Laser-assisted VUV plasma light source with long-pass filtering

Abstract

A laser-assisted broadband light source is disclosed. The light source may include a gas confinement structure containing a mixture of a first noble gas and a second noble gas. The light source may include a laser pump source for generating an optical pump to maintain a plasma within the gas confinement structure. The first noble gas absorbs broadband light in a first wavelength band and a second wavelength band. The light source may include a filter positioned within the gas confinement structure and configured to absorb the broadband light emitted by the plasma having a wavelength below a selected wavelength threshold. The absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical elements from damage.

Description

CROSS-REFERENCE TO RELATED APPLICATION

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

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

[0003] Laser-assisted plasma light sources (LSPs) are widely used in broadband inspection equipment for semiconductor inspection and imaging. Generally, the near-infrared (NIR) light from the pump laser is focused onto a gas-containing vessel, where a plasma is ignited and maintained by absorbing 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 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 a similar assembly. The various plasma vessels have high internal pressures, reaching tens or even hundreds of atmospheres during operation. This high-pressure gas contained within the vessel is critical to LSP operation.The plasma light is collected through transparent walls or windows of the vessel and used as an illumination source for inspection tools.

[0004] Various versions of such sources have been developed. Most of these sources are designed for operation in the visible (VIS) or ultraviolet (UV) spectral range. When these sources are used to generate light in the vacuum ultraviolet (VUV) spectral range, particularly in the range from approximately 125 to 150 nm, the choice of practical designs is relatively limited and restricted to relatively low pump powers. A typical source for generating VUV light consists of a metal chamber with several windows that couple the laser light into and out of the chamber. While various materials can be used for the laser windows, few alternatives exist for VUV generation. The most commonly used materials are MgF2, with a transmission cutoff wavelength of approximately 115 nm, or CaF2, with a transmission cutoff wavelength of approximately 125 nm.

[0005] One of the most significant limitations for the operation of such a VUV source is the optical damage to the MgF2 windows and downstream optics. All transmitting and reflecting optics are rapidly damaged when exposed to shortwave radiation. Bulk MgF2 material and reflector coatings containing MgF2 are severely damaged by radiation at or near the absorption band edge of 115 nm. Currently, there are few options to prevent optical damage to these materials, which is present in both the light source and the downstream optics. Transmissive optical components directly irradiated by the plasma and the downstream optics suffer significant damage in the bulk material due to the wavelengths that penetrate and are absorbed by the bulk material. Reflective components are also rapidly damaged by wavelengths shorter than 125 nm.Wavelengths longer than about 117 nm propagate over greater distances in MgF2 and damage downstream optics. Damage to optical components is greatly reduced when the radiation wavelengths are longer than about 125 nm.

[0006] Another significant issue is the thermal management of windows in a high-power LSP, which is due to the overheating of MgF2 windows due to the absorption of light by MgF2. This overheating compromises the structural strength of the MgF2 window. Moving the window further away from the plasma facilitates cooling but increases the structural stress on the window.

[0007] Therefore, it would be desirable to provide a broadband VUV light source that overcomes the above-mentioned limitations. OVERVIEW

[0008] A laser-assisted broadband light source is disclosed. In some aspects, the laser-assisted broadband light source comprises: a gas confinement structure containing a mixture of a first noble gas and a second noble gas; a laser pump source configured to generate an optical pump to maintain a plasma within the gas confinement structure, the plasma generating broadband light, wherein the first noble gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band;and a filter positioned within the gas confinement structure and configured to absorb a portion of the broadband light emitted by the plasma having a wavelength below a selected wavelength threshold, wherein the absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical elements from damage.;

[0009] A characterization system is disclosed. In some aspects, the characterization system comprises: a broadband light source including: a gas confinement structure containing a mixture of a first noble gas and a second noble gas; a laser pump source configured to generate an optical pump to maintain a plasma within the gas confinement structure, the plasma generating broadband light; wherein the first noble gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band;and a filter positioned within the gas confinement structure and configured to absorb a portion of the broadband light emitted by the plasma having a wavelength below a selected wavelength threshold, wherein the absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength to protect one or more downstream optical elements from damage; and a light collection element configured to collect at least a portion of the broadband light emitted by the plasma; a set of illumination optics configured to direct broadband light from the broadband light source onto one or more samples; a set of collection optics configured to collect the light emanating 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 comprises: enclosing a mixture of a first noble gas and a second noble gas within a gas confinement structure; generating an optical pump and directing the optical pump into the gas confinement structure to maintain a plasma within the gas confinement structure to generate broadband light; providing long-pass filtering of the broadband light, wherein providing long-pass filtering of the broadband light comprises: absorbing a portion of the broadband light within a first wavelength band and a second wavelength band by the first noble gas; and absorbing a portion of the broadband light having a wavelength below a selected wavelength threshold by a filter.

[0011] It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily limiting of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate the subject matter of the disclosure. Together, the descriptions and drawings serve to explain the principles of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures. Fig. 1 shows a simplified schematic view of a broadband laser-assisted plasma (LSP) light source with long-pass filtering according to one or more embodiments of the present disclosure. Fig. 2 shows a data diagram illustrating the transmission properties of the LSP broadband light source materials in the case of a CaF2 filter and an Ar / Kr gas mixture according to one or more embodiments of the present disclosure. Fig. 3 shows a simplified schematic view of the LSP broadband light source with a filter tube according to one or more embodiments of the present disclosure. Fig. 4 shows a simplified schematic view of the LSP broadband light source with an elliptical reflector arrangement according to one or more embodiments of the present disclosure. Fig. 5 shows a simplified schematic view of an LSP broadband light source with a pressurized reflector assembly according to one or more embodiments of the present disclosure. Fig. 6 shows a simplified schematic view of a characterization system including the LSP broadband light source according to one or more alternative and / or additional embodiments of the present disclosure. Fig. 7 shows a process flow diagram illustrating a method for generating VUV light with the LSP broadband light source with long-pass filtering according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0013] 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 reference to certain embodiments and their specific features. The embodiments shown herein are to be considered as illustrative rather than restrictive. It should be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the disclosure.

[0014] With reference to the Fig. generally describes a laser-based broadband VUV light source with long-pass filtering in accordance with one or more embodiments of the present disclosure.

[0015] Embodiments of the present disclosure relate to an LSP broadband light source with long-pass filtering. In embodiments, the long-pass filtering is achieved by the selected combination of gas mixtures and materials used in one or more filters (e.g., filter plate, filter tube, etc.). The filter material used in a particular filter is selected to cause substantial absorption of light with wavelengths below a certain threshold. For example, in the case of a CaF2 filter, the CaF2 filter may cause strong absorption of light below about 125 nm. In combination with the filter material, a gas mixture surrounding the filter is selected such that the one or more components of the gas mixture exhibit one or more strong absorption lines near i) an absorption edge of the one or more downstream optics (e.g.,window, mirror, lens) to protect the one or more downstream optics from degradation; and / or ii) an absorption edge of the filter itself to protect the filter from degradation. For example, if MgF2 optics (e.g., MgF2 window, MgF2-coated mirror, MgF2 lens) are to be protected, a CaF2 filter can be used, which provides strong absorption of light below 125 nm, generally protecting MgF2 optics. Furthermore, the addition of Kr in the plasma-generating gas mixture of the light source provides a strong absorption band around the CaF2 absorption edge, thereby protecting the CaF2 filter itself. The absorption band of the Kr gas can be broadened by increasing the partial pressure of the Kr gas in the gas mixture. U.S. Application No. 18 / 438,025, filed February 9, 2024, is incorporated herein by reference in its entirety.

[0016] Fig. 1 shows a simplified schematic view of an LSP broadband light source 100 with long-pass filtering according to one or more embodiments. In embodiments, the light source 100 includes a gas confinement structure 102 containing a mixture of a first noble gas (e.g., Kr) and a second noble gas (e.g., Ar). In embodiments, the light source 100 includes a filter 104 (e.g., CaF2 filter) disposed within the gas confinement structure 102. In embodiments, the light source 100 includes a laser pump source 106 configured to generate an optical pump 108. The laser pump source 106 and one or more focusing optics 107 can direct and focus the optical pump 108 through an input optical window 110 to maintain a plasma 112 within the gas confinement structure 102 to generate broadband light 113.The laser pump source 106 may comprise any laser known in the field 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 comprise, 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 embodiments, the first noble gas absorbs a portion of the broadband light 113a within a first wavelength band and a second wavelength band. The filter 104 may absorb a portion of the broadband light 113b whose wavelength is below a selected wavelength threshold. The absorption of the broadband light 113 (combination of 113a and 113b) by the first noble gas and the filter 104 provides long-pass filtering of the broadband light 113 below the selected wavelength to protect one or more downstream optical elements 111 (e.g., lenses, mirrors, windows) from damage. The one or more downstream optical elements 111 may include, but are not limited to, one or more windows 114, one or more lenses 116, or one or more mirrors 118 (e.g., MF2-coated aluminum mirrors).In embodiments, the output filtered broadband light 117 is transmitted out of the gas confinement structure 102 through an optical output window 114 (e.g., a MgF2 window).

[0018] In embodiments described in Fig. 1, the light source 100 comprises one or more optical collection elements for collecting the filtered broadband light 117 and for transmitting the filtered broadband light 117 through an optical output window (e.g., a MgF2 window) to one or more downstream optical elements outside the gas confinement structure 102. The light source 100 may, for example, include a collection mirror, such as a retroreflector. Several collection arrangements are shown in the Fig. 3, Fig. 4 and Fig. 5 described in more detail.

[0019] It should be noted that the first noble gas and the filter material can be selected to achieve the desired long-pass filter properties, and that a variety of combinations of first noble gas and filter tube material are possible. It should be noted that the scope of the present disclosure should not be construed as being limited to any particular noble gas or filter material. For example, in a first combination, the first noble gas may be krypton, the second noble gas may be argon, and the filter material may be CaF2. Such a combination is particularly useful for protecting MgF2-based optical elements (e.g., exit windows, convex lenses, mirrors, etc.) from broadband radiation. Fig. Figure 2 shows a data diagram illustrating the transmission characteristics of the materials of the light source 100 in the case of a CaF2 filter and an Ar / Kr gas mixture. Krypton has an absorption line centered at approximately 124 nm and therefore blocks radiation at the absorption edge of CaF2 at 123 nm, thus protecting the CaF2 filter itself from damage. A small amount of Kr can be added to Ar with the thin CaF2 filter. Kr absorbs a portion of the light in the spectral bands at 116 nm and 124 nm (see, for example, Fig. 2). The absorption in these spectral bands is caused by very strong absorption transitions from the ground state, which tend to broaden into the red from the main wavelength of the transition at 123.58 nm and 116.49 nm. By using a thin CaF2 filter, the remaining light below 125 nm is removed. The CaF2 absorption edge of filter 104 is protected by the Kr absorption (see, for example, Fig. 2). It should be noted that the CaF2 filter does not need to bear any structural load and can be made thin enough to transmit most light >125 nm. Kr provides a sharper absorption edge than a simple CaF2 filter and also shields CaF2 from much of the radiation near the CaF2 absorption edge, reducing damage to the material. The transmission edge can have a longer wavelength if the partial pressure of the first noble gas is increased. For example, the resulting long-pass filter has a transmission edge at about 125 nm, which can be tuned into the red region by increasing the Kr partial pressure. The filtered broadband light 117 emitted by the light source 100 is less harmful to MgF2 and is also not absorbed by MgF2 windows or other optics because the absorption edge of MgF2 is at about 116 nm, i.e., below the 125 nm limit of the filtered broadband light 117.Furthermore, structural components of the gas confinement structure 102 can be positioned at relatively large distances from the plasma 112, reducing radiant heat load, facilitating cooling, and lowering operating temperatures. Low temperatures of the windows and chamber walls reduce noise from refraction.

[0020] Another example: In a second combination, the first noble gas can contain xenon (e.g., xenon mixed with argon) and the filter 104 can contain sapphire. In this case, damage to the bulk sapphire is reduced by the Xe absorption line at 146.96 nm, which coincides with the absorption edge of the sapphire, thus protecting the sapphire filter tube from damage. It should be noted that the gas mixture in the gas confinement structure is not limited to Kr / Ar or Xe / Ar. The gas can contain, for example, a few percent of Kr in Ar, pure Kr, an Ar / Kr / Xe mixture, pure Xe, etc. The addition of Xe blocks emission below approximately 132–136 nm and in the range from 144 to approximately 150–160 nm, depending on the Xe partial pressure. The use of 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 gas can be used in combination with a filter tube made of crystalline quartz, fused silica, CaF2 or sapphire, and the output and laser windows can be made of fused silica, sapphire, MgF2 or CaF2.

[0021] It is pointed out that the Fig. 1 does not limit the scope of the present disclosure and that the light source 100 and the filter 104 may be arranged in a variety of suitable configurations.

[0022] In embodiments, the partial pressure of the first noble gas in the regions upstream and downstream of the filter 104 can be independently controlled, such that there is little or no pressure difference across the filter 104. For example, with a Kr / Ar gas mixture and a CaF2 filter, the Kr partial pressure in the regions upstream and downstream of the CaF2 filter can be independently controlled, with little or no pressure difference across the CaF2 filter. For example, the gas mixture upstream of the CaF2 filter can have a Kr partial pressure of 1 bar and an Ar partial pressure of 99 bar, while the gas mixture downstream of the filter can have a Kr partial pressure of 100 bar. This is useful for increasing the brightness of the LSP, since adding too much Kr (or Xe) to Ar results in a less luminous plasma with lower spectral radiance.

[0023] Fig. Figure 3 shows a simplified schematic view of the light source 100 equipped with a filter 104 formed in a tubular structure to form a filter tube 304, in accordance with one or more embodiments of the present disclosure. It should be noted that the various implementations and components previously described herein with respect to the Fig. 1-2 are to be interpreted as referring to the Fig. 3 unless otherwise noted. In this embodiment, 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 304 and generate broadband light 113. Subsequently, the gas mixture 103 and the filter tube 304 can filter the broadband light to generate the filtered broadband light 117. In this embodiment, the light source 100 includes one or more collection optical elements for collecting the filtered broadband light 117 and transmitting the filtered broadband light 117 through an output optical window to one or more downstream optical elements external to the gas confinement structure 102. For example, the light source 100 can include, but is not limited to, a collection mirror, such as a retroreflector 119.

[0024] Fig. Figure 4 shows a simplified schematic view of the light source 100 with a reflector assembly 400 according to one or more embodiments of the present disclosure. It should be noted that the various implementations and components previously described herein with respect to the Fig. 1 to 3 are to be interpreted as referring to the Fig. 4, unless otherwise stated. In this embodiment, the optical collection element comprises a reflector array. For example, the optical collection element may comprise, but is not limited to, an elliptical reflector array 402, and the filter may comprise a filter tube 304 positioned within the reflector array 400, with the plasma 112 formed within the volume of the filter tube 304.

[0025] Fig. Figure 5 shows a simplified schematic view of the light source 100 with a pressurized reflector assembly 500 according to one or more embodiments of the present disclosure. It should be noted that the various embodiments and components previously described herein with respect to the Fig. 1 to 4 are to be interpreted as referring to Fig. 5, unless otherwise noted. In this embodiment, the optical collection element comprises a composite reflector assembly. The optical collection element may include, for example, but is not limited to, an elliptical reflector and a hemispherical retroreflector coupled to the top of the elliptical reflector to form a pressure chamber. In this embodiment, the gas mixture (e.g., Kr / Ar) is located within the pressurized reflector assembly 500, which serves as the gas containment structure and collection optics. The filter tube 304 may be located within the pressure chamber 500, with the plasma 112 forming within the volume of the filter tube 304.

[0026] Fig. Figure 6 shows a simplified schematic view of an optical characterization system 600 with the compact LSP broadband light source according to one or more alternative and / or additional embodiments. In embodiments, the system 600 includes the LSP light source 100, an illumination arm 603, a collection arm 605, a detector assembly 614, and a controller 618 with one or more processors 620 and a memory 622.

[0027] It is noted that system 600 may include any imaging, inspection, metrology, lithography, or other characterization system known in the art. In this regard, system 600 may be configured to perform inspection, optical metrology, lithography, and / or any form of imaging on a sample 607. Sample 607 may include any sample known in the art, including, but not limited to, a wafer, a reticle, a photomask, and the like. It is noted that system 600 may include one or more of the various embodiments of LSP light source 100 described in the present disclosure.

[0028] In one embodiment, the sample 607 is mounted on a stage assembly 612 to enable movement of the sample 607. The stage assembly 612 may comprise any stage assembly 612 known in the art, including, but not limited to, an XY stage, an R-θ stage, and the like. In another embodiment, the stage assembly 612 is capable of adjusting the height of the sample 607 during inspection or imaging to maintain focus on the sample 607.

[0029] In one embodiment, the illumination arm 603 is configured to direct broadband light 117 from the broadband LSP light source 100 onto the sample 607. The illumination arm 603 may include any number and type of optical components known in the art. In one embodiment, the illumination arm 603 includes one or more optical elements 602, a beam splitter 604, and an objective lens 606. In this regard, the illumination arm 603 may be configured to focus broadband light 117 from the broadband LSP light source 100 onto the surface of the sample 607.The one or more optical elements 602 may comprise 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 that the collection point may comprise, but is not limited to, one or more of the optical elements 602, a beam splitter 604, or an objective lens 606.

[0030] In one embodiment, system 600 includes a collection arm 605 configured to collect the light reflected, scattered, diffracted, and / or emitted by sample 607. In another embodiment, collection arm 605 may direct and / or focus the light from sample 607 onto a sensor 616 of a detector array 614. It should be noted that sensor 616 and detector array 614 may include any sensor and detector array known in the art. Sensor 616 may include, but is not limited to, a CCD sensor or a CCD-TDI sensor. Further, sensor 616 may include, but is not limited to, a line sensor or an electron bombardment line sensor.

[0031] In one embodiment, the detector array 614 is communicatively coupled to a controller 618 that includes one or more processors 620 and a memory 622. For example, the one or more processors 620 may be communicatively coupled to the memory 622, with the one or more processors 620 configured to execute a set of program instructions stored in the memory 622. In one embodiment, the one or more processors 620 are configured to analyze the output of the detector array 614. In one embodiment, the set of program instructions is configured to cause the one or more processors 620 to analyze one or more properties of the sample 607.In another embodiment, the set of program instructions is configured to cause the one or more processors 620 to modify one or more properties of the system 600 to maintain focus on the sample 607 and / or the sensor 616. For example, the one or more processors 620 may be configured to adjust the objective lens 606 or one or more optical elements 602 to focus the broadband light 117 of the broadband light source 100 onto the surface of the sample 607. As another example, the one or more processors 620 may be configured to adjust the objective lens 606 and / or one or more optical elements 610 to collect light from the surface of the sample 607 and focus the collected light onto the sensor 616.

[0032] It should be noted that the system 600 can be configured in any optical configuration known in the art, including, but not limited to, a dark-field configuration, a bright-field orientation, and the like. The system 600 can be configured as any type of metrology tool known in the art, such as a spectroscopic ellipsometer with one or more illumination angles, a spectroscopic ellipsometer for measuring Mueller matrix elements (e.g., using rotating compensators), 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.

[0033] Further details of various embodiments of the optical characterization system 600 are described in U.S. Patent 7,957,066 B2, published June 7, 2011, entitled "Split Field Inspection System Using Small Catadioptric Objectives"; published U.S. patent application 2007 / 0002465, titled "Beam Delivery System for Laser Dark-Field Illumination in a Catadioptric Optical System," published January 4, 2007; U.S. Patent 5,999,310, titled "Ultrabroadband UV Microscope Imaging System with Wide Range Zoom Capability," published December 7, 1999; U.S. Patent 7,525,649, titled "Surface Inspection System Using Laser Line Illumination with Two Dimensional Imaging," issued April 28, 2009; published US patent application 2013 / 0114085 entitled “Dynamically Adjustable Semiconductor Metrology System” by Wang et al. and published on 9.May 2013; U.S. Patent 5,608,526, entitled "Focused Beam Spectroscopic Ellipsometry Method and System," by Piwonka-Corle et al., issued March 4, 1997; and U.S. Patent 6,297,880, entitled "Apparatus for Analyzing MultiLayer Thin Film Stacks on Semiconductors," by Rosencwaig et al., issued October 2, 2001, each of which is incorporated herein by reference in its entirety.

[0034] The one or more processors 620 of the present disclosure may include one or more processing elements known in the art. In this sense, the one or more processors 620 may include any microprocessor-like device configured to execute software algorithms and / or instructions. In one embodiment, the one or more processors 620 may be a desktop computer, a mainframe computer, a workstation, an imaging computer, a parallel processor, or other computer system (e.g., a networked computer) configured to execute a program configured to operate the system 600 and / or the broadband light source 100 as described in the present disclosure.It should be appreciated that the steps described in the present 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 include any device having one or more processing elements that execute program instructions from a non-transitory storage medium 622. Moreover, various subsystems of the various disclosed systems may include processor or logic elements capable of performing at least a portion of the steps described in the present disclosure. Therefore, the above description should not be construed as limiting the present disclosure, but merely as an illustration.

[0035] The storage medium 622 may comprise any storage medium known in the art suitable for storing program instructions that can be executed by the associated one or more processors 620. For example, the storage medium 622 may be a non-volatile storage medium. The storage medium 622 may comprise, for example, but is not limited to, read-only memory, random access memory, a magnetic or optical storage device (e.g., a hard disk), magnetic tape, a solid-state drive, and the like. In another embodiment, the memory 622 is configured to store one or more results and / or outputs of the various steps described herein. It should further be noted that the memory 622 may be housed in a common controller housing with the one or more processors 620.In an alternative embodiment, the memory 622 may be located remotely with respect to the physical location of the processors 620. For example, the one or more processors 620 may access remote storage (e.g., a server) accessible over a network (e.g., Internet, intranet, and the like). In another embodiment, the storage medium 622 includes program instructions for causing the one or more processors 620 to perform the various steps described in the present disclosure.

[0036] Fig.7 shows a process flow diagram illustrating a method 700 for generating VUV light with a long-pass filtered LSP broadband light source according to one or more alternative and / or additional embodiments. It is noted here that the steps of method 700 may be implemented in whole or in part by LSP broadband light source 100. However, it is further recognized that method 700 is not limited to LSP broadband light source 100, as additional or alternative system-level embodiments may perform all or a portion of the steps of method 700.

[0037] In step 702, method 700 includes enclosing a mixture of a first noble gas and a second noble gas in a gas confinement structure. In step 704, method 700 includes generating an optical pump and directing the optical pump into the gas confinement structure to maintain a plasma within the gas confinement structure to generate broadband light. In step 706, method 700 includes long-pass filtering the broadband light via the first noble gas and the filter to provide broadband light having a wavelength below a selected wavelength threshold. Method 700 may include, in an additional step 706a, absorbing a portion of the broadband light within a first wavelength band and a second wavelength band by the first noble gas.In a further step 706b, the method 700 includes absorbing a portion of the broadband light with a wavelength below a selected wavelength threshold via a filter. In step 708, the method 700 includes transmitting the filtered broadband light out of the gas confinement structure via an optical output window.

[0038] Those skilled in the art will recognize that the components, operations, devices, objects, and the accompanying discussions described herein are used as examples for conceptual clarity, and that various configuration changes are contemplated. Accordingly, the specific examples provided herein and the accompanying discussion are intended to be representative of their more general classes. In general, each specific example is intended to be representative of its class, and the exclusion of specific components (e.g., operations), devices, and objects should not be construed as limiting.

[0039] Regarding the use of terms in the plural and / or singular, the skilled person may translate from the plural to the singular and / or from the singular to the plural depending on the context and / or application. The various singular / plural permutations are not explicitly listed here for the sake of clarity.

[0040] The subject matter described herein sometimes illustrates various components contained within or connected to other components. It should be understood that such illustrated architectures 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 to achieve the same functionality is effectively "connected" such that the desired functionality is achieved. Therefore, two components combined herein to achieve a particular functionality may be considered "connected together" such that the desired functionality is achieved, regardless of architectures or intermediary components.Likewise, two components connected in this manner may be considered "connected" or "coupled" to achieve the desired functionality, and two components that can be connected in this manner may also be considered "coupleable" to achieve the desired functionality. Specific examples of "coupleable" include, but are not limited to, physically mating and / or physically interacting components, and / or wirelessly interacting and / or wirelessly interacting components, and / or logically interacting and / or logically interacting components.

[0041] Furthermore, the invention is defined by the appended claims. Those skilled in the art will understand that the terms used herein, and particularly in the appended claims (e.g., in the parts of the appended claims), are generally to be understood as "open" terms (e.g., the term "including" should be interpreted as "including, but not limited to," the term "having" as "having at least," the term "comprises" as "comprises, but is not limited to," and the like). Those skilled in the art will appreciate that when a particular number of introduced claim features are intended, this intention will be expressly mentioned in the claim, and when such a mention is missing, no such intention exists. For clarity, in the claims below, for example, the introductory terms "at least one" and "one or more" may be used to introduce claim features.However, the use of such expressions should not be interpreted as meaning that the introduction of a claim feature by the indefinite articles “a” or “an” limits a particular claim containing such an introduced claim feature to inventions containing only such a feature, even if the same claim contains the introductory expressions “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and / or “an” should normally be interpreted to mean “at least one” or “one or more”). The same applies to the use of definite articles to introduce claim formulations. Even if a specific number of introduced claim features is explicitly mentioned, the skilled person will recognize that such a list should normally be interpreted to mean at least the stated number (e.g.,the mere mention of "two features" without other modifiers usually means at least two features or two or more features). Furthermore, where a convention is used analogously to "at least one of A, B, and C, and the like," such a construction is generally meant in the sense in which a person skilled in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include, but is not limited to, systems having A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like). Where a convention is used analogously to "at least one of A, B, or C, and the like," such a construction is generally meant in the sense in which a person skilled in the art would understand the convention (e.g.,(For example, "a system having at least one of A, B, or C" would include systems having A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like.) Those skilled in the art will appreciate that virtually any disjunctive word and / or disjunctive sentence containing two or more alternative terms, whether in the description, claims, or drawings, should be understood to include the possibility of including one of the terms, any of the terms, or both. For example, the phrase "A or B" should be understood to include the possibility of "A" or "B" or "A and B."

[0042] It is believed that the present disclosure and many of the attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in the form, construction, and arrangement of components may be made without departing from the disclosed subject matter or without sacrificing all of its essential advantages. The described form is merely illustrative, and it is the intent of the following claims to encompass and embrace such changes. Moreover, the invention is defined by the appended claims. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 63 / 445,307

[0001] US 63 / 446,911

[0001] US 7,957,066 B2

[0033] US 2007 / 0002465

[0033] US 5,999,310

[0033] US 7,525,649

[0033] US 2013 / 0114085

[0033] US 5,608,526

[0033] US 6,297,880

[0033]

Claims

[1] Laser-based broadband light source comprising: a gas inclusion structure containing a mixture of a first noble gas and a second noble gas, a laser pump source configured to generate an optical pump for maintaining a plasma within the gas confinement structure, the plasma generating broadband light, wherein the first noble gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band; and a filter positioned within the gas confinement structure and configured to absorb a portion of the broadband light emitted by the plasma having a wavelength below a selected wavelength threshold, wherein the absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage. [2] A broadband light source according to claim 1, wherein the absorption of the broadband light at the first wavelength by the first noble gas protects the filter from deterioration. [3] Broadband light source according to claim 1, wherein a transmission edge of the long-pass filtering is tunable by adjusting a partial pressure of the first noble gas within the gas inclusion structure. [4] A broadband light source according to claim 3, wherein the transmission edge shifts to a longer wavelength when the partial pressure of the first noble gas is increased. [5] A broadband light source according to claim 1, wherein the first noble gas comprises at least one of the gases krypton or xenon. [6] A broadband light source according to claim 1, wherein the second noble gas comprises argon. [7] Broadband light source according to claim 1, wherein the filter is formed from at least one of the materials CaF2 or sapphire. [8] A broadband light source according to claim 1, wherein the first noble gas comprises krypton, the second noble gas comprises argon and the filter is formed of CaF2. [9] A broadband light source according to claim 1, wherein the first noble gas comprises xenon, the second noble gas comprises argon and the filter is formed of sapphire. [10] A broadband light source according to claim 1, wherein the filter comprises at least one of the following elements: a plate or a tube. [11] A broadband light source according to claim 1, further comprising: an optical collection element configured to collect at least a portion of the broadband light emitted by the plasma and to direct the portion of the broadband light to the one or more downstream optical elements. [12] Broadband light source according to claim 11, wherein the optical collecting element comprises a mirror and / or a lens. [13] A broadband light source according to claim 12, wherein the optical collection element comprises a reflector arrangement. [14] A broadband light source according to claim 13, wherein the optical collection element comprises an elliptical reflector arrangement. [15] A broadband light source according to claim 13, wherein the optical collection element comprises a composite reflector assembly having an elliptical reflector assembly and a hemispherical retroreflector. [16] Broadband light source according to claim 1, wherein the one or more downstream optical elements are formed from MgF2. [17] Broadband light source according to claim 1, wherein the one or more downstream optical elements comprise one or more transmissive optical elements and / or one or more reflective optical elements. [18] A broadband light source according to claim 17, wherein the one or more downstream optical elements comprise at least one of the following elements: a window, a lens, or a mirror. [19] Characterization system comprising: a broadband light source with: a gas inclusion structure containing a mixture of a first noble gas and a second noble gas, a laser pump source configured to generate an optical pump for maintaining a plasma within the gas confinement structure, the plasma generating broadband light; wherein the first noble gas absorbs a portion of the broadband light within a first wavelength band and a second wavelength band; a filter disposed within the gas confinement structure and configured to absorb a portion of the broadband light emitted by the plasma having a wavelength below a selected wavelength threshold, wherein the absorption of the broadband light by the first noble gas and the filter provides long-pass filtering of the broadband light below the selected wavelength threshold to protect one or more downstream optical elements from damage; and an optical collection element configured to collect broadband light emitted by the plasma and to direct the broadband light to the one or more downstream optical elements; a set of illumination optics configured to direct broadband light from the broadband light source onto one or more samples; a set of collection optics configured to collect the light emanating from the one or more samples; and a detector array. [20] The characterization system of claim 19, wherein the absorption of the broadband light at the first wavelength by the first noble gas protects the filter from degradation. [21] The characterization system of claim 19, wherein a transmission edge of the long-pass filtering is tunable by adjusting a partial pressure of the first noble gas within the gas inclusion structure. [22] The characterization system of claim 21, wherein the transmission edge shifts to a longer wavelength as the partial pressure of the first noble gas is increased. [23] The characterization system of claim 19, wherein the first noble gas comprises krypton and / or xenon. [24] The characterization system of claim 19, wherein the second noble gas comprises argon. [25] The characterization system of claim 19, wherein the filter is formed from a CaF2 or a sapphire filter. [26] The characterization system of claim 19, wherein the first noble gas comprises krypton, the second noble gas comprises argon, and the filter is formed of CaF2. [27] The characterization system of claim 19, wherein the first noble gas comprises xenon, the second noble gas comprises argon, and the filter is formed of sapphire. [28] The characterization system of claim 19, wherein the filter comprises at least one of the following elements: a plate or a tube. [29] The characterization system of claim 19, wherein the optical collection element comprises a mirror and / or a lens. [30] The characterization system of claim 29, wherein the optical collection element comprises a reflector array. [31] The characterization system of claim 30, wherein the optical collection element comprises an elliptical reflector array. [32] The characterization system of claim 30, wherein the optical collection element comprises a composite reflector assembly including an elliptical reflector assembly and a hemispherical retroreflector. [33] The characterization system of claim 19, wherein the one or more downstream optical elements are formed from MgF2. [34] The characterization system of claim 19, wherein the one or more downstream optical elements comprise one or more transmissive optical elements and / or one or more reflective optical elements. [35] The characterization system of claim 19, wherein the one or more downstream optical elements comprise at least one of the following elements: a window, a lens, or a mirror. [36] Procedure comprising: enclosing a mixture of a first noble gas and a second noble gas in a gas confinement structure; Generating an optical pump and directing the optical pump into the gas confinement structure to maintain a plasma within the gas confinement structure to generate broadband light; and Providing long-pass filtering of the broadband light, wherein providing long-pass filtering of the broadband light comprises: Absorbing a portion of the broadband light within a first wavelength band and a second wavelength band by the first noble gas; and Absorbing a portion of the broadband light with a wavelength below a selected wavelength threshold via a filter.

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