Deep ultraviolet laser using strontium tetraborate for frequency conversion.

Abstract

A nonlinear crystal 300 having stacked strontium tetraborate (SrB4O7) crystal plates 335-1 / 2 / 3 / 4 arranged together to generate a periodic structure for quasi-phase matching (QPM) is used in the final frequency doubling stages 230A,B of the laser assembly 200A to generate laser output light 239A,B having a wavelength in the range of approximately 180 nm to 200 nm. One or more frequency conversion stages 220A, 220B-1, 221B, 222B are used to frequency double, downconvert and / or sum one or more fundamental laser beams 211A,B to produce intermediate frequency light 212A, 214B having a corresponding wavelength in the range of about 360 nm to 400 nm, and then a final frequency conversion stage 130 utilizes a nonlinear crystal 300 to frequency double the intermediate frequency light 212A, 214B to produce the desired high power laser output light 239A,B. Methods, inspection systems, lithography systems and cutting systems incorporating the laser assembly are also described.

Description

[Technical field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 288,560, filed December 11, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present application relates to lasers capable of producing light having deep ultraviolet (DUV) and vacuum ultraviolet (VUV) wavelengths, particularly lasers capable of producing light in the range of about 180 nm to 200 nm, and systems using such lasers. Systems incorporating the disclosed lasers can be configured to inspect samples, such as photomasks, reticles, and semiconductor wafers. Systems incorporating the disclosed lasers can be configured as lithography systems for pattern exposure on substrates, such as semiconductor wafers, for cutting or drilling substrates, and for ablation or cutting of biological tissue, such as in corrective eye surgery. [Background technology]

[0003] As the dimensions of semiconductor devices decrease, the size of the smallest particle or pattern defect that can cause device malfunction also decreases. This creates a need to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. The intensity of light scattered by a particle smaller than the wavelength of the light generally increases as a higher power of the particle's size. For example, the total scattered intensity of light from an isolated small spherical particle increases proportionally to the sixth power of the diameter of the sphere and inversely proportional to the fourth power of the wavelength. Due to the higher intensity of the scattered light, shorter wavelengths generally provide better sensitivity for detecting small particles and defects than longer wavelengths.

[0004] The intensity of light scattered by small particles and defects is generally very low, so high illumination intensities are required to generate signals that can be detected in a very short time. Average source power levels of 1 W or more may be required to generate such signals. At these high average power levels, high pulse repetition rates are desirable because the higher the repetition rate, the lower the energy per pulse, which in turn reduces the risk of damage to the system optics and the object being inspected. Illumination needs for inspection and metrology are often best met by continuous wave (CW) sources. CW sources provide a constant power level, avoiding peak power damage issues and allowing images or data to be captured continuously. However, in many cases, mode-locked lasers (also called quasi-CW lasers) with repetition rates of about 50 MHz or higher can be useful because the higher repetition rate means that the energy per pulse can be small enough to avoid damage in many metrology and inspection applications. Compared to CW lasers at the same average power level, mode-locked lasers with higher peak powers can allow for more efficient and simpler frequency conversion.

[0005] Wavelengths around 193 nm are particularly useful because they are close to the shortest wavelengths that can propagate in dry air over reasonable distances (e.g., about 1 m). Wavelengths below about 190 nm are strongly absorbed by oxygen and are usually referred to as VUV wavelengths. ArF excimer lasers (also called exciplex lasers) produce wavelengths around 193 nm and have been used in the semiconductor and medical industries for over 20 years. However, ArF excimer lasers have several disadvantages. They have a maximum pulse repetition rate of about 100 kHz. Fluorine is corrosive, and the laser requires frequent maintenance. The pulse length can vary from a few ns to about 100 ns, depending on the details of the laser cavity design. Some applications, such as cutting and ablation of materials, require short pulses (e.g., less than about 10 ps) to minimize heating damage to materials adjacent to the material being removed.

[0006] US Patent No. 5,999,333, issued to Mead et al., describes a solid-state laser configured to generate wavelengths around 193 nm. The laser is complex and therefore inefficient in converting the energy of the fundamental laser to light of the output wavelength. It has five frequency conversion stages, e.g., an OPO (optical parametric oscillator), two frequency doubling stages, and two frequency summing stages. The DUV frequency conversion stages may use BBO (beta-barium borate) or CLBO (cesium lithium borate) crystals. Both materials have their own challenges when used in this application. BBO has a relatively low damage threshold when exposed to high intensity DUV radiation. CLBO may have a higher damage threshold than BBO, but is hygroscopic and requires great care during handling, processing, and operation. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Pat. No. 5,742,626 [Patent Document 2] U.S. Pat. No. 1,092,1261 [Patent Document 3] U.S. Pat. No. 1,136,032 [Patent Document 4] U.S. Patent No. 6,201,601 [Patent Document 5] U.S. Patent No. 6,271,916 [Patent Document 6] U.S. Patent No. 7,525,649 [Patent Document 7] U.S. Patent No. 7,817,260 [Patent Document 8] U.S. Patent No. 8,298,335 [Patent Document 9] U.S. Patent No. 8,824,514 [Patent Document 10] U.S. Patent No. 8,976,343 [Patent Document 11] U.S. Patent No. 9,023,152 [Patent Document 12] U.S. Patent No. 9,461,435 [Patent Document 13] U.S. Patent No. 9,059,560 [Patent Document 14] U.S. Patent No. 9,293,882 [Patent Document 15] U.S. Pat. No. 9,660,409 [Patent Document 16] U.S. Patent No. 9,250,178 [Patent Document 17] U.S. Patent No. 9,459,215 [Patent Document 18] U.S. Pat. No. 9,509,112 [Patent Document 19] U.S. Pat. No. 10,044,166 [Patent Document 20] U.S. Pat. No. 10,283,366 [Patent Document 21] U.S. Pat. No. 1,118,0866 [Patent Document 22] U.S. Patent No. 9,255,887 [Patent Document 23] U.S. Pat. No. 9,645,287 [Patent Document 24] U.S. Patent No. 9,709,510 [Patent Document 25] U.S. Patent No. 9,726,617 [Non-patent literature]

[0008] [Non-Patent Document 1] P. Trabs, F. Noack, AS Aleksandrovsky, AI Zaitsev, NV Radionov, and V. Petrov, “Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV,” Opt. Express 23, 10091 (2015) Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, a need has arisen for lasers that overcome the limitations of the conventional approaches discussed above. [Means for solving the problem]

[0010] A characterization system is disclosed in accordance with one or more embodiments of the present disclosure. In some embodiments, the characterization system comprises a light source configured to generate light having a wavelength in the range of 180 nm to 200 nm. In some embodiments, the characterization system comprises an optical system configured to direct the light onto a sample. In some embodiments, the light source comprises a first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency, one or more intermediate frequency conversion stages cooperatively configured to generate intermediate frequency light using the fundamental laser beam, the intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between 360 nm and 400 nm, and a final frequency doubling stage configured to pass the intermediate frequency light into a nonlinear crystal. In some embodiments, the nonlinear crystal comprises a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate. In various embodiments, the SBO crystal plates are arranged together to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light such that the light emerging from the nonlinear crystal includes a laser output having an output frequency and a corresponding wavelength within a range of about 180 nm to about 200 nm. The characterization system can be an inspection system, a metrology system, or a lithography system.

[0011] According to one or more embodiments of the present disclosure, a laser assembly for generating a laser output light is disclosed. The laser assembly of the embodiments includes a first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency. The laser assembly of the embodiments includes one or more intermediate frequency conversion stages cooperatively configured to generate intermediate frequency light using the fundamental laser beam, the intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength of about 360 nm to about 400 nm. The laser assembly of the embodiments includes a final frequency doubling stage configured to pass the intermediate frequency light into a nonlinear crystal. In the embodiments, the nonlinear crystal includes a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second SBO crystal plate. In various embodiments, the SBO crystal plates are arranged together to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light, such that the light emerging from the nonlinear crystal comprises a laser output light having the output frequency and a corresponding wavelength within a range of about 180 nm to about 200 nm.

[0012] According to one or more embodiments of the present disclosure, a method for generating a laser output is disclosed. In some embodiments, the method generates intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between about 360 nm and 400 nm. In some embodiments, the method utilizes a final frequency doubling stage to pass the intermediate frequency light through a nonlinear crystal. In some embodiments, the nonlinear crystal includes a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration, with each first SBO crystal plate adjacent to at least one second SBO crystal plate. In some embodiments, the plurality of SBO crystal plates are arranged together to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light such that light emerging from the nonlinear crystal includes a laser output having an output frequency and a corresponding wavelength within a range of about 180 nm to about 200 nm.

[0013] 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.

[0014] Those skilled in the art will be able to better appreciate the numerous advantages of the present disclosure by referring to the accompanying drawings. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic block diagram illustrating a characterization system configured to inspect or measure a sample in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 1 is a schematic block diagram illustrating an overview of a laser assembly in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 1 is a schematic block diagram illustrating an overview of a laser assembly in accordance with one or more embodiments of the present disclosure. [Diagram 3] 1 is a schematic diagram depicting a final frequency doubling stage utilized in the laser assemblies according to one or more embodiments of the present disclosure. [Figure 4] 1 is a schematic diagram depicting first and final frequency doubling stages utilized in the laser assemblies according to one or more embodiments of the present disclosure. [Diagram 5] 1 is a schematic diagram illustrating a frequency summing and final frequency doubling stage utilized in the laser assemblies according to one or more embodiments of the present disclosure. [Figure 6] FIG. 1 is a schematic diagram depicting an amplification stage utilized within an optical parametric system (OPS) utilized in the laser assemblies according to one or more embodiments of the present disclosure. [Figure 7] FIG. 1 is a schematic diagram depicting an OPS utilized in the laser assemblies according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Reference will now be made in detail to the disclosed subject matter, as illustrated in the accompanying drawings. The present disclosure will be specifically shown and described with reference to certain embodiments and their individual features. The embodiments described herein should be understood as illustrative and not limiting. As will be readily apparent to those skilled in the art, various changes and modifications in form and detail can be made without departing from the spirit and scope of the present disclosure.

[0017] Embodiments of the present disclosure are directed to CW, mode-locked, or short-pulse lasers that generate radiation near a wavelength of 193 nm and that avoid many or all of the shortcomings of conventional 193 nm lasers and are suitable for use in systems configured to inspect substrates, patternwise expose photoresist on substrates, or drill, cut, or ablate materials, including biological tissue. Embodiments of the present disclosure are directed to lasers that utilize two to four frequency conversion stages and are configured to generate high power laser output light having a wavelength near 193 nm (e.g., wavelengths of about 180 nm to about 200 nm) while avoiding the problems and shortcomings of conventional approaches. In the following description, when a wavelength is mentioned without qualification, the wavelength can be considered to be the wavelength in a vacuum.

[0018] The frequency conversion stages generate intermediate frequency light having a wavelength of about 360 nm to 400 nm (e.g., a wavelength around 386 nm); the final frequency doubling stage among them has a nonlinear crystal having stacked strontium tetraborate (SrB4O7) crystal plates forming a periodic structure, which is arranged together to perform quasi-phase matching (QPM) and is suitable for frequency doubling the intermediate frequency light to generate a laser output light having a wavelength of about 180 nm to 200 nm (e.g., a wavelength around 193 nm).

[0019] In one embodiment, the interlocking arrangement involves physically stacking individual SBO crystal plates and alternately inverting the c-axes of successively arranged SBO crystal plates (e.g., rotating the crystal axis of a given SBO crystal plate approximately 180° relative to the crystal axis of its adjacent SBO crystal plate(s)) to form a periodic structure akin to a periodically poled crystalline material (e.g., each SBO crystal plate forms a physical pole within the periodic structure). Each nonlinear crystal is further configured for use in a given optical system by orienting the SBO crystal plates so that their inverted crystal axes are aligned substantially parallel to the polarization direction of the incident light passing through the SBO crystal stack in the optical system, and forming the SBO crystal plates so that the thickness of at least one SBO crystal plate provides a pole spacing (i.e., the distance that the light travels between the opposite surfaces of each plate along the light propagation direction) that is substantially equal to an odd multiple of the critical length, thereby enabling quasi-phase matching between the input light frequency and the output frequency (the second harmonic of the input frequency). By configuring two or more SBO crystal plates in this manner, the nonlinear crystals made according to the present invention can achieve frequency doubling to generate light having a wavelength near 193 nm while avoiding the above-mentioned problems and shortcomings of the conventional approach.

[0020] According to specific disclosed embodiments described below, the present invention is directed to improvements in inspection and metrology systems used in the semiconductor manufacturing industry, and in particular to a laser assembly for such inspection and metrology systems capable of generating laser light having a source power level of about 1 W or more and an emission wavelength in the range of about 180 nm to about 200 nm, for example a wavelength near 193 nm. In one practical embodiment, at least one fundamental laser and one to three intermediate frequency conversion stages are provided within a final frequency doubling stage of the associated laser assembly using a nonlinear crystal, with each fundamental laser individually generating a fundamental laser beam having a corresponding fundamental frequency, and the intermediate frequency conversion stages are configured to cooperate to convert the fundamental laser beam(s) to intermediate frequency light having an associated intermediate frequency corresponding to a wavelength of about 360 nm to 400 nm. The final frequency doubling stage is configured to direct the intermediate frequency light into the inverted SBO crystal plates that form the nonlinear crystal, and the polarization direction (electric field direction) of the light is approximately parallel to the c-axis of the crystal axes of each plate, thereby achieving QPM of the intermediate frequency light and the laser output light in the periodic structure of the stacked SBO crystal plates.

[0021] In a laser assembly and associated methods described herein with reference to a first specific embodiment, a laser output having an output frequency and a wavelength of about 193 nm is generated by generating a fundamental light having a fundamental frequency corresponding to a fundamental wavelength in the range of about 720 nm to about 800 nm, using the fundamental light to generate a second harmonic of the fundamental light, and using the second harmonic as an intermediate frequency light to pass to a final frequency doubling stage. In one aspect of this first embodiment, the final frequency doubling stage is configured to frequency double the second harmonic light; to that end, the stage is configured to include a nonlinear crystal, and the nonlinear crystal is configured to generate a fourth harmonic having a frequency equal to four times the fundamental frequency. To generate a fourth harmonic output light of about 193 nm, two or more stacked SBO crystal plates in the linear crystal have their c-crystal axes oriented approximately parallel to the polarization direction of the second harmonic input light and inverted. The thickness of each plate along the light propagation direction (i.e., the spacing between the poles of the periodic structure) is approximately equal to an odd multiple of a quasi-phase matching critical length approximately equal to 0.85 μm (e.g., in the range of 0.80 μm to 0.90 μm), thereby achieving QPM for the second harmonic frequency and the fourth harmonic frequency, thereby generating a laser output light having an output wavelength of about 193 nm.

[0022] A laser assembly and associated methods described herein with reference to a second specific embodiment includes generating a first fundamental light having a first fundamental frequency corresponding to a first fundamental wavelength in the range of about 1000 nm to about 1100 nm, using the first fundamental light to generate a second harmonic of the first fundamental light, generating light having a third frequency using an optical parametric oscillator (OPO), adding the second harmonic light to the light having the third frequency, and using the sum of the second harmonic light and the third frequency light as intermediate frequency light for passing to a final frequency doubling stage. In one aspect of this second embodiment, the final frequency doubling stage is configured to frequency double the intermediate light; to that end, the stage is configured to include a nonlinear crystal, and the nonlinear crystal is configured to generate a second harmonic of the intermediate light having a frequency equal to four times the first fundamental frequency plus two times the third frequency. To produce a laser output light of about 193 nm, the linear crystal has two or more SBO crystal plates stacked with their c crystal axes oriented and inverted substantially parallel to the polarization direction of the second harmonic input light, and the thickness of each plate along the light propagation direction (i.e., the spacing between the poles of the periodic structure) is approximately equal to an odd integer multiple of a quasi-phase matching critical length approximately equal to 0.85 μm (e.g., in the range of 0.80 μm to 0.90 μm), thereby achieving QPM for the intermediate frequency and the second harmonic frequency, thereby producing a laser output light having an output wavelength of about 193 nm.

[0023] In one embodiment, a laser described herein that produces an emitted light having a wavelength of about 180 nm to 200 nm is incorporated into an inspection system configured to inspect a sample, such as a wafer, reticle, or photomask. In an alternative embodiment, a laser described herein that produces an emitted light having a wavelength of about 193 nm is incorporated into a lithography system configured to expose a pattern on a photoresist coated on a substrate, such as a semiconductor wafer. In yet another embodiment, a laser described herein that produces an emitted light having a wavelength of about 180 nm to 200 nm is incorporated into a system configured to cut, drill, or ablate a material, such as biological tissue.

[0024] The use of strontium tetraborate as an optical coating is discussed in U.S. Patent No. 6,313,636, issued on February 16, 2021, and U.S. Patent No. 6,313,636, issued on June 14, 2022, which are incorporated herein by reference in their entireties. This application is also related to the following U.S. patent documents, all of which are incorporated herein by reference: U.S. Patent No. 6,313,636 to Vaez-Iravani et al., U.S. Patent No. 6,313,636 to Marxer et al., U.S. Patent No. 6,313,636 to Leong et al., U.S. Patent No. 6,313,636 to Chuang et al., U.S. Patent No. 6,313,636 to Armstrong, U.S. Patent No. 6,313,636 to Dribinski, U.S. Patent No. 6,313,636 to Dribinski et al., U.S. Patent No. 6,313,636 to Chuang ... and U.S. Patent No. 6,313,636 to Chuang et al., which are incorporated herein by reference in their entireties.

[0025] 1 illustrates a characterization system 100 in accordance with one or more embodiments of the present disclosure. The characterization system 100 may be configured to inspect or measure a sample 108. The characterization system 100 may comprise an inspection system or a metrology system. The characterization system 100 may also be configured to cut, drill or ablate material from the sample 108, or to expose a pattern of light onto a photoresist on the sample 108.

[0026] The sample 108 may include any sample known in the art, such as, but not limited to, a wafer, a reticle, a photomask, etc. In one embodiment, the sample 108 is positioned on a stage assembly 112 to facilitate movement of the sample 108. The stage assembly 112 may include any stage assembly known in the art, such as, but not limited to, an XY stage, an Rθ stage, etc. In another embodiment, the stage assembly 112 may adjust the height of the sample 108 during inspection to maintain focus on the sample 108. In yet another embodiment, a lens, such as the objective lens 150, may be moved up and down during inspection to maintain focus on the sample 108.

[0027] The characterization system 100 includes an illumination source 102 incorporating a laser 200-0 having an emission frequency ω 1 corresponding to a wavelength in the range of about 180 nm to about 200 nm. OUT Output light L having OUT 2A-7. Illumination source 102 may include additional light sources, such as lasers operating at longer or shorter wavelengths or broadband light sources. Characterization system 100 may include one or more optical elements. For example, but not limited to, light L may be included in one or more optical elements of characterization system 100. OUTThe optical elements may include a beam splitter, mirrors, lenses, apertures, and wave plates configured to dim and direct the light L onto the sample 108. The optical elements may be configured to illuminate an area, line, or spot on the sample 108. According to one embodiment, the beam splitter or mirror 134, mirrors 137 and 138, and lens 152 are configured to illuminate the sample 108 from below, thereby directing the light L within the sample. INT In another embodiment, the beam splitters or mirrors 134 and 135, the mirror 136, and the lens 151 can be configured to transmit light L at an oblique angle of incidence. Obl The specular reflected light L is configured to illuminate the sample 108 (e.g., at an angle of incidence greater than 60° relative to the normal to the sample surface). Spec In yet another embodiment, the optical system 103 may direct the illumination light L onto the top surface of the sample 108. IN Collaboratively organize to direct the

[0028] When illuminating the sample 108 in one or more of the above modes, the optical system 103 further comprises a filter 104 for detecting light L reflected, scattered, diffracted, transmitted, and / or emitted by the sample 108. R / S / T Collect the light L R / S / T 1 and 2. The detector assembly 104 is configured to focus the light onto a sensor 106 of the detector assembly 104. It is noted that the sensor 106 and detector assembly 104 may include any sensor 106 known in the art, including, but not limited to, a charge-coupled device (CCD) detector, a complementary metal-oxide semiconductor (CMOS) detector, a time-delay integration (TDI) detector, a photomultiplier tube (PMT), an avalanche photodiode (APD), a line sensor, an electron-bombarded line sensor, and the like. The detector assembly 104 is communicatively coupled to a controller 114.

[0029] The controller 114 is configured to store and / or analyze data from the detector assembly 104 under the control of program instructions 118 stored on the carrier medium 116. The controller 114 may be further configured to control other elements of the characterization system 100, such as the stage 112, the illumination source 102, and the optical system 103.

[0030] In one embodiment, the optical system 103 includes an illumination tube lens 132. The illumination tube lens 132 can be configured to image the illumination pupil aperture 131 onto a pupil in the objective lens 150. For example, the illumination tube lens 132 can be configured so that the illumination pupil aperture 131 and the pupil in the objective lens 150 are conjugate with each other. In one embodiment, the illumination pupil aperture 131 can be configured by switching and inserting different apertures into the illumination pupil aperture 131. In another embodiment, the illumination pupil aperture 131 can be configured by adjusting the diameter or shape of the opening of the illumination pupil aperture 131. In this case, the sample 108 can be illuminated with different angular ranges depending on the characterization (e.g., metrology or inspection) performed under the control of the controller 114. The illumination pupil aperture 131 can be configured to image the illumination light L. IN The light source may have a polarizing element for controlling the polarization state of the light.

[0031] In some embodiments, the optical element or elements 103 include a collection tube lens 122. For example, the collection tube lens 122 can be configured to image the pupil in the objective lens 150 onto the collection pupil aperture 121. For example, the collection tube lens 122 can be configured so that the collection pupil aperture 121 and the pupil in the objective lens 150 are conjugate with each other. In some embodiments, the collection pupil aperture 121 can be configured by switching in and out different apertures at the collection pupil aperture 121 location. In other embodiments, the collection pupil aperture 121 can be configured by adjusting the diameter or shape of the opening of the collection pupil aperture 121. In this case, illumination at different angular ranges reflected or scattered by the sample 108 can be directed to the detector assembly 104 under the control of the controller 114. The collection pupil aperture 121 can be configured to focus light L of a particular polarization. R / S / T The light may include a polarizing element so that the light may be transmitted to the sensor 106.

[0032] According to certain other embodiments, the illumination pupil aperture 131 and / or the collection pupil aperture 121 can have a programmable aperture. Programmable apertures are generally described in U.S. Patent No. 5,399,433 issued to Brunner on Feb. 9, 2016, entitled “2D programmable aperture mechanism,” and U.S. Patent No. 5,399,433 issued to Brunner on May 9, 2017, entitled “Flexible optical aperture mechanisms,” both of which are incorporated herein by reference in their entireties. Methods for selecting an inspection aperture configuration are generally described in U.S. Patent No. 5,393,433 to Kolchin et al., entitled “Determining a configuration for an optical element positioned in a collection aperture during wafer inspection,” which issued on July 18, 2017, and U.S. Patent No. 5,393,433 to Kolchin et al., entitled “Apparatus and methods for finding a best aperture and mode to enhance defect detection,” which issued on August 8, 2017, both of which are incorporated herein by reference in their entireties.

[0033] The various optical elements and modes of operation depicted in Figure 1 are merely intended to illustrate how laser 200-0 may be used in characterization system 100 and are not intended to limit the scope of the present disclosure. An actual characterization system 100 may be equipped with a subset or superset of the modes and optics depicted in Figure 1. Additional optical elements and subsystems may be incorporated as needed based on a particular application.

[0034] 2A is a schematic block diagram illustrating a laser assembly 200A configured to generate a wavelength in the range of about 180 nm to about 200 nm (e.g., about 193 nm) according to a first specific exemplary embodiment of the present invention. The laser assembly 200A includes a first fundamental laser 210A and two frequency doubling (conversion) stages (e.g., one intermediate frequency doubling stage 220A and a final frequency doubling stage 230A) that are cooperatively configured to generate a laser output 239A having a wavelength in the range of about 180 nm to about 200 nm. The first fundamental laser 210A generates a first fundamental wavelength in the range of about 720 nm to about 800 nm and a corresponding first fundamental frequency ω 1A The first frequency doubling stage 220A receives the first fundamental light 211A and generates a fundamental light 211A having a first fundamental frequency ω 1A The second harmonic frequency 2ω is equal to twice 1A A final (second) frequency doubling stage 230A receives the second harmonic light (intermediate frequency light) 212A and produces a second fundamental frequency ω 1A The emission frequency ω is equal to four times OUTA The laser produces a laser output beam 239A having a magnitude of

[0035] In FIG. 2A, a first fundamental laser 210A is configured to emit a first fundamental frequency ω 1A In one embodiment, the first fundamental laser 210A is configured to generate a first fundamental light 211A (simply referred to as "fundamental") at a first fundamental frequency ω 1AThe first fundamental laser 210A is configured to generate the first fundamental light 211A at 193 nm. In one embodiment, the first fundamental laser 210A is realized using a titanium sapphire (Ti sapphire) laser emission medium. Suitable fundamental lasers operating at wavelengths around 800 nm are commercially available. To generate enough light at a wavelength of about 193 nm to inspect semiconductor wafers, reticles, or photomasks, the first fundamental laser 210A should generate tens or hundreds of watts of fundamental light 211A. Other applications may not require this much power or may require more power. Depending on the pulse width and repetition rate requirements imposed on the laser 200A, the first fundamental laser can be configured as a Q-switched laser, a mode-locked laser, or a CW laser.

[0036] The first frequency doubling stage 220A is configured to generate second harmonic light 212A from the first fundamental light 211A. In one embodiment, the first frequency doubling stage 220A incorporates a lithium triborate (LBO) nonlinear crystal configured for critical phase matching of the first fundamental frequency and the second harmonic frequency. The first frequency doubling stage 220A may include other components as required, such as a prism to separate the second harmonic light 212A from the unconsumed fundamental light. The first frequency doubling stage 220A may include a cavity that resonates at the first fundamental frequency to increase conversion efficiency.

[0037] The final frequency doubling stage 230A is configured to generate a laser output light 239A from the second harmonic light 212A. The final frequency doubling stage 230A incorporates a nonlinear crystal 300 configured to double the frequency of the second harmonic light 212A and to emit light 303 including light at the frequency of the laser output light 239A and the unconsumed second harmonic light. The nonlinear crystal 300 comprises a stack of SBO plates. For illustrative purposes, FIG. 2A depicts four such plates 335-1, 335-2, 335-3, and 335-4 stacked on top of each other. It should be noted that in some embodiments, tens or hundreds or thousands of plates may be stacked. In FIG. 2A, the plates are depicted as being in contact with each other. The plates may be in contact with each other (e.g., the plates may be in optical contact with each other) or there may be a small air gap between the plates, e.g., an air gap having a width close to or less than the thickness of one plate. The thickness of each plate is selected to allow quasi-phase matching during frequency doubling of the second harmonic light 212A. The crystallographic c-axes of adjacent plates (e.g., plates 335-1 and 335-2) are oriented in opposite directions. These and other important aspects of the nonlinear crystal are discussed in more detail below in conjunction with FIG.

[0038] The final frequency doubling stage 230A may optionally include other optical components, such as a prism to separate the laser output light 239A from the unconsumed fundamental second harmonic light, and may include a cavity to recycle the second harmonic frequency, thereby increasing the conversion efficiency.

[0039] According to an alternative embodiment, a single cavity may incorporate both the first frequency doubling stage 220A and the final frequency doubling stage 230A, these and other important aspects of this embodiment are described in more detail below in conjunction with FIG.

[0040] 2B is a schematic block diagram illustrating a laser assembly 200B configured to generate a wavelength in the range of about 180 nm to about 200 nm (e.g., about 193 nm) according to one or more embodiments of the present disclosure. The laser assembly 200B includes a first fundamental laser 210B-1 and four frequency conversion stages (e.g., first frequency doubling stage 220B-1, optical parametric system 221B, first frequency doubling stage 220B-1, frequency summing stage 222B, and final frequency doubling stage 230B) that are cooperatively configured to generate a laser output having a wavelength in the range of about 180 nm to about 200 nm. The first fundamental laser 210B-1 generates a first fundamental wavelength in the range of about 1.0 μm to about 1.1 μm and a corresponding first fundamental frequency ω 1B The first frequency doubling stage 220B-1 receives the first fundamental light 211B-1 and generates a fundamental light 211B-1 having a first fundamental frequency ω 1B The second harmonic frequency 2ω is equal to twice 1B OPS 221B is configured to generate third frequency light 213B having a frequency ω3 and a corresponding wavelength in the range of about 1.2 μm to about 2.0 μm. Frequency summing stage 222B receives light from OPS 221B and first frequency doubling stage 220B-1 and generates a second harmonic light 212B-1 having a frequency ω3 and a corresponding wavelength in the range of about 1.2 μm to about 2.0 μm. Frequency summing stage 222B receives light from OPS 221B and first frequency doubling stage 220B-1 and generates a third frequency light 213B having a frequency equal to the sum of the frequencies of its inputs (i.e., 2ω 1B The final (second) frequency doubling stage 230B receives the intermediate frequency light 214B and generates an output frequency ω 1 equal to twice the intermediate frequency (ω 2 +ω 3 =ω 4 ). OUTA =4ω 1B A laser emission light 239A having +2ω3=2ω4 and a corresponding wavelength within the range of about 180 nm to about 200 nm is generated.

[0041] In FIG. 2B, the first fundamental laser 210B-1 is configured using known techniques to generate the first fundamental light 211B-1 at a first fundamental frequency ω1. In one embodiment, the first fundamental laser 210B-1 is configured to generate the first fundamental light 211B-1 at a first fundamental frequency ω1 corresponding to a wavelength in the range of about 1.0 μm to about 1.1 μm (e.g., a wavelength of about 1064 nm). In one embodiment, the first fundamental laser 210B-1 is realized using one of a Nd-doped yttrium aluminum garnet (YAG) laser emitting medium, a Nd-doped yttrium orthovanadate laser emitting medium, and a ytterbium-doped fiber laser emitting medium. Suitable fundamental lasers operating at wavelengths around 1064 nm are commercially available. To generate enough light at a wavelength of about 193 nm to inspect a semiconductor wafer or reticle, the first fundamental laser 210B-1 should generate tens or hundreds of watts of fundamental light 211B-1. Other applications may not require as much power or may require more power. Depending on the pulse width and repetition rate requirements imposed on the laser 200B, the first fundamental laser 210B-1 can be configured as a Q-switched laser, a mode-locked laser, or a CW laser.

[0042] The first frequency doubling stage 220B-1 is configured to generate second harmonic light 212B-1 from the first fundamental light 211B-1. In one embodiment, the first frequency doubling stage 220B-1 incorporates a lithium triborate (LBO) nonlinear crystal configured for critical phase matching to generate the second harmonic frequency from the first fundamental frequency. The first frequency doubling stage 220B-1 may include other components as required, such as a prism to separate the second harmonic light 212B-1 from the unconsumed first fundamental light. The first frequency doubling stage 220B-1 may include a cavity configured to recycle the first fundamental frequency to increase conversion efficiency.

[0043] The OPS 221B can be configured to generate third frequency light 213B having a frequency ω3 and a corresponding wavelength in the range of about 1.2 μm to about 2.0 μm, for example, about 1.4 μm. The OPS 221B can use any known OPO configuration and / or optical parametric generator (OPG) configuration and / or amplifier stage (e.g., fiber amplifier, thin disk amplifier, cavity amplifier, rod amplifier, optical parametric amplifier (OPA) or multi-pass amplifier) ​​appropriate for the pulse width, pulse repetition rate, power and wavelength of the third frequency light 213B. The OPS 221B can use any suitable nonlinear crystal for frequency conversion, including periodically polarizable materials. Suitable nonlinear crystals include lithium niobate and stoichiometric strontium tantalate. Three alternative configurations for pumping the OPS 221B are depicted in FIG. 2B using dashed lines.

[0044] In one embodiment, the first fundamental light 211B-1 is split into two parts by a beam splitter 224B-1 (essential only in this embodiment). A first part of the first fundamental light is directed to a first frequency doubling stage 220B-1 to generate second harmonic light 212B-1. A second part of the first fundamental light is directed to an OPS 221B as pump light. An advantage of this exemplary embodiment is that the OPS 221B can be configured as an optical parametric oscillator (OPO) and thus can convert the energy of the pump light (the second part of the first fundamental light 211B-1) into third frequency light with high efficiency; this is achieved by converting the energy of the pump light (the second part of the first fundamental light 211B-1) into third frequency light at a frequency ω 1B However, the idler (e.g. ω 1BBecause the idler frequency (light having a frequency equal to -ω3) has a relatively low frequency and therefore a relatively long wavelength, suitable materials for the nonlinear crystals used in the OPS221B configured as an OPO may be expensive or difficult to obtain; this is because the crystal should preferably be reasonably transparent at the idler frequency to minimize thermal effects, such as thermal dephasing or thermal lensing. For example, if the first fundamental light has a wavelength of about 1064 nm and the third frequency light has a wavelength of about 1400 nm, then the idler would have a wavelength of about 4.4 μm.

[0045] In the second embodiment, the second harmonic light 212B-1 is split into two parts by a beam splitter 224B-2. The first part of the second harmonic light is directed to the frequency summing stage 222. The second part of the second harmonic light is directed to the OPS 221B as pump light. The advantage of this embodiment is that it eliminates the idler (e.g., 2ω 1B Since the unwanted light having a frequency equal to -ω3 has a relatively high frequency and therefore a relatively short wavelength, suitable materials for the nonlinear crystals used in the OPS 221B configured as an OPO may be inexpensive and easy to obtain. For example, if the first fundamental light 211B-1 has a wavelength of about 1064 nm (and therefore the second harmonic light 212B-1 has a wavelength of about 532 nm) and the third frequency light has a wavelength of about 1400 nm, then the idler will have a wavelength of about 860 nm. However, this second embodiment is not as efficient as the first exemplary embodiment in converting the energy of the pump light (the second portion of the first fundamental light 211B-1) into the third frequency light; this is because the unwanted light having a frequency equal to -ω3 has a relatively high frequency and therefore a relatively short wavelength. 1B and ω3, ω 1B and ω3 are separated by a large distance, so more energy is injected into the idler.

[0046] In a third embodiment, the laser 200B includes a second fundamental laser 210B-2 configured to generate a second fundamental light 211B-2 having a frequency ω2. The second fundamental light 211B-2 is directed to the OPS 221B as pump light. The second fundamental laser 210B-2 can use any convenient laser emitting medium capable of generating a wavelength suitable for pumping the OPS 221B, including the exemplary laser emitting materials listed above in connection with the first fundamental laser 210B-1. The second fundamental laser 210B-2 can be configured to generate a wavelength near 1064 nm, or to generate a wavelength near 532 nm, for example by frequency doubling the output of a laser generating a wavelength near 1064 nm. This third embodiment, although at first glance more complicated than either the first or second embodiment, may be preferred when many watts of power are desired in the laser output beam 239B; this is because first fundamental lasers 210B-1 with sufficient power may not be readily available or may be very expensive.

[0047] Frequency summing stage 222B receives light from OPS 221B and first frequency doubling stage 220B-1 and has a frequency equal to the sum of its inputs (i.e., 2ω 1B The frequency summing stage 222B is configured to generate intermediate frequency light 214B having an intermediate frequency (equal to ω3 +ω4). The intermediate frequency light 214B should have a wavelength between about 360 nm and 400 nm. In one embodiment, the frequency summing stage 222B incorporates a lithium triborate (LBO) nonlinear crystal configured for critical phase matching to generate the intermediate frequency by summing the second harmonic frequency and the third frequency. The frequency summing stage 222B may include other optical components as needed, such as a prism to separate the intermediate light 214B from the unconsumed second harmonic light and the unconsumed third frequency light. The frequency summing stage 222B may include a resonant cavity configured to recycle the third frequency or the second harmonic frequency to increase conversion efficiency.

[0048] The final frequency doubling stage 230B is configured to generate a laser output light 239B from the intermediate frequency light 214B. The final frequency doubling stage 230B may be configured similarly to the final frequency doubling stage 230A described above with reference to FIG. 2A. The final frequency doubling stage 230B incorporates a nonlinear crystal 300 configured to double the frequency of the intermediate frequency light 214B and to emit light 303 including the laser output light 239B and light at the frequency of the unused intermediate frequency light. The nonlinear crystal 300 comprises a stack of SBO plates. For illustrative purposes, FIG. 2B depicts four such plates 335-1, 335-2, 335-3, and 335-4 stacked on top of each other. It should be noted that in a practical embodiment, tens, hundreds, or thousands of plates may be stacked. In FIG. 2B, the plates are depicted as being in contact with each other. The plates may be in contact or may have a small air gap between them, e.g., an air gap having a width approaching or less than the thickness of one plate. The thickness of each plate is selected to provide quasi-phase matching for frequency doubling of the second harmonic light 214B. The crystallographic c-axes of adjacent plates (e.g., plates 335-1 and 335-2) are oriented in opposite directions. These and other important aspects of the nonlinear crystal are discussed in more detail below in conjunction with FIG. 3.

[0049] The final frequency doubling stage 230B may optionally include other optical components, such as a prism to separate the laser output light 239B from the unused intermediate frequency light. The final frequency doubling stage 230B may include a cavity configured to recycle the intermediate frequency to increase conversion efficiency.

[0050] The frequency summing stage 222B and the final frequency doubling stage 230B may be incorporated into a single cavity; the cavity may be configured to receive the optical frequency ω3 from the OPS 221B and / or the optical frequency ω2 from the first frequency doubling stage 220B-1. 1B At and / or 2ω 1B It is configured to increase the conversion efficiency by recycling light at the intermediate frequency of intermediate frequency light 214B, which is equal to +ω3=ω4. These and other important aspects of the cavity are described in more detail below in conjunction with FIG.

[0051] Figure 3 shows the frequency ω x 3 shows details of a nonlinear crystal 300 having four stacked SBO plates 335-1 to 335-4 configured to frequency double the incident light 301 having a frequency of 100 Hz. The incident light 301 may correspond to the second harmonic light 212A in the laser 200A of FIG. 2A or the intermediate frequency light 214B in the laser 200B of FIG. 2B. The nonlinear crystal 300 shown in FIG. 3 has four stacked SBO crystal plates 335-1 to 335-4 and exhibits a periodic structure, but the total number of SBO plates may be two or less, more than ten, or more than one hundred. There may be an odd number or an even number of plates. The thickness of each of the SBO plates 335-1 to 335-4 may be hundreds of nanometers to tens of micrometers. Specifically, the SBO plate thickness Λ along the propagation direction of the light 301A inside the crystal plate is given by: Λ=mL c (Equation 1) where m is an odd number (e.g. 1, 3, 5, 7…) and L c is the quasi phase matching (QPM) critical length L c =π / Δk (Equation 2) And the definition of Δk is Δk=k(2ω x )-2k(ω x ) (Equation 3) where k(ω) is the wave vector of light of frequency ω in the nonlinear crystal 300. k(ω)=(ωn(ω)) / c (Equation 4) where n(ω) is the refractive index of the nonlinear crystal for the corresponding polarization at frequency ω, and c is the speed of light in vacuum.

[0052] The quasi-phase matching critical length L when doubling the frequency of the incident light 301 with a wavelength of 386.8 nm cm is about 0.85 μm (e.g., thickness of 0.8 μm to 0.9 μm). A reasonable m in the range of 1 to about 999 may be used to achieve a slab thickness that is convenient for handling and processing. This example QPM critical length for frequency doubling light having a wavelength of 386.8 nm to produce light having a wavelength of 193.4 nm was calculated based on the associated SBO refractive index using the Sellmeier model published by IEEE Transactions on Physics and Physics, Vol. 13, No. 1, pp. 1111-1115, 2003, which is incorporated herein by reference in its entirety. Furthermore, variations in the impurity levels in the SBO crystal and the presence of defects in the crystal may slightly alter the value of the refractive index of the crystal. One skilled in the art would know how to use the above equation to calculate the QPM critical length for a particular input and output frequency given the exact refractive index of the crystal.

[0053] In Figure 3, the frequency ω x An incident light 301 of a frequency ω is incident on an incident surface 335-IN of a nonlinear crystal 300. The polarization direction of the incident light 301 is depicted by a dashed arrow 302. SBO plates 335-1 to 335-4 are stacked one on top of the other, and the incident surface 335-IN and the exit surface 335-OUT are arranged to be in the form of a polarization direction of the light having a frequency ω x The SBO crystal is oriented at approximately the Brewster angle θ with respect to the incident light 301 at 386 nm, resulting in minimal reflection losses without the use of anti-reflective coatings. The Brewster angle is approximately equal to 60.3° with respect to the surface normal N for wavelengths near 386 nm polarized nearly parallel to the c-axis of the SBO crystal, and approximately equal to 61.9° with respect to the surface normal N for wavelengths near 193 nm with the same polarization direction. Since reflection losses are small for any angle within a few degrees (e.g., within ±2°) of the Brewster angle, any angle of incidence near 61° results in negligible reflection losses for both the incoming and outgoing light. Furthermore, because the precision of the angle is not critical to reflection losses, small adjustments can be made to the nonlinear crystal 300 (e.g., small adjustments to the angle of incidence θ) to adjust the optical path length Λ within the SBO plate, thereby achieving more precise QPM even when the plate thickness is not precisely as expected due to manufacturing variations. The light 303 emerging from the stack of SBO plates has a frequency of 2ω xThe second harmonic of the incident light and frequency ω x 301 , which includes the unconsumed incident light. In some embodiments, the SBO plates are in optical contact with each other to minimize the reflection loss at each interface. In other embodiments, a small air gap may be provided between the plates. Orienting the input and output faces near the Brewster angle with respect to the incident light 301 can make the reflection loss at each interface between the air and SBO plates very small. However, due to refraction at each SBO-to-air interface, the incident light and its second harmonic may travel along slightly different directions at any air gap. If the difference accumulates across multiple plates, the conversion efficiency may decrease. To minimize this effect, the air gap between the plates should be kept preferably below a few hundred nanometers, and the diameter of the incident light beam should be as large as possible (e.g., about 100 μm or a few hundred micrometers) in line with the requirement of achieving high conversion efficiency at high power density.

[0054] To generate a periodic structure for QPM, the SBO plates 335-1 to 335-4 are rotated relative to one another such that their corresponding c-crystal axes are inverted relative to one another as shown in the two insets in FIG. 3. The two insets show the surface normal N of the SBO plates of thickness Λ (where Λ is the spacing between the poles in the crystal) and the direction of light 301A propagation inside the SBO plates. This physical arrangement of the crystal plates enables QPM. This can be considered analogous to using PPLN (periodically poled lithium niobate) for QPM, except that lithium niobate is a ferroelectric crystal and can be periodically poled. In contrast, SBO is non-ferroelectric, so the crystal plates must be physically arranged to generate a periodic structure for QPM. Furthermore, periodic poling requires the application of an electric field parallel to the crystal axes of the ferroelectric crystal, so that the polarization direction is necessarily aligned with the crystal axes. In contrast, the SBO crystal plate of the present disclosure may be cut and polished in any orientation relative to the crystal axis, allowing the plate to be cut and oriented at the Brewster's angle for light incident on the plate.

[0055] According to one embodiment, the crystal axes of the SBO plates 335-1 to 335-4 are oriented so that the light 301A propagating through the SBO plates propagates substantially perpendicular to the c-axis and the polarization direction (electric field direction) of the light 301A is substantially parallel to the c-axis. 33 It is possible to take advantage of the fact that is the largest nonlinear optical coefficient of SBO, and thus maximize the conversion efficiency. For example, as depicted in FIG. 3, the crystal axes of the SBO plate 335-2 can be oriented so that the light 301A propagates approximately parallel to the a-axis of the SBO crystal. Alternatively, the crystal axes can be oriented so that the light 301A propagates parallel to the b-axis or at an angle in the a-plane of the crystal. In other words, the crystal axes depicted in the two insets in FIG. 3 can be rotated around the c-axis. When the incident surface 335-IN of the SBO plate 335-4 is oriented at Brewster's angle with respect to the incident light 301, the propagation direction of the light 301A in the plate 335-4 is approximately 29.7° with respect to the surface normal N.

[0056] There are many ways to fabricate and assemble the nonlinear crystal 300. If only a few plates are needed for the laser (e.g., high conversion efficiency is not required), it may be convenient to polish the plates to the desired thickness and stack them in the appropriate orientation. If hundreds (or more) of plates are needed to achieve the required conversion efficiency, other fabrication methods may be more convenient. For example, interdigitated nonlinear crystals and methods of fabrication are disclosed in U.S. Patent Application No. 17 / 555,404, filed December 18, 2021, entitled "Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings," which is incorporated herein by reference in its entirety.

[0057] 4 is a schematic diagram illustrating a first and final frequency doubling stage cavity 400 having the first and final frequency doubling stages 220A and 230A of FIG. 2A according to an embodiment of the present disclosure. 1A) enters the bowtie ring cavity, which includes an input coupler 432C-1, curved mirrors 432C-2, 432C-3, and 432C-4, a first frequency doubling stage nonlinear crystal 441 (e.g., LBO), and an SBO nonlinear crystal 300, via an input coupler 432C-1. 1A The beam (having a frequency of ω 1A The unconsumed incident light 438C exits the cavity after passing through or near mirror 432C-2. The output from the first frequency doubling crystal 441 includes the generated second harmonic light (at a frequency of 2ω 1A The second harmonic light 433C is reflected by mirrors 432C-2 and 432C-3 and passes through the SBO nonlinear crystal 300 (shown in FIG. 3). The light appearing at the exit face 444 of the SBO nonlinear crystal 300 is 436C (having a frequency of 2ω 1A and 4ω 1A (It has a frequency of 2ω of 436C. 1A The portion having the frequency 2ω 1A The beam having frequency 4ω passes through beam splitter (BS) 437C, is reflected by mirror 432C-4 and input coupler 432C-1, and recirculates within the cavity. 1A The portion having the radiative absorption coefficient 437c is reflected by the surface of the beam splitter (BS) 437C and travels outside the cavity as output light 439C.

[0058] In some embodiments, the first frequency doubling crystal 441 is configured to operate at a frequency ω 1A and / or the incident light 221A and the unconsumed incident light 438C of frequency 2ω 1A The entrance surface 445 and the exit surface 446 are provided with anti-reflective coatings configured to transmit the circulated light 433C and 442C.

[0059] In some embodiments, the SBO plate 300 is configured such that the entrance surface 443 and the exit surface 444 are irradiated with a frequency of 2ω 1AThe BS 437C is configured to be oriented at approximately the Brewster angle with respect to the circulating lights 433C and 442C of the BS 437C. The polarization directions of the lights 433C and 442C are depicted by arrows 402. Additionally, the BS 437C can be configured to laterally displace the light 442C circulating within its cavity by an amount that substantially offsets the lateral displacement of the light 433C caused by the two or more SBO plates 300, thus maintaining a substantially symmetric bowtie cavity and simplifying the optical alignment of the cavity.

[0060] In some embodiments, the SBO plate 300 is 1A The entrance face 443 is provided with an anti-reflective coating configured to transmit circulating light 443C and 442C having a frequency of 4ω 1A Through the output light 439C of 1A The entrance surface 443 and the exit surface 444 are configured such that they have an anti-reflective coating that is configured to transmit circulating light 443C and 442C having a frequency of 2ω. 1A It is not necessary to orient the circular lights 433C and 442C at Brewster's angle.

[0061] In some embodiments, BS437C may comprise SBO crystal, SBO glass, or CaF2 crystal. Since SBO has good deep UV transparency and high damage threshold, SBO can be successfully used as a substrate material for BS437C to ensure long lifetimes despite high power levels of the unconsumed incident light 442C circulating in the cavity. If 437C comprises SBO crystal, its thickness and / or the orientation of its crystal c-axis can be appropriately configured to minimize any frequency doubling of the unconsumed circulating light 442C passing therethrough. BS437C may comprise a dichroic beam splitter, prism, or other wavelength separation member.

[0062] According to an alternative embodiment, instead of orienting the entrance faces 445, 443 and the exit faces 446, 444 at Brewster's angle, the entrance faces of the first frequency doubling crystal 441 and / or the two or more SBO crystal plates 300 can be coated with a suitable anti-reflective coating.

[0063] In an alternative embodiment, the frequency doubling crystal 441 is located outside the cavity, and the incident light 211A is first passed through the frequency doubling crystal 441 before being introduced into the cavity via the input coupler 432C-1. 1A Circulating light 433C and 442C having the wavelengths .gtoreq. ...

[0064] Although the first and final frequency doubling stage cavities 400 are depicted in FIG. 4 as having four curved mirrors, other combinations of mirrors and / or lenses can be used to refocus the light circulating in the cavities. In an alternative embodiment, the first and final frequency doubling stage cavities 400 can be delta cavities, standing wave cavities, or other shaped cavities instead of bowtie cavities. In an alternative embodiment, any combination of frequencies involved can be cycled. Any of these cavities can be stabilized with standard PDH (Pond-Drever-Hall) or HC (Hensch-Kuyo) locking techniques. The length of the resonant cavity can be adjusted to maintain resonance by adjusting the position of one of the mirrors (e.g. mirror 432C-4 in FIG. 4) or the position of the prism under the action of a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator.

[0065] 5 is a schematic diagram of a frequency summing stage and final frequency doubling stage cavity 500 that may be used in frequency summing stage 222B and final frequency doubling stage 230B of FIG. 2B according to an embodiment of the present disclosure. Incident light 511A (light 213B (ω3) and light 212B-1 (2ω 1B)) enters a bow-tie ring cavity, which includes an input coupler 532C-1, curved mirrors 532C-2, 532C-3, 532C-4, a frequency summing stage nonlinear crystal 541 (e.g., LBO) and an SBO nonlinear crystal 300, via an input coupler 532C-1. 1B (including that which contains ω3 and 2ω4) enters the bowtie ring cavity, passes through or near mirror 532C-1, and passes through summing crystal 541. 1B The unconsumed input light 538C exits the cavity after passing through or near mirror 532C-2. The output from the summing crystal 541 contains the generated output light (at frequency ω4=2ω3+4ω 1B The output light 533C is reflected by mirrors 532C-2 and 532C-3 and passes through the SBO nonlinear crystal 300 (see FIG. 3). The light appearing at the output face 544 of the SBO nonlinear crystal 300 is 536C (having frequencies ω4 and ω4). The portion of 536C having frequency ω4, i.e., the unconsumed light 542C (having frequency ω4), passes through the beam splitter (BS) 537C, is reflected by mirror 532C-4 and input coupler 532C-1, and recirculates within the cavity. The portion of light 536C having frequency ω4 is reflected by the surface of the beam splitter (BS) 537C and passes out of the cavity as output light 539C.

[0066] In some embodiments, the first frequency summing crystal 541 is configured to sum the frequencies ω and 2ω 1B 5, such that input surface 545 and output surface 546 are provided with anti-reflective coatings configured to transmit input light 511A and unconsumed input light 538C, and / or circulated light 533C and 542C of frequency ω4.

[0067] In some embodiments, the SBO plate 300 is configured such that the entrance face 543 and the exit face 544 are oriented at approximately the Brewster angle with respect to the circulating light 533C and 542C of frequency ω4. The polarization directions of the light 533C and 542C are depicted by arrows 502. Furthermore, the BS 537C can be configured to laterally displace the light 542C circulating within its cavity by an amount that substantially offsets the lateral displacement of the light 533C caused by the two or more SBO plates 300, thus maintaining a substantially symmetric bowtie cavity and simplifying the optical alignment of the cavity.

[0068] In one embodiment, SBO plate 300 is configured with anti-reflective coatings on input surface 543 and output surface 544, and the anti-reflective coatings are configured to pass circulating light 533C and 542C, both of which have frequency ω4; the pass angle is the angle that input surface 543 and output surface 544 make with respect to circulating light 533C and 542C, which is not necessarily the Brewster angle for circulating light 533C and 542C of frequency ω4.

[0069] In some embodiments, the BS537C may comprise SBO crystal, SBO glass, or CaF2 crystal. Since SBO has good deep UV transparency and a high damage threshold, it can be successfully used as a substrate material for the BS537C to ensure a long lifetime despite the high power levels of the unconsumed incident light 542C circulating within the cavity. If the 537C comprises an SBO crystal, its thickness and / or the orientation of its crystal c-axis can be appropriately configured to minimize any frequency doubling of the unconsumed incident light 542C passing therethrough. The BS537C may comprise a dichroic beam splitter, prism, or other wavelength separation member.

[0070] According to an alternative embodiment, instead of orienting the entrance faces 545, 543 and the exit faces 546, 544 at Brewster's angle, the entrance faces of the first frequency doubling crystal 541 and / or the two or more SBO crystal plates 300 can be coated with a suitable anti-reflective coating.

[0071] In an alternative embodiment, frequency summing crystal 541 is located outside the cavity and incident light 511A passes first through frequency summing crystal 541 before entering the cavity via input coupler 532C-1. In this embodiment, circulating light 533C and 542C having frequency ω4 may originate outside the cavity and be optically coupled into the cavity via input coupler 532C-1.

[0072] Although the frequency summing and final frequency doubling stage cavity 500 is depicted in FIG. 5 as having four curved mirrors, other combinations of mirrors and / or lenses can be used to refocus the light circulating in the cavity. In an alternative embodiment, the frequency summing and final frequency doubling stage cavity 500 can be a delta cavity, a standing wave cavity, or other shaped cavity instead of a bowtie cavity. In an alternative embodiment, any combination of frequencies involved can be cycled. Any of these cavities can be stabilized with standard PDH or HC locking techniques. The length of the resonant cavity can be adjusted to maintain resonance by adjusting the position of one of the mirrors (e.g., mirror 532C-4 in FIG. 5) or the position of the prism under the action of a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator.

[0073] FIG. 6 is a schematic diagram of an amplifier stage 600 used in the OPS 221B of FIG. 2B according to an embodiment of the present disclosure. The incident light 213B (ω3) is generated by an OPO or OPG. The incident light 213B (ω3) and the CW pump light 611 enter a bowtie ring cavity, which includes an input coupler 632C-1, a curved mirror 632C-2, flat mirrors 632C-3 and 632C-4, and a gain medium 643, via input coupler 632C-1. The incident light 213B (having a frequency ω3) enters the bowtie ring cavity, passes through or near mirror 632C-1, and passes through gain medium 643 (e.g., a Nd-doped YAG rod). The CW pump light 611 is at a frequency and power suitable for pumping the gain medium 643, which may cause stimulated emission of light at frequency ω3 that constitutes the incident light 213B. The CW pump light 611 is 211B-1 (ω 1B ), 211B-2(ω2) or 212B-1(2ω 1B ) The output from gain medium 643 includes amplified input light 633C (having frequency ω3). Output light 633C is partially reflected by output coupler 632C-2, mirror 632C-3, mirror 632C-4, and input coupler 632C-1, and thus recirculates within the cavity. A portion of circulated light 633C is optically coupled out of the cavity via output coupler 632C-2 as output light 213C (having frequency ω3).

[0074] In some embodiments, the gain medium 643 is configured with anti-reflective coatings for the incident light 213B and the CW pump light 611 in the input face 645 and output face 646. In an alternative embodiment, the gain medium 643 is oriented at approximately the Brewster's angle with respect to the circulating light 633C (ω3).

[0075] In some embodiments, a diode emitting pump light 611 is configured to side pump the gain medium 643. Such a diode may be positioned parallel to the incident light 213B on one or more sides of the gain medium 643, causing the pump light 611 to be emitted perpendicular to the incident light 213B into the gain medium 643. The pump light 611 in this embodiment may be coherent, non-coherent, quasi-CW, CW or pulsed.

[0076] Although amplifier stage 600 is depicted in FIG. 6 as having two curved and two flat mirrors, other combinations of mirrors and / or lenses can be used to refocus the light circulating in the cavity. According to an alternative embodiment, amplifier stage 600 can have a delta cavity, a standing wave cavity, or other shaped cavity instead of a bowtie cavity. Any of these cavities can be stabilized with standard PDH or HC locking techniques. The length of the resonant cavity can be adjusted to maintain resonance by adjusting the position of one of the mirrors (e.g., mirror 632C-4 in FIG. 6) or the position of the prism under the action of a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator.

[0077] 7 is a schematic diagram illustrating an example of an OPS 700 that may be used as OPS 221B in FIG. 2B according to an embodiment of the present disclosure. y , 211B-1(ω 1B ), 212B-1(2ω 1B ) and 211B-2 (ω2), enters a bowtie ring cavity that includes an input coupler 732C-1, curved mirrors 732C-2 and 732C-3, an output coupler 732C-4, an OPO crystal 744 (e.g., PPLN), a gain medium 743, and a frequency selection device 747 via an input coupler 732C-1. y ω ) enters the bowtie ring cavity, passes through or near mirror 732C-1, and passes through OPO crystal 744. yUnconsumed input light 748 exits the cavity after passing through or near mirror 732C-2. The output from OPO crystal 744 includes generated circulating light (having frequency ω3) 733C. Circulating light 733C is reflected by mirror 732C-2, passes through frequency selective device 747 (e.g., a transmissive volume Bragg grating), is reflected by mirror 732C-3, and passes through gain medium 743. Amplified circulating light 736C (having frequency ω3) emerging at output face 742 of the gain medium is partially reflected by output coupler 732C-4. Output light 213B transmitted through output coupler 732C-4 has frequency ω3. Amplified circulating light 736C partially reflected by output coupler 732C-4 is reflected by input coupler 732C-1, thus recirculating through the cavity. CW pump light 711 is optically coupled into the cavity via mirror 732C-3 and passes through gain medium 743. Unused CW pump light 712 exits the cavity via output coupler 732C-4. CW pump light 711 has a frequency and power suitable to cause stimulated emission in gain medium 743 of light at frequency ω3 that constitutes circulating light 733C.

[0078] In some embodiments, OPO crystal 744 and gain medium 743 are configured with anti-reflective coatings on input faces 745, 741 and output faces 746, 742 for circulating light 733C and 736C and anti-reflective coatings for incident light 713 (towards OPO crystal 744) or CW pump light 711 (towards gain medium 743). In an alternative embodiment, gain medium 743 is oriented at approximately Brewster's angle with respect to circulating light 733C, 736C (ω3) and CW pump light 711. In an alternative embodiment, OPO crystal 744 is oriented at approximately Brewster's angle with respect to circulating light 733C, 736C (ω3) and incident light 713 (ω3).

[0079] In some embodiments, diodes emitting pump light 711 are configured to side pump the gain medium 743. Such diodes may be positioned parallel to the incident light 713 on one or more sides of the gain medium 743, causing the pump light 711 to radiate perpendicular to the incident light 713 into the gain medium 743. The pump light 711 in this embodiment may be coherent, non-coherent, quasi-CW, CW, or pulsed.

[0080] In some embodiments, frequency selection device 747 can be configured to control the wavelength or bandwidth of circulated light 733C, amplified circulated light 736C, and output light 213B by transmission or reflection gratings or other frequency selection methods. In some embodiments, frequency selection device 747 is configured as an intra-cavity (e.g., linear cavity) reflective frequency selection device (e.g., a reflective volume Bragg grating) that reflects circulated light 733C or amplified circulated light 736C instead of a cavity mirror.

[0081] Although OPS 700 is depicted in FIG. 7 as having four curved mirrors, other combinations of mirrors and / or lenses can be used to refocus the light circulating within the cavity. In an alternative embodiment, the frequency summing stage and OPS 700 can include a delta cavity, a standing wave cavity, or other shaped cavity instead of a bowtie cavity. Any of these cavities can be stabilized with standard PDH or HC locking techniques. The cavity length can be adjusted to maintain resonance by adjusting the position of one of the mirrors (e.g., mirror 732C-4 in FIG. 7) or the position of the prism using control signals (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator.

[0082] The figures above are not meant to represent the actual physical layout of components. The figures above show the main optical modules involved in the process, but not all optical elements. Those skilled in the art will understand from the figures and the accompanying description how to construct a laser that produces wavelengths around 193 nm. As will be appreciated, more or fewer optical components can be used to direct the light, if necessary. Lenses and / or curved mirrors can be used to focus the beam waist to a focal point of substantially circular or elliptical cross section in or near the nonlinear crystal, if appropriate. Prisms, beam splitters, gratings, or diffractive optical elements can be used to manipulate or separate the various wavelengths at the output of each frequency conversion stage, if necessary. Prisms, coated mirrors, or other elements can be used to appropriately combine the various wavelengths at the input to the frequency conversion stage. Beam splitters or coated mirrors can be used appropriately to split a set of wavelengths into two beams. Filters may be used to block or separate unwanted wavelengths at the output of any stage. Waveplates may be used to rotate the polarization as needed. Other optical elements may be used accordingly. As will be appreciated by those skilled in the art, various compromises and substitutions may be made in implementing the lasers disclosed herein.

[0083] While the present invention, as described herein, readily achieves the generation of laser emission light at a desired wavelength of about 193 nm using a variety of fundamental wavelengths, other wavelengths within a few or tens of nanometers of the desired wavelength may be generated by varying the wavelength of the first fundamental laser (laser 200A or 200B) or by varying the wavelength of the light emitted by the OPS (laser 200B).Unless otherwise specified in the appended claims, such lasers and systems utilizing them are deemed to be within the scope of the present invention.

[0084] Lasers with sub-200 nm wavelengths are not commercially available with sufficient power levels, are unreliable, or are expensive to operate. In particular, there is no prior art other than excimer lasers that generate optical powers of 1 W or more in the wavelength range of about 180 nm to 200 nm. Embodiments of the present invention generate wavelengths around 193 nm, which provides better sensitivity for detecting small particles and defects than longer wavelengths. The laser of the present invention does not use toxic or corrosive gases, making it easier and cheaper to operate and maintain.

[0085] As one of ordinary skill in the art will readily appreciate, the lasers of the present invention have many potential applications in addition to their use in semiconductor inspection and metrology. For example, lasers operating at wavelengths near 193.4 nm can be used in lithography systems configured to patternwise expose photoresist coated on a substrate, such as a semiconductor wafer. For example, lasers operating at wavelengths between about 180 nm and 200 nm can be used in systems configured to cut or ablate biological tissue. The lasers described herein can be configured to produce very short pulses at the emitted wavelength, allowing preferential removal of material by ablation instead of heating, with less damage to surrounding material. For example, such lasers can be used in laser eye surgery or laser vision correction. Although the present invention has been described in terms of certain specific embodiments, it will be apparent to one of ordinary skill in the art that the unique features of the present invention are equally applicable to other embodiments, all of which are considered to be within the scope of the present disclosure.

[0086] With respect to the use of substantially all plural and / or singular terms herein, one of ordinary skill in the art may translate the plural to the singular and / or the singular to the plural as appropriate to the context and / or application. In the interest of clarity, the present application has not explicitly set forth the various singular / plural permutations.

[0087] Given the above description, one of ordinary skill in the art can make and use the present disclosure as presented in the context of a particular application and its requirements. The directional terms used herein, such as "top", "bottom", "up", "down", "upwards", "upwards", "downwards", "downwards" and "downwards", are intended to provide relative positions for descriptive purposes and are not intended to specify an absolute reference coordinate system. Various modifications to the preferred embodiment will be apparent to one of ordinary skill in the art, and the general principles defined herein may be applied to other embodiments. Thus, the present disclosure is not intended to be limited to the specific embodiments shown and described, but rather is to be accorded the widest scope consistent with the principles and novel features of the present disclosure.

[0088] Further, as can be understood, the present invention is defined by the claims. As can be understood by those skilled in the art, generally, the terms used in this application, particularly in the claims (e.g., the body of the claims) are generally intended to be "open" terms (e.g., the term "including" should be interpreted as "including but not limited to", the term "having" should be interpreted as "having at least", the term "including" should be interpreted as "including but not limited to", etc.). As can also be understood by those skilled in the art, if a specific number of claim-introducing features is intended, then the intent is clearly stated in the claim, and the absence of such features indicates no intent. For example, as an aid to understanding, the following appended claims may incorporate the introductory phrases "at least one" and "one or more". However, the use of the indefinite article "a" or "an" should not be interpreted as if it implies that the introduction of a claim-introducing feature with the indefinite article "a" or "an" implies that all individual claims containing the indefinite article are limited to inventions containing only one of the feature, even when the indefinite article "one or more" or "at least one" coexists with the indefinite article, e.g., "a" or "an" in the very claim (e.g., "a" and / or "an" should usually be interpreted as meaning "at least one" or "one or more"), and the same is true with respect to the introduction of a claim-introducing feature with the use of the definite article. In addition, even if a specific number of features is specified in a claim, the number should usually be interpreted as meaning at least the specified number, as would be recognized by a person skilled in the art (e.g., the bare expression "two features" without other modifiers usually means at least two features or more than two features).Furthermore, in instances where a convention similar to "at least one of A, B, and C, etc." is used, the syntax is generally intended to conform to the sense in which a person of ordinary skill 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, a system having only A, only B, only C, both A and B, both A and C, both B and C, and / or a system having all three of A, B, and C, etc.). In instances where a convention similar to "at least one of A, B, or C, etc." is used, the syntax is generally intended to conform to the sense in which a person of ordinary skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include, but is not limited to, a system having only A, only B, only C, both A and B, both A and C, both B and C, and / or a system having all three of A, B, and C, etc.). As those of ordinary skill in the art will also appreciate, nearly all disjunctive conjunctions and / or disjunctive phrases presenting two or more alternative terms, whether in the specification, claims, or drawings, should be understood to contemplate the inclusion of either term, either term, or both terms. For example, the phrase "A or B" will be understood to encompass the possibilities of "A" or "B" or "A and B."

[0089] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will also be apparent that various changes in the form, construction and arrangement of the parts may be made without departing from the disclosed subject matter or diminishing all of its essential advantages. The form described is merely illustrative, and it is the intent of the following claims to encompass and embrace all such modifications. Moreover, it is the appended claims which define the invention.

Claims

1. 1. An inspection system comprising: a light source configured to generate light having a wavelength in the range of 180 nm to 200 nm; an optical system configured to direct the light onto a sample; wherein the light source comprises: a first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency; one or more intermediate frequency conversion stages cooperatively configured to generate intermediate frequency light using the fundamental laser beam, the intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between 360 nm and 400 nm; a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal; Equipped with the nonlinear crystal includes a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate; the plurality of SBO crystal plates are arranged together to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light such that light emerging from the nonlinear crystal comprises a laser output having an output frequency corresponding to a wavelength within a range of about 180 nm to about 200 nm; Inspection system.

2. 2. The inspection system of claim 1, wherein the plurality of SBO crystal plates are configured such that a first crystal axis of the first SBO crystal plate is inverted with respect to a second crystal axis of the second SBO crystal plate.

3. 2. The inspection system of claim 1, wherein the emission frequency corresponds to a wavelength of about 193 nm.

4. 2. The inspection system of claim 1, wherein the first fundamental laser is configured to generate a wavelength between about 760 nm and about 800 nm, and the one or more intermediate frequency conversion stages are configured to generate the intermediate frequency light by doubling the frequency of the first fundamental laser beam.

5. 5. The inspection system of claim 4, wherein the first fundamental laser comprises a Ti:sapphire laser.

6. 2. The inspection system of claim 1, wherein the first fundamental laser is configured to generate a first fundamental laser beam having a wavelength of about 1 μm to about 1.1 μm, and the one or more intermediate frequency conversion stages include: a first frequency doubling stage optically coupled to the first fundamental laser and configured to generate second harmonic light by doubling the frequency of at least a first portion of the first fundamental laser beam; an optical parametric system (OPS) configured to generate third frequency light having a wavelength between about 1.2 μm and about 2.0 μm; a frequency summing stage optically coupled to the first frequency doubling stage and to the OPS and configured to generate the intermediate frequency light by summing at least a first portion of the second harmonic light and the third frequency light; An inspection system comprising:

7. 7. The inspection system of claim 6, wherein the one or more intermediate frequency conversion stages are further configured as a pump to direct a second portion of the first fundamental laser beam to the OPS.

8. 7. The inspection system of claim 6, wherein the one or more intermediate frequency conversion stages are further configured as a pump to direct a second portion of the second harmonic light to the OPS.

9. 7. The inspection system of claim 6, further comprising a second fundamental laser configured to generate a second fundamental laser beam, the second fundamental laser optically coupled to the OPS to optically pump the OPS with the second fundamental laser beam.

10. 2. The inspection system of claim 1, wherein the first fundamental laser beam comprises one of a Nd-doped yttrium aluminum garnet (YAG) laser, a Nd-doped yttrium orthovanadate laser, and a Yb-doped fiber laser.

11. 10. The inspection system of claim 1, further comprising: a first optical system configured to direct the laser output light to the sample; and a second optical system configured to collect at least one of light transmitted, reflected, and scattered by the substrate and to direct the collected light to a sensor.

12. 12. The inspection system of claim 11, further comprising a computer operatively connected to the sensor, the computer configured to determine the presence or absence of defects on the substrate by analyzing signals from the sensor.

13. 2. The inspection system of claim 1, wherein an optical parametric system (OPS) including at least one of an optical parametric oscillator (OPO), an optical parametric generator (OPG), and an amplifier stage is used to generate the third frequency light.

14. 14. The inspection system of claim 13, wherein the amplification stage comprises at least one of a fiber amplifier, a thin disk amplifier, a cavity amplifier, a rod amplifier, an optical parametric amplifier (OPA), and a multi-pass amplifier.

15. 2. The inspection system of claim 1, wherein the OPS comprises an optical parametric generator configured to generate a third frequency light, the OPG optically coupled to a cavity, the cavity coupled to an amplifier configured to amplify the third frequency light.

16. 2. The inspection system of claim 1, wherein the OPS comprises an OPO cavity configured to recirculate light having the third frequency, the cavity comprising an amplifier configured to amplify the third frequency light.

17. 2. The inspection system of claim 1, wherein the final frequency doubling stage further comprises a cavity in which the plurality of SBO plates reside, the cavity being configured to recirculate light having the intermediate frequency.

18. 2. The inspection system of claim 1, wherein the frequency summation stage further comprises a cavity configured to recirculate light having the intermediate frequency, and wherein the plurality of SBO plates are contained within the cavity.

19. 1. A laser assembly configured to generate a laser emission light, comprising: a first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency; one or more intermediate frequency conversion stages cooperatively configured to generate intermediate frequency light using the fundamental laser beam, the intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength of about 360 nm to about 400 nm; a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal; Equipped with the nonlinear crystal includes a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second SBO crystal plate; the plurality of SBO crystal plates are arranged together to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light such that light emerging from the nonlinear crystal comprises a laser output having an output frequency corresponding to a wavelength within a range of about 180 nm to about 200 nm; Laser assembly.

20. 20. The laser assembly of claim 19, wherein the plurality of SBO crystal plates are configured such that a first crystal axis of the first SBO crystal plate is inverted relative to a second crystal axis of the second SBO crystal plate.

21. 20. The laser assembly of claim 19, wherein the emission frequency corresponds to a wavelength of about 193 nm.

22. 20. The laser assembly of claim 19, wherein the first fundamental laser is configured to generate a wavelength between about 760 nm and about 800 nm, and the one or more intermediate frequency conversion stages are configured to generate the intermediate frequency light by doubling the frequency of the first fundamental laser beam.

23. 23. The laser assembly of claim 22, wherein the first fundamental laser comprises a Ti:sapphire laser.

24. 20. The laser assembly of claim 19, wherein the first fundamental laser is configured to generate a first fundamental laser beam having a wavelength between about 1 μm and about 1.1 μm, and the one or more intermediate frequency conversion stages further comprise: a first frequency doubling stage optically coupled to the first fundamental laser and configured to generate second harmonic light by doubling the frequency of at least a first portion of the first fundamental laser beam; an optical parametric system (OPS) configured to generate third frequency light having a wavelength between about 1.2 μm and about 2.0 μm; a frequency summing stage optically coupled to the first frequency doubling stage and to the OPS and configured to generate the intermediate frequency light by summing at least a first portion of the second harmonic light and the third frequency light; A laser assembly comprising:

25. 25. The laser assembly of claim 24, wherein the one or more intermediate frequency conversion stages are further configured as an optical pump to direct a second portion of the first fundamental laser beam to the OPS.

26. 25. The laser assembly of claim 24, wherein the one or more intermediate frequency conversion stages are further configured as an optical pump to direct a second portion of the second harmonic light to the OPS.

27. 25. The laser assembly of claim 24, further comprising a second fundamental laser configured to generate a second fundamental laser beam, the second fundamental laser optically coupled to the OPS to optically pump the OPS with the second fundamental laser beam.

28. 25. The laser assembly of claim 24, wherein the first fundamental laser beam comprises one of a Nd-doped yttrium aluminum garnet (YAG) laser, a Nd-doped yttrium orthovanadate laser, and a Yb-doped fiber laser.

29. 20. The laser assembly of claim 19, wherein generating the third frequency light utilizes an optical parametric system (OPS) comprising at least one of an optical parametric oscillator (OPO), an optical parametric generator (OPG), and an amplification stage.

30. 30. The laser assembly of claim 29, wherein the amplification stage comprises at least one of a fiber amplifier, a thin-disk amplifier, a cavity amplifier, a rod amplifier, an optical parametric amplifier (OPA), and a multi-pass amplifier.

31. 20. The laser assembly of claim 19, wherein the OPS comprises an optical parametric generator configured to generate light at a third frequency, the OPG optically coupled to a cavity, the cavity being coupled to an amplifier configured to amplify the light at the third frequency.

32. 20. The laser assembly of claim 19, wherein the OPS comprises an OPO cavity configured to recycle light having the third frequency, the cavity comprising an amplifier configured to amplify the third frequency light.

33. 20. The laser assembly of claim 19, wherein the final frequency doubling stage further comprises a cavity configured to recycle light having the intermediate frequency, the plurality of SBO plates being within the cavity.

34. 20. The laser assembly of claim 19, wherein the frequency summation stage further comprises a cavity configured to recycle light having the intermediate frequency, and wherein the plurality of SBO plates reside within the cavity.

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