CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 20215365.6 which was filed on 18 December 2020, and which is incorporated herein in its entirety by reference.

FIELD OF INVENTION

[0002] The present invention relates to a cleaning apparatus for cleaning a component of a lithographic apparatus, a cleaning tip for cleaning a component of a lithographic apparatus, a method of cleaning a surface of a lithographic apparatus, an apparatus for preventing re-contamination of a surface of a lithographic apparatus, a method of preventing re-contamination of a surface of a lithographic apparatus, as well as the use of such apparatus or methods in a lithographic apparatus or method.

BACKGROUND

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

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0005] In operation, contamination, possibly in the form of particles, may be deposited on surfaces within a lithographic apparatus. Such surfaces may be optical elements, which are elements which interact with the radiation beam, such as mirrors, reticles or pellicles. In such cases where there is contamination on such surfaces, this may adversely affect the performance of the lithographic apparatus. It is therefore desirable to clean any contaminated surfaces within the lithographic apparatus, or surfaces introduced into lithographic apparatus, particularly any surfaces which interact with the radiation beam. [0006] Existing methods of cleaning surfaces of lithographic apparatuses, particularly pellicles, is by the providing electrodes near the pellicle and inducing vibrations in the pellicle by applying pulses of voltage to the electrodes to dislodge any particle contamination on the pellicle. Pellicles are relatively fragile and may be damaged if the forces associated with the vibrations are too great. [0007] The present invention has been devised to provide an improved or alternative system for cleaning one or more particle-sensitive element, namely elements whose performance is impacted by the presence of contaminants, such as optical elements particularly pellicles, of a lithographic apparatus.

SUMMARY OF THE INVENTION

[0008] According to a first aspect of the present invention, there is provided a cleaning apparatus for cleaning a component of a lithographic apparatus, said apparatus comprising a first cleaning surface configured to physically interact with a contaminant particle located on a surface to be cleaned to remove the contaminant particle from the surface to be cleaned.

[0009] Existing cleaning methods rely on vibrating the surface which is in need of cleaning to dislodge any particles on the surface. Whilst this is able to remove some particles, the particles are not actively or controllably removed, rather they are thrown off in a relatively uncontrolled manner and may be re-deposited to the surface from which they have just been removed. As a result, the cleaning efficiency for non-conductive particles is not always optimal. It is also possible to displace particles by using streams of gas, but these methods are similarly relatively uncontrolled and the displaced particles may ultimately simply be moved to another surface of the lithographic apparatus. In the present invention a cleaning surface is configured to physically interact with a contaminant particle located on the surface to be cleaned to remove the contaminant particle. In particular, the first cleaning surface is able to come into physical contact with a particle rather than simply shaking the particle loose of attempting to blow it away with a gas. This allows for a much more controlled cleaning of the surface and avoids the problem of the contaminants simply being moved to another surface. The component being cleaned may be a pellicle or other optical element, such as a mirror. Due to the optical elements being very sensitive, it has not previously been possible to clean the surfaces of such optical elements by physically interacting a cleaning surface with such elements, but rather non-contact cleaning methods. As such, physical interaction may also be referred to as contacting or physically contacting. [00010] The first cleaning surface may have a controllable adhesion. In other words, the adhesion characteristics of the first cleaning surface may be selectively altered in order to increase or decrease the degree of adhesiveness of the surface. In this way, it is possible for the adhesion to be increased so that contaminant particles can be removed from the surface and adhered to the first cleaning surface and then they can later be easily removed from the first cleaning surface by decreasing the adhesion of the first cleaning surface. As such, the cleaning surface can be re-used to clean further contaminant particles from the surface.

[00011] The apparatus may comprise an electrical bias source in electrical communication with the first cleaning surface and configured to provide an electrical bias to the first cleaning surface to thereby control the adhesion of the first cleaning surface. As such, when an electrical bias is applied to the first cleaning surface, this increases the degree to which contaminant particles adhere to the surface. Since this relies on the application of electrical bias, the adhesive characteristics of the first cleaning surface can be controlled and selectively increased or decreased (returned to the state with no applied electrical bias). The contaminant particles can therefore be removed from the apparatus and deposited where required in order to avoid recontamination of the apparatus. In addition, the application of electrical bias can allow more strongly bound contaminant particles to be removed than would otherwise be the case. The electrical bias provided may be up to 200 V, up to 100 V, up to 50 V, up to 30 V, or up to 10 V. It has been found that the application of an electrical bias of 30 V results in an increase of 100 fold in the adhesion of particles compared to the non-biased state.

[00012] The first cleaning surface may comprise an electrically conductive material coated with a dielectric material. The electrically conductive material allows the first cleaning surface to be electrically biased and the dielectric material may be polarized such that it is able to interact and pick up contaminant particles from the surface in need of cleaning.

[00013] The electrically conductive material may comprise a plurality of nanotubes and/or an electrically conductive aerogel. The plurality of nanotubes may comprise a plurality of carbon nanotubes. The dielectric material may be aluminium oxide, although other dielectric materials such as metal oxides or dielectric polymers may be used. The surface of the first cleaning surface may be soft in that it is at least partially deformed by the contaminant particles. In this way, when the first cleaning surface is brought into contact with the contaminant particle, it is able to deform around the contaminant particle to provide a greater surface area over which the cleaning surface and the particle can interact. This increases the adhesion of the particle to the cleaning surface and allows the particle to be removed. Carbon nanotubes are useful in this context as they are electrically conductive but are also flexible and can bend and move to accommodate any contaminant particles. As such, the first cleaning surface may comprise carbon nanotubes coating with aluminium oxide.

[00014] The first cleaning surface may be substantially planar or may be non-planar. The first cleaning surface may be cylindrical. The first cleaning surface may be shaped to complement the shape of the surface to be cleaned. A lot of surfaces which are to be cleaned are planar and so the first cleaning surface may similarly be planar to match the shape. Other surfaces to be cleaned may be curved and so the first cleaning surface may be similarly shaped. In cases where the first cleaning surface is cylindrical, the cylinder may be rotatable such that as the first cleaning surface is passed over the surface to be cleaned it can be rotated at a similar rate such that a particle-free portion of the first cleaning surface is always closest to the surface to be cleaned, in a similar manner as a sticky roller is used to clean fluff, hair, and dust from clothing.

[00015] The apparatus may comprise a second cleaning surface disposed opposite the first cleaning surface and spaced apart from the first cleaning surface. Each of the features described in respect of the first cleaning surface are equally applicable to the second, and any further, cleaning surface. The electrically biased cleaning surfaces exert forces on the surface which is being cleaned. In certain cases, the surface being cleaned is very thin, such as a pellicle, and so if the forces are too great, the surface may be damaged. By providing cleaning surfaces on both sides, the forces are balanced. [00016] The first and/or second cleaning surfaces may be comprised in an array of cleaning surfaces. In certain cases, it may be desirable to clean a large area and so a plurality of cleaning surfaces may be provided.

[00017] The apparatus may comprise a controller in electrical communication with the first and/or second arrays of cleaning surfaces. The controller may be configured to control the electrical bias applied to the cleaning surfaces. Similarly, where there are no arrays and just individual cleaning surfaces, a controller may be provided. It may be desirable to turn on and turn off cleaning surfaces in the array depending on the location of the contaminant particles. By turning on individual cleaning surfaces, local particle contaminations can be attracted to the cleaning surfaces but the surface to be cleaned itself is only attracted locally and not over significant areas. Opposing cleaning surfaces can be configured to have one turned on and the opposing cleaning surface turned off, such that the particles are attracted in only one direction. The pattern of cleaning surfaces being turned on and turned off can be controlled such that there is overall balance between the forces on one side and the forces on the other side across the surface, with only local differences in force. In one configuration, alternate cleaning surfaces on one side can be turned on in the pattern of a chessboard with the opposite configuration on the opposite side.

[00018] The first cleaning surface may comprise an atomic-force microscopy (AFM) tip. For an even more specific cleaning of a surface, such as a pellicle, an AFM tip may be provide which is able to be very carefully controlled to remove particles contaminating a surface. The AFM tip may serve a dual purpose of detecting the presence of a particle on a surface and then also interacting with the particle to remove it from the surface. The same tip or a different tip may be used for each step. For example, the AFM tip could be pressed down into the particle and subsequently lifted up. The AFM tip could be used to tap the particle away or could be used to push or pull the particle along the surface. The AFM tip could be used like a hammer to tap the opposite side of a surface from a particle to displace the particle on the opposite side.

[00019] If used in the hammer mode, the AFM tip may be located on the opposite side of the particle while the tip adhesion to the pellicle may be made minimal for instance by making the tip of ceramic with low or medium Hamaker constant. The speed v of the AFM tip with respect to the pellicle plane at the moment of contact may be in the range of 0.3 m/s < v < 30 m/s, preferably from 1 to 10 m/s. The AFM tip displacement may be from 1 nm to 100 nm, more preferably from 5 nm to 50 nm abd even more preferably from 7 nm to 30 nm with respect to the pellicle plane to prevent pellicle rupture by over-strain. The kick to AFM tip may be provided when the tip is in contact or prior to the contact with pellicle, in the vicinity of the particle and on the opposite side of the pellicle. After the pellicle is knocked off, the AFM tip is pulled from the pellicle and moved to the next particle. The AFM tip in hammer mode may have a Young modulus >1 GPa. The AFM tip may be non-sticky. Preferably the AFM tip in hammer mode has curved surface that is comparable or smaller than the typical particles (1 um). [00020] The AFM tip could be reconditioned for further use by removing any attached particle by dipping the AFM tip onto an adhesive material, such as polyurethane or gold, to remove the particle from the AFM tip. A pair of AFM tips may be provided to act as tweezers or pliers to pinch or flick a particle off the surface to be cleaned.

[00021] The AFM tip may comprise a material selected to enhance adhesion with the particle. The AFM tip used in a pick-up mode may comprise a material with a high Hamaker constant, such as diamond, silicon, gold, or combinations thereof. These materials increase the van der Waals forces between the tip and the particle. The AFM tip may comprise a material which is deformable, such as gold, silver, a rare earth methal or indium. By deforming, the tip may conform to the particle to provide a greater surface area for contact between the tip and the particle. The AFM tip may comprise a material selected to provide a triboelectrical charging effect, such as Teflon ® or polyurethane. The AFM tip may be rounded or may be angular. The AFM tip may be shaped to narrow down to a point.

[00022] The AFM tip may comprise an electrically conductive layer coated with a dielectric material. The electrically conductive material may comprise a plurality of nanotubes and/or an electrically conductive aerogel. The plurality of nanotubes may comprise a plurality of carbon nanotubes. The dielectric material may be a metal oxide, such as aluminium oxide, or a dielectric polymer. As such, the AFM tip may comprise the same material as described above in respect of the first cleaning surface.

[00023] The cleaning apparatus may comprise a particle contamination locating element configured to detect the presence and/or location of contamination particles on the surface to be cleaned. The particle contamination locating element may comprise one or more elements which locate particles via optical techniques, such as scattering techniques, scanning electron microscopy, and/or scanning AFM. Since the number of particles contaminating a surface is likely quite low, it may be more efficient to locate the particles and then specifically control the apparatus to focus on such particles rather than moving the cleaning apparatus over the whole surface to be cleaned.

[00024] The cleaning apparatus may further comprise a conditioning surface configured to remove any contaminant particles from the first cleaning surface or any other cleaning surface. Since it is undesirable for the particles to be released freely as they may re-contaminate the apparatus, a conditioning surface may be provided which acts as a store of the particles which have been removed. The conditioning surface is also able to clean the cleaning surfaces themselves so that the cleaning surfaces can be used again.

[00025] According to a second aspect of the present invention, there is provided a cleaning tip for cleaning a component of a lithographic apparatus, the cleaning tip comprising an AFM tip, wherein said AFM tip comprises a material selected to at least temporarily retain a contaminant particle. The AFM tip may comprise any of the materials describes in respect of the AFM tip in the first aspect of the present invention. As such, the cleaning tip may comprise i) a material with a high Hamaker constant; ii) a deformable material;, iii) a material selected to provide a triboelectrical charging effect; or an electrically conductive layer material coated with a dielectric material. The electrically conductive material may comprise a plurality of nanotubes, such as a plurality of carbon nanotubes, and/or an electrically conductive aerogel. The dielectric material may be a metal oxide, such as aluminium oxide, or a dielectric polymer.

[00026] According to a third aspect of the present invention, there is provided a method for cleaning a surface of a lithographic apparatus, said method comprising: i) providing a cleaning apparatus having a first cleaning surface, ii) bringing the first cleaning surface to the surface of the lithographic apparatus in need of cleaning, iii) causing any particle contamination on the surface of the lithographic apparatus in need of cleaning to be picked up by or moved by the first cleaning surface; and iv) moving the first cleaning surface away from the surface of the lithographic apparatus. The method according to the third aspect of the present invention allows for very selective removal of contaminant particles from the surface to be cleaned, which may be a pellicle. The first cleaning surface is able to remove the particle contamination by physically picking up the particles or physically pushing them along the surface being cleaned. This differs from existing cleaning methods which impart sufficient momentum to the particles to release them from the surface.

[00027] The method may comprise applying an electrical bias to the first cleaning surface in order to increase the adhesion of the first cleaning surface. Specifically, the bias is applied between the first cleaning surface and the pellicle surface, or between the first cleaning surface and a second (cleaning) surface. If the pellicle is grounded, the bias may be applied with respect to the ground. As with the first aspect of the present invention, the first cleaning surface may be configured to have a surface with an adjustable adhesion characteristic. By applying an electrical bias to the first cleaning surface, it is possible to selectively remove particles and then release them at a desired time and/or location.

[00028] The electrical bias may be turned off once the first cleaning surface has been moved away from the surface of the lithographic apparatus in order to allow any contaminant particles to be removed from the first cleaning surface. This frees up the first cleaning surface to remove more particles from the surface in need of cleaning.

[00029] The step of moving the first cleaning surface away from the surface of the lithographic apparatus further comprises moving the first cleaning surface to a conditioning surface and transferring any contaminant particles from the first cleaning surface to the conditioning surface. Again, as described in respect of the first aspect of the present invention, this provides a store for the particles which have been removed and prevents them re-contaminating the lithographic apparatus.

[00030] The method may comprise determining the position of one or more particle contaminants. This may be achieved by any suitable method, such as optical techniques, such as scattering techniques, scanning electron microscopy or atomic force microscopy. The first cleaning surface may be moved to the predetermined position of the one or more particle contaminants. This allows for more efficient cleaning since only the areas which comprise particle contaminants are cleaned by the apparatus rather than simply scanning the cleaning apparatus over the entire surface to be cleaned. [00031] The method may comprise providing a second cleaning surface opposite to the first cleaning surface and disposing the surface of the lithographic apparatus in need of cleaning between the first and second cleaning surfaces. As described in respect of the first aspect of the present invention, by having cleaning surfaces disposed on both side of the surface in need of cleaning, it is possible to balance the forces acting upon the surface, which may be a pellicle, in order to avoid causing damage to the surface. This also allows to apply higher pressing force onto the particle: without support from the back side by the second cleaning surface the pellicle may yield and the force applied to the particle by the first cleaning surface may be limited.

[00032] The first and second cleaning surfaces may have different properties: the tip that contacts the particle may be deformable, sticky (e.g. with a high Hamaker constant) and may comprise nanotubes and other structures that promote adhesion, while the second cleaning surface may be the opposite: it may be hard / non-deformable and it may have low Hamaker constant (e.g. to simplify the breaking contact with pellicle after the particle is removed).

[00033] The first and/or the second cleaning surfaces may be comprised in an array of cleaning surfaces and comprise a controller configured to control the provision of electrical bias to the cleaning surfaces.

[00034] The first cleaning surface may be in the form of a cylinder and wherein the cylinder rotates to remove particle contamination from the surface of the lithographic apparatus in need of cleaning. The cylinder preferably rotates at the same rate as the cleaning surface moves across the surface in need of cleaning.

[00035] The cleaning apparatus used in the method according to the third aspect of the present invention may be the cleaning apparatus according to the first aspect of the present invention.

[00036] According to a fourth aspect of the present invention, there is provided a lithographic apparatus comprising the cleaning apparatus of the first aspect of the present invention.

[00037] According to a fifth aspect of the present invention, there is provided an apparatus for preventing re-contamination of a surface of a lithographic apparatus, the apparatus comprising at least one plate configured to capture particles released from the surface of the lithographic apparatus, said at least one plate comprising an electrically conductive material coated with a dielectric material.

[00038] One of the problems with existing cleaning methods and apparatus is that any dislodged particles are able to re-contaminate the apparatus. The present invention allows for particles which have been released from a surface to be captured and retained by another surface in order to avoid recontamination.

[00039] The at least one plate may be in electrical communication with an electrical bias source configured to apply an electrical bias to the at least one plate. Again, as described in respect of the other aspects of the present invention, it is possible to control the adhesive characteristics, i.e. the degree to which particles adhere to a surface, of a surface by applying an electrical bias. [00040] The electrically conductive material may comprise a plurality of nanotubes and/or an electrically conductive aerogel. The plurality of nanotubes may comprise a plurality of carbon nano tubes. The dielectric material may be a metal oxide, such as aluminium oxide, or a dielectric polymer.

[00041] The apparatus may further comprise first and second electrostatic electrodes spaced apart to receive a surface in need of cleaning and configured to vibrate a surface disposed between the first and second electrostatic electrodes to dislodge any particle contaminants.

[00042] According to a sixth aspect of the present invention, there is provided a method for preventing re-contamination of a surface of a lithographic apparatus, said method comprising: i) providing an apparatus having at least one plate configured to capture particles released from the surface of the lithographic apparatus; ii) applying an electrical bias to the at least one plate; and iii) displacing any particle contaminants from the surface of the lithographic apparatus and capturing them on the at least one plate.

[00043] The apparatus may be the apparatus according to the fifth aspect of the present invention.

[00044] According to a seventh aspect of the present invention, there is provided the use of an apparatus according to the first or fifth aspects of the present invention, a cleaning tip according to the second aspect of the present invention, a method according to the third or sixth aspects of the present invention, in a lithographic apparatus or method.

[00045] It will be appreciated that features described in respect of one aspect or embodiment may be combined with any features described in respect of another aspect or embodiment and all such combinations are expressly considered and disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[00046] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[00047] Figure 1 depicts a lithographic apparatus which may include the lithography apparatus component according to an embodiment of the invention;

[00048] Figures 2a to c are a schematic depiction of a cleaning apparatus according to one embodiment of the present invention removing a particle contaminant from a surface;

[00049] Figures 3a to c are a schematic depiction of a cylindrical cleaning surface in accordance with an embodiment of the present invention;

[00050] Figure 4 is a schematic depiction of an apparatus for preventing re-contamination of a surface according to the present invention;

[00051] Figures 5a and b are schematic depictions of arrays of cleaning surfaces according to an embodiment of the present invention; and [00052] Figures 6a to d are schematic depictions of AFM tips according to embodiments of the present invention.

[00053] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The following detailed description will describe the invention in relation to pellicles, but it will be appreciated that it is applicable to the cleaning of other surfaces of a lithographic apparatus, in particular the surface of optical elements like mirrors, or reticles and reticle, wafer, or substrate stages. Indeed the present invention may be applied to any surface of a lithographic apparatus which is in need of cleaning.

DETAILED DESCRIPTION

[00054] Figure 1 shows a lithographic apparatus according to the present invention including a pellicle 15. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W. In this embodiment, the pellicle 15 is depicted in the path of the radiation and protecting the patterning device MA. It will be appreciated that the pellicle 15 may be located in any required position and may be used to protect any of the mirrors in the lithographic apparatus.

[00055] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[00056] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. A laser, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam into a fuel, such as tin (Sn) which is provided from a fuel emitter. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region. The laser beam is incident upon the tin at the plasma formation region. The deposition of laser energy into the tin creates a plasma at the plasma formation region. Radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of ions of the plasma.

[00057] The EUV radiation is collected and focused by a near normal incidence radiation collector (sometimes referred to more generally as a normal incidence radiation collector). The collector may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region, and a second focal point may be at an intermediate focus, as discussed below.

[00058] The laser may be separated from the radiation source SO. Where this is the case, the laser beam may be passed from the laser to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser and the radiation source SO may together be considered to be a radiation system.

[00059] Radiation that is reflected by the collector forms a radiation beam B. The radiation beam B is focused at a point to form an image of the plasma formation region, which acts as a virtual radiation source for the illumination system IL. The point at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus is located at or near to an opening in an enclosing structure of the radiation source.

[00060] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11.

[00061] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT, which may also be referred to as a substrate stage. The lithography apparatus component according to the present invention may comprise the substrate stage WT and/or the support structure MT for the patterning device MA. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 13, 14 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors). [00062] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[00063] Figures 2a to 2c depict a cleaning surface according to an embodiment of the present invention cleaning a contamination particle 18 from the surface of a pellicle 15. The cleaning surface 20 comprises a conductive electrode 16 and a plurality of carbon nanotubes 17 extending from the conductive electrode 16. The carbon nanotubes 17 include a dielectric material coating. In Figure 2b, an electrical bias V has been applied to the plurality of carbon nanotubes 17, which have transferred the particle 18 from the pellicle 15 to the carbon nanotubes 17. The carbon nanotubes are flexible and so may be pressed against the particle 18 to ensure transfer of the particle 18 away from the pellicle 15. As the carbon nanotubes 17 are pressed into the particle, the electrical bias applied thereto increases the adhesiveness of the carbon nanotubes and allows the contaminant particle 18 to be retained by the forest of carbon nanotubes 17. Once the particle 18 has been removed from the pellicle 15, the cleaning surface can be moved away from the pellicle 15 and the electrical bias can be removed in order to remove the particle 18. As such, the embodiment of Figure 2 allows a particle to be removed from a surface by the selective provision of an electrical bias. The dielectric coating on the carbon nanotubes

17 allows the nano tubes to be polarized and attract a particle 18.

[00064] Figures 3a to 3c depict an embodiment of the present invention in which the cleaning surface comprises a cylinder. The cylindrical conductive electrode 16 comprises a plurality of carbon nanotubes coated with a dielectric material 17 disposed on the surface. The cylindrical electrode 16 rotates as it is moved along the surface of the pellicle 15. An electrical bias V is applied to the electrode 16 and a particle 18 is removed from the pellicle 15. Once the cleaning surface has been taken away from the pellicle 15, the electrical bias can be removed and the particle 18 can be taken away so that the cleaning surface can be used again.

[00065] Figure 4 is a schematic depiction of an apparatus for preventing re-contamination of a surface of a lithographic apparatus. The apparatus comprises a pair of electrostatic electrodes 19 disposed opposing one another with a pellicle 15 in between. Radiofrequency (RF) or pulsed voltage is provided to the electrodes 19 which induces vibrations in the pellicle 15. The vibrations dislodge any particle contaminants 18 from the pellicle 15. There are also provided electrically conductive electrodes 16 comprising a plurality of carbon nanotubes 17 which are electrically biased. As such, the particles

18 removed from the pellicle 15 are captured by the carbon nano tubes 17 and prevented from recontaminating the pellicle 15. Whilst a specific configuration of conductive electrodes 16 is depicted, it will be appreciated that they may be located as any desired location, preferably at a location where dislodged particles 18 are most likely to move to. An insulation layer may be provided between the electrically conductive electrode 16 and the electrostatic electrodes 19. The electrostatic removal of particles requires high voltages in the tens of kilovolts. The apparatuses and methods according to the present invention are able to operate at less than 200 V, and closer to around 30 V, meaning that there is a greater degree of safety and ease in using the present invention to clean a surface of a lithographic apparatus.

[00066] Figure 5a is a schematic of a cleaning apparatus comprising an array of cleaning surfaces. The array of cleaning surfaces 20 can comprise any number or arrangement of cleaning surfaces 20, but for the sake of example, is depicted as comprising four cleaning surfaces 20 in a 2 x 2 grid. The array of cleaning surfaces 20 is able to move relative to the pellicle 15 to allow the entire surface of the pellicle 15 to be cleaned. In embodiments, there is a complementary array of cleaning elements 20 on the opposite side of the pellicle 15. The individual cleaning elements 20 may be controlled by a controller (not shown) to turn them on and off. The individual cleaning elements 20 may be turned on (shown in hatched shading) or may be turned off (shown with no shading). The corresponding cleaning elements 20 on the opposite side of the pellicle 15 may have the opposite condition, either on or off. As such, although there are locally unbalanced forces on the pellicle 15, over the larger surface of the pellicle 15, there is net zero force. This is more clearly shown in Figure 5b in which a side view of the pellicle 15 is provided and the opposing cleaning elements 20 are shown to be in opposite on/off conditions. In embodiments where the location of the particle contaminants has been determined, the array of cleaning elements may be moved to that specific location. Otherwise, the array of cleaning elements may be scanned across the surface of the pellicle 15.

[00067] Figure 6a is a schematic depiction of an AFM tip 21 according to an embodiment of the present invention. The AFM tip 21 includes the plurality of carbon nanotubes coated in a dielectric material as described herein. As such, the AFM tip 21 is able to controllably pick up particles from a surface and remove them. The shape of the AFM tip 21 in Figure 6a is similar to the usual shape of AFM tips used in atomic force microscopy. Figure 6b depicts an embodiment of an AFM tip which has a broader cleaning surface 20 and Figure 6c depicts an embodiment in which there is an array of cleaning surfaces 20. Finally, Figure 6d depicts an embodiment in which the AFM tip does not include a specific surface coating. In such a configuration, the AFM tip can be pressed onto particles or used to push particles along the surface to be cleaned.

[00068] In summary, the present invention provides a cleaning apparatus which is able to transfer particle contamination from surfaces of a lithographic apparatus, such as pellicles, by controlling the adhesion characteristics of the surface. The adhesion characteristics may be controlled by the application of an electrical bias. The adhesion may be effected by the presence of a plurality of electrically conductive nanotubes coated with a dielectric material. In addition, the use of atomic force microscopy tips may allow very specific removal of particle contamination from a surface. The present invention has the advantage of allowing very controlled removal of contaminants, the use of very low voltages compared to existing techniques, and the prevention of re-contamination of surfaces.

[00069] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. [00070] 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 and clauses set out below.

1. A cleaning apparatus for cleaning a component of a lithographic apparatus, said apparatus comprising a first cleaning surface configured to physically interact with a contaminant particle located on a surface to be cleaned to remove the contaminant particle from the surface to be cleaned.

2. A cleaning apparatus according to clause 1, wherein the first cleaning surface has a controllable adhesion.

3. A cleaning apparatus according to clause 2, further comprising an electrical bias source in electrical communication with the first cleaning surface and configured to provide an electrical bias to the first cleaning surface to thereby control the adhesion of the first cleaning surface, optionally wherein the electrical bias is up to 200 V, up to 100 V, up to 50 V, up to 30 V, or up to 10 V.

4. A cleaning apparatus according to any preceding clause, wherein the first cleaning surface comprises an electrically conductive material coated with a dielectric material.

5. A cleaning apparatus according to clause 4, wherein the electrically conductive material comprises a plurality of nano tubes and/or an electrically conductive aerogel.

6. A cleaning apparatus according to clause 5, wherein the plurality of nanotubes comprises a plurality of carbon nano tubes.

7. A cleaning apparatus according to any of clauses 4 to 6, wherein the dielectric material comprises aluminium oxide.

8. A cleaning apparatus according to any preceding clause, wherein the first cleaning surface is substantially planar or is non-planar.

9. A cleaning apparatus according to clause 8, wherein when the first cleaning surface is non- planar, the first cleaning surface is cylindrical or is shaped to complement the shape of the surface to be cleaned.

10. A cleaning apparatus according to any preceding clause, wherein the apparatus comprises a second cleaning surface disposed opposite the first cleaning surface and spaced apart from the first cleaning surface.

11. A cleaning apparatus according to any preceding clause, wherein first cleaning surface is comprised in an array of cleaning surfaces and/or the or a second cleaning surface is comprised in an array of cleaning surfaces.

12. A cleaning apparatus according to clause 11 further comprising a controller in electrical communication with first and/or second arrays of cleaning surfaces, the controller being configured to control the electrical bias applied to the cleaning surfaces.

13. A cleaning apparatus according to any preceding clause, wherein the first cleaning surface comprises an atomic-force microscopy (AFM) tip. 14. A cleaning apparatus according to clause 13, wherein the AFM tip comprises a material with a Hamaker constant A>10 ¹⁹ J, optionally wherein the AFM tip comprises diamond, silicon carbide, gold or combinations thereof, or wherein the AFM tip comprises a deformable material, optionally gold, silver, a rare earth metal or indium, or wherein the AFM tip comprises a material selected to provide a triboelectrical charging effect, optionally Teflon® or polyurethane, and/or wherein the AFM tip is rounded or angular.

15. A cleaning apparatus according to clause 13, wherein the AFM tip comprises an electrically conductive layer coated with a dielectric material, optionally wherein the electrically conductive material comprises a plurality of nanotubes and/or an electrically conductive aerogel, optionally wherein the plurality of nanotubes is a plurality of carbon nanotubes.

16. A cleaning apparatus according to any preceding clause further comprising a particle contamination locating element configured to detect the presence and/or location of particle contamination on the surface to be cleaned.

17. A cleaning apparatus according to clause 16, wherein the particle contamination locating element comprises one or more elements which located particles via optical techniques, scanning electron microscopy and/or scanning AFM.

18. A cleaning apparatus according to any preceding clause, wherein the apparatus further comprises a conditioning surface configured to remove any contaminant particles from the first cleaning surface or any other cleaning surface.

19. A cleaning tip for cleaning a component of a lithographic apparatus, the cleaning tip comprising an AFM tip, wherein said AFM tip further comprises a material selected to at least temporarily retain a contaminant particle.

20. A cleaning tip according to clause 19, wherein the cleaning tip comprises: i) a material with a high Hamaker constant; ii) a deformable material; iii) a material selected to provide a triboelectrical charging effect; or iv) an electrically conductive material coated with a dielectric material, optionally wherein the electrically conductive material comprises a plurality of nanotubes and/or an electrically conductive aerogel, optionally wherein the plurality of nanotubes is a plurality of carbon nanotubes.

21. A method for cleaning a surface of a lithographic apparatus, said method comprising: i) providing a cleaning apparatus having a first cleaning surface; ii) bringing the first cleaning surface to the surface of the lithographic apparatus in need of cleaning; iii) causing any particle contamination on the surface of the lithographic apparatus in need of cleaning to be picked up by or moved by the first cleaning surface; and iv) moving the first cleaning surface away from the surface of the lithographic apparatus. 22. The method according to clause 21, wherein the method comprises applying an electrical bias to the first cleaning surface in order to increase the adhesion of the first cleaning surface.

23. The method according to clause 22, wherein the electrical bias is turned off once the first cleaning surface has been moved away from the surface of the lithographic surface to allow any contaminant particles to be removed from the first cleaning surface.

24. The method according to any of clauses 21 to 23, wherein the step of moving the first cleaning surface away from the surface of the lithographic apparatus further comprises moving the first cleaning surface to a conditioning surface and transferring any contaminant particles from the first cleaning surface to the conditioning surface.

25. The method according to any of clauses 21 to 24, the method further comprising determining the position of one or more particle contaminants.

26. The method according to clause 25, wherein the first cleaning surface is moved to the predetermined position of the one or more particle contaminants.

27. The method according to any of clauses 21 to 26, the method further comprising providing a second cleaning surface opposite to the first cleaning surface and disposing the surface of the lithographic apparatus in need of cleaning between the first and second cleaning surfaces.

28. The method according to clause 27, wherein the first cleaning surface and/or the second cleaning surface are comprised in an array of cleaning surfaces and comprise a controller configured to control the provision of electrical bias to the cleaning surfaces.

29. The method according to any of clauses 21 to 28, wherein the first cleaning surface is in the form of a cylinder and the cylinder rotates in order to remove particle contamination from the surface of the lithographic apparatus in need of cleaning.

30. The method according to any of clauses 21 to 29, wherein the cleaning apparatus is the cleaning apparatus of any of clauses 1 to 18.

31. A lithographic apparatus comprising the cleaning apparatus of any of clauses 1 to 18.

32. An apparatus for preventing re-contamination of a surface of a lithographic apparatus, the apparatus comprising at least one plate configured to capture particles released from the surface of the lithographic apparatus, said at least one plate comprising an electrically conductive material coated with a dielectric material.

33. The apparatus according to clause 32, wherein the at least one plate is in electrical communication with an electrical source configured to apply an electrical bias to the at least one plate.

34. The apparatus according to clause 32 or clause 33, wherein the electrically conductive material comprises a plurality of nanotubes and/or an electrically conductive aerogel, optionally wherein the plurality of nanotubes is a plurality of carbon nanotubes, optionally wherein the dielectric material is aluminium oxide.

35. The apparatus according to any of clauses 32 to 34, further comprising first and second electrostatic electrodes spaced apart to receive a surface in need of cleaning and configured to vibrate a surface disposed between the first and second electrostatic electrodes to dislodge any particle contaminants.

36. A method for preventing re-contamination of a surface of a lithographic apparatus, said method comprising: i) providing an apparatus having at least one plate configured to capture particles released from the surface of the lithographic apparatus; ii) applying an electrical bias to the at least one plate; and iii) displacing any particle contaminants from the surface of the lithographic apparatus and capturing them on the at least one plate.

37. The method according to clause 36, wherein the apparatus is the apparatus according to any of clauses 32 to 35.

38. The use of the apparatus of any of clauses 1 to 18 or 32 to 35, an AFM tip according to any of clauses 19 and 20 or a method according to any of clauses 21 to 30 or 36 and 37 in a lithographic apparatus or method.

39. A cleaning apparatus according to clause 3, wherein the electrical bias source in electrical communication with the first cleaning surface is configured to provide an electrical bias to the first cleaning surface with respect to the component of the lithographic apparatus component or another surface.

40. A cleaning apparatus according to clause 13, wherein the AFM tip comprises a deformable material with a Young modulus U < 100 MPa.

41. A cleaning apparatus according to clause 10, wherein the second cleaning surface comprises a material with a Young modulus U >1 GPa) and/or has Hamaker constant A<10 ¹⁹ J and/or the AFM tip of the second cleaning surface has a smaller size and sharper shape than the AFM tip of the first cleaning surface.

42. A cleaning apparatus according to clause 10, wherein the second cleaning surface is arranged to contact the pellicle in a dynamic way with a speed from 0.3 to 30 m/s, preferable with a speed from 1-10 m/s.

43. A cleaning apparatus according to clause 42, wherein the second cleaning surface is arranged such as to displace pellicle with less than 1 pm, preferable less than 100 nm, and wherein the contact of second cleaning surface with pellicle is in vicinity of a particle to be removed, where optionally the same or another particle may be simultaneously or subsequently be in contact with the first cleaning surface or in vicinity of the fist cleaning surface.

44. A cleaning apparatus according to clause 10, wherein the second cleaning surface is arranged such as to provide the pellicle a movement with a speed v<0.1 m/s, wherein the second cleaning surface acts as a support to allow higher pressure force from the first cleaning surface when brought in contact with the particle. 45. A cleaning apparatus according to clause 10, wherein the first and second cleaning surfaces are substantially aligned and the distance between the AFM tips facing the pellicle is less than 100 um, preferably less than 10 pm.

46. A cleaning apparatus according to clause 10 arranged such that the pellicle can be provided between the first and second cleaning surface such as to bring the particle to be cleaned with the AFM tips to at least 10 pm or more.

47. A method according to clause 27, wherein the second cleaning surface is arranged such as to provide the pellicle a movement with a speed v<0.1 m/s, wherein the second cleaning surface acts as a support to allow higher pressure force from the first cleaning surface when brought in contact with the particle.

48. A method according to clause 27, wherein the second cleaning surface is arranged to contact the pellicle in a dynamic way with a speed from 0.3 to 30 m/s, preferable with a speed from 1-10 m/s.

49. A method according to clause 48, wherein the second cleaning surface is arranged such as to displace pellicle with less than 1 pm, preferable less than 100 nm, and wherein the contact of second cleaning surface with pellicle is in vicinity of a particle to be removed, where optionally the same or another particle may be simultaneously or subsequently be in contact with the first cleaning surface or in vicinity of the fist cleaning surface.

50. A method according to any one of clauses 47 to 49, wherein the second cleaning surface follows the first cleaning surface in the plane of the pellicle.