Removing molecules from surfaces in EUV lithography tools

Abstract

A lithographic apparatus configured to project EUV radiation onto a substrate location, the lithographic apparatus comprising: an illumination system having a plurality of reflective surfaces for directing the EUV radiation along a beam path; and an IR source configured to generate IR radiation, wherein the IR source and the illumination system are arranged such that the IR radiation is projected along the beam path.

Description

[Technical Field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to European Application No. 23154189.7, filed January 31, 2023, and European Application No. 23186901.7, filed July 21, 2023, which are incorporated herein by reference in their entireties.

[0002] [Technical field] The present invention relates to lithographic apparatus, in particular to extreme ultraviolet (EUV) lithographic apparatus. [Background technology]

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, also referred to as a mask or reticle, may be used to generate the circuit pattern formed on an individual layer of the IC. This pattern may be transferred onto a target portion (e.g. comprising part of a die, a single die, or several dies) on the 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 decrease, lithography becomes an even more important factor in enabling finer ICs and other devices and / or structures to be manufactured.

[0005] A theoretical approximation of the limit of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1). CD=k1*λ / NA (1) where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process-dependent adjustment factor also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. According to equation (1), a reduction in the minimum printable size of a feature can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA, or by decreasing the value of k1.

[0006] To shorten the exposure wavelength and reduce the minimum printable size, it has been proposed to use extreme ultraviolet (EUV) radiation sources. EUV radiation is electromagnetic radiation having a wavelength in the range of 10-20 nm (e.g., in the range of 13-14 nm). It has further been proposed that EUV radiation with wavelengths smaller than 10 nm (e.g., in the range of 5-10 nm, such as 6.7 nm or 6.8 nm) could be used. Such radiation is referred to as 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 or free-electron lasers provided by electron storage rings. Summary of the Invention [Problem to be solved by the invention]

[0007] Once generated, EUV radiation is directed by a number of mirrors through the lithography apparatus to a patterning surface of a patterning device that imparts the EUV radiation with a desired pattern. Oxidation of the mirrors can undesirably reduce the mirror's reflectivity to EUV radiation. Molecules such as water droplets on the surface of the mirrors contribute to such oxidation.

[0008] The water droplets can be removed by reducing the pressure and waiting for them to desorb and leave the environment, which can take a long time.

[0009] It is therefore an object of the present invention to provide a more effective technique for removing molecules, such as in water droplets, from surfaces in EUV lithography tools. [Means for solving the problem]

[0010] According to an aspect of the present invention, there is provided a lithographic apparatus configured to project EUV radiation onto a substrate location, the lithographic apparatus comprising: an illumination system comprising a plurality of reflective surfaces for directing the EUV radiation along a beam path; and an IR source configured to generate IR radiation, wherein the IR source and the illumination system are arranged such that the IR radiation is projected along the beam path.

[0011] According to another aspect of the invention, there is provided a method for removing molecules from a reflective surface of a lithographic apparatus configured to project EUV radiation along a beam path towards a substrate location, the reflective surface being for directing the EUV radiation along the beam path, the method comprising generating IR radiation and projecting the IR radiation along the beam path.

[0012] According to another aspect of the present invention, there is provided a vent gas generator for generating a vent gas for venting a vacuum environment, the vent gas generator comprising: a gas dryer configured to receive an input gas and output a dry gas having fewer than one water molecule per billion molecules of gas; and a molecular catcher configured to receive the dry gas, catch water molecules from the dry gas, and output a vent gas free of the caught water molecules.

[0013] According to another aspect of the invention, there is provided a method for generating a vent gas for venting a vacuum environment, the method comprising: treating an input gas to output a dry gas having less than one water molecule per billion molecules of gas; and capturing water molecules from the dry gas to output a vent gas free of the captured water molecules.

[0014] There are several methods that may be utilized to reduce the amount of water on the mirror: Uses IR light to stimulate water desorption. Use temperature control to stimulate water desorption in a manner that results in a small net heat input (e.g., using liquid flow directly beneath the mirror surface along with high and low temperature sequences). Use getters (molecular catchers) to capture water before it can reach the optical element.

[0015] Another option is not to reduce the amount of water on the mirror, but to prevent it from appearing in the first place, for example by using ultra-dry XCDA (hereafter referred to as XXCDA), which is similar to a getter that prevents water from reaching the mirror.

[0016] The aforementioned method for reducing the amount of water on the mirrors leads to less water on the mirrors after a given period of time during which the method is applied. This may reduce the time required to reach a predetermined maximum acceptable level of adsorbed water before EUV light can be switched on and production can begin, which also depends on the amount of water present at the start of the cleaning action. If the amount of adsorbed water is low to begin with, the method leads to the required specification in less time, increasing the useful time for exposing substrates. Thus, it is important to prevent water from entering the vacuum environment of the lithography apparatus when it is opened. When providing XCDA to vent and flush the vacuum environment of the lithography apparatus, there is a risk that the gas will not be dry enough to prevent a first monolayer (ML) of water from adsorbing to the interior surfaces (mirrors, walls) of the lithography apparatus. For this reason, the XCDA can be further dried by about 3-4 orders of magnitude to control the amount of water provided in the vacuum environment of the lithography apparatus. [Brief explanation of the drawings]

[0017] Embodiments of the present invention are now described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0018] FIG. 1 depicts a schematic diagram of a lithographic apparatus.

[0019] FIG. 2 shows a more detailed view of the lithographic apparatus schematically.

[0020] FIG. 3 shows a schematic arrangement for IR radiation to follow a beam path in a lithographic apparatus.

[0021] FIG. 4 is a graph showing the relationship between radiation wavelength and absorption by water.

[0022] FIG. 5 shows, in a schematic manner, another arrangement for IR radiation to follow a beam path in a lithographic apparatus.

[0023] FIG. 6 shows, schematically, another arrangement for IR radiation to follow a beam path in a lithographic apparatus.

[0024] FIG. 7 shows a schematic diagram of a vent gas generator.

[0025] The features shown in the figures are not necessarily to scale and are not limited to the size and / or arrangement shown. The figures are understood to include optional features that are not essential to the invention. Furthermore, not all features of a device are shown in each figure, and a figure may show only some of the components relevant to describing a particular feature. DETAILED DESCRIPTION OF THE INVENTION

[0026] 1 depicts a lithographic apparatus 100 including a source collector module SO according to one embodiment of the present 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., mask table) MT configured to support a patterning device (e.g., mask or reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; · a substrate table (e.g., wafer table) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0027] 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.

[0028] 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 whether or not the patterning device is held in a vacuum environment. The support structure MT may 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.

[0029] The term "patterning device" should be interpreted broadly to refer to any device that can be used to create a pattern in the cross-section of a radiation beam B, for example so as to create a pattern in a target portion C of a substrate W. The pattern created in 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.

[0030] 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 that can be individually tilted so as to reflect an incoming beam in different directions. The tilted mirrors form a pattern in a radiation beam that is reflected by the mirror matrix.

[0031] 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 used and other factors such as the use of a vacuum. The use of a vacuum environment for EUV radiation is desirable as other gases may excessively absorb the radiation, and therefore a vacuum environment may be provided for the entire beam path by means of a vacuum wall and vacuum pumps.

[0032] As here depicted, lithographic apparatus 100 is of a reflective type (eg, employing a reflective mask).

[0033] 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 additional support structures MT) may be used in parallel, or preparation steps may be performed 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.

[0034] Referring to FIG. 1 , the illumination system IL receives a beam of extreme ultraviolet radiation from a source collector module SO. Methods for generating EUV light include, but are not necessarily limited to, converting a material having at least one element (e.g., xenon, lithium, or tin) into a plasma state having one or more emission lines in the EUV range. In one such method, often referred to as laser-produced plasma (“LPP”), the required plasma can be generated by irradiating a fuel, such as droplets, streams, or clusters of material having the required line-emitting elements, with a laser beam. The source collector module SO may be part of an EUV radiation system that includes a laser (not shown in FIG. 1 ) for providing a laser beam that excites the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, that is collected using a radiation collector disposed within the source collector module. For example, when a CO laser is used to provide the laser beam for fuel excitation, the laser and source collector module SO may be separate entities.

[0035] In such cases, the laser is not to be understood as forming part of lithographic apparatus 100, and the radiation beam B is passed from the laser to the source collector module SO by a beam delivery system, for example comprising suitable directing mirrors and / or beam expanders. In other cases, for example where the source is a discharge produced plasma EUV generator, often referred to as a DPP source, the source may be an integral part of the source collector module SO.

[0036] The illumination system IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Typically, at least the outer and / or inner radial extent (commonly referred to as "σ-outer" and "σ-inner", respectively) of the intensity distribution in a pupil plane of the illumination system IL may 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 adjust the radiation beam B so that it has a desired uniformity and intensity distribution in its cross-section.

[0037] The radiation beam B is incident on a patterning device (e.g., mask) MA, which is held on a support structure (e.g., mask table) MT, and is patterned by the patterning device MA. After reflecting from the patterning device (e.g., mask) MA, the radiation beam B passes through a projection system PS, which focuses the radiation beam B onto a target portion C of a substrate W. The substrate table WT may be accurately driven (e.g., to position different target portions C in the path of the radiation beam B) by a second positioner PW and a position sensor PS2 (e.g., an interferometric device, a linear encoder or a capacitive sensor). Similarly, the first positioner PM and another position sensor PS1 may 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 substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[0038] The controller 500 controls the overall operation of the lithographic apparatus 100, and in particular executes the operational processes described further below. The controller 500 may be embodied as a suitably programmed general-purpose computer having a central processing unit, volatile and non-volatile storage means, one or more input and output devices such as a keyboard and a screen, one or more network connections, and one or more interfaces with various parts of the lithographic apparatus 100. It is understood that a one-to-one relationship between the control computer and the lithographic apparatus 100 is not necessary. In one embodiment, a single computer of the present invention can control multiple lithographic apparatuses 100. In one embodiment, multiple network computers of the present invention may be used to control a single lithographic apparatus 100. The controller 500 may be configured to control one or more associated processing and substrate handling devices in a lithocell or cluster of which the lithographic apparatus 100 is a part. The controller 500 may also be configured to be subordinate to a supervisory control system of the lithocell or cluster and / or an overall control system of the fab.

[0039] 2 shows lithographic apparatus 100 in more detail, including a source collector module SO, an illumination system IL, and a projection system PS. An EUV radiation-emitting plasma 210 may be formed by a plasma source. The EUV radiation may be generated by a gas or vapor (e.g., Xe gas, Li vapor, or Sn vapor) from which the radiation-emitting plasma 210 is generated to emit radiation in the EUV range of the electromagnetic spectrum. In one embodiment, a plasma of excited tin (Sn) is provided to generate the EUV radiation.

[0040] Radiation emitted by the radiation-emitting plasma 210 passes from the source chamber 211 into the collector chamber 212 .

[0041] The collector chamber 212 may include a radiation collector CO. Radiation passing through the radiation collector CO may be focused to a virtual source point IF. The virtual source point IF is commonly referred to as an intermediate focus, and the source collector module SO is arranged such that the virtual source point IF is located on or near an opening 221 in the enclosure structure 220. The virtual source point IF is an image of the radiation-emitting plasma 210.

[0042] The radiation then passes through an illumination system IL, which may include a faceted field mirror device 22 and a faceted pupil mirror device 24, which are provided to provide a desired angular distribution of the unpatterned beam 21 at the patterning device MA and a desired uniformity of the radiation intensity at the patterning device MA. Upon reflection of the unpatterned beam 21 off the patterning device MA, which is held by a support structure MT, a patterned beam 26 is formed, which is imaged by the projection system PS, via reflective elements 28, 30, onto a substrate W held by a substrate table WT. A pellicle 80 may be used to protect the patterning device from contaminants.

[0043] In general, more elements than those shown may be present in the illumination system IL and projection system PS. Additionally, more mirrors than shown may be present. For example, 1-6 additional reflective elements may be present in the projection system PS relative to that shown in Figure 2.

[0044] Alternatively, the source collector module SO may be part of an LPP radiation system.

[0045] As shown in Figure 1, in one embodiment, 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 a substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto a substrate W. The pattern is for EUV radiation in the radiation beam B.

[0046] The intervening space between the projection system PS and the substrate table WT may be at least partly evacuated, and may be bounded at the position of the projection system PS by a solid surface through which the utilized radiation is directed towards the substrate table WT.

[0047] Figure 3 shows a schematic diagram of an arrangement for IR radiation to follow a beam path 44 of EUV radiation in a lithographic apparatus 100. Figure 3 shows a schematic diagram of a source collector module SO and an illumination system IL of the lithographic apparatus 100. The lithographic apparatus 100 may have features such as those described above and / or shown in Figures 1 or 2, for example. For ease of understanding, the optical axis O is shown in Figure 3 as a straight line passing through the source collector module SO and the illumination system IL. Of course, the optical axis may be folded, for example, by one or more reflective surfaces as shown in Figure 2.

[0048] Lithographic apparatus 100 is configured to project EUV radiation onto a substrate position. During the exposure process, a substrate may be placed at the substrate position. The EUV radiation may be projected in the form of a radiation beam B. The radiation beam B may be projected along a beam path 44. The beam path 44 may extend from where the radiation beam B is generated to the substrate position.

[0049] In an embodiment, the illumination system IL comprises a plurality of reflective surfaces 41 for directing the EUV radiation along a beam path 44. The reflective surfaces 41 may be surfaces of respective reflectors 40. Figure 3 shows schematically one such reflector 40 comprising a reflective surface 41. The reflective surface 41 faces the beam path 44. When the EUV radiation is being projected towards the substrate position, a radiation beam B of EUV radiation reflects from the reflective surface 41. In an embodiment, the reflector 40 of the illumination system IL comprises, for example, a faceted field mirror device 22 and / or a faceted pupil mirror device 24 as shown in Figure 2.

[0050] In an embodiment, lithographic apparatus 100 further comprises a reflective surface for directing the EUV radiation down-beam of the illumination system IL. For example, in an embodiment, projection system PS comprises reflective elements 28, 30 as shown in Figure 2. In an embodiment, lithographic apparatus 100 further comprises a reflective surface for directing the EUV radiation up-beam of the illumination system IL. For example, in an embodiment, source collector module comprises one or more reflective elements as shown in Figure 2.

[0051] 2, in one embodiment, the source collector module SO comprises an EUV redirector 42 configured to redirect EUV radiation generated in the radiation-emitting plasma 210. The redirected EUV radiation follows a beam path 44.

[0052] As shown in FIG. 3 , in one embodiment, the source collector module SO comprises a beam stop 43. The shroud 43 is configured to block drive radiation that drives the generation of EUV radiation. The drive radiation may be directed toward the radiation-emitting plasma 210 to drive the emission of EUV radiation. During the generation of EUV radiation, it may be undesirable for the drive radiation to follow the beam path 44. The beam stop 43 is configured to reduce or prevent the drive radiation (which may typically have a longer wavelength than the EUV radiation) from following the beam path 44. Other features shown in FIG. 3 may be as described above with respect to FIG. 2 .

[0053] As shown in FIG. 3 , in one embodiment, lithographic apparatus 100 includes an IR source 50. IR source 50 is configured to generate IR radiation 51. IR radiation 51 is intended to promote desorption of molecules, such as a water film, from the surface of lithographic apparatus 100. Such a water film may be formed of one or more monolayers of water molecules. For example, the water molecules may absorb the IR radiation 51 to gain energy, which may increase the likelihood that the water molecules will leave the surface of lithographic apparatus 100.

[0054] 3, in one embodiment, an IR source 50 and an illumination system IL are provided such that IR radiation 51 is projected along a beam path 44. Beam path 44 is the beam path 44 along which a radiation beam B of EUV radiation is projected during the exposure process. The radiation following beam path 44 strikes a reflective surface 41 that directs the EUV radiation beam along path 44. As IR radiation 51 follows beam path 44, IR radiation 51 may promote desorption of molecules from multiple (optionally substantially all) reflective surfaces 41, which direct the EUV radiation towards the substrate location.

[0055] By projecting the IR radiation 51 along the beam path 44, the IR radiation 51 may reach the reflective surface 41 more efficiently. The reflective surface 41 is a surface that is prone to becoming less reflective to EUV radiation when oxidation occurs. The oxidation may be caused by molecules, such as water molecules, adhering to the reflective surface 41. The oxygen that causes the oxidation may exist in the form of water. The water molecules may be decomposed by EUV radiation to produce highly reactive atomic oxygen. An embodiment of the present invention is expected to improve the efficiency of maintaining the reflectivity of the reflective surface 41. By maintaining their reflectivity, the lifetime of the reflector 40 may be extended. The cost of maintaining the lithography apparatus 100 may be reduced. The uptime of the lithography apparatus may be increased.

[0056] In one embodiment, lithographic apparatus 100 is configured to operate in different modes. In an exposure mode or EUV mode, lithographic apparatus 100 is configured to project EUV radiation towards a substrate location. In exposure mode, IR radiation 51 is not projected along beam path 44. In a maintenance mode or drying mode, lithographic apparatus 100 is configured to project IR radiation 51 along beam path 44. In maintenance mode, EUV radiation is not projected along beam path 44, or is projected only along a portion of beam path 44.

[0057] As mentioned above, using a vacuum is desirable for EUV radiation because other gases may excessively absorb the radiation. Thus, a vacuum environment may be provided for the entire beam path 44 through a vacuum wall and a vacuum pump. During the lifetime of lithographic apparatus 100, the vacuum environment may be vented over time. After venting, a vacuum pump may be used to reduce the pressure again to the required vacuum before lithographic apparatus 100 is used for another exposure process.

[0058] It takes time to reach the required pressure. In particular, it is desirable to reduce the partial pressure of water molecules so as to mitigate oxidation / damage of reflective surface 41 (e.g., to the mirror coating). An embodiment of the present invention is expected to reduce the amount of water present in lithographic apparatus 100 without significantly increasing the time it takes before lithographic apparatus 100 can be used to project EUV radiation. An embodiment of the present invention is expected to reduce the amount of time required for the partial pressure of one or more types of molecules (e.g., water molecules) to reach a target level for the exposure process.

[0059] In one embodiment, IR source 50 is configured to generate IR radiation 51 having a wavelength in the range of about 1 μm to about 11 μm. In one embodiment, IR source 50 is configured to generate IR radiation 51 having a wavelength such that the IR radiation 51 promotes desorption of water molecules from a surface, such as reflective surface 41 of lithographic apparatus 100. IR radiation 51 having a wavelength in the range of about 1 μm to about 11 μm is expected to be well absorbed by water molecules to promote their desorption.

[0060] FIG. 4 is a graph showing the relationship between radiation wavelength and absorption by water molecules. The X-axis represents the wavelength of radiation. The Y-axis represents the absorption by water molecules. As shown in FIG. 4, IR radiation having a wavelength in the range of about 1 μm to about 11 μm may be well absorbed by water molecules. The graph shown in FIG. 4 shows the absorption of radiation by free water molecules (i.e., water molecules not on a surface). The shape of water molecules on a surface may be different from the shape of free water molecules. The absorption of radiation by water molecules on a surface may be different from the absorption of radiation by free water molecules. Nevertheless, it is expected that IR radiation having a wavelength in the range of about 1 μm to about 11 μm will be well absorbed by water molecules on a surface.

[0061] Water molecules attach to surfaces, such as the reflective surface 41, and to other water molecules through hydrogen bonds. Typical strengths of hydrogen bonds between water molecules are on the order of >0.1 eV. Photons with wavelengths of 10.6 μm (or shorter) have energies of 0.117 eV (or greater), and are therefore expected to gain enough energy to break hydrogen bonds in a single collision. IR radiation 51 with wavelengths in the range of about 1 μm to about 11 μm can assist in the desorption of water during pump-down.

[0062] The surface of the lithographic apparatus 100 may have multiple monolayers of water attached to it. The IR radiation 51 may promote the desorption of the water monolayers. The IR radiation 51 breaks the hydrogen bonds between the monolayers so that each monolayer can be desorbed.

[0063] The lowest monolayer may be bonded to the reflective surface 41 rather than to the other monolayers. In one embodiment, one or more of the reflective surfaces 41 comprises a capping layer. The lowest monolayer may be adsorbed onto the capping layer. In one embodiment, the capping layer comprises a metal, such as ruthenium. The bond strength between water and the metal capping layer may be different from the bond strength between water molecules. For example, the bond strength between water and ruthenium is about 0.7 eV. In one embodiment, the IR radiation 51 has a wavelength such that the photons have a polarization of at least 0.7 eV. This allows the photons to desorb the lowest monolayer in a single collision.

[0064] However, it is not necessary for the IR radiation 51 to have photons with an energy of 0.5 eV. The IR radiation 51 may also promote desorption by a multi-photon process instead of by a single collision. For example, if the relaxation of the vibrational intensity is slower than the time between successive photon collisions, desorption by a multi-photon process may occur. However, if the relaxation of the vibrational intensity is faster than the time between successive photon collisions, desorption by a multi-photon process may not occur. Alternatively, the IR radiation 51 may remove the higher monolayers of water but leave the lowest monolayer on the surface. Removal of all but one monolayer is expected to reduce the possibility of undesired oxidation (compared to leaving more than one monolayer of water on the surface).

[0065] An embodiment of the present invention is expected to reduce the power required for the IR source 50 to remove water from the surfaces of the lithographic apparatus 100. Lower power is expected to reduce the likelihood of undesired heating of components (particularly optical elements) of the lithographic apparatus 100. Photon energy used to desorb molecules is not used to undesiredly heat the mirrors. Such undesired heating requires time for cooling, which can increase downtime of the lithographic apparatus 100. An embodiment of the present invention is expected to reduce downtime of the lithographic apparatus 100. Undesired heating can make it more difficult to accurately control the temperature of components of the lithographic apparatus 100. An embodiment of the present invention is expected to increase the accuracy with which the temperature of components of the lithographic apparatus 100 is controlled.

[0066] In one embodiment, IR source 50 is configured to generate IR radiation 51 having a wavelength of approximately 10.6 μm. IR source 50 may have a dual purpose. IR source 50 may be configured to generate drive radiation for driving the generation of EUV radiation. This reduces the number of IR sources required. One such embodiment is described in more detail below with reference to FIG. 5.

[0067] Alternatively, the IR source 50 may be separate from the source of the drive radiation. Such an arrangement is shown, for example, in Figures 3 and 6. By providing separate IR sources for the drive radiation and the IR radiation 51, the IR radiation 51 may more easily have a different wavelength than the drive radiation. This allows the wavelength of the IR radiation 51 to be selected to optimize its ability to desorb water molecules without significantly heating the surface of the lithographic apparatus 100.

[0068] In one embodiment, the IR source 50 is configured to generate IR radiation 51 having a wavelength smaller than that of the drive radiation (e.g., smaller than 10.6 μm). By providing a smaller wavelength, the energy of the photons increases, thereby increasing the likelihood of breaking the hydrogen bonds in the water molecules.

[0069] By providing a wavelength of at least 1 μm, the level of absorption of IR radiation 51 by water molecules may be increased, as shown in FIG. 4 . By providing IR radiation 51 with higher absorption by water molecules, the power of the IR radiation may be reduced. By providing a wavelength of at least 1 μm, the photons have sufficiently low energy that there is less chance of undesired cracking of water molecules. Undesired cracking of water molecules can lead to undesired oxidation. One embodiment of the present invention is expected to reduce oxidation on surfaces of lithographic apparatus 100.

[0070] In one embodiment, the IR source 50 is configured to generate IR radiation 51 having a wavelength of about 2 μm to about 5 μm, optionally about 2.5 μm to about 3.5 μm. Such wavelengths have high absorption by water molecules, such that a low-power IR source 50 may be used. On the other hand, such wavelengths have higher energy than longer wavelengths, such that hydrogen bonds may be broken more easily and / or hydrogen bonds with higher energy may be broken.

[0071] In one embodiment, the IR source 50 is configured to generate IR radiation 51 having a wavelength such that the IR radiation 51 does not significantly heat the reflector 40. For example, the IR radiation 51 may have a wavelength that is not well absorbed by the material of the reflector 40. The IR radiation 51 may be absorbed to some extent by the reflector 40. However, in one embodiment, the heat associated with the IR radiation 51 does not travel deep within the reflector 40 and does not provide a significant heating effect.

[0072] In one embodiment, the reflective surface 41 has a ruthenium capping layer that is highly reflective of IR radiation 51. By using IR radiation, the extent to which the radiation heats the reflector 40 to promote molecular desorption may be reduced.

[0073] As shown in FIG. 3 , in one embodiment, the lithographic apparatus 100 includes a molecule catcher 232. The molecule catcher 232 is configured to catch molecules from the reflective surface 41. In one embodiment, the molecule catcher 232 is configured to catch water molecules from a surface different from the reflective surface 41. The molecule catcher 232 may catch water molecules that impinge on the molecule catcher 232, regardless of the origin of the water molecules. The molecule catcher 232 (which may alternatively be referred to as a getter) is configured to collect water molecules and retain (i.e., not release) these water molecules so that they cannot be adsorbed onto the mirror. The release of water molecules from the molecule catcher 232 may be performed without EUV radiation, for example, by heating the molecule catcher 232. The molecule catcher 232 is configured to reduce the likelihood that water molecules desorbed from the surface will be re-adsorbed onto the surface.

[0074] As described above, molecules desirably desorb from a surface, such as the reflective surface 41. The desorbed molecules may be expected to travel in a substantially straight line until they strike another surface. One possibility is that the molecules leave the vacuum environment, for example, by reaching an opening in a vacuum pump. Another possibility is that the molecules strike another surface of the lithographic apparatus 100 and remain in the vacuum environment. The molecules may desorb and adsorb multiple times before leaving the vacuum environment. The molecule catcher 232 is configured to catch and hold the molecules so that they may remain in the vacuum environment without oxidizing. Molecules adsorbed on the molecule catcher 232 may not pose oxidation problems.

[0075] In one embodiment, the molecule catcher 232 comprises a surface for catching molecules and a heat remover configured to remove heat from the surface. The surface may be a cold surface. In one embodiment, the molecule catcher 232 comprises a cryopump.

[0076] 3, in one embodiment, the molecular catcher 232 is positioned outside the EUV beam path 44. The molecular catcher 232 does not interfere with the passage of EUV radiation during use of the lithography apparatus 100. It is desirable for the molecular catcher 232 to cover a large solid angle from the reflective surface 41. By covering a larger solid angle, the likelihood that water molecules desorbed from the reflective surface 41 will hit the molecular catcher 232 is increased. In one embodiment, the molecular catcher 232 is provided to cover most of the total wall area.

[0077] As shown in Figure 3, in an embodiment, a molecular catcher 232 is arranged on an inner surface of the housing 230 of the illumination system IL. In an embodiment, a molecular catcher 222 is also arranged on an inner surface of the closure structure 220 of the source collector module SO. Molecules adsorbed on the molecular catcher are less likely to desorb. The partial pressure of the molecules is reduced. In an embodiment, one or more molecular catchers are provided on other parts of the illumination system IL and / or projection system PS.

[0078] In one embodiment, the lithographic apparatus 100 is maintained by removing the layer of molecules adsorbed on the molecular catcher. This may be done periodically or when it is determined that a significant accumulation of molecules has occurred on the molecular catcher. The molecular catcher may also be located at other positions on the lithographic apparatus 100. One or more molecular catchers may be provided near each reflective surface 41 in the beam path 44.

[0079] In one embodiment, the IR source 50 is configured to generate pulsed IR radiation 51. The heat associated with a pulse of radiation (compared to continuous radiation) may be expected to conduct more shallowly within the body. For example, the heat associated with a pulse of IR radiation 51 may conduct more shallowly within the reflector 40 upon which the radiation is incident. This may reduce undesired heating of the reflector 40. The heat conducted within the mirror depends on the radiation power and duration. Because the pulses have a short duration, they also heat the mirror less.

[0080] In one embodiment, the IR radiation 51 heats the reflective surface 41. Heating the reflective surface 41 may promote desorption of water molecules from the reflective surface 41. The residence time of the water may be reduced. Desirably, heating only the reflective surface 41 without significantly heating the bulk of the reflector 40 stimulates desorption without the drawback of having to return the mirror to a suitable temperature for use with EUV light. Heating the reflective surface 41 provides heat for the water molecules to overcome the binding energy that would keep them adsorbed. In one embodiment, the IR radiation 51 heats the reflective surface 41 to a depth of at most 100 nm, optionally at most 10 nm.

[0081] A relatively small amount of heat is transferred to the reflector 40 by the incident IR radiation 51. As a result, the amount of time required to cool the reflector 40 back down to its target temperature for operation in the exposure process is relatively short.

[0082] In one embodiment, the IR source 50 is configured such that the IR radiation is pulsed with a duty cycle of at most 50%, optionally at most 20%, optionally at most 10%, optionally at most 5%, and optionally at most 2%. A lower duty cycle correlates with a higher peak power for the total amount of energy delivered over time averaged.

[0083] In one embodiment, the IR source 50 is configured such that the pulses have a duration of at most 100 ns, optionally at most 10 ns, optionally at most 1 ns, optionally at most 100 ps, ​​optionally at most 10 ps, ​​optionally at most 1 ps, optionally at most 100 fs, optionally at most 10 fs. Shorter pulses may be expected to reduce the depth to which heat penetrates.

[0084] By using pulses of radiation compared to continuous radiation, the rate of desorption of water molecules for a given amount of energy is expected to increase. This may allow the energy of the IR radiation to be reduced without decreasing the rate of desorption, or may allow the rate of desorption to be increased without increasing the energy of the IR radiation. This is because pulses of IR radiation result in temporarily higher temperatures, especially at the incident surface, resulting in a greater rate of desorption. The increase in desorption rate more than compensates for the lower desorption rate in the time between pulses. This is because the desorption rate exhibits exponential behavior as a function of temperature.

[0085] In one embodiment, lithographic apparatus 100 includes at least one thermal conditioner. The thermal conditioner is configured to thermally condition at least one of the respective reflective surfaces 41 by conduction. For example, the thermal conditioner may include conditioning channels through the body. A conditioning fluid, such as water or glycol, may flow through the conditioning channels. The temperature of the body may be regulated by controlling the temperature of the conditioning fluid. The conditioning fluid may be used to cool reflective surface 41 to an operating temperature, i.e., the temperature at which reflective surface 41 should be during the exposure process.

[0086] In one embodiment, the thermal conditioner comprises a conditioning channel located at most 10 mm, optionally at most 5 mm, optionally at most 2 mm, and optionally at most 1 mm from the reflective surface 41. By locating the conditioning channel close to the surface to be conditioned, the temperature of the surface can be controlled more precisely and with lower overall heat transfer.

[0087] In one embodiment, a thermal conditioner is used to heat the reflective surface 41. The reflective surface 41 may be heated prior to the process of reducing the pressure in the vacuum environment. Heating may promote desorption of molecules. The thermal conditioner may then be used to cool the reflective surface 41 back down to the operating temperature. In one embodiment, the cooling process comprises using a conditioning fluid at a first temperature, followed by using a conditioning fluid at a second temperature. The second temperature may be higher than the first temperature. For example, the method may comprise a first step of heating a surface portion of the mirror, followed by a second step of cooling to the operating temperature using a temperature lower than the operating temperature, followed by a third step of temperature conditioning using a cooling water temperature at (or slightly lower than) the operating temperature. Using a lower temperature in the second step accelerates the cooling.

[0088] Alternatively, the thermal conditioner may not be used to heat the reflective surface 41. The thermal conditioner may be used to cool the reflective surface 41 to the extent that the reflective surface 41 is heated by the application of IR radiation 51.

[0089] In one embodiment, the heat input during the heating step is substantially equal to the heat extracted during cooling.

[0090] 3, in one embodiment, an IR source 50 is provided such that IR radiation 51 is coupled into beam path 44 down beam of source collector module SO. The source collector module SO may be of a standard type. An embodiment of the present invention is expected to reduce the cost of manufacturing lithographic apparatus 100, allowing IR radiation 51 to follow beam path 44.

[0091] As shown in Figure 3, in one embodiment, an IR source 50 is located outside the vacuum environment through which beam path 44 extends. As shown in Figure 3, in one embodiment, IR radiation 51 enters via an illumination system IL. IR radiation 51 couples into beam path 44 within the illumination system IL.

[0092] 3, in one embodiment, the lithographic apparatus 100 includes a window 55. The window 55 is configured to transmit IR radiation 51. The window 55 is configured to maintain a vacuum in the vacuum environment. The window 55 is substantially gas-tight. The window 55 may be provided with an openable hatch, for example, to allow an engineer to service the illumination system IL.

[0093] 3, in one embodiment, IR source 50 is configured to generate a collimated beam of IR radiation 51. IR source 50 may be a laser. As shown in FIG. 3, in one embodiment, lithographic apparatus 100 includes a focusing lens 52 configured to focus IR radiation 51 to a focal point 53.

[0094] 3, in one embodiment, lithographic apparatus 100 includes an optical joiner 54 (which may also be referred to as a mirror or reflector). Optical joiner 54 is configured to couple IR radiation 51 into beam path 44 in illumination system IL. Optical joiner 54 allows IR source 50 to be positioned away from beam path 44. This reduces the complexity of lithographic apparatus 100.

[0095] As shown in FIG. 3 , in one embodiment, the optical joiner 54 includes an IR mirror. The IR mirror may be movable between a retracted position outside the beam path 44 and a coupling position in the beam path 44. FIG. 3 shows the IR mirror in the coupling position (i.e., in the beam path 44). When the IR mirror is in the coupling position, the IR mirror couples IR radiation 51 into the beam path 44. When IR radiation 51 is not needed (e.g., during an exposure process), the IR mirror may be driven to the retracted position. In the retracted position, the IR mirror does not couple IR radiation into the beam path 44. The movement of the IR mirror is indicated in FIG. 3 by a double-headed arrow. In one embodiment, the IR mirror is a 45° flat mirror. The IR mirror may be angled 45° with respect to the optical axis O. It is not necessary for the IR mirror to be tilted at 45°. In another embodiment, the IR mirror is tilted at another angle. The angle may be selected to facilitate positioning of the IR source 50 and / or the focusing lens 52.

[0096] In one embodiment, focusing lens 52 is configured to position focal point 53 at a distance from optical axis O equal to the distance from a virtual EUV source point IF to the IR mirror. In one embodiment, the focal length of focusing lens 52 is selected so that substantially the full numerical aperture of illumination system IL receives IR radiation 51. This is shown in FIG. 3 , where IR radiation 51 fills the cone of beam path 44. This helps ensure that IR radiation 51 reaches surfaces where molecules may be decomposed by the EUV radiation during the exposure process.

[0097] Figure 5 schematically shows another arrangement for IR radiation 51 to follow beam path 44 in lithographic apparatus 100. As shown in Figure 5, in one embodiment, an IR source 50 is provided such that IR radiation 51 is coupled into beam path 44 in source collector module SO. This may allow IR radiation 51 to reach one or more reflective surfaces in source collector module SO. This may allow a drive laser for source collector module SO to be used as IR source 50.

[0098] In one embodiment, the source collector module SO is switchable between an EUV mode, in which the drive radiation drives the generation of EUV radiation, and a dry mode for projecting IR radiation 51 along the beam path.

[0099] 5, in one embodiment, the source collector module SO comprises a beam stop 43 movable between an EUV position (e.g., as shown in FIG. 3) to block drive radiation in the beam path 44 and a dry position (e.g., as shown in FIG. 5) that allows IR radiation 51 to follow the beam path 44. The beam stop 43 can be retracted.

[0100] As shown in Figure 5, in one embodiment, lithographic apparatus 100 includes a primary lens 56 configured to focus IR radiation onto a focal plane 53. As shown in Figure 5, the IR beam diverges after focal plane 53. A focusing lens 52 is positioned in the down beam of focal plane 53. In one embodiment, focusing lens 52 has a focal length such that IR radiation 51 is focused to a virtual source point IF. This increases the degree to which IR radiation 51 resembles EUV radiation, following beam path 44.

[0101] In one embodiment, the source collector module SO includes a focusing lens 52. In one embodiment, the focusing lens 52 is movable between an EUV position outside the beam path 44 and a dry position for directing the IR radiation 51 along the beam path 44. The focusing lens 52 can be retracted, for example, by a lens retractor 46. The focusing lens 52 is out of the EUV optical path when exposing the substrate W. In one embodiment, a screen is provided to protect the focusing lens 52 from being damaged by the EUV-induced plasma. For example, the focusing lens 52 may be located in a closed storage compartment.

[0102] Other features shown in FIG. 5 may be as described above with respect to FIG.

[0103] Figure 6 shows schematically another arrangement for IR radiation 51 to follow beam path 44 in lithographic apparatus 100. As shown in Figure 6, the IR source 50 may be separate from the drive laser for source collector module SO. This makes it easier to use photons with greater energy than the drive radiation.

[0104] As shown in Figure 6, in one embodiment, IR radiation 51 is coupled into the system via an additional mirror 57. This mirror 57 may be a simple flat mirror, as shown in Figure 6. In one embodiment, mirror 57 may be retractable.

[0105] Other features shown in FIG. 6 may be as described above with respect to FIG. 3 or FIG.

[0106] As mentioned above, the vacuum environment may be vented over time during the lifetime of lithographic apparatus 100. After venting, a vacuum pump may be utilized to reduce the pressure again to the required vacuum before lithographic apparatus 100 is used for another exposure process. In one embodiment, the vacuum environment is vented with a vent gas.

[0107] 7 shows a schematic of a vent gas generator 60. In one embodiment, the vent gas generator 60 is for generating a vent gas for venting the vacuum environment of the lithographic apparatus 100. The vacuum environment may include a beam path of the EUV radiation. For example, a reflective surface for directing the EUV radiation may be within the vacuum environment of the lithographic apparatus 100.

[0108] In one embodiment, the vent gas generator 60 is configured to output a vent gas for venting the vacuum environment. For example, the vent gas generator may be configured to be in fluid communication with the vacuum environment when it is desired to vent the vacuum environment. At other times, the vacuum environment may be substantially sealed to prevent the vent gas from entering the vacuum environment. In one embodiment, a vacuum valve is between the vent gas generator 60 and the vacuum environment of the lithographic apparatus 100. The vacuum valve is controllable to switch between a state in which the vent gas generator 60 is in fluid communication with the vacuum environment and a state in which the vacuum environment is substantially sealed to prevent the vent gas from entering the vacuum environment.

[0109] 7, in one embodiment, vent gas generator 60 comprises gas dryer 61. In one embodiment, gas dryer 61 is configured to receive input gas 67. In one embodiment, input gas 67 comprises air. In one embodiment, input gas 67 may be relatively particle-free air, for example, of the type used for clean rooms.

[0110] In one embodiment, the gas dryer 61 is configured to output a dry gas 68, such as dried XCDA, nitrogen, or hydrogen. The dry gas 68 may be a purified gas having a relatively low density of impurities. The dry gas may also be referred to as a clean gas. In one embodiment, the dry gas has less than one water molecule per billion molecules of the dry gas. In one embodiment, the dry gas has less than one molecule per billion molecules of one or more of sulfur, refractory compounds, amines, carboxylic acids, ammonia, NOx, siloxanes, phthalates, and total organic carbon (C7+).

[0111] In one embodiment, the gas dryer 61 is configured to process the input gas 67 to produce a dry gas 68. The gas dryer 61 may be a gas purifier configured to remove impurities from the input gas 67.

[0112] As shown in FIG. 7 , in one embodiment, the vent gas generator 60 includes a molecular catcher 232. The molecular catcher 232 is configured to receive the dry gas 68. For example, in one embodiment, the molecular catcher 232 is in fluid communication with the gas dryer 61. The molecular catcher 232 may be located at or near the outlet of the gas dryer 61. The molecular catcher 232 is configured to catch water molecules from the dry gas 68. In one embodiment, the molecular catcher 232 is configured to output a vent gas 69. The vent gas 69 is generated from the dry gas 68 from which the water molecules have been caught. The molecular catcher 232 is configured to output a vent gas 69 that is free of water molecules. Of course, some water molecules may remain in the vent gas 69 output by the molecular catcher 232. However, the number of water molecules relative to the number of gas molecules provided in the vent gas 69 is smaller than that of the dry gas 68. The vent gas may be used to vent a vacuum environment.

[0113] In one embodiment, the molecular catcher 232 comprises a surface configured to catch water molecules. The molecular catcher 232 may have a low-temperature surface. Temperature affects the adsorption behavior of the surface. To reach a sufficiently low temperature, a heat remover may be provided. In one embodiment, the molecular catcher 232 comprises a heat remover 64 configured to remove heat from the surface. In one embodiment, the surface that catches water molecules comprises an inner surface of the molecular catcher 232. Removing heat from the surface may increase the likelihood that water molecules will adsorb onto the surface. The molecular catcher 232 may be referred to as a cold trap.

[0114] By capturing water molecules from the dry gas 68 before the vacuum environment is vented, the partial pressure of water molecules entering the vacuum environment may be reduced. By reducing the entry of water molecules into the vacuum environment, the time it takes to reduce the partial pressure of water in the vacuum environment to a target level (e.g., for an exposure operation) may be reduced. An embodiment of the present invention is expected to increase the throughput of the lithographic apparatus 100. An embodiment of the present invention is expected to achieve higher productivity because less time is spent reducing the pressure of the vacuum environment.

[0115] As shown in FIG. 7 , in one embodiment, the molecular catcher 232 comprises a maze-like path. The molecular catcher 232 is configured to direct the dry gas 68 along the maze-like path 63 before being output from the molecular catcher 232. The surface configured to catch water molecules may be an internal surface defining the maze-like path 63. The molecular catcher 232 may be configured such that the surface is maze-like. By providing a maze-like (or winding) path, the surface area that the dry gas 68 can contact may be increased. Increasing the internal surface area may increase the likelihood that molecules of the dry gas 68 interact with the surface before exiting the molecular catcher 232. This may increase the proportion of water molecules that are captured (adsorbed) on the walls of the molecular catcher 232.

[0116] As shown in FIG. 7 , in one embodiment, a heat remover 64 is coupled to the molecular catcher 232. For example, the vent gas generator 60 may include a thermal connection 65 configured to thermally connect the molecular catcher 232 to the heat remover 64. In one embodiment, the heat remover 64 includes a cooling device. The heat remover 64 is configured to function as a heat sink. In one embodiment, the heat remover 64 is configured to maintain a surface at a temperature of at most 200 K, optionally at most 150 K, optionally at most 120 K, optionally at most 100 K, and optionally at most 80 K. For example, liquid nitrogen may be used as a cooling fluid for the heat remover 64. The liquid nitrogen may have a temperature of 77 K. The heat remover 64 is configured to keep the walls, i.e., the interior surfaces, of the molecular catcher 232 cool.

[0117] 7, in one embodiment, the molecular catcher 232 includes insulation 62. The insulation 62 may be configured to thermally isolate the molecular catcher 232 from the external environment, which may allow the molecular catcher 232 to operate at atmospheric pressure.

[0118] In one embodiment, the insulation 62 comprises a thermally insulating material. The insulation 62 may cover the molecular catcher 232. In an alternative embodiment, the insulation 62 may take the form of a vacuum shield around the molecular catcher 232. By providing the insulation 62, the power required to maintain the surface of the molecular catcher 232 at a sufficiently cool level may be reduced. By providing the insulation 62, ice formation on the outside of the molecular catcher 232 may be reduced or prevented.

[0119] 7, in one embodiment, the vent gas generator 60 includes a heater 66. The heater 66 is configured to heat the vent gas 69 from the molecular catcher 232 and output heated vent gas 70. The heated vent gas 70 is for venting the vacuum environment. The vent gas 69 output from the molecular catcher 232 may be cold. The cold vent gas 69 may be heated to a target temperature before being supplied to the vacuum environment of the lithographic apparatus 100. For example, the target temperature may be room temperature.

[0120] In one embodiment, the molecular catcher 232 is configured to have a surface at a temperature below 90.2 K. By using a molecular catcher 232 at a temperature below 90.2 K, the condensation of oxygen may be promoted, which helps reduce the oxygen content of the gas due to condensation.

[0121] In one embodiment, the vent gas 69 is 2.5x10 13 The molecular catcher 232 is configured to reduce the water content of the dry gas 68 so that it has fewer than one water molecule for every other molecule. -11 The present invention provides a water partial pressure below 100 mbar. By providing such a low water partial pressure, the vent gas used to vent the vacuum environment may be expected to result in less than one monolayer of water molecules on surfaces within the vacuum environment. By reducing the formation of a monolayer of water molecules in the vacuum environment, the time it takes to reduce the water partial pressure to the target partial pressure may be reduced. The likelihood of EUV radiation cracking water molecules, leading to oxidizing oxygen molecules, may be reduced.

[0122] In one embodiment, the molecular catcher 232 may be regenerated, for example, at regular intervals. By regenerating the molecular catcher 232, the molecular catcher 232 may be maintained. Regeneration of the molecular catcher 232 may comprise removing adsorbed water molecules from the surface of the molecular catcher 232. For example, the molecular catcher 232 may be heated to promote evaporation of the water molecules from the surface. Between regeneration intervals of the molecular catcher 232, the water molecules are expected to remain adsorbed on the surface of the molecular catcher 232. While the molecules remain adsorbed on the surface of the molecular catcher 232, the water molecules are prevented from causing problems in a vacuum environment.

[0123] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, including the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0124] Where the context allows, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present 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, acoustic, or other forms of transmission signals (e.g., carrier waves, infrared signals, digital signals, etc.), and the like. Furthermore, firmware, software, routines, and instructions may be described as performing particular actions. However, it should be understood that such description is merely for convenience and that such actions may actually be brought about by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., causing actuators or other devices to interact with the physical world.

[0125] 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 also be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may be generally referred to as lithography tools.

[0126] 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 understood that the invention is not limited to optical lithography, where the context permits.

[0127] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced otherwise than as described. For example, in an alternative embodiment, IR source 50 may be arranged to apply IR radiation to a surface of lithographic apparatus 100 rather than to reflective surface 41 in beam path 44. For example, it may be desirable to remove water molecules from the walls of lithographic apparatus 100.

[0128] The foregoing description is intended to be illustrative and 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 that follow.

[0129] The present invention can also be described by the following items.

[0130] Item 1: 1. A lithographic apparatus configured to project EUV radiation onto a substrate location, the apparatus comprising: an illumination system comprising a plurality of reflective surfaces for directing the EUV radiation along a beam path; an IR source configured to generate IR radiation; Equipped with the IR source and the illumination system are arranged such that the IR radiation is projected along the beam path; Lithography equipment.

[0131] Item 2: Item 2. The lithographic apparatus of item 1, wherein the IR source is configured to generate IR radiation having a wavelength in the range of about 1 μm to about 11 μm, optionally about 2 μm to about 5 μm, optionally about 2.5 μm to about 3.5 μm.

[0132] Item 3: Item 3. A lithographic apparatus according to item 1 or 2, comprising a molecule catcher configured to catch molecules from the reflective surface.

[0133] Item 4: Item 4. The lithographic apparatus of item 3, wherein the molecule catcher comprises a surface for catching the molecule and a heat remover configured to remove heat from the surface.

[0134] Item 5: 5. The lithographic apparatus of any of items 1 to 4, wherein the IR source is configured to generate pulsed IR radiation.

[0135] Item 6: Item 6. The lithographic apparatus of item 5, wherein the IR source is configured to generate pulsed IR radiation having a duty cycle of at most 50%, optionally at most 20%, optionally at most 10%, optionally at most 5%, optionally at most 2%.

[0136] Item 7: 7. The lithographic apparatus according to item 5 or 6, wherein the pulses have a duration of at most 100 ns, optionally at most 10 ns, optionally at most 1 ns, optionally at most 100 ps, ​​optionally at most 10 ps, ​​optionally at most 1 ps, optionally at most 100 fs, optionally at most 10 fs.

[0137] Item 8: Item 8. A lithographic apparatus according to any of items 1 to 7, comprising at least one thermal conditioner configured to thermally condition at least one of the reflective surfaces, respectively, by conduction.

[0138] Item 9: Item 9. The lithographic apparatus of item 8, wherein the thermal conditioner comprises a conditioning channel located at most 10 mm, optionally at most 5 mm, optionally at most 2 mm, optionally at most 1 mm from the reflective surface.

[0139] Item 10: 10. The lithographic apparatus of item 8 or 9, wherein the thermal conditioner is configured to heat the reflective surface so as to stimulate molecules on the reflective surface for desorption, and subsequently cool the reflective surface to an operating temperature.

[0140] Item 11: Item 11. A lithographic apparatus according to any of items 1 to 10, comprising a source collector module configured to generate the EUV radiation, the beam path extending from the source collector module.

[0141] Item 12: Item 12. The lithographic apparatus of item 11, wherein the IR source is arranged to couple the IR radiation into the beam path downbeam of the source collector module.

[0142] Item 13: Item 13. A lithographic apparatus according to item 11 or 12, wherein the IR source is located outside a vacuum environment through which the beam path extends.

[0143] Item 14: Item 14. A lithographic apparatus according to any of items 11 to 13, comprising an optical joiner configured to couple the IR radiation into the beam path of the illumination system.

[0144] Item 15: Item 15. The lithographic apparatus of item 14, wherein the optical joiner comprises an IR mirror movable between a retracted position outside the beam path and a coupling position in the beam path.

[0145] Item 16: Item 12. The lithographic apparatus of item 11, wherein the IR source is arranged to couple the IR radiation into the beam path in the source collector module.

[0146] Item 17: Item 17. The lithographic apparatus of item 16, wherein the source collector module is switchable between an EUV mode in which drive radiation drives the generation of the EUV radiation and a dry mode in which the IR radiation is projected along the beam path.

[0147] Item 18: Item 18. The lithographic apparatus of item 17, wherein the source collector module comprises a beam stop movable between an EUV position for blocking the drive radiation in the beam path and a dry position for allowing the IR radiation to follow the beam path.

[0148] Item 19: Item 19. The lithographic apparatus of item 17 or 18, wherein the source collector module comprises a focusing lens movable between an EUV position outside the beam path and a dry position for directing the IR radiation along the beam path.

[0149] Item 20: 20. A lithographic apparatus according to any of items 17 to 19, wherein the IR source is configured to generate the drive radiation.

[0150] Item 21: 20. A lithographic apparatus according to any of items 17 to 19, comprising a drive source separate from the IR source, configured to generate the drive radiation.

[0151] Item 22: a vacuum environment containing the beam path; a vent gas generator for generating a vent gas for venting the vacuum environment, a gas dryer configured to receive an input gas and output a dry gas having less than one water molecule per billion molecules of gas; a molecular catcher configured to receive the dry gas, catch water molecules from the dry gas, and output vent gas free of the caught water molecules; a vent gas generator comprising: 22. A lithographic apparatus according to any of items 1 to 21, comprising:

[0152] Item 23: Item 23. The lithographic apparatus of item 22, wherein the molecule catcher comprises a surface configured to catch water molecules and a heat remover configured to remove heat from the surface.

[0153] Item 24: Item 24. A lithographic apparatus according to item 23, wherein the molecular catcher is arranged such that the surface is labyrinthine.

[0154] Item 25: 25. The lithographic apparatus of any of items 22 to 24, wherein the vent gas generator comprises a heater configured to heat the vent gas from the molecule catcher and output the heated vent gas to vent the vacuum environment.

[0155] Item 26: 1. A method for removing molecules from a reflective surface of a lithographic apparatus configured to project EUV radiation along a beam path to a substrate position, the method comprising: the reflective surface is for directing the EUV radiation along the beam path; generating IR radiation; projecting the IR radiation along the beam path; A method for providing the above.

[0156] Item 27: 27. The method of claim 26, wherein the radiation has a wavelength in the range of about 1 μm to about 11 μm, optionally about 2 μm to about 5 μm, optionally about 2.5 μm to about 3.5 μm.

[0157] Item 28: 28. The method of claim 26 or 27, comprising catching molecules from the reflective surface with a molecule catcher.

[0158] Item 29: 29. The method of claim 28, comprising removing heat from the surface of the molecular catcher.

[0159] Item 30: 30. The method of any of items 26 to 29, wherein the IR radiation is pulsed.

[0160] Item 31: 31. The method of claim 30, wherein the IR radiation is pulsed with a duty cycle of at most 50%, optionally at most 20%, optionally at most 10%, optionally at most 5%, optionally at most 2%.

[0161] Item 32: 32. The method of claim 30 or 31, wherein the pulses have a duration of at most 100 ns, optionally at most 10 ns, optionally at most 1 ns, optionally at most 100 ps, ​​optionally at most 10 ps, ​​optionally at most 1 ps, optionally at most 100 fs, optionally at most 10 fs.

[0162] Item 33: 33. The method of any of items 26 to 32, comprising thermally conditioning at least one of the reflective surfaces by conduction.

[0163] Item 34: 34. The method of claim 33, wherein the thermal conditioning is by a conditioning channel located at most 10 mm, optionally at most 5 mm, optionally at most 2 mm, optionally at most 1 mm from the reflective surface.

[0164] Item 35: 35. The method of claim 33 or 34, wherein the thermal conditioning comprises heating the reflective surface to stimulate molecules on the reflective surface for desorption, followed by cooling the reflective surface to an operating temperature.

[0165] Item 36: 36. The method of claim 35, wherein the heat input during the heating step is substantially equal to the amount of heat extracted during the cooling step.

[0166] Item 37: 37. The method of any of items 26 to 36, comprising generating the EUV radiation in a source collector module, the beam path extending from the source collector module.

[0167] Item 38: Item 38. The method of item 37, wherein the IR radiation is coupled into the beam path downbeam of the source collector module.

[0168] Item 39: 39. The method according to item 37 or 38, wherein the IR radiation is generated outside a vacuum environment through which the beam path extends.

[0169] Item 40: 40. The method of claim 38 or 39, comprising driving the IR mirror between a retracted position outside the beam path and a coupling position in the beam path, in which the IR mirror is provided for coupling the IR radiation into the beam path.

[0170] Item 41: Item 38. The method of item 37, wherein the IR radiation is coupled into the beam path within the source collector module.

[0171] Item 42: Item 42. The method of item 41, comprising switching the source collector module between an EUV mode in which drive radiation drives the generation of the EUV radiation and a dry mode to project the IR radiation along the beam path.

[0172] Item 43: Item 43. The method of item 42, comprising driving a beam stop of the source collector module between an EUV position to block the drive radiation in the beam path and a dry position to allow the IR radiation to follow the beam path.

[0173] Item 44: Item 44. The method of item 42 or 43, comprising driving a focusing lens of the source collector module between an EUV position outside the beam path and a dry position for directing the IR radiation along the beam path.

[0174] Item 45: 45. The method of any of items 26 to 44, comprising venting a vacuum environment containing the beam path before projecting the IR radiation along the beam path.

[0175] Item 46: treating an input gas to output a dry gas having less than one water molecule per billion molecules of gas; capturing water molecules from the dry gas to output vent gas free of the captured water molecules; generating a vent gas for venting the vacuum environment by Item 46. The method according to item 45.

[0176] Item 47: Item 47. The method according to item 46, wherein the water molecules are caught by a molecular catcher from which heat is removed.

[0177] Item 48: Item 48. The method of item 47, wherein the dry air is directed through the surface in a labyrinth shape while the water molecules are captured.

[0178] Item 49: Item 49. The method of any of items 46 to 48, comprising heating the vent gas to output heated vent gas for venting the vacuum environment.

[0179] Item 50: 1. A vent gas generator for generating a vent gas for venting a vacuum environment, comprising: a gas dryer configured to receive an input gas and output a dry gas having less than one water molecule per billion molecules of gas; a molecular catcher configured to receive the dry gas, catch water molecules from the dry gas, and output vent gas free of the caught water molecules; A vent gas generator comprising:

[0180] Item 51: Item 51. The gas modifier of item 50, wherein the molecular catcher comprises a surface configured to catch water molecules and a heat remover configured to remove heat from the surface.

[0181] Item 52: Item 52. The gas modifier of item 51, wherein the molecular catcher is provided so that the surface is labyrinth-shaped.

[0182] Item 53: 53. The gas modifier of any of items 50 to 52, comprising a heater configured to heat the vent gas from the molecular catcher and output heated vent gas.

[0183] Item 54: 1. A method for generating a vent gas for venting a vacuum environment, comprising: treating an input gas to output a dry gas having less than one water molecule per billion molecules of gas; capturing water molecules from the dry gas to output vent gas free of the captured water molecules; A method for providing the above.

[0184] Item 55: 55. The method of claim 54, wherein the water molecules are caught by a molecular catcher from which heat is removed.

[0185] Item 56: Item 56. The method of item 55, wherein the dry air is directed through the surface in a labyrinth while the water molecules are captured.

[0186] Item 57: 57. The method of any of items 54 to 56, comprising heating the vent gas to output heated vent gas.

Claims

1. 1. A lithographic apparatus configured to project EUV radiation onto a substrate location, the apparatus comprising: an illumination system comprising a plurality of reflective surfaces for directing the EUV radiation along a beam path; an IR source configured to generate IR radiation; Equipped with the IR source and the illumination system are arranged such that the IR radiation is projected along the beam path; Lithography equipment.

2. 10. The lithographic apparatus of claim 1, wherein the IR source is configured to generate IR radiation having a wavelength in the range of about 1 μm to about 11 μm, optionally about 2 μm to about 5 μm, optionally about 2.5 μm to about 3.5 μm.

3. A lithographic apparatus according to claim 1 or 2, comprising a molecule catcher configured to catch molecules from the reflective surface.

4. The lithographic apparatus of claim 3 , wherein the molecule catcher comprises a surface for catching the molecule and a heat remover configured to remove heat from the surface.

5. The lithographic apparatus of claim 1 , wherein the IR source is configured to generate pulsed IR radiation.

6. A lithographic apparatus according to any one of the preceding claims, comprising at least one thermal conditioner configured to thermally condition at least one of the reflective surfaces, respectively, by conduction.

7. The lithographic apparatus of claim 6 , wherein the thermal conditioner is configured to heat the reflective surface so as to stimulate molecules on the reflective surface for desorption, and subsequently cool the reflective surface to an operating temperature.

8. 8. A lithographic apparatus according to any preceding claim, comprising a source collector module configured to generate the EUV radiation, the beam path extending from the source collector module.

9. a vacuum environment containing the beam path; a vent gas generator for generating a vent gas for venting the vacuum environment, a gas dryer configured to receive an input gas and output a dry gas having less than one water molecule per billion molecules of gas; a molecular catcher configured to receive the dry gas, catch water molecules from the dry gas, and output vent gas free of the caught water molecules; a vent gas generator comprising: A lithographic apparatus according to any one of claims 1 to 8, comprising:

10. 10. A lithographic apparatus according to claim 9, wherein the molecule catcher comprises a surface configured to catch water molecules and a heat remover configured to remove heat from the surface.

11. The lithographic apparatus of claim 10 , wherein the molecular catcher is provided such that the surface is labyrinth-shaped.

12. 1. A method for removing molecules from a reflective surface of a lithographic apparatus configured to project EUV radiation along a beam path to a substrate location, the method comprising: the reflective surface is for directing the EUV radiation along the beam path; generating IR radiation; projecting the IR radiation along the beam path; A method for providing the above.

13. The method of claim 12, wherein the radiation has a wavelength in the range of about 1 μm to about 11 μm, optionally about 2 μm to about 5 μm, optionally about 2.5 μm to about 3.5 μm.

14. 14. The method of claim 12 or 13, comprising catching molecules from the reflective surface with a molecule catcher.

15. 15. The method of claim 14, comprising removing heat from a surface of the molecular catcher.

16. 16. The method of any of claims 12 to 15, wherein the IR radiation is pulsed.

17. 17. The method of any of claims 12 to 16, comprising thermally conditioning at least one of the reflective surfaces by conduction.

18. 20. The method of claim 17, wherein the thermally conditioning comprises heating the reflective surface to stimulate molecules on the reflective surface for desorption, followed by cooling the reflective surface to an operating temperature.

19. 1. A vent gas generator for generating a vent gas for venting a vacuum environment, comprising: a gas dryer configured to receive an input gas and output a dry gas having less than one water molecule per billion molecules of gas; a molecular catcher configured to receive the dry gas, catch water molecules from the dry gas, and output vent gas free of the caught water molecules; A vent gas generator comprising:

20. 1. A method for generating a vent gas for venting a vacuum environment, comprising: treating an input gas to output a dry gas having less than one water molecule per billion molecules of gas; capturing water molecules from the dry gas to output vent gas free of the captured water molecules; A method for providing the above.

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