High-power, compact VUV laser-sustained plasma light source

Abstract

The compact LSP broadband light source includes a gas confinement structure containing a mixture of a first noble gas and a second noble gas, a filter tube disposed within the gas confinement structure, an input window, and a laser pump source. The laser pump source directs an optical pump through the input window to maintain a plasma within the filter tube. The first noble gas absorbs broadband light within a first wavelength band and a second wavelength band. The filter tube absorbs broadband light having wavelengths below a selected threshold. The absorption of the broadband light by the first noble gas and the filter tube provides long-pass filtering to protect one or more downstream optical elements. The gas confinement structure includes an output optical window for transmission of the filtered broadband light. The gas confinement structure includes a gas inlet and an outlet for generating a reverse vortex flow pattern within the filter tube.

Description

[Technical Field]

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

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

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

[0004] Various versions of such illumination sources have been developed. Most of these illumination sources are designed to operate in the visible (VIS) or ultraviolet (UV) spectral regions. When these illumination sources are used to generate light in the vacuum ultraviolet (VUV) spectral region, particularly in the range of approximately 125–150 nm, the practical design options are relatively limited and the pump power is relatively low. A typical illumination source for generating VUV light includes a metal chamber with multiple windows that couple laser light into and out of the chamber. While various materials can be used for the laser windows, there are few options for VUV generation. MgF2, with a transmission cutoff wavelength of approximately 115 nm, or CaF2, with a transmission cutoff wavelength of approximately 125 nm, are the most widely used.

[0005] There is a significant amount of short-wavelength radiation emitted from LSPs, which has been shown to degrade MgF2, especially in the presence of radiation below 125 nm. The options for efficient mirrors that can be used in the VUV wavelength range are also limited (e.g., aluminum protected by an MgF2 coating), and these too have been shown to be rapidly damaged by light below 125 nm. Damage to optical components is significantly reduced when the irradiation wavelength is longer than approximately 125 nm. The high pressure in the plasma chamber imposes practical limitations on the size of the window; if the window is close to the plasma, it is rapidly damaged by plasma radiation; if the window is farther 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 be thicker and bulkier, which is difficult to achieve due to the low strength of optical materials available for the VUV. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent Application Publication No. 2021 / 0231292 [Patent Document 2] European Patent No. 3357081 Summary of the Invention [Problem to be solved by the invention]

[0007] It would therefore be desirable to provide a VUV broadband light source that overcomes the limitations outlined above. [Means for solving the problem]

[0008] a filter tube disposed within the gas confinement structure; an input optical window; a laser pump source configured to generate an optical pump, the laser pump source configured to direct the optical pump through the input optical window to sustain a plasma within the filter tube, the plasma generating broadband light, the first noble gas absorbing portions of the broadband light within first and second wavelength bands, the filter tube configured to absorb portions of the broadband light having wavelengths below a selected wavelength threshold, the absorption of the broadband light by the first noble gas and the filter tube providing long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; an output optical window configured to transmit the filtered broadband light out of the gas confinement structure; a gas inlet; and a gas outlet configured to generate a reverse vortex flow pattern within the filter tube.

[0009] A characterization system is disclosed. In some embodiments, the characterization system includes a broadband light source, the broadband light source including a gas confinement structure containing a mixture of a first noble gas and a second noble gas, a filter tube disposed within the gas confinement structure, an input optical window, a laser pump source configured to generate an optical pump, the laser pump source configured to direct the optical pump through the input optical window to maintain a plasma in the filter tube, the plasma generating broadband light, the first noble gas absorbing a portion of the broadband light within a first wavelength band and a second wavelength band, the filter tube configured to absorb a portion of the broadband light having a wavelength below a selected wavelength threshold, and the absorption of the broadband light by the first noble gas and the filter tube. The collection includes a broadband light source comprising a laser pump source that provides long-pass filtering of broadband light below a selected wavelength threshold to protect one or more downstream optical elements from damage, an output optical window configured to transmit the filtered broadband light out of the gas confinement structure, a gas inlet, and a gas outlet, the gas inlet and gas outlet configured to generate a reverse vortex flow 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 emanating from the one or more samples, and a detector assembly.

[0010] In some embodiments, a method for generating VUV broadband light includes: containing a mixture of a first noble gas and a second noble gas in a gas confinement structure; generating a reverse vortex pattern in a filter tube in the gas confinement structure; generating an optical pump and directing the optical pump into the filter tube of the gas confinement structure through an input optical window of the gas confinement structure to maintain a plasma in the filter tube of the gas confinement structure to generate the broadband light; filtering the broadband light with the first noble gas and the filter tube to filter out the broadband light having wavelengths below a selected wavelength threshold; and transmitting the filtered broadband light out of the gas confinement structure through an output optical window.

[0011] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of this 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 explanation of the drawings]

[0012] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying drawings.

[0013] [Figure 1] 1 shows a simplified schematic diagram of a compact laser-sustained plasma (LSP) broadband light source in accordance with one or more embodiments of the present disclosure. [Figure 2] 10 shows a data graph illustrating the material transmission characteristics of an LSP broadband light source for a CaF2 filter tube and a gas mixture of Ar / Kr, in accordance with one or more embodiments of the present disclosure. [Figure 3A] 1 shows a simplified schematic diagram of a compact LSP broadband light source in accordance with one or more alternative and / or additional embodiments of the present disclosure. [Figure 3B] 1 shows a simplified schematic diagram of a compact LSP broadband light source in accordance with one or more alternative and / or additional embodiments of the present disclosure. [Figure 4] 1 shows a simplified schematic diagram of a compact LSP broadband light source in accordance with one or more alternative and / or additional embodiments of the present disclosure. [Figure 5A] 1 shows a simplified schematic diagram of a compact LSP broadband light source according to one or more alternative and / or additional embodiments. [Figure 5B] 10 illustrates possible pupil intensity distributions for shaping the plasma of an LSP broadband light source in accordance with one or more alternative and / or additional embodiments. [Figure 6] 1 shows a simplified schematic diagram of a compact LSP broadband light source in accordance with one or more alternative and / or additional embodiments of the present disclosure. [Figure 7] 1 shows a simplified schematic diagram of a compact LSP broadband light source in accordance with one or more alternative and / or additional embodiments of the present disclosure. [Figure 8] FIG. 10 shows a simplified schematic diagram of a characterization system incorporating a compact LSP broadband light source in accordance with one or more alternative and / or additional embodiments of the present disclosure. [Figure 9] FIG. 1 shows a process flow diagram illustrating a method for producing VUV light using a compact LSP broadband light source, in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0014] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with reference to certain embodiments and certain features thereof. The embodiments described herein are to be construed as illustrative and not limiting. It will be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made therein without departing from the spirit and scope of the present disclosure.

[0015] 1-8, a compact laser-sustained plasma broadband light source in accordance with one or more embodiments of the present disclosure will be described.

[0016] FIG. 1 shows a simplified schematic diagram of a compact LSP broadband light source 100 according to one or more embodiments. In an embodiment, the light source 100 includes a gas confinement structure 102 containing a mixture of a first noble gas and a second noble gas. In an embodiment, the light source 100 includes a filter tube 104 (e.g., a CaF2 filter tube) disposed within the gas confinement structure 102. In an embodiment, the light source 100 includes a gas inlet 116 and a gas outlet 118 configured to establish a reverse vortex flow pattern 120 within the filter tube 104. 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 may focus the optical pump 108 through an input optical window 110 to maintain a plasma 112 within the filter tube 104 and generate broadband light 113. The laser pump source 106 may include any laser known in the art of plasma-based broadband light generation. In embodiments, 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-disk laser, a frequency-doubled laser, or a diode laser. The laser pump source 106 may be configured to emit light in the visible, IR (e.g., NIR), or ultraviolet ranges.

[0017] In an embodiment, the first noble gas absorbs portions of the broadband light 113 within a first wavelength band and a second wavelength band. The filter tube 104 may absorb portions of the broadband light 113 having wavelengths below a selected wavelength threshold. The absorption of the broadband light by the first noble gas and the filter tube 104 results in 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 out of the gas confinement structure 102 by an output optical window 114 (e.g., an MgF2 window). In an embodiment, the light source 100 includes one or more collection optics for collecting the filtered broadband light 117 and transmitting the filtered broadband light 117 via the output optical window (e.g., an MgF2 window) to one or more downstream optical elements outside the gas confinement structure 102. 1, light source 100 may include a collecting mirror, such as, but not limited to, retroreflector 119. As another example, light source 100 may include a collecting lens (e.g., as shown in FIG. 3A).

[0018] The reverse vortex pattern 120 of the gas flow inside the filter tube 104 allows the optical pump 108 to avoid propagating through the plasma plume and other regions of gas with high-temperature gradients. Rather, the optical pump 108 propagates through cooler regions of gas with low refractive index gradients. This arrangement also results in low noise. In embodiments, unlike previous reverse vortex designs in which a high-NA laser pump was used to pump the plasma, the optical pump of the present disclosure can be focused with a relatively small NA, freeing up the available solid angle for plasma light collection. Plasma growth in the direction of laser propagation is mitigated by the axial velocity of the reverse vortex gas flow. LSP sources implementing a reverse vortex gas flow configuration are described in U.S. Pat. No. 11,690,162 B2, issued June 27, 2023; U.S. Pat. No. 11,776,804 B2, issued October 3, 2023; and U.S. Patent Application Publication No. 2023 / 0053035 A1, each of which is incorporated by reference in its entirety.

[0019] It should be noted that the first noble gas and the filter tube material can be selected to achieve the desired long-pass filtering characteristics, including various combinations of first noble gases and filter tube materials. The scope of the present disclosure should not be construed as limited to any particular noble gas or filter tube material. For example, in a first combination, the first noble gas may include krypton, the second noble gas may include argon, and the filter tube material may include CaF2. Such a combination is particularly useful for protecting MgF2-based optical elements (e.g., output windows, focusing lenses, etc.) from broadband output. Figure 2 shows a data graph illustrating the transmission characteristics of the materials of the light source 100 for a CaF2 filter tube and an Ar / Kr gas mixture. Krypton has an absorption line centered at approximately 123.58 nm, thus blocking radiation at the 123 nm absorption edge of CaF2, thereby protecting the CaF2 filter tube from degradation. The spectrum of light transmitted through the Ar / Kr gas mixture and CaF2 filter tube has a sharp cutoff at approximately 125 nm. This cutoff can be tuned by a few nm by changing the partial pressure of Kr in the gas confinement structure 102. The filtered broadband light 117 emitted from the light source 100 does not damage MgF2 and is not absorbed by the MgF2 window or other optical components, as the absorption edge of MgF2 is at approximately 116 nm, below the 125 nm cutoff of the filtered broadband light 117. As a result, the radiative heat load on the MgF2 window is significantly reduced, the MgF2 window can be made smaller and thinner, and the MgF2 window can be placed much closer to the plasma 112, thereby reducing the pressure forces required to hold the MgF2 window in place. Similarly, the laser window and any other optical components can be placed closer to the plasma and reduced in size, as described further herein. Additionally, the structural components of the gas confinement structure 102 can be positioned a relatively large distance from the plasma 112, thereby reducing the radiative heat load, making them easier to cool, and lowering their operating temperatures. Low temperatures of the windows and chamber walls reduce noise due to refraction.

[0020] As another example, in a second combination, the first noble gas may include xenon (e.g., xenon mixed with argon) and the filter tube 104 may include sapphire. In this case, the 146.96 nm absorption line of xenon, which coincides with the absorption edge of sapphire, reduces sapphire bulk damage, 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 may include a few percent Kr in Ar, pure Kr, an Ar / Kr / Xe mixture, and pure Xe. The addition of Xe blocks emissions below approximately 132-136 nm and from 144 to approximately 150-160 nm, depending on the partial pressure of the Xe. Using different gas mixtures and gas combinations allows for protection of different filter tube and output window materials. For example, Ar mixed with a few percent of Kr and Xe gas may be used in combination with a filter tube of crystalline quartz, fused silica, CaF2, or sapphire, and the output window and laser window may be made of fused silica, sapphire, MgF2, or CaF2.

[0021] In an embodiment, the gas confinement structure 102 includes a tube cover 122 disposed above the filter tube 104. The tube cover 122 may separate the collection of gas within the filter tube 104 from the remainder of the gas confinement structure 102. In an embodiment, the seal between the interior volume of the filter tube 104 and the remainder of the gas confinement structure 102 is not airtight. In contrast to other reverse vortex flow sources in which a tube is used to contain high pressure, neither the tube nor the cover in the disclosed embodiments are subject to structural loads and may be relatively thin. The filter tube 104 and tube cover are used to separate the collection of gas within the filter tube, where the high-velocity reverse vortex gas flow 120 is formed, from the remainder of the gas confinement structure 102. The filter tube 104 and tube cover 122 are configured to i) form a small cylindrical collection of gas so that the reverse vortex flow can properly function, ii) filter out light having wavelengths shorter than a selected wavelength (e.g., approximately 125 nm), and iii) transmit light having wavelengths longer than a selected wavelength (e.g., approximately 125 nm).

[0022] Due to its proximity to the plasma 112, the CaF2 tube absorbs wavelengths shorter than about 123 nm. Following the example above, the heat load of a 2 cm tube is 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 high-velocity, high-pressure tangential gas flow of the reverse vortex 120. Typical heat transfer coefficients in 100 atm Ar with a velocity of 100 m / s are several thousand W / m 2 / K, the temperature rise of the filter tube 104 is not expected to exceed a few tens of degrees Celsius.

[0023] In an embodiment, gas confinement structure 102 includes one or more water cooling channels 128. Water cooling channels 128 may function to remove heat from the main structural elements of gas confinement structure 102 that are exposed to laser radiation from pump source 106 and broadband light from plasma 112.

[0024] In an embodiment, gas confinement structure 102 includes gas purge inlet 124 and gas purge outlet 126. Gas purge inlet 124 and gas purge outlet 126 may provide a low flow purge of high pressure gas within gas confinement structure 102, which may regulate gas flow within the interior of gas confinement structure 102.

[0025] 3A shows a simplified schematic diagram of a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. In an embodiment, one or more optical elements may be disposed within the gas confinement structure 102. For example, the laser focusing lens 202 may be integrated with the cover of the filter tube 104. As another example, the focusing lens 204 may be disposed within the gas confinement structure 102.

[0026] Because embodiments of the present disclosure help reduce damage and radiative heat loads to the optical elements of the light source 100, the optical elements of the light source 100 can be made smaller and positioned closer to the plasma 112 than would typically be appropriate. In the case of MgF2 optical elements, the low damage rate of MgF2 optical elements irradiated by light filtered by a filter tube (e.g., a CaF2 filter tube) and a first noble gas (e.g., Kr) extends the life of the optical elements, allowing for longer maintenance / replacement intervals and, as a result, reducing the frequency with which the gas confinement structure 102 needs to be opened. For example, the laser focusing lens 202 can be integrated with a tube cover disposed on top of the filter tube 104. As another example, the laser focusing lens 202 can be integrated with a laser high-pressure window. In embodiments, the focusing lens 204 can be positioned inside the pressurized volume of the gas confinement structure 102. The focusing lens 204 can be used to focus the filtered plasma light 117 through the smaller-sized high-pressure output window 114. In an embodiment, the retroreflector 119 may be disposed within a pressurized volume of the gas confinement structure 102 .

[0027] FIG. 3B shows a simplified schematic diagram of a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Note that the various implementations and components of FIGS. 1-3A should be construed as extended to FIG. 3B unless otherwise noted. In this embodiment, light source 100 implements a collector mirror 304. In an embodiment, collector mirror 304 may be disposed within the pressurized volume of gas confinement structure 102. Collector mirror 304 may be used to focus filtered plasma light 117 through a smaller-sized high-pressure output window 114.

[0028] FIG. 4 shows a simplified schematic diagram of a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Note that the various implementations and components of FIGS. 1-3B should be construed as applying to FIG. 4 unless otherwise specified. In an embodiment, the light source 100 can operate without the tube cover previously described herein. In this embodiment, the filter tube 104 extends upward to the laser input window 110 to form a restricted flow space in which gas flow can vary axially. Removal of the tube cover can reduce protection for the laser window 110, and the coverless light source 100 is an option in situations where damage to the laser window 110 is of little concern. For example, the coverless light source 100 can be an option in situations where the laser window 110 is formed from sapphire, when damage to the sapphire does not significantly impede laser transmission.

[0029] FIG. 5A shows a simplified schematic diagram of a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Note that the various implementations and components of FIGS. 1-4 should be construed as applying to FIG. 5A unless otherwise specified. Note that pump beams of various shapes may be implemented. As shown in FIG. 5A, the optical pump 108 may be focused in the plane of the drawing or may extend in the focusing direction. In this sense, an elongated LSP may be formed by the optical pump 108 being focused in a straight line in the focusing direction (e.g., tight focusing in a direction perpendicular to the plane of FIG. 5A). FIG. 5B shows possible pupil intensity distributions 502, 504, 506 for shaping the plasma 112. The laser pupil distribution may be bell-shaped 502, an inverted donut-like bell-shaped 504, or a flat-top-shaped 506.

[0030] FIG. 6 shows a simplified schematic diagram of a compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. Note that the various implementations and components of FIGS. 1-5B should be construed as applying to FIG. 6 unless otherwise noted. In an embodiment, an optical pump 108 laser enters the gas confinement structure 102 through a laterally located input optical window 110 and enters the filter tube 104 through the side of the filter tube 104. In this embodiment, filtered broadband light 117 can be collected via a laterally located output window 114 and / or a tube cover 122.

[0031] FIG. 7 illustrates a simplified schematic axial view of a multi-pass compact LSP broadband light source 100 according to one or more alternative and / or additional embodiments. It should be noted that the various implementations and components of FIGS. 1-6 should be construed as applying to FIG. 7 unless otherwise specified. In embodiments, a multi-pass focusing arrangement can be implemented by locating the optical components 702a, 702b, and 702c required for focusing inside or outside the gas confinement structure 102. Multi-pass focusing increases the collected radiance. An example of four-pass focusing is provided in FIG. 7. The large solid angle available for plasma light focusing allows for various mirror placements, and lower damage to optical components allows for mirrors to be placed relatively close to the plasma 112, reducing the overall size of the light source 100.

[0032] 8 shows a simplified schematic diagram of an optical characterization system 800 incorporating a compact LSP broadband light source according to one or more alternative and / or additional embodiments. In an embodiment, system 800 includes LSP light source 100, an illumination arm 803, a collection arm 805, a detector assembly 814, and a controller 818 including one or more processors 820 and a memory 822.

[0033] It is noted herein that system 800 may include 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 include any sample known in the art, including, but not limited to, wafers, reticles, photomasks, and the like. It is noted that system 800 may incorporate one or more of the various embodiments of LSP light source 100 described throughout this disclosure.

[0034] In one embodiment, the sample 807 is placed 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, etc. In another embodiment, the stage assembly 812 may adjust the height of the sample 807 during inspection or imaging to maintain focus on the sample 807.

[0035] In one embodiment, the illumination arm 803 is configured to direct the broadband light 117 from the broadband LSP light source 100 to the sample 807. The illumination arm 803 may include any number and type of optical components known in the art. In one embodiment, the illumination arm 803 includes one or more optical elements 802, a beam splitter 804, and an objective lens 806. In this regard, the illumination arm 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, etc. It should be noted that, as used herein, a focusing point may include, but is not limited to, one or more of the optical element 802, the beam splitter 804, or the objective lens 806.

[0036] In one embodiment, system 800 includes a collection arm 805 configured to collect light reflected, scattered, diffracted, and / or emitted from sample 807. In another embodiment, collection arm 805 may direct and / or focus light from sample 807 to sensor 816 of detector assembly 814. Note that sensor 816 and detector assembly 814 may include any sensor and detector assembly known in the art. Sensor 816 may include, but is not limited to, a CCD sensor or a CCD-TDI sensor. Additionally, sensor 816 may include, but is not limited to, a line sensor or an electron bombardment line sensor.

[0037] In one embodiment, the detector assembly 814 is communicatively coupled to a controller 818 that includes 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, where the one or more processors 820 are configured to execute a set of program instructions stored in 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 is 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 is 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 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 one or more optical elements 810 to collect illumination from the surface of the sample 807 and focus the collected illumination onto the sensor 816.

[0038] It should be noted that system 800 may be configured in any optical configuration known in the art, including, but not limited to, a dark-field configuration, a bright-field arrangement, etc. System 800 may be configured as any type of metrology tool known in the art, such as, but not limited to, a spectroscopic ellipsometer with one or more illumination angles, a spectroscopic ellipsometer for measuring Mueller matrix elements (e.g., using a rotating 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.

[0039] Further details of various embodiments of the optical characterization system 800 can be found in U.S. Pat. No. 7,957,066 B2, entitled “Split Field Inspection System Using Small Catadioptric Objectives,” issued June 7, 2011; U.S. Patent Application Publication No. 2007 / 0002465, entitled “Beam Delivery System for Laser Dark-Field Illumination in a Catadioptric Optical System,” published January 4, 2007; U.S. Pat. No. 5,999,310, entitled “Ultra-broadband UV Microscope Imaging System with Wide Range Zoom Capability,” issued December 7, 1999; U.S. Pat. No. 7,525,649, entitled “Surface Inspection System Using Laser Line Illumination with Two Dimensional Imaging,” issued April 28, 2009; and Wang et al., “Dynamically Adjustable Semiconductor Metrology,” published May 9, 2013. No. 5,608,526, entitled "Focused Beam Spectroscopic Ellipsometry Method and System," by Piwonka-Corle et al., issued March 4, 1997; and U.S. Pat. No. 6,297,880, entitled "Apparatus for Analyzing Multi-Layer Thin Film Stacks on Semiconductors," by Rosencwaig et al., issued October 2, 2001, each of which is incorporated herein by reference in its entirety.

[0040] 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 comprise a desktop computer, mainframe computer system, workstation, image computer, parallel processor, or other computer system (e.g., networked computers) configured to execute programs configured to operate the system 800 and / or broadband LSP light source 100 as described throughout this disclosure. It should be appreciated that the steps described throughout this disclosure may be performed by a single computer system or, alternatively, by multiple computer systems. In general, the term “processor” may be broadly defined to encompass any device having one or more processing elements that executes program instructions from a non-transitory memory medium 822. Furthermore, the various subsystems of the various disclosed systems may include processors or logic elements suitable for performing at least some of the steps described throughout this disclosure. Accordingly, the above description should not be construed as a limitation on the present disclosure, but merely as illustrative.

[0041] 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 a non-transitory memory medium. For example, the memory medium 822 may include, but is not limited to, read-only memory, random-access memory, magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state 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 is further noted that the memory 822 may be housed within a common controller housing along with one or more processors 820. In an alternative embodiment, the memory 822 may be located remotely relative to the physical location of the processor(s) 820. For example, the one or more processors 820 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.). In another embodiment, the memory medium 822 holds program instructions for causing the one or more processors 820 to perform the various steps described throughout this disclosure.

[0042] 9 shows a process flow diagram illustrating 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 is noted herein that the steps of method 900 may be performed in whole or in part by broadband LSP light source 100. However, it is further recognized that method 900 is not limited to broadband LSP light source 100 in that additional or alternative system-level embodiments may perform all or some of the steps of method 900.

[0043] In step 902, the method 900 includes containing a mixture of a first noble gas and a second noble gas within a gas confinement structure. In step 904, the method 900 includes generating a reverse vortex flow pattern within a filter tube within the gas confinement structure. In step 906, the method 900 includes generating an optical pump and directing the optical pump into the filter tube of the gas confinement structure through an input optical window of the gas confinement structure to maintain a plasma within the filter tube of the gas confinement structure to generate broadband light. In step 908, the method 900 includes filtering the broadband light with the first noble gas and the filter tube to filter out broadband light having wavelengths below a selected wavelength threshold. In step 910, the method 900 includes transmitting the filtered broadband light out of the gas confinement structure through an output optical window.

[0044] Those skilled in the art will recognize that the components, operations, devices, objects, and accompanying discussion described herein are used as examples to clarify the concepts, and that various configuration modifications are contemplated. Thus, as used herein, the specific examples set forth and the accompanying discussion are intended to represent their more general classes. In general, the use of any specific example is intended to represent that class, and the exclusion of specific components (e.g., operations), devices, and objects should not be construed as limiting.

[0045] With respect to the use of substantially all plural and / or singular terms herein, those of ordinary skill in the art can convert from plural to singular and / or from singular to plural as appropriate to the context and / or application. The various singular / plural permutations are not expressly set forth herein for the sake of clarity.

[0046] The subject matter described herein sometimes depicts various components contained within or connected to other components. It should be understood that the architectures depicted are merely exemplary, and that in fact many other architectures may be implemented that achieve the same functionality. In a conceptual sense, any arrangement of components that achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components herein that combine to achieve a particular function can be considered to be “associated” with each other such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components so associated can also be considered to be “connected” or “coupled” with each other to achieve the desired functionality, and any two components that can be associated in this way can also be considered to be “couplable” with each other to achieve the desired functionality. Specific examples of combinability include, but are not limited to, physically interlockable and / or physically interacting components, wirelessly interactable and / or wirelessly interacting components, and / or logically interacting and / or logically interacting components.

[0047] It should further be understood that the present invention is defined by the appended claims. In general, it will be understood by those skilled in the art that the terms used herein, and particularly in the appended claims (e.g., the body of the appended claims), are generally intended as "open" terms (e.g., the term "comprising" should be interpreted as "including, but not limited to," the term "having" should be interpreted as "having at least," the term "comprising" should be interpreted as "including, but not limited to," etc.). Where a specific number of introduced claim recitations are intended, such intention will be explicitly set forth in the claim; in the absence of such recitation, it will be further understood by those skilled in the art that no such intention exists. For example, as an aid to understanding, the following appended claims may include the use of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed as implying that introducing a claim recitation with the indefinite article "a" or "an" limits any particular claim containing a claim recitation so introduced to inventions containing only one such recitation, even when the same claim includes the introductory phrase "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should typically be interpreted to mean "at least one" or "one or more"), and the same applies to the use of definite articles used to introduce claim recitations. Additionally, even when a specific number of introduced claim recitations is explicitly recited, those skilled in the art will recognize that such a recitation should typically be interpreted to mean at least the recited number (e.g., the unmodified recitation of "two recitations" without any other modifier typically means at least two recitations, or two or more recitations).Furthermore, in instances where phrases similar to "at least one of A, B, and C, etc." are used, such configurations are generally intended in the sense that one of ordinary skill in the art would understand the phrase (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, systems having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). In instances where phrases similar to "at least one of A, B, or C, etc." are used, such configurations are generally intended in the sense that one of ordinary skill in the art would understand the phrase (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, systems having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). It will be further understood by those skilled in the art that substantially all disjunctive words and / or phrases presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibility of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" is understood to include the possibilities of "A" or "B" or "A and B."

[0048] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in form, construction, and arrangement of the elements may be made without departing from the disclosed subject matter or sacrificing all of its material advantages. The forms described are illustrative only, and it is the intent of the following claims to embrace and include such modifications. It will further be understood that the invention is defined by the appended claims.

Claims

1. 1. A laser-sustained plasma broadband light source comprising: a gas containment structure containing a mixture of a first noble gas and a second noble gas; a filter tube disposed within the gas containment structure; an input optical window; a laser pump source configured to generate an optical pump, the laser pump source configured to direct the optical pump through the input optical window to maintain a plasma in the filter tube, the plasma generating broadband light; the first noble gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; a laser pump source, wherein the filter tube is configured to absorb a portion of the broadband light having wavelengths below a selected wavelength threshold, the absorption of the broadband light by the first noble gas and the filter tube providing long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; an output optical window configured to transmit the filtered broadband light out of the gas confinement structure; A gas inlet; a gas outlet, wherein the gas inlet and the gas outlet are configured to create a reverse vortex flow pattern within the filter tube; 1. A laser-sustained plasma broadband light source comprising:

2. 10. The broadband light source of claim 1, wherein the absorption of broadband light in the first wavelength band by the first noble gas protects the filter tube from degradation.

3. 10. The broadband light source of claim 1, wherein the transmission edge of the longpass filtering is adjustable by adjusting the partial pressure of the first noble gas within the gas confinement structure.

4. 4. The broadband light source of claim 3, wherein the transmission edge shifts to greater wavelengths as the partial pressure of the first noble gas is increased.

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

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

7. The filter tube is made of CaF ２ 10. The broadband light source of claim 1, wherein the broadband light source is formed from at least one of a quartz or sapphire filter.

8. The first noble gas comprises krypton, the second noble gas comprises argon, and the filter tube comprises CaF ２ 10. The broadband light source of claim 1 formed from

9. 10. 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 from sapphire.

10. The output optical window is made of MgF ２ 10. The broadband light source of claim 1 formed from

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

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

13. The one or more downstream optical elements may be MgF ２ 12. The broadband light source of claim 11 formed from

14. The broadband light source of claim 11 , 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.

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

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

17. 17. The broadband light source of claim 16, wherein at least one of a collecting mirror or a collecting lens is disposed within the gas confinement structure.

18. 1. A characterization system comprising:

1. A broadband light source, comprising: a gas containment structure containing a mixture of a first noble gas and a second noble gas; a filter tube disposed within the gas containment structure; input optical window, a laser pump source configured to generate an optical pump, the laser pump source configured to direct the optical pump through the input optical window to maintain a plasma in the filter tube, the plasma generating broadband light; the first noble gas absorbs portions of the broadband light within a first wavelength band and a second wavelength band; a laser pump source, wherein the filter tube is configured to absorb a portion of the broadband light having wavelengths below a selected wavelength threshold, the absorption of the broadband light by the first noble gas and the filter tube providing long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; an output optical window configured to transmit the filtered broadband light out of the gas confinement structure; Gas inlet, and a gas outlet, wherein the gas inlet and the gas outlet are configured to create a counter vortex flow pattern within the filter tube. a broadband light source comprising: a set of illumination optics configured to direct the filtered broadband light from the broadband light source onto one or more samples; a set of collection optics configured to collect light emanating from the one or more samples; Detector assembly and A characterization system comprising:

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

20. 20. The characterization system of claim 18, wherein the transmission edge of the longpass filtering is adjustable by adjusting the partial pressure of the first noble gas within the gas confinement structure.

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

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

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

24. The filter tube is made of CaF ２ 20. The characterization system of claim 18, wherein the characterization system is formed from at least one of a filter of silicon dioxide or sapphire.

25. The first noble gas comprises krypton, the second noble gas comprises argon, and the filter tube comprises CaF ２ 20. The characterization system of claim 18 formed from

26. 20. 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 from sapphire.

27. The output optical window is made of MgF ２ 20. The characterization system of claim 18 formed from

28. collection optics configured to collect at least a portion of the broadband light emitted from the plasma and direct the at least a portion of the broadband light to the one or more downstream optical elements; The characterization system of claim 18 further comprising:

29. 30. The characterization system of claim 28, wherein the collection optics includes at least one of a mirror or a lens.

30. The one or more downstream optical elements may be MgF ２ 30. The characterization system of claim 28 formed from:

31. 30. The characterization system of claim 28, 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.

32. 32. The characterization system of claim 31 , wherein the one or more downstream optical elements include 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 disposed within the gas confinement structure.

34. 34. The characterization system of claim 33, wherein at least one of a collecting mirror or a collecting lens is disposed within the gas confinement structure.

35. 1. A method for producing VUV broadband light, comprising: Containing a mixture of a first noble gas and a second noble gas within a gas confinement structure; creating a reverse vortex flow pattern within a filter tube within the gas containment structure; generating an optical pump and directing the optical pump into the filter tube of the gas confinement structure through an input optical window of the gas confinement structure to sustain a plasma within the filter tube of the gas confinement structure to generate broadband light; filtering the broadband light with the first noble gas and the filter tube to filter the broadband light having wavelengths below a selected wavelength threshold; transmitting the filtered broadband light out of the gas confinement structure by an output optical window; 1. A method for producing VUV broadband light, comprising:

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