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] This 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 approximately 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 exposing patterns on substrates, such as semiconductor wafers, for cutting or drilling substrates, or for ablation or cutting of biological tissue, such as in corrective eye surgery. [Background technology]

[0003] As semiconductor device dimensions shrink, the size of the smallest particle or pattern defect capable of causing device malfunction also shrinks. This has created 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 high-order 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 sphere's diameter and inversely proportionally to the fourth power of its wavelength. Due to the higher scattered light intensity, shorter wavelengths generally offer better sensitivity for detecting small particles and defects than longer wavelengths.

[0004] Because the intensity of light scattered by small particles and defects is typically very low, high illumination intensities are required to generate signals that can be detected within very short periods of time. Generating such signals may require average source power levels of 1 W or higher. At these high average power levels, high pulse repetition rates are desirable because higher repetition rates result in lower energy per pulse, which in turn reduces the risk of damage to the system optics and the object being inspected. Inspection and metrology illumination needs are often best met by continuous-wave (CW) sources. CW sources offer a constant power level, avoiding peak-power damage issues and allowing for continuous image or data capture. However, in many cases, mode-locked lasers (also known as quasi-CW lasers) with repetition rates of approximately 50 MHz or higher can be useful because the high 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 with the same average power level, mode-locked lasers with higher peak powers can enable more efficient and simpler frequency conversion.

[0005] Wavelengths near 193 nm are particularly useful because they are near the shortest wavelengths that can propagate over reasonable distances (e.g., about 1 m) in dry air. Wavelengths below about 190 nm are strongly absorbed by oxygen and are commonly referred to as VUV wavelengths. ArF excimer lasers (also known as exciplex lasers) generate wavelengths near 193 nm and have been used in the semiconductor and medical industries for over 20 years. However, ArF excimer lasers have several drawbacks. Their maximum pulse repetition rate is approximately 100 kHz. Fluorine is corrosive, and these lasers require frequent maintenance. Their pulse lengths can range from a few nanoseconds to approximately 100 nanoseconds, depending on the design of the laser cavity. Some applications, such as cutting and ablation, require short pulses (e.g., less than approximately 10 ps) to minimize thermal damage to materials adjacent to the material being removed.

[0006] U.S. Patent No. 5,999,499 to Mead et al. describes a solid-state laser configured to generate wavelengths near 193 nm. This laser is complex and therefore inefficient at converting fundamental laser energy to light at the output wavelength. It includes five frequency conversion stages, including an OPO (optical parametric oscillator), two frequency doubling stages, and two frequency summing stages. These DUV frequency conversion stages may use BBO (beta-barium borate) or CLBO (cesium lithium borate) crystals. Both materials present challenges for use 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 its hygroscopic nature requires significant care during handling, processing, and operation. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 5,742,626 [Patent Document 2] U.S. Patent No. 10,921,261 [Patent Document 3] U.S. Patent 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. Patent 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. Patent No. 9,509,112 [Patent Document 19] U.S. Patent No. 10,044,166 [Patent Document 20] U.S. Patent No. 10,283,366 [Patent Document 21] U.S. Patent No. 1,118,0866 [Patent Document 22] U.S. Patent No. 9,255,887 [Patent Document 23] U.S. Patent 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, there is a need for lasers that overcome the limitations of the conventional approaches mentioned 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 includes 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 includes an optical system configured to direct the light onto a sample. In some embodiments, the light source includes 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 through a nonlinear crystal. In some embodiments, the nonlinear crystal includes multiple strontium tetraborate (SBO) crystal plates arranged in a stacked configuration, each adjacent to at least one second SBO crystal plate. In various embodiments, the SBO crystal plates are configured together to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light so that the light emerging from the nonlinear crystal comprises 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 configured as an inspection system, a metrology system, or a lithography system.

[0011] In accordance with one or more embodiments of the present disclosure, a laser assembly for generating laser output light is disclosed. In some embodiments, the laser assembly includes a first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency. In some embodiments, the laser assembly includes one or more intermediate frequency conversion stages cooperatively configured to use the fundamental laser beam to generate intermediate frequency light, the intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between about 360 nm and about 400 nm. In some embodiments, the laser assembly includes a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal. In some embodiments, the nonlinear crystal includes multiple strontium tetraborate (SBO) crystal plates arranged in a stacked configuration, each 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 so that the light emerging from the nonlinear crystal comprises a laser output light having the output frequency and a corresponding wavelength in the range of about 180 nm to about 200 nm.

[0012] According to one or more embodiments of the present disclosure, a method for generating laser output light is disclosed. In some embodiments, the method generates intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between approximately 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 SBO crystal plates are configured 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 light having an output frequency and a corresponding wavelength within a range of approximately 180 nm to approximately 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 explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic block diagram depicting 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. [Figure 3] 1 is a schematic diagram depicting a final frequency doubling stage utilized in the laser assemblies in accordance with one or more embodiments of the present disclosure. [Figure 4] 1 is a schematic diagram depicting the first and final frequency doubling stages utilized in the laser assemblies according to one or more embodiments of the present disclosure. [Figure 5] 1 is a schematic diagram depicting a frequency summing and final frequency doubling stage utilized in the laser assemblies in accordance with one or more embodiments of the present disclosure. FIG. [Figure 6] FIG. 1 is a schematic diagram depicting an amplification stage utilized within an optical parametric system (OPS) utilized in the laser assemblies in accordance with one or more embodiments of the present disclosure. [Figure 7] 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 INVENTION

[0016] Reference will now be made in detail to the disclosed subject matter, as illustrated in the accompanying drawings. The present disclosure will be particularly shown and described with reference to certain embodiments and their particular features. The embodiments described herein are to 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 therein 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 wavelengths near 193 nm (e.g., wavelengths between about 180 nm and about 200 nm) while avoiding the problems and shortcomings of conventional approaches. Note that, in the following description, when wavelengths are referred to without qualification, they can be considered to be wavelengths in a vacuum.

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

[0019] In some embodiments, the interlocking arrangement involves physically stacking individual SBO crystal plates and alternately flipping the c-axes of successive 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 (e.g., each SBO crystal plate forms a physical pole within the periodic structure), similar to a periodically poled crystalline material. 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 incident light passing through the SBO crystal stack in the optical system, and by forming the SBO crystal plates so that the thickness of at least one SBO crystal plate provides a pole-to-pole spacing (i.e., the distance light travels between the opposing surfaces of each plate along the light propagation direction) 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 coordinating two or more SBO crystal plates in this manner, frequency doubling can be achieved using nonlinear crystals fabricated in accordance with the present invention to generate light having a wavelength near 193 nm while avoiding the problems and drawbacks of conventional approaches.

[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 greater and an emission wavelength in the range of about 180 nm to about 200 nm, e.g., a wavelength near 193 nm. In one practical embodiment, within the final frequency doubling stage of the associated laser assembly, in which a nonlinear crystal is utilized, there is provided at least one fundamental laser and one to three intermediate frequency conversion stages, each of which individually generates a fundamental laser beam having a corresponding fundamental frequency, and the intermediate frequency conversion stages are cooperatively configured to convert the fundamental laser beam(s) into 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. This achieves QPM between 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 related 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 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 intermediate frequency light through 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 do so, 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 beam at approximately 193 nm, the linear crystal includes two or more stacked SBO crystal plates with their c-axes oriented substantially parallel to the polarization direction of the second-harmonic input beam and inverted. The thickness of each plate along the light propagation direction (i.e., the spacing between the poles of the periodic structure) is set to be approximately equal to an odd-integer multiple of the quasi-phase-matching critical length, which is approximately equal to 0.85 μm (e.g., in the range of 0.80 μm to 0.90 μm). This achieves QPM between the second-harmonic frequency and the fourth-harmonic frequency, thereby generating a laser beam with an output wavelength of approximately 193 nm.

[0022] A laser assembly and related methods described herein with reference to a second specific embodiment include 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 passing through 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 do so, the stage is configured to include a nonlinear crystal, and the nonlinear crystal is configured to generate a second harmonic of the intermediate light, the second harmonic having a frequency equal to four times the first fundamental frequency plus two times the third frequency. To generate a laser output beam at approximately 193 nm, the linear crystal comprises two or more stacked SBO crystal plates with their c-axes inverted and oriented substantially parallel to the polarization direction of the incident second-harmonic beam. The thickness of each plate along the light propagation direction (i.e., the spacing between the poles of the periodic structure) is set to approximately equal an odd integer multiple of the 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 between the intermediate frequency and the second-harmonic frequency, thereby generating a laser output beam with an output wavelength of approximately 193 nm.

[0023] In one embodiment, a laser described herein that generates 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 generates emitted light having a wavelength of about 193 nm is incorporated into a lithography system configured to patternwise expose a photoresist coated on a substrate, such as a semiconductor wafer. In yet another embodiment, a laser described herein that generates 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. 6,233,699, issued February 16, 2021, and U.S. Patent No. 6,233,699, issued June 14, 2022, which are incorporated herein by reference in their entireties. This application is also related to the following U.S. patents, all of which are incorporated herein by reference: U.S. Patent No. 6,233,699 to Vaez-Iravani et al., U.S. Patent No. 6,233,699 to Marxer et al., U.S. Patent No. 6,233,699 to Leong et al., U.S. Patent No. 6,233,699 to Chuang et al., U.S. Patent No. 6,233,699 and U.S. Patent No. 6,233,699 to Armstrong, U.S. Patent No. 6,233,699 to Genis, U.S. Patent No. 6,233,699 to Dribinski, U.S. Patent Nos. 6,233,699 and 6,233,709 to Dribinski et al., U.S. Patent Nos. 6,233,699 and 6,233,709 to Chuang et al., and U.S. Patent No. 6,233,699 to Chuang et al.

[0025] 1 illustrates a characterization system 100 according to 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 a 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 a focus on the sample 108.

[0027] The characterization system 100 includes an illumination source 102 incorporating a laser 200-0 emitting light at an emission frequency ω 0 corresponding to a wavelength in the range of about 180 nm to about 200 nm. OUT The output light L OUT 2A-7. The illumination source 102 may include additional light sources, such as lasers operating at longer or shorter wavelengths or broadband light sources. The characterization system 100 may include one or more optical elements. For example, but not limited to, one or more optical elements of the characterization system 100 may include light L 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 may be configured to transmit light L at an oblique angle of incidence. Obl The specularly 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 can be configured to direct the illumination light L onto the top surface of the sample 108. IN Collaboratively organize to direct

[0028] When illuminating the sample 108 in one or more of the above modes, the optical system 103 further comprises a filter 106 for detecting light L reflected, scattered, diffracted, transmitted, and / or emitted by the sample 108. R / S / T Gathering the light L R / S / T The detector assembly 104 is configured to focus the light onto a sensor 106 of the detector assembly 104. Note 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, etc. 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 different apertures into place of 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 at different angular ranges depending on the characterization (e.g., metrology or inspection) being 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, one or more optical 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 its collection pupil aperture 121 and the pupil in the objective lens 150 are conjugate to each other. In some embodiments, the collection pupil aperture 121 can be configured by switching different apertures into place of the collection pupil aperture 121. 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, different angular ranges of illumination 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 allow light of a particular polarization L R / S / T The sensor 106 may include a polarizing element so that the light can be transmitted to the sensor 106.

[0032] According to certain other embodiments, illumination pupil aperture 131 and / or collection pupil aperture 121 can comprise a programmable aperture. Programmable apertures are generally described in U.S. Patent No. 5,622,492 to Brunner, entitled "2D programmable aperture mechanism," issued on February 9, 2016, and U.S. Patent No. 5,622,492 to Brunner, entitled "Flexible optical aperture mechanisms," issued on May 9, 2017, 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. 6,233,624 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. 6,233,624 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 intended merely 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) cooperatively configured to generate laser output light 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 generates a second harmonic light 212A having the first fundamental frequency ω 1A The output frequency ω is equal to four times OUTA The laser beam 239A has a wavelength of 1000 nm.

[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 corresponding to a wavelength of about 720 nm to about 800 nm (for example, a wavelength of about 774 nm). 1AThe first fundamental laser 210A is configured to generate first fundamental light 211A at a wavelength of approximately 193 nm. In one embodiment, the first fundamental laser 210A is implemented using a titanium sapphire (Ti:sapphire) laser emission medium. Suitable fundamental lasers operating at wavelengths around 800 nm are commercially available. To generate sufficient light at a wavelength of approximately 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 require less or more power. Depending on the pulse width and repetition rate requirements imposed on the laser 210A, 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 between the first fundamental frequency and the second harmonic frequency. The first frequency doubling stage 220A may optionally include additional components, such as a prism, to separate the second harmonic light 212A from the unconsumed fundamental light. The first frequency doubling stage 220A may also include a cavity resonating at the first fundamental frequency to increase conversion efficiency.

[0037] The final frequency doubling stage 230A is configured to generate 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. In some embodiments, dozens, hundreds, or even thousands of plates may be stacked. In FIG. 2A, the plates are depicted as abutting each other. The plates may also be in contact with each other (e.g., the plates may be optically in contact with each other), or a small air gap, e.g., an air gap having a width approaching or less than the thickness of a plate, may be provided between the plates. The thickness of each plate is selected to allow quasi-phase matching during frequency doubling of second-harmonic light 212A. The crystal 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 connection with FIG. 3.

[0038] Final frequency doubling stage 230A may optionally include other optical components, such as a prism to separate laser output light 239A from the unconsumed fundamental second harmonic light, and may include a cavity to recycle the second harmonic frequency, thereby increasing 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 connection 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) 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 ... 1B The final (second) frequency doubling stage 230B receives the intermediate frequency light 214B and outputs an output frequency ω ≡ twice the intermediate frequency (ω ≡ ω +ω ≡ ... OUTA =4ω 1B The laser beam 239A is generated having +2ω3=2ω4 and a corresponding wavelength within the range of approximately 180 nm to approximately 200 nm.

[0041] 2B, first fundamental laser 210B-1 is configured using known techniques to generate first fundamental light 211B-1 at a first fundamental frequency ω1. In one embodiment, first fundamental laser 210B-1 is configured to generate 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, first fundamental laser 210B-1 is implemented using one of a Nd-doped yttrium aluminum garnet (YAG) laser emitting medium, a Nd-doped yttrium orthovanadate laser emitting medium, and an 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, first fundamental laser 210B-1 should generate tens or hundreds of watts of fundamental light 211B-1. Other applications may require less or even more power. Depending on the pulse width and repetition rate requirements imposed on laser 210B, 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 optionally include additional components, 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, e.g., about 1.4 μm. The OPS 221B can use any known OPO 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 poled materials. Suitable nonlinear crystals include lithium niobate and stoichiometric strontium tantalate. Three alternative configurations for pumping the OPS 221B are depicted in Figure 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). The first part of the first fundamental light is directed to a first frequency doubling stage 220B-1 to generate second harmonic light 212B-1. The 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 crystal used in an OPS221B configured as an OPO can 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 approximately 1064 nm and the third frequency light has a wavelength of approximately 1400 nm, the idler will have a wavelength of approximately 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. An advantage of this embodiment is that the idler (e.g., 2ω 1B Because the unwanted pump light (having a frequency equal to -ω3) has a relatively high frequency and therefore a relatively short wavelength, materials suitable for the nonlinear crystals used in OPS 221B configured as an OPO may be inexpensive and readily available. For example, if first fundamental light 211B-1 has a wavelength of about 1064 nm (and therefore second-harmonic light 212B-1 has a wavelength of about 532 nm) and the third-frequency light has a wavelength of about 1400 nm, 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 first fundamental light 211B-1) into the third-frequency light; this is because the unwanted pump 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, so more energy will be injected into the idler.

[0046] In a third embodiment, laser 200B includes second fundamental laser 210B-2 configured to generate second fundamental light 211B-2 having a frequency ω2. Second fundamental light 211B-2 is directed to OPS 221B as pump light. Second fundamental laser 210B-2 can use any convenient lasing medium capable of generating wavelengths suitable for pumping OPS 221B, including the exemplary lasing materials listed above in connection with first fundamental laser 210B-1. Second fundamental laser 210B-2 can be configured to generate wavelengths near 1064 nm or near 532 nm, for example, by frequency doubling the output of a laser generating wavelengths near 1064 nm. This third embodiment, although at first glance more complex than either the first or second embodiments, may be preferable 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 operates at a frequency equal to the sum of the frequencies of its inputs (i.e., 2ω 1B The frequency summing stage 222B is configured to generate intermediate frequency light 214B having an intermediate frequency (ω3 = ω4). The intermediate frequency light 214B should have a wavelength between approximately 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 optionally include other optical components, 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 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 containing 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. Note that in an actual embodiment, tens, hundreds, or even thousands of plates may be stacked. In FIG. 2B, the plates are depicted as abutting each other. The plates may be in contact, or there may be a small air gap between them, 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 for frequency doubling of the second harmonic light 214B. The crystal 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 connection with FIG. 3.

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

[0050] The frequency summing stage 222B and the final frequency doubling stage 230B may be combined into a single cavity; the cavity may be configured to receive the light at frequency ω3 from OPS 221B and / or at frequency ω2 from first frequency doubling stage 220B-1. 1B At, and / or 2ω 1B The cavity is configured to increase 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 discussed in more detail below in connection with FIG.

[0051] Figure 3 shows the frequency ω x 3 illustrates details of a nonlinear crystal 300 having four stacked SBO plates 335-1 to 335-4 configured to frequency-double incident light 301 having a frequency of 1 / 2. The incident light 301 may correspond to second-harmonic light 212A in the laser 200A of FIG. 2A or intermediate-frequency light 214B in the laser 200B of FIG. 2B. The nonlinear crystal 300 illustrated in FIG. 3 has four stacked SBO crystal plates 335-1 to 335-4 and exhibits a periodic structure. However, the total number of SBO plates may be two or less, more than ten, or more than one hundred. There may be an odd or even number of plates. The thickness of each SBO plate 335-1 to 335-4 may be from several hundred nanometers to several tens of micrometers. Specifically, the SBO plate thickness Λ along the propagation direction of light 301A within the crystal plate is given by Λ=mL c (Equation 1) where m is an odd number (e.g., 1, 3, 5, 7, etc.) 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) and 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 a vacuum.

[0052] Quasi-phase matching critical length L when doubling the frequency of incident light 301 with a wavelength of 386.8 nm cis approximately 0.85 μm (e.g., a thickness of 0.8 μm to 0.9 μm). A reasonable m in the range of 1 to approximately 999 may 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 refractive index of SBO using the Sellmeier model published by [Reference 1], which is incorporated herein by reference in its entirety. Furthermore, variations in the impurity level in the SBO crystal and the presence of defects in the crystal may slightly alter the refractive index of the crystal. Those skilled in the art will understand 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 above the other, and the incident surface 335-IN and the output surface 335-OUT are arranged to be polarized by a frequency ω x The incident light 301 is oriented at approximately the Brewster angle θ relative to the surface normal N, resulting in minimal reflection losses without the use of anti-reflective coatings. The Brewster angle is approximately equal to 60.3° relative to the surface normal N for wavelengths near 386 nm polarized approximately parallel to the c-axis of the SBO crystal, and approximately equal to 61.9° relative to the surface normal N for wavelengths near 193 nm with the same polarization direction. Because reflection losses are small at 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 incident and output light. Furthermore, because angular precision is not critical to reflection losses, small adjustments to the nonlinear crystal 300 (e.g., small adjustments to the angle of incidence θ) can be made 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 incident light second harmonic and frequency ω x This includes unconsumed incident light. In some embodiments, the SBO plates are in optical contact with each other to minimize reflection losses at each interface. In other embodiments, small air gaps may be provided between the plates. By orienting the input and output surfaces near the Brewster angle with respect to the incident light 301, reflection losses at each air-SBO plate interface can be minimized. 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 this difference accumulates across multiple plates and becomes large enough, it can degrade the conversion efficiency. 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., approximately 100 μm or a few hundred micrometers) in line with the requirement for high conversion efficiency at high power density.

[0054] To generate a periodic structure for QPM, the SBO plates 335-1 through 335-4 are rotated relative to one another so that their corresponding c-axes are inverted relative to one another, as shown in the two insets in Figure 3. The two insets show the surface normal N of the SBO plates of thickness Λ (where Λ is the spacing between poles within the crystal) and the propagation direction of light 301A within the SBO plates. This physical arrangement of the 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 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 crystallographic axes of the ferroelectric crystal, which ensures that the polarization direction is aligned with the crystallographic axes. In contrast, the SBO crystal plate disclosed herein can be cut and polished in any orientation relative to the crystal axis, allowing the plate to be cut and oriented at the Brewster 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 This can take advantage of the fact that SBO has the largest nonlinear optical coefficient, i.e., π / 2, which is the maximum nonlinear optical coefficient of SBO, thereby maximizing the conversion efficiency. For example, as depicted in FIG. 3, the crystal axes of SBO plate 335-2 can be oriented so that light 301A propagates approximately parallel to the a-axis of the SBO crystal. Alternatively, the crystal axes can be oriented so that light 301A propagates parallel to the b-axis or at an angle within the a-b 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 SBO plate 335-4 is oriented at the Brewster angle with respect to the incident light 301, the propagation direction of light 301A within plate 335-4 is approximately 29.7° with respect to the surface normal N.

[0056] There are many ways to fabricate and assemble nonlinear crystal 300. If only a few plates are needed for a laser (e.g., if high conversion efficiency is not required), it may be convenient to polish the plates to the desired thickness and then 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, U.S. Patent Application No. 17 / 555,404, filed December 18, 2021, entitled "Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings," discloses interdigitated nonlinear crystals and methods of fabrication, and 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 a bow-tie 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 ) enters the bowtie ring cavity, passes through or near mirror 432C-1, and passes through first frequency doubling crystal 441. 1A 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 surface 444 of the SBO nonlinear crystal 300 is 436C (having a frequency of 2ω 1A and 4ω 1A (It has a frequency of 2ω) 1A The portion having the unconsumed light 442C (frequency 2ω 1A The beam splitter (BS) 437C passes through the beam splitter (BS) 437C, is reflected by the mirror 432C-4 and the input coupler 432C-1, and recirculates within the cavity. 1A The part having the value is reflected by the surface of the beam splitter (BS) 437C and goes out of the cavity as the 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 unconsumed incident light 438C at 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 incident surface 443 and the exit surface 444 are reflective to the frequency 2ω 1AThe BS 437C is configured to be oriented at approximately the Brewster angle with respect to the circulating lights 433C and 442C. The polarization directions of lights 433C and 442C are depicted by arrows 402. Furthermore, 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 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 surface 443 is provided with an anti-reflective coating configured to transmit circulating light 443C and 442C at a frequency 4ω 1A Through the emitted light 439C and both of the frequency 2ω 1A The entrance surface 443 and the exit surface 444 are configured to have an anti-reflective coating configured to transmit circulated light 443C and 442C at a frequency 2ω. 1A It is not necessary to orient the circular light 433C and 442C at Brewster's angle.

[0061] In some embodiments, BS437C can comprise SBO crystal, SBO glass, or CaF crystal. Because SBO has good deep-UV transparency and a high damage threshold, it can be successfully used as a substrate material for BS437C, ensuring long lifetimes despite high power levels of unconsumed incident light 442C circulating within the cavity. When 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 can also comprise a dichroic beam splitter, prism, or other wavelength separation element.

[0062] According to an alternative embodiment, instead of orienting the entrance faces 445, 443 and 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-reflection coating.

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

[0064] While 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 within the cavities. In alternative embodiments, 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 alternative embodiments, any combination of frequencies involved can be cycled. Any of these cavities can be stabilized using standard PDH (Pond-Drever-Hall) or HC (Hensch-Kuyaw) locking techniques. By adjusting the position of one of the mirrors (e.g., mirror 432C-4 in FIG. 4) or the position of the prism via a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator, the length of the resonant cavity can be adjusted to maintain resonance.

[0065] 5 is a schematic diagram illustrating 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 cavity comprising an input coupler 532C-1, curved mirrors 532C-2, 532C-3, and 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 ) enters the bowtie cavity, passes through or near mirror 532C-1, and passes through frequency summing crystal 541. 1B The unconsumed incident 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 ω = 2ω + 4ω 1B The output light 533C includes a portion of the beam splitter (BS) 537C having frequency ω4 (unconsumed light 542C). The output light 533C is reflected by mirrors 532C-2 and 532C-3 and passes through the SBO nonlinear crystal 300 (shown in FIG. 3). The light appearing at the output face 544 of the SBO nonlinear crystal 300 is 536C (with frequencies ω4 and ω4). The portion of 536C having frequency ω4, i.e., the unconsumed light 542C (with 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 exits the cavity as output light 539C.

[0066] In some embodiments, the first frequency summing crystal 541 is configured to operate at frequencies ω and 2ω 1B 5B, the incident surface 545 and the exit surface 546 are configured to have anti-reflective coatings configured to transmit incident light 511A and unconsumed incident light 538C of frequency ω4, and / or circulated light 533C and 542C of frequency ω4.

[0067] In some embodiments, SBO plate 300 is configured so that input face 543 and output face 544 are oriented approximately at the Brewster angle with respect to circulating light 533C and 542C at frequency ω. The polarization directions of light 533C and 542C are depicted by arrows 502. Furthermore, BS 537C can be configured to laterally displace light 542C circulating within its cavity by an amount that substantially offsets the lateral displacement of light 533C induced by two or more SBO plates 300, thus maintaining a substantially symmetric bowtie cavity and simplifying 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 transmit circulating light 533C and 542C, both of which have frequency ω4; the transmit 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 can comprise SBO crystal, SBO glass, or CaF2 crystal. Because SBO has good deep-UV transparency and a high damage threshold, it can be successfully used as a substrate material for the BS537C, ensuring long lifetimes despite the high power levels of the unconsumed incident light 542C circulating within the cavity. When 537C 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 incident light 542C passing therethrough. The BS537C can also comprise a dichroic beam splitter, prism, or other wavelength separation element.

[0070] According to an alternative embodiment, instead of orienting the entrance faces 545, 543 and 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-reflection 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 would originate outside the cavity and be optically coupled into the cavity via input coupler 532C-1.

[0072] Although FIG. 5 depicts the frequency summing and final frequency doubling stage cavity 500 as having four curved mirrors, other combinations of mirrors and / or lenses can be used to refocus the light circulating within the cavity. In alternative embodiments, the frequency summing and final frequency doubling stage cavity 500 can comprise a delta cavity, a standing wave cavity, or other cavity shape instead of a bowtie cavity. In alternative embodiments, any combination of frequencies involved can be cycled. Any of these cavities can be stabilized using 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 via a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator.

[0073] FIG. 6 is a schematic diagram illustrating an amplifier stage 600 used in the OPS 221B of FIG. 2B according to an embodiment of the present disclosure. Incident light 213B (ω) is generated by an OPO or OPG. Incident light 213B (ω) and CW pump light 611 enter a bow-tie cavity, which includes input coupler 632C-1, curved mirror 632C-2, flat mirrors 632C-3 and 632C-4, and gain medium 643, via input coupler 632C-1. Incident light 213B (having frequency ω) enters the bow-tie cavity, passes through or near mirror 632C-1, and then passes through gain medium 643 (e.g., an Nd-doped YAG rod). The CW pump light 611 has a frequency and power suitable for pumping gain medium 643, which can induce stimulated emission of light at frequency ω, constituting incident light 213B. The CW pump light 611 is 211B-1(ω 1B ), 211B-2(ω2) or 212B-1(2ω 1B ) can be associated with one of the frequency components ω3 and ω4. 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, thereby recirculating 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 angle with respect to the circulating light 633C (ω3).

[0075] In some embodiments, diodes emitting pump light 611 are configured to side-pump the gain medium 643. Such diodes 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 including two curved and two flat mirrors, other combinations of mirrors and / or lenses can be used to refocus the light circulating within the cavity. According to an alternative embodiment, amplifier stage 600 can include a delta cavity, a standing wave cavity, or other cavity shapes instead of a bowtie cavity. Any of these cavities can be stabilized using standard PDH or HC locking techniques. By adjusting the position of one of the mirrors (e.g., mirror 632C-4 in FIG. 6) or the position of the prism via a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator, the length of the resonant cavity can be adjusted to maintain resonance.

[0077] FIG. 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 (which includes one of ω2) enters a bow-tie cavity via input coupler 732C-1, which includes input coupler 732C-1, curved mirrors 732C-2 and 732C-3, output coupler 732C-4, OPO crystal 744 (e.g., PPLN), gain medium 743, and frequency selective device 747. y ) enters the bowtie ring cavity, passes through or near mirror 732C-1, and passes through OPO crystal 744. yUnconsumed incident 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, thereby recirculating within 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 are positioned parallel to the incident light 713 on one or more sides of the gain medium 743, causing the pump light 711 to be emitted perpendicular to the incident light 713 into the gain medium 743. The pump light 711 in this embodiment can be coherent, non-coherent, quasi-CW, CW, or pulsed.

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

[0081] While 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 cavity shapes instead of a bowtie cavity. Any of these cavities can be stabilized using standard PDH or HC locking techniques. 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, the cavity length can be adjusted to maintain resonance.

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

[0083] While the present invention, as described herein, readily achieves the generation of laser output light at a desired wavelength of approximately 193 nm using various fundamental wavelengths, other wavelengths within a few or tens of nanometers of this desired wavelength can also 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 considered 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, no prior art exists other than excimer lasers that generate optical powers of 1 W or more in the wavelength range from approximately 180 nm to 200 nm. Embodiments of the present invention generate wavelengths near 193 nm, providing better sensitivity for detecting small particles and defects than longer wavelengths. Because the lasers of the present invention do not use toxic or corrosive gases, they are easier and cheaper to operate and maintain.

[0085] Those skilled in the art will readily appreciate that 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 substrates, such as semiconductor wafers. For example, lasers operating at wavelengths between approximately 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 generate very short pulses at the emitted wavelength, allowing for preferential removal of material by ablation rather than heating, thereby causing less damage to surrounding materials. For example, such lasers can be used in laser eye surgery or laser vision correction. While the present invention has been described in connection with certain specific embodiments, those skilled in the art will recognize 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, those skilled in the art will be able to translate from the plural to the singular and / or from the singular to the plural as appropriate to the context and / or use. In the interest of clarity, the various singular / plural permutations have not been explicitly set forth herein.

[0087] Given the foregoing description, one skilled in the art will be able to make and use the present disclosure as presented in the context of a particular application and its requirements. Directional terms used herein, such as "top," "bottom," "up," "down," "upward," "upward," "downward," "downward," and "downward," are intended to indicate relative positions for descriptive purposes and are not intended to specify an absolute reference frame. Various modifications to the preferred embodiment will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments. Therefore, 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] Furthermore, as will be understood, the present invention is defined by the appended claims. As will be understood by those skilled in the art, the terms used in this application, particularly in the appended claims (e.g., the body of the appended claims), are generally intended to be "open" (e.g., the term "comprising" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "including" should be interpreted as "including but not limited to," etc.). As will also be understood by those skilled in the art, if a specific number of claim-introducing features is intended, that intention will be clearly stated in the claim, and the absence of such features indicates no intention. For example, as an aid to understanding, the appended claims below may incorporate the introductory phrases "at least one" and "one or more" to introduce claim features. However, the use of the indefinite article "a" or "an" should not be construed as implying that the introduction of a claim feature with the indefinite article "a" or "an" implies that all individual claims containing that claim feature are limited to inventions containing only one of that feature, nor should such construing be done when the indefinite article "a" or "an" coexists with the indefinite article in the very same claim (e.g., "a" and / or "an" should generally be interpreted as meaning "at least one" or "one or more"). The same is true for the introduction of claim features with the definite article. In addition, even when a specific number of claim features is specified, that number should generally be interpreted to mean at least that specified number, as would be recognized by a person skilled in the art (e.g., the bare phrase "two features" without any other modifier generally means at least two features or more than two features).Furthermore, 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 way a person skilled in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include, but is not limited to, 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.). 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 way a person skilled 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 will also be understood by those skilled in the art, 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 can be made in the form, construction and arrangement of the parts without departing from the disclosed subject matter or diminishing all of its essential advantages. The described form is merely illustrative, and it is the intent of the following claims to encompass and embrace all such modifications. It is the appended claims which further 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; Equipped with The light source is a first fundamental laser configured to generate a fundamental laser beam having a fundamental frequency; one or more intermediate frequency conversion stages configured to generate intermediate frequency light having a wavelength between 360 nm and 400 nm from the fundamental laser beam; a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal; Equipped with the nonlinear crystal has a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each of at least one first SBO crystal plate is adjacent to at least one second SBO crystal plate, and the crystal axis of the first SBO crystal plate is inverted relative to the crystal axis of the second SBO crystal plate; the plurality of SBO crystal plates are configured 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 light having an output frequency corresponding to a wavelength within a range of about 180 nm to about 200 nm; Inspection system.

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

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

4. 4. The inspection system of claim 3, wherein the first fundamental laser comprises a Ti:sapphire laser.

5. 10. The inspection system of claim 1, 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; the one or more intermediate frequency conversion stages: 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 the OPS, the frequency summing stage 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:

6. 6. The inspection system of claim 5, 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.

7. 6. The inspection system of claim 5, 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.

8. 6. The inspection system of claim 5, 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.

9. 10. 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.

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

11. 11. The inspection system of claim 10, further comprising a computer coupled to the sensor, the computer configured to determine the presence or absence of defects on the sample by analyzing signals from the sensor.

12. 6. The inspection system of claim 5, wherein the OPS comprises at least one of an optical parametric oscillator (OPO), an optical parametric generator (OPG), and an amplifier stage.

13. 13. The test system of claim 12, 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.

14. 6. The inspection system of claim 5, wherein the OPS comprises an optical parametric generator (OPG) configured to generate the third frequency light, the OPG optically coupled to a cavity, and the cavity coupled to an amplifier configured to amplify the third frequency light.

15. 6. The inspection system of claim 5, wherein the OPS comprises a cavity configured to recycle the third frequency light, the cavity comprising an amplifier configured to amplify the third frequency light.

16. 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 the intermediate frequency light.

17. 6. The inspection system of claim 5, wherein the frequency summation stage further comprises a cavity configured to recirculate the intermediate frequency light, and wherein the plurality of SBO plates reside within the cavity.

18. 1. A laser assembly configured to generate a laser output 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 configured to generate intermediate frequency light having a wavelength between 360 nm and 400 nm from the fundamental laser beam; a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal; Equipped with the nonlinear crystal has a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each of at least one first SBO crystal plate is adjacent to at least one second SBO crystal plate, and the crystal axis of the first SBO crystal plate is inverted relative to the crystal axis of the second SBO crystal plate; the plurality of SBO crystal plates are configured 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 light having an output frequency corresponding to a wavelength within a range of about 180 nm to about 200 nm; Laser assembly.

19. 20. The laser assembly of claim 18, wherein the emission frequency corresponds to a wavelength of about 193 nm.

20. 20. The laser assembly of claim 18, wherein the first fundamental laser is configured to produce a wavelength between about 760 nm and about 800 nm, and the one or more intermediate frequency conversion stages are configured to produce the intermediate frequency light by doubling the frequency of the first fundamental laser beam.

21. 21. The laser assembly of claim 20, wherein the first fundamental laser comprises a Ti:sapphire laser.

22. 19. The laser assembly of claim 18, 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: 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 the OPS, the frequency summing stage 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:

23. 23. The laser assembly of claim 22, 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.

24. 23. The laser assembly of claim 22, 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.

25. 23. The laser assembly of claim 22, 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.

26. 23. The laser assembly of claim 22, 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.

27. 23. The laser assembly of claim 22, wherein the OPS comprises at least one of an optical parametric oscillator (OPO), an optical parametric generator (OPG), and an amplifier stage.

28. 28. The laser assembly of claim 27, 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.

29. 23. The laser assembly of claim 22, wherein the OPS comprises an optical parametric generator configured to generate the third frequency light, the OPG optically coupled to a cavity, the cavity coupled to an amplifier configured to amplify the third frequency light.

30. 23. The laser assembly of claim 22, wherein the OPS comprises a cavity configured to recycle the third frequency light, the cavity comprising an amplifier configured to amplify the third frequency light.

31. 20. The laser assembly of claim 18, wherein the final frequency doubling stage further comprises a cavity configured to recycle the intermediate frequency light, the cavity containing the plurality of SBO crystal plates.

32. 23. The laser assembly of claim 22, wherein the frequency summing stage further comprises a cavity configured to recycle the intermediate frequency light, the cavity containing the plurality of SBO crystal plates.

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