Counterflow gas nozzle for contamination mitigation in extreme ultraviolet inspection systems.

Abstract

The present invention relates to a system and method for reducing and reducing contamination of one or more elements of a coating inspection system. Specifically, a counterflow of purge gas through a counterflow nozzle can be used to reduce the presence of contaminants in one or more parts of the inspection system. The system can include a source chamber, one or more vacuum chambers, an intermediate focusing box having an aperture, an illumination source designed to generate and direct illumination through the aperture in an illumination direction, and a counterflow nozzle designed to direct a counterflow of purge gas into the source chamber in a direction opposite to the illumination direction.

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates generally to the field of semiconductor inspection, and more particularly to an extreme ultraviolet based inspection system with contamination mitigation. [Background technology]

[0002] As the demand for lithography-based device structures having ever-shrinking features continues to increase, so does the need for improved illumination sources for use in the lithography and inspection of the associated reticles that lithographically print these ever-shrinking devices. Particular such illumination sources utilized in lithography and inspection systems are extreme ultraviolet (EUV) light, vacuum ultraviolet (VUV) light, deep ultraviolet (DUV) light, or ultraviolet (UV) light-based inspection systems.

[0003] Inspection systems, particularly their optical systems, often require operation in a clean vacuum environment. However, contaminants that tend to contaminate the vacuum environment cannot be completely removed from the system. This is true, for example, when components of the inspection system, such as certain optical surfaces, contain contaminants. As a result, the inspection system's optical system (often located within a vacuum chamber) is exposed to partial pressures of contaminants such as hydrocarbons and gas-phase HO. These contaminants, when exposed to radiation within the system (e.g., as may be the case during an illumination process), result in the growth of carbon and / or oxide optical surfaces of the system, such as mirrors. In the case of mirrors, contamination causes a decrease in reflectivity and a phase shift in the light incident on a given mirror. Both of these effects, if unchecked, can cause degradation of optical components over time, leading to failure of the optical system. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 8,711,346 Summary of the Invention [Problem to be solved by the invention]

[0005] It would therefore be desirable to provide a method and system that cures the above-identified deficiencies of the prior art. [Means for solving the problem]

[0006] An optical system is disclosed according to one or more embodiments of the present disclosure. In one embodiment, the optical system includes a source chamber (a chamber of an illumination source). In another embodiment, the optical system includes one or more vacuum chambers containing a set of imaging optics. In another embodiment, the optical system includes an intermediate focus housing having an aperture, the intermediate focus housing coupled to the source chamber and the one or more vacuum chambers. In another embodiment, the optical system includes an illumination source configured to generate illumination and direct the illumination through one or more portions of the source chamber. In another embodiment, the optical system includes a set of illumination optics configured to receive at least a portion of the illumination from the illumination source and direct at least a portion of the illumination in an illumination direction to the sample via an illumination axis, the illumination axis configured such that at least a portion of the illumination is transmitted through the aperture. In another embodiment, the optical system includes a purge gas inlet coupled to the intermediate focus housing, the purge gas inlet configured to deliver purge gas to one or more purge gas ducts. In another embodiment, the optical system includes a counterflow nozzle coupled to the intermediate focus housing and at least partially disposed within the source chamber, the counterflow nozzle configured to receive a purge gas and direct a counterflow of the purge gas into the source chamber in a direction opposite to the illumination direction through one or more purge gas ducts, the counterflow of the purge gas configured to mitigate contamination within the one or more vacuum chambers by reducing permeation of one or more volatile species of contaminants into the one or more vacuum chambers.

[0007] An apparatus according to one or more embodiments of the present disclosure is disclosed. In one embodiment, the apparatus includes an intermediate focus housing coupled to each of the source chamber and one or more vacuum chambers, the intermediate focus housing including an opening. In another embodiment, the apparatus includes a purge gas inlet coupled to the intermediate focus housing, the purge gas inlet configured to deliver purge gas to one or more purge gas ducts. In another embodiment, the apparatus includes a counterflow nozzle coupled to the intermediate focus housing and disposed at least partially within the source chamber, the counterflow nozzle configured to receive purge gas and direct a counterflow of the purge gas into the source chamber in a direction opposite to the illumination direction through the one or more purge gas ducts. The counterflow of the purge gas is configured to reduce contamination in the one or more vacuum chambers by reducing permeation of one or more volatile species of contaminants into the one or more vacuum chambers.

[0008] According to one or more embodiments of the present disclosure, a method for reducing contamination of an optical system is disclosed. In one embodiment, the method includes generating illumination and directing at least a portion of the illumination toward one or more portions of a source chamber in the direction of the illumination. In another embodiment, the method includes directing at least a portion of the illumination through an opening in an intermediate focus housing, the intermediate focus housing being disposed between the source chamber and one or more vacuum chambers. In another embodiment, the method includes supplying a purge gas to a counter-flow nozzle coupled to the intermediate focus housing and disposed at least partially within the source chamber. In another embodiment, the method includes generating a counter-flow of the purge gas through the counter-flow nozzle.

[0009] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. [Brief explanation of the drawings]

[0010] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying drawings. [Figure 1A] FIG. 1A illustrates a conceptual diagram of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1B illustrates a block diagram of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 1C] FIG. 1C illustrates a conceptual diagram of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 2A depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 2B depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 2C] FIG. 2C depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 3A] FIG. 3A depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 3B] FIG. 3B depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 3C] FIG. 3C depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 4A] FIG. 4A depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 4B] FIG. 4B depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 4C] FIG. 4C depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 5A] FIG. 5A depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 5B] FIG. 5B depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 5C] FIG. 5C depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 6A] FIG. 6A depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 6B] FIG. 6B depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 6C] FIG. 6C depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 7A] FIG. 7A depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 7B] FIG. 7B depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 7C] FIG. 7C depicts a counter-flow nozzle of an optical system in accordance with one or more embodiments of the present disclosure. [Figure 8] FIG. 8 is a process flow diagram illustrating steps in a method for reducing contamination in an optical system in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present disclosure has been particularly shown and described with respect to certain embodiments and particular features thereof. The embodiments described herein are to be construed as illustrative and not restrictive, and it will be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made therein without departing from the spirit and scope of the present disclosure. Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings.

[0012] Embodiments of the present disclosure are directed to systems and methods for cleaning optical surfaces in an overlay inspection system. In particular, systems and methods are disclosed for optical path coupling of light in a projection imaging system for in-situ chemical cleaning thereof.

[0013] It is recognized herein that contamination of optical surfaces in an inspection system can occur when volatile organic contaminant species adsorb to the optical surface such that the volatile organic contaminant species can interact with energetic radiation (e.g., EUV or VUV light) to produce carbonaceous buildup in areas exposed to the energetic radiation. The presence of even a small amount of buildup (e.g., buildup having a height of only a few nanometers) can cause significant degradation of the performance of the optical elements of the inspection system.

[0014] Contaminant buildup on optical surfaces of an inspection system can be removed by one or more photochemical reactions. For example, deposited carbonaceous buildup can be removed through one or more reactions with ionic or free radical species (e.g., oxygen (1D) and other excited states of oxygen, as well as excited states of nitrogen, fluorine, etc.). Ionic or free radical species can be generated when a precursor gas (e.g., ozone) reacts with photons of sufficient energy (e.g., EUV, VUV, DUV, or UV photons). Thus, irradiation of an optical surface with sufficient energy illumination in the presence of a precursor gas can cause removal of contaminants that have accumulated on the optical surface.

[0015] Alternatively, contamination within the inspection system may be removed or manipulated from optical elements to reduce the need for decontamination. For example, some techniques for reducing contamination within optical systems involve the use of optical windows to prevent the transmission of contaminants within the optical system. However, optical windows are not suitable for use in certain types of optical systems, including, but not limited to, EUV systems, due to the high likelihood of EUV absorption by the optical windows. As another example, other techniques for reducing contamination within optical systems involve the use of gas “curtains” configured to prevent the movement of particulate matter within the optical system through larger particulate contaminant defects by cross-flow gas “curtains” within one or more regions of the optical system. However, such techniques are not effective against all types of contamination, such as the diffusion of gaseous hydrocarbon particulates within the system 100. In this sense, it would be desirable to have systems and methods configured to address the shortcomings of current technology, including reducing contamination by gaseous hydrocarbon particulates while providing an unobstructed path for illumination so that the inspection process can be performed.

[0016] As described in more detail herein, embodiments of the present disclosure may utilize counter-flow of purge gas through counter-flow nozzles to reduce the presence of contaminants within one or more portions of an inspection system.

[0017] 1A-1C generally illustrate an embodiment of an optical system 100 in accordance with one or more embodiments of the present disclosure.

[0018] FIG. 1A illustrates a conceptual diagram of an optical system 100 in accordance with one or more embodiments of the present disclosure.

[0019] In one embodiment, the system 100 includes a source chamber 102. The source chamber 102 may be coupled to one or more vacuum chambers 104. The source chamber 102 may have a source chamber pressure, and the one or more vacuum chambers may have a vacuum chamber pressure. In some embodiments, the vacuum chamber pressure may be less than the source chamber pressure. For example, the source chamber pressure may be about 10 -2 The vacuum chamber pressure can be about 10 torr. -8 Torr. Each of the source chamber 102 and the one or more vacuum chambers 104 may contain a mixture of one or more gases, including, but not limited to, oxygen, nitrogen, argon, gaseous water, and / or xenon. The source chamber 102 may also contain gaseous hydrocarbon contaminants, which may result in undesirable carbonaceous buildup on one or more elements of the system 100. The source chamber pressure may be maintained by one or more vacuum pumps coupled to the source chamber 102 (not shown). The vacuum chamber pressure may be maintained by one or more vacuum pumps coupled to one or more vacuum chambers (e.g., vacuum pump 130). It should be noted that the one or more vacuum pumps may be configured to generate a high-vacuum environment suitable for inspection (e.g., EUV inspection) within the source chamber 102 and / or one or more vacuum chambers 104. Alternatively, the one or more vacuum pumps may be configured to generate a low-vacuum environment within the source chamber 102 and / or one or more vacuum chambers 104.

[0020] One or more vacuum pumps of the present disclosure, including but not limited to vacuum pump 130, may comprise a pumping system suitable for establishing and maintaining a low pressure or vacuum environment within source chamber 102 and / or one or more vacuum chambers 104. For example, the one or more vacuum pumps may include a turbo pump and / or a lutos pump supporting a dry pumping unit and including an exhaust system (not shown).

[0021] In one embodiment, the system 100 includes an illumination source 112 configured to generate illumination 118. In another embodiment, the system 100 includes a set of illumination optics 105 configured to direct at least a portion of light toward one or more samples 120. For example, the illumination source 112 may include, but is not limited to, a broadband light source (e.g., a light source suitable for generating EUV, VUV, DUV, and / or UV light) or a narrowband illumination source (e.g., a laser source). The illumination source 112 may include a laser source configured to generate illumination having at least one of a wavelength of about 355 nanometers, about 266 nanometers, or about 193 nanometers. The illumination source 112 can be configured to generate and direct illumination 118 in an illumination direction (e.g., as shown in FIG. 1A ).

[0022] The illumination source 112 can be disposed within the source chamber 102. Alternatively, in some embodiments, the illumination source 112 may be disposed outside the source chamber 102 and configured to direct illumination 118 into or through one or more portions of the source chamber 102.

[0023] In one embodiment, the illumination optics 105 can be configured to direct illumination 118 emanating from the illumination source 112 along an illumination axis 121 (e.g., an illumination path) to one or more samples 120 disposed on a stage 123. The one or more samples 120 can include, but are not limited to, wafers (e.g., semiconductor wafers) or reticles. The illumination optics 105 can be located within the source chamber 102. Alternatively, in some embodiments, the illumination optics can be located outside the source chamber 102 and configured to direct illumination 118 through the source chamber 102 to one or more portions of the system 100.

[0024] Illumination source 112 may be configured to generate illumination such that illumination 118 is uniformly irradiated illumination. In another embodiment, illumination source 112 may be configured to generate illumination such that illumination 118 includes one or more scanning rasters.

[0025] In another embodiment, the system 100 may include a set of imaging optics 106 configured to receive illumination from the surface of the one or more samples 120 and direct the illumination from the one or more samples 120 to a detector 138 via an imaging axis 127 (e.g., an illumination collection path). For example, the imaging optics 106 may include one or more projection optical systems suitable for collecting light scattered, reflected, diffracted, or otherwise emitted from the sample 120 and directing the light to one or more detectors 138 (e.g., CCDs, TDI-CCDs, PMTs, etc.). In another embodiment, the system 100 includes a controller (as shown in FIG. 1B ) for receiving and / or analyzing signals indicative of the illumination collected from the sample 120 via the imaging optics 106 and the detector 138.

[0026] It should be noted that, as illustrated by Figure 1A, embodiments of the present disclosure may be configured to perform one or more inspection processes. For example, as shown in Figure 1A, optical system 100 may be configured to perform one or more inspection processes on specimen 120.

[0027] The system 100 may include an intermediate focus housing 108 coupled to each of the source chamber 102 and the one or more vacuum chambers 104. The intermediate focus housing may include an aperture 110 configured to allow transmission of light (e.g., illumination 118) and / or gas between the source chamber 102 and the one or more vacuum chambers 104. The aperture 110 may have a diameter configured to allow transmission of radiation and / or gas from one side of the aperture 110 (e.g., the source chamber 102) to the other side of the aperture 110 (e.g., the one or more vacuum chambers 104). For example, in some embodiments, the aperture 110 may have a diameter of about 1 millimeter to about 2 millimeters. The aperture 110 may be configured such that an area of ​​the aperture 110 is open to allow fluid communication of gas between the source chamber 102 and the one or more vacuum chambers 104. In this sense, the aperture 110 may be configured to reduce absorption of illumination that might otherwise occur (e.g., as in the case of an optical window in an EUV optical system).

[0028] Although not shown, the intermediate focus housing 108 may include multiple components that, when combined, form the intermediate focus housing 108. For example, the intermediate focus housing 108 may include, but is not limited to, an outer housing component, a gate valve component configured to protrude into the intermediate focus housing 108 so that air flow through an opening may be blocked, an inner gas nozzle component, and / or an outer shroud component. For purposes of this disclosure, it is specifically intended that references to the design and / or configuration of the intermediate focus housing 108, in some embodiments, should be interpreted as references to the design and / or configuration of any one of the outer housing component, the gate valve component, the inner gas nozzle component, and / or the outer shroud component of the intermediate focus housing.

[0029] The intermediate focus housing 108 may be coupled to or formed to include a counter-flow nozzle 124. The counter-flow nozzle 124 may be partially disposed within the source chamber 102. The counter-flow nozzle 124 may be of any shape or configuration known in the art to be suitable for the purposes contemplated by the present disclosure. For example, in some embodiments, the counter-flow nozzle 124 may be diverging conically and / or cylindrically shaped.

[0030] The counter-flow nozzle 124 may include one or more purge gas ducts 126 configured to transmit purge gas through the counter-flow nozzle 124 and into the source chamber 102. The one or more purge gas ducts 126 may be formed in one or more portions of the counter-flow nozzle 124 and / or the intermediate focus housing 108. The purge gas may be directed to the one or more purge gas ducts 126 from a purge gas source 128 via a purge gas inlet 122. The purge gas inlet 122 may be coupled to the purge gas source 128 and at least one of the intermediate focus housing 108, the counter-flow nozzle 124, or the one or more purge gas ducts 126.

[0031] The purge gas may include any gas known in the art to be suitable for the purposes contemplated by the present disclosure. For example, the purge gas may include, but is not limited to, oxygen, nitrogen, fluorine, xenon, argon, or any combination thereof. The purge gas source 128 may include one or more pumps (e.g., turbomolecular pumps).

[0032] The purge gas can have a purge gas pressure. In some embodiments, the purge gas pressure can be greater than the vacuum chamber pressure of one or more vacuum chambers 104. The purge gas pressure can be approximately equal to the source chamber pressure of the source chamber 102. In this sense, with respect to transmission of illumination 118 along the illumination direction, the optical system 100 can be configured such that minimal gas flow can occur in the illumination direction (e.g., from the source chamber 102 to one or more vacuum chambers 104) (e.g., as a result of the relative values ​​of the purge gas pressure, the vacuum chamber pressure, and / or the source chamber pressure).

[0033] It should be noted that, although not shown in either case, system 100 can include one or more devices and / or components configured to manipulate the flow of gas within the system. Additionally, in some embodiments, system 100 may include one or more elements configured to create and / or maintain a vacuum environment within one or more portions of system 100. For example, in some embodiments, system 100 may include a skimmer nozzle 131 disposed within one or more portions of one or more vacuum chambers.

[0034] 1B shows a block diagram of an optical system 100 in accordance with one or more embodiments of the present disclosure. In some embodiments, the system 100 may include a controller 140 communicatively coupled to one or more other elements of the system 100 (e.g., the illumination source 112, the detector 138, the one or more vacuum pumps 130, or the purge gas source 128). The controller 140 may include one or more processors 132 communicatively coupled to a memory 134. The one or more processors 132 may be configured to execute a set of program instructions stored in the memory 134 to acquire inspection and / or measurement data from one or more components of the system 100 (e.g., the detector 138) and / or to control one or more portions of the system 100 (e.g., the illumination source 112, the detector 138, the one or more vacuum pumps 130, the purge gas source 128, the stage 123, etc.).

[0035] 1C illustrates an optical system 100 according to one or more embodiments of the present disclosure. As shown in FIG. 1C and as described above, the counter-flow nozzle 124 can include one or more purge gas ducts 126 configured to deliver purge gas through the counter-flow nozzle 124 and into the source chamber 102. The purge gas source 128 can be configured to direct the purge gas through the purge gas inlet 122 to the one or more purge gas ducts 126. The purge gas can then be directed through a portion of the counter-flow nozzle 124 into the source chamber 102. In this sense, the system 100 can be configured such that the flow of purge gas 125 is directed into the source chamber 102 (e.g., via the purge gas inlet 122 and one or more purge gas ducts 126) in a direction opposite to the illumination direction. For example, the system 100 (e.g., the illumination source 112, the illumination optics 105, etc.) can be configured such that the illumination direction is toward the one or more vacuum chambers 104 and away from the source chamber 102.

[0036] System 100 may be configured to mitigate contamination within one or more portions of system 100. For example, as shown in FIG. 1B , flow of purge gas 125 may be configured to prevent the transmission of one or more contaminants 136 from source chamber 102 to one or more vacuum chambers 104 (where an inspection process may be performed). As another example, contaminants 136 that may be present within source chamber 102 and would otherwise be susceptible to fluid transmission to one or more vacuum chambers 104 (e.g., through opening 110 in intermediate focus housing 108, etc.) may be prevented from flowing into one or more vacuum chambers 104 by flow of purge gas 125. In this sense, system 100 may be configured such that illumination 118 is directed through opening 110 while transmission of contaminants 136 through opening 110 is mitigated.

[0037] Although shown with reference to an embodiment of system 100 having a single vacuum chamber 104, in some embodiments, system 100 may include multiple vacuum chambers 104. The multiple vacuum chambers 104 may be coupled such that any substance within any one of the multiple vacuum chambers may be in fluid communication throughout the interior of any other of the multiple vacuum chambers. One or more elements of system 100 may be disposed within the multiple vacuum chambers 104. For example, imaging optics 106, detector 138, sample 120, and / or stage 123 may be disposed within one or more vacuum chambers 104.

[0038] It should be noted that one or more aspects of the intermediate focus housing 108, the counter-flow nozzle 124, the one or more purge gas ducts 126, the purge gas inlet 122, and / or the purge gas source 128 may be configured and / or designed to achieve a laminar flow (e.g., flow 125) of purge gas within the counter-flow nozzle 124 and / or the source chamber 102. For example, any of the intermediate focus housing 108, the counter-flow nozzle 124, the one or more purge gas ducts 126, the purge gas inlet 122, and / or the purge gas source 128 may be configured to include orifices, chambers, passages, ducts, etc. that reduce turbulence within the flow of purge gas.

[0039] Furthermore, it should be noted that one or more aspects of the intermediate focus housing 108, the counterflow nozzle 124, the one or more purge gas ducts 126, the purge gas inlet 122, and / or the purge gas source 128 may be configured and / or designed to achieve a desired level of protection from contaminants for the one or more vacuum chambers 104. For example, the intermediate focus housing 108, the counterflow nozzle 124, the one or more purge gas ducts 126, the purge gas inlet 122, and / or the purge gas source 128 may be configured to achieve a protection factor for the system 100.

[0040] The protection factor of system 100 can be expressed as the ratio of the partial pressure of a gas in one portion of system 100 to the partial pressure of the gas in another portion of system 100. For example, the protection factor of system 100 can be expressed as the ratio of the partial pressure of a gas in source chamber 102 (PPsc) to the partial pressure of the gas in one or more vacuum chambers 104 (PPvc). It should be noted that for purposes of the present disclosure, the protection factor of system 100 can be expressed as the ratio of the partial pressure of any gas in source chamber 102 (PPsc) to the partial pressure of the same gas in one or more vacuum chambers 104 (PPvc), including, but not limited to, when such gases include argon, xenon, gaseous hydrocarbons (e.g., carbonaceous contaminants), and / or gaseous water.

[0041] In some embodiments, the protection factor of system 100 may include a function of one or more variables related to the operation of system 100. For example, the protection factor may be a function of one or more aspects of the purge gas utilized in the system. Specifically, the protection factor may be a function of purge gas flow 125, which may be affected by the design and / or configuration of intermediate focus housing 108, counterflow nozzle 124, one or more purge gas ducts 126, purge gas inlet 122, and / or purge gas source 128.

[0042] 2A-7C generally illustrate one or more aspects of various embodiments of the counter-flow nozzle 124, in accordance with one or more embodiments of the present disclosure. The various embodiments of the counter-flow nozzle 124 illustrated in the accompanying drawings may be configured such that a desired protection factor is achieved within the system 100 (e.g., as a result of one or more design and / or configuration considerations regarding the intermediate focus housing 108, the counter-flow nozzle 124, the one or more purge gas ducts 126, the purge gas inlet 122, and / or the purge gas source 128 of a particular embodiment).

[0043] 2A-2C, counter-flow nozzle 124a is shown. Figure 2A shows a cross-sectional view of a portion of system 100a, including intermediate focus housing 108, counter-flow nozzle 124a, and a portion of source chamber 102. Figure 2B shows a cross-sectional view of counter-flow nozzle 124a (e.g., as viewed from one end of counter-flow nozzle 124a).

[0044] The geometry of the counterflow nozzle 124a and / or one or more purge gas ducts 126a may be configured to provide desirable flow characteristics (e.g., a desirable protection factor) for the gas (e.g., purge gas) flowing through the counterflow nozzle 124a. For example, as shown in FIG. 2C, the purge gas flow 125a from the counterflow nozzle 124a can have laminar and / or turbulent flow characteristics as a result of one or more aspects of the design and / or configuration of the counterflow nozzle 124a and / or one or more purge gas ducts 126a. In this sense, the design and / or configuration of the counterflow nozzle 124a and / or one or more purge gas ducts 126a can provide a particular protection factor for the system 100a with the counterflow nozzle 124a in operation. As an illustrative example, the protection factor for the system 100a including the counterflow nozzle 124a, where the purge gas includes argon, may be equivalent to approximately 860,000.

[0045] 3A-3C, counter-flow nozzle 124b is shown. Figure 3A shows a cross-sectional view of a portion of system 100b, including intermediate focus housing 108, counter-flow nozzle 124b, and a portion of source chamber 102. Figure 3B shows a cross-sectional view of counter-flow nozzle 124b (e.g., as viewed from one end of counter-flow nozzle 124b).

[0046] The geometry of the counterflow nozzle 124b and / or one or more purge gas ducts 126b may be configured to provide desirable flow characteristics (e.g., a desirable protection factor) for the gas (e.g., purge gas) flowing through the counterflow nozzle 124b. For example, as shown in FIG. 3C , the flow 125b of purge gas from the counterflow nozzle 124b can have laminar and / or turbulent flow characteristics as a result of one or more aspects of the design and / or configuration of the counterflow nozzle 124b and / or one or more purge gas ducts 126b. In this sense, the design and / or configuration of the counterflow nozzle 124b and / or one or more purge gas ducts 126b can provide a particular protection factor for the system 100b having the counterflow nozzle 124b during operation. As an illustrative example, the protection factor for the system 100b including the counterflow nozzle 124b and in which the purge gas comprises argon may be equivalent to approximately 1,700,000.

[0047] 4A-4C, counter-flow nozzle 124c is shown. Figure 4A shows a cross-sectional view of a portion of system 100c, including intermediate focus housing 108, counter-flow nozzle 124c, and a portion of source chamber 102. Figure 4B shows a cross-sectional view of counter-flow nozzle 124c (e.g., as viewed from one end of counter-flow nozzle 124c).

[0048] The geometry of the counterflow nozzle 124c and / or one or more purge gas ducts 126c may be configured to provide desirable flow characteristics (e.g., a desirable protection factor) for the gas (e.g., purge gas) flowing through the counterflow nozzle 124c. For example, as shown in FIG. 4C, the flow 125c of purge gas from the counterflow nozzle 124c can have laminar and / or turbulent flow characteristics as a result of one or more aspects of the design and / or configuration of the counterflow nozzle 124c and / or one or more purge gas ducts 126c. In this sense, the design and / or configuration of the counterflow nozzle 124c and / or one or more purge gas ducts 126c can provide a particular protection factor for the system 100c having the counterflow nozzle 124c during operation. As an illustrative example, the protection factor for the system 100c including the counterflow nozzle 124c and in which the purge gas comprises argon may be equivalent to approximately 1,900,000.

[0049] 5A-5C, counter-flow nozzle 124d is shown. Figure 5A shows a cross-sectional view of a portion of system 100d, including intermediate focus housing 108, counter-flow nozzle 124d, and a portion of source chamber 102. Figure 5B shows a cross-sectional view of counter-flow nozzle 124d (e.g., as viewed from one end of counter-flow nozzle 124d).

[0050] The geometry of the counter-flow nozzle 124d and / or one or more purge gas ducts 126d may be configured to provide desirable flow characteristics (e.g., a desirable protection factor) for the gas (e.g., purge gas) flowing through the counter-flow nozzle 124d. For example, as shown in FIG. 5C, the flow 125d of purge gas from the counter-flow nozzle 124d can have laminar and / or turbulent flow characteristics as a result of one or more aspects of the design and / or configuration of the counter-flow nozzle 124d and / or one or more purge gas ducts 126d. In this sense, the design and / or configuration of the counter-flow nozzle 124d and / or one or more purge gas ducts 126d can provide a particular protection factor for the system 100d having the counter-flow nozzle 124d during operation. As an illustrative example, the protection factor for the system 100d including the counter-flow nozzle 124d and in which the purge gas comprises argon may be equivalent to approximately 1,200,000.

[0051] 6A-6C, counter-flow nozzle 124e is shown. Figure 6A shows a cross-sectional view of a portion of system 100e, including intermediate focus housing 108, counter-flow nozzle 124e, and a portion of source chamber 102. Figure 6B shows a cross-sectional view of counter-flow nozzle 124e (e.g., as viewed from one end of counter-flow nozzle 124e).

[0052] The geometry of the counter-flow nozzle 124e and / or one or more purge gas ducts 126e may be configured to provide desirable flow characteristics (e.g., a desirable protection factor) for the gas (e.g., purge gas) flowing through the counter-flow nozzle 124e. For example, as shown in FIG. 6C , the flow 125e of purge gas from the counter-flow nozzle 124e can have laminar and / or turbulent flow characteristics as a result of one or more aspects of the design and / or configuration of the counter-flow nozzle 124e and / or one or more purge gas ducts 126e. In this sense, the design and / or configuration of the counter-flow nozzle 124e and / or one or more purge gas ducts 126e can provide a particular protection factor for the system 100e having the counter-flow nozzle 124e during operation. As an illustrative example, the protection factor for a system 100e including the counter-flow nozzle 124e and in which the purge gas comprises argon may be equivalent to approximately 1,000,000.

[0053] 7A-7C, counter-flow nozzle 124f is shown. Figure 7A shows a cross-sectional view of a portion of system 100f, including intermediate focus housing 108, counter-flow nozzle 124f, and a portion of source chamber 102. Figure 7B shows a cross-sectional view of counter-flow nozzle 124f (e.g., as viewed from one end of counter-flow nozzle 124f).

[0054] The geometry of the counter-flow nozzle 124f and / or one or more purge gas ducts 126f may be configured to provide desirable flow characteristics (e.g., a desirable protection factor) for the gas (e.g., purge gas) flowing through the counter-flow nozzle 124f. For example, as shown in FIG. 7C , the flow 125f of purge gas from the counter-flow nozzle 124f may have laminar and / or turbulent flow characteristics as a result of one or more aspects of the design and / or configuration of the counter-flow nozzle 124f and / or one or more purge gas ducts 126f. In this sense, the design and / or configuration of the counter-flow nozzle 124f and / or one or more purge gas ducts 126f may result in a particular protection factor for the system 100f having the counter-flow nozzle 124f during operation. As an illustrative example, the protection factor for a system 100f including the counter-flow nozzle 124f and in which the purge gas comprises argon may be equivalent to approximately 4,600,000.

[0055] It should be noted that, for purposes of this disclosure, the specific examples of protection factors for systems 100 having various embodiments of counter-flow nozzles 124 described herein are intended to be illustrative, not limiting. In particular, it should be noted that the protection factor for any given embodiment of system 100 will depend on several variables related to system 100, including, but not limited to, the diameter of openings 110, the area of ​​openings 110, the temperature of the purge gas, the velocity of one or more particles in the purge gas, the conductance of the purge gas at openings 110, the location of one or more purge gas ducts 126 within counter-flow nozzle 124, the length of counter-flow nozzle 124, etc.

[0056] FIG. 8 shows a process flow diagram illustrating steps in a method 800 for reducing contamination in an optical system in accordance with one or more embodiments of the present disclosure.

[0057] Illumination is generated in step 802. For example, illumination source 112 may generate one or more beams of radiation (e.g., illumination 118).

[0058] In step 804, at least a portion of the illumination is directed in an illumination direction through at least a portion of the source chamber. For example, in step 804, illumination 118 may be directed into or through one or more portions of source chamber 102. As another example, illumination may be directed along illumination axis 121 to, by, or through one or more illumination optics 105.

[0059] At least a portion of the illumination is directed through an opening in the intermediate focus housing in an illumination direction in step 806. For example, the set of illumination optics 105 can direct illumination 118 through an opening 110 in the intermediate focus housing 108.

[0060] In step 808, purge gas is supplied to a counter-flow nozzle coupled to the intermediate focus housing and disposed at least partially within the source chamber. For example, purge gas source 128 can supply purge gas via purge gas inlet 122 to counter-flow nozzle 124, which is coupled to and in fluid communication with at least a portion of the intermediate focus housing.

[0061] In step 810, a counterflow of purge gas through the counterflow nozzle is generated. For example, the purge gas may be supplied in flow 125 to one or more purge gas ducts 126 and transmitted through the counterflow nozzle 124 to the source chamber 102. The purge gas flow 125 may be configured to prevent transmission of one or more contaminants 136 from the source chamber 102 to one or more vacuum chambers 104 (where an inspection process may be performed). As another example, contaminants 136 that may be present in the source chamber 102 and would otherwise be susceptible to fluid transmission to one or more vacuum chambers 104 (e.g., through opening 110 in the intermediate focus housing 108, etc.) may be prevented from flowing into the one or more vacuum chambers 104 by the purge gas flow 125. In this sense, the method 800 (e.g., via at least step 810) may be configured such that the illumination 118 is directed through the opening 110 while transmission of the contaminants 136 through the opening 110 is mitigated.

[0062] It should be noted that the scope of the present disclosure is not limited to optical system 100. Rather, systems incorporating various aspects of the present disclosure may include any other optical systems known in the art, including characterization systems, metrology systems, and / or lithography systems.

[0063] For example, in one embodiment, optical system 100 is configured as a wafer inspection system or a reticle inspection system. In this regard, optical system 100 may include any wafer or reticle inspection optical architecture known in the art that is suitable for operation in the EUV and / or VUV spectral range. It is further recognized that optical system 100 may be configured as an EUV mask blank inspection system (mask inspection system). EUV-based mask blank inspection is generally described in U.S. Pat. No. 8,711,346 (Stokowski, April 29, 2014), which is incorporated herein by reference in its entirety. EUV-based mask blank inspection is generally described in U.S. Patent Application Publication No. 13 / 417,982 (Xiong et al., March 12, 2012), which is incorporated herein by reference in its entirety. EUV-based reticle inspection is generally described in U.S. Patent Application Publication No. 13 / 905,449 (Nasser)-Ghodsi et al., filed May 30, 2013, the entire contents of which are incorporated herein by reference.

[0064] As another example, in another embodiment, not shown, optical system 100 can be configured as an EUV lithography system. In one embodiment, the optical lithography system (not shown) can include a set of illumination optics configured to direct output light from illumination source 112 onto an EUV-compatible lithography mask (e.g., an EUV reflective mask). In another embodiment, the lithography system includes a set of projection optics configured to receive illumination reflected from the mask and direct the illumination reflected from the mask onto one or more wafers positioned on a wafer stage. The optical lithography system can include any EUV lithography system known in the art. EUV-based lithography is generally described in U.S. Patent Application Publication No. 13 / 419,042 (Wang, March 13, 2012), the entire contents of which are incorporated herein by reference.

[0065] It is contemplated herein that various embodiments of the present disclosure may be implemented in combination or independently. For example, the embodiments depicted in Figures 1A through 7C should be construed as being applicable in conjunction with one another in some embodiments.

[0066] The one or more processors 132 of the controller 140 may include any processor or processing element known in the art. For purposes of this disclosure, the term “processor” or “processing element” may be broadly defined to encompass any device having one or more processing or logic elements (e.g., one or more microprocessor devices, one or more application-specific integrated circuit (ASIC) devices, one or more field-programmable gate arrays (FPGAs), or one or more digital signal processors (DSPs)). In this sense, the one or more processors 132 may include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in memory). In one embodiment, the one or more processors 132 may be embodied as a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, a network computer, or any other computer system configured to execute programs that operate or are configured to operate with the optical system 100, as described throughout this disclosure.

[0067] Additionally, different components of the system 100, including, but not limited to, the illumination source 112, the detector 138, the one or more vacuum pumps 130, and / or the purge gas source 128, may include processors or logic elements suitable for performing at least some of the steps described in this disclosure. Accordingly, the above description should not be construed as a limitation on the embodiments of the present disclosure, but merely as examples. Furthermore, the steps described throughout this disclosure may be performed by a single controller 140 or, alternatively, by multiple controllers. Furthermore, the controller 140 may include one or more controllers housed within a common housing or multiple housings. In this manner, any controller or combination of controllers may be packaged separately as a module suitable for integration into the optical system 100. Furthermore, the controller 140 may analyze data received from the detector 138 and provide the data to additional components within or external to the system 100.

[0068] The memory 134 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 132. For example, the memory 134 may include a non-transitory storage medium. As another example, the memory 134 may include, but is not limited to, read-only memory (ROM), random access memory (RAM), magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state drives, etc. Furthermore, it should be noted that the memory 134 may be housed within a common controller housing along with one or more processors 132. In one embodiment, the memory 134 may be located remotely relative to the physical locations of the one or more processors 132 and the controller 140. For example, one or more processors 132 of the controller 140 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.).

[0069] In one embodiment, although not shown, a user interface may be communicatively coupled to controller 140. In one embodiment, the user interface may include, but is not limited to, one or more desktops, laptops, tablets, etc. In another embodiment, the user interface includes a display used to display data of system 100 to a user. The display of the user interface may include any display known in the art. For example, the display may include, but is not limited to, a liquid crystal display (LCD), an organic light-emitting diode (OLED)-based display, or a CRT display. Those skilled in the art will recognize that any display device capable of integration with a user interface is suitable for implementation in the present disclosure. In another embodiment, a user may input selections and / or commands in response to data displayed to the user via a user input device of the user interface.

[0070] In another embodiment, the controller 140 is communicatively coupled to one or more elements of the system 100. In this regard, the controller 140 can transmit and / or receive data from any component of the system 100. Additionally, the controller 140 can direct or otherwise control any component of the system 100 by generating one or more control signals for the associated component. For example, the controller 140 may be communicatively coupled to at least one of the illumination source 112, the detector 138, the one or more vacuum pumps 130, and / or the purge gas source 128 and may be configured to control any one or more of the illumination source 112, the detector 138, the one or more vacuum pumps 130, and / or the purge gas source 128 by generating one or more control signals.

[0071] All of the methods described herein may include storing results of one or more steps of a method embodiment in memory 134. The results may include any of the results described herein and may be stored in any manner known in the art. The memory may include any memory described herein or any other suitable storage medium known in the art. After the results are stored, they can be accessed in memory, used by any of the method or system embodiments described herein, formatted for display to a user, used by another software module, method, or system, etc. Furthermore, the results may be stored “permanently,” “semi-permanently,” “temporarily,” or for a period of time. For example, the memory may be random access memory (RAM), and the results may not necessarily persist in memory indefinitely.

[0072] It is further contemplated that each of the above-described method embodiments may include any other step(s) of any other method(s) described herein. In addition, each of the above-described method embodiments may be performed by any of the systems described herein.

[0073] Those skilled in the art will recognize that the component operations, devices, objects, and accompanying discussion described herein are used as examples for conceptual clarity, and that various configuration modifications are contemplated. Thus, as used herein, the specific examples described and accompanying discussion are intended to be representative of their more general classes. In general, the use of any specific example is intended to represent that class, and the non-inclusion of specific components, operations, devices, and objects should not be construed as limiting.

[0074] As used herein, directional terms such as "top", "bottom", "over", "under", "upper", "upward", "lower", "down", and "downward" are intended to provide relative positions for purposes of description and are not intended to specify an absolute frame of reference. Various modifications to the described embodiments will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments.

[0075] With respect to the use herein of any plural and / or singular terms, those skilled in the art will be able to convert from the plural to the singular and / or from the singular to the plural as appropriate to the context and / or application. The various singular / plural permutations need not necessarily be expressly set forth herein for ease of understanding.

[0076] The subject matter described herein illustrates different components that are, in some cases, included within or connected to other components. It should be understood that such depicted architectures are merely exemplary, and that in fact many other architectures that achieve the same functionality may be implemented. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components herein that combine to achieve a particular function can be considered to be “associated” with each other such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components so associated can also be considered to be “connected” or “coupled” to each other to achieve the desired functionality, and any two components that can be so associated can also be considered to be “couplable” with each other to achieve the desired functionality. Specific examples of what can be coupled include, but are not limited to, physically coupleable and / or physically interacting components and / or wirelessly interacting and / or wirelessly interacting components and / or logically interacting and / or logically interacting components.

[0077] It should further be understood that the invention disclosed and described herein is defined by the appended claims. In general, those skilled in the art will understand that the terms used herein, and particularly in the appended claims (e.g., the body of the appended claims), are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but not limited to," etc.). Those skilled in the art will further understand that if a specific number of introduced claim recitations is intended, such intention will be expressly recited in the claim; in the absence of such recitation, no such intention exists. For example, as an aid to understanding, the following appended claims may include the use of the introductory phrases "at least one" and "one or more" to introduce the claim recitations. However, the use of such phrases should not be interpreted as meaning that the introduction of a claim recitation with the indefinite article "a" or "an" limits any particular claim containing such an introduced claim recitation to an invention containing only one such recitation. The same applies to the use of express articles used to introduce claim recitations, even when the same claim includes the introductory phrase "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should typically be interpreted to mean "at least one" or "one or more"). Those skilled in the art will also recognize that even when a specific number of introduced claim recitations is explicitly recited, such a recitation should typically be interpreted to mean at least the recited number (e.g., a bare recitation of "two recitations" without other modifiers typically means at least two recitations, or two or more recitations).Furthermore, in instances where a conventional expression similar to "such as at least one of A, B, and C" is used, such configuration is generally intended in the sense that one of ordinary skill in the art would understand the conventional expression (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, a system having only A, a system having only B, a system having only C, a system having both A and B, a system having both A and C, a system having both B and C, and / or a system having both A, B, and C, etc.). In instances where a conventional expression similar to "such as at least one of A, B, or C" is used, such configuration is generally intended in the sense that one of ordinary skill in the art would understand the conventional expression (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, a system having only A, a system having only B, a system having only C, a system having both A and B, a system having both A and C, a system having both B and C, and / or a system having both A, B, and C, etc.). Those of ordinary skill in the art will further understand that virtually any disjunctive word and / or phrase presenting two or more alternative terms, wherever it appears in the description, claims, or drawings, should be understood to contemplate the possibility of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" would be understood to include the possibilities of "A" or "B" or "A and B."

[0078] It is believed that the present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes can be made in the form, construction, and arrangement of the elements without departing from the disclosed subject matter or sacrificing all of its material advantages. The described forms are merely illustrative, and it is the intent of the following claims to embrace and include such modifications. It is further understood that the invention is defined by the appended claims.

Claims

1. 1. An optical system comprising: a source chamber; one or more vacuum chambers containing a set of imaging optics; an intermediate focus housing having an opening and coupled to the source chamber and one or more vacuum chambers; an illumination source configured to generate illumination and direct the illumination through one or more portions of the source chamber; a set of illumination optics configured to receive at least a portion of the illumination from the illumination source and direct at least a portion of the illumination in an illumination direction to the sample via an illumination axis, the illumination axis configured to transmit at least a portion of the illumination through the aperture; a purge gas inlet coupled to the intermediate focus housing, the purge gas inlet configured to deliver purge gas to one or more purge gas ducts; a counter-flow nozzle coupled to the intermediate focus housing and at least partially disposed within the source chamber, the counter-flow nozzle configured to receive a purge gas and direct a counter-flow of the purge gas into the source chamber in a direction opposite to the illumination direction through the one or more purge gas ducts formed within the counter-flow nozzle, the one or more purge gas ducts configured to provide a laminar counter-flow of purge gas, the counter-flow of purge gas configured to reduce permeation of one or more volatile species of contaminants into the one or more vacuum chambers, thereby mitigating contamination within the one or more vacuum chambers; Including, the purge gas duct includes a passage having an annular cross section extending in the illumination direction and an annular opening on the counterflow nozzle side, and directs the purge gas toward the illumination source, thereby making the counterflow of the purge gas a laminar flow; Optical system.

2. The optical system of claim 1 , wherein the set of imaging optics is configured to receive at least a portion of illumination from a sample along an imaging axis.

3. The optical system of claim 1 , further comprising a detector configured to receive at least a portion of the illumination from the sample through the set of imaging optics.

4. The optical system of claim 1 , wherein the optical system is configured as a mask inspection system.

5. The optical system of claim 1 , wherein the source chamber has a source chamber pressure and the one or more vacuum chambers each have a pressure less than the source chamber pressure.

6. The optical system of claim 1 , wherein the illumination source is configured to generate at least one of extreme ultraviolet, vacuum ultraviolet, deep ultraviolet, or ultraviolet light.

7. An optical system as described in claim 5, further comprising one or more vacuum pumps configured to generate a selected pressure within each of the one or more vacuum chambers.

8. The optical system of claim 7 , wherein the one or more vacuum pumps include one or more turbomolecular pumps.

9. The optical system of claim 1 , wherein the set of illumination optics is configured to direct at least a portion of the illumination through the aperture.

10. The optical system of claim 1 , wherein the source chamber is filled with at least one of argon or xenon.

11. The optical system of claim 1 , wherein the purge gas comprises argon.

12. The optical system of claim 1 , wherein the counter-flow nozzle comprises a plurality of nozzle elements.

13. The optical system of claim 1 , wherein the counterflow nozzle is configured to increase a protection factor of the optical system.

14. an intermediate focus housing including an opening and coupled to each of the source chamber and the one or more vacuum chambers; an illumination source configured to generate illumination and direct the illumination through one or more portions of the source chamber; a purge gas inlet coupled to the intermediate focus housing, the purge gas inlet configured to deliver purge gas to one or more purge gas ducts; a counter-flow nozzle coupled to the intermediate focus housing and at least partially disposed within the source chamber, the counter-flow nozzle configured to receive a purge gas and direct a counter-flow of the purge gas into the source chamber in a direction opposite to an illumination direction through the one or more purge gas ducts, the one or more purge gas ducts being formed within the counter-flow nozzle and configured to provide a laminar counter-flow of the purge gas, the counter-flow of the purge gas being configured to reduce permeation of one or more volatile species of contaminants into the one or more vacuum chambers, thereby mitigating contamination within the one or more vacuum chambers; Including, the purge gas duct includes a passage having an annular cross section extending in the illumination direction and an annular opening on the counterflow nozzle side, and directs the purge gas toward the illumination source, thereby making the counterflow of the purge gas a laminar flow; Device.

15. 1. A method for reducing contamination in an optical system, comprising: generating illumination and directing at least a portion of the illumination toward one or more portions of the source chamber; directing at least a portion of the illumination through an opening in an intermediate focus housing, the intermediate focus housing being disposed between the source chamber and one or more vacuum chambers; supplying a purge gas to a counter-flow nozzle coupled to the intermediate focus housing and disposed at least partially within the source chamber; generating a laminar counterflow of purge gas through the counterflow nozzle in a direction opposite to the illumination direction; Including, the purge gas flows toward the source chamber from a purge gas duct including a passage having an annular cross section extending in the illumination direction and an annular opening on the counterflow nozzle side, thereby forming a laminar flow of the counterflow of the purge gas; method.

16. 16. The method of claim 15, wherein generating a countercurrent flow of purge gas comprises reducing permeation of one or more volatile species of contaminants into the one or more vacuum chambers.

17. The method of claim 15 , wherein the illumination comprises at least one of extreme ultraviolet, vacuum ultraviolet, deep ultraviolet, or ultraviolet.

18. 16. The method of claim 15, wherein supplying a purge gas to the counter-flow nozzle coupled to the intermediate focus housing and disposed at least partially within the source chamber comprises supplying a purge gas to the intermediate focus housing through a purge gas inlet, and further comprises supplying a purge gas to one or more purge gas ducts of the intermediate focus housing.

19. The method of claim 15 , wherein the purge gas comprises argon.

20. The method of claim 15 , wherein the one or more purge gas ducts are configured to generate a laminar flow of purge gas.

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