Laser closed power loop using an acousto-optic modulator for power modulation.

Abstract

A laser beam power modulation system includes an acousto-optic modulator (AOM) that receives a laser beam and separates the laser beam into a main beam and multiple diffracted beams based on an input signal, the power of the main beam being determined by the input signal. The system also includes a slit for passing the main beam and discarding the multiple diffracted beams, a controller for generating a control signal based at least in part on feedback indicative of the power of the main beam or the power of beams generated using the main beam, and a driver for generating the input signal based at least in part on the control signal.

Description

[Technical field]

[0001] The present disclosure relates to laser power control, and more specifically, to the use of acousto-optic modulators (AOMs) for laser power modulation. [Background technology]

[0002] Related Applications This application claims priority to U.S. Provisional Patent Application No. 62 / 886,477, filed August 14, 2019, which is incorporated by reference in its entirety for all purposes.

[0003] As semiconductor design rules become tighter, defect sizes become correspondingly smaller. Defect inspection tools, also called particle inspection tools or simply inspection tools, are required to detect the smallest particles in advanced design layers (e.g., design layers with the tightest design rules). To enable the detection of smaller particles, the laser power density requirements for laser scattering-based inspection tools continue to increase. Therefore, new inspection tools have higher laser power and smaller focused spot size.

[0004] High power lasers used in inspection tools typically have two or three harmonic frequency conversion stages. These lasers are typically operated at constant power, i.e. a closed control loop varies the power of the input stage to keep the power output of the final stage of frequency conversion constant. This can be achieved by varying the current of the pump diodes or by attenuating the power using a combination of half wave plates and polarizing beam cubes with motorized rotary mounts. In the first case, Nd:YAG or Nd:YVO 4 The power of the input stage is changed by changing the pump diode current of the fundamental wavelength of the laser, which has an associated settling time that adversely affects the operation of the system. In the second case, the power of the input stage is changed by rotating a half-wave plate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] US Patent Application Publication No. 2017 / 0050266 [Patent Document 2] US Patent Application Publication No. 2005 / 0270631 [Patent Document 3] US Patent Application Publication No. 2014 / 0204963 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the reliability of the motorized rotary mount that rotates the half-wave plate limits the reliability of the overall system. Moving parts are undesirable.

[0007] Therefore, there is a need for improved systems and methods for controlling laser power. [Means for solving the problem]

[0008] In some embodiments, a laser beam power modulation system includes an acousto-optic modulator (AOM) that receives a laser beam and separates the laser beam into a main beam and a plurality of diffracted beams based on an input signal, the power of the main beam being determined by the input signal. The system also includes a slit for passing the main beam and discarding the plurality of diffracted beams, a controller for generating a control signal based at least in part on feedback indicative of the power of the main beam or a power of a beam generated using the main beam, and a driver for generating the input signal based at least in part on the control signal.

[0009] In some embodiments, a method for modulating laser beam power includes splitting a laser beam into a main beam and a plurality of diffracted beams using an AOM, the power of the main beam being determined by an input signal from a driver, the method also includes discarding the plurality of diffracted beams while passing the main beam, generating a control signal based at least in part on feedback indicative of the power of the main beam or a power of a beam generated using the main beam, providing the control signal to the driver, and using the driver to generate the input signal based at least in part on the control signal.

[0010] For a better understanding of the various implementations described herein, the following detailed description should be read in conjunction with the following drawings, which may not be to scale. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 illustrates a laser beam power modulation system comprising an acousto-optic modulator (AOM) according to some embodiments. [Diagram 2] FIG. 2 illustrates a laser beam power modulation system including the laser beam power modulation system of FIG. 1 and further including a harmonic frequency conversion stage, according to some embodiments. [Diagram 3] FIG. 3 illustrates an example of the laser beam power modulation system of FIG. 2 for providing a laser beam to an inspection tool according to some embodiments. [Figure 4] FIG. 2 is a block diagram of a UV sensor, according to some embodiments. [Diagram 5] FIG. 2 is a block diagram of a green sensor according to some embodiments. [Figure 6] 1 is a graph illustrating an example of the power of a green primary beam versus control signal voltage, according to some embodiments. [Figure 7] 1 is a graph showing the results of a multi-day test in which a driver supplies RF power to an AOM to maintain the power of a laser beam. [Figure 8] 1 is a flowchart of a method for modulating laser beam power using an AOM, according to some embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Like reference numbers indicate corresponding parts throughout the drawings and specification.

[0013] Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments described herein. However, it will be apparent to those skilled in the art that the various embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been described in detail as not to unnecessarily obscure aspects of these embodiments.

[0014] FIG. 1 is a diagram illustrating a laser beam power modulation system 100, according to some embodiments. The system 100 includes a laser 102 that generates a laser beam 104. In some embodiments, the laser 102 is a green laser, and the laser beam is green (e.g., has a frequency in the range of 500-565 nm). For example, the laser 102 is a 532 nm mode-locked laser. In some embodiments, the laser 102 includes a laser cavity and a second harmonic generator (SHG) that doubles the frequency of the laser light from the laser cavity. For example, the laser cavity includes a Nd:YVO4 laser lasing at a fundamental wavelength of 1064 nm. 4 It contains a crystal whose 1064 nm light is converted by SHG to 532 nm light, resulting in a laser beam 104 .

[0015] The laser beam 104 is received by an acousto-optic modulator (AOM) 106, which splits it into a main beam 128 (i.e., a zeroth order beam) and a number of diffracted beams 130 (e.g., + / -1st order diffracted beams, + / -2nd order diffracted beams, etc.) based on an input signal received from a driver 114. The AOM 106 performs this splitting by diffracting a portion of the laser beam 104 into the diffracted beams 130 based on an input signal from the driver 114. Thus, when the AOM 106 is operating, the power of the main beam 128 is less than the power of the laser beam 104. The input signal specifies the strength of the diffraction that the AOM 106 performs, and therefore the amount of splitting that it performs.

[0016] In some embodiments, the driver 114 is a radio frequency (RF) driver and the input signal is an RF signal. The power of the main beam 128 depends on the input signal, as does the power of the multiple diffracted beams 130. For example, increasing the RF power of the input signal increases the diffraction performed by the AOM 106, and therefore decreases the power of the main beam 128. That is, more of the laser beam 104 is diffracted into the multiple diffracted beams 130. Similarly, decreasing the RF power of the input signal decreases the diffraction performed by the AOM 106, and therefore increases the power of the main beam 128. That is, less of the laser beam 104 is diffracted into the multiple diffracted beams 130.

[0017] The slit 124 acts as a filter that passes the main beam 128 and discards (i.e., filters out) the multiple diffracted beams 130. In some embodiments, the slit 124 includes a pair of mirrors with a gap between them. The gap passes the main beam 128 (i.e., the main beam 128 passes through the gap), while the mirrors reflect the multiple diffracted beams 130 to the beam dump 126.

[0018] In some embodiments, one or more mirrors direct the main beam 128 and the multiple diffracted beams 130 toward the slit 124. In the example of FIG. 1, the one or more mirrors include a first mirror (M1) 118 and a second mirror (M2) 120. The first mirror 118 reflects the main beam 128 and the multiple diffracted beams 130 and directs them toward the second mirror 120. The second mirror 120 reflects the main beam 128 and the multiple diffracted beams 130 and directs them toward the slit 124. The first mirror 118 and the second mirror 120 are disposed between the AOM 106 and the slit 124 along the optical paths of the main beam 128 and the multiple diffracted beams 130. The first mirror 118 is disposed between the AOM 106 and the second mirror 120 along the optical paths of the main beam 128 and the multiple diffracted beams 130.

[0019] In some embodiments, the system 100 further includes an attenuator 108 for attenuating the main beam 128. The attenuator 108 may also attenuate the multiple diffracted beams 130. For example, the attenuator 108 directs a portion of the light from the main beam 128 and the multiple diffracted beams 130 toward the beam dump 116. The attenuator 108 may be disposed between the AOM 106 and one or more mirrors that direct the main beam 128 and the multiple diffracted beams 130 toward the slit 124. For example, the attenuator 108 is disposed between the AOM 106 and the first mirror 118.

[0020] In some embodiments, a sensor 122 (e.g., green sensor 502, FIG. 5) receives light from the main beam 128 that leaks through a corresponding one of the one or more mirrors. The corresponding mirror reflects the main beam 128 but not completely, and therefore leaks (i.e., passes) some of the light from the main beam 128. In the example of FIG. 1, the corresponding mirror is the second mirror 120. That is, light from the main beam 128 leaks through the second mirror 120 and is provided to the sensor 122. The sensor 122 may include an aperture (e.g., aperture 504, FIG. 5) to filter out light from the multiple diffracted beams 130 that leaks through the second mirror 120 so that the sensor 122 senses only light from the main beam 128. The power of the light that leaks through the corresponding mirror and is sensed by the sensor 122 is related to the power of the main beam 128 (e.g., is a percentage of the power of the main beam 128). By sensing this light, the sensor 122 generates feedback indicative of the power of the main beam 128 and provides this feedback to the controller 110.

[0021] Controller 110 (e.g., a microcontroller) generates control signal 112 based at least in part on feedback from sensor 122. In some embodiments, controller 110 filters out noise (e.g., noise in the kilohertz range) from the feedback in the process of generating control signal 112. Controller 110 provides control signal 112 to driver 114, which generates an input signal it provides to AOM 106 based at least in part on control signal 112. For example, control signal 112 specifies the RF power of the input signal. In some embodiments, control signal 112 specifies a DC voltage (V DC ).

[0022] The laser beam 128 that passes through the slit 124 may be provided to subsequent optics for use in an optical system.

[0023] FIG. 2 is a diagram illustrating a laser beam power modulation system 200 according to some embodiments, which includes the laser beam power modulation system 100 (FIG. 1) and further includes a harmonic frequency conversion stage. A harmonic generator 210 converts a first portion of the main beam 128 into a converted beam 212, which is a harmonic of the main beam 128. Because the conversion efficiency of the harmonic generator 210 is less than 100 percent, the harmonic generator 210 also passes a second portion 214 of the main beam 128. The harmonic generator 210 includes a nonlinear optical crystal (e.g., a cesium-lithium-borate (CLBO) crystal). In some embodiments, the harmonic generator 210 is a fourth harmonic generator. That is, the frequency of the converted beam 212 can be four times the fundamental frequency of the laser beam emitted by the laser cavity of the laser 102 and twice the frequency of the main beam 128. In some embodiments, the primary beam 128 is green and the converted beam 212 is ultraviolet (UV). For example, the primary beam 128 has a wavelength of 532 nm and the converted beam 212 has a wavelength of 266 nm.

[0024] In some embodiments, a beam reduction telescope (BRT) 202 focuses the primary beam 128 provided by the system 100 onto the harmonic generator 210. The beam reduction telescope is disposed in the path of the primary beam 128 between the slit 124 and the harmonic generator 210. The beam reduction telescope 202 may change the shape of the beam spot of the primary beam 128 from circular to elliptical. The beam reduction telescope 202 may include a spherical lens 204 and a pair of cylindrical lenses 206 and 208, with the spherical lens 204 located closest to the slit 124.

[0025] Because the converted beam 212 is a harmonic of the primary beam 128, it has a different frequency than the primary beam 128 and, therefore, a different frequency than the second portion 214 of the primary beam 128. A frequency separator 216 receives the converted beam 212 and the second portion 214 of the primary beam, passes the converted beam 212 and discards the second portion 214 of the primary beam. After passing through the frequency separator 216, the converted beam 212 may be provided to an illumination system 234.

[0026] In some embodiments, a sensor 232 (e.g., UV sensor 402, FIG. 4) receives light from the converted beam 212 that leaks out of the frequency separator 216. The power of the light that leaks out of the frequency separator 216 and is sensed by the sensor 232 is related to the power of the converted beam 212 (e.g., is a percentage of the power of the converted beam 212). By sensing this light, the sensor 232 generates feedback indicative of the power of the converted beam 212 and provides this feedback to the controller 110. The controller 110 may generate the control signal 112 based at least in part on the feedback from the sensor 232. For example, the controller 110 may generate the control signal 112 using feedback from both the sensor 232 and the sensor 122, or may generate the control signal 112 using feedback from the sensor 232 but without feedback from the sensor 122. Alternatively, the controller 110 may generate the control signal 112 using feedback from the sensor 122 but without feedback from the sensor 232. In some embodiments, the controller 110 filters out noise (eg, noise in the kilohertz range) from the feedback from the sensor 232 and / or the sensor 122 in the process of generating the control signal 112.

[0027] In some embodiments, the frequency separator 216 includes a Perrin-Broca prism 222, a third mirror (M3) 224, and a fourth mirror (M4) 226. The third mirror 224 is disposed between the Perrin-Broca prism 222 and the fourth mirror 226 in the beam path of the converted beam 212. The Perrin-Broca prism 222 spatially separates the converted beam 212 and the second portion of the main beam 214. The front surface of the Perrin-Broca prism 222 also reflects a portion of the second portion of the main beam 214 and directs it toward the beam dump 220. The third mirror 224, which is a dichroic, reflects the converted beam 212 while passing the second portion of the main beam 214 to the beam dump 228 for disposal.

[0028] The third mirror 224 reflects and directs the converted beam 212 towards the fourth mirror 226, which reflects and thus passes the converted beam 212 beyond the frequency separator 216. The third mirror 224 may also reflect a portion (e.g., <5%) of the second portion 214 of the main beam on a spatially independent path towards the fourth mirror 226. The fourth mirror 226, which may be dichroic, reflects and directs a portion (e.g., <5%) of the light from this second portion of the main beam towards the beam dump 230. Thus, the frequency separator 216 substantially discards the second portion 214 of the main beam (e.g., green light that was not converted to UV by the harmonic generator 210) while passing the converted beam 212 (e.g., UV light generated by the harmonic generator 210).

[0029] 3 is a diagram illustrating a laser beam power modulation system 300 according to some embodiments in which the laser beam power modulation system 200 (FIG. 2) provides a converted beam 212 to a laser scattering inspection tool 302. The inspection tool 302 uses the converted beam 212 to inspect a substrate (e.g., a semiconductor wafer or a reticle) for defects (e.g., particles). In the system 300, the illumination system 234 is the illumination system of the inspection tool 302.

[0030] In some embodiments, the inspection tool 302 performs an initial inspection with a low power beam to determine whether a large defect (e.g., a particle) is present on the substrate before irradiating the substrate with the high power converted beam 212. A large defect is a defect whose size meets a threshold (e.g., exceeds or is equal to or greater than the threshold). The inspection tool 302 may use a photomultiplier tube (PMT) as a sensor for imaging with the low power beam and may use a separate sensor (e.g., a time delay integration (TDI) sensor) to detect defects with the converted beam 212. The presence of a large defect may be determined based on the amplitude of the PMT signal (e.g., whether the amplitude of the PMT signal meets a threshold). In response to determining that a defect whose size meets the threshold is present on the substrate, the inspection tool 302 provides (e.g., asserts) a signal 304 to the controller 110. The signal 304 (e.g., when asserted) may be referred to as a fast laser power modulation (LPM) trigger and indicates that the size of the defect meets the threshold. In response to signal 304 indicating that the size of the defect meets a threshold, controller 110 asserts control signal 306, which is provided to driver 114. The driver 114 generates an input signal to provide to AOM 106 based on control signal 306 in addition to control signal 112. In response to assertion of control signal 306, the AOM 106 temporarily reduces the power of the main beam 128. For example, in response to assertion of control signal 306, driver 114 temporarily increases the RF power of the input signal, causing AOM 106 to diffract laser beam 104 more, thus attenuating main beam 128. This temporary attenuation of main beam 128 attenuates converted beam 212, which prevents ablation of the defect by converted beam 212.

[0031] FIG. 4 is a block diagram of a UV sensor 402, according to some embodiments. The UV sensor 402 is one example of a sensor 232 (FIGS. 2-3). UV light 400 (e.g., leakage light from the converted beam 212, FIGS. 2-3) incident on the UV sensor 402 is attenuated by a diffuser 404 and then filtered by an ND filter 406 and a bandpass filter 408 before being detected by a UV detector 410. The bandpass filter 408, according to some embodiments, passes (i.e., allows light to pass) at the wavelength of the converted beam 212 (e.g., 266 nm). The UV detector 410 generates a feedback signal 412 based on the detected light and provides the feedback signal 412 to the controller 110 (FIGS. 2-3).

[0032] 5 is a block diagram of a green sensor 502, according to some embodiments. The green sensor 502 is an example of a sensor 122 (FIGS. 1-3). An aperture 504 limits the green light 500 that the green sensor 502 may receive. For example, the aperture 504 allows leakage light from the main beam 128 to enter the green sensor 502 and blocks leakage light from the multiple diffracted beams 130 from entering the green sensor 502. The green light 500 that passes through the aperture 504 is attenuated by a diffuser 506 and filtered by an ND filter 508 before being detected by a green detector 510. The green detector 510 generates a feedback signal 512 based on the detected light and provides the feedback signal 512 to the controller 110 (FIGS. 1-3).

[0033] FIG. 6 is a graph showing an example of the power 604 of the green main beam 128 versus the control signal voltage 602, according to some embodiments. The control signal voltage 602 is an example of the control signal 112 (FIGS. 1-3). The voltage dependence of the power 604 can be determined by fitting an appropriate curve 606 (e.g., a polynomial as shown in FIG. 6) to each data point. The controller 110 can use this curve 606 (i.e., curve fitting) to control the power 604 by controlling the AOM 106. For example, the controller 110 can set the control signal voltage 602 to maintain the power 604 at a nearly constant value (e.g., within 1 percent). To maintain the power 604 of the main beam 128 at a desired value, power (e.g., RF power) can be constantly provided from the driver 114 to the AOM 106 via the input signal described above (FIGS. 1-3). A similar curve can be generated for the converted beam 212. 7 shows the results of a multi-day test in which the driver 114 provides 80 MHz RF power to the AOM 106 to maintain the power 702 of the converted beam 212 (266 nm UV in this test) at 4 W. In this test, the sensor 122 was placed behind the first mirror 118 instead of the second mirror 120. The results show a typical power noise of less than 1 percent. Effective control of the main beam 128 and the converted beam 212 is achieved without the use of moving parts (e.g., rotating mounts) and without modulating the pump diodes in the laser 102.

[0034] FIG. 8 is a flow chart of a method 800 of modulating the power of a laser beam, according to some embodiments. In the method 800, a laser beam (e.g., laser beam 104, FIGS. 1-3) is generated (802) (e.g., by laser 102, FIGS. 1-3). Using an AOM (e.g., AOM 106, FIGS. 1-3), the laser beam is split (804) into a main beam (e.g., main beam 128, FIGS. 1-3) and multiple diffracted beams (e.g., multiple diffracted beams 130, FIGS. 1-3). The power of the main beam is determined by an input signal (e.g., an RF signal) from a driver (e.g., driver 114, FIGS. 1-3).

[0035] The diffracted beams are discarded (806) while the main beam passes through. In some embodiments, one or more mirrors (e.g., first mirror 118 and second mirror 120, FIGS. 1-3) are used (808) to direct the main beam and the diffracted beams towards a slit (e.g., slit 124, FIGS. 1-3) that passes the main beam and discards the diffracted beams.

[0036] In some embodiments, a first portion of the main beam is converted (810) into a converted beam (e.g., converted beam 212, FIGS. 2-3) that is a harmonic of the main beam (e.g., the fourth harmonic of the fundamental wavelength of the laser cavity used in step 802) (e.g., has twice the frequency of the main beam), while a second portion of the main beam (e.g., second portion 214, FIGS. 2-3) passes (e.g., through harmonic generator 210, FIGS. 2-3). The second portion of the main beam is then discarded, while the converted beam passes. In some embodiments, the converted beam is separated (812) from the second portion of the main beam using a frequency separator (e.g., frequency separator 216, FIGS. 2-3). The frequency separator discards the second portion of the main beam, while passing the converted beam. In some embodiments, the converted beam is provided (814) to an illumination system of a defect inspection tool (eg, illumination system 234 of inspection tool 302, FIG. 3).

[0037] A control signal is generated (816) based at least in part on feedback indicative of the power of the main beam or a beam generated using the main beam (e.g., the converted beam of step 810). For example, controller 110 (FIGS. 1-3) generates control signal 112. In some embodiments, the feedback is generated (818) based at least in part on light from the converted beam leaking through the frequency separator to a first sensor (e.g., sensor 232, FIGS. 2-3). This is one example of generating a control signal at least in part on feedback indicative of the power of a beam generated using the main beam. In some embodiments, the feedback is generated (820) based at least in part on light from the main beam leaking through a corresponding one of a plurality of mirrors (e.g., second mirror 120, FIGS. 1-3) to a second sensor (e.g., sensor 122, FIGS. 1-3). This is one example of generating a control signal at least in part on feedback indicative of the power of the main beam.

[0038] In some embodiments, the control signal is generated based at least in part on the feedback indicative of the power of the main beam and the feedback indicative of the power of the beam (e.g., the converted beam) generated using the main beam. For example, the control signal may be generated using the feedback of step 818 and the feedback of step 820 (i.e., steps 818 and 820 are both included in method 800). The main beam may be green, the converted beam may be ultraviolet, a first sensor may be a UV sensor (e.g., UV sensor 402, FIG. 4), and a second sensor may be a green sensor (e.g., green sensor 502, FIG. 5).

[0039] In some embodiments, noise (e.g., noise in the kilohertz range) is filtered out (822) from the feedback to generate the control signal. Noise contributions include inherent laser noise, noise due to harmonic conversion, sensor noise (e.g., noise due to the first and / or second sensors of steps 818 and 820), and noise due to modulation of the AOM driver (e.g., DC modulation). The filtering may be performed using two loops, an inner loop and an outer loop. The inner loop averages the feedback signal and stores the results in an array. The outer loop sorts the array and periodically selects the median beam power of the array. A value for the control signal may be determined based on this median value.

[0040] The control signal is provided to the driver 824. The input signal is generated based at least in part on the control signal using the driver 826. The input signal is provided to the AOM.

[0041] In some embodiments, the control signal of step 816 is a first control signal, and passing the converted beam in step 810 includes providing the converted beam to an illumination system of a defect inspection tool (e.g., illumination system 234 of inspection tool 302, FIG. 3). Method 800 may further include asserting a second control signal (e.g., control signal 306, FIG. 3) in response to a signal from the defect inspection tool indicating that a size of the defect on the substrate meets a threshold (e.g., signal 304, FIG. 3). In step 826, the input signal is generated based on the first control signal and the second control signal. In response to assertion of the second control signal, the AOM temporarily reduces the power of the main beam, resulting in a corresponding attenuation of the converted beam. For example, in response to assertion of the second control signal, the driver temporarily increases the RF power of the input signal, causing the AOM to diffract the laser beam more and thus attenuate the main beam.

[0042] Controller 110 (FIGS. 1-3) includes a processor circuit and may include embedded non-volatile memory. The embedded non-volatile memory stores instructions (e.g., one or more programs) for the processor circuit to execute. For example, controller 110 may be a microcontroller with embedded flash memory or other non-volatile memory. Alternatively, the controller is coupled to a separate non-volatile memory that stores instructions for the processor circuit to execute. The non-volatile memory, whether embedded in or coupled to controller 110, is one example of a non-transitory computer-readable storage medium. When the processing circuit executes the instructions, the instructions cause controller 110 to perform the functions described herein for controller 110 (e.g., generating control signals, which may include filtering feedback).

[0043] The above description refers to specific embodiments for purposes of explanation. However, the above illustrative discussion is not intended to be exhaustive or to limit the scope of the claims to the precise form disclosed herein. Numerous modifications and variations are possible in light of the above teachings. The above embodiments were selected to best explain the principles underlying the claims and their practical application, and thus to allow those skilled in the art to most effectively use these embodiments with various modifications appropriate to the particular use contemplated.

Claims

1. 1. A laser beam power modulation system comprising: an acousto-optic modulator for receiving a laser beam and separating the laser beam into a main beam and a plurality of diffracted beams based on an input signal, the power of the main beam being determined by the input signal; a slit for passing the main beam and discarding the plurality of diffracted beams; a controller for generating a control signal based at least in part on feedback indicative of a power of the main beam and a power of a beam generated using the main beam; a driver for generating the input signal based at least in part on the control signal; a harmonic generator for converting a first portion of the main beam into a converted beam that is a harmonic of the main beam and for passing a second portion of the main beam; a frequency separator for receiving the converted beam and the second portion of the main beam, passing the converted beam, and discarding the second portion of the main beam; Equipped with the converted beam is a beam generated using the main beam, a sensor for receiving light from the converted beam that leaks out of the frequency separator and for providing the feedback indicative of the power of a beam generated using the main beam; the controller generates the control signal based at least in part on the feedback indicative of a power of a beam generated using the main beam; the frequency separator providing the converted beam to an illumination system of a defect inspection tool; the control signal is a first control signal, the controller asserts a second control signal in response to a signal from the defect inspection tool indicating that a size of a defect on the substrate meets a threshold; the driver generates the input signal based on the first control signal and the second control signal, and the acousto-optic modulator temporarily reduces the main beam power in response to assertion of the second control signal. system.

2. one or more mirrors for directing the main beam and the plurality of diffracted beams towards the slit; a sensor for receiving light from the main beam that leaks from a corresponding one of the one or more mirrors and providing the feedback indicative of a power of the main beam based on the received light; Further equipped with The system of claim 1 , wherein the controller generates the control signal based at least in part on the feedback indicative of a power of the main beam.

3. the one or more mirrors include a first mirror and a second mirror; the first mirror is disposed in the optical paths of the main beam and the plurality of diffracted beams between the acousto-optic modulator and the second mirror; The system of claim 2 , wherein the second mirror is the corresponding mirror from which light leaks out of the main beam received by the sensor.

4. The system of claim 2 , further comprising an attenuator disposed in the optical path of the main beam and the plurality of diffracted beams between the acousto-optic modulator and the one or more mirrors.

5. the driver is a radio frequency (RF) driver; The system of claim 1 , wherein the input signal is an RF signal.

6. The system of claim 1 , wherein the control signal is a direct current (DC) voltage.

7. The system of claim 6 , wherein the controller filters out noise from the feedback to generate the control signal.

8. 10. The system of claim 1, further comprising a beam dump, the slit including a pair of mirrors for directing the plurality of diffracted beams toward the beam dump, the pair of mirrors separated by a gap for passing the main beam.

9. The system of claim 1 , wherein the harmonic generator is a fourth harmonic generator.

10. 10. The system of claim 1, further comprising a beam reducing telescope disposed in the optical path of the main beam between the slit and the harmonic generator for focusing the main beam onto the harmonic generator.

11. The system of claim 1 , wherein said primary beam is green, said converted beam is ultraviolet, and said sensor is an ultraviolet sensor.

12. The frequency separator comprises: a Perrin-Broca prism for separating the converted beam and the second portion of the main beam; a first dichroic mirror for reflecting the converted beam and passing at least a portion of the second portion of the main beam; a second mirror for reflecting the converted beam and for leaking light from the converted beam to the sensor; Equipped with 2. The system of claim 1, wherein the first dichroic mirror is disposed in a beam path of the converted beam between the Perrin-Broca prism and the second mirror.

13. the sensor is a first sensor, one or more mirrors for directing the main beam and the plurality of diffracted beams towards the slit; a second sensor for receiving light from the main beam that leaks from a corresponding one of the one or more mirrors and providing the feedback indicative of a power of the main beam based on the received light; Further equipped with 2. The system of claim 1, wherein the controller generates the control signal based at least in part on the feedback indicative of a power of the main beam and the feedback indicative of a power of a beam generated using the main beam.

14. 14. The system of claim 13, wherein the primary beam is green and the converted beam is ultraviolet, the first sensor is an ultraviolet sensor and the second sensor is a green sensor.

15. Using an acousto-optic modulator to split a laser beam into a main beam and a plurality of diffracted beams, the power of the main beam being determined by an input signal from a driver; discarding the plurality of diffracted beams while passing the main beam; generating a control signal based at least in part on feedback indicative of a power of the main beam and a power of a beam generated using the main beam; providing said control signal to said driver; generating, using the driver, the input signal based at least in part on the control signal; converting a first portion of the main beam into a converted beam that is a harmonic of the main beam while passing a second portion of the main beam; passing the converted beam while discarding the second portion of the main beam; Including, the converted beam is a beam generated using the main beam, generating the control signal based at least in part on the feedback indicative of a power of a beam generated using the main beam; passing the converted beam while discarding the second portion of the main beam includes separating the converted beam and the second portion of the main beam using a frequency separator; generating the feedback indicative of a power of a beam generated using the main beam based on light from the converted beam leaking through the frequency separator to a sensor; passing the transformed beam includes providing the transformed beam to an illumination system of a defect inspection tool; the control signal is a first control signal, asserting a second control signal in response to a signal from the defect inspection tool indicating that a size of a defect on a substrate meets a threshold; the input signal is generated based on the first control signal and the second control signal; The method of claim 1, wherein the acousto-optic modulator temporarily reduces the power of the main beam in response to assertion of the second control signal.

16. discarding the plurality of diffracted beams while passing the main beam includes using one or more mirrors to direct the main beam and the plurality of diffracted beams towards a slit, the slit passing the main beam and discarding the plurality of diffracted beams; generating the control signal based at least in part on the feedback indicative of a power of the main beam; 16. The method of claim 15, further comprising generating the feedback indicative of a power of the main beam based on light from the main beam leaking through a corresponding one of the plurality of mirrors to a sensor.

17. The method of claim 15 , wherein generating the control signal includes filtering out noise from the feedback.

18. the sensor is a first sensor, discarding the plurality of diffracted beams while passing the main beam includes using one or more mirrors to direct the main beam and the plurality of diffracted beams towards a slit, the slit passing the main beam and discarding the plurality of diffracted beams; generating the control signal based at least in part on the feedback indicative of a power of the main beam and the feedback indicative of a power of a beam generated using the main beam; 16. The method of claim 15, further comprising generating the feedback indicative of a power of the main beam based on light from the main beam leaking through a corresponding one of the plurality of mirrors to a second sensor.

19. 20. The method of claim 18, wherein the primary beam is green and the converted beam is ultraviolet, the first sensor is an ultraviolet sensor and the second sensor is a green sensor.

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