Frequency conversion using stacked strontium tetraborate plates

Abstract

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

Description

Technical Field

[0001] The present application relates to a laser capable of generating light having VUV wavelengths, and particularly relates to a laser capable of generating light in a range of approximately 125 nm to 183 nm, and an inspection system that uses such a laser to inspect, for example, photomasks, reticles, and semiconductor wafers.

Background Art

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

[0003] This application is further related to the following U.S. patent documents, all of which are incorporated herein by reference: U.S. Patent No. 6,201,601 by Vaez-Iravani et al., U.S. Patent No. 6,271,916 by Marxer et al., U.S. Patent No. 7,525,649 by Leong et al., U.S. Patent No. 7,817,260 by Chuang et al., U.S. Patent No. 8,298,335 and 8,824,514 by Armstrong, U.S. Patent No. 8,976,343 by Genis, and Dri These include U.S. Patent No. 9,023,152 by Dribinski, U.S. Patents No. 9,461,435 and 9,059,560 by Dribinski et al., U.S. Patents No. 9,293,882 and 9,660,409 by Chuang, U.S. Patents No. 9,250,178, 9,459,215, 9,509,112, 10,044,166 and 10,283,366 by Chuang et al., and U.S. Patent Application No. 2014 / 0305367 published by Dribinski et al.

[0004] As the dimensions of semiconductor devices shrink, the size of the smallest particles or pattern defects that can cause device failure also shrinks. Therefore, there is a need to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. The intensity of light scattered by a particle is smaller than the wavelength of that light and is generally scaled as a higher-order power of the particle's size (for example, the total scattering intensity of light from an isolated small spherical particle is proportional to the sixth power of the sphere's diameter and inversely proportional to the fourth power of the wavelength). Because the intensity of scattered light increases, shorter wavelengths are generally more sensitive to detecting smaller particles and defects than longer wavelengths.

[0005] Because the intensity of light scattered from small particles and defects is generally very low, high illumination intensity is required to generate a signal that can be detected in a very short time. An average light source power level of 0.3W or higher may be required. At these high average power levels, a high pulse repetition rate is desirable because a higher repetition rate results in lower energy per pulse, thus reducing the risk of damage to system optical components or the item being inspected. The need for illumination for inspection and measurement is generally best suited to continuous wave (CW) light sources. CW light sources have a constant power level, thus avoiding peak power damage problems and allowing for continuous acquisition of images or data. However, in some cases, a mode-locked laser with a repetition rate of approximately 50 MHz or higher may be useful because a high repetition rate means that the energy per pulse can be kept sufficiently low to avoid damage relevant to specific measurement and inspection applications.

[0006] Pulsed lasers for generating VUV light are well known in this art. Prior art lasers for generating light at 133 nm are well known (e.g., Non-Patent Documents 1 and 2). Unfortunately, such lasers have low laser pulse repetition rates and low average power levels, making them unsuitable for inspection applications.

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

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

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

[0010] [Non-Patent Document 1] GWFaris and MJDyer, "Two-photon excitation of neon at 133 nm," Optics Letters, Vol. 18, p. 382 (1993). [Non-Patent Document 2] A. Tunnermann, C. Momma, K. Mossavi, C. Windolph, and B. Wellegehausen, "Generation of tunable short pulse VUV radiation by four-wave mixing in Xenon with femtosecond KrF-excimer laser pulses," IEEE Journal of Quantum Electronics, Vol. 29, p. 1233 (1993). [Overview of the project] [Problems that the invention aims to solve]

[0011] Therefore, there is a need for mode-locked or CW lasers that generate radiation in the vacuum ultraviolet (VUV) range, particularly in the range shorter than 133 nm, and are suitable for use in the inspection of photomasks, reticles, and / or wafers. The practical application of lasers that enable mode-locked or CW output at higher power levels around 133 nm could enable more accurate and faster inspection and measurement, potentially contributing to state-of-the-art semiconductor manufacturing.

[0012] Furthermore, the need arises to provide inspection systems and related laser systems that can generate mode-locked or CW laser light with output VUV wavelengths in the range of approximately 125 nm to 183 nm, thereby avoiding some or all of the above problems and drawbacks. [Means for solving the problem]

[0013] The present invention relates to a nonlinear crystal comprising stacked strontium tetraborate (SrB4O7) crystal plates that forms a periodic structure capable of achieving pseudo-phase matching (QPM) suitable for frequency conversion of applied light, thereby facilitating the generation of DUV and VUV laser light at high power and photon energy levels, while avoiding the aforementioned problems and drawbacks associated with prior art approaches. SBO crystals exhibit attractive features (e.g., a wide transparency range, excellent damage resistance and chemical stability, high microhardness, and a high diagonal element value d33 compared to the band gap value), which avoid many of the aforementioned problems and drawbacks associated with prior art approaches. However, SBO single crystals also exhibit low birefringence, making frequency conversion by critical or noncritical phase matching impossible. The present invention avoids the low birefringence of SBO by coordinating the SBO crystal plates to form a periodic structure that achieves QPM for one or more input light frequencies (intermediate light beams), so that the light emanating from the nonlinear crystal contains laser output light having a desired DUV / VUV output frequency. In one embodiment, the cooperative configuration involves physically stacking separate SBO crystal plates, resulting in alternating inversions of the crystal axes of the sequentially arranged plates (i.e., the crystal axis of a given SBO crystal plate is substantially rotated 180° with respect to the crystal axis of an adjacent SBO crystal plate(s) that share a common interface(s) with the given plate in the stack), thereby forming a periodic structure similar to that of a periodically polarized crystalline material (i.e., each SBO crystal plate forms a physical pole of the periodic structure). Each nonlinear crystal is further configured for use in a given optical system, which is done by orienting the SBO crystal plates so that the inverted crystal axes are aligned perpendicular to the polarization direction of light as light passes through the SBO crystal stack in the optical system, and by forming the SBO crystal plates such that the thickness of at least one SBO crystal plate forms a gap between poles (i.e., the distance light travels between opposing surfaces of each plate in the direction of light propagation), which is substantially equal to an odd multiple of a critical length that enables pseudo-phase matching between one or more input optical frequencies and output frequencies.By cooperatively configuring two or more SBO crystal plates in this manner, the nonlinear crystal produced according to the present invention facilitates frequency conversion (e.g., frequency doubling of one input optical frequency, or frequency summing of two or more input optical frequencies) required to generate DUV and VUV wavelengths at high power levels (i.e., several milliwatts (mW) to several watts (W) or more) and high photon energy levels (e.g., 7.00 eV at 177 nm and 9.32 eV at 133 nm), while avoiding the aforementioned problems and drawbacks associated with prior art approaches. Although primarily described with particular reference to practical applications including the generation of CW laser light, the nonlinear crystals disclosed herein can be used in other optical systems and for other purposes, including the generation of pulsed laser light, without departing from the spirit and scope of the present invention.

[0014] In the specifically disclosed embodiments described below, the present invention relates to improvements to inspection systems used in the semiconductor manufacturing industry, and more particularly to laser assemblies for inspection systems that have a light source power level of 0.3 W or more and that can generate mode-locked or continuous-wave (CW) laser light having an output wavelength in the range of about 125 nm to about 183 nm. In actual embodiments, each nonlinear crystal is utilized in the final frequency conversion stage of the associated laser assembly, which further includes at least one fundamental wave laser and two or more intermediate frequency conversion stages, where each fundamental wave laser each generates a fundamental light beam having a corresponding fundamental frequency (e.g., having a wavelength between about 1 μm and 1.1 μm), and the intermediate frequency conversion stages are collectively configured to convert the fundamental light beam(s) into at least one intermediate light beam having a corresponding intermediate frequency. The final frequency conversion stage is configured to guide the intermediate light beam(s) through inverted SBO crystal plates forming a nonlinear crystal, so that the polarization direction (electric field direction) of the light is substantially parallel to the c-axis (or a-axis) of the crystal axis of each plate, and the periodic structure of the stacked SBO crystal plates achieves the QPM of the intermediate light beam(s). In certain embodiments, the final frequency conversion stage includes a number of mirrors, which are configured to receive and circulate (e.g., by one or more matching lenses) at least one of the intermediate light beams (e.g., in a bow-tiling cavity configuration), so that a beam waist of the circulated light is generated in the nonlinear crystal (i.e., inward or proximal). In one embodiment, the final frequency conversion stage utilizes a beam splitter (e.g., an SBO crystal, SBO glass, or CaF2 crystal) which splits the outgoing light (i.e., light leaving / exiting the nonlinear crystal) so that the reflected (first) portion of the outgoing light forms a desired laser output light beam having an output wavelength in the range of approximately 125 nm to approximately 183 nm, and the unreflected (second) portion of the outgoing light, including the unconsumed input light, is passed through the beam splitter for circulation by a cavity mirror. Note that in the following description, where wavelength is mentioned unconditionally, it can be assumed that the wavelength is the wavelength in a vacuum.

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

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

[0017] Referring to a second specific embodiment, according to the laser assembly and associated method described herein, a laser output light having an output frequency having a wavelength of about 177 nm is produced by generating a first fundamental light having a first fundamental frequency, utilizing the first fundamental light to generate a second harmonic of the first fundamental light, summing the second harmonic of the first fundamental light with a second fundamental light having a second fundamental frequency, and using the summing product as intermediate light passed to the final frequency conversion stage. In one embodiment, each of the first and second fundamental frequencies has a corresponding wavelength in the range of about 1000 nm to about 1100 nm, so that the wavelength of the intermediate light beam (i.e., the summing product) is approximately equal to the third harmonic of the first fundamental frequency. According to one aspect of the second embodiment, nearly sixth-harmonic output light can be generated from an intermediate (nearly third-harmonic) light beam, by configuring the final frequency conversion stage as a frequency doubling cavity resonating at the third-harmonic frequency, and constructing an SBO crystal plate of stacked linear crystals having an interpole spacing substantially equal to twice an odd multiple of the pseudo-phase matching critical length (i.e., in the range of 0.59 μm to 0.61 μm), thereby realizing QPM at the third and sixth harmonic frequencies, and thereby generating laser output light with an output wavelength of approximately 177 nm.

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

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

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

[0021] In one embodiment, an inspection system configured to inspect a sample such as a wafer, a reticle, or a photomask includes one of the lasers described herein, which generates an output wavelength of about 177 nm, 152 nm, or about 133 nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] [Figure 1] It is a simplified block diagram showing an exemplary laser assembly according to a generalized exemplary embodiment of the present invention. [Figure 2A] It is a simplified block diagram showing a simplified laser assembly according to a first embodiment of the present invention. [Figure 2B] It is a simplified block diagram showing a simplified laser assembly according to a specific embodiment of the present invention. [Figure 3]This is a simplified diagram showing an exemplary final frequency doubling stage used in a laser assembly of a first and second specific embodiment according to an exemplary embodiment of the present invention. [Figure 4] This is a simplified diagram showing an exemplary nonlinear crystal configured for use in the final frequency doubling stage of Figure 3. [Figure 5A] This is a simplified block diagram showing a simplified laser assembly according to a third specific embodiment of the present invention. [Figure 5B] This is a simplified block diagram showing a simplified laser assembly according to a fourth specific embodiment of the present invention. [Figure 5C] This is a simplified block diagram showing a simplified laser assembly according to a fifth specific embodiment of the present invention. [Figure 6] This is a simplified diagram showing an exemplary final frequency doubling stage used in a laser assembly of a third, fourth, and fifth specific embodiment according to an exemplary embodiment of the present invention. [Figure 7] This is a simplified diagram showing an exemplary nonlinear crystal configured for use in the final frequency doubling stage of Figure 8. [Figure 8] This is a simplified diagram illustrating an exemplary inspection system comprising dark-field and bright-field inspection modes, utilizing one of the laser assemblies described herein, according to another specific embodiment of the present invention. [Figure 9A] This figure shows a dark-field inspection system utilizing one of the laser assemblies described herein, according to another specific embodiment of the present invention. [Figure 9B] This figure shows a dark-field inspection system utilizing one of the laser assemblies described herein, according to another specific embodiment of the present invention. [Figure 10] This figure shows an alternative dark-field inspection system configured to inspect an unpatterned wafer using one of the laser assemblies described herein, according to another specific embodiment of the present invention. [Modes for carrying out the invention]

[0023] This invention relates to improvements to lasers for semiconductor inspection systems. The following description is provided to enable those skilled in the art to construct and use the invention as provided for a particular application and its requirements. As used herein, terms indicating direction, such as “up,” “left,” “right,” “horizontal,” and “down,” are intended to provide relative positions for explanatory purposes and not to indicate an absolute coordinate system. Various modifications to the described embodiments will be apparent to those skilled in the art, and the general principles set forth herein may be applied to other embodiments. Thus, the invention is not intended to be limited to the specific embodiments shown and described, but rather to the broadest range consistent with the principles and novel features disclosed herein.

[0024] The variation in the second-order susceptibility of ascentric crystals can lead to changes in the quasi-phase matching (QPM) conditions, which may be useful for frequency conversion. In the VUV spectral region below approximately 150 nm, transparent optical crystals that couple non-zero second-order nonlinearity with sufficient birefringence remain unclear. Several attempts to fabricate QPM structures have been reported, for example, by electric field polarization of ferroelectric BaMgF4 with mm2 symmetry (EGVillora, K. Shimamura, K. Sumiya, and H. Ishibashi, "Birefringent-and quasi phase-matching with BaMgF4 for vacuum-UV / UV and mid-IR all solid-state lasers", Optics Express Vol. 17, p. 12362 (2009)), or by mechanical twinning of crystalline quartz (SiO2) with 3 times 32 symmetry (S. Kurimura, M. Harada, K. Muramatsu, M. Ueda, M. Adachi, T. Yamada, and T. Ueno, "Quartz revisits nonlinear optics: twinned crystal for quasi-phase matching [Invited]", Optical Materials According to Express, Vol. 1, p. 1367 (2011), both materials exhibit low nonlinear coefficients, and the shortest wavelength demonstrated to date is 194 nm.

[0025] Strontium tetraborate (SrB4O7, SBO) crystallizes in an orthorhombic system with point group mm² and space group Pnm21, with unit cell sizes a=4.4255 Å, b=10.709 Å, and c=4.2341 Å (YSOseledchik, ALProsvirnin, AIPisarevskiy, VVStarshenko, VVOsadchuk, SPBelokrys, NVSvitanko, ASKorol, SAKrikunov, and AFSelevich, "New nonlinear optical crystals: strontium and lead tetraborates," Optical Materials, Vol. 4, p. 669 (1995)). All boron atoms are coordinated to tetrahedra, and oxygen atoms are common to three tetrahedra. Despite the three-dimensional network of tetrahedra, the borate network appears as a layered structure due to the relatively few links in the c-direction of the unit cell.

[0026] SBO exhibits very small birefringence (<0.005) and is not ferroelectric. Non-phase-matched second harmonic generation (SHG) has been implemented using SBO for diagnostics, but the efficiency is very low when only one coherence length is used, and the practical detection limit has been estimated to be 2 μJ at 267 nm for a 120 fs pulse (V. Petrov, F. Noack, D. Shen, F. Pan, G. Shen, X. Wang, R. Komatsu, and V. Alex, "Application of the nonlinear crystal SrB4O7 for ultrafast diagnostics converting to wavelengths as short as 125 nm", Optical Letters, Vol. 29, p. 373 (2004)).

[0027] SBO exhibits unique optical and mechanical properties. The transparency range of SBO is 130–3200 nm in wavelength (YSOseledchik et al., cited above). SBO also exhibits a high diagonal element value d33 (1.5–3.5 pm / V) compared to the bandgap value. The photodamage threshold is very high compared to other materials such as MgF2 (14.7 GW / cm²). 2 ). The microhardness of SBO is also high (1750 kg / mm² in the x-direction). 2 , 1460 kg / mm ​​in the y direction 2 , 1350 kg / mm² in the z direction 2 Due to its high photodamage threshold and microhardness, SBO crystals can withstand extreme conditions when exposed to DUV and VUV radiation. DUV and VUV lasers may have high power levels ranging from several milliwatts (mW) to several watts (W) or more, and high photon energies (e.g., 9.32 eV at 133 nm, 8.16 eV at 152 nm). A wide transparency range, excellent damage resistance and chemical stability, and a high diagonal element value d33 are features that make SBO very attractive for frequency conversion to generate DUV and VUV wavelengths. However, its low birefringence means that frequency doubling by critical or noncritical phase matching is not possible.

[0028] Trabs et al. (P. Trabs, F. Noack, ASA Leksandrovsky, AIZaitsev, NVRadionov, and V. Petrov, "Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV," Optics Express, Vol. 23, p. 10091 (2015)) reported generating second harmonics from ultrashort laser pulses via random pseudo-phase matching in the VUV using an SBO crystal. The second harmonic generation method described by Trabs et al. is unsuitable for semiconductor measurement and inspection systems of light sources because the frequency conversion process is inefficient, making it impractical to generate watts of second harmonic laser power using this method, and furthermore, it requires ultrashort laser pulses.

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

[0030] Referring to the upper left portion of Figure 1, the fundamental wave laser 110 is configured to generate fundamental light beams 119-1, 119-2…119-n (collectively referred to as 119) having corresponding fundamental frequencies ω1~ωn, with each frequency having a corresponding fundamental wavelength between approximately 1 μm and 1.1 μm. In some embodiments, all fundamental light beams 119 have substantially the same wavelength (for example, fundamental frequency ω1 is substantially equal to fundamental frequency ω2). Specific fundamental wave laser types are referred to in the specific embodiments provided below.

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

[0032] Referring to the lower half of Figure 1, the laser assembly 100 also includes a final frequency conversion stage 130 which passes an intermediate light beam 129 (ωX or ωX and ωy) through a nonlinear crystal 135 and directs the laser output light beam 139 out of the laser assembly 100, configured for use in one or more inspection systems described below with reference to Figures 8 to 10, for example. In one embodiment, the intermediate light beam 129-1 has a frequency ωx as described in the specific embodiment shown below and enters a bow-tiling cavity formed by an input coupler mirror 132-1, a plane mirror 132-2, two curved mirrors 132-3 and 132-4, an SBO linear crystal 135, and a beam splitter 137. For illustrative purposes, a portion of the light transmitted by the bow-tiling cavity from the input / coupler mirror 132-1 to the linear crystal 135 is shown as a circulating light portion 133, which consists of both an intermediate light beam 129-1 and an unconsumed circulating light portion 138-1 (generated as described below), where both light portions 133 and 138-1 have a frequency ωX. In one embodiment, a mode-matching lens 131 is used to focus the intermediate light beam 129-1 through the input coupler / mirror 132-1, and the bow-tiling cavity formed by the mirrors 132-1 to 132-4 is configured such that, otherwise, the light portion 133 is directed onto the nonlinear crystal 135 at a selected angle θ with respect to the surface normal N (i.e., perpendicular to the input surface 135-IN), and the beam waist of the light portion 133 (i.e., containing the intermediate light beam 129-1) is generated in the nonlinear crystal 135 (i.e., inward or adjacent to it). When an intermediate light beam 129-2 having frequency ωy is used as described in the relevant specific embodiments shown below, the intermediate light beam 129-2 enters a bow-tiling cavity that passes near (but does not necessarily pass through) the curved mirror 132-3, is substantially directed at a selected angle θ on the input surface 135-IN, and passes through the linear crystal 135.As shown in this exemplary configuration, the final frequency conversion stage 130 is configured to pass the in-crystal light 134 (i.e., only the optical portion 133, or both the optical portion 133 and the intermediate light beam 129-2) through the nonlinear crystal 135 so that the outgoing light 136 (i.e., all the light leaving the linear crystal 135) is directed to the input surface 137-IN of the beam splitter 137. The beam splitter 137 is configured to split the outgoing light 136 so that the unconsumed input light 138-1 with frequency ωx is passed to the mirror 132-4 for recirculation within the bowtie cavity, and the laser output light 139 with output frequency ωOUT is directed out of the laser assembly 100. As shown, in some embodiments, the beam splitter 137 is also configured to reflect the unconsumed input light 138-1 with frequency ωy out of the bowtie cavity. The beam splitter 137 may be implemented using one of the following: an SBO single crystal, SBO glass, or a CaF2 crystal.

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

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

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

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

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

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

[0039] According to the exemplary embodiment shown in Figure 2A, each of the frequency doubling stages 120A-1 and 120A-2 comprises an external resonant cavity containing at least three optical mirrors and a nonlinear crystal positioned inside, respectively. The cavity can be stabilized using standard PDH (pound-drever-hole), HC (Hensch-Couillaud), or other locking techniques. The length of the cavity is adjusted to maintain resonance by adjusting the position of the mirrors or prisms with a control signal. The first frequency doubling stage 120A-1 receives and converts a first fundamental light 119A at a first fundamental frequency ω1, and generates a second harmonic light 121A at twice the first fundamental frequency (2ω1). The second frequency doubling stage 120A-2 receives and converts the second harmonic light 121A, generating the fourth harmonic light 129A at four times the first fundamental frequency (4ω1).

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

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

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

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

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

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

[0046] Any of the frequency conversion stages may be enclosed in one or more protective environments, as described, for example, in U.S. Patent No. 8,298,335 by Armstrong, entitled “Enclosure for controlling the environment of optical crystals,” which is incorporated herein by reference. In particular, since the final frequency doubling stage 130A generates VUV wavelengths, this stage needs to be in an environment with very low concentrations of oxygen and water (preferably less than a few ppm). Preferably, the final frequency doubling stage is kept in an environment purged with pure nitrogen or argon. Note that a single protective environment may enclose multiple stages or a single stage.

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

[0048] Furthermore, it should be noted that any of the intermediate frequency conversion stages described herein may advantageously utilize nonlinear crystals doped or treated with deuterium, hydrogen, and / or fluorine. Such crystals may be prepared, processed, or treated by any of the processes or methods described in Dribinski's U.S. Patent No. 9,023,152, Chuang et al.'s U.S. Patents No. 9,250,178, 9,459,215, and 10,283,366, and published U.S. Patent Application No. 2014 / 0305367, titled "Passivation of Nonlinear Optical Crystals," filed by Dribinski et al. on April 8, 2014. These patents and applications are incorporated herein by reference. Doped or treated crystals may be particularly useful in stages involving deep UV wavelengths, including the second frequency doubling stage 120A-2 in Figure 2A.

[0049] Figure 2B is a simplified block diagram showing an exemplary laser assembly 100B configured to generate wavelengths in the range of about 170 nm to about 180 nm (e.g., about 177 nm) according to a second specific embodiment of the present invention. The laser assembly 100B includes a first fundamental laser 110B-1, a second fundamental laser 110B-2, a frequency doubling (conversion) stage 120B-1, a frequency summing (conversion) stage 120B-2, and a final frequency doubling stage 130B, which together generate laser output light 139B with an output frequency ωOUTB having wavelengths in the range of about 170 nm to about 180 nm. The first fundamental laser 110B-1 is configured to generate fundamental light 119B-1, which has a first fundamental wavelength in the range of about 1000 nm to about 1100 nm (i.e., about 1 μm to 1.1 μm) and a corresponding first fundamental frequency ω1. The second fundamental laser 110B-2 is configured to generate fundamental light 119B-2, which has a second fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm (i.e., approximately 1 μm to 1.1 μm) and a corresponding second fundamental frequency ω2. The frequency doubling stage 120B-1 receives the first fundamental light 119B-1 and generates second harmonic light 121B having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The frequency summing stage 120B-2 sums the second harmonic 121B with the second fundamental light 119B-2 to generate an intermediate light beam 129B with a total frequency 2ω1 + ω2. If the frequencies of the first fundamental laser 10B-1 and the second fundamental laser 110B-2 are the same (ω1=ω2), the intermediate light beam 129B is the third harmonic of the fundamental light (3ω1 or 3ω2). The final frequency doubling stage 130B receives the intermediate light beam 129B and generates the final output light 139B, which has an output frequency ωOUTB equal to twice the total frequency 2ω1+ω2, i.e., 4ω1+2ω2. If the frequencies of the first fundamental laser 110B-1 and the second fundamental laser 110B-2 are the same (ω1=ω2), the output frequency ωOUTB of the final laser output light 139B is the sixth harmonic of the fundamental light (6ω1 or 6ω2).

[0050] Referring to Figure 2B, the first and second fundamental lasers 110B-1 and 110B-2 are configured as described above with reference to the fundamental laser 110A in Figure 2A. In an alternative embodiment, the second fundamental laser 110B-2 may be omitted, and the output of the first fundamental laser 110B-1 may be divided into two parts: a first part directed to the first frequency doubling stage 120B-1 and a second part directed to the frequency summing stage 120B-2 together with the second harmonic light 121B. In this alternative embodiment, ω2 = ω1 is necessarily the case.

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

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

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

[0054] Figure 3 is a simplified diagram showing an exemplary final frequency doubling stage 130C used in the 133nm laser assembly 100A in Figure 2A and the 177nm laser assembly 100B in Figure 2B, according to exemplary embodiments of the present invention. Input light 129C having frequency ωx (for example, ωx = 4ω1 when stage 130C is used in the 133nm laser 100A, or ωx = 2ω1 + ω2 when stage 130B is used in the 177nm laser 100B) enters via input coupler 132C-1 into a bow-tiling cavity comprising an input coupler 132C-1, a planar mirror 132C-2, curved mirrors 132C-3, 132C-4, and a nonlinear crystal 135B (including input surface 135C-IN and output surface 135C-OUT) having two or more SBO crystal plates, and is recirculated to enhance power. The emitted light 136C is output from the nonlinear crystal 135C through the output surface 135C-OUT and includes the unconsumed input light 138C and the generated laser output light 139C having an output frequency ωOUTC equal to twice the frequency of the input light 129C (i.e., frequency ωOUTC can be equal to either the 8th harmonic output light 139A in Figure 2A or the 6th harmonic output light 139B in Figure 2B). The laser output light 139C is reflected from the surface of the beam splitter (BS) 137C and directed out of the cavity.

[0055] Preferably, the nonlinear crystal 135C is configured such that its input surface 135C-IN and output surface 135C-OUT are oriented at approximately a Brewster angle with respect to the circulating input light 133C. The polarization direction of the circulating input light 133C is indicated by arrow 329C. Furthermore, BS137C may be configured to laterally displace the circulating input light 133C in the cavity by an amount that substantially offsets the lateral displacement of the input light caused by the nonlinear crystal 135C, thereby maintaining a substantially symmetrical bowtie cavity and simplifying the optical alignment of the cavity.

[0056] In one embodiment, BS137C may include an SBO crystal, SBO glass, or CaF2 crystal. Because SBO has high transmittance to deep UV and a high damage threshold, SBO is advantageously used as a substrate material for BS137C, and can ensure a long lifespan despite the high power levels of unconsumed input light 138C circulating within the cavity. If BS137C includes an SBO crystal, its thickness and / or the orientation of its crystal axis may be configured to minimize frequency doubling of the unconsumed input light 138C passing through it. BS137C may include a dichroic beam splitter, a prism, or other components for wavelength separation. In one embodiment, a nonlinear crystal 135C is configured such that the output light 139C has orthogonal polarization with respect to the circulating input light 133C. In this embodiment, BS137C may include a polarizing beam splitter configured to transmit the unconsumed input light 138C and reflect the output light 139C. In this embodiment, it is not possible to utilize the maximum nonlinear coefficient d33, so it should be noted that this configuration trades off a decrease in conversion efficiency with the convenience of using a polarizing beam splitter. In one embodiment, the BS137C is oriented such that the surface of the unconsumed input light 138C is substantially p-polarized with respect to its surface, and the surface is approximately at a Brewster angle with respect to the unconsumed input light.

[0057] As shown in Figure 3, the input light (ωx) 129C is focused by one or more lenses 131C before entering the cavity and matches the eigenmode of a resonant cavity having a beam waist inside or near the nonlinear crystal 135C. In a preferred embodiment, the one or more lenses 131C include one or more cylindrical lenses, each containing SBO glass or crystal and configured to operate at approximately a Brewster angle with respect to the incident light 129C to minimize reflection losses without the use of anti-reflective coatings. SBO is a suitable material for such lenses due to its high damage threshold at UV and DUV wavelengths. Unconsumed input light 138C(ωx) passing through BS137C is reflected by mirrors 132C-4 and circulates within the cavity to increase its intensity. If the enhanced input light (ωx) power density is sufficiently high, the conversion efficiency from input light (ωx) to output light 139C(2ωx) can be very high, up to or even exceeding 50%. The output light 139C(2ωx), having a wavelength of approximately 177nm or 133nm, exits the cavity after being reflected from BS137C.

[0058] In an alternative embodiment, the input surface of the nonlinear crystal 135C may be coated with an appropriate anti-reflective coating instead of orienting the input surface 135C-IN and the output surface 135C-OUT to the Brewster angle.

[0059] Figure 3 shows the final frequency doubling stage 130C as including a cavity comprising two planar mirrors and two curved mirrors, but other combinations of mirrors and / or lenses may be used to refocus the light circulating within the cavity. In alternative embodiments, the final frequency doubling stage 130C may include a delta cavity, a standing wave cavity, or a cavity of other shape instead of a bowtie cavity. When a standing wave cavity is used, the eighth harmonic is generated in the same direction as the incident fourth harmonic light. Any of these cavities can be stabilized using standard PDH or HC locking techniques. The length of the cavity is adjusted to maintain resonance by adjusting the position of one of the mirrors (e.g., mirror 132C-2 in Figure 3) or the position of the prism via a control signal (not shown) connected to a piezoelectric transducer (PZT), a voice coil, or another actuator. It should be noted that when the final frequency doubling stage 130C is used in a pulsed laser, a cavity is not required, and the input light 129C can be directed towards the nonlinear crystal 135C and focused into or near the nonlinear crystal 135C by any suitable combination of lenses and / or mirrors.

[0060] Figure 4 shows details of a nonlinear crystal 135C containing four stacked SBO plates 135C-1 to 135C-4, which is configured to double the frequency of the sum product of the second harmonic of the first fundamental light and the second fundamental light in the case of a 177nm laser 100B, or to double the frequency of the fourth harmonic to generate the eighth harmonic in the case of a 133nm laser 100A. There may be an odd or even number of plates. Figure 4 shows a nonlinear crystal 135C having a periodic structure containing four stacked SBO crystal plates 135C-1 to 135C-4, but the total number of SBO plates may be around two or more than ten. The thickness of each of the SBO plates 135C-1 to 135C-4 may be 10 microns to 2 millimeters. Specifically, the thickness Λ of the SBO plate is given by Λ = mLc, where m is an odd integer (i.e., 1, 3, 5, 7…) and the pseudo-phase-matching critical length Lc = π / Δk. In the final frequency doubling stage 130B of the 177nm laser 100B, the pseudo-phase-matching critical length Lc is approximately 0.60 μm, while in the final frequency doubling stage 130A of the 133nm laser 100A, the pseudo-phase-matching critical length Lc is approximately 0.13 μm. An appropriate m may be several hundred or several thousand in order to achieve a slab thickness that is convenient for handling and processing. A typical QPM critical length for generating 133nm light by frequency doubling 266nm light was calculated from the refractive index of SBO at wavelengths of 133nm and 266nm using the Sellmeier model published by Trabs et al. (see above). Since Trabs et al. did not generate wavelengths shorter than 160 nm, the extrapolated refractive index at 133 nm may be inaccurate. Those skilled in the art will understand how to calculate the QPM critical length when a more accurate refractive index is given.

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

[0062] To create the periodic structure of the QPM, SBO plates 135C-1 to 135C-4 are arranged with one rotated relative to the other, resulting in their corresponding c-crystal axes being inverted relative to each other, as shown in the two insets of Figure 4. The surface normal N of the SBO plate with thickness Λ (where Λ is the distance between the poles of the crystal) and the direction of light propagation 133C inside the SBO plate are shown in the two insets. The physical arrangement of the crystal plates makes the QPM possible. This can be considered analogous to using PPLN (periodically polarized lithium niobate) for the QPM, but differs in that lithium niobate is a ferroelectric crystal and can be periodically polarized. In contrast, since SBO is non-ferroelectric, it is necessary to physically arrange the crystal plates to create the periodic structure of the QPM. Furthermore, since periodic polarization requires the application of an electric field parallel to the crystal axis of the ferroelectric crystal, the polarization direction inevitably coincides with the crystal axis. In contrast, the SBO crystal plates disclosed herein can be cut and polished in any direction with respect to the crystal axis, thereby allowing the crystal plates to be cut and directed at a Brewster angle with respect to light incident on the plate.

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

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

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

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

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

[0068] Figure 6 is a simplified diagram showing an exemplary final frequency total stage 130G used in the 152nm laser assembly of Figure 5A, the 152nm laser assembly of Figure 5B, and the 177nm laser assembly of Figure 5C, according to exemplary embodiments of the present invention. The input light 129G (with frequency ωx, for example, ωx=2ω1+ω2 when stage 130G is used with the 152nm laser 100D in Figure 5A, ωx=2ω1 when stage 130G is used with the 152nm laser 100E in Figure 5B, or ωx=2ω2 when stage 130G is used with the 177nm laser 100F in Figure 5C) enters a bow-tiling cavity comprising an input coupler 132G-1, a planar mirror 132G-2, curved mirrors 132G-3 and 132G-4, and a nonlinear crystal 135G (including the input surface 135G-IN and the output surface 135G-OUT) via the input coupler 132G-1, is recirculated to enhance power. The input light (second intermediate light beam) 129G-2 with frequency ωy (for example, ωy=4ω1 when stage 130G is used with the 152nm laser 100D in Figure 5A, or ωy=4ω1+ω2 when stage 130G is used with the 152nm laser 100E in Figure 5B, or ωy=4ω1 when stage 130G is used with the 177nm laser 100F in Figure 5C) enters the bow-tiling cavity near (but not necessarily passing through) mirror 132G-2 and passes through the nonlinear crystal 135G. The output light 136G, which is output from the nonlinear crystal 135G via the output surface 135C-OUT, includes unconsumed input light 138G-1 with frequency ωx, unconsumed input light 138G-2 with frequency ωy, and generated laser output light 139G with output frequency ωOUTG, which is equal to the sum of frequencies ωX and ωy of the intermediate (input) light beams 129G-1 and 129G-2 (i.e., frequency ωOUTG may be substantially equal to either the 7th harmonic output light 139D and 139E in Figure 5A or Figure 5B, or the 6th harmonic output light 139F in Figure 5C). The laser output light 139G is reflected from the input surface of the beam splitter (BS) 137G and directed out of the cavity.Unconsumed input light 138G-1 with frequency ωx passes through beam splitter 137G and optional beam splitter 325 (if present), increasing the intensity of light 133G that is reflected and circulated by mirrors 132G-4 and 132G-1. Unconsumed input light 138G-2 with frequency ωy is reflected from beam splitter 137G or from the optional (second) beam splitter 325 before leaving the cavity. The polarization direction of the circulating input light 133G is indicated by arrow 329G.

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

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

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

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

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

[0074] The above diagram is not intended to represent the actual physical layout of the components. The above diagram shows the main optical modules included in the process, but does not show all optical elements. Those skilled in the art will understand from the above diagram and related explanation how to construct 177 nm, 152 nm, and 133 nm lasers. It should be understood that more or fewer optical components can be used to direct the light where needed. Lenses and / or curved mirrors can be used to focus the beam waist to a focal point in a substantially circular or elliptical cross-section inside or near a suitable nonlinear crystal. Prisms, beam splitters, gratings, or diffractive optical elements can be used to manipulate or separate different wavelengths at the output of each frequency conversion stage, as needed. Prisms, coated mirrors, or other elements can be used to combine different wavelengths at the input to the frequency conversion stage, as needed. A beam splitter or coated mirror can be appropriately used to split one wavelength into two beams. Filters can be used to block or separate undesirable wavelengths at the output of any stage. Waveplates can be used to rotate the polarization, as needed. Other optical elements may be used as appropriate. Those skilled in the art will understand the various trade-offs and alternatives possible in the implementation of 177nm, 152nm, and 133nm lasers.

[0075] In the various alternative embodiments described above, the first fundamental laser may be configured to produce first fundamental light at a first fundamental frequency ω1, where the corresponding wavelength is equal to one of approximately 1070 nm, 1064 nm, 1053 nm, 1047 nm, and 1030 nm. If used, the second fundamental laser may be configured to produce second fundamental light at a second fundamental frequency ω2, where the corresponding wavelength is equal to one of approximately 1070 nm, 1064 nm, 1053 nm, 1047 nm, and 1030 nm. The various harmonic frequencies referred to herein are based on multiples of the corresponding fundamental frequencies. The exact wavelength of light produced by a given fundamental laser depends on several factors, including the exact composition of the laser medium, the operating temperature of the laser medium, and the design of the optical cavity. Two lasers using the same laser line of a given laser medium may operate at wavelengths differing by as much as tenths of a nm or several nanometers due to the aforementioned and other factors. Those skilled in the art will understand how to select appropriate first and second fundamental wavelengths from any one or two fundamental wavelengths to generate a desired output wavelength.

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

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

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

[0079] Another aspect of the present invention is an inspection or measurement system for wafers, reticles, or photomasks incorporating at least one of the 177 nm, 152 nm, and 133 nm lasers of the present invention as described above. Embodiments of such systems are shown in Figures 8, 9, and 10.

[0080] This laser may be used in an inspection system having dark-field and bright-field inspection modes, as shown in Figure 8. This figure and system are described in U.S. Patent No. 7,817,260 by Chuang et al., which is incorporated herein by reference as if it were fully described herein. Figure 8 shows a reflectance-refractory imaging system 800 incorporating perpendicular incidence laser illumination. The illumination block of system 800 includes a laser 801, a fitting optical component 802 for controlling the illumination beam size and profile on the surface being inspected, an aperture and window 803 in a mechanism housing 804, and a prism 805 for reorienting the laser along the optical axis perpendicularly incident to the surface of the sample 808. The prism 805 further directs specular reflections from the surface features of the sample 808 and reflections from the optical surface of the objective 806 along the optical path to the imaging plane 809. The lens of the objective 806 may be arranged in the common form of a reflectance-refractory objective, a focusing lens group, and a zoom tube lens section 807. In a preferred embodiment, laser 801 may be implemented by one of the above-mentioned lasers.

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

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

[0083] The beam shaping optical component 903 is oriented such that its main surface is substantially parallel to the sample surface 911, and as a result, the illumination line 905 is formed on the surface 911 at the focal plane of the beam shaping optical component 903. In addition, the light beam 902 and the focused beam 904 are directed at non-orthogonal incidence angles to the surface 911. In particular, the light beam 902 and the focused beam 904 can be directed at angles from about 1° to about 85° from the direction perpendicular to the surface 911. Thus, the illumination line 905 is substantially within the incidence plane of the focused beam 904.

[0084] The focusing system 910 includes a lens 912 for collecting scattered light from the illumination line 905 and a lens 913 for focusing the light exiting the lens 912 onto a device such as a charge-coupled device (CCD) 914 comprising an array of photosensitive detectors. In one embodiment, the CCD 914 may include a linear array of detectors. In such a case, the linear array of detectors in the CCD 914 may be oriented parallel to the illumination line 905. In another embodiment, the CCD 914 may include a two-dimensional array of detectors arranged as a rectangular array whose major axis is parallel to the illumination line 905. For example, the CCD 914 may include a rectangular array of about 1000 to 8000 detectors × about 50 to 250 detectors. In one embodiment, multiple focusing systems may be included, each of which includes similar components but with different orientations.

[0085] For example, Figure 9B shows an exemplary array of focusing systems 931, 932, and 933 for a surface inspection apparatus (wherein its illumination system, such as illumination system 901, is not shown for simplification). A first optical component in focusing system 931 collects light scattered from the surface of sample 911 in a first direction. A second optical component in focusing system 932 collects light scattered from the surface of sample 911 in a second direction. A third optical component in focusing system 933 collects light scattered from the surface of sample 911 in a third direction. Note that the first, second, and third optical paths are at different reflection angles with respect to the surface of sample 911. Using the stand 921 supporting sample 911, relative motion can be induced between the optical components and sample 911, thereby allowing the entire surface of sample 911 to be scanned.

[0086] This laser may also be used in inspection systems for unpatterned wafers, such as inspection system 1000 shown in Figure 10. Such inspection systems may incorporate oblique and / or perpendicular incident illumination as well as a large focusing solid angle for scattered light, as shown in these figures. Illumination source 1100 incorporates at least one of the lasers described herein to generate VUV light to illuminate wafer 1122 at a desired angle, so that reflected light is not focused by the system of imaging focusing optical components 1108. Optical components 1106 may be configured to generate a desired illumination pattern. Scattered light from wafer 1122 can be focused by the system of imaging focusing optical components 1108, which is configured to direct the light towards an afocal lens system 1110. In one embodiment, a focusing lens mask system 1112 can split the light into multiple channels and send them to a TDI sensor 1118. One embodiment may include an intensifier 1114 and / or a sensor relay 1116. The TDI sensor 1118 and / or intensifier 1114 may be configured to transmit signals to an image processing computer 1120 to generate a wafer image and / or a list of defects or particles on the surface of the wafer 1122. An additional description of the elements of Figure 10 can be found in U.S. Patent No. 9,891,177B2 by Vazhaeparambil et al. Further details of the non-patterned wafer inspection system are described in U.S. Patents Nos. 6,201,601 and 6,271,916. All of these patents are incorporated by reference as if they were fully described herein.

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

Claims

1. A laser assembly for inspection that generates a laser output light beam having an output frequency with corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, One or more fundamental wave lasers, each configured to generate a fundamental light beam having a corresponding fundamental frequency, A plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using one or more fundamental light beams, wherein each of the one or more intermediate light beams has a related intermediate frequency, The final frequency conversion stage is configured to pass one or more intermediate light beams through a nonlinear crystal, Here, the nonlinear crystal consists of multiple strontium tetraborate (SrB) crystals arranged in a stacked configuration. ４ O ７ (SBO) crystal plates are included, and each SBO crystal plate forms an interface surface with at least one adjacent crystal plate. Here, the plurality of SBO crystal plates are coordinately configured to form a periodic structure that realizes pseudo-phase matching (QPM) of one or more intermediate light beams, so that the light emanating from the nonlinear crystal includes the laser output light beam having the output frequency. A laser assembly wherein the plurality of SBO crystal plates are configured such that the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of the adjacent SBO crystal plate, and the spacing between poles of the periodic structure in the direction of light propagation is equal to an odd multiple of the critical length that enables the pseudo-phase matching (QPM) between one or more input optical frequencies and output frequencies.

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

3. The aforementioned final frequency conversion stage further, The configuration is such that the beam waist of the one or more intermediate light beams is generated in the nonlinear crystal, and the mirrors are configured to receive and circulate the one or more intermediate light beams. A beam splitter is positioned to receive the light emitting from the nonlinear crystal, and is configured to reflect a first portion of the emitted light to form the laser output light beam, and to pass a second portion of the emitted light through the plurality of mirrors so as to be circulated by the plurality of mirrors, The laser assembly according to claim 1, including the following:

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

5. The one or more fundamental wave lasers are configured to generate fundamental light having a fundamental frequency between 1 μm and 1.1 μm, with the corresponding fundamental wavelength being. The aforementioned plurality of intermediate frequency conversion stages are A first frequency doubling stage is coupled to receive the fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the fundamental frequency, The system includes a second frequency doubling stage, which is coupled to the first frequency doubling stage to receive the second harmonic light and is configured to generate the intermediate light beam as a fourth harmonic light having a fourth harmonic frequency equal to four times the fundamental frequency, The laser assembly according to claim 1, wherein the final frequency conversion stage is configured to frequency-duplicate the fourth harmonic light so that the output frequency of the laser output light becomes an eighth harmonic frequency equal to eight times the fundamental frequency.

6. The laser assembly according to claim 5, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the nonlinear crystal, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the eighth harmonic frequency that forms the laser output light beam, and the unreflective portion of the emitted light passing through the beam splitter includes the unused portion of the fourth harmonic frequency.

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

8. The one or more fundamental wave lasers are: A first fundamental wave laser configured to generate a first fundamental light having a first fundamental frequency between 1 μm and 1.1 μm, and A second fundamental wave laser configured to generate a second fundamental light having a second fundamental frequency between 1 μm and 1.1 μm, the corresponding fundamental wavelength is: The aforementioned plurality of intermediate frequency conversion stages are A frequency doubling stage is coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency, The frequency summing stage is coupled to the frequency doubling stage to receive the second harmonic light and the second fundamental light, and is configured to generate the intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency, The laser assembly according to claim 1, wherein the final frequency conversion stage is configured to frequency-duplicate the third harmonic light so that the output frequency of the laser output light becomes a sixth harmonic frequency substantially equal to six times the first fundamental frequency.

9. The laser assembly according to claim 8, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the nonlinear crystal, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the sixth harmonic frequency that forms the laser output light beam, and the unreflective portion of the emitted light passing through the beam splitter includes the unused portion of the third harmonic frequency.

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

11. The one or more fundamental wave lasers are: A first fundamental wave laser configured to generate a first fundamental light having a first fundamental frequency between 1 μm and 1.1 μm, and A second fundamental wave laser configured to generate a second fundamental light having a second fundamental frequency between 1 μm and 1.1 μm, the corresponding fundamental wavelength is: The aforementioned plurality of intermediate frequency conversion stages are A first frequency doubling stage is coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency, A frequency summing stage is coupled to the first frequency doubling stage to receive the first portion of the second harmonic light and the second fundamental light, and is configured to generate the first intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency, The system includes a second frequency doubling stage coupled to the first frequency doubling stage to receive a second portion of the second harmonic light, and configured to generate a second intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental frequency, The laser assembly according to claim 1, wherein the final frequency conversion stage is configured to sum the third harmonic light received from the frequency summing stage and the fourth harmonic light received from the second frequency doubling stage, so that the output frequency of the laser output light becomes a seventh harmonic frequency substantially equal to seven times the first fundamental frequency.

12. The laser assembly according to claim 11, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the nonlinear crystal, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the seventh harmonic frequency that forms the laser output light beam, and the unreflected portion of the emitted light passing through the beam splitter includes the unused portion of the third harmonic frequency.

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

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

15. The laser assembly according to claim 14, wherein the final frequency conversion stage further includes a beam splitter, which is positioned to receive light emanating from the nonlinear crystal, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the seventh harmonic frequency that forms the laser output light beam, and the unreflective portion of the emitted light passing through the beam splitter includes the unused portion of the second harmonic frequency.

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

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

18. The laser assembly according to claim 17, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the nonlinear crystal, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the sixth harmonic frequency that forms the laser output light beam, and the unreflective portion of the emitted light passing through the beam splitter includes the unused portion of the second harmonic frequency.

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

20. A method for generating an inspection laser output light beam having an output frequency with corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, One or more fundamental light beams are generated such that each of the fundamental light beams has a corresponding fundamental frequency with a corresponding fundamental wavelength of approximately 1 μm to 1.1 μm. This method utilizes a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using one or more fundamental light beams, wherein each of the one or more intermediate light beams has a related intermediate frequency. This includes using a final frequency conversion stage to pass one or more intermediate light beams through a nonlinear crystal, Here, the nonlinear crystal consists of multiple strontium tetraborate (SrB) crystals arranged in a stacked configuration. ４ O ７ (SBO) crystal plates are included, and each SBO crystal plate forms an interface surface with at least one adjacent crystal plate. Here, the plurality of SBO crystal plates are coordinately configured to form a periodic structure that realizes pseudo-phase matching (QPM) of one or more intermediate light beams, so that the light emanating from the nonlinear crystal includes the laser output light beam having the output frequency. The plurality of SBO crystal plates are configured such that the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of the adjacent SBO crystal plate, and the spacing between poles of the periodic structure in the direction of light propagation is equal to an odd multiple of the critical length that enables the pseudo-phase matching (QPM) between one or more input optical frequencies and output frequencies.

21. An inspection system configured to inspect a sample using a laser output light beam having an output frequency with corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, wherein the laser output light beam is One or more fundamental wave lasers, each configured to generate a fundamental light beam having a corresponding fundamental frequency, A plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using one or more fundamental light beams, wherein each of the one or more intermediate light beams has a corresponding intermediate frequency, A laser assembly including a final frequency conversion stage configured to pass one or more intermediate light beams through a nonlinear crystal generates the light generated by the laser assembly, Here, the nonlinear crystal consists of multiple strontium tetraborate (SrB) crystals arranged in a stacked configuration. ４ O ７ (SBO) crystal plates are included, and each SBO crystal plate forms an interface surface with at least one adjacent crystal plate. Here, the plurality of SBO crystal plates are coordinately configured to form a periodic structure that realizes pseudo-phase matching (QPM) of one or more intermediate light beams, so that the light emanating from the nonlinear crystal includes the laser output light beam having the output frequency. An inspection system comprising a plurality of SBO crystal plates, wherein the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of the adjacent SBO crystal plate, and the spacing between poles of the periodic structure in the direction of light propagation is equal to an odd multiple of the critical length that enables the pseudo-phase matching (QPM) between one or more input optical frequencies and output frequencies.

22. A nonlinear crystal configured to convert one or more input optical frequencies into output frequencies having corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, A plurality of strontium tetraborate SrB ４ O ７ (SBO) crystal plates, comprising a plurality of SBO crystal plates arranged in a stacked configuration, each of said SBO crystal plates forming an interface with at least one adjacent said crystal plate, Here, the plurality of SBO crystal plates are configured such that the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of at least one adjacent SBO crystal plate. Herein, the thickness of at least one of the plurality of SBO crystal plates generates an inter-pole spacing substantially equal to an odd multiple of the critical length, enabling pseudo-phase matching between the one or more input optical frequencies and the output frequencies, thereby the light emanating from the nonlinear crystal includes inspection laser output light having the output frequencies.

23. A laser assembly for inspection that generates a laser output light beam having an output frequency with corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, One or more fundamental wave lasers, each configured to generate a fundamental light beam having a corresponding fundamental frequency, A plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using one or more fundamental light beams, wherein each of the one or more intermediate light beams has a related intermediate frequency, The final frequency conversion stage is configured to pass one or more intermediate light beams through a plurality of strontium tetraborate SrB4O7 (SBO) crystal plates, The plurality of SBO crystal plates are coordinately configured to form a periodic structure that realizes pseudo-phase matching (QPM) of one or more intermediate light beams, so that the light emanating from the plurality of SBO crystal plates includes the laser output light beam having the output frequency. A laser assembly wherein the plurality of SBO crystal plates are configured such that the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of the adjacent SBO crystal plate, and the spacing between poles of the periodic structure in the direction of light propagation is equal to an odd multiple of the critical length that enables the pseudo-phase matching (QPM) between one or more input optical frequencies and output frequencies.

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

25. The aforementioned final frequency conversion stage further, The configuration is such that the beam waist of the one or more intermediate light beams is generated at the incident surface of the plurality of SBO crystal plates, and the mirrors are configured to receive and circulate the one or more intermediate light beams. A beam splitter is configured to receive the light emanating from the plurality of SBO crystal plates, to reflect a first portion of the emitted light to form the laser output light beam, and to allow a second portion of the emitted light to pass through and be circulated by the plurality of mirrors, The laser assembly according to claim 23, including the following:

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

27. The one or more fundamental wave lasers are configured to generate fundamental light having a fundamental frequency between 1 μm and 1.1 μm, with the corresponding fundamental wavelength being. The aforementioned plurality of intermediate frequency conversion stages are A first frequency doubling stage is coupled to receive the fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the fundamental frequency, The system includes a second frequency doubling stage, which is coupled to the first frequency doubling stage to receive the second harmonic light and is configured to generate the intermediate light beam as a fourth harmonic light having a fourth harmonic frequency equal to four times the fundamental frequency, The laser assembly according to claim 23, wherein the final frequency conversion stage is configured to frequency-duplicate the fourth harmonic light so that the output frequency of the laser output light becomes an eighth harmonic frequency equal to eight times the fundamental frequency.

28. The laser assembly according to claim 27, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the plurality of SBO crystal plates, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the eighth harmonic frequency that forms the laser output light beam, and the unreflected portion of the emitted light passing through the beam splitter includes the unused portion of the fourth harmonic frequency.

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

30. The one or more fundamental wave lasers are: A first fundamental wave laser configured to generate a first fundamental light having a first fundamental frequency between 1 μm and 1.1 μm, and A second fundamental wave laser configured to generate a second fundamental light having a second fundamental frequency between 1 μm and 1.1 μm, the corresponding fundamental wavelength is: The aforementioned plurality of intermediate frequency conversion stages are A frequency doubling stage is coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency, The frequency summing stage is coupled to the frequency doubling stage to receive the second harmonic light and the second fundamental light, and is configured to generate the intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency, The laser assembly according to claim 23, wherein the final frequency conversion stage is configured to frequency-duplicate the third harmonic light so that the output frequency of the laser output light becomes a sixth harmonic frequency substantially equal to six times the first fundamental frequency.

31. The laser assembly according to claim 30, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the plurality of SBO crystal plates, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the sixth harmonic frequency that forms the laser output light beam, and the unreflected portion of the emitted light passing through the beam splitter includes the unused portion of the third harmonic frequency.

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

33. The one or more fundamental wave lasers are: A first fundamental wave laser configured to generate a first fundamental light having a first fundamental frequency between 1 μm and 1.1 μm, and A second fundamental wave laser configured to generate a second fundamental light having a second fundamental frequency between 1 μm and 1.1 μm, the corresponding fundamental wavelength is: The aforementioned plurality of intermediate frequency conversion stages are A first frequency doubling stage is coupled to receive the first fundamental light and configured to generate second harmonic light having a second harmonic frequency equal to twice the first fundamental frequency, A frequency summing stage is coupled to the first frequency doubling stage to receive the first portion of the second harmonic light and the second fundamental light, and is configured to generate the first intermediate light beam as third harmonic light having a third harmonic frequency substantially equal to three times the first fundamental frequency, The system includes a second frequency doubling stage coupled to the first frequency doubling stage to receive a second portion of the second harmonic light, and configured to generate a second intermediate light beam as fourth harmonic light having a fourth harmonic frequency equal to four times the first fundamental frequency, The laser assembly according to claim 23, wherein the final frequency conversion stage is configured to sum the third harmonic light received from the frequency summing stage and the fourth harmonic light received from the second frequency doubling stage, so that the output frequency of the laser output light becomes a seventh harmonic frequency substantially equal to seven times the first fundamental frequency.

34. The laser assembly according to claim 33, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the plurality of SBO crystal plates, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the seventh harmonic frequency that forms the laser output light beam, and the unreflected portion of the emitted light passing through the beam splitter includes the unused portion of the third harmonic frequency.

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

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

37. The laser assembly according to claim 36, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the plurality of SBO crystal plates, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the seventh harmonic frequency that forms the laser output light beam, and the unreflective portion of the emitted light passing through the beam splitter includes the unused portion of the second harmonic frequency.

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

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

40. The laser assembly according to claim 39, wherein the final frequency conversion stage further includes a beam splitter, which is arranged to receive light emanating from the plurality of SBO crystal plates, and is configured such that the reflective portion of the emitted light reflected from the surface of the beam splitter includes the sixth harmonic frequency that forms the laser output light beam, and the unreflected portion of the emitted light passing through the beam splitter includes the unused portion of the second harmonic frequency.

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

42. A method for generating an inspection laser output light beam having an output frequency with corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, One or more fundamental light beams are generated such that each of the fundamental light beams has a corresponding fundamental frequency with a corresponding fundamental wavelength of approximately 1 μm to 1.1 μm. This method utilizes a plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using one or more fundamental light beams, wherein each of the one or more intermediate light beams has a related intermediate frequency. Using the final frequency conversion stage, one or more intermediate light beams are converted to multiple strontium tetraborate (SrB) ４ O ７ (SBO) Includes passing through an optical element including a crystal plate, The plurality of SBO crystal plates are coordinately configured to form a periodic structure that realizes pseudo-phase matching (QPM) of one or more intermediate light beams, so that the light emitted from the optical element includes the laser output light beam having the output frequency. The plurality of SBO crystal plates are configured such that the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of the adjacent SBO crystal plate, and the spacing between poles of the periodic structure in the direction of light propagation is equal to an odd multiple of the critical length that enables the pseudo-phase matching (QPM) between one or more input optical frequencies and output frequencies.

43. An inspection system configured to inspect a sample using a laser output light beam having an output frequency with corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, wherein the laser output light beam is One or more fundamental wave lasers, each configured to generate a fundamental light beam having a corresponding fundamental frequency, A plurality of intermediate frequency conversion stages collectively configured to generate one or more intermediate light beams using one or more fundamental light beams, wherein each of the one or more intermediate light beams has a corresponding intermediate frequency, A laser assembly including a final frequency conversion stage configured to pass one or more intermediate light beams through an optical element generates the light generated by the laser assembly, The optical element comprises multiple strontium tetraborate SrB ４ O ７ The optical element includes (SBO) crystal plates, wherein the plurality of SBO crystal plates are coordinately configured to form a periodic structure that realizes pseudo-phase matching (QPM) of the one or more intermediate light beams, so that the light emitted from the optical element includes the laser output light beam having the output frequency. An inspection system comprising a plurality of SBO crystal plates, wherein the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of the adjacent SBO crystal plate, and the spacing between poles of the periodic structure in the direction of light propagation is equal to an odd multiple of the critical length that enables the pseudo-phase matching (QPM) between one or more input optical frequencies and output frequencies.

44. An optical element that converts one or more input optical frequencies into output frequencies having corresponding wavelengths in the range of approximately 125 nm to approximately 183 nm, Multiple strontium tetraborate SrB ４ O ７ The (SBO) crystal plate is included, and the first crystal axis of each SBO crystal plate is inverted with respect to the second crystal axis of at least one adjacent SBO crystal plate. An optical element in which at least one of the plurality of SBO crystal plates has a thickness substantially equal to an odd multiple of the critical length, enabling pseudo-phase matching of one or more input optical frequencies and output frequencies, thereby including an inspection laser output light beam having the output frequency as light emanating from the optical element.

Images