Tunable DUV Laser Assembly

Abstract

The tunable laser assembly selectively generates laser output light at two or more output wavelengths in the range of 184 nm to 200 nm using a fundamental wavelength of 1 μm to 1.1 μm by directing fundamental light to different regions of a fanned periodically polled nonlinear crystal to generate corresponding different down-converted signals and mixing the different down-converted signals with a fifth harmonic of the fundamental wavelength using different nonlinear adding crystals. Each nonlinear adding crystal has a crystal axis aligned at an angle to the light propagation direction to facilitate efficient transmission and summing with the fifth harmonic and associated down-converted signals. In response to a user-selected output wavelength, the frequency control system positions the fanned periodically polled nonlinear crystal to generate a corresponding down-converted signal frequency and positions the associated nonlinear adding crystal to receive the fifth harmonic and the corresponding down-converted signal.

Description

[Technical Field]

[0001] This application relates to lasers capable of generating light having deep ultraviolet (DUV) wavelengths, and more particularly to tunable lasers capable of generating light having two or more wavelengths in the range of approximately 184 nm to approximately 200 nm, and to inspection systems using such lasers to inspect, for example, photomasks, reticles, and semiconductor wafers. [Background technology]

[0002] Related Applications This application claims priority to U.S. Provisional Patent Application No. 63 / 209,413, entitled "DUV Tunable Laser Using OPO," filed June 11, 2021, and incorporated herein by reference.

[0003] This application is related to U.S. Patent No. 10,199,149 to Chuang et al., U.S. Patent No. 9,748,729 to Chuang et al., U.S. Patent No. 9,529,182 to Chuang et al., and U.S. Patent No. 9,608,399 to Chuang et al., all of which are incorporated herein by reference.

[0004] As the dimensions of semiconductor devices decrease, the size of the smallest particle or pattern defect that can damage the device also decreases. Thus, a need arises to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. Generally, the intensity of light scattered by particles smaller than the wavelength of light increases as the power of the particle's size increases (e.g., the total scattered light intensity of a small, loose, spherical particle increases proportionally to the sixth power of the spherical diameter and inversely proportional to the fourth power of the wavelength). Because of the increased scattered light intensity, shorter wavelengths generally offer greater sensitivity for detecting small particles and defects than longer wavelengths.

[0005] The intensity of light scattered from small particles and defects is typically low, requiring high illumination intensities to generate signals that can be detected in very short periods of time. Average source power levels of 0.3 W or higher may be required. At these high average power levels, high pulse repetition rates are desirable because the higher the repetition rate, the lower the energy per pulse and therefore the lower the risk of damage to the system optics and the item being inspected. In general, continuous-wave (CW) sources best satisfy the illumination needs and metrology of an inspection. CW sources have a constant power level, which avoids the damage issues associated with peak power and allows for continuous acquisition of images or data. However, in some cases, mode-locked lasers with repetition rates of approximately 50 MHz or higher may be useful because the high repetition rate means that the energy per pulse can be low enough to avoid damage in certain metrology and inspection applications.

[0006] Pulsed lasers that generate deep ultraviolet (DUV) light are known in the art. Prior art excimer lasers that generate 193 nm light are well known. However, such lasers are not well suited for inspection applications because of their low pulse repetition rate and the use of corrosive gases that are harmful to the laser medium, resulting in a high cost of ownership. A few solid-state and fiber-based lasers that generate light close to 193 nm output are known in the art. Exemplary lasers use two different fundamental wavelengths (e.g., U.S. Patent No. 6,229,999 to Lei et al.) or the eighth harmonic of the fundamental (e.g., U.S. Patent No. 6,229,999 to Tokuhisa et al.), both of which require expensive or non-mass-produced lasers or materials. Another approach (e.g., U.S. Patent No. 6,229,999 to Mead et al.) has not resulted in a commercial product with the stable output and high power required for semiconductor inspection applications (typically, approximately 0.3 W or higher is required for a laser that can operate continuously for three months or more between inspection events). Furthermore, most of these lasers have very low power outputs and are limited to laser pulse repetition rates of a few MHz or less. Chuang et al. (US Pat. No. 5,629,493) disclose a 183 nm mode-locked laser and an associated inspection system.

[0007] Pulsed sources have instantaneous peak power levels that are much higher than the time-average power levels of CW sources. The very high peak power of the laser pulses can cause damage to the optics and the sample being measured, i.e., the wafer, because most damage processes are nonlinear and depend predominantly on the peak power rather than the average power. The higher the pulse repetition rate, the lower the instantaneous peak power per pulse for the same time-average power level.

[0008] None of the existing conventional laser assemblies allow users to selectively adjust (i.e., change or tune) the laser output light between two significantly different DUV wavelengths (i.e., wavelengths that differ by more than 5 nanometers). Adjusting a single laser assembly to produce two or more different DUV wavelengths (e.g., so that the laser outputs light having a wavelength of approximately 184 nm during a first period and a wavelength of approximately 194 nm during a second period) would significantly improve the inspection system's ability to detect specific defects that cannot be detected or accurately identified using a single DUV output wavelength. That is, when inspecting a wafer / reticle using two different DUV wavelengths, the shape, size, and / or material of a given defect may produce substantially more reflection at the first DUV wavelength than at the second DUV wavelength. Similarly, the reflectivity of the pattern surrounding the defect may differ at the two DUV wavelengths. Therefore, selecting the appropriate wavelengths allows for improved contrast between the defect and the pattern, further improving defect detection. Because conventional DUV laser assemblies only produce a single wavelength (e.g., approximately 193 nm), wafer / reticle inspection systems using such lasers may not be able to detect or accurately identify critical defects on the wafer / reticle. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] US Patent Application Publication No. 2014 / 0111799 [Patent Document 2] U.S. Patent No. 7,623,557 [Patent Document 3] U.S. Patent No. 5,742,626 [Patent Document 4] US Patent Application Publication No. 2016 / 0099540 Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, a need arises for a user-tunable mode-locked or CW laser that produces laser output light at two or more DUV wavelengths, particularly in the range of about 184 nm to about 200 nm, and a need arises for an inspection system and related method that produces mode-locked or CW tunable laser light having two or more output DUV wavelengths in the range of about 184 nm to about 200 nm, and that avoids some or all of the problems and difficulties discussed above. [Means for solving the problem]

[0011] The present invention relates to improvements to inspection systems used in the semiconductor manufacturing industry, and more particularly, to a tunable laser assembly and related method for such inspection systems. The tunable laser assembly and related method utilize a tunable fan-shaped periodically poled nonlinear optical crystal (fan-shaped PPNLC) combined with a set of two or more replaceable nonlinear addition crystals. The set of two or more replaceable nonlinear addition crystals is operably controlled by a frequency control system to generate laser light having a source power level of 0.3 W or greater and a user-selected output frequency / wavelength that varies within a range of approximately 184 nm to approximately 200 nm (e.g., in response to an output frequency signal provided by a user). The tunable laser assembly utilizes one or more fundamental lasers to generate fundamental light having a wavelength of approximately 1 μm to 1.1 μm and utilizes a fifth harmonic generator to convert a portion of the fundamental light to fifth harmonic light. The fan-shaped PPNLC is movably positioned relative to a second portion of the fundamental light, configured to generate two or more different down-conversion frequencies when the second fundamental light portion is directed to two or more corresponding different regions of the fan-shaped PPNLC crystal. The nonlinear add crystals are provided with corresponding different crystal axis orientations to facilitate efficiently summing the fifth-harmonic light with a wide range of different down-conversion frequencies (e.g., the crystal axis orientation of a first add crystal is configured to efficiently handle a relatively low down-conversion frequency, and the crystal axis orientation of a second add crystal is configured to efficiently handle a relatively high down-conversion frequency). In response to different output frequency signal values ​​provided by a user, the frequency control system positions / orients the various crystals so that the fan-shaped PPNLC generates down-converted light at the selected down-conversion frequency. The down-conversion frequency, when summed with the fifth-harmonic light by the selected corresponding nonlinear add crystal, generates DUV laser light at a selected DUV wavelength within a range of approximately 184 nm to approximately 200 nm.The use of a fan-shaped PPNLC in combination with two or more replaceable nonlinear addition crystals allows the tunable laser assembly to switch the laser output light between two or more different DUV wavelengths that change by 5 nm or more during wafer / reticle inspection, thereby significantly improving detection of specific defects (e.g., by increasing the contrast between each defect and the surrounding defect-free pattern), and thus allowing the user to detect / analyze defects that may not be detectable using a single DUV frequency (e.g., only 184 nm light, or only 194 nm light). Note that when wavelengths are referred to without limitation in the following description, it may be assumed that the wavelengths in question are wavelengths in vacuum.

[0012] According to exemplary embodiments of the present invention, fifth-harmonic light is generated by a fifth-harmonic generation stage configured to mix fundamental light with fourth-harmonic light generated by a fourth-harmonic generation module. In some embodiments, the fourth-harmonic generation module is implemented using two frequency doubling stages. In one embodiment, the first frequency doubling stage generates second-harmonic light using a lithium triborate (LBO) crystal. LBO crystals can be substantially non-critically phase-matched (with appropriate selection of crystal planes) at temperatures between room temperature and about 200°C to generate second-harmonic light in the wavelength range of about 515 nm to about 535 nm. In alternative embodiments, the first frequency doubling stage may include a cesium lithium borate (CLBO) crystal or a beta barium borate (BBO) crystal, either of which can be critically phase-matched to generate second-harmonic light in the wavelength range of about 515 nm to about 535 nm. In other alternative embodiments, the first frequency doubling stage may include KTiOPO4 (KTP), periodically poled lithium niobate (PPLN), periodically poled stoichiometric lithium tantalate (PPSLT), or other nonlinear crystals for frequency conversion. In one embodiment, the second frequency doubling stage generates fourth-harmonic light using critical phase matching in CLBO, BBO, or other nonlinear crystal materials. In a preferred embodiment, the second frequency doubling stage includes a hydrogen- or heavy water-treated CLBO crystal. In alternative embodiments, the fifth harmonic generation stage is configured to circulate the fundamental light without a cavity using a nonlinear crystal, as in a pulsed laser, or in a cavity configured to resonate at the fundamental frequency using a cavity for a CW laser, thereby directly mixing the fundamental light and the fourth harmonic light by passing the circulated fundamental light through a nonlinear crystal, and directing the fourth harmonic light so that it also passes through the nonlinear crystal (i.e., without circulating in a cavity) in a manner that combines the fourth harmonic light and the circulated first fundamental light to generate the fifth harmonic light. These particular apparatus provide a fifth harmonic generator that can generate fifth harmonic light at power levels suitable for inspection systems.

[0013] According to an embodiment, the tunable laser assembly includes an optical parameter system (OPS). The OPS is configured to cycle the down-conversion frequency through the fan-shaped PPNLC to further enhance the power of the down-converted light passed through a set of nonlinear add crystals. In an alternative embodiment, the OPS is implemented as either an optical parametric oscillator (OPO) or an optical parametric amplifier (OPA) configured to generate a down-converted signal at a down-conversion wavelength between about 1350 nm and about 3300 nm, which, when mixed with fifth-harmonic light based on a fundamental frequency of 1064 nm, results in the tunable laser assembly generating laser output light at a wavelength between about 184 nm and about 200 nm. In one embodiment, the OPS includes both a PPNLC and a reflective wavelength selector (e.g., a volume Bragg grating, a distributed Bragg reflector, or a Littrow grating). In another embodiment, the OPS includes both a PPNLC and a transmissive wavelength selector (e.g., a transmission grating or an etalon). In some embodiments, small adjustments to the wavelength of the laser output may be made by adjusting the temperature of the wavelength selector to change the down-conversion frequency.

[0014] According to an embodiment of the present invention, DUV laser output light is generated by mixing down-converted laser light with fifth-harmonic light using a selected nonlinear adder crystal of a nonlinear adder crystal set. Frequency addition is preferably performed within a cavity through which the down-converted signal circulates. In a CW laser, the cavity is configured to resonate at the down-conversion frequency and direct the circulated down-converted signal into the selected nonlinear adder crystal. In a pulsed laser, the cavity length is configured so that the circulating pulse overlaps the input pulse. In either case, the fifth-harmonic light is directed through the selected nonlinear adder crystal in a manner that combines the fifth-harmonic light and the circulated down-converted laser light (i.e., without circulating). By providing a cavity configured to circulate light at the down-conversion frequency, the power of light at the down-conversion frequency within the nonlinear adder crystal is significantly increased compared to without the cavity, thereby improving conversion efficiency.

[0015] According to another embodiment, the tunable laser utilizes a single cavity device configured to selectively recycle unconsumed down-converted light exiting one of the nonlinear adder crystals, whereby the unconsumed down-converted light is redirected to the fan-shaped PPNLC. By arranging a single set of mirrors to recycle light at the down-converted frequency and directing the down-converted light along a continuous path to both the nonlinear adder crystal set and the fan-shaped PPNLC, the single cavity device simplifies the laser and improves conversion efficiency compared to multiple cavity embodiments.

[0016] In one embodiment, an inspection system configured to inspect a specimen such as a wafer, reticle, or photomask includes one of the lasers described herein that produce an input wavelength in the range of approximately 184 nm to approximately 200 nm. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a simplified block diagram illustrating a simplified tunable laser assembly according to a general embodiment of the present invention. [Figure 1A] FIG. 2 is a simplified perspective view showing a portion of the tunable laser assembly of FIG. 1 in operation. [Figure 1B] FIG. 2 is a simplified perspective view showing a portion of the tunable laser assembly of FIG. 1 in operation. [Figure 2] 1 is a block diagram illustrating a tunable laser assembly in accordance with certain exemplary embodiments of the present invention. [Figure 3] FIG. 3 is a simplified diagram illustrating an example OPO configured to generate a down-converted signal used in a pulsed laser version of the tunable assembly of FIG. 2 in accordance with an example embodiment of the present invention. [Figure 4] 3 is a simplified diagram illustrating an example OPO / OPA configured to generate a down-converted signal for use in a CW laser version of the tunable assembly of FIG. 2 in accordance with another example embodiment of the present invention. [Figure 5] 3 is a simplified diagram illustrating an exemplary frequency summing stage utilized in the variable assembly of FIG. 2, in accordance with an exemplary embodiment of the present invention. [Figure 6] FIG. 10 is a simplified diagram illustrating a partially tunable laser assembly according to an alternative embodiment in which the fan-shaped PPNLC and the nonlinear add crystal set are included in a single cavity device. [Figure 7A] 7A-7C are simplified diagrams illustrating a portion of the tunable laser assembly of FIG. 6 in various operating states. [Figure 7B] 7A-7C are simplified diagrams illustrating a portion of the tunable laser assembly of FIG. 6 in various operating states. [Figure 7C] 7A-7C are simplified diagrams illustrating a portion of the tunable laser assembly of FIG. 6 in various operating states. [Figure 7D] 7A-7C are simplified diagrams illustrating a portion of the tunable laser assembly of FIG. 6 in various operating states. [Figure 8] FIG. 2 is a simplified diagram illustrating an exemplary inspection system with dark-field and bright-field inspection modes utilizing one of the laser assemblies described herein, in accordance with another specific embodiment of the present invention. [Figure 9A] 1 illustrates a dark field inspection system, each utilizing one of the laser assemblies described herein, according to another specific embodiment of the present invention. [Figure 9B] 1 illustrates a dark field inspection system, each utilizing one of the laser assemblies described herein, according to another specific embodiment of the present invention. [Figure 10] 1 illustrates an alternative dark-field inspection system configured to inspect an unpatterned wafer using one of the laser assemblies described herein, according to another specific embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to an improved laser for a semiconductor inspection system. The following description is presented to enable one skilled in the art to make and use the invention in the context of a particular application and its requirements. As used herein, directional terms such as "top," "left," "right," "horizontal," and "bottom" are intended to provide relative positions for descriptive purposes and are not intended to indicate an absolute framework of reference. Various modifications to the described embodiments will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0019] FIG. 1 illustrates a variable output frequency ω 100 having a corresponding wavelength selectively tunable in a range from approximately 184 nm to approximately 200 nm, according to a general embodiment. OUT 1 is a simplified block diagram illustrating a tunable laser assembly 100 configured to generate pulsed or CW deep ultraviolet (DUV) output light 159 having a wavelength of approximately 1350 nm to approximately 3300 nm. Generally, the tunable laser assembly 100 comprises one or more fundamental lasers 110, a fifth harmonic generator 130, a fan-shaped periodically poled nonlinear crystal (PPNLC) 145, an optional wavelength selector 147, a set of nonlinear addition crystals 155, and a frequency control system 160. In the exemplary embodiment described below, the fundamental laser 110 is configured to generate first and second fundamental frequencies ω and ω having an infrared fundamental wavelength of approximately 1064 nm, and the fan-shaped PPNLC 145 generates down-converted frequencies ω and ω having corresponding wavelengths between approximately 1350 nm and approximately 3300 nm. x and the nonlinear summing crystal set 155 is configured to generate a down-converted signal 149 having an output frequency ω having a wavelength in the range of approximately 184 nm to approximately 200 nm. OUT The laser output 159 includes at least two nonlinear addition crystals cooperatively configured to generate a laser output 159 having a wavelength of 1000 nm.

[0020] Fundamental laser 110 is configured using known techniques to generate fundamental light beams 119-1 and 119-2 (simply referred to in the industry as "fundamental") having corresponding fundamental frequencies ω1 and ω2 with corresponding fundamental wavelengths of approximately 1 μm to 1.1 μm. In an exemplary embodiment, fundamental laser 110 is implemented using one of a Nd:YAG (neodymium-doped yttrium aluminum garnet) laser medium, a Nd-doped yttrium orthovanadate laser medium, and a yttrium-doped (Yb-doped) fiber laser medium, or with a fiber amplifier. To generate sufficient light at wavelengths of approximately 184 nm to approximately 200 nm for semiconductor wafer or reticle inspection, fundamental laser 110 must generate fundamental light beams 119-1 and 119-2 with average powers of tens or hundreds of watts or more. Suitable fundamental lasers are commercially available as pulsed (mode-locked or quasi-CW) from Coherent Inc. (including the Paladin family of models with 80 MHz and 120 MHz repetition rates), Newport Corporation (including the Explorer family of models), and other manufacturers. The laser power levels of such fundamental lasers can range from milliwatts to tens of watts or more. In an alternative exemplary embodiment, the fundamental laser 110 is implemented by a laser using a Nd:YLF (neodymium-doped yttrium lithium fluoride) laser medium that generates fundamental laser light at a fundamental wavelength of approximately 1053 nm or approximately 1047 nm. In yet another exemplary embodiment, the fundamental laser 110 may be implemented by an ytterbium-doped fiber laser that generates fundamental laser light at a fundamental wavelength of approximately 1030 nm. In some embodiments, light generated by a single fundamental laser may be split into two light portions 119-1 and 119-2, with the second fundamental frequency ω2 necessarily equal to the first fundamental frequency ω1.

[0021] The fifth harmonic generator 130 is configured to generate a fifth harmonic light beam (fifth harmonic) 139 equal to five times the fundamental frequency ω1 (i.e., having a corresponding fifth harmonic wavelength of about 200 nm to about 220 nm) using known techniques (e.g., those described below with reference to FIG. 2).

[0022] The fan-shaped PPNLC 145 is configured to receive the second fundamental light portion 119-2 and to receive the selected down-converted frequency ω x 1 , in one embodiment, the fan-shaped PPNLC 145 is a crystalline structure mounted on a suitable platform 146 (e.g., an X-θ table) and defines parallel XZ planes (using the XYZ coordinate system shown) and has first and second opposing end faces 145-E1 and 145-E2, and first and second opposing side faces 145-S1 and 145-S2 extending parallel (e.g., along the Y axis) between the end faces 145-E1 and 145-E2. The fan-shaped PPNLC 145 is fabricated using known techniques to include a fan-shaped periodic poling configuration that gradually increases from a minimum (first) poling period Λ1 at the first side edge 145-S1 to a maximum (second) poling period Λ2 (e.g., indicated by a shaded region) at the opposing (second) side edge 145-S1. As discussed below with reference to FIGS. 1A and 1B, this fan-shaped periodic poling configuration results in a selective down-conversion frequency ω that depends on the poling period experienced by the second fundamental light portion 119-2 as it passes through the fan-shaped PPNLC 145 between the end face 145-E1 and the end face 145-E2. x to generate a down-converted signal 149. In alternative embodiments, the fan-shaped PPNLC 145 may include one or more of lithium niobate (PPLN), magnesium oxide-doped lithium niobate (Mg:LN), stoichiometric lithium tantalate (SLT), magnesium oxide-doped stoichiometric lithium tantalate (Mg:SLT), and potassium titanyl phosphate (KTP).

[0023] In some embodiments, the down-conversion frequency ω x is the selected down-conversion frequency ω xThe wavelength selector 147 is transmitted through or reflected by a wavelength selector 147 configured to reject frequencies outside a narrow useful bandwidth centered nearby. In the case shown in FIG. 1 and described below with reference to FIG. 4, the wavelength selector 147 may include one or more transmissive elements positioned in the path of the down-converted light 149 (i.e., such that the down-converted light 149 transmitted from the fan-shaped PPNLC 145 passes through the wavelength selector 147 to the nonlinear adding crystal set 155). In other cases (e.g., shown in FIG. 3 and discussed below), the wavelength selector 147 may include one or more reflective elements positioned out of the path between the fan-shaped PPNLC 145 and the nonlinear adding crystal set 155 (i.e., such that the down-converted signal 149 is transmitted from the fan-shaped PPNLC 145 to the nonlinear adding crystal set 155 by reflection from the wavelength selector 147). In still other embodiments, such as those described below with reference to FIG. 6, the wavelength selector 147 may include one or more transmissive elements and one or more reflective elements. Furthermore, the wavelength selector 147 must ensure that the effective bandwidth of the light leaving the fan-shaped PPNLC 145 is sufficiently narrow to accommodate the selective down-conversion frequency ω x In either case, the down-converted signal 149 is transmitted from the fan-shaped PPNLC 145 and directed to a set of nonlinear summing crystals 155.

[0024] 1, the fifth harmonic wave 139 and the down-converted signal 149 are directed substantially collinearly (i.e., at a relative angle β of 5° or less) to a nonlinear summing crystal set 155. In a typical embodiment, the nonlinear summing crystal set 155 includes two (first and second) nonlinear summing crystals 155-1 and 155-2 selectively positioned to selectively receive the fifth harmonic wave 139 and the down-converted signal 149 (i.e., such that the first nonlinear summing crystal 155-1 receives the fifth harmonic wave and the down-converted signal, as shown in FIG. 1A, or such that the second nonlinear summing crystal 155-2 receives the fifth harmonic wave and the down-converted signal, as shown in FIG. 1B). As shown in the bubble on the left side of Figure 1, in a preferred embodiment, the first and second nonlinear addition crystals 155-1 and 155-2 are maintained in a fixed relationship (e.g., by fixed mounting on an XY-θ table or other selectively movable frame 156) so that both crystals are parallel to the light propagation (Y-axis) direction (i.e., the direction in which the down-converted light 149 is directed toward the set 155). With this arrangement, during one operating mode (described below with reference to FIG. 1A), the nonlinear adding crystal 155-1 is positioned so that the fifth harmonic 139 and the down-converted signal 149 enter the crystal 155-1 through input face 155-11 and travel along optical path LP1 to output face 155-12, and during a second operating mode (described below with reference to FIG. 1B), the nonlinear adding crystal 155-2 is positioned so that the fifth harmonic 139 and the down-converted signal 149 enter the crystal 155-2 through input face 155-21 and travel along optical path LP2 to output face 155-22.

[0025] According to an embodiment of the present invention, both the nonlinear adding crystals 155-1 and 155-2 comprise the same nonlinear crystal material, but each nonlinear adding crystal is fabricated (e.g., cut) with a different crystal axis orientation relative to the input and output faces to facilitate phase matching across correspondingly different ranges of signal and output frequencies. In a preferred embodiment, the nonlinear adding crystals 155-1 and 155-2 comprise one of an annealed cesium lithium borate (CLBO) crystal, a hydrogen-treated CLBO crystal, a deuterium-treated CLBO crystal, and a strontium tetraborate (SBO) crystal. As shown in the left-hand bubble in FIG. 1 , the first nonlinear adding crystal 155-1 has a corresponding (first) crystal axis A1 oriented at an angle Φ1 with respect to the light propagation direction LP1, and the second nonlinear adding crystal 155-2 has a corresponding (second) crystal angle A2 oriented at a corresponding (second) angle Φ2 with respect to the light propagation direction LP2. In a preferred embodiment, nonlinear addition crystals 155-1 and 155-2 are fabricated so that angle Φ1 differs from angle Φ2 by at least 5 degrees when positioned in a parallel relationship as shown in Figure 1. By fabricating nonlinear addition crystals 155-1 and 155-2 using known techniques so that crystal axes A1 and A2 are aligned at appropriate angles Φ1 and Φ2 relative to the direction of light propagation, nonlinear addition crystals 155-1 and 155-2 facilitate efficiently summing fifth harmonic light 139 with two or more different down-converted signal frequencies that could not normally be efficiently handled by a single nonlinear addition crystal, thereby enabling tunable laser assembly 100 to generate DUV output light at significantly different DUV frequencies within the range of about 184 nm to about 200 nm.

[0026] The frequency control system 160 controls the user-selected variable output frequency ω OUT(e.g., as defined by output frequency signal OF) and cooperatively adjust (set) the relative positions and orientations of fan-shaped PPNLC 145, optional wavelength selector 147, and nonlinear adder crystal 155, such that tunable laser assembly 100 generates DUV output light 159 at two or more different DUV output frequencies specified by a user. In an exemplary embodiment, frequency control system 160 performs the cooperative adjustments by sending coordinated control signals CS-1 and CS-2 to corresponding first and second controllers 161 and 162, which apply associated wavelength adjustments W1 and W2 to fan-shaped PPNLC 145, optional wavelength selector 147, and nonlinear adder crystal 155. For example, FIG. 1A illustrates how frequency control system (FCS) 160 controls tunable laser assembly 100 to generate a relatively high DUV output light frequency ω OUT1 1 illustrates relevant portions of tunable laser assembly 100 during a first operating period T1 (denoted by "100(T1)") when receiving output frequency signal OF including an exemplary (first) output frequency signal value (i.e., "OF=1") that conveys a user's desire to generate a frequency control signal CS11 and a frequency control signal CS21. In response to the first output frequency signal value ("OF=1"), frequency control system 160 generates first control signals CS11 and CS21. The control signal CS11 causes the controller 161 (FIG. 1) to apply a first wavelength adjustment W11 to position the fan-shaped PPNLC 145 and any wavelength selector 147 (not shown in FIG. 1A) such that the fundamental light portion 119-2 passes through a first region R1 (e.g., such that the fundamental light portion 119-2 passes between opposing edges along a path closer to the side edge 145-S1 than to the opposing side edge 145-S2), thereby causing the fan-shaped PPNLC 145 and any wavelength selector 147 (not shown) to receive a relatively high (first) down-conversion frequency ω x1 Further, control signal CS12 causes controller 162 (FIG. 1) to generate a wavelength tuning W21 that operates frame 156 such that both fifth harmonic light 139 and down-converted signal 149-1 are operatively focused into nonlinear adding crystal 155-1, thereby adjusting the wavelength W21 to the user-selected DUV wavelength ω OUT11B illustrates the case where FCS 160 receives output frequency signal OF that includes a different (second) output frequency signal value (i.e., "OF=2") that conveys a user's desire to generate a different output optical frequency with tunable laser assembly 100 (e.g., the user desires a relatively low DUV frequency ω OUT2 1B illustrates relevant portions of the tunable laser assembly 100 during a second operating period T2 (indicated by "100(T2)") when it is desired to perform an analysis using DUV output light 159-2 having a second output frequency value. In response to the second output frequency value, the FCS 160 generates a second (different) set of control signals CS12 and CS22 that produce corresponding adjustments W11 and W12. The control signals / adjustments CS12 / W12 reposition the fan-shaped PPNLC 145 and any wavelength selector 147 (not shown in FIG. 1B) to pass the fundamental light portion 119-2 through a second region R2 (e.g., such that the fundamental light portion 119-2 passes along a path closer to side edge 145-S2 than to the opposing side edge 145-S1), thereby causing the fan-shaped PPNLC 145 and any wavelength selector 147 (not shown) to receive the relatively lower (second) down-conversion frequency ω x2 and the control signal / regulation CS22 / W22 operates the frame 156 so that both the fifth harmonic light 139 and the down-converted signal 149-2 are operatively focused into the nonlinear addition crystal 155-2, thereby generating a corresponding (second) down-converted signal 149-2 having a frequency of 5th harmonic ω . OUT2 The output light 159-2 has a DUV output.

[0027] In addition to the positioning shown in FIGS. 1A and 1B, frequency control system 160 is further configured to operatively control (e.g., via first controller 161) wavelength selector 147 such that when the output frequency signal has a first value (OF=1), wavelength selector 147 selects a first down-conversion frequency ω x1 The wavelength selector 147 is configured to reject frequencies outside the useful bandwidth centered around, and when the output frequency signal has a second value (OF=2), the wavelength selector 147 selects a second down-converted frequency ω x2The wavelength selector 147 may be configured to filter out frequencies outside of a useful bandwidth centered nearby. Operable control of the wavelength selector 147 may include, for example, translating or rotating the wavelength selector 147 and varying the operating temperature of the wavelength selector 147.

[0028] 2 is a simplified block diagram illustrating a tunable laser assembly 100A configured to generate DUV output light 159A, according to a specific embodiment. Generally, laser assembly 100A includes a first fundamental laser 110A-1, a second fundamental laser 110A-2, a fifth harmonic generator 130A, an optical parameter system (OPS) 140A, and a frequency summing stage 150A. Referring to the lower portion of FIG. 2, OPS 140A includes a fan-shaped PPNLC 145A and a wavelength selector 147A configured to receive and process second fundamental light 119A-2, similar to the manner described with reference to fan-shaped PPNLC 145 and wavelength selector 147 (FIG. 1). In an alternative specific embodiment, OPS 140A down-converts a second fundamental light 119A-2 to a down-converted frequency ω 2 , which is lower than the second fundamental frequency ω 2 . x 2. The fifth harmonic light 139A is implemented as either an optical parametric oscillator (OPO) or an optical parametric amplifier (OPA) configured to down-convert a portion of the second fundamental light 119A-2 to a down-converted signal 149A having a sum 5ω1+ω2. Further details regarding the OPS 140A of specific embodiments are provided below with reference to specific embodiments described with reference to Figures 3, 4, and 6. Referring to the lower right portion of Figure 2, the frequency summing stage 150A utilizes a nonlinear summing crystal set 155A to sum the fifth harmonic light 139A received from the fifth harmonic generator 150A with the down-converted signal 149A received from the OPS 140A to obtain the sum 5ω1+ω2. x Output frequency ω equal to OUT, and produces a final output light 159A having a wavelength between approximately 1350 nm and approximately 3300 nm. Further details of frequency summation stage 150A are described below with reference to Figures 5 and 6. Tunable laser assembly 100A also includes a frequency control system (not shown) that is configured and functions similarly to frequency control system 160 described above with reference to Figure 1 and further described below with reference to Figures 3, 4, 5, and 6. In some preferred embodiments, first and second fundamental frequencies ω1 and ω2 have corresponding wavelengths of approximately 1064 nm, and OPS 140A down-converts the first and second fundamental frequencies ω1 and ω2 to produce a final output light 159A having a corresponding wavelength between approximately 1350 nm and approximately 3300 nm. x and the laser assembly 100A is configured to generate a down-converted signal 149A having an output frequency ω having a wavelength in the range of approximately 184 nm to approximately 200 nm. OUT The laser generates a laser output beam 159A having a wavelength of 1000 nm.

[0029] The first and second fundamental lasers 110A-1 and 110A-2 are configured to generate fundamental light having a fundamental wavelength in the range of approximately 1000 nm to approximately 1100 nm (i.e., approximately 1 μm to 1.1 μm), as described above with reference to Figure 1. In one embodiment, the first fundamental laser 110A-1 generates the first fundamental light 119A-1 and the second fundamental laser 110A-2 generates the second fundamental light 119A-2 such that the first fundamental frequency ω1 of the first fundamental light 119A-1 is substantially equal to the second fundamental frequency ω2 of the second fundamental light 119A-2 (e.g., both the first fundamental light 119A-1 and the second fundamental light 119A-1 have wavelengths of approximately 1064 nm).

[0030] 2, the fifth harmonic generator 130A is configured to receive the first fundamental light 119A-1 and includes a beam splitter 210, two frequency doubling stages 220 and 230 that collectively form a fourth harmonic generation module 240, and a fifth harmonic generation stage 250. The beam splitter 210 is configured to split the first fundamental light 119A-1 into a first fundamental portion 212 and a second fundamental portion 213, both of which have a first fundamental frequency ω1. The first frequency doubling stage 220 receives the first fundamental portion 212 and generates second harmonic light 221 having a second harmonic frequency 2ω1 equal to twice the first fundamental frequency ω1. The second frequency doubling stage 230 receives the second harmonic light 221 and generates fourth harmonic light 241 having a fourth harmonic frequency 4ω1 equal to four times the first fundamental frequency ω1. The fifth harmonic generation stage 250 is configured to receive and mix the second fundamental portion 213 and the fourth harmonic light 241 to generate fifth harmonic light 251 at a fifth harmonic frequency 5ω1 equal to five times the first fundamental frequency ω1. In some embodiments, the tunable laser assembly 100A is configured as a CW laser assembly, and the fifth harmonic generation stage 250 implements frequency mixing in a resonant cavity configured to circulate only the second fundamental light portion 213 to improve power. In other embodiments, the tunable laser assembly 100A is configured as a pulsed laser assembly, and the fifth harmonic generation stage 250 generates the fifth harmonic light by mixing the fourth harmonic light 241 and the second fundamental wave light portion 213 without using a resonant cavity.

[0031] In a specific embodiment in which the tunable laser assembly 100A is implemented as a pulsed laser assembly, the unconsumed portion of the fundamental light exiting the first frequency doubling stage 220 may be separated from the second harmonic light 221 by a beam splitter or prism (not shown) and directed as the second fundamental light portion 213 toward the fifth harmonic generation stage 250. In one embodiment (not shown), the unconsumed fundamental is not separated from the second harmonic light 221 but co-propagates with the second harmonic light 221 through the second frequency doubling stage 230 and arrives at the fifth harmonic generation stage 250 at substantially the same location as the fourth harmonic light 241. One advantage of separating the unconsumed fundamental from the second harmonic light 221 is that an appropriate time delay can be applied to either the unconsumed fundamental portion or the fourth harmonic light 241, so that the two laser pulses arrive at the fifth harmonic generation stage 250 at substantially the same time. A further advantage is that the optical elements used to direct / focus the light, such as mirrors, lenses and prisms (not shown), can be optimized separately for the appropriate wavelength in each path.

[0032] In some embodiments, each frequency doubling stage 220 and 230 may include an external resonant cavity with at least three optical mirrors and a nonlinear crystal arranged inside. The cavity may be stabilized using standard Pound-Dreaver-Hall (PDH), Hensch-Couillaud (HC), or other locking techniques. Adjusting the position of the mirrors or prisms with a control signal adjusts the cavity length and maintains resonance.

[0033] In a preferred embodiment, the first frequency doubling stage 220 comprises a lithium triborate (LBO) crystal. Lithium triborate crystals can be substantially non-critically phase matched (with appropriate selection of crystal planes) at temperatures between room temperature and about 200°C to generate second-harmonic waves in the wavelength range of about 515 nm to about 535 nm. In alternative embodiments, the first frequency doubling stage 220 may comprise a cesium lithium borate (CLBO) crystal or a beta barium borate (BBO) crystal. Any of these can be critically phase matched to generate second-harmonic waves in the wavelength range of about 515 nm to about 535 nm. In other alternative embodiments, the first frequency doubling stage 220 may comprise KTiOPO4 (KTP), periodically poled lithium niobate (PPLN), periodically poled stoichiometric lithium tantalate (PPSLT), or other nonlinear crystals for frequency conversion. The second frequency doubling stage 230 may use critical phase matching with CLBO, BBO, or other nonlinear crystals. In a preferred embodiment, the second frequency doubling stage 230 includes a hydrogen- or deuterium-treated CLBO crystal.

[0034] In one embodiment, at least one of the fifth harmonic generator 130A and the frequency summing stage 150A includes an annealed, hydrogen-treated, or deuterated cesium lithium borate (CLBO) crystal configured to be nearly non-critically phase-matched to generate wavelengths in the range of approximately 184 nm to approximately 200 nm by mixing the fifth harmonic light 139A with the down-converted light 149A. Due to the nearly non-critical phase-matching, the frequency mixing is very efficient (e.g., the non-linear coefficient is approximately 1 pm V). -1 The annealed CLBO crystals have a low temperature (e.g., less than about 30 mrad) and a low walk-off angle (e.g., less than about 30 mrad). In a preferred embodiment, the annealed CLBO crystals are held at a constant temperature near 50°C. In another preferred embodiment, the annealed CLBO crystals for the fifth harmonic generator are held at a constant temperature of approximately 80°C or less, and the annealed CLBO crystals for the frequency summing stage are held at a constant temperature of approximately 30°C or less.

[0035] FIG. 3 illustrates the down-conversion frequency ω used to implement OPS 140A (FIG. 2) when tunable laser assembly 100 or 100A is implemented as a pulsed laser assembly. x 2. The OPS 140A-1 is configured to receive the second fundamental light (second portion) 119A-2 from the second fundamental laser 110A-2 (FIG. 2) and down-convert the portion of the second fundamental light 119A-2 to a frequency ω x The OPS 140A-1 generates a down-converted signal 149A-1 having the following wavelength selector: OPS 140A-1 includes a focusing lens 301, a first focusing mirror 302, a fan-shaped PPNLC 145A-1, a second focusing mirror 303, a reflective diffraction grating (wavelength selector) 147A-1, and an output coupler 306, which are configured to form an optical cavity in which light is reflected between the diffraction grating 147A-1 and the output coupler 306 via focusing mirrors 302 and 303, mirror 304, and the fan-shaped PPNLC 145A-1, as shown. The second fundamental light 119A-2 is focused by the focusing lens 301 and passes through the focusing mirror 302 to enter the OPS 140A-1. The focusing lens 301 functions to focus the fundamental light 119A-2 to a point close to the center of the fan-shaped PPNLC 145A-1. The fan-shaped PPNLC 145A-1 may include any of the embodiments described above with reference to the fan-shaped PPNLC 145, and may convert the second fundamental light at frequency ω2 to the signal frequency ω x The fan-shaped PPNLC 145A-1 is designed for quasi-phase matching to generate light at the signal frequency ω. Any remaining second fundamental light 119A-2 that has not been converted to light at the signal frequency by the fan-shaped PPNLC 145A-1 passes through the focusing mirror 303 and may be removed. The focusing mirror 303 should preferably also transmit the idler frequency generated within the fan-shaped PPNLC 145A-1. In one embodiment, the focusing mirror 303 is configured to transmit the idler frequency ω x and is positioned to direct light of the signal frequency generated within or passing through the fan-shaped PPNLC 145A-1 to the output coupler 306 by reflection from a mirror 304. The output coupler 306 is configured to be highly reflective to light of the signal frequency ω xIt transmits a first fraction of light incident on it (e.g., approximately 20%) and reflects a second fraction (e.g., approximately 80%) at a signal frequency ω x A second fraction of the light is reflected by mirror 304 onto focusing mirror 303, which redirects the light through sectorial PPNLC 145A-1 onto focusing mirror 302, which then redirects the light onto diffraction grating 147A-1. Diffraction grating 147A-1 acts as a wavelength selector, selecting the desired signal frequency ω using known techniques. x For example, grating 147A-1 may reflect a range of wavelengths with a FWHM of approximately 0.2 nm or a FWHM of approximately 0.5 nm. Grating 147A-1 is important in determining the wavelength of laser output 159A (see, e.g., FIG. 2) because the wavelength of laser output 159A is determined by the fifth harmonic of the fundamental and the signal frequency ω x In one embodiment, grating 147A-1 comprises a volume Bragg grating. In a preferred embodiment, grating 147A-1 is maintained at a constant temperature to ensure that its center wavelength remains constant. In one embodiment, the signal frequency ω x , and therefore the wavelength of the laser output light can be adjusted to the down-converted signal frequency ω by adjusting the temperature of the diffraction grating 147A-1. x In other embodiments, the diffraction grating 147A-1 comprises one of a distributed Bragg reflector and a Littrow grating. The signal frequency may be adjusted by rotating the diffraction grating 147A-1 to select the wavelengths that are reflected back toward the focusing mirror 302 and the fan-shaped PPLNC 145A-1.

[0036] signal frequency ω x After reflecting off of the diffraction grating 147A-1, the down-converted light of signal frequency ω returns to the focusing mirror 302, which redirects the down-converted light to the fan-shaped PPNLC 145A-1. xThe optical path length traveled by light at the signal frequency ω, traveling from sector-shaped PPNLC 145A-1 to focusing mirror 303, output coupler 306, returning to focusing mirror 303, through sector-shaped PPNLC 145A-1, focusing mirror 302, mirror 304, diffraction grating 147A-1, returning to focusing mirror 302, and returning to sector-shaped PPNLC 145A-1, is x 1. It is necessary to ensure that each pulse of light at the signal frequency re-arrives at the fan-shaped PPNLC 145A-1 substantially simultaneously with the pulse of input laser light 119A-2. With this arrangement, the pulses of input laser light 119A-2 and light at the signal frequency substantially co-propagate through the fan-shaped PPNLC 145A-1 to down-convert the input laser light to the frequency ω x In a preferred embodiment, the optical path length is such that the signal frequency ω x The mismatch in arrival times between the pulses of light in this laser beam and the pulses of input laser beam 119A-2 should be less than about 10% of the pulse width of input laser beam 119A-2.

[0037] In one embodiment, focusing mirrors 302 and 303 are configured to include focal lengths that are set so that, after completing the just-described round trip, the pulses of light at the down-converted frequency re-arrive at nonlinear crystal 310 and are focused near the center of fan-shaped PPNLC 145A-1 and substantially spatially overlap with the pulses of input laser light 119A-2. In an alternative embodiment, diffraction grating 147A-1 and / or output coupler 306 may be used instead of or in addition to focusing mirrors 302 and 303 to focus the pulses of light at the down-converted frequency ω x In another embodiment, one or more lenses may be used instead of or in addition to the focusing mirror to refocus the down-converted frequency.

[0038] Note that additional mirrors and / or prisms can be incorporated to down-convert the frequency ω x The relative positions of output coupler 306 and diffraction grating 305 may be swapped as long as the appropriate reconfiguration is performed to redirect the light. The configuration shown in Figure 3 is exemplary and intended to illustrate the principles of operation.

[0039] In one embodiment, one or more controllers 161A-1 are utilized to control the position, orientation, and / or temperature of the sectorial PPNLC 145A-1 and the diffraction grating 147A-1 (e.g., by wavelength tuning W1A-11 and W1A-12) to generate a selected down-conversion frequency ω x generates the down-converted signal 149A-1.

[0040] FIG. 4 illustrates the down-conversion frequency ω used to implement OPS 140A (FIG. 2) when tunable laser assembly 100 or 100A is implemented as a CW laser assembly. x 2. The OPS 140A-2 receives the second fundamental light (second portion) 119A-2 from the second fundamental laser 110A-2 (FIG. 2) and converts the portion of the second fundamental light 119A-2 to a frequency ω x The OPS 140A-2 includes a first focusing mirror 401, a fan-shaped PPNLC 145A-2, a second focusing mirror 402, a wavelength selector 147A-2, a third focusing mirror 403, and a fourth focusing mirror 404 operatively configured to form an optical cavity in which light is reflected between the focusing mirrors 401, 402, 403, and 404 to transmit through the wavelength selector 147A-2 and the fan-shaped PPNLC 145A-2, as shown. The second fundamental wave light 119A-2 enters the OPS 140A-2 by passing through the focusing mirror 401. Focusing or mode-matching optics (not shown) may be disposed in the optical path of the input laser light 119A-2 to focus the input light 119A-2 to a point near the center of the fan-shaped PPNLC 145A-2. The fan-shaped PPNLC 145A-2 may include any of the embodiments described above with reference to the fan-shaped PPNLC 145, and may convert the second fundamental light at frequency ω2 to the signal frequency ω x Any remaining second fundamental wave light 119A-2 that has not been converted to light of the signal frequency by the fan-shaped PPNLC 145A-2 passes through the focusing mirror 402 and may be removed.

[0041] Wavelength selector 147A-2 selects the desired signal frequency ω x Wavelength selector 147A-2 is configured to be highly transmissive for a narrow frequency range centered at . Wavelength selector 147A-2 is important in determining the wavelength of laser output 159A (see, e.g., FIG. 2) because the wavelength of laser output 100A is determined by the fifth harmonic of the fundamental and the down-converted signal frequency ω x In one embodiment, wavelength selector 147A-2 includes a transmissive diffraction grating. In another embodiment, wavelength selector 147A-2 includes an etalon.

[0042] In an alternative embodiment, the transmissive wavelength selector 147A-2 may be replaced with a reflective element that replaces one of the mirrors that form the cavity, such as mirror 403 or mirror 404. In this embodiment, the wavelength selector selects the desired signal frequency ω x In this embodiment, the wavelength selector may include a reflective grating, such as a reflective volume Bragg grating or a Littrow grating.

[0043] In an embodiment, wavelength selector 147A-2 is held at a constant temperature to ensure its center wavelength remains constant. In one embodiment, the down-conversion frequency ω can be adjusted by adjusting the temperature of wavelength selector 147A-2 or by rotating the wavelength selector element. x Adjustments can be made to

[0044] signal frequency ω x The down-converted light 149A-2 of signal frequency ω is first reflected by focusing mirror 402, then by focusing mirror 403 to pass through wavelength selector 147A-2, and is then reflected by focusing mirrors 404 and 401 to be redirected to fan-shaped PPNLC 145A-2. xDown-converted light 149A-2 exits OPS 140A-2 through mirror 402. Focusing mirrors 401, 402, 403, and 404 form a cavity, allowing the down-converted light to circulate within the cavity and increase in power. In another embodiment, one or more lenses may be used instead of or in addition to the focusing mirrors to focus the light at the signal frequency.

[0045] In one embodiment, one or more controllers 161A-2 are utilized to control the position, orientation, and / or temperature of the fan-shaped PPNLC 145A-2 and the diffraction grating 147A-2 (e.g., by wavelength tuning W1A-21 and W1A-22) to generate a selected down-conversion frequency ω x generates the down-converted signal 149A-2.

[0046] Using known techniques, the length of the OPS cavity can be adjusted, for example by moving mirror 403 or mirror 404, to achieve a frequency ω x A resonance condition may be maintained within the cavity to circulate the down-converted signal.

[0047] Figure 5 shows the selected output frequency ω OUT3A is a simplified diagram illustrating an exemplary frequency summing stage 150A configured to generate DUV output light 159A at 1000 MHz, representing an exemplary embodiment that may be used with tunable laser assembly 100A (FIG. 2). Frequency summing stage 150A receives down-converted signal 149A from OPS 140A (e.g., either OPS 140A-1 shown in FIG. 3 or OPS 140A-2 shown in FIG. 4) and fifth-harmonic light 139A from fifth-harmonic generator 130A (FIG. 2). Frequency summing stage 150A includes a bowtie ring cavity formed by several reflective optical elements, including input coupler 503, flat mirror 504, and curved mirrors 505 and 506. Down-converted signal 149A enters the cavity through input coupler 503 and is redirected by mirrors 504 and 505 through a selected one of nonlinear adding crystals 155A-1 and 155A-2 (e.g., through input face 155A-11 and output face 155A-12 of crystal 155A-1 as shown in FIG. 5). Fifth harmonic light 139A enters the bowtie ring cavity, passes near (but does not pass through) mirror 505, and is directed toward a selected one of nonlinear adding crystals 155A-1 and 155A-2 (e.g., crystal 155A-1). Light emerging from output face 155A-12 of the selected nonlinear adding crystal 155A-1 is tuned to a user-selected output frequency ω OUT The output light 581 includes selected DUV output light 159A, an unconsumed portion 514 of down-converted light 149A, and an unconsumed portion 516 of fifth-harmonic light 139A. Output light 581 reflects from the surface of beam splitter (BS) 515 and is directed out of the cavity. DUV output light 159A exits the cavity after reflecting from beam splitter (BS) 515, which is positioned between output face 155A-12 and mirror 506. Unconsumed fifth-harmonic light portion 516 is also directed out of the cavity, for example, by reflecting from a second beam splitter 525, which is positioned between beam splitter 515 and mirror 506. Beam splitters 515 and 525 recirculate within the cavity to convert the down-converted frequency ω x 5A is configured to pass an unconsumed portion 514 of the down-converted signal 149A to increase the optical power at the

[0048] In one embodiment, one or more second controllers 162A are utilized to control the position, orientation, and / or temperature of the nonlinear addition crystals 155A-1 and 155A-2 (e.g., by tuning wavelength W2A-1) to generate a selected DUV output frequency ω OUTA and controls the position, orientation, and / or temperature of BS515 (eg, by wavelength tuning W2A-2) to direct DUV output signal 159A outside frequency summation stage 150A.

[0049] In some embodiments, the nonlinear addition crystals 155A-1 and 155A-2 comprise annealed (deuterium-treated or hydrogen-treated) cesium lithium borate (CLBO) crystals maintained (e.g., by the second controller 162) at a constant temperature T of approximately 30°C or less via a suitable temperature control system (e.g., a thermoelectric cooler). In one embodiment, the temperature of the nonlinear addition crystals 155A-1 and 155A-2 may be lower than 0°C, for example, approximately −5°C or −10°C. For Type I matching of CLBO at a temperature of approximately 30°C using a second fundamental or signal wavelength near 1342 nm and a fifth harmonic having a wavelength near 209.4 nm, the phase matching angle is approximately 79°. For Type I matching of CLBO at a temperature of approximately 30°C using a second fundamental or signal wavelength near 1300 nm and a fifth harmonic having a wavelength near 213 nm, the phase matching angle is approximately 81°. Both of these examples demonstrate that near-noncritical phase matching can be achieved with high efficiency and low walk-off to generate wavelengths ranging from approximately 184 nm to approximately 200 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 to select different combinations of wavelength, temperature, and angle to achieve phase matching.

[0050] In one embodiment, the BS 515 may comprise strontium tetraborate (SBO) crystal, SBO glass, or CaF crystal. Because SBO has good deep UV transmittance and a high damage threshold, SBO may be advantageously used as the substrate material for the BS 515 to ensure long lifetimes despite the high power levels of the unconsumed down-converted light 514 circulating within the cavity. If the BS 515 comprises an SBO crystal, its thickness and / or crystal c-axis orientation may be configured to minimize any frequency doubling of the unconsumed down-converted light 514 passing therethrough. The BS 515 may also include a polarizing beam splitter, a dichroic beam splitter, a prism, or other components to separate wavelengths. In one embodiment, the BS 515 has a surface oriented such that the unconsumed down-converted light 514 is substantially P-polarized and is at approximately the Brewster angle with respect to the unconsumed input light.

[0051] In another embodiment, the BS 515 may be further configured to reflect both the DUV output light 159A and the unconsumed fifth harmonic light portion 516 out of the cavity, and the second beam splitter 525 may be omitted.

[0052] In one embodiment, the down-converted light 149A (ω x ) is focused by one or more lenses 501 before entering the cavity to align the eigenmodes of the resonant cavity with a beam waist in or near the selected nonlinear adding crystal 155A-1. In an alternative embodiment, instead of orienting the input and output faces 155A-11, 155A-12 at Brewster's angle, a suitable anti-reflection coating may be applied to the input face 155A-11 of the nonlinear adding crystal 155A-1.

[0053] Although FIG. 5 illustrates the cavity of final frequency summing stage 150A as formed by two flat mirrors and two curved mirrors, other combinations of mirrors and / or lenses may be used to refocus the down-converted light circulating within the cavity. In alternative embodiments, the final frequency summing stage may comprise a delta cavity, a standing wave cavity, or a cavity of another shape instead of a bowtie cavity. Any of these cavities may be stabilized using standard PDH or HC locking techniques. The cavity length is adjusted to maintain resonance by adjusting the position of one of the mirrors (such as mirror 504 in FIG. 5) or the position of the prism via a control signal (not shown) connected to a piezoelectric transformer (PZT), voice coil, or other actuator.

[0054] In the case of a pulsed laser assembly, mixing of down-converted signal 149A with fifth-harmonic light 139A may be implemented cavity-free by eliminating mirrors 504, 505, and 506. Nonlinear summing crystals 155A-1 and 155A-2 are selectively positioned to selectively receive both fifth-harmonic light 139A (i.e., from fifth-harmonic generator 150A in FIG. 2) and down-converted signal 149A (from OPO 140A in FIG. 2) so that both are approximately collinearly incident on nonlinear summing crystal 155A-1 and focused into corresponding beam waists located within or proximal to nonlinear summing crystal 155A-1 (beam waist not shown).

[0055] FIG. 6 shows the down-conversion frequency ω through both the fan-shaped PPNLC 145B and the nonlinear adder crystal set 155B. x1 illustrates a portion of a tunable laser assembly 100B according to another embodiment, in which a fan-shaped PPNLC 145B and a nonlinear add crystal set 155B are included in a single cavity device 170B configured to selectively recycle light from the fan-shaped PPNLC 145B and the nonlinear add crystal set 155B. That is, in contrast to the two separate cavities utilized by the OPS 145A and frequency add stage 150A of assembly 100A (FIG. 2), unconsumed down-converted light 627 exiting the nonlinear add crystal set 155B is recirculated (back) to the fan-shaped PPNLC 145B. The unconsumed down-converted light 627 is thereby amplified to generate a down-converted signal 149B, which is directed to the nonlinear add crystal set 155B by various mirrors and optical elements forming the single cavity device 170B. In addition to the single cavity device 170B, the tunable laser assembly 100B includes one or more fundamental lasers (not shown) and a fifth harmonic generator (not shown). One or more fundamental lasers and fifth-harmonic generators provide fundamental light 119B and fifth-harmonic light 139B to cavity device 170B in a manner similar to that described above with reference to Figures 1 and 2. Other than operating within a single cavity as described below, fan-shaped PPNLC 145B is constructed and functions substantially as described with reference to the above-described embodiments. Nonlinear addition crystal set 155B differs from the above-described embodiments in that it includes three nonlinear addition crystals 155B-1, 155B-2, and 155B-3 selectively positioned within the path of light generated by single cavity device 170B, as described below with reference to Figures 7A-7D, but otherwise functions similarly to sets 155 and 155A, respectively, described above with reference to Figures 1 and 2.

[0056] In the exemplary embodiment shown in FIG. 6 , single cavity device (cavity) 170B is formed by mirrors 601, 602, 606, and 620, reflective element mirror (i.e., mirror or wavelength selector) 147B-1, and output coupler / mirror 621, which down-convert the laser beam to a frequency ω 1 along an optical path with two laser beam waists, one occurring in PPNLC 145B and the other occurring in one of nonlinear addition crystals 155B-1, 155B-2, and 155B-3 (e.g., in nonlinear addition crystal 155B-3 shown in FIG. 6 ). x 6, the fundamental light 119B-2 at fundamental frequency ω2 is directed to mirror 601 and focused into the fan-shaped PPNLC 145B to provide a down-converted signal frequency ω x The first mirror 601 is also configured to reflect the unconsumed down-converted light 627, such that the unconsumed down-converted light 627 received from mirror 606 forms a beam waist within the PPNLC 145B. To facilitate the fundamental light 119B-2 entering cavity 170B through mirror 601 (as shown), the mirror 601 may be coated with a coating that transmits light having a frequency close to the fundamental frequency ω, while the down-converted light 627 may be coated with a coating that transmits light having a frequency close to the fundamental frequency ω, while the down-converted light 119B-2 may be coated with a coating that transmits light having a frequency close to the fundamental frequency ω. x ) by reflecting light having a frequency close to the fundamental wave light 119B-2, while transmitting or passing the down-converted frequency ω xMirror 602 may be configured to reflect the recirculated signal light 627. In an alternative embodiment, fundamental light 119B-2 could be injected into cavity 170B through another mirror, such as mirror 606. In this case, mirror 601 must be configured to reflect both fundamental light 119B-2 and recirculated signal light 627. In one embodiment, mirror 602 is configured to selectively reflect down-converted signal 149B and to allow the remaining fundamental light 631 having the fundamental frequency ω2 (i.e., the fundamental light not consumed in producing down-converted light 149B) to exit cavity 170B. Down-converted light 149B is directed from mirror 602 to reflective element reflective grating / mirror 147B-1, which redirects down-converted light 149B toward mirror 620. Mirror 620 is configured to redirect down-converted light 149B received from mirror 147B-1 through one of nonlinear adding crystals 155B-1, 155B-2, and 155B-3. Mirror 621 is configured to reflect unconsumed down-converted light 627 exiting nonlinear adding crystals 155B-1 through 155B-3, and mirror 606 is configured to receive and reflect unconsumed down-converted light 627 to mirror 601. While cavity 170B is illustrated as including six mirrors, more or fewer mirrors may be used. When tunable laser assembly 100B is configured as a CW laser, it is preferable to keep the cavity length short; therefore, only curved mirrors such as 601, 602, 620, and 621 may be used. If the tunable laser assembly 100B is configured as a pulsed laser, the cavity length must match the spacing between pulses (or an integer fraction or multiple of that length), and therefore the cavity must be large (e.g., if the laser repetition rate is 80 MHz, the cavity length must be 3.747 m or 1.874 m). Such cavities may use six or more mirrors to fit into a convenient footprint. The CW cavity may be configured as a single-mode cavity (which will produce a very narrow bandwidth signal) or a multimode cavity (e.g., a few modes or hundreds of modes).Generally, multimode cavities are more stable than single-mode cavities, but provide a larger bandwidth for the signal light 149B and 627. For use in semiconductor inspection systems, a pulsed laser with a large number of modes (approximately 100 or more) or a high repetition rate would generally be preferred.

[0057] In one embodiment, the down-conversion frequency ω of the signal light 149B emitted from the fan-shaped PPNLC 145B is x is determined by quasi-phase matching (QPM) due to the poling length of the section of the fan-shaped PPNLC 145B through which the fundamental wave light 119B-2 and the recirculated signal light 627 pass. As discussed above, the section (area) of the fan-shaped PPNLC 145B through which the down-converted light passes is selectively adjusted by the controller 161B through wavelength tuning W1B-1 (e.g., translation of the nonlinear crystal 145B in the +X-axis direction and −X-axis direction shown in FIG. 6 ) in combination with wavelength selection by the wavelength selector 147B-2 and / or the reflective element 147B-1. Typically, only one of the reflective element 147B-1 and the wavelength selector 147B-2 is configured to select a wavelength (i.e., when a transmissive wavelength selector 147B-2 is used, the reflective element 147B-1 may be implemented using a mirror; conversely, when the reflective element 147B-1 is implemented as a wavelength selector using, for example, a reflective diffraction grating, the wavelength selector 147B-2 may be omitted). Wavelength selector 147B-2 may include one of an etalon (Fabry-Perot interferometer) and a transmission grating (such as a volume Bragg grating or a mechanically ruled grating). When implemented as a wavelength selector, reflective element 147B-1 may be adjusted by controller 161B by wavelength adjustment W1B-2 (as shown by the dotted arrow) to rotate the grating to point the desired wavelength toward mirror 620, or by adjusting the temperature T in the case of a volume Bragg grating. When reflective element 147B-1 is a mirror, wavelength selector 147B-2 may be adjusted by controller 161B by wavelength adjustment W1B-3 (e.g., by rotation as shown by the arrow and / or adjusting the operating temperature T). In either case, reflective element 147B-1 and / or wavelength selector 147B-2 direct the desired signal frequency ω to mirror 620.x and undesired wavelengths, including the idler frequency, may be directed out of the cavity by elements 147B-1 and / or 147B-2, absorbed, or otherwise blocked. Additionally, one or more of the mirrors in the cavity may be configured to transmit the idler frequency with high efficiency (e.g., about 90% or greater efficiency), while reflecting signal light 149B such that substantially all of the idler frequency leaves the cavity after reflecting off multiple mirrors.

[0058] Similar to the fan-shaped PPNLC element described above, the fan-shaped PPNLC 145B is polled in a fan-shaped configuration with a range of polling periods spanning the desired signal frequency range. Suitable fan-shaped PPNLC elements are commercially available, for example, from HC Photonics (Hsinchu, Taiwan). When the input light 119B-2 has a wavelength near 1064.5 nm, the signal frequency range corresponding to signal wavelengths of 1356 to 2129 nm requires a QPM poling period of 25.86 μm to 32.15 μm if the fan-shaped PPNLC 145B contains an MgO-doped lithium niobate crystal, or a QPM poling period of 25.66 μm to 32.32 μm if the fan-shaped PPNLC 145B contains a stoichiometric lithium titanate crystal. Note that these crystal materials have strong absorption at wavelengths longer than approximately 4 μm; therefore, these materials may not be useful for signal frequencies corresponding to signal wavelengths shorter than approximately 1450 nm. This is because the idler has a corresponding wavelength longer than 4 μm, and heat dissipated in the fan-shaped PPNLC 145B may degrade phase matching and / or cause thermal lensing effects.

[0059] The cavity 170B is configured such that the down-converted signal light 149B and the fifth harmonic light 139B are directed along a summing optical path to a nonlinear summing crystal set 155B, producing an output frequency ω OUTB6, the fifth harmonic light 139B enters the cavity 170B by passing through a mirror 620 or by exiting from its edge. The fifth harmonic light 139B travels at a slight angle (e.g., less than 5°) relative to the signal light 149B such that the fifth harmonic light 139B and the signal light 149B traverse near the center of a selected nonlinear addition crystal (e.g., crystal 155B-3 shown in FIG. 6). The desired output frequency ω OUTB The laser output light 159B and unconsumed fifth harmonic light 616 exit the cavity 170B by passing near or through a mirror / output coupler 621.

[0060] For tunable laser assembly 100B to generate output light over a wide range of wavelengths, such as wavelengths from about 193 nm to about 184 nm, nonlinear adding crystal set 155B may need to include more than two nonlinear adding crystals because the required angular adjustment range may be as much as 25°. Rotating a single nonlinear adding crystal through such a large angle may result in reflection losses that prevent the generation of laser output light with the required power. Furthermore, such large rotation angles may result in too much displacement of the unconsumed down-converted light 627 by the nonlinear adding crystal to be compensated for by adjusting one or more of the mirrors that form cavity 170B. Therefore, in one embodiment, nonlinear adding crystal set 155B includes three nonlinear adding crystals 159B-1 through 159B-3. Each of these crystals is fabricated (cut) so that its different crystal axes form correspondingly different angles with respect to the add light propagation direction and are phase-matched over different ranges of signal and output frequencies (i.e., similar to those described above with reference to FIG. 1). To facilitate moving / positioning the most appropriate nonlinear addition crystal 159B-1, 159B-2, or 159B-3 into the addition light path to generate DUV output light at a user-selected wavelength, the nonlinear addition crystals 159B-1 through 159B-3 are fixedly mounted to a frame 156B (e.g., an XY-θ table, linear stage, or another suitable mechanism) controlled by a second controller 162B (i.e., by wavelength tuning W2B-1), as described below with reference to, for example, FIGS. 7A through 7D. The frame 156B should be temperature-controlled and rotatable to provide optimization of the phase-matching conditions for frequency addition. In other embodiments, two, four, or more nonlinear addition crystals may be utilized.

[0061] Referring to the upper portion of FIG. 6, nonlinear addition crystal set 155B includes three nonlinear crystals 155B-1, 155B-2, and 155B-3 selectively positioned (moved) within a substantially co-located optical path formed by converted signal light 149B and fifth-harmonic light 139B. A second controller 162B forms another part of the frequency control system of tunable laser assembly 100B. In one embodiment, nonlinear addition crystals 155B-1 through 155B-3 are maintained in a fixed relationship by being fixedly mounted on an XY-θ table (frame) 156B. The operation of table (frame) 156B is controlled by controller 162B via wavelength tuning W2B-1.

[0062] 7A-7D illustrate a portion of tunable laser assembly 100B when configured in four different operating states (i.e., four different time periods). As described above with reference to FIG. 1, the four different operating states correspond to four different user-provided output frequency control signal values ​​submitted to the frequency control system of tunable laser assembly 100B prior to each illustrated operating state. For illustrative purposes, FIGS. 7A-7D illustrate a portion of tunable laser assembly 100B including mirrors 620 and 621 and nonlinear addition crystals 155B-1-155B-3. Although not shown in FIGS. 7A-7D, it will be understood that the frequency control system controls the illustrated movement of nonlinear addition crystals 155B-1-155B-3 via controller 162B, as well as the transmission of light through fan-shaped PPNLC 145B (FIG. 6) via controller 161B, so that different down-converted light frequencies are directed to selected nonlinear addition crystals 155B-1-155B-3.

[0063] 7A, 7B, and 7C illustrate example operating states that may be utilized in response to significantly different user-selected output frequencies. For example, as illustrated in FIG. 7A, during a first time period T1 (e.g., in response to a first output frequency value OF=1), the frequency control system passes fundamental light portion 119B-2 through a first region of fan-shaped PPNLC 145B, thereby generating a first down-converted frequency ω xB1When the first down-converted signal 149B-1 is then summed with the fifth harmonic light 139B in a selected nonlinear summing crystal 155B-1, the tunable laser assembly 100 produces a first user-selected output wavelength ω OUTB1 7B, during a second time period T2 (e.g., in response to a second output frequency value OF=2), the frequency control system passes the fundamental light portion 119B-2 through a second region of the fan-shaped PPNLC 145B, thereby generating a corresponding down-converted frequency ω xB2 When the down-converted light 149B-2 is summed with the fifth harmonic light 139B in the selected nonlinear adding crystal 155B-2, the tunable laser assembly 100 produces a second user-selected output wavelength ω OUTB2 Finally, as shown in FIG. 7C, during a third time period T3 (e.g., in response to a third output frequency value OF=3), the frequency control system passes the fundamental light portion 119B-2 through a third region of the fan-shaped PPNLC 145B, thereby generating a corresponding down-converted frequency ω xB3 When the down-converted light 149B-3 is summed with the fifth harmonic light 139B in the selected nonlinear adding crystal 155B-3, the tunable laser assembly 100B produces a third user-selected output wavelength ω OUTB3 The output light 159B-3 has a DUV output light 159B-3.

[0064] 7D illustrates the tunable laser assembly 100B in an exemplary fourth operating state (e.g., responsive to a fourth output frequency value OF=4). The exemplary fourth operating state corresponds to a DUV output frequency ω OUTB3 7C. During the fourth time period T4, as in the previous example, the frequency control system passes the fundamental light portion 119B-2 through a corresponding fourth region of the fan-shaped PPNLC 145B, thereby providing a down-converted frequency ω xB3 The corresponding down-conversion frequency ω differs from xB4Furthermore, the frequency control system adjusts the input frequency ω by changing the angle between the axis of the selected nonlinear addition crystal 155B-3 and the light propagation direction of the down-converted light 149B-4 (i.e., compared to the phase-matching angle achieved in FIG. 7C ). xB4 and 5ω1 and output frequency ω OUTB4 . These changes in operating conditions cause the tunable laser assembly 100B to achieve a third user-selected output wavelength, ω OUTB3 A fourth user-selected output wavelength ω that differs by a relatively small amount from OUTB4 The nonlinear adding crystal 155B-1 to 155B-3 generates DUV output light 159B-4 having a phase-matching angle of 100°. In some embodiments, the temperature of the nonlinear adding crystal 155B-3 is controlled (e.g., by a heater, not shown) to maintain optimal phase-matching conditions. In a preferred embodiment, the nonlinear adding crystal 155B-3 may be tuned by rotating the crystal and adjusting its temperature to maintain phase-matching. Adjusting the signal frequency may involve corresponding adjustments to the temperature and / or angle of the nonlinear adding crystal. Rotating the nonlinear adding crystal changes the direction in which the DUV output light 159B-4 leaves the cavity. Adjustable downstream optics (not shown) may be provided to compensate for this change. The nonlinear adding crystals 155B-1 to 155B-3 preferably have input and output faces oriented near the Brewster's angle with respect to the add light propagation direction to minimize reflection losses. Adjusting the orientation of the add crystals by a small angle (e.g., ±5°) does not result in significant reflection losses because the down-converted signal light and the fifth-harmonic light remain near the Brewster's angle.

[0065] Because adjusting one or more of the components, such as fan-shaped PPNLC 145B, diffraction gratings 147B-1 and 147B-2, and nonlinear adder crystals 155B-1 through 155B-3, may result in small displacements or changes in the direction of recirculating signal light 627, one or more of the other cavity mirrors (or other optical compensators included in the cavity, not shown) may be selectively controlled by the assembly's control system (e.g., using controllers 161B or 162B) to maintain good alignment of recirculating signal light 627 within cavity 170B. If cavity 170B is configured as a CW cavity, known cavity locking techniques may be used to adjust one of the mirrors or other cavity components with a suitable controller (e.g., a piezoelectric transformer or voice coil) to achieve and maintain the desired cavity length.

[0066] Table 1 (provided below) lists exemplary wavelengths and phase matching angles for an embodiment in which three CLBO crystals are used to implement laser assembly 100B to facilitate the generation of laser output light having an output wavelength range of 184.8 nm to 193.5 nm. In this example, the first CLBO crystal, used to implement nonlinear adding crystal 155B-1, would be cut with its axis at an angle of 68.4° relative to the direction of light propagation. The second CLBO crystal, implementing nonlinear adding crystal 155B-2, would be cut with its axis at an angle of 61.1°. The third CLBO crystal, implementing nonlinear adding crystal 155B-3, would be cut with its axis at an angle of 54.4°. The table also includes examples of the shortest and longest laser output wavelengths that can be produced using each CLBO crystal, using the fifth harmonic of 212.9 nm. Each crystal would need to be tuned over a range of less than ±4° from nominal to accommodate the different output wavelengths, as described above with reference to FIG. 7D. If each crystal is cut so that the input and output faces are at Brewster's angle at the nominal wavelength, then rotating them by less than 4° from nominal to match a different wavelength will result in only a small increase in reflectivity, and therefore cavity loss.

[0067] [Table 1]

[0068] Another embodiment of the present invention is a wafer, reticle, or photomask inspection or metrology system incorporating at least one laser according to the present invention having an output wavelength in the range of approximately 184 nm to approximately 200 nm. Embodiments of such a system are shown in Figures 8, 9, and 10.

[0069] This laser may be used in an inspection system employing dark-field and bright-field inspection modes, as shown in FIG. 8. This diagram and system are described in U.S. Pat. No. 7,817,260 to Chuang et al., which is incorporated by reference herein as if fully set forth. FIG. 8 illustrates a catadioptric imaging system 800 incorporating normal-incidence laser illumination. The illumination block of system 800 includes laser 801, adaptive optics 802 that control the size and shape of the illumination beam at the surface being inspected, an aperture window 803 within mechanism housing 804, and a prism 805 that redirects the laser along its optical axis toward the surface of sample 808 at normal incidence. Prism 805 also directs specular reflections from surface features of sample 808 and reflections from the optical surfaces of objective lens 806 along an optical path to image plane 809. The lens for objective lens 806 may be provided in the general form of a catadioptric objective, a focusing lens group, and a magnifying barrel section 807. In a preferred embodiment, the laser 801 may be implemented by one of the lasers described above.

[0070] This laser may be used in a dark-field inspection system using oblique line illumination, as shown in Figures 9A and 9B. The inspection system may have two or three different focusing systems, including off-axis, near-normal focusing, as shown. The dark-field inspection system may also include normal-incidence line illumination (not shown). Further details, including a description of the system shown in Figures 9A and 9B, can be found in U.S. Patent No. 7,525,649 to Leong et al., which is incorporated herein by reference as if fully set forth.

[0071] 9A shows a surface inspection apparatus 900. The surface inspection apparatus 900 includes an illumination system 901 and a focusing system 910 for inspecting multiple areas of a surface 911. As shown in FIG. 9A, a laser system 920 directs a light beam 902 to beam-forming optics 903. In a preferred embodiment, the laser system 920 includes at least one of the laser assemblies described above (i.e., laser assembly 100, 100A, or 100B). The first beam-forming optics 903 may be configured to receive a beam from the laser system. The beam is focused onto the surface 911.

[0072] Beam-forming optics 903 are oriented such that their major plane is substantially parallel to specimen surface 911, such that illumination ray 905 is formed on surface 911 at the focal plane of beam-forming optics 903. Furthermore, light ray 902 and focused ray 904 are directed at surface 911 at non-orthogonal angles of incidence. Specifically, light ray 902 and focused ray 904 may be directed at surface 911 at an angle between about 1° and about 85° from normal. In this manner, illumination ray 905 is substantially within the plane of incidence of focused ray 904.

[0073] Light collection system 910 includes a lens 912 that collects light scattered from illumination beam 905 and a lens 913 that focuses light exiting lens 912 onto a device, such as a charge-coupled device (CCD) 914, that includes an array of photosensitive detectors. In one embodiment, CCD 914 may include a linear array of detectors. In such a case, the linear array of detectors within CCD 914 may be oriented parallel to illumination beam 905. In another embodiment, CCD 914 may include a two-dimensional array of detectors arranged in a rectangular array with the long axis parallel to illumination beam 905. For example, CCD 914 may include a rectangular array of approximately 1000-8000 detectors by approximately 50-250 detectors. In one embodiment, multiple light collection systems may be included, each with similar components but oriented differently.

[0074] For example, Figure 9B shows an exemplary arrangement of light collection systems 931, 932, and 933 for a surface inspection apparatus (e.g., an illumination system similar to that of illumination system 901 is not shown for simplicity). A first optical system of light collection system 931 collects light scattered in a first direction from the surface of sample 911. A second optical system of light collection system 932 collects light scattered in a second direction from the surface of sample 911. A third optical system of light collection system 933 collects light scattered in a third direction from the surface of sample 911. Note that the first, second, and third paths are at different reflection angles with respect to the surface of sample 911. A platform 912 supporting the sample 911 may be used to create relative motion between the optics and the sample 911, thereby scanning the entire surface of the sample 911.

[0075] The lasers described herein may also be used in inspection systems for unpatterned wafers, such as the inspection system 1000 shown in FIG. 10 . Such inspection systems may incorporate grazing and / or normal incidence illumination for scattered light as shown in these figures, as well as a large collection solid angle. The illumination source 1100 incorporates either the laser assembly 100, 100A, or 100B to generate DUV or VUV light and illuminate the wafer 1122 at a desired angle to prevent reflected light from being collected by the imaging and collection optics 1108 system. The optics 1106 may be configured to generate a desired illumination pattern. Scattered light from the wafer 1122 may be collected by the imaging and collection optics 1108 system, which is configured to direct the light to the afocal lens system 1110. In one embodiment, the collection lens mask system 1112 may split the light into multiple channels and provide them to the TDI sensor 1118. One embodiment may include an intensifier 1114 and / or a sensor repeater 1116. The TDI sensor 1118 and / or intensifier 1114 may be configured to send signals to an image processing computer 1120. The image processing computer 1120 may be configured to generate a wafer image and / or a list of defects or particles on the surface of the wafer 1122. Further description of the elements of FIG. 10 may be found in U.S. Patent No. 9,891,177 B2 to Vazhaeparambil et al. Further details of unpatterned wafer inspection systems may be found in U.S. Patent Nos. 6,201,601 and 6,271,916, all of which are incorporated by reference herein as if fully set forth.

[0076] Although the present invention has been described in connection with certain specific embodiments, it will be apparent to those skilled in the art that the inventive features of the present invention are applicable to other embodiments, which are intended to be encompassed within the scope of the present invention.

Claims

1. A tunable laser assembly that generates deep ultraviolet (DUV) output light having a variable output frequency with a corresponding wavelength selectively tunable in a range of 184 nm to 200 nm, comprising: one or more fundamental lasers each configured to generate fundamental light having at least one corresponding fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a fifth harmonic generator configured to receive a first portion of the fundamental light and generate fifth harmonic light having a fifth harmonic frequency equal to five times the fundamental frequency; a fan-shaped periodically poled nonlinear crystal configured to receive a second portion of the fundamental light; first and second nonlinear addition crystals; a frequency control system configured to cooperatively adjust the relative positions and orientations of the fan-shaped periodically polled nonlinear crystal and the first and second nonlinear addition crystals in response to an output frequency signal, thereby when the output frequency signal has a first value, the frequency control system passes the second portion through a first region of the fan-shaped periodically polled nonlinear crystal, thereby generating a first down-converted signal having a first down-conversion frequency; the frequency control system positions the first nonlinear addition crystal to receive both the fifth harmonic light and the first down-converted signal, thereby generating the DUV output light having a first wavelength within the range; When the output frequency signal has a second value, the frequency control system passes the second portion through a second region of the fan-shaped periodically polled nonlinear crystal, thereby generating a second down-converted signal having a second down-converted frequency, and the frequency control system positions the second nonlinear addition crystal to receive both the fifth harmonic light and the second down-converted signal, thereby generating the DUV output light having a second wavelength within the range.

2. the fan-shaped periodically poled nonlinear crystal has opposing first and second side edges and opposing first and second end edges, and includes a poling period that varies stepwise from a first period at the first side edge to a second period at the second side edge, the first period being greater than the second period; when the frequency control system passes the second portion through the first region, the second portion passes between the opposing first and second edges along a path that is closer to the second side edge than to the first side edge; 2. The tunable laser assembly of claim 1, wherein when the frequency control system passes the second portion through the second region, the second portion passes between the opposing first and second edges along a path that is closer to the first side edge than to the second side edge.

3. 3. The tunable laser assembly of claim 2, wherein the fan-shaped periodically poled nonlinear crystal comprises one of lithium niobate (PPLN), magnesium oxide-doped lithium niobate (Mg:LN), stoichiometric lithium tantalate (SLT), magnesium oxide-doped stoichiometric lithium tantalate (Mg:SLT), and potassium titanyl phosphate (KTP).

4. the first nonlinear addition crystal has a first crystal axis, and the second nonlinear addition crystal has a second crystal axis; When the output frequency signal has the first value, the frequency control system positions the first nonlinear adding crystal such that the first down-converted signal passes through the first nonlinear adding crystal in a first optical propagation direction that forms a first angle with respect to the first crystal axis; When the output frequency signal has the second value, the frequency control system positions the second nonlinear adding crystal such that the second down-converted signal passes through the second nonlinear adding crystal in a second optical propagation direction that forms a second angle with respect to the second crystal axis; 10. The tunable laser assembly of claim 1, wherein the first angle differs from the second angle by at least 5 degrees.

5. 10. The tunable laser assembly of claim 1, wherein each of the first and second nonlinear addition crystals comprises one of an annealed cesium lithium borate (CLBO) crystal and a strontium tetraborate (SBO) crystal.

6. the first and second nonlinear addition crystals are fixedly connected to a frame; The frequency control system includes: a first controller configured to move the fan-shaped periodically polled nonlinear crystal, thereby passing the second portion through the first region when the output frequency signal has a first value, and passing the second portion through the second region when the output frequency signal has a second value; a second controller configured to move the frame to a first position such that the first down-converted signal passes through the first nonlinear summing crystal when the output frequency signal has the first value, and the second down-converted signal passes through the second nonlinear summing crystal when the output frequency signal has the second value; 10. The tunable laser assembly of claim 1, comprising:

7. 7. The tunable laser assembly of claim 6, further comprising a wavelength selector arranged to receive the down-converted signal, wherein the frequency control system is further configured to control the wavelength selector such that the wavelength selector is configured to remove frequencies outside an effective bandwidth centered about the first down-converted frequency when the output frequency signal has the first value, and the wavelength selector is configured to remove frequencies outside the effective bandwidth centered about the second down-converted frequency when the output frequency signal has the second value.

8. an optical parameter system (OPS) optically coupled to receive the second portion of the fundamental light and configured to direct the second portion of the fundamental light to at least one of the fan-shaped periodically polled nonlinear crystal and a wavelength selector; a frequency summing stage optically coupled to receive the down-converted signal from the OPS and the fifth harmonic light from the fifth harmonic generator, the frequency summing stage configured to direct the down-converted signal and the fifth harmonic light to one of the first and second summing crystals; The tunable laser assembly of claim 1 further comprising:

9. The fifth harmonic generator a beam splitter configured to receive and split the first portion of the fundamental light into a first fundamental portion and a second fundamental portion; a fourth harmonic generation module optically coupled to receive the first fundamental portion and configured to generate fourth harmonic light having a fourth harmonic frequency equal to four times the fundamental frequency; a fifth harmonic generation stage optically coupled to receive both the second fundamental portion and the fourth harmonic light, and configured to generate the fifth harmonic light by mixing the fourth harmonic light and the fundamental portion; 9. The tunable laser assembly of claim 8, comprising:

10. 9. The tunable laser assembly of claim 8, wherein the OPS further comprises a first focusing mirror and a second focusing mirror, the first focusing mirror and the second focusing mirror being operatively configured to form a cavity with the wavelength selector, in which light is reflected between the wavelength selector and the first and second focusing mirrors, whereby the reflected light passes through the fan-shaped periodically poled nonlinear crystal.

11. 11. The tunable laser assembly of claim 10, wherein the wavelength selector comprises one of a volume Bragg grating, a distributed Bragg reflector, and a Littrow grating.

12. 12. The tunable laser assembly of claim 11, further comprising means for maintaining said wavelength selector at a constant temperature.

13. 9. The tunable laser assembly of claim 8, wherein the OPS further comprises a first focusing mirror, a second focusing mirror, a third mirror, and a fourth mirror, the first focusing mirror, the second focusing mirror, the third mirror, and the fourth mirror being operatively configured to form a cavity within which light is directed to both the fan-shaped periodically poled nonlinear crystal and the wavelength selector.

14. 14. The tunable laser assembly of claim 13, wherein the wavelength selector includes one of a transmission grating and an etalon.

15. 9. The tunable laser assembly of claim 8, wherein the frequency summing stage further comprises a plurality of reflective optical elements collectively configured to form a cavity that circulates an unconsumed portion of light at the down-conversion frequency whereby the unconsumed portion is combined with the down-converted signal received from the OPS before being directed to the one of the first and second summing crystals, each of the first and second summing crystals comprising one of an annealed cesium lithium borate (CLBO) crystal and a strontium tetraborate (SBO) crystal.

16. 2. The tunable laser assembly of claim 1, further comprising a single cavity device configured to circulate unconsumed down-converted light exiting one of the first and second nonlinear addition crystals, whereby the unconsumed down-converted light is directed to the fan-shaped periodically poled nonlinear crystal.

17. The single cavity device comprises: a first mirror configured to reflect the unconsumed down-converted light into the fan-shaped periodically poled nonlinear crystal; a second mirror configured to receive light from the fanned periodically poled nonlinear crystal and reflect a portion of the received light having the down-converted frequency, the reflected portion of the light comprising the down-converted signal generated by the fanned periodically poled nonlinear crystal; and a third mirror configured to receive a portion of the reflected light from the second mirror; and a fourth mirror configured to reflect a portion of the reflected light received from the third mirror to the one of the first and second nonlinear addition crystals; a fifth mirror configured to reflect the unconsumed down-converted light exiting the one of the first and second nonlinear addition crystals; and a sixth mirror configured to reflect the unconsumed down-converted light received from the fifth mirror to the first mirror; 17. The tunable laser assembly of claim 16, comprising:

18. the tunable laser assembly further comprising a third frequency summing crystal; 2. The tunable laser assembly of claim 1, wherein the frequency control system is further configured to cooperatively adjust relative positions and orientations of the fan-shaped periodically polled nonlinear crystal and the first, second, and third nonlinear addition crystals in response to the output frequency signal, so that when the output frequency signal has a third value, the frequency control system passes the second portion through a third region of the fan-shaped periodically polled nonlinear crystal, thereby generating a third down-converted signal having a third down-conversion frequency, and the frequency control system positions the third nonlinear addition crystal to receive both the fifth harmonic light and the third down-converted signal, thereby generating the DUV output light having a third output wavelength that differs from both the first output wavelength and the second output wavelength by at least 5 nm.

19. 1. An inspection system comprising: a laser assembly configured to generate laser output light having an output wavelength in the range of 184 nm to 200 nm; a first optical system configured to direct the laser output light from the laser assembly to an object to be inspected; a second optical system configured to collect an image portion of the laser output light affected by the object under inspection and direct the image portion to one or more sensors; Equipped with The laser assembly includes: one or more fundamental lasers each configured to generate fundamental light having at least one corresponding fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; a fifth harmonic generator configured to receive a first portion of the fundamental light and generate fifth harmonic light having a fifth harmonic frequency equal to five times the fundamental frequency; a fan-shaped periodically poled nonlinear crystal configured to receive a second portion of the fundamental light; first and second nonlinear addition crystals; a frequency control system configured to cooperatively adjust the relative positions of the fan-shaped periodically polled nonlinear crystal and the nonlinear summation crystal in response to an output frequency signal, thereby when the output frequency signal has a first value, the frequency control system passes the second portion through a first region of the fan-shaped periodically polled nonlinear crystal, thereby generating a first down-converted signal having a first down-conversion frequency; the frequency control system positions the first nonlinear addition crystal to receive both the fifth harmonic light and the first down-converted signal, thereby generating deep ultraviolet (DUV) output light having a first wavelength within the range; When the output frequency signal has a second value, the frequency control system passes the second portion through a second region of the fan-shaped periodically polled nonlinear crystal, thereby generating a second down-converted signal having a second down-converted frequency, and the frequency control system positions the second nonlinear addition crystal to receive both the fifth harmonic light and the second down-converted signal, thereby generating the DUV output light having a second wavelength within the range.

20. 1. A method of operating a tunable laser assembly such that the tunable laser assembly generates first deep ultraviolet (DUV) output light having a first DUV wavelength during a first time period and generates second DUV output light having a second DUV wavelength during a second time period, both of the first and second DUV wavelengths being in a range of 184 nm to 200 nm, the method comprising: generating fundamental light having a fundamental frequency with a corresponding fundamental wavelength between 1 μm and 1.1 μm; generating fifth harmonic light having a fifth harmonic frequency equal to five times the fundamental frequency using a first portion of the fundamental light; directing a second portion of the fundamental light to a first region of the fan-periodically poled nonlinear crystal during the first time period, so that the fan-periodically poled nonlinear crystal generates a first down-converted signal having a first down-converted frequency with a corresponding wavelength of 1350 nm to 3300 nm, and directing both the first down-converted signal and the fifth harmonic light to a first nonlinear addition crystal, so that the first nonlinear addition crystal generates the first DUV output light having the first DUV wavelength; during the second time period, directing the second portion of the fundamental light to a second region of the fan-periodically poled nonlinear crystal so that the fan-periodically poled nonlinear crystal produces a second down-converted signal having a second down-converted frequency with a corresponding wavelength of 1350 nm to 3300 nm, and directing both the second down-converted signal and the fifth harmonic light to the second nonlinear adding crystal so that the second nonlinear adding crystal produces the second DUV output light having the second DUV wavelength; the first nonlinear addition crystal has a first crystal axis and is positioned such that, during the first period, the first down-converted signal passes through the first nonlinear addition crystal in a first optical propagation direction that forms a first angle with respect to the first crystal axis; the second nonlinear addition crystal has a second crystal axis and is positioned such that, during the second period, the second down-converted signal passes through the second nonlinear addition crystal in a second optical propagation direction that forms a second angle with respect to the second crystal axis; The method, wherein the first angle differs from the second angle by at least 5 degrees.

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