183 Nanolaser and inspection system

Abstract

Embodiments of the present disclosure relate to a 183 nm laser and inspection system. A laser assembly for generating a laser output light at an output wavelength of approximately 183 nm includes a fundamental laser, an optical parametric system (OPS), a fifth harmonic generator, and a mixing module. The fundamental laser generates a fundamental light at a fundamental frequency. The OPS generates a down-converted signal at a down-converted frequency. The fifth harmonic generator generates a fifth harmonic of the fundamental light. The mixing module mixes the down-converted signal and the fifth harmonic to generate the laser output light at a frequency equal to a sum of the fifth harmonic frequency and the down-converted frequency. The OPS generates the down-converted signal by generating a down-converted seed signal at the down-converted frequency and then mixing the down-converted seed signal and a portion of the fundamental light. At least one of the frequency mixing, the frequency conversion, or the harmonic generation utilizes a CLBO crystal that is annealed, deuterium treated, or hydrogen treated.

Description

[0001] This application is a divisional application of the invention patent application filed on October 2, 2015, with application number "201580053715.6" and invention title "183 nm Laser and Inspection System".

[0002] Priority application

[0003] This application claims priority to U.S. Provisional Patent Application No. 62 / 059,368, filed October 3, 2014, entitled “183nm Laser and Inspection System” by Chuang et al.

[0004] Related applications

[0005] This application relates to U.S. Patent Application No. 13 / 797,939, filed March 12, 2013, and incorporated herein by reference, entitled "Solid-State Laser and Inspection System Using 193nm Laser". Technical Field

[0006] This invention relates to a laser, and more particularly, to a solid-state or fiber laser that generates radiation close to 183 nm and is suitable for use in the inspection of photomasks, reticles, and / or wafers. The laser is preferably a pulsed laser, such as a Q-switched laser or a mode-locked laser. The invention further relates to an inspection system using a laser operating at wavelengths close to 183 nm. Background Technology

[0007] Excimer lasers for generating light at 193 nm are well known in this technology. Unfortunately, such lasers are not well-suited for testing applications due to their low laser pulse repetition rate and the use of toxic and corrosive gases in their laser media (which leads to high holding costs).

[0008] Solid-state and fiber lasers for generating light close to 193 nm are also known. Exemplary lasers use two different fundamental wavelengths (e.g., US 2014 / 0111799 by Lei et al.) or the eighth harmonic of the fundamental wavelength (e.g., US 7,623,557 by Tokuhisa et al.), either of which requires expensive or mass-producible lasers or materials. Another approach (US 5,742,626 by Mead et al.) has not yet produced commercially available products with stable output and high power (typically around 1 W or more in lasers capable of continuous operation for three months or more between service events) required for applications such as semiconductor testing. Furthermore, most of these lasers have extremely low power output and are limited to laser pulse repetition rates of a few MHz or less.

[0009] As semiconductor device sizes shrink, the size of the largest particle or pattern defect that can cause device malfunction also decreases. Therefore, there is a need to detect smaller particles and defects on both patterned and unpatterned semiconductor wafers. The intensity of light scattered by particles (where the particles are smaller than the wavelength of the light) typically scales proportionally to a power of the particle size (for example, the total scattered intensity of light from isolated spherical particles is proportional to the sixth power of the diameter of the sphere and inversely scaled to the fourth power of the wavelength). Due to the increasing intensity of the scattered light, shorter wavelengths generally provide superior sensitivity for detecting small particles and defects compared to longer wavelengths.

[0010] Because the intensity of light scattered from small particles and defects is typically extremely low, high illumination intensity is required to generate signals that can be detected in a very short time. An average light source power level of 1W or greater may be necessary. At these high average power levels, a high pulse repetition rate is desirable because a higher repetition rate results in lower energy per pulse, and therefore a lower risk of damaging the optics or items being inspected. A high repetition rate is also desirable in high-speed inspection because it (e.g., approximately 50 MHz or higher) allows for the collection of many pulses for each image, resulting in lower sensitivity to pulse-to-pulse intensity variations.

[0011] Therefore, there is a need for lasers, preferably solid-state or fiber lasers that produce radiation shorter than 193 nm and are suitable for use in the inspection of photomasks, photomasks, and / or wafers. Clearly, such inspections at high speeds typically require a minimum laser pulse repetition rate of several MHz (e.g., in some cases greater than 50 MHz). Summary of the Invention

[0012] This invention relates to a laser assembly and associated method for generating 183nm laser light using a fundamental frequency laser by generating and mixing a fifth harmonic of the fundamental frequency laser with a down-converted signal, wherein the down-converted signal is generated by generating a low-power down-converted seed signal having a desired down-converted frequency, and then mixing the down-converted seed signal with a portion of the fundamental frequency laser to generate the down-converted signal at a peak power level ten times or more than ten times greater than the down-converted seed signal. In addition to the efficiency associated with generating the 183nm output laser light using fifth harmonic light, the two-step method according to the invention for generating the down-converted signal provides several advantages over conventional methods. First, the initial step of generating the low-power down-converted seed signal facilitates the avoidance of deformation and damage to these components by minimizing the exposure of the optical components used to generate the higher-power down-converted signal to a high-power idle frequency with a wavelength longer than about 4μm (which is absorbed by most nonlinear crystals in a manner that leads to deformation and / or damage). Secondly, generating a down-converted seed signal at relatively low power facilitates greater control over the down-converted frequency, which in turn facilitates fine-tuning of the 183nm laser output light. Another advantage of the invention is that it facilitates the manufacture of 183nm laser assemblies using various components, thereby providing manufacturing flexibility by allowing manufacturers to select and utilize readily available and / or relatively inexpensive components during manufacturing. For example, various described embodiments generate 183nm laser output light by mixing a selected fundamental frequency (e.g., having a corresponding fundamental wavelength of approximately 1064nm or approximately 1030nm) with a corresponding down-converted signal frequency (e.g., having a corresponding down-converted wavelength in the range of approximately 1250nm to approximately 1420nm or in the range of approximately 1400nm to approximately 1830nm). Fundamental lasers capable of generating at least one of these fundamental frequencies, with various combinations of power and repetition rate, are generally readily available at a reasonable price. Because the Optical Parameter System (OPS) generates the down-converted signal in a manner that facilitates control over the frequency of the down-converted signal, this invention allows manufacturers to select the lowest-priced or most readily available fundamental frequency laser for a given manufacturing operation for which they are fully confident that the 183nm laser output light will be produced.

[0013] According to an embodiment of the present invention, a laser assembly includes a fundamental frequency laser, an optical parameter system (OPS), a fifth harmonic generator, and a mixer module. The fundamental frequency laser is configured to generate fundamental frequency light having a fundamental wavelength (e.g., equal to one of approximately 1064 nm, approximately 1053 nm, approximately 1047 nm, or approximately 1030 nm) and a corresponding fundamental frequency. The OPS is optically coupled to the fundamental frequency laser such that the OPS receives a first portion of the fundamental frequency light and is configured to generate light having a desired down-conversion frequency ω. sThe down-converted signal. In one embodiment, the required down-converted frequency (ω) s The frequency is lower than the fundamental frequency (ω) and higher than 50% of the fundamental frequency (i.e., 0.5ω < ω). s <ω). The fifth harmonic generator receives a second portion of the fundamental frequency light, optionally also receiving the fourth harmonic, and is configured to generate fifth harmonic light (i.e., having a fifth harmonic frequency (5ω) equal to five times the fundamental frequency). The mixer module is optically coupled to receive the down-converted signal from the OPS and the fifth harmonic light from the fifth harmonic generator, and is configured to generate 183nm laser output light by operably mixing the down-converted signal and the fifth harmonic light. According to the present invention, the OPS comprises: a down-converted seed signal generator (e.g., a seed laser or an optical parameter oscillator) configured to generate a down-converted seed signal at a desired down-converted frequency and at a relatively low (first) peak power level; and an optical parameter amplifier (OPA) configured such that the down-converted seed signal is mixed with a portion of the fundamental frequency light by passing through a nonlinear crystal once, thereby generating the down-converted signal at the down-converted frequency and at a (second) peak power level ten times (or more than ten times) higher than the down-converted seed signal. The OPS is further configured to generate the down-converted signal at an appropriate down-converted frequency and peak power level, such that the sum of the down-converted frequency and the fifth harmonic frequency produces laser output light in the range of approximately 180 nm to approximately 185 nm.

[0014] In an alternative embodiment, the fundamental frequency laser is configured to generate fundamental frequency light at a fundamental frequency having a corresponding wavelength equal to one of approximately 1064 nm, approximately 1053 nm, approximately 1047 nm, and approximately 1030 nm, and the OPS is configured to generate a down-converted signal at a down-converted signal frequency and a corresponding wavelength, the down-converted signal being mixed with the fifth harmonic of the fundamental frequency (e.g., approximately 1250 nm to 1420 nm for a fundamental wavelength of approximately 1064 nm) to produce laser output light at approximately 183 nm. By another example, when the fundamental wavelength is approximately 1030 nm, a down-converted signal with a wavelength of approximately 1400 nm to 1830 nm is generated, and for a fundamental frequency laser with a wavelength of approximately 1047 nm or approximately 1053 nm, a down-converted signal with a wavelength between approximately 1290 nm and 1580 nm is generated. In an alternative embodiment, the laser assembly used to generate the approximately 183 nm output wavelength described herein utilizes a fundamental frequency laser of a Q-switched laser, a mode-locked laser, or a quasi-continuous-wave laser. Due to the use of near-critical phase matching in the final mixing module, the final conversion stage is efficient and relatively insensitive to small misalignments, thereby allowing stable output from power levels in the range of approximately 1 W to 20 W or greater.

[0015] In one embodiment, at least one of the fifth harmonic generator and the mixer module comprises an annealed, hydrogen-treated, or deuterated lithium cesium borate (CLBO) crystal configured for near-noncritical phase matching to generate a wavelength close to 183 nm by mixing a wavelength between about 206 nm and 213 nm with an infrared wavelength. Due to the near-noncritical phase matching, the mixing is highly efficient (e.g., the nonlinear coefficient can be approximately or slightly greater than 1 pm V). -1 Furthermore, the walk-off angle is small (e.g., less than about 30 mrad). In a preferred embodiment, the annealed CLBO crystal is maintained at a constant temperature close to 50°C.

[0016] According to an exemplary embodiment, a down-converted seed signal generator is configured to generate a down-converted seed signal at a lower (first) average power level in the range of 1mW to 500mW, and the OPA is configured to generate a higher power down-converted signal at a (second) power level in the range of 1W to 20W (or higher). In one exemplary embodiment, the down-converted seed signal generator of the OPA is implemented using a diode laser that directly generates the down-converted seed signal, and in other exemplary embodiments, the down-converted seed signal generator is implemented using an optical parametric oscillator (OPO) configured to generate the down-converted seed signal by means of converting a portion of the fundamental frequency light. In two exemplary embodiments, the OPA of the optical parameter system includes: a beam combiner configured to combine a first fundamental frequency light portion with a down-converted seed signal; a nonlinear crystal configured to amplify the down-converted seed signal by stimulated down-conversion of the first portion of the fundamental frequency light; and a beam splitter (wavelength separator) configured to separate the down-converted signal from the desired frequency. In the present preferred embodiment, the nonlinear crystal used in the OPS (e.g., in the OPA and optionally in the OPO) is implemented using a periodically polarized nonlinear optical crystal (e.g., a periodically polarized nonlinear optical crystal formed of lithium niobate (LN), magnesium oxide-doped lithium niobate (Mg:LN), stoichiometric lithium tantalate (SLT), magnesium oxide-doped stoichiometric lithium tantalate (Mg:SLT), or potassium titanate phosphate (KTP).

[0017] According to an alternative embodiment of the invention, 183nm laser output light is generated by mixing the fifth harmonic light with the down-converted signal in a manner similar to that described above. However, in this case, the down-converted signal is generated by down-converting the second harmonic of the fundamental frequency laser (i.e., instead of down-converting the light at the fundamental frequency). When a fundamental frequency laser with a wavelength of 1064nm is used, the second harmonic light includes light in the visible green spectrum (i.e., the second harmonic light has a wavelength of 532nm). This avoids the heating problems associated with generating a 1.3μm down-converted signal from the 1064nm fundamental frequency light (i.e., deformation / damage to the nonlinear crystal in the OPS due to absorption of idle signals with wavelengths greater than 4μm) by using a "green pumped" OPO to generate the down-converted signal, thus eliminating the need for generating the lower power seed signal used in the embodiments described above. However, the generation of a 1.3μm downconverted signal by downconverting 532nm light causes other problems that limit the type of nonlinear crystal that can be used in “green pumped” OPOs (i.e., LBOs are currently the preferred nonlinear crystal), and the downconversion process is relatively inefficient.

[0018] This document also discloses systems and methods for inspecting articles (e.g., semiconductor wafers, photomasks, or photoresists). These systems and methods involve using a laser in the final frequency summing stage to generate an output wavelength close to 183 nm using near-critical phase matching.

[0019] Besides its shorter wavelength, the 183nm laser of this invention has several advantages over a 193nm laser. Compared to lasers that generate 193nm as a sixth or eighth harmonic, the 183nm laser of this invention has the advantage of using an easily obtainable fundamental frequency wavelength in the power range of tens to hundreds of W. An advantage over lasers that generate 193nm by mixing the fifth harmonic with the signal frequency is that the mixer module of the 183nm laser is more efficient because the CLBO is near-critically phase-matched for generating 183nm from a fifth harmonic wavelength in the range of approximately 206nm to approximately 213nm. This allows the signal frequency and the fifth harmonic to be more efficiently converted to the final output and also makes the mixer module more stable. Another advantage is that for signal frequencies with corresponding wavelengths between approximately 1.25 μm and approximately 1.83 μm, significantly more energy enters the signal compared to the idle frequency, thereby causing more efficient conversion of the fundamental frequency power (almost the same amount of power must enter both the signal and idle frequencies compared to signal wavelengths closer to 2.1 μm). Attached Figure Description

[0020] Figure 1A and 1B This is a simplified block diagram illustrating an exemplary 183nm laser assembly according to an alternative exemplary embodiment of the invention.

[0021] Figure 2 This demonstrates an embodiment of the invention for use with Figure 1A A simplified block diagram of an exemplary fifth harmonic generator in a 183nm laser assembly.

[0022] Figure 3 Demonstrating alternative embodiments according to the present invention Figure 1A The 183nm laser assembly produces and in Figure 1A A table showing the exemplary wavelength of 183nm laser output light produced by mixing within a 183nm laser assembly.

[0023] Figure 4 This demonstrates an embodiment of the invention for use with Figure 1A A simplified block diagram of an exemplary mixer module in a 183nm laser assembly.

[0024] Figure 5 This illustrates an optional use of embodiments of the invention. Figure 1AA simplified block diagram of an amplifier module in a 183nm laser assembly to increase the power of the fundamental frequency laser.

[0025] Figure 6A and 6B This illustrates a configuration according to a specific alternative embodiment of the invention to generate a product for Figure 1A A simplified block diagram of an exemplary optical parameter system for a down-converted signal in a 183nm laser assembly.

[0026] Figure 7 This demonstrates a photomask, photomask, or wafer inspection system that simultaneously detects two image or signal channels on a single sensor.

[0027] Figure 8 The description includes a demonstrative testing system containing multiple objectives and one of the modified 193nm lasers described above.

[0028] Figure 9 This describes the addition of normal incident laser dark field illumination to a catadioptric imaging system.

[0029] Figure 10A This describes a surface inspection device that includes an illumination system and a light-gathering system for inspecting surfaces.

[0030] Figure 10B This describes an exemplary light-collecting system array used in surface inspection equipment.

[0031] Figure 11 This describes a surface inspection system that can be used to inspect for anomalies on a surface.

[0032] Figure 12 This describes an inspection system configured to use both normal and tilted illumination beams to perform anomaly detection.

[0033] Figure 13 This describes an exemplary pulse multiplier for use with the 183nm laser described above in a testing or measurement system.

[0034] Figure 14 This is a simplified block diagram illustrating a 183nm laser assembly according to another alternative embodiment of the invention. Detailed Implementation

[0035] This invention relates to improvements to inspection systems used in the semiconductor manufacturing industry, and more particularly, to laser assemblies for such inspection systems, said laser assemblies being able to generate lasers having an average output wavelength in the range of about 180 nm to about 185 nm (e.g., about 183 nm) and an average source power level of 1 W or greater, in a manner that avoids problems associated with prior art methods. The following description is presented to enable those skilled in the art to make and use the invention as provided in the context of the specific applications and their claims. It should be noted in the following description that, where wavelengths are mentioned without limitation, it is assumed that the wavelengths are in vacuum.

[0036] Figure 1A and 1B This is a simplified block diagram illustrating 183nm laser assemblies 100A and 100B according to alternative exemplary embodiments of the present invention. Although laser assemblies 100A and 100B differ in some respects, each laser assembly 100A and 100B utilizes substantially the same set of core optical components; that is, each of laser assemblies 100A and 100B includes a fundamental frequency laser 102, an optical parameter system (OPS) 116, and a fifth harmonic generator (for reasons explained below) Figure 1A Use "103" and in Figure 1B (Identified using "157"), and a mixing module 104, the components being arranged and configured to generate laser output light 140 having a frequency in the range of approximately 180 nm to approximately 185 nm, and most preferably approximately 183 nm. It should be noted that these core components are... Figure 1A and 1BEach of these components is identified by the same or similar reference numerals to indicate that these core components are configured and function in the same or similar manner in each of the two exemplary embodiments. Specifically, in each embodiment, the fundamental frequency laser 102 is configured to generate fundamental frequency light 128 having a fundamental wavelength (e.g., approximately 1064 nm) and a corresponding fundamental frequency ω. Similarly, in each embodiment, an OPS 116 is optically coupled to the fundamental frequency laser 102 such that the OPS 116 receives a portion 127 of the fundamental frequency light 128 as input light, and the OPS 116 is configured to generate a down-converted signal 129. In a similar manner, a fifth harmonic generator 103 is optically coupled to the fundamental frequency laser 102 such that the fifth harmonic generator 103 receives at least a portion 130 of the fundamental frequency light 128 as input light, and the fifth harmonic generator 103 is configured to generate fifth harmonic light 134 at a fifth harmonic frequency 5ω, which is five times the fundamental frequency ω. The mixer module 104 is optically coupled to receive both the down-converted signal 129 from the OPS 116 and the fifth harmonic light 134 from the fifth harmonic generator 103 as input light, and is configured to generate laser output light 140 by mixing the down-converted signal 129 and the fifth harmonic light 134.

[0037] According to one aspect of the invention, the OPS 116 utilizes a down-converted seed signal generator 117 (e.g., a diode laser or OPO) and an optical parameter amplifier (OPA) 119 to generate a signal at the down-converted frequency ω. s The down-converted signal 129, when mixed with the fifth harmonic light 134 in the mixer module 104, produces laser output light 140 at the desired wavelength (i.e., in the range of approximately 180 nm to approximately 185 nm). Specifically, the down-converted seed signal generator 117 is configured to generate laser output light 140 having the same down-converted frequency ω as the down-converted signal 129. s However, the down-converted seed signal 118 has a lower (first) peak power level that is substantially lower than the peak power level of the down-converted signal 129. As used herein, the phrase "down-converted" is intended to indicate the down-conversion frequency ω of the down-converted signal 129. s It is a frequency lower than the fundamental frequency ω of the fundamental frequency laser signal 128. In a specific embodiment, the down-conversion frequency ω s It is also higher than 50% (1 / 2) of the fundamental frequency ω (i.e., 0.5ω < ω). s<ω). OPA 119 is configured to mix the down-converted seed signal 118 with the fundamental frequency light portion 127 to generate a down-converted signal 129 at the desired (second) peak power level (i.e., more than ten times the first peak power level). One advantage of generating a higher-power down-converted signal 129 by mixing the lower-power down-converted seed signal 118 with the fundamental frequency light is that it is much easier to control the stability and bandwidth of the low-power laser. Therefore, generating a down-converted seed signal 118 at a lower (first) peak power level promotes the down-conversion frequency ω of the down-converted signal 129. s Greater control is achieved. Another advantage of using a lower-power down-converted seed signal 118 to generate a higher-power down-converted signal 129 is that this method facilitates the generation of the down-converted signal 129 by passing the down-converted seed signal 118 and the fundamental frequency portion 127 through the OPA 119 only once, which (as explained in additional details below) minimizes the distortion of the down-converted signal 129 caused by the idle frequency when using a higher-power down-converted signal to generate the 183nm laser output light 140.

[0038] The following refers to laser assembly 100A ( Figure 1A The detailed description below provides additional details on the functional layout and operation of each of the core components mentioned above. Unless otherwise specified, refer to the following text. Figure 1A The additional details provided apply to the corresponding core components used in laser assembly 100B, and therefore for the sake of brevity, from Figure 1B The description (below) omits repetition of additional details.

[0039] refer to Figure 1A In addition to the core components mentioned above, the laser assembly 100A utilizes a beam splitter 120 optically coupled between the fundamental frequency laser 102 and the OPS 116 and fifth harmonic generator 103. Specifically, the fundamental frequency laser 102 generates fundamental frequency light 128 that is guided onto the beam splitter 120, which divides the fundamental frequency light 128 into two portions: a first portion 127 guided to the OPS 116 in a first (e.g., downward) direction, and a second portion 130 guided to the fifth harmonic generator 103 in a second (e.g., horizontal) direction. The OPS 116 uses an OPA 119 to downconvert the fundamental frequency light portion 127 and generate a beam with a downconverted frequency ω. s The down-converted signal 129 is transmitted to the mixer module 104. The fifth harmonic generator module 103 converts the fundamental frequency light portion 130 and transmits the fifth harmonic light 134 to the mixer module 104. The mixer module 104 mixes the down-converted signal 129 and the fifth harmonic light 134 to generate the laser output light 140.

[0040] refer to Figure 1A The left portion uses known techniques to configure a fundamental laser 102 to generate fundamental light 128 (referred to in the industry simply as "fundamental") at a fundamental frequency within the fundamental frequency bandwidth (range) Δω. In one embodiment, the fundamental laser 102 is configured to generate fundamental light 128 at a fundamental frequency ω corresponding to an infrared wavelength of approximately 1064 nm. In exemplary embodiments, the fundamental laser 102 is implemented using one of an Nd:YAG (neodymium-doped yttrium aluminum garnet) laser medium, an Nd-doped yttrium orthovanadate laser medium, or a ytterbium-doped fiber laser. Suitable fundamental lasers are commercially available from Coherent Inc. (including models in the Paladin series with repetition rates of 80 MHz and 120 MHz), Newport Corporation (including models in the Explorer series), and other manufacturers as pulsed (Q-switched, mode-locked, or quasi-CW). The power level of the laser used in such fundamental frequency lasers can range from several milliwatts to tens of watts or more. In an alternative exemplary embodiment, the fundamental frequency laser 102 is implemented by a laser using an Nd:YLF (neodymium-doped yttrium fluoride lithium) laser medium that produces fundamental frequency laser light at a fundamental frequency wavelength close to 1053 nm or 1047 nm. In yet another exemplary embodiment, the fundamental frequency laser 102 may be implemented by a ytterbium-doped fiber laser that produces fundamental frequency laser light at a fundamental frequency wavelength close to 1030 nm.

[0041] refer to Figure 1ATo the right of the fundamental frequency laser 102, a beam splitter 120 is used to divide the fundamental frequency light 128 into fundamental frequency light portions 127 and 130, which are respectively directed to the OPS 116 and the fifth harmonic generator module 103. In a preferred embodiment, the beam splitter 120 includes a standard gauge or other wavelength selective device that selects the first and second portions from the fundamental frequency wavelength such that the second portion 130 includes a wavelength range within the fundamental frequency wavelength bandwidth that is narrower than the first portion 127. Using the wavelength selective device for the beam splitter 120 allows the output bandwidth of the laser to be controlled independently of the bandwidth of the fundamental frequency laser 102. Further details on how the wavelength selective device can be used to control the output bandwidth of a deep UV laser (e.g., a laser that generates wavelengths close to 183 nm) can be found in U.S. Patent Application 14 / 300,227, filed June 9, 2014, by Deng et al., which is incorporated herein by reference. In one embodiment, the 183nm laser assembly 100A is configured to operate at a repetition rate higher than 1MHz, which is important for high-speed inspection applications. To achieve this high repetition rate operation, the fundamental laser 102 is implemented using a mode-locked or quasi-CW fundamental laser operating at a repetition rate greater than or about 50MHz. This is particularly advantageous for high-speed inspection of semiconductor wafers, photomasks, and photomasks because using such a high repetition rate allows for high-speed image acquisition and reduces the peak power per pulse (and thus results in less damage to optics and the items being inspected) compared to lower repetition rate lasers of the same power. Although the invention is described herein using various fundamental wavelengths that facilitate the generation of laser output light 140 at the desired 183nm wavelength, other wavelengths within a few nanometers of 183nm can be generated using different fundamental wavelengths (i.e., when mixed with appropriate signal frequencies). Unless otherwise specified in the appended claims, such lasers and systems utilizing such lasers are considered to be within the scope of this invention.

[0042] lie in Figure 1A The OPS 116 below the beam splitter 120 is configured to receive the first portion 127 of the fundamental frequency light 128 and down-convert it, such that this down-conversion produces a signal at the desired down-converted frequency ω. s The down-converted signal 129 (i.e., the mixed down-converted signal 129 and the fifth harmonic light 134 produce an output laser 140 at approximately 183 nm). In an alternative embodiment, OPS 116 includes an optical parameter oscillator (OPO), an optical parameter amplifier (OPA), or a combination of both OPO and OPA.

[0043] According to one aspect of the invention, OPS 116 further includes a wavelength selectivity device 117, such as a volume Bragg grating or a narrowband stable seed diode, which, in conjunction with OPO or OPA operation, determines the frequency ω of the down-converted signal 129. sThe bandwidth, wherein a selective specific wavelength is selected for a given particular embodiment based on the frequency / wavelength of the fundamental light 128 and the desired wavelength of the laser output light 140. For example, when the fundamental laser 102 generates fundamental light 128 at a wavelength of approximately 1064 nm (e.g., a wavelength between approximately 1064 nm and approximately 1065 nm), then the wavelength selection device 117 is implemented by a specific wavelength selection device that causes OPS 116 to generate a down-converted signal 129 at a frequency corresponding to a wavelength between approximately 1250 nm and approximately 1420 nm, such that when mixed with the fifth harmonic light 134 generated by the fifth harmonic generation module 103 based on the 1064 nm fundamental frequency, the laser assembly 100A generates laser output light 140 at a wavelength between approximately 182 nm and approximately 185 nm. In another example, when the fundamental frequency laser 102 generates fundamental frequency light 128 at a wavelength of approximately 1053 nm (i.e., a wavelength between approximately 1053 nm and approximately 1054 nm), then the wavelength selective device 117 is implemented by another specific wavelength selective device, which causes the OPS 116 to generate a down-converted signal 129 at a frequency corresponding to a wavelength between approximately 1290 nm and approximately 1520 nm, so as to generate laser output light 140 at a wavelength between approximately 181 nm and approximately 185 nm. In another example, when the fundamental frequency laser 102 generates fundamental frequency light 128 at a wavelength of approximately 1047 nm (i.e., a wavelength between approximately 1047 nm and approximately 1048 nm), then the wavelength selective device 117 is implemented by another specific wavelength selective device, which causes OPS 116 to generate a down-converted signal 129 at a frequency corresponding to a wavelength between approximately 1290 nm and approximately 1580 nm, so as to generate laser output light 140 at a wavelength between approximately 180 nm and approximately 185 nm. In the final example, when the fundamental frequency laser 102 generates fundamental frequency light 128 at a wavelength of approximately 1030 nm (i.e., a wavelength between approximately 1029 nm and approximately 1031 nm), then the wavelength selectivity device 117 is implemented by another specific wavelength selectivity device, which causes OPS 116 to generate a down-converted signal 129 at a frequency corresponding to a wavelength between approximately 1400 nm and approximately 1830 nm, in order to generate laser output light 140 at a wavelength between approximately 179 nm and approximately 185 nm. Given these exemplary values, those skilled in the art will understand how to select an appropriate wavelength selectivity device for a given fundamental frequency and laser output wavelength.

[0044] Refer again Figure 1AThe second portion 130 of the fundamental frequency light 128 is guided from the beam splitter 120 toward the fifth harmonic generation module 103, which is configured to generate a fifth harmonic light 134 having a frequency five times the fundamental frequency ω by means of converting the fundamental frequency portion 130. If the bandwidth of the second fundamental frequency portion 130 is narrower than the bandwidth of the fundamental frequency light 128 (i.e., because the beam splitter 120 includes a wavelength selective device), then the fifth harmonic light 134 will also have a narrower bandwidth than if it were generated directly from the fundamental frequency light 128 without using a wavelength selective device.

[0045] Figure 2 A fifth harmonic generator module 103 according to a presently preferred embodiment is shown, comprising a first frequency doubling module (second harmonic generation) 202, an optional beam splitter / prism 212, a second frequency doubling module (fourth harmonic generation) 203, an optional beam splitter / combiner 213, a frequency summing module (fifth harmonic generation) 204, and an optional beam splitter or wavelength splitter 214. Generally, the fifth harmonic generator module 103 is used to generate fifth harmonic light 134 by using the frequency doubling modules 202 and 203 to convert a portion of the input signal at the fundamental frequency ω (i.e., the second fundamental frequency portion 130) to generate a fourth harmonic laser 203A at four times (4ω) the fundamental frequency, and then using the frequency summing module 204 to mix the fourth harmonic laser 203A with the unconsumed portion of the input light. According to the current preferred embodiment, at least one of the first frequency doubling module 202, the second frequency doubling module 203, and the frequency summing module 204 is implemented using an annealed CLBO crystal, a deuterated CLBO crystal, or a hydrogen-treated CLBO crystal.

[0046] The fifth harmonic generator module 103 generates the fourth harmonic laser 203A by generating the second harmonic laser 202A using a first frequency doubling module 202, and then doubling the second harmonic laser 202A using a second frequency doubling module 203. (Reference) Figure 2On the left side, the first frequency multiplier module 202 receives and converts the fundamental frequency portion 130 at the fundamental frequency ω to form a second harmonic light 202A at twice the fundamental frequency (2ω). The second frequency multiplier module 203 receives and converts the second harmonic light 202A to form a fourth harmonic light 203A at four times the fundamental frequency (4ω). The unconsumed portion 202B of the fundamental frequency light 130 leaving the first frequency multiplier module 202 can be separated from the second harmonic light 202A by a beam splitter or prism 212 and guided toward the frequency summing module 204. In one embodiment (not shown), the unconsumed fundamental frequency portion 202B does not separate from the second harmonic 202A and co-propagates with the second harmonic light 202A through the second frequency multiplier module 203 to arrive at the frequency summing module 204 substantially in sync with the fourth harmonic 203A. One advantage of separating the unconsumed fundamental frequency portion 202B from the second harmonic light 202A is that an appropriate time delay can be applied to either the unconsumed fundamental frequency portion 202B or the fourth harmonic light 203A, so that the two laser pulses arrive at the frequency summing module 204 substantially simultaneously. Another advantage is that the optics used for guiding and / or focusing the light (e.g., mirrors, lenses, and prisms (not shown)) can be individually optimized in each path for the appropriate wavelength.

[0047] In one embodiment, the unconsumed second harmonic portion 203B (i.e., a portion of the second harmonic light not used within the second frequency doubling module 203) is separated from the fourth harmonic 203A by an optional beam splitter / combiner 213. The beam splitter / combiner 213 may include one or more beam splitters and / or one or more prisms. The beam splitter / combiner 213 may (if needed) combine the unconsumed fundamental frequency portion 202B with the fourth harmonic 203A, such that they propagate together to the frequency summing module 204.

[0048] refer to Figure 2 On the right side, the frequency summing module 204 generates fifth harmonic light 134 by summing the fourth harmonic light 203A and the unconsumed fundamental frequency light portion 202B. An optional beam splitter or wavelength splitter 214 is used in some embodiments to separate any unconsumed fundamental frequency portion and the fourth harmonic 204B from the fifth harmonic light 134. The beam splitter 214 may include a prism, a polarizing beam splitter, a dichroic beam splitter, or a combination of optical elements.

[0049] In a preferred embodiment, the second harmonic generation module 202 includes a lithium triborate (LBO) crystal for frequency conversion. In other embodiments, the second harmonic generation module 202 includes a CLBO, BBO, or other nonlinear crystal for frequency conversion. In a preferred embodiment of the fifth harmonic generator 103, the fourth harmonic generation module 203 includes a CLBO crystal for frequency conversion. In other embodiments, the fourth harmonic generation module 203 may include a BBO or other nonlinear crystal for frequency conversion. In a preferred embodiment of the fifth harmonic generator 103, the frequency summing module 203 includes a CLBO crystal for frequency summing. In other embodiments, the frequency summing module 204 may include a BBO or other nonlinear crystal for frequency summing.

[0050] Figure 3 A table showing exemplary wavelength ranges (in nm) for the 183 nm laser shown in Figure 1 is provided. For each fundamental frequency laser type, exemplary short-wavelength and long-wavelength fundamental frequency portions are shown, along with the wavelengths corresponding to the harmonics and the down-converted signal required for the desired output wavelength (183 nm in the examples shown in the table). The precise wavelength of a fundamental frequency laser depends on many factors, including the precise composition of the laser medium, the operating temperature of the laser medium, and the design of the optical cavity. Two lasers using the same laser line with a given laser medium can operate with wavelengths differing by a few tenths of a nanometer or several nanometers due to the foregoing and other factors. Those skilled in the art will understand how to select an appropriate wavelength for the down-converted signal to produce the desired output wavelength from any fundamental frequency wavelength close to those listed in the table. Similarly, if the desired output wavelength differs from 183 nm by several nanometers, then the desired output wavelength can also be achieved by appropriately adjusting the wavelength used for the down-converted signal.

[0051] Figure 4 Demonstrates a laser assembly 100A according to a preferred embodiment ( Figure 1A The mixing module 104 used in the process is described. The mixing module 104 includes a nonlinear crystal 402, which in a preferred embodiment comprises an annealed (deuterated or hydrogen-treated) lithium cesium borate (CLBO) crystal including an input surface 442 and a relative output surface 452. The nonlinear crystal 402 is positioned to receive both fifth harmonic light 134 (i.e., from the fifth harmonic generator 103) and a down-converted signal 129 (from the OPS generator 116) at the input surface 442, such that the signal 129 and the light 134 enter the nonlinear crystal 402 substantially collinearly (e.g., in direction 410, by...). Figure 4(As indicated by the dashed arrow in the diagram), and focused onto the corresponding beam waist (not shown) located inside or near crystal 402. For a Type I match in a CLBO at approximately 50°C with a down-converted signal having a wavelength close to 1433 nm and a fifth harmonic having a wavelength close to 206 nm, the phase matching angle is approximately 74.9°. For a Type I match in a CLBO at approximately 50°C with a down-converted signal having a wavelength close to 1274 nm and a fifth harmonic having a wavelength close to 213 nm, the phase matching angle is approximately 85.7°. Both of these examples demonstrate that near-noncritical phase matching with high efficiency and low walk-off can be achieved for generating wavelengths close to 183 nm. These wavelength combinations are merely examples and are not intended to limit the scope of the invention. Those skilled in the art will understand how different combinations of wavelength, temperature, and angle can be selected to achieve phase matching.

[0052] In some embodiments, the input surface 442 of crystal 402 is truncated and positioned approximately at a Brewster angle relative to the fifth harmonic light 134 (i.e., relative to direction 410 and the polarization of the fifth harmonic light 134). This angle minimizes reflection of the fifth harmonic wavelength and thus facilitates avoiding the need for an anti-reflective coating on the input surface 442 in some embodiments. In other embodiments, an anti-reflective coating (not shown) is applied to surface 442 to reduce reflected light at the fifth harmonic and / or signal wavelengths. The output surface 452 of crystal 402 may be coated or uncoated. In one embodiment, the output surface 452 of crystal 402 is truncated and maintained at a Brewster angle relative to the laser output light 140 and is uncoated. It should be noted that if type I phase matching is used, the polarization of the laser output light 140 is preferably perpendicular to the polarization of the input wavelength (i.e., the fifth harmonic light 134 and the down-converted signal 129), and therefore the Brewster angle output surface 452 must be appropriately truncated. The advantage of not coating the output surface 452 is that the coating has a shorter lifespan when exposed to intense UV radiation.

[0053] Refer again Figure 4 In a preferred embodiment, the mixer module 104 may use one or more optical elements (optical devices) 405 to separate the desired output wavelength (i.e., the laser output light 140 at approximately 183 nm) from other undesired wavelengths 451 (e.g., the unconsumed portion of the fifth harmonic light 134 and / or the unconsumed portion of the down-converted signal 129). Optical devices 405 may include beam splitters, prisms, gratings, or other optical elements. In some embodiments, the combination of the angle of departure from the output surface 452 of the crystal 402 can achieve sufficient separation of the laser output 140 from other wavelengths, thus eliminating the need for optical devices 405.

[0054] In a preferred embodiment of the 183nm laser, a substantial portion or almost all of the fifth harmonic light 134 is consumed in the crystal 402 due to the use of a high-power down-converted signal 129. Although this may result in a lower overall conversion efficiency from the fundamental light 128 (in FIG. 1) to the laser output light 140, using a laser with higher power at the signal wavelength and lower power at the fifth harmonic for a given output power can result in a longer lifespan and require less frequent maintenance, because deep UV light (e.g., fourth and fifth harmonics) can easily cause damage to the optics within the laser and light pollution.

[0055] It should be noted that in any of the embodiments described, mirrors, prisms, periscopes, etc., may be used as needed to guide the fundamental wavelength or other wavelengths. For example, prisms, beam splitters, beam combiners, and dichroically coated mirrors may be used as needed to separate and combine beams. Various combinations of mirrors and beam splitters can be used to separate and route various wavelengths between different frequency conversion stages in any suitable order. The facet of the frequency conversion crystal, prism, beam splitter, or lens may be truncated at an angle approximately equal to the Brewster angle of the incident wavelength to minimize or control reflections without the use of an anti-reflective coating. This truncation is particularly advantageous for surfaces where UV radiation is incident, as anti-reflective coatings can degrade upon exposure to UV, and thus degrade the reliability of the laser (in the case of use on these surfaces). Waveplates (including Brewster angle waveplates or retarders) or other optical elements may be used as needed to rotate the polarization of any of the wavelengths to align the polarization with the appropriate crystal axis of the next frequency conversion or mixing stage. The use of Brewster angle optics in DUV lasers is described in more detail in Armstrong’s U.S. Patent 8,711,470, entitled “High Damage Threshold Frequency Conversion System.” This patent is incorporated herein by reference.

[0056] The above description and associated figures illustrate various lasers for generating light with a wavelength of approximately 183 nm. Some specific wavelengths and wavelength ranges are described to illustrate embodiments. Other laser embodiments, similar to those described above, are possible and within the scope of this invention, capable of generating wavelengths several nanometers shorter or longer than 183 nm.

[0057] The diagrams described above are not intended to represent the actual physical layout of the components. The diagrams illustrate the main optical modules involved in the process, but not every single optical element. Those skilled in the art will understand how to construct a 183nm laser based on the diagrams and associated descriptions described above. It should be understood that more or fewer optical components may be used to guide the light as needed. Where appropriate, lenses and / or curved mirrors may be used to focus the beam waist to a focal point within or near a substantially circular or elliptical cross-section of the nonlinear crystal. Prisms, beam splitters, gratings, or diffractive optical elements may be used, as needed, to guide or separate different wavelengths at the output of each frequency converter or mixer module. Prisms, coated mirrors, or other elements may be used, where appropriate, to combine different wavelengths at the input to the frequency converter and mixer. Beam splitters or coated mirrors may be used, where appropriate, to split a wavelength into two beams. Filters may be used to block or separate unwanted wavelengths at any stage of the output. Waveplates may be used to rotate polarization, as needed. Other optical elements may be used, where appropriate. In some cases, it is acceptable to allow unconsumed light from one frequency conversion stage to pass to the next stage (even though the light is not needed in the subsequent stage). This is acceptable if the power density is low enough not to cause damage and if there is minimal interference with the desired frequency conversion process (e.g., due to lack of phase matching at the crystal corners or due to light polarization). Those skilled in the art will understand the various trade-offs and alternatives possible in implementations of 183nm lasers.

[0058] In a preferred embodiment, the first frequency doubling module 202 that generates the second harmonic ( Figure 2 The first frequency doubling module 202 may comprise a lithium triborate (LBO) crystal, which may be substantially non-critically phase-matched (depending on the appropriate selection of the crystal plane) at a temperature between room temperature and about 200°C to generate second harmonics in a wavelength range between about 515 nm and about 532 nm. In other embodiments, the first frequency doubling module 202 may comprise a lithium cesium borate (CLBO) crystal or a barium β-borate (BBO) crystal, either of which may be critically phase-matched to generate second harmonics in a wavelength range between about 515 nm and about 532 nm.

[0059] The second harmonic generation module 203 (which generates the fourth harmonic) Figure 2 The frequency doubling module 203 and the frequency summing module 204, which generate the fifth harmonic, can use critical phase matching in CLBO, BBO, or other nonlinear crystals. In a preferred embodiment, both the frequency doubling module 203 and the frequency summing module 204 comprise a CLBO crystal.

[0060] Frequency conversion stage (included in) Figure 1A , 2Any of the methods and systems disclosed in U.S. Patent No. 8,873,596, entitled "Laser With High Quality, Stable Output Beam, And Long Life High Conversion Efficiency Non-Linear Crystal," by Dribinski et al., may advantageously use some or all of the methods and systems disclosed therein. This patent is incorporated herein by reference.

[0061] Frequency conversion stage (included in) Figure 1A , 2 Any of the following (as shown in 4) may comprise one or more protective environments, such as those described in Armstrong’s U.S. Patent 8,298,335 entitled “Enclosure for controlling the environment of optical crystals”. This patent is incorporated herein by reference. It should be noted that a single protective environment may enclose multiple levels or a single level.

[0062] Frequency conversion stage (included in) Figure 1A , 2Any of the following (and those shown in 4) may be incorporated into: any of the methods or systems described in U.S. Patent 8,298,335 to Debinsky 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 8,824,514 to Armstrong entitled "Measuring crystal site lifetime in a non-linear optical crystal"; any of the apparatus and methods described in U.S. Patent No. 8,976,343 to Genis entitled "Laser crystal degradation compensation"; and any of the preferred shift directions to prolong the life and minimize disturbances of scanning non-linear optical crystals, filed by Genis on June 19, 2013. Any of the systems and methods described in U.S. Provisional Patent Application No. 61 / 837,053, entitled “Perturbations of a scanning nonlinear optical crystal”; and any of the systems and methods described in U.S. Provisional Patent Applications Nos. 61 / 666,675 and 61 / 762,269, filed by Armstrong et al. on June 29, 2012, and February 7, 2013, respectively, entitled “Scan rate for continuous motion of a crystal in a frequency-converted laser”. The laser may further incorporate any of the systems and methods described in U.S. Patent No. 8,686,331, entitled “Dynamic wavefront control of a frequency-converted laser system” by Armstrong.All of these patents, applications and provisional applications are incorporated herein by reference.

[0063] Further attention should be paid to the frequency conversion stage (included in...) Figure 1A , 2 Any of the following (as shown in 4) can advantageously utilize nonlinear crystals doped or treated with deuterium, hydrogen, and / or fluorine. Such crystals can be formed, disposed of, or treated by any of the processes or methods described in U.S. Patent No. 9,023,152, filed September 3, 2010, by Debinsky et al., or in U.S. Patent Application No. 13 / 488,635, filed June 1, 2012, by Zhuang et al., and U.S. Patent Application No. 14 / 248,045, filed April 8, 2014, by Debinsky et al. (both are co-filed). These patents and applications are incorporated herein by reference. The doped or treated crystals are particularly useful in those stages involving deep UV wavelengths (including frequency doubling module 203, frequency summing module 204, and mixing module 104).

[0064] In some embodiments, one or more amplifiers may be used to increase the power at the fundamental frequency wavelength in order to generate sufficient power at that wavelength. If two or more amplifiers are used, it is preferable to use a seed laser to seed all amplifiers so that they all output synchronized laser pulses at the same wavelength. Figure 5 This describes an exemplary amplifier module 500 in which a seed laser 503 can generate a stabilized narrowband seed laser 504 at a desired fundamental wavelength (e.g., approximately 1064 nm, approximately 1053 nm, approximately 1047 nm, or approximately 1030 nm). In some embodiments, the seed laser 503 is one of an Nd-doped YAG laser, an Nd-doped yttrium orthovanadate laser, an Nd-doped YLF laser, a fiber laser, or a stabilized diode laser. The seed light 504 travels to a first amplifier 507, which amplifies the light to a higher power level to generate the fundamental light 128. In one embodiment, the first amplifier 507 includes Nd-doped YAG or Nd-doped yttrium orthovanadate. In one embodiment, an amplifier pump 505 includes a laser capable of pumping the first amplifier 507. In some embodiments, this pumping can be performed using one or more diode lasers operating at a wavelength of approximately 808 nm or approximately 888 nm. In other embodiments, the first amplifier 507 may include a Yb-doped fiber amplifier.

[0065] Figure 5Exemplary additional components that can be used in some embodiments of amplifier module 500 are also described. Since the OPO / OPA 116, the first frequency multiplier module 202, and the frequency summing module 204 (Figures 1 and 2) receive the fundamental frequency laser wavelength as input and depend on the required output power close to a 183nm wavelength, it may be necessary to conveniently generate more fundamental frequency laser light in a single amplifier with the desired bandwidth, stability, and beam quality. In practice, increasing the power output of an optical amplifier can lead to increased bandwidth, beam quality degradation due to thermal lensing or other effects, reduced stability, and / or shortened lifespan.

[0066] Therefore, in some embodiments of amplifier module 500, the first amplifier 507 and the additional second amplifier 517 can be used to generate two fundamental frequency laser outputs 128 and 528, respectively, wherein the fundamental frequency light 128 is utilized as mentioned above, and the light 528 can be directed to a different frequency conversion stage (not shown) instead of (for example) 127 (in Figure 1A (in China) or 202B (in Figure 2 (In the middle). The second amplifier 517 may be substantially the same as the first amplifier 507. In one embodiment, the amplifier pump 515 includes a laser capable of pumping the second amplifier 517. The amplifier pump 515 may be substantially the same as the amplifier pump 505. Obviously, the same seed laser 503 can be used to seed the two lasers to ensure that outputs 128 and 528 are at the same wavelength and are synchronized. Beam splitters or prisms 511 and mirrors or prisms 512 can divide the seed light 504 and direct a portion of it to the second amplifier 517.

[0067] Figure 6A and 6B OPS 116D and OPS 116E according to two alternative exemplary embodiments are shown respectively. (Refer to the above text.) Figure 1A As mentioned, OPS 116 includes a down-converted seed signal generator (DCSSG) 117 that generates a lower-power down-converted seed signal 118. Then, an optical parametric amplifier (OPA) 119 combines the lower-power down-converted seed signal 118 with the fundamental frequency optical portion 127 to generate a higher-power down-converted signal 129. The higher-power down-converted signal 129 is then transmitted to a mixer module 104 to be mixed with the fifth harmonic light 134. As stated in the following exemplary embodiments, OPS 116D and OPS 116E utilize a similar OPA structure but with two different DCSSG arrangements. Specifically, in OPS 116D ( Figure 6A In the case of using a seed laser to directly generate a down-converted seed signal, the OPS 116E ( Figure 6BAn optical parametric oscillator is used to generate a down-converted seed signal by converting a portion of the fundamental frequency laser. The advantages of each of these methods are stated in the following description.

[0068] refer to Figure 6A The OPS 116D typically includes a down-converted signal seed generator (DCSSG) 117D implemented using a seed laser 603 and an OPA 119D. The OPA 119D includes a beam combiner 611, a nonlinear crystal 607, and a beam splitter 621. The seed laser 603 is configured to directly generate signals at the desired down-converted signal frequency ω. s The down-converted seed light 118D is directed to the beam combiner 611 in the OPA 119D. The beam combiner 611 is configured and positioned to receive both the fundamental frequency portion 127 (input laser) at the fundamental frequency ω and the down-converted seed light 118D, and combines (i.e., directs along a collinear path) the fundamental frequency portion 127 and the down-converted seed light 118D so that they enter the nonlinear crystal 607. The nonlinear crystal 607 is configured to amplify the down-converted seed signal 118 by stimulated down-conversion of the fundamental frequency portion 127, and emits the amplified signal toward the beam splitter (wavelength splitter) 621. The beam splitter 621 is configured to separate the down-converted signal 129 from other frequencies present in the amplified signal received from the nonlinear crystal 607, and directs the down-converted signal 129 to the mixer module (not shown). Each of these components is described in additional detail in the following paragraphs.

[0069] In a preferred embodiment, the seed laser 603 is implemented using a diode laser or a low-power fiber laser and is configured to generate a signal at the down-converted signal frequency ω. s A seed laser 604 is then used to seed the signal at the said frequency for a down-conversion process. The seed laser 603 only needs to have an average power of approximately 1 mW to several hundred mW. In a preferred embodiment, the seed laser 603 is stabilized by using (for example) a grating and by temperature stabilization. The seed laser frequency and bandwidth determine the frequency and bandwidth of the down-converted signal 129. The advantage of using a seed laser is that controlling the stability and bandwidth of a lower-power laser is much easier compared to a high-power laser. Stabilizing a narrow-bandwidth seed laser determines the bandwidth and stability of the down-converted signal 129. In one embodiment, the seed laser 603 generates polarized light that is substantially perpendicular to the polarization of the fundamental frequency light (i.e., the input laser 127), and then introduces said polarized light into a nonlinear converter 607.

[0070] In one embodiment, the beam combiner 611 (e.g., a prism) includes a dichroic coating that effectively reflects a first wavelength while transmitting a second wavelength, such that the fundamental frequency portion 127 and the transmitted seed laser 118D travel substantially collinearly through the nonlinear converter 607. For example, as in... Figure 6A As indicated, beam combiner 611 reflects the fundamental frequency light portion 127 and transmits the seed laser 118D, such that the two are emitted substantially collinearly through nonlinear converter 607, as shown. In an alternative embodiment (not shown), the beam combiner is configured and arranged to transmit the fundamental frequency light portion and reflect the seed laser, such that the two travel substantially collinearly through the nonlinear converter.

[0071] In one embodiment, the nonlinear crystal 607 uses the input laser frequency ω and the down-converted signal frequency ω. s Any suitable nonlinear optical crystal or periodically polarized nonlinear optical crystal can be implemented, whether phase-matched or quasi-phase-matched. In a preferred embodiment, the nonlinear crystal 607 comprises one of periodically polarized lithium niobate, periodically polarized magnesium oxide-doped lithium niobate, periodically polarized stoichiometric lithium tantalate (PPSLT), periodically polarized magnesium oxide-doped stoichiometric lithium tantalate, and periodically polarized potassium titanate phosphate (PPKTP).

[0072] In one embodiment, beam splitter 621 (e.g., prism) is configured and positioned using known techniques to separate the down-converted signal 129 from the undesired frequency 623 (e.g., unconsumed fundamental frequency and idle frequency). In one embodiment (not shown), the unconsumed fundamental frequency can be set to match the time delay of the next incoming laser pulse of the fundamental light portion 127 and recycle back to the input of nonlinear converter 607.

[0073] Figure 6B The OPS 116E according to the second exemplary embodiment generates a signal at the desired down-converted frequency ω by means of a portion of the converted fundamental frequency laser. sThe high-power down-converted signal 129 (e.g., greater than about 3W). The OPS116E typically includes: a beam splitter 631 configured to split the fundamental frequency optical portion 127 at the fundamental frequency ω into a first sub-portion 127A and a second sub-portion 127B; an optical parameter oscillator (OPO; i.e., a down-converted seed signal generator) 117E configured to generate a down-converted seed signal 118E by means of converting the fundamental frequency photonic portion 127A; and an OPA 119E configured to mix the down-converted seed signal 118E with the (second) fundamental frequency photonic portion 127B. The OPO 117E includes a first focusing mirror 632, a nonlinear crystal 633, a second focusing mirror 634, a wavelength selector 637, and an output coupler 636, which, as shown, are operably configured to form an optical cavity in which light is reflected between the wavelength selector 637 and the output coupler 636 by means of the focusing mirrors 632 and 634 and the nonlinear crystal 633. Similar to the OPS 116D (… Figure 6A The OPA 119E includes a beam combiner 640, a nonlinear crystal 641, and a wavelength splitter 642. Each of these components is described in additional detail in the following paragraphs.

[0074] refer to Figure 6B On the left side, in one embodiment, the fundamental frequency portion (input laser) 127 at the fundamental frequency ω is divided by a beam splitter 631 such that sub-portion 127A, guided to OPO 117E, contains less than 50% of the energy of the input laser 127, and sub-portion 127B, guided to OPO 119E, contains more than 50% of the energy of the input laser 127. Sub-portion 127A enters OPO 117E by means of a focusing lens 632. A focusing or mode-matching optics (not shown) may be placed in the optical path of the input laser 127 before OPO 117E to focus sub-portion 127A near the center of the nonlinear crystal 633.

[0075] The nonlinear crystal 633 is designed for phase matching or quasi-phase matching to generate signals at frequency ω from a sub-part 127A at frequency ω. s The light is transmitted through the nonlinear crystal 633. In one embodiment, the nonlinear crystal 633 comprises a periodically polarized material, such as periodically polarized lithium niobate (PPLN) or periodically polarized stoichiometric lithium tantalate (PPSLT). Any input laser light that is not converted into signal frequency light by the nonlinear crystal 633 passes through the focusing lens 634 and can be discarded. The focusing lens 634 should preferably also transmit idle frequencies formed in the nonlinear crystal 633.

[0076] In one embodiment, the focusing lens 634 is configured in pairs at the signal frequency ω. sThe light is highly reflective and is arranged to guide light at or through the nonlinear crystal 633 to the output coupler 636 at the frequency of the signal. The output coupler 636 transmits light incident thereon at the signal frequency ω. s The first portion of the light (e.g., approximately 20%) is reflected, and the second portion of the light (e.g., approximately 80%) is reflected. At the signal frequency ω... s The second portion of the light is reflected back to focusing lens 634, which then guides the light through nonlinear crystal 633 back to focusing lens 632, which in turn guides the light back to wavelength selector 637.

[0077] Wavelength selector 637 is configured using known techniques to select the desired signal frequency ω. s The narrow frequency range centered on the signal is highly reflective. For example, wavelength selector 637 can reflect a wavelength range of approximately 0.2 nm FWHM. Wavelength selector 637 is important for determining the wavelength of laser output 140 (see, for example, Figure 1a) because the wavelength of laser output 140 corresponds to the fifth harmonic of the fundamental frequency wavelength and the signal frequency ω. s The wavelengths of the laser output 140 are determined by the temperature of the laser. In one embodiment, the wavelength selector 637 includes a volumetric Bragg grating. In a preferred embodiment, the wavelength selector 637 is kept at a constant temperature to ensure that its center wavelength remains constant. In one embodiment, the wavelength of the laser output 140 can be slightly adjusted by adjusting the temperature of the wavelength selector 637 to change the signal frequency ω. s .

[0078] At signal frequency ω s The down-converted light, after being reflected from the wavelength selector 637, returns to the focusing mirror 632, which guides it back to the nonlinear crystal 633. At the signal frequency ω... s The optical path length followed by light from nonlinear crystal 633 to focusing lens 634 to output coupler 636, back to focusing lens 634, through nonlinear crystal 633 to focusing lens 632, to wavelength selector 637, back to focusing lens 632, and back to nonlinear crystal 633 should be such that at signal frequency ω... s Each pulse of light returns to the nonlinear crystal 633 substantially simultaneously with the pulse of the input laser 127. This arrangement ensures that the pulses of the input laser 127 and the light at the signal frequency substantially co-propagate through the nonlinear crystal 633 to enable the input laser to propagate to the signal frequency ω. s The light is subjected to stimulated down-conversion. In a preferred embodiment, the optical path length should be such that at the signal frequency ω... s The mismatch between the arrival time of the light pulse and the pulse of the input laser 127 is less than about 10% of the width of the pulse of the input laser 127.

[0079] In one embodiment, focusing lenses 632 and 634 are configured to include a focal length set such that pulses of light at the signal frequency return to the nonlinear crystal 633 after a complete round trip as described above, focusing near the center of the nonlinear crystal and substantially spatially overlapping with the pulses of the input laser 127. In an alternative embodiment, wavelength selector 637 and / or output coupler 636 may replace or supplement focusing lenses 632 and 634 to focus at the signal frequency ω. s The light. In another embodiment, one or more lenses may be used in place of a focusing lens or as a supplement to a focusing lens to refocus the signal frequency.

[0080] It should also be noted that the relative positions of the output coupler 636 and the wavelength selector 637 can be interchanged, provided that appropriate layout changes are made to incorporate additional mirrors and / or prisms to select the wavelength at the signal frequency ω. s The second part 127B of the light and input laser is redirected to the beam combiner 640. Figure 6B The layout shown is intended to be illustrative to explain the principles of operation.

[0081] Other known OPO configurations in this technology can replace OPO 117E. For example, a ring cavity OPO or a butterfly cavity OPO can be used. Other modifications can be made to OPO 117E without departing from the scope of the invention. For example, instead of wavelength selector 637, a mirror can be used, and a transmissive wavelength selector (not shown) can be included at the signal frequency ω. s In the optical path. Additional plane mirrors or prisms may be included in the OPO 117E to (for example) achieve the desired optical path length while maintaining a compact overall size.

[0082] For high-power laser outputs of 140 (e.g., 1W or greater), the signal wavelength ω is generated directly from the fundamental frequency laser rather than from the second harmonic of the fundamental frequency light. s This is preferred because less power is wasted, and therefore a lower power fundamental frequency laser 102 can be used for a given output power (e.g., Figure 1A Generally speaking, an OPO can generate a signal frequency ω. s High average output power (e.g., several watts or more), such as the high average output power required to generate a laser output 140 of about 1W or more. This invention is directed to generating a laser output 140 with a wavelength between about 180nm and 185nm from a fundamental wavelength close to 1μm. This requires a signal frequency ω corresponding to a wavelength between about 1.2μm and about 1.6μm. s (exist Figure 3(Some example wavelength combinations are shown in the diagram). The generation of this short wavelength relative to the fundamental laser wavelength implies that the idler frequency, which forms simultaneously with the signal frequency, must have a long wavelength, for example, longer than approximately 4 μm. Readily available high-gain, high-quality nonlinear crystals (e.g., PPLN and PPSLT) suitable for generating signal wavelengths between approximately 1.2 μm and approximately 1.6 μm from wavelengths close to 1 μm are strongly absorbent at wavelengths longer than approximately 4 μm. If the OPO 117E is used to generate high power at signal frequencies within the desired range, the idler frequency will also contain significant power. Because the idler frequency is absorbed by the nonlinear crystal 633, a significant temperature gradient will form within the nonlinear crystal 633 at higher idler power. These temperature gradients locally alter the optical properties of the nonlinear crystal 633, resulting in the generation of signals at frequencies ω... s The irregular distribution curve of light may lead to unstable operation of OPO117E.

[0083] In this invention, OPO 117E is operated to generate a signal at frequency ω. s These problems are overcome by the relatively low output power (e.g., hundreds of mW average power). At this output power, local heating of the nonlinear crystal 633 is minimal and the OPO 117E can operate stably with a good distribution curve for the downconverted seed signal 118E. The nonlinear crystal 633 can be selected to maximize conversion efficiency with less concern for damage or thermal properties by using a long-length material (e.g., PPLN or PPSLT) with a high nonlinear coefficient, for example.

[0084] In this invention, the signal generated by OPO 117E is at frequency ω. s The light 118E is amplified by OPA 119E to the required power level, as shown by the down-converted signal 129. Beam combiner 640 combines the second portion of the input laser 127 with the light from OPA 117E at the signal frequency ω. s The optical path length from beam splitter 631 to beam combiner 640 should be such that the pulse of the input laser is at the signal frequency ω. s The light pulses arrive at beam combiner 640 substantially simultaneously. Additional mirrors, prisms, or other optical components may be placed in the optical path between 631 and 640 and / or between 636 and 640 to ensure that the pulses arrive at 640 substantially simultaneously. Lenses, curved mirrors, or other optical elements (not shown) may be used in either optical path as needed to ensure that the second portion of the input laser 127 is at the signal frequency ω. s The light rays essentially overlap in space and both are focused near the center of the nonlinear crystal 641.

[0085] Beam combiner 640 directs the optical pulse to nonlinear crystal 641. Nonlinear crystal 641 amplifies the signal at frequency ω by stimulated downconversion of the second fundamental frequency photonic portion 127B. s The wavelength splitter 642 separates the down-converted signal 129 from any unconsumed input laser 643 and any idle frequency. The wavelength splitter 642 may include a polarization beam splitter (in cases where the down-converted signal 129 has a polarization different from the input laser), a dichroic mirror, a Pellin-Broca prism, or any other suitable wavelength splitter known in this art. The nonlinear crystal 641 may include components for the input laser frequency ω and the down-converted signal frequency ω. s Any suitable nonlinear optical crystal, whether phase-matched or quasi-phase-matched, or a periodically polarized nonlinear optical crystal. In a preferred embodiment, the nonlinear crystal 641 comprises a PPSLT or a periodically polarized Mg-doped SLT. These materials are particularly suitable for operation at higher power levels.

[0086] Because the down-converted signal 129 passes through the nonlinear crystal 641 only once, the thermal gradient in the crystal 641 results in a lower optical distribution profile than would be achieved in an OPO configured to produce similar output power. That is, if the OPA119E were replaced by an OPO (e.g., configured as, for example, OPO 117E), then a signal frequency ω would be required. s The light passes through its nonlinear crystal multiple times (e.g., nonlinear crystal 633 in OPO 117E), resulting in significant heating due to the idle frequency. Therefore, by utilizing a two-step method of first generating a low-power seed signal and then mixing the seed signal with a portion of the fundamental frequency light to generate a down-converted signal 129 at the desired frequency and power level, the present invention overcomes the obvious limitation of using only an OPO to generate a high-power down-converted signal 129.

[0087] refer to Figure 1B As mentioned above, laser assembly 100B and laser assembly 100A ( Figure 1A The similarities lie in that both laser assemblies include: a fundamental frequency laser 102 configured to generate fundamental frequency light 128 having a fundamental frequency wavelength ω; an OPS 116 optically coupled to receive a portion 127 of the fundamental frequency light 128 and generate a down-converted signal 129; a fifth harmonic generation module 157; and a mixer module 104 configured to receive and mix the down-converted signal 129 with a fifth harmonic laser 134 from the fifth harmonic generator 157 to generate laser output light 140. Additionally, the OPS 116 generates light at the down-converted wavelength ω using a DCSSG 117. sThe lower power is downconverted by the seed signal 118 and then mixed with the baseband optical portion 127 to generate the downconverted signal 129.

[0088] Laser assembly 100B and laser assembly 100A ( Figure 1A The first difference between them is that all the fundamental frequency light 128 generated by the fundamental frequency laser 102 is emitted to the second harmonic generation module 153, and portions 127 and 130 of the unused fundamental frequency light 182 leaving the second harmonic generation module 153 are supplied to the OPS 116 and the fifth harmonic generation module 157. This method illustrates a beneficial alternative to the case where the fundamental frequency laser 102 outputs a second fundamental frequency light and unused fundamental frequency light (i.e., where the fundamental frequency laser actually includes the second harmonic generation module 153). To facilitate this substitution, a first beam splitter 181 is used to separate the second harmonic light 189 from the second harmonic generation module 102 from the unused fundamental frequency light 182, such that the second harmonic light 189 is emitted to a fourth harmonic generation module 155, and the unused fundamental frequency light 182 is emitted to a second beam splitter 183, which generates portions 127 and 130 that are respectively guided to OPS 116 and the fifth harmonic module 157.

[0089] Apart from the differences mentioned above, the operation of laser assembly 100B is essentially the same as that of laser assembly 100A. The second harmonic generation module 153 and the first frequency doubling module 202 ( Figure 2 They function essentially similarly and can be configured similarly. The fourth harmonic generation module 155 and the second harmonic generation module 203 ( Figure 2 They function essentially similarly and can be configured similarly. The fifth harmonic generation module 157 and the frequency summing module 204 ( Figure 2 They function essentially similarly and can be configured similarly. In other words, modules 153, 155, and 157 perform essentially the same functions as the fifth harmonic generation module 103, but with different baseband routes between the modules.

[0090] Figures 7 to 12 The description may include systems using one of the 183nm lasers described above. These systems can be used in photomask, mask, or wafer inspection and measurement applications.

[0091] Figure 7A photomask, photomask, or wafer inspection system 700 is demonstrated that simultaneously inspects two image or signal channels using a single sensor 770. An illumination source (laser assembly) 709 is configured to produce laser output light 710 having an output wavelength in the range of approximately 180 nm to approximately 185 nm (e.g., 183 nm) as described herein. The illumination source 709 may further include a pulse repetition rate multiplier and / or coherence reduction scheme. The two image / signal channels may include reflected and transmitted light when the object under inspection (which is mounted on a stage 730) is transparent (e.g., a photomask or photomask), or may include two different illumination modes, such as incident angle, polarization state, wavelength range, or a combination thereof.

[0092] As in Figure 7 As shown, the inspection system 700 includes illumination relay (first) optics 715 and 720, which are optical systems configured using known techniques to relay illumination (laser output light) 710 from source 709 to an object under inspection mounted on stage 730. The object under inspection may be a photomask, photomask, semiconductor wafer, or other article under inspection. The inspection system 700 also includes image relay (second) optics 740, 755, and 760, which are optical systems configured using known techniques to relay a portion 710' of the illumination 710 that affects (i.e., reflects, scatters, and / or transmits) the object under inspection to sensor 770. Data of the detected signals or images corresponding to the two channels are displayed as data 780 and transmitted to a computer (not shown) for processing.

[0093] Further details of a photomask or photomask inspection system that can be configured to measure transmitted and reflected light from a photomask or photomask are described in U.S. Patent 5,563,702 to Emery et al., U.S. Patent 7,352,457 to Kvamme et al., and U.S. Patent 7,528,943 to Brown et al., all of which are incorporated herein by reference.

[0094] Figure 8This describes an exemplary inspection system 800 comprising multiple objectives and one of the 183nm laser assemblies described above. In system 800, illumination from laser source 801 is sent to multiple sections of an illumination subsystem. The first section of the illumination subsystem comprises elements 802a to 806a. Lens 802a focuses the light from laser source 801. The light from lens 802a is then reflected from mirror 803a. Mirror 803a is placed in this position for illustrative purposes, but can be positioned elsewhere. The light from mirror 803a is then collected by lens 804a, forming an illumination pupil plane 805a. An aperture, filter, or other device for modifying the light may be placed in pupil plane 805a, depending on the needs of the inspection mode. The light from pupil plane 805a then passes through lens 806a and forms illumination field plane 807.

[0095] The second section of the illumination subsystem comprises elements 802b to 806b. Lens 802b focuses light from laser source 801. Light from lens 802b is then reflected from mirror 803b. Light from mirror 803b is then collected by lens 804b, forming illumination pupil plane 805b. Depending on the needs of the inspection mode, apertures, filters, or other devices for modifying the light may be placed in pupil plane 805b. Light from pupil plane 805b then passes through lens 806b and forms illumination field plane 807. Light from the second section is then redirected by mirrors or reflective surfaces so that the illumination field light energy at illumination field plane 807 is composed of the combined illumination sections.

[0096] The field plane light is then collected by lens 809 before being reflected away from beam splitter 810. Lenses 806a and 809 form an image of a first illumination pupil plane 805a at objective pupil plane 811. Similarly, lenses 806b and 809 form an image of a second illumination pupil plane 805b at objective pupil plane 811. Objective lens 812 (or alternatively 813) then acquires the pupil light and forms an image of illumination field 807 at sample 814. Objective lens 812 or objective lens 813 can be positioned close to sample 814. Sample 814 can be moved on a stage (not shown) that positions the sample in the desired location. Light reflected and scattered from sample 814 is collected by high NA reflective objective lens 812 or objective lens 813. After the reflected pupil is formed at objective pupil plane 811, the light energy passes through beam splitter 810 and lens 815 before forming internal field 816 in the imaging subsystem. This internal imaging field is an image of sample 814 and the corresponding illumination field 807. This field can be spatially separated into multiple fields corresponding to the illumination field. Each of these fields can support a separate imaging mode. For example, one imaging mode can be a bright-field imaging mode, while another can be a dark-field imaging mode.

[0097] Mirror 817 can be used to redirect one of these fields. The redirected light then passes through lens 818b before forming another imaging pupil 819b. This imaging pupil is the image of pupil 811 and the corresponding illumination pupil 805b. An aperture, filter, or other device for modifying the light may be placed in the pupil plane 819b, depending on the needs of the inspection mode. Light from the pupil plane 819b then passes through lens 820b and forms an image on sensor 821b. In a similar manner, light passing through mirror or reflective surface 817 is collected by lens 818a and forms imaging pupil 819a. Light from imaging pupil 819a is then collected by lens 820a before forming an image on detector 821a. The light imaged on detector 821a can be used for imaging modes different from the light imaged on sensor 821b.

[0098] The illumination subsystem used in system 800 consists of a laser source 801, light-collecting optics 802 to 804, a beam shaping assembly positioned close to the pupil plane 805, and relay optics 806 and 809. An internal field plane 807 is located between lenses 806 and 809. In a preferred configuration, the laser source 801 may comprise one of the 183nm lasers described above.

[0099] Regarding laser source 801, although it is described as a single uniform block with two transmission points or transmission angles, this actually means a laser source capable of providing two illumination channels. For example, the two illumination channels are, for example, a first channel of light energy of laser energy at a first frequency (e.g., a deep UV wavelength close to 183 nm) passing through elements 802a to 806a, and a second channel of light energy of laser energy at a second frequency (e.g., different harmonics from the same laser, such as the 4th or 5th harmonic, or light from a different laser) passing through elements 802b to 806b.

[0100] Although the light energy from laser source 801 is shown to be emitted at 90-degree intervals, and components 802a to 806a and 802b to 806b are oriented at 90-degree angles, in reality light can be emitted in various directions and is not necessarily two-dimensional, and the components can be oriented in a way different from that shown. Figure 8 Therefore, it is simply a representation of the components used, and the angles or distances shown are neither proportional nor specifically required by the design.

[0101] Elements positioned close to the pupil plane 805a / 805b can be used in current systems employing the aperture shaping concept. Using this design, uniform or near-uniform illumination, as well as individual point illumination, ring illumination, quadruple illumination, or other desired patterns, can be achieved.

[0102] Various implementation schemes for the objective lens can be employed in a general imaging subsystem. A single fixed objective lens can be used. This single objective lens can support all desired imaging and inspection modes. This design can be achieved if the imaging system supports a relatively large field size and a relatively high numerical aperture. The numerical aperture can be reduced to the desired value by using internal apertures placed at the pupil planes 805a, 805b, 819a, and 819b.

[0103] Other than that Figure 8 Multiple objectives are used as shown. For example, although two objectives 812 and 813 are shown, any number is possible. Each objective in this design can be optimized for each wavelength produced by the laser source 801. These objectives 812 and 813 can have a fixed position or be moved to a position close to the sample 814. To move multiple objectives closer to the sample, a rotating stage can be used, as is common on standard microscopes. Other designs for moving objectives closer to the sample are available, including, but not limited to, lateral translation of the objectives on the stage and lateral translation of the objectives along an arc using a goniometer. Additionally, any combination of fixed objectives and multiple objectives on a rotating stage can be implemented according to the system of the invention.

[0104] The maximum numerical aperture of this configuration can approach or exceed 0.97, but may be smaller in some instances. The wide range of illumination and focusing angles possible in this high-NA catadioptric imaging system, along with its large field size, allows the system to simultaneously support multiple inspection modes. As understood from the preceding paragraphs, multiple imaging modes can be implemented using a single optical system or machine in conjunction with the illumination setup. The high NA revealed by the illumination and focusing allows for the use of the same optical system to implement imaging modes, thereby enabling imaging optimization for different types of defects or samples.

[0105] The imaging subsystem also includes an intermediate image-forming optics 815. The purpose of the image-forming optics 815 is to form an internal image 816 of the sample 814. At this internal image 816, a mirror 817 can be placed to redirect light corresponding to one of the examination modes. Redirecting light at this location is possible because the light used for the imaging mode is spatially separated. The image-forming optics 818 (818a and 818b) and 820 (820a and 820b) can be implemented in several different forms, including zoom zoom devices, multiple telephoto lens barrels with focusing optics, or multiple image-forming zoom barrels. Additional details regarding system 800 are described in U.S. Patent No. 7,957,066, issued June 7, 2011 and incorporated herein by reference.

[0106] Figure 9This describes an exemplary catadioptric imaging system 900 configured with both bright-field and dark-field inspection modes. System 900 may incorporate two illumination sources: a laser 901 and a broadband illumination module 920. In one embodiment, laser 901 may comprise a 183nm laser as described herein.

[0107] In dark-field mode, light from laser 901 is directed to adaptation optics 902, which controls the size and profile of the laser illumination beam on the surface being inspected. Mechanical housing 904 includes apertures and windows 903 and a prism 905 to redirect incident laser light along the optical axis normal to the surface of sample 908. Prism 905 also directs specular reflections from surface features of sample 908 away from objective lens 906. Objective lens 906 collects the light scattered by sample 908 and focuses it onto sensor 909. Lenses for objective lens 906 may be provided in the general form of a catadioptric objective lens 912, a focusing lens group 913, and a barrel lens section 914, optionally including scaling capabilities.

[0108] In bright-field mode, the broadband illumination module 920 directs broadband light to a beam splitter 910, which reflects the light toward a focusing lens group 913 and a catadioptric objective 912. The catadioptric objective 912 illuminates the sample 908 with broadband light. Light reflected or scattered from the sample 908 is collected by objective 906 and focused onto sensor 909. The broadband illumination module 920 includes, for example, a laser-pumped plasma source or an arc lamp. The broadband illumination module 920 may also include an autofocus system to provide signals to control the height of the sample 908 relative to the catadioptric objective 912.

[0109] System 900 is further described in detail in U.S. Patent 7,345,825 entitled "Beamdelivery system for laser dark-field illumination in a catadioptric optical system" by Zhuang et al., U.S. Patent 8,665,536 entitled "External beam delivery system for laser dark-field illumination in a catadioptric optical system" by Armstrong, and U.S. Patent 8,896,917 entitled "External beamdelivery system using catadioptric objective with aspheric surfaces" by Armstrong, all of which are incorporated herein by reference.

[0110] Figure 10A This describes a surface inspection device 1000, comprising an illumination system 1001 and a light-gathering system 1010, used for inspecting the area of ​​surface 1011. For example... Figure 10A As shown, laser system 1020 guides beam 1002 through lens 1003. In a preferred embodiment, laser system 1020 includes one of the 183nm lasers described above, an annealed crystal, and a housing for maintaining the annealed state of the crystal during standard operation by protecting it from moisture or other environmental contamination. A first beam shaping optics may be configured to receive the beam from the laser and focus the beam onto an elliptical cross-section in or near the beam waist of the crystal.

[0111] Lens 1003 is oriented such that its principal plane is substantially parallel to the sample surface 1011, and thus illumination ray 1005 is formed on surface 1011 within the focal plane of lens 1003. Furthermore, beam 1002 and focused beam 1004 are guided to surface 1011 at a non-orthogonal angle of incidence. Specifically, beam 1002 and focused beam 1004 can be guided to surface 1011 at an angle between approximately 1 degree and approximately 85 degrees with respect to the normal direction. In this manner, illumination ray 1005 is substantially within the plane of incidence of focused beam 1004.

[0112] The light-collecting system 1010 includes a lens 1012 for collecting light scattered from the illumination line 1005, and a lens 1013 for focusing the light emitted from the lens 1012 onto a device including a photodetector array (e.g., a charge-coupled device (CCD) 1014). In one embodiment, the CCD 1014 may include a linear detector array. In such cases, the linear detector array within the CCD 1014 may be oriented parallel to the illumination line 1015. In one embodiment, the CCD 1014 may be an electron-bombarded CCD or a linear array of burst light detectors. In one embodiment, multiple light-collecting systems may be included, each of which includes similar but oriented components.

[0113] For example, Figure 10B An exemplary array of light-collecting systems 1031, 1032, and 1033 for a surface inspection apparatus is described (their illumination systems, such as those similar to illumination system 1001, are not shown for simplicity). A first optics element in light-collecting system 1031 collects light scattered from the surface of sample 1011 along a first direction. A second optics element in light-collecting system 1032 collects light scattered from the surface of sample 1011 along a second direction. A third optics element in light-collecting system 1033 collects light scattered from the surface of sample 1011 along a third direction. It should be noted that the first, second, and third paths are at different angles of incidence to the surface of sample 1011. A platform 1035 supporting sample 1011 can be used such that relative movement between the optics and sample 1011 allows scanning of the entire surface of sample 1011. U.S. Patent 7,525,649, issued to Leong et al. on April 28, 2009 and incorporated herein by reference, further describes the surface inspection apparatus 1000 and several other light-gathering systems in detail.

[0114] Figure 11 This describes a surface inspection system 1100 that can be used to inspect for anomalies on surface 1101. In this embodiment, surface 1101 may be partially illuminated by a substantially static illumination device of a laser system 1130, including one of the 183nm lasers described above. The output of the laser system 1130 may be continuously passed through a polarizing optics 1121, a beam expander and aperture 1122, and a beam shaping optics 1123 to expand and focus the beam.

[0115] The focused laser beam 1102 is then reflected by the beam folding assembly 1103 and the beam deflector 1104 to guide the beam 1105 toward the surface 1101 to illuminate the surface. In a preferred embodiment, the beam 1105 is substantially normal to or perpendicular to the surface 1101, but in other embodiments the beam 1105 may be at an angle to the surface 1101.

[0116] In one embodiment, the beam 1105 is substantially perpendicular to or normal to surface 1101, and the beam deflector 1104 reflects the specular reflection of the beam from surface 1101 towards the beam turning assembly 1103, thereby acting as a shield to prevent the specular reflection from reaching the detector. The direction of the specular reflection is along line SR, which is normal to the sample surface 1101. In one embodiment where the beam 1105 is normal to surface 1101, this line SR is aligned with the direction of the illumination beam 1105, wherein this common reference line or direction is referred to herein as the axis of the inspection system 1100. When the beam 1105 is at an angle to surface 1101, the direction of the specular reflection SR will not be consistent with the direction in which the beam 1105 enters; in this example, the line SR indicating that the direction of the surface is normal is referred to as the principal axis of the light-collecting portion of the inspection system 1100.

[0117] Light scattered by small particles is collected by mirror 1106 and directed toward aperture 1107 and detector 1108. Light scattered by large particles is collected by lens 1109 and directed toward aperture 1110 and detector 1111. It should be noted that some large particles will scatter light that is also collected and directed to detector 1108, and similarly, some small particles will scatter light that is also collected and directed to detector 1111, but this light will have a relatively low intensity compared to the intensity of the scattered light that the respective detector is designed to detect. In one embodiment, detector 1111 may comprise an array of photosensitive elements, each of which is configured to detect a corresponding portion of a magnified image of the illumination line. In one embodiment, the inspection system may be configured to detect defects on an unpatterned wafer. Inspection system 1100 is further described in detail in U.S. Patent 6,271,916, issued August 7, 2001 to Marx et al. and incorporated herein by reference.

[0118] Figure 12 This describes an inspection system 1200 configured to perform anomaly detection using both normal and tilted illumination beams. In this configuration, a laser system 1230, including one of the 183nm lasers described above, provides a laser beam 1201. A lens 1202 focuses the beam 1201 through a spatial filter 1203, and a lens 1204 collimates the beam and transmits it to a polarization beam splitter 1205. The beam splitter 1205 transmits a first polarization component to the normal illumination channel and a second polarization component to the tilted illumination channel, wherein the first and second components are orthogonal. In the normal illumination channel 1206, the first polarization component is focused by an optics device 1207 and reflected by a mirror 1208 toward the surface of a sample 1209. Radiation scattered by the sample 1209 is collected by a parabolic mirror 1210 and focused onto a detector or photomultiplier tube 1211.

[0119] In the tilted illumination channel 1212, the second polarization component is reflected by the beam splitter 1205 to the mirror 1213, which reflects this beam through the half-wave plate 1214 and focuses it onto the sample 1209 by the optics 1215. Radiation originating from the tilted illumination beam in the tilted channel 1212 and scattered by the sample 1209 is collected by the parabolic mirror 1210 and focused onto the detector or photomultiplier tube 1211. The detector or photomultiplier tube 1211 has a pinhole or slit entrance. The pinhole or slit and the illumination spot (from the normal and tilted illumination channels on the surface 1209) are preferably located at the focal point of the parabolic mirror 1210.

[0120] Parabolic mirror 1210 collimates the scattered radiation from sample 1209 into a collimated beam 1216. The collimated beam 1216 is then focused by objective lens 1217 and passes through analyzer 1218 to photomultiplier tube 1211. It should be noted that curved mirror surfaces with shapes other than parabolic can also be used. Instrument 1220 provides relative motion between the beam and sample 1209 such that the light spot is scanned across the surface of sample 1209. U.S. Patent 6,201,601, issued March 13, 2001 to Vaez-Iravani et al. and incorporated herein by reference, further details the testing system 1200.

[0121] Figure 13 This describes an exemplary pulse multiplier 1300 used in an inspection or measurement system (such as one of the inspection systems described above) with the 183nm laser described above. The pulse multiplier 1300 is configured to generate a pulse train from each input pulse 1301 from the 183nm laser (not shown). The input pulse 1301 is incident on a beam splitter 1307. A portion of each pulse is emitted by the beam splitter 1307 along the output direction 1302 and partially enters the ring cavity. As explained in U.S. Patent Application 13 / 711,593 (hereinafter referred to as '593 application), filed December 11, 2012, and incorporated herein by reference, entitled "Semiconductor inspection and metrology system using laser pulse multiplier," when used as a pulse rate multiplier, if the ring cavity and beam splitter 1307 are non-destructive, then the beam splitter 1307 should preferably transmit approximately one-third of the energy of each laser pulse and reflect approximately two-thirds back into the ring cavity. As explained in '593 application, these transmission and reflection values ​​may be modified to account for beam splitter and cavity losses in order to maintain substantially equal energy output pulses in the pulse rate multiplier.

[0122] After the laser pulse enters the annular cavity, it is reflected by curved mirror 1305 and guided towards curved mirror 1306. Mirror 1306 redirects the light back towards mirror 1305. This process involves multiple reflections from the two mirrors (…). Figure 13 In the example shown, after two reflections from each mirror, the pulse passes through compensator plate 1308 and returns to beam splitter 1307. Compensator plate 1308 is intended to compensate for the displacement of the laser pulse as it passes through beam splitter 1307 inside the annular cavity. Preferably, compensator plate 1308 has substantially the same thickness and refractive index as beam splitter 1307. If compensator plate 1308 and beam splitter 1307 are placed in the same portion of the annular cavity optical path (as shown), then compensator plate 1308 should preferably be oriented at an equal angle but opposite direction to beam splitter 1307 relative to the optical path. Alternatively, compensator plate 1308 may be suitably oriented in another portion of the annular cavity.

[0123] As explained in application '593, the annular cavity without beam splitter 1307 and compensator plate 1308 is similar to the annular cavity described in Herriott et al.'s "Off-axis Spherical Mirror Interferometers" (Applied Optics 3, #4, pp. 523-526 (1964)) and in Herriott et al.'s "Folded Optical Delay Lines" (Applied Optics 4, #8, pp. 883-889 (1965)). As described in these references, the number of reflections from each mirror depends only on the radius of curvature d of the gap between the two mirrors and not on the precise angle at which the light enters the annular cavity. For example, if the radius of curvature of the two mirrors is d (i.e., the focal length of each mirror is d / 2), then after two reflections from each mirror, each pulse will be refocused and will return to its starting point ( Figure 13(Beam splitter 1307 in the image). Heriot et al. (1964) gave values ​​for the focal length (and therefore the radius of curvature) of the mirrors as multiples of d for 2, 3, 4, 6, 12, and 24 reflections from each mirror. Other numbers of reflections are possible, as explained by Heriot et al. (1964). As described by Heriot et al. (1964), the reflections do not necessarily lie in a single plane; they depend on the number of reflections and the angle at which light enters the mirror 1305 from the beam splitter 1307. More than two reflections from each mirror make the cavity more compact than a cavity using two reflections from each mirror. However, since some light is lost in each mirror reflection, two reflections per mirror would be preferred when the mirror reflection loss is not so small (e.g., at deep UV wavelengths, for example), but more than two reflections per mirror are usable when the loss per reflection is small (e.g., at infrared, visible, or near UV wavelengths). It should be noted that the length of the annular cavity, and therefore the focusing of the annular cavity, can be adjusted by adjusting the distance d.

[0124] As the laser pulse travels back to beam splitter 1307 after crossing the cavity, a portion of the pulse is reflected out of the annular cavity along direction 1302, and a portion is emitted back into the annular cavity. Pulse multiplier 1300 refocuses the laser pulse (regardless of the beam waist position of the input laser pulse) such that the output pulse exiting along direction 1302 will exhibit a divergence and beam waist position that are substantially similar to the input pulse. In some preferred embodiments of pulse multiplier 1300, the input laser pulse from direction 1301 is substantially collimated to minimize the power density incident on beam splitter 1307. The output laser pulse is then also substantially collimated.

[0125] Periodically, a new input pulse 1301 is provided by a laser to a pulse multiplier 1300. In one embodiment, the laser can generate laser pulses of approximately 0.015 nanoseconds (ns) at a repetition rate of approximately 80 MHz, and the cavity can double said repetition rate. It should be noted that the optical path length of the annular cavity, and therefore the delay of the annular cavity, can be controlled by selecting the distance d between mirrors 1305 and 1306 and the radius of curvature, which controls the number of reflections while ensuring refocusing of the laser pulse.

[0126] The optical path length of the ring cavity can be slightly greater or slightly less than the nominal length calculated directly from the pulse interval divided by the multiplication factor. This results in not all pulses arriving at the polarization beam splitter exactly at the same time, thus slightly widening the output pulse. For example, when the input pulse repetition rate is 80 MHz, the cavity delay for a frequency multiplication of ×2 will nominally be 6.25 ns. In one embodiment, a cavity length corresponding to a delay of 6.27 ns can be used so that pulses with multiple reflections do not arrive exactly at the same time as the incoming pulse. Furthermore, a cavity length of 6.27 ns for an 80 MHz input pulse repetition rate can advantageously widen the pulse and reduce the pulse height. Other pulse multipliers with different input pulse rates or different multiplication factors can have different cavity delays.

[0127] Further details of pulse multipliers and alternative pulse multipliers suitable for use with 183nm lasers in inspection and metrology systems can be found in the aforementioned '593 application, U.S. Patent Application No. 13 / 487,075, filed June 1, 2012, entitled "Semiconductor Inspection and Metrology System Using Laser Pulse Multiplier," by Zhuang et al., and U.S. Patent Application No. 14 / 596,738, filed January 14, 2015, entitled "Laser Pulse Multiplication Using Prisms," by Zhuang et al. All of these applications are incorporated herein by reference.

[0128] In addition to the solution described above, which uses fundamental frequency light to generate a down-converted signal to produce 183nm laser output light, it is also possible to generate a suitable down-converted signal by down-converting second harmonic light. For example, Figure 14 The display includes Figure 1A and 1B The embodiments utilize several of the same components (and are therefore identified using the same reference numerals) of the laser assembly 1400. Specifically, the laser assembly 1400 includes a fundamental frequency laser 102 configured to generate fundamental frequency light 128 having a fundamental frequency wavelength ω, and a beam splitter 120 divides the fundamental frequency light 128 into portions 127 and 130, wherein portion 130 is directed to a fifth harmonic generator 103C. It should be noted that, alternatively, a similar approach may be used. Figure 1B The method depicted in the diagram obtains a portion 130 of the fundamental frequency light from the unconsumed fundamental frequency light output from the second harmonic generation module 153, which is then guided to the fifth harmonic generator. Additionally, similar to... Figure 1BThe method shown in the diagram includes a laser assembly 1400 comprising a second harmonic generation module 153 and a fourth harmonic generation module 155 that generate second harmonic light 175 and fourth harmonic light 162 emitted to a fifth harmonic generation module 103C. Finally, the laser assembly 1400 includes an OPS 116C for generating light at a down-converted frequency ω. s The down-converted signal 129 is used to generate laser output light 140 in the range of approximately 180 nm to approximately 185 nm when the down-converted signal 129 is subsequently mixed with the fifth harmonic light 134 in the mixer module 104.

[0129] According to an embodiment of the present invention, the laser assembly 1400 is different from... Figure 1A and 1B An embodiment of the OPS116C comprises a “green-pumped” optical parametric oscillator (OPO) 117C that receives and down-converts a second harmonic light portion 177 (which is divided from the output of the second harmonic generation module 153 by means of a beam splitter 174). At the various frequencies of the aforementioned common fundamental frequency laser (i.e., having corresponding wavelengths in the range from 1030 nm to 1064 nm), the second harmonic frequency 2ω of the second harmonic light portion 177 has a corresponding wavelength in the range of 515 nm to 532 nm, which is typically in the range associated with visible green light (i.e., 495 nm to 570 nm). Thus, the OPO 117C is “green-pumped” in the sense that its input is light in the visible green spectrum. Figure 14 As indicated by the dashed box in the lower left section, the OPO117C is otherwise constructed and configured in a manner similar to the OPO 117E (discussed above) to downconvert the second harmonic optical portion 177 to a suitable downconverted signal frequency (e.g., 532 nm to approximately 1.3 μm). That is, except for the nonlinear crystal 633C (discussed below), the optical components forming the continuous-wave single-resonant OPO arrangement utilized by the OPO 117C are substantially the same as those described above with reference to the OPO 117E, and thus, for simplicity, their description is not repeated here. The advantage of this method is that it avoids the need for a low-power seed signal (i.e., because downconversion of 532 nm light does not produce frequencies absorbed by most nonlinear crystals), thus simplifying the OPS 116C, as it only uses the OPO and the optional beam splitter 642C (which can be used to remove unwanted frequencies from the downconverted signal 129, such as in…). Figure 14 As shown in the figure, the down-converted signal 129 is generated.

[0130] Although the green pumped OPO method used in laser assembly 1400 has been successfully applied to generate lasers at down-converted frequencies ω using green pumped OPO 117C,s The down-converted signal 129 (e.g., 1.3 μm) is required to generate the 183 nm output laser, but using second harmonic (green) light to generate the down-converted signal 129 limits the types of nonlinear crystals that can be used in OPO117C, and the conversion efficiency of green light is lower than that of lower fundamental frequencies. That is, at high power levels, compared to higher frequencies (e.g., OPO 117E; see...) Figure 6B Many of the preferred nonlinear crystals (e.g., PPSLTs) used together are damaged by two-photon absorption of light in the visible green spectrum (e.g., 532 nm). To address this problem, the green-pumped OPO 117C preferably uses a lithium triborate (LBO) crystal to implement the nonlinear crystal 633C, because the LBO crystal has a larger band gap than lithium niobate or SLT, and therefore is not subject to damage caused by high power at the green light frequency. However, even when an LBO crystal (or another green light-tolerant crystal) is used in the OPO 117C, the downconversion of the green light produces an unwanted approximately 900 nm photon per 1.3 μm photon, thus more than half of the power delivered to the OPO 117C is lost, making the laser assembly 1400 less efficient than laser assemblies 100A and 100B (described above).

[0131] According to yet another possible embodiment, a result similar to Figure 1A The laser assembly shown in the paper replaces the OPS 116 with a conventional OPO utilizing lithium indium selenide (LISE) crystals. The inventors believe this method should work because LISE crystals are considered to have weak absorption at frequencies around 6 μm and therefore should not significantly deform or undergo damage due to heating. However, LISE crystals are novel, and the availability of sufficiently high-quality LISE crystals is currently uncertain.

[0132] The 183nm laser described herein can be used in conjunction with optics in inspection or measurement systems to shape pulses, reduce coherence, or reduce speckle. Further details of the pulse shaping, coherence, and speckle reduction apparatus and methods are disclosed in U.S. Patent No. 9,080,990, issued July 14, 2015, and U.S. Patent No. 9,080,991, also issued July 14, 2015. Both patents are incorporated herein by reference.

[0133] The various embodiments of the structures and methods described herein are merely illustrative of the principles of the invention and are not intended to limit the scope of the invention to the specific embodiments described. For example, nonlinear crystals other than CLBO, LBO, or BBO or periodically polarized materials can be used in some stages of frequency conversion, harmonic generation, and mixing stages.

Claims

1. A laser assembly for generating laser output light having an output wavelength in the range of 180 nm to 185 nm, the laser assembly comprising: A fundamental frequency laser, configured to produce fundamental frequency light with a fundamental frequency; An optical parameter system (OPS) coupled to the fundamental frequency laser such that the OPS receives a first portion of the fundamental frequency light, and the OPS is configured to generate a down-converted signal having a down-converted frequency less than the fundamental frequency. A fifth harmonic generator coupled to the fundamental frequency laser such that the fifth harmonic generator receives a second portion of the fundamental frequency light, and the fifth harmonic generator is configured to generate fifth harmonic light having a fifth harmonic frequency equal to five times the fundamental frequency. and A mixing module, optically coupled to receive the down-converted signal from the OPS and the fifth harmonic light from the fifth harmonic generator, and configured to generate the laser output light by mixing the down-converted signal and the fifth harmonic light. The OPS mentioned therein includes: A downconverted seed signal generator is configured to generate a downconverted seed signal having the downconverted frequency and a first power level. and An optical parametric amplifier (OPA) is configured such that the down-converted seed signal and a portion of the fundamental frequency light are mixed once by passing through a nonlinear crystal, wherein the nonlinear crystal is configured such that the mixing produces the down-converted signal at a second power level greater than ten times the first power level. The OPS is configured such that the sum of the down-conversion frequency and the fifth harmonic frequency produces laser output light in the range between 180 nm and 185 nm. The fifth harmonic generator and at least one of the mixer module include one of an annealed CLBO crystal, a deuterated CLBO crystal, and a hydrogen-treated CLBO crystal, the at least one being configured to approach non-critical phase matching to allow stable output of a power level in the range of 1 W to 20 W or a power level greater than 1 W to 20 W.

2. The laser assembly of claim 1, wherein the fundamental frequency laser is configured to generate the fundamental frequency light at the fundamental frequency having a corresponding wavelength of one of 1064 nm, 1053 nm, 1047 nm and 1030 nm.

3. The laser assembly of claim 1, wherein the OPA comprises: A beam combiner configured to combine the first portion of the fundamental frequency light with the down-converted seed signal; A nonlinear crystal configured to receive the down-converted seed signal and amplify the down-converted seed signal by stimulated down-conversion of the first portion of the fundamental frequency light; and A beam splitter configured to separate the down-converted signal from other frequencies received from the nonlinear crystal and to direct the down-converted signal to the mixer module.

4. The laser assembly of claim 3, wherein the down-converted seed signal generator is configured to generate the down-converted seed signal of the first power level in the range of 1 mW to 500 mW, and wherein the OPA is configured to generate the down-converted signal of the second power level in the range of 1 W to 20 W.

5. The laser assembly of claim 3, wherein the beam combiner is further configured to reflect the first wavelength while transmitting the second wavelength, such that the fundamental frequency portion is emitted collinearly with the down-converted seed signal through the nonlinear crystal.

6. The laser assembly of claim 5, wherein the nonlinear crystal comprises one of the following: periodically polarized lithium niobate PPLN, periodically polarized magnesium oxide-doped lithium niobate, periodically polarized stoichiometric lithium tantalate PPSLT, periodically polarized magnesium oxide-doped stoichiometric lithium tantalate, and periodically polarized potassium titanate phosphate PPKTP.

7. The laser assembly of claim 1, wherein the down-converted seed signal generator of the OPS comprises a diode laser.

8. The laser assembly according to claim 1, The OPS further includes a beam splitter configured to split the first portion of the fundamental frequency light into a first sub-portion and a second sub-portion, and The down-converted seed signal generator includes an optical parametric oscillator (OPO) configured to generate the down-converted seed signal by converting the first sub-part. The OPA is configured to mix the down-converted seed signal with the second sub-part.

9. The laser assembly according to claim 8, The down-converted signal has a down-converted wavelength corresponding to the down-converted frequency. The OPO includes a first focusing mirror, a nonlinear crystal, a second focusing mirror, a wavelength selector, and an output coupler operably configured to form a cavity, in which light is reflected between the wavelength selector and the output coupler by means of the first and second focusing mirrors and the nonlinear crystal. The wavelength selector is configured to have high reflectivity for light having a wavelength centered at 0.2 nm, which is the down-converted wavelength. The output coupler is configured to transmit a portion of the light reflected between the wavelength selector and the output coupler as the down-converted seed signal.

10. The laser assembly of claim 9, wherein the OPS comprises: A beam combiner configured to combine the second sub-section with the down-converted seed signal; and A nonlinear crystal configured to amplify the down-converted seed signal by stimulated down-conversion of the second sub-section. The optical path length from the beam splitter to the beam combiner is set such that the pulse of the second sub-part arrives at the beam combiner simultaneously with the pulse of the down-converted seed signal.

11. The laser assembly of claim 1, wherein the optical parameter system is configured such that the down-converted signal has a signal wavelength in the range of 1250 nm to 1830 nm.

12. The laser assembly of claim 11, wherein the fundamental frequency laser is configured such that the fundamental frequency has a corresponding wavelength of 1064.4 nm, and wherein the optical parameter system is configured such that the down-converted signal has a signal wavelength in the range of 1250 nm to 1420 nm.

13. The laser assembly of claim 11, wherein the fundamental frequency laser is configured such that the fundamental frequency has a corresponding wavelength of 1030 nm, and wherein the optical parameter system is configured such that the down-converted signal has a signal wavelength in the range of 1400 nm to 1830 nm.

14. The laser assembly of claim 11, wherein the fundamental frequency laser comprises one of a mode-locked laser, a quasi-continuous wave laser, a laser diode, and a fiber laser.

15. An inspection system comprising: A laser assembly configured to produce laser output light having an output wavelength in the range of 180 nm to 185 nm; A first optical device is configured to guide the laser output light from the laser assembly to the object being inspected; A second optical device is configured to collect an image portion of the laser output light affected by the object under inspection, and to guide the image portion to one or more sensors. The laser assembly includes: A fundamental frequency laser, configured to produce fundamental frequency light with a fundamental frequency; An optical parameter system (OPS) coupled to the fundamental frequency laser such that the OPS receives a first portion of the fundamental frequency light, and the OPS is configured to generate a down-converted signal having a down-converted frequency less than the fundamental frequency. A fifth harmonic generator coupled to the fundamental frequency laser such that the fifth harmonic generator receives a second portion of the fundamental frequency light, and the fifth harmonic generator is configured to generate fifth harmonic light having a fifth harmonic frequency equal to five times the fundamental frequency; and A mixing module, optically coupled to receive the down-converted signal from the OPS and the fifth harmonic light from the fifth harmonic generator, and configured to generate the laser output light by mixing the down-converted signal and the fifth harmonic light. The OPS mentioned therein includes: A down-converted seed signal generator, configured to generate a down-converted seed signal having the said down-converted frequency and a first power level; and An optical parametric amplifier (OPA) is configured to mix the down-converted seed signal with a portion of the fundamental frequency light, such that the mixing produces the down-converted signal at a second power level greater than ten times the first power level. The OPS is configured such that the sum of the down-conversion frequency and the fifth harmonic frequency produces laser output light in the range between 180 nm and 185 nm. The fifth harmonic generator includes at least one of an annealed CLBO crystal, a deuterated CLBO crystal, and a hydrogen-treated CLBO crystal, wherein the at least one is configured to approach non-critical phase matching to allow stable output of a power level in the range of 1 W to 20 W or a power level greater than 1 W to 20 W.

16. The inspection system of claim 15, wherein the inspection system includes a dark field inspection system.

17. The inspection system of claim 15, further comprising at least one of an acousto-optic modulator and an electro-optic modulator configured to reduce the coherence of the laser output light directed to the object under inspection.

18. The inspection system of claim 15, further comprising a pulse rate multiplier configured to increase the pulse repetition rate of the laser assembly.

19. The inspection system of claim 15, wherein the second optics is configured to simultaneously guide the reflected image portion and the transmitted image portion to a single sensor.

20. The inspection system of claim 15, wherein the first optical device comprises one or more components configured to guide the laser output light such that the laser output light forms an illumination line on the object being inspected.

21. The inspection system of claim 15, wherein the first optical device comprises one or more components configured to guide the laser output light such that the laser output light forms a plurality of simultaneously illuminated spots on the object being inspected.

22. The inspection system of claim 15, wherein the optical parameter system is configured such that the down-converted signal has a signal wavelength in the range of 1250 nm to 1830 nm.

23. A method for generating light with wavelengths between 180 nm and 185 nm, the method comprising: It generates fundamental frequency light at the fundamental frequency corresponding to the fundamental wavelength of 1064.4 nm; The first part of the fundamental frequency light is downconverted, so that the downconversion generates a downconverted signal with a corresponding downconverted wavelength between 1250 nm and 1420 nm. The second part of the fundamental frequency light is converted such that the conversion produces fifth harmonic light at the fifth harmonic frequency of the fundamental frequency; and The fifth harmonic light is mixed with the down-converted signal such that the mixture produces an output laser with an output wavelength corresponding to the sum of the down-converted frequency and the fifth harmonic frequency. The down-conversion of the first portion of the fundamental frequency light includes: Generate a down-converted seed signal having the down-converted frequency and the first power level; and The down-converted seed signal is mixed with the first portion of the fundamental frequency light, such that the mixing of the down-converted seed signal produces the down-converted signal at a second power level ten times greater than the first power level. The mixing of the fifth harmonic light and the down-converted signal includes passing the fifth harmonic light and the down-converted signal through at least one of an annealed CBLO crystal, a deuterated CLBO crystal, and a hydrogen-treated CLBO crystal, wherein the at least one is configured to approach non-critical phase matching to allow stable output of a power level in the range of 1 W to 20 W or a power level greater than 1 W to 20 W.

24. A method for generating light with wavelengths between 180 nm and 185 nm, the method comprising: It generates fundamental frequency light at a fundamental frequency corresponding to the fundamental wavelength of 1030 nm; The first part of the fundamental frequency light is downconverted, so that the downconversion generates a downconverted signal with a corresponding downconverted wavelength between 1400 nm and 1830 nm. The second part of the fundamental frequency light is converted such that the conversion produces fifth harmonic light at the fifth harmonic frequency of the fundamental frequency; and The fifth harmonic light is mixed with the down-converted signal such that the mixture produces an output laser with an output wavelength corresponding to the sum of the down-converted frequency and the fifth harmonic frequency. The down-conversion of the first portion of the fundamental frequency light includes: Generate a down-converted seed signal having the down-converted frequency and the first peak power level; and The down-converted seed signal is mixed with the first portion of the fundamental frequency light, such that the mixing of the down-converted seed signal produces the down-converted signal at a second peak power level that is ten times greater than the first peak power level. The mixing of the fifth harmonic light and the down-converted signal includes passing the fifth harmonic light and the down-converted signal through at least one of an annealed CBLO crystal, a deuterated CLBO crystal, and a hydrogen-treated CLBO crystal, wherein the at least one is configured to approach non-critical phase matching to allow stable output of a power level in the range of 1 W to 20 W or a power level greater than 1 W to 20 W.

25. A method for testing a sample, the method comprising: It generates fundamental frequency light at a fundamental frequency corresponding to the fundamental wavelength of 1030 nm; The first part of the fundamental frequency light is downconverted, so that the downconversion generates a downconverted signal with a corresponding downconverted wavelength between 1400 nm and 1830 nm. The second part of the fundamental frequency light is converted such that the conversion produces fifth harmonic light at the fifth harmonic frequency of the fundamental frequency; The fifth harmonic light is mixed with the down-converted signal such that the mixture produces an output laser with an output wavelength corresponding to the sum of the down-converted frequency and the fifth harmonic frequency; Guide the output laser onto the object being inspected; and Collect an image portion of the output laser light affected by the object being inspected, and guide the image portion to one or more sensors. The down-conversion of the first portion of the fundamental frequency light includes: Generate a down-converted seed signal having the down-converted frequency and the first power level; and The down-converted seed signal is mixed with the first portion of the fundamental frequency light, such that the mixing of the down-converted seed signal produces the down-converted signal at a second power level ten times greater than the first power level. The mixing of the fifth harmonic light and the down-converted signal includes passing the fifth harmonic light and the down-converted signal through at least one of an annealed CBLO crystal, a deuterated CLBO crystal, and a hydrogen-treated CLBO crystal, wherein the at least one is configured to approach non-critical phase matching to allow stable output of a power level in the range of 1 W to 20 W or a power level greater than 1 W to 20 W.

26. A laser assembly for generating laser output light having an output wavelength in the range of 180 nm to 185 nm, the laser assembly comprising: A fundamental frequency laser configured to generate fundamental frequency light with a fundamental wavelength of 1064 nm and a corresponding fundamental frequency; A second harmonic generator coupled to the fundamental frequency laser such that the second harmonic generator receives a first portion of the fundamental frequency light, the second harmonic generator being configured to generate second harmonic light having a second harmonic frequency equal to twice the fundamental frequency. The fourth harmonic generation module is coupled to receive a first portion of the second harmonic light and generate fourth harmonic light having a fourth harmonic frequency equal to four times the fundamental frequency. An optical parameter system (OPS) coupled to the fundamental laser such that the OPS receives a second portion of the second harmonic light, and the OPS is configured to generate a down-converted signal having a down-converted frequency less than the fundamental frequency. A fifth harmonic generator coupled to the fundamental frequency laser such that the fifth harmonic generator receives a second portion of the fundamental frequency light and the fourth harmonic light, and the fifth harmonic generator is configured to generate fifth harmonic light having a fifth harmonic frequency equal to five times the fundamental frequency. and A mixing module, optically coupled to receive the down-converted signal from the OPS and the fifth harmonic light from the fifth harmonic generator, and configured to generate the laser output light by mixing the down-converted signal and the fifth harmonic light. The fundamental frequency laser is configured to generate the fundamental frequency light such that the second portion of the second harmonic light includes visible green light. The OPS includes a green pump-excited optical parametric oscillator configured to generate the down-converted signal by down-converting the second portion of the second harmonic light, and The OPS is configured such that the sum of the down-conversion frequency and the fifth harmonic frequency produces laser output light in the range of 180 nm to 185 nm. The down-converted signal has a down-converted wavelength corresponding to the down-converted frequency. The green pump-driven OPS includes a first focusing mirror, a nonlinear crystal, a second focusing mirror, a wavelength selector, and an output coupler operably configured to form a cavity in which light is reflected between the wavelength selector and the output coupler by means of the first and second focusing mirrors and the nonlinear crystal. The wavelength selector is configured to have high reflectivity for light having a wavelength centered at 0.2 nm, which is the down-converted wavelength. The output coupler is configured to transmit a portion of the light reflected between the wavelength selector and the output coupler as the down-converted signal, and The nonlinear crystal mentioned above includes lithium triborate (LBO) crystal.

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