VUV laser-sustained plasma light source with long-pass filtering.

Abstract

A laser-sustained broadband light source is disclosed. The light source may include a gas containment structure containing a mixture 103 of a first noble gas and a second noble gas. The light source may include a laser pump source 106 for generating an optical pump 108 to sustain a plasma 112 within the gas containment structure. The first noble gas absorbs broadband light 113a within a first wavelength band and a second wavelength band. The light source may include a filter 104 located within the gas containment structure and configured to absorb broadband light 113b emitted by the plasma having wavelengths below a selected wavelength threshold. Absorption of the broadband light 113b by the first noble gas and the filter 104 provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical elements from damage.

Description

[Technical Field]

[0001] This application claims priority to U.S. Provisional Application No. 63 / 445,307, filed February 14, 2023, and U.S. Provisional Application No. 63 / 446,911, filed February 20, 2023, which are incorporated herein by reference in their entireties.

[0002] The present invention relates generally to plasma-based radiation sources, and more particularly to high-power vacuum ultraviolet (VUV) laser-sustained plasma (LSP) light sources with long-pass filtering. [Background technology]

[0003] Laser-sustained plasma (LSP) light sources are widely used in broadband inspection tools used for semiconductor inspection and imaging. Typically, near-infrared (NIR) continuous-wave (CW) pump laser light is focused into a gas-filled vessel, where a plasma is ignited and sustained by absorption of the pump laser radiation. This vessel can be a lamp (e.g., a glass bulb with or without electrodes used for plasma ignition), a cell (e.g., an optomechanical assembly with transparent walls that allow laser and plasma radiation to enter and exit the cell), a chamber (e.g., a metal vessel with transparent windows for laser light input and plasma light output), or a similar assembly. Various plasma vessels have high internal pressures, reaching tens of atmospheres or even over a hundred atmospheres, during operation. This high-pressure gas contained within the vessel is critical to the operation of the LSP. The plasma light is focused through the vessel's transparent walls or windows and used as an illumination source for the inspection tool.

[0004] Many different types of such light sources have been developed. Most of these sources are designed to operate in the visible (VIS) or ultraviolet (UV) spectral regions. When using these sources to generate light in the vacuum ultraviolet (VUV) spectral region, particularly in the region between approximately 125 and 150 nm, practical design options are relatively limited, limiting them to relatively low pump powers. A typical light source for generating VUV light includes a metal chamber with multiple windows that couple laser light into and out of the chamber. While various materials are available for the laser windows, there are fewer options for VUV generation. The most widely used are MgF2, with a transmission cutoff wavelength of approximately 115 nm, or CaF2, with a transmission cutoff wavelength of approximately 125 nm.

[0005] One of the most significant limitations in the operation of such VUV sources is optical damage to MgF2 windows and downstream optics. All transmissive and reflective optics are rapidly damaged by exposure to short-wavelength radiation. Bulk MgF2 materials and MgF2-containing reflector coatings are severely damaged by radiation at or near the 115 nm absorption band edge. Currently, few options exist to avoid optical damage to these materials, both in the light source and downstream optics. Transmissive optical components and downstream optics directly irradiated by the plasma suffer significant damage due to wavelengths that penetrate into and are absorbed by the bulk material. Reflective components are also rapidly damaged by wavelengths shorter than approximately 125 nm. Wavelengths longer than approximately 117 nm propagate longer distances in MgF2, damaging downstream optics. If the irradiation wavelength is longer than approximately 125 nm, damage to optical components is significantly reduced.

[0006] Another important issue concerns the thermal management of windows in high-power LSPs, due to overheating of the MgF2 windows caused by light absorption by the MgF2. Overheating compromises the structural strength of the MgF2 windows. Moving the windows away from the plasma makes them easier to cool, but this also increases the structural load on the windows. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent No. 3357081 [Patent Document 2] U.S. Patent Application Publication No. 2021 / 0231292 [Patent Document 3] U.S. Patent Application Publication No. 2019 / 0033204 Summary of the Invention [Problem to be solved by the invention]

[0008] It would therefore be desirable to provide a VUV broadband light source that overcomes the limitations outlined above. [Means for solving the problem]

[0009] A laser-sustained broadband light source is disclosed that, in some embodiments, includes: a gas containment structure containing a mixture of a first noble gas and a second noble gas; a laser pump source configured to generate an optical pump to sustain a plasma within the gas containment structure, the plasma generating broadband light and the first noble gas absorbing portions of the broadband light within a first wavelength band and a second wavelength band; and a filter located within the gas containment structure and configured to absorb portions of the broadband light emitted by the plasma having wavelengths below a selected wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage.

[0010] A characterization system is disclosed that includes: a broadband light source including a gas containment structure containing a mixture of a first noble gas and a second noble gas; a laser pump source configured to generate an optical pump to sustain a plasma within the gas containment structure, wherein the plasma generates the broadband light and the first noble gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; a filter located within the gas containment structure and configured to absorb portions of the broadband light emitted by the plasma having wavelengths below a selected wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; and a collector element configured to collect at least a portion of the broadband light emitted from the plasma; a set of illumination optics configured to direct the broadband light from the broadband light source to one or more samples; a set of collection optics configured to collect light emitted from the one or more samples; and a detector assembly.

[0011] A method for generating VUV broadband light is disclosed. In some embodiments, the method includes: containing a mixture of a first noble gas and a second noble gas in a gas containment structure; generating and directing an optical pump into the gas containment structure to maintain a plasma in the gas containment structure to generate the broadband light; and performing long-pass filtering of the broadband light, the performing long-pass filtering including absorbing portions of the broadband light within a first wavelength band and a second wavelength band via the first noble gas and absorbing portions of the broadband light having wavelengths below a selected wavelength threshold via a filter.

[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 present disclosure. The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate the subject matter of the present disclosure, and together with the description and drawings serve to explain the principles of the present disclosure.

[0013] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the following drawings. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a simplified schematic diagram of a laser-sustained plasma (LSP) broadband light source with long-pass filtering in accordance with one or more embodiments of the present disclosure. [Figure 2] 1 is a data graph depicting the material transmission characteristics of an LSP broadband light source with a CaF 2 filter and a gas mixture of Ar / Kr, in accordance with one or more embodiments of the present disclosure. [Figure 3] FIG. 1 is a simplified schematic diagram of an LSP broadband light source with a filter tube in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 1 is a simplified schematic diagram of an LSP broadband light source with an ellipsoidal reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 5] FIG. 1 is a simplified schematic diagram of an LSP broadband light source with a pressurized reflector assembly in accordance with one or more embodiments of the present disclosure. [Figure 6] FIG. 10 is a simplified schematic diagram of a characterization system incorporating an LSP broadband light source in accordance with one or more alternative and / or additional embodiments of the present disclosure. [Figure 7] FIG. 1 is a process flow diagram depicting a method of producing VUV light using an LSP broadband light source with long-pass filtering, in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0015] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with reference to certain embodiments and detailed features thereof. The embodiments set forth herein are to be construed as illustrative and not limiting. 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.

[0016] Referring generally to Figures 1-6, a VUV laser-sustained plasma broadband light source with longpass filtering is illustrated in accordance with one or more embodiments of the present disclosure.

[0017] Embodiments of the present disclosure are directed to an LSP broadband light source with long-pass filtering. In embodiments, long-pass filtering is achieved by a selected combination of gas mixtures and materials used in one or more filters (e.g., filter sheets, filter tubes, etc.). The filter materials used in a particular filter are selected to achieve substantial absorption of light having wavelengths below a selected threshold. For example, in the case of a CaF2 filter, the CaF2 filter can achieve strong absorption of light below approximately 125 nm. In combination with the filter material, the gas mixture surrounding the filter is selected so that one or more components of the gas mixture exhibit one or more strong absorption lines: i) near the absorption edge of one or more downstream optics (e.g., windows, mirrors, lenses) to protect one or more downstream optics from degradation, and / or ii) near the absorption edge of the filter itself to protect the filter from degradation. For example, in environments where it is desired to protect MgF2 optics (e.g., MgF2 windows, MgF2-coated mirrors, MgF2 lenses), a CaF2 filter that achieves strong absorption of light below 125 nm may be used, which generally protects the MgF2 optics. Additionally, the inclusion of Kr in the plasma-generating gas mixture of the light source provides a strong additional absorption band around the CaF absorption edge, thereby protecting the CaF filter itself. The absorption band of Kr gas can be broadened by increasing the partial pressure of Kr gas in the gas mixture. U.S. Application No. 18 / 438,025, filed February 9, 2024, is incorporated herein by reference in its entirety.

[0018] FIG. 1 shows a simplified schematic diagram of an LSP broadband light source 100 with long-pass filtering, according to one or more embodiments. In an embodiment, the light source 100 includes a gas containment structure 102 containing a gas mixture of a first noble gas (e.g., Kr) and a second noble gas (e.g., Ar). In an embodiment, the light source 100 includes a filter 104 (e.g., a CaF2 filter) located within the gas containment structure 102. In an embodiment, the light source 100 includes a laser pump source 106 configured to generate an optical pump 108. The laser pump source 106 and one or more focusing optics 107 can direct and focus the optical pump 108 through an input optical window 110 to maintain a plasma 112 within the gas containment structure 102 to generate broadband light 113. The laser pump source 106 can include any laser known in the art of plasma-based broadband light generation. In an embodiment, the laser pump source 106 can include one or more continuous-wave (CW) pump lasers and / or one or more pulsed lasers. For example, the laser pump source 106 may include, but is not limited to, a fiber laser, a thin-disk laser, a frequency-doubled laser, or a diode laser. The laser pump source 106 may be configured to emit light in the visible, IR (e.g., NIR), or ultraviolet ranges.

[0019] In an embodiment, the first noble gas absorbs a portion 113a of the broadband light within a first wavelength band and a second wavelength band. The filter 104 can absorb a portion 113b of the broadband light having wavelengths below a selected wavelength threshold. The absorption of the broadband light 113 (the combination of 113a and 113b) by the first noble gas and the filter 104 provides long-pass filtering of the broadband light 113 below the selected wavelength to protect one or more downstream optical elements 111 (e.g., lenses, mirrors, windows) from damage. The one or more downstream optical elements 111 may include, but are not limited to, one or more windows 114, one or more lenses 116, or one or more mirrors 118 (e.g., MF2-coated aluminum mirrors). In an embodiment, the output filtered broadband light 117 is transmitted out of the gas containment structure 102 through the output optical window 114 (e.g., an MgF2 window).

[0020] 1, light source 100 includes one or more collection optical elements for collecting filtered broadband light 117 and transmitting filtered broadband light 117 through an output optical window (e.g., an MgF2 window) to one or more downstream optical elements outside gas containment structure 102. For example, light source 100 may include a collection mirror, such as, but not limited to, a retroreflector. Multiple collection arrangements are described in further detail with respect to FIGS. 3, 4, and 5.

[0021] It should be noted that the first noble gas and filter material can be selected to achieve the desired long-pass filtering characteristics, including various combinations of the first noble gas and the filter tube material. The scope of the present disclosure should not be construed as limited to any particular noble gas or filter material. By way of example, in a first combination, the first noble gas may include krypton, the second noble gas may include argon, and the filter material may include CaF2. Such a combination is particularly useful for protecting MgF2-based optical elements (e.g., output windows, focusing lenses, mirrors, etc.) from broadband output. Figure 2 is a data graph depicting the transmission characteristics of the materials of the light source 100 for a CaF2 filter and an Ar / Kr gas mixture. Krypton has an absorption line centered at approximately 124 nm, thereby blocking radiation at the 123 nm absorption edge of CaF2, thereby protecting the CaF2 filter itself from damage. For a thin CaF2 filter, a small amount of Kr can be added to the Ar. Kr absorbs a certain amount of light in the spectral bands around 116 nm and 124 nm (see, e.g., Figure 2). The absorption in these spectral bands is caused by very strong absorption transitions emanating from the ground state, which tend to red-broaden from the dominant transition wavelengths of 123.58 nm and 116.49 nm. The inclusion of a thin CaF2 filter eliminates residual light below 125 nm. The CaF2 absorption edge of filter 104 is shielded by the Kr absorption (see, e.g., Figure 2). Note that the CaF2 filter does not need to carry any structural load and can be thin enough to transmit most light above 125 nm. Kr exhibits a steeper absorption edge than a simple CaF2 filter and also shields the CaF2 from most of the radiation near the CaF2 absorption edge, thereby reducing damage to the bulk of the material. The transmission edge shifts to longer wavelengths as the partial pressure of the first noble gas increases. For example, the resulting long-pass filter has a transmission edge of about 125 nm that can be tuned to the red by increasing the Kr partial pressure.The filtered broadband light 117 emitted from the light source 100 does not significantly damage MgF2 and is not absorbed by the MgF2 window or other optics because the absorption edge of MgF2 is about 116 nm, which is below the 125 nm cutoff of the filtered broadband light 117. Furthermore, the structural components of the gas containment structure 102 can be located at a relatively large distance from the plasma 112, thereby reducing radiative heat loads, making them easier to cool, and resulting in lower operating temperatures. Lower window and chamber wall temperatures reduce refraction noise.

[0022] As another example, in a second combination, the first noble gas may include xenon (e.g., xenon mixed with argon) and the filter 104 may include sapphire. In this case, the 146.96 nm absorption line of Xe coincides with the absorption edge of sapphire, thereby reducing bulk damage to the sapphire and protecting the sapphire filter tube. Note that the gas mixture within the gas containment structure is not limited to Kr / Ar or Xe / Ar. For example, the gas may include a few percent Kr in Ar, pure Kr, an Ar / Kr / Xe mixture, or pure Xe. The addition of Xe blocks radiation in the bands below approximately 132-136 nm and from 144 to approximately 150-160 nm, depending on the Xe partial pressure. Various gas mixtures and gas combinations can be used to protect various filter tube and output window materials. For example, a gas mixture of Ar with a few percent Kr and Xe may be used in combination with a filter tube of crystalline quartz, fused silica, CaF2, or sapphire, and the output and laser windows may be made of fused silica, sapphire, MgF2, or CaF2.

[0023] It should be noted that the configuration depicted in FIG. 1 is not intended to limit the scope of the present disclosure, and that light source 100 and filter 104 may be arranged in a variety of suitable configurations.

[0024] In embodiments, the partial pressure of the first noble gas may be independently controlled in the regions before and after the filter 104 so that there is little or no pressure difference between them. For example, in the case of a Kr / Ar gas mixture and a CaF2 filter, the Kr partial pressure may be independently controlled in the regions before and after the CaF2 filter so that there is little or no pressure difference between them. For example, before the CaF2 filter, the gas mixture may include a Kr partial pressure of 1 bar and an Ar partial pressure of 99 bar, while after the filter, the gas mixture may include a Kr partial pressure of 100 bar. This is useful for increasing the brightness of the LSP, because adding too much Kr (or Xe) to Ar reduces the spectral radiance and makes the plasma dimmer.

[0025] FIG. 3 shows a simplified schematic diagram of a light source 100 including a filter 104 formed in a tubular structure to form a filter tube 304, in accordance with one or more embodiments of the present disclosure. Note that various implementations and components previously described herein with respect to FIGS. 1-2 should be construed as extending to the system of FIG. 3 unless otherwise indicated. In this embodiment, a laser pump source 106 and one or more focusing optics 107 can direct and focus an optical pump 108 through an input optical window 110 to maintain a plasma 112 within the filter tube 304 to generate broadband light 113. The gas mixture 103 and filter tube 304 can then filter the broadband light to generate an output of filtered broadband light 117. In this embodiment, the light source 100 includes one or more focusing optics for collecting the filtered broadband light 117 and transmitting the filtered broadband light 117 through an output optical window to one or more downstream optical elements outside the gas containment structure 102. For example, light source 100 may include a collecting mirror, such as, but not limited to, retroreflector 119 .

[0026] 4 shows a simplified schematic diagram of a light source 100 having a reflector assembly 400 in accordance with one or more embodiments of the present disclosure. Note that the various implementations and components previously described herein with respect to FIGS. 1-3 should be construed as extending to the system of FIG. 4 unless otherwise noted. In this embodiment, the collection optics includes a reflector assembly. For example, but not limited to, the collection optics may include an ellipsoidal reflector assembly 402, and the filter may comprise a filter tube 304 located within the reflector assembly 400, with the plasma 112 formed within the volume of the filter tube 304.

[0027] FIG. 5 shows a simplified schematic diagram of a light source 100 having a pressurized reflector assembly 500 in accordance with one or more embodiments of the present disclosure. Note that the various implementations and components previously described herein with respect to FIGS. 1-4 should be construed as extending to the system of FIG. 5 unless otherwise indicated. In this embodiment, the collection optics includes a composite reflector assembly. For example, the collection optics may include, but are not limited to, an ellipsoidal reflector and a hemispherical retroreflector coupled to the top of the ellipsoidal reflector to form a pressurized chamber. In this embodiment, a gas mixture (e.g., Kr / Ar) is enclosed within the pressurized reflector assembly 500, which functions as a gas containment structure and collection optics. A filter tube 304 can be placed within the pressurized chamber 500, forming a plasma 112 within the volume of the filter tube 304.

[0028] 6 shows a simplified schematic diagram of an optical characterization system 600 incorporating a compact LSP broadband light source according to one or more alternative and / or additional embodiments. In an embodiment, system 600 includes LSP light source 100, an illumination arm 603, a collection arm 605, a detector assembly 614, and a controller 618 including one or more processors 620 and a memory 622.

[0029] It is noted herein that system 600 may comprise any imaging, inspection, metrology, lithography, or other characterization system known in the art. In this regard, system 600 may be configured to perform any form of inspection, optical metrology, lithography, and / or imaging on sample 607. Sample 607 may include any sample known in the art, including, but not limited to, a wafer, a reticle / photomask, etc. It is noted that system 600 may incorporate one or more of the various embodiments of LSP light source 100 described throughout this disclosure.

[0030] In one embodiment, the sample 607 is disposed on a stage assembly 612 to facilitate movement of the sample 607. The stage assembly 612 may include any stage assembly 612 known in the art, including, but not limited to, an XY stage, an R-Theta stage, etc. In another embodiment, the stage assembly 612 may adjust the height of the sample 607 to maintain focus on the sample 607 during inspection or imaging.

[0031] In one embodiment, the illumination arm 603 is configured to direct the broadband light 117 from the broadband LSP light source 100 to the sample 607. The illumination arm 603 may include any number and type of optical components known in the art. In one embodiment, the illumination arm 603 includes one or more optical elements 602, a beam splitter 604, and an objective lens 606. In this regard, the illumination arm 603 may be configured to focus the broadband light 117 from the broadband LSP light source 100 onto the surface of the sample 607. The one or more optical elements 602 may include any optical element or combination of optical elements known in the art, including, but not limited to, one or more mirrors, one or more lenses, one or more polarizers, one or more gratings, one or more filters, one or more beam splitters, etc. It should be noted that, as used herein, a focusing point may include, but is not limited to, one or more of the optical element 602, the beam splitter 604, or the objective lens 606.

[0032] In one embodiment, system 600 includes a collection arm 605 configured to collect light reflected, scattered, diffracted, and / or emitted from sample 607. In another embodiment, collection arm 605 can direct and / or focus light from sample 607 to a sensor 616 of a detector assembly 614. Note that sensor 616 and detector assembly 614 can include any sensor and detector assembly known in the art. Sensor 616 can include, but is not limited to, a CCD sensor or a CCD-TDI sensor. Additionally, sensor 616 can include, but is not limited to, a line sensor or an electron-bombarded line sensor.

[0033] In one embodiment, the detector assembly 614 is communicatively coupled to a controller 618 that includes one or more processors 620 and a memory 622. For example, the one or more processors 620 may be communicatively coupled to the memory 622, where the one or more processors 620 are configured to execute a set of program instructions stored in the memory 622. In one embodiment, the one or more processors 620 are configured to analyze the output of the detector assembly 614. In one embodiment, the set of program instructions is configured to cause the one or more processors 620 to analyze one or more characteristics of the sample 607. In another embodiment, the set of program instructions is configured to cause the one or more processors 620 to modify one or more characteristics of the system 600 to maintain focusing on the sample 607 and / or the sensor 616. For example, the one or more processors 620 may be configured to adjust the objective lens 606 or one or more optical elements 602 to focus the broadband light 117 from the broadband LSP light source 100 onto the surface of the sample 607. As another example, the one or more processors 620 may be configured to adjust the objective lens 606 and / or one or more optical elements 610 to collect illumination from the surface of the sample 607 and focus the collected illumination onto the sample 616.

[0034] It should be noted that system 600 may be configured in any optical configuration known in the art, including, but not limited to, a dark-field configuration, a bright-field configuration, etc. System 600 may be configured as any type of metrology tool known in the art, such as, but not limited to, a spectroscopic ellipsometer with one or more illumination angles, a spectroscopic ellipsometer for measuring elements of a Mueller matrix (e.g., using a rotational compensator), a short-wavelength ellipsometer, an angle-resolved ellipsometer (e.g., a beam profile ellipsometer), a spectroscopic reflectometer, a single-wavelength reflectometer, an angle-resolved reflectometer (e.g., a beam profile reflectometer), an imaging system, a pupil imaging system, a spectroscopic imaging system, or a scatterometer.

[0035] Additional details of various embodiments of the optical characterization system 600 may be found in published U.S. Pat. No. 7,957,066 B2, entitled “Split Field Inspection System Using Small Catadioptric Objectives,” issued June 7, 2011; U.S. Patent Application Publication No. 2007 / 0002465, entitled “Beam Delivery System for Laser Dark-Field Illumination in a Catadioptric Optical System,” published January 4, 2007; U.S. Pat. No. 5,999,310, entitled “Ultra-broadband UV Microscope Imaging System with Wide Range Zoom Capability,” issued December 7, 1999; U.S. Pat. No. 7,525,649, entitled “Surface Inspection System Using Laser Line Illumination with Two Dimensional Imaging,” issued April 28, 2009; and U.S. Pat. No. 7,525,649, entitled “Surface Inspection System Using Laser Line Illumination with Two Dimensional Imaging,” by Wang et al., published May 9, 2013. No. 5,608,526, entitled "Focused Beam Spectroscopic Ellipsometry Method and System," by Piwonka-Corle et al., issued March 4, 1997; and U.S. Patent No. 6,297,880, entitled "Apparatus for Analyzing Multi-Layer Thin Film Stacks on Semiconductors," by Rosencwaig et al., issued October 2, 2001, all of which are incorporated herein by reference in their entireties.

[0036] The one or more processors 620 of the present disclosure may include any one or more processing elements known in the art. In this sense, the one or more processors 620 may include any microprocessor-type device configured to execute software algorithms and / or instructions. In one embodiment, the one or more processors 620 may comprise a desktop computer, mainframe computer system, workstation, image computer, parallel processor, or other computer system (e.g., networked computer) configured to execute programs configured to operate the system 600 and / or broadband LSP light source 100 as described throughout this disclosure. It should be appreciated that the steps described throughout this disclosure may be performed by a single computer system or, alternatively, by multiple computer systems. In general, the term “processor” may be broadly defined to encompass any device having one or more processing elements that execute program instructions from a non-transitory memory medium 622. Furthermore, different subsystems of the various disclosed systems may include processors or logic elements suitable for performing at least some of the steps described throughout this disclosure. Accordingly, the above description should be construed as merely illustrative and not as limiting on the present disclosure.

[0037] The memory medium 622 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 620. For example, the memory medium 622 may include a non-transitory memory medium. For example, the memory medium 622 may include, but is not limited to, read-only memory, random-access memory, magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state devices, etc. In another embodiment, the memory 622 is configured to store one or more results and / or outputs of the various steps described herein. It is further noted that the memory 622 may be housed together with one or more processors 620 within a common controller housing. In an alternative embodiment, the memory 622 may be located remotely relative to the physical location of the processors 620. For example, one or more processors 620 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.). In another embodiment, the memory medium 622 holds program instructions for causing one or more processors 620 to perform the various steps described throughout this disclosure.

[0038] 7 is a process flow diagram depicting a method 700 of generating VUV light using an LSP broadband light source with long-pass filtering, according to one or more alternative and / or additional embodiments. Note that it is noted herein that the steps of method 700 may be performed in whole or in part by broadband LSP light source 100. However, it is further recognized that method 700 is not limited to broadband LSP light source 100, in that additional or alternative system-level embodiments may perform all or some of the steps of method 700.

[0039] In step 702, the method 700 includes enclosing a mixture of a first noble gas and a second noble gas within a gas containment structure. In step 704, the method 700 includes generating and directing an optical pump into the gas containment structure to maintain a plasma within the gas containment structure to generate broadband light. In step 706, the method 700 includes long-pass filtering the broadband light via the first noble gas and a filter to provide broadband light having a wavelength below a selected wavelength threshold. The method 700 may include, in an additional step 706a, absorbing portions of the broadband light within the first and second wavelength bands via the first noble gas. The method 700 includes, in an additional step 706b, absorbing portions of the broadband light having wavelengths below the selected wavelength threshold via the filter. In step 708, the method 700 includes transmitting the filtered broadband light out of the gas containment structure through an output optical window.

[0040] Those skilled in the art will recognize that the components, operations, devices, objects, and accompanying discussion described herein are used as examples for conceptual clarity and that various configuration modifications are contemplated. Consequently, as used herein, the specific examples set forth and the accompanying discussion are intended to be representative of their more general classes. In general, the use of any specific example is intended to be representative of its class, and the absence of a particular component (e.g., operation), device, or object should not be construed as limiting.

[0041] With respect to the use of virtually any plural and / or singular term herein, those skilled in the art can translate from plural to singular and / or from singular to plural as appropriate to the context and / or application, and the various singular / plural permutations have not been explicitly set forth herein for the sake of clarity.

[0042] The subject matter described herein sometimes illustrates various components contained within or connected to other components. It should be understood that the depicted configurations are merely exemplary and that many other configurations that achieve the same functionality can actually be implemented. In a conceptual sense, any configuration of components that achieves the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components combined herein to achieve a particular functionality can be considered to be “associated with” each other such that the desired functionality is achieved, regardless of the configuration or intervening components. Similarly, any two components so associated can also be considered to be “connected” or “coupled” to each other such that the desired functionality is achieved, and any two components so associateable can also be considered to be “couplable” to each other such that the desired functionality is achieved. Specific examples of being couplable include, but are not limited to, physically joinable and / or physically interacting components, wirelessly interactable and / or wirelessly interacting components, and / or logically interacting and / or logically interacting components.

[0043] It should further be understood that the present invention is defined by the appended claims. In general, those skilled in the art will understand that the terms used herein, 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 "including, but not limited to," etc.). Those skilled in the art will further understand that where a specific number is intended by the introductory claim recitation, such intention will be expressly stated in the claim; in the absence of such recitation, no such intention is present. 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 claim recitation. However, the use of such phrases should not be construed as suggesting that introducing a claim statement with the indefinite article "a" or "an" limits any particular claim containing such an introduced claim statement to an invention containing only one such statement; this is true 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"). The same applies to the use of definite articles used to introduce claim statements. Moreover, those skilled in the art will recognize that when a specific number of introduced claim statements is explicitly recited, such a statement should typically be interpreted to mean at least the recited number (e.g., a minimal recitation of "two statements" without other modifiers typically means at least two statements, or more than two statements).Furthermore, when a conventional expression similar to "at least one of A, B, and C, etc." is used, such a structure is generally intended to have the meaning as 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" would include, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or both A, B, and C, etc.). When a conventional expression similar to "at least one of A, B, or C, etc." is used, such a structure is generally intended to have the meaning as 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" would include, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or both A, B, and C, etc.). Those skilled in the art will further appreciate that virtually any disjunctive word and / or phrase presenting two or more alternative terms, whether in the specification, claims, or drawings, should be understood to contemplate the possibility of including one of those terms, either of those terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

[0044] It will be 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 in form, construction, and arrangement of the elements may be made without departing from the disclosed subject matter or sacrificing all of its advantages. The described embodiments are merely illustrative, and it is intended that the following claims encompass and include all such modifications. It is to be further understood that the invention is defined by the appended claims.

Claims

1. 1. A laser-sustained broadband light source comprising: a gas containment structure containing a mixture of a first rare gas and a second rare gas; a laser pump source configured to generate an optical pump for sustaining a plasma within the gas containment structure, the plasma generating broadband light; a laser pump source, wherein the first noble gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; a filter located within the gas containment structure and configured to absorb a portion of the broadband light emitted by the plasma having wavelengths below a selected wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; and 1. A laser-sustained broadband light source comprising:

2. 2. The broadband light source of claim 1, wherein said absorption of said broadband light of said first wavelength by said first noble gas protects said filter from degradation.

3. 2. A broadband light source as defined in claim 1, wherein the transmission edge of said long-pass filtering is adjustable by adjusting the partial pressure of said first rare gas within said gas containment structure.

4. 4. A broadband light source as defined in claim 3, wherein said transmission edge shifts to greater wavelengths as said partial pressure of said first noble gas increases.

5. 2. The broadband light source of claim 1, wherein the first noble gas comprises at least one of krypton or xenon.

6. 2. The broadband light source of claim 1, wherein the second noble gas comprises argon.

7. 10. The broadband light source of claim 1, wherein the filter is made of CaF ２ or sapphire.

8. 10. The broadband light source of claim 1, wherein the first noble gas comprises krypton, the second noble gas comprises argon, and the filter comprises CaF ２ A broadband light source comprising:

9. 2. A broadband light source as defined in claim 1, wherein said first noble gas comprises xenon, said second noble gas comprises argon, and said filter is formed from sapphire.

10. 10. The broadband light source of claim 1, wherein the filter comprises at least one of a sheet or a tube.

11. 10. The broadband light source of claim 1, a collection optical element configured to collect at least a portion of the broadband light emitted from the plasma and direct the portion of the broadband light to the one or more downstream optical elements; A broadband light source further comprising:

12. 12. The broadband light source of claim 11, wherein the collection optical element comprises at least one of a mirror or a lens.

13. 13. The broadband light source of claim 12, wherein the collection optic comprises a reflector assembly.

14. 14. The broadband light source of claim 13, wherein the concentrating optical element comprises an ellipsoidal reflector assembly.

15. 14. The broadband light source of claim 13, wherein the concentrating optical element comprises a compound reflector assembly comprising an ellipsoidal reflector assembly and a hemispherical reflector.

16. 10. The broadband light source of claim 1, wherein the one or more downstream optical elements are MgF ２ A broadband light source comprising:

17. 10. The broadband light source of claim 1, wherein the one or more downstream optical elements comprise one of one or more transmissive optical elements or one or more reflective optical elements.

18. 20. The broadband light source of claim 17, wherein the one or more downstream optical elements comprise at least one of a window, a lens, or a mirror.

19. 1. A characterization system comprising:

1. A broadband light source, comprising: a gas containment structure containing a mixture of a first noble gas and a second noble gas; a laser pump source configured to generate an optical pump for sustaining a plasma within the gas containment structure, the plasma generating broadband light; a laser pump source, wherein the first noble gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; a filter located within the gas containment structure and configured to absorb a portion of the broadband light emitted by the plasma having wavelengths below a selected wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; and a collection optical element configured to collect broadband light emitted from the plasma and direct the broadband light to the one or more downstream optical elements; a broadband light source comprising: a set of illumination optics configured to direct broadband light from the broadband light source onto one or more samples; a set of collection optics configured to collect light emitted from the one or more samples; a detector assembly; A characteristic evaluation system comprising:

20. 20. The characterization system of claim 19, wherein the absorption of broadband light at the first wavelength by the first noble gas protects the filter from degradation.

21. 20. The characterization system of claim 19, wherein the transmission edge of the longpass filtering is adjustable by adjusting the partial pressure of the first noble gas within the gas containment structure.

22. 22. The characterization system of claim 21, wherein the transmission edge shifts to greater wavelengths as the partial pressure of the first noble gas increases.

23. 20. The characterization system of claim 19, wherein the first noble gas comprises at least one of krypton or xenon.

24. 20. The characterization system of claim 19, wherein the second noble gas comprises argon.

25. 20. The characterization system of claim 19, wherein the filter is made of CaF ２ 10. A characterization system comprising at least one of a silicon dioxide filter and a sapphire filter.

26. 20. The characterization system of claim 19, wherein the first noble gas comprises krypton, the second noble gas comprises argon, and the filter comprises CaF ２ A characteristic evaluation system comprising:

27. 20. The characterization system of claim 19, wherein the first noble gas comprises xenon, the second noble gas comprises argon, and the filter is formed from sapphire.

28. 20. The characterization system of claim 19, wherein the filter comprises at least one of a sheet or a tube.

29. 20. The characterization system of claim 19, wherein the collection optical element comprises at least one of a mirror or a lens.

30. 30. The characterization system of claim 29, wherein the collection optics comprises a reflector assembly.

31. 31. The characterization system of claim 30, wherein the collection optic comprises an ellipsoidal reflector assembly.

32. 31. The characterization system of claim 30, wherein the collection optical element comprises a compound reflector assembly comprising an ellipsoidal reflector assembly and a hemispherical reflector.

33. 20. The characterization system of claim 19, wherein the one or more downstream optical elements are MgF ２ A characteristic evaluation system comprising:

34. 20. The characterization system of claim 19, wherein the one or more downstream optical elements comprise one of one or more transmissive optical elements or one or more reflective optical elements.

35. 20. The characterization system of claim 19, wherein the one or more downstream optical elements comprise at least one of a window, a lens, or a mirror.

36. Containing a mixture of a first noble gas and a second noble gas in a gas containment structure; generating an optical pump and directing the optical pump into the gas containment structure to maintain a plasma within the gas containment structure to generate broadband light; and realizing long-pass filtering of the broadband light, the realizing long-pass filtering of the broadband light comprising: absorbing a portion of the broadband light within a first wavelength band and a second wavelength band via the first noble gas; and absorbing a portion of the broadband light having wavelengths below a selected wavelength threshold through a filter. and A method comprising:

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