VUV laser sustaining plasma light source with long pass filtering

Abstract

The invention discloses a laser maintained broadband light source. The light source may include a gas containment structure containing a mixture of a first inert gas and a second inert gas. The light source may include a laser pumping source to generate optical pumping to maintain plasma within the gas containment structure. The first inert gas absorbs broadband light in a first wavelength band and a second wavelength band. The light source may include a filter positioned within the gas containment structure and configured to absorb the broadband light emitted by the plasma having a wavelength below a selected wavelength threshold. Absorbing broadband light by the first inert gas and the filter provides long pass filtering of broadband light below the selected wavelength to protect one or more downstream optical elements from damage.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 445,307, filed Feb. 14, 2023, and U.S. Provisional Application No. 63 / 446,911, filed Feb. 20, 2023, the entire disclosures of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present invention generally relates to plasma-based radiation sources, and more particularly, to a high-power vacuum ultraviolet (VUV) laser sustained plasma (LSP) source having long-pass filtering. BACKGROUND OF THE INVENTION

[0004] Laser sustained plasma (LSP) sources are widely used in broadband inspection tools used in semiconductor inspection and imaging. Generally, a near-infrared (NIR) continuous wave (CW) pump laser is focused into a gas containment 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 for plasma ignition), or a cell (e.g., an opto-mechanical assembly with transparent walls to allow laser and plasma radiation to enter and exit the cell), or a chamber (e.g., a metal container with transparent windows for laser input and plasma light output), or a similar assembly. Various plasma containers have high internal pressures, which reach dozens or over a hundred atmospheres during operation. This high-pressure gas contained in the container is crucial for LSP operation. Plasma light is collected through the transparent wall or window of the container and used as an illumination source for the inspection tool.

[0005] Various versions of such sources have been developed. Most of these sources are designed to operate in the visible (VIS) or ultraviolet (UV) spectral regions. When these sources are used to generate light in the vacuum ultraviolet (VUV) spectral region and particularly in the range from approximately 125 nm to approximately 150 nm, the choices of practical configurations are relatively few and are limited to relatively low pump powers. Typical sources for generating VUV light include metal chambers having multiple windows for coupling the laser in and out of the chamber. While different materials can be used for the laser windows, the choices for VUV generation are few. The most widely used are MgF2 having a transmission cutoff wavelength of approximately 115 nm or CaF2 having a transmission cutoff wavelength of approximately 125 nm.

[0006] One of the most important limitations in operating this VUV source is optical damage to the MgF2 window and downstream optics. All transmissive and reflective optics are quickly damaged when exposed to short-wavelength radiation. Bulk MgF2 material and reflector coatings containing MgF2 are severely damaged by radiation at or near the 115 nm absorption band edge. Currently, it is nearly impossible to avoid optical damage to these materials present in both the light source and downstream optics. The transmissive optical components are directly irradiated by the plasma and the downstream optics experience significant damage to the bulk material caused by wavelengths that penetrate into the bulk and are absorbed by the bulk. Reflective components are also quickly damaged by wavelengths shorter than about 125 nm. Wavelengths longer than approximately 117 nm propagate through MgF2 over longer distances to cause damage to downstream optics. If the irradiation wavelength is longer than about 125 nm, the damage to the optical components is significantly reduced.

[0007] Another important issue relates to the thermal management of the window within the high-power LSP, which is attributed to overheating of the MgF2 window caused by absorption of light by MgF2. Overheating jeopardizes the structural integrity of the MgF2 window. Moving the window farther from the plasma makes cooling easier, but it increases the structural load on the window.

[0008] Accordingly, it is desirable to provide a VUV broadband light source that overcomes the limitations outlined above. Summary of the Invention

[0009] Disclosed is a laser sustained broadband light source. In some aspects, the laser sustained broadband light source includes: a gas containment structure that contains a mixture of a first inert gas and a second inert gas; a laser pump source that is configured to generate an optical pump to sustain a plasma within the gas containment structure, wherein the plasma generates broadband light, wherein the first inert gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; and a filter that is positioned within the gas containment structure and is 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 inert gas and the filter provides a long-pass filter of the broadband light below the selected wavelength to protect one or more downstream optical elements from damage.

[0010] Disclosed is a characterization system. In some aspects, the characterization system includes: a broadband light source, which includes: a gas containment structure that contains a mixture of a first inert gas and a second inert gas; a laser pump source configured to generate an optical pump to maintain a plasma within the gas containment structure, wherein the plasma generates broadband light; wherein the first inert gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; and a filter positioned 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 inert gas and the filter provides a long-pass filter of the broadband light below the selected wavelength to protect one or more downstream optical components 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] Disclosed is a method of generating VUV broadband light. In some aspects, the method includes: containing a mixture of a first inert gas and a second inert gas within 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; providing a long-pass filter of the broadband light, wherein providing the long-pass filter of the broadband light includes: absorbing portions of the broadband light within a first wavelength band and a second wavelength band via the first inert gas; and absorbing portions of the broadband light having wavelengths below a selected wavelength threshold via a filter.

[0012] It should be understood that 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 specification, illustrate the subject matter of the present disclosure. The description and the drawings together serve to explain the principles of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Those skilled in the art can better understand many advantages of the present disclosure by referring to the accompanying drawings.

[0014] Figure 1 Simplified schematic illustration of a laser sustained plasma (LSP) broadband light source with long-pass filtering according to one or more embodiments of the present disclosure.

[0015] Figure 2 Data plot illustrating the transmission characteristics of materials of an LSP broadband light source in the case of a CaF2 filter and a gas mixture of Ar / Kr according to one or more embodiments of the present disclosure.

[0016] Figure 3 Simplified schematic diagram of an LSP broadband light source with a filter tube according to one or more embodiments of the present disclosure.

[0017] Figure 4 Simplified schematic diagram of an LSP broadband light source with an elliptical reflector assembly according to one or more embodiments of the present disclosure.

[0018] Figure 5 Simplified schematic diagram of an LSP broadband light source with a pressurized reflector assembly according to one or more embodiments of the present disclosure.

[0019] Figure 6 Simplified schematic diagram of a characterization system incorporating an LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure.

[0020] Figure 7 Process flow diagram depicting a method of generating VUV light using an LSP broadband light source with long - pass filtering according to one or more embodiments of the present disclosure. Detailed Description

[0021] Reference will now be made in detail to the disclosed subject matter illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to specific embodiments and their specific features. The embodiments set forth herein are to be considered illustrative rather than restrictive. Those skilled in the art will readily appreciate that various changes and modifications can be made to the form and details without departing from the spirit and scope of the present disclosure.

[0022] Generally referring to Figures 1 - 6 , a VUV laser - sustained plasma broadband light source with long - pass filtering according to one or more embodiments of the present disclosure is described.

[0023] Embodiments of the present disclosure relate to an LSP broadband light source with long - pass filtering. In an embodiment, the long - pass filtering is accomplished through a selected combination of a gas mixture and materials in one or more filters (e.g., filter plates, filter tubes, etc.). The filter material used in a particular filter is selected to provide substantial absorption of light having wavelengths below a selected threshold. For example, in the case of a CaF2 filter, the CaF2 filter can provide strong absorption of light below approximately 125 nm. The combination of the gas mixture surrounding the filter and the filter material is selected such that one or more components of the gas mixture exhibit one or more strong absorption lines that are close to i) the absorption edges of one or more downstream optical components (e.g., windows, mirrors, lenses) to protect one or more downstream optical devices from degradation; and / or close to ii) the absorption edge of the filter itself to protect the filter from degradation. For example, in a setup where it is desired to protect MgF2 optical devices (e.g., MgF2 windows, MgF2 - coated mirrors, MgF2 lenses), a CaF2 filter can be used, which provides strong absorption of light below 125 nm to substantially protect the MgF2 optical devices. Additionally, including Kr within the gas mixture that generates the plasma in the light source further provides a strong absorption band around the CaF2 absorption edge, thereby protecting the CaF2 filter itself. The absorption band of the Kr gas can be broadened by increasing the partial pressure of the Kr gas in the gas mixture. The entire content of U.S. Application No. 18 / 438,025, filed on February 9, 2024, is incorporated herein by reference.

[0024] Figure 1 A simplified schematic diagram illustrating 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 - containing structure 102 that houses a mixture of a first inert gas (e.g., Kr) and a second inert gas (e.g., Ar). In an embodiment, the light source 100 includes a filter 104 (e.g., a CaF2 filter) positioned within the gas - containing 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 sustain a plasma 112 within the gas - containing structure 102 to generate broadband light 113. The laser pump - source 106 can include any laser known in the art for 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 can include, but is not limited to, fiber lasers, thin - disk lasers, frequency - doubled lasers, or diode lasers. The laser pump - source 106 can be configured to emit light in the visible, IR (e.g., NIR), or ultraviolet light regions.

[0025] In an embodiment, a first inert gas absorbs portions 113a of broadband light within a first wavelength band and a second wavelength band. The filter 104 may absorb portion 113b of the broadband light having wavelengths below a selected wavelength threshold. Absorption of the broadband light 113 (the combination of 113a and 113b) by the first inert gas and the filter 104 provides a 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., aluminum mirrors coated with MF2). In an embodiment, the output filtered broadband light 117 is transmitted from the gas containment structure 102 through an output optical window 114 (e.g., a MgF2 window).

[0026] In an embodiment, although Figure 1 not shown in, the light source 100 includes one or more light collecting optical elements for collecting the filtered broadband light 117 and transmitting the filtered broadband light 117 through an output optical window (e.g., a MgF2 window) to one or more downstream optical elements outside the gas containment structure 102. For example, the light source 100 may include a light collecting mirror, such as, but not limited to, a retroreflector. Multiple light collecting arrangements are described in more detail with respect to Figure 3 , 4 and 5.

[0027] It should be noted that the materials of the first inert gas and the filter may be selected to achieve the desired long-pass filtering characteristics and may include various combinations of first inert gases and filter tube materials. It should be noted that the scope of the present disclosure should not be construed as being limited to any particular inert gas or filter material. For example, in a first combination, the first inert gas may include krypton, the second inert gas may include argon, and the material of the filter may include CaF2. This combination is particularly useful for protecting MgF2-based optical elements (e.g., output windows, light collecting lenses, mirrors, etc.) from broadband output. Figure 2 A data plot illustrating the transmission characteristics of the materials of the light source 100 in the case of a CaF2 filter and an Ar / Kr gas mixture is described. Krypton has an absorption line centered at approximately 124 nm and thus blocks radiation at the 123 nm absorption edge of CaF2, thereby protecting the CaF2 filter itself from damage. A small amount of Kr may be added to Ar using a thin CaF2 filter. Kr absorbs a certain amount of light within the approximate 116 nm and 124 nm spectral bands (e.g., see Figure 2 ). Absorption within these spectral bands is caused by very strong absorption transitions from the ground state, which tend to broaden from the main transition wavelengths of 123.58 nm and 116.49 nm to the red. The inclusion of a thin CaF2 filter removes the remaining light below 125 nm. The 123 nm absorption edge of the filter 104 is protected by Kr absorption (e.g., seeFigure 2 ) Note that the CaF2 filter does not have to bear the structural load and can be made thin enough to transmit most of the light greater than 125 nm. Kr provides a steeper absorption edge than a simple CaF2 filter alone and also protects the CaF2 from most of the radiation near the CaF2 absorption edge, which reduces damage to the bulk of the material. The transmission edge can be shifted to a larger wavelength as the partial pressure of the first inert gas increases. For example, the resulting long-pass filter has a transmission edge at approximately 125 nm that can be tuned to red by increasing the Kr partial pressure. The filtered broadband light 117 emitted from the light source 100 neither damages the MgF2 nor is absorbed by the MgF2 window or other optical components, given that the MgF2 absorption edge is at approximately 116 nm, which is below the 125 nm cut-off of the filtered broadband light 117. Additionally, the structural components of the gas containment structure 102 can be placed at a relatively large distance from the plasma 112, thereby reducing the radiative heat load and making it easier to cool and operating at a lower temperature. The low temperature of the window and chamber walls reduces noise from refraction.

[0028] As another example, in a second combination, the first inert gas can include xenon (e.g., xenon mixed with argon) and the filter 104 can include sapphire. In this case, the overall damage to the sapphire is reduced due to the Xe 146.96 nm absorption line that coincides with the sapphire absorption edge, thereby protecting the sapphire filter tube from damage. Note that the gas mixture within the gas containment structure is not limited to Kr / Ar or Xe / Ar. For example, the gas can include a few percent of Kr in Ar, pure Kr, an Ar / Kr / Xe mixture, pure Xe, etc. Adding Xe blocks emissions in the frequency bands below approximately 132 nm to approximately 136 nm and 144 nm to approximately 150 nm to approximately 160 nm, depending on the Xe partial pressure. Utilizing different gas mixtures and gas combinations allows for the protection of different filter tubes and output window materials. For example, Ar mixed with a few percent of Kr and Xe gas can be used in combination with crystalline quartz, fused silica, CaF2, or sapphire filter tubes and the output and laser windows can be made of fused silica, sapphire, MgF2, or CaF2.

[0029] Note that Figure 1 the configurations depicted in do not limit the scope of the present disclosure and the light source 100 and the filter 104 can be arranged in various suitable configurations.

[0030] In an embodiment, the partial pressure of the first inert gas can be independently controlled in the regions in front of and behind the filter 104 such that there is little or no pressure difference across the filter 104. For example, in the case of a Kr / Ar gas mixture and a CaF2 filter, the Kr partial pressure can be independently controlled in the regions in front of and behind the CaF2 filter and there is little or no pressure difference across the CaF2 filter. For instance, in front of the CaF2 filter, the gas mixture can include a 1 bar Kr partial pressure and a 99 bar Ar partial pressure, and behind the filter, the gas mixture can include a 100 bar partial pressure of Kr. This is useful for increasing the LSP brightness because when too much Kr (or Xe) is added to Ar, it results in a darker plasma with a lower spectral radiance.

[0031] Figure 3 Simplified schematic illustration of a light source 100 equipped with a filter 104 formed as a tubular structure to form a filter tube 304, according to one or more embodiments of the present disclosure. It should be noted that unless otherwise mentioned, the various embodiments and components described previously with respect to Figures 1 - 2 should be interpreted as extending to Figure 3 . In this embodiment, the laser pump source 106 and one or more focusing optics 107 can direct and focus the optical pump 108 through the input optical window 110 to maintain a plasma 112 within the filter tube 304 to generate broadband light 113. Then, the gas mixture 103 and the filter tube 304 can filter the broadband light to produce an output of filtered broadband light 117. In this embodiment, the light source 100 includes one or more light collecting optical elements for collecting the filtered broadband light 117 and transmitting the filtered broadband light 117 through the output optical window to one or more downstream optical elements outside the gas containment structure 102. For example, the light source 100 can include a light collecting mirror, such as but not limited to a retroreflector 119.

[0032] Figure 4 Simplified schematic illustration of a light source 100 having a reflector assembly 400, according to one or more embodiments of the present disclosure. It should be noted that unless otherwise mentioned, the various embodiments and components described previously with respect to Figures 1 - 3 should be interpreted as extending to Figure 4 . In this embodiment, the light collecting optical element includes a reflector assembly. For example, the light collecting optical element can include but is not limited to an elliptical reflector assembly 402 and the filter can include a filter tube 304 positioned within the reflector assembly 400, where a plasma 112 is formed within the volume of the filter tube 304.

[0033] Figure 5 Simplified schematic illustration of a light source 100 having a pressurized reflector assembly 500, according to one or more embodiments of the present disclosure. It should be noted that unless otherwise mentioned, the various embodiments and components described previously with respect to Figures 1 - 4 should be interpreted as extending toFigure 5 。In this embodiment, the light-collecting optical element includes a compound reflector assembly. For example, the light-collecting optical element may include, but is not limited to, an elliptical reflector and a hemispherical retroreflector coupled to an upper portion of the elliptical reflector to form a pressurized chamber. In this embodiment, a gas mixture (e.g., Kr / Ar) is contained within a pressurized reflector assembly 500 that serves as both a gas containment structure and a light-collecting optical device. The filter tube 304 may be positioned within the pressurized chamber 500, where a plasma 112 is formed within the volume of the filter tube 304.

[0034] Figure 6 A simplified schematic illustration of an optical characterization system 600 incorporating a compact LSP broadband light source in accordance with one or more alternative and / or additional embodiments is described. In an embodiment, the system 600 includes an LSP light source 100, an illumination branch 603, a light-collecting branch 605, a detector assembly 614, and a controller 618 including one or more processors 620 and a memory 622.

[0035] It should be noted herein that the system 600 may include any imaging, inspection, metrology, lithography, or other characterization system known in the art. In this regard, the system 600 may be configured to perform inspection, optical metrology, lithography, and / or any form of imaging on a sample 607. The sample 607 may include any sample known in the art, including but not limited to wafers, reticles, photomasks, and the like. It should be noted that the system 600 may incorporate one or more of the various embodiments of the LSP light source 100 described throughout this disclosure.

[0036] In one embodiment, the sample 607 is placed 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 X-Y stage, an R-θ stage, and the like. In another embodiment, the stage assembly 612 is capable of adjusting the height of the sample 607 during inspection or imaging to maintain focus on the sample 607.

[0037] In one embodiment, the illumination branch 603 is configured to direct broadband light 117 from the broadband LSP light source 100 to the sample 607. The illumination branch 603 can include any number and type of optical components known in the art. In one embodiment, the illumination branch 603 includes one or more optical elements 602, a beam splitter 604, and an objective lens 606. In this regard, the illumination branch 603 can 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 can 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, and the like. It should be noted herein that the light collection position can include but is not limited to one or more of the optical elements 602, the beam splitter 604, or the objective lens 606.

[0038] In one embodiment, the system 600 includes a light collection branch 605 that is configured to collect light reflected, scattered, diffracted, and / or emitted from the sample 607. In another embodiment, the light collection branch 605 can direct and / or focus the light from the sample 607 onto the sensor 616 of the detector assembly 614. It should be noted that the sensor 616 and the detector assembly 614 can include any sensor and detector assembly known in the art. The sensor 616 can include but is not limited to a CCD sensor or a CCD-TDI sensor. Additionally, the sensor 616 can include but is not limited to a line sensor or an electron bombardment line sensor.

[0039] 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 can be communicatively coupled to the memory 622, where the one or more processors 620 are configured to execute a set of program instructions stored on 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 in order to maintain focus on the sample 607 and / or the sensor 616. For example, the one or more processors 620 can be configured to adjust the objective lens 606 or the one or more optical elements 602 in order 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 can be configured to adjust the objective lens 606 and / or the one or more optical elements 610 in order to collect illumination from the surface of the sample 607 and focus the collected illumination onto the sensor 616.

[0040] Note that system 600 can be configured with any optical configuration known in the art, including but not limited to dark field configurations, bright field orientations, and the like. System 600 can 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 Mueller matrix elements (e.g., using a rotating compensator), a single wavelength ellipsometer, an angular resolved ellipsometer (e.g., a beam profile ellipsometer), a spectroscopic reflectometer, a single wavelength reflectometer, an angular resolved reflectometer (e.g., a beam profile reflectometer), an imaging system, a pupil imaging system, a spectral imaging system, or a scatterometer.

[0041] Additional details of various embodiments of the optical characterization system 600 are described in the following: U.S. Patent No. 7,957,066 B2, titled "Split Field Inspection System Using Small Catadioptric Objectives," issued on June 7, 2011; U.S. Patent Application Publication No. 2007 / 0002465, titled "Beam Delivery System for Laser Dark-Field Illumination in a Catadioptric Optical System," published on January 4, 2007; U.S. Patent No. 5,999,310, titled "Ultra-broadband UV Microscope Imaging System with Wide Range Zoom Capability," issued on December 7, 1999; U.S. Patent No. 7,525,649, titled "Surface Inspection System Using Laser Line Illumination with Two Dimensional Imaging," issued on April 28, 2009; U.S. Patent Application Publication No. 2013 / 0114085, titled "Dynamically Adjustable Semiconductor Metrology System," published on May 9, 2013, by Wang et al.; U.S. Patent No. 5,608,526, titled "Focused Beam Spectroscopic Ellipsometry Method and System," issued on March 4, 1997, by Piwonka-Corle et al.; and U.S. Patent No. 6,297,880, titled "Apparatus for Analyzing Multi-Layer Thin Film Stacks on Semiconductors," issued on October 2, 2001, by Rosencwaig et al., the entire contents of each of which are incorporated herein by reference.

[0042] One or more processors 620 of the present disclosure may include any one or more processing elements known in the art. In this sense, one or more processors 620 may include any microprocessor-type device configured to execute software algorithms and / or instructions. In one embodiment, one or more processors 620 may consist of: a desktop computer, a main computer system, a workstation, an image computer, a parallel processor, or other computer systems (e.g., network computers) configured to execute programs configured to operate the operating system 600 and / or the broadband LSP light source 100, as described throughout the present disclosure. It should be recognized that the steps described throughout the present disclosure may be implemented by a single computer system or, alternatively, multiple computer systems. Generally, the term "processor" may be broadly defined to cover any device having one or more processing elements that execute program instructions from a non-transitory memory medium 622. Additionally, different subsystems of the various disclosed systems may include processors or logic elements suitable for implementing at least a portion of the steps described throughout the present disclosure. Thus, the above description should not be construed as limiting the present disclosure but is for illustrative purposes only.

[0043] 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, memory medium 622 may include a non-transitory memory medium. For instance, 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 tapes, solid-state drives, and the like. In another embodiment, memory 622 is configured to store one or more results and / or outputs of the various steps described herein. It should be further noted that memory 622 may be housed in a common controller housing with one or more processors 620. In an alternative embodiment, memory 622 may be remotely located relative to the physical location of processor 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 internal network, and the like). In another embodiment, memory medium 622 maintains program instructions for causing one or more processors 620 to implement the various steps described throughout the present disclosure.

[0044] Figure 7 A process flow diagram illustrating a method 700 for generating VUV light using an LSP broadband light source with long-pass filtering according to one or more alternative and / or additional embodiments is depicted. It should be noted herein that the steps of method 700 may be implemented in whole or in part by the broadband LSP light source 100. However, it should be further recognized that method 700 is not limited to the broadband LSP light source 100, as additional or alternative system-level embodiments may implement all or part of the steps of method 700.

[0045] In step 702, method 700 includes containing a mixture of a first inert gas and a second inert gas within a gas containment structure. In step 704, method 700 includes 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. In step 706, method 700 includes long-pass filtering the broadband light via the first inert gas and a filter to provide broadband light having wavelengths below a selected wavelength threshold. In an additional step 706a, method 700 may include absorbing portions of the broadband light within a first wavelength band and a second wavelength band via the first inert gas. In an additional step 706b, method 700 includes absorbing portions of the broadband light having wavelengths below the selected wavelength threshold via the filter. In step 708, method 700 includes transmitting the filtered broadband light from the gas containment structure via an output optical window.

[0046] Those skilled in the art will recognize that the components, operations, devices, objects, and their accompanying discussions described herein are used as examples for clarity of concept and various configuration modifications are contemplated. Thus, as used herein, the specific examples and accompanying discussions set forth are intended to represent their more general classes. In general, the use of any specific example is intended to represent its class, and the inclusion or exclusion of specific components (e.g., operations), devices, and objects should not be construed as limiting.

[0047] Regarding the use of substantially any plural and / or singular terms herein, those skilled in the art may convert the plural to the singular and / or the singular to the plural, depending on the context and / or application. For clarity, various singular / plural permutations are not explicitly set forth herein.

[0048] The subject matter described herein sometimes illustrates different components contained within or connected to other components. It should be understood that such depictions of architectures are for illustrative purposes only, and in fact, many other architectures may be implemented to achieve the same functionality. In a conceptual sense, any arrangement of components that achieves the same functionality is effectively "associated" such that the desired functionality is achieved. Thus, regardless of the architecture or intermediate components, any two components that are combined herein to achieve a particular functionality can be considered to be "associated" with each other such that the desired functionality is achieved. Similarly, any two components so associated can also be considered to be "connected" or "coupled" to each other to achieve the desired functionality, and any two components that can be so associated can also be considered to be "couplable" to each other to achieve the desired functionality. Specific examples of couplable include, but are not limited to, components that can physically mate and / or physically interact and / or components that can wirelessly interact and / or wirelessly interact and / or components that can logically interact and / or can logically interact.

[0049] In addition, it should be understood that the present invention is defined by the appended claims. Those skilled in the art should understand that, generally speaking, the terms used herein and especially in the appended claims (e.g., the body of the appended claims) are generally intended to be "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" and the like). Those skilled in the art should further understand that if a specific number of claim recitations is intended to be introduced, then this intention should be explicitly recited in the claim, and if there is no such recitation, then there is no such intention. For example, for purposes of aiding understanding, the following appended claims may contain the use of introductory phrases such as "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed as implying that the introduction of a claim recitation by the indefinite article "a" or "an" will limit any particular claim containing such introduced claim recitation to an invention having only one such recitation, even if the same claim contains an introductory phrase such as "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should generally be interpreted as meaning "at least one" or "one or more"); the same applies to the use of definite articles for introducing claim recitations. In addition, even if a specific number of claim recitations is explicitly recited, those skilled in the art should recognize that such recitation will generally be interpreted as meaning at least the recited number (e.g., a bare recitation of "two recitations" without any other modifiers generally means at least two recitations or two or more recitations). In addition, in examples where a convention similar to "at least one of A, B, and C and the like" is used, this construction is generally intended to have the meaning that those skilled in the art would typically understand the convention to have (e.g., "a system having at least one of A, B, and C" will include but not be limited to a system having only A, only B, only C, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like). In examples where a convention similar to "at least one of A, B, or C and the like" is used, this construction is generally intended to have the meaning that those skilled in the art would typically understand the convention to have (e.g., "a system having at least one of A, B, or C" will include but not be limited to a system having only A, only B, only C, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like). Those skilled in the art should further understand that, whether in the detailed description, the claims, or the drawings, almost any disjunctive term and / or phrase presenting two or more alternatives should be understood as contemplating the possibility of including one of the included items, either of the two items, or both items. For example, the phrase "A or B" should be understood as including the possibilities of "A" or "B" or "A and B".

[0050] It is believed that the present disclosure and many of its attendant advantages will be understood from the foregoing description, and it should be understood that various changes may be made in the form, construction, and arrangement of the components without departing from the disclosed subject matter or sacrificing all of its material advantages. The described form is for purposes of illustration only, and the appended claims are intended to cover and embrace such changes. Additionally, it should be understood that the invention is defined by the appended claims.

Claims

1. A laser - sustained broadband light source, comprising: A gas - containing structure that contains a mixture of a first inert gas and a second inert gas; A laser pump source configured to generate an optical pump to sustain a plasma within the gas - containing structure, wherein the plasma generates broadband light, Wherein the first inert gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; And A filter positioned within the gas - containing structure and configured to absorb portions of the broadband light emitted by the plasma having wavelengths below a selected wavelength threshold, wherein the absorption of the broadband light by the first inert gas and the filter provides a long - pass filter of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage.

2. The broadband light source according to claim 1, wherein the absorption of the broadband light of the first wavelength by the first inert gas protects the filter from degradation.

3. The broadband light source according to claim 1, wherein the transmission edge of the long - pass filter can be tuned by adjusting the partial pressure of the first inert gas within the gas - containing structure.

4. The broadband light source according to claim 3, wherein the transmission edge shifts to a larger wavelength as the partial pressure of the first inert gas increases.

5. The broadband light source according to claim 1, wherein the first inert gas includes at least one of krypton or xenon.

6. The broadband light source according to claim 1, wherein the second inert gas includes argon.

7. The broadband light source according to claim 1, wherein the filter is formed of at least one of CaF2 or sapphire.

8. The broadband light source according to claim 1, wherein the first inert gas includes krypton, the second inert gas includes argon, and the filter is formed of CaF2.

9. The broadband light source according to claim 1, wherein the first inert gas includes xenon, the second inert gas includes argon, and the filter is formed of sapphire.

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

11. The broadband light source according to claim 1, further comprising: A light - collecting 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.

12. The broadband light source according to claim 11, wherein the light - collecting optical element includes at least one of a mirror or a lens.

13. The broadband light source according to claim 12, wherein the light - collecting optical element includes a reflector assembly.

14. The broadband light source according to claim 13, wherein the light - collecting optical element includes an elliptical reflector assembly.

15. The broadband light source according to claim 13, wherein the light - collecting optical element includes a composite reflector assembly, the composite reflector assembly including an elliptical reflector assembly and a hemispherical retroreflector.

16. The broadband light source according to claim 1, wherein the one or more downstream optical elements are formed of MgF2.

17. The broadband light source according to claim 1, wherein the one or more downstream optical elements include at least one of one or more transmissive optical elements or one or more reflective optical elements.

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

19. A characterization system, comprising: A broadband light source, comprising: A gas containment structure that contains a mixture of a first inert gas and a second inert gas; A laser pump source configured to generate an optical pump to maintain a plasma within the gas containment structure, wherein the plasma generates broadband light; Wherein the first inert gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; A filter positioned 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 inert gas and the filter provides a long-pass filter of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; and A light 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 set of illumination optics configured to direct broadband light from the broadband light source to one or more samples; A set of light collection optics configured to collect light emitted from the one or more samples; and A detector assembly.

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

21. The characterization system according to claim 19, wherein the transmission edge of the long-pass filter can be tuned by adjusting the partial pressure of the first inert gas within the gas containment structure.

22. The characterization system according to claim 21, wherein the transmission edge shifts to a greater wavelength as the partial pressure of the first inert gas increases.

23. The characterization system according to claim 19, wherein the first inert gas includes at least one of krypton or xenon.

24. The characterization system according to claim 19, wherein the second inert gas includes argon.

25. The characterization system according to claim 19, wherein the filter is formed of at least one of a CaF2 or a sapphire filter.

26. The characterization system according to claim 19, wherein the first inert gas includes krypton, the second inert gas includes argon, and the filter is formed of CaF2.

27. The characterization system according to claim 19, wherein the first inert gas includes xenon, the second inert gas includes argon, and the filter is formed of sapphire.

28. The characterization system according to claim 19, wherein the filter includes at least one of a sheet or a tube.

29. The characterization system according to claim 19, wherein the light collection optical element includes at least one of a mirror or a lens.

30. The characterizing system according to claim 29, wherein the light collecting optical element comprises a reflector assembly.

31. The characterizing system according to claim 30, wherein the light collecting optical element comprises an elliptical reflector assembly.

32. The characterizing system according to claim 30, wherein the light collecting optical element comprises a compound reflector assembly, the compound reflector assembly comprising an elliptical reflector assembly and a hemispherical retroreflector.

33. The characterizing system according to claim 19, wherein the one or more downstream optical elements are formed of MgF2.

34. The characterizing system according to claim 19, wherein the one or more downstream optical elements comprise at least one of one or more transmissive optical elements or one or more reflective optical elements.

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

36. A method, comprising: containing a mixture of a first inert gas and a second inert gas within 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 providing long-pass filtering of the broadband light, wherein providing long-pass filtering of the broadband light comprises: absorbing, via the first inert gas, portions of the broadband light within a first wavelength band and a second wavelength band; and absorbing, via a filter, portions of the broadband light having wavelengths below a selected wavelength threshold.

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