Frequency conversion using stacked strontium tetraborate plates

Abstract

To fill the need for a mode-locked or CW laser that generates radiation in the range shorter than 133 nm and is suitable for use in inspection of photomasks, reticles, and / or wafers.SOLUTION: A nonlinear crystal including stacked Strontium tetraborate SrB4O7 crystal plates is configured to create a periodic structure for quasi-phase matching and is used in a final frequency conversion stage of the laser assembly to generate laser output light having a wavelength in a range of 125 nm to 183 nm. One or more fundamental light beams having fundamental wavelengths between 1 and 1.1 μm are doubled and / or summed using a plurality of intermediate frequency conversion stages to generate one or more intermediate light beam frequencies, and then the final frequency conversion stage utilizes a nonlinear crystal to either double a single intermediate light beam frequency or to sum two intermediate light beam frequencies to generate a desired laser output light at high power and photon energy levels.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application relates to lasers capable of generating light having VUV wavelengths, and in particular to lasers capable of generating light in the range of approximately 125 nm to 183 nm, and to inspection systems using such lasers to inspect, for example, photomasks, reticles, and semiconductor wafers. [Background technology]

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 038,134, entitled "177 nm and 133 nm CW Lasers Using Stacked Strontium Tetraborate Plates," filed June 12, 2020, and incorporated herein by reference. This application further claims priority to U.S. Provisional Patent Application No. 63 / 076,391, entitled "152 nm and 177 nm CW Lasers Using Stacked Strontium Tetraborate Plates," filed September 10, 2020, and incorporated herein by reference.

[0003] This application is further related to the following U.S. patent documents, all of which are incorporated herein by reference: U.S. Patent No. 6,201,601 to Vaez-Iravani et al.; U.S. Patent No. 6,271,916 to Marxer et al.; U.S. Patent No. 7,525,649 to Leong et al.; U.S. Patent No. 7,817,260 to Chuang et al.; U.S. Patent Nos. 8,298,335 and 8,824,514 to Armstrong; U.S. Patent No. 8,976,343 to Genis; and U.S. Patent No. 6,824,514 to Dri. Nos. 9,461,435 and 9,059,560 to Dribinski et al.; U.S. Patent Nos. 9,293,882 and 9,660,409 to Chuang; U.S. Patent Nos. 9,250,178, 9,459,215, 9,509,112, 10,044,166, and 10,283,366 to Chuang et al.; and U.S. Patent Application No. 2014 / 0305367 published by Dribinski et al.

[0004] As semiconductor device dimensions shrink, the size of the smallest particle or pattern defect that can cause device failure also shrinks. Therefore, a need arises to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. The intensity of light scattered by a particle is smaller than the wavelength of the light and generally scales as 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). Due to the increased intensity of scattered light, shorter wavelengths generally have higher 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 intensities are required to generate signals that can be detected in very short periods of time. Average source power levels of 0.3 W or higher 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 system optics or the item being inspected. Illumination needs for inspection and metrology are generally best served by continuous-wave (CW) sources. Because CW sources have a constant power level, they avoid peak-power damage issues and allow for continuous acquisition of images or data. However, in some cases, mode-locked lasers with repetition rates of approximately 50 MHz or higher may be useful, as the high repetition rate means that the energy per pulse can be low enough to avoid damage for certain metrology and inspection applications.

[0006] Pulsed lasers for generating VUV light are well known in the art. Prior art lasers for generating light at 133 nm are well known (e.g., Non-Patent Document 1 and Non-Patent Document 2). Unfortunately, such lasers are not well suited for inspection applications due to the low repetition rate of the laser pulses and the low average power levels.

[0007] However, mode-locked and CW lasers with wavelengths in the VUV range are either not commercially available at sufficient power levels or are very unreliable. There was no prior art for generating mode-locked or CW light at powers greater than about 0.3 W in the wavelength range up to about 133 nm.

[0008] Pulsed sources have instantaneous peak power levels that are much higher than the time-average power levels of CW sources. The very high peak power of the laser pulses can damage the optical components and the sample or wafer being measured because most damage mechanisms are nonlinear and depend more strongly on peak power than on average power. The higher the pulse repetition rate, the lower the instantaneous peak power per pulse for the same time-average power level. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent Application Publication No. 2018 / 0188633 [Non-patent literature]

[0010] [Non-Patent Document 1] G.W. Faris and M.J. Dyer, "Two-photon excitation of neon at 133 nm," Optics Letters, Vol. 18, p. 382 (1993). [Non-patent document 2] A. Tu(umlaut)nnermann, 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, Vol. 29, p. 1233 (1993). Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, a need has arisen for mode-locked or CW lasers that generate radiation in the vacuum ultraviolet (VUV) range, particularly in the range below 133 nm, and that are suitable for use in photomask, reticle, and / or wafer inspection. Lasers capable of mode-locked or CW output near 133 nm and at higher power levels could potentially enable more accurate and faster inspection and measurement, contributing to cutting-edge semiconductor manufacturing.

[0012] Furthermore, a need arises to provide an inspection system and associated laser system that can generate mode-locked or CW laser light having an output VUV wavelength, such as in the range of about 125 nm to about 183 nm, that avoids some or all of the above problems and drawbacks. [Means for solving the problem]

[0013] The present invention generally relates to a nonlinear crystal comprising stacked strontium tetraborate (SrBO) crystal plates that form a periodic structure capable of achieving quasi-phase matching (QPM) suitable for frequency conversion of applied light, thereby facilitating the generation of DUV and VUV laser light at high power and photon energy levels while avoiding the problems and drawbacks described above associated with prior art approaches. SBO crystals exhibit attractive features (e.g., a wide transparency range, excellent damage tolerance and chemical stability, high microhardness, and a high diagonal element value d33 relative to the bandgap value), which avoid many of the problems and drawbacks described above associated with prior art approaches. However, SBO single crystals also exhibit low birefringence, making frequency conversion by critical or noncritical phase matching impossible. The present invention circumvents the low birefringence of SBO by cooperatively configuring SBO crystal plates to form a periodic structure that achieves QPM of one or more input optical frequencies (intermediate optical beams), such that the light exiting the nonlinear crystal contains laser output light having the desired DUV / VUV output frequencies. In one embodiment, the cooperative configuration involves physically stacking separate SBO crystal plates such that the crystal axes of successively arranged crystal plates are alternately inverted (i.e., the crystal axis of a given SBO crystal plate is rotated substantially 180° relative to the crystal axis of adjacent SBO crystal plate(s) that share a common interface(s) with the given plate in the stack), thereby forming a periodic structure similar to a periodically poled crystalline material (i.e., each SBO crystal plate forms a physical pole of the periodic structure). Each nonlinear crystal is further configured for use in a given optical system by orienting the SBO crystal plates so that the inverted crystal axis is aligned perpendicular to the polarization direction of the light as it passes through the SBO crystal stack in the optical system, and by forming the SBO crystal plates so that the thickness of at least one SBO crystal plate forms a spacing between the poles (i.e., the distance that light travels between the opposing surfaces of each plate in the direction of light propagation) that is substantially equal to an odd multiple of the critical length that enables quasi-phase matching of one or more input optical frequencies with the output frequency.By coordinating two or more SBO crystal plates in this manner, nonlinear crystals produced in accordance with the present invention facilitate the frequency conversion (e.g., frequency doubling of one input optical frequency or frequency summing of two or more input optical frequencies) required to generate DUV and VUV wavelengths at high power levels (e.g., from several milliwatts (mW) to several watts (W) or more) and high photon energy levels (e.g., 7.00 eV at 177 nm and 9.32 eV at 133 nm), while avoiding the problems and drawbacks described above associated with prior art approaches. While primarily described with particular reference to practical applications involving the generation of CW laser light, the nonlinear crystals disclosed herein can be used in other optical systems and for other purposes, including the generation of pulsed laser light, without departing from the spirit and scope of the present invention.

[0014] In the specifically disclosed embodiments described below, the present invention relates to improvements in inspection systems utilized in the semiconductor manufacturing industry, and in particular to laser assemblies for such inspection systems capable of generating mode-locked or continuous wave (CW) laser light having a source power level of 0.3 W or greater and an output wavelength ranging from about 125 nm to about 183 nm. In practical embodiments, each nonlinear crystal is utilized in a final frequency conversion stage of an associated laser assembly further comprising at least one fundamental laser and two or more intermediate frequency conversion stages, where each fundamental laser generates a fundamental optical beam having a corresponding fundamental frequency (e.g., having a wavelength between about 1 μm and 1.1 μm), and the intermediate frequency conversion stages are collectively configured to convert the fundamental optical beam(s) into at least one intermediate optical beam having an associated intermediate frequency. The final frequency conversion stage is configured to guide the intermediate light beam(s) through inverted SBO crystal plates forming a nonlinear crystal, so that the polarization direction (electric field direction) of the light is substantially parallel to the c-axis (or a-axis) of the crystal axis of each plate, and the periodic structure of the stacked SBO crystal plates achieves QPM of the intermediate light beam(s). In certain embodiments, the final frequency conversion stage includes multiple mirrors configured to receive and circulate (e.g., by one or more matching lenses) at least one of the intermediate light beams (e.g., in a bow-tie cavity configuration), so that a beam waist of the circulated light is generated at (i.e., inside or proximal to) the nonlinear crystal. In one embodiment, the final frequency conversion stage utilizes a beam splitter (e.g., SBO crystal, SBO glass, or CaF crystal) that splits the output light (i.e., light leaving / exiting the nonlinear crystal) so that a reflected (first) portion of the output light forms the desired laser output light beam having an output wavelength in the range of about 125 nm to about 183 nm, and an unreflected (second) portion of the output light, comprising unconsumed input light, is passed by the beam splitter for circulation by the cavity mirrors. Note that in the following description, when wavelength is referred to unconditionally, it may be assumed that the wavelength is the wavelength in a vacuum.

[0015] In a specifically disclosed embodiment, the present invention relates to an improved laser system for inspection systems utilized in the semiconductor manufacturing industry, particularly a laser assembly for such inspection systems, having a source power level of 0.3 W or greater and capable of generating laser light having an output wavelength in the range of about 128 nm to about 134 nm (e.g., about 133 nm), about 147 nm to about 155 nm (e.g., about 152 nm), or about 170 nm to about 180 nm (e.g., about 177 nm). In some specific embodiments disclosed herein, the nonlinear crystal includes SBO crystal layers cooperatively configured to frequency-double a single intermediate light beam having a UV wavelength near 355 nm or a DUV wavelength near 266 nm to generate laser light having a VUV wavelength near 177 nm or near 133 nm, respectively. In another embodiment disclosed herein, the linear crystal includes SBO crystal layers cooperatively configured to frequency-sum two intermediate light beams to generate laser light having a desired VUV wavelength. For example, in one embodiment disclosed herein, the linear crystal includes SBO crystal layers cooperatively configured to frequency-sum a first intermediate optical beam having a UV wavelength near 355 nm with a second intermediate optical beam having a deep UV wavelength near 266 nm to generate laser light having a VUV output wavelength near 152 nm. In another embodiment disclosed herein, the linear crystal includes SBO crystal layers cooperatively configured to frequency-sum a first intermediate optical beam having a visible wavelength near 532 nm with a second intermediate optical beam having a deep UV wavelength near 213 nm to generate laser light having a VUV wavelength near 152 nm. In yet another alternative embodiment, the linear crystal includes SBO crystal layers cooperatively configured to frequency-sum a first intermediate optical beam having a visible wavelength near 532 nm with a second intermediate optical beam having a deep UV wavelength near 266 nm to generate CW laser light having a VUV output wavelength near 177 nm.In certain embodiments, a beam splitter is utilized to direct (pass) the selected interharmonic frequency back to the final frequency conversion cavity for circulation and to redirect (reflect) the desired output frequency away from the laser assembly, and the crystal plates are formed with corresponding thicknesses such that the inter-pole spacing in each periodic structure (i.e., the distance light travels between the opposing surfaces of each plate) is substantially equal to an odd integer multiple of the associated QPM critical length.

[0016] With reference to a first specific embodiment, in accordance with the laser assembly and associated methods described herein, laser output light having an output frequency with a wavelength of approximately 133 nm is generated by generating fundamental light having a fundamental frequency with a corresponding fundamental wavelength in a range of approximately 1000 nm to approximately 1100 nm, using the fundamental light to generate a second harmonic of the fundamental light, using the second harmonic to generate a fourth harmonic of the first fundamental light, and using the fourth harmonic as intermediate light and passing it to a final frequency conversion stage. According to one aspect of the first embodiment, the final frequency conversion stage is configured to frequency double the fourth harmonic light, for example, by configuring the stage to include a cavity resonating at the fourth harmonic frequency and configuring a linear crystal to generate eighth harmonic light having a frequency equal to eight times the fundamental frequency. In some embodiments, the final frequency conversion stage utilizes a beam splitter to reflect the eighth harmonic portion of the light exiting the linear crystal as laser output light and pass the unconsumed fourth harmonic portion of the light exiting the linear crystal for circulation in the final stage. To generate eighth harmonic output light at approximately 133 nm, the linear crystal includes two or more stacked SBO crystal plates with inverted crystal axes oriented substantially parallel to the polarization direction of the fourth harmonic input light, where the thickness of each plate in the light propagation direction (i.e., the spacing between the poles of the periodic structure) is substantially equal to an odd integer multiple of the quasi-phase-matching critical length, approximately equal to 0.13 μm (i.e., in the range of 0.11 μm to 0.15 μm), to achieve QPM of the fourth and eighth harmonic frequencies, thereby generating laser output light with an output wavelength of approximately 133 nm.

[0017] With reference to a second specific embodiment, in accordance with the laser assembly and associated methods described herein, laser output light having an output frequency having a wavelength of approximately 177 nm is generated by generating a first fundamental light having a first fundamental frequency, utilizing the first fundamental light to generate a second harmonic of the first fundamental light, summing the second harmonic of the first fundamental light with a second fundamental light having a second fundamental frequency, and utilizing the summing product as intermediate light that is passed to a final frequency conversion stage. In one embodiment, the first and second fundamental frequencies each have a corresponding wavelength within a range of approximately 1000 nm to approximately 1100 nm, such that the wavelength of the intermediate light beam (i.e., the summing product) is approximately equal to the third harmonic of the first fundamental frequency. According to one aspect of the second embodiment, approximately sixth-harmonic output light can be generated from the intermediate (approximately third-harmonic) light beam by configuring the final frequency conversion stage as a frequency doubling cavity resonating at the third-harmonic frequency and configuring stacked SBO crystal plates of linear crystals with a pole-to-pole spacing substantially equal to twice an odd multiple of the quasi-phase-matching critical length substantially equal to 0.60 μm (i.e., in the range of 0.59 μm to 0.61 μm) to achieve QPM of the third-harmonic frequency and the sixth-harmonic frequency, thereby generating laser output light with an output wavelength of approximately 177 nm.

[0018] According to a third specific embodiment, laser output light is generated with an output wavelength of approximately 152 nm by configuring a final frequency conversion stage to sum the third and fourth harmonics of the fundamental frequency to generate a seventh harmonic of the fundamental frequency. In this case, the third harmonic frequency is generated using two (first and second) fundamental light beams having substantially equal (first and second) fundamental frequencies, using the first fundamental frequency to generate a second harmonic and then summing a first portion of the second harmonic with the second fundamental frequency, and the fourth harmonic is generated by doubling the second portion of the second harmonic. The final frequency conversion (summing) stage may be configured as a cavity resonating at the third harmonic frequency, which serves as the first intermediate light beam circulated through the nonlinear crystal. The fourth harmonic acts as a second intermediate light beam that is provided directly to the input face of the nonlinear crystal, so that both the third and fourth harmonic lights pass along parallel paths through the nonlinear crystal. According to one aspect of the third embodiment, approximately seventh harmonic output light having an output wavelength of approximately 152 nm is generated by configuring stacked SBO crystal plates of the linear crystal so that the spacing between the poles of the periodic structure is based on a critical length substantially equal to 0.30 μm (i.e., in the range of 0.29 μm to 0.31 μm).

[0019] According to a fourth specific embodiment, laser output light is generated with an output wavelength of approximately 152 nm by configuring a final frequency conversion stage to sum the second and fifth harmonics of the fundamental frequency. A first frequency doubling stage is utilized to generate second-harmonic light having a frequency twice the fundamental frequency, and a first portion of this second-harmonic light forms a first intermediate light beam that is provided to the final frequency conversion (summing) stage. The fifth-harmonic light forms a second intermediate light beam that is provided to the final frequency conversion (summing) stage and is generated by frequency doubling a second portion of the second-harmonic light to generate fourth-harmonic light and then summing the fourth-harmonic light with the fundamental frequency. The final frequency conversion (summing) stage may include a cavity configured to resonate at the second-harmonic frequency, and the fifth-harmonic light is directed parallel to the second-harmonic light through an input facet of the nonlinear crystal. According to one aspect of the fourth embodiment, near-seventh harmonic output light having an output wavelength of about 152 nm is generated by configuring linear crystal stacked SBO crystal plates such that the spacing between the poles of the periodic structure is based on a critical length substantially equal to 0.34 μm (i.e., in the range of 0.33 μm to 0.35 μm).

[0020] According to a fifth specific embodiment, laser output light is generated with an output wavelength of approximately 177 nm by configuring a final frequency conversion stage to sum the second and fourth harmonics of a fundamental frequency. A first frequency doubling stage is used to generate a first intermediate light beam having a frequency substantially equal to twice the fundamental frequency. A second frequency doubling stage is used to generate second harmonic light having a frequency that is twice the fundamental frequency, and a third frequency doubling stage is used to receive the second harmonic light and generate fourth harmonic light (a second intermediate light beam) having a frequency that is four times the fundamental frequency. The final frequency conversion (summing) stage may include a cavity configured to resonate at the second harmonic frequency, and the fourth harmonic light is directed parallel to the second harmonic through an input face of the nonlinear crystal. According to one aspect of the fifth embodiment, near sixth harmonic output light having an output wavelength of about 177 nm is generated by constructing linear crystal stacked SBO crystal plates with a critical length substantially equal to 0.66 μm (i.e., in the range of 0.65 μm to 0.67 μm).

[0021] 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 generates an output wavelength of about 177 nm, 152 nm, or about 133 nm. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a simplified block diagram illustrating an exemplary laser assembly in accordance with a generalized exemplary embodiment of the present invention. [Figure 2A] FIG. 1 is a simplified block diagram illustrating a simplified laser assembly according to a first embodiment of the present invention. [Figure 2B] FIG. 1 is a simplified block diagram illustrating a simplified laser assembly in accordance with certain embodiments of the present invention. [Figure 3]1 is a simplified diagram illustrating an exemplary final frequency doubling stage utilized in the laser assemblies of the first and second specific embodiments, in accordance with exemplary embodiments of the present invention. FIG. [Figure 4] 4 is a simplified diagram illustrating an exemplary nonlinear crystal configured for use in the final frequency doubling stage of FIG. 3. [Figure 5A] FIG. 10 is a simplified block diagram illustrating a simplified laser assembly according to a third specific embodiment of the present invention. [Figure 5B] FIG. 10 is a simplified block diagram illustrating a simplified laser assembly according to a fourth specific embodiment of the present invention. [Figure 5C] FIG. 10 is a simplified block diagram illustrating a simplified laser assembly according to a fifth specific embodiment of the present invention. [Figure 6] 10 is a simplified diagram illustrating an exemplary final frequency doubling stage utilized in the third, fourth, and fifth specific embodiment laser assemblies, according to exemplary embodiments of the present invention. [Figure 7] 9 is a simplified diagram illustrating an exemplary nonlinear crystal configured for use in the final frequency doubling stage of FIG. 8. [Figure 8] FIG. 1 is a simplified diagram illustrating an exemplary inspection system with dark-field and bright-field inspection modes utilizing one of the laser assemblies described herein, in accordance with another specific embodiment of the present invention. [Figure 9A] FIG. 1 illustrates a dark field inspection system utilizing one of the laser assemblies described herein, in accordance with another specific embodiment of the present invention. [Figure 9B] FIG. 1 illustrates a dark field inspection system utilizing one of the laser assemblies described herein, in accordance with another specific embodiment of the present invention. [Figure 10] FIG. 10 illustrates an alternative dark-field inspection system configured to inspect an unpatterned wafer using one of the laser assemblies described herein, in accordance with another specific embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention relates to an improvement in a laser for a semiconductor inspection system. The following description is presented to enable one skilled in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as "top," "left," "right," "horizontal," and "downward" are intended to provide relative positions for purposes of description and are not intended to indicate an absolute coordinate system. 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. Thus, the present invention 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.

[0024] The second-order susceptibility variation of acentric crystals leads to modifications of the quasi-phase matching (QPM) conditions, which can be useful for frequency conversion. In the VUV spectral region below about 150 nm, transparent optical crystals that combine non-zero second-order nonlinearity with sufficient birefringence are still not evident. Several attempts to fabricate QPM structures have been reported, for example, by electric field polarization of ferroelectric BaMgF4 with mm2 symmetry (E.G. Villora, K. Shimamura, K. Sumiya, and H. Ishibashi, "Birefringent- and quasi phase-matching with BaMgF4 for vacuum-UV / UV and mid-IR all solid-state lasers," Optics Express, Vol. 17, p. 12362 (2009)) or by mechanical twinning of crystalline quartz (SiO2) with trigonal 32 symmetry (S. Kurimura, M. Harada, K. Muramatsu, M. Ueda, M. Adachi, T. Yamada, and T. Ueno, "Quartz revisits nonlinear optics: twinned crystal for quasi-phase matching [Invited]," Optical Materials Express, Vol. 1, p. 1367 (2011)), both materials exhibit low nonlinear coefficients and the shortest wavelength demonstrated so far is 194 nm.

[0025] Strontium tetraborate, SrBO (SBO), crystallizes in the orthorhombic system, point group mm, space group Pnm, with unit cell dimensions a = 4.4255 Å, b = 10.709 Å, and c = 4.2341 Å (YS Oseledchik, AL Prosvirnin, AI Pisarevskiy, VV Starshenko, VV O Sadchuk, SP Belokrys, NVS Vitanko, A.S. Korol, S.A. Krikunov, and A.F. Selevich, "New nonlinear optical crystals: strontium and lead tetraborates," Optical Materials, Vol. 4, p. 669 (1995)). All boron atoms are tetrahedrally coordinated, and oxygen atoms are common to three tetrahedra. Despite the three-dimensional network of tetrahedra, the borate network appears as a layered structure due to the relatively few links in the c direction of the unit cell.

[0026] SrBO exhibits very small birefringence (<0.005) and is not ferroelectric. Non-phase-matched second-harmonic generation (SHG) has been implemented using SBO for diagnostic purposes, but the efficiency is very low when using only one coherence length, and the practical detection limit was estimated to be 2 μJ at 267 nm for a 120 fs pulse (V. Petrov, F. Noack, D. Shen, F. Pan, G. Shen, X. Wang, R. Komatsu, and V. Alex, "Application of the nonlinear crystal SrBO for ultrafast diagnostics converting to wavelengths as short as 125 nm," Optical Letters, Vol. 29, p. 373 (2004)).

[0027] SBO exhibits unique optical and mechanical properties. Its transparency ranges from 130 to 3200 nm in wavelength (Y.S.O.Seledchik et al., op. cit.). SBO also exhibits high diagonal element values ​​d33 (1.5 to 3.5 pm / V) (compared to the bandgap value). Its optical damage threshold is very high (14.7 GW / cm) compared to other materials such as MgF2. 2 ) The microhardness of SBO is also high (1750 kg / mm ​​in the x direction). 2 , 1460 kg / mm ​​in the y direction 2 , 1350 kg / mm ​​in the z direction 2 ). The high optical damage threshold and microhardness allow SBO crystals to withstand the extreme conditions of exposure to DUV and VUV radiation. DUV and VUV lasers may have high power levels, from several milliwatts (mW) to several watts (W) or more, and high photon energies (e.g., 9.32 eV at 133 nm and 8.16 eV at 152 nm). The wide transparency range, excellent damage tolerance and chemical stability, and high diagonal element value d33 are features that make SBO very attractive for frequency conversion to generate DUV and VUV wavelengths. However, the low birefringence means that frequency doubling by critical or noncritical phase matching is not possible.

[0028] Trabs et al. (P. Trabs, F. Noack, A.S. Leksandrovsky, A.I. Zaitsev, N.V. Radionov, and V. Petrov, "Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV," Optics Express, Vol. 23, p. 10091 (2015)) reported the generation of second-harmonic waves from ultrashort laser pulses in the VUV via random quasi-phase matching using an SBO crystal. The second-harmonic generation method described by Trabs et al. is unsuitable for semiconductor metrology and inspection systems because the efficiency of the frequency conversion process is low, making it impractical to generate many watts of second-harmonic laser power using this method, and because ultrashort laser pulses are required.

[0029] 1 shows a laser assembly 100 for generating a laser output light beam 139 having an output frequency ω with a corresponding wavelength in the range of about 125 nm to about 183 nm. The laser assembly 100 generally includes one or more fundamental lasers 110, two or more intermediate frequency conversion stages 120, and a final frequency conversion stage 130.

[0030] Referring to the upper left portion of FIG. 1, fundamental lasers 110 are configured to generate fundamental optical beams 119-1, 119-2...119-n (collectively designated as 119), each having a corresponding fundamental frequency ω1 through ωn, with each frequency having a corresponding fundamental wavelength between approximately 1 μm and 1.1 μm. In some embodiments, all fundamental optical beams 119 have substantially the same wavelength (e.g., fundamental frequency ω1 is substantially equal to fundamental frequency ω2). Specific fundamental laser types are noted in the specific embodiments provided below.

[0031] The intermediate frequency conversion stages 120 are optically coupled to receive one or more fundamental optical beams 119 (or light from associated intermediate frequency conversion stages) and are collectively configured to generate one or more intermediate optical beams 129. In some specific embodiments, the intermediate optical beams 129 include a single (first) intermediate optical beam 129-1 having an associated intermediate frequency ωX. In other specific embodiments, the intermediate optical beams 129 include both the intermediate optical beam 129-1 and a second intermediate optical beam 129-2 having an associated intermediate frequency ωy. FIG. 1 is not intended to limit the scope of the appended claims to require that all intermediate frequency conversion stages 120 receive the fundamental optical beam 119. For example, in the specific example below, a given “downstream” intermediate frequency conversion stage can receive second-, third-, or fourth-harmonic light generated by one or more “upstream” intermediate frequency conversion stages, where the upstream intermediate frequency conversion stages are optically coupled between the fundamental laser 110 and a given downstream stage.

[0032] 1, laser assembly 100 also includes a final frequency conversion stage 130, which passes intermediate optical beam 129 (ωX or ωX and ωy) through a nonlinear crystal 135 and directs laser output optical beam 139 out of laser assembly 100 for use in, for example, one or more of the inspection systems described below with reference to FIGS. 8-10. In one embodiment, intermediate optical beam 129-1 has a frequency ωx, as described in the specific embodiment below, and enters a bow-tied cavity formed by input coupler mirror 132-1, flat mirror 132-2, two curved mirrors 132-3 and 132-4, SBO linear crystal 135, and beam splitter 137. For purposes of illustration, the portion of light transmitted by the bow-tiling cavity from input / coupler mirror 132-1 to linear crystal 135 is shown as circulating light portion 133, which is composed of both intermediate light beam 129-1 and unconsumed circulating light portion 138-1 (generated as described below), where both light portions 133 and 138-1 have frequency ω. In one embodiment, mode-matching lens 131 is utilized to focus intermediate light beam 129-1 through input coupler / mirror 132-1, and the bow-tiling cavity formed by mirrors 132-1 through 132-4 is otherwise configured such that light portion 133 is directed onto nonlinear crystal 135 at a selected angle θ relative to surface normal N (i.e., perpendicular to input face 135-IN), and such that the beam waist of light portion 133 (i.e., including intermediate light beam 129-1) is generated at (i.e., inside or adjacent to) nonlinear crystal 135. When an intermediate light beam 129-2 having a frequency ωy is used as described in the relevant specific embodiment below, the intermediate light beam 129-2 passes through the linear crystal 135, substantially directed at a selected angle θ onto the input face 135-IN by entering the bow-tiling cavity passing near (but not necessarily through) the curved mirror 132-3.As shown in this exemplary arrangement, the final frequency conversion stage 130 is configured to pass intra-crystal light 134 (i.e., only light portion 133, or both light portion 133 and intermediate light beam 129-2) through the nonlinear crystal 135 and direct output light 136 (i.e., all light exiting the linear crystal 135) to an input face 137-IN of a beam splitter 137. The beam splitter 137 is configured to split the output light 136 so that unconsumed input light 138-1 having a frequency ωx is passed to mirror 132-4 to recirculate within the bowtie cavity and laser output light 139 having an output frequency ωout is directed out of the laser assembly 100. As shown, in some embodiments, the beam splitter 137 is also configured to reflect unconsumed input light 138-1 having a frequency ωy out of the bowtie cavity. The beam splitter 137 may be implemented using one of SBO single crystal, SBO glass, or CaF2 crystal.

[0033] The SBO crystal plates 135-1 and 135-2 are configured to cooperate to form a periodic structure that achieves quasi-phase matching (QPM) of the intermediate light beam 129 with the laser output 139 (i.e., between ω and either only the frequency ω as shown, or both the frequencies ω and ω as illustrated in some of the specific examples below), so that the light portion 136 exiting the output face 135-OUT of the nonlinear crystal 135 comprises the laser output light beam 139 with the desired output frequency ω. Referring to the bottom bubble section of Figure 1, the SBO crystal plates 135-1 and 135-2 are configured such that the crystal axes a1-b1-c1 of the SBO crystal plate 135-1 are inverted (i.e., rotated substantially 180°) with respect to the crystal axes a2-b2-c2 of the adjacent SBO crystal plate 135-2. In addition, the nonlinear crystal 135 is configured within the final frequency conversion stage 130, and one or both intermediate light beams 129 propagate in a direction parallel to the a-axes of both SBO crystal plates (i.e., parallel to the axis a1 of the SBO crystal plate 135-1 and parallel to the axis a2 of the SBO crystal plate 135-2), and the spacing between the poles of at least one SBO crystal plate (i.e., either the spacing Λ1, which is the distance that light travels between the opposing surfaces of the SBO crystal plate 135-1, or the spacing Λ2, which is the distance that light travels between the opposing surfaces of the SBO crystal plate 135-2) is determined by:

[0034] Λ=mLc (Equation 1) where m is an odd integer (1, 3, 5, 7, etc.) and the quasi-phase-matching critical length Lc=π / Δk′, where Δk is defined as:

[0035] Δk=k(ωOUT)-k(ωx)-k(ωy) (Equation 2) where k(ω) is the wave vector of light of frequency ω in nonlinear crystal 135. In embodiments where only intermediate light beam 129-1 is present, ωy in this equation must be replaced by ωx, i.e.,

[0036] Δk=k(ωOUT)-2k(ωx) (Equation 3). The spacing between poles in each periodic structure is sometimes referred to herein as a thickness, because spacing Λ1 is substantially equal to the physical thickness T1 of SBO crystal plate 135-1, and spacing Λ2 is substantially equal to the physical thickness T2 of SBO crystal plate 135-2. Note that thicknesses T1 and T2 are measured parallel to the light propagation direction of intra-crystal optical portion 134 between opposing plate surfaces. In one embodiment, nonlinear crystal 135 is produced by polishing a large SBO plate to the desired thickness, then dividing it into individual smaller pieces, and assembling the pieces in the correct orientation (described below) relative to each other to form stacked SBO crystal plates, with the connection between adjacent SBO crystal plates being achieved by optically contacting their polished surfaces together. In this case, all SBO crystal plates forming a given nonlinear crystal have the same thickness (e.g., thickness T1 is equal to thickness T2), and therefore the stacked crystal plates form a periodic structure in which the spacing between each pole is the same (e.g., spacing Λ1 is substantially equal to spacing Λ2). In another alternative embodiment, the crystal axes of the SBO crystal plates 135-1 and 135-2 can be oriented so that light 134 propagates parallel to the b-axis or at an angle within the ab-plane of the two crystal plates.

[0037] 2A is a simplified block diagram illustrating an exemplary laser assembly 100A configured to generate a wavelength in the range of about 128 nm to about 134 nm (e.g., about 133 nm) according to a first specific exemplary embodiment of the present invention. The laser assembly 100A includes a first fundamental laser 110A and three frequency doubling (conversion) stages (i.e., two intermediate frequency doubling stages 120A-1 and 120A-2 and a final frequency doubling stage 130A), which are cooperatively configured to generate laser output light having a wavelength in the range of about 128 nm to about 134 nm. The first fundamental laser 110A is configured to generate fundamental light 119A, which has a first fundamental wavelength in the range of about 1000 nm to about 1100 nm (i.e., between about 1 μm and 1.1 μm) and a corresponding first fundamental frequency ω1. The first intermediate frequency doubling stage 120A-1 receives the first fundamental light 119A and generates second harmonic light 121A having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The second intermediate frequency doubling stage 120A-2 receives the second harmonic light 121A and generates intermediate light beam 129A as fourth harmonic light having a fourth harmonic frequency 4ω1 equal to four times the first fundamental frequency ω1. The final (third) frequency doubling stage 130A receives the fourth harmonic light (intermediate light beam) 129A and generates laser output light 139A having an output frequency ωOUTA equal to eight times the first fundamental frequency ω1.

[0038] Referring to FIG. 2A , the first fundamental laser 110A is configured to generate a first fundamental light 119A (simply referred to in the industry as a “fundamental wave”) at a first fundamental frequency ω1 using well-known techniques. In one embodiment, the first fundamental laser 110A is configured such that the first fundamental light 119A is generated at a first fundamental frequency ω1 corresponding to an infrared wavelength of approximately 1064 nm. In an exemplary embodiment, the first fundamental laser 110A is implemented using one of a Nd:YAG (neodymium-doped yttrium aluminum garnet) laser medium, a Nd-doped yttrium orthovanadate (Nd:YVO4) laser medium, or an ytterbium-doped fiber laser medium. Suitable fundamental lasers are commercially available from Coherent Inc., IPG Photonics Corporation, and other manufacturers. Such manufacturers also sell lasers that generate light having wavelengths around 532 nm, i.e., the laser includes a first fundamental laser 110A and a first frequency doubling stage 120A-1. To generate enough light at a wavelength of about 133 nm for inspecting semiconductor wafers or reticles, the first fundamental laser 110A must generate tens or hundreds of watts or more of fundamental light 119A.

[0039] According to the exemplary embodiment of FIG. 2A, each of frequency doubling stages 120A-1 and 120A-2 comprises an external resonant cavity, each including at least three optical mirrors and a nonlinear crystal disposed therein. The cavity may be stabilized using standard Pound-Drever-Hall (PDH), Hensch-Couillaud (HC), or other locking techniques. The cavity length is adjusted to maintain resonance by adjusting the position of the mirrors or prisms via a control signal. The first frequency doubling stage 120A-1 receives and converts first fundamental light 119A at a first fundamental frequency ω1 to generate second-harmonic light 121A at twice the first fundamental frequency (2ω1). The second frequency doubling stage 120A-2 receives and converts the second harmonic light 121A to generate fourth harmonic light 129A at four times the first fundamental frequency (4ω1).

[0040] In some alternative embodiments (not shown), a first frequency doubling module may be coupled to the first fundamental laser to generate second harmonic light 121A using intracavity frequency doubling with an NLO crystal placed inside the fundamental solid-state laser cavity.

[0041] 2A, which generates the second-harmonic light 121A, may include a lithium triborate (LBO) crystal, which may be substantially non-critically phase-matched (due to appropriate selection of crystal planes) at temperatures between room temperature and about 200° C. to generate second-harmonic light in the wavelength range between about 515 nm and about 535 nm. In alternative embodiments, the first frequency doubling stage 120A-1 may include a cesium lithium borate (CLBO) crystal or a beta barium borate (BBO) crystal, either of which may be critically phase-matched to generate second-harmonic light in the wavelength range between about 515 nm and about 535 nm. In another alternative embodiment, the first frequency doubling stage 120A-1 may include KTiOPO4 (KTP), periodically poled lithium niobate (PPLN), periodically poled lithium tantalate (PPSLT), or other nonlinear crystals for frequency conversion.

[0042] The second frequency doubling stage 120A-2, which generates the fourth harmonic, may use critical phase matching in CLBO, BBO, or other nonlinear crystals. In a preferred embodiment, the second frequency doubling stage 120A-2 includes a hydrogen- or deuterium-treated CLBO crystal.

[0043] In an alternative embodiment, the second frequency doubling stage 120A-2, which generates the fourth harmonic, may use quasi-phase matching (QPM) in stacked SBO plates configured as shown in FIG. 4 below. The critical length of QPM for generating 532 nm to 266 nm in SBO is approximately 2.80 μm (i.e., in the range of 2.79 μm to 2.90 μm). Because the critical length is longer than the critical length for generating shorter wavelengths, the thickness of the SBO plate in the light propagation direction (Λ in FIG. 4) may be equal to the critical length or a small odd integer (e.g., 3 to 19) times the critical length.

[0044] Further details on how the fourth harmonic of a CW fundamental IR laser can be generated with high power, low noise, and high stability can be found in U.S. Patent Nos. 9,293,882 and 9,660,409 to Chuang and 9,509,112 and 10,044,166 to Chuang et al., which are incorporated herein by reference.

[0045] Referring to FIG. 2A , the final frequency doubling stage 130A receives the fourth harmonic light 129A and generates eighth harmonic light 139A having an eighth harmonic frequency 8ω1 equal to eight times the first fundamental frequency ω1. In a preferred embodiment, the final frequency doubling stage 130A of FIG. 2A, which generates the eighth harmonic light 139A, may include two or more SBO crystal plates configured for quasi-phase matching (QPM). For example, in the case of two SBO crystal plates, the crystal plates are rotated 180° relative to each other so that their crystal axes are inverted relative to each other. This physical arrangement of the crystal plates enables QPM. This can be considered similar to using PPLN (periodically poled lithium niobate) for QPM, except that lithium niobate is a ferroelectric crystal and can be periodically poled. In contrast, SBO is non-ferroelectric, so the crystal plates must be physically arranged to create the periodic structure for QPM. Furthermore, because periodic poling requires applying an electric field parallel to the crystallographic axes of the ferroelectric crystal, the polarization direction necessarily coincides with the crystallographic axes. In contrast, the SBO crystal plates disclosed herein can be cut and polished in any direction relative to the crystallographic axes, allowing the crystal plates to be cut and oriented, for example, at the Brewster angle relative to the light incident on the plate. See Figures 4 and 7 and the related discussion below.

[0046] Any of the frequency conversion stages may be enclosed in one or more protective environments, such as those described in U.S. Patent No. 8,298,335 by Armstrong, entitled "Enclosure for controlling the environment of optical crystals," which is incorporated herein by reference. In particular, because the final frequency doubling stage 130A produces VUV wavelengths, this stage must be in an environment with very low concentrations of oxygen and water (preferably concentrations of a few ppm or less). Preferably, the final frequency doubling stage is maintained in an environment purged with pure nitrogen or argon. It should be noted that a single protective environment can surround multiple stages or a single stage.

[0047] Any of the frequency conversion stages may incorporate any of the methods or systems described in U.S. Patent Nos. 9,461,435 and 9,059,560, entitled "Alleviation of laser-induced damage in optical materials by suppression of transient color center formation and control of phonon population," to Dribinski et al.; any of the apparatus or methods described in U.S. Patent No. 8,824,514, entitled "Measuring crystal site lifetime in a non-linear optical crystal," to Armstrong; or any of the apparatus and methods described in U.S. Patent No. 8,976,343, entitled "Laser crystal degradation compensation," to Genis, all of which are incorporated herein by reference.

[0048] Additionally, it should be noted that any of the intermediate frequency conversion stages described herein may advantageously use nonlinear crystals doped or treated with deuterium, hydrogen, and / or fluorine. Such crystals may be made, fabricated, or treated by any of the processes or methods described in U.S. Patent No. 9,023,152 to Dribinski, U.S. Patent Nos. 9,250,178, 9,459,215, and 10,283,366 to Chuang et al., and published U.S. patent application Ser. No. 2014 / 0305367, entitled "Passivation of Nonlinear Optical Crystals," filed April 8, 2014, by Dribinski et al. These patents and applications are incorporated herein by reference. Doped or treated crystals may be particularly useful in stages involving deep UV wavelengths, including the second frequency doubling stage 120A-2 of FIG. 2A.

[0049] 2B is a simplified block diagram illustrating an exemplary laser assembly 100B configured to generate a wavelength in the range of about 170 nm to about 180 nm (e.g., about 177 nm) according to a second specific embodiment of the present invention. The laser assembly 100B includes a first fundamental laser 110B-1, a second fundamental laser 110B-2, a frequency doubling (conversion) stage 120B-1, a frequency summation (conversion) stage 120B-2, and a final frequency doubling stage 130B, collectively configured to generate laser output light 139B with an output frequency ω having a wavelength in the range of about 170 nm to about 180 nm. The first fundamental laser 110B-1 is configured to generate fundamental light 119B-1, which has a first fundamental wavelength in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to 1.1 μm) and a corresponding first fundamental frequency ω. The second fundamental laser 110B-2 is configured to generate fundamental light 119B-2, which has a second fundamental wavelength in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to 1.1 μm) and a corresponding second fundamental frequency ω2. The frequency doubling stage 120B-1 receives the first fundamental light 119B-1 and generates second harmonic light 121B having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The frequency summing stage 120B-2 sums the second harmonic 121B with the second fundamental light 119B-2 to generate an intermediate light beam 129B having a sum frequency 2ω1 + ω2. When the first fundamental laser 110B-1 and the second fundamental laser 110B-2 have the same frequency (ω1 = ω2), the intermediate light beam 129B is the third harmonic (3ω1 or 3ω2) of the fundamental light. The final frequency doubling stage 130B receives the intermediate light beam 129B and generates final output light 139B having an output frequency ωOUTB equal to twice the total frequency 2ω1 + ω2, i.e., 4ω1 + 2ω2. When the first fundamental laser 110B-1 and the second fundamental laser 110B-2 have the same frequency (ω1 = ω2), the output frequency ωOUTB of the final laser output light 139B is the sixth harmonic (6ω1 or 6ω2) of the fundamental light.

[0050] 2B, first and second fundamental lasers 110B-1 and 110B-2 are configured as described above with reference to fundamental laser 110A of FIG. 2A. In an alternative embodiment, second fundamental laser 110B-2 may be omitted, and the output of first fundamental laser 110B-1 may be split into two portions: a first portion directed to first frequency doubling stage 120B-1, and a second portion directed to frequency summation stage 120B-2 along with second-harmonic light 121B. In this alternative embodiment, ω2 = ω1.

[0051] According to the exemplary embodiment of FIG. 2B, first frequency doubling stage 120B-1 is configured as described above with reference to stages 120A-1 and 120A-2 of FIG. 2A.

[0052] In one embodiment, the frequency summation stage 120B-2 sums the second harmonic 121B with the second fundamental light 119B-2 using a lithium triborate (LBO) crystal, a cesium lithium borate (CLBO) crystal, or a beta barium borate (BBO) crystal.

[0053] In a preferred embodiment, the final frequency doubling stage 130B includes two or more SBO crystal plates configured for quasi-phase matching (QPM) in a manner similar to that described above with reference to the final frequency doubling stage 130A of Figure 2A. The differences between the final frequency doubling stages 130A and 130B are described below with reference to Figures 3 and 4.

[0054] 3 is a simplified diagram illustrating an exemplary final frequency doubling stage 130C utilized in the 133 nm laser assembly 100A of FIG. 2A and the 177 nm laser assembly 100B of FIG. 2B, in accordance with an exemplary embodiment of the present invention. Input light 129C having a frequency ω (e.g., ω=4ω when stage 130C is used in the 133 nm laser 100A, or ω=2ω+ω when stage 130B is used in the 177 nm laser 100B) enters a bow-tied cavity comprising an input coupler 132C-1, a flat mirror 132C-2, curved mirrors 132C-3 and 132C-4, and a nonlinear crystal 135B (including an input face 135C-IN and an output face 135C-OUT) having two or more SBO crystal plates via input coupler 132C-1 and is recirculated to enhance power. Output light 136C is output from nonlinear crystal 135C through output face 135C-OUT and includes unconsumed input light 138C and generated laser output light 139C having an output frequency ωOUTC equal to twice the frequency of input light 129C (i.e., frequency ωOUTC can be equal to either eighth harmonic output light 139A in FIG. 2A or sixth harmonic output light 139B in FIG. 2B). Laser output light 139C is reflected from the surface of beam splitter (BS) 137C and directed out of the cavity.

[0055] Preferably, nonlinear crystal 135C is configured such that input face 135C-IN and output face 135C-OUT are oriented at approximately the Brewster angle with respect to circulating input light 133C. The polarization direction of circulating input light 133C is indicated by arrow 329C. Furthermore, BS 137C can be configured to laterally displace circulating input light 133C within the cavity by an amount that substantially offsets the lateral displacement of the input light caused by nonlinear crystal 135C, thereby maintaining a substantially symmetric bowtie cavity and simplifying optical alignment of the cavity.

[0056] In one embodiment, the BS 137C may include SBO crystal, SBO glass, or CaF crystal. Because SBO has high deep UV transmittance and a high damage threshold, SBO may be advantageously used as the substrate material for the BS 137C, ensuring a long lifetime despite the high power level of the unconsumed input light 138C circulating within the cavity. When the BS 137C includes an SBO crystal, its thickness and / or the orientation of its crystal axis may be configured to minimize frequency doubling of the unconsumed input light 138C passing through it. The BS 137C may include a dichroic beam splitter, a prism, or other components for separating wavelengths. In one embodiment, the nonlinear crystal 135C is configured so that the output light 139C has an orthogonal polarization to the circulating input light 133C. In this embodiment, the BS 137C may include a polarizing beam splitter configured to transmit the unconsumed input light 138C and reflect the output light 139C. Note that in this embodiment, it is not possible to utilize the maximum nonlinear coefficient d33, so this configuration trades off reduced conversion efficiency for the convenience of using a polarizing beam splitter. In one embodiment, BS 137C has its surfaces oriented such that unconsumed input light 138C is substantially p-polarized with respect to their surfaces, and the surfaces are at approximately the Brewster angle with respect to the unconsumed input light.

[0057] According to FIG. 3, input light (ωx) 129C is focused by one or more lenses 131C before entering the cavity to match an eigenmode of the resonant cavity with a beam waist within or near the nonlinear crystal 135C. In a preferred embodiment, the one or more lenses 131C include one or more cylindrical lenses, each of which comprises SBO glass or crystal and is configured to operate at approximately the Brewster angle relative to the incident light 129C to minimize reflection losses without the use of anti-reflection coatings. SBO is a suitable material for such lenses due to its high damage threshold at UV and deep UV wavelengths. Unconsumed input light 138C (ωx) passing through BS 137C is reflected by mirror 132C-4 and circulates within the cavity, increasing its intensity. If the enhanced input light (ωx) power density is sufficiently strong, the conversion efficiency from input light (ωx) to output light 139C (ωx) can be very high, possibly up to or exceeding 50%. Output light 139C (2ωx) with wavelengths near 177 nm or near 133 nm exits the cavity after reflecting from BS 137C.

[0058] In an alternative embodiment, the input face of the nonlinear crystal 135C may be coated with a suitable anti-reflection coating instead of orienting the input face 135C-IN and output face 135C-OUT at Brewster's angle.

[0059] Although FIG. 3 illustrates the final frequency doubling stage 130C as including a cavity with two flat mirrors and two curved mirrors, other combinations of mirrors and / or lenses may be used to refocus the light circulating within the cavity. In alternative embodiments, the final frequency doubling stage 130C may include a delta cavity, a standing wave cavity, or a cavity of another shape instead of a bowtie cavity. If a standing wave cavity is used, the eighth harmonic is generated in the same direction as the incident fourth harmonic light. Both of these cavities can be stabilized using standard PDH or HC locking techniques. The cavity length is adjusted to maintain resonance by adjusting the position of one of the mirrors (such as mirror 132C-2 in FIG. 3) or the position of the prism via a control signal (not shown) connected to a piezoelectric transducer (PZT), voice coil, or another actuator. It should be noted that if the final frequency doubling stage 130C is used in a pulsed laser, no cavity is required and the input light 129C can be directed to the nonlinear crystal 135C and focused within or near the nonlinear crystal 135C by any suitable combination of lenses and / or mirrors.

[0060] FIG. 4 shows details of a nonlinear crystal 135C including four stacked SBO plates 135C-1 through 135C-4, which are configured to frequency-doubling the sum of the second harmonic of the first fundamental and the second fundamental in the case of the 177 nm laser 100B, or to frequency-doubling the fourth harmonic to generate the eighth harmonic in the case of the 133 nm laser 100A. There may be an odd or even number of plates. While FIG. 4 shows a nonlinear crystal 135C having a periodic structure including four stacked SBO crystal plates 135C-1 through 135C-4, the total number of SBO plates may be as few as two or more than ten. The thickness of each of the SBO plates 135C-1 through 135C-4 may be between 10 microns and 2 millimeters. Specifically, the thickness Λ of the SBO slab is given by Λ = mLc, where m is an odd integer (i.e., 1, 3, 5, 7, etc.), and the quasi-phase-matching critical length Lc = π / Δk. For the final frequency-doubling stage 130B of the 177 nm laser 100B, the quasi-phase-matching critical length Lc is approximately 0.60 μm, while for the final frequency-doubling stage 130A of the 133 nm laser 100A, the quasi-phase-matching critical length Lc is approximately 0.13 μm. A suitable m may be on the order of hundreds or even thousands to achieve a slab thickness that is convenient for handling and processing. A typical QPM critical length for generating 133 nm light by frequency doubling 266 nm light was calculated from the refractive indices of SBO at wavelengths of 133 nm and 266 nm using the Sellmeier model published by Trabs et al. (ibid.). did not generate wavelengths shorter than 160 nm, so the extrapolated refractive index at 133 nm may be inaccurate. Those skilled in the relevant art will understand how to calculate the QPM critical length given a more accurate refractive index.

[0061] Referring to FIG. 4, input light 133C at frequency ωx is incident on input surface 135C-IN of nonlinear crystal 135C. The polarization direction of input light 133C is indicated by the dashed arrow. SBO plates 135C-1 through 135C-4 are stacked one on top of the other, so that the input surface 135C-IN and output surface 135C-OUT are oriented at approximately the Brewster angle θ with respect to the circulating light 133C at frequency ωx, minimizing reflection losses without the use of anti-reflection coatings. The Brewster angle is approximately 60.5±1° with respect to the surface normal N for wavelengths longer than approximately 210 nm. Light 136C exiting the stack of SBO plates includes the second harmonic 2ωx of the input light and the unconsumed input light ωx.

[0062] To create the periodic structure of the QPM, the SBO plates 135C-1 to 135C-4 are rotated relative to each other, resulting in their corresponding c-crystal axes being inverted relative to each other, as shown in the two insets of Figure 4. The surface normal N of the SBO plate with thickness Λ (Λ is the spacing between the crystal poles) and the propagation direction of light 133C inside the SBO plate are shown in the two insets. The physical arrangement of the crystal plates enables QPM. This can be considered similar to using PPLN (periodically poled lithium niobate) for QPM, except that lithium niobate is a ferroelectric crystal and can be periodically poled. In contrast, SBO is non-ferroelectric, so the crystal plates must be physically arranged to create the periodic structure of the QPM. Furthermore, periodic poling requires applying an electric field parallel to the crystal axes of the ferroelectric crystal, so the polarization direction necessarily coincides with the crystal axes. In contrast, the SBO crystal plates disclosed herein can be cut and polished in any direction relative to the crystal axis, allowing the plate to be cut and oriented at the Brewster angle relative to the light incident on the plate.

[0063] In a preferred embodiment, the crystal axes of the SBO plates 135C-1 to 135C-4 are oriented so that light 133C propagating through the SBO plates propagates substantially perpendicular to the c-axis with the polarization direction (electric field direction) of light 133C substantially parallel to the c-axis. This maximizes conversion efficiency by utilizing the largest nonlinear optical coefficient, d33, of SBO. For example, as shown in FIG. 4, the crystal axes of the SBO plate 135C-1 can be oriented so that light 133C propagates substantially parallel to the a-axis of the SBO crystal. Alternatively, the crystal axes can be oriented so that light 133C propagates parallel to the b-axis (not shown) or at an angle within the a-plane of the crystal. In other words, the crystal axes shown in the two insets of FIG. 4 can be rotated around the c-axis. When the input surface of the SBO plate 135C-1 is oriented at Brewster's angle with respect to the input light 133C, the propagation direction of the light within the plate 135C-1 is approximately 29.5±1° with respect to the surface normal N.

[0064] 5A is a simplified block diagram illustrating an exemplary laser assembly 100D according to a third specific exemplary embodiment of the present invention. The laser assembly 100D includes a first fundamental laser 110D-1, a second fundamental laser 110D-2, three intermediate frequency conversion stages (i.e., a first frequency doubling stage 120D-1, a frequency summing stage 120D-2, and a second frequency doubling stage 120D-3), and a final frequency summing (or conversion) stage 130D, which are cooperatively configured to generate laser output light 139D having a wavelength in the range of about 147 nm to about 155 nm (e.g., about 152 nm). The first fundamental laser 110D-1 is configured in the manner described above and generates (first) fundamental light 119D-1 having a first fundamental wavelength in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to 1.1 μm) and a corresponding first fundamental frequency ω1. The second fundamental laser 110D-2 is also configured in the manner described above and generates (second) fundamental light 119D-2 having a second fundamental wavelength in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to 1.1 μm) and a corresponding second fundamental frequency ω2. The first frequency doubling stage 120D-1 receives the first fundamental light 119D-1 and generates second harmonic light 121D having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The beam splitter 124D splits the second harmonic light 121D into two portions: a first portion 121D-1 and a second portion 121D-2. The first portion 121D-1 of the second harmonic light 121D is received by the frequency summation stage 120D-2, which sums the first portion 121D-1 with the second fundamental light 119D-2 to generate a first intermediate optical beam 129D-1 having a corresponding frequency ωx equal to the sum frequency 2ω1 + ω2. For convenience, this sum frequency is referred to herein as substantially equal to the third harmonic (because ω1 and ω2 are similar or nearly equal). That is, when the frequencies of the first fundamental laser 110D-1 and the second fundamental laser 110D-2 are substantially the same (i.e., ω1 = ω2), the frequency ωx of the first intermediate optical beam 129D-1 is substantially equal to the third harmonic of either the fundamental optical frequency ω1 or ω2 (i.e., ωx ≈ 3ω1 or ωx ≈ 3ω2).The frequency summing stage 120D-2 is configured in a manner similar to that described above for the frequency summing stage 120B-2 with reference to FIG. 2B. The second portion 121D-2 of the second harmonic light 121D is passed to the second frequency doubling stage 120D-3, which is configured to generate a second intermediate light beam 129D-2 having a corresponding frequency ω equal to four times the first fundamental frequency ω (i.e., ω=4ω). According to the exemplary embodiment of FIG. 1, each of the frequency doubling stages 120D-1 and 120D-3 includes an external resonant cavity including at least three optical mirrors and a nonlinear crystal disposed therein, in a manner similar to that described above with reference to the second frequency doubling stage 120A-2 of FIG. 2A. Final frequency summing stage 130D uses the techniques described herein to sum first and second intermediate light beams 129D-1 and 129D-2 (i.e., ωx + ωy) to generate laser output light 139D with an output frequency ωOUTD equal to 6ω1 + ω2, which is herein considered substantially equivalent to seventh harmonic light (i.e., because ωx + ωy = 6ω1 + ω2 ≈ 7ω1 when ω1 and ω2 are similar or approximately equal), which has a wavelength of approximately 152 nm in the preferred embodiment. In an alternative embodiment, second fundamental laser 110D-2 may be omitted, and the output of first fundamental laser 110D-1 may be split into two portions: a first portion directed to first frequency doubling stage 120D-1, and a second portion directed to frequency summing stage 120D-2 along with second harmonic light 121D-1. In this alternative embodiment, ω2 = ω1.

[0065] 5B is a simplified block diagram illustrating an exemplary laser assembly 100E configured to generate a wavelength in the range of about 147 nm to about 155 nm (e.g., about 152 nm) according to a fourth specific exemplary embodiment of the present invention. The laser assembly 100E includes a first fundamental laser 110E-1, a second fundamental laser 110E-2, three intermediate frequency conversion stages (i.e., a first frequency doubling stage 120E-1, a second frequency doubling stage 120E-2, and a first frequency summing stage 120E-3), and a final frequency summing (conversion) stage 130E, and generates laser output light with an output frequency ωOUTE having a wavelength in the range of about 147 nm to about 155 nm (e.g., about 152 nm). Fundamental lasers 110E-1 and 110E-2 are configured in the manner described above to generate fundamental light 119E-1 and 119E-2, respectively, having fundamental wavelengths in the range of about 1000 nm to about 1100 nm (i.e., between about 1 μm and about 1.1 μm) and corresponding fundamental frequencies ω1 and ω2. First frequency doubling stage 120E-1 receives first fundamental light 119E-1 and generates second-harmonic light 121E-1 having a second-harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. Beam splitter 124E splits second-harmonic light 121E-1 into two portions: first portion 121E-11 and second portion 121E-12. The first portion 121E-11 of the second harmonic light 121E-1 is utilized as the first intermediate optical beam 129E-1 having a corresponding frequency ωx and is passed directly to the final frequency summing stage 130E. The second frequency doubling stage 120E-2 receives the second portion 121E-12 of the second harmonic light 121E-1 and generates the fourth harmonic light 121E-2 having a fourth harmonic frequency 4ω1 equal to four times the first fundamental frequency ω1. The first frequency summing stage 120E-3 sums the fourth harmonic light 121E-2 with the second fundamental light 119E-2 to generate the second intermediate optical beam 129E-2 having a corresponding frequency ωy equal to the sum frequency 4ω1 + ω2.For convenience, this sum frequency is referred to herein as the fifth harmonic light (i.e., because when ω1 and ω2 are similar or approximately equal, the sum of the fourth harmonic of the first fundamental frequency and the second fundamental frequency is substantially equal to the fifth harmonic of the first fundamental frequency, i.e., ωy = 4ω1 + ω2 ≈ 5ω1). Final frequency summation stage 130 sums first and second intermediate light beams 129E-1 and 129E-2 to generate laser output light 139E with an output frequency ωOUTE equal to the sum frequency 6ω1 + ω2, which for convenience is referred to herein as substantially equal to the seventh harmonic of the first fundamental frequency ω1 (i.e., when ω1 ≈ ω2, ωx + ωy = 6ω1 + ω2 ≈ 7ω1), and in a preferred embodiment has a wavelength of approximately 152 nm. In an alternative embodiment, the second fundamental laser 110E-2 may be omitted, and the output of the first fundamental laser 110E-1 is split into two portions: a first portion directed to the first frequency doubling stage 120E-1, and a second portion directed to the first frequency summation stage 120E-3 along with the fourth-harmonic light 121E-2. In this alternative embodiment, ω2 = ω1 necessarily.

[0066] The first frequency summing stage 120E-3 may be configured to sum the fourth-harmonic light 121E-2 with the second fundamental light 119E-2 using CLBO or hydrogen- or deuterium-treated CLBO in a near-noncritical phase-matching configuration. Alternatively, the first frequency summing stage 120E-3 may use quasi-phase matching (QPM) with stacked SBO plates configured as shown in FIG. 7 below. The critical length of QPM for summing 266 nm and 1064 nm wavelengths in SBO to generate 213 nm is approximately 1.81 μm (i.e., in the range of 1.80 μm to 1.82 μm). Because this critical length is longer than the critical length for generating shorter wavelengths, the thickness of the SBO plate in the light propagation direction (Λ in FIG. 7) may be equal to the critical length or a small odd integer (e.g., 3 to 19) multiple of the critical length.

[0067] 5C is a simplified block diagram illustrating an exemplary laser assembly 100F according to a fifth specific exemplary embodiment of the present invention. The laser assembly 100F includes a first fundamental laser 110F-1, a second fundamental laser 110F-2, three intermediate frequency conversion stages (i.e., a first frequency doubling stage 120F-1, a second frequency doubling stage 120F-2, and a third frequency doubling stage 120F-3), and a final frequency summation (conversion) stage 130F, which are cooperatively configured to generate laser output light having a wavelength in the range of about 170 nm to about 180 nm (e.g., about 177 nm). The fundamental lasers 110F-1 and 110F-2 are configured in the manner described above to generate fundamental light 119F-1 and 119F-2, respectively, having fundamental wavelengths in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to about 1.1 nm) and corresponding fundamental frequencies ω1 and ω2, respectively. The first frequency doubling stage 120F-1 receives the second fundamental light 119F-2 and generates a first intermediate light beam 129F-1 having a frequency ωx equal to the second harmonic of the second fundamental frequency ω2 (i.e., equal to twice the second fundamental frequency ω2). The second frequency doubling stage 120F-2 receives the first fundamental light 119F-1 and generates second harmonic light 121F having a frequency equal to the second harmonic of the first fundamental frequency ω1 (i.e., equal to twice the first fundamental frequency ω1). The third frequency doubling stage 120F-3 receives the second harmonic light 121F and generates a second intermediate light beam 129F-2 having a frequency ωy with a fourth harmonic frequency 4ω1 equal to four times the first fundamental frequency ω1. The final frequency summation stage 130F sums the first intermediate light beam 129F-1 (i.e., the second harmonic 2ω2) and the second intermediate light beam 129F-2 (i.e., the fourth harmonic 4ω1) to generate laser output light 139F having an output frequency ωOUTF, which is substantially equal to six times the first fundamental frequency (i.e., because ωx+ωy=4ω1+2ω2≒6ω1 when ω1 is approximately equal to ω2), which in a preferred embodiment has a wavelength of approximately 177 nm.In an alternative embodiment, the second fundamental laser 110F-2 and the first frequency doubling stage 120F-1 may be omitted, and the output 121F of the second frequency doubling stage 120F-2 may be split into two portions: a first portion directed to the third frequency doubling stage 120F-3, and a second portion directed to the final frequency summation stage 130F along with the fourth harmonic light 129F-2. In this alternative embodiment, necessarily ω2 = ω1.

[0068] FIG. 6 is a simplified diagram illustrating an exemplary final frequency summing stage 130G utilized in the 152 nm laser assembly of FIG. 5A, the 152 nm laser assembly of FIG. 5B, and the 177 nm laser assembly of FIG. 5C, in accordance with an exemplary embodiment of the present invention. Input light 129G (having a frequency ωx, e.g., ωx=2ω1+ω2 when stage 130G is used in 152 nm laser 100D of FIG. 5A, ωx=2ω1 when stage 130G is used in 152 nm laser 100E of FIG. 5B, or ωx=2ω2 when stage 130G is used in 177 nm laser 100F of FIG. 5C) enters a bow-tiing cavity comprising input coupler 132G-1, flat mirror 132G-2, curved mirrors 132G-3 and 132G-4, and nonlinear crystal 135G (including input face 135G-IN and output face 135G-OUT) via input coupler 132G-1 and is recirculated to increase power. Input light (second intermediate light beam) 129G-2 having frequency ωy (e.g., ωy=4ω1 when stage 130G is used in 152 nm laser 100D of FIG. 5A, or ωy=4ω1+ω2 when stage 130G is used in 152 nm laser 100E of FIG. 5B, or ωy=4ω1 when stage 130G is used in 177 nm laser 100F of FIG. 5C) enters the bow-tiling cavity near (but not necessarily through) mirror 132G-2 and passes through nonlinear crystal 135G. Output light 136G from nonlinear crystal 135G via output face 135C-OUT includes unconsumed input light 138G-1 having frequency ωx, unconsumed input light 138G-2 having frequency ωy, and generated laser output light 139G having output frequency ωOUTG, which is equal to the sum of the frequencies ωX and ωy of intermediate (input) light beams 129G-1 and 129G-2 (i.e., frequency ωOUTG may be substantially equal to either seventh-harmonic output light 139D and 139E in FIG. 5A or 5B, or sixth-harmonic output light 139F in FIG. 5C). Laser output light 139G is reflected from the input face of beam splitter (BS) 137G and directed out of the cavity.Unconsumed input light 138G-1 with frequency ω passes through beam splitter 137G and optional beam splitter 325 (if present) and is reflected by mirrors 132G-4 and 132G-1 to increase the intensity of recycled light 133G. Unconsumed input light 138G-2 at frequency ω is reflected from beam splitter 137G or from optional (second) beam splitter 325 before exiting the cavity. The polarization direction of the recycled input light 133G is indicated by arrow 329G.

[0069] Frequency summation stage 130G may be modified using any of the features and alternatives described above with reference to frequency doubling stage 130C of Figure 3. For example, stage 130G may utilize one or more lenses 131G to focus input light 129C-1 having frequency ω as described above, and may further utilize one or more lenses 308 to focus input light 129G-2 as it enters the cavity near mirror 132G-3, where both one or more lenses 131G and one or more lenses 308 are configured as described above with reference to lens 131C (Figure 3). Additionally, beam splitter 137G may be configured as described above with reference to beam splitter 137C of Figure 3. It should be noted that if the final frequency summation stage 130G is used in a pulsed laser, no cavity is required and the input lights 129G-1 and 129G-2 are made collinear (or nearly collinear, such as within 5° of each other) and directed and focused onto or near the nonlinear crystal 135G by any suitable combination of lenses and / or mirrors.

[0070] FIG. 7 illustrates an exemplary nonlinear crystal 135G, including four stacked SBO plates 135G-1 through 135G-4 configured to sum input light 133G at frequency ωx and input light 129G-2 at frequency ωy. While nonlinear crystal 135G is depicted as including four plates, the total number of plates may be, for example, as few as two or more than ten, with an even number of plates (as shown) or an odd number of SBO crystal plates. The thickness of each of SBO plates 135G-1 through 135G-4 may be from about 10 microns to about 2 millimeters. Specifically, the thickness Λ of the SBO plates is given by Λ = mLc, where m = 1, 3, 5, 7, ..., and the quasi-phase-matching critical length Lc = π / Δk. When the polarizations of input light 133G, input light 129G-2, and output light 136G are all aligned substantially parallel to the c-axis of the SBO crystal plate to utilize the largest nonlinear coefficient (d33) of SBO, the quasi-phase-matching critical length Lc is approximately 0.30 μm when final frequency summation stage 130G is used to generate 152 nm laser output light 139D of FIG. 5A, approximately 0.34 μm when final frequency summation stage 130G is used to generate 152 nm laser output light 139E of FIG. 5B, and approximately 0.66 μm when final frequency summation stage 130G is used to generate 177 nm laser output light 139F of FIG. 5C. A suitable m may be on the order of hundreds or thousands to achieve a slab thickness that is convenient for handling and processing. Other combinations of the polarization directions of input light 129G-1, input light 129G-2, and output light 139G are possible and have different phase-matching critical lengths. Those skilled in the art will understand how to calculate the phase-matching critical length from the refractive index of SBO. The conversion efficiency of such polarization combinations will generally be lower than when all polarizations are parallel to the c-axis because other nonlinear coefficients of SBO are smaller than d33. However, such configurations may be preferable for other reasons, such as simplifying the combining or separating of wavelengths based on their polarization.

[0071] Referring to FIG. 7, input light 133G having frequency ωx and input light beam 129G-2 having frequency ωy are incident on input face 135G-IN of nonlinear crystal 135G, which in the illustrated example is implemented using the exposed surface of SBO crystal plate 135G-4. The polarization direction of input light 133G (and input light 129G-2) is indicated by arrow 329G. The angle β between the propagation direction of input light 133G and the propagation direction of input light 129G-2 should be small, e.g., less than 5°, preferably about 2° or less. In a preferred embodiment, nonlinear crystal 135G is configured so that input face 135G-IN and output face 135G-OUT are oriented at approximately the Brewster angle θ with respect to input light 133G having frequency ωx, minimizing reflection losses without the use of anti-reflection coatings. The output light 136G includes output light 139G having a total frequency ωx+ωy, unconsumed input light 138G-1 having frequency ωx, and unconsumed input light 138G-2 having frequency ωy.

[0072] To create a periodic structure for QPM, two or more SBO plates 135G-1 through 135G-4 are rotated relative to one another, resulting in corresponding c-crystal axes being flipped relative to each other, as shown in the two insets of FIG. 7 . The surface normal N of the SBO plate with thickness Λ and the propagation direction of light 133G inside the SBO plate are shown in the two insets. This physical arrangement of the crystal plates enables QPM. In a preferred embodiment, the thickness of each plate is substantially equal to Λ = mLc, where m = 1, 3, 5, 7, ... as explained above. In the context of QPM, substantially equal means equal to within about 20% or about 10% of the quasi-phase-matching critical length Lc. In one embodiment, a large SBO plate is polished to the desired thickness and then cut into individual smaller pieces, which are then assembled in the correct orientation relative to each other to form the nonlinear crystal 135G.

[0073] In a preferred embodiment, the crystal axes of the SBO crystal plates 135G-1 to 135G-4 are oriented so that input light 133G and input light 129G-2 passing through the SBO plates propagate substantially perpendicular to the c-axis with the polarization direction (electric field direction) of light 133G and 129G-2 substantially parallel to the c-axis, thereby maximizing conversion efficiency by utilizing the largest nonlinear optical coefficient, d33, of SBO. For example, as shown in FIG. 7, the crystal axes of the SBO plate 132G-4 can be oriented so that light 133G and light 129G-2 propagate substantially parallel to the a-axis of the SBO crystal. Alternatively, the crystal axes can be oriented so that light 133G and light 129G-2 propagate parallel to the b-axis or at an angle within the a-plane of the crystal. When the input surface of SBO plate 132G-4 is oriented at Brewster angle θ with respect to polarization direction 329G of input light 133G / 129G-2, the propagation direction of light inside plate 132G-4 is approximately 29.5±1° with respect to surface normal N. Note that input lights 133C and 129G-2, as explained above, are incident on input surface 135G-IN at angles of a few degrees relative to each other, and therefore they propagate approximately parallel to each other within SBO plates 132G-1 through 132G-4 and can be considered to propagate substantially unidirectionally.

[0074] The above diagram is not intended to represent the actual physical layout of components. The above diagram illustrates the major optical modules involved in the process, but does not show all optical elements. Those skilled in the art will understand from the above diagram and associated description how to construct 177 nm, 152 nm, and 133 nm lasers. It should be understood that more or fewer optical components may be used to direct light where needed. Lenses and / or curved mirrors may be used to focus the beam waist to a substantially circular or elliptical cross-section focal point within or near an appropriate nonlinear crystal. Prisms, beam splitters, gratings, or diffractive optical elements may be used to manipulate or separate different wavelengths at the output of each frequency conversion stage, as needed. Prisms, coated mirrors, or other elements may be used to combine different wavelengths at the input to a frequency conversion stage, as needed. Beam splitters or coated mirrors may be used appropriately to split one wavelength into two beams. Filters may be used to block or separate undesired wavelengths at the output of any stage. Wave plates may be used to rotate polarization as needed. Other optical elements may be used as appropriate. Those skilled in the art will appreciate the various tradeoffs and alternatives possible in implementing 177 nm, 152 nm, and 133 nm lasers.

[0075] In various alternative embodiments described above, the first fundamental laser may be configured to generate first fundamental light at a first fundamental frequency ω1 with a corresponding wavelength equal to any one of approximately 1070 nm, approximately 1064 nm, approximately 1053 nm, approximately 1047 nm, and approximately 1030 nm. If used, the second fundamental laser may be configured to generate second fundamental light at a second fundamental frequency ω2 with a corresponding wavelength equal to any one of approximately 1070 nm, approximately 1064 nm, approximately 1053 nm, approximately 1047 nm, and approximately 1030 nm. The various harmonic frequencies referred to herein are based on multiples of the corresponding fundamental frequency. The exact wavelength of light generated by a given fundamental laser depends on many factors, including the exact 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 up to tenths of a nanometer or several nanometers, depending on the aforementioned and other factors. Those skilled in the art will understand how to select appropriate first and second fundamental wavelengths to generate a desired output wavelength from any one or two fundamental wavelengths.

[0076] Although the present invention is described herein using various fundamental wavelengths that facilitate generating laser output light at desired wavelengths of about 177 nm, about 152 nm, or about 133 nm, other wavelengths within a few nanometers of these desired wavelengths can be generated using different fundamental wavelengths. Unless otherwise stated in the appended claims, such lasers and systems utilizing such lasers are considered to be within the scope of the present invention.

[0077] Compared to pulsed lasers, CW light sources have a constant power level, which avoids the problem of damage caused by peak power and allows for continuous image or data acquisition. Furthermore, the bandwidth of the generated CW light is several orders of magnitude narrower than that of a typical mode-locked laser. Therefore, the design of the corresponding illumination or detection optical system can be much simpler, with better performance and lower system costs. However, some inspection and metrology applications can tolerate the higher bandwidth and peak power levels of pulsed lasers. Pulsed lasers are simpler than CW lasers because a resonant cavity is not required for the frequency conversion stage. Therefore, both CW and pulsed lasers are within the scope of the inventions disclosed herein and can be used as appropriate.

[0078] Lasers with wavelengths below 200 nm are either not commercially available at sufficient power levels or are very unreliable. In particular, there is no prior art that generates light with powers greater than several hundred milliwatts in the wavelength range between approximately 125 nm and 183 nm. Embodiments of the present invention generate short wavelengths down to approximately 133 nm, and therefore have better sensitivity for detecting small particles and defects than longer wavelengths.

[0079] Another aspect of the present invention is a wafer, reticle, or photomask inspection or metrology system incorporating at least one of the 177 nm, 152 nm, and 133 nm lasers of the present invention described above. An embodiment of such a system is shown in Figures 8, 9, and 10.

[0080] This laser can be used in an inspection system with dark-field and bright-field inspection modes, as shown in FIG. 8. This diagram and system are described in U.S. Patent No. 7,817,260 to Chuang et al., which is incorporated by reference as if fully set forth herein. FIG. 8 shows a catadioptric imaging system 800 incorporating normal-incidence laser illumination. The illumination block of system 800 includes laser 801, adaptive optics 802 for controlling the illumination beam size and profile on the surface being inspected, an opening and window 803 in mechanism housing 804, and a prism 805 for redirecting the laser along an optical axis at normal incidence to the surface of sample 808. Prism 805 also directs specular reflections from surface features of sample 808 and reflections from optical surfaces of objective 806 along an optical path to image plane 809. The lenses of objective 806 can be arranged in the general form of a catadioptric objective, a focusing lens group, and a zoom tube lens section 807. In a preferred embodiment, the laser 801 may be implemented by one of the lasers described above.

[0081] This laser can be used in a dark-field inspection system with oblique line illumination, as shown in Figures 9A and 9B. The inspection system can have two or three different focusing systems, including off-axis and near-normal focusing as shown. The dark-field inspection system can also include normal-incidence line illumination (not shown). Further details, including a description of the system shown in Figures 9A and 9B, can be found in U.S. Patent No. 7,525,649 to Leong et al., which is incorporated by reference as if fully set forth herein.

[0082] 9A illustrates a surface inspection apparatus 900 including an illumination system 901 and a collection system 910 for inspecting a region of a surface 911. As shown in FIG. 9A, a laser system 920 directs a light beam 902 through beam shaping optics 903. In a preferred embodiment, the laser system 920 includes at least one of the lasers described above. The first beam shaping optics 903 may be configured to receive the beam from the laser system, where the beam is focused onto the surface 911.

[0083] The beam shaping optics 903 are oriented such that their major surfaces are substantially parallel to the sample surface 911, resulting in an illumination line 905 being formed on the surface 911 in the focal plane of the beam shaping optics 903. In addition, the light beam 902 and the focused beam 904 are directed at a non-orthogonal angle of incidence with respect to the surface 911. In particular, the light beam 902 and the focused beam 904 may be directed at an angle between about 1° and about 85° from normal to the surface 911. In this manner, the illumination line 905 is substantially in the plane of incidence of the focused beam 904.

[0084] The light collection system 910 includes a lens 912 for collecting scattered light from the illumination line 905 and a lens 913 for focusing the light emerging from the lens 912 onto a device such as a charge-coupled device (CCD) 914 comprising an array of light-sensitive detectors. In one embodiment, the CCD 914 may include a linear array of detectors. In such a case, the linear array of detectors within the CCD 914 may be oriented parallel to the illumination line 905. In another embodiment, the CCD 914 may include a two-dimensional array of detectors arranged in a rectangular array with its long axis parallel to the illumination line 905. For example, the CCD 914 may include a rectangular array of approximately 1,000-8,000 detectors by approximately 50-250 detectors. In one embodiment, multiple light collection systems may be included, each containing similar components but with different orientations.

[0085] For example, FIG. 9B shows an exemplary array of light collection systems 931, 932, and 933 for a surface inspection apparatus (wherein its illumination system, e.g., one similar to illumination system 901, is not shown for simplicity). A first optical component in light collection system 931 collects light scattered in a first direction from the surface of sample 911. A second optical component in light collection system 932 collects light scattered in a second direction from the surface of sample 911. A third optical component in light collection system 933 collects light scattered in a third direction from the surface of sample 911. Note that the first, second, and third optical paths are at different reflection angles relative to the surface of sample 911. A stage 921 supporting sample 911 can be used to induce relative motion between the optical components and sample 911, so that the entire surface of sample 911 can be scanned.

[0086] This laser may also be used in inspection systems for unpatterned wafers, such as the inspection system 1000 shown in FIG. 10 . Such inspection systems may incorporate oblique and / or normal incidence illumination and a large collection solid angle for scattered light, as shown in these figures. The illumination source 1100 incorporates at least one of the lasers described herein and generates VUV light to illuminate the wafer 1122 at a desired angle to prevent reflected light from being collected by the imaging collection optics 1108 system. The optics 1106 may be configured to generate a desired illumination pattern. Scattered light from the wafer 1122 may be collected by the imaging collection optics 1108 system configured to direct the light to the afocal lens system 1110. In one embodiment, the collection lens mask system 1112 can split the light into multiple channels to send to the TDI sensor 1118. One embodiment may include an intensifier 1114 and / or a sensor relay 1116. The TDI sensor 1118 and / or intensifier 1114 may be configured to send signals to an image processing computer 1120, which may be configured to generate a wafer image and / or a list of defects or particles on the surface of the wafer 1122. Additional description of the elements of Figure 10 can be found in U.S. Patent No. 9,891,177 B2 to Vazhaeparambil et al. Further details of unpatterned wafer inspection systems are described in U.S. Patent Nos. 6,201,601 and 6,271,916, all of which are incorporated by reference as if fully set forth herein.

[0087] Although the present invention has been described with respect to certain specific embodiments, it will be apparent to those skilled in the art that the inventive features of the present invention are equally applicable to other embodiments, all of which are intended to be within the scope of the present invention.

Claims

1. 1. An inspection laser assembly for generating a laser output light beam having an output frequency with a corresponding wavelength in a range of about 125 nm to about 183 nm, comprising: one or more fundamental lasers each configured to generate a fundamental light beam having a corresponding fundamental frequency; a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using the one or more fundamental light beams, each of the one or more intermediate light beams having an associated intermediate frequency; a final frequency conversion stage configured to pass the one or more intermediate optical beams through a nonlinear crystal; wherein the nonlinear crystal comprises a plurality of strontium tetraborate (SrB) crystals arranged in a stacked configuration. ４ O ７ (SBO) crystal platelets, each of the SBO crystal platelets forming an interface surface with at least one adjacent one of the crystal platelets; wherein the plurality of SBO crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the one or more intermediate light beams, such that light exiting the nonlinear crystal comprises the laser output light beam having the output frequency; a laser assembly in which the plurality of SBO crystal plates are configured such that a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of an adjacent SBO crystal plate, and a spacing between poles of the periodic structure in a direction of light propagation is equal to an odd multiple of a critical length that enables the quasi-phase matching (QPM) between one or more input optical frequencies and an output frequency;

2. 2. The laser assembly of claim 1, wherein the final frequency conversion stage is configured such that the one or more intermediate light beams propagate through the plurality of SBO crystal plates in a direction parallel to one of (i) an a-axis and (ii) a b-axis of the first crystal axis and the second crystal axis.

3. The final frequency conversion stage further comprises: a plurality of mirrors configured to receive and circulate the one or more intermediate optical beams such that a beam waist of the one or more intermediate optical beams occurs at the nonlinear crystal; a beam splitter positioned to receive the light exiting the nonlinear crystal and configured to reflect a first portion of the exiting light to form the laser output light beam and to pass a second portion of the exiting light to be recycled by the plurality of mirrors; The laser assembly of claim 1 , comprising:

4. The beam splitter is made of SBO crystal, SBO glass, or CaF ２ The laser assembly of claim 3 , including one of the crystals.

5. the one or more fundamental lasers are configured to generate fundamental light having a fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the fundamental frequency; a second frequency doubling stage coupled to receive the second harmonic light from the first frequency doubling stage and configured to generate the intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the fundamental frequency; 2. The laser assembly of claim 1, wherein the final frequency conversion stage is configured to frequency double the fourth harmonic light such that the output frequency of the laser output light is an eighth harmonic frequency equal to eight times the fundamental frequency.

6. 6. The laser assembly of claim 5, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the nonlinear crystal and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the eighth harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the fourth harmonic frequency.

7. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization direction of the fourth harmonic light passing through the first and second SBO crystal plates; said output frequency being substantially equal to 133 nm; 6. The laser assembly of claim 5, wherein at least one of the first and second SBO crystal plates has a thickness such that a spacing between poles of the periodic structure is substantially equal to an odd integer multiple of a critical length of 0.13 μm, enabling quasi-phase matching of the fourth harmonic frequency and the eighth harmonic frequency.

8. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a frequency summing stage coupled to receive the second harmonic light and the second fundamental light from the frequency doubling stage and configured to generate the intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency; 2. The laser assembly of claim 1, wherein the final frequency conversion stage is configured to frequency double the third harmonic light such that the output frequency of the laser output light is a sixth harmonic frequency substantially equal to six times the first fundamental frequency.

9. 9. The laser assembly of claim 8, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the nonlinear crystal and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the sixth harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the third harmonic frequency.

10. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization direction of the third harmonic light passing through the first and second SBO crystal plates; said output frequency being substantially equal to 177 nm; 9. The laser assembly of claim 8, wherein at least one of the first and second SBO crystal plates has a thickness such that a spacing between poles of the periodic structure is substantially equal to an odd integer multiple of a critical length of 0.60 μm, enabling quasi-phase matching of the third harmonic frequency and the sixth harmonic frequency.

11. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a frequency summing stage coupled to receive a first portion of the second harmonic light and the second fundamental light from the first frequency doubling stage, the frequency summing stage configured to generate a first of the intermediate light beams as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency; a second frequency doubling stage coupled to receive a second portion of the second harmonic light from the first frequency doubling stage and configured to generate a second of the intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental frequency; 2. The laser assembly of claim 1, wherein the final frequency conversion stage is configured to sum the third harmonic light received from the frequency summation stage and the fourth harmonic light received from the second frequency doubling stage, such that the output frequency of the laser output light is a seventh harmonic frequency substantially equal to seven times the first fundamental frequency.

12. 12. The laser assembly of claim 11, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the nonlinear crystal and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the seventh harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the third harmonic frequency.

13. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second crystal axes are substantially parallel to the polarization directions of the third and fourth harmonic light passing through the first and second SBO crystal plates; said output frequency being substantially equal to 152 nm; 12. The laser assembly of claim 11, wherein at least one of the first and second SBO crystal plates has a thickness such that a spacing between poles of the periodic structure is substantially equal to an odd integer multiple of a critical length of 0.30 μm, enabling quasi-phase matching of the third harmonic frequency, the fourth harmonic frequency, and the seventh harmonic frequency.

14. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a second frequency doubling stage coupled to receive a first portion of the second harmonic light from the first frequency doubling stage and configured to generate fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental frequency; a frequency summing stage coupled to receive the fourth harmonic light and the second fundamental light from the second frequency doubling stage and configured to generate fifth harmonic light having a fifth harmonic frequency substantially equal to five times the first fundamental frequency; 2. The laser assembly of claim 1, wherein the final frequency conversion stage is configured to sum the fifth harmonic light with a second portion of the second harmonic light from the first frequency doubling stage, such that the output frequency of the laser output light is substantially a seventh harmonic frequency of the first fundamental frequency.

15. 15. The laser assembly of claim 14, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the nonlinear crystal and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the seventh harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the second harmonic frequency.

16. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization directions of the third and fourth harmonic light passing through the first and second SBO crystal plates; 15. The laser assembly of claim 14, wherein at least one of the first and second SBO crystal plates has a thickness such that a spacing between poles of the periodic structure is substantially equal to an odd integer multiple of a critical length of 0.34 μm, enabling quasi-phase matching of the second harmonic frequency, the fifth harmonic frequency, and the seventh harmonic frequency.

17. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the second fundamental light and configured to generate a first of the intermediate light beams having a second harmonic frequency equal to twice the second fundamental frequency; a second frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a third frequency doubling stage coupled to receive the second harmonic light from the second frequency doubling stage and configured to generate a second of the intermediate light beams having a fourth harmonic frequency equal to four times the first fundamental frequency; 2. The laser assembly of claim 1, wherein the final frequency conversion stage is configured to sum the first intermediate optical beam and the second intermediate optical beam such that the output frequency of the laser output light has a sixth harmonic frequency substantially equal to six times the first fundamental frequency.

18. 18. The laser assembly of claim 17, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the nonlinear crystal and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the sixth harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the second harmonic frequency.

19. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization directions of the second and fourth harmonic light passing through the first and second SBO crystal plates; 18. The laser assembly of claim 17, wherein at least one of the first and second SBO crystal plates has a thickness such that a spacing between poles of the periodic structure is substantially equal to an odd integer multiple of a critical length of 0.66 μm, enabling quasi-phase matching of the second harmonic frequency, the fourth harmonic frequency, and the sixth harmonic frequency.

20. 1. A method for generating an inspection laser output light beam having an output frequency with a corresponding wavelength in a range of about 125 nm to about 183 nm, comprising: generating one or more fundamental light beams, each of said fundamental light beams having a corresponding fundamental frequency with a corresponding fundamental wavelength between about 1 μm and 1.1 μm; utilizing a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using the one or more fundamental light beams, each of the one or more intermediate light beams having an associated intermediate frequency; and passing the one or more intermediate optical beams through a nonlinear crystal utilizing a final frequency conversion stage; wherein the nonlinear crystal comprises a plurality of strontium tetraborate (SrB) crystals arranged in a stacked configuration. ４ O ７ (SBO) crystal platelets, each of the SBO crystal platelets forming an interface surface with at least one adjacent one of the crystal platelets; wherein the plurality of SBO crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the one or more intermediate light beams, such that light exiting the nonlinear crystal comprises the laser output light beam having the output frequency; The plurality of SBO crystal plates are configured such that a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of an adjacent SBO crystal plate, and a spacing between poles of the periodic structure in the light propagation direction is equal to an odd multiple of a critical length that enables the quasi-phase matching (QPM) between one or more input optical frequencies and an output frequency.

21. 1. An inspection system configured to inspect a sample using a laser output light beam having an output frequency with a corresponding wavelength in a range of about 125 nm to about 183 nm, wherein the laser output light beam comprises: one or more fundamental lasers each configured to generate a fundamental light beam having a corresponding fundamental frequency; a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using the one or more fundamental light beams, each of the one or more intermediate light beams having an associated intermediate frequency; a final frequency conversion stage configured to pass the one or more intermediate light beams through a nonlinear crystal; wherein the nonlinear crystal comprises a plurality of strontium tetraborate (SrB) crystals arranged in a stacked configuration. ４ O ７ (SBO) crystal platelets, each of the SBO crystal platelets forming an interface surface with at least one adjacent one of the crystal platelets; wherein the plurality of SBO crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the one or more intermediate light beams, such that light exiting the nonlinear crystal comprises the laser output light beam having the output frequency; the plurality of SBO crystal plates are configured such that a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of an adjacent SBO crystal plate, and a spacing between poles of the periodic structure in a light propagation direction is equal to an odd multiple of a critical length that enables the quasi-phase matching (QPM) between one or more input optical frequencies and an output frequency.

22. 1. A nonlinear crystal configured to convert one or more input optical frequencies to output frequencies having corresponding wavelengths in a range from about 125 nm to about 183 nm, Multiple strontium tetraborates (SrB) ４ O ７ a plurality of (SBO) crystal plates arranged in a stacked configuration such that each of the SBO crystal plates forms an interface with at least one adjacent one of the crystal plates; wherein the plurality of SBO crystal plates are configured such that a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of the at least one adjacent SBO crystal plate; wherein the thickness of at least one of the plurality of SBO crystal plates creates a pole-to-pole spacing substantially equal to an odd multiple of the critical length, enabling quasi-phase matching of the one or more input optical frequencies with the output frequency, whereby light exiting the nonlinear crystal comprises test laser output light having the output frequency.

23. 1. An inspection laser assembly for generating a laser output light beam having an output frequency with a corresponding wavelength in a range of about 125 nm to about 183 nm, comprising: one or more fundamental lasers each configured to generate a fundamental light beam having a corresponding fundamental frequency; a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using the one or more fundamental light beams, each of the one or more intermediate light beams having an associated intermediate frequency; a final frequency conversion stage configured to pass the one or more intermediate light beams through a plurality of strontium tetraborate SrBO (SBO) crystal plates; the plurality of SBO crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the one or more intermediate light beams, such that light exiting the plurality of SBO crystal plates comprises the laser output light beam having the output frequency; a laser assembly in which the plurality of SBO crystal plates are configured such that a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of an adjacent SBO crystal plate, and a spacing between poles of the periodic structure in a direction of light propagation is equal to an odd multiple of a critical length that enables the quasi-phase matching (QPM) between one or more input optical frequencies and an output frequency;

24. 24. The laser assembly of claim 23, wherein the final frequency conversion stage is configured such that the one or more intermediate light beams propagate through the plurality of SBO crystal plates in a direction parallel to one of (i) an a-axis and (ii) a b-axis of the first crystal axis and the second crystal axis.

25. The final frequency conversion stage further comprises: a plurality of mirrors configured to receive and circulate the one or more intermediate light beams such that beam waists of the one or more intermediate light beams occur at the incidence surfaces of the plurality of SBO crystal plates; a beam splitter positioned to receive the light exiting the plurality of SBO crystal plates, the beam splitter configured to reflect a first portion of the exiting light to form the laser output light beam and to transmit a second portion of the exiting light to be recycled by the plurality of mirrors; 24. The laser assembly of claim 23, comprising:

26. The beam splitter is made of SBO crystal, SBO glass, or CaF ２ 26. The laser assembly of claim 25, including one of the crystals.

27. the one or more fundamental lasers are configured to generate fundamental light having a fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the fundamental frequency; a second frequency doubling stage coupled to receive the second harmonic light from the first frequency doubling stage and configured to generate the intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the fundamental frequency; 24. The laser assembly of claim 23, wherein the final frequency conversion stage is configured to frequency double the fourth harmonic light such that the output frequency of the laser output light is an eighth harmonic frequency equal to eight times the fundamental frequency.

28. 28. The laser assembly of claim 27, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the plurality of SBO crystal plates and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the eighth harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the fourth harmonic frequency.

29. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization direction of the fourth harmonic light passing through the first and second SBO crystal plates; said output frequency being substantially equal to 133 nm; 28. The laser assembly of claim 27, wherein at least one of the first and second SBO crystal plates has a thickness substantially equal to an odd multiple of a critical length to enable quasi-phase matching of the fourth harmonic frequency and the eighth harmonic frequency.

30. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a frequency summing stage coupled to receive the second harmonic light and the second fundamental light from the frequency doubling stage and configured to generate the intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency; 24. The laser assembly of claim 23, wherein the final frequency conversion stage is configured to frequency double the third harmonic light such that the output frequency of the laser output light is a sixth harmonic frequency substantially equal to six times the first fundamental frequency.

31. 31. The laser assembly of claim 30, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the plurality of SBO crystal plates and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the sixth harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the third harmonic frequency.

32. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization direction of the third harmonic light passing through the first and second SBO crystal plates; said output frequency being substantially equal to 177 nm; 31. The laser assembly of claim 30, wherein at least one of the first and second SBO crystal plates has a thickness substantially equal to an odd multiple of a critical length to enable quasi-phase matching of the third harmonic frequency and the sixth harmonic frequency.

33. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a frequency summing stage coupled to receive a first portion of the second harmonic light and the second fundamental light from the first frequency doubling stage, the frequency summing stage configured to generate a first of the intermediate light beams as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency; a second frequency doubling stage coupled to receive a second portion of the second harmonic light from the first frequency doubling stage and configured to generate a second of the intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental frequency; 24. The laser assembly of claim 23, wherein the final frequency conversion stage is configured to sum the third harmonic light received from the frequency summation stage and the fourth harmonic light received from the second frequency doubling stage, such that the output frequency of the laser output light is a seventh harmonic frequency substantially equal to seven times the first fundamental frequency.

34. 34. The laser assembly of claim 33, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the plurality of SBO crystal plates and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the seventh harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the third harmonic frequency.

35. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second crystal axes are substantially parallel to the polarization directions of the third and fourth harmonic light passing through the first and second SBO crystal plates; said output frequency being substantially equal to 152 nm; 34. The laser assembly of claim 33, wherein at least one of the first and second SBO crystal plates has a thickness substantially equal to an odd multiple of a critical length to enable quasi-phase matching of the third harmonic frequency, the fourth harmonic frequency, and the seventh harmonic frequency.

36. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a second frequency doubling stage coupled to receive a first portion of the second harmonic light from the first frequency doubling stage and configured to generate fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental frequency; a frequency summing stage coupled to receive the fourth harmonic light and the second fundamental light from the second frequency doubling stage and configured to generate fifth harmonic light having a fifth harmonic frequency substantially equal to five times the first fundamental frequency; 24. The laser assembly of claim 23, wherein the final frequency conversion stage is configured to sum the fifth harmonic light with a second portion of the second harmonic light from the first frequency doubling stage, such that the output frequency of the laser output light is substantially a seventh harmonic frequency of the first fundamental frequency.

37. 37. The laser assembly of claim 36, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the plurality of SBO crystal plates and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the seventh harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the second harmonic frequency.

38. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization directions of the third and fourth harmonic light passing through the first and second SBO crystal plates; 37. The laser assembly of claim 36, wherein at least one of the first and second SBO crystal plates has a thickness substantially equal to an odd multiple of a critical length to enable quasi-phase matching of the second harmonic frequency, the fifth harmonic frequency, and the seventh harmonic frequency.

39. The one or more fundamental lasers a first fundamental laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; The plurality of intermediate frequency conversion stages include: a first frequency doubling stage coupled to receive the second fundamental light and configured to generate a first of the intermediate light beams having a second harmonic frequency equal to twice the second fundamental frequency; a second frequency doubling stage coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency; a third frequency doubling stage coupled to receive the second harmonic light from the second frequency doubling stage and configured to generate a second of the intermediate light beams having a fourth harmonic frequency equal to four times the first fundamental frequency; 24. The laser assembly of claim 23, wherein the final frequency conversion stage is configured to sum the first intermediate optical beam and the second intermediate optical beam such that the output frequency of the laser output light has a sixth harmonic frequency substantially equal to six times the first fundamental frequency.

40. 40. The laser assembly of claim 39, wherein the final frequency conversion stage further comprises a beam splitter positioned to receive light exiting the plurality of SBO crystal plates and configured such that a reflected portion of the output light reflected from a surface of the beam splitter comprises the sixth harmonic frequency forming the laser output light beam, and such that an unreflected portion of the output light passing through the beam splitter comprises an unconsumed portion of the second harmonic frequency.

41. the plurality of SBO crystal plates are configured such that a first crystal axis of a first SBO crystal plate is inverted with respect to a second crystal axis of an adjacent second SBO crystal plate; the crystal axes of the first and second SBO crystal plates are oriented such that the crystal c-axes of both the first and second SBO crystal plates are substantially parallel to the polarization directions of the second and fourth harmonic light passing through the first and second SBO crystal plates; 40. The laser assembly of claim 39, wherein at least one of the first and second SBO crystal plates has a thickness substantially equal to an odd multiple of a critical length to enable quasi-phase matching of the second harmonic frequency, the fourth harmonic frequency, and the sixth harmonic frequency.

42. 1. A method for generating an inspection laser output light beam having an output frequency with a corresponding wavelength in a range of about 125 nm to about 183 nm, comprising: generating one or more fundamental light beams, each of said fundamental light beams having a corresponding fundamental frequency with a corresponding fundamental wavelength between about 1 μm and 1.1 μm; utilizing a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using the one or more fundamental light beams, each of the one or more intermediate light beams having an associated intermediate frequency; A final frequency conversion stage is utilized to convert the one or more intermediate light beams into a plurality of strontium tetraborate (SrB ４ O ７ passing the light through an optical element comprising a (SBO) crystal plate; the plurality of SBO crystal plates are cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the one or more intermediate light beams, such that light exiting the optical element comprises the laser output light beam having the output frequency; The plurality of SBO crystal plates are configured such that a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of an adjacent SBO crystal plate, and a spacing between poles of the periodic structure in the light propagation direction is equal to an odd multiple of a critical length that enables the quasi-phase matching (QPM) between one or more input optical frequencies and an output frequency.

43. 1. An inspection system configured to inspect a sample using a laser output light beam having an output frequency with a corresponding wavelength in a range of about 125 nm to about 183 nm, the laser output light beam comprising: one or more fundamental lasers each configured to generate a fundamental light beam having a corresponding fundamental frequency; a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using the one or more fundamental light beams, each of the one or more intermediate light beams having an associated intermediate frequency; a final frequency conversion stage configured to pass the one or more intermediate light beams through an optical element; The optical element comprises a plurality of strontium tetraborate (SrB ４ O ７ (SBO) crystal plates, the plurality of SBO crystal plates cooperatively configured to form a periodic structure that achieves quasi-phase matching (QPM) of the one or more intermediate light beams, such that light exiting the optical element comprises the laser output light beam having the output frequency; the plurality of SBO crystal plates are configured such that a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of an adjacent SBO crystal plate, and a spacing between poles of the periodic structure in a light propagation direction is equal to an odd multiple of a critical length that enables the quasi-phase matching (QPM) between one or more input optical frequencies and an output frequency.

44. an optical element that converts one or more input optical frequencies to output frequencies having corresponding wavelengths in a range of about 125 nm to about 183 nm; Multiple strontium tetraborates (SrB) ４ O ７ (SBO) crystal plates, wherein a first crystal axis of each SBO crystal plate is inverted with respect to a second crystal axis of at least one adjacent SBO crystal plate; an optical element, wherein at least one of the plurality of SBO crystal plates has a thickness substantially equal to an odd multiple of a critical length, and enables quasi-phase matching of one or more input optical frequencies with an output frequency, whereby light exiting the optical element comprises an inspection laser output light beam having the output frequencies.

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