Lithographic apparatus and device manufacturing method with preventing contaminant particles from being deposited on a sensitive component, such as a patterning surface of a patterning device

Abstract

Disclosed is a lithographic apparatus including: an illumination system for providing a beam of EUV radiation along a beam path; a holder for a patterning device configured to impart a pattern to the beam of radiation, the patterning device comprising a patterning surface with a pattern thereon; and an electron beam source configured to emit electrons toward the patterning surface and / or a part of the beam path adjacent the patterning surface.

Description

LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURING METHOD WITH PREVENTING CONTAMINANT PARTICLES FROM BEING DEPOSITED ON A SENSITIVE COMPONENT, SUCH AS A PATTERNING SURFACE OF A PATTERNING DEVICECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 614,199 which was filed on 22 December 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present description relates to a lithographic apparatus and a method of manufacturing a device.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and / or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and / or structures to be manufactured.

[0005] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):where is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, kl is a process-dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from Equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength Z. by increasing the numerical aperture NA or by decreasing the value of kl .

[0006] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within therange of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser- produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.SUMMARY

[0007] Contaminant particles, in particular particles of tin (Sn) emitted by a plasma source, may be present in the environment at or surrounding the patterning device. Particles originating from the source may travel towards the patterning device, either via a direct path or indirectly having bounced off mirror devices that direct the exposure radiation to the patterning device. Consequently, it is likely that contaminant particles within the environment at or surrounding the patterning device will be deposited onto the patterning surface. The presence of contaminant particles on the patterning surface can cause imaging errors, which reduces the yield of the lithographic process.

[0008] An aim is to improve the operation of an EUV lithographic process by preventing contaminant particles from being deposited on a sensitive component, such as a patterning surface of a patterning device.

[0009] According to an aspect of the present invention, there is provided a lithographic apparatus having a protection system comprising: an electron emission surface located adjacent a travel path of particles toward a component to be protected; an electro-magnetic radiation source configured to irradiate the electron emission surface with electromagnetic radiation so as to cause emission of electrons by the photo-electric effect; and a field generator located along the travel path toward the component to be protected relative to the electron emission surface and configured to generate an electromagnetic field, wherein particles travelling toward the component to be protected are charged by the electrons emitted by the electron emission surface and deflected by the electromagnetic field.

[0010] According to an aspect of the present invention, there is provided a method of protecting a component of a lithographic apparatus, the method comprising: illuminating an electron emission surface with electromagnetic radiation to create an electron cloud on a travel path of particles toward the component to be protected; and generating an electromagnetic field in a field region, wherein a particle travelling toward the component to be protected is charged by the electron cloud and deflected by the electromagnetic field.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with referenceto the accompanying schematic drawings, in which corresponding reference symbols indicate corresponding parts.

[0012] Figure 1 schematically depicts a lithographic apparatus.

[0013] Figure 2 schematically depicts a more detailed view of the lithographic apparatus.

[0014] Figure 3 schematically depicts a protection arrangement of an embodiment.

[0015] Figure 4 schematically depicts the protection arrangement of Figure 3 in more detail.

[0016] The features shown in the Figures are not necessarily to scale, and the size and / or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature.DETAILED DESCRIPTION

[0017] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module SO according to one embodiment of the invention. The apparatus 100 comprises: an illumination system (or illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation). a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0018] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0019] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

[0020] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section such as to create a pattern in atarget portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.

[0021] Examples of patterning devices include masks, programmable mirror arrays, and programmable liquid-crystal display (LCD) panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

[0022] The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0023] As here depicted, the lithographic apparatus 100 is of a reflective type (e.g., employing a reflective mask).

[0024] The lithographic apparatus 100 may be of a type having two (dual stage) or more substrate tables WT (and / or two or more support structures MT). In such a “multiple stage” lithographic apparatus the additional substrate tables WT (and / or the additional support structures MT) may be used in parallel, or preparatory steps may be carried out on one or more substrate tables WT (and / or one or more support structures MT) while one or more other substrate tables WT (and / or one or more other support structures MT) are being used for exposure.

[0025] Referring to Figure 1, the illumination system IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV radiation include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1 , for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module SO may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0026] In such cases, the laser is not considered to form part of the lithographic apparatus 100 and the radiation beam B is passed from the laser to the source collector module SO with the aid of a beam delivery system comprising, for example, suitable directing mirrors and / or a beam expander. In other cases the source may be an integral part of the source collector module SO, for example when the sourceis a discharge produced plasma EUV generator, often termed as a DPP source.

[0027] The illumination system IL may comprise an adjuster configured to adjust the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illumination system IL can be adjusted. In addition, the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section.

[0028] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PSI can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W may be aligned using mask alignment marks MR1, MR2 and substrate alignment marks Pl, P2.

[0029] A controller 500 controls the overall operations of the lithographic apparatus 100 and in particular performs one or more operations of a process described further below. Controller 500 can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and non-volatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus 100. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus 100 is not necessary. In an embodiment, one computer can control multiple lithographic apparatuses 100. In an embodiment, multiple networked computers can be used to control one lithographic apparatus 100. The controller 500 may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus 100 forms a part. The controller 500 can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and / or an overall control system of a fab.

[0030] Figure 2 shows the lithographic apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. An EUV radiation emitting plasma 210 may be formed by a plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0031] The radiation emitted by the radiation emitting plasma 210 is passed from a source chamber211 into a collector chamber 212.

[0032] The collector chamber 212 may include a radiation collector CO. Radiation that traverses the radiation collector CO can be focused along an axis O to a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module SO is arranged such that the virtual source point IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0033] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the unpatterned beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the unpatterned beam 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT.

[0034] More elements than shown may generally be present in the illumination system IL and the projection system PS. Further, there may be more mirrors present than those shown in the Figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in Figure 2.

[0035] Alternatively, the source collector module SO may be part of an LPP radiation system.

[0036] As depicted in Figure 1, in an embodiment the lithographic apparatus 100 comprises an illumination system IL and a projection system PS. The illumination system IL is configured to emit a radiation beam B. The projection system PS is separated from the substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto the substrate W. The pattern is for EUV radiation of the radiation beam B.

[0037] The space intervening between the projection system PS and the substrate table WT can be at least partially evacuated. The intervening space may be delimited at the location of the projection system PS by a solid surface from which the employed radiation is directed toward the substrate table WT.

[0038] In use of the lithographic apparatus, a patterning device MA is clamped to a support structure MT. As described above, the support structure MT may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. Both the patterning device MA and support structure MT may be contained within a patterning device environment. The patterning device environment may be separated from an external environment surrounding the lithographic apparatus 100 and / or other components within the lithographic apparatus such that gases and contaminant particles P are substantially prevented from entering the patterning device environment.

[0039] The patterning device environment may be partially evacuated of gas. That is, the pressure within the patterning device environment may be less than ambient pressure. This is to limit the attenuation of EUV radiation as it travels through the patterning device environment. Even though thepressure within the patterning device environment is less than ambient pressure, it is not a perfect vacuum, so gas particles are present in the patterning device environment.

[0040] Contaminant particles P may also be present in the patterning device environment 90. Despite the separation of the patterning device environment from the external environment and / or other components within the lithographic apparatus, it is possible that some contaminant particles P may enter the patterning device environment. In particular, tin (Sn) particles emitted by a plasma source may travel from the intermediate focus IF to the patterning device environment. A common mitigation against Sn particles from the source is to provide a mechanical barrier on the direct path from the intermediate focus IF to the patterning device. However, Sn particles can still reach the patterning device environment by bouncing off parts of the lithographic apparatus, in particular the mirror devices of the illumination system IL. Contaminant particles may therefore become attached to the patterning surface giving rise to imaging defects until they can be removed. However, removal risks damaging the patterning surface and there will be a significant reduction in yield until the contaminant particles are removed. Therefore, it is highly desirable to prevent contaminant particles contacting the patterning surface at all. Whilst contaminant particles attached to the patterning surface are an especially severe problem, contaminant particles can cause problems for other components of the lithographic apparatus.

[0041] The possible paths of contaminant particles from intermediate focus IF to patterning device MA are shown in Figure 3 which is a schematic diagram of some elements of the illumination system IL and projection system PS. It will be seen that contaminant particles can take two main routes to reach the patterning device MA. Some particles may follow the path of the exposure radiation, indicated by the solid line arrows, making several collisions with mirror devices Ml- M3 to reach patterning device MA. Some particles may even bounce off the patterning device MA and a mirror device M4 of the projection system. It is not possible to put a physical obstruction in this path, or even a significant transverse gas flow to deflect particles, as this would interfere with the exposure radiation. Although a direct path from intermediate focus IF to patterning device MA can be blocked by a physical obstruction, contaminant particles can still take a semi-direct path, indicated by dashed line arrows, to reach the patterning device after one or a few collisions with other parts of the apparatus, such as parts of the walls of the vacuum chamber.

[0042] To address a problem of contaminant particles reaching the patterning device MA, it is proposed to provide a protection system 140 upbeam (i.e. nearer the source along the path of the exposure radiation) of the patterning device MA. The protection system may also be referred to as a contamination control system. Desirably, protection system 140 is located around the beam of exposure radiation as closely as possible without obstructing any part of the exposure radiation. The protection system may be in the general form of a cone surrounding the beam. As will be discussed further below, parts of the protection system may be integrated into other components of the lithographic apparatus, such as parts of the wall of the vacuum chamber.

[0043] Figure 4 is a more detailed schematic view of the protection system 140. It comprises anelectron emission surface 101 which is located adjacent a travel path of particles 109 toward a component to be protected, in this example the pattering device MA. An electromagnetic radiation source 102 is arranged to irradiate the electron emission surface 101 with electromagnetic radiation 106 so as to cause emission of electrons 107 by the photo-electric effect. This creates an electron cloud 120 through which the contaminant particles 109 travel, becoming charged through collisions with free electrons in the electron cloud. Desirably, a voltage source 108 is provided to apply a negative bias to the electron emission surface 101 so as to enhance the emission of electrons and prevent the electron emission surface 101 becoming positively charged.

[0044] A field generator 103 - 105 is located further along the travel path toward the component to be protected relative to the electron emission surface 101 and is configured to generate an electromagnetic field in field region 130. In this example, the field generator comprises electrodes 103, 104 connected to a potential source 105 so as to generate a transverse electric field 110. The field generator may also be configured to generate a magnetic field or a combination of magnetic and electric fields.

[0045] As a result of the electromagnetic field generated by the field generator, particles 109 travelling toward the component to be protected that have been charged by the electrons 107 emitted by the electron emission surface 101 are deflected so as not to reach the component to be protected, e.g. patterning device MA. A particle catcher 111 may be provided to catch the deflected particles and ensure that they do not contaminate sensitive parts of the lithographic apparatus. Particle catcher 111 may comprise a rough surface, e.g. with a roughness Ra approximately equal to the diameter of the particles to be captured.

[0046] It will be appreciated that, due to the small amounts of gas in the illumination system and the ionizing effect of the exposure radiation, there will be free electrons in other parts of the illumination system and the contaminant particles may already be charged by the time they reach the protection system 140. However, the protection system is intended to significantly increase the charge on the contaminant particles so that the deflection thereof in the electromagnetic field is greater. The electron cloud desirably has a higher electron density than any other part of the illumination system.

[0047] The protection system 140 has several parameters which enable the protection effect to be adjusted according to circumstances, e.g. the size and velocity of contaminant particles occurring in use of the lithographic apparatus. These parameters may include: the length of the electron cloud 120 in the direction of travel of the contaminant particles; the density of the electron cloud; the strength of the electromagnetic field; the length of the field region 130 in the direction of travel of the contaminant particles; and the distance between the protection system 140 and the component to be protected. In one example, the contaminant particles may have average diameters in the range of 10s to 100s of nm; the size and density of the electron cloud may be sufficient that the particles pick up approximately 5-20 electrons per nm of radius; and the field may be an electric field of order of 10s to 100s Vm ’. Such a field may be generated with electrodes 103, 104 spaced about 10 cm apart and having a potential difference of a few 10s of V. Additional sources of electrons, e.g. electron guns or corona dischargers,may also be provided but the photoelectric effect provides a particularly efficient mechanism for generating free electrons.

[0048] In an EUV lithographic apparatus, EUV radiation is typically generated in pulses. That is, there are periods when EUV radiation is generated, and periods when it is not. Pulses of EUV radiation are typically generated at a rapid frequency. This frequency may be, for example, approximately 50 kHz, approximately 60 kHz, or approximately 100 kHz. Because the EUV beam is powerful and the energy of its photons is very high, contaminant particles that are hit by pulses of the exposure radiation will emit electrons due to the photoelectric effect and become positively charged. This counteracts the effect of the electron cloud in charging the contaminant particles negatively. However, because the duty cycle of the exposure radiation is low (i.e. periods when the beam is off are much longer than periods when it is on) most contaminant particles will not be hit by a pulse of radiation; contaminant particles mostly travel parallel to the beam path but not always within the beam path. In addition, a contaminant particle that is positively charged by the exposure radiation may still be deflected by the electromagnetic field 110.

[0049] To increase efficiency of the protection system 140 it is desirable that the electron emission surface is made of a material that has a high photoelectric conversion efficiency (i.e. emits a large number of electrons for a given energy input). Suitable materials from which the electron emission surface 101 may be formed include: Ni, Ta, Ca, potassium bromide (KBr), cesium telluride (CsTe), cesium iodide (CsI) and rubidium telluride (RbTe). The electron emission surface 101 may be provided as a self-supporting plate made from or having a coating of one of the aforementioned materials. Alternatively, the electron emission surface 101 may be provided as a layer or coating of a component of the lithographic apparatus, such as a structural component, for example a part of the wall of a vacuum vessel. The use of a coating provides flexibility as to the location and size of the electron emission surface(s) to help ensure that a large number of free electrons can be generated using the photo-electric affect. In some cases, a heat treatment at a temperature of 100 to 200 °C can be used to enhance the photo-electric conversion efficiency.

[0050] The radiation source 102 emits electromagnetic radiation having a photon energy higher than the work function cp of the electron emission surface 101, where: he<P = ~ -''■cut-off in which h is Planck’s constant, c is the speed of light and Acut-oyy is the wavelength below which no photoelectric effect is observed.

[0051] Desirably the wavelength of the radiation source 102 is chosen according to the material of the electron emission surface 101 such that the efficiency of electron emission is high and the emitted electrons have kinetic energy to reach the electron cloud. The energy of emitted electrons may beboosted by a negative potential applied to the electron emission surface 101. The wavelength of the electromagnetic radiation may be less than 400 nm. Desirably the radiation of the electromagnetic radiation is not such as would affect (expose) the radiation sensitive layer that is applied to substrates to be exposed in case any of this radiation reaches the substrate being exposed. The wavelength of the electromagnetic radiation may be greater than 200 nm, desirably greater than 250 nm. More desirably, where the electron emission surface 101 is formed from one of the materials listed above, the wavelength of the electromagnetic radiation may be in the range of 250 nm to 360 nm or lower. The radiation source may be a laser, a laser diode, a light emitting diode or other radiation source. The radiation source need not be monochromatic. The radiation source 102 can be located at any convenient location which can be provided with power and has a line of sight to the electron emission surface 101.

[0052] Although one radiation source 102 and one electron emission surface 101 are depicted in Figure 4, it will be appreciated that there may be many radiation sources and / or many electron emission surfaces as is convenient. There need not be a 1:1 ratio of radiation sources and electron emission surfaces; one radiation source may irradiate many electron emission surfaces or one electron emission surface may be irradiated by many radiation sources. Areas of irradiation of different radiation sources may overlap.

[0053] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0054] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0055] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus,or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools.

[0056] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography.

[0057] Aspects of the invention are described in the following numbered clauses.1. A lithographic apparatus having a protection system comprising: an electron emission surface located adjacent a travel path of particles toward a component to be protected; an electro-magnetic radiation source configured to irradiate the electron emission surface with electromagnetic radiation so as to cause emission of electrons by the photo-electric effect; and a field generator located along the travel path toward the component to be protected relative to the electron emission surface and configured to generate an electromagnetic field, whereby particles travelling toward the component to be protected are charged by the electrons emitted by the electron emission surface and deflected by the electromagnetic field.2. The lithographic apparatus of clause 1, wherein the field generator is configured to generate an electric field.3. The lithographic apparatus of clause 2, wherein the electric field is generally transverse to the travel path.4. The lithographic apparatus of clause 1, clause 2 or clause 3, wherein the electron emission surface is formed of a material selected from the group consisting of: Ni, Ta, Ca, potassium bromide (KBr), cesium telluride (CsTe), cesium iodide (CsI) and rubidium telluride (RbTe).5. The lithographic apparatus of any of the preceding clauses, wherein the electron emission surface comprises a coating formed on a structural component of the lithographic apparatus.6. The lithographic apparatus of any of the preceding clauses, wherein the electromagnetic radiation source comprises a laser, a laser diode or a light emitting diode.7. The lithographic apparatus of any of the preceding clauses, wherein the electromagnetic radiation source is configured to emit radiation of a wavelength for which the electron emission surface has a high photoelectric conversion efficiency.8. The lithographic apparatus of any of the preceding clauses, wherein the electro-magnetic radiation source is configured to emit radiation of a wavelength less than 400 nm.9. The lithographic apparatus of any of the preceding clauses, wherein the electro-magnetic radiation source is configured to emit radiation of a wavelength greater than 200 nm, desirably greater than 250 nm.10. The lithographic apparatus of any of the preceding clauses, wherein the electron emission surface is connected to ground.11. The lithographic apparatus of any of the preceding clauses, further comprisingan illumination system for providing a beam of EUV radiation along a beam path; and a holder for a patterning device configured to impart a pattern to the beam of radiation, the patterning device comprising a patterning surface with a pattern thereon, wherein the patterning device is the component to be protected.12. A method of protecting a component of a lithographic apparatus, the method comprising: illuminating an electron emission surface with electromagnetic radiation to create an electron cloud on a travel path of particles toward the component to be protected; and generating an electromagnetic field in a field region, whereby particles travelling toward the component to be protected are charged by the electron cloud and thereby the particle is deflected by the electromagnetic field.13. The method according to clause 12, wherein the field region is closer to the component to be protected than is the electron cloud14. A device manufacturing method comprising: directing a beam of EUV radiation along a beam path to a patterning surface of a patterning device; and protecting the patterning device by the method of clause 13 or clause 14.

[0058] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A protection system for a lithographic apparatus, the protection system comprising: an electron emission surface configured to be located adjacent a travel path of particles toward a component to be protected; an electro-magnetic radiation source configured to irradiate the electron emission surface with electromagnetic radiation so as to cause emission of electrons by the photo-electric effect; and a field generator configured to be located along the travel path toward the component to be protected relative to the electron emission surface and configured to generate an electromagnetic field, wherein particles travelling toward the component to be protected are charged by the electrons emitted by the electron emission surface and deflected by the electromagnetic field.

2. The protection system of claim 1 , wherein the field generator is configured to generate an electric field.

3. The protection system of claim 2, configured to provide the electric field generally transverse to the travel path.

4. The protection system of any of claims 1-3, wherein the electron emission surface is formed of a material selected from the group consisting of: Ni, Ta, Ca, potassium bromide (KBr), cesium telluride (CsTe), cesium iodide (CsI) and rubidium telluride (RbTe).

5. The protection system of any of the preceding claims, wherein the electron emission surface comprises a coating formed on a structural component of the lithographic apparatus.

6. The protection system of any of the preceding claims, wherein the electromagnetic radiation source comprises a laser, a laser diode or a light emitting diode.

7. The protection system of any of the preceding claims, wherein the electromagnetic radiation source is configured to emit radiation of a wavelength for which the electron emission surface has a high photoelectric conversion efficiency.

8. The protection system of any of the preceding claims, wherein the electro-magnetic radiation source is configured to emit radiation of a wavelength less than 400 nm.

9. The protection system of any of the preceding claims, wherein the electro-magnetic radiation source is configured to emit radiation of a wavelength greater than 200 nm, desirably greaterthan 250 nm.

10. The protection system of any of the preceding claims, wherein the electron emission surface is configured to be connected to ground.

11. A lithographic apparatus comprising: an illumination system to provide a beam of EUV radiation along a beam path; a holder for a patterning device configured to impart a pattern to the beam of radiation, the patterning device comprising a patterning surface with a pattern thereon; and the protection system of any of the preceding claims, wherein the patterning device is the component to be protected by the protection system.

12. A method of protecting a component of a lithographic apparatus, the method comprising: illuminating an electron emission surface with electromagnetic radiation to create an electron cloud on a travel path of particles toward the component to be protected; and generating an electromagnetic field in a field region, wherein particles travelling toward the component to be protected are charged by the electron cloud and deflected by the electromagnetic field.

13. The method according to claim 12, wherein the field region is closer to the component to be protected than is the electron cloud.

14. A device manufacturing method comprising: directing a beam of EUV radiation along a beam path to a patterning surface of a patterning device; and protecting the patterning device by the method of claim 12 or claim 13.

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