System and method for growing quasi-phase matched strontium tetraborate and lithium triborate crystals for frequency conversion

Abstract

A method for growing a periodically poled nonlinear crystal may include placing a seed crystal into a melt to form a seed crystal melt mixture, where the seed crystal may include at least one of strontium tetraborate (SBO) or lithium triborate (LBO), and where the melt includes at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. The method may further include heating the seed melt mixture to a predetermined temperature until the periodically poled nonlinear crystal is formed.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Serial No. 63 / 521,880, filed on June 20, 2023, which is incorporated herein by reference in its entirety. Technical Field

[0003] Generally, the present disclosure relates to a system and method for growing nonlinear optical crystals, and more particularly, to a system and method for growing periodically poled nonlinear optical crystals. Background Art

[0004] As semiconductor device sizes shrink, the size of the smallest particles or pattern defects that can cause device failure also shrinks. Consequently, there is a need to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. The intensity of light scattered by particles smaller than the wavelength of the light is generally proportional to a high power of the particle's size (e.g., the total scattered intensity of light from an isolated small spherical particle is proportional to the sixth power of the sphere's diameter and inversely proportional to the fourth power of the wavelength). Because the intensity of the scattered light increases, shorter wavelengths generally provide better sensitivity for detecting small particles and defects than longer wavelengths.

[0005] Because the intensity of light scattered from small particles and defects is generally very low, high illumination intensity is required to produce a signal that can be detected in a very short time. For example, average light source power levels of 1 W or greater may be required. At these high average power levels, high pulse repetition rates are desirable because the higher the repetition rate, the lower the energy per pulse, and therefore the lower the risk of damage to the system optics or the item being inspected. The illumination requirements for inspection and metrology are often best met by continuous wave (CW) light sources. CW light sources have a constant power level, which avoids peak power damage issues and allows for continuous acquisition of images or data. However, in many cases, mode-locked lasers (also called quasi-CW lasers) with repetition rates of approximately 50 MHz or higher may be useful because the high repetition rate means the energy per pulse can be low enough to avoid damage for many metrology and inspection applications. The higher peak power of a mode-locked laser can allow for more efficient and simpler frequency conversion than a CW laser with the same average power level.

[0006] Pulsed lasers for generating vacuum ultraviolet (VUV) light are known in the art. Background lasers for generating 133 nm light are well known (see, for example, GW Faris and MJ Dyer, "Two-photon excitation of neon at 133 nm," Opt. Lett. 18, 382 (1993), and A. Tünnermann, C. Momma, K. Mossavi, C. Windolph, and B. Wellegehausen, "Generation of tunable short pulse VUV radiation by four-wave mixing in xenon with femtosecond KrF-excimer laser pulses," IEEE Journal of Quantum Electronics 29, 1233 (1993)). Unfortunately, these lasers are not well suited for inspection applications due to their low laser pulse repetition rate and their use of toxic and corrosive gases in the lasing medium, which results in a high cost of ownership.

[0007] Solid-state deep ultraviolet (DUV) lasers are desirable because of their higher possible repetition rates, CW generation capabilities, and the absence of the need for toxic liquids or gases. Several crystals exist that can be used for DUV frequency conversion. For example, beta-barium borate (BBO) and cesium lithium borate (CLBO) crystals are common crystals used for ultraviolet (UV) frequency conversion. Both materials have some ability to phase-match UV light, but have various disadvantages for high-power VUV frequency conversion. BBO has a relatively low damage threshold when exposed to high-intensity DUV radiation. In addition, BBO is non-transmissive below approximately 190 nm. CLBO can have a higher damage threshold than BBO, but it is hygroscopic, requiring extreme caution during handling, processing, and operation. In addition, CLBO exhibits increased absorption for wavelengths shorter than approximately 185 nm.

[0008] Other less common crystals have been explored for DUV frequency conversion. For example, potassium beryllium fluoroborate (KBBF) (KBe2BO3F2) and other beryllium fluoroborate (ABe2BO3F2)-containing materials (where A = Na, K, Rb, Cs, Tl, NH4) have absorption edges between 147 nm and 155 nm, suitable nonlinear coefficients, and sufficiently large birefringence to make phase matching possible for 161 nm to 202 nm. Second harmonic generation of 200 nm light has been demonstrated in KBBF at 1.2 W power, however, transparency begins to decrease for wavelengths shorter than 200 nm, making high-power generation less feasible for shorter wavelengths. In addition, the largest KBBF crystal growth reported was 3.7 mm (Wang, XY; Yan, X.; Luo, SY; Chen, CT “Flux Growth of Large KBBF Crystals by Localized Spontaneous Nucleation”, Journal of Crystal Growth, 318, 610-612 (2011)), which limits the application potential of this material.

[0009] Other DUV-transmitting crystals exist, namely strontium beryllium borate (SBBO) (Sr2Be2B2O7), strontium pentafluoroaluminate (SrAlF5), and boron phosphate (BPO4), among others, but these crystals suffer from unstable crystal structures, growth difficulties, and toxic precursors, or require further development of growth methods and study of damage thresholds and nonlinear processes.

[0010] Other nonlinear crystals that are transparent in the DUV do not have a large enough birefringence to allow birefringence phase matching in the DUV. However, quasi-phase matching is possible for many of these crystals. For example, barium magnesium fluoride (BaMgF2) and strontium magnesium tetrafluoride (SrMgF4) have high transmittance down to about 125 nm and are ferroelectric and can therefore be periodically poled to achieve quasi-phase matching, but the nonlinear coefficients in these materials are too small to overcome losses from surface scattering or absorption in the material. Furthermore, periodic poling using the ferroelectric properties of the crystal does not always produce perfectly straight boundaries between polarization domains, which is acceptable for infrared (IR) or visible light quasi-phase matching, but is detrimental in VUV / DUV quasi-phase matching because of the smaller poling period resulting from the larger mismatch in refractive index between the shorter wavelengths involved, as found in the Sellmeier refractive index model of the transparent region of a dielectric nonlinear frequency conversion crystal. There are currently no commercially available periodically poled crystals for VUV / DUV frequency conversion of any size.

[0011] To achieve sufficiently high conversion efficiencies (requiring tens to thousands of poling periods), it is necessary to form VUV / DUV-transmissive frequency conversion crystals grown with sufficiently large dimensions (approximately 5 mm to 10 mm clear aperture). However, unless the material is ferroelectric at the necessary dimensions, there is currently no high-quality growth method for periodically poled VUV / DUV nonlinear bulk crystals.

[0012] Therefore, there is a need for a commercially scalable method for growing periodically poled nonlinear crystals that produce DUV radiation at wavelengths approaching 120 nm to 200 nm and that avoid many or all of the disadvantages of background art crystals and are suitable for use in systems configured for examining samples, for exposing patterns in photoresist on a substrate, or for drilling, cutting, or ablating materials including biological tissue. Summary of the Invention

[0013] A method for growing a periodically poled nonlinear crystal according to one or more embodiments of the present disclosure is disclosed. In one embodiment, the method includes placing a seed crystal into a melt to form a seed-melt mixture, wherein the seed crystal may include at least one of strontium tetraborate (SBO) or lithium triborate (LBO), and wherein the melt includes at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. In one embodiment, the method includes heating and cooling the seed-melt mixture to a predetermined temperature until the periodically poled nonlinear crystal is formed.

[0014] A periodically poled nonlinear crystal according to one or more embodiments of the present disclosure is disclosed. In one embodiment, the periodically poled nonlinear crystal includes a plurality of crystal plates, the plurality of crystal plates being arranged in a stacked configuration, wherein the plurality of crystal plates include at least a first crystal plate and a second crystal plate, wherein the first crystal plate is adjacent to the second crystal plate, wherein the plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) plates or one or more lithium triborate (LBO) plates, wherein the plurality of crystal plates are configured to form a periodic structure, wherein the periodic structure achieves quasi-phase matching (QPM) of light.

[0015] An optical system according to one or more embodiments of the present disclosure is disclosed. In one embodiment, the optical system includes an illumination source configured to generate illumination having a wavelength between 120 nm and 200 nm. In one embodiment, the optical system includes an optical subsystem configured to direct the illumination from the illumination source onto a sample. In one embodiment, the illumination source includes a first fundamental frequency laser configured to generate a fundamental frequency laser beam having a corresponding fundamental frequency and a fundamental wavelength between 720 nm and 800 nm. In an embodiment, the illumination source includes two or more frequency doubling stages, the two or more frequency doubling stages including at least an intermediate frequency doubling stage and a final frequency doubling stage, wherein the intermediate frequency doubling stage is configured to receive the first fundamental frequency and generate second harmonic light having a second harmonic frequency, wherein the final frequency doubling stage is configured to output light from the second harmonic light generating laser, wherein the final frequency doubling stage includes a nonlinear crystal configured to double the frequency of the second harmonic light, wherein the nonlinear crystal includes a plurality of crystal plates disposed in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, wherein the plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates, and wherein the plurality of crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the first fundamental frequency and the second harmonic frequency.

[0016] A laser assembly according to one or more embodiments of the present disclosure is disclosed. In one embodiment, the laser assembly includes a first fundamental frequency laser configured to generate a fundamental frequency laser beam having a corresponding fundamental frequency and a fundamental frequency wavelength between 720 nm and 800 nm. In an embodiment, the laser assembly includes two or more frequency doubling stages, wherein the two or more frequency doubling stages include at least an intermediate frequency doubling stage and a final frequency doubling stage, wherein the intermediate frequency doubling stage is configured to receive the first fundamental frequency and generate second harmonic light having a second harmonic frequency, wherein the final frequency doubling stage is configured to generate laser output light from the second harmonic light, wherein the final frequency doubling stage includes a nonlinear crystal configured to double the frequency of the second harmonic light, wherein the nonlinear crystal includes a plurality of crystal plates disposed in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, wherein the plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates, and wherein the plurality of crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the first fundamental frequency and the second harmonic frequency.

[0017] A method for growing a periodically poled nonlinear crystal according to one or more embodiments of the present disclosure is disclosed. In one embodiment, the method includes placing a periodically poled seed crystal in contact with a melt mixture from a platinum (Pt) nozzle connected to a Pt crucible containing the melt mixture, wherein the periodically poled seed crystal includes at least one of strontium tetraborate (SBO) or lithium triborate (LBO), wherein the melt includes at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. In one embodiment, the method includes pulling the periodically poled seed crystal away from the Pt nozzle at a predetermined speed while maintaining contact with the melt until the periodically poled nonlinear crystal is formed.

[0018] 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 invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Those skilled in the art may better appreciate the numerous advantages of the present disclosure by referring to the accompanying drawings.

[0020] Figure 1 is a simplified block diagram of an optical system according to one or more embodiments of the present disclosure.

[0021] Figure 2 is a simplified block diagram depicting a laser assembly according to one or more embodiments of the present disclosure.

[0022] Figure 3 is a simplified diagram depicting a furnace for growing periodically poled strontium tetraborate (SBO) according to one or more embodiments of the present disclosure.

[0023] Figure 4A is a simplified diagram depicting a periodically poled SBO seed according to one or more embodiments of the present disclosure.

[0024] Figure 4B is a simplified diagram depicting a periodically poled SBO crystal according to one or more embodiments of the present disclosure.

[0025] Figure 5 is a simplified diagram depicting a periodically poled SBO crystal grown from a periodically poled SBO seed according to one or more embodiments of the present disclosure.

[0026] Figure 6 is a flow chart depicting a method for growing a periodically poled nonlinear crystal according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0027] Reference will now be made in detail to the disclosed subject matter as illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to specific embodiments and specific features thereof. The embodiments set forth herein are to be considered illustrative rather than restrictive. It will be apparent to those skilled in the art that various changes and modifications in form and details may be made without departing from the spirit and scope of the present disclosure.

[0028] Embodiments of the present disclosure are directed to improvements in the nonlinear crystal growth of periodically poled strontium tetraborate (SBO) (SrB4O7) and lithium triborate (LBO) (LiB3O5) for use in semiconductor optical systems.

[0029] SBO is a material of increasing interest in DUV frequency generation. The space group of SBO is Pnm21 space group and the point group is mm2, which indicates non-zero d 33 Nonlinear coefficients can exist and be used to perform quasi-phase matching. For SBO, the natural crystallographic coordinates are and (Oseledchik, YS et al., "New nonlinear optical crystals: strontium and lead tetraborates," Optical Materials 4, 669-674 (1995), incorporated herein by reference in its entirety.) The crystallographic coordinates in the rectangular reference system are X, Y, and Z, and X, Y, and Z correspond to a, b, and c. The optical coordinates are x, y, and z, and x, -y, and z correspond to b, a, and c, respectively. z The z optical coordinate of the refractive index is along the 21-axis of symmetry of the SBO. In the case of quasi-phase matching (QPM) of the SBO, the alternating axis is the c-axis in order to obtain high d 33 Nonlinear coefficient. For the frequency doubling from 800nm to 400nm, this d 33The nonlinear coefficient was measured to be 1.5 pm / V (Petrov, V. et al., "Application of the nonlinear crystal SrB4O7 for ultrafast diagnostics converting to wavelengths as short as 125 nm," Optics Express, 29, 4 (2004), which is incorporated herein by reference in its entirety). In addition, SBO has DUV transparency for wavelengths as short as 125 nm and has demonstrated frequency conversion to this wavelength. SBO has a power of 16.4 J / cm at 266 nm. 2 The UV light-induced damage threshold is significantly higher than that of calcium fluoride (CaF2) (11.4 J / cm 2 ) and silicon dioxide (4.8J / cm 2 ) has a UV light-induced damage threshold (Tanaka et al., "High surface laser-induced damage threshold of SBO single crystals under 266-nm (DUV) laser irradiation", Optics Express, 28, 20 29239 (2020), which is incorporated herein by reference in its entirety). Although biaxial, SBO is almost isotropic (Oseledchik, YS et al., "New nonlinear optical crystals: strontium and lead tetraborates", Optical Materials 4, 669-674 (1995)), and therefore birefringence phase matching is not possible for frequency conversion in DUV. Due to the high d 33 The nonlinear coefficient, therefore SBO, is the coordinate for quasi-phase matching, where the fundamental and second harmonics are polarized parallel to each other and parallel to the c-axis. The phase mismatch caused by the different refractive indices of the fundamental and higher harmonics is compensated by flipping the direction of the c-crystal axis of the material by 180 degrees, so that the phase difference between the harmonics is mitigated by the different signs of the nonlinear coefficient.

[0030] It should be noted that SBO exhibits unique optical and mechanical properties. The transparency range of SBO is from 130 nm to 3200 nm in wavelength. See YS Oseledchik, AL Prosvirnin, AI Pisarevskiy, VV Starshenko, VV Osadchuk, SP Belokrys, NVS Vitanko, A S Korol, S A Krikunov, and A F Selevich, "New nonlinear optical crystals: strontium and lead tetraborates," Optical Materials 4, 669 (1995), which is incorporated herein by reference in its entirety. This wide transparency range covers the VUV, DUV, visible light, and near infrared (IR) wavelength ranges. The VUV and DUV ranges are of particular interest in semiconductor inspection and metrology. It should also be noted that the transmittance is high. For example, for a wavelength range from about 250 nm to about 2500 nm, the transmittance exceeds 80%. This high transmittance makes SBO a good candidate for frequency generation, particularly in the UV wavelength range. If SBO is grown under optimal conditions, a good transmission curve can be obtained: for wavelengths longer than 200 nm, the transmittance can reach more than 80%, and for wavelengths between 130 nm and 200 nm, it can reach more than 50%. The dielectric and optical properties of strontium tetraborate glass are described by M.V. Shankar and K.B.R. Barma in "Dielectric and Optical Properties of Strontium Tetraborate Glass," Journal of Materials Science Letters, 15 (1996) 858-860, which is incorporated herein by reference in its entirety.

[0031] In this paper, LBO is a well-studied and commercially available nonlinear optical material. LBO belongs to the Pna21 space group and the mm2 point group, which indicates that d 31 d 32 d 33 d 24 and d 15, nonlinear coefficients may exist and can be used to perform quasi-phase matching. For LBO, following the conventions of Roberts, D., "Simplified characterization of uniaxial and biaxial nonlinearoptical crystals: a plea for standardization of nomenclature and conventions" (IEEE Journal of Quantum Electronics, 28, 10 (1992), Table 1 "IEEE / ANSI" header), which is incorporated herein by reference in its entirety, the natural crystallographic coordinates are and The crystallographic coordinates in the rectangular reference system are X, Y, and Z, and X, Y, and Z correspond to a, b, and c. The optical coordinates are x, y, and z, and x, y, and z correspond to b, c, and a, respectively. y The y optical coordinate of the refractive index is along the 21-axis of symmetry of LBO. In the case of QPM of LiB3O5, the alternating axis is the c-axis in order to obtain high d 31 and d 15 Nonlinear coefficient. d 31 d 32 and d 33 The nonlinear coefficients were measured to be 0.85 pm / V, -0.67 pm / V, and 0.04 pm / V, respectively, at 1064 nm ("Simplified Characterization of Uniaxial and Biaxial Nonlinear Optical Crystals: A Plea for Standardization of Nomenclature and Conventions," Roberts, D.A., IEEE Journal of Quantum Electronics, 28, 10 (1992), incorporated herein by reference in its entirety). Assuming Kleinman symmetry and neglecting absorption, d 24 The nonlinear coefficient is equal to d 32 Nonlinear coefficient, and d 15 The nonlinear coefficient is equal to d 31 Nonlinear coefficient. LBO has a transparency range of 160nm to 2300nm and has sufficient birefringence to perform phase-matched frequency conversion for wavelengths as short as about 266nm. For wavelengths less than 200nm, birefringence phase matching is not possible, but quasi-phase matching can be used. 33The nonlinear coefficient is too small for effective conversion in practice, but d 31 and d 15 The nonlinear coefficient is large enough for practical quasi-phase matching. 31 The I-type phase matching of the nonlinear coefficient is that the fundamental frequency is polarized parallel to the crystal axis and the second harmonic is polarized parallel to the c crystal axis. 15 Type II phase matching of the nonlinear coefficients, where one or a portion of the fundamental beam is polarized parallel to the c-crystal axis, and one or a portion of the fundamental beam is polarized parallel to the a-crystal axis, and a second harmonic is generated that is polarized parallel to the a-crystal axis. The phase mismatch caused by the different refractive indices of the fundamental and higher harmonics is compensated by flipping the direction of the c-crystal axis of the material by 180 degrees, so that the phase difference between the harmonics is mitigated by the different signs of the nonlinear coefficients. The damage threshold of LBO is approximately 18 J / cm at 355 nm. 2 , which is higher than BBO.

[0032] Ferroelectric materials, such as periodically poled lithium niobate (PPLN) or magnesium barium fluoride (MgBaF2), can have their crystal axes flipped by applying an electrostatic field, allowing for the straightforward design of quasi-phase-matched materials. As previously mentioned, MgBaF2 has a small nonlinear coefficient, making it unsuitable for quasi-phase matching, and PPLN is opaque in the deep UV. SBO and LBO are not known to exhibit ferroelectric effects along the c-axis, so alternative methods must be used to quasi-phase-match SBO and LBO. For example, one approach starts with a large single crystal of SBO, as generally discussed in the following U.S. Patents: U.S. Patent No. 11,543,732, entitled “Frequency Conversion Using Stacking Strontium Tetraborate Plates,” issued on January 3, 2023; U.S. Patent No. 11,237,455, entitled “Frequency Conversion Using Stacked Strontium Tetraborate Plates,” issued on February 1, 2022; and U.S. Patent No. 11,237,455, entitled “Frequency Conversion Using Stacked Strontium Tetraborate Plates,” issued on January 31, 2023. Gratings); and U.S. Patent No. 11,899,338, issued on February 13, 2024, entitled “Deep Ultraviolet Laser Using Strontium Tetraborate for Frequency Conversion,” all of which are incorporated herein by reference in their entirety. This single crystal is polished or etched into thin slabs of the correct thickness for quasi-phase matching, with each slab oriented with its c-axis flipped relative to each other, with an air gap between each slab, or optically contacted. Another approach to forming quasi-phase-matched SBOs uses the Czochralski method (Aleksandrovsky, AS, Vyunishev, AM, and Zaitsev, AI, "Applications of Random Nonlinear Photonic Crystals Based on Strontium Tetraborate," Crystals, 2, 1393-1409 (2012), incorporated by reference in its entirety).As discovered by Aleksandrovsky et al., the c-axis of SBO switches naturally during Czochralski growth. This study found it difficult to control the period of each layer during the growth process and explored using randomly spaced layer production. Random quasi-phase matching would be less efficient than quasi-phase matching (except in the statistically unlikely case where the thicknesses of the randomly grown layers are each an odd multiple of the coherence length) and would give unpredictable results that vary between crystals, which is undesirable in commercial lasers.

[0033] High quality and quantity of SBO grown using the Kyropoulos method using a twin-type stirring blade has been demonstrated by Tanaka, Y. et al., "Kyropoulos growth of a 300g SBO single crystal using a twin-type stirring blade," Japanese Journal of Applied Physics 61, 075503 (2022), which is incorporated herein by reference in its entirety. Because the phase of SBO at atmospheric pressure exists in a small region of the phase diagram, careful control is required to grow stoichiometric SrBO. A six-zone resistive heater furnace for Kyropoulos growth can be used, with growth carried out at temperatures between 995°C and 1005°C (e.g., 1002°C). Growth rates of 0.1 mm / day to 5 mm / day have been recorded using a twin-type stirring blade and a Pt crucible rotating at 15 rpm. By starting with a quasi-phase-matched SBO seed crystal in polished optical contact, Kilopoulos growth will propagate the crystal pattern of the seed crystal in a similar manner and result in larger quasi-phase-matched SBO crystals that can be diced and stacked to multiply the quasi-phase-matched layers.

[0034] LBO can be grown into large, high-quality embryonic crystals using a high-temperature solution top-seeding method. Crystals can be grown using a flux method using boron trioxide (BO) as a self-flux, but with the addition of molybdenum trioxide (MoO) to reduce the viscosity of the flux. Crystals weighing up to 2 kg have been reported. Details of this growth method can be found in, for example, Hu, ZG et al., "Large LBO Crystal Growth at 2 kg-level," Journal of Crystal Growth 335 (2011), which is incorporated herein by reference in its entirety.

[0035] Figure 1A simplified block diagram of an optical system 100 is illustrated, in accordance with one or more embodiments of the present disclosure. The optical system 100 can be configured as an inspection system or a metrology system for inspecting a sample 108 and / or obtaining optical metrology measurements from the sample 108. The optical system 100 can include a semiconductor fabrication system. For example, the optical system 100 can include a fabrication system that can be configured to cut, drill, or ablate material from the sample 108, or to expose a pattern in a photoresist on the sample 108.

[0036] The sample 108 may include any sample known in the art, such as, but not limited to, a wafer, a reticle, a photomask, and the like. In one embodiment, the sample 108 may be placed 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-theta stage, and the like. In one 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 one embodiment, a lens (e.g., objective lens 150) may be moved up and down during inspection to maintain focus on the sample 108.

[0037] In an embodiment, the optical system 100 includes an illumination source 102 incorporating a laser 200-0 that generates a light having an output frequency ω OUT and output light L of a corresponding wavelength in a range between about 120 nm and about 200 nm OUT Details of laser 200-0 can be found at Figure 2 and the description of Table 1. Laser 200-0 incorporates at least one of an SBO quasi-phase-matched crystal and an LBO quasi-phase-matched crystal grown using the methods described herein. Illumination source 102 may include additional light sources, such as lasers operating at longer or shorter wavelengths or broadband light sources.

[0038] In an embodiment, the optical system 100 includes a OUT One or more optical components, such as, but not limited to, beam splitters, mirrors, lenses, apertures, and wave plates, are configured to illuminate and direct light onto sample 108. The optical components may be configured to illuminate an area, line, or point on sample 108. In an embodiment, beam splitter or mirror 134, mirrors 137 and 138, and lens 152 are configured to illuminate sample 108 from below so that light L INT Transmission through the sample enables inspection or measurement of the sample 108. In an embodiment, beam splitters or mirrors 134 and 135, mirror 136, and lens 151 are configured to use light L at an oblique angle of incidence (e.g., an angle of incidence greater than 60°) relative to the normal to the sample surface. Obl The sample 108 is illuminated. In this embodiment, the specularly reflected light L SpecIn an embodiment, the optical device 103 is collectively configured to convert the illumination light L IN Guided to the top surface of the sample 108.

[0039] When the sample 108 is illuminated in one or more of the modes described above, the optical device 103 is also configured to collect light L that is reflected, scattered, diffracted, transmitted, and / or emitted from the sample 108. R / S / T And the light L R / S / T The light is directed and focused onto the sensor 106 of the detector assembly 104. It should be noted herein that the sensor 106 and the detector assembly 104 can include any sensor 106 known in the art. For example, the sensor 106 can include, but is not limited to, a charge coupled device (CCD) detector, a complementary metal oxide semiconductor (CMOS) detector, a time delayed integration (TDI) detector, a photomultiplier tube (PMT), an avalanche photodiode (APD), a line sensor, an electron bombardment line sensor, and the like. The detector assembly 104 can be communicatively coupled to the controller 114.

[0040] Controller 114 may be 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 inspection system 100, such as platform assembly 112, illumination source 102, and optics 103.

[0041] In an embodiment, the optical device 103 includes an illumination tube lens 132. The illumination tube lens 132 may be configured to image an illumination pupil aperture 131 onto a pupil within the objective lens 150. For example, the illumination tube lens 132 may be configured such that the illumination pupil aperture 131 and the pupil within the objective lens 150 are conjugate to each other. In an embodiment, the illumination pupil aperture 131 may be configured by switching different apertures into the position of the illumination pupil aperture 131. In an 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, the sample 108 may be illuminated at different ranges of angles depending on the characterization (e.g., measurement or inspection) performed under the control of the controller 114. The illumination pupil aperture 131 may also include a plurality of apertures for controlling the illumination light L. IN The polarization state of the polarized component.

[0042] In an 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 an embodiment, the collection pupil aperture 121 can be configured by switching different apertures into the position of the collection pupil aperture 121. In an 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 different angular ranges 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 to be transmitted to the sensor 106 .

[0043] In embodiments, the illumination pupil aperture 131 and / or the collection pupil aperture 121 may comprise a programmable aperture. Programmable apertures are generally discussed in U.S. Patent No. 9,255,887, entitled “2D programmable aperture mechanism,” issued to Brunner on February 9, 2016, and U.S. Patent No. 9,645,287, entitled “Flexible optical aperture mechanisms,” issued to Brunner on May 9, 2017, both of which are incorporated herein by reference in their entirety. Methods for selecting an aperture configuration for inspection are generally described in: U.S. Patent No. 9,709,510, entitled “Determining a configuration for an optical element positioned in a collection aperture during wafer inspection,” issued to Kolchin et al. on July 18, 2017; and U.S. Patent No. 9,726,617, entitled “Apparatus and methods for finding a best aperture and mode to enhance defect detection,” issued to Kolchin et al. on August 8, 2017, both of which are incorporated herein by reference in their entirety.

[0044] Figure 1The various optical elements and operating modes depicted in FIG. 1 are merely illustrative of how the laser 200-0 may be used in the inspection system 100 and are not intended to limit the scope of the present disclosure. Figure 1 200-0. Additional optical elements and subsystems may be incorporated as needed for a particular application. The related references cited above and other references cited herein disclose many other important details of systems that may incorporate laser 200-0.

[0045] Figure 2 is a simplified block diagram depicting a laser assembly 200 configured to produce wavelengths in the range of approximately 120 nm to approximately 200 nm (eg, approximately 193 nm) according to an embodiment of the present disclosure.

[0046] In an embodiment, the laser assembly 200 includes a first fundamental frequency laser 210 and two frequency conversion (doubling) stages (i.e., an intermediate frequency conversion (doubling) stage 220 and a final frequency conversion (doubling) stage 230) that are cooperatively configured to generate laser output light 239 having a wavelength in the range of approximately 120 nm to approximately 200 nm. The first fundamental frequency laser 210 is configured to generate a first fundamental frequency wavelength in the range of approximately 720 nm to approximately 800 nm and a corresponding first fundamental frequency ω. y The first fundamental frequency light 211. The first frequency conversion (doubling) stage 220 receives the first fundamental frequency light 211 and generates a light having a frequency equal to the first fundamental frequency ω y Twice the second harmonic frequency ω x The final (second) frequency conversion (doubling) stage 230 receives the second harmonic light (intermediate frequency light) 212 and generates a second harmonic light having a frequency equal to the first fundamental frequency ω y Four times the output frequency ω OUT The laser output light 239.

[0047] refer to Figure 2 The first fundamental frequency laser 210 is configured using any suitable technique to generate a first fundamental frequency ω y In an embodiment, the first fundamental frequency laser 210 is configured such that the first fundamental frequency light 211 is emitted at a first fundamental frequency ω corresponding to a wavelength between about 720 nm and about 800 nm (e.g., a wavelength of about 774 nm). yIn an embodiment, the first fundamental frequency laser 210 is implemented using a titanium-sapphire (Ti-sapphire) laser medium. In order to generate enough light at a wavelength of about 193 nm to inspect semiconductor wafers, photomasks or photomasks, it is contemplated herein that the first fundamental frequency laser 210 should generate tens or hundreds of watts of fundamental frequency light 211. 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 200, the first fundamental frequency laser can be configured as a Q-switched laser, a mode-locked laser or a CW laser.

[0048] The first frequency conversion (doubling) stage 220 can be configured to generate the second harmonic light 212 from the first fundamental frequency light 211. In an embodiment, the first frequency conversion (doubling) stage 220 incorporates a lithium triborate (LBO) nonlinear crystal configured to critically phase-match the first fundamental frequency with the second harmonic frequency. The first frequency conversion (doubling) stage 220 can include other components as needed, such as a prism for separating the second harmonic light 212 from the unconsumed fundamental frequency light. The first frequency conversion (doubling) stage 220 can include a cavity that resonates at the first fundamental frequency to increase conversion efficiency.

[0049] The final frequency conversion (doubling) stage 230 can be configured to generate laser output light 239 from the second harmonic light 212. The final frequency conversion (doubling) stage 230 can incorporate a nonlinear crystal 400B configured to double the frequency of the second harmonic light 212 and output light 435-OUT that includes light at the frequency of the laser output light 239 and the unconsumed second harmonic light. The nonlinear crystal 400B can include a stack of SBO plates or LBO plates. It should be noted herein that for illustration purposes, Figure 2 Four such plates 435B-1, 435B-2, 435B-3, and 435B-4 are depicted stacked upon one another. However, it should be noted herein that in practical embodiments, there may be tens or hundreds or thousands of stacked plates. Figure 2 The plates are depicted as touching each other. The thickness of each plate is selected to achieve quasi-phase matching to double the frequency of the second harmonic light 212. Adjacent plates (e.g., plates 435B-1 and 435B-2) have crystal c-axes oriented in opposite directions relative to each other. Figure 4B These and other important aspects of nonlinear crystals are described in detail.

[0050] The final frequency conversion (doubling) stage 230 may include other optical components as needed, such as a prism to separate the laser output light 239 from the unconsumed fundamental and second harmonic light. The final frequency conversion (doubling) stage 230 may include a cavity to recycle the second harmonic frequency to increase conversion efficiency.

[0051] In an embodiment, a single cavity may include both the first frequency doubling stage 220 and the final frequency doubling stage 230. In an embodiment, the first fundamental frequency laser 210 comprises a laser having an output frequency of approximately 1000 nm (e.g., 1064 nm or 1030 nm). In an embodiment, the first frequency conversion stage 220 comprises a sum frequency generation stage, a sum difference generation stage, an optical parametric oscillation stage, or an optical parametric amplification stage. In an embodiment, the final frequency conversion stage 230 comprises a sum frequency generation stage having a frequency conversion crystal 400B, such as Figure 4B As further described in the description of . Frequency conversion pathways are generally discussed in U.S. Patent No. 11,543,732, entitled “Frequency Conversion Using Stacking strontium tetraborate Plates,” issued on January 3, 2023; U.S. Patent No. 11,237,455, entitled “Frequency Conversion Using Stacked strontium tetraborate Plates,” issued on February 1, 2022; U.S. Patent No. 11,567,391, entitled “Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings,” issued on January 31, 2023; and U.S. Patent No. 11,567,391, entitled “Deep Ultraviolet Laser Using strontium tetraborate for Frequency Conversion,” issued on February 13, 2024. No. 11,899,338, all of which are incorporated herein by reference in their entirety.

[0052] Figure 3 is a simplified block diagram depicting a furnace 301 for growing SBO from a seed crystal 305 using the top seeded Kilopoulos method.

[0053] In one embodiment, the furnace 301 may comprise a five-, six-, or more-zone resistance-heated furnace comprising a platinum crucible 303 having a diameter of approximately 150 mm and a height of approximately 150 mm. The platinum crucible 303 may be larger or smaller depending on the final size of the crystal being grown and should be approximately twice the diameter of the desired final crystal size. A seed crystal 305 may be secured to an alumina cylinder 302 to prevent the seed crystal 305 from falling into the melt 304, and a cooling liquid or gas is also supplied to the seed crystal 305 to prevent melting during growth. In one embodiment, the melt 304 is composed of strontium carbonate (SrCO3) and boron trioxide (BO3) powder, with boron trioxide acting as a self-fluxing agent and comprising approximately 67% of the melt. L-shaped or dual-shaped stirring blades promote mixing of the melt. The melt 304 temperature should be maintained near 1000°C and can be measured by one or more thermocouples 306, and the crucible rotation should be approximately 10 to 20 rpm. It should be noted that in this article, Figure 4A The seed crystal 305 is further described.

[0054] Another method that can be used to grow periodically poled SBO crystals from periodically poled seeds is hydrothermal growth. Hydrothermal growth of single crystal SBO has been demonstrated by Mcmillen, C. in the May 2007 paper "Hydrothermal Crystal Growth of Oxides for Optical Applications," Chapter 4, which is incorporated herein by reference in its entirety. SBO growth was demonstrated using an Sr:B ratio of 1:14.6. The mineralizer used was 1 M sodium hydroxide (NaOH) at 565°C and 20 kpsi, and boron was added via (NH4)2B 10 O 168H2O is introduced at a high density. Alternatively, a mineralizer can be used that includes one or more of NaOH, sodium chloride (NaCl), strontium dichloride (SrCl2), potassium chloride (KCl), strontium hydroxide (Sr(OH)2), cesium fluoride (CsF), lithium chloride (LiCl), potassium hydroxide (KOH), LiOH, and water (H2O) (Carla Heyward, August 2013, "Hydrothermal Crystal Growth of Metal Borates for Optical Applications," incorporated herein by reference in its entirety). Sr(OH)2 was used as the strontium source. In this paper, seedless SBO crystals up to 1.5 mm in size were grown in 0.25-inch OD silver ampoules. A temperature gradient of 10°C to 120°C was maintained across the autoclave and measured with the aid of a thermocouple attached to the exterior. Hydrothermal methods can eliminate the problems caused by the high viscosity of the B2O3 melt used in the Kiropoulos method for growing borate crystals and avoid inclusions caused by viscosity-reducing additives such as MoO3. 1 to 5 wt% is an ideal solubility value for hydrothermal growth of large crystals. Other sources can be used as feedstock for hydrothermal growth of SBO, such as glassy crystals containing strontium and borates, Sr(OH)2, B2O3, H3BO3, and SrCO3.

[0055] Another method for growing periodically poled SBO crystals from periodically poled seeds uses edge-fed film growth (EFG) or micro-pulldown. In EFG, a stoichiometric melt of SBO is contained in a crucible made of platinum or another inert material, and one end of a mold or narrow tube is immersed in the melt. Capillary action pulls the melt to the unsubmerged end of a slit, mold, or tube, where it contacts the surface of a periodically poled SBO seed crystal. The seed crystal is slowly pulled away from the slit, mold, or tube, and as the melt adjacent to the seed crystal cools, it crystallizes with the same periodic pole pattern as the seed crystal. As the seed crystal is pulled away, capillary forces continue to supply the melt. In this way, the shape of the crystal can be controlled by the shape of the mold or tube. This method is currently used to produce high-quality sapphire crystals. The micro-pulldown method can use a periodically poled SBO or LBO seed crystal positioned below a nozzle connected to a stoichiometric melt crucible at a controlled temperature. The nozzle and crucible can be made of platinum or another inert material. A seed crystal at a lower temperature than the melt contacts the melt drop at the end of the nozzle and pulls the seed crystal downward away from the nozzle at a controlled speed. As the seed crystal moves away from the nozzle, the melt touching the seed crystal cools and crystallizes with the same periodic polarization pattern as the seed crystal. The nozzle replenishes the melt. The micro-pull-down method has been demonstrated by Maeda, K. et al. in "Fabrication of Quasi-Phase-Matching Structure during Paraelectric Borate Crystal Growth" (Applied Physics Letters 6, 015501 (2013)) using polarized lithium tetraborate seeds, which is incorporated herein by reference in its entirety. Single crystal SBO has been grown using a micro-pull-down method by Machida, T. et al. in “Growth of Transparent SrB4O7 Crystal Fiber by the μ-PD Method” (Journal of the Japan Society for Materials Research, 42, 123-126 (2017)), which is incorporated herein by reference.

[0056] Periodically poled LBO crystals can be grown using a high temperature solution top seeding method using periodically poled LBO seeds, as described herein with respect to Figure 4AAs discussed. Crystals can be grown by a flux method using B2O3 as a self-flux or using MoO3 additives to reduce the viscosity of the flux. Details of this growth method can be found in the literature, for example, in "Large LBO Crystal Growth at 2kg-level" by Hu, ZG et al. (Journal of Crystal Growth, 335 (2011)), which is incorporated by reference in its entirety.

[0057] In embodiments, other crystal growth methods such as the Czochralski method, other pulling methods, or other flux or melt growth methods may be used.

[0058] Figure 4B is a simplified block diagram depicting an SBO or LBO periodically poled crystal. Specifically, Figure 4B The nonlinear crystal 400B includes a crystal configured to have a frequency ω x The frequency of the input light 401 is doubled by the four stacked SBO or LBO plates 435B-1 to 435B-8. Figure 4B The diagram illustrates a nonlinear crystal 400B having a periodic structure comprising eight stacked SBO or LBO crystal plates 435B-1 to 435B-8, but the total number of SBO or LBO plates can be as few as two, more than ten, or more than 100. There can be an odd or even number of plates. The thickness of each of the SBO or LBO plates 435B-1 to 435B-4 can be hundreds of nanometers to hundreds of micrometers. Specifically, the thickness Λ of the SBO or LBO plate in the propagation direction of light 401A within the crystal plate is given by the following equation:

[0059] Λ=mL c , (Equation 1)

[0060] Where m is an odd integer (e.g., 1, 3, 5, 7, ...), and L c is the quasi-phase matching (QPM) critical length

[0061]

[0062] where in the case of second harmonic generation, Δk is defined by the following equation

[0063] Δk=k(2ω x )–2k(ω x ), (Equation 3)

[0064] And in the case of sum frequency generation, Δk is defined by the following equation

[0065] Δk=k(ω3)–k(ω1)–k(ω2), (Equation 4)

[0066] where k(ω) is the wave vector of light with frequency ω in the nonlinear crystal 400B, which is given by

[0067]

[0068] and where n(ω) is the refractive index of the nonlinear crystal appropriately polarized at frequency ω and c is the speed of light in a vacuum. In the case of sum frequency generation, ω1+ω2=ω3, where ω3 is the frequency of the generated light.

[0069] In a non-limiting example, the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength of 193 nm. In an additional non-limiting example, the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength between 172 nm and 178 nm. In an additional non-limiting example, the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength between 147 nm and 153 nm. In an additional non-limiting example, the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength between 129 nm and 134 nm.

[0070] In a non-limiting example, the crystal plate thickness is an odd multiple of at least one of: 700 nm to 860 nm, 435 nm to 620 nm, 510 nm to 690 nm, 200 nm to 380 nm, 200 nm to 320 nm, or 80 nm to 175 nm. In an additional non-limiting example, the crystal plate thickness is an odd multiple of at least one of: 700 nm to 920 nm, 420 nm to 646 nm, and 460 nm to 730 nm.

[0071] To double the frequency of the input light 401 having a wavelength of 386.8 nm, the quasi-phase matching critical length L for SBO is c is about 0.85 μm (e.g., a thickness between 0.8 μm and 0.9 μm). In LiB3O5, for type I quasi-phase matching (386.8 nm light polarized along the a crystallographic axis and 193.4 nm light polarized along the c crystallographic axis), the critical length L c is about 0.9 μm (e.g., a thickness between 0.85 μm and 0.95 μm). For type II quasi-phase matching (half of the 386.8 nm light is polarized along the a crystallographic axis and half along the c crystallographic axis, and the 193.4 nm light is polarized along the c crystallographic axis), the critical length L cis about 0.72 μm (e.g., a thickness between 0.7 μm and 0.8 μm). Suitable m may be in the range of 1 to about 999 to achieve a slab thickness that is convenient for handling and processing. This exemplary QPM critical length for generating light having a wavelength of 193.4 nm by frequency doubling light having a wavelength of 386.8 nm is calculated from the correlated refractive index of SBO using the Sellmeier model published by P. Trabs, F. Noack, A.S. Aleksandrovsky, A.I. Zaitsev, N.V. Radionov, and V. Petrov in “Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV”, Optics Express 23, 10091 (2015) and is calculated using the Sellmeier model published by K. Kato in “Temperature-tuned 90° phase-matching properties of LiB3O5”. The Selmayer models published in "(IEEE Journal of Quantum Electronics, 30(12), 2950-2952 (1994)", which are all incorporated by reference in their entirety, are calculated based on the relevant refractive indices of LBO. The accuracy of these Selmayer models is uncertain. In addition, different impurity levels in SBO or LBO crystals or the presence of defects within the crystal can slightly change the value of the refractive index of that crystal. It is contemplated herein that one skilled in the art will understand how to use the above equations to calculate the QPM critical length for a specific input frequency and output frequency given the accurate refractive index of the crystal.

[0072] refer to Figure 4B , with frequency ω x Input light 401, which may consist of one or more wavelengths depending on whether second harmonic generation or sum frequency generation is desired, is incident on the input surface 435-IN of the nonlinear crystal 400B. The SBO or LBO plates 435B-1 to 435B-8 are optically contacted on top of each other so that the input surface 435-IN and the output surface 435-OUT are relative to each other with a frequency ω. x The propagation direction of the input light 401 is at an angle θ BOrientation. When the thickness of the slab is not exactly as expected due to manufacturing variability, small adjustments to the orientation of the nonlinear crystal 400B (i.e., small adjustments to the angle of incidence θ) can be made to adjust the path length Λ of the light in the SBO slab to achieve QPM more accurately. In addition, the temperature of the stack of slabs can be fine-tuned to shift the temperature-dependent refractive index and correct for thickness variation related issues. The light 403 exiting the stack of SBO or LBO slabs includes: in the case of second harmonic generation 2ω x input light at a frequency of ω and, in the case of sum frequency generation, the second harmonic of the sum of the two fundamental frequencies; and ω x The seed slabs will be in optical contact, minimizing reflection or scattering losses between each slab. The native cross section of the crystal will be continuous and therefore have no reflection or scattering losses at the interface because the refractive indices will be matched.

[0073] In an embodiment, Figure 4B The angle θ shown in B The Brewster angle is approximately 60.3° relative to the surface normal N for wavelengths parallel to the c-axis of the SBO crystal near 386 nm, and is approximately 61.9° relative to the surface normal N for wavelengths near 193 nm with the same polarization direction. The polarization direction of input light 401 into the SBO periodically poled crystal is illustrated by dashed arrow 402. Reflection losses are low at any angle within a few degrees (e.g., within ±2°) of the Brewster angle, so there will be very low reflection losses for any angle of incidence near 61° for both input and output light.

[0074] In an embodiment, the frequency ω x Input light 401 can be coupled into the stack by a prism. In one such embodiment, additional material from the growth process at either end of the stack can be cut to allow light to couple in at the Brewster angle and then travel through the unpolarized material before reaching the polarized material.

[0075] In embodiments, anti-reflective coatings for input light 401 and output light 403 may be applied to input surface 435-IN and / or output surface 435-OUT to minimize reflections of light frequencies involved in the conversion.

[0076] refer to Figure 4ATo form a periodic structure for quasi-phase matching of seed crystal 400A, SBO or LBO plates 435B-1 to 435B-4 are positioned so that one is rotated relative to the other, causing their corresponding c-crystal axes to be reversed relative to each other (as shown in the two insets of FIG. 4 , in the case of SBO). The two insets show the surface normal N of an SBO plate having a thickness Λ (where Λ is the spacing between the poles in the crystal along the propagation direction of light 401A within the crystal) and the propagation direction of light 401A within the SBO plate. This physical arrangement of the crystal plates allows for quasi-phase matching. This can be considered similar to using PPLN (periodically poled lithium niobate) for quasi-phase matching, except that lithium niobate is a ferroelectric crystal and can be periodically poled via an applied electric field. In contrast, SBO and LBO are not ferroelectric along the c-axis, so the crystal plates need to be physically arranged to form a periodic structure for quasi-phase matching.

[0077] In an embodiment, the crystal axes of the SBO plates 435B-1 to 435B-4 are oriented so that the light 401 propagating inside the SBO plates propagates substantially perpendicular to the c-axis, wherein the polarization direction (electric field direction) of the light 401A is substantially parallel to the c-axis. This takes advantage of the maximum nonlinear coefficient d in the SBO. 33 , and thus maximize the conversion efficiency. In the preferred embodiment using LBO, there is an additional constraint on LBO, which is because the maximum nonlinear coefficient d 31 , the fundamental frequency polarization direction of the light should be substantially parallel to the a-axis in the case of Type I phase matching, or parallel to the c-axis and the a-axis in the case of Type II phase matching, and the generated harmonic polarization should be substantially parallel to the c-axis in plates 435B-1 to 435B-4 for Type I phase matching and for Type II phase matching. Therefore, the LBO crystal plate must be cut perpendicular to the b-axis in the ca plane. Since both the fundamental frequency and the generated frequency have polarizations substantially parallel to the c-axis, the SBO crystal can be cut perpendicular to the a-axis in the cb plane, perpendicular to the b-axis in the ac plane, or at any angle as long as the c-axis is contained in the plane. For example, in an embodiment, the crystal axes of the SBO plates 435B-1 to 435B-4 can be oriented so that the propagation direction of the light 401 is substantially parallel to the a-axis of the SBO crystal. In an embodiment, the crystal axes can be oriented so that the light 401A propagates parallel to the b-axis, or propagates at an angle within the ab plane of the crystal. In other words, Figure 4A The crystal axes depicted in the two illustrations in FIG can be rotated about the c-axis. If the input surface of SBO plate 435B-1 is oriented at the Brewster angle relative to input light 401, then for a fundamental wavelength of approximately 386 nm, the propagation direction of light 401 within plates 435B-1 to 435B-4 will be approximately 29.7° relative to the surface normal N.

[0078] It is contemplated herein that there are many ways to make and assemble the nonlinear seed crystal 400A. 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, optionally, bring the plates into optical contact with the proper orientation. When hundreds (or more) of plates are needed to achieve the desired conversion efficiency, other fabrication methods may be more convenient. For example, U.S. Patent 11,567,391, entitled "Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings" and filed on December 18, 2021, generally discusses interdigitated nonlinear crystal gratings and methods for making the same, which U.S. Patent is incorporated herein by reference in its entirety. SBO and LBO slabs can be etched in the correct orientation, removed from their substrates, and brought into optical contact.

[0079] Another method for making a seed crystal is described by Maeda, K. et al. in "Fabrication of Quasi-Phase-Matching Structure during Paraelectric Borate Crystal Growth," Appl. Phys. Lett. 6, 015501 (2013), which is incorporated herein by reference in its entirety. This method uses a thin crystal with two different adjacent crystal domains. A platinum (Pt) wire melts a small portion of the crystal, close to the size of the Pt wire, in one domain, and the wire is slowly pulled until it touches the second domain and then withdraws into the first domain. The melted material "behind" the wire will recrystallize into the second domain orientation, forming second domain fingers within the first domain. This process can be repeated to produce periodically spaced fingers with the correct spacing for quasi-phase matching. This crystal can then be used as a seed crystal to grow a larger crystal in a pulling, solution grown, melt grown, or hydrothermal method.

[0080] Figure 5 The final crystal 500 grown from the seed crystal 400A is depicted. Figure 5In the figure, only four slabs are shown in the seed crystal, but the seed crystal may contain between two and thousands of slabs. Once grown, the grown crystal 500 can be cut into a more desired configuration by dicing the grown crystal parallel to the minimum dimension of each slab and stacking the slabs. In embodiments, the grown crystal 500 can be diced and stacked in other configurations. The grown crystal 500 has been diced and stacked to form a crystal having more slabs 501-A, 501-B, 501-C, 501-D, which can be used as the seed crystal 400A in further growth. Thus, the seed crystal can be repeatedly grown, diced, stacked, and used as a seed crystal to produce a grown crystal having thousands of cycles for quasi-phase matching.

[0081] Figure 6 6 is a flow chart depicting a method 600 for growing a periodically poled nonlinear crystal. At step 602, a seed crystal may be placed into a melt to form a seed-melt mixture, wherein the seed crystal includes at least one of strontium tetraborate (SBO) or lithium triborate (LBO), wherein the melt includes at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. At step 604, the seed-melt mixture may be heated and cooled to a predetermined temperature until a periodically poled nonlinear crystal is formed, as shown in FIG. Figure 5 As shown in .

[0082] According to an exemplary embodiment of the present disclosure, Table 1 contains Figure 1 and 2 Table of exemplary coherence lengths required for quasi-phase matching of SBO or LBO crystals for wavelengths produced by laser assemblies 200-0 and 200A to produce laser output light L having a wavelength in the range of approximately 125 nm to 140 nm (e.g., approximately 133 nm), 147 nm to 155 nm (e.g., approximately 152 nm) OUT239. Laser output light 161 having a wavelength in the range of approximately 170 nm to 180 nm (e.g., approximately 177 nm) and a wavelength in the range of approximately 190 nm to 195 nm (e.g., approximately 193 nm). For fundamental laser types, exemplary fundamental wavelengths and wavelengths corresponding to harmonics are shown. The precise wavelength of the fundamental laser depends on many factors, including the precise composition of the laser medium, the operating temperature of the laser medium, and the design of the optical cavity. Two lasers using the same laser line of a given laser medium may operate at wavelengths that differ by a fraction or several nm due to these and other factors. One skilled in the art will understand how to select appropriate first and second fundamental wavelengths to produce a desired output wavelength close to any of the fundamental wavelengths listed in the table. Similarly, if the desired output wavelength is a few nm away from 133 nm, a few nm away from 152 nm, a few nm away from 177 nm, or a few nm away from 193 nm, the desired output wavelength can also be achieved by appropriately adjusting the wavelength used for the first fundamental wavelength or the second fundamental wavelength.

[0083]

[0084]

[0085] Table 1

[0086] Although the present disclosure is described herein using various fundamental wavelengths that facilitate the generation of laser output light having a desired wavelength between approximately 120 nm and 200 nm, other wavelengths within a few nanometers or tens of nanometers of this desired wavelength can be generated by varying the wavelength of the first fundamental laser (i.e., laser assembly 200A). Unless otherwise specified in the appended claims, such lasers and systems utilizing such lasers are considered to be within the scope of the present disclosure.

[0087] Lasers with sub-200 nm wavelengths at sufficient power levels are not commercially available or are unreliable or expensive to operate. Periodically poled SBO and LBO crystals are not commercially available. In particular, there is no prior art for growing periodically poled crystals with high purity, high damage threshold, nonlinear coefficient, and high transparency in the sub-200 nm region from periodically poled seeds.

[0088] Lasers with sub-200 nm wavelengths at sufficient power levels are not commercially available or are unreliable or expensive to operate. Aside from excimer lasers, there is no prior art to generate 1 W or greater of optical power in the wavelength range between approximately 120 nm and 200 nm with a multi-month lifetime. Embodiments of the present disclosure generate wavelengths between 120 nm and 200 nm, thus providing better sensitivity for detecting small particles and defects than longer wavelengths. The lasers of the present disclosure do not use toxic or corrosive gases and are therefore easier and less expensive to operate and maintain.

[0089] Those skilled in the art will readily appreciate that in addition to using the laser crystals described herein in semiconductor inspection and metrology, there are many possible applications for the lasers. For example, a laser operating at a wavelength close to 193.4 nm can 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 approximately 120 nm and 200 nm can be used in a system configured to cut or ablate biological tissue. The lasers described herein can be configured to produce very short pulses at an output wavelength, which can enable material to be removed preferentially by ablation rather than heating, thereby causing less damage to surrounding materials. For example, such lasers can be used in laser eye surgery or laser vision correction. Although the present disclosure has been described with respect to certain specific embodiments, those skilled in the art will appreciate that the features of the present disclosure may also be applicable to other embodiments, all of which are intended to be within the scope of the present disclosure.

[0090] All methods described herein may include storing the result of one or more steps of method embodiments in a memory. The result may include any one of the results described herein and may be stored in any manner known in the art. The memory may include any memory described herein or any other suitable storage medium known in the art. After the result has been stored, the result may be accessed in the memory and used by any one of the methods or system embodiments described herein, formatted to be displayed to the user, used by another software module, method or system, or the like. In addition, the result may be stored "permanently", "semi-permanently", "temporarily" or for a period of time. For example, the memory may be a random access memory (RAM), and the result may not necessarily remain in the memory indefinitely.

[0091] It is further contemplated that each of the embodiments of the method described above may include any (any) other steps of any (any) other method described herein. In addition, each of the embodiments of the method described above may be performed by any of the systems described herein.

[0092] Those skilled in the art will recognize that the components, operations, devices, and articles described herein and the accompanying discussions are used as examples for the purpose of conceptual clarity, and that various configuration modifications are contemplated. Therefore, as used herein, the specific examples and accompanying discussions set forth are intended to represent their more general categories. In general, the use of any specific example is intended to represent its category, and the omission of specific components, operations, devices, and articles should not be considered limiting.

[0093] As used herein, directional terms such as "top," "bottom," "above," "below," "upper," "upward," "lower," "downward," and the like are intended to provide relative positions for purposes of description and are not intended to specify an absolute reference frame. Various modifications to the described embodiments will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments.

[0094] With respect to the use of substantially any plural and / or singular terms herein, those skilled in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. For clarity, the various singular / plural permutations are not explicitly set forth herein.

[0095] The subject matter described herein sometimes illustrates different components contained within or connected to other components. It should be understood that these depicted architectures are merely exemplary, and in fact many other architectures that achieve the same functionality can be implemented. In a conceptual sense, any component arrangement that achieves the same functionality is effectively "associated" so that the desired functionality is achieved. Therefore, any two components that are combined to achieve a specific functionality herein can be considered to be "associated" with each other so that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components that are so associated can also be considered to be "connected" or "coupled" to each other to achieve the desired functionality, and any two components that can be so associated can also be considered to be "coupleable" to each other to achieve the desired functionality. Specific examples of coupleable include, but are not limited to, components that can physically cooperate and / or physically interact, and / or components that can interact and / or interact wirelessly, and / or components that interact and / or interact logically.

[0096] In addition, it should be understood that the present disclosure is defined by the appended claims. Those skilled in the art will understand that, in general, the terms used herein and particularly in the appended claims (e.g., the bodies of the appended claims) are generally intended to be "open-ended" 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," etc.). Those skilled in the art will further understand that if it is intended that the described technical solution be recited as a specific number, such intention will be explicitly described in the technical solution, and in the absence of such description, such intention does not exist. For example, as an aid to understanding, the following appended claims may contain the use of the descriptive phrases "atleast one" and "one or more" to introduce the technical solution description. However, the use of such phrases should not be construed as implying that the introduction of a claim by the indefinite article "a" or "an" limits any particular claim containing such introduced claim to a disclosure containing only one such claim, even when the same claim includes the illustrative 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 definite articles to introduce claim claims. In addition, even if a specific number of introduced claim claims is explicitly recited, those skilled in the art will recognize that such recitation should generally be construed to mean at least the recited number (e.g., an explicit recitation of "two claims," without other modifiers, generally means at least two claims, or two or more claims). Furthermore, in those instances where a convention similar to “at least one of A, B, and C, etc.” is used, generally, such construction is intended to mean 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, both A and B, both A and C, both B and C, and / or both A, B, and C, etc.). In those instances where a convention similar to “at least one of A, B, or C, etc.” is used, generally, such construction is intended to mean 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, both A and B, both A and C, both B and C, and / or both A, B, and C, etc.).Those skilled in the art will further understand that any transitional word and / or phrase that essentially represents two or more alternative terms (whether in the specification, claims, or drawings) should be understood to encompass the possibility of including one, either, or both of the terms. For example, the phrase "A or B" will be understood to include the possibility of "A" or "B" or "A and B."

[0097] It is believed that the present disclosure and its many attendant advantages will be understood from the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of components without departing from the disclosed subject matter or sacrificing all of its essential advantages. The described form is illustrative only, and the appended claims are intended to cover and encompass such changes. Furthermore, it should be understood that the present disclosure is defined by the appended claims.

Claims

1. A method for growing a periodically poled nonlinear crystal, comprising: placing a periodically poled seed into a melt to form a seed-melt mixture, wherein the seed comprises at least one of strontium tetraborate (SBO) or lithium triborate (LBO), wherein the melt comprises at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O; and The seed melt mixture is heated and cooled to one or more predetermined temperatures until the periodically poled nonlinear crystal is formed.

2. The method of claim 1, wherein the melt is contained in a platinum crucible in a furnace, wherein the seed crystal is secured to an alumina tube.

3. The method of claim 2, wherein the melt comprises strontium carbonate and boron trioxide. The method according to claim 3 , wherein the predetermined temperature is between 995° C. and 1005° C.

5. The method of claim 3, wherein the boron trioxide acts as a self-fluxing agent and constitutes 67% of the melt.

6. The method of claim 1, wherein the melt comprises strontium hydroxide.

7. The method of claim 1 , wherein the melt comprises: Boron oxide is at least one of a self-flux agent or molybdenum trioxide.

8. The method according to claim 1, further comprising: polishing the crystal thin slab; and forming the periodically poled crystal by contacting the polished thin slabs together with alternating c-axis orientations, wherein the crystalline thin slab comprises at least one of strontium tetraborate or lithium triborate.

9. A periodically poled nonlinear seed crystal, comprising: a plurality of crystal plates disposed in a stacked configuration, wherein the plurality of crystal plates include at least a first crystal plate and a second crystal plate, wherein the first crystal plate is adjacent to the second crystal plate, wherein the plurality of crystal plates comprises at least one of one or more strontium tetraborate (SBO) plates or one or more lithium triborate (LBO) plates, The plurality of crystal plates are configured to form a periodic structure, wherein the periodic structure achieves quasi-phase matching (QPM) of light.

10. The crystal of claim 9, wherein a first crystal axis of the first crystal plate is reversed relative to a second crystal axis of the second crystal plate.

11. The crystal of claim 9, wherein the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength of 193 nm.

12. The crystal of claim 9, wherein the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength between 172 nm and 178 nm.

13. The crystal of claim 9, wherein the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength between 147 nm and 153 nm.

14. The crystal of claim 9, wherein the crystal plate thicknesses and orientations of the plurality of crystal plates are configured to achieve phase matching to produce a wavelength between 129 nm and 134 nm.

15. The crystal of claim 9, wherein the crystal plate thickness is an odd multiple of at least one of: 700nm to 860nm, 435nm to 620nm, 510nm to 690nm, 200nm to 380nm, 200nm to 320nm or 80nm to 175nm, The c-crystal axis of the first crystal plate is reversed relative to the c-crystal axis of the second crystal plate.

16. The crystal of claim 9, wherein the crystal plate thickness is an odd integer multiple of at least: 700nm to 920nm, 420nm to 646nm and 460nm to 730nm, The c-crystal axis of the first crystal plate is reversed relative to the c-crystal axis of the second crystal plate.

17. An optical system comprising: an illumination source configured to generate illumination having a wavelength between 120 nm and 200 nm; and an optical subsystem configured to direct the illumination from the illumination source onto a sample, Wherein the illumination source comprises: a first fundamental frequency laser configured to generate a fundamental frequency laser beam having a corresponding fundamental frequency and a fundamental wavelength between 720 nm and 800 nm; and two or more frequency doubling stages, the two or more frequency doubling stages including at least an intermediate frequency doubling stage and a final frequency doubling stage, the intermediate frequency doubling stage configured to receive the first fundamental frequency and generate second harmonic light having a second harmonic frequency, the final frequency doubling stage configured to output light from the second harmonic light generating laser, the final frequency doubling stage including a nonlinear crystal configured to double the frequency of the second harmonic light, wherein the nonlinear crystal comprises a plurality of crystal plates disposed in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, the plurality of crystal plates comprising at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates, and The plurality of crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) between the first fundamental frequency and the second harmonic frequency.

18. A laser assembly comprising: a first fundamental frequency laser configured to generate a fundamental frequency laser beam having a corresponding fundamental frequency and a fundamental wavelength between 720 nm and 800 nm; and two or more frequency doubling stages, the two or more frequency doubling stages including at least an intermediate frequency doubling stage and a final frequency doubling stage, the intermediate frequency doubling stage configured to receive the first fundamental frequency and generate second harmonic light having a second harmonic frequency, the final frequency doubling stage configured to output light from the second harmonic light generating laser, the final frequency doubling stage including a nonlinear crystal configured to double the frequency of the second harmonic light, wherein the nonlinear crystal comprises a plurality of crystal plates, the plurality of crystal plates being arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, the plurality of crystal plates comprising at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates, and wherein the plurality of crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the first fundamental frequency and the second harmonic frequency.

19. A method for growing a periodically poled nonlinear crystal, comprising: placing a periodically poled seed crystal in contact with a melt mixture from a platinum (Pt) nozzle connected to a Pt crucible containing the melt mixture, wherein the periodically poled seed crystal comprises at least one of strontium tetraborate (SBO) or lithium triborate (LBO), wherein the melt comprises at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O; and The periodically poled seed crystal is pulled away from the Pt nozzle at a predetermined speed while maintaining contact with the melt until the periodically poled nonlinear crystal is formed.

20. The method of claim 19, wherein the melt is contained in a platinum crucible in a furnace, wherein the melt is maintained at a predetermined temperature.

21. The method of claim 20, wherein the predetermined temperature is 995°C to 1005°C.

22. The method of claim 19, wherein the melt comprises stoichiometric Sr, B, and O.

23. The method of claim 19, wherein the melt comprises stoichiometric Li, B, and O.

24. The method of claim 19, wherein the nozzle comprises a narrow tube, a slit, or a die, wherein capillary action transports the melt from the Pt crucible to the seed crystal.

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