrons being incident on the shielding blade.

[0092] In another alternative embodiment, the electron beam source 300 may be positioned at approximately the same height (i.e., z-direction position) as the shielding blade 20. The electron beam source 300 may be positioned to one side of the shielding 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 toward the patterning device MA. The advantage of this arrangement is that the patterning device MA does not move toward and away from the electron beam source 300, and thus the current provided to the patterning device by the electron beam can remain generally constant. When using this arrangement, a single electron beam source may be provided.

[0093] In Figure 5Further alternative embodiments of the present invention are depicted. In such further embodiments, the patterning device MA, the support structure NT, and the masking blade 20 are as further described above in connection with other embodiments. For ease of illustration, the gas supply system, the beam attenuator, and the patterning device exchange system are omitted from Figure 5 The depicted height of the patterning device environment 25 is reduced compared to the depiction in other figures of the present application, bringing the relative positions of the depicted device elements closer to those of a physical lithography device (although Figure 5 still being schematic). Figure 5 An EUV radiation beam B is depicted, which is different from other embodiments. In

[0094] The exposure zone 31 can be seen in Figure 5 The exposure zone 31 is the location where the EUV beam B intersects the patterning device MA in use.

[0095] The bottom of the housing 24, which includes the patterning device environment 25, includes a bottom plate that can be formed by the upper surfaces of different device elements. This is schematically depicted in Figure 5 as a stepped bottom plate 402. An opening 26 is provided in the stepped bottom plate 402 of the housing 24. The opening includes a pair of walls 404. The walls 404 are inclined to form an inwardly tapering space through which the EUV radiation beam B can pass and through which reflected EUV radiation (not depicted) can also pass.

[0096] An electron beam source 400 (in the scanning direction (y - direction)) is provided to one side of the opening 26. The electron beam source 400 can be located at the bottom of the housing 24. The electron beam source 400 can be positioned adjacent to one of the walls 404 of the opening 26. The electron beam source 400 can form part of the bottom plate of the housing 24 (e.g., the stepped bottom plate 402 formed by the upper surfaces of the device elements).

[0097] The electron beam source 400 emits an electron beam 401. The electron beam 401 is angled or inclined with respect to the vertical direction (z - direction) such that the electron beam generally points towards the exposure zone 31. This can be achieved by arranging the electron beam source 400 at an angle or inclined with respect to the vertical direction (as depicted) or by configuring the electron beam source to emit the electron beam at an angle (as further explained below). The electron beam 401 has a certain divergence such that the size of the cross - sectional area of the electron beam increases with the distance from the electron beam source 400. The electron beam 401 can extend fully across the exposure zone 31 in the scanning direction (y - direction) of the lithography device (as depicted). The mid - portion of the electron beam 401, schematically depicted by the dashed line 403, can overlap with the exposure zone 31.

[0098] The electron beam 401 can be directly incident on the patterning device MA in the exposure section 31. This advantageously means that the patterning device MA does not move towards and away from the electron beam 401 during use. Instead, the electron beam 401 remains continuously incident on the patterning device MA during the lithographic exposure. The current emitted from the electron beam source 401 to the patterning device MA can remain substantially constant during the lithographic exposure, or it can be modulated by means of bias or RF power modulation. A constant current emitted by the electron beam source is more robust and reliable, and it is the preferred embodiment.

[0099] The spacing between the electron beam source 400 and the patterning device MA can be less than 10 cm. The spacing can be, for example, less than 4 cm. The spacing can be, for example, approximately 3 cm. It is advantageous to arrange the electron source 400 at such a relatively close spacing to the patterning device MA because it allows electrons to be provided with an energy of less than 13.6 eV. The spacing is small enough such that an energy of less than 13.6 eV is sufficient for most electrons to reach the patterning device MA. Advantageously, the electrons do not have sufficient energy to ionize hydrogen atoms via electron collisions in the patterning device environment 25.

[0100] Although the spacing is small enough such that most electrons with an energy of less than 13.6 eV will reach the patterning device, electrons with higher energies can be used. For example, electrons with an energy of 30 eV or higher can be used. Electrons with an energy of up to 100 eV can be used.

[0101] The electron beam source 400 can provide an electron beam current of at least 100 μA. The electron beam source 400 can provide an electron beam current of up to approximately 10 mA. This can also apply to other embodiments. For any embodiment in which the electron beam is modulated (the electron beam can be constant or modulated), these electron beam current values are time-averaged currents. A current of at least 100 μA can be sufficient to maintain a negative voltage at the patterning device MA. Currents greater than 10 mA can be provided. However, a current of up to 10 mA will achieve the function of maintaining a negative voltage at the patterning device MA, and increasing the current beyond 10 mA will impose unnecessary stress on the environment (the plasma generated by the electron beam).

[0102] Typically, disposing the electron beam source 400 at the bottom of the housing 24 and adjacent to the wall 404 of the opening 26 can advantageously provide a line of sight from the electron beam source to the exposure area 31 in the presence of a spacing, where the spacing allows the electrons to have an energy of about (for example) 10 eV, for example up to 100 eV, where the electron beam current is (for example) up to 10 mA, while providing a sufficient negative potential to the patterning device during exposure. Electron energies above 100 eV and / or electron beam currents above 10 mA can be used, but these will impose unnecessary stress on the environment (the plasma generated by the electron beam) without providing any benefits.

[0103] Multiple electron beam sources (for this embodiment or for other embodiments) can be provided. The multiple electron beam sources (not depicted) can be distributed in the x - direction such that the electron beams 401 are incident at different x - direction positions on the patterning device MA. The electron beam sources 401 can have, for example, a small enough spacing such that adjacent electron beams overlap with each other when they are incident on the patterning device MA. The multiple electron beam sources 400 can be distributed to provide overlapping electron beams such that the electron band extends fully across the patterning device MA in the x - direction.

[0104] The electron beam source (for this embodiment or for other embodiments) can be configured to output an electron beam having an elliptical cross - sectional shape. For example, referring to Figure 3 , the single lens 136 can have a rectangular cross - section (elongated in the x - direction) such that it provides an elliptical cross - section to the electron beam. Other electrode configurations can be used. The elliptical beam can have an aspect ratio greater than 1, preferably greater than 2, and optimally greater than 3. An aspect ratio greater than 3 is optimal because this spreads the electron beam to a greater extent in the x - direction on the patterning device MA (compared to an aspect ratio less than 3). This, in turn, can provide full 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).

[0105] Over time, the electrodes of the electron beam source of embodiments of the present invention (for example, the electrode 122 that provides RF excitation) can be damaged due to sputtering by hydrogen or helium ions, which can have an energy of up to 100 eV. To reduce the incidence of such damage, gas can be supplied to the plasma electrode. The gas can be a single element or a mixture of elements. The gas can include at least one of H2 and He.

[0106] To reduce the damage to the electrodes, the electrodes can be in contact with one of the following: stainless steel, tungsten, molybdenum, tantalum metal or alloy, which can resist sputtering by hydrogen or helium ions with an energy of up to 100 eV.

[0107] The electron beam source may include at least one permanent magnet. The permanent magnet (not depicted) may be disposed around the plasma chamber 120 and configured to confine the electrons of the plasma 121. A magnetic shielding material such as Mu-metal may be located around the electron beam source to shield other parts of the lithographic apparatus from the magnetic field provided by the permanent magnet.

[0108] The contamination control system of a lithographic patterning device according to any one of the preceding technical solutions, wherein the outer wall of the electron beam source is made of one of the following: stainless steel, tungsten, molybdenum, tantalum metal or alloy, which can resist sputtering of the hydrogen EUV plasma.

[0109] 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 up to 100 mA. The wall of the plasma chamber 120 (and the electrode 122, if the electrode is present) may be connected to a DC power supply configured to provide up to -1 kV (to support electron beam extraction) or up to +1 kV (to support positively charged ion beam extraction).

[0110] In Figure 5 the depicted embodiment, the electron beam 401 is generally symmetric 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 depicted), the electron beam may be tilted relative to the body of the electron beam source. For example, the electron beam may have an inclination angle of up to 20°. The inclination of the electron beam may be arranged by offsetting the opening 134 in the grounded partition wall 132 of the electron beam source (see Figure 3 ) relative to the central axis of the electron beam source (e.g., the axis defined by the RF antenna 123). The inclination of the electron beam may be assisted by tilting the grounded partition wall 132 and by tilting the wall of the plasma chamber in which the opening 124 is provided. Alternatively, an auxiliary electrode or magnet may be placed near the extraction grounded electrode opening 134 and may tilt the electron beam (101).

[0111] Typically, the electron beam source can be set at any position within or adjacent to 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 patterning device MA will move towards and away from the electron beam source, and thus the current supplied to the patterning device by the electron beam source will change. The effectiveness of maintaining a negative voltage at the patterning device face by the electron beam current may change accordingly. To avoid this changing effectiveness, the first electron beam source can be offset from the exposure zone 31 in the Y direction, and the second electron beam source can be offset from the exposure zone 31 in the -Y direction. Alternatively, a single electron beam source can be provided, but it has a higher output electron beam current (the current can be selected such that the patterning device face always has a negative voltage).

[0112] In an embodiment (not depicted), the electron beam source can be set on either side of the exposure zone 31. In so doing, the current supplied to the patterning device can remain substantially constant. However, the implementation of such an embodiment can be more complex and more expensive than the single electron beam source embodiment.

[0113] During unloading of the patterning device MA from the support structure MT, it may be desirable to avoid the rear side of the patterning device MA having a negative potential. Unloading (and loading) the patterning device MA from (to) the support structure MT by the patterning device exchange system 102 can be referred to as manipulating the patterning device. The patterning device MA can act as one plate of a capacitor, where the support structure MT acts as the other plate of the capacitor, and the capacitance of such a capacitor will rapidly decrease as the patterning device moves away from the support structure. If the rear side of the patterning device is negatively charged, the decreasing capacitance will cause the voltage to increase. There is a risk that the voltage on the rear side of the patterning device reaches the kV level and may cause a discharge. This is undesirable because it may damage the lithography equipment. Embodiments of the present invention can neutralize or reduce the negative potential on the rear side of the patterning device.

[0114] In an embodiment, a positive ion current can be delivered to the rear side of the patterning device MA. This positive ion current can neutralize the charge on the patterning device MA. The positive ion current can be obtained by reversing the polarity applied to the electron beam source 100 (such that positively charged ions are delivered near the mask changer 102). Since the rear side of the patterning device MA is negatively charged, positive ions will be attracted even in the absence of a line of sight to the rear side of the patterning device. The controller can control the electron beam source such that positive ions are delivered when the lithographic apparatus is not performing a lithographic exposure, for example when the patterning device MA is unloaded from the support structure. Alternatively, the electron beam source 100, which operates in a normal electron emission mode during patterning device loading / unloading, can ionize hydrogen in the environment 25 and provide the necessary positive ions near the mask changer 102.

[0115] In an embodiment, an electron beam source (or a different electron source) can be used to assist an ionizer in generating a plasma in the patterning device environment. The ionizer can be, for example, a coil configured to cause hydrogen ionization in the patterning device environment. The electrons provided by the electron source can be accelerated and thus contribute to the ionization of hydrogen molecules, thereby providing a "kick-start" to the ionization provided by the ionizer. Once an initial plasma has been generated, the plasma can become self-sustaining (assuming the ionizer continues to operate). The plasma will flow to the rear side of the patterning device and neutralize the charge on the rear side of the patterning device MA. Thus, when the patterning device MA moves away from the support structure MT, the plasma reduces the risk of discharge. An electrostatic mirror can be used to help direct the plasma to the rear side of the patterning device.

[0116] Other techniques different from the extraction of electrons from a micro-plasma source can also provide an electron beam suitable for controlling the potential of the patterning device. The electron source can be a heating filament that provides thermionic emission. The electron source can be, for example, a cold emission source (e.g., a sharp tip that is held in an electric field strong enough to cause electron emission from the tip). The electron source can be a DUV LED (e.g., having a wavelength in the range of 200 to 300 nm) that is configured to irradiate a surface from which photoemission occurs (i.e., a surface having a photoemission energy lower than the energy of the incident photons). Any other technique can form an electron beam with a desired energy (e.g., 10 eV,..., 100 eV) with the assistance of a power supply.

[0117] The ionizer can be, for example, an induction coil. The induction coil can have a volume of several mm 3 such as less than 1 cm 3 . The induction coil can be positioned within the patterning device environment. Multiple induction coils can be positioned within the patterning device environment.

[0118] The controller can control the ionizer such that when the lithographic apparatus is not performing a lithographic exposure, for example when unloading the patterning device MA from the support structure, the ionizer is switched on. This also applies to the electron source.

[0119] The use of the terms "electron beam" and "electron beam source" does not necessarily mean that electrons are provided as a collimated beam. As further described above, significant divergence of the electrons may occur.

[0120] The electron beam source can be configured such that when the lithographic apparatus exposes a substrate, most of the electrons generated by the beam source and maintained in the patterning device environment can have an energy of less than 13.6 eV.

[0121] Multiple electron beam sources can be provided.

[0122] In this document, a reference to being grounded can be interpreted as referring to electrical grounding (which is alternatively referred to as electrical earth or simply as earth).

[0123] In this document, the electron beam source can be configured to direct an electron beam to a patterning device supported by a support structure. At certain instances, there may be no patterning device on the support structure. In such a case, the configuration of the electron beam source remains unchanged (although the electron beam can be switched off). Such a possible event can be covered by referring to the area of the support structure that holds the patterning device. Thus, for example, the line of sight can 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 can be positioned at a distance of up to 11 cm from the area of the support structure that holds the patterning device (the thickness of the patterning device is approximately 1 cm).

[0124] The method according to an embodiment of the invention can be performed by a computing device. The device can include a central processing unit ("CPU") connected to a memory. The methods described herein can be implemented as program code (software) stored on a memory including one or more storage media and arranged for execution on a processor including one or more processing units. The storage media can be integrated in the CPU and / or separate from the CPU. The program code, which can be referred to as instructions, is configured to be retrieved from the memory and executed on the processor to perform operations according to the embodiments discussed herein. Alternatively, it is not excluded that some or all of the functionality of the CPU is implemented in dedicated hardware circuitry or configurable hardware circuitry such as an FPGA.

[0125] The computing device can include an input device configured to enable a user to input data into a software program running on the CPU. The input device can include a mouse, keyboard, touch screen, microphone, etc. The computing device can also include an output device configured to output measurement results to the user.

[0126] Although specific reference may be made herein 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. Other possible applications include the manufacture of integrated optical systems, the guidance and detection of patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0127] Although specific reference may be made herein to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may operate under vacuum conditions or ambient (non-vacuum) conditions.

[0128] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it should be appreciated that, where the context allows, the invention is not limited to optical lithography and may be used in other applications such as imprint lithography.

[0129] 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 instructions 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, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only and that such actions are in fact performed by computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc., and that such performance may cause actuators or other devices to interact with the physical world.

[0130] Although specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative, not limiting. Thus, it will be apparent to those skilled in the art that the invention may be modified as described without departing from the scope of the claims set forth below.

Claims

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

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

3. The contamination control system for a lithographic pattern forming apparatus according to claim 2, wherein, During EUV exposure, for all positions of the patterning device, there exists a line of sight extending from the electron beam source to the patterning device.

4. The contamination control system for a lithography pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is configured to emit electrons having an energy of at least 1 eV.

5. The contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is configured to emit electrons having an energy up to 13.5 eV.

6. The contamination control system for a lithographic pattern forming apparatus according to any one of claims 1 to 4, wherein, The electron beam source is configured to emit electrons having an energy up to 100 eV.

7. The contamination control system for a lithographic pattern forming apparatus according to any one of claims 1 to 4, wherein, The electron beam source is positioned at a distance greater than 10 cm from the patterning device, and wherein the electron beam source is configured to emit electrons having an energy of at least 10 eV.

8. The contamination control system for a lithographic pattern forming apparatus according to any one of claims 1 to 6, wherein, The electron beam source is positioned at a distance of 10 cm or less from the patterning device.

9. The contamination control system for a lithographic pattern forming apparatus according to claim 8, wherein, The electron beam source is positioned at the bottom of a housing that defines a patterning device environment, and the electron beam source is positioned adjacent to a wall of an opening provided in the housing.

10. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is configured to emit electrons having a current of at least 100 μA.

11. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is configured to emit electrons having a current up to 10 mA.

12. The contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is configured to output an electron beam having an elliptical cross-sectional shape.

13. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source plasma electrode is in contact with one of the following: stainless steel, tungsten, molybdenum, tantalum metal or alloy, which can resist sputtering by hydrogen or helium ions having an energy up to 100 eV.

14. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source includes at least one permanent magnet and a magnetic shielding material such as a high-permeability alloy.

15. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source has a housing that is grounded and in electrical contact with the frame of the lithography tool.

16. The contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source housing is made of one of the following: stainless steel, tungsten, molybdenum, tantalum metal or alloy, which can resist sputtering by hydrogen EUV plasma.

17. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, 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. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is connected to a supply source of at least one of H2 and He.

19. A contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is configured to direct the electron beam to an exposure zone within the patterning device environment.

20. The contamination control system for a lithographic pattern forming apparatus according to any one of claims 1 to 18, wherein, The electron beam source is configured to direct the electron beam to a position offset from the exposure zone.

21. The contamination control system for a lithographic pattern forming apparatus according to claim 20, wherein, An additional electron beam source is configured to direct an additional electron beam to a position offset in the opposite direction with respect to the exposure zone.

22. The contamination control system for a lithographic pattern forming apparatus according to any one of claims 1 to 20, wherein, The electron beam source is one of a plurality of electron beam sources.

23. The contamination control system for a lithographic pattern forming apparatus according to any one of the preceding claims, wherein, The electron beam source is configured to maintain the patterning device at at least a time-averaged negative potential during EUV exposure.

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

25. A lithographic apparatus, comprising a contamination control system for a patterning device according to any one of the preceding claims, and further comprising a shadow blade, a patterning device exchange system, and a housing, wherein a patterning device environment is located within the housing.

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

27. A lithographic apparatus according to claim 25, wherein, The electron beam source is positioned at a height corresponding to the height of the shadow blade, and the electron beam source is offset in a direction orthogonal to the scanning direction of the patterning device support structure.

28. A lithographic apparatus according to claim 25, wherein, The electron beam source is positioned adjacent to the patterning device exchange system.

29. A lithographic apparatus, comprising an electron source and an ionizer located in a patterning device environment.

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

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

32. The method according to claim 31, wherein, Supplying a gas to a plasma electrode of the electron beam source, the gas comprising at least one of H2 and He.

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

34. A method for controlling charge on a rear surface of a lithographic patterning apparatus, the method comprising: When the lithographic patterning device is manipulated by the patterning device exchange system, directing an electron beam or a positive ion beam towards a rear surface of the lithographic patterning device.

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