Frequency conversion using opposed interdigital nonlinear crystal gratings.

Abstract

A nonlinear crystal grating assembly includes two integral nonlinear crystal grating structures having inverted crystal axes and parallel spaced apart mesas of a predetermined mesa width, arranged such that when assembled in a facing comb configuration, the mesas of the two grating structures form an alternating grating pattern aligned with the propagation direction of the input light, thereby creating a quasi-phase-matched (QPM) periodic structure. The nonlinear crystal grating structures are formed using strontium tetraborate, lithium triborate, or another nonlinear crystal material. The nonlinear crystal grating assembly is utilized in a laser assembly, where the fundamental wavelength is doubled and / or summed using an intermediate frequency conversion stage, and then a final frequency conversion stage utilizes the nonlinear crystal grating assembly to double or sum one or more intermediate optical beam frequencies to generate laser output light at high power and photon energy levels. Methods and inspection systems are also described.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 282,706, entitled “Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings,” filed November 24, 2021, and incorporated herein by reference.

[0002] This disclosure is related 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, 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 U.S. Provisional Patent Application No. 17 / 239,561, entitled "Frequency Conversion Using Stacked Strontium Tetraborate Plates," filed April 23, 2021. All of these applications are incorporated herein by reference.

[0003] This application also relates to 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, U.S. Patent No. 9,023,152 to Dribinski ... and U.S. Patent No. 9,023,152 to Dribinski. Nos. 9,461,435 and 9,059,560 to Ski et al., 9,293,882 and 9,660,409 to Chuang, and 9,250,178, 9,459,215, 9,509,112, 10,044,166, 10,283,366, and 11,180,866 to Chuang et al., all of which are incorporated herein by reference.

[0004] This application relates to lasers capable of producing light having wavelengths in the deep UV (DUV) or vacuum UV (VUV), and more particularly to lasers capable of producing light in the range of approximately 125 nm to 300 nm, and inspection systems that use such lasers to inspect, for example, photomasks, reticles, and semiconductor wafers. [Background technology]

[0005] As the dimensions of semiconductor devices decrease, so does the size of the smallest particles and pattern defects that can damage the devices. Thus, a need arises to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. In general, the intensity of light scattered by a particle smaller than the wavelength of light increases as a high multiplier 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 scales inversely as the fourth power of the wavelength). Because of the increased intensity of the scattered light, shorter wavelengths generally provide greater sensitivity for detecting small particles and defects than longer wavelengths.

[0006] Typically, the intensity of light scattered from small particles and defects is very low, so high illumination intensities are required to produce detectable signals in very short times. Average source power levels of 0.3 W or more may be required. At these high average power levels, high pulse repetition rates are desirable because the higher the repetition rate, the lower the energy per pulse and therefore the lower the risk of damage to the optics of the system or the item being inspected. Generally, the illumination needs in inspection and metrology are best met by continuous wave (CW) sources. CW sources have a constant power level, which avoids the problem of damage from peak powers and also allows images or data to be acquired continuously. However, in many cases, mode-locked lasers (sometimes called quasi-CW lasers) with repetition rates of around 50 MHz or more may be useful because the high repetition rate means that the energy per pulse can be low enough to avoid damage in certain metrology and inspection applications. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] US Patent Application Publication No. 2015 / 0155680 [Patent Document 2] US Patent Application Publication No. 2014 / 0362880 Summary of the Invention [Problem to be solved by the invention]

[0008] Thus, a need arises for mode-locked or CW lasers that produce radiation in the VUV range and are suitable for use in photomask, reticle, and / or wafer inspection. The practical manufacture of lasers capable of mode-locked or CW output near 133 nm at higher power levels would enable more accurate and faster inspection and metrology, contributing to cutting-edge semiconductor manufacturing.

[0009] Furthermore, lasers that generate radiation in the DUV wavelengths (e.g., wavelengths between 190 nm and 300 nm) at high power levels and long service intervals (e.g., months or years) typically use cesium lithium borate (CLBO) as a nonlinear crystal for frequency conversion. However, CLBO is hygroscopic and must be protected from moisture during handling, storage, and operation, resulting in significant complexity and cost in the manufacturing, shipping, and operation processes. Other nonhygroscopic nonlinear crystals with high damage thresholds do not phase match at important wavelengths in the DUV wavelength range and therefore cannot be used with critical phase matching or noncritical phase matching at those wavelengths. Thus, a need also arises for mode-locked or CW lasers that generate radiation in the DUV range without using hygroscopic nonlinear crystals.

[0010] A need also arises for an inspection system and associated laser system capable of generating mode-locked or CW laser light having an output in a DUV or VUV wavelength range, such as from about 125 nm to about 300 nm, that avoids some or all of the problems and drawbacks discussed above. [Means for solving the problem]

[0011] The present invention generally relates to a nonlinear crystal grating assembly for use in a laser assembly (laser) configured to generate a laser output light beam having an output frequency with a corresponding output wavelength in the range of approximately 125 nm to approximately 300 nm. Each nonlinear crystal grating assembly (grating assembly) includes two integral nonlinear crystal grating structures, each machined to include a row of parallel, spaced apart mesa-type structures (mesas). The mesas are shaped and configured to facilitate assembly in an opposed interdigital configuration (e.g., whereby a mesa of one grating structure fits into a groove formed between two adjacent mesas of another grating structure, and vice versa). Additionally, both grating structures are formed (e.g., etched or cut) such that each width of each mesa is substantially equal to an odd integer multiple of a critical length, measured in the propagation direction of the light beam, required to achieve quasi-phase matching (QPM) of an input light beam with the output frequency of a desired laser output light beam. Also, the grating structures are formed with inverted crystal axes (e.g., the first optical axis is aligned parallel to the propagation direction, and the second and third optical axes are rotated substantially 180° relative to one another), such that the first grating structure has an "upright" (first) crystal axis relative to the "inverted" (second) crystal axis of the second grating structure. When the grating structures are assembled in a counter-interdigital configuration and the grating assembly is operatively arranged to receive one or more incident light beams, the incident (intermediate) light beam is substantially parallel to the second optical axis of the first and second crystal axes, and the mesas of the two grating structures collectively form a grating pattern, with the light beam passing alternately through the upright and inverted mesas (i.e., passing through the upright mesa of one grating structure, then through the inverted mesa of the other grating structure, then through the second upright mesa of the first grating structure, etc.).By forming and assembling the grating structures in this manner, the grating assembly provides a periodic structure capable of achieving QPM suitable for frequency conversion of applied light, thereby facilitating the production of DUV and VUV laser light at high power and photon energy levels, while avoiding the above-mentioned problems and shortcomings associated with prior art approaches. Moreover, by forming the two grating structures such that when the two grating structures are arranged in a facing interdigital configuration, the upright and inverted mesas form a desired alternating grating pattern, the present invention significantly simplifies the associated manufacturing and assembly steps, and thus reduces the cost of producing laser assemblies using the crystalline grating assembly of the present invention.

[0012] In one embodiment, each of the grating structures used to form the nonlinear crystal grating assemblies of the present invention comprises a single (monolithic) strontium tetraborate SrBO (SBO) crystal. SBO crystals exhibit attractive characteristics, such as a wide transmission range, high damage resistance and chemical stability, high microhardness, and high oblique d-axis angle compared to the bandgap value. 33nonlinear optical component value), thereby avoiding many of the above-mentioned problems and shortcomings associated with prior art approaches. In an alternative embodiment, the nonlinear crystal is lithium triborate LiB3O5 (LBO), beta barium borate β-BaB2O4 (BBO), or another nonlinear crystal material that is transparent to the input and output frequencies and has at least one nonlinear coefficient that is fairly high (approximately 1 pm / V or greater). SBO crystals exhibit low birefringence, which precludes frequency conversion by critical or noncritical phase matching. LBO is widely used to frequency double infrared wavelengths (e.g., 1064 nm) to generate second harmonics in the green portion of the visible spectrum. LBO has a high damage threshold and transmits light at short wavelengths, around 160 nm. However, the UV refractive index of LBO does not allow for critical or noncritical phase matching to frequency double green light, for example, at wavelengths around 532 nm. The present invention circumvents the phase matching limitations of SBO, LBO, and other nonlinear crystals by forming a grating pattern (i.e., a series of aligned, periodically successive upright and inverted mesas having a first / upright crystal axis and a second / inverted crystal axis, respectively) using the opposed comb configuration described herein to achieve QPM of one or more input optical frequencies of an intermediate light beam that is directed to an input face of a grating assembly at approximately the Brewster angle, such that the light exiting the output face of the grating assembly comprises laser output light having a desired DUV or VUV output frequency.

[0013] In a practical embodiment, the grating structures are formed by etching or scoring (scribing) periodically spaced rectangular grooves or cavities into a single nonlinear crystal by standard fabrication techniques. The nonlinear crystal material remaining between each groove is referred to herein as a mesa or plate, and each grating structure consists of a horizontal row of parallel spaced mesas. The parallel spaced mesas extend vertically from a horizontal base and are separated by intervening grooves. As described above, the width of each mesa of both grating structures is substantially equal to an odd multiple of the critical length to enable QPM of the desired output and input optical frequencies. The width of each groove is greater than the width of each mesa, such that when the grating structures are assembled in an opposed comb configuration, the vertical faces of the mesas of the first grating structure are separated by a small gap distance from the opposing vertical faces of the mesas of the second grating structure. In one embodiment, the depth of the rectangular grooves (i.e., the height of each mesa) is at least 10 microns, preferably at least 50 μm, or at least 100 μm. By forming the grating structures in this manner, the mesas of the two grating structures form an alternating grating pattern of opposing combs, which facilitates the use of the grating assemblies produced according to the present invention to perform the frequency conversion (e.g., frequency doubling of one input optical frequency or frequency summing of two input optical frequencies) required to generate DUV and VUV wavelengths at high power levels (i.e., hundreds of milliwatts (mW) to several watts (W) or more) and high photon energy levels (e.g., 4.66 eV at 266 nm, 7.00 eV at 177 nm, 8.16 eV at 152 nm, 9.32 eV at 133 nm), while avoiding the above-mentioned problems and shortcomings associated with the prior art approaches.

[0014] In the embodiments specifically described below, the present invention is directed to improvements in inspection systems utilized in the semiconductor manufacturing industry, and in particular 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 in the range of approximately 125 nm to approximately 300 nm. In a practical embodiment, each nonlinear crystal grating assembly is utilized in a frequency conversion stage of an associated laser assembly that also includes at least one fundamental laser and one or more intermediate frequency conversion stages, each fundamental laser generating a respective fundamental light beam having a corresponding fundamental frequency (e.g., having a wavelength of approximately 1 μm to 1.1 μm), and the intermediate frequency conversion stages collectively configured to convert the fundamental light beams to at least one intermediate light beam having an associated intermediate frequency. In at least one embodiment, the final frequency conversion stage is configured to guide the intermediate light beams to the grating assembly, such that the polarization direction (electric field direction) of the light through each crystal mesa (plate) is substantially parallel to one axis (e.g., axis A2) of the crystal material, and the propagation direction of the light is substantially parallel to another axis (e.g., axis A1) of the crystal, and the alternating upright / inverted periodic configuration formed by the opposing comb mesas achieves QPM of the output wavelengths and the intermediate light beams. In a specific embodiment, the final frequency conversion stage includes a plurality of mirrors operatively configured to receive and circulate (e.g., by one or more matching lenses) at least one of the intermediate light beams (e.g., in the form of a bow-tie cavity formation), such that a beam waist of the circulated light appears at (i.e., within or immediately adjacent to) the grating assembly. In one embodiment, the final frequency conversion stage utilizes a beam splitter (e.g., SBO crystal, SBO glass, or CaF2 crystal). The beam splitter is configured to split the output light (i.e., the light leaving / exiting the grating assembly) so that a reflected (first) portion of the output light forms a desired laser output light beam having an output wavelength in the range of approximately 125 nm to approximately 300 nm, and a non-reflected (second) portion of the output light, which comprises unconsumed input light, is transmitted by the beam splitter to be recycled by the cavity mirrors.

[0015] In various disclosed embodiments, the present invention is directed to improved laser assemblies for inspection systems utilized in the semiconductor manufacturing industry, and in particular laser assemblies for such inspection systems capable of generating laser light having a source power level of 0.3 W or greater and an output wavelength in the range of approximately 125 nm to approximately 300 nm. In some embodiments, the nonlinear crystal grating assembly is configured to frequency double a single intermediate light beam having a visible wavelength near 532 nm, a UV wavelength near 355 nm, or a DUV wavelength near 266 nm to generate laser light having a DUV wavelength near 266 nm, a VUV wavelength near 177 nm, or a VUV wavelength near 133 nm, respectively. In other embodiments, the nonlinear crystal grating assembly is configured to frequency add a first intermediate light beam having a UV wavelength near 355 nm to a second intermediate light beam having a DUV wavelength near 266 nm to generate laser light having a VUV output wavelength near 152 nm. In other embodiments disclosed herein, the nonlinear crystal grating assembly is configured to frequency-add a first intermediate optical beam having a visible wavelength near 532 nm with a second intermediate optical beam having a DUV wavelength near 213 nm to generate laser light having a VUV wavelength near 152 nm. In yet another alternative embodiment, the nonlinear crystal grating assembly is configured to frequency-add a first intermediate optical beam having a visible wavelength near 532 nm with a second intermediate optical beam having a DUV wavelength near 266 nm to generate CW laser light having a VUV output wavelength near 177 nm. In all of the above embodiments, the integral nonlinear crystal grating structure forming the nonlinear crystal grating assembly is fabricated to include a mesa having a mesa width substantially equal to an odd integer multiple of the associated QPM critical lengths of the input and output optical frequencies.

[0016] According to another embodiment, a laser assembly is configured to generate laser output light having an output wavelength of approximately 133 nm by configuring a final frequency conversion stage to double the fourth harmonic of the fundamental frequency using the nonlinear crystal grating assembly of the present invention to generate an eighth harmonic of the fundamental frequency.

[0017] According to another embodiment, a laser assembly is configured to generate laser output light having an output wavelength of approximately 177 nm by configuring a final frequency conversion stage to double the third harmonic of the fundamental frequency using the nonlinear crystal grating assembly of the present invention to generate a sixth harmonic of the fundamental frequency.

[0018] According to another embodiment, a laser assembly is configured to generate laser output light having an output wavelength of approximately 266 nm by configuring a final frequency conversion stage to sum two second harmonics of the fundamental frequency (or double a single second harmonic) using the nonlinear crystal grating assembly of the present invention to generate a fourth harmonic of the fundamental frequency.

[0019] According to another embodiment, a laser assembly is configured to generate laser output light having 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 using the nonlinear crystal grating assembly of the present invention to generate a seventh harmonic of the fundamental frequency.

[0020] According to another embodiment, a laser assembly is configured to generate laser output light having 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 using the nonlinear crystal grating assembly of the present invention.

[0021] According to another embodiment, a laser assembly is configured to generate laser output light having an output wavelength of approximately 177 nm by configuring a final frequency conversion stage to sum the second and fourth harmonics of the fundamental frequency using the nonlinear crystal grating assembly of the present invention.

[0022] In other embodiments, the inspection system is configured to inspect a specimen, e.g., a wafer, reticle, or photomask, using one of the lasers described herein that generate output wavelengths of, for example, approximately 266 nm, 177 nm, 152 nm, or approximately 133 nm. [Brief description of the drawings]

[0023] [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. 2 is a simplified block diagram illustrating a simplified laser assembly according to a first specific embodiment of the present invention. [Figure 2B] FIG. 4 is a simplified block diagram illustrating a simplified laser assembly according to a second exemplary embodiment of the present invention. [Figure 3A] FIG. 2 is a simplified schematic diagram illustrating an exemplary final frequency doubling stage utilized in the laser assemblies of the first and second specific embodiments in accordance with an exemplary embodiment of the present invention. [Figure 3B] 11A-11C are simplified schematic diagrams illustrating exemplary final frequency summing stages utilized in the laser assemblies of the third, fourth, and fifth specific embodiments in accordance with exemplary embodiments of the present invention. [Figure 4A] 1A and 1B are perspective views illustrating an example integral nonlinear crystal grating structure for use in a final frequency conversion stage of a laser assembly, and an example simplified nonlinear crystal grating assembly formed by the integral nonlinear crystal grating structure. [Figure 4B]1A and 1B are perspective views illustrating an example integral nonlinear crystal grating structure for use in a final frequency conversion stage of a laser assembly, and an example simplified nonlinear crystal grating assembly formed by the integral nonlinear crystal grating structure. [Figure 4C] 1A and 1B are perspective views illustrating an example integral nonlinear crystal grating structure for use in a final frequency conversion stage of a laser assembly, and an example simplified nonlinear crystal grating assembly formed by the integral nonlinear crystal grating structure. [Diagram 5] 4D is a modified cross-sectional view showing a partial final frequency conversion stage including the exemplary nonlinear crystal of FIG. 4C during operation. [Figure 6A] FIG. 2 is a perspective view illustrating a nonlinear crystal grating assembly according to another exemplary embodiment. [Figure 6B] FIG. 2 is a perspective view illustrating a nonlinear crystal grating assembly according to another exemplary embodiment. [Figure 6C] FIG. 2 is a perspective view illustrating a nonlinear crystal grating assembly according to another exemplary embodiment. [Figure 7A] FIG. 13 is a simplified block diagram illustrating a simplified laser assembly in accordance with an additional specific embodiment of the present invention. [Figure 7B] FIG. 13 is a simplified block diagram illustrating a simplified laser assembly in accordance with an additional specific embodiment of the present invention. [Figure 7C] FIG. 13 is a simplified block diagram illustrating a simplified laser assembly in accordance with an additional specific embodiment of the present invention. [Figure 8] FIG. 2 is a simplified diagram illustrating an example 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. 2 is a schematic diagram illustrating 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. 2 is a schematic diagram illustrating 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. 1 is a schematic diagram illustrating 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 PREFERRED EMBODIMENTS

[0024] 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 present invention in the context of a particular application and its requirements. The directional terms used herein, such as "top", "left", "right", "horizontal", "vertical" and "lower", are intended to provide relative positions for purposes of explanation and are not intended to specify an absolute frame of reference. Various modifications of 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 specific embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0025] The second-order susceptibility change of an acentric crystal leads to a modification of the quasi-phase matching (QPM) conditions that may be favorable for frequency conversion. For the VUV spectral region below about 150 nm, there are as yet no known transparent optical crystals that combine non-zero second-order nonlinearity with sufficient birefringence. For example, several attempts have been reported to fabricate QPM structures by electric field poling 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,” Opt. Express 17, 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],” Opt. Mat. Express 1, 1367 (2011)). However, both materials exhibit low nonlinear coefficients and the shortest wavelength demonstrated so far is 194 nm.

[0026] Strontium tetraborate SrB4O7 (SBO) crystallizes in the orthorhombic system with point group mm2, space group Pnm21, and unit cell dimensions a = 4.4255 Å, b = 10.709 Å, and c = 4.2341 Å (YS Oseledchik, AL Prosvirnin, AI Pisarevskiy, VV Starshenko, VVO Sadchuk, SP Belokrys, NVS Vitanko, AS Korol, SAKrikunov, and AF Selevich, “New nonlinear optical crystals: strontium and lead tetraborates,” Opt. Mater. 4, 669 (1995)). All boron atoms are coordinated tetrahedra, with oxygen atoms common to three tetrahedra. Despite being a three-dimensional network of tetrahedra, the boron network appears as a layer-like structure because there are relatively few links in the c direction of the unit cell.

[0027] SBO exhibits very small birefringence (<0.005) and is not ferroelectric. Non-phase-matched second harmonic generation (SHG) has been implemented with SBO for diagnostics, but the efficiency is extremely low when only one coherence length is utilized, and the practical detection limit was estimated to be 2 μJ for 120 fs pulses at 267 nm (V. Petrov, F. Noack, D. Shen, F. Pan, G. Shen, X. Wang, R. Komatsu, and V. Alex, “Application of the nonlinear crystal SrB4O7 for ultrafast diagnostics converting to wavelengths as short as 125 nm,” Opt. Lett. 29, 373 (2004)).

[0028] SBO exhibits unique optical and mechanical properties. The transmission range of SBO is from 130 to 3200 nm (YS Oseledchik et al., op. cit.). SBO also exhibits a high degree of optical transparency in the oblique direction. 33It also shows high values ​​of the components (1.5-3.5 pm / V) (compared to the band gap value). The optical damage threshold is very high (approximately 15 GW / cm) compared to other materials, e.g. MgF2. 2 ). The microhardness of SBO is also high (1750 kg / mm 2 , 1460 kg / mm ​​in the y direction 2 , 1350kg / mm ​​in z direction 2 ). The high optical damage threshold and microhardness enable SBO crystals to withstand extreme conditions when exposed to DUV and VUV radiation. DUV and VUV lasers can 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 transmission range, high damage tolerance and chemical stability, and oblique d 33 The high values ​​of the components are the features that make SBOs very attractive for frequency conversion to produce DUV and VUV wavelengths, but the low birefringence means that frequency doubling by critical or non-critical phase matching is not possible.

[0029] Trabs et al. (P.Trabs, F.Noack, ASA Leksandrovsky, AIZAitsev, NVRadionov, and V.Petrov, “Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV”, Opt.Express 23, 10091 (2015)) reported the use of SBO crystals to generate second harmonics in the VUV from ultrashort laser pulses by random quasi-phase matching. The second harmonic generation method described by Trabs et al. is not suitable for light source semiconductor metrology and inspection systems because the frequency conversion process has low efficiency, making it infeasible to generate watts of second harmonic laser power using this method, and it requires ultrashort laser pulses.

[0030] Lithium triborate LiB3O5 (LBO) crystallizes in the orthorhombic system with a point group of mm2, space group of Pna21, and unit cell dimensions of a = 8.4473 Å, b = 7.3788 Å, and c = 5.1395 Å (C. Chen, Y. Wu, A. Jiang, B. Wu, G. You, R. Li, and S. Lin, “New nonlinear optical crystal: LiB3O5,” J. Opt. Soc. Am. B6, 616-621 (1989)). It has a high optical damage threshold (45 GW / cm for a 1.1 ns pulse at 1064 nm). 2 ), which is transparent over a wide wavelength range from 160 nm to 2.6 μm. The maximum nonlinear optical coefficient is d 31 (approximately -1.1 pm / V) and d 32 (approximately 1.2 pm / V). The nonlinear optical coefficient d 33 is much smaller and less useful for nonlinear frequency conversion. LBO is biaxially birefringent and can generate, for example, second and third harmonics at wavelengths near 1.064 μm by critically or noncritically phasematched second and sum frequency generation. However, LBO is not suitable for generating DUV and VUV wavelengths, e.g., 266 nm or 177 nm, by either critically or noncritically phasematching.

[0031] The output frequency ω in the wavelength range of approximately 125 nm to approximately 300 nm corresponding to FIG. OUT 1 shows a laser assembly 100 for generating a laser output light beam 139 having a frequency of 100 Hz to 100 Hz. Generally, the laser assembly 100 comprises one or more fundamental lasers 110, one or more intermediate frequency conversion stages 120, and a final frequency conversion stage 130.

[0032] Referring to the upper left portion of FIG. 1, the fundamental laser 110 has corresponding fundamental angular frequencies ω1 to ω nEach of the fundamental lasers 119-1, 119-2...119-n (collectively denoted as 119) has a fundamental wavelength of about 1 μm to 1.1 μm. In some embodiments, all of the fundamental laser beams 119 have substantially the same wavelength (e.g., fundamental frequency ω1 is substantially equal to fundamental frequency ω2). Specific fundamental laser types are mentioned in the specific embodiments provided below. Note that when wavelengths are mentioned without limitation in the following description, the wavelengths may be assumed to be wavelengths in a vacuum.

[0033] The intermediate frequency conversion stages 120 are optically coupled to receive one or more of the fundamental light beams 119 (or light from an associated intermediate frequency conversion stage) and are collectively configured to generate one or more intermediate light beams 129. In some particular embodiments, the intermediate light beams 129 have an associated intermediate frequency ω x In another particular embodiment, the intermediate light beam 129 includes a single (first) intermediate light beam 129-1 having an associated intermediate frequency ω y 1 includes both the fundamental laser 110 and a second intermediate light beam 129-2 having a fundamental light beam 119. 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 light beam 119. For example, in the specific examples described below, a given "downstream" intermediate frequency conversion stage may receive second, third, or fourth harmonic light generated by one or more "upstream" intermediate frequency conversion stages optically coupled between the fundamental laser 110 and the given downstream stage.

[0034] Referring to the lower half of FIG. 1, the laser assembly 100 outputs an intermediate light beam 129 (ω x or ω x and ω y) through a nonlinear crystal grating assembly 150 and out of the laser assembly 100 to direct the laser output light beam 139. In one embodiment, the intermediate light beam 129-1 has a frequency ω x and enters a bowtie ring cavity formed by input coupler mirror 132-1, flat mirror 132-2, two curved mirrors 132-3 and 132-4, grating assembly 150, and beam splitter 137. For purposes of illustration, the portion of light transmitted by the bowtie ring cavity from input / coupler mirror 132-1 to grating assembly 150 is shown as circulated light portion 133. Circulated light portion 133 is comprised of both intermediate light beam 129-1 and unconsumed circulated light portion 138-1 (generated as described below). Both light portions 133 and 138-1 have a frequency ω x The bow-tiling cavity formed by mirrors 132-1 through 132-4 is configured such that optical portion 133 is guided along an optical path through grating assembly 150. In one embodiment, a mode-matching lens 131 is utilized to focus intermediate optical 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 optical portion 133 is the beam waist of optical portion 133 (i.e., including intermediate optical beam 129-1) that originates at (i.e., within or in the immediate vicinity of) grating assembly 150. yWhen intermediate light beam 129-2 having a frequency ω is used as described in the relevant specific embodiment described below, intermediate light beam 129-2 enters the bowtie ring cavity, passes near (but not necessarily through) curved mirror 132-3, and is thereby directed to input face 153-IN at a substantially selected angle θ through grating assembly 150. As shown in this exemplary arrangement, 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) to grating assembly 150, and output light 136 (i.e., all light outputting from grating assembly 150) is directed to input face 137-IN of beam splitter 137. Beam splitter 137 is configured to split output light 136, thereby converting it to a frequency ω x Unconsumed input light 138-1 having a frequency of ω OUT A laser output light 139 having a frequency ω y The beam splitter 137 is also configured to reflect the unconsumed input light 138-2 having a wavelength of 1000 nm out of the bowtie cavity. The beam splitter 137 may be implemented using one of a single SBO crystal, SBO glass, or CaF2 crystal. Note that if the final frequency doubling stage 130 is used with a pulsed laser, no cavity is required (i.e., the mirrors 132-1, 132-2, 132-3, and 132-4 may be omitted), and the intermediate light beam 129-1 may be guided to the input face 153-IN of the grating assembly 150 by any suitable combination of lenses and / or mirrors and focused within or adjacent to the input face 153-IN.

[0035] The nonlinear crystal grating assembly 150 includes an integral nonlinear crystal grating structure 160 and an integral nonlinear crystal grating structure 170 that are fixedly connected to one another (e.g., by an external frame, not shown) in a facing interdigital configuration. As described in more detail below, each integral nonlinear crystal grating structure (grating structure) 160 and 170 includes a plurality of parallel spaced apart mesas (plates), but for clarity and to simplify the description below, each of the grating structures 160 and 170 is illustrated in FIG. 1 with only one mesa. Specifically, the grating structure 160 includes a mesa 162 that is integrally connected to and protruding (extending) from a base 161, and the grating structure 170 includes a mesa 172 that is integrally connected to and protruding from a base 171. As used herein, the terms "integral" and "integrally connected" are used to describe grating structures formed by removing or otherwise processing material from a single nonlinear crystal (e.g., both base 161 and mesa 162 are part of a single SBO crystal). In an actual embodiment, as described below with reference to Figures 4C and 6C, each of grating structures 160 and 170 includes a plurality of counter-comb mesas integrally connected to and extending from bases 161 and 171, respectively. As used herein, the term "counter-comb configuration" means that grating structure 160 is positioned relative to grating structure 170 such that the mesas of grating structure 160 extend into spaces disposed between the mesas of grating structure 170, such that light passes alternately through the alternating counter-comb grating patterns formed by aligned portions of the counter-comb mesas of grating structures 160 and 170 (see embodiment described below with reference to Figure 5 for further clarification).

[0036] As described in more detail below, the mesas 162 and 172 are rectangular structures having opposing planar input and output faces. In operation, a light beam 133 passes through the input and output faces. Referring to the leg section of FIG. 1, the light beam 133 is guided to the planar input face 163-1, a first intra-crystalline light portion 134-1 passes through the mesa 162, a second intra-crystalline light portion 134-2 emerges from the output face 163-2 through the intervening space 152 and enters the planar input face 173-1, a third intra-crystalline light portion 134-3 passes through the mesa 172, and the emitted light 136 is transmitted out of the grating assembly 150 by the output face 173-2. In one embodiment, the input / output faces of the outermost mesas form the input / output faces of the grating assembly 150. 1 includes only two mesas, so that input face 163-1 of mesa 162 serves as input face 153-IN of grating assembly 150, and output face 173-2 of mesa 172 serves as output face 153-OUT of grating assembly 150 (in a practical embodiment, the output and input faces of the grating assembly would not appear on adjacent mesas). Thus, when grating structures 160 and 170 are arranged in the requisite opposed-comb configuration, and grating assembly 150 is otherwise operatively arranged within final conversion stage 130, light passes through grating assembly 150 by impinging on input face 153-IN, passes through mesas 162 and 172 and intervening gap 152, and then exits grating assembly 150 through output face 153-OUT.

[0037] The nonlinear crystal grating assembly 150 achieves QPM of the incident light (e.g., beam 133 and / or beam 129) when the grating structures 160 and 170 are formed with mesas having appropriate widths in the light propagation direction PD, and the grating structures 160 and 170 are formed with inverted crystal axes aligned with respect to the light propagation direction PD and polarization direction 329 of the incident light as described below. With reference to the callout section of FIG. 1, both the grating structures 160 and the grating structures 170 are fabricated such that the respective mesas 162 and 172 have widths T1 (e.g., distances between input faces 163-1 / 173-1 and output faces 163-2 / 173-2) that are substantially equal to an odd integer multiple of a critical length Λ1 measured in the propagation direction PD of the intermediate light beam 129 / 133. The critical length Λ1 is described in further detail below with reference to FIGS. 4A-4C. Also, as shown in the callout section, the crystalline material used to form the grating structure 160 includes an associated first crystalline axis (indicated by optical axes A11, A12, and A13), and the crystalline material used to form the grating structure 170 includes an associated second crystalline axis (indicated by optical axes A21, A22, and A23). To achieve QPM, the first optic axes A11 and A21 of both grating structures 160 and 170 are aligned parallel to the light propagation direction PD, and both second optic axes A12 and A22 are aligned parallel to the light polarization direction 329. Furthermore, as described above, the first and second crystalline axes of the grating structures 160 and 170 are flipped (rotated 180° with respect to each other). For purposes of explanation, the term "upright" is assigned to indicate the arbitrarily selected first crystal axis A11 / A12 / A13 shown in FIG. 1 that satisfies the optical axis orientation requirement (i.e., the optical axis A11 is aligned parallel to the light propagation direction PD, and the optical axis A12 is parallel to the light polarization direction 329). In contrast, the term "inverted" is assigned to three possible orientations of the second crystal axis A21 / A22 / A23 that are rotated 180° with respect to the first crystal axis A11 / A12 / A13 and that satisfy the optical axis orientation requirement (i.e., in each of the three orientations, the optical axis A21 is aligned parallel to the light propagation direction PD, and the optical axis A22 is parallel to the light polarization direction 329).1, and thus the terms "upright" and "inverted" are assigned arbitrarily and are used only to indicate the 180° rotation between the first crystal axis A11 / A12 / A13 and the second crystal axis A21 / A22 / A23. When the grating structures 160 and 170 are formed as described above and arranged in an opposed interdigital configuration, the mesas 162 and 172 (along with the intervening gap 152) provide a 180° offset between the intermediate light beam 129 and the laser output 139 (i.e., ω, as illustrated in the specific examples described below). OUT and frequency ω x between ω and ω, or as described in some of the specific examples below. x and ω y collectively form a periodic structure that achieves quasi-phase matching (QPM) such that the light portion 136 emerging from the output face 153-OUT of the nonlinear crystal grating assembly 150 has a desired output frequency ω OUT The laser output light beam 139 has a magnitude of

[0038] 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) 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 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 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 an 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 an output frequency ω1 equal to eight times the first fundamental frequency ω1. OUTA The laser generates a laser output beam 139A having a

[0039] 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 "fundamental") at a first fundamental frequency ω1 using 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, Trumpf LLC, and other manufacturers. Such manufacturers also sell lasers that generate light having wavelengths around 532 nm. That is, the laser includes a first fundamental laser 110A and a first frequency doubling stage 120A-1. To generate sufficient light at a wavelength of approximately 133 nm for inspecting a semiconductor wafer or reticle, the first fundamental laser 110A should generate fundamental light 119A of tens or hundreds of watts or more.

[0040] According to the exemplary embodiment of FIG. 2A, each of the frequency doubling stages 120A-1 and 120A-2 includes an external resonant cavity including at least three optical mirrors and a nonlinear crystal disposed therein. The cavity can be stabilized with standard PDH (Ponder-Drever-Hall), HC (Hensch-Couillet), or other locking techniques. The length of the cavity is adjusted to maintain resonance by adjusting the position of the mirror or prism with a control signal. The first frequency doubling stage 120A-1 receives and converts the first fundamental light 119A at a first fundamental frequency ω1 to generate a 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 a fourth harmonic light 129A at four times the first fundamental frequency (4ω1).

[0041] In some other embodiments (not shown), a first frequency doubling module may be combined with a first fundamental laser to generate second harmonic light 121A using intracavity frequency doubling in which an NLO crystal is placed within the fundamental solid-state laser cavity.

[0042] In at least one embodiment, the first frequency doubling stage 120A-1 of FIG. 2A that generates the second harmonic light 121A may include a lithium triborate (LBO) crystal. The LBO crystal may be substantially non-critically phase matched (for appropriate selection of crystal planes) at temperatures between room temperature and about 200° C. to generate the second harmonic light in the wavelength range of about 515 nm to 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 the second harmonic light in the wavelength range of about 515 nm to about 535 nm. In other alternative embodiments, the first frequency doubling stage 120A-1 may include KTiOPO4 (KTP), periodically poled lithium niobate (PPLN), periodically poled stoichiometric lithium tantalate (PPSLT), or other nonlinear crystals for frequency conversion.

[0043] 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.

[0044] In an alternative embodiment, the second frequency doubling stage 120A-2, which generates the fourth harmonic, may include a nonlinear crystal grating assembly of the type described herein for implementing QPM. In one embodiment, the grating assembly is formed using SBO crystal. 33When the nonlinear crystal is configured such that the polarizations of both the input and output light are parallel to the c-axis of the SBO crystal to exploit the nonlinear optical coefficient, the critical length of the QPM for generating light having a wavelength of 266 nm from light having a wavelength of 532 nm in SBO is approximately 2.59 μm (i.e., in the range of 2.5 μm to 2.7 μm). Since this critical length is longer than the critical length for generating shorter wavelengths, the thickness of the SBO mesa in the light propagation direction may be equal to the critical length or a small odd multiple (e.g., 3 to 19 times) of the critical length. In another embodiment, the grating assembly is made of an LBO crystal. When the polarization of the input light having a wavelength of 532 nm is parallel to the b-axis and the polarization of the output light having a wavelength of 266 nm is parallel to the c-axis of the LBO crystal, the critical length of the QPM for generating 266 nm from 532 nm in LBO is approximately 3.81 μm (i.e., in the range of 3.78 μm to 3.84 μm).

[0045] 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 U.S. Patent Nos. 9,509,112 and 10,044,166 to Chuang et al., which are incorporated herein by reference.

[0046] Referring to FIG. 2A, the final frequency doubling stage 130A receives the fourth harmonic light 129A and generates the eighth harmonic light 139A having an eighth harmonic frequency 8ω1 equal to eight times the first fundamental frequency ω1. In at least one embodiment, the final frequency doubling stage 130A of FIG. 2A that generates the eighth harmonic light 139A may comprise a nonlinear crystal grating assembly of the type described herein, including an opposed comb grating configuration to achieve QPM. Any of the frequency conversion stages may be housed in one or more more protective environments, such as those described in U.S. Patent No. 8,298,335 to 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 generates VUV wavelengths, this stage needs to be in an environment with very low concentrations of oxygen and water (preferably at concentrations of a few ppm or less). Preferably, the final frequency doubling stage is kept in an environment purged with pure nitrogen or argon, although multiple stages or a single stage may be housed in a single protective environment.

[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, both to Dribinski et al., entitled "Alleviation of laser-induced damage in optical materials by suppression of transient color centers formation and control of phonon population," any of the apparatus or methods described in U.S. Patent No. 8,824,514 to Armstrong, entitled "Measuring crystal site lifetime in a non-linear optical crystal," and any of the apparatus and methods described in U.S. Patent No. 8,976,343 to Genis, entitled "Laser crystal degradation compensation," all of which are incorporated herein by reference.

[0048] Furthermore, any of the intermediate frequency conversion stages referred to 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 U.S. Patent Application Publication No. 2014 / 0305367, entitled "Passivation of Nonlinear Optical Crystals," filed April 8, 2014 to Dribinski et al., which are incorporated herein by reference. Doped or treated crystals may be particularly useful in stages with 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 approximately 170 nm to approximately 180 nm (e.g., approximately 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 summing (conversion) stage 120B-2, and a final frequency doubling stage 130B, which generate an output frequency ω 1 in the wavelength range of approximately 170 nm to approximately 180 nm. OUTB The first fundamental laser 110B-1 is configured to generate fundamental light 119B-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 110B-2 is configured to generate fundamental light 119B-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 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 light 121B to the second fundamental light 119B-2 to generate an intermediate light beam 129B having a sum frequency 2ω1+ω2. If the frequencies of the first fundamental laser 110B-1 and the second fundamental laser 110B-2 are the same (ω1=ω2), then the intermediate light beam 129B is the third harmonic of the fundamental light (3ω1 or 3ω2). The final frequency doubling stage 130B receives the intermediate light beam 129B and outputs an output frequency ω1 equal to twice the sum frequency 2ω1+ω2, i.e., equal to 4ω1+2ω2. OUTB When the frequencies of the first fundamental wave laser 110B-1 and the second fundamental wave laser 110B-2 are the same (ω1=ω2), the output frequency ω OUTB is the sixth harmonic of the fundamental light (6ω1 or 6ω2).

[0050] 2B, the first and second fundamental lasers 110B-1 and 110B-2 are configured as described above with reference to the fundamental laser 110A of FIG. 2A. In an alternative embodiment, the second fundamental laser 110B-2 may be omitted and the output of the first fundamental laser 110B-1 may be split into two parts, a first part that is guided to the first frequency doubling stage 120B-1 and a second part that is guided to the frequency summing stage 120B-2 together with the second harmonic light 121B. In this alternative embodiment, it follows that ω2=ω1.

[0051] According to the exemplary embodiment of FIG. 2B, the 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 summing stage 120B-2 sums the second harmonic light 121B to 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 at least one embodiment, the final frequency doubling stage 130B comprises a nonlinear crystal grating assembly including an opposed comb grating configuration 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. Differences between the final frequency doubling stage 130A and the final frequency doubling stage 130B are described below with reference to Figures 3A and 3B.

[0054] 3A 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. x ω x= 4ω1, or ω if stage 130B is used with 177 nm laser 100B. x =2ω1+ω2) enters a bow-tie cavity that includes input coupler 132C-1, flat mirror 132C-2, and curved mirrors 132C-3, 132C-4, which collectively form optical elements configured to steer input light beam 129C along an optical path through nonlinear crystal grating assembly 150C (i.e., to steer input light beam 129C to input face 153C-IN in a selected propagation direction). Output light 136C is output from nonlinear crystal grating assembly 150C through output face 153C-OUT and is coupled to unconsumed input light 138C and an output frequency ω OUTC (i.e., frequency ω OUTC and a generated laser output light 139C having a power of 1.5 W. (which may be equal to either eighth harmonic output light 139A of FIG. 2A or sixth harmonic output light 139B of FIG. 2B). Unconsumed input light 138C passes through beam splitter (BS) 137C and is recirculated to increase power. Laser output light 139C is reflected from the surface of beam splitter (BS) 137C and guided out of the cavity.

[0055] Preferably, the nonlinear crystal grating assembly 150C is configured such that the input face 153C-IN and the output face 153C-OUT are oriented at approximately the Brewster's angle with respect to the circulating input light 133C. The polarization direction of the circulating input light 133C is indicated by arrow 329C. Additionally, the BS 137C may be configured to laterally displace the circulating input light 133C within the cavity by an amount that offsets the lateral displacement of the input light caused by the grating assembly 150C, 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 CaF2 crystal. Because SBO has high deep UV transmission and a high damage threshold, SBO may be advantageously used as the substrate material of the BS 137C to ensure a long service life despite the high power level of the unconsumed input light 133C circulating in the cavity. If the BS 137C includes an SBO crystal, its thickness and / or the orientation of its crystal axis may be configured to minimize any frequency doubling of the unconsumed input light 133C passing therethrough. The BS 137C may include a dichroic beam splitter, a prism, or other components that separate wavelengths. In one embodiment, the grating assembly 150C is configured such 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. In one embodiment, BSs 137C have surfaces oriented such that unconsumed input light 138C is substantially p-polarized relative to their surfaces and such that their surfaces are at approximately the Brewster's angle relative to the unconsumed input light.

[0057] According to FIG. 3A, the input light (ω x ) 129C is focused by one or more lenses 131C before entering the cavity and matching an eigenmode of the resonant cavity. It has a beam waist within or in close proximity to the nonlinear crystal grating assembly 150C. In at least one embodiment, the one or more lenses 131C include one or more cylindrical lenses comprising SBO glass or SBO crystal and configured to operate at approximately Brewster's angle with respect to the incident light 129C and minimize reflection losses without the use of anti-reflection coatings. SBO is a suitable material for such lenses because it has a high damage threshold at UV and DUV wavelengths. Unconsumed input light 138C (ω ) through BS 137C x ) is reflected by mirror 132C-4 and circulates within the cavity, increasing its intensity. x When the power density of the input light (ωx ) output light 139C(2ω x ) can be very efficient, up to 50% or even higher. x ) exits the cavity after being reflected from BS137C.

[0058] In an alternative embodiment, instead of orienting the input face 153C-IN and output face 153C-OUT at Brewster's angle, the input face 153C-IN of the grating assembly 150C may be coated with a suitable anti-reflection coating.

[0059] Although FIG. 3A illustrates the final frequency doubling stage 130C as including a cavity including two flat mirrors and two curved mirrors, other combinations of mirrors and / or lenses may be used to refocus the light circulating in the cavity. For example, in alternative embodiments, instead of a bowtie cavity, the final frequency doubling stage 130C may include a delta cavity, a standing wave cavity, or a cavity of other shape. If a standing wave cavity is used, the output light may be generated in the same direction as the input light. Any of these cavities can be stabilized using standard PDH or HC locking techniques. The length of the cavity is adjusted to maintain resonance by adjusting the position of one of the mirrors (e.g., mirror 132C-2 in FIG. 3A) or the position of the prism with 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 with a pulsed laser, no cavity is required and the input light 129C may be guided to the grating assembly 150C by any suitable combination of lenses and / or mirrors and focused within or adjacent to the grating assembly 150C.

[0060] 3B shows a simplified diagram of an exemplary final frequency summing stage 130D utilized in the 152 nm laser assembly 100H of FIG. 7A, the 152 nm laser assembly 100I of FIG. 7B, and the 177 nm laser assembly 100J of FIG. 7C, in accordance with an exemplary embodiment of the present invention. x For example, if stage 130D is used in the 152 nm laser 100H of FIG. x = 2ω1 + ω2, and when stage 130D is used with 152 nm laser 100I of FIG. x =2ω1, or if stage 130D is used with 177 nm laser 100J of FIG. 7C, ω x =2ω2) enters a bow-tie cavity that includes input coupler 132D-1, flat mirror 132D-2, curved mirrors 132D-3, 132D-4, and nonlinear crystal grating assembly 150D (including input face 153D-IN and output face 153D-OUT) and passes through input coupler 132D-1 for recirculation to increase power. Grating assembly 150D is configured using an opposed comb configuration as described herein. Input light (second intermediate light beam) 129D-2 has a frequency of ω y (For example, when stage 130D is used with 152 nm laser 100H of FIG. 7A, ω y = 4ω1, or if stage 130D is used with 152 nm laser 100I of FIG. 7B, ω y = 4ω1 + ω2, or if stage 130D is used with the 177 nm laser 100J of FIG. 7C, ω y = 4ω1), enters the bowtie ring cavity, passes near (but not necessarily through) mirror 132D-2, and passes through grating assembly 150D. Output light 136D is output from nonlinear crystal grating assembly 150D through output face 153D-OUT and has a frequency of ω x Unconsumed input light 138D-1 having frequency ω y and the intermediate (input) light beams 129D-1 and 129D-2 having frequencies ω x and ω y The output frequency ω is equal to the sum ofOUTD (i.e., frequency ω OUTD (Wherein, ω can be substantially equal to either seventh harmonic output light 139H and 139I of FIG. 7A or FIG. 7B, or sixth harmonic output light 139J of FIG. 7C). Laser output light 139D is reflected from the input face of beam splitter 137D and guided out of the cavity. x Unconsumed input light 138D-1 having frequency ω passes through beam splitter 137D and optional beam splitter 325 (if present) and is reflected by mirrors 132D-4 and 132D-1, enhancing the intensity of circulated light 133D. y Unconsumed input light 138D-2 exits the cavity after being reflected from either beam splitter 137D or optional (second) beam splitter 325. The polarization direction of circulating input light 133D is indicated by arrow 329D.

[0061] Frequency summing stage 130D may be modified using any of the features and alternatives described above with reference to frequency doubling stage 130C of FIG. 3A. For example, stage 130D may utilize one or more lenses 131D to multiply the frequency ω x and one or more lenses 308 are also utilized to focus input light 129D-1 having a .gtoreq..times ...

[0062] 4A-4C respectively illustrate a first nonlinear crystal grating structure 160E, a second nonlinear crystal grating structure 170E, and a nonlinear crystal grating assembly 150E fabricated by assembling the grating structures 160E and 170E in an opposed interdigital configuration. Grating assembly 150E represents an exemplary embodiment that may be configured for use in the second frequency doubling stage 120A-2 and / or final frequency doubling stage 130A of FIG. 2A, the final frequency doubling stage 130B of FIG. 2B, the second frequency doubling stage 120H-3 and / or final frequency doubling stage 130H of FIG. 7A (described below), the first frequency doubling stage 120I-1 and / or the second frequency doubling stage 120I-2 and / or final frequency summing stage 130I of FIG. 7B (described below), and / or the third frequency doubling stage 120J-3 and / or final frequency summing stage 130J of FIG. 7C (described below). Grating assembly 150E is illustrated and described with reference to five mesas and three grooves for clarity and brevity and is not intended to represent an actual embodiment (see Figures 6A-6C, described below).

[0063] Referring to FIG. 4A, the grating structure 160E includes parallel mesas 162E-1 and 162E-2. The parallel mesas 162E-1 and 162E-2 extend from a base 161E and are separated by a groove 165E. In a practical embodiment, the grating structure 160E is formed by etching or scoring (scribing) rectangular grooves 165E (i.e., open channels or cavities) in a single nonlinear crystal material using standard fabrication techniques. For example, the rectangular grooves 165E can be formed using a dry etching process, such as reactive ion etching (RIE), electron cyclotron resonance plasma etching (ECR), or inductively coupled plasma etching (ICP). Alternatively, the grooves may be cut using a ruling machine (such as one used to cut diffraction gratings) or a diamond lathe. The grating structure 160E is formed such that the mesas 162E-1 and 162E-2 have rectangular cross sections and parallel opposing flat surfaces. That is, mesa 162E-1 includes opposed planar surfaces 163E-11 and 163E-12, and mesa 162E-2 includes opposed planar surfaces 163E-21 and 163E-22. Each planar surface 163E-11, 163E-12, 163E-21, and 163E-22 defines an associated vertical plane parallel to a YZ plane defined by the reference XYZ axes shown in FIG. 4A. Additionally, each mesa 162E-1 and 162E-2 is formed with a width T1 (measured between associated opposed planar surfaces of the X axis) determined as described below with reference to FIG. The depth D1 of groove 165E (i.e., the height of mesas 162E-1 and 162E-2) is at least 10 microns, preferably at least 50 μm, or at least 100 μm, and the width T2 of groove 165E (i.e., the distance on the X-axis between face 163E-12 of mesa 162E-1 and face 163E-21 of mesa 162E-2) is greater than mesa width T1 by about 100 nm to 1 μm.

[0064] 4B, the grating structure 170E includes parallel mesas 172E-1, 172E-2, and 172E-3 that extend from a base 171E (including 171E-1 and 171E-2) and are separated by grooves 175E-1 and 173E-2. Also, the grating structure 170E is formed by etching or cutting rectangular grooves 175E-1 and 175E-2 into the second single nonlinear crystal material using standard fabrication techniques, so that the mesas 172E-1 through 172E-3 are formed with corresponding plate-like rectangular cross sections and parallel opposing flat surfaces (i.e., mesa 172E-1 includes opposing flat surfaces 173E-11 and 173E-12, mesa 172E-2 includes planar end faces 173E-21 and 173E-22, and mesa 172E-3 includes planar end faces 173E-31 and 173E-32). Also, each of the mesas 172E-1 through 172E-3 is formed with the same width T1 as the mesas 162E-1 and 162E-2, and the grooves 175E-1 and 175E-2 have the same depth D1 as that of the groove 165E.

[0065] 4C shows the grating assembly 150E after the grating structures 160E and 170E are assembled in a counter-interdigitated configuration. The mesa 162E-1 is disposed in the groove 175E-1, the mesa 162E-2 is disposed in the groove 175E-2, and the mesa 172E-2 is disposed in the groove 165E. The fabrication of the grating structures 160E and 170E is coordinated such that when assembled in a counter-interdigitated configuration, the grating structure 160E is formed with associated upright crystal axes A11 / A12 / A13 aligned as shown on the left side of FIG. 4C, and the grating structure 170E is formed with one of the three associated inverted crystal axes A21 / A22 / A23 as shown on the right side of FIG. 4C. It should be noted that the larger width T2 (i.e., compared to the width T1 of the mesa) of the grooves 165E, 175E-1, and 175E-2 creates gaps (spaces) 152E-1 to 152E-4 between the opposing flat surfaces of the corresponding mesas when the grating structures 160E and 170E are assembled in an opposing comb configuration. That is, the gap 152E-1 is formed between the flat surface 163E-11 (see FIG. 4A) of the mesa 162E-1 and the flat surface 173E-12 (see FIG. 4B) of the mesa 172E-1. Similarly, the gap 152E-2 is formed between the opposing flat surfaces of the mesas 162E-1 and 172E-2, the gap 152E-3 is formed between the opposing flat surfaces of the mesas 162E-2 and 172E-2, and the gap 152E-4 is formed between the opposing flat surfaces of the mesas 162E-2 and 172E-3. In some embodiments, grating assembly 150E is disposed within an enclosure or chamber that contains a partial vacuum or an inert gas (e.g., argon) to prevent harmful contaminants from collecting on the opposing surfaces that border gaps 152E-1 through 152E-4.

[0066] It should be noted that the grating assembly 150E is illustrated as having equal mesa widths T1 and equal groove widths T2 for the upright grating structure 160E and the inverted grating structure 170E, but this is solely for convenience of manufacture. As can be readily appreciated from this disclosure, each mesa must have a width equal to an odd multiple of the critical length for QPM, but the width need not be the same odd multiple for each mesa, and each groove must be wide enough to receive a corresponding mesa of another grating with a small gap on either side. The nonlinear crystal grating assembly of the present invention is configured for use in a given optical system such that the crystal axis of the inverted grating structure 170E is inverted (i.e., rotated substantially 180°) relative to the crystal axis of the upright grating structure 160E, as shown in the upper and lower left insets of FIG. 5 and described in the associated description below. This physical arrangement of the crystal plates enables QPM. This might be thought of as analogous to using PPLN (periodically poled lithium niobate) for QPM, except that lithium niobate is a ferroelectric crystal and can be periodically poled, whereas SBO and LBO are non-ferroelectric, so we need to physically arrange the crystal mesas to create the periodic structure for QPM.

[0067] FIG. 5 illustrates the final frequency conversion stage 130E with optical elements (not shown) configured to guide the input light beam 133E and the optional second input light beam 133E-2 to the grating assembly 150E as described herein and in more detail above with reference to FIGS. 4A-4C. For illustrative purposes, base portions of the grating structures 160E and 170E are illustrated in dashed (hidden) form to highlight the opposing comb alternating grating pattern 155E formed by the successively aligned mesas 172E-1, 162E-1, 172E-2, 162E-2, and 172E-3 through which the input light 133E passes. In an alternative embodiment, the opposing comb alternating grating pattern 155E is adapted to convert the frequency ωx or a frequency ω x of input light 133E and frequency ω y The input light 133E may be generated, for example, as a sum product of the second harmonic of the first fundamental light and the second fundamental light in the case of the 177 nm laser 100B (described above with reference to FIG. 2B), or as a fourth harmonic of the fundamental light in the case of the 133 nm laser 100A (described above with reference to FIG. 2A). In other examples, the input lights 133E and 133E-2 may be generated as an addition product of the second harmonic of the first fundamental light and the second fundamental light, and as a fourth harmonic of the first fundamental light, in the case of the 152 nm laser 100H (described above with reference to FIG. 7A), as a second harmonic of the first fundamental light and as a addition product of the fourth harmonic of the first fundamental light and the second fundamental light, in the case of the 152 nm laser 100I (described above with reference to FIG. 7B), or as a second harmonic of the second fundamental light and a fourth harmonic of the first fundamental light, in the case of the 177 nm laser 100J (described above with reference to FIG. 7C). Although FIG. 5 illustrates grating assembly 150E as having a periodic structure including an opposed comb grating including five mesas (i.e., mesas 172E-1, 162E-1, 172E-2, 162E-2, and 172E-3), the total number of mesas may be greater than ten, and the total number of gaps will be less than the number of mesas.

[0068] FIG. 5 shows that the gap 152E-1 formed between the mesas 162E-1 and 172E-1 has a gap width T21, and the gap 152E-2 between the mesas 162E-1 and 172E-2 has a gap width T22. Since the groove 175E-1 has a groove width T2 and the mesa 162E-1 has a mesa width T1, the groove width T2 is equal to the sum of the mesa width T1 and the gap widths T21 and T22. As explained above, preferably, the gap widths T21 and T22 should be about 100 nm to 1 μm, respectively. Although the gap widths T21 and T22 do not have to be equal, it will generally be easier to assemble the grating structures 160E and 170E if the mesas of each grating structure are approximately centered on the grooves of the other grating structure (i.e., T21 and T22 are approximately equal). Preferably, the groove width T2 is wider than the mesa width T1 by an amount that provides sufficient clearance to place the two grating structures together without sticking during the assembly process. For example, the width of each groove may be approximately 200 nm to 2 μm wider than the width of a mesa, so that the grating structures can be assembled with gaps on either side of each mesa having gap widths T21 and T22 of approximately 100 nm to 1 μm. If the groove width T2 is too small (e.g., so that the gap widths T21 and T22 are less than about 10 nm), van der Waals force interactions between the two opposing mesa surfaces will cause the two grating structures to stick together before the stacking process is complete. Conversely, if the groove width T2 is too large (e.g., greater than 1 μm), the frequency ω x and (if present) ω y The light of frequency ω OUTE The light does not travel exactly parallel to the light of the other frequencies. Across many such gaps in an assembly containing many mesas, the displacement (walk-off) between the different frequencies of light would accumulate, thereby limiting the conversion efficiency.

[0069] 5, the upright crystal axes A11 / A12 / A13 of the mesas 162E-1 and 162E-2 and the inverted crystal axes A21 / A22 / A22 of the mesas 172E-1, 172E-2, and 172E-3 are configured as shown. Both grating structures 160E and 170E are formed and configured within the final frequency conversion stage 130E such that one or both of the intermediate optical beams 133E and 133E-2 propagate in a direction parallel to the first optical axes A11 and A21, and the polarization direction 329E is parallel to the second optical axes A12 and A22. The second optical axis has a higher nonlinear coefficient than the other optical axes of the nonlinear crystal material, and the mesas 162E-1, 162E-2, 172E-1, 172E-2, and 172E-3 are configured to rotate the optical beam 133E (ω x ) is formed having a thickness T1 substantially equal to an odd integer multiple of the critical length Λ1 measured in the propagation direction PD of the desired laser output light beam 139E and the input light beams 133E and 133E-2 (ω x ,ω y ) is required to achieve quasi-phase matching (QPM) of the mesa 133E. The critical length Λ1 is the distance traveled by the optical beam 133E through each mesa and is determined by the following equation: Λ1=m1L c (Equation 1) where m1 is an odd number (e.g., 1, 3, 5, 7...) and L c is the critical length for quasi-phase matching.

number

number

[0070] In one embodiment, the grating assembly 150E is constructed from SBO crystalline material. In the final frequency doubling stage 130B of the 177 nm laser 100B (FIG. 2B), the quasi-phase matching critical length L c is about 0.6 μm, whereas in the final frequency doubling stage 130A of the 133 nm laser 100A (FIG. 2A), the quasi-phase matching critical length L c is approximately 0.13 μm. An exemplary QPM critical length for frequency doubling of 266 nm light to produce 133 nm light was calculated from the refractive index of SBO at wavelengths of 133 nm and 266 nm using the Sellmeier model published by Trabs et al. (cited above). Trabs et al. did not produce any wavelengths shorter than 160 nm, so the extrapolated reflectivity at 133 nm may be inaccurate. The quasi-phase-matching critical length L c is approximately 0.30 μm when final frequency summing stage 130H is used to generate 152 nm laser output light 139H as described below with reference to Figure 7A, approximately 0.34 μm when final frequency summing stage 130I is used to generate 152 nm laser output light 139I as described below with reference to Figure 7B, and approximately 0.66 μm when final frequency summing stage 130J is used to generate 177 nm laser output light 139J as described below with reference to Figure 7C. Those skilled in the art will know how to calculate the QPM critical length for any given combination of input and output frequencies, given the exact refractive indices.

[0071] In another embodiment, the grating assembly 150E is constructed from LBO crystalline material. In the final frequency doubling stage 130B of the 177 nm laser 100B (FIG. 2B), the quasi-phase matching critical length L c is about 0.6 μm when the polarization direction 329E of the input light 133E is parallel to the c-axis (optical axis A12 / A22). In the final frequency summing stage 130J of the 177 nm laser 100J (FIG. 7C, described below), the quasi-phase matching critical length L c is the input light 133E(ω x =2ω2) is parallel to the a-axis (optical axis A11 / A21), and the polarization direction 329E of the input light 133E-2 (ω y=4ω1) is parallel to the c-axis (optical axis A12 / A22) and the polarization direction of the output light 136E is parallel to the a-axis (optical axis A11 / A21), then it is about 0.86 μm. The value of the refractive index at 177 nm wavelength of LBO used to calculate these critical lengths may not be accurate, so this estimate of the QPM critical length may not be accurate either. Other combinations of polarization orientations are possible for frequency doubling and frequency summing using LBO. Those skilled in the art will understand how to use the above equation to calculate the QPM critical length for a particular input and output light polarization combination, given the exact refractive index.

[0072] As shown in Figure 5, x The input light beam 133E of frequency ω is incident on the input face 153E-IN of the grating assembly 150E, which in this embodiment is implemented by the planar face 173E-11 of the mesa 173E-1. If the grating assembly 150E is configured for frequency doubling, the input light beam 133E-2 is omitted. If the grating assembly 150E is configured for frequency adding, the input light beam of frequency ω y An input light beam 133E-2 having a frequency ω is also incident on the input face 153E-IN. The preferred polarization direction of the input light 133E is indicated by dashed arrow 329E. The polarization direction of the input light 133E-2 depends on the type of nonlinear crystal and the frequency conversion scheme selected. See above for some examples. The angle β between the propagation directions of the input light 133E and the input light 133E-2 should be small, for example less than 5°, preferably about 2° or less. The grating assembly 150E has an input face 153E-IN and an output face 153E-OUT with a frequency ω xThe mesa is oriented at approximately the Brewster angle θ with respect to the circulating light 133E of the 133A, and is configured to minimize reflection losses without the use of anti-reflection coatings. For SBO, the Brewster angle for light polarized parallel to the c-axis (optical z-axis) is approximately 60.5±1° with respect to the surface normal N for UV and visible wavelengths longer than about 210 nm. For LBO, the Brewster angle for light polarized parallel to the b-axis (optical z-axis) is approximately 58.5±0.2° for wavelengths between 532 nm and 355 nm. Alternatively, an anti-reflection coating may be coated on the mesa surface after forming the opposed comb grating assembly to reduce light losses. When coating the mesa with an anti-reflection coating, the width of the mesa must be adjusted to account for the different optical path lengths of different frequencies of light through the coating.

[0073] In the case of frequency doubling, the output light 136E is equal to the second harmonic of the input light (i.e., 2ω x ) Output frequency ω OUTE and an output light 139E having an input frequency ω x In the case of frequency addition, the output light 136E includes the unconsumed input light 138E of the sum of the two input frequencies ω x +ω y Output frequency ω equal to OUTE and output light 139E having frequency ω x and unconsumed input light 138E-1 having frequency ω y and unconsumed input light 138E-2 having

[0074] In at least one embodiment, the crystal axes are oriented such that the light propagating inside the mesas of the grating structures 160E and 170E propagates substantially parallel to one optical axis (identified as A11 and A21 in FIG. 4C), and the polarization direction 329E (electric field direction) of the light 133E is substantially parallel to another optical axis (identified as A12 and A22 in FIG. 4C). For example, to exploit the maximum nonlinear optical coefficient of the selected nonlinear crystal material, or in another example to minimize the difference in refractive index between the input and output frequencies and thus maximize the critical length, the output polarization may be parallel to the optical axis A12 / A22 or parallel to the optical axis A13 / A23. For example, both SBO and LBO crystals have a point group of mm2. The c-axis corresponds to an axis that does not have mirror symmetry and must be inverted. The other axes may need to have specific orientations for the polarization of the input and output light depending on the frequency conversion scheme selected. For example, in at least one embodiment utilizing SBO, the maximum nonlinear coefficient of the crystal, d 33 To take advantage of this, the input and output polarizations need to be parallel to the crystal c-axis. In this example, the optical axis A12 / A22 would correspond to the crystal c-axis, and the upright and inverted grating crystals could be rotated about their a- or b-axes relative to each other. In another example, in an embodiment utilizing LBO, the maximum nonlinear coefficient is d 32 and d 24 (which must be equal due to the symmetry of the crystals). For frequency doubling, the input polarization must be parallel to the b-axis and the output polarization must be parallel to the c-axis, so the crystals of the upright and inverted grating structures must be rotated substantially 180° about the a-axis relative to each other. These are only examples of possible crystal axis orientations for two specific materials and are not intended to limit the scope of the invention. Those skilled in the art will understand how to select the appropriate crystal axis orientations of the upright and inverted gratings for any selected frequency conversion application of a particular nonlinear crystal.

[0075] When the input face 153E-IN of the grating structure 170E is oriented at Brewster's angle with respect to the input light 133E, the propagation direction PD of light within the mesa 172E-1 will be approximately 29.5° with respect to the surface normal N if the grating structure 170E is fabricated from an SBO crystal, and approximately 31.5° with respect to the surface normal N if the grating structure 170E is fabricated from an LBO crystal.

[0076] 6A-6C respectively illustrate a nonlinear crystal grating structure 160F according to another embodiment, a nonlinear crystal grating structure 170F, and a nonlinear crystal grating assembly 150F fabricated by assembling the grating structures 160F and 170F in an opposed comb configuration. The grating structure 160F includes a row of parallel rectangular mesas 162F-1-162F-N. The parallel rectangular mesas 162F-1-162F-N extend vertically downward (i.e., in the Y-axis direction) from a horizontally oriented base 161F. The grating structure 170F includes a row of parallel rectangular mesas 172F-1-172F-N-1. The parallel rectangular mesas 172F-1-172F-N-1 extend vertically upward from a base 171F. Each adjacent pair of mesas is separated by an intervening rectangular groove (e.g., mesas 162F-1 and 162F-2 are separated by groove 165F-1, and mesas 162F-N-1 and 162F-N are separated by groove 165F-N-1. Similarly, mesas 172F-1 and 172F-2 are separated by groove 175F-1, and mesas 172F-N-1 and 172F-N-2 are separated by groove 175F-N-2). In one embodiment, grating structures 160F and 170F are etched, cut, or otherwise machined into corresponding grating structures as described herein, such that input light beam 133F (ω x) and by generating mesas having a mesa width substantially equal to an odd multiple of the critical length required to achieve QPM of the laser output light beam 139F. When assembled as shown in FIG. 6C, the grating assembly 150F includes an alternating opposed-comb grating pattern 155F formed by successively aligned mesas 162F-1, 172F-1...172F-N-1, and 162F-N through which the input light 133F passes. In some practical embodiments, the total number of mesas and grooves forming the alternating opposed-comb grating pattern 155F exceeds 10 (e.g., the total number of mesas may be a hundred or hundreds, or about a thousand). The large number of mesas facilitates higher energy conversion. Thus, the grating assembly 150F may be advantageously utilized in any of the various frequency conversion stages described herein with reference to the grating assembly 150E (FIG. 4C).

[0077] 7A is a simplified block diagram illustrating an exemplary laser assembly 100H according to another specific exemplary embodiment of the present invention. The laser assembly 100H includes a first fundamental laser 110H-1, a second fundamental laser 110H-2, three intermediate frequency conversion stages (i.e., a first frequency doubling stage 120H-1, a frequency summing stage 120H-2, and a second frequency doubling stage 120H-3), and a final frequency summing (conversion) stage 130H, which generate a wavelength ω in the range of approximately 147 nm to approximately 155 nm (e.g., approximately 152 nm). OUTHThe first fundamental laser 110H-1 is configured to generate a (first) fundamental light 119H-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 as described above. The second fundamental laser 110H-2 is configured to generate a (second) fundamental light 119H-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 as described above. The first frequency doubling stage 120H-1 receives the first fundamental light 119H-1 and generates a second harmonic light 121H having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The beam splitter 124H splits the second harmonic light 121H into two parts, a first part 121H-1 and a second part 121H-2. The first part 121H-1 of the second harmonic light 121H is received by the frequency summing stage 120H-2, which sums the first part 121H-1 with the second fundamental light 119H-2 to produce a corresponding frequency ω x For convenience, this sum frequency is referred to herein as being substantially equal to the third harmonic (because ω1 and ω2 are similar or nearly equal). That is, if the frequencies of the first fundamental laser 110H-1 and the second fundamental laser 110H-2 are substantially the same (i.e., ω1=ω2), then the frequency ω of the first intermediate optical beam 129H-1 will be x is substantially equal to the third harmonic of either the fundamental optical frequency ω or ω (i.e., ω x ≒3ω1 or ω x ≈3ω2). The frequency summing stage 120H-2 is configured similarly as described above for the frequency summing stage 120B-2 with reference to FIG. 2B. The second portion 121H-2 of the second harmonic light 121H is sent to a second frequency doubling stage 120H-3. The second frequency doubling stage 120H-3 outputs a corresponding frequency ω y(i.e. ω y 7A, each of the frequency doubling stages 120H-1 and 120H-3 comprises an external resonant cavity including at least three optical mirrors and a nonlinear crystal arranged therein in a similar manner as described above with reference to the second frequency doubling stage 120A-2 of FIG. 2A. The final frequency summation stage 130H is configured to generate the first and second intermediate optical beams 129H-1 and 129H-2 (i.e., ω x +ω y ) to get the output frequency ω equal to 6ω1+ω2 OUTH Laser output light 139H is referred to herein as being substantially equivalent to seventh harmonic light (i.e., if ω1 and ω2 are similar or approximately equal, then ω x +ω y = 6ω1 + ω2 ≈ 7ω1), which in at least one embodiment has a wavelength of approximately 152 nm. In an alternative embodiment, the second fundamental laser 110H-2 may be omitted and the output of the first fundamental laser 110H-1 may be split into two parts, a first part that is guided to the first frequency doubling stage 120H-1 and a second part that is guided to the frequency summing stage 120H-2 together with the second harmonic light 121H-1. In this alternative embodiment, it follows that ω2 = ω1.

[0078] 7B is a simplified block diagram illustrating an exemplary laser assembly 100I configured to generate a wavelength in the range of approximately 147 nm to approximately 155 nm (e.g., approximately 152 nm) in accordance with another specific exemplary embodiment of the present invention. The laser assembly 100I includes a first fundamental laser 110I-1, a second fundamental laser 110I-2, three intermediate frequency conversion stages (i.e., a first frequency doubling stage 120I-1, a second frequency doubling stage 120I-2, and a first frequency summing stage 120I-3), and an output frequency ω 1 in the range of approximately 147 nm to approximately 155 nm (e.g., approximately 152 nm). OUTIThe first frequency doubling stage 120I-1 receives the first fundamental light 119I-1 and generates second harmonic light 121I-1 having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The beam splitter 124I splits the second harmonic light 121I-1 into two parts, a first part 121I-11 and a second part 121I-12. The fundamental lasers 110I-1 and 110I-2 are configured to generate fundamental lights 119I-1 and 119I-2, respectively, having fundamental wavelengths in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to 1.1 μm) and corresponding fundamental frequencies ω1 and ω2, respectively, as described above. The first frequency doubling stage 120I-1 receives the first fundamental light 119I-1 and generates second harmonic light 121I-1 having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The beam splitter 124I splits the second harmonic light 121I-1 into two parts, a first part 121I-11 and a second part 121I-12. The first portion 121I-11 of the second harmonic light 121I-1 has a corresponding frequency ω x The second frequency doubling stage 120I-2 receives the second portion 121I-12 of the second harmonic light 121I-1 to generate fourth harmonic light 121I-2 having a fourth harmonic frequency 4ω1 equal to four times the first fundamental frequency ω1. The first frequency summing stage 120I-3 sums the fourth harmonic light 121I-2 with the second fundamental light 119I-2 to generate a corresponding frequency ω1 equal to the sum frequency 4ω1+ω2. y For convenience, this sum frequency is referred to herein as the fifth harmonic light (i.e., if ω1 and ω2 are similar or approximately equal, then the sum of the fourth harmonic of the first fundamental frequency and the second fundamental frequency will be substantially equal to the fifth harmonic of the first fundamental wavelength, i.e., ω y Final frequency summation stage 130I sums first and second intermediate light beams 129I-1 and 129I-2 using techniques described herein to produce an output frequency ω OUTE For convenience, this is referred to herein as being substantially equal to the seventh harmonic of the first fundamental frequency ω (i.e., if ω≈ω, then ω x +ωy = 6ω1 + ω2 ≈ 7ω1). In at least one embodiment, this has a wavelength of approximately 152 nm. In an alternative embodiment, the second fundamental laser 110I-2 may be omitted and the output of the first fundamental laser 110I-1 may be split into two parts, a first part that is guided to the first frequency doubling stage 120I-1 and a second part that is guided together with the fourth harmonic light 121I-2 to the first frequency summing stage 120I-3. In this alternative embodiment, it follows that ω2 = ω1.

[0079] The first frequency summing stage 120I-3 may be configured to sum the fourth harmonic light 121I-2 to the second fundamental light 119I-2 using CLBO, or hydrogen- or deuterium-treated CLBO in a nearly non-critical phase-matched configuration. Alternatively, the first frequency summing stage 120I-3 may achieve quasi-phase matching (QPM) using a nonlinear crystal grating assembly of the type described herein. In one embodiment, the nonlinear crystal grating assembly is constructed from an SBO crystal. The critical length of QPM for generating 213 nm by summing 266 nm and 1064 nm in SBO is approximately 1.81 μm (i.e., in the range of 1.80 μm to 1.82 μm). Since this critical length is longer than the critical length for generating shorter wavelengths, the thickness of the SBO mesa in the light propagation direction may be equal to the critical length or may be equal to a small odd multiple (e.g., 3 to 9 times) of the critical length. In another embodiment, the nonlinear crystal is an LBO crystal.

[0080] 7C is a simplified block diagram illustrating an exemplary laser assembly 100J according to another specific exemplary embodiment of the present invention. The laser assembly 100J includes a first fundamental laser 110J-1, a second fundamental laser 110J-2, three intermediate frequency conversion stages (i.e., a first frequency doubling stage 120J-1, a second frequency doubling stage 120J-2, and a third frequency doubling stage 120J-3), and a final frequency summing (conversion) stage 130J, which generate a wavelength ω in the range of approximately 170 nm to approximately 180 nm (e.g., approximately 177 nm). OUTJThe fundamental lasers 110J-1 and 110J-2 comprise one or more nonlinear crystal grating assemblies and are configured to generate fundamental lights 119J-1 and 119J-2, respectively, having fundamental wavelengths in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to 1.1 μm) and corresponding fundamental frequencies ω1 and ω2, respectively, as described above. The first frequency doubling stage 120J-1 receives the second fundamental light 119J-2 and multiplies it by a frequency ω x The second frequency doubling stage 120J-2 receives the first fundamental light 119J-1 and generates a second harmonic light 121J 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 120J-3 receives the second harmonic light 121J and generates a fourth harmonic light 121J having a frequency ω1 equal to four times the first fundamental frequency ω1. y The final frequency summation stage 130J sums the first intermediate optical beam 129J-1 (i.e., the second harmonic 2ω2) and the second intermediate optical beam 129J-2 (i.e., the fourth harmonic 4ω1) using techniques described herein to generate an output frequency ω OUTF (i.e., if ω is approximately equal to ω, then x +ω y = 4ω1 + 2ω2 ≈ 6ω1). In at least one embodiment, this has a wavelength of approximately 177 nm. In an alternative embodiment, the second fundamental laser 110J-2 and the first frequency doubling stage 120J-1 may be omitted, and the output 121J of the second frequency doubling stage 120J-2 may be split into two parts, a first part that is guided to the third frequency doubling stage 120J-3, and a second part that is guided to the final frequency summing stage 130J together with the fourth harmonic light 129J-2. In this alternative embodiment, it follows that ω2 = ω1.

[0081] The above diagrams are not intended to represent the actual physical arrangement of components. The above diagrams show the main optical modules involved in the process, but do not show all optical elements. Those skilled in the art will understand how to construct 177 nm, 152 nm, 133 nm, and similar lasers from the above diagrams and their associated descriptions. It will be understood that more or fewer optical components may be used to guide the light, if desired. Lenses and / or curved mirrors may be used to focus the beam waist to a substantially circular or elliptical cross-sectional focal point within or immediately adjacent to the nonlinear crystal, if appropriate. 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, if desired. Prisms, coated mirrors, or other elements may be used to suitably combine different wavelengths at the input to the frequency conversion stage. Beam splitters or coated mirrors may be used suitably 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 the polarization, if desired. Other optical elements may be used suitably. Those skilled in the art understand the various tradeoffs and options that are possible in implementing 177 nm, 152 nm, 133 nm, and similar lasers.

[0082] In various alternative embodiments described above, the first fundamental laser may be configured to generate a first fundamental light at a first fundamental frequency ω1 having a corresponding wavelength equal to one of approximately 1070 nm, approximately 1064 nm, approximately 1053 nm, approximately 1047 nm, and approximately 1030 nm. When used, the second fundamental laser may be configured to generate a second fundamental light at a second fundamental frequency ω2 having a corresponding wavelength equal to 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 corresponding multiples of the 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 a few tenths of a nm or a few nm due to the above and other factors. Those skilled in the art will understand how to select appropriate first and second fundamental wavelengths to produce a desired output wavelength from any one or two fundamental wavelengths.

[0083] Although the invention is described herein with various fundamental wavelengths that facilitate production of laser output light at desired wavelengths of approximately 177 nm, approximately 152 nm, or approximately 133 nm, different fundamental wavelengths can be used to produce other wavelengths within a few nanometers of these desired wavelengths. Unless otherwise specified in the appended claims, such lasers, as well as systems utilizing such lasers, are considered within the scope of the invention.

[0084] Compared with pulsed lasers, CW light sources have a constant power level, which avoids the problem of damage caused by peak power. Also, the bandwidth of the generated CW light is orders of magnitude narrower than a typical mode-locked laser, so the design of the corresponding illumination or detection optical system can be less complex, with higher performance and lower system cost. However, some monitoring and metrology applications can tolerate the higher bandwidth and peak power levels of pulsed lasers. Pulsed lasers are simpler than CW lasers because they do not require a resonant cavity in the frequency conversion stage. Therefore, both CW and pulsed lasers are within the scope of the invention disclosed herein and can be used as appropriate.

[0085] High repetition rate and CW lasers using wavelengths shorter than less than 200 nm are not commercially available at sufficient power levels or are unreliable. Specifically, there are no lasers currently available to generate light with powers of hundreds of mW or more in the wavelength range of approximately 125 nm to 190 nm. An embodiment of the present invention generates short wavelength light down to approximately 133 nm, thus providing higher sensitivity for detecting smaller particles and defects than longer wavelengths. 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 according to the present invention described above. Aspects of such a system are shown in Figures 8, 9A, 9B, and 10.

[0086] This laser can be used in an inspection system using dark-field and bright-field inspection modes as shown in FIG. 8. This figure and system are described in U.S. Patent No. 7,817,260 to Chuang et al. and are 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 the system 800 includes a laser 801, adaptive optics 802 that control the size and profile of the illumination beam at the surface being inspected, apertures and windows 803 in a mechanical housing 804, and a prism 805 that redirects the laser along its optical axis upon normal incidence on the surface of the sample 808. The prism 805 also guides specular reflections of surface features of the sample 808 and reflections from the optical surfaces of the objective lens 806 along the optical path to the image plane 809. Lenses for the objective lens 806 can be provided in the general form of a catadioptric objective lens, a focusing lens group, and a zoom tube lens section 807. In at least one embodiment, the laser 801 may be implemented by one of the lasers described above.

[0087] 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 may have two or three different focusing systems, including off-axis focusing and near normal focusing, as shown. The dark field inspection system may 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.

[0088] 9A illustrates a surface inspection apparatus 900 that includes an illumination system 901 and a focusing system 910 for inspecting an area of ​​a surface 911. As shown in FIG. 9A, a laser system 920 directs a light beam 902 through beam forming optics 903. In at least one embodiment, the laser system 920 includes at least one of the lasers described above. The first beam forming optics 903 can be configured to receive a beam from the laser system. The beam is focused onto the surface 911.

[0089] The beam forming optics 903 is oriented such that its principal plane is substantially parallel to the sample surface 911, such that an illumination line 905 is formed at the surface 911 in the focal plane of the beam forming optics 903. Furthermore, the light beam 902 and the focused beam 904 are guided to the surface 911 at a non-orthogonal angle of incidence. Specifically, the light beam 902 and the focused beam 904 may be guided to the surface 911 at an angle between about 1° and about 85° from the normal direction. In this manner, the illumination line 905 is substantially in the plane of incidence of the focused beam 904.

[0090] The light collection system 910 includes a lens 912 for collecting light scattered from the illumination line 905, and a lens 913 for focusing the light from the lens 912 onto a device including an array of light-sensitive detectors, such as a charge-coupled device (CCD) 914. In one embodiment, the CCD 914 may include a linear array of detectors. In such a case, the linear array of detectors in 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 as a rectangular array, with their long axes parallel to the illumination line 905. For example, the CCD 914 may include a rectangular array of approximately 1000-8000 detectors by approximately 50-250 detectors. In one embodiment, multiple light collection systems may be included, each of which includes similar components but with different orientations.

[0091] For example, FIG. 9B shows an exemplary array of light collection systems 931, 932, and 933 for a surface inspection apparatus (illumination systems similar to that of illumination system 901, for example, are not shown for simplicity). A first optical system of light collection system 931 collects light scattered in a first direction from the surface of sample 921. A second optical system of light collection system 932 collects light scattered in a second direction from the surface of sample 921. A third optical system of light collection system 933 collects light scattered in a third direction from the surface of sample 921. Note that the first, second, and third paths are at different reflection angles with respect to the surface of sample 921. A stage 922 supporting sample 921 can be used to provide relative motion between the optical system and sample 921, thereby scanning the entire surface of sample 921.

[0092] The laser may also be used in an inspection system for unpatterned wafers, such as the inspection system 1000 shown in FIG. 10. Such an inspection system may incorporate oblique and / or normal incidence illumination of scattered light and a large collection solid angle as shown in these figures. The illumination source 1100 incorporates at least one of the laser assemblies described herein that generate DUV or VUV light to illuminate the wafer 1122 at a desired angle and 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 may split the light into multiple channels to send to the TDI sensor 1118. One embodiment may include an intensifier 1114 and / or a sensor repeater 1116. The TDI sensor 1118 and / or intensifier 1114 may be configured to send signals to an image processing computer 1120. The image processing computer 1120 may be configured to generate a wafer image of the wafer 1122 and / or a list of defects or particles on the surface of the wafer 1122. Further description of the elements of FIG. 10 may be found in U.S. Patent No. 9,891,177 B2 to Vazhaeparambil et al. Further details of unpatterned wafer inspection systems may be found 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.

[0093] Although the present invention has been described in connection with 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 encompassed within the scope of the present invention.

Claims

1. A laser assembly for generating a laser output optical beam having an output frequency in the range of approximately 125 nm to approximately 300 nm corresponding output wavelength, One or more fundamental wave lasers each configured to generate a fundamental optical beam having a corresponding fundamental wave frequency, One or more intermediate frequency conversion stages collectively configured to generate one or more intermediate optical beams using the one or more fundamental optical beams, wherein each of the one or more intermediate optical beams has a related intermediate frequency and a corresponding intermediate wavelength, one or more intermediate frequency conversion stages, A final frequency conversion stage comprising a non-linear crystal grating assembly and a plurality of optical elements configured to guide the one or more intermediate optical beams to the non-linear crystal grating assembly such that the one or more intermediate optical beams propagate through the non-linear crystal grating assembly in a propagation direction, Comprising, The non-linear crystal grating assembly, A first integral non-linear crystal grating structure including a first base and a plurality of parallel first mesas protruding from the first base, A second integral non-linear crystal grating structure including a second base and a plurality of parallel second mesas protruding from the second base, Comprising, The first and second integral non-linear crystal grating structures are fixedly arranged in an opposing comb-like configuration and are arranged to alternately pass through the first and second mesas while the one or more intermediate optical beams pass through the non-linear crystal grating assembly, The width of each of the first mesas and each of the second mesas is substantially equal to an odd multiple of a critical length required to achieve quasi-phase matching (QPM) of the one or more intermediate optical beams and the laser output optical beam, A laser assembly, wherein a first crystal axis of the first integral non-linear crystal grating structure is inverted with respect to a corresponding second crystal axis of the second integral non-linear crystal grating structure.

2. The laser assembly according to claim 1, wherein the first and second integral non-linear crystal grating structures comprise strontium tetraborate crystals.

3. The final frequency conversion stage is configured such that the one or more intermediate optical beams are incident on the input surface of the nonlinear crystal grating assembly at approximately the Brewster angle, the laser assembly according to claim 1.

4. The final frequency conversion stage is configured such that the first optical axes of both the first and second crystal axes are parallel to the propagation direction, the laser assembly according to claim 1.

5. The final frequency conversion stage is further configured such that the polarization of the intermediate optical beam is substantially parallel to the second optical axis of the first and second crystal axes, and the second optical axis has a higher nonlinear coefficient than the first optical axis and the third optical axis of the first and second crystal axes, the laser assembly according to claim 4.

6. The plurality of parallel first mesas of the first integral nonlinear crystal grating structure include a first mesa and a second mesa separated by a first groove, The plurality of parallel second mesas of the second integral nonlinear crystal grating structure include a third mesa, a fourth mesa, and a fifth mesa, the third mesa being separated from the fourth mesa by a second groove, and the fourth mesa being separated from the fifth mesa by a third groove, When the first and second integral nonlinear crystal grating structures are fixedly arranged in the opposed comb configuration, the first mesa is disposed in the second groove, the second mesa is disposed in the second groove, and the fourth mesa is disposed in the first groove, the laser assembly according to claim 1.

7. The width of the first groove is about 100 nm to 1 μm larger than the width of the fourth mesa, the laser assembly according to claim 6.

8. Each of the first, second, and third grooves has a depth of at least 10 μm, the laser assembly according to claim 7.

9. The final frequency conversion stage further includes a plurality of mirrors configured to receive and circulate the one or more intermediate optical beams such that the beam waist of the one or more intermediate optical beams appears on the input surface of the nonlinear crystal grating assembly A beam splitter arranged to receive light emitted from the non-linear crystal, wherein the beam splitter reflects a first portion of the emitted light to form the laser output light beam, and a second portion of the emitted light is configured to pass through the beam splitter and be circulated by the plurality of mirrors. The laser assembly according to claim 1, characterized by comprising the above.

10. The one or more fundamental wave lasers are configured to generate fundamental light having a fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm. The one or more intermediate frequency conversion stages 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 wave 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 wave frequency. Comprising The final frequency conversion stage is configured to frequency double the fourth harmonic light, whereby the output frequency of the laser output light is equal to eight times the fundamental wave frequency. The laser assembly according to claim 1.

11. The first and second integral non-linear crystal grating structures include a strontium tetraborate crystal formed such that the crystal c-axes of the first and second crystal axes are substantially parallel to the polarization direction of the fourth harmonic light. The output frequency corresponds to a wavelength substantially equal to 133 nm. The width of each of the first mesa and each of the second mesa is substantially equal to an odd multiple of a critical length in the range of 0.11 μm to 0.15 μm to enable quasi-phase matching between the fourth harmonic frequency and the eighth harmonic frequency. The laser assembly according to claim 10.

12. The one or more fundamental wave lasers A first fundamental wave laser configured to generate first fundamental light having a first fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm. A second fundamental wave laser configured to generate second fundamental light having a second fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm. Comprising The one or more intermediate frequency conversion stages A frequency doubling stage configured to be coupled to receive the first fundamental optical wave and generate a second harmonic optical wave having a second harmonic frequency equal to twice the first fundamental wave frequency; A frequency addition stage configured to be coupled to receive the second harmonic optical wave from the frequency doubling stage and the second fundamental optical wave and generate the intermediate optical beam as a third harmonic optical wave having a third harmonic frequency substantially equal to three times the first fundamental wave frequency; Comprising; The final frequency conversion stage is configured to frequency double the third harmonic optical wave, whereby the output frequency of the laser output light is substantially equal to six times the first fundamental wave frequency. The laser assembly according to claim 1.

13. The first and second integral non-linear crystal grating structures include a strontium tetraborate crystal formed such that the crystal c-axis of the first and second crystal axes is substantially parallel to the polarization direction of the third harmonic optical wave; The output frequency corresponds to a wavelength substantially equal to 177 nm; The width of each of the first mesa and each of the second mesa is substantially equal to an odd multiple of a critical length in the range of 0.58 μm to 0.62 μm to enable quasi-phase matching between the third harmonic frequency and the sixth harmonic frequency. The laser assembly according to claim 12.

14. The one or more fundamental wave lasers are configured to generate a fundamental optical wave having a fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm; The one or more intermediate frequency conversion stages Comprise a first frequency doubling stage configured to be coupled to receive the fundamental optical wave and generate a second harmonic optical wave having a second harmonic frequency equal to twice the fundamental wave frequency; The final frequency conversion stage is configured to frequency double the second harmonic optical wave, whereby the output frequency of the laser output light is equal to four times the fundamental wave frequency. The laser assembly according to claim 1.

15. The first and second integral non-linear crystal grating structures include a strontium tetraborate crystal formed such that the crystal c-axis of the first and second crystal axes is substantially parallel to the polarization direction of the second harmonic optical wave; The output frequency corresponds to a wavelength substantially equal to 266 nm; The width of each of the first mesa and each of the second mesa is substantially equal to an odd multiple of a critical length in the range of 2.5 μm to 2.7 μm to enable pseudo-phase matching between the second harmonic frequency and the fourth harmonic frequency, the laser assembly according to claim 14.

16. 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 in the range of 1 μm to 1.1 μm; A second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength in the range of 1 μm to 1.1 μm; Comprising The plurality of intermediate frequency conversion stages 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 addition stage coupled to receive the fourth harmonic light from the second frequency doubling stage and the second fundamental light and configured to generate fifth harmonic light having a fifth harmonic frequency substantially equal to five times the first fundamental frequency; Comprising The final frequency conversion stage is configured to add the fifth harmonic light to a second portion of the second harmonic light from the first frequency doubling stage, whereby the output frequency of the laser output light is substantially equal to the seventh harmonic frequency of the first fundamental frequency, the laser assembly according to claim 1.

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 in the range of 1 μm to 1.1 μm; A second fundamental laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength in the range of 1 μm to 1.1 μm; Comprising The plurality of intermediate frequency conversion stages 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 addition stage configured to receive a first portion of the second harmonic light from the first frequency doubling stage and the second fundamental light, and generate a first intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental wave frequency; A second frequency doubling stage configured to receive a second portion of the second harmonic light from the first frequency doubling stage and generate a second intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental wave frequency; Comprising: The final frequency conversion stage is configured to add the third harmonic light received from the frequency addition stage and the fourth harmonic light received from the second frequency doubling stage, whereby the output frequency of the laser output light is substantially equal to seven times the first fundamental wave frequency. The laser assembly according to claim 1.

18. The one or more fundamental wave lasers A first fundamental wave laser configured to generate a first fundamental light having a first fundamental wave frequency with a corresponding fundamental wave wavelength of 1 μm to 1.1 μm; A second fundamental wave laser configured to generate a second fundamental light having a second fundamental wave frequency with a corresponding fundamental wave wavelength of 1 μm to 1.1 μm; Comprising: The plurality of intermediate frequency conversion stages A first frequency doubling stage configured to receive the second fundamental light and generate a first intermediate light beam having a second harmonic frequency equal to twice the second fundamental wave frequency; A second frequency doubling stage configured to receive the first fundamental light and generate second harmonic light having a second harmonic frequency equal to twice the first fundamental wave frequency; A third frequency doubling stage configured to receive the second harmonic light from the second frequency doubling stage and generate a second intermediate light beam having a fourth harmonic frequency equal to four times the first fundamental wave frequency; Comprising: The final frequency conversion stage is configured to add the first intermediate light beam and the second intermediate light beam, whereby the output frequency of the laser output light has a sixth harmonic frequency substantially equal to six times the first fundamental wave frequency. The laser assembly according to claim 1.

19. An inspection system configured to inspect a sample using a laser output optical beam having an output frequency in a range of approximately 125 nm to approximately 300 nm for a corresponding output wavelength, wherein the laser output optical beam is generated by a laser assembly, and the laser assembly comprises one or more fundamental wave lasers each configured to generate a fundamental optical beam having a corresponding fundamental wave frequency, one or more intermediate frequency conversion stages collectively configured to generate one or more intermediate optical beams using the one or more fundamental optical beams, wherein each of the intermediate optical beams has a related intermediate frequency, a non-linear crystal grating assembly, and a final frequency conversion stage comprising a plurality of optical elements configured to guide the one or more intermediate optical beams to the non-linear crystal grating assembly such that the one or more intermediate optical beams propagate through the non-linear crystal grating assembly in a propagation direction, comprising the non-linear crystal grating assembly comprises a first integral non-linear crystal grating structure including a first base and a plurality of parallel first mesas protruding from the first base, a second integral non-linear crystal grating structure including a second base and a plurality of parallel second mesas protruding from the second base, comprising the first and second integral non-linear crystal grating structures are fixedly arranged in an opposing comb-like configuration and are arranged to alternately pass through the first and second mesas while the one or more intermediate optical beams pass through the non-linear crystal grating assembly, the width of each of the first mesas and each of the second mesas is substantially equal to an odd multiple of a critical length required to achieve quasi-phase matching (QPM) between the one or more intermediate optical beams and the laser output optical beam, the inspection system, wherein a first crystal axis of the first integral non-linear crystal grating structure is inverted with respect to a corresponding second crystal axis of the second integral non-linear crystal grating structure.

20. A non-linear crystal grating assembly configured to convert one or more input optical beams into laser output light, wherein the one or more input optical beams have corresponding fundamental frequencies and are guided through the non-linear crystal grating assembly in a predetermined propagation direction, and the laser output light has an output frequency corresponding to an output wavelength in the range of approximately 125 nm to approximately 300 nm, and the non-linear crystal grating assembly comprises a first integral non-linear crystal grating structure including a first base and a plurality of parallel first mesas protruding from the first base; a second integral non-linear crystal grating structure including a second base and a plurality of parallel second mesas protruding from the second base; and the first and second integral non-linear crystal grating structures are fixedly arranged in a opposed comb configuration and are arranged to alternately pass through the first and second mesas while the one or more intermediate optical beams pass through the non-linear crystal grating assembly along the predetermined propagation direction, the width of each of the first mesas and the width of each of the second mesas are substantially equal to an odd multiple of a critical length required to achieve quasi-phase matching (QPM) between the one or more input optical beams and the laser output optical beam, a non-linear crystal grating assembly, characterized in that a first crystal axis of the first integral non-linear crystal grating structure is inverted with respect to a corresponding second crystal axis of the second integral non-linear crystal grating structure.

21. A method for generating a laser output optical beam having an output frequency corresponding to an output wavelength in the range of approximately 125 nm to approximately 300 nm, comprising generating the one or more fundamental optical beams such that each of the one or more fundamental optical beams has a corresponding fundamental frequency with a corresponding fundamental wavelength in the range of about 1 µm to 1.1 µm; utilizing one or more intermediate frequency conversion stages collectively configured to generate one or more intermediate optical beams using the one or more fundamental optical beams, each of the one or more intermediate optical beams having a related intermediate frequency; guiding the one or more intermediate optical beams to the non-linear crystal grating assembly such that the one or more intermediate optical beams pass through the non-linear crystal grating assembly in a propagation direction using a final frequency conversion stage; and The non-linear crystal grating assembly is, a first integrated non-linear crystal grating structure including a first base and a plurality of parallel first mesas protruding from the first base, a second integrated non-linear crystal grating structure including a second base and a plurality of parallel second mesas protruding from the second base, comprising, the first and second integrated non-linear crystal grating structures are fixedly arranged in a counter-comb configuration, and are arranged to alternately pass through the first and second mesas while the one or more intermediate optical beams pass through the non-linear crystal grating assembly, the width of each of the first mesas and each of the second mesas is substantially equal to an odd multiple of a critical length required to achieve quasi-phase matching (QPM) between the one or more intermediate optical beams and the laser output optical beam, A method characterized in that a first crystal axis of the first integrated non-linear crystal grating structure is inverted with respect to a corresponding second crystal axis of the second integrated non-linear crystal grating structure. **Claim 22**: A laser assembly for generating a laser output optical beam having an output frequency in the range of approximately 125 nm to approximately 300 nm for a corresponding output wavelength, one or more fundamental wave lasers each configured to generate a fundamental optical beam having a corresponding fundamental wave frequency, one or more intermediate frequency conversion stages collectively configured to generate one or more intermediate optical beams using the one or more fundamental optical beams, wherein each of the one or more intermediate optical beams has a related intermediate frequency and a corresponding intermediate wavelength, one or more intermediate frequency conversion stages, a final frequency conversion stage configured such that the one or more intermediate optical beams pass through a non-linear crystal grating assembly, comprising, the non-linear crystal grating assembly includes the upright grating and the inverted grating interleaved in a counter-comb configuration such that the one or more intermediate optical beams pass through the upright grating and the inverted grating, each of the upright grating and the inverted grating includes two parallel mesas separated by grooves, each of the upright grating and the inverted grating is, The width of each of the mesas in the propagation direction of the one or more intermediate optical beams passing through each mesa is substantially equal to an odd multiple of a critical length required to achieve quasi-phase matching (QPM) of the one or more intermediate optical beams and the laser output optical beam having the output frequency. The width of each of the grooves is greater than the width of each of the mesas. A laser assembly, wherein a first crystal axis of the upright grating is configured to be inverted with respect to a corresponding second crystal axis of the inverted grating. **Claim 23**: The one or more fundamental wave lasers A first fundamental wave laser configured to generate first fundamental light having a first fundamental wave frequency with a corresponding fundamental wave wavelength of 1 μm to 1.1 μm. A second fundamental wave laser configured to generate second fundamental light having a second fundamental wave frequency with a corresponding fundamental wave wavelength of 1 μm to 1.1 μm. comprising The one or more intermediate frequency conversion stages 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 wave frequency. A frequency addition stage coupled to receive the second harmonic light from the frequency doubling stage and the second fundamental light and configured to generate the intermediate optical beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental wave frequency. comprising The final frequency conversion stage is configured to frequency double the third harmonic light, whereby the output frequency of the laser output light is substantially equal to six times the first fundamental wave frequency. The laser assembly according to claim 22. **Claim 24**: The laser assembly according to claim 23, wherein the upright grating and the inverted grating comprise a lithium triborate crystal. **Claim 25**: The non-linear crystal grating assembly is configured such that a first crystal axis of the upright grating, a crystal b-axis of the upright grating, is oriented to be substantially parallel to a polarization direction of the third harmonic light passing through the upright grating. The output frequency corresponds to a wavelength substantially equal to 177 nm. The laser assembly according to claim 24, wherein each of the mesas has a thickness substantially equal to an odd multiple of a critical length in the range of 0.58 μm to 0.62 μm in order to enable pseudo-phase matching between the third harmonic frequency and the output frequency. **Claim 26**: The non-linear crystal grating assembly is configured such that the crystal b-axis of the grating arranged in the opposed comb-like configuration is oriented substantially parallel to the polarization direction of the second harmonic light passing through the upright grating. The output frequency corresponds to a wavelength substantially equal to 266 nm. The laser assembly according to claim 24, wherein each of the mesas has a thickness substantially equal to an odd multiple of a critical length in the range of 3.3 μm to 3.7 μm in order to enable pseudo-phase matching between the second harmonic frequency and the output frequency. **Claim 27**: The one or more fundamental wave lasers are configured to generate fundamental light having a fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm. The one or more intermediate frequency conversion stages are coupled to receive the fundamental light and include a first frequency doubling stage configured to generate second harmonic light having a second harmonic frequency equal to twice the fundamental wave frequency. The laser assembly according to claim 22, wherein the final frequency conversion stage is configured to frequency double the second harmonic light, whereby the output frequency of the laser output light is equal to four times the fundamental wave frequency. **Claim 28**: The laser assembly according to claim 27, wherein the upright grating and the inverted grating include a lithium triborate crystal. **Claim 29**: The one or more fundamental wave lasers include a first fundamental wave laser configured to generate first fundamental light having a first fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm, and a second fundamental wave laser configured to generate second fundamental light having a second fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 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 wave frequency. ​ Coupled to receive the 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. Coupled to receive the fourth harmonic light from the second frequency doubling stage and the second fundamental light, and configured to generate fifth harmonic light having a fifth harmonic frequency substantially equal to five times the first fundamental frequency. Comprising The final frequency conversion stage is configured to add the fifth harmonic light to the second portion of the second harmonic light from the first frequency doubling stage, whereby the output frequency of the laser output light is substantially equal to the seventh harmonic frequency of the first fundamental frequency. The laser assembly according to claim 22, characterized in that.

30. The crystal axes of the upright grating and the inverted grating are oriented such that the crystal c-axes of both the upright grating and the inverted grating are substantially parallel to the polarization directions of the second harmonic light and the fifth harmonic light passing through the upright grating and the inverted grating. The output frequency corresponds to a wavelength substantially equal to 152 nm. At least one of the upright grating and the inverted grating has a mesa thickness substantially equal to an odd multiple of a critical length in the range of 0.31 μm to 0.37 μm to enable quasi-phase matching of the second harmonic frequency, the fifth harmonic frequency, and the seventh harmonic frequency. The laser assembly according to claim 29, characterized in that.

31. The one or more fundamental wave lasers A first fundamental wave laser configured to generate first fundamental light having a first fundamental frequency with a corresponding fundamental wavelength of 1 μm to 1.1 μm. A second fundamental wave laser configured to generate second fundamental light having a second fundamental frequency with a corresponding fundamental wavelength of 1 μm to 1.1 μm. Comprising The plurality of intermediate frequency conversion stages 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. Coupled to receive a first portion of the second harmonic light from the first frequency doubling stage and the second fundamental light, and configured to generate a first intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental wave frequency. A frequency addition stage; Coupled to receive a second portion of the second harmonic light from the first frequency doubling stage, and a second frequency doubling stage configured to generate a second intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental wave frequency; Comprising; The final frequency conversion stage is configured to add the third harmonic light received from the frequency addition stage and the fourth harmonic light received from the second frequency doubling stage, whereby the output frequency of the laser output light is substantially equal to seven times the first fundamental wave frequency. The laser assembly according to claim 22, characterized in that

32. The upright grating and the inverted grating are oriented such that the crystal c-axes of both the first crystal axis and the second crystal axis are substantially parallel to the polarization directions of the third harmonic light and the fourth harmonic light passing through the upright grating and the inverted grating; The output frequency corresponds to a wavelength substantially equal to 152 nm; At least one of the upright grating and the inverted grating has a mesa thickness substantially equal to an odd multiple of a critical length in the range of 0.27 μm to 0.33 μm to enable quasi-phase matching of the third harmonic frequency, the fourth harmonic frequency, and the seventh harmonic frequency. The laser assembly according to claim 31, characterized in that

33. The one or more fundamental wave lasers A first fundamental wave laser configured to generate a first fundamental light having a first fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm; A second fundamental wave laser configured to generate a second fundamental light having a second fundamental wave frequency with a corresponding fundamental wave wavelength in the range of 1 μm to 1.1 μm; Comprising; The plurality of intermediate frequency conversion stages Coupled to receive the second fundamental light, and a first frequency doubling stage configured to generate a first intermediate light beam having a second harmonic frequency equal to twice the second fundamental wave frequency; A second frequency doubling stage that is coupled to receive the first fundamental optical wave and is configured to generate a second harmonic optical wave having a second harmonic frequency equal to twice the first fundamental wave frequency; A third frequency doubling stage that is coupled to receive the second harmonic optical wave from the second frequency doubling stage and is configured to generate the second intermediate optical beam having a fourth harmonic frequency equal to four times the first fundamental wave frequency; Comprising: The final frequency conversion stage is configured to add the first intermediate optical beam and the second intermediate optical beam, whereby the output frequency of the laser output light has a sixth harmonic frequency substantially equal to six times the first fundamental wave frequency. The laser assembly according to claim 22.

34. The crystal axes of the upright grating and the inverted grating are such that the crystal c-axes of both the upright grating and the inverted grating are substantially parallel to the polarization directions of the second harmonic light and the fourth harmonic light passing through the upright grating and the inverted grating. The laser assembly according to claim 29, wherein the output frequency corresponds to a wavelength substantially equal to 177 nm.

35. Both the upright grating and the inverted grating are one of a strontium tetraborate crystal and a lithium triborate crystal, At least one of the upright grating and the inverted grating has a mesa thickness substantially equal to an odd multiple of a critical length in the range of 0.5 μm to 0.7 μm to enable quasi-phase matching of the second harmonic frequency, the fourth harmonic frequency, and the sixth harmonic frequency. The laser assembly according to claim 29.

36. The crystal axes of the upright grating and the inverted grating are such that the crystal c-axes of both the upright grating and the inverted grating are substantially parallel to the polarization directions of the second harmonic light and the fourth harmonic light passing through the upright grating and the inverted grating. The output frequency corresponds to a wavelength substantially equal to 177 nm, Both the upright grating and the inverted grating include a lithium triborate crystal. The laser assembly according to claim 29, wherein at least one of the upright grating and the reversal grating has a mesa thickness substantially equal to an odd multiple of a critical length in the range of 0.8 μm to 0.9 μm in order to enable pseudo-phase matching of the second harmonic frequency, the fourth harmonic frequency, and the sixth harmonic frequency.

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