Highly efficient laser sustained plasma system

Abstract

To provide a multi-pass laser-sustained plasma system for generating broadband light.SOLUTION: A system includes: a pump source 104 configured to generate a pumping beam 101; a gas containment structure for containing a gas 107; an optical assembly which includes optical elements configured to perform one or more passes of the pumping beam through a portion of the gas to sustain a broadband-light-emitting plasma; and a collection assembly configured to direct at least a portion of broadband radiation emitted by the plasma to downstream optical elements, where the collection assembly collects at least a portion of broadband radiation emitted by the plasma and directs it back to the plasma.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] This application relates generally to laser-sustained plasma light sources, and more particularly to multi-pass laser-sustained plasma light sources. [Background technology]

[0002] Priority This application is related to the application(s) listed below ("related applications") and claims the benefit of the earliest effective filing date(s) from said application (e.g., claims an earliest effective priority date to a non-provisional application or claims the benefit under 35 U.S.C. §119(e) to any and all parent, grandparent, great-grandparent, etc. applications of the related applications).

[0003] Related Applications Due to non-statutory requirements of the USPTO, this application constitutes a regular (non-provisional) patent application of U.S. Provisional Patent Application No. 62 / 183,069, entitled “MULTIPASS LASER-SUSTAINED PLASMA PUMP GEOMETRIES,” filed June 22, 2015 in the names of Ilya Bezel, Anatoly Shchemelinin Eugene Shifrin and Matthew Derstine as inventors, which is incorporated herein by reference in its entirety.

[0004] Due to non-statutory requirements of the USPTO, this application constitutes a regular (non-provisional) patent application of U.S. Provisional Patent Application No. 62 / 290,593, entitled “OPTICAL WAFER INSPECTOR,” filed February 3, 2016, in the names of Matthew Derstine and Ilya Bezel as inventors, which is incorporated herein by reference in its entirety.

[0005] As the demand for integrated circuits with ever-smaller device features continues to increase, there is an increasing need for improved illumination sources with which to inspect these ever-shrinking devices. One such illumination source includes a laser-sustained plasma source. Laser-sustained plasma sources can produce high-power broadband light. Laser-sustained sources work by focusing a laser beam into a gas volume to excite a gas, such as argon or xenon, into a plasma state capable of emitting light. This effect is typically referred to as "pumping" the plasma.

[0006] Plasmas sustained by continuous wave (CW) radiation often operate at relatively low densities where the plasma is substantially transparent to the pump light used. As a result, the brightness of the broadband light emitted by the plasma is much lower than the theoretical blackbody limit. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] US Patent Application Publication No. 2012 / 0091893 [Patent Document 2] US Patent Application Publication No. 2012 / 0119116 [Patent Document 3] US Patent Application Publication No. 2012 / 0068091 Summary of the Invention [Problem to be solved by the invention]

[0008] Several approaches have been used to address this problem of low-range optical systems. One approach involves addressing the low density of the plasma by using higher pressure lamps. This approach has had limited success as it results in plasma growth. Another approach involves pumping the plasma with a low numerical aperture (NA) laser to generate an elongated plasma. By collecting broadband light along the length, brightness approaching that of a blackbody can theoretically be achieved. Separate pumping and collection geometries have been proposed to address issues related to the mismatch between laser absorption and DUV / VUV emission. In both implementations, the elongated plasma emits a large amount of light that has not been used. It is therefore desirable to provide a system and method that corrects the disadvantages of the conventional approaches as identified above. [Means for solving the problem]

[0009] In accordance with one or more embodiments of the present disclosure, a multi-pass laser-sustained plasma system for generating broadband light is disclosed. In one embodiment, the system includes a pump source configured to generate a pump beam. In another embodiment, the system includes a multi-pass optical assembly. In another embodiment, the multi-pass optical assembly includes one or more optical elements configured to perform multiple passes of the pump beam through the portion of the gas to sustain the broadband light-emitting plasma. In another embodiment, the one or more optical elements are configured to collect an unabsorbed portion of the pump beam transmitted through the plasma and direct the collected unabsorbed pump beam portion back to the portion of the gas.

[0010] According to one or more embodiments of the present disclosure, a multi-pass laser-sustained plasma system for generating broadband light is disclosed. In one embodiment, the system includes a pump source configured to generate a pump beam. In another embodiment, the system includes a gas containment structure for containing the gas. In another embodiment, the system includes an optical assembly. In another embodiment, the optical subsystem includes one or more optical elements configured to perform one or more passes of the pump beam through the portion of the gas to maintain the broadband light-emitting plasma. In another embodiment, the system includes a collection assembly including one or more collection optics. In another embodiment, the system includes one or more collection optics configured to direct at least a portion of the broadband radiation emitted by the plasma to one or more downstream optical elements. In another embodiment, the system includes one or more collection optics configured to collect and direct at least a portion of the broadband radiation emitted by the plasma back to the plasma.

[0011] In accordance with one or more embodiments of the present disclosure, a multi-pass laser-sustained plasma system for generating broadband light is disclosed. In one embodiment, the system includes a pump source configured to generate a pump beam. In another embodiment, the system includes a reflector assembly configured to contain a gas. In another embodiment, the reflector assembly includes one or more inlet openings for receiving the pump beam from the pump source. In another embodiment, the pump beam sustains a plasma within a portion of the gas. In another embodiment, the plasma emits broadband radiation. In another embodiment, an inner surface of the reflector assembly is configured to collect at least a portion of the broadband radiation emitted by the plasma and direct the collected broadband radiation back to the plasma.

[0012] 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 this specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. [Brief description of the drawings]

[0013] The advantages of the present disclosure may be better understood by those skilled in the art with reference to the following drawings. [Figure 1A] FIG. 1 is a simplified schematic diagram of a multi-pass laser-sustained plasma system in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1 is a simplified schematic diagram of a multi-pass laser-sustained plasma system in accordance with one or more embodiments of the present disclosure. [Figure 1C] FIG. 1 is a simplified schematic diagram of a multi-pass laser-sustained plasma system in accordance with one or more embodiments of the present disclosure. [Figure 1D] FIG. 1 is a simplified schematic diagram of a multi-pass laser-sustained plasma system in accordance with one or more embodiments of the present disclosure. [Figure 1E] FIG. 1 is a simplified schematic diagram of a multi-pass laser-sustained plasma system in accordance with one or more embodiments of the present disclosure. [Figure 1F] FIG. 1 is a simplified schematic diagram of a multi-pass laser-sustained plasma system in accordance with one or more embodiments of the present disclosure. [Figure 1G] FIG. 2 is a diagram of a multi-pass optical assembly with one or more off-axis mirrors in accordance with one or more embodiments of the present disclosure. [Figure 1H] 1 illustrates a multi-pass optical assembly arranged in an optical threading configuration in accordance with one or more embodiments of the present disclosure. [Figure 1I] 1 illustrates a multi-pass optical assembly according to one or more embodiments of the present disclosure arranged to reduce the NA of the illumination with one or more successive passes through the plasma. [Figure 1J] FIG. 1 illustrates control of plasma shape using a multi-pass optical assembly in accordance with one or more embodiments of the present disclosure. [Figure 1K] FIG. 1 illustrates control of plasma shape using a multi-pass optical assembly in accordance with one or more embodiments of the present disclosure. [Figure 1L] FIG. 2 is a diagram of a multi-pass optical assembly with one or more retroreflector assemblies in accordance with one or more embodiments of the present disclosure. [Figure 1M] FIG. 1 is a diagram of a multi-pass optical assembly including one or more retroreflector assemblies arranged in a resonator configuration in accordance with one or more embodiments of the present disclosure. [Figure 1N] FIG. 1 is a diagram of a multi-pass optical assembly including one or more retroreflector assemblies arranged in a resonator configuration in accordance with one or more embodiments of the present disclosure. [Figure 1O] FIG. 2 is a diagram of a multi-pass optical assembly with one or more polarizing elements in accordance with one or more embodiments of the present disclosure. [Figure 1P] FIG. 2 is a diagram of a multi-pass optical assembly equipped with one or more wavefront sensors for measuring aberrations in accordance with one or more embodiments of the present disclosure. [Figure 1Q] FIG. 2 is a diagram of a multi-pass optical assembly equipped with one or more wavefront sensors for measuring aberrations in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 2C] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 2D] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 2E] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 2F] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 2G] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 2H] FIG. 1 is a diagram of a multipath system with a reflector assembly in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Reference will now be made to the details of the disclosed subject matter, which are illustrated in the accompanying drawings.

[0015] While specific embodiments of the present invention have been illustrated, it will be apparent that various modifications and embodiments of the present invention can be made by those skilled in the art without departing from the scope and spirit of the above disclosure. Accordingly, the scope of the present invention is intended to be limited only by the claims appended hereto.

[0016] 1A-2H, a highly efficient laser-sustained plasma (LSP) light source is disclosed according to one or more embodiments of the present disclosure. An embodiment of the present disclosure is directed to a multi-pass laser-sustained plasma system including multiple discrete optical elements that recirculate pump illumination and / or broadband light. An additional embodiment of the present disclosure is directed to a multi-pass laser-sustained plasma system including an elliptical reflector assembly that recirculates pump illumination and / or broadband light. Recirculating the pump illumination and / or plasma emission broadband back to the plasma to further pump the plasma improves the efficiency of the embodiments of the present disclosure compared to non-recirculating systems. Additionally, the combination of multiple images of the plasma using discrete optical elements or reflector assemblies of the present disclosure may improve the brightness of the broadband output.

[0017] Generation of light-sustained plasma is also generally described in U.S. Patent No. 7,435,982, issued October 14, 2008, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent No. 7,786,455, issued August 31, 2010, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent No. 7,989,786, issued August 2, 2011, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent No. 8,182,127, issued May 22, 2012, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent No. 8,309,943, issued November 13, 2012, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent No. 8,525,138, issued February 9, 2013, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent No. 8,921,814, issued December 30, 2014, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent No. 9,318,311, issued April 19, 2016, which is incorporated herein by reference in its entirety. Generation of plasma is also generally described in U.S. Patent Application Publication No. 2014 / 029154, filed March 25, 2014, which is incorporated herein by reference in its entirety. Transverse pumping of light-sustained plasma is also generally described in U.S. Patent Application Publication No. 2015 / 0282288, filed March 31, 2015, which is incorporated herein by reference in its entirety. In a general sense, the various embodiments of the present disclosure are intended to be construed as extending to any plasma-based light source known in the art. Optical systems for use in the context of plasma generation are also generally described in U.S. Patent No. 7,705,331, issued April 27, 2010, which is incorporated herein by reference in its entirety.

[0018] 1A-1O illustrate a multi-pass system 100 for efficiently generating an LSP plasma in accordance with one or more embodiments of the present disclosure. In one embodiment, as shown in FIG. 1A, the system 100 includes a pump source 104 (e.g., one or more lasers) configured to generate a beam of pumping illumination 101 to sustain a plasma 106. The pump beam 101 is adapted to sustain a plasma 106 within a volume of a gas 107. The plasma 106 emits broadband radiation 115 in response to optical pumping from the pump beam 101. In another embodiment, the system 100 includes a multi-pass optical assembly 102. In one embodiment, the multi-pass optical assembly 102 includes one or more optical elements 110a-110f configured to perform multiple passes 108a-108d of the pump beam through portions of the gas 107 to sustain the plasma 106. In another embodiment, one or more optical elements 110a-110f are configured to collect an unabsorbed portion of the pump beam transmitted through the plasma 106 and direct the collected unabsorbed portion of the pump beam back to the plasma 106 and / or gas 107 near the plasma 106. As previously described herein, the multi-pass optical assembly 102 is particularly useful in low pressure settings where absorption of pump illumination by the plasma 106 is low. The one or more optical elements 110a-110f of the multi-pass assembly 102 provide a "recirculation" of the pump illumination such that the unabsorbed portion of the pump beam redirects the unabsorbed pump beam back to the plasma 106 and / or gas 107 near the plasma 106.

[0019] In one embodiment, as shown in FIG. 1A, the multi-pass optical assembly 102 includes two or more optical elements 110a-110f. The two or more optical elements 110a-110f may include any optical element known in the art capable of collecting plasma illumination transmitted through the plasma 106 and redirecting pump illumination back to the plasma 106 and / or portions of the gas 107 proximate the plasma 106. For example, the two or more optical elements 110a-110f may include, but are not limited to, one or more reflective optical elements (e.g., mirrors) and / or one or more transmissive optical elements (e.g., lenses). For example, as depicted in FIG. 1A, the two or more optical elements 110a-110f may include, but are not limited to, a set of mirrors 110a-110f. For example, the set of mirrors 110a-110f may include, but are not limited to, one or more turning mirrors. The set of mirrors 110a-110f may include any type of mirror known in the art. For example, the set of mirrors 110a-110f may include one or more of the following: one or more elliptical mirrors, one or more parabolic mirrors, one or more spherical mirrors, or one or more planar mirrors.

[0020] As shown in FIG. 1A, the pump beam 101 performs a first pass 108a through the plasma 106. A first portion of the first pass 108a of the pump beam 101 is absorbed by the plasma 106 and / or the gas 107 in close proximity to the plasma 106. A second portion of the first pass 108a of the beam 101 is transmitted through the semi-transparent plasma 106 and collected by the mirror 110a. The mirror 110a then directs the collected illumination to the second mirror 110b, which then performs the second pass 108b of the pump beam 101 through the plasma 106. Again, a first portion of the second pass 108b of the beam 101 is absorbed by the plasma 106 and / or the gas 107 in close proximity to the plasma 106, while a second portion of the second pass 108b of the beam 101 is transmitted through the plasma 106. This process is repeated multiple times via mirrors 110c-110c to perform passes 108c and 108d of the pump beam 101 through the plasma 106. It should be noted that the scope of the present disclosure is not limited to the number or arrangement of mirrors depicted in FIG. 1A, which is provided for illustrative purposes only. It should be recognized herein that the multipass assembly 102 may include any number of optical elements (e.g., reflective and / or transmissive) arranged and oriented in any manner to achieve multiple passes of the pump illumination through the plasma 106.

[0021] In another embodiment, pump source 104 includes one or more lasers. Pump source 104 may include any laser system known in the art. For example, pump source 104 may include any laser system known in the art capable of emitting radiation in the infrared, visible, and / or ultraviolet portions of the electromagnetic spectrum.

[0022] In one embodiment, the pump source 104 may include a laser system configured to emit a continuous wave (CW) laser beam. For example, the pump source 104 may include one or more CW infrared laser sources. For example, in a setting where the gas 107 is or includes argon, the pump source 104 may include a CW laser (e.g., a fiber laser or a disk Yb laser) configured to emit radiation at 1069 nm. It should be noted that this wavelength is particularly useful for pumping argon gas because it matches the 1068 nm absorption line of argon. It should be noted that the above description of a CW laser is not limiting herein and any laser known in the art may be implemented in the context of this application.

[0023] In another embodiment, the pump source 104 may include one or more lasers configured to provide laser light at a substantially constant power to the plasma 106. In another embodiment, the pump source 104 may include one or more modulated lasers configured to provide modulated laser light to the plasma 106. In another embodiment, the pump source 104 may include one or more pulsed lasers configured to provide pulsed laser light to the plasma.

[0024] In another embodiment, the pump source 104 may include one or more diode lasers. For example, the pump source 104 may include one or more diode lasers that emit radiation at wavelengths corresponding to any one or more absorption lines of the species of the gas 107. The diode lasers of the pump source 104 may be selected such that the wavelength of the diode laser is tuned to any absorption line (e.g., ionization transition line) of any plasma or any absorption line (e.g., highly excited neutral transition line) of any plasma generating gas known in the art. Thus, the selection of a given diode laser (or set of diode lasers) will depend on the type of gas 107 used by the system 100.

[0025] In another embodiment, the pump source 104 may include an ion laser. For example, the pump source 104 may include any noble gas ion laser known in the art. For example, in the case of an argon-based plasma, the pump source 104 used to pump the argon ions may include an Ar+ laser. In another embodiment, the pump source 104 may include one or more frequency converted laser systems. For example, the pump source 104 may include a Nd:YAG laser or a Nd:YLF laser with a power level in excess of 100 watts. In another embodiment, the pump source 104 may include a disk laser. In another embodiment, the pump source 104 may include a fiber laser. In another embodiment, the pump source 104 may include a broadband laser.

[0026] In another embodiment, pump source 104 may include one or more non-laser sources. Pump source 104 may include any non-laser light source known in the art. For example, pump source 104 may include any non-laser source known in the art capable of emitting radiation, either discretely or continuously, in the infrared, visible, or ultraviolet portions of the electromagnetic spectrum.

[0027] In another embodiment, the pump source 104 may include two or more light sources. In one embodiment, the pump source 104 may include two or more lasers. For example, the pump source 104 (or multiple "sources") may include multiple diode lasers. In another embodiment, each of the two or more lasers may emit a laser beam matched to a different absorption line of the gas or plasma in the system 100.

[0028] 1B illustrates a multi-pass assembly 102 comprised of a single reflective surface, in accordance with one or more embodiments of the present disclosure. In one embodiment, the multi-pass assembly 102 includes a reflective surface having a first portion and at least a second portion arranged to collect the unabsorbed portion of the pump beam 101 and redirect the collected unabsorbed portion of the pump beam back to the plasma 106 and / or gas 107 proximate to the plasma 106. In this embodiment, the reflective surface may be comprised of a single mirror having a complex shape, whereby the first portion of the mirror and at least the second portion of the mirror function as separate mirrors to perform multiple passes of the pump illumination through the plasma 106. The single reflective surface may be constructed to function in a similar manner to any of the embodiments of the present disclosure that include separate mirrors. In this regard, the single reflective surface may include multiple portions that replicate the separate mirrors of the various embodiments of the present disclosure. For example, as shown in FIG. 1B, the reflective surface may include, but is not limited to, multiple portions 113a-113f that serve to collect unabsorbed portions of pump illumination transmitted through the plasma 106 and redirect the collected pump illumination back to the plasma 106 and / or gas 107 proximate to the plasma 106. The shape and configuration of the single reflective surface in FIG. 1B should not be construed as a limitation to the present disclosure and is provided merely for illustrative purposes. It should be noted herein that the configuration of the reflective portions of the single reflective surface in FIG. 1B may be extended to any shape, number and / or location of multiple reflective portions (e.g., 113a-113f). In another embodiment, the single reflective surface may be formed from multiple pieces (e.g., a first half and a second half) that fit together. In another embodiment, the single reflective surface may be configured to act as a gas containment element or vessel for containing the gas 107.

[0029] 1C illustrates a system 100 with a set of collection optics 103 in accordance with one or more embodiments of the present disclosure. In one embodiment, the set of collection optics 103 includes one or more collection optics 111a-111f configured to direct broadband radiation 115 emitted by the plasma 106 to one or more downstream optics 105, which may be, but are not limited to, a homogenizer, a lens, a mirror, a filter, and / or an aperture. In this sense, the collection optics 111a-111f act to combine multiple images of the plasma 106, which results in a broadband output with increased brightness.

[0030] In one embodiment, the collecting optics 111a-111f may be positioned on the same plane as the multi-pass optics 110a-110f, as depicted in FIG. 1C. In this embodiment, the collecting optics 111a-111f are interposed between the multi-pass optics 110a-110f so as not to interfere with the lines of the sine of the multi-pass optics 110a-110f. When the broadband light 115 is emitted by the plasma 106, a portion of the light 115 is collected by the first collecting element 111a and directed back to the plasma 106 and / or gas 107 proximate to the plasma 106 via the second collecting element 111b. This process may be repeated any number of times via additional collecting optics 111c-111f to perform additional passes through the plasma 106 and / or gas 107 proximate to the plasma 106.

[0031] It should be noted that only a single reflected path is shown for the collected broadband radiation 115, starting from collection optics 111a. It should be recognized herein that the broadband radiation may be initially collected (prior to redirection) by any one of collection optics 111b-111f. It should also be noted that broadband light that begins its reflected path at any of elements 111b-111f experiences fewer passes through the plasma 106 than broadband light that begins at collection element 111a.

[0032] The number and configuration of collecting optics 111a-111f in Figure 1C should not be construed as a limitation on the scope of the present disclosure. Rather, the configuration depicted in Figure 1C is provided for illustrative purposes only. It should be noted that collecting optics 103 may be of any optical configuration suitable for collecting broadband light 115 emitted by plasma 106 and combining multiple images of the plasma to increase the brightness of the broadband output.

[0033] In one embodiment, not shown, collecting optics 103 may be positioned outside the plane defined by multi-pass optical elements 102. In this embodiment, collecting optics 111a-111f may be positioned in an optical plane different from the optical plane defined by multi-pass optical elements 110a-110f. For example, the planes of collecting optics 111a-111f may be positioned at 90° relative to the plane defined by multi-pass optical elements 110a-110f.

[0034] In another embodiment, not shown, the collection optics 103 may include a single collector element. For example, the single collector element may be comprised of a single reflective surface similar to the embodiment of FIG. 1B and configured to collect broadband light 115 emitted by the plasma 106. The reflective surface of the collector element may then serve to combine multiple images of the plasma 106 to provide a broadband output of increased brightness. In one embodiment, the multipass assembly 102 may be disposed within the interior volume of the single collector element.

[0035] In another embodiment, the multipass assembly 102 and the collection assembly 103 may be integrated into a single unit. In this regard, a single reflective surface may perform multiple passes of the pump beam 101 through the plasma 106 and / or gases near the plasma 106 while also serving to combine multiple images of the plasma 106 to increase the overall brightness of the broadband output. In this regard, a single reflective unit serves as both the multipass assembly 102 and the collection assembly 103. One such configuration includes an elliptical reflector assembly as discussed further herein (see FIGS. 2A-2H).

[0036] 1D illustrates a system 100 configured in a single pump pass mode in accordance with one or more embodiments of the present disclosure. For example, as shown in FIG. 1D, the pump illumination 104 passes through the plasma 106 once. In another embodiment, the collection optical assembly 103 may collect the broadband light 115 emitted by the pump illumination and redirect it back into the plasma 106.

[0037] Collector assembly 103 (or an integrated multipass / collector assembly) may collect one or more visible, NUV, UV, DUV, VUV, and / or EUV radiation emitted by plasma 106 and direct broadband illumination 115 to one or more downstream optics. In this regard, collector assembly 103 may provide visible, NUV, UV, DUV, VUV, and / or EUV radiation to downstream optics of any optical characterization system known in the art, such as, but not limited to, an inspection tool or a metrology tool. In this regard, broadband output 115 may be coupled to illumination optics of an inspection tool and / or metrology tool.

[0038] 1C , in one embodiment, one or more of the optical elements 110a-110f of the multipass assembly 102 and / or one or more of the collecting elements 111a-111f of the collection assembly 103 selectively transmit one or more selected wavelengths of light. For example, one or more of the optical elements 110a-110f and / or one or more of the collecting elements 111a-111f are transmissive to one or more spectral regions of the broadband radiation 115 emitted by the plasma 106. For example, one or more of the optical elements 110a-110f may be transmissive to one or more spectral regions of the broadband radiation 115 of interest to be collected by the collection assembly 103. In one embodiment, one or more of the optical elements 110a-110f of the multipass assembly 102 and / or one or more of the collecting elements 111a-111f of the collection assembly 103 selectively absorb one or more selected wavelengths of light. For example, one or more of the optical elements 110a-110f and / or one or more of the collecting elements 111a-111f may absorb one or more spectral regions of the broadband radiation 115 emitted by the plasma 106. In another embodiment, one or more of the optical elements 110a-110f of the multi-pass assembly 102 and / or one or more of the collecting elements 111a-111f of the collection assembly 103 selectively reflect one or more selected wavelengths of light. For example, one or more of the optical elements 110a-110f and / or one or more of the collecting elements 111a-111f may reflect one or more spectral regions of the broadband radiation 115 emitted by the plasma 106 back to the plasma 106. Additionally, one or more of the optical elements 110a-110f of the multipass assembly 102 and / or one or more of the collecting elements 111a-111f of the collection assembly 103 may reflect one or more selected wavelengths of the broadband light 115 back to the plasma 106, while selectively transmitting one or more additional selected wavelengths of the broadband light 115 (such that the additional selected wavelengths of light are not reflected back to the plasma). For example, one or more of the optical elements 110a-110f of the multipass assembly 102 may absorb one or more spectral regions of the broadband radiation 115 that are intended for collection by the collection assembly 103.

[0039] It should be noted herein that the volume of gas 107 from which the plasma 106 is initiated and sustained may be contained in any gas containment structure or vessel known in the art for plasma generation.

[0040] FIG. 1E illustrates a system 100 with a plasma sphere and / or cell 140 for containing the gas 107 according to one embodiment of the present disclosure. In one embodiment, the gas containment structure is a plasma sphere. The use of plasma spheres is described at least in U.S. Patent Application No. 11 / 695,348, filed April 2, 2007, U.S. Patent Application No. 11 / 395,523, filed March 31, 2006, and U.S. Patent Application No. 13 / 647,680, filed October 9, 2012, each of which have been previously incorporated herein by reference in their entirety. In another embodiment, the gas containment structure is a plasma cell. In the case of a plasma cell, the plasma cell may include, but is not limited to, a transmission element disposed in combination with one or more flanges for containing the gas 107. The use of flanged plasma cells is described at least in U.S. patent application Ser. No. 14 / 231,196, filed Mar. 31, 2014, and U.S. patent application Ser. No. 14 / 288,092, filed May 27, 2014, each of which was previously incorporated herein by reference in its entirety.

[0041] FIGURE 1F illustrates a system 100 with a chamber 150 for containing a gas 107, according to one embodiment of the disclosure. As shown in FIGURE 1E, the multi-pass optical assembly 102 and / or the collection assembly 103 may be disposed within the chamber 150. In another embodiment, the chamber 150 includes one or more entrance windows 142 for receiving pumping illumination 101 from the pump source 104. In another embodiment, the chamber 150 includes one or more exit windows 144 for transmitting broadband light 115 to one or more downstream optical elements 105. The use of a gas chamber as a gas containment structure is described in U.S. patent application Ser. No. 12 / 787,827, filed May 26, 2010, U.S. patent application Ser. No. 14 / 660,846, filed March 17, 2015, U.S. patent application Ser. No. 14 / 670,210, filed March 26, 2015, and U.S. patent application Ser. No. 14 / 224,945, filed March 25, 2014, each of which is incorporated herein by reference in its entirety.

[0042] The transparent portions (e.g., transparent elements, spheres, or windows) of the gas containment structure of the system 100 may be formed from any material known in the art that is at least partially transparent to the broadband light 115 generated by the plasma 106 and / or pump illumination 101. In one embodiment, one or more transparent portions (e.g., transparent elements, spheres, or windows) of the gas containment structure may be formed from any material known in the art that is at least partially transparent to EUV radiation, VUV radiation, DUV radiation, UV radiation, NUV radiation, and / or visible light generated within the gas containment structure. Additionally, one or more transparent portions of the gas containment structure may be formed from any material known in the art that is at least partially transparent to IR radiation, visible light, and / or UV light from the pump source 104. In another embodiment, one or more transparent portions of the gas containment structure may be formed from any material known in the art that is transparent to both radiation from the pump source 104 (e.g., an IR source) and radiation emitted by the plasma 106 (e.g., EUV, VUV, DUV, UV, NUV radiation and / or visible light).

[0043] In some embodiments, the transmissive portion(s) of the gas containment structure may be formed from a low OH content fused silica glass material. In another embodiment, the transmissive portion(s) of the gas containment structure may be formed from a high OH content fused silica glass material. For example, the transmissive elements, spheres or windows of the gas containment structure 140, 150 may include, but are not limited to, SUPRASIL1, SUPRASIL2, SUPRASIL300, SUPRASIL310, HERALUX PLUS, HERALUX-VUV, etc. In another embodiment, the transmissive elements, spheres or windows of the gas containment structure 140, 150 may include, but are not limited to, calcium fluoride, magnesium fluoride, lithium fluoride, crystalline quartz, and sapphire. Various glasses suitable for implementing the gas containment structures of the present disclosure are discussed in detail in A. Schreiber et al., Radiation Resistance of Quartz Glass for VUV Discharge Lamps, J. Phys. D: Appl. Phys. 38 (2005), 3242-3250, which is incorporated herein by reference in its entirety.

[0044] In one embodiment, the gas containment structure 140 and / or 150 may contain any selected gas known in the art suitable for generating a plasma when pump illumination is absorbed (e.g., argon, xenon, mercury, etc.). In one embodiment, pump illumination 101 from the pump source 104 is focused into the volume of gas 107, whereby energy is absorbed (via one or more selected absorption lines) by the gas or plasma within the gas containment structure, thereby "pumping" the gas species to generate and / or sustain the plasma 106. In another embodiment, not shown, the gas containment structure may include a set of electrodes for initiating the plasma 106 within the interior volume of the gas containment structure, whereby illumination from the pump source 104 sustains the plasma 106 after being ignited by the electrodes.

[0045] It is contemplated herein that the system 100 may be used to activate and / or maintain the plasma 106 in a variety of gas environments. In one embodiment, the gas used to activate and / or maintain the plasma 106 may include an inert gas (e.g., a noble or non-noble gas) or a non-inert gas (e.g., mercury). In another embodiment, the gas used to activate and / or maintain the plasma 106 may include a mixture of gases (e.g., a mixture of inert gases, a mixture of inert and non-inert gases, or a mixture of non-inert gases). For example, gases suitable for implementation in the system 100 of the present disclosure may include, but are not limited to, Xe, Ar, Ne, Kr, He, N2, H2O, O2, H2, D2, F2, CH4, one or more metal halides, halogens, Hg, Cd, Zn, Sn, Ga, Fe, Li, Na, Ar:Xe, ArHg, KrHg, XeHg, and any mixtures thereof. The present invention should be interpreted to extend to any optically pumped plasma generating system, and further to any type of gas suitable for sustaining a plasma within a gas containment structure.

[0046] FIG. 1G illustrates a multi-pass optical assembly 102 with one or more off-axis mirrors according to one or more embodiments of the present disclosure. In one embodiment, one or more of the mirrors are off-axis parabolic mirrors. In one embodiment, one or more of the mirrors are off-axis elliptical mirrors. In one embodiment, one or more of the mirrors are flat mirrors. In the example depicted in FIG. 1G, the multi-pass optical assembly 102 includes off-axis parabolic mirrors 110a, 110b, off-axis elliptical mirror 110c, and flat mirror 110d. It should be noted that illumination passing between off-axis parabolic mirrors 110a, 110b is substantially collimated. In contrast, illumination reflected from off-axis elliptical mirror 110c is focused to plasma 106 (via reflection from flat mirror 110d). It should be noted that the number, type, and arrangement of mirrors 110a-110d in FIG. 1G are not limitations on the scope of the present disclosure and are provided for illustration purposes only.

[0047] 1H illustrates a multi-pass optical assembly 102 arranged in an optical threading configuration, according to one or more embodiments of the present disclosure. In one embodiment, one or more higher passes 108c may be threaded between optical elements associated with a lower pass 108b. For example, the multi-pass optical assembly 102 may include a first set of optical elements 110a, 110b associated with a first pass 108b of the pump beam 101 through the plasma 106 and / or gas 107 proximate to the plasma 106. Additionally, the multi-pass optical assembly 102 may include an additional set of optical elements 110c, 110d associated with an additional pass 108c of the pump beam 101 through the plasma 106 and / or gas 107 proximate to the plasma 106. In another embodiment, the first set of optical elements 110a, 110b and the additional set of optical elements 110c, 110d are arranged such that the illumination of the additional path 108c of the pump beam 101 traverses the region 117 between the mirrors 110a and 110b of the first set of optical elements. It should be noted that the configuration depicted in FIG. 1H is provided for illustration only and not as a limitation to the scope of the present disclosure. It should be recognized herein that the embodiment of FIG. 1H may be extended to any setting where illumination from a higher reflection is threaded through a lower optical element as shown in FIG. 1H.

[0048] It should be noted that the embodiment depicted in FIG. 1H is particularly useful in settings where NA (numerical aperture) space is constrained, as is common in LSP applications. For example, such constraints on available NA space may result from a desire to separate the NA of pump illumination from the NA of collection illumination for plasma performance (or other technical reasons). Furthermore, it is generally desirable to use as much of the NA space for plasma pumping as possible to minimize the size of the plasma. Furthermore, it is typically desirable to have a particular pump intensity distribution, such as, but not limited to, a uniform intensity distribution, within the NA space. Implementations of the optical threading embodiments described above, in which illumination from a higher pass is directed between optical elements (e.g., steering mirrors) of earlier lower passes, are particularly useful. Such configurations help maximize the NA space available for LSP pumping.

[0049] FIG. 1I illustrates a multi-pass optical assembly 102 configured such that the NA of the illumination is reduced by one or more successive passes through the plasma 106, according to one or more embodiments of the present disclosure. For example, one or more optical elements 110a and 110b of the multi-pass assembly 102 may be arranged such that the NA of the pump beam 101 during the second pass 108b through the plasma 106 and / or gas 107 proximate to the plasma 106 is smaller than the NA of the pump beam 101 during the first pass 108a through the plasma 106 and / or gas proximate to the plasma 106. It should be noted that due to absorption of the pump beam 101 along each successive path, the focus NA of the successive higher passes may be reduced to maintain roughly the same intensity distribution in NA space as the lower passes. Such a configuration allows the pump illumination to be more uniformly distributed within the NA space designed for plasma pumping. In another embodiment, the optical elements of the multi-pass optical assembly 102 may be arranged to establish a variation in NA for successive passes of the pump beam 101 to produce a selected pump intensity distribution in NA space.

[0050] 1J and 1K illustrate control of the shape of the plasma 106 using a multi-pass optical assembly 102 according to one or more embodiments of the present disclosure. One or more optical elements of the multi-pass optical assembly 102 may be arranged to control the shape of the plasma 106 by controlling the position of each focal point associated with each pass of the pump beam 101 through the plasma 106 and / or gas 107 proximate to the plasma 106. In one embodiment, as previously indicated herein, the one or more optical elements 110a, 110b are arranged such that the focal points of each of the passes of the pump beam 101 substantially overlap to form a spot. In another embodiment, as shown in FIG. 1J, the one or more optical elements 110a, 110b are arranged to displace the focal point of a first pass 108a of the pump beam relative to the focal point of at least one additional pass 108b of the pump beam to form an elongated plasma region 121. 1K, one or more optical elements 110a, 110b are positioned to focus illumination from each successive path 108a, 108b into a line, thereby forming an elongated plasma region 121. In one embodiment, one or more of the optical elements of the multipass assembly 102 may be positioned coplanar with the pump source 104. In another embodiment, one or more of the optical elements of the multipass assembly 102 are positioned outside the plane defined by the pump source 104, thereby forming a three-dimensional distribution of the pump beam 101 in NA space.

[0051] It should be noted that the above arrangements and plasma shapes are provided for illustrative purposes only and are not intended to be a limitation on the scope of the present disclosure. The positioning of the focal point of the multi-pass may be controlled in any suitable manner to form a plasma region having a suitable shape.

[0052] 1L illustrates a multi-pass optical assembly 102 according to one or more embodiments of the present disclosure with one or more retroreflector assemblies. In one embodiment, one or more of the optical elements of the multi-pass optical assembly 102 include one or more retroreflector assemblies. For example, the multi-pass optical assembly 102 may include, but is not limited to, a retroreflector 116. The retroreflector 116 may include, but is not limited to, a spherical mirror. In this embodiment, following a first pass of the pump illumination 101 through the plasma 106, the mirrors 110a, 110b collect and direct the unabsorbed pump illumination 101 back through the plasma 106 to perform a second pass. The retroreflector 116 then receives the unabsorbed pump illumination from the second pass and reflects the unabsorbed pump illumination back through the plasma 106 to perform a third pass of the pump illumination 101 through the plasma 106. It should be noted that the use of a retroreflector effectively doubles the number of passes through a given optical system.

[0053] 1M-1N illustrate a multi-pass optical assembly 102 including one or more retroreflector assemblies arranged in a resonator configuration, according to one or more embodiments of the present disclosure. In one embodiment, as shown in FIG. 1M, the multi-pass optical assembly 102 may include, but is not limited to, two or more retroreflectors 116a, 116b arranged in a resonator configuration. In this regard, the first retroreflector 116a and the second retroreflector 116b form a resonator. In this embodiment, following a first pass of the pump illumination 101 through the plasma 106, the mirrors 110a, 110b collect the unabsorbed pump illumination 101 and direct it back through the plasma 106 to perform a second pass. The first retroreflector 116a (e.g., a spherical mirror) then receives the unabsorbed pump illumination from the second pass and reflects the pump illumination back through the plasma 106 to perform a third pass of the pump illumination 101 through the plasma 106. Mirrors 110b and 110a then direct the light from the third pass back through the plasma 106 to perform a fourth pass through the plasma 106. A second retroreflector 116b then receives the light from the fourth pass to repeat the optical path sequence.

[0054] In one embodiment, as shown in FIG. 1N, the multi-pass optical assembly 102 may include, but is not limited to, two or more retroreflectors 116a, 116b arranged in a simple resonator configuration. In this regard, the first retroreflector 116a and the second retroreflector 116b form a simple resonator. In this embodiment, following a first pass of the pump illumination 101 through the plasma 106, the first retroreflector 116a (e.g., a spherical mirror) receives the unabsorbed pump illumination from the first pass and reflects the pump illumination back through the plasma 106 to perform a second pass of the pump illumination 101 through the plasma 106. The second retroreflector 116b then receives the light from the second pass to repeat the optical path sequence. It should be noted that the number and arrangement of retroreflectors presented above are not limitations on the scope of the present disclosure and are provided for illustrative purposes only.

[0055] 10 illustrates a multi-pass optical assembly 102 with one or more polarizing elements according to one or more embodiments of the present disclosure. In one embodiment, the multi-pass optical assembly 102 includes a polarizer 125 and a quarter-wave wave 126 to perform polarization rotation within the multi-pass assembly 102. In one embodiment, the pump illumination 101 is horizontally polarized and passes through the polarizer 125 to a lens 124 to focus the illumination 101 onto the plasma 106. Mirrors 110a and 110b then collect the unabsorbed illumination and redirect it back through the plasma 106 to perform a second pass. The unabsorbed illumination from the second pass then passes through the quarter-wave wave 126 to the retroreflector 116a and reflects back through the quarter-wave wave 126, flipping the polarization to vertical polarization. The vertically polarized pump illumination then travels back through the optical assembly, making a third and fourth pass, returning to polarizer 125, which reflects the vertically polarized illumination to retroreflector 116b (e.g., a planar mirror). Retroreflector 116b then reflects the illumination back to polarizer 125, which transmits the illumination back through optical assembly 102. The number and arrangement of retroreflectors and polarization / waveplate elements presented above are not limitations on the scope of the disclosure and are provided for illustrative purposes only. In another embodiment, polarization rotation may be performed in multi-pass optical assembly 102 using multiple reflections from mirrors located outside of the plane defined by the pump illumination from pump source 104.

[0056] 1P-1Q show a multi-pass optical assembly 102 with one or more wavefront sensors for measuring aberrations in the multi-pass optical assembly, according to one or more embodiments of the present disclosure. It should be noted that in the multi-pass assembly 102, aberrations may accumulate in the transmitted pump laser wavefront after multiple passes through the plasma 106. The aberrations may result from the difference in refractive index between the plasma and the surrounding gas. These aberrations prevent effective focusing of the laser into subsequent passes. In one embodiment, the multi-pass optical assembly 102 may be configured to correct or at least reduce these aberrations. In one embodiment, the multi-pass assembly 102 includes one or more wavefront sensors 128 and one or more adaptive optics 118 communicatively coupled to the wavefront sensors 128. In another embodiment, the one or more wavefront sensors 128 may collect illumination along one or more of the pumps of the multi-pass assembly 102 to measure the aberrations. The wavefront sensor 128 may then transmit this measurement to one or more adaptive optics 118, which may then adjust the wavefront of the beam reflecting from the adaptive optics 118 to compensate for the aberrations measured by the wavefront sensor 128. The adaptive optics may include any adaptive optics known in the art, such as, but not limited to, a digital micromirror array device.

[0057] 1Q, the adaptive optics may include an adaptive retroreflector 130 (e.g., an adaptive spherical mirror). In this regard, the adaptive retroreflector 130 may act to reflect pump illumination back to the plasma 106 while also acting to correct for aberrations measured by the one or more wavefront sensors 128.

[0058] In another embodiment, not shown, the system 100 may include one or more controllers communicatively coupled to the one or more wavefront sensors 128 and the one or more adaptive optics 118 and / or 130. In this regard, the one or more controllers may receive wavefront data from the one or more wavefront sensors 128 and analyze the data for aberrations. The controller may then send one or more control signals to the one or more adaptive optics to instruct the one or more adaptive optics to compensate for the measured aberrations. The controller may include one or more processors and one or more memories for storing program instructions. The program instructions may be preprogrammed to cause the one or more processors to perform steps associated therewith.

[0059] 2A-2H show a multi-pass system 100 with a reflector assembly 202 according to one or more embodiments of the present disclosure. The reflector assembly 202 of this embodiment may provide for radiance closer to the blackbody limit. Collection of multiple passes through the plasma allows optimization of the gas pressure as if the plasma were much larger. As a result, the absorption / emissivity of the desired broadband output wavelength in the plasma for each pass may be reduced. If a comparable geometry is used for the pumping illumination, the absorption of the pumping illumination may be reduced as well, allowing for lower gas pressures. Operation of the system 100 at lower gas pressures helps reduce "gas wiggle" that results from density and temperature gradients and leads to deviations in reflectivity.

[0060] Furthermore, light emitted from the plasma 106 of the reflector assembly 202 at angles that do not direct it to an output port is always focused back to (or close to) the plasma(s). As a result, light that would normally be lost is partially used to pump the plasma(s). Such a configuration results in improved efficiency of the system 100 compared to a single pass system, allowing a lower power pump source 104 to achieve a given broadband radiance.

[0061] 2A-2B show cross-sectional views of the reflector assembly 202 of the system 100 in accordance with one or more embodiments of the present disclosure. It should be noted that Figure 2A represents a cross-section perpendicular to the cross-section of Figure 2B.

[0062] In one embodiment, the system 100 includes a pump source 104 and a reflector assembly 202. The pump source 104 may include one or more illumination sources. For example, the pump source 104 may include a single illumination source or multiple discrete illumination sources (see FIGS. 2G-2H). It should be noted that the pump source 104 may be comprised of any of the pump sources 104 previously discussed herein. In one embodiment, the reflector assembly 202 serves as a pressure vessel for containing the gas 107. It should be noted that the reflector assembly 202 may include any of the gases described throughout this disclosure, including, but not limited to, Ar, Xe, Ne, N2, H2, or mixtures thereof. In another embodiment, the reflector assembly 202 may maintain the pressure of the gas 107 at a pressure between 1 atm and 100 atm.

[0063] In another embodiment, the wavelength of the pump illumination may be adjusted to match an absorption line of the gas 107 such that the gas efficiently absorbs the pump illumination 101. This approach is particularly useful in low density settings.

[0064] In another embodiment, the reflector assembly 202 includes one or more entrance apertures 204 for receiving the pump beam 101 (see FIG. 2A) from the illumination source 104. In another embodiment, the reflector assembly 202 includes one or more exit apertures 206 for passing at least a portion of the broadband radiation 115 (see FIG. 2B) from the interior volume of the reflector assembly to one or more downstream optical elements 105.

[0065] In one embodiment, the reflector assembly 202 is an ellipsoidal shaped shell having a reflective inner surface 203. In one embodiment, as shown in FIG. 2A, the pump illumination 101 enters the reflector assembly 202 and is focused by the reflective inner surface 203 to a first focal point 201a of the reflector assembly 202. In response to the excitation energy from the pump illumination 101, a plasma is sustained (activated and / or maintained) at the first focal point 201a. The broadband light 115 is then emitted from the plasma 106, as shown in FIG. 2B. In another embodiment, the inner reflective surface of the reflector assembly 202 is configured to collect at least a portion of the broadband radiation emitted from the plasma and direct the collected broadband radiation back to the plasma 106. This process is repeated until a given portion of the broadband light 115 is absorbed by the plasma 106 or exits through the exit aperture 206. In this regard, the internal reflective surface 203 of the reflector assembly 202 acts to combine two or more images of the plasma 106 such that the output portion of the broadband radiation 115 has a brightness greater than the brightness of a single image of the plasma 106. It should be noted that the path of the redirected broadband light 115 may first pass through a second focal point 201b before reaching the first focal point 201a.

[0066] In one embodiment, the elliptical reflector assembly 202 includes a single elliptical shell. In another embodiment, the elliptical reflector assembly 202 includes a partial elliptical shell. It should be noted that the light coupled through the exit aperture 206 comes from a limited footprint on an ellipse. Therefore, a fully elliptical shell is not always necessary. In this case, a partial elliptical shell formed from multiple elements that cover the portion of the ellipse that is illuminated may be implemented. For example, a partial elliptical shell may include any two or more sections of a full elliptical surface.

[0067] In one embodiment, the output angle and separation of the focal points 201a, 201b may be selected such that the middle portion of the ellipse does not receive broadband light. In this case, the reflector assembly 202 may include two elliptical collectors that represent the top and bottom of a full ellipse. Such a configuration also allows for the elliptical collector to be surrounded with a second, larger ellipse, thereby directing laser light into the plasma 106 from all directions, providing a smaller, hotter plasma 106. Using an elliptical collector rather than a full ellipse relaxes the requirements of the coating process for the internal reflective surfaces.

[0068] The entrance aperture 204 and / or the exit aperture 206 may be fitted with one or more windows. The entrance aperture 204 and / or the exit aperture 206 windows may be formed from any material known in the art. For example, the entrance aperture 204 window and / or the exit aperture 206 window may comprise one or more of the following materials: sapphire, fused silica, crystalline quartz, magnesium fluoride, calcium fluoride, lithium fluoride. In another embodiment, the entrance aperture 204 window may be curved to shape and / or focus the pump illumination 101.

[0069] The pump source 104 may include one or more illumination sources. For example, the pump source 104 may include a single illumination source or multiple discrete illumination sources (see FIGS. 2G-2H). It should be noted that the pump source 104 may be comprised of any of the one or more pump sources 104 previously discussed herein. In another embodiment, the pump source 104 may include a directly coupled pump source (e.g., a directly coupled laser diode). In another embodiment, the pump source 104 may include a fiber coupled pump source.

[0070] 2A, the pump illumination 101 may make a single pass through the plasma 106 located at a first focal point 201a of the elliptical reflector assembly 202. In this embodiment, the reflector assembly 202 may include an additional exit aperture for passing unabsorbed pump illumination transmitted through the plasma 106 to a region external to the reflector assembly 202.

[0071] 2A, the reflector assembly 202 may be configured to recirculate the pump illumination 101. In this regard, the internal reflective surface 203 of the reflector assembly 202 is configured to collect the unabsorbed portion of the pump beam 101 transmitted through the plasma 106 located at the first focal point 201a and direct the collected unabsorbed portion of the pump beam back to the plasma 106 or to the gas 107 near the plasma 106. It should be noted that the path of the redirected pump illumination 101 may first pass through a second focal point 201b before reaching the first focal point 201a.

[0072] In another embodiment, not shown, the system 100 may include one or more plasma ignition units. For example, the system 100 may include one or more electrode assemblies disposed within the reflector assembly 202 to ignite the plasma 106. For example, the system 100 may include one or more electrode assemblies disposed within the reflector assembly 202 to ignite the plasma 106. As another example, the system 100 may include one or more RF plasma generators to ignite the plasma 106. As another example, the system 100 may activate the plasma 106 using laser light from the pump source 104 or using laser light from an additional laser source dedicated to plasma ignition.

[0073] In another embodiment, the gas 107 may be pre-ionized prior to injection into the ellipsoidal reflector assembly 202. For example, the gas 107 may be pre-ionized by one or more of electrical ionization, laser ionization, or RF ionization. For example, the system 100 may include one or more of an additional set of electrodes, an additional laser ignition unit, or an RF ignition unit configured to pre-ionize the gas 107 before it enters the interior volume of the reflector assembly 202.

[0074] In one embodiment, as shown in FIGS. 2A and 2B, the inner surface of the elliptical reflector assembly is configured to form a single plasma 106 at one of the focal points 201a of the elliptical reflector assembly 202.

[0075] In another embodiment, the major axis of the elliptical reflector assembly may be disposed vertically. In another embodiment, the major axis of the elliptical reflector assembly may be disposed horizontally. In another embodiment, the major axis of the elliptical reflector assembly may be disposed at a selected angle relative to the vertical or horizontal direction. Furthermore, the selection of the focal point at which the single plasma 106 is formed may be based on one or more factors. For example, if the major axis of the elliptical reflector assembly is disposed vertically, the single plasma 106 may be formed at either the top focal point 201a or the bottom focal point 201b based on the shape of the gas flow (see FIGS. 2D-2E). It should be noted that the location of the plasma 106 (focal point 201a or focal point 201b) may be controlled by where the plasma is initiated (e.g., via an electrode, an RF ignition unit, or a laser ignition unit).

[0076] In another embodiment, two plasma regions 106a, 106b may be formed by an elliptical reflector assembly 202, as shown in FIG. 2C. For example, a first plasma 106a is formed at a first focal point 201a of the elliptical reflector assembly 202, and a second plasma is formed at a second focal point 201b of the elliptical reflector assembly 202. Such a dual plasma configuration reduces the number of reflections on the round trip through the plasma. The dual plasma configuration of FIG. 2C is particularly advantageous for spectral types (e.g., 100-200 nm) where the reflective surface 203 is a less than ideal reflector. In these settings, the amount of broadband light 115 collected by the system 100 is greater when the number of reflections on the round trip through the plasmas 106a, 106b is reduced.

[0077] In another embodiment, in the case of two plasmas 106a, 106b, the two plasmas 106a, 106b may be activated sequentially. For example, in a vertical configuration, the lower plasma 106b may be activated by any of the means described above (e.g., electrical, RF or laser). The plume from the first plasma 106a may then act to ignite the upper plasma 106b.

[0078] 2D and 2E show a reflector assembly 202 with a gas circulation assembly 209 according to one or more embodiments of the present disclosure. It should be noted that the limited output ports / paths in the reflector assembly 202 lead to a large portion of the energy in the injected pump illumination beam 101 being absorbed by the gas 107 and / or the internal reflective surface 203. As a result, one or more cooling techniques may be implemented to maintain the temperature of the reflector assembly 202 at or below the temperature required for high quality plasma operation. In one embodiment, the gas circulation assembly 209 includes a gas input port 210 and a gas output port 212 for circulating gas through the reflector assembly 202.

[0079] As shown in FIGS. 2D-2F, the gas input port 210 provides a flow of gas to the plasma 106a and / or 106b. The gas output port 212 then exhausts the gas 107 from the reflector assembly 202. Although the embodiments of FIGS. 2D and 2E depict the gas input port 210 and the gas output port 212 arranged along the same direction, this is not a limitation to the present disclosure. In another embodiment, the gas input port 210 and the gas output port 212 may be arranged such that the flow of gas entering the reflector assembly 202 is oriented at a selected angle relative to the flow of gas exiting the reflector assembly 202. In another embodiment, the flow of gas may be biased toward and coupled to a heat sink external to the reflector assembly 202. In another embodiment, the reflector assembly 202 includes one or more convention devices and / or heat sinks configured to cool the reflector assembly 202 by convention. Reflux-based cooling is generally described in U.S. patent application Ser. No. 14 / 288,092, filed May 27, 2014, previously incorporated by reference herein in its entirety.

[0080] In one embodiment, as shown in FIG. 2F, the gas inlet / outlet ports 210, 212 may act as plasma ignition electrodes. For example, the system 100 may include one or more electrical feedthroughs 224. The one or more electrical feedthroughs 224 may act to insulate the metal gas inlet / outlet tubes 210, 212. Additionally, the gas inlet / outlet tubes 210, 212 may be electrically coupled to a high voltage source 225 to provide electrical energy to the gas 107 for igniting the plasma 106. Additionally, the system 100 may include one or more electrical insulation sections 220 to insulate the remainder of the gas circulation system 209 from the charged gas inlet / outlet tubes 210, 212.

[0081] In another embodiment, the reflector assembly 202 is a multi-piece reflector assembly. In this regard, the elliptical shell of the assembly 202 may be formed from two or more pieces. For example, as shown in FIG. 2F, the reflector assembly 202 may include a top portion 230 and a bottom portion 232 that, when mated together, form the elliptical shell of the reflector assembly 202.

[0082] In another embodiment, the individual components 230 and / or 232 may be cooled. For example, the top 230 and / or bottom 232 of the reflector assembly 202 may be liquid cooled. For example, the top 230 and / or bottom 232 of the reflector assembly 202 may include a liquid flow system (e.g., one or more tubes) for passing a cooling liquid (e.g., water) through the bodies of the components 230, 232. Additionally, the liquid flow system may be coupled to an external heat sink, such that heat is transferred from the top 230 and / or bottom 232 of the reflector assembly 202 to the heat sink by liquid flowing in the flow system.

[0083] 2G-2H show a reflector assembly 202 with multiple illumination sources 104a-104d, in accordance with one or more embodiments of the present disclosure. Figures 2G and 2H show top and side views, respectively, of the reflector assembly 202. It should be noted herein that the use of multiple discrete sources 104a-104d can help achieve high NA plasma pumping at the reflector assembly 202.

[0084] In one embodiment, the reflector assembly 202 includes a single inlet port 204 that receives light from discrete pump sources 104 a - 104 d distributed around the reflector assembly 202 .

[0085] In another embodiment, the reflector assembly 202 includes multiple inlet ports 204 distributed about the reflector assembly 202. In one embodiment, the multiple inlet ports 204 may be positioned at different angles on opposite sides of the elliptical reflector assembly 202 such that the footprints associated with each source 204 are interleaved. Such a configuration results in pump illumination having a more uniform intensity deviation with a solid angle.

[0086] In one embodiment, the multiple inlet ports 204 may be arranged such that the solid angle subtended by each illumination reflection is smaller than the previous reflection. Such a configuration may help maintain the intensity of the illumination as power is absorbed by the plasma 106 as the pumping illumination 101 traverses the plasma 106.

[0087] 2A-2F, in one embodiment, the reflector assembly 202 includes a single aperture used as the laser entrance and broadband output. In this embodiment, the pumping illumination 101 may enter the reflector assembly 202 through the aperture opposite the output broadband light 115. The pumping illumination 101 and the output broadband light 115 may be separated using a cold mirror.

[0088] The subject matter described herein sometimes shows different components contained within or connected to another component. It should be understood that such depicted architectures are merely exemplary, and that in fact many alternative architectures that achieve the same functionality may be implemented. In a conceptual sense, any configuration of components to achieve the same functionality is effectively "coupled" such that the desired functionality is achieved. Thus, two components herein that are coupled to achieve a particular functionality may be considered to be "coupled" with each other such that the desired functionality is achieved regardless of the architecture or intermediate components. Similarly, any two components that are so coupled may be considered to be "connected" or "coupled" with each other to achieve the desired functionality, and any two components capable of such coupling may likewise be considered to be "coupleable" with each other to achieve the desired functionality. Particular examples of coupleable include, but are not limited to, physically interactable and / or physically interacting components and / or wirelessly interactable and / or wirelessly interacting components and / or logically interactable and / or logically interacting components.

[0089] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in the form, configuration and arrangement of parts may be made without departing from the disclosed subject matter or sacrificing all of its substantial advantages. The forms described are merely illustrative, and it is the intent of the following claims to embrace and include all such modifications. Moreover, it should be understood that the invention is defined by the appended claims.

Claims

1. a pump source configured to generate a pump beam; a gas containment structure for containing the gas; an optical assembly including one or more optical elements configured to perform one or more passes of the pump beam through a portion of the gas to maintain a broadband light-emitting plasma, the optical assembly being positioned to collect an unabsorbed portion of the pump beam that has passed through the plasma and to redirect the collected unabsorbed portion of the pump beam back into the gas; a collection assembly including one or more collection optics configured to direct at least a portion of the broadband radiation emitted by the plasma to one or more downstream optical elements and configured to collect and direct at least a portion of the broadband radiation emitted by the plasma back to the plasma; Equipped with the optical assembly and the collection assembly are integrated into a single elliptical reflector assembly; maintaining a plasma with the combination of the optical assembly and the collection assembly; a first plasma is formed at a first focal point of the single ellipsoidal reflector assembly; a second plasma is formed at a second focal point of the single ellipsoidal reflector assembly different from the first focal point, and the first plasma and the second plasma are operated sequentially. system.

2. Ignition means for optionally forming the first plasma and the second plasma. The system of claim 1 further comprising:

3. 1. A system comprising: a pump source configured to generate a pump beam; a gas containment structure for containing the gas; a multi-pass optical assembly including one or more optical elements configured to perform multiple passes of the pump beam through the portion of the gas to maintain a broadband light-emitting plasma, the multi-pass optical assembly being positioned to collect an unabsorbed portion of the pump beam that is transmitted through the plasma and to direct the collected unabsorbed portion of the pump beam back into the portion of the gas; Equipped with The one or more optical elements include a plurality of focusing mirrors arranged in an optical threading configuration, and one or more optical elements of a higher path of the multi-pass optical assembly are configured to direct a pump beam from the higher path between two or more optical elements of a lower path of the multi-pass optical assembly and arranged to generate an intensity distribution in a subsequent path that is substantially the same as the intensity distribution of a first path.

4. 1. A system comprising: a pump source configured to generate a pump beam; a gas containment structure for containing the gas; A system comprising an optical assembly including a multi-pass optical assembly and a collection assembly, the multi-pass optical assembly including one or more optical elements configured to perform multiple passes of the pump beam through a portion of the gas to maintain a broadband light-emitting plasma, the collection assembly including one or more collecting optical elements, the one or more collecting optical elements configured to collect at least a portion of broadband radiation emitted by the plasma and return it to the plasma, and interspersed between the one or more optical elements of the multi-pass optical assembly.

5. The system of claim 4 , wherein the collection assembly is further configured to combine two or more images of the plasma.

6. 6. The system of claim 5, wherein the collection assembly is further configured to combine two or more images of the plasma such that an output portion of the broadband radiation has a brightness greater than a brightness of a single image of the plasma.

7. 1. A system comprising: an illumination source configured to generate a pump beam; 1. A system comprising: a reflector assembly configured to contain a gas, the reflector assembly including one or more entrance openings for receiving the pump beam from the illumination source, the pump beam maintaining a plasma within a portion of the gas, the plasma emitting broadband radiation, an inner surface of the reflector assembly configured to collect at least a portion of the broadband radiation emitted by the plasma and direct the collected broadband radiation back to the plasma, the reflector assembly configured to collect an unabsorbed portion of the pump beam that was transmitted through a portion of the gas and direct the collected unabsorbed portion of the pump beam back to the portion of the gas.

8. The system of claim 7 , wherein the reflector assembly is further configured to combine two or more images of the plasma.

9. 10. The system of claim 8, wherein the reflector assembly is further configured to combine two or more images of the plasma such that an output portion of the broadband radiation has a brightness greater than a brightness of a single image of the plasma.

10. The system of claim 7 , wherein the reflector assembly is an elliptical reflector assembly.

11. The system of claim 10 , wherein an inner surface of the elliptical reflector assembly is configured to focus a portion of the broadband radiation to one or more focal points of the elliptical reflector assembly.

12. The system of claim 10 , wherein the plasma is formed at a single focal point of the elliptical reflector assembly.

13. The system of claim 10 , wherein a first plasma is formed at a first focal point of the elliptical reflector assembly and a second plasma is formed at a second focal point of the elliptical reflector assembly.

14. The system of claim 10 , wherein the elliptical reflector assembly is a partial elliptical reflector assembly.

15. The system of claim 7 , wherein the reflector assembly is a multi-component reflector assembly.

16. The system of claim 7 , wherein the one or more entrance apertures of the reflector assembly include a plurality of entrance apertures for receiving the pump beam from a plurality of illumination sources.

17. 8. The system of claim 7, wherein the one or more entrance openings of the reflector assembly include one or more windows formed from at least one of sapphire, fused silica, quartz, magnesium fluoride, calcium fluoride, or lithium fluoride.

18. The system of claim 7 , wherein the reflector assembly includes one or more exit apertures for passing at least a portion of the broadband radiation from an interior volume of the reflector assembly to one or more downstream optical elements.

19. 20. The system of claim 18, wherein one of the one or more exit apertures of the reflector assembly includes one or more windows formed from at least one of sapphire, fused silica, quartz, magnesium fluoride, calcium fluoride, and lithium fluoride.

20. The system of claim 7 further comprising a gas recirculation assembly.

21. The gas recirculation assembly comprises: A gas input port; a gas output port for flowing the gas through an interior volume of the reflector assembly via the gas recirculation assembly; The system of claim 20, comprising:

22. The system of claim 7 , wherein the reflector assembly includes one or more electrical feedthroughs.

23. The system of claim 7 , wherein the illumination source comprises one or more lasers.

24. 24. The system of claim 23, wherein the one or more lasers include at least one of one or more infrared lasers, one or more visible lasers, or one or more ultraviolet lasers.

25. 24. The system of claim 23, wherein the one or more lasers include at least one of a diode laser, a disk laser, or a fiber laser.

26. 24. The system of claim 23, wherein the one or more lasers include at least one of a narrowband laser or a broadband laser.

27. 24. The system of claim 23, wherein the one or more lasers are configured to operate in at least one of a continuous wave (CW) mode, a pulsed mode, or a modulated mode.

28. The system of claim 7 , wherein the gas comprises at least one of an inert gas, a non-inert gas, and a mixture of two or more gases.

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