Deep UV laser for frequency conversion using strontium tetraborate

Abstract

A nonlinear crystal 300 comprising stacked strontium tetraborate SrB4O7 (SBO) crystal slabs 335-1, 335-2, 335-3, 335-4 cooperatively configured to produce a periodic structure for quasi-phase matching (QPM) is used in the final frequency doubling stage 230A, 230B of the laser assemblies 200A, 200B to produce laser output light 239A, 239B having a wavelength in the range of approximately 180 nm to 200 nm. One or more elementary laser beams 211A, 211B are frequency doubled, down-converted, and / or summed using one or more frequency conversion stages 220A, 220B-1, 221B, 222B 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 the nonlinear crystal 300 to double the frequency of the intermediate frequency light 212A, 214B to produce the desired laser output light 239A, 239B at high power. Methods, inspection systems, lithography systems, and cutting systems incorporating the laser assembly are also described.

Description

[0001] CROSS REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 288,560, filed on December 11, 2021, the entirety of which is incorporated herein by reference. Technical Field

[0003] The present application relates to lasers capable of producing light having deep ultraviolet (DUV) and vacuum ultraviolet (VUV) wavelengths, and in particular to lasers capable of producing light in the range of approximately 180 nm to 200 nm and systems using such lasers. Systems incorporating the lasers disclosed herein can be configured to inspect samples such as photomasks, reticles, and semiconductor wafers. Systems incorporating the lasers disclosed herein can be configured as lithography systems for exposing patterns on substrates such as semiconductor wafers, can be configured for cutting or drilling substrates, or can be configured for ablating or cutting biological tissue, such as in corrective eye surgery. Background Art

[0004] As the size of semiconductor devices decreases, the size of the smallest particle or pattern defect that can cause the device to malfunction also decreases. Therefore, there is a need to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and masks. The intensity of light scattered by particles smaller than the wavelength of the light is generally proportional to a high power of the size of the particle. For example, the total scattered intensity of light from an isolated small spherical particle is proportional to the sixth power of the sphere diameter and inversely proportional to the fourth power of the wavelength. Since the intensity of the scattered light increases, shorter wavelengths will generally provide better sensitivity for detecting small particles and defects than longer wavelengths.

[0005] Since the intensity of light scattered from small particles and defects is usually very low, high illumination intensity is required to generate a signal that can be detected in a very short time. An average light source power level of 1W or more may be required to generate this signal. At these high average power levels, a high pulse repetition rate can be expected because the higher the repetition rate, the lower the energy per pulse and the lower the risk of damaging the system optics or the inspected item. Continuous wave (CW) light sources are usually the best for the illumination needs of inspection and metrology. CW light sources have a constant power level, which avoids the problem of peak power damage and allows images or data to be acquired continuously. However, in many cases, a mode-locked laser (also known as a quasi-CW laser) with a repetition rate of about 50MHz or more may be useful because a high repetition rate means that the energy per pulse may be low enough to avoid damaging many metrology and inspection applications. Compared with a CW laser of the same average power level, the higher peak power of a mode-locked laser allows more efficient and simpler frequency conversion.

[0006] In particular, wavelengths close to 193 nm are useful because such wavelengths are close to the shortest wavelengths that can propagate a reasonable distance (e.g., about 1 m) in dry air. Wavelengths shorter than about 190 nm are strongly absorbed by oxygen and are generally referred to as VUV wavelengths. ArF excimer lasers (also known as exciplex lasers) produce wavelengths close to 193 nm and have been used in the semiconductor and medical industries for more than 20 years. However, ArF excimer lasers have several disadvantages. The maximum pulse repetition rate is about 100 kHz. Fluorine is corrosive and causes the laser to require frequent maintenance. Depending on the details of the laser cavity design, the pulse length can range from a few ns to about 100 ns. Some applications such as cutting and ablating materials require short pulses (e.g., about 10 ps or shorter) to minimize thermal damage to materials adjacent to the material being removed.

[0007] U.S. Pat. No. 5,742,626 to Mead et al. describes a solid-state laser configured to produce a wavelength close to 193 nm. This laser is complex and therefore has low efficiency in converting the energy of the basic laser into light at the output wavelength. It includes five frequency conversion stages, such as an OPO (optical parametric oscillator), two frequency doubling stages, and two frequency summing stages. The DUV frequency conversion stage may use BBO (beta barium borate) or CLBO (cesium lithium borate) crystals. Both materials have disadvantages when used in this application. BBO has a relatively low damage threshold when exposed to high intensity DUV radiation. CLBO may have a higher damage threshold than BBO, but is hygroscopic, so extreme care is required during handling, processing, and operation.

[0008] Therefore, there is a need for a laser that overcomes the limitations of previous approaches as described above. Summary of the invention

[0009] According to one or more embodiments of the present disclosure, a characterization system is disclosed. In an embodiment, the characterization system includes a light source configured to generate light having a wavelength in a range between 180 nm and 200 nm. In an embodiment, the characterization system includes an optical system configured to direct the light onto a sample. In an embodiment, 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, which are collectively 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 360 nm and 400 nm; and a final frequency doubling stage, which is configured to pass the intermediate frequency light through a nonlinear crystal. In an embodiment, the nonlinear crystal includes a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second SBO crystal plate. In an embodiment, the plurality of SBO crystal slabs are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light, such that light exiting the nonlinear crystal comprises laser output light having an output frequency and a corresponding wavelength in the range of approximately 180 nm to approximately 200 nm. The characterization system may be configured as an inspection system, a metrology system, or a lithography system.

[0010] According to one or more embodiments of the present disclosure, a laser assembly for generating laser output light is disclosed. In an embodiment, the laser assembly includes a first fundamental frequency laser configured to generate a fundamental laser beam having a corresponding fundamental frequency. In an embodiment, the laser assembly includes one or more intermediate frequency conversion stages collectively 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 approximately 360 nm and approximately 400 nm. In an embodiment, the laser assembly includes a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal. In an embodiment, the nonlinear crystal includes a plurality of strontium tetraborate (SBO) crystal slabs disposed in a stacked configuration such that each first SBO crystal slab is adjacent to at least one second SBO crystal slab. In an embodiment, the plurality of SBO crystal slabs are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light, such that light exiting the nonlinear crystal includes laser output light having the output frequency and a corresponding wavelength in the range of approximately 180 nm to approximately 200 nm.

[0011] According to one or more embodiments of the present disclosure, a method for generating laser output light is disclosed. In an embodiment, the method includes generating intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between about 360 nm and 400 nm. In an embodiment, the method includes passing the intermediate frequency light through a nonlinear crystal using a final frequency doubling stage. In an embodiment, the nonlinear crystal includes a plurality of strontium tetraborate (SBO) crystal slabs disposed in a stacked configuration such that each first SBO crystal slab is adjacent to at least one second SBO crystal slab. In an embodiment, the plurality of SBO crystal slabs are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate frequency light, such that light exiting the nonlinear crystal includes laser output light having an output frequency and a corresponding wavelength in a range of approximately 180 nm to approximately 200 nm.

[0012] It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and do not necessarily limit the present disclosure. The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate the subject matter of the present disclosure. The description and drawings together serve to explain the principles of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Numerous advantages of the present disclosure may be better appreciated by those skilled in the art by referring to the accompanying drawings.

[0014] Figure 1 The Description depicts a simplified block diagram of a characterization system configured to inspect or measure a sample in accordance with one or more embodiments of the present disclosure.

[0015] Figure 2A and 2B The Description depicts a simplified block diagram of a simplified laser assembly in accordance with one or more embodiments of the present disclosure.

[0016] Figure 3 The Description depicts a simplified diagram of a final frequency doubling stage utilized in a laser assembly according to one or more embodiments of the present disclosure.

[0017] Figure 4 The description depicts a simplified diagram of the first and final frequency doubling stages utilized in a laser assembly according to one or more embodiments of the present disclosure.

[0018] Figure 5 The description depicts a simplified diagram of the frequency summing and final frequency doubling stages utilized in a laser assembly in accordance with one or more embodiments of the present disclosure.

[0019] Figure 6 The description depicts a simplified diagram of amplification stages utilized in an optical parametric system (OPS) for use in a laser assembly, according to one or more embodiments of the present disclosure.

[0020] Figure 7 The description depicts a simplified diagram of an OPS utilized in a laser assembly according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0021] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to certain embodiments and specific features thereof. The embodiments set forth herein are to be understood to be illustrative and not restrictive. It should be readily apparent to one of ordinary skill in the art that various changes and modifications may be made in form and detail without departing from the spirit and scope of the present disclosure.

[0022] Embodiments of the present disclosure relate to a CW, mode-locked or short-pulse laser that produces radiation at a wavelength close to 193 nm and avoids many or all of the disadvantages of prior art 193 nm lasers and is suitable for use in systems configured for inspecting substrates, exposing patterns into photoresists on substrates, or drilling, cutting or ablating materials containing biological tissue. Embodiments of the present disclosure relate to lasers that utilize two to four frequency conversion stages and are configured to produce laser output light having a wavelength close to 193 nm (e.g., a wavelength between about 180 nm and about 200 nm) at high power while avoiding the above-mentioned problems and disadvantages associated with prior art methods. It should be noted that in the following description, where wavelength is mentioned unconditionally, the wavelength may be assumed to be the wavelength in a vacuum.

[0023] The frequency conversion stage generates intermediate frequency light having a wavelength between approximately 360 nm and 400 nm (e.g., a wavelength close to 386 nm), and includes a final frequency doubling stage incorporating a nonlinear crystal, the nonlinear crystal comprising stacked strontium tetraborate (SrB4O7) crystal slabs forming a periodic structure, the crystal slabs cooperatively configured to achieve quasi-phase matching (QPM) suitable for frequency doubling of the intermediate frequency light to generate laser output light having a wavelength between approximately 180 nm and 200 nm (e.g., a wavelength close to 193 nm).

[0024] In one embodiment, the cooperative configuration includes physically stacking individual SBO crystal slabs such that the c-axes of sequentially arranged crystal slabs are alternately inverted (e.g., the crystal axis of a given SBO crystal slab is rotated 180° relative to the crystal axis of (several) adjacent SBO crystal slabs), thereby forming a periodic structure similar to a periodically poled crystalline material (e.g., wherein each SBO crystal slab forms a physical pole in the periodic structure). Each nonlinear crystal is further configured for use in a given optical system by the following steps: orienting the SBO crystal slabs such that the inverted crystal axis is aligned substantially parallel to the polarization direction of the input light as the input light passes through the SBO crystal stack within the optical system; and forming the SBO crystal slabs such that the thickness of at least one SBO crystal slab produces a spacing between the poles substantially equal to an odd-integer multiple of the critical length (i.e., the distance that light travels between the opposing surfaces of each slab along the light propagation direction) to achieve quasi-phase matching of the input light frequency and the output frequency (the second harmonic of the input frequency). By cooperatively configuring two or more SBO crystal slabs in this manner, the nonlinear crystal produced in accordance with the present invention facilitates frequency doubling to produce light having a wavelength approaching 193 nm while avoiding the above-mentioned problems and disadvantages associated with prior art approaches.

[0025] In the specific disclosed embodiments described below, the present invention is directed to improvements in inspection and metrology systems utilized in the semiconductor manufacturing industry, and in particular to laser assemblies for such inspection and metrology systems, the laser assemblies being capable of producing laser light having a source power level of about 1 W or more and having an output wavelength in the range of approximately 180 nm to approximately 200 nm, such as a wavelength close to 193 nm. In a practical embodiment, a nonlinear crystal is utilized in a final frequency doubling stage of an associated laser assembly that also includes at least one fundamental laser and one to three intermediate frequency conversion stages, wherein each fundamental laser respectively produces a fundamental laser beam having a corresponding fundamental frequency, and the intermediate frequency conversion stages are collectively configured to convert the fundamental laser beam(s) into intermediate frequency light having an associated intermediate frequency corresponding to a wavelength between about 360 nm and 400 nm. The final frequency doubling stage is configured to guide the intermediate frequency light through the inverted SBO crystal slabs forming a nonlinear crystal, so that the polarization direction (electric field direction) of the light is substantially parallel to the c-axis of the crystal axis of each slab, whereby the periodic structure of the stacked SBO crystal slabs achieves QPM of the intermediate frequency light and the laser output light.

[0026] According to the laser assembly and associated method described herein with reference to a first specific embodiment, a laser output light with an output frequency having a wavelength of approximately 193 nm is produced by the following steps: generating a fundamental light having a fundamental frequency and a corresponding fundamental wavelength in the range of approximately 720 nm to approximately 800 nm; utilizing the fundamental light to generate a second harmonic of the fundamental light; utilizing the second harmonic as intermediate frequency light, which is then passed to a final frequency doubling stage. According to aspects of the first embodiment, the final frequency doubling stage is configured to frequency double the second harmonic light by configuring the stage to include a nonlinear crystal, the nonlinear crystal being configured to generate fourth harmonic light having a frequency equal to four times the fundamental frequency. In order to generate fourth harmonic output light at approximately 193 nm, the linear crystal includes two or more stacked SBO crystal slabs having inverted c-crystal axes oriented substantially parallel to the polarization direction of the second harmonic input light, wherein the thickness of each slab along the light propagation direction (i.e., the spacing between the poles of the periodic structure) is substantially equal to an odd multiple of the quasi-phase matching critical length, which is approximately equal to 0.85 μm (e.g., in a range between 0.80 μm and 0.90 μm), to achieve QPM of the second harmonic frequency and the fourth harmonic frequency, thereby generating laser output light with an output wavelength of approximately 193 nm.

[0027] According to the laser assembly and associated methods described herein with reference to a second specific embodiment, a laser output light with an output frequency having a corresponding wavelength of approximately 193 nm is produced by the following steps: generating a fundamental light having a first fundamental frequency and a corresponding first fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm; utilizing the first fundamental light to generate a second harmonic of the first fundamental light; using an optical parametric oscillator (OPO) to generate light having a third frequency; summing the second harmonic light together with the light having the third frequency; and utilizing the sum of the second harmonic light and the third frequency light as intermediate frequency light, which is then passed to a final frequency doubling stage. According to aspects of the second embodiment, the final frequency doubling stage is configured to frequency double the intermediate light by configuring the stage to include a nonlinear crystal, the nonlinear crystal being configured to generate a second harmonic of the intermediate light having a frequency equal to four times the first fundamental frequency plus twice the third frequency. In order to generate laser output light at approximately 193 nm, the linear crystal includes two or more stacked SBO crystal slabs having inverted c-crystal axes oriented substantially parallel to the polarization direction of second harmonic input light, wherein the thickness of each slab along the light propagation direction (i.e., the spacing between the poles of the periodic structure) is substantially equal to an odd multiple of the quasi-phase matching critical length, which is approximately equal to 0.85 μm (e.g., in the range between 0.80 μm and 0.90 μm), to achieve QPM of an intermediate frequency and its second harmonic frequency, thereby generating laser output light with an output wavelength of approximately 193 nm.

[0028] In one embodiment, an inspection system configured to inspect a sample, such as a wafer, reticle, or photomask, includes one of the lasers described herein that produces output light having a wavelength between approximately 180 nm and 200 nm. In an alternative embodiment, a photolithography system configured to expose a pattern onto a photoresist coated on a substrate, such as a semiconductor wafer, includes one of the lasers described herein that produces output light having a wavelength of approximately 193 nm. In yet another embodiment, a system configured to cut, drill, or ablate a material, such as biological tissue, includes one of the lasers described herein that produces output light having a wavelength between approximately 180 nm and 200 nm.

[0029] The use of strontium tetraborate as an optical coating is discussed in U.S. Pat. No. 10,921,261, issued on February 16, 2021, and U.S. Pat. No. 11,360,032, issued on June 14, 2022, the entireties of which are incorporated herein by reference. This application also relates to the following U.S. patents, all of which are incorporated herein by reference: U.S. Pat. No. 6,201,601 to Vaez-Iravani et al.; U.S. Pat. No. 6,271,916 to Marxer et al.; U.S. Pat. No. 7,525,649 to Leong et al.; U.S. Pat. No. 7,817,260 to Chuang et al.; U.S. Pat. Nos. 8,298,335 and 8,824,514 to Armstrong; U.S. Pat. No. 8,094,667 to Genis et al. ,976,343; U.S. Patent 9,023,152 to Dribinski; U.S. Patents 9,461,435 and 9,059,560 to Dribinski et al.; U.S. Patents 9,293,882 and 9,660,409 to Chuang; U.S. Patents 9,250,178, 9,459,215, 9,509,112, 10,044,166 and 10,283,366 to Chuang et al.; and U.S. Patent 11,180,866 to Chuang et al.

[0030] Figure 1A characterization system 100 is described in accordance with one or more embodiments of the present disclosure. The characterization system 100 can be configured to inspect or measure a sample 108. The characterization system 100 can include an inspection system or a metrology system. The characterization system 100 can also be configured to cut, drill, or ablate material from the sample 108, or expose a pattern to a photoresist on the sample 108.

[0031] The sample 108 may include any sample known in the art, such as, but not limited to, a wafer, a reticle, a photomask, or the like. In one embodiment, the sample 108 is disposed 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, including, but not limited to, an XY stage, an R-θ stage, and the like. In another embodiment, the stage assembly 112 is capable of adjusting the height of the sample 108 during inspection to maintain focus on the sample 108. In yet another embodiment, a lens such as an objective lens 150 may be moved up and down during inspection to maintain focus on the sample 108.

[0032] Characterization system 100 includes an illumination source 102 incorporating a laser 200-0 that generates an optical signal having an output frequency ω OUT and the output light L corresponding to the wavelength in the range between approximately 180 nm and approximately 200 nm OUT Details of the laser 200-0 can be found at Figures 2A to 7 The illumination source 102 may include additional light sources, such as lasers or broadband light sources operating at longer or shorter wavelengths. The characterization system 100 includes one or more optical components. For example, the one or more optical elements of the characterization system 100 may include, but are not limited to, a light source configured to transmit light L OUT The optical components may be configured to illuminate an area, line, or point on the sample 108. In one embodiment, the beam splitter or mirror 134, mirrors 137 and 138, and lens 152 are configured to illuminate the sample 108 from below to achieve a desired illumination effect by directing light L INT The sample 108 is transmitted through to inspect or measure the sample. In another embodiment, beam splitters or mirrors 134 and 135, mirror 136, and lens 151 are configured to use light L at an oblique incident angle (e.g., an incident angle greater than 60° relative to the normal of the sample surface) to detect or measure the sample. Obl The sample 108 is illuminated. In this embodiment, the specularly reflected light L Spec In another embodiment, the optical device 103 is configured to block or discard the illumination light L IN Guided to the top surface of the sample 108 .

[0033] When the sample 108 is illuminated in one or more of the above-described modes, the optical device 103 is also configured to collect light L reflected, scattered, diffracted, transmitted, and / or emitted from the sample 108. R / S / T And the light L R / S / T The detector assembly 104 is directed and focused to the sensor 106 of the detector assembly 104. It should be noted herein that the sensor 106 and the detector assembly 104 may include any sensor 106 known in the art. The sensor may include, but is 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 bombardment line sensor, or the like. The detector assembly 104 is communicatively coupled to the controller 114.

[0034] Controller 114 is configured to store and / or analyze data from detector assembly 104 under the control of program instructions 118 stored on carrier medium 116. Controller 114 may further be configured to control other elements of characterization system 100, such as stage 112, illumination source 102, and optics 103.

[0035] In one embodiment, the optics 103 includes an illumination tube lens 132. The illumination tube lens 132 may be configured to image the illumination pupil aperture 131 onto a pupil within the objective 150. For example, the illumination tube lens 132 may be configured such that the illumination pupil aperture 131 and the pupil within the objective 150 are conjugate to each other. In one embodiment, the illumination pupil aperture 131 may be configured by switching different apertures into the position of the illumination pupil aperture 131. In another embodiment, the illumination pupil aperture 131 may be configured by adjusting the diameter or shape of the opening of the illumination pupil aperture 131. In this regard, depending on the characterization (e.g., measurement or inspection) performed under the control of the controller 114, the sample 108 may be illuminated by a different range of angles. The illumination pupil aperture 131 may also include a plurality of apertures for controlling the illumination pupil aperture 131. IN The polarization state of the polarized component.

[0036] In one embodiment, the 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 within the objective lens 150 to the collection pupil aperture 121. For example, the collection tube lens 122 can be configured so that the collection pupil aperture 121 and the pupil within the objective lens 150 are conjugate to each other. In one embodiment, the collection pupil aperture 121 can be configured by switching different apertures into the position of the collection pupil aperture 121. In another embodiment, 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 regard, illumination of a different range of angles reflected or scattered from the sample 108 can be directed to the detector assembly 104 under the control of the controller 114. The collection pupil aperture 121 can also include a polarization element so that light L of a particular polarization can be selected. R / S / T for transmission to the sensor 106 .

[0037] In another embodiment, the illumination pupil aperture 131 and / or the collection pupil aperture 121 may include a programmable aperture. Programmable apertures are generally discussed in U.S. Pat. No. 9,255,887, issued on Feb. 9, 2016 to Brunner, entitled “2D programmable aperture mechanism”; and U.S. Pat. No. 9,645,287, issued on May 9, 2017 to Brunner, entitled “Flexible optical aperture mechanisms”, both of which are incorporated herein by reference in their entirety. Methods for selecting an aperture configuration for inspection are generally discussed in the following patents: U.S. Patent No. 9,709,510, entitled “Determining a configuration for an optical element positioned in a collection aperture during wafer inspection,” issued on July 18, 2017, to Kolchin et al.; and U.S. Patent No. 9,726,617, entitled “Apparatus and methods for finding a best aperture and mode to enhance defect detection,” issued on August 8, 2017, to Kolchin et al., the entire texts of which are incorporated herein by reference.

[0038] Figure 1 The various optical elements and operating modes depicted in the figure are merely used to illustrate how the laser 200-0 can be used in the characterization system 100 and are not intended to limit the scope of the present disclosure. Figure 1 Additional optical elements and subsystems may be incorporated as desired for a particular application.

[0039] Figure 2A 1 is a simplified block diagram showing a laser assembly 200A configured to produce a wavelength in the range of approximately 180 nm to approximately 200 nm (e.g., approximately 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., an intermediate frequency doubling stage 220A and a final frequency doubling stage 230A) that are cooperatively configured to produce a laser output light 239A having a wavelength in the range of approximately 180 nm to approximately 200 nm. The first fundamental frequency laser 210A is configured to produce a first fundamental wavelength in the range of approximately 720 nm to approximately 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 light having a frequency equal to the first fundamental frequency ω 1A Twice the second harmonic frequency 2ω 1A The final (second) frequency doubling stage 230A receives the second harmonic light (intermediate frequency light) 212A and generates a second harmonic light having a frequency equal to the first fundamental frequency ω 1A Four times the output frequency ω OUTA Laser output light 239A.

[0040] refer to Figure 2A , using known techniques to configure the first basic laser 210A to emit light at a first fundamental frequency ω 1A In one embodiment, the first basic laser 210A is configured to generate a first fundamental light 211A (referred to as “basic”) at a first fundamental frequency ω corresponding to a wavelength between approximately 720 nm and approximately 800 nm (e.g., a wavelength of approximately 774 nm). 1AThe first elementary laser 210A is provided at a wavelength of approximately 193 nm to generate the first elementary light 211A. In one embodiment, the first elementary laser 210A is implemented using titanium sapphire (Ti sapphire) laser media. Suitable elementary lasers operating at wavelengths close to 800 nm are commercially available. In order to generate enough light at a wavelength of approximately 193 nm for inspecting semiconductor wafers, photomasks or photomasks, the first elementary laser 210A should generate tens or hundreds of watts of elementary light 211A. Other applications may not require so much power or may require more power. Depending on the pulse width and repetition rate requirements for the laser 200A, the first elementary laser can be configured as a Q-switched laser, a mode-locked laser or a CW laser.

[0041] The first frequency doubling stage 220A is configured to generate the second harmonic light 212A from the first fundamental light 211A. In one embodiment, the first frequency doubling stage 220A incorporates a lithium triborate (LBO) nonlinear crystal configured for critical phase matching of the first fundamental frequency and the second harmonic frequency. If necessary, the first frequency doubling stage 220A may include other components, such as a prism for separating the second harmonic light 212A from the unconsumed fundamental light. The first frequency doubling stage 220A may include a cavity resonating at the first fundamental frequency to increase conversion efficiency.

[0042] Final frequency doubling stage 230A is configured to generate laser output light 239A from second harmonic light 212A. Final frequency doubling stage 230A incorporates a nonlinear crystal 300 configured to double the frequency of second harmonic light 212A and output light 303, which includes light at the frequency of laser output light 239A and unconsumed second harmonic light. Nonlinear crystal 300 includes a stack of SBO slabs. For illustration purposes, Figure 2A Four such plates 335-1, 335-2, 335-3, and 335-4 are depicted stacked one upon another. It should be noted that in embodiments, there may be dozens, hundreds, or thousands of stacked plates. Figure 2A The plates are depicted in contact with each other. The plates may be in contact with each other (e.g., the plates may be in optical contact with each other) or there may be a small air gap between the plates having a width similar to or less than the thickness of one plate. The thickness of each plate is selected to achieve quasi-phase matching for doubling the frequency of the second harmonic light 212A. Adjacent plates (e.g., plates 335-1 and 335-2) have their c crystal axes oriented in opposite directions relative to each other. These and other important aspects of nonlinear crystals are discussed below with respect to Figure 3 Describe in detail.

[0043] Final frequency doubling stage 230A may include other optical components if necessary, such as a prism for separating the laser output light 239A from the unconsumed fundamental second harmonic light. Final frequency doubling stage 230A may include a cavity for recycling the second harmonic frequency to increase conversion efficiency.

[0044] In an alternative embodiment, a single cavity may include both the first frequency doubling stage 220A and the final frequency doubling stage 230A. These and other important aspects of this embodiment will be discussed below with respect to Figure 4 Describe in detail.

[0045] Figure 2B 1 is a simplified block diagram showing a laser assembly 200B configured to generate a wavelength in the range of approximately 180 nm to approximately 200 nm (e.g., approximately 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., a first frequency doubling stage 220B-1, an optical parameter system 221B, a frequency summing stage 222B, and a final frequency doubling stage 230B) that are cooperatively configured to generate a laser output light having a wavelength in the range of approximately 180 nm to approximately 200 nm. The first fundamental laser 210B-1 is configured to generate a first fundamental wavelength in the range of approximately 1.0 μm to approximately 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 light having a frequency equal to the first fundamental frequency ω 1B Twice the second harmonic frequency 2ω 1B OPS 221B is configured to generate third frequency light 213B having a frequency of ω3 and a corresponding wavelength in the range of approximately 1.2 μm to approximately 2.0 μm. Frequency summing stage 222B is configured to receive light from OPS 221B and from first frequency doubling stage 220B-1 and generate a sum having a frequency equal to its input (i.e., equal to 2ω 1B The final (second) frequency doubling stage 230B receives the intermediate frequency light 214B and generates an output frequency ω equal to twice the intermediate frequency. OUTA =4ω 1B +2ω3=2ω4 and the corresponding wavelength of laser output light 239A is in the range of approximately 180 nm to approximately 200 nm.

[0046] refer to Figure 2B, the first elementary laser 210B-1 is configured using known techniques to generate the first elementary light 211B-1 at a first elementary frequency ω1. In one embodiment, the first elementary laser 210B-1 is configured so as to generate the first elementary light 211B-1 at a first elementary frequency ω1 corresponding to a wavelength in a range between approximately 1.0 μm and approximately 1.1 μm, for example, a wavelength of approximately 1064 nm. In one embodiment, the first elementary laser 210B-1 is implemented using one of a Nd-doped yttrium aluminum garnet (YAG) laser medium, a Nd-doped yttrium vanadate laser medium, and a ytterbium-doped fiber laser medium. Suitable elementary lasers operating at wavelengths close to 1064 nm are commercially available. In order to generate enough light at a wavelength of approximately 193 nm for inspecting semiconductor wafers or masks, the first elementary laser 210B-1 should generate tens or hundreds of watts of elementary light 211B-1. Other applications may not require so much power or may require more power. Depending on the pulse width and repetition rate requirements of the laser 200B, the first elementary laser 210B- 1 may be configured as a Q-switched laser, a mode-locked laser, or a CW laser.

[0047] 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 a second harmonic frequency from the first fundamental frequency. The first frequency doubling stage 220B-1 may include other components, such as a prism for separating the second harmonic light 212B-1 from the unconsumed first fundamental light, if necessary. The first frequency doubling stage 220B-1 may include a cavity configured to recycle the first fundamental frequency to increase conversion efficiency.

[0048] OPS221B is configured to generate a third frequency light 213B having a frequency ω3 and a corresponding wavelength in the range of approximately 1.2 μm to approximately 2.0 μm, such as a wavelength of approximately 1.4 μm. OPS221B may use any known OPO configuration and / or optical parameter generator (OPG) configuration and / or amplification stage (such as a fiber amplifier, a thin disk amplifier, a cavity amplifier, a rod amplifier, an optical parameter amplifier (OPA), or a multi-pass amplifier) ​​suitable for the pulse width, pulse repetition rate, power, and wavelength of the third frequency light 213B. OPS221B may use any suitable nonlinear crystal for frequency conversion, including periodically poled materials. Suitable nonlinear crystals include lithium niobate and stoichiometric strontium tantalate. Figure 2B Three alternative configurations for pumping the OPS221B are depicted using dashed lines.

[0049] In one embodiment, the first fundamental light 211B-1 is split into two parts by a beam splitter 224B-1 (which only needs to be included in this embodiment). The first part of the first fundamental light is guided to the first frequency doubling stage 220B-1 to generate the second harmonic light 212B-1. The second part of the first fundamental light is guided to the 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) to efficiently convert the energy of the pump light (the second part of the first fundamental light 211B-1) into the third frequency light, because the frequency ω 1B and ω3 may be fairly close. However, due to the idler signal (e.g., having a frequency equal to ω) generated in the OPS221B configured as an OPO 1B -ω3 frequency) has a relatively low frequency and therefore a relatively long wavelength, and materials suitable for nonlinear crystals used in an OPS221B configured as an OPO may be expensive or difficult to obtain because the crystal should preferably be reasonably transparent at the frequency of the idler signal in order to minimize thermal effects such as thermal dephasing or thermal lensing. For example, if the first fundamental light has a wavelength of approximately 1064nm and the third frequency light has a wavelength of approximately 1400nm, then the idler signal will have a wavelength of approximately 4.4μm.

[0050] In the second embodiment, the second harmonic light 212B-1 is divided into two parts by the beam splitter 224B-2. The first part of the second harmonic light is guided to the frequency summing stage 222B. The second part of the second harmonic light is guided to the OPS 221B as pump light. The advantage of this embodiment is that due to the idler signal (e.g., having a power equal to 2ω) generated in the OPS 221B configured as an OPO, 1B Since the unwanted light of the frequency 2ω3 (the unwanted light of the frequency 211B-1) has a relatively high frequency and thus a relatively short wavelength, materials suitable for nonlinear crystals used in OPS 221B configured as an OPO may be inexpensive and readily available. For example, if the first fundamental light 211B-1 has a wavelength of approximately 1064 nm (and thus the second harmonic light 212B-1 has a wavelength of approximately 532 nm) and the third frequency light has a wavelength of approximately 1400 nm, then the idler signal will have a wavelength of approximately 860 nm. However, this second embodiment will be less efficient than the first exemplary embodiment in converting the energy of the pump light (the second portion of the first fundamental light 211B-1) into the third frequency light because the frequency 2ω3 is relatively high and thus the second harmonic light 212B-1 has a wavelength of approximately 532 nm. 1B and ω3 is greater than ω 1B and ω3 are further apart, resulting in more energy going into the idler signal.

[0051] In a third embodiment, the laser 200B includes a second elementary laser 210B-2 configured to generate a second elementary light 211B-2 having a frequency of ω2. The second elementary light 211B-2 is directed to the OPS 221B as pump light. The second elementary laser 210B-2 may use any convenient laser medium capable of generating a suitable wavelength for pumping the OPS 221B, including the exemplary laser materials listed above with respect to the first elementary laser 210B-1. The second elementary laser 210B-2 may be configured to generate a wavelength close to 1064 nm or close to 532 nm by, for example, frequency doubling the output of a laser generating a wavelength close to 1064 nm. Although the third embodiment may appear more complex than either of the first two embodiments, it may be preferred when the laser output light 239B requires multi-watt power, because the first elementary laser 210B-1 with sufficient power may not be readily available or may be very expensive.

[0052] The frequency summing stage 222B is configured to receive light from OPS 221B and the first frequency doubling stage 220B-1 and produce a sum having a frequency equal to its input (ie, equal to 2ω 1B The intermediate frequency light 214B is an intermediate frequency light having an intermediate frequency of 1 / 400 nm (1 / 400 nm). 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 from the sum of the second harmonic frequency and the third frequency. The frequency summing stage 222B may include other optical components, such as a prism for separating the intermediate light 214B from the unconsumed second harmonic light and the unconsumed third frequency light, if necessary. The frequency summing stage 222B may include a cavity configured to recirculate the third frequency or the second harmonic frequency to increase conversion efficiency.

[0053] 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 similar to the above reference Figure 2A The final frequency doubling stage 230A described above is configured. The final frequency doubling stage 230B incorporates a nonlinear crystal 300 configured to double the frequency of the intermediate frequency light 214B and output light 303, which includes light at the frequency of the laser output light 239B and unconsumed intermediate frequency light. The nonlinear crystal 300 includes a stack of SBO slabs. For illustration purposes, Figure 2B Four such plates 335-1, 335-2, 335-3, and 335-4 are depicted stacked one upon another. It should be noted that in actual embodiments, there may be dozens, hundreds, or thousands of stacked plates. Figure 2BThe plates are depicted in contact with each other. The plates may be in contact or there may be a small air gap between the plates having a width similar to or less than the thickness of one plate. The thickness of each plate is selected to achieve quasi-phase matching for doubling the frequency of the second harmonic light 214B. Adjacent plates (e.g., plates 335-1 and 335-2) have c-crystal axes oriented in opposite directions from each other. These and other important aspects of nonlinear crystals will be discussed below with respect to Figure 3 Describe in detail.

[0054] Final frequency doubling stage 230B may include other optical components, if necessary, such as a prism for separating laser output light 239B from unconsumed intermediate frequency light. Final frequency doubling stage 230B may include a cavity configured to recycle the intermediate frequency to increase conversion efficiency.

[0055] The frequency summing stage 222B and the final frequency doubling stage 230B may be included in a single cavity configured to allow the light at frequency ω3 from OPS 221B and / or at frequency 2ω from the first frequency doubling stage 220B-1 to be 1B and / or in a state with a value equal to 2ω 1B +ω3=ω4 intermediate frequency light 214B is recycled to increase conversion efficiency. These and other important aspects of the cavity will be discussed below with respect to Figure 5 Describe in detail.

[0056] Figure 3 The instructions include being configured so that a frequency ω x Detail of the nonlinear crystal 300 of four stacked SBO slabs 335-1 to 335-4 that double the frequency of the input light 301. The input light 301 may correspond to Figure 2A The second harmonic light 212A in the laser 200A may correspond to Figure 2B The intermediate frequency light 214B in the laser 200B. Figure 3 A nonlinear crystal 300 having a periodic structure including four stacked SBO crystal slabs 335-1 to 335-4 is illustrated, but the total number of SBO slabs may be as few as two, may be more than ten, or may be more than 100. There may be an odd or even number of slabs. The thickness of each of the SBO slabs 335-1 to 335-4 may be hundreds of nanometers to hundreds of micrometers. Specifically, the SBO slab thickness Λ in the propagation direction of light 301A inside the crystal slab is given by the following equation:

[0057] Λ=mL c , (Equation 1)

[0058] Where m is an odd integer (e.g., 1, 3, 5, 7, ...) and L c is the quasi-phase matching (QPM) critical length

[0059]

[0060] where Δk is defined by the following equation

[0061] Δk=k(2ω x )–2k(ω x ), (Equation 3)

[0062] where k(ω) is the wave vector of light with frequency ω in the nonlinear crystal 300 given by,

[0063]

[0064] and where n(ω) is the refractive index of the nonlinear crystal appropriately poled at frequency ω and c is the speed of light in vacuum.

[0065] In order to double the frequency of the input light 301 having a wavelength of 386.8 nm, the quasi-phase matching critical length L c is about 0.85 μm (eg, a thickness between 0.8 μm and 0.9 μm). Reasonable m may range from 1 to about 999 to achieve a plate thickness that is convenient for handling and processing. This exemplary QPM critical length for producing light having a wavelength of 193.4 nm by frequency doubling light having a wavelength of 386.8 nm was calculated from the relevant refractive index of the SBO using the Sellmeier model published in "Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV" by P. Trabs, F. Noack, AS Aleksandrovsky, AI Zaitsev, NV Radionov, and V. Petrov, Opt. Express 23, 10091 (2015), which is incorporated herein by reference in its entirety. Furthermore, varying levels of impurities in the SBO crystal or the presence of defects within the crystal may slightly change the value of the refractive index of the crystal. Those skilled in the relevant art will understand how to use the above equation to calculate the QPM critical length for specific input and output frequencies given the exact refractive index of the crystal.

[0066] refer to Figure 3 , the frequency is ω xAn input light 301 of ω is incident on the input surface 335-IN of the nonlinear crystal 300. The polarization direction of the input light 301 is illustrated by the dashed arrow 302. The SBO slabs 335-1 to 335-4 are stacked on top of each other so that the input surface 335-IN and the output surface 335-OUT are ω-1 and ω-2 with respect to the frequency ω. x Input light 301 is oriented approximately at Brewster's angle θ to minimize reflection losses without the use of antireflection coatings. For wavelengths near 386 nm polarized parallel to the c-axis of the SBO crystal, the Brewster angle is approximately equal to 60.3° relative to the surface normal N, and for wavelengths near 193 nm with the same polarization direction, the Brewster angle is approximately equal to 61.9° relative to the surface normal N. Reflection losses are low at any angle within a few degrees of the Brewster angle (e.g., within ±2°), so for any angle of incidence near 61°, reflection losses for both the input and output light will be very low. In addition, since the exact angle is not critical to reflection losses, small adjustments can be made to the orientation of the nonlinear crystal 300 (e.g., small adjustments to the angle of incidence θ) to adjust the path length of the light Λ in the SBO slab to more accurately achieve QPM when the thickness of the slab is not exactly the expected thickness due to manufacturing variability. Light 303 exiting the SBO slab stack includes light at 2ω x The second harmonic of the input light at the frequency of ω x In one embodiment, the reflection losses at each interface are minimized by optically contacting the SBO plates together. In another embodiment, there may be a small air gap between the plates. By orienting the input and output surfaces close to the Brewster angle relative to the input light 301, the reflection losses at each interface between the air and the SBO plates can be very low. However, due to refraction at each SBO to air interface, the input light and the second harmonic of the input light may travel in slightly different directions in any air gap. On many plates, this displacement may accumulate into a significant offset and may reduce conversion efficiency. To minimize this effect, the air gap between the plates should preferably be kept at a few hundred nanometers or narrower, and the beam diameter of the input light should be as large as possible (e.g., about 100 μm or a few hundred microns), consistent with the desire for high power density to achieve high conversion efficiency.

[0067] To generate a periodic structure for QPM, SBO plates 335-1 to 335-4 are placed, with one SBO plate rotated relative to the other SBO plates so that their corresponding c-crystal axes are inverted relative to each other, as shown in FIG. Figure 3. The surface normal N of an SBO slab of thickness Λ (where Λ is the spacing between poles in the crystal) and the propagation direction of light 301A inside the SBO slab are shown in the two illustrations. This physical arrangement of the crystal slab allows QPM. This can be considered similar to using PPLN (periodically poled lithium niobate) for QPM, except that lithium niobate is a ferroelectric transistor and can be periodically polarized. In contrast, SBO is non-ferroelectric, so we need to physically arrange the crystal slab to produce a periodic structure for QPM. In addition, periodic poling requires the application of an electric field parallel to the crystal axis of the ferroelectric transistor, so the polarization direction must be aligned with the crystal axis. In contrast, the SBO crystal slab disclosed herein can be cut and polished in any orientation relative to the crystal axis, thus allowing the crystal slab to be cut and oriented at a Bruce angle relative to light incident on the crystal slab.

[0068] In one embodiment, the crystal axes of the SBO slabs 335-1 to 335-4 are oriented so that the light 301A propagating inside the SBO slab propagates approximately perpendicular to the c-axis, wherein the polarization direction (electric field direction) of the light 301A is substantially parallel to the c-axis to utilize the d 33 As the maximum nonlinear optical coefficient of SBO and thus maximizes the conversion efficiency. Figure 3 As depicted in , the crystal axes of SBO slab 335-2 may be oriented so that light 301A propagates substantially parallel to the a-axis of the SBO crystal. Alternatively, the crystal axes may be oriented so that light 301A propagates parallel to the b-axis, or at an angle within the ab plane of the crystal. In other words, Figure 3 The crystal axes depicted in the two insets in FIG. 335 can be rotated about the c-axis. If the input surface 335-IN of the SBO slab 335-4 is oriented at a Brewster angle relative to the input light 301, then the propagation direction of the light 301A within the slab 335-4 will be approximately 29.7° relative to the surface normal N.

[0069] There are many ways to manufacture and assemble the nonlinear crystal 300. When only a few plates are needed for a laser (e.g., when high conversion efficiency is not required), it may be convenient to polish the plates to the desired thickness and then stack the plates in the appropriate orientation. When hundreds (or more) of plates are needed to achieve the desired conversion efficiency, other manufacturing methods may be more convenient. For example, U.S. patent application No. 17 / 555,404, entitled "Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings" and filed on December 18, 2021, discloses interdigitated nonlinear crystal gratings and methods for manufacturing the same, and the entire text of the patent application is incorporated herein by reference.

[0070] Figure 4 The first and final frequency doubling stage cavities 400 (including Figure 2A A simplified diagram of the first frequency doubling stage 220A and the final frequency doubling stage 230A of the input light 211A (ω 1A ) enters the bow-tie ring cavity through the input coupler 432C-1, and the bow-tie ring cavity includes the input coupler 432C-1, curved mirrors 432C-2, 432C-3, 432C-4, and the first frequency doubling stage nonlinear crystal 441 (for example, LBO), and the SBO nonlinear crystal 300. The input light 211A (having a frequency ω 1A ) enters the bow-tie annular cavity, passes near or through the mirror 432C-1 and passes through the first frequency doubling crystal 441. The frequency is ω 1A The unconsumed input light 438C leaves the cavity after passing near or through mirror 432C-2. The output from the first frequency doubling crystal 441 includes the generated second harmonic light (having a frequency of 2ω 1A )433C. The second harmonic light 433C is reflected from the mirror 432C-2 and the mirror 432C-3, and is transmitted through the SBO nonlinear crystal 300 ( Figure 3 The light at the output surface 444 of the SBO nonlinear crystal 300 is 436C (having a frequency of 2ω 1A and 4ω 1A ). 436C has a frequency of 2ω 1A The portion is the unconsumed light 442C (with a frequency of 2ω 1A ), which passes through beam splitter (BS) 437C and reflects from mirror 432C-4 and input coupler 432C-1 and recirculates in the cavity. 1A Part of the light is reflected from the surface of the beam splitter (BS) 437C and directed out of the cavity as output light 439C.

[0071] In an embodiment, the first frequency doubling crystal 441 is configured such that the input surface 445 and the output surface 446 include an anti-reflection coating configured to transmit a frequency of ω 1A The input light 221A and the unconsumed input light 438C and / or the frequency is 2ω 1A Circulation light 433C and 442C.

[0072] In an embodiment, the SBO plate 300 is configured such that the input surface 443 and the output surface 444 are 2ω with respect to the frequency 1AThe circulating lights 433C and 442C are oriented approximately at the Brewster angle. The polarization directions of the lights 433C and 442C are illustrated by arrows 402. In addition, the BS 437C can be configured to laterally shift the light 442C circulating in the cavity by an amount that substantially cancels the lateral displacement of the light 433C caused by the two or more SBO slabs 300 so as to maintain a substantially symmetrical bow-tie cavity and simplify optical alignment of the cavity.

[0073] In one embodiment, the SBO panel 300 is configured such that the input surface 443 includes a surface configured to transmit the circulating light 433C and 442C (both with a frequency of 2ω 1A ) and the output surface 444 includes an anti-reflection coating configured to transmit a frequency of 4ω 1A The output light 439C and the transmitted circulating light 433C and 442C (both with a frequency of 2ω 1A ) anti-reflection coating. The input surface 443 and the output surface 444 are not necessarily relative to the frequency 2ω 1A The circulating light 433C and 442C are oriented at the Brewster angle.

[0074] In one embodiment, BS 437C may include SBO crystal, SBO glass, or CaF2 crystal. Since SBO has good deep UV transmittance and has a high damage threshold, SBO may be advantageously used as the substrate material for BS 437C to ensure long life despite the high power levels of unconsumed input light 442C circulating in the cavity. If 437C includes an SBO crystal, its thickness and / or the orientation of its c-crystal axis may be configured so as to minimize any frequency doubling of unconsumed circulating light 442C passing therethrough. BS 437C may include a dichroic beam splitter, prism, or other component to separate the wavelengths.

[0075] In alternative embodiments, rather than orienting input surfaces 445, 443 and output surfaces 446, 444 at the Brewster angle, the input surface of first doubler crystal 441 and / or two or more SBO crystal slabs 300 may be coated with a suitable antireflection coating.

[0076] In an alternative embodiment, the frequency doubling crystal 441 is external to the cavity and the input light 211A first passes through the frequency doubling crystal 441 before entering the cavity through the input coupler 432C-1. 1A The circulating light 433C and 442C will be generated outside the cavity and optically coupled into the cavity through the input coupler 432C-1.

[0077] although Figure 4The first and final frequency doubling stage cavities 400 are depicted as including four curved mirrors, but other combinations of mirrors and / or lenses may be used to refocus the light circulating in the cavities. In alternative embodiments, the first and final frequency doubling stage cavities 400 may include triangular cavities, standing wave cavities, or other shaped cavities instead of bow tie cavities. In alternative embodiments, any combination of the frequencies involved may be circulated. Any of these cavities may utilize standard PDH (Pound-Drever-Hall) or HC ( By adjusting one of the mirrors (e.g., Figure 4 By adjusting the position of the mirror 432C-4) or the position of the prism, the length of the resonant cavity can be adjusted to maintain resonance.

[0078] Figure 5 The present invention is to show the embodiment of the present invention. Figure 2B Simplified diagram of the frequency summing stage and final frequency doubling stage cavity 500 used in the frequency summing stage 222B and the final frequency doubling stage 230B. Input light 511A (from Figure 2B The light 213B (ω3) and the light 212B-1 (2ω 1B )) enters the bow-tie ring cavity through the input coupler 532C-1, and the bow-tie ring cavity includes the input coupler 532C-1, curved mirrors 532C-2, 532C-3, 532C-4, a frequency summing stage nonlinear crystal 541 (for example, LBO) and an SBO nonlinear crystal 300. The input light 511A (including frequencies ω3 and 2ω 1B ) enters the bow-tie annular cavity, passes near or through mirror 532C-1 and passes through frequency summing crystal 541. The frequencies are ω3 and 2ω 1B The unconsumed input light 538C leaves the cavity after passing near or through mirror 532C-2. The output from the frequency summing crystal 541 includes the generated output light (having a frequency ω4 = 2ω3 + 4ω 1B The output light 533C is reflected from the mirror 532C-2 and the mirror 532C-3 and is transmitted through the SBO nonlinear crystal 300 (eg Figure 34). The light at the output surface 544 of the SBO nonlinear crystal 300 is 536C (having frequencies 2ω4 and ω4). The portion of 536C having frequency ω4 is unconsumed light 542C (having frequency ω4), which passes through a beam splitter (BS) 537C and reflects from a mirror 532C-4 and an input coupler 532C-1, recycling in the cavity. The portion of light 536C having frequency 2ω4 reflects from the surface of the beam splitter (BS) 537C and is directed out of the cavity as output light 539C.

[0079] In an embodiment, the first frequency summing crystal 541 is configured such that the input surface 545 and the output surface 546 include an anti-reflection coating configured to transmit frequencies ω3 and 2ω 1B The input light 511A and the unconsumed input light 538C and / or the circulating lights 533C and 542C with a frequency of ω4.

[0080] In an embodiment, the SBO slab 300 is configured such that the input surface 543 and the output surface 544 are oriented approximately at Brewster's angle relative to the circulating light 533C and 542C at a frequency of ω4. The polarization directions of the light 533C and 542C are illustrated by arrows 502. In addition, the BS 537C can be configured to laterally shift the light 542C circulating in the cavity by an amount that substantially cancels out the lateral displacement of the light 533C caused by two or more SBO slabs 300 so as to maintain a substantially symmetrical bow-tie cavity and simplify optical alignment of the cavity.

[0081] In one embodiment, the SBO slab 300 is configured such that the input surface 543 and the output surface 544 include anti-reflection coatings configured to transmit the circulating lights 533C and 542C (both with a frequency of ω4) at an angle formed by the orientation of the input surface 543 and the output surface 544 relative to the circulating lights 533C and 542C, which is not necessarily the Brewster angle relative to the circulating lights 533C and 542C with a frequency of ω4.

[0082] In one embodiment, BS 537C may include SBO crystal, SBO glass, or CaF2 crystal. Since SBO has good deep UV transmittance and has a high damage threshold, SBO may be advantageously used as the substrate material for BS 537C to ensure long life despite the high power levels of unconsumed input light 542C circulating in the cavity. If 537C includes an SBO crystal, its thickness and / or the orientation of its c-crystal axis may be configured so as to minimize any frequency doubling of unconsumed input light 542C passing therethrough. BS 537C may include a dichroic beam splitter, prism, or other component to separate the wavelengths.

[0083] In alternative embodiments, rather than orienting input surfaces 545, 543 and output surfaces 546, 544 at the Brewster angle, the input surface of first doubler crystal 541 and / or two or more SBO crystal slabs 300 may be coated with a suitable anti-reflection coating.

[0084] In an alternative embodiment, the frequency summing crystal 541 is external to the cavity and the input light 511A first passes through the frequency summing crystal 541 before entering the cavity through the input coupler 532C-1. In this embodiment, circulating light 533C and 542C having frequency ω4 will be generated outside the cavity and optically coupled into the cavity through the input coupler 532C-1.

[0085] although Figure 5 The frequency summing and final frequency doubling stage cavity 500 is depicted as including four curved mirrors, but other combinations of mirrors and / or lenses may be used to refocus the light circulating in the cavity. In alternative embodiments, the frequency summing and final frequency doubling stage cavity 500 may include a triangular cavity, a standing wave cavity, or other shaped cavity, rather than a bow tie cavity. In alternative embodiments, any combination of the frequencies involved may be circulated. Any of these cavities may be stabilized using standard PDH or HC locking techniques. By adjusting one of the mirrors (e.g., a PZT) through a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator, the frequency summing and final frequency doubling stage cavity 500 may be stabilized using standard PDH or HC locking techniques. Figure 5 By adjusting the position of the mirror 532C-4) or the position of the prism, the length of the resonant cavity can be adjusted to maintain resonance.

[0086] Figure 6 The present invention is to show the embodiment of the present invention. Figure 2B Simplified diagram of the amplifier stage 600 utilized in the OPS 221B of FIG. The input light 213B (ω3) is generated by an OPO or an OPG. The input light 213B (ω3) and the CW pump light 611 enter the bow-tie annular cavity through the input coupler 632C-1, and the bow-tie annular cavity includes the input coupler 632C-1, the curved mirror 632C-2, the plane mirrors 632C-3, 632C-4 and the gain medium 643. The input light 213B (including the frequency ω3) enters the bow-tie annular cavity, passes near or through the mirror 632C-1 and passes through the gain medium 643 (e.g., an Nd-doped YAG rod). The CW pump light 611 is a frequency and power suitable for pumping the gain medium 643 to achieve stimulated emission of light with a frequency ω3 including the input light 213B. The CW pump light 611 may correspond to Figure 2B 211B-1(ω 1B ), 211B-2(ω2) or 212B-1(2ω 1B). The output from the gain medium 643 includes the amplified input light (having frequency ω3) 633C. The output light 633C is partially reflected from the output coupler 632C-2, the mirror 632C-3, the mirror 632C-4, and the input coupler 632C-1, and is thus recycled within the cavity. A portion of the recycled light 633C is optically coupled out of the cavity through the output coupler 632C-2 as the output light 213C (having frequency ω3).

[0087] In an embodiment, gain media 643 is configured such that input surface 645 and output surface 646 contain anti-reflection coatings for input light 213B and CW pump light 611. In an alternative embodiment, gain media 643 is oriented at approximately Brewster's angle relative to circulating light 633C (ω3).

[0088] In an embodiment, the diodes emitting pump light 611 are configured to side-pump the gain medium 643. These diodes are positioned on one or more sides of the gain medium 643 parallel to the input light 213B so as to emit the pump light 611 perpendicular to the input light 213B into the gain medium 643. In this embodiment, the pump light 611 can be coherent, incoherent, quasi-CW, CW, or pulsed.

[0089] although Figure 6 The amplifier stage 600 is depicted as including two curved mirrors and two flat mirrors, but other combinations of mirrors and / or lenses may be used to refocus the light circulating in the cavity. In alternative embodiments, the amplifier stage 600 may include a triangular cavity, a standing wave cavity, or other shaped cavity instead of a bow tie cavity. Any of these cavities may be stabilized using standard PDH or HC locking techniques. By adjusting one of the mirrors (e.g., a standing wave cavity) via a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator, the amplifier stage 600 may be stabilized using standard PDH or HC locking techniques. Figure 6 By adjusting the position of the mirror 632C-4) or the position of the prism, the length of the resonant cavity can be adjusted to maintain resonance.

[0090] Figure 7 The present invention is used to demonstrate the embodiment of the present invention. Figure 2B Simplified diagram of an exemplary OPS 700 of the OPS 221B. Input light 713 (ω y , including from Figure 2B 211B-1(ω 1B )、212B-1(2ω 1B) or 211B-2(ω2)) enters the bow-tie ring cavity through the input coupler 732C-1, and the bow-tie ring cavity includes the input coupler 732C-1, curved mirrors 732C-2 and 732C-3, the output coupler 732C-4, the OPO crystal 744 (e.g., PPLN), the gain medium 743, and the frequency selection device 747. The input light 713 (including the frequency ω y ) enters the bow-tie annular cavity, passes near or through mirror 732C-1 and passes through OPO crystal 744. The frequency is ω y The unconsumed input light 748 leaves the cavity after passing near or through the mirror 732C-2. The output from the OPO crystal 744 includes the generated circulating light (having a frequency ω3) 733C. The circulating light 733C is reflected from the mirror 732C-2 and passes through the frequency selection device 747 (e.g., a transmission volume Bragg grating), reflected from the mirror 732C-3, and passes through the gain medium 743. The amplified circulating light 736C (having a frequency ω3) at the output surface 742 of the gain medium is partially reflected from the output coupler 732C-4. The output light 213B transmitted through the output coupler 732C-4 has a frequency ω3. The amplified circulating light 736C partially reflected from the output coupler 732C-4 is reflected from the input coupler 732C-1 and thereby recirculated within the cavity. The CW pump light 711 is optically coupled into the cavity through the mirror 732C-3 and passes through the gain medium 743. Unused CW pump light 712 leaves the cavity through output coupler 732C-4. CW pump light 711 has a frequency and power suitable for gain medium 743 to achieve stimulated emission of light having a frequency of ω3, including circulating light 733C.

[0091] In an embodiment, the OPO crystal 744 and the gain medium 743 are configured such that the input surfaces 745, 741 and the output surfaces 746, 742 contain anti-reflection coatings for the circulating lights 733C and 736C and the input light 713 (for the OPO crystal 744) or the CW pump light 711 (for the gain medium 743). In an alternative embodiment, the gain medium 743 is oriented at approximately the Brewster angle relative to the circulating lights 733C, 736C (ω3) and the CW pump light 711. In an alternative embodiment, the OPO crystal 744 is oriented at approximately the Brewster angle relative to the circulating lights 733C, 736C (ω3) and the input light 713 (ω3).

[0092] In an embodiment, the diodes emitting pump light 711 are configured to side-pump the gain medium 743. The diodes are positioned on one or more sides of the gain medium 743 parallel to the input light 713 so as to emit the pump light 711 into the gain medium 743 perpendicular to the input light 713. In this embodiment, the pump light 711 can be coherent, incoherent, quasi-CW, CW, or pulsed.

[0093] In an embodiment, the frequency selection device 747 may include a transmissive or reflective grating or other frequency selection method to control the wavelength or bandwidth of the circulating light 733C, the amplified circulating light 736C, and the output light 213B. In an embodiment, the frequency selection device 747 is configured as a reflective frequency selection device (e.g., a reflective volume Bragg grating) in a cavity (e.g., a linear cavity) to reflect the circulating light 733C or the amplified circulating light 736C instead of a cavity mirror.

[0094] although Figure 7 OPS 700 is depicted as including four curved mirrors, but other combinations of mirrors and / or lenses may be used to refocus the light circulating in the cavity. In alternative embodiments, the frequency summing stage and OPS 700 may include a triangular cavity, a standing wave cavity, or other shaped cavity instead of a bow tie cavity. Any of these cavities may be stabilized using standard PDH or HC locking techniques. By adjusting one of the mirrors (e.g., a standing wave cavity) via a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or other actuator, the frequency summing stage and OPS 700 may include a triangular cavity, a standing wave cavity, or other shaped cavity instead of a bow tie cavity. Any of these cavities may be stabilized using standard PDH or HC locking techniques. Figure 7 By adjusting the position of the mirror 732C-4) or the position of the prism, the cavity length is adjusted to maintain resonance.

[0095] The above figure is not intended to represent the actual physical layout of the components. The above figure shows the main optical modules involved in the procedure, but does not show every optical element. A person skilled in the relevant art will understand from the above figure and its associated description how to build a laser that produces a wavelength close to 193nm. It should be understood that more or fewer optical components can be used to guide the light to where it is needed. Lenses and / or curved mirrors can be used in appropriate cases to focus the beam waist to the focus of a substantially circular or elliptical cross-section inside or near the nonlinear crystal. When necessary, prisms, beam splitters, gratings or diffractive optical elements can be used to manipulate or separate different wavelengths at the output of each frequency conversion stage. Prisms, coated mirrors or other elements can be used when appropriate to combine different wavelengths at the input of the frequency conversion stage. A beam splitter or coated mirror can be used when appropriate to split one wavelength into two beams. Filters can be used to block or separate undesirable wavelengths at the output of any stage. Wave plates can be used to rotate polarization as needed. Other optical elements can be used when appropriate. Those skilled in the relevant art will appreciate that various tradeoffs and alternatives are possible in the implementation of the lasers disclosed herein.

[0096] Although the present invention is described herein using various basic wavelengths that facilitate the production of laser output light at a desired wavelength of approximately 193 nm, other wavelengths within a few or tens of nanometers of this desired wavelength can be produced by changing the wavelength of the first basic laser (laser 200A or 200B) or by changing the wavelength of the light output by the OPS (laser 200B). Unless otherwise specified in the appended claims, such lasers and systems utilizing such lasers are considered to be within the scope of the present invention.

[0097] Lasers with wavelengths in the sub-200 nm range are not commercially available at sufficient power levels or are unreliable or expensive to operate. In particular, other than excimer lasers, there is no other prior art for producing 1 W or more of optical power in the wavelength range between approximately 180 nm and 200 nm. Embodiments of the present invention produce wavelengths close to 193 nm, thus providing better sensitivity for detecting small particles and defects compared to longer wavelengths. The lasers of the present invention do not use toxic or corrosive gases, and are therefore easier and cheaper to operate and maintain.

[0098] Those skilled in the relevant art will readily appreciate that the inventive lasers described herein have many possible applications in addition to use in semiconductor inspection and metrology. For example, a laser operating at a wavelength close to 193.4 nm may be used in a lithography system configured to expose a pattern into a photoresist coated on a substrate such as a semiconductor wafer. In another example, a laser operating at a wavelength between about 180 nm and 200 nm may be used in a system configured to cut or ablate biological tissue. The lasers described herein may be configured to produce very short pulses at an output wavelength, which may enable preferential removal of material by ablation rather than by heating, thereby causing less damage to surrounding materials. For example, such lasers may be used in laser eye surgery or laser vision correction. Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are also applicable to other embodiments, all of which are intended to fall within the scope of the present disclosure.

[0099] With respect to the use of substantially any plural and / or singular terms herein, those skilled in the art may translate from the plural to the singular and / or from the singular to the plural, depending on the context and / or application. For clarity, the various singular / plural arrangements are not explicitly set forth herein.

[0100] This specification is presented to enable one of ordinary skill in the art to make and use the disclosure in the context of a particular application and its requirements. As used herein, directional terms, such as "top", "bottom", "above", "below", "up", "upward", "down", "under", and "downward" are intended to provide relative positions for descriptive purposes and are not intended to specify absolute reference coordinates. Various modifications to the preferred embodiments will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the disclosure is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0101] In addition, it should be understood that the present invention is defined by the appended claims. Those skilled in the art will understand that, in general, the terms used herein and especially in the appended claims (e.g., the body of the appended claims) are generally intended to be "open" terms (e.g., the term "including" should be interpreted as "including but not limited to", the term "having" should be interpreted as "having at least", the term "includes" should be interpreted as "including but not limited to" and the like). Those skilled in the art will further understand that if a specific number of introduced claim statements is intended, then such intention will be expressly stated in the claims, and in the absence of such a statement, then such intention is not present. For example, to aid understanding, the following appended claims may contain the use of the introductory phrases "at least one" and "one or more" to introduce claim statements. However, the use of such phrases should not be construed as implying that the introduction of a claim recitation by the indefinite article "a" or "an" limits any particular claim containing such introduced claim recitation to an invention containing only one such recitation, even when the same claim contains the introductory phrases "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should generally be construed to mean "at least one" or "one or more"); the same applies to the use of the definite article used to introduce a claim recitation. In addition, even if a specific number of introduced claim recitations is explicitly recited, one skilled in the art will recognize that such recitation should generally be construed to mean at least the recited number (e.g., the simple recitation of "two recitations" without other modifiers generally means at least two recitations, or two or more recitations). Furthermore, where a convention similar to “at least one of A, B, and C, and the like” is used, it is generally expected that one skilled in the art will understand the meaning of the convention (e.g., “a system having at least one of A, B, and C” will include, but is not limited to, systems having only A, only B, only C, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like). Where a convention similar to “at least one of A, B, or C, and the like” is used, it is generally expected that one skilled in the art will understand the meaning of the convention (e.g., “a system having at least one of A, B, or C” will include, but is not limited to, systems having only A, only B, only C, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like). Those skilled in the art will further understand that virtually any separate word and / or phrase presenting two or more alternatives, whether in this specification, claims or drawings, should be understood to consider the possibility of including one of the items, either of the items, or both of the items.For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

[0102] It is believed that from the foregoing description, the present disclosure and its many attendant advantages will be understood, and it will be apparent that various changes may be made to the form, construction, and arrangement of components without departing from the disclosed subject matter or without sacrificing all of its substantial advantages. The described form is merely illustrative, and the following claims are intended to cover and encompass such changes. Furthermore, it should be understood that the invention is defined by the appended claims.

Claims

1. A testing system, comprising: a light source configured to generate light having a wavelength in a range between 180 nm and 200 nm; and an optical system configured to direct the light onto a sample, The light source comprises: a first elementary laser configured to generate an elementary laser beam having a corresponding fundamental frequency; one or more intermediate frequency conversion stages collectively configured to generate intermediate frequency light using the primary laser beam, the intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between 360 nm and 400 nm; a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal, wherein the nonlinear crystal comprises a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second SBO crystal plate, and wherein the plurality of SBO crystal slabs are cooperatively configured to form a periodic structure that achieves quasi-phase-matched QPM of the intermediate frequency light, such that light exiting the nonlinear crystal comprises laser output light having an output frequency and a corresponding wavelength in a range of approximately 180 nm to approximately 200 nm. 2 . The inspection system of claim 1 , wherein the plurality of SBO crystal plates are configured such that a first crystal axis of the first SBO crystal plate is inverted relative to a second crystal axis of the second SBO crystal plate.

3. The inspection system of claim 1, wherein the output frequency corresponds to a wavelength of approximately 193 nm.

4. The inspection system of claim 1, wherein the first primary laser is configured to produce a wavelength between approximately 760 nm and approximately 800 nm, and wherein the one or more intermediate frequency conversion stages are configured to double the frequency of the first primary laser beam to produce the intermediate frequency light.

5. The inspection system of claim 4, wherein the first primary laser comprises a titanium sapphire laser.

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

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

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

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

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

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

12. The inspection system of claim 11, further comprising a computer operably connected to the sensor and configured to determine the presence or absence of a defect on the sample by analyzing a signal from the sensor.

13. The inspection system of claim 6, wherein generating the third frequency light utilizes the optical parametric system OPS, the optical parametric system comprising at least one of an optical parametric oscillator OPO, an optical parametric generator OPG, and an amplification stage.

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

15. The inspection system of claim 6, wherein the OPS comprises an optical parameter generator (OPG) configured to generate a third frequency light, the OPG being optically coupled to a cavity connected to an amplifier configured to amplify the third frequency light.

16. The inspection system of claim 6, wherein the OPS comprises an OPO cavity configured to recirculate the third frequency light, wherein the cavity comprises an amplifier configured to amplify the third frequency light.

17. The inspection system of claim 1, wherein the final frequency doubling stage further comprises a cavity containing the plurality of SBO crystal slabs and configured to recycle the intermediate frequency light.

18. The inspection system of claim 6, wherein the frequency summing stage further comprises a cavity configured to recirculate the intermediate frequency light and containing the plurality of SBO crystal slabs.

19. A laser assembly configured to generate laser output light, comprising: a first elementary laser configured to generate an elementary laser beam having a corresponding fundamental frequency; one or more intermediate frequency conversion stages collectively configured to generate intermediate frequency light using the primary laser beam, the intermediate frequency light having an associated intermediate frequency and a corresponding intermediate wavelength between approximately 360 nm and approximately 400 nm; a final frequency doubling stage configured to pass the intermediate frequency light through a nonlinear crystal, wherein the nonlinear crystal comprises a plurality of strontium tetraborate (SBO) crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second SBO crystal plate, and wherein the plurality of SBO crystal slabs are cooperatively configured to form a periodic structure that achieves quasi-phase-matched QPM of the intermediate frequency light, such that light exiting the nonlinear crystal comprises laser output light having an output frequency and a corresponding wavelength in a range of approximately 180 nm to approximately 200 nm.

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

21. The laser assembly of claim 19, wherein the output frequency corresponds to a wavelength of approximately 193 nm.

22. The laser assembly of claim 19, wherein the first elementary laser is configured to produce a wavelength between approximately 760 nm and approximately 800 nm, and wherein the one or more intermediate frequency conversion stages are configured to double the frequency of the first elementary laser beam to produce the intermediate frequency light.

23. The laser assembly of claim 22, wherein the first elementary laser comprises a titanium sapphire laser.

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

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

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

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

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

29. The laser assembly of claim 24, wherein generating the third frequency light utilizes the optical parametric system OPS, the optical parametric system comprising at least one of an optical parametric oscillator OPO, an optical parametric generator OPG, and an amplification stage.

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

31. The laser assembly of claim 24, wherein the OPS comprises an optical parameter generator (OPG) configured to generate a third frequency light, the OPG optically coupled to a cavity connected to an amplifier configured to amplify the third frequency light.

32. The laser assembly of claim 24, wherein the OPS comprises an OPO cavity configured to recirculate the third frequency light, wherein the cavity comprises an amplifier configured to amplify the third frequency light.

33. The laser assembly of claim 19, wherein the final frequency doubling stage further comprises a cavity configured to recycle the intermediate frequency light and containing the plurality of SBO crystal slabs.

34. The laser assembly of claim 24, wherein the frequency summing stage further comprises a cavity configured to recycle the intermediate frequency light and containing the plurality of SBO crystal slabs.

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