High power compact VUV laser maintained plasma light source

Abstract

A compact LSP broadband light includes a gas containment structure containing a mixture of a first inert gas and a second inert gas, a filter tube positioned within the gas containment structure, an input window, and a pumping source. The laser pumping source directs an optical pump through the input window to maintain plasma within the filter tube. The first inert gas absorbs broadband light in first and second wavelength bands. The filter tube absorbs broadband light having a wavelength below a selected threshold. The absorption of broadband light by the first inert gas and the filter tube provides long pass filtering to protect one or more downstream optical elements. The gas containment structure includes an output optical window for transmitting filtered broadband light. The gas containing structure comprises a gas inlet and a gas outlet which are used for generating a reverse vortex mode in the filter pipe.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 445,307, filed on February 14, 2023, and U.S. Provisional Application No. 63 / 446,911, filed on February 20, 2023, the entireties of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates generally to plasma-based radiation sources, and more particularly to a high-power compact vacuum ultraviolet (VUV) laser-sustained plasma (LSP) light source. Background Art

[0004] Laser-sustained plasma (LSP) light 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-containing vessel, where the 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), a cell (e.g., an optomechanical assembly with transparent walls to allow laser and plasma radiation to enter and exit the cell), a chamber (e.g., a metal vessel with transparent windows for laser input and plasma light output), or similar assemblies. Various plasma vessels have high internal pressures, reaching tens or exceeding a hundred atmospheres during operation. This high-pressure gas contained within the vessel is crucial to LSP operation. The plasma light is collected through the transparent walls or windows of the vessel and used as the 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 of approximately 125 nm to 150 nm, practical configuration options are relatively limited and limited by relatively low pump powers. Typical sources for generating VUV light include a metal chamber with multiple windows for coupling laser light into and out of the chamber. While different materials can be used for the laser windows, the options for generating VUV are limited. The most widely used are MgF2, which has a transmission cutoff wavelength of approximately 115 nm, or CaF2, which has a transmission cutoff wavelength of approximately 125 nm.

[0006] There is a large amount of short wavelength radiation emitted from the LSP, which has been shown to cause MgF2 degradation, especially when radiation less than 125nm is present. The selection of efficient mirrors available for the VUV wavelength range is also limited (e.g., aluminum protected by a MgF2 coating), and they have also been shown to be quickly damaged by light less than 125nm. If the irradiation wavelength is longer than about 125nm, the damage to the optical components is greatly reduced. The high pressure in the plasma chamber actually limits the size of the window: if the window is close to the plasma, it is quickly damaged by the plasma radiation; if the window is far away from the plasma, it must be larger for the same collection NA. However, a larger window must withstand the same pressure, and therefore it must become thicker and more bulky, which is difficult to achieve due to the poor strength of optical materials available for VUV.

[0007] Therefore, it is desirable to provide a VUV broadband light source that overcomes the above limitations. Summary of the Invention

[0008] A laser-sustained broadband light source is disclosed. In some aspects, the laser-sustained broadband light source comprises: a gas containment structure containing a mixture of a first noble gas and a second noble gas; a filter tube positioned within the gas containment structure; an input optical window; a laser pump source configured to generate optical pumping, wherein the laser pump source is configured to direct the optical pumping through the input optical window to sustain a plasma within the filter tube, wherein the plasma generates broadband light; wherein the first noble gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band; wherein the filter tube is configured to absorb a portion of the broadband light having a wavelength below a selected wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter tube provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical components from damage; an output optical window configured to transmit the filtered broadband light from the gas containment structure; a gas inlet; and a gas outlet, wherein the gas inlet and the gas outlet are configured to generate a counter-vortex pattern within the filter tube.

[0009] A characterization system is disclosed. In some aspects, the characterization system includes: a broadband light source comprising: a gas containment structure containing a mixture of a first noble gas and a second noble gas; a filter tube positioned within the gas containment structure; an input optical window; a laser pump source configured to generate an optical pump, wherein the laser pump source is configured to direct the optical pump through the input optical window to maintain a plasma within the filter tube, wherein the plasma generates broadband light; wherein the first noble gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band; wherein the filter tube is configured to absorb a portion of the broadband light having a wavelength below a selected wavelength. a portion of a wavelength below a wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter tube provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical components from damage; an output optical window configured to transmit the filtered broadband light from the gas containment structure; a gas inlet; and a gas outlet, wherein the gas inlet and the gas outlet are configured to generate a reverse vortex pattern within the filter tube; a set of illumination optics configured to direct the filtered 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.

[0010] A method for generating VUV broadband light is disclosed. In some aspects, the method includes: containing a mixture of a first noble gas and a second noble gas within a gas containment structure; generating a counter-rotating vortex pattern within a filter tube within the gas containment structure; generating an optical pump and directing the optical pump through an input optical window of the gas containment structure and into the filter tube of the gas containment structure to maintain a plasma within the filter tube of the gas containment structure to generate broadband light; filtering the broadband light through the first noble gas and the filter tube to filter the broadband light having wavelengths below a selected wavelength threshold; and transmitting the filtered broadband light from the gas containment structure through an output optical window.

[0011] It should be understood that both the foregoing general description and the following detailed description are exemplary and illustrative only and are not necessarily limiting 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. Together, the description and the drawings serve to explain the principles of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Those skilled in the art may better appreciate the numerous advantages of the present disclosure by referring to the accompanying drawings.

[0013] Figure 1 A simplified schematic diagram illustrating a compact laser-sustained plasma (LSP) broadband light source according to one or more embodiments of the present disclosure.

[0014] Figure 2 Graphs depicting data on the transmission characteristics of materials for an LSP broadband light source in the presence of a CaF2 filter tube and a gas mixture of Ar / Kr according to one or more embodiments of the present disclosure are shown.

[0015] Figure 3A A simplified schematic diagram illustrating a compact LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure.

[0016] Figure 3B A simplified schematic diagram illustrating a compact LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure.

[0017] Figure 4 A simplified schematic diagram illustrating a compact LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure.

[0018] Figure 5A A simplified schematic diagram illustrating a compact LSP broadband light source according to one or more alternative and / or additional embodiments.

[0019] Figure 5B Possible pupil intensity distributions for plasma shaping of an LSP broadband light source according to one or more alternative and / or additional embodiments are described.

[0020] Figure 6 A simplified schematic diagram illustrating a compact LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure.

[0021] Figure 7 A simplified schematic diagram illustrating a compact LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure.

[0022] Figure 8 A simplified schematic diagram illustrating a characterization system incorporating a compact LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure.

[0023] Figure 9 A process flow diagram is described depicting a method for generating VUV light with a compact LSP broadband light source according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0024] 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 specific features thereof. The embodiments described herein are to be considered illustrative rather than restrictive. It will be readily apparent to those skilled in the art that various changes and modifications in form and details may be made without departing from the spirit and scope of the present disclosure.

[0025] Generally speaking Figures 1 to 8, according to one or more embodiments of the present disclosure, a compact laser-sustained plasma broadband light source is described.

[0026] Figure 1 A simplified schematic diagram illustrates a compact LSP broadband light source 100 according to one or more embodiments. In one embodiment, the light source 100 includes a gas containment structure 102 containing a mixture of a first noble gas and a second noble gas. In one embodiment, the light source 100 includes a filter tube 104 (e.g., a CaF filter tube) positioned within the gas containment structure 102. In one embodiment, the light source 100 includes a gas inlet 116 and a gas outlet 118 configured to establish a counter-vortex pattern 120 within the filter tube 104. In one 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 filter tube 104 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 one embodiment, the laser pump source 106 may 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-slice 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 regions.

[0027] In an embodiment, the first noble gas absorbs a portion of the broadband light 113 within the first wavelength band and the second wavelength band. The filter tube 104 may absorb a portion of the broadband light 113 having a wavelength below a selected wavelength threshold. The absorption of the broadband light by the first noble gas and the filter tube 104 provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical elements (e.g., lenses, mirrors, windows) from degradation. In an embodiment, the filtered broadband light 117 is transmitted from the gas containment structure 102 through the output optical window 114 (e.g., a MgF2 window). In an embodiment, the light source 100 includes one or more collecting optical elements for collecting the filtered broadband light 117 and transmitting the filtered broadband light 117 through the output optical window (e.g., a MgF2 window) to one or more downstream optical elements outside the gas containment structure 102. For example, as Figure 1 As shown in FIG, the light source 100 may include a light collecting reflector, such as but not limited to a retroreflector 119. As another example, the light source 100 may include a light collecting lens (e.g., Figure 2 ).

[0028] The counter-vortex pattern 120 of the gas flow within the filter tube 104 allows the optical pump 108 to avoid propagating through the plasma plume and other regions of the gas with high temperature gradients. Rather, the optical pump 108 propagates through cold regions of the gas with low refractive gradients. This arrangement also results in low noise. In embodiments, unlike previous counter-vortex designs in which high-NA laser pumping is used to pump the plasma, the optical pump of the present disclosure can be focused with a relatively small NA to free up a solid angle available for plasma light collection. Plasma growth along the direction of laser propagation is mitigated by the axial velocity of the counter-vortex gas flow. LSP sources implementing a counter-vortex gas flow configuration are described in the following: U.S. Patent No. 11,690162 B2, issued on June 27, 2023; U.S. Patent No. 11,776,804 B2, issued on October 3, 2023; and U.S. Patent Publication No. 2023 / 0053035 A1, each of which is incorporated by reference in its entirety.

[0029] It should be noted that the first noble gas and the filter tube material can be selected to achieve the desired longpass filtering characteristics and can include various first noble gas and filter tube material combinations. It should be noted that the scope of this disclosure should not be interpreted as limited to any particular noble gas or filter tube material. For example, in a first combination, the first noble gas can include krypton, the second noble gas can include argon, and the filter tube material can include CaF2. This combination is particularly useful for protecting MgF2-based optical components (e.g., output windows, collecting lenses, etc.) from broadband output. Figure 2 A data graph depicting the transmission properties of the material of light source 100 for the case of a CaF2 filter tube and an Ar / Kr gas mixture is illustrated. Krypton gas has an absorption line centered at approximately 123.58 nm and therefore blocks radiation at the CaF2 123 nm absorption edge, thereby protecting the CaF2 filter tube from degradation. The spectrum of light transmitted through the Ar / Kr gas mixture and the CaF2 filter tube has a sharp cutoff at approximately 125 nm. This cutoff can be tuned by a few nanometers by varying the Kr partial pressure within gas containment structure 102. The filtered broadband light 117 emitted from 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 cutoff of the filtered broadband light 117. Consequently, the radiative heat load on the MgF2 window is significantly reduced, and it can be made smaller and thinner, and placed closer to the plasma 112 to reduce the forces from the pressure it must withstand. Similarly, the laser window and any other optical components can be placed closer to the plasma and reduced in size, as discussed further herein. 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 windows and chamber walls reduces noise from refraction.

[0030] As another example, in a second combination, the first inert gas may comprise xenon (e.g., xenon mixed with argon) and the filter tube 104 may comprise sapphire. In this case, overall sapphire damage is reduced due to the Xe 146.96 nm absorption line coinciding with the sapphire absorption edge, thereby protecting the sapphire filter tube from damage. It should be noted that the gas mixture within the gas containment structure is not limited to Kr / Ar or Xe / Ar. For example, the gas may comprise a few percent of Kr in Ar, pure Kr, an Ar / Kr / Xe mixture, pure Xe, and so on. The addition of Xe blocks emission in the bands below approximately 132 nm to 136 nm and from 144 nm to approximately 150 nm to 160 nm, depending on the Xe partial pressure. Utilizing different gas mixtures and gas combinations allows for the protection of different filter tube and output window materials. For example, Ar mixed with a few percent of Kr and Xe gases 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.

[0031] In one embodiment, gas containment structure 102 includes a cap 122 positioned above filter tube 104. Cap 122 isolates the gas volume within filter tube 104 from the remainder of gas containment structure 102. In one embodiment, the seal between the volume within filter tube 104 and the remainder of gas containment structure 102 is not airtight. Compared to other reverse vortex light sources where the tube is used to contain high pressure, neither the tube nor the cap of the disclosed embodiments bears structural loads and can be relatively thin. Filter tube 104 and the cap serve to isolate the gas volume within the filter tube, where rapid reverse vortex gas flow 120 is formed, from the remainder of gas containment structure 102. Filter tube 104 and cap 122 are configured to: i) form a small cylindrical gas volume for proper reverse vortex operation; filter out light having wavelengths shorter than a selected wavelength (e.g., approximately 125 nm); and transmit light having wavelengths longer than a selected wavelength (e.g., approximately 125 nm).

[0032] The CaF2 tube is in close proximity to the plasma 112 and absorbs wavelengths shorter than about 123 nm. Following the above example, the heat load for a 2 cm tube is about 10 W / cm 2 The tube temperature is not expected to rise significantly because the inner surface of the tube is cooled very efficiently by the fast high pressure tangential airflow of the counter-vortex 120. Typical heat transfer coefficients for 100 atmospheres Ar with a velocity of 100 m / s are several thousand W / m 2 / K, so it is expected that the temperature rise of the filter tube 104 does not exceed tens of degrees Celsius.

[0033] In an embodiment, the gas containment structure 102 includes one or more water cooling channels 128. The water cooling channels 128 can be used to remove heat from major structural elements of the gas containment structure 102 that are exposed to laser radiation from the pump source 106 and broadband light from the plasma 112.

[0034] In one embodiment, gas containment structure 102 includes a gas purge inlet 124 and a gas purge outlet 126. Gas purge inlet 124 and gas purge outlet 126 can provide a low flow purge of high pressure gas within gas containment structure 102, which can organize gas flow within gas containment structure 102.

[0035] Figure 3A A simplified schematic diagram illustrates a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. In one embodiment, one or more optical elements can be positioned within the gas containment structure 102. For example, the laser focusing lens 202 can be integrated with the cover of the filter tube 104. As another example, the collecting lens 204 can be positioned within the gas containment structure 102.

[0036] Because embodiments of the present disclosure are used to reduce damage and radiant heat loads on optical components of light source 100, optical components of light source 100 can be made smaller and placed in closer proximity to plasma 112 than is typically appropriate. In the case of MgF2 optical components, the low damage rate of MgF2 optical components irradiated by light filtered by a filter tube (e.g., a CaF2 filter tube) and a first inert gas (e.g., Kr) extends the life of the optical components and allows for longer maintenance / replacement intervals, thereby reducing the frequency with which gas containment structure 102 needs to be opened. For example, laser focusing lens 202 can be integrated with a cap positioned atop filter tube 104. As another example, laser focusing lens 202 can be integrated with a laser high-voltage window. In embodiments, collecting lens 204 can be positioned within the pressurized volume of gas containment structure 102. Collecting lens 204 can be used to focus filtered plasma light 117 through a smaller high-voltage output window 114. In embodiments, retroreflector 119 can be positioned within the pressurized volume of gas containment structure 102.

[0037] Figure 3B A simplified schematic diagram illustrating a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Figures 1 to 3A The various implementations and components should be interpreted as extending to Figure 3B In this embodiment, the light source 100 implements a collecting reflector 304. In an embodiment, the collecting reflector 304 can be placed within the pressurized volume of the gas containment structure 102. The collecting reflector 304 can be used to focus the filtered plasma light 117 through the smaller size high pressure output window 114.

[0038] Figure 4 A simplified schematic diagram illustrating a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Figures 1 to 3B The various embodiments and components should be interpreted as applicable to Figure 4 In an embodiment, the light source 100 can be operated without the tube cover described above. In this embodiment, the filter tube 104 extends upward to the laser input window 110 to form a confined flow space in which the gas flow can change axial direction. Removing the tube cover can reduce the protection of the laser window 110. In settings where damage to the laser window 110 is not a concern, an uncovered light source 100 is an option. For example, in settings where the laser window 110 is formed of sapphire, an uncovered light source 100 may be an option because damage to the sapphire will not significantly interfere with laser transmission.

[0039] Figure 5A A simplified schematic diagram illustrating a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Figures 1 to 4 The various embodiments and components should be interpreted as applicable to Figure 5A It should be noted that various shapes of pump beams can be implemented. Figure 5A As shown in FIG, the optical pump 108 can be focused on the plane of the figure and can remain extended along the collection direction. In this sense, the optical pump 108 can be focused along the collection direction to a line (e.g., along the normal to Figure 5A The optical pump 108 (tightly focused in the direction of the plane) forms an elongated LSP. Figure 5B Possible pupil intensity distributions 502, 504, 506 are illustrated for shaping the plasma 112. The laser pupil distribution can be bell-shaped 502, inverted donut-shaped bell-shaped 504, or flat-top shaped 506.

[0040] Figure 6 A simplified schematic diagram illustrating a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Figures 1 to 5B The various embodiments and components should be interpreted as applicable to Figure 6 In one embodiment, optical pump 108 laser light enters gas containment structure 102 through side-positioned input optical window 110 and enters filter tube 104 through a side portion of filter tube 104. In this embodiment, filtered broadband light 117 can be collected through side-positioned output window 114 and / or through tube cap 122.

[0041] Figure 7 A simplified schematic axial view illustrating a multi-pass compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Figures 1 to 6The various embodiments and components should be interpreted as applicable to Figure 7 In an embodiment, a multi-pass light collection arrangement may be implemented with the optics 702a, 702b, 702c required for light collection placed inside the gas containment structure 102 or outside the gas containment structure 102. Multi-pass light collection increases the amount of radiation collected. Figure 7 An example of four-way light collection is presented in . The large solid angle available for plasma light collection allows for a variety of mirror arrangements, and the low damage to optical components allows the mirrors to be placed relatively close to the plasma 112 to reduce the overall size of the light source 100.

[0042] Figure 8 A simplified schematic diagram illustrating an optical characterization system 800 incorporating a compact LSP broadband light source according to one or more alternative and / or additional embodiments. In one embodiment, the system 800 includes the LSP light source 100, an illumination branch 803, a collection branch 805, a detector assembly 814, and a controller 818 including one or more processors 820 and a memory 822.

[0043] It should be noted herein that system 800 may comprise any imaging, inspection, metrology, lithography, or other characterization system known in the art. In this regard, system 800 may be configured to perform inspection, optical metrology, lithography, and / or any form of imaging on sample 807. Sample 807 may comprise any sample known in the art, including, but not limited to, a wafer, a reticle, a photomask, and the like. It should be noted that system 800 may incorporate one or more of the various embodiments of the LSP light source 100 described throughout this disclosure.

[0044] In one embodiment, the sample 807 is disposed on a stage assembly 812 to facilitate movement of the sample 807. The stage assembly 812 may include any stage assembly 812 known in the art, including but not limited to an XY stage, an R-theta stage, and the like. In another embodiment, the stage assembly 812 is capable of adjusting the height of the sample 807 during inspection or imaging to maintain focus on the sample 807.

[0045] In one embodiment, the illumination branch 803 is configured to direct broadband light 117 from the broadband LSP light source 100 to the sample 807. The illumination branch 803 may include any number and type of optical components known in the art. In one embodiment, the illumination branch 803 includes one or more optical elements 802, a beam splitter 804, and an objective lens 806. In this regard, the illumination branch 803 may be configured to focus the broadband light 117 from the broadband LSP light source 100 onto the surface of the sample 807. The one or more optical elements 802 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, and the like. It should be noted herein that the light collection location may include, but is not limited to, one or more of the optical element 802, the beam splitter 804, or the objective lens 806.

[0046] In one embodiment, system 800 includes a light collection branch 805 configured to collect light reflected, scattered, diffracted, and / or emitted from sample 807. In another embodiment, light collection branch 805 can direct and / or focus light from sample 807 to sensor 816 of detector assembly 814. It should be noted that sensor 816 and detector assembly 814 can include any sensor and detector assembly known in the art. Sensor 816 can include, but is not limited to, a CCD sensor or a CCD-TDI sensor. Additionally, sensor 816 can include, but is not limited to, a line sensor or an electron bombardment line sensor.

[0047] In one embodiment, the detector assembly 814 is communicatively coupled to a controller 818 comprising one or more processors 820 and a memory 822. For example, the one or more processors 820 may be communicatively coupled to the memory 822, wherein the one or more processors 820 are configured to execute a set of program instructions stored on the memory 822. In one embodiment, the one or more processors 820 are configured to analyze the output of the detector assembly 814. In one embodiment, the set of program instructions are configured to cause the one or more processors 820 to analyze one or more characteristics of the sample 807. In another embodiment, the set of program instructions are configured to cause the one or more processors 820 to modify one or more characteristics of the system 800 to maintain focus on the sample 807 and / or the sensor 816. For example, the one or more processors 820 may be configured to adjust the objective lens 806 or the one or more optical elements 802 to focus the broadband light 117 from the broadband LSP light source 100 onto the surface of the sample 807. As another example, the one or more processors 820 may be configured to adjust the objective lens 806 and / or the one or more optical elements 810 in order to collect illumination from the surface of the sample 807 and focus the collected illumination onto the sensor 816 .

[0048] It should be noted that system 800 can be configured in any optical configuration known in the art, including but not limited to dark field configurations, bright field orientations, and the like. System 800 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 rotational compensator), a single 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 spectral imaging system, or a scatterometer.

[0049] Additional details of various embodiments of the optical characterization system 800 are described in the following: U.S. Published Patent No. 7,957,066 B2, entitled “Split Field Inspection System Using Small Catadioptric Objectives,” published on June 7, 2011; U.S. Published Patent Application No. 2007 / 0002465, entitled “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, entitled “Ultra-broadband UV Microscope Imaging System with Wide Range Zoom Capability,” published on December 7, 1999; and U.S. Patent No. 5,999,310, entitled “Surface Inspection System Using Laser Line Illumination and Two-Dimensional Imaging,” published on April 28, 2009. with Two Dimensional Imaging”; U.S. Published Patent Application No. 2013 / 0114085, titled “Dynamically Adjustable Semiconductor Metrology System,” published by Wang et al. on May 9, 2013; U.S. Patent No. 5,608,526, titled “Focused Beam Spectroscopic Ellipsometry Method and System,” published by Piwonka-Corle et al. on March 4, 1997; and U.S. Patent No. 6,297,880, titled “Apparatus for Analyzing Multi-Layer Thin Film Stacks on Semiconductors,” published by Rosencwaig et al. on October 2, 2001, the entireties of each of which are incorporated herein by reference.

[0050] The one or more processors 820 of the present disclosure may include any one or more processing elements known in the art. In this sense, the one or more processors 820 may include any microprocessor-type device configured to execute software algorithms and / or instructions. In one embodiment, the one or more processors 820 may be composed of a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, or other computer system (e.g., a network computer) configured to execute a program configured to operate the system 800 and / or the broadband LSP light source 100, as described throughout this disclosure. It should be recognized 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 822. Furthermore, different subsystems of the various disclosed systems may include processors or logic elements suitable for performing at least a portion of the steps described throughout this disclosure. Therefore, the above description should not be construed as limiting the present disclosure but is merely illustrative.

[0051] The memory medium 822 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 820. For example, the memory medium 822 may include non-transitory memory media. For example, the memory medium 822 may include, but is not limited to, read-only memory, random access memory, magnetic or optical storage devices (e.g., disks), tape, solid-state drives, and the like. In another embodiment, the memory 822 is configured to store one or more results and / or outputs of the various steps described herein. It should be further noted that the memory 822 may be housed in a common controller housing with the one or more processors 820. In alternative embodiments, the memory 822 may be remotely located relative to the physical location of the processors 820. For example, the one or more processors 820 may access remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, and the like). In another embodiment, the memory medium 822 maintains program instructions for causing the one or more processors 820 to perform the various steps described throughout this disclosure.

[0052] Figure 9 A process flow diagram is described depicting a method 900 for generating VUV light using a compact LSP broadband light source according to one or more alternative and / or additional embodiments. It should be noted herein that the steps of method 900 may be implemented in whole or in part by the broadband LSP light source 100. However, it should be further appreciated that method 900 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 900.

[0053] In step 902, method 900 includes containing a mixture of a first noble gas and a second noble gas within a gas containment structure. In step 904, method 900 includes generating a counter-vortex pattern within a filter tube within the gas containment structure. In step 906, method 900 includes generating an optical pump and directing the optical pump through an input optical window of the gas containment structure and into the filter tube of the gas containment structure to sustain a plasma within the filter tube of the gas containment structure to generate broadband light. In step 908, method 900 includes filtering the broadband light through the first noble gas and the filter tube to filter the broadband light having a wavelength below a selected wavelength threshold. In step 910, method 900 includes transmitting the filtered broadband light from the gas containment structure through an output optical window.

[0054] Those skilled in the art will recognize that the components, operations, devices, objects, and their accompanying discussions described herein are used as examples for conceptual clarity and contemplate various configuration modifications. Therefore, as used herein, the specific examples and accompanying discussions set forth are intended to represent their more general class. In general, the use of any particular example is intended to represent its class, and the absence of a specific component (e.g., operation), device, or object should not be considered limiting.

[0055] With respect to the use of substantially any plural and / or singular terms herein, those skilled in the art can translate the plural to the singular and / or the singular to the plural depending on the context and / or application. For clarity, various singular / plural arrangements are not explicitly set forth herein.

[0056] The subject matter described herein sometimes illustrates different components contained within or connected to other components. It should be understood that such depicted architectures are for exemplary purposes only, and in fact, many other architectures that achieve the same functionality can be implemented. In a conceptual sense, any arrangement of components that achieve the same functionality is effectively "associated" so that the desired functionality is achieved. Therefore, regardless of the architecture or intermediate components, any two components combined to achieve a specific functionality herein can be considered to be "associated" with each other so 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 "coupling" to each other to achieve the desired functionality. Specific examples of coupling include, but are not limited to, physically compatible and / or physically interactive components and / or wirelessly interactive and / or wirelessly interactive components and / or logically interactive and / or logically interactive components.

[0057] In addition, it should be understood that the present invention is defined by the appended claims. It should be understood by those skilled in the art that, in general, 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). It should be further understood by those skilled in the art that if a specific number of claim recitations is intended to be introduced, then such intention should be explicitly stated in the claim, and if such statement is not made, then such intention does not exist. For example, to aid understanding, the following appended claims may contain the use of the introductory phrases "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" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even if the same claim contains 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 generally be construed to mean "at least one" or "one or more"); the same applies to the use of definite articles to introduce claim recitations. In addition, even if a specific number of introduced claim recitations is explicitly recited, those skilled in the art will recognize that such recitation should generally be construed to mean at least the number of recitations (e.g., the bare recitation of "two recitations" without other modifiers generally means at least two recitations or two or more recitations). Additionally, 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 be generally understood by those skilled in the art for the meaning of the convention (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, 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 be generally understood by those skilled in the art for the meaning of the convention (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, and the like). Those skilled in the art will further understand that, whether in the detailed description, claims, or drawings, virtually any disjunctive term and / or phrase presenting two or more alternatives should be understood to contemplate the possibility of including one, either, or both of the terms. For example, the phrase "A or B" should be understood to include the possibility of "A" or "B" or "A and B."

[0058] It is believed that the present disclosure and its many attendant advantages will be understood from the foregoing description, and it will be appreciated that various changes may be made in the form, construction, and arrangement of components without departing from the disclosed subject matter or sacrificing all of its material advantages. The forms described are for illustration only, and the appended claims are intended to cover and encompass such changes. Furthermore, it will be understood that the invention is defined by the appended claims.

Claims

1. A laser-maintained plasma broadband light source comprising: a gas containment structure containing a mixture of a first inert gas and a second inert gas; a filter tube positioned within the gas containment structure; Input optical window; a laser pump source configured to generate an optical pump, wherein the laser pump source is configured to direct the optical pump through the input optical window to sustain a plasma within the filter tube, wherein the plasma generates broadband light; wherein the first inert gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band; wherein the filter tube is configured to absorb a portion of the broadband light having wavelengths below a selected wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter tube provides long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical components from damage; an output optical window configured to transmit filtered broadband light from the gas containment structure; air intake; and An air outlet, wherein the air inlet and the air outlet are configured to generate a counter-rotating vortex flow pattern within the filter tube.

2. The broadband light source of claim 1, wherein absorption of the broadband light at the first wavelength by the first noble gas protects the filter tube from degradation. 3 . The broadband light source of claim 1 , wherein a transmission edge of the longpass filter is tunable via adjusting a partial pressure of the first noble gas within the gas containment structure. 4 . The broadband light source of claim 3 , wherein the transmission edge shifts to a longer wavelength as the partial pressure of the first noble gas increases.

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

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

8. The broadband light source of claim 1, wherein the first noble gas comprises krypton, the second noble gas comprises argon, and the filter tube is formed of CaF2.

9. The broadband light source of claim 1, wherein the first noble gas comprises xenon, the second noble gas comprises argon, and the filter tube is formed of sapphire.

10. The broadband light source of claim 1, wherein the output optical window is formed of MgF2.

11. The broadband light source of claim 1 , further comprising: Light collection optics are 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 of claim 11, wherein the light collecting optical element comprises at least one of a mirror or a lens.

13. The broadband light source of claim 11, wherein the one or more downstream optical elements are formed of MgF2.

14. The broadband light source of claim 11, wherein the one or more downstream optical elements comprise at least one of one of one or more transmissive optical elements or one or more reflective optical elements.

15. The broadband light source of claim 14, wherein the one or more downstream optical elements comprise at least one of a window, a lens, or a reflector.

16. The broadband light source of claim 1, wherein one or more optical elements are positioned within the gas containment structure.

17. The broadband light source of claim 16, wherein at least one of a collecting reflector or a collecting lens is positioned within the gas containment structure.

18. A characterization system comprising: A broadband light source comprising: a gas containment structure containing a mixture of a first inert gas and a second inert gas; a filter tube positioned within the gas containment structure; Input optical window; a laser pump source configured to generate an optical pump, wherein the laser pump source is configured to direct the optical pump through the input optical window to sustain a plasma within the filter tube, wherein the plasma generates broadband light; wherein the first inert gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band; wherein the filter tube is configured to absorb a portion of the broadband light having a wavelength below a selected wavelength threshold, wherein absorption of the broadband light by the first noble gas and the filter tube provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical components from damage; an output optical window configured to transmit filtered broadband light from the gas containment structure; air intake; and an air outlet, wherein the air inlet and the air outlet are configured to generate a counter-rotating vortex pattern within the filter tube; a set of illumination optics configured to direct filtered 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 Detector assembly.

19. The characterization system of claim 18, wherein absorption of broadband light at the first wavelength by the first noble gas protects the filter tube from degradation.

20. The characterization system of claim 18, wherein a transmission edge of the longpass filter is tunable via adjusting a partial pressure of the first noble gas within the gas containment structure.

21. The characterization system of claim 20, wherein the transmission edge shifts to longer wavelengths as the partial pressure of the first noble gas increases.

22. The characterization system of claim 18, wherein the first noble gas comprises at least one of krypton or xenon.

23. The characterization system of claim 18, wherein the second inert gas comprises argon.

24. The characterization system of claim 18, wherein the filter tube is formed of at least one of CaF2 or a sapphire filter.

25. The characterization system of claim 18, wherein the first noble gas comprises krypton, the second noble gas comprises argon, and the filter tube is formed of CaF2.

26. The characterization system of claim 18, wherein the first noble gas comprises xenon, the second noble gas comprises argon, and the filter tube is formed of sapphire.

27. The characterization system of claim 18, wherein the output optical window is formed of MgF2.

28. The characterization system of claim 18, further comprising: Light collection optics are 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.

29. The characterization system of claim 28, wherein the light collecting optical element comprises at least one of a mirror or a lens.

30. The characterization system of claim 28, wherein the one or more downstream optical elements are formed of MgF2.

31. The characterization system of claim 28, wherein the one or more downstream optical elements comprise at least one of one of one or more transmissive optical elements or one or more reflective optical elements.

32. The characterization system of claim 31 , wherein the one or more downstream optical elements comprise at least one of a window, a lens, or a mirror.

33. The characterization system of claim 18, wherein one or more optical elements are positioned within the gas containment structure.

34. The characterization system of claim 33, wherein at least one of a collecting reflector or a collecting lens is positioned within the gas containment structure.

35. A method of generating VUV broadband light, comprising: Containing a mixture of a first inert gas and a second inert gas in a gas containment structure; generating a reverse vortex flow pattern within a filter tube within the gas containment structure; generating an optical pump and directing the optical pump through an input optical window of the gas containment structure and into the filter tube of the gas containment structure to sustain a plasma within the filter tube of the gas containment structure to generate broadband light; filtering the broadband light through the first inert gas and the filter tube to filter the broadband light having a wavelength below a selected wavelength threshold; and Filtered broadband light is transmitted from the gas containment structure through an output optical window.

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