Patent Publication Number: US-2023161221-A1

Title: Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/555,404, entitled “Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings”, which was filed on Dec. 18, 2021, which claims priority from U.S. Provisional Patent Application No. 63/282,706, entitled “Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings”, which was filed on Nov. 24, 2021, and is incorporated by reference herein. 
     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”, which was filed on Jun. 12, 2020, to U.S. Provisional Patent Application No. 63/076,391, entitled “152 nm and 177 nm CW Lasers Using Stacked Strontium Tetraborate Plates”, which was filed on Sep. 10, 2020, and to U.S. patent application Ser. No. 17/239,561, entitled “Frequency Conversion Using Stacked Strontium Tetraborate Plates”, which was filed on Apr. 23, 2021. All these applications are incorporated by reference herein. 
     This application is also related to the following U.S. patent documents, all of which are incorporated by reference herein: U.S. Pat. No. 6,201,601 to Vaez-Iravani et al., U.S. Pat. No. 6,271,916 to Marxer et al., U.S. Pat. No. 7,525,649 to Leong et al., U.S. Pat. No. 7,817,260 to Chuang et al., U.S. Pat. Nos. 8,298,335 and 8,824,514 to Armstrong, U.S. Pat. No. 8,976,343 to Genis, U.S. Pat. No. 9,023,152 to Dribinski, U.S. Pat. Nos. 9,461,435 and 9,059,560 to Dribinski et al., U.S. Pat. Nos. 9,293,882 and 9,660,409 to Chuang, U.S. Pat. Nos. 9,250,178, 9,459,215, 9,509,112, 10,044,166, 10,283,366 and 11,180 866 to Chuang et al. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present application relates to lasers capable of generating light having deep UV (DUV) or vacuum UV (VUV) wavelengths, and more particularly to lasers capable of generating light in the range of approximately 125 nm to 300 nm and inspection systems that use such lasers to inspect, e.g., photomasks, reticles, and semiconductor wafers. 
     Related Art 
     As semiconductor devices&#39; dimensions shrink, the size of the smallest particle or pattern defect that can cause a device to fail also shrinks. Hence a need arises for detecting smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. The intensity of light scattered by particles smaller than the wavelength of that light generally scales as a high power of the dimensions of that particle (for example, the total scattered intensity of light from an isolated small spherical particle scales proportional to the sixth power of the diameter of the sphere and inversely proportional to the fourth power of the wavelength). Because of the increased intensity of the scattered light, shorter wavelengths will generally provide better sensitivity for detecting small particles and defects than longer wavelengths. 
     Since the intensity of light scattered from small particles and defects is generally very low, high illumination intensity is required to produce a signal that can be detected in a very short time. Average light source power levels of 0.3 W or more may be required. At these high average power levels, a high pulse repetition rate is desirable as the higher the repetition rate, the lower the energy per pulse and hence the lower the risk of damage to the system optics or the article being inspected. The illumination needs for inspection and metrology are generally best met by continuous wave (CW) light sources. A CW light source has a constant power level, which avoids the peak power damage issues and also allows for images or data to be acquired continuously. However, in many cases, mode-locked lasers (sometimes referred to as quasi-CW lasers) with repetition rates of about 50 MHz or higher may be useful because the high repetition rate means that the energy per pulse can be low enough to avoid damage for certain metrology and inspection applications. 
     Therefore, a need arises for a mode-locked or CW laser that generates radiation in the VUV range and is suitable for use in inspection of photomasks, reticles, and/or wafers. If a laser enabling mode-locked or CW output at near 133 nm at higher power level can be practically produced, it could enable more accurate and faster inspection and metrology and contribute to cutting-edge semiconductor production. 
     Furthermore, lasers that generate radiation at DUV wavelengths (such as wavelengths between 190 nm and 300 nm) with high power levels and long intervals between service events (such as 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 humidity during handling, storage, and operation, resulting in considerable complexity and cost in manufacturing, shipping, and operating processes. Other non-hygroscopic nonlinear crystals with high damage thresholds do not phase match at important wavelengths in the DUV wavelength range and so cannot be used with critical or noncritical phase matching for such wavelengths. Therefore, a need also arises for a mode-locked or CW laser that generates radiation in the DUV range without using hygroscopic nonlinear crystals. 
     A need also arises for an inspection system and associated laser system that is capable of generating mode-locked or CW laser light having an output at a DUV or VUV wavelength such as in the range of approximately 125 nm to approximately 300 nm and avoids some, or all, of the above problems and disadvantages. 
     SUMMARY OF THE DISCLOSURE 
     The present invention generally relates to nonlinear crystal grating assemblies for use in laser assemblies (lasers) that are configured to generate 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 respectively processed to include a row of parallel, spaced-apart mesa-type structures (mesas) that are shaped and configured to facilitate assembly into an interdigitated configuration (e.g., such that the mesas of one grating structure fit into grooves formed between two adjacent mesas of the other grating structure, and vice versa). Both grating structures are also formed (e.g., etched or cut) such that each a width of each mesa is substantially equal to an odd integer multiple of a critical length, measured in the propagation direction of light beam, that is required to achieve quasi-phase-matching (QPM) of the incident light beams and the output frequency of a desired laser output light beam. The grating structures are also formed with crystal axes that are inverted (e.g., with the first optical axes aligned parallel with the propagation direction and the second and third optical axes rotated by substantially 180° with respect to each other) 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 into the interdigitated configuration and the grating assembly is operably positioned to receive one or more incident light beams the incident (intermediate) light beam is substantially parallel to the second optical axes of the first and second crystal axes, the mesas of the two grating structures collectively form a grating pattern in which the light beams alternately pass through upright and inverted mesas (i.e., through the upright mesa of one grating structure, then through an inverted mesa of the other grating structure, then through a 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 generation of DUV and VUV laser light at high power and photon energy levels while avoiding the above-mentioned problems and disadvantages associated with prior art approaches. Moreover, by forming the two grating structures such that the upright and inverted mesas form the desired alternating grating pattern when the two grating structures are disposed in the interdigitated configuration, the present invention greatly simplifies the associated manufacturing and assembly processes, and thus reduces the cost of manufacturing the laser assemblies using the crystal grating assemblies of the present invention. 
     In one embodiment, each of the grating structures used to form the nonlinear crystal grating assemblies of the present invention comprise single (integral) strontium tetraborate SrB 4 O 7  (SBO) crystals. SBO crystals exhibit attractive features (e.g., broad transparency range, good damage resistivity and chemical stability, high microhardness, and a high diagonal d 33  nonlinear optical element value compared to the band-gap value) that avoid many of the above-mentioned problems and disadvantages associated with prior art approaches. In an alternative embodiment, the nonlinear crystal is a lithium triborate LiB 3 O 5  (LBO), beta barium borate β-BaB 2 O 4  (BBO), or another nonlinear crystal material that is transparent for input and output frequencies and having at least one nonlinear coefficient that is reasonably large (approximately 1 pm/V or larger). SBO crystals exhibit low birefringence that makes frequency conversion by critical or non-critical phase matching impossible. LBO is widely used for doubling the frequency of an infrared wavelength (such as 1064 nm) to create a second harmonic in the green part of the visible spectrum. LBO has high damage threshold and transmits light at wavelengths as short as about 160 nm. However, the UV refractive indices of LBO are such that critical and noncritical phase matching are not possible for, for example, doubling the frequency of green light at a wavelength near 532 nm. The present invention circumvents the phase-matching limitations of SBO, LBO and other nonlinear crystals by way of using the interdigitated configuration described herein to form grating pattern (i.e., a sequentially aligned periodic series of upright and inverted mesas respectively having the first/upright and second/inverted crystal axes) that achieves QPM of one or more input light frequencies of intermediate light beams directed onto the input surface of the grating assembly at approximately Brewster&#39;s angle such that light exiting the output surface of the grating assembly includes laser output light having a desired DUV or VUV output frequency. 
     In practical embodiments, the grating structures are formed by etching or cutting (ruling) periodically spaced rectangular grooves or cavities in a single piece of nonlinear crystal by means of standard fabrication techniques. The residual nonlinear crystal material between each groove is referred to herein as a mesa or plate, whereby each grating structure consists of a horizontal row of parallel spaced-apart mesas that extend vertically from a horizontal base and are separated by intervening grooves. As mentioned above, the width of each mesa of both grating structures is substantially equal to an odd integer multiple of a critical length to enable QPM of the input light frequency and the desired output frequency. The width of each groove is larger than the width of each mesa such that when the grating structures are assembled into the interdigitated configuration, vertical planar surfaces of the mesas of the first grating structure are separated by small gap distances from opposing vertical planar surfaces 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 ten 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 interdigitated alternating grating pattern that facilitates the use of grating assemblies produced in accordance with the present invention to perform frequency conversion (e.g., frequency doubling of one input light frequency or frequency summing of two input light frequencies) that is required to generate DUV and VUV wavelengths at high power levels (i.e., from several hundred milli-watts (mW) to several watts (W) or more) and high photon energy levels (for example 4.66 eV at 266 nm, 7.00 eV at 177 nm, 8.16 eV at 152 nm, and 9.32 eV at 133 nm) while avoiding the above-mentioned problems and disadvantages associated with prior art approaches. 
     In specifically embodiments described below, the present invention is directed to improvements in inspection systems utilized in the semiconductor fabrication industry, and in particular to laser assemblies for such inspection systems that are capable of generating mode-locked or continuous wave (CW) laser light having a light source power level of 0.3 W or more and having 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, where each fundamental laser respectively generates a fundamental light beam having a corresponding fundamental frequency (e.g., having wavelengths between about 1 μm and 1.1 μm), and the intermediate frequency conversion stages are collectively configured to convert the fundamental light beam(s) into at least one intermediate light beam having an associated intermediate frequency. In at least one embodiment, the final frequency conversion stage is configured to direct the intermediate light beam(s) through a grating assembly such that a polarization direction (electric field direction) of the light passing through each crystal mesa (plate) is substantially parallel to one axis of the crystal material (e.g., axis A2), and such that a propagation direction of the light is substantially parallel to another axis of the crystal (e.g., axis A1), whereby the alternating upright/inverted periodic configuration formed by the interdigitated mesas achieves QPM of the intermediate light beam and the output wavelength. In a specific embodiment, the final frequency conversion stage includes multiple mirrors operably configured (e.g., in a bow-tie ring cavity formation) to receive and circulate at least one of the intermediate light beams (e.g., by way of one or more matching lenses) such that a beam waist of the circulated light occurs at (i.e., inside or proximate to) the grating assembly. In one embodiment the final frequency conversion stage utilizes a beam splitter (e.g., SBO crystal, SBO glass, or CaF 2  crystal) that is configured to split the exiting light (i.e., light leaving/exiting the grating assembly) such that a reflected (first) portion of the exiting light forms the desired laser output light beam having an output wavelength in the range of approximately 125 nm to approximately 300 nm, and such that the non-reflected (second) portion of the exiting light comprising unconsumed input light is passed by the beam splitter for circulation by the cavity mirrors. 
     In various disclosed embodiments, the present invention is directed to improved laser assemblies for inspection systems utilized in the semiconductor fabrication industry, and in particular to laser assemblies for such inspection systems that are capable of generating laser light having a light source power level of 0.3 W or more and having an output wavelength in the range of in the range of approximately 125 nm to approximately 300 nm. In some embodiments, a 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, a nonlinear crystal grating assembly is configured to frequency-sum a first intermediate light beam having a UV wavelength near 355 nm with 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, a nonlinear crystal grating assembly is configured to frequency-sum a first intermediate light beam having visible wavelength near 532 nm with a second intermediate light beam having a DUV wavelength near 213 nm to generate laser light having a VUV wavelength near 152 nm. In yet another alternative embodiment, a nonlinear crystal grating assembly is configured to frequency-sum a first intermediate light beam having a visible wavelength near 532 nm with a second intermediate light beam having a DUV wavelength near 266 nm to generate CW laser light having a VUV output wavelength near 177 nm. In all the above-mentioned embodiments, the integral nonlinear crystal grating structures forming the nonlinear crystal grating assembly are fabricated to include mesas having mesa widths substantially equal to an odd integer multiple of an associated QPM critical length for the input and output light frequencies. 
     In accordance with another embodiment, a laser assembly is configured to generate laser output light with an output wavelength of approximately 133 nm by creating an eighth harmonic of a fundamental frequency by configuring the final frequency conversion stage to double a fourth harmonic of the fundamental frequency using a nonlinear crystal grating assembly of the present invention. 
     In accordance with another embodiment, a laser assembly is configured to generate laser output light with an output wavelength of approximately 177 nm by creating a sixth harmonic of a fundamental frequency by configuring the final frequency conversion stage to double a third harmonic of the fundamental frequency using a nonlinear crystal grating assembly of the present invention. 
     In accordance with another embodiment, a laser assembly is configured to generate laser output light with an output wavelength of approximately 266 nm by creating a fourth harmonic of a fundamental frequency by configuring the final frequency conversion stage to sum two second harmonics (or double a single second harmonic) of the fundamental frequency using a nonlinear crystal grating assembly of the present invention. 
     In accordance with another embodiment, a laser assembly is configured to generate laser output light with an output wavelength of approximately 152 nm by creating a seventh harmonic of a fundamental frequency by configuring the final frequency conversion stage to sum third and fourth harmonics of the fundamental frequency using a nonlinear crystal grating assembly of the present invention. 
     In accordance with another embodiment, a laser assembly is configured to generate laser output light with an output wavelength of approximately 152 nm by configuring the final frequency conversion stage to sum second and fifth harmonics of a fundamental frequency using a nonlinear crystal grating assembly of the present invention. 
     In accordance with another embodiment, a laser assembly is configured to generate laser output light with an output wavelength of approximately 177 nm by configuring the final frequency conversion stage to sum second and fourth harmonics of a fundamental frequency using a nonlinear crystal grating assembly of the present invention. 
     In other embodiments, an inspection system is configured to inspect a sample such as a wafer, reticle or photomask using one of the lasers described herein that generates an output wavelength of, for example, approximately 266 nm, 177 nm, 152 nm or approximately 133 nm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a simplified block diagram showing an exemplary laser assembly according to a generalized exemplary embodiment of the present invention. 
         FIGS.  2 A and  2 B  are simplified block diagrams respectively showing simplified laser assemblies according to first and second specific embodiments of the present invention. 
         FIG.  3 A  is a simplified diagram showing an exemplary final frequency doubling stage utilized in the laser assemblies of the first and second specific embodiments according to an exemplary embodiment of the present invention. 
         FIG.  3 B  is a simplified diagram showing an exemplary final frequency summing stage utilized in the laser assemblies of the third, fourth and fifth specific embodiments according to an exemplary embodiment of the present invention. 
         FIGS.  4 A,  4 B and  4 C  are perspective views showing exemplary integral nonlinear crystal grating structures and an exemplary simplified nonlinear crystal grating assembly formed by the integral nonlinear crystal grating structures for use in the final frequency conversion stages of a laser assembly. 
         FIG.  5    is a modified cross-sectional view showing a partial final frequency conversion stage including the exemplary nonlinear crystal of  FIG.  4 C  during operation. 
         FIGS.  6 A,  6 B and  6 C  are perspective views showing a nonlinear crystal grating assembly according to another exemplary embodiment. 
         FIGS.  7 A,  7 B and  7 C  are simplified block diagrams respectively showing simplified laser assemblies according to additional specific embodiments of the present invention. 
         FIG.  8    is a simplified diagram showing an exemplary inspection system with dark-field and bright field inspection modes that utilizes one of the laser assemblies described herein in accordance with another specific embodiment of the present invention. 
         FIGS.  9 A and  9 B  illustrates a dark-field inspection system that utilize one of the laser assemblies described herein in accordance with another specific embodiment of the present invention. 
         FIG.  10    illustrates an alternative dark-field inspection system configured for inspecting unpatterned wafers using one of the laser assemblies described herein in accordance with another specific embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention relates to an improvement in lasers for semiconductor inspection systems. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “top”, “left”, “right”, “horizontal”, “vertical” and “downward” are intended to provide relative positions for purposes of description and are not intended to designate an absolute frame of reference. Various modifications to the described embodiments will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. 
     Second-order susceptibility variation in acentric crystals leads to modification of the quasi-phase-matching (QPM) conditions which can be useful for frequency conversion. For the VUV spectral region below about 150 nm, there is not yet a known transparent optical crystal that combines non-zero second order nonlinearity with sufficient birefringence. Some attempts to fabricate QPM structures have been reported, for instance, by electric-field poling of the ferroelectric BaMgF 4  which has mm2 symmetry (E. G. Villora, K. Shimamura, K. Sumiya, and H. Ishibashi, “Birefringent- and quasi phase-matching with BaMgF 4  for vacuum-UV/UV and mid-IR all solid-state lasers,” Opt. Express 17, 12362 (2009)), or by mechanical twinning of crystalline quartz (SiO2) which has 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. 
     Strontium tetraborate SrB 4 O 7  (SBO) crystallizes in the orthorhombic system, point group mm2, space group Pnm2 1 , with unit cell dimensions a=4.4255 Å, b=10.709 Å, and c=4.2341 Å (Y. S. Oseledchik, A. L. Prosvirnin, A. I. Pisarevskiy, V. V. Starshenko, V. V. Osadchuk, S. P. Belokrys, N. V. Svitanko, A. S. Korol, S. A. Krikunov, and A. F. Selevich, “New nonlinear optical crystals: strontium and lead tetraborates,” Opt. Mater. 4, 669 (1995)). All boron atoms are coordinated tetrahedrally and an oxygen atom is common to three tedrahedra. Despite the three-dimensional network of tetrahedral, the borate network appears as a layer-like structure since there are relatively fewer links in the c direction of the unit cell. 
     SBO exhibits very small birefringence (&lt;0.005) and is not ferroelectric. Non-phase-matched second-harmonic generation (SHG) has been implemented using SBO for diagnostics, but the efficiency is extremely low when only one coherence length is utilized and a practical detection limit was estimated to be 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 SrB 4 O 7  for ultrafast diagnostics converting to wavelengths as short as 125 nm,” Opt. Lett. 29, 373 (2004)). 
     SBO exhibits unique optical and mechanical properties. The transparency range of SBO is 130-3200 nm in wavelength (Y. S. Oseledchik et al., op. cit.). SBO also exhibits a high (1.5-3.5 μm/V) value of the diagonal d 33  element (compared to the band-gap value). The optical damage threshold is very high (approximately 15 GW/cm 2 ) compared with other materials such as MgF 2 . The microhardness of SBO is also high (1750 kg/mm 2  in the x direction, 1460 kg/mm 2  in the y direction and 1350 kg/mm 2  in the z direction). The high optical damage threshold and microhardness allow SBO crystals to withstand extreme conditions when exposed to DUV and VUV radiation. DUV and VUV lasers may have high power levels from several milli-watts (mW) to several watts (W) or more, and high photon energy (for example, 9.32 eV at 133 nm and 8.16 eV at 152 nm). The broad transparency range, the good damage resistivity and chemical stability, and high value of the diagonal d 33  element are features that make SBO very attractive for frequency conversion to generate DUV and VUV wavelengths. However, the low birefringence means that frequency doubling by critical or non-critical phase matching are not possible. 
     Trabs et al. (P. Trabs, F. Noack, A. S. Aleksandrovsky, A. I. Zaitsev, N. V. Radionov, and V. Petrov, “Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV”, Opt. Express 23, 10091 (2015)) reported using an SBO crystal to generate second harmonics in the VUV from ultrashort laser pulses through random quasi phase matching. The second harmonic generation method described by Trabs et al. is unsuitable for a light source semiconductor metrology and inspection systems because the frequency conversion process has low efficiency making it impractical to use this method to generate Watts of second harmonic laser power, and also because it requires ultrashort laser pulses. 
     Lithium triborate LiB 3 O 5  (LBO) crystallizes in the orthorhombic system, point group mm2, space group Pna2 1 , with unit cell dimensions 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: LiB 3 O 5 ,” J. Opt. Soc. Am. B 6, 616-621 (1989)). The optical damage threshold is high (45GW/cm 2  for 1.1 ns pulses at 1064 nm), and it is transparent over a broad wavelength range from 160 nm to 2.6 μm. The largest nonlinear optical coefficients are d 31  (approximately −1.1 μm/V) and d 32  (approximately 1.2 μm/V). The diagonal nonlinear optical coefficient d 33  is much smaller and is less useful for nonlinear frequency conversion. LBO is biaxially birefringent and can generate, for example, the second and third harmonics of a wavelength near 1.064 μm by critically or noncritically phase-matched second harmonic and sum frequency generation. However, LBO is not suitable for generating DUV and VUV wavelengths such as 266 nm or 177 nm by either critical or noncritical phase matching. 
       FIG.  1    shows a laser assembly  100  for generating a laser output light beam  139  having an output frequency ω out  with a corresponding wavelength in the range of approximately 125 nm to approximately 300 nm. Laser assembly  100  generally includes one or more fundamental lasers  110 , one or more intermediate frequency conversion stages  120  and a final frequency conversion stage  130 . 
     Referring to the upper left portion of  FIG.  1   , fundamental lasers  110  are respectively configured to generate fundamental light beams  119 - 1 ,  119 - 2  . . .  119 - n  (collectively indicated as  119 ) having corresponding fundamental angular frequencies ω 1  to ω 2 , where each frequency has a corresponding fundamental wavelength between about 1 μm and 1.1 μm. In some embodiments all fundamental light beams  119  have substantially the same wavelength (e.g., fundamental frequency ω 1  is substantially equal to fundamental frequency ω 2 ). Specific fundamental laser types are mentioned in the specific embodiments provided below. Note that in the following description, where a wavelength is mentioned without qualification, that wavelength may be assumed to be the wavelength in vacuum. 
     Intermediate frequency conversion stages  120  are optically coupled to receive one or more of 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 specific embodiments intermediate light beams  129  comprise a single (first) intermediate light beam  129 - 1  having an associated intermediate frequency ω x . In other specific embodiments intermediate light beams  129  include both intermediate light beam  129 - 1  and a second intermediate light beam  129 - 2  having an associated intermediate frequency ω y .  FIG.  1    is not intended to limit the appended claims such that all intermediate frequency conversion stages  120  are required to receive a fundamental light beam  119 . For example, in the specific examples below set forth 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 that is/are optically coupled between a fundamental laser  110  and the given downstream stage. 
     Referring to the lower half of  FIG.  1   , laser assembly  100  also includes a final frequency conversion stage  130  configured to pass intermediate light beams  129  (ω x  or ω x  and ω y ) through a nonlinear crystal grating assembly  150 , and to direct laser output light beam  139  out of laser assembly  100  for use, e.g., in one or more of the inspection systems described below with reference to  FIGS.  8 - 10   . In one embodiment, intermediate light beam  129 - 1 , which has a frequency ω x  as described in the specific embodiments set forth below, enters a bow-tie ring cavity formed by an input coupler mirror  132 - 1 , a flat mirror  132 - 2 , two curved mirrors  132 - 3  and  132 - 4 , grating assembly  150  and a beam splitter  137 . For descriptive purposes, a portion of the light transmitted by the bow-tie ring cavity from input/coupler mirror  132 - 1  to grating assembly  150  is indicated as circulated light portion  133 , which is composed of both intermediate light beam  129 - 1  and unconsumed circulated light portion  138 - 1  (generated as described below), where both light portions  133  and  138 - 1  have frequency ω x . The bow-tie ring cavity formed by mirrors  132 - 1  to  132 - 4  is configured such that light portion  133  is directed along an optical path that passes through grating assembly  150 . In one embodiment, a mode matching lens  131  is utilized to focus intermediate light beam  129 - 1  though input coupler/mirror  132 - 1 , and the bow-tie ring cavity formed by mirrors  132 - 1  to  132 - 4  is otherwise configured such that light portion  133  is a beam waist of light portion  133  (i.e., including intermediate light beam  129 - 1 ) that occurs at (i.e., inside or proximate to) grating assembly  150 . When intermediate light beam  129 - 2  having frequency ω y  is used as described in relevant specific embodiments set forth below, intermediate light beam  129 - 2  enters the bow-tie ring cavity passing close to (but not necessarily through) curved mirror  132 - 3  such that it is directed substantially at selected angle θ onto input surface  153 -IN and passes through grating assembly  150 . As illustrated 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 ) through grating assembly  150 , with exiting light  136  (i.e., the total light exiting grating assembly  150 ) being directed onto an input surface  137 -IN of a beam splitter  137 . Beam splitter  137  is configured to split exiting light  136  such that unconsumed input light  138 - 1  having frequency ω x  is passed to mirror  132 - 4  for recirculation within the bow-tie cavity and such that laser output light  139  having output frequency ω OUT  is directed out of laser assembly  100 . As indicated, in some embodiments beam splitter  137  is also configured to reflect unconsumed input light  138 - 2  having frequency ω y  out of the bow-tie cavity. Beam splitter  137  may be implemented using one of a single SBO crystal, SBO glass or a CaF 2  crystal. Note that when final frequency doubling stage  130  is used in a pulsed laser, no cavity is needed (i.e. mirrors  132 - 1 ,  132 - 2 ,  132 - 3  and  132 - 4  may be omitted), and intermediate light beam  129 - 1  may be directed to, and focused in or proximate to, input surface  153 -IN of grating assembly  150  by any suitable combination of lenses and/or mirrors. 
     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 each other (e.g., by way of an external frame, not shown) in an interdigitated configuration. As described in additional detail below, each integral nonlinear crystal grating structure (grating structure)  160  and  170  includes multiple parallel spaced-apart mesas (plates), but for clarity and to simplify the following description, each of grating structures  160  and  170  is depicted in  FIG.  1    using only one mesa. Specifically, grating structure  160  includes a mesa  162  that is integrally connected to and protruding (extending) from a base  161 , and grating structure  170  includes a mesa  172  that is integrally connected to and protruding from a base  171 . As used herein, the term “integral” and the phrase “integrally connected” are used to describe grating structures formed by removing material from or otherwise processing a single nonlinear crystal (e.g., both base  161  and mesa  162  are parts of a single SBO crystal). In practical embodiments, as described below with reference to  FIGS.  4 C and  6 C , each of grating structures  160  and  170  include multiple interdigitated mesas that are integrally connected to and extend from bases  161  and  171 , respectively. As used herein, the phrase “interdigitated configuration” means that grating structure  160  is disposed with respect to grating structure  170  such that the mesas of grating structure  160  extend into spaces disposed between the mesas of grating structure  170 , whereby light alternately passes through an interdigitated alternating grating pattern formed by aligned portions of the interdigitated mesas of grating structures  160  and  170  (for further clarity see the embodiment described below with reference to  FIG.  5   ). 
     As described in additional detail below, mesas  162  and  172  are rectangular structures having opposing planar input and output surfaces through which light beam  133  is passed during operation. Referring to the bubble section at the bottom of  FIG.  1   , light beam  133  is directed onto planar input surface  163 - 1 , a first intra-crystal light portion  134 - 1  passes through mesa  162 , a second intra-crystal light portion  134 - 2  exiting through output surface  163 - 2  passes through an intervening space  152  onto planar input surface  173 - 1 , a third intra-crystal light portion  134 - 3  passes through mesa  172  and exiting light  136  is transmitted out of grating assembly  150  by way of output surface  173 - 2 . In one embodiment the input/output surfaces of the outermost mesas form input/output surfaces for grating assembly  150 . Because the simplified example shown in  FIG.  1    only includes two mesas, input surface  163 - 1  of mesa  162  serves as input surface  153 -IN of grating assembly  150 , and output surface  173 - 2  of mesa  172  serves as output surface  153 -OUT of grating assembly  150  (in practical embodiments the grating assembly input and output surfaces would not occur on adjacent mesas). Accordingly, when grating structures  160  and  170  are disposed in the requisite interdigitated configuration and grating assembly  150  is otherwise operably arranged within final conversion stage  130 , light passes through grating assembly  150  by striking input surface  153 -IN, passing through mesas  162  and  172  and an intervening gap  152 , and then exiting grating assembly  150  through output surface  153 -OUT. 
     Nonlinear crystal grating assembly  150  achieves QPM of incident light (e.g., beam  133  and/or beam  129 ) when grating structures  160  and  170  are formed with mesas having a proper width in light propagation direction PD, and when grating structures  160  and  170  are formed with inverted crystal axes that are aligned as described below with respect to the light propagation direction PD and a polarization direction  329  of the incident light. Referring to the bubble section in  FIG.  1   , both grating structure  160  and grating structure  170  are fabricated such that each mesa  162  and  172  has a width T1 (e.g., a distance between input surfaces  163 - 1 / 173 - 1  and output surfaces  163 - 2 / 173 - 2 ) substantially equal to an odd integer multiple of a critical length Λ1 measured in a propagation direction PD of intermediate light beams  129 / 133 . Critical length Λ1 is described in additional detail below with reference to  FIGS.  4 A- 4 C . As also indicated in the bubble section, the crystal material used to form grating structure  160  includes an associated first crystal axis (indicated by optical axes A11, A12 and A13), and the crystal material used to form grating structure  170  includes an associated second crystal axis (indicated by optical axes A21, A22 and A23). To achieve QPM, the first optical axes A11 and A21 of both grating structures  160  and  170  are aligned parallel to light propagation direction PD and both second optical axes A12 and A22 are aligned parallel to light polarization direction  329 . In addition, as mentioned above, the first and second crystal axes of grating structures  160  and  170  are inverted (rotated 180° relative to each other). For descriptive purposes, the term “upright” is assigned to indicate the arbitrarily selected first crystal axis A11/A12/A13 depicted in  FIG.  1    that meets the optical axis orientation requirements (i.e., with optical axis A11 aligned parallel to light propagation direction PD and optical axis A12 parallel to light polarization direction  329 ). In contrast, the term “inverted” is assigned to the three possible orientations of second crystal axis A21/A22/A23 that are both rotated 180° relative to first crystal axis A11/A12/A13 and meet the optical axis orientation requirements (i.e., in each of the three orientations optical axis A21 is aligned parallel to light propagation direction PD and optical axis A22 parallel to light polarization direction  329 ). Note that grating structure  160  may be formed using any of the three crystal axis orientations A21/A22/A23 shown in  FIG.  1   , and therefore the terms “upright” and “inverted” are assigned arbitrarily and solely used to indicate the 180° rotation between first crystal axis A11/A12/A13 and second crystal axis A21/A22/A23. When grating structures  160  and  170  are formed in the manner described above and disposed in the interdigitated configuration, mesas  162  and  172  (along with intervening gap  152 ) collectively form a periodic structure that achieves quasi-phase-matching (QPM) of intermediate light beams  129  with laser output  139  (i.e., between ω OUT  and frequency ω x , as described in the specific examples set forth below, or both frequencies cox and ω y , as described in some of the specific examples set forth below) such that light portion  136  exiting output surface  153 -OUT of nonlinear crystal grating assembly  150  includes laser output light beam  139  having a desired output frequency ω OUT . 
       FIG.  2 A  is a simplified block diagram showing an exemplary laser assembly  100 A configured to generate a wavelength in the range of approximately 128 nm to approximately 134 nm (e.g., approximately 133 nm) according to a first specific exemplary embodiment of the present invention. Laser assembly  100 A comprises a first fundamental laser  110 A and three frequency doubling (conversion) stages (i.e., two intermediate frequency doubling stages  120 A- 1  and  120 A- 2 , and a final frequency doubling stage  130 A) that are cooperatively configured to generate laser output light having a wavelength in the range of approximately 128 nm to approximately 134 nm. The first fundamental laser  110 A is configured to generate fundamental light  119 A having a first fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm (i.e., between about 1 μm and 1.1 μm) and a corresponding first fundamental frequency ω 1 . First intermediate frequency doubling stage  120 A- 1  receives the first fundamental light  119 A and generates the second harmonic light  121 A with a second harmonic frequency ω 1  equal to twice the first fundamental frequency ω 1 . Second intermediate frequency doubling stage  120 A- 2  receives the second harmonic light  121 A and generates an intermediate light beam  129 A as fourth harmonic light with the fourth harmonic frequency 4ω 1  equal to four times the first fundamental frequency ω 1 . Final (third) frequency doubling stage  130 A receives the fourth harmonic light (intermediate light beam)  129 A and generates laser output light  139 A with an output frequency ω OUTA  that is equal to eight times the first fundamental frequency ω 1 . 
     Referring to  FIG.  2 A , the first fundamental laser  110 A is configured using known techniques to generate the first fundamental light  119 A (referred to simply as the “fundamental” in the industry) at first fundamental frequency ω 1 . In one embodiment, the first fundamental laser  110 A is configured such that the first fundamental light  119 A is generated at a first fundamental frequency ω 1  corresponding to an infra-red wavelength of approximately 1064 nm. In an exemplary embodiment, the first fundamental laser  110 A is implemented using one of a Nd:YAG (neodymium-doped yttrium aluminum garnet) lasing medium, a Nd-doped yttrium orthovanadate (Nd:YVO 4 ) lasing medium, or an ytterbium-doped fiber lasing medium. Suitable fundamental lasers are commercially available from Coherent Inc., IPG Photonics Corporation, Trumpf GmbH and other manufacturers. Such manufacturers also sell lasers generating light having a wavelength near 532 nm, i.e., the laser includes first fundamental laser  110 A and the first frequency doubling stage  120 A- 1 . In order to generate sufficient light at a wavelength of approximately 133 nm for inspecting semiconductor wafers or reticles, first fundamental laser  110 A should generate tens or hundreds of Watts or more of fundamental light  119 A. 
     According to an exemplary embodiment in  FIG.  2 A , each of the frequency doubling stages  120 A- 1  and  120 A- 2  comprises an external resonant cavity including at least three optical mirrors and a nonlinear crystal arranged therein, respectively. The cavities can be stabilized with standard PDH (Pound-Drever-Hall), HC (Hänsch-Couillaud) or other locking techniques. The cavity length is adjusted to maintain resonance by adjusting the position of a mirror or prism through a control signal. The first frequency doubling stage  120 A- 1  receives and converts first fundamental light  119 A at the first fundamental frequency ω 1  to generate the second harmonic light  121 A at two times the first fundamental frequency (2ω 1 ). Second frequency doubling stage  120 A- 2  receives and converts second harmonic light  121 A to generate fourth harmonic light  129 A at four times the first fundamental frequency (4ω 1 ). 
     In some other embodiments (not shown), the first frequency doubling module may be combined with the first fundamental laser to use intra-cavity frequency doubling with the NLO crystal placed inside the fundamental solid-state laser cavity to generate the second harmonic light  121 A. 
     In at least one embodiment, the first frequency doubling stage  120 A- 1  in  FIG.  2 A  that generates the second harmonic light  121 A can include a Lithium triborate (LBO) crystal, which can be substantially non-critically phase-matched (for an appropriate choice of crystal plane) at temperatures between room temperature and about 200° C. for producing a second harmonic in a wavelength range between about 515 nm and about 535 nm. In alternative embodiments, the first frequency doubling stage  120 A- 1  may include a Cesium Lithium Borate (CLBO) crystal or a beta-Barium Borate (BBO) crystal, either of which can be critically phase matched for generating a second harmonic in a wavelength range between about 515 nm and about 535 nm. In other alternative embodiments, the first frequency doubling stage  120 A- 1  may include a KTiOPO 4  (KTP), periodically poled lithium niobate (PPLN), periodically poled stoichiometric lithium tantalate (PPSLT), or other nonlinear crystal for frequency conversion. 
     The second frequency doubling stage  120 A- 2  that generates the fourth harmonic may use critical phase matching in CLBO, BBO or other nonlinear crystal. In preferred embodiments, the second frequency doubling stage  120 A- 2  includes a hydrogen-treated or deuterium-treated CLBO crystal. 
     In an alternative embodiment, the second frequency doubling stage  120 A- 2  that generates the fourth harmonic may include a nonlinear crystal grating assembly of the type described herein to implement QPM. In one embodiment, the grating assembly is formed using SBO crystal. The critical length for 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 a range from 2.5 μm to 2.7 μm) when the nonlinear crystal is configured so that the polarizations of both the input and output light are parallel to the c axis of the SBO crystal to take advantage of the large d 33  nonlinear optical coefficient. Since the critical length is longer than the critical lengths for generating shorter wavelengths, the SBO mesa thickness in the light propagation direction may be equal to the critical length or may be equal to a small, odd integer (such as between 3 and 19) times the critical length. In another embodiment, the grating assembly is composed of LBO crystal. The critical length for QPM for generating 266 nm from 532 nm in LBO is approximately 3.81 μm (i.e. in a range from 3.78 μm to 3.84 μm) 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. 
     Further details of how a fourth harmonic of a CW fundamental IR laser can be generated with high power, low noise, and good stability, can be found in U.S. Pat. Nos. 9,293,882 and 9,660,409, to Chuang, and U.S. Pat. Nos. 9,509,112 and 10,044,166 to Chuang et al. These patents are incorporated herein by reference. 
     Referring to  FIG.  2 A , the final frequency doubling stage  130 A receives the fourth harmonic light  129 A and generates the eighth harmonic light  139 A with the eighth harmonic frequency 8ω 1  equal to eight times the first fundamental frequency ω 1 . In at least one embodiment, the final frequency doubling stage  130 A in  FIG.  2 A  that generates the eighth harmonic light  139 A can include a nonlinear crystal grating assembly of the type described herein comprising an interdigitated grating configuration to achieve QPM. Any of the frequency conversion stages may be enclosed in one or more protective environments, such as those described in U.S. Pat. No. 8,298,335, entitled “Enclosure for controlling the environment of optical crystals”, by Armstrong. This patent is incorporated by reference herein. In particular, since the final frequency doubling stage  130 A generates a VUV wavelength, this stage needs to be in an environment with very low oxygen and water concentrations (preferably a few ppm or lower concentrations). Preferably the final frequency doubling stage is kept in an environment that is purged with pure nitrogen or argon. Note that a single protective environment may enclose multiple stages or a single stage. 
     Any of the frequency conversion stages may incorporate any of the methods or systems described in U.S. Pat. Nos. 9,461,435 and 9,059,560, both entitled “Alleviation of laser-induced damage in optical materials by suppression of transient color centers formation and control of phonon population”, to Dribinski et al., any of the apparatus or methods described in U.S. Pat. No. 8,824,514, entitled “Measuring crystal site lifetime in a non-linear optical crystal”, to Armstrong, and any of the apparatus and methods described in U.S. Pat. No. 8,976,343, entitled “Laser crystal degradation compensation” to Genis. All of these patents are incorporated herein by reference. 
     Further note that any of the intermediate frequency conversion stages mentioned herein may advantageously use deuterium, hydrogen and/or fluorine doped or treated non-linear crystals. Such crystals may be created, processed or treated by any of the processes or methods described in U.S. Pat. No. 9,023,152 to Dribinski, U.S. Pat. Nos. 9,250,178, 9,459,215 and 10,283,366 to Chuang et al., and Published U.S. Patent Application 2014/0305367, entitled “Passivation of Nonlinear Optical Crystals”, and filed on Apr. 8, 2014 by Dribinski et al. These patents and applications are incorporated herein by reference. The doped or treated crystals may be particularly useful in those stages involving deep UV wavelengths, including the second frequency doubling stage  120 A- 2  in  FIG.  2 A . 
       FIG.  2 B  is a simplified block diagram showing an exemplary laser assembly  100 B 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. Laser assembly  100 B comprises a first fundamental laser  110 B- 1 , a second fundamental laser  110 B- 2 , a frequency doubling (conversion) stage  120 B- 1 , a frequency summing (conversion) stage  120 B- 2 , and a final frequency doubling stage  130 B that are collectively configured to generate laser output light  139 B with an output frequency ω OUTS  having a wavelength in the range of approximately 170 nm to approximately 180 nm. The first fundamental laser  110 B- 1  is configured to generate fundamental light  119 B- 1  having a first fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm (i.e., between about 1 μm and 1.1 μm) and a corresponding first fundamental frequency ω 1 . The second fundamental laser  110 B- 2  is configured to generate fundamental light  119 B- 2  having a second fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm (i.e., between about 1 μm and 1.1 μm) and a corresponding second fundamental frequency 6ω 2 . Frequency doubling stage  120 B- 1  receives the first fundamental light  119 B- 1  and generates the second harmonic light  121 B with a second harmonic frequency 2ω 1  equal to twice the first fundamental frequency ω 1 . Frequency summing stage  120 B- 2  sums the second harmonic  121 B with the second fundamental light  119 B- 2  and generates intermediate light beam  129 B having summing frequency 2ω 1 +ω 2 . If the frequencies of the first fundamental laser  110 B- 1  and the second fundamental laser  110 B- 2  are the same (ω 1 =ω 2 ), the intermediate light beam  129 B is the third harmonic (3ω 1  or 3ω 2 ) of the fundamental light. Final frequency doubling stage  130 B receives the intermediate light beam  129 B and generates final output light  139 B with output frequency ω OUTB  equal to twice the summing frequency 2ω 1 +ω 2 , i.e. equal to 4ω 1 +2ω 2 . If the frequencies of the first fundamental laser  110 B- 1  and the second fundamental laser  110 B- 2  are the same (ω 1 =ω 2 ), then output frequency ω OUTB  of final laser output light  139 B is the sixth harmonic (6ω 1  or 6ω 2 ) of the fundamental light. 
     Referring to  FIG.  2 B , the first and second fundamental lasers  110 B- 1  and  110 B- 2  are configured as described above with reference to fundamental laser  110 A in  FIG.  2 A . In an alternative embodiment, second fundamental laser  110 B- 2  may be omitted, and the output of first fundamental laser  110 B- 1  may be divided into two portions: a first portion directed to first frequency doubling stage  120 B- 1 , and a second portion directed to frequency summing stage  120 B- 2  along with second harmonic light  121 B. In this alternative embodiment, necessarily ω 2 =ω 1 . 
     According to the exemplary embodiment in  FIG.  2 B , the first frequency doubling stage  120 B- 1  is configured as described above with reference to stages  120 A- 1  and  120 A- 2  of  FIG.  2 A . 
     In one embodiment, frequency summing stage  120 B- 2  sums the second harmonic  121 B with the second fundamental light  119 B- 2  using a Lithium triborate (LBO) crystal, a Cesium Lithium Borate (CLBO) crystal or a beta-Barium Borate (BBO) crystal. 
     In at least one embodiment, final frequency doubling stage  130 B includes a nonlinear crystal grating assembly including an interdigitated grating configuration that is configured for quasi-phase-matching (QPM) in a manner similar to that described above with reference to final frequency doubling stage  130 A of  FIG.  2 A . Differences between final frequency doubling stages  130 A and  130 B are set forth below with reference to  FIGS.  3 A and  3 B . 
       FIG.  3 A  is a simplified diagram showing an exemplary final frequency doubling stage  130 C utilized in the 133 nm laser assembly  100 A of  FIG.  2 A  and in the 177 nm laser assembly  100 B of  FIG.  2 B  according to exemplary embodiments of the present invention. Input light  129 C with frequency ω x  (for example, ω x =4ω 1  when stage  130 C is used in the 133 nm laser  100 A, or ω x =2ω 1 +ω 2  when stage  130 B is used in the 177 nm laser  100 B) enters a bow-tie ring cavity comprising input coupler  132 C- 1 , flat mirror  132 C- 2 , curved mirrors  132 C- 3 ,  132 C- 4  that collectively form optical elements configured to direct input light beam  129 C along an optical path that passes through a nonlinear crystal grating assembly  150 C (i.e., such that input light beam  129 C is directed in a selected propagation direction onto input surface  153 C-IN). Exiting light  136 C, which is output from nonlinear crystal grating assembly  150 C through output surface  153 C-OUT, comprises unconsumed input light  138 C and generated laser output light  139 C with an output frequency ω OUTC  that is equal to twice the frequency of the input light  129 C (i.e., frequency ω OUTC  can be either equal to the eighth harmonic output light  139 A of  FIG.  2 A  or the sixth harmonic output light  139 B of  FIG.  2 B ). Unconsumed input light  138 C passes through beam splitter (BS)  137 C and is recirculated to enhance the power. The laser output light  139 C is reflected from the surface of beam splitter (BS)  137 C and directed out of the cavity. 
     Preferably, nonlinear crystal grating assembly  150 C is configured so that input surface  153 C-IN and output surface  153 C-OUT are oriented approximately at Brewster&#39;s angle relative to the circulating input light  133 C. The polarization direction of the circulating input light  133 C is illustrated by arrow  329 C. Furthermore, BS  137 C may be configured to laterally displace the circulating input light  133 C in the cavity by an amount that substantially offsets the lateral displacement of the input light caused by grating assembly  150 C, so as to maintain a substantially symmetric bow-tie cavity and simplify optical alignment of the cavity. 
     In one embodiment, BS  137 C may comprise an SBO crystal, SBO glass or a CaF 2  crystal. Since SBO has good deep UV transmission and has a high damage threshold, SBO may advantageously be used as a substrate material for the BS  137 C to ensure long life in spite of the high-power level of the unconsumed input light  133 C circulating in the cavity. If BS  137 C comprises an SBO crystal, its thickness and/or the orientation of its crystal axes may be configured so as to minimize any frequency doubling of the unconsumed input light  133 C passing through it. BS  137 C may comprise a dichroic beam splitter, prism, or other component to separate the wavelengths. In one embodiment, grating assembly  150 C is configured so that output light  139 C has orthogonal polarization relative to circulating input light  133 C. In this embodiment, BS  137 C may comprise a polarizing beam splitter configured to transmit unconsumed input light  138 C and reflect output light  139 C. In one embodiment, BS  137 C has its surfaces oriented so that the unconsumed input light  138 C is substantially p-polarized relative to those surfaces and the surfaces are at approximately Brewster&#39;s angle relative to that unconsumed input light. 
     According to  FIG.  3 A , the input light (ω x )  129 C is focused by one or more lenses  131 C before entering the cavity to match the intrinsic mode of the resonant cavity that has a beam waist inside or proximate to nonlinear crystal  150 C. In at least one embodiment, one or more lenses  131 C include one or more cylindrical lenses comprising SBO glass or crystal and configured to operate at approximately Brewster&#39;s angle relative to the incoming light  129 C so as to minimize reflection losses without using an antireflection coating. SBO is a suitable material for such lenses as it has high damage threshold at UV and DUV wavelengths. Unconsumed input light  138 C (ω x ) passing through BS  137 C gets reflected by mirror  132 C- 4  and circulates inside the cavity to build up the intensity. If the enhanced input light (ω x ) power density is intense enough, the conversion efficiency from the input light (ω x ) to output light  139 C (2ω x ) may be very high, up to or even higher than 50%. Output light  139 C (2ω x ) with a wavelength near 177 nm or near 133 nm exits the cavity after reflection from the BS  137 C. 
     In an alternative embodiment, input surface  153 C-IN of grating assembly  150 C may be coated with an appropriate anti-reflection coating instead of orienting input surface  153 C-IN and output surface  153 C-OUT at Brewster&#39;s angle. 
     Although  FIG.  3 A  depicts final frequency doubling stage  130 C as including a cavity comprising 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 an alternative embodiment, final frequency doubling stage  130 C may comprise a delta cavity, a standing-wave cavity, or other shaped cavity instead of a bow-tie cavity. 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 with standard PDH or HC locking techniques. The cavity length is adjusted to maintain resonance by adjusting the position of one of the mirrors (such as mirror  132 C- 2  in  FIG.  3 A ) or the position of a prism, through a control signal (not shown) connected to a piezo-electric transducer (PZT), voice coil or another actuator. Note that when final frequency doubling stage  130 C is used in a pulsed laser, no cavity is needed, and input light  129 C may be directed to, and focused in or proximate to, grating assembly  150 C by any suitable combination of lenses and/or mirrors. 
       FIG.  3 B  is a simplified diagram showing an exemplary final frequency summing stage  130 D utilized in the 152 nm laser assembly  100 H of  FIG.  7 A , in the 152 nm laser assembly  100 I of  FIG.  7 B  and in the 177 nm laser assembly  100 J of  FIG.  7 C  according to exemplary embodiments of the present invention. Input light  129 D- 1  (with frequency ω x , for example ω x =2ω 1 +ω 2  when stage  130 D is used in the 152 nm laser  100 H of  FIG.  7 A , ω x =2ω 1  when stage  130 D is used in the 152 nm laser  100 I of  FIG.  7 B , or ω x =2ω 2  when stage  130 D is used in the 177 nm laser  100 J of  FIG.  7 C ) enters a bow-tie ring cavity comprising input coupler  132 D- 1 , flat mirror  132 D- 2 , curved mirrors  132 D- 3 ,  132 D- 4  and a nonlinear crystal grating assembly  150 D (including an input surface  153 D-IN and an output surface  153 D-OUT) through input coupler  132 D- 1  and is recirculated to enhance the power. Grating assembly  150 D is configured using the interdigitated configuration described herein. Input light (second intermediate light beam)  129 D- 2  with frequency ω y  (e.g., ω y =4ω 1  when stage  130 D is used in the 152 nm laser  100 H of  FIG.  7 A , or ω y =4ω 1 +ω 2  when stage  130 D is used in the 152 nm laser  100 I of  FIG.  7 B , or ω y =4ω 1  when stage  130 D is used in the 177 nm laser  100 J of  FIG.  7 C ) enters the bow-tie ring cavity passing close to (but not necessarily through) mirror  132 D- 2  and passes through grating assembly  150 D. Exiting light  136 D, which is output from nonlinear crystal grating assembly  150 D through output surface  153 D-OUT, comprises unconsumed input light  138 D- 1  with frequency ω x , unconsumed input light  138 D- 2  with frequency ω y  and the generated laser output light  139 D with an output frequency ω OUTD  that is equal to a sum of the frequencies wand ω y  of intermediate (input) light beams  129 D- 1  and  129 D- 2  (i.e., frequency ω OUTD  can be either substantially equal to the seventh harmonic output light  139 H and  139 I of  FIG.  7 A or  7 B , or the sixth harmonic output light  139 J of  FIG.  7 C ). The laser output light  139 D is reflected from the input surface of beam splitter  137 D and directed out of the cavity. Unconsumed input light  138 D- 1  with frequency ω x  passes through beam splitter  137 D and optional beam splitter  325  (if present) and is reflected by mirrors  132 D- 4  and  132 D- 1  to enhance the intensity of circulated light  133 D. Unconsumed input light  138 D- 2  of frequency ω y  exits the cavity after being reflected either from beam splitter  137 D or from an optional (second) beam splitter  325 . The polarization direction of the circulating input light  133 D is illustrated by arrow  329 D. 
     Frequency summing stage  130 D may be modified using any of the features and alternatives described above with reference to frequency doubling stage  130 C of  FIG.  3 A . For example, stage  130 D utilizes one or more lenses  131 D to focus input light  129 D- 1  with frequency ω x  as described above, and also utilizes one or more lenses  308  to focus input light  129 D- 2  as it enters the cavity near mirror  132 D- 3 , where both one or more lenses  131 D and one or more lenses  308  are configured as described above with reference to lenses  131 C ( FIG.  3 A ). Furthermore, beam splitter  137 D may be configured as described above with reference to beam splitter  137 C of  FIG.  3 A . Note that when final frequency summing stage  130 D is used in a pulsed laser, no cavity is needed, and input lights  129 D- 1  and  129 D- 2  may be made colinear (or nearly colinear such as within 5° of one another), directed to, and focused in or proximate to, grating assembly  150 D by any suitable combination of lenses and/or mirrors. 
       FIGS.  4 A to  4 C  respectively depict a first nonlinear crystal grating structure  160 E, a second nonlinear crystal grating structure  170 E, and a nonlinear crystal grating assembly  150 E produced by assembling grating structures  160 E and  170 E in an interdigitated configuration. Grating assembly  150 E represents an exemplary embodiment that may be configured for use in 2 nd  frequency doubling stage  120 A- 2  and/or in final frequency doubling stage  130 A of  FIG.  2 A , in final frequency doubling stage  130 B of  FIG.  2 B , in 2 nd  frequency doubling stage  120 H- 3  and/or in final frequency summing stage  130 H of  FIG.  7 A  (described below), in 1 st  frequency doubling stage  120 I- 1  and/or in 2 nd  frequency doubling stage  120 I- 2  and/or in final frequency summing stage  130 I of  FIG.  7 B  (described below), and/or in 3 rd  frequency doubling stage  120 J- 3  and/or final frequency summing stage  130 J of  FIG.  7 C  (described below). Grating assembly  150 E is depicted and described with reference to five mesas and three grooves for clarity and brevity and is not intended to represent a practical embodiment (see  FIGS.  6 A- 6 C , described below). 
     Referring to  FIG.  4 A , grating structure  160 E includes parallel mesas  162 E- 1  and  162 E- 2  that extend from base  161 E and are separated by a groove  165 E. In practical embodiments, grating structure  160 E is formed by etching or cutting (ruling) rectangular groove  165 E (i.e., an open-ended channel or a cavity) in a single piece of nonlinear crystal material using standard fabrication techniques. For example, dry etching processes such as reactive ion etching (RIE), electron cyclotron resonance plasma etching (ECR) or inductively coupled plasma etching (ICP) can be used to form the rectangular groove  165 . Alternatively, a ruling engine (such as one used for cutting diffraction gratings) or a diamond turning machine may be used to cut the grooves. Grating structure  160 E is formed such that mesas  162 E- 1  and  162 E- 2  have rectangular cross-sections and parallel opposing planar surfaces. That is, mesa  162 E- 1  includes opposing planar surfaces  163 E- 11  and  163 E- 12  and mesa  162 E- 2  includes opposing planar surfaces  163 E- 21  and  163 E- 22 , where each planar surface  163 E- 11 ,  163 E- 12 ,  163 E- 21  and  163 E- 22  defines an associated vertical plane that is parallel to Y-Z plane defined by the reference X-Y-Z axis shown in  FIG.  4 A . Each mesa  162 E- 1  and  162 E- 2  is also formed with a width T1 (as measured in the X-axis between the associated opposing planar surfaces) that is determined as set forth below with reference to  FIG.  5   . A depth D1 of groove  165 E (i.e., a height of mesas  162 E- 1  and  162 E- 2 ) is at least ten microns, preferably at least 50 μm or at least 100 μm, and a width T2 of groove  165 E (i.e., the X-axis distance between surface  163 E- 12  of mesas  162 E- 1  and surface  163 E- 21  of mesa  162 E- 2 ) is greater than mesa width T1 by between about 100 nm and 1 μm. 
     Referring to  FIG.  4 B , grating structure  170 E includes parallel mesas  172 E- 1 ,  172 E- 2  and  172 E- 3  that extend from base  171 E (including sections  171 E- 1  and  171 E- 2 ) and are separated by a grooves  175 E- 1  and  173 E- 2 . Grating structure  170 E is also formed by etching or cutting rectangular grooves  175 E- 1  and  175 E- 2  into a second single piece of nonlinear crystal material using standard fabrication techniques such that mesas  172 E- 1  to  172 E- 3  are formed with corresponding plate-like rectangular cross-sections and parallel opposing planar surfaces (i.e., mesa  172 E- 1  includes opposing planar surfaces  173 E- 11  and  173 E- 12 , mesa  172 E- 2  includes a planar end surface  173 E- 21  and  173 E- 22 , and mesa  172 E- 3  includes a planar end surface  173 E- 31  and  173 E- 32 ). Each mesa  172 E- 1  to  172 E- 3  is also formed with the same width T1 of mesas  162 E- 1  and  162 E- 2 , and grooves  175 E- 1  and  175 E- 2  have the same depth D1 as that of groove  165 E. 
       FIG.  4 C  shows grating assembly  150 E after grating structures  160 E and  170 E have been assembled into an interdigitated configuration in which mesa  162 E- 1  is disposed in groove  175 E- 1 , mesa  162 E- 2  is disposed in groove  175 E- 2  and mesa  172 E- 2  is disposed in groove  165 E. The fabrication of grating structures  160 E and  170 E is coordinated such that grating structure  160 E is formed with an associated upright crystal axis A11/A12/A13 that is aligned as indicated in the left side of  FIG.  4 C  when assembled in the interdigitated configuration and grating structure  170 E is formed with any of the three associated inverted crystal axes A21/A22/A23 indicated in the right side of  FIG.  4 C . Note that the larger width T2 of grooves  165 E,  175 E- 1  and  175 E- 2  (i.e., in comparison to mesa width T1) produces gaps (spaces)  152 E- 1  to  152 E- 4  between corresponding opposing planar mesa surfaces when grating structures  160 E and  170 E are assembled in the interdigitated configuration. That is, gap  152 E- 1  is formed between planar surface  163 E- 11  (see  FIG.  4 A ) of mesa  162 E- 1  and planar surface  173 E- 12  (see  FIG.  4 B ) of mesa  172 E- 1 . Similarly, gap  152 E- 2  is formed between opposing planar surfaces of mesas  162 E- 1  and  172 E- 2 , gap  152 E- 3  is formed between opposing planar surfaces of mesas  162 E- 2  and  172 E- 2 , and gap  152 E- 4  is formed between opposing planar surfaces of mesas  162 E- 2  and  172 E- 3 . In some embodiments grating assembly  150 E is disposed in a housing or chamber that contains a partial vacuum or an inert gas (e.g., argon) to prevent the collection of harmful contaminants on the opposing surfaces bordering gaps  152 E- 1  to  152 E- 4 . 
     Note that although grating assembly  150 E depicts upright grating structure  160 E and inverted grating structure  170 E as having the equal mesa widths T1 and equal groove widths T2, this is merely for convenience of fabrication. As can be readily understood from this disclosure, each mesa needs to have a width equal to an odd integer multiple of the critical length for QPM though it does not need to be the same odd integer multiplier for each mesa, and each groove needs to be wide enough to accept the corresponding mesa of the other grating with a small clearance (gap) on either side. Nonlinear crystal grating assemblies of the present invention are configured for use in a given optical system such that the crystal axes of inverted grating structure  170 E are inverted (i.e., rotated by substantially 180°) with respect to the crystal axes of the upright grating structure  160 E as shown in the upper and lower-left insets of  FIG.  5    and explained in the associated description below. This physical arrangement of the crystal plates allows for QPM. This may be considered as analogous to using PPLN (periodically poled lithium niobate) for QPM except that Lithium Niobate is a ferroelectric crystal and can be periodically poled. In contrast, SBO and LBO are non-ferroelectric, so we need to physically arrange the crystal mesas to create a periodic structure for QPM. 
       FIG.  5    depicts a final frequency conversion stage  130 E including optical elements (not shown) that are configured as described herein to direct input light beam  133 E and optional second input light beam  133 E- 2  into grating assembly  150 E, which is described in additional detail above with reference to  FIGS.  4 A to  4 C . For descriptive purposes, the base portions of grating structures  160 E and  170 E are depicted in dashed-line (hidden) form, thereby emphasizing the interdigitated alternating grating pattern  155 E formed by sequentially aligned mesas  172 E- 1 ,  162 E- 1 ,  172 E- 2 ,  162 E- 2  and  172 E- 3  through which input light  133 E passes. In alternative embodiments, interdigitated alternating grating pattern  155 E is configured to double the frequency ω x  of the input light  133 E or configured to sum input light  133 E of frequency ω x  and input light  133 E- 2  of frequency ω y . The input light  133 E may, for example, be generated as the summing product of a second harmonic of a first fundamental light and a second fundamental light in the case of the 177 nm laser  100 B (described above with reference to  FIG.  2 B ), or as the fourth harmonic of a fundamental light in the case of the 133 nm laser  100 A (described above with reference to  FIG.  2 A ). The input lights  133 E and  133 E- 2  may, in other examples, be generated as the summing product of a second harmonic of a first fundamental light and a second fundamental light and as the fourth harmonic of a first fundamental light in the case of 152 nm laser  100 H (described above with reference to  FIG.  7 A ), as a second harmonic of a first fundamental light and as the summing product of a fourth harmonic of the first fundamental light and a second fundamental light in the case of 152 nm laser  100 I (described above with reference to  FIG.  7 B ), or as a second harmonic of a second fundamental light and fourth harmonic of a first fundamental light in the case of 177 nm laser  100 J (described above with reference to  FIG.  7 C ). Although  FIG.  5    illustrates grating assembly  150 E as having a periodic structure including an interdigitated grating comprising five mesas (i.e., mesas  172 E- 1 ,  162 E- 1 ,  172 E- 2 ,  162 E- 2  and  172 E- 3 ), the total number of mesas may be more than ten and the total number of gaps will be one less than the number of mesas. 
       FIG.  5    shows that gap  152 E- 1  formed between mesas  162 E- 1  and  172 E- 1  has a gap width T21, and that gap  152 E- 2  between mesas  162 E- 1  and  172 E- 2  has a gap width T22. With groove  175 E- 1  having groove width T2 and mesa  162 E- 1  having mesa width T1, is follows that groove width T2 is equal to the sum of mesa width T1 and gap widths T21 and T22. As explained above, preferably gap widths T21 and T22 should each be between about 100 nm and 1 μm. Gap widths T21 and T22 need not be equal, though generally it will be easier to assemble grating structures  160 E and  170 E if the mesas of each grating structure are approximately centered in the grooves of the other grating structure (i.e., if T21 and T22 are approximately equal). Preferably groove width T2 is wider than the mesa width T1 by an amount that provides enough clearance for the two grating structures to be placed together without getting stuck during the assembly process. For example, the width of each groove may be between approximately 200 nm and 2 μm wider than the width of one mesa so that the grating structures can be assembled with gaps on either side of each mesa having gap widths T21 and T22 of between about 100 nm and 1 μm. If groove width T2 is too small (for example, such that gap widths T21 and T22 are less than about 10 nm), Van der Waals force interactions between the two opposing mesa surfaces may cause the two grating structures to stick together before the assembly process is complete. Conversely, if groove width T2 is too large (e.g., larger than 1 μm, as the light at frequencies ω x  and (if present) ω y  do not travel exactly parallel to light at frequency ω OUTE  in the gaps due to refraction at the output surface of each mesa. Over many such gaps in an assembly including many mesas, there will be a cumulative displacement (walk off) between the light at the different frequencies which will limit the conversion efficiency. 
     Referring to the bubble sections at the bottom of  FIG.  5   , the upright crystal axes A11/A12/A13 of mesas  162 E- 1  and  162 E- 2  and the inverted crystal axes A21/A22/A22 of mesas  172 E- 1 ,  172 E- 2  and  172 E- 3  are configured as shown. Both grating structures  160 E and  170 E are formed and configured within final frequency conversion stage  130 E such that one or both intermediate light beams  133 E and  133 E- 2  propagates in a direction parallel to first optical axes A11 and A21, with a polarization direction  329 E parallel to second optical axes A12 and A22, wherein second optical axis has a higher nonlinear coefficient than the other optical axes of the nonlinear crystal material, and mesas  162 E- 1 ,  162 E- 2 ,  172 E- 1 ,  172 E- 2  and  172 E- 3  are formed with a thickness T1 substantially equal to an odd integer multiple of a critical length Λ1, measured in the propagation direction PD of light beam  133 E (ω x ), that is required to achieve quasi-phase-matching (QPM) of the incident light beams  133 E and  133 E- 2  (ω x , ω y ) and the output frequency ω OUTE  of a desired laser output light beam  139 E. Critical length Λ1 is the distance traveled by light beam  133 E through each mesa, is determined by: 
       Λ1= m   1   L   c ,  (Equation 1)
 
     where m 1  is an odd integer (e.g., 1,3,5,7 . . . ) and L c  is a quasi-phase-matching critical length: 
     
       
         
           
             
               
                 
                   
                     
                       L 
                       c 
                     
                     = 
                     
                       π 
                       
                         Δ 
                         ⁢ 
                         k 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     where Δk is defined by: 
       Δ k=k (ω OUT )−2 k (ω x ) or  (Equation 3a)
 
       Δ k=k (ω OUT )− k (ω x )− k (ω y ),  (Equation 3b)
 
     where k(ω) is the wavevector of light of frequency co in nonlinear crystal: 
     
       
         
           
             
               
                 
                   
                     
                       k 
                       ⁡ 
                       ( 
                       ω 
                       ) 
                     
                     = 
                     
                       
                         ω 
                         ⁢ 
                         
                           n 
                           ⁡ 
                           ( 
                           ω 
                           ) 
                         
                       
                       c 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     and where n(ω) is the refractive index of the nonlinear crystal material for the appropriate polarization at frequency ω and c is the velocity of light in vacuo. Note that equation 3a is applicable when final frequency conversion stage  130 E is configured for frequency doubling, and equation 3b is applicable when final frequency stage  130 E is configured for frequency summing. The polarization of the light at output frequency ω OUTE  may be selected by the appropriate choice of L c  and crystal orientation if grating assembly  150 E is oriented so that the polarization directions parallel to optical axes A12/A22 and to optical axes A13/A23 have different refractive indices. The choice between whether light at output frequency ω OUTE  is polarized parallel to axes A12/A22 or to axes A13/23 can be made based on, for example, which polarization combination has the larger conversion efficiency per mesa based on nonlinear coefficients and refractive indices of the crystal material used to form grating structures  160 E and  170 E. Note that critical length Λ1 may be substantially equal to the physical mesa width T1. Although it is convenient to make the thicknesses of two mesas equal to one another as depicted, it is not necessary that they be equal as long as each mesa thickness is equal to an odd integer multiple of the critical length for phase matching as shown in Equation 1 above. Note that the correspondence between the optical axis labels A11/A12/A13 and A21/A22/A23 and the crystallographic axes of grating structures  160 E and  170 E depends on the nonlinear crystal material used (for example, SBO or LBO) and the selected polarization states for the input and output light. The lower right bubble in  FIG.  5    depicts three different rotations of inverted crystal axis A21/A22/A23 relative to upright crystal axis A11/A12/A13 corresponding to rotation about each of the optical axes. For any given nonlinear crystal and chosen frequency conversion scheme, only one or two of these rotations will work. The rotation must invert the sign of the nonlinear coefficient to achieve QPM and must preserve the relative alignment of the input and output light polarizations relative to the crystal axes (see below). 
     In one embodiment, grating assembly  150 E is made from SBO crystal material. For the final frequency doubling stage  130 B of 177 nm of laser  100 B ( FIG.  2 B ) the quasi-phase-matching critical length L c  is about 0.6 μm, whereas for the final frequency doubling stage  130 A of 133 nm laser  100 A ( FIG.  2 A ) the quasi-phase-matching critical length L c  is about 0.13 μm. The exemplary QPM critical length for generating 133 nm light by frequency-doubling 266 nm light was calculated from the refractive indices of SBO at wavelengths of 133 nm and 266 nm using the Sellmeier model published by Trabs et al. (cited above). Since Trabs et al. did not generate any wavelengths shorter than 160 nm, the extrapolated refractive index at 133 nm may be inaccurate. The quasi-phase-matching critical length L c  is about 0.30 μm when the final frequency summing stage  130 H is utilized to generate the 152 nm laser output light  139 H as described below with reference to  FIG.  7 A , about 0.34 μm when the final frequency summing stage  130 I is utilized to generate the 152 nm laser output light  139 I as described below with reference to  FIG.  7 B , and about 0.66 μm when the final frequency summing stage  130 J is utilized to generate the 177 nm laser output light  139 J as described below with reference to  FIG.  7 C . One skilled in the relevant arts would understand how to calculate a QPM critical length for any given combination of input and output frequencies given accurate refractive indices. 
     In another embodiment, grating assembly  150 E is made from LBO crystal material. For the final frequency doubling stage  130 B ( FIG.  2 B ) of 177 nm laser  100 B the quasi-phase-matching critical length L c  is about 0.6 μm when the polarization direction  329 E of the input light  133 E is parallel to the c axis (optical axis A12/A22). For the final frequency summing stage  130 J of 177 nm laser  100 J ( FIG.  7 C , described below) the quasi-phase-matching critical length L c  is about 0.86 μm when the polarization direction  329 E of input light  133 E (ω x =2ω 2 ) is parallel to the a axis (optical axis A11/A21), the polarization direction of the input light  133 E- 2  (ω y =4ω 1 ) is parallel to the c axis (optical axis A12/A22), and the polarization direction of the output light  136 E is parallel to the a axis (optical axis A11/A21). The refractive index value 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. Other combinations of polarization orientations are possible for frequency doubling and frequency summing with LBO. One skilled in the relevant arts would understand how to calculate a QPM critical length for a specific input and output light polarization combination given accurate refractive indices using the above equations. 
     As indicated in  FIG.  5   , input light  133 E of frequency ω x  is incident on input surface  153 E-IN of grating assembly  150 E, which in the present embodiment is implemented by planar surface  173 E- 11  of mesa  173 E- 1 . If grating assembly  150 E is configured for frequency doubling, then input light beam  133 E- 2  is omitted. If grating assembly  150 E is configured for frequency summation, input light beam  133 E- 2  with frequency ω y  is also incident on input surface  153 E-IN. A preferred polarization direction of the input light  133 E is illustrated by the dashed-line-arrow  329 E. The polarization direction of input light  133 E- 2  depends on the nonlinear crystal type and the chosen frequency conversion scheme. See above for some examples. The angle β between the propagating directions of input light  133 E and input light  133 E- 2  should be small, such as less than 5°, preferably about 2° or less. Grating assembly  150 E is configured such that input surface  153 E-IN and output surface  153 E-OUT are oriented approximately at Brewster&#39;s angle θ relative to the circulating light  133 E of frequency ω x  so as to minimize reflection losses without using an antireflection coating. For SBO, Brewster&#39;s 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, Brewster&#39;s 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, after forming the interdigitated grating assembly, an antireflection coating may be coated on the mesa surfaces to reduce light loss. If the mesas are coated with an antireflection coating, then the width of the mesas must be adjusted to account for the different optical path lengths of the different light frequencies passing through the coating. 
     In case of frequency doubling, exiting light  136 E comprises output light  139 E having an output frequency ω OUTE  equal to the second harmonic of the input light (i.e., 2ω x ) and unconsumed input light  138 E- 1  at input frequency ω x . In case of frequency summing, exiting light  136 E comprises output light  139 E having an output frequency ω OUTE  equal to the sum of the two input frequencies ω x +ω y , unconsumed input light  138 E- 1  with frequency ω x , and unconsumed input light  138 E- 2  with frequency ω y . 
     In at least one embodiment, the crystal axes are oriented such that light propagating inside the mesas of grating structures  160 E and  170 E propagates substantially parallel to one optical axis (labeled A11 and A21 in  FIG.  4 C ) with a polarization direction  329 E (electric field direction) of light  133 E substantially parallel to another optical axis (labeled A12 and A22 in  FIG.  4 C ). The output polarization may be parallel to optical axis A12/A22 or parallel to optical axis A13/A23, for example, to take advantage of the largest non-linear optical coefficient of the selected nonlinear crystal material or, in another example, to minimize the refractive index difference between the input and output frequencies and hence maximize the critical length. For example, SBO and LBO crystals both have an mm2 point group. The c axis, which corresponds to the axis without mirror symmetry, must be inverted. Other axes may need to have specific orientations relative to the polarizations of the input and output light depending on the chosen frequency conversion scheme. For example, in at least one embodiment utilizing SBO, the input and output polarizations should be parallel to the crystal c axis in order to take advantage of d 33 , which is the largest nonlinear coefficient of the crystal. In this example, optical axis A12/A22 would correspond to the crystal c axis, and the crystals of the upright and inverted gratings could be rotated about either the a or the b axis with respect to one another. In another example, in an embodiment utilizing LBO, the largest nonlinear coefficients are d 32  and d 24  (which must be equal because of the symmetry of the crystal). For frequency doubling, the input polarization should be parallel to the b axis and the output polarization should 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 with respect to one another. These are merely examples of possible crystal axis orientations for two specific materials and are not meant to limit the scope of the invention. One skilled in the relevant arts would understand how to select the appropriate crystal axis orientations for the upright and inverted gratings for any chosen frequency-conversion application of a specific nonlinear crystal. 
     If the input surface  153 E-IN of grating structure  170 E is oriented at Brewster&#39;s angle with respect to input light  133 E, then propagation direction PD of the light within mesa  172 E- 1  will be approximately 29.5° relative to surface normal N if grating structure  170 E is fabricated from an SBO crystal and will be approximately 31.5° relative to surface normal N if grating structure  170 E is fabricated from an LBO crystal. 
       FIGS.  6 A to  6 C  respectively depict a nonlinear crystal grating structure  160 F, a nonlinear crystal grating structure  170 F, and a nonlinear crystal grating assembly  150 F produced by assembling grating structures  160 F and  170 F in an interdigitated configuration according to another embodiment. Grating structure  160 F includes a row of parallel rectangular mesas  162 F- 1  to  162 F-N that extend vertically downward (i.e., in the Y-axis direction) from a horizontally oriented base  161 F and grating structure  170 F includes a row of parallel rectangular mesas  172 F- 1  to  172 F-N- 1  that extend vertically upward from a base  171 F. Each adjacent pair of mesas is separated by an intervening rectangular groove (e.g., mesas  162 F- 1  and  162 F- 2  are separated by groove  165 F- 1  and mesas  162 F-N- 1  and  162 F-N are separated by groove  165 F-N- 1 ; similarly, mesas  172 F- 1  and  172 F- 2  are separated by groove  175 F- 1  and mesas  172 F-N- 1  and  172 F-N- 2  are separated by groove  175 F-N- 2 ). In one embodiment, grating structures  160 F and  170 F are formed by etching, cutting or otherwise processing corresponding grating structures in the manner described herein to generate mesas having mesa widths that are substantially equal to an odd integer multiple of a critical length required to achieve QPM of input light beam  133 F (ω x ) and laser output light beam  139 F, and such that grating structure  160 F has a crystal axis that is inverted (i.e., rotated by substantially 180°) with respect to a corresponding crystal axis of grating structure  170 F. When assembled as indicated in  FIG.  6 C , grating assembly  150 F includes an interdigitated alternating grating pattern  155 F formed by sequentially aligned mesas  162 F- 1 ,  172 F- 1  . . .  172 F-N- 1  and  162 F-N through which input light  133 F passes. In some practical embodiments, the total number of mesas and grooves forming interdigitated alternating grating pattern  155 F is more than ten (e.g., the total number of mesas may be one or a few hundred or about one thousand). A larger number of mesas facilitates higher energy conversion. As such, grating assembly  150 F may be beneficially utilized in any of the various frequency conversion stages described herein with reference to grating assembly  150 E ( FIG.  4 C ). 
       FIG.  7 A  is a simplified block diagram showing an exemplary laser assembly  100 H according to another specific exemplary embodiment of the present invention. Laser assembly  100 H includes a first fundamental laser  110 H- 1 , a second fundamental laser  110 H- 2 , three intermediate frequency conversion stages (i.e., a first frequency doubling stage  120 H- 1 , a frequency summing stage  120 H- 2  and a second frequency doubling stage  120 H- 3 ) and a final frequency summing (conversion) stage  130 H that are cooperatively configured to generate laser output light  139 H having a wavelength ω OUTH  in the range of approximately 147 nm to approximately 155 nm (e.g., approximately 152 nm). First fundamental laser  110 H- 1  is configured in the manner described above to generate (first) fundamental light  119 H- 1  having a first fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm (i.e., between about 1 μm and 1.1 μm) and a corresponding first fundamental frequency ω 1 . Second fundamental laser  110 H- 2  is also configured in the manner described above to generate (second) fundamental light  119 H- 2  having a second fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm (i.e., between about 1 μm and 1.1 μm) and a corresponding second fundamental frequency ω 2 . First frequency doubling stage  120 H- 1  receives the first fundamental light  119 H- 1  and generates second harmonic light  121 H with a second harmonic frequency 2ω 1  equal to twice the first fundamental frequency ω 1 . A beam splitter  124 H separates the second harmonic light  121 H into two portions: a first portion  121 H- 1  and a second portion  121 H- 2 . First portion  121 H- 1  of second harmonic light  121 H is received by frequency summing stage  120 H- 2 , which sums first portion  121 H- 1  with second fundamental light  119 H- 2  to generate a first intermediate light beam  129 H- 1  having a corresponding frequency ω x  that is equal to the summing frequency 2ω 1 +ω 2 . For convenience, this summing frequency is referred to herein as substantially equal to a third harmonic (since ω 1  and ω 2  are similar or approximately equal). That is, when the frequencies of the first fundamental laser  110 H- 1  and the second fundamental laser  110 H- 2  are substantially the same (i.e., ω 1 =ω 2 ) then frequency ω x  of first intermediate light beam  129 H- 1  is substantially equal to the third harmonic of either fundamental light frequencies ω 1  or ω 2  (i.e., ω x ≈3ω 1  or ω x ≈3 ω 2 ). Frequency summing stage  120 H- 2  is configured in a manner similar to that described above for frequency summing stage  120 B- 2  with reference to  FIG.  2 B . Second portion  121 H- 2  of second harmonic light  121 H is passed to second frequency doubling stage  120 H- 3 , which is configured to generate a second intermediate light beam  129 H- 2  having corresponding frequency ω y  equal to equal to four times the first fundamental frequency ω 1  (i.e., ω y =4ω 1 ). According to the exemplary embodiment in  FIG.  7 A , each of the frequency doubling stages  120 H- 1  and  120 H- 3  comprises an external resonant cavity including at least three optical mirrors and a nonlinear crystal arranged therein in a manner similar to that described above with reference to second frequency doubling stage  120 A- 2  in  FIG.  2 A . Final frequency summing stage  130 H uses techniques described herein to sum the first and second intermediate light beams  129 H- 1  and  129 H- 2  (i.e., ω x +ω y ) and to generate laser output light  139 H with an output frequency ω OUTH  that is equal to 6ω 1 +ω 2 , which is referred to herein as substantially equivalent to seventh harmonic light (i.e., because when ω 1  and ω 2  are similar or approximately equal, ω x +ω y =6ω 1 +ω 2 ≈7ω 1 ), which in at least one embodiment has a wavelength of approximately 152 nm. In an alternative embodiment, second fundamental laser  110 H- 2  may be omitted, and the output of first fundamental laser  110 H- 1  may be divided into two portions: a first portion directed to first frequency doubling stage  120 H- 1 , and a second portion directed to frequency summing stage  120 H- 2  along with second harmonic light  121 H- 1 . In this alternative embodiment, necessarily ω 2 =ω 1 . 
       FIG.  7 B  is a simplified block diagram showing an exemplary laser assembly  100 I configured to generate a wavelength in the range of approximately 147 nm to approximately 155 nm (e.g., approximately 152 nm) according to another specific exemplary embodiment of the present invention. Laser assembly  100 I comprises a first fundamental laser  110 I- 1 , a second fundamental laser  110 I- 2 , three intermediate frequency conversion stages (i.e., a first frequency doubling stage  120 I- 1 , a second frequency doubling stage  120 I- 2 , and a first frequency summing stage  120 I- 3 ) and a final frequency summing (conversion) stage  130 I to generate laser output light  139 I with an output frequency ω OUTI  having a wavelength in the range of approximately 147 nm to approximately 155 nm (e.g., approximately 152 nm). Fundamental lasers  110 I- 1  and  110 I- 2  are configured in the manner described above to respectively generate fundamental light  119 I- 1  and  119 I- 2  having fundamental wavelengths in the range of approximately 1000 nm to approximately 1100 nm (i.e., between about 1 μm and 1.1 μm) and corresponding fundamental frequencies ω 1  and ω 2 , respectively. First frequency doubling stage  120 I- 1  receives first fundamental light  119 I- 1  and generates the second harmonic light  121 I- 1  with a second harmonic frequency ω 1  equal to twice the first fundamental frequency ω 1 . Beam splitter  124 I separates second harmonic light  121 I- 1  into two portions: a first portion  121 I- 11  and a second portion  121 I- 12 . First portion  121 I- 11  of second harmonic light  121 I- 1  is utilized as a first intermediate light beam  129 I- 1  having a corresponding frequency ω x  that is passed directly to final frequency summing stage  130 I. Second frequency doubling stage  120 I- 2  receives second portion  121 I- 12  of second harmonic light  121 I- 1  and generates fourth harmonic light  121 I- 2  with a fourth harmonic frequency 4ω 1  equal to four times the first fundamental frequency ω 1 . First frequency summing stage  120 I- 3  sums the fourth harmonic light  121 I- 2  with the second fundamental light  119 I- 2  and generates a second intermediate light beam  129 I- 2  having a corresponding frequency ω y  equal to the summing frequency 4ω 1 +ω 2 . For convenience, this summing frequency is referred to herein as fifth harmonic light (i.e., because when ω 1  and ω 2  are similar or approximately equal, the sum of the fourth harmonic of the first fundamental frequency and the second fundamental frequency is substantially equal to the fifth harmonic of the first fundamental frequency, or ω y =4ω 1 +ω 2 ≈5ω 1 ). Final frequency summing stage  130 I uses techniques described herein to sum the first and second intermediate light beams  129 I- 1  and  129 I- 2  and generates laser output light  139 I with an output frequency ω OUTE  being equal to summing frequency 6ω 1 +ω 2 , which is referred to for convenience herein as substantially equal to the seventh harmonic of first fundamental frequency ω 1  (i.e., if ω 1 ≈ω 2 , then ω x +ω y =6ω 1 +ω 2 ≈7ω 1 ), which in at least one embodiment has a wavelength of approximately 152 nm. In an alternative embodiment, second fundamental laser  110 I- 2  may be omitted, and the output of first fundamental laser  110 I- 1  may be divided into two portions: a first portion directed to first frequency doubling stage  120 I- 1 , and a second portion directed to first frequency summing stage  120 I- 3  along with fourth harmonic light  121 I- 2 . In this alternative embodiment, necessarily ω 2 =ω 1 . 
     The first frequency summing stage  120 I- 3  may be configured to use CLBO or hydrogen or deuterium-treated CLBO in a nearly non-critical phase matched configuration to sum fourth harmonic light  121 I- 2  with second fundamental light  119 I- 2 . Alternatively, the first frequency summing stage  120 I- 3  may use a nonlinear crystal grating assembly of the type described herein to achieve quasi-phase-matching (QPM). In one embodiment, the nonlinear crystal grating assembly is made of SBO crystal. The critical length for QPM for generating 213 nm by summing 266 nm and 1064 nm in SBO is approximately 1.81 μm (i.e. in a range from 1.80 μm to 1.82 μm). Since this critical length is longer than the critical lengths for generating shorter wavelengths, the SBO mesa thickness in the light propagation direction may be equal to the critical length or may be equal to a small, odd integer (such as between three and nine) times the critical length. In another embodiment, the nonlinear crystal is an LBO crystal. 
       FIG.  7 C  is a simplified block diagram showing an exemplary laser assembly  100 J according to another specific exemplary embodiment of the present invention. Laser assembly  100 J comprises a first fundamental laser  110 J- 1 , a second fundamental laser  110 J- 2 , three intermediate frequency conversion stages (i.e., a first frequency doubling stage  120 J- 1 , a second frequency doubling stage  120 J- 2 , and a third frequency doubling stage  120 J- 3 ), and a final frequency summing (conversion) stage  130 J that are cooperatively configured to generate laser output light  139 J having a wavelength ω OUTJ  in the range of approximately 170 nm to approximately 180 nm (e.g., approximately 177 nm). Fundamental lasers  110 J- 1  and  110 J- 2  include one or more nonlinear crystal grating assemblies and are configured in the manner described above to respectively generate fundamental light  119 J- 1  and  119 J- 2  having fundamental wavelengths in the range of approximately 1000 nm to approximately 1100 nm (i.e., between about 1 μm and 1.1 μm) and corresponding fundamental frequencies ω 1  and ω 2 , respectively. First frequency doubling stage  120 J- 1  receives second fundamental light  119 J- 2  and generates a first intermediate light beam  129 J- 1  having a frequency ω x  equal to a second harmonic of the second fundamental frequency ω 2  (i.e., equal to twice the second fundamental frequency ω 2 ). Second frequency doubling stage  120 J- 2  receives first fundamental light  119 J- 1  and generates second harmonic light  121 J having a frequency equal to a second harmonic of first fundamental frequency ω 1  (i.e., equal to twice the first fundamental frequency ω 1 ). Third frequency doubling stage  120 J- 3  receives second harmonic light  121 J and generates a second intermediate light beam  129 J- 2  having a frequency ω y  with a fourth harmonic frequency 4ω 1  equal to four times the first fundamental frequency ω 1 . Final frequency summing stage  130 J uses techniques described herein to sum first intermediate light beam  129 J- 1  (i.e., second harmonic 2ω 2 ) and second intermediate light beam  129 J- 2  (i.e., fourth harmonic 4ω 1 ) and generates laser output light  139 J having an output frequency ω OUTF  that is substantially equal to six times the first fundamental frequency (i.e., because when ω 1  approximately equals ω 2 , ω x +ω y =4ω 1 +2ω 2 ≈6ω 1 ), which in at least one embodiment has a wavelength of approximately 177 nm. In an alternative embodiment, second fundamental laser  110 J- 2  and 1 st  frequency doubling stage  120 J- 1  may be omitted, and the output  121 J of second frequency doubling stage  120 J- 2  may be divided into two portions: a first portion directed to third frequency doubling stage  120 J- 3 , and a second portion directed to final frequency summing stage  130 J along with fourth harmonic light  129 J- 2 . In this alternative embodiment, necessarily ω 2 =ω 1 . 
     The above-described figures are not meant to represent the actual physical layout of the components. The above-described figures show the main optical modules involved in the process, but do not show every optical element. One skilled in the appropriate arts would understand how to build the 177 nm, 152 nm, 133 nm and similar lasers from the above-described figures and their associated descriptions. It is to be understood that more or fewer optical components may be used to direct the light where needed. Lenses and/or curved mirrors may be used to focus the beam waist to foci of substantially circular or elliptical cross sections inside or proximate to the non-linear crystals where appropriate. Prisms, beam-splitters, gratings or diffractive optical elements may be used to steer or separate the different wavelengths at the outputs of each frequency conversion stage when needed. Prisms, coated mirrors, or other elements may be used to combine the different wavelengths at the inputs to the frequency conversion stages as appropriate. Beam splitters or coated mirrors may be used as appropriate to divide one wavelength into two beams. Filters may be used to block or separate undesired wavelengths at the output of any stage. Waveplates may be used to rotate the polarization as needed. Other optical elements may be used as appropriate. One skilled in the appropriate arts would understand the various tradeoffs and alternatives that are possible in the implementation of the 177 nm, 152 nm, 133 nm and similar lasers. 
     In the various alternative embodiments described above, the first fundamental laser may be configured to generate first fundamental light at first fundamental frequency ω 1  having a corresponding wavelength equal to one of approximately 1070 nm, approximately 1064 nm, approximately 1053 nm, approximately 1047 nm, and approximately 1030 nm. If used, the second fundamental laser may be configured to generate second fundamental light at second fundamental frequency ωt 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 mentioned herein are based on corresponding multiples of the fundamental frequencies. The exact wavelength of light generated by a given fundamental laser depends on many factors including the exact composition of the lasing medium, the operating temperature of the lasing medium, and the design of the optical cavity. Two lasers using the same laser line of a given lasing medium may operate at wavelengths that differ by a few tenths of 1 nm or a few nm due to the aforementioned and other factors. One skilled in the appropriate arts would understand how to choose the appropriate first and second fundamental wavelengths in order to generate the desired output wavelength from any one or two fundamental wavelengths. 
     Although the present invention is described herein using various fundamental wavelengths that facilitate generating laser output light at desired wavelengths of approximately 177 nm, approximately 152 nm or approximately 133 nm, other wavelengths within a few nanometers of these desired wavelengths can be generated using different fundamental wavelengths. Unless otherwise specified in the appended claims, such lasers and systems utilizing such lasers are considered within the scope of this invention. 
     Compared to pulsed lasers, a CW light source has a constant power level, which avoids the peak power damage issues. Also, the bandwidth of the generated CW light is several orders of magnitude narrower than typical mode-locked lasers, so the design of the corresponding illumination or detection optical system can be less complex with better performance and lower system cost. However, some inspection and metrology applications can tolerate the higher bandwidth and peak power levels of a pulsed laser. A pulsed laser is simpler than a CW laser as resonant cavities are not needed for the frequency conversion stages. Hence both CW and pulsed lasers are within the scope of the invention disclosed herein and may be used as appropriate. 
     CW lasers and lasers with high-repetition rates with a wavelength shorter than sub-200 nm are not commercially available at sufficient power level or are unreliable. In particular, there are no currently available lasers for generating light of hundreds of mW of power or greater in a wavelength range between approximately 125 nm and 190 nm. The embodiments of the present invention generate short wavelength light down to approximately 133 nm, therefore provide better sensitivity for detecting small particles and defects than longer wavelengths. Another aspect of the invention is a wafer, reticle or photomask inspection or metrology system that incorporates at least one of the inventive 177 nm, 152 nm and 133 nm lasers described above. Aspects of such systems are illustrated in  FIGS.  8 ,  9 A,  9 B and  10   . 
     This laser may be used in an inspection system with dark-field and bright-field inspection modes as shown in  FIG.  8   . This figure and the system are explained in U.S. Pat. No. 7,817,260 to Chuang et al., which is incorporated by reference as if fully set forth herein.  FIG.  8    illustrates a catadioptric imaging system  800  incorporating normal incidence laser illumination. The illumination block of system  800  includes a laser  801 , adaptation optics  802  to control the illumination beam size and profile on the surface being inspected, an aperture and window  803  in a mechanical housing  804 , and a prism  805  to redirect the laser along the optical axis at normal incidence to the surface of a sample  808 . Prism  805  also directs the specular reflection from surface features of sample  808  and reflections from the optical surfaces of an objective  806  along the optical path to an image plane  809 . Lenses for objective  806  can be provided in the general form of a catadioptric objective, a focusing lens group, and a zooming tube lens section  807 . In at least one embodiment, laser  801  can be implemented by the one of above-described lasers. 
     This laser may be used in a dark-field inspection system with oblique line illumination as shown in  FIGS.  9 A and  9 B . This inspection system may have two or three different collection systems including off axis and near normal collection as shown. This dark field inspection system may also include normal incidence line illumination (not shown). More details including an explanation of the system shown in  FIGS.  9 A and  9 B  can be found in U.S. Pat. No. 7,525,649 to Leong et al., which is incorporated by reference as if fully set forth herein. 
       FIG.  9 A  illustrates a surface inspection apparatus  900  that includes illumination system  901  and collection system  910  for inspecting areas of surface  911 . As shown in  FIG.  9 A , a laser system  920  directs a light beam  902  through beam shaping optics  903 . In at least one embodiment, the laser system  920  includes at least one of the above-described lasers. First beam shaping optics  903  can be configured to receive a beam from the laser system, which is focused onto surface  911 . 
     Beam shaping optics  903  is oriented so that its principal plane is substantially parallel to a sample surface  911  and, as a result, illumination line  905  is formed on surface  911  in the focal plane of beam shaping optics  903 . In addition, light beam  902  and focused beam  904  are directed at a non-orthogonal angle of incidence to surface  911 . In particular, light beam  902  and focused beam  904  may be directed at an angle between about 1° and about 85° from a normal direction to surface  911 . In this manner, illumination line  905  is substantially in the plane of incidence of focused beam  904 . 
     Collection system  910  includes lens  912  for collecting light scattered from illumination line  905  and lens  913  for focusing the light coming out of lens  912  onto a device, such as charge coupled device (CCD)  914 , comprising an array of light sensitive detectors. In one embodiment, CCD  914  may include a linear array of detectors. In such cases, the linear array of detectors within CCD  914  can be oriented parallel to illumination line  905 . In another embodiment, CCD  914  may include a two-dimensional array of detectors, arranged as a rectangular array with its long axis parallel to illumination line  905 . For example, CCD  914  may comprise a rectangular array of approximately 1000 to 8000 detectors by approximately 50 to 250 detectors. In one embodiment, multiple collection systems can be included, wherein each of the collection systems includes similar components, but differ in orientation. 
     For example,  FIG.  9 B  illustrates an exemplary array of collection systems  931 ,  932 , and  933  for a surface inspection apparatus (wherein its illumination system, e.g., similar to that of illumination system  901 , is not shown for simplicity). First optics in collection system  931  collect light scattered in a first direction from the surface of sample  921 . Second optics in collection system  932  collect light scattered in a second direction from the surface of sample  921 . Third optics in collection system  933  collect light scattered in a third direction from the surface of sample  921 . Note that the first, second, and third paths are at different angles of reflection to said surface of sample  921 . A platform  922  supporting sample  921  can be used to cause relative motion between the optics and sample  921  so that the whole surface of sample  921  can be scanned. 
     This laser may also be used in inspection systems for un-patterned wafers such as inspection system  1000  shown in  FIG.  10   . Such an inspection system may incorporate oblique and/or normal incidence illumination and a large collection solid angle for the scattered light as shown in these figures. Illumination source  1100  incorporates at least one of the laser assemblies described herein that generates DUV or VUV light to illuminate wafer  1122  at a desirable angle to ensure that reflected light is not collected by a system of imaging collection optics  1108 . Optics  1106  may be configured to generate the desired illumination pattern. Scattered light from the wafer  1122  may be collected by a system of imaging collection optics  1108  configured to direct the light into an afocal lens system  1110 . In one embodiment collection lens mask system  1112  may divide the light into a plurality of channels for delivery to a TDI sensor  1118 . One embodiment may include an intensifier  1114  and/or a sensor relay  1116 . TDI sensor  1118  and/or intensifier  1114  may be configured to transmit signals to image processing computer  1120 , which may be configured to generate a wafer image and/or a list of defects or particles on the surface of wafer  1122 . Additional explanation of the elements of  FIG.  10    can be found in U.S. Pat. No. 9,891,177B2 to Vazhaeparambil et al. Further details on un-patterned wafer inspection systems can be found in U.S. Pat. Nos. 6,201,601 and 6,271,916. All of these patents are incorporated by reference as if fully set forth herein. 
     Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention.