Patent Publication Number: US-9413134-B2

Title: Multi-stage ramp-up annealing for frequency-conversion crystals

Description:
RELATED APPLICATION 
     The present application is a divisional of U.S. Application Ser. No. 14/507,758, entitled “Laser With High Quality, Stable Output Beam, And Long Life High Conversion Efficiency Non-Linear Crystal”, and filed Oct. 6, 2014, which is a divisional of U.S. application Ser. No. 13/412,564, entitled “Laser With High Quality, Stable Output Beam, And Long Life High Conversion Efficiency Non-Linear Crystal”, and filed Mar. 5, 2012, now U.S. Pat. No. 8,873,596, and claims priority of U.S. Provisional Application 61/510,633, entitled “Mode-Locked UV Laser With High Quality, Stable Output Beam, Long-Life High Conversion Efficiency Non-Linear Crystal And A Wafer Inspection Using A Mode-Locked UV Laser”, and filed on Jul. 22, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to lasers and in particular to lasers using non-linear crystals to obtain shorter wavelength radiation from the longer wavelength radiation by a frequency-conversion process. 
     2. Related Art 
     Each successive node of semiconductor manufacturing requires detection of smaller defects and particles on the wafer. Therefore, yet higher power and shorter wavelength UV (ultraviolet) lasers for wafer inspection are always in demand. Because the defect or particle size is reduced, the fraction of the light reflected or scattered by that defect or particle is also typically reduced. As a result, an improved signal-to-noise ratio is required to detect smaller defects and particles. If a brighter light source is used to illuminate the defect or particle, then more photons will be scattered or reflected and the signal-to-noise ratio can be improved as long as other noise sources are controlled. Using shorter wavelengths can further improve the sensitivity to smaller defects, because the fraction of light scattered by a particle smaller than the wavelength of light increases as the wavelength decreases. 
     In general, semiconductor wafer inspection and metrology needs can be met by continuous wave (CW) lasers with high beam quality (e.g. with M 2  close to 1, wherein M 2  is the ratio of the beam parameter product of the beam to that of an ideal Gaussian beam of the same wavelength). If CW lasers of sufficient power and beam quality are not available, then the next best alternative is generally a high repetition rate laser, e.g. with a repetition rate of approximately 50 MHz or higher. Such high repetition rates are possible with mode-locked lasers (which is a type of pulsed laser). Q-switched lasers have repetition rates that are much lower (lower than 10 MHz, usually lower than 1 MHz). Generally, mode-locked lasers are capable of emitting extremely short pulses on the order of picoseconds or even femtoseconds. A mode-locked laser induces a fixed phase relationship between the modes of its resonant cavity such that interference between those modes causes the laser light to be produced as pulses. 
     Beam quality (e.g. as measured by M 2 ) is important in semiconductor inspection and metrology applications because a laser beam must be focused to a small spot (or line) to detect small defects or particles and/or to measure small areas. If the beam quality is poor, then the focused spot (or line) on the wafer is not Gaussian in profile and the tails of that profile contain more energy than ideal. Those bigger tails result in at least some of the signal being collected from outside the area of interest, thereby reducing the contrast of the signal from the area of interest. 
     Non-linear crystals can be used to create a UV laser beam by generating a harmonic of a long wavelength beam or by mixing two laser beams of different frequencies to create a frequency equal to the sum (or difference) of the two frequencies. Because the harmonic generation and the mixing process are non-linear processes, higher incident power density typically results in a more efficient conversion process and higher output power. 
     However, increasing the incident laser power on a non-linear crystal can have undesirable side effects. Specifically, a high power level may change the refractive index of the crystal (photorefraction). Because the focused laser spot in the crystal has an approximately Gaussian profile, the intensity is different at different locations within the crystal. Therefore, the change in the refractive index varies with location in the crystal. This refractive index gradient in the crystal can distort the output beam, thereby resulting in astigmatism. As the quality of the output laser beam worsens, the spot or the line on the wafer generated by that beam becomes broader and thus less efficient for detecting small particles or defects. Although small amounts of astigmatism may be approximately corrected by optics placed in the beam path after the crystal, such correction will only be approximate and will only be effective if the initial astigmatism level is very low. 
     Another undesirable side effect of higher incident power level on the crystal is that permanent damage may occur in the crystal over time. With accumulated exposure, this damage can result in generally decreasing power intensity as well as generally increasing astigmatism. Therefore, correcting the astigmatism with optics would require frequent compensating adjustments, which would be impractical in commercial applications. Moreover, the astigmatism may also rapidly increase to the level where accurate compensation is not possible even with adjustment. 
     Generating a shorter output wavelength can also accelerate the degradation of the crystal because the output photons are more energetic and therefore can change characteristics of, or even permanently damage, the crystal. Thus, at shorter output wavelengths, astigmatism and other adverse beam quality and intensity effects may also increasingly occur. 
     The optimum power density in the non-linear crystal is a balance between maximum conversion efficiency (which usually requires as high a power density as possible), and minimizing color center formation, photorefraction, and two-photon absorption (all of which are minimized by lowering the power density) while maintaining a good beam profile. 
     Notably, photorefraction and two-photon absorption may cause a temporary change in optical properties, which persists for at least the duration of the incident laser pulse and, typically, for a short time thereafter. When the laser repetition rate is low, as in a Q-switched laser, there may be sufficient time between one pulse and the next for these changes to the crystal to partially or fully relax back to the original state. This relaxation may be faster if the crystal is operating at a high temperature (such as between 120-150° C., which is a typical temperature range for standard operation). Applications in the semiconductor inspection and metrology typically are better served by very high repetition rates (such as 50 MHz, 100 MHz, or higher) as can be achieved by mode-locked lasers. However, such high repetition rates typically do not allow time for the changes in crystal properties to relax substantially from one pulse to the next. 
     Non-linear crystals, such as CLBO (cesium lithium borate) or CBO (cesium borate), can be used to create deep UV light from the 2 nd  harmonic of a visible laser light input. For example, 266 nm wavelength light can be created from a 532 nm laser beam using CLBO. In another embodiment, light near 213 nm wavelength can be created by mixing, for example, 266 nm and 1064 nm wavelengths. The maximum power level at which such crystals can be operated is limited by defects and impurities in the crystals. 
     Impurities in a crystal or defects in its crystal lattice can degrade the lifetime of the crystal or create color centers that become locations at which changes in the crystal optical properties happen faster than elsewhere in the crystal. Thus, to the extent possible, the highest purity of starting material should be used for fabricating the crystal. 
     Notably, impurities, such as water, can be incorporated into a crystal during its growing process or even during normal operation (when used in an inspection system) even if not present in the starting material. These impurities can adversely impact the crystal lifetime at high power densities. Unfortunately, improving the purity of the starting material does not reduce impurities that are incorporated during operation. 
     One known technique for reducing or slowing deterioration in the crystal is to operate the crystal at a high temperature (typically between 120-150° C.), which generates higher energy electrons in the crystal. These higher energy electrons are able to move around more easily, thereby cancelling out some of the light-induced changes in the short term. This technique is most useful for low repetition rate lasers because there is a relatively long time interval between pulses (which allows recovery from the effects of one pulse). This high operating temperature can also help prevent absorption of water by the crystal while it is in use. 
     Although operating the crystal at a high temperature can provide more electrons with high enough mobility to neutralize some changes in the crystal, it also increases the energy of defect states in the crystal. Thus, the high operating temperature can slow some defect mechanisms while accelerating others. Notably, the increased temperature is less effective at reducing short-term changes in the crystal when the repetition rate is high. 
     Another known technique to deal with crystal damage is to use one location in the crystal for a period of time, then move to a new location before there is too much degradation of the output beam quality and/or intensity. Frequent adjustment of the location in the crystal used for frequency conversion means that for a significant fraction of the operating time, the laser is being adjusted, realigned, or is stabilizing after an adjustment. Even if the adjustment and realignment are automated, there may be times when the laser is not operating to full specification while it stabilizes after adjustment. In inspection and measurement applications in industries like the semiconductor industry where manufacturing facilities run 24 hours a day, this interruption to, or reduction in, operating time is a significant disadvantage. If the damage rate is high, even with frequent and automatic adjustments of the conversion location in the crystal, the crystal may still last only a few days or a few weeks before needing to be replaced. Such a short time interval between service events is unacceptable in the semiconductor industry. 
     Therefore, a need arises for a high power laser system that includes a frequency-conversion crystal (i.e. a non-linear crystal capable of generating harmonics of the fundamental laser wavelength), yet can ensure a high quality, stable laser beam and a long crystal lifetime. 
     SUMMARY OF THE INVENTION 
     A mode-locked laser system operable at low temperature is described. As used herein, a “low” temperature is approximately 50° C. and below (wherein a conventional standard operating temperature is at least 100° C., and typically between 120° C.-150° C.). In one embodiment, the low temperature may be between 30° C. and −10° C. This laser system can include an annealed, frequency-conversion crystal and a housing to protect the crystal from impurities during standard operation at the low temperature, and thereby maintain the annealed condition of the crystal. In general, the crystal has a length approximately equal to the smaller of twice the Rayleigh range in a non-walk-off direction and a length of twice a beam waist radius in a walk-off direction divided by a walk-off angle in radians. In one embodiment, the crystal can have an increased length (such as 12.5 mm-long or 15 mm-long CLBO compared with a typical 10 mm-long CLBO used for converting 532 nm to 266 nm). First beam shaping optics can be configured to focus a beam from a light source to an elliptical cross section at a beam waist located in or proximate to the crystal. A harmonic separation block can divide an output from the crystal into beams of different frequencies separated in space. In one embodiment, the mode-locked laser system can further include second beam shaping optics configured to convert an elliptical cross section of the desired frequency beam into a beam with a desired aspect ratio, such as a circular cross section. 
     In one embodiment, the elliptical cross section in the crystal can have an aspect ratio between 2:1 and 6:1. The first beam shaping optics can include at least one of a prism, a cylindrical curvature element, a radially-symmetric curvature element, and a diffractive element. The harmonic separation block can include a prism, such as a Pellin-Broca prism. The crystal can be a CLBO (cesium lithium borate) crystal, a CBO (cesium borate) crystal, a BBO (β barium borate) crystal, an LBO (lithium triborate) crystal, a lithium niobate crystal, a KDP (potassium dihydrogen phosphate) crystal or another non-linear optical crystal. 
     A method of operating a laser system at low temperature is also described. In this method, a beam from a light source can be focused into an elliptical cross section at a beam waist in or proximate to an annealed, frequency-conversion crystal. From an output of the crystal, a desired frequency beam can be separated from any undesired frequency beam. The method can further include maintaining an annealed condition of the crystal during standard operation at the low temperature and/or converting the elliptical cross section into a circular cross section. 
     A method of annealing a frequency-conversion crystal is also described. In this method, the temperature of the crystal can be increased to about 150° C. over about 2 hours. The temperature can be held near 150° C. for about 10 hours. Then, the temperature can be increased to about 200° C. over about 1 hour. At this point, the temperature can be held between 150-200° C. for about 100 hours. Finally, the temperature can be decreased down to room temperature over about 3 hours. 
     Another method of annealing a frequency-conversion crystal is also described. In this method, during the first annealing step described above, a determination is made whether —OH bonds absorption is at a first level. If not, then the temperature is continued to be held near 150° C. If so, then the temperature is increased to about 200° C. over about 1 hour. During the second annealing step described above, a determination is made whether the —OH bonds absorption is at a second level. If not, then the temperature is continued to be held between 150-200° C. If so, then the temperature is decreased down to room temperature over about 3 hours. In one embodiment, determining absorption can be performed using FTIR (Fourier transform infrared spectroscopy). For example, FTIR can monitor the —OH bonds near 3580 cm −1  in an infra-red spectrum. 
     A multi-stage ramp-up annealing process is also described. In this process, a first ramp-up stage can increase a temperature to a first predetermined temperature. The temperature can be held at the first predetermined temperature for a first predetermined period. Then, a second ramp-up stage can increase the temperature to a second predetermined temperature, the second predetermined temperature being higher than the first predetermined temperature. The temperature can be held at the second predetermined temperature for a second predetermined period. Finally, the temperature can be decreased to room temperature. 
     Another multi-stage ramp-up annealing process is also described. In this process, a first ramp-up stage can increase a temperature to a first predetermined temperature. The temperature can be held at the first predetermined temperature until —OH bonds absorption is at a first level. Then a second ramp-up stage can increase the temperature to a second predetermined temperature, which is higher than the first predetermined temperature. The temperature can be held at the second predetermined temperature until the —OH bonds absorption is at a second level. Finally, the temperature can be decreased to room temperature. 
     An optical system for detecting contaminants and defects on a test surface is also described. This optical system can include the improved laser system for generating a laser beam and optics directing the laser beam along a path onto the test surface, and producing an illuminated spot thereon. A detector and ellipsoidal mirrored surface are also provided. The mirrored surface and detector have an axis of symmetry about a line perpendicular to the test surface. The mirrored surface defines an input aperture positioned proximate to the test surface to receive scattered light there through from the surface and an exit aperture. The mirrored surface is substantially rotationally symmetric about the axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about the axis of symmetry light that passes through the input aperture to the detector. The exit aperture is located opposite to the input aperture. 
     An optical system for detecting anomalies of a sample is also described. This optical system can include the improved laser system for generating first and second beams. First optics can direct the first beam of radiation along a first path onto a first spot on a surface of the sample. Second optics can direct the second beam of radiation along a second path onto a second spot on a surface of the sample. The first and second paths are at different angles of incidence to said surface of the sample. Collection optics can include a curved mirrored surface for receiving scattered radiation from the first or the second spot on the sample surface and originating from the first or second beam and focusing the scattered radiation to a first detector. The first detector can provide a single output value in response to the radiation focused onto it by said curved mirrored surface. An instrument can cause relative motion between the two beams and the sample so that the spots are scanned across the surface of the sample. 
     A surface inspection apparatus is also described. This apparatus can include the improved laser system for generating a beam of radiation. An illumination system can be configured to focus the beam of radiation at a non-normal incidence angle relative to a surface to form an illumination line on the surface substantially in a plane of incidence of the focused beam. The plane of incidence is defined by the focused beam and a direction that is through the focused beam and normal to the surface. A collection system can be configured to image the illumination line. The collection system can include an imaging lens for collecting light scattered from a region of the surface comprising the illumination line, a focusing lens for focusing the collected light, and a device comprising an array of light sensitive elements. Each light sensitive element of the array of light sensitive elements can be configured to detect a corresponding portion of a magnified image of the illumination line. 
     A pulse multiplier including the improved laser system for generating an input laser pulse is also described. This pulse multiplier can further include a polarizing beam splitter that receives the input laser pulse. A wave plate can receive light from the polarized beam splitter and generate a first set of pulses and a second set of pulses, the first set of pulses having a different polarization than the second set of pulses. A set of mirrors can create a ring cavity including the polarizing beam splitter and the wave plate. The polarizing beam splitter can transmit the first set of pulses as an output of the pulse multiplier and reflect the second set of pulses into the ring cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a simplified, laser system in accordance with the present invention. 
         FIG. 1B  illustrates an exemplary Gaussian laser beam with beam waist indicated. 
         FIGS. 1C, 1D, and 1E  illustrate exemplary housings that could implement a housing for protecting the frequency-conversion crystal from impurities, thereby maintaining the annealed condition of the crystal whether the laser is operating or not operating. 
         FIG. 1F  illustrates an exemplary multi-step frequency conversion. 
         FIGS. 2A, 2B, and 2C  illustrate exemplary anamorphic optics for creating an elliptical cross section of a light source output. 
         FIGS. 3A and 3B  illustrate exemplary annealing processes that can be used on a frequency-conversion crystal. 
         FIGS. 4, 5, 6, and 7  illustrates exemplary inspection systems including the improved laser system. 
         FIG. 8  illustrates an exemplary pulse multiplier configured to generate pulse trains from each input laser pulse generated by the improved laser system. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In accordance with an improved laser system and operation, a mode-locked laser system includes an annealed, frequency-conversion crystal and an input light source that creates an elliptical cross-section beam waist located in or proximate to the crystal during normal operation. In some embodiments, the frequency-conversion crystal is maintained at a low temperature. As used herein, a “low” temperature is approximately 50° C. and below. In one embodiment, the low temperature may be between 30° C. and −10° C. This combination of components and operation can ensure beam quality, beam stability, and crystal lifetime. 
     In accordance with some embodiments of a laser system, a “long” crystal (described below) can be included. In a typical frequency conversion stage in a laser, the input laser beam is focused to an approximately circular cross section beam waist in or proximate to the frequency-conversion crystal. The longer the crystal, the more of input wavelength(s) will be converted to the output wavelength because the input laser beam spends more time inside the crystal. However, too long a crystal degrades the beam quality because of walk-off between the input and output wavelengths. One factor determining the maximum useable conversion crystal length is the walk-off angle (which depends on the crystal material and the input and output wavelengths) and the beam diameter. In applications for inspection and measurement of semiconductors, good beam quality is required, so typically the maximum useable crystal length is approximately equal to the laser beam-waist diameter divided by the walk-off angle in radians. Expressed as an equation, if the radius of the beam waist is w 0 , the walk-off angle in radians is α wo , and the crystal length L, then the maximum useable crystal length is approximately given by: 
     
       
         
           
             
               L 
               max 
             
             ≈ 
             
               
                 2 
                 ⁢ 
                 
                   w 
                   0 
                 
               
               
                 α 
                 
                   w 
                   0 
                 
               
             
           
         
       
     
     For example, for conversion of a 532 nm input laser beam to 266 nm in a CLBO crystal, the walk-off angle is approximately 33 mrad (with a weak dependence on operating temperature). If the beam waist radius were approximately 175 μm, then the maximum crystal length would be approximately 11 mm. A 10 mm CLBO crystal length would typically be used for such an application. Although increasing the beam waist radius would allow a longer crystal to be used, the overall conversion efficiency would be reduced because the conversion efficiency per unit length depends on the power density which scales inversely with the square of w 0 . 
     Very small values of of w 0  are generally not desirable because too high a power density damages the crystal rapidly. Furthermore, small values of w 0  reduce the Rayleigh range of the focused laser beam so making the system more sensitive to misalignment, and, if the crystal length is greater than about twice the Rayleigh range, result in only a part of the crystal length being efficiently used for frequency conversion. The Rayleigh range of a laser beam with an approximately cross-section beam and good beam quality (i.e. M 2  close to 1.0 as discussed below) is given by: 
     
       
         
           
             R 
             = 
             
               
                 π 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   w 
                   0 
                   2 
                 
               
               λ 
             
           
         
       
     
     Often the crystal length is chosen to be approximately equal to the smaller of twice the Rayleigh range and L max  above, if large enough crystals of good enough quality are available at a reasonable cost. Thus, in general, the crystal has a length approximately equal to a smaller of twice a Rayleigh range and a length of twice a beam waist radius divided by a walk-off angle in radians. 
     In some embodiments of the invention, the beam is focused into an elliptical cross section at the beam waist in or proximate to the crystal, as described below. The ellipse is elongated in the walk-off direction relative to the non-walk-off direction. An increased beam waist radius in the walk-off direction allows a longer crystal to be used since L max  depends only on the value of w 0  in the walk-off direction. 
     The power density at an elliptical focus is inversely proportional to the product of the beam waist radii (or inversely proportional to the beam waist radius squared for a circular cross-section beam waist). Instead of a circular focus, it is possible to use an elliptical focus with two beam waist radii chosen such that the power density is substantially equivalent to that of the circular focus, resulting in conversion efficiency per unit length and damage rate being substantially similar between the circular and elliptical focus cases. However, because the elliptical focus has a larger beam waist radius in the walk-off direction, the crystal length may be longer resulting in an overall conversion efficiency increase. Alternatively, the elliptical focus beam waist radii could be chosen so as to slightly reduce the power density (for example to approximately 70% of the power density of the circular beam waist) so as to reduce the rate of damage to the crystal, while using a long enough crystal (twice as long in this example) to result in substantially equal overall conversion efficiency. Note also that if the ellipse has too large an aspect ratio, then the smaller w 0  value in the more tightly focused direction will lead to a smaller Rayleigh range in that direction and so limit the maximum useful crystal length. One skilled in the appropriate art would understand these and other trade-offs between overall conversion efficiency, damage rate, crystal length and cost, and can select an appropriate beam waist profile. 
     Since the beam waist radius in the non-walk-off direction is smaller than the walk-off beam waist radius when an elliptical focus is used in accordance with embodiments of the present invention, the Rayleigh range in the non-walk-off direction is shorter than the Rayleigh range in the walk-off direction. Therefore, the maximum useful crystal length is approximately determined by the smaller of twice the Rayleigh range in the non-walk-off direction and the length defined by twice a beam waist radius in the walk-off direction divided by a walk-off angle in radians. 
     For example, in a frequency conversion stage that converts 532 nm to 266 nm using a CLBO crystal, instead of a circular beam waist of approximately 175 μm in radius, an elliptical beam waist of approximately 125 μm in radius (non-walk-off direction) by 250 μm in radius (walk-off direction) could be used. The power density in these two cases is substantially similar, but the elliptical focus would allow a maximum crystal length of approximately 15 mm, versus a maximum crystal length of approximately 11 mm for the circular focus. The overall conversion efficiency would be about 50% greater for the elliptical focus if used with the longer crystal. 
       FIG. 1A  illustrates a simplified, improved laser system  100 . In laser system  100 , the output of a light source  101  can be focused to an elliptical cross-section Gaussian beam waist in or proximate to a frequency-conversion crystal  103  (also called crystal  103  for ease of reference) using beam shaping optics  102 . As used herein, the term “proximate to” is preferably less than half of the Rayleigh range from the center of crystal  103 . Note that crystal  103  may be a long crystal, as discussed above, or a standard-sized crystal. In one preferred embodiment, the aspect ratio between the Gaussian widths of the principle axes of the ellipse could be between about 2:1 and about 6:1. Note that the principle axes  104 A and  104 B of an ellipse  104  are perpendicular to one another and define the longest length and the widest width of ellipse  104 . In other preferred embodiments the ratio between the principle axes of the ellipse could be between about 2:1 and about 10:1. In one embodiment, the wider Gaussian width is substantially aligned with the walk-off direction of the frequency-conversion crystal (e.g. to within about 10° of alignment). 
       FIG. 1B  illustrates an exemplary Gaussian laser beam  108  (with exaggerated tails for clarity), wherein line  104 C indicates a 2× beam waist (also called a focal region).  FIGS. 1C, 1D, and 1E  illustrate exemplary housings that could implement housing  107 , which can protect frequency-conversion crystal  103  during standard operation. In one embodiment, housing  107  can protect crystal  103  from impurities, thereby facilitating maintaining its annealed condition even with a low standard operating temperature, i.e. less than about 50° C. Note that a crystal exposed to impurities over time will begin to deteriorate and may revert back to substantially an un-annealed state. U.S. patent application Ser. No. 12/154,337, entitled “Enclosure For Controlling The Environment of Optical Crystals”, filed May 6, 2008, describes these housings in greater detail, and is incorporated by reference herein. In other embodiments, housing  107  may be a larger structure including crystal  103  and other components of a laser system. In one embodiment, housing  107  is large enough to house all components of the laser system. Note that the larger the housing, the more precautions needed for maintenance and repair of the laser system (to protect crystal  103  from degradation and maintain its annealed condition). Therefore, in one embodiment, housing  107  is preferably smaller, rather than larger. 
     Beam shaping optics  102  can include anamorphic optics, which can change the cross section of output from light source  101 . Anamorphic optics can include, for example, at least one of a prism, a cylindrical curvature element, a radially-symmetric curvature element, and a diffractive element. In one embodiment, light source  101  can include a laser producing a frequency in the visible range, e.g. 532 nm, to be doubled inside crystal  103 . In other embodiments, light source  101  can include a laser source producing two or more frequencies to be combined inside crystal  103  to generate a sum or difference frequency. 
       FIG. 1F  illustrates an exemplary frequency conversion that generates a 4 th  harmonic generation from, for example, a 1064 nm fundamental wavelength. The fundamental wavelength, which is generated by a laser  190 , can be converted to a 2 nd  harmonic  196  (i.e. 532 nm wavelength) using a 2 nd  harmonic generation technique  191  and a frequency separation technique  192 . Frequency separation technique  192  can separate the unwanted fundamental (i.e. an unconsumed fundamental  197 ) from 2 nd  harmonic  196 . In one embodiment, 2 nd  harmonic generation technique  191  and frequency separation technique  192  can be implemented using the above-described beam shaping optics  102 , frequency conversion crystal  103 , and harmonic separation block  105 . 
     2 nd  harmonic  196 , in turn, can be the source for a 4 th  harmonic  198  (i.e. 266 nm wavelength), which can be created by using a 4 th  harmonic generation technique  193  and a frequency separation technique  194 . Frequency separation technique  194  separates an unwanted 2 nd  harmonic, i.e. unconsumed 2 nd  harmonic  199 , from 4 th  harmonic  198 . In one embodiment, 4 th  harmonic generation technique  193  and frequency separation technique  194  can be implemented using the above-described beam shaping optics  102 , frequency conversion crystal  103 , and harmonic separation block  105 . In another embodiment, more than one of the frequency conversion and separation stages (such as the 2 nd  and 4 th  harmonic conversion stages in the example) can be implemented using the above-described beam shaping optics  102 , frequency conversion crystal  103 , and harmonic separation block  105 . 
     Note that the exemplary frequency conversion of  FIG. 1F  is illustrative only and not a limitation. Other harmonics may be created by, for example, mixing the fundamental with the 2 nd  harmonic to create a 3 rd  harmonic, or mixing the fundamental with the 4 th  harmonic (or mixing 2 nd  and 3 rd  harmonics) to create a 5 th  harmonic. Any, or all, of the conversion stages that generate ultraviolet (UV) wavelengths can use the beam shaping optics, the frequency conversion crystal, and the harmonic separation block. Note that although  FIG. 1F  is drawn as if the harmonic separation blocks reflect the unwanted frequencies and the wanted frequencies travel substantially undeviated, this is merely an illustration that that harmonic separation blocks separate the frequencies and not meant to limit which frequencies are reflected or transmitted. 
       FIGS. 2A, 2B, and 2C  illustrate exemplary anamorphic optics.  FIG. 2A  illustrates two prisms  201  and  202  that can be configured to operate near Brewster&#39;s angle. In one embodiment, prisms  201  and  202  can be tuned to adjust the ellipticity of the beam without introducing any power in the system.  FIG. 2B  illustrates a cylindrical telescope implemented with two lenses  203  and  204 . Note that in a cylindrical telescope, collimation can be set to compensate for most astigmatism.  FIG. 2C  illustrates an afocal cylindrical zoom telescope implemented with three lenses  205 ,  206 , and  207 . An afocal cylindrical zoom telescope can be used to tune the beam diameter and set focusing. Note that although the anamorphic optics of  FIGS. 2A, 2B, and 2C  include two or three components, other embodiments may include any number of components. Indeed, the optics configurations of  FIGS. 2A-2C  are illustrative only and not limiting. Referring back to  FIG. 1 , beam shaping optics  102  generate an elliptical cross section beam output, which can then be focused at the beam waist in or proximate to crystal  103  using known spherical or cylindrical optics (not shown). 
     Harmonic separation block  105  can include a prism for beam separation. In one embodiment, an uncoated Pellin-Broca prism can create two beams such that one beam (i.e. the beam with the desired frequency) has minimal losses due to Fresnel reflections and the other beam (i.e. the beam with the undesired frequency) suffers significant losses due to Fresnel reflections. Notably, the output beam generated by the Pellin-Broca prism is at 90 degrees to the input beam, which allows for convenient set-up and removal of the unwanted beam (i.e. the unwanted frequency). Note that other embodiments may include generating multiple beams with desired wavelengths and/or other means for removing one or more unwanted beams having undesired frequencies. Moreover, harmonic separation block  105  may generate any number of harmonics, e.g. 2 nd , 3 rd , 4 th  or 5 th  harmonics, and/or the sum or difference frequencies of such harmonics and/or the fundamental. 
     Notably, beam shaping optics  106  can be positioned after crystal  103  to create a substantially circular symmetric beam profile for the output beam of laser system  100 . In one embodiment, optics similar to those of beam shaping optics  102  (circular beam to elliptical beam) may be used to convert the output beam from an elliptical cross section to a circular cross section, as would be understood by those skilled in the art. However, note that in the walk-off plane (which is perpendicular to the ordinary axis and causes distortion to the beam), the divergence of the output beam is substantially similar to the divergence of the input beam, but may be a little larger due to distortion of the beam profile caused by crystal  103 . In contrast, in the non-walk-off direction (i.e. at the ordinary axis), the width of the output beam will be reduced by a factor of approximately √{square root over (2)} because of the non-linear (approximately quadratic) conversion process. Additionally, the optics for implementing beam shaping optics  106  should also account for the shorter wavelength of the output beam. Notably, higher laser powers and/or shorter laser wavelengths allow detection of smaller defects at higher speeds. A high quality beam close to M 2 =1.0 can be tightly focused to a small spot or narrow line, thereby facilitating the detection of small defects. Note that an M 2  value of 1.0 corresponds to a laser beam with an ideal Gaussian cross section. In one preferred embodiment, beam shaping optics  106  can create an output beam with an M 2  value that is less than about 1.2. 
     Although M 2  is one measure of beam quality, the beam quality may be further quantified by its astigmatism. In accordance with another aspect of laser system  100 , beam shaping optics  102  (and beam shaping optics  106 , when used) can produce a UV output laser beam with an astigmatism that causes no more than 10% of Rayleigh range shift in relative waist position for the two axes. 
     Note that in some applications, an elliptical output beam may better match the application for laser system  100 . For example, where the UV illumination beam is incident on the wafer at an oblique angle, an elliptical laser beam would allow a circular spot. For such an application, anamorphic optics may be unnecessary for beam shaping optics  106 . Alternatively, in other embodiments, the anamorphic optics of beam shaping optics  106  may change the output elliptical cross section to a different elliptical cross section than that output by beam shaping optics  102 . In that embodiment, beam quality can be measured by M x   2  and M y   2 , where x and y represent the principal axes of the ellipse (i.e.  104 A and  104 B in  FIG. 1 ). In this case, both M x   2  and M y   2  should be less than 1.2. 
     In one embodiment, to reduce the defects or impurities (such as H 2 O and/or OH) in crystal  103 , an improved annealing process can be performed. In general, annealing can be performed in an environment with less than about 20 ppm water. In one embodiment, the environment can contain clean, dry air or a dry, inert gas (such as N 2  or Ar). 
     In one multi-stage ramp-up annealing process  300  shown in  FIG. 3A , the temperature can be slowly raised from room temperature to about 150° C. over a time interval of about 2 hours in step  301 . Step  302  can hold the temperature near 150° C. for about 10-20 hours. Step  303  can then increase the temperature to about 200° C. over another approximately 1 hour period. If the initial crystal has particularly high water content, then step  303  may perform this temperature ramping slower. Then, step  304  may hold the temperature between about 150-200° C. for at least 100 hours (in some embodiments, for approximately 200 or 300 hours). On the other hand, if the initial water content is lower, then a faster temperature ramp (step  303 ) and a shorter anneal time (step  304 ) of, for example, 48 hours can be used. Step  305  can decrease the temperature down to room temperature over about 3 hours. Note that the times associated with steps  301 - 305  are for crystals with linear dimensions in the range of about 5-15 mm. Because the process of removing the water from the bulk of the crystal is essentially a diffusion process, crystals with larger dimensions (and hence lower surface area to volume ratios) can require longer anneal times. In contrast, crystals with smaller dimensions (and hence larger surface area to volume ratios) may be adequately annealed in a shorter time. 
     Moreover, the temperature ramp rate can be adjusted to ensure that no mechanical damage to the crystal occurs, whether due to water initially being driven off too fast or due to mechanical stresses induced by the temperature changes. In general, crystals with higher levels of water or with larger mechanical dimensions may need slower temperature ramps and/or longer hold times near 150° C. than smaller or drier crystals. 
     In another multi-stage ramp-up annealing process  310  shown in  FIG. 3B , FTIR (Fourier transform infrared spectroscopy) may be used to monitor the absorption of —OH bonds (including H 2 O) near 3580 cm −1  in the infra-red spectrum. For example, FTIR monitoring can be done on a crystal while it is undergoing annealing (i.e. holding the temperature near 150° C.) in step  302 A. Step  302 B can determine whether the absorption is at a first level, such as the —OH absorption peak being reduced by approximately 20% from its initial value. If not, then the process continues with step  302 A. If so, then the process proceeds to the annealing of step  302 A. Additionally, FTIR measurement can be made on a crystal during annealing (i.e. holding the temperature between about 150-200° C.) in step  304 A. Step  304 B can determine whether the absorption is at a second level, such as the —OH absorption peak area has been reduced to approximately 5% of its original value. If not, then the process continues with step  304 A. If so, then the process proceeds to the ramp-down of step  305 . 
     Annealing processes  300  and  310  can be used for various frequency-conversion crystals including, for example, those made of CLBO, CBO, BBO, LBO (lithium triborate), lithium niobate, KDP, or other non-linear materials. In some embodiments, these crystals materials may contain other elements, such as dopants. Annealing processes  300  and  310  may be particularly useful for hygroscopic materials, such as CBO and CLBO. Even for some non-hygroscopic materials annealing may be useful to reduce surface or bulk contaminants. 
     Laser operating temperatures well below 150° C. are particularly useful when used with a high grade frequency-conversion crystal with very few defects or impurities. Notably, a crystal having minimal defects/impurities can be used at significantly lower temperatures than normal operation, thereby increasing crystal lifetime. For example, in one embodiment, crystal  103  (which was annealed using one of the above-described processes) can be used at a temperature close to, or even below, room temperature, such as an operating temperature of between about 10° C. and about 50° C. In one preferred embodiment, the operating temperature of the crystal is about 30° C. In another preferred embodiment, the crystal can be kept in a controlled environment with no water vapor and then used in very low temperatures, such as at about 0° C., −10° C. or colder. In one embodiment, a thermo-electric (or Peltier) cooler can be used to maintain and control the temperature of crystal  103 . 
     Increasing the lifetime of the conversion crystal advantageously decreases the frequency of maintenance of the laser system and increases the percentage of the time that the inspection or metrology tool incorporating the laser system is operating productively. Increased lifetime for the crystal also means a lower operating cost for the laser system because the time interval between each service event for the laser system can be increased. 
     The laser system described herein can advantageously incorporate the coherence reducing schemes disclosed in U.S. patent application Ser. No. 13/073,986 (the &#39;986 application), filed Mar. 28, 2011, now published as US20110228263, which is incorporated by reference herein. As described in the &#39;986 application, a combination of one or more dispersive elements with one or more electro-optic modulators can be used to reduce coherence by modulating the light and by mixing spatial and temporal characteristics of the light. 
       FIG. 4  illustrates a surface inspection system  400  that can be used for inspecting anomalies on a surface  401 . In this embodiment, surface  401  can be illuminated by a substantially stationary illumination device portion of system  400  comprising a laser beam generated by laser system  100  (see  FIG. 1 ). The output of laser system  100  can be consecutively passed through polarizing optics  421 , a beam expander and aperture  422 , and beam-forming optics  423  to expand and focus the beam. 
     The focused laser beam  402  is then reflected by a beam folding component  403  and a beam deflector  404  to direct the beam  405  towards surface  401  for illuminating the surface. In the preferred embodiment, beam  405  is substantially normal or perpendicular to surface  401 , although in other embodiments beam  405  may be at an oblique angle to surface  401 . 
     In one embodiment, beam  405  is substantially perpendicular or normal to surface  401  and beam deflector  404  reflects the specular reflection of the beam from surface  401  towards beam turning component  403 , thereby acting as a shield to prevent the specular reflection from reaching the detectors. The direction of the specular reflection is along line SR, which is normal to surface  401 . In one embodiment where beam  405  is normal to surface  401 , this line SR coincides with the direction of illuminating beam  405 , where this common reference line or direction is referred to herein as the axis of inspection system  400 . Where beam  405  is at an oblique angle to surface  401 , the direction of specular reflection SR would not coincide with the incoming direction of beam  405 ; in such instance, the line SR indicating the direction of the surface normal is referred to as the principal axis of the collection portion of inspection system  400 . 
     Light scattered by small particles is collected by mirror  406  and directed towards aperture  407  and detector  408 . Light scattered by large particles is collected by lenses  409  and directed towards aperture  410  and detector  411 . Note that some large particles will scatter light that is also collected and directed to detector  407 , and similarly some small particles will scatter light that is also collected and directed to detector  411 , but such light is of relatively low intensity compared to the intensity of scattered light the respective detector is designed to detect. In one embodiment, inspection system can be configured for use in detecting defects on unpatterned wafers. 
       FIG. 5  illustrates an inspection system  500  configured to implement anomaly detection using both normal and oblique illumination beams. In this configuration, laser system  100  ( FIG. 1 ) can provide a laser beam  501 . A lens  502  focuses the beam  501  through a spatial filter  503  and lens  504  collimates the beam and conveys it to a polarizing beam splitter  505 . Beam splitter  505  passes a first polarized component to the normal illumination channel and a second polarized component to the oblique illumination channel, where the first and second components are orthogonal. In the normal illumination channel  506 , the first polarized component is focused by optics  507  and reflected by mirror  508  towards a surface of a sample  509 . The radiation scattered by sample  509  is collected and focused by a paraboloidal mirror  510  to a photomultiplier tube  511 . 
     In the oblique illumination channel  512 , the second polarized component is reflected by beam splitter  505  to a mirror  513  which reflects such beam through a half-wave plate  514  and focused by optics  515  to sample  509 . Radiation originating from the oblique illumination beam in the oblique channel  512  and scattered by sample  509  is collected by paraboloidal mirror  510  and focused to photomultiplier tube  511 . Photomultiplier tube  511  has a pinhole entrance. The pinhole and the illuminated spot (from the normal and oblique illumination channels on surface  509 ) are preferably at the foci of the paraboloidal mirror  510 . 
     The paraboloidal mirror  510  collimates the scattered radiation from sample  509  into a collimated beam  516 . Collimated beam  516  is then focused by an objective  517  and through an analyzer  518  to the photomultiplier tube  511 . Note that curved mirrored surfaces having shapes other than paraboloidal shapes may also be used. An instrument  520  can provide relative motion between the beams and sample  509  so that spots are scanned across the surface of sample  509 . 
       FIG. 6  illustrates another surface inspection apparatus  600  that includes illumination system  601  and collection system  610  for inspecting areas of surface  20 . As shown in  FIG. 6 , laser system  100  ( FIG. 1 ) is configured to direct light beam  602  through lens  603 . Lens  603  is oriented so that its principal plane is substantially parallel to surface  611  and, as a result, illumination line  605  is formed on surface  611  in the focal plane of lens  603 . In addition, light beam  602  and focused beam  604  are directed at a non-orthogonal angle of incidence to surface  611 . In particular, light beam  602  and focused beam  604  may be directed at an angle between about 1 degree and about 85 degrees from a normal direction to surface  611 . In this manner, illumination line  605  is substantially in the plane of incidence of focused beam  604 . 
     Collection system  610  includes lens  612  for collecting light scattered from illumination line  605  and lens  613  for focusing the light coming out of lens  612  onto a device, such as charge coupled device (CCD)  614 , comprising an array of light sensitive detectors. In one embodiment, CCD  614  may include a linear array of detectors. In such cases, the linear array of detectors within CCD  614  can be oriented parallel to illumination line  615 . In one embodiment, multiple collection systems can be included, wherein each of the collection systems includes similar components, but differ in orientation. For example,  FIG. 7  illustrates an exemplary array of collection systems  701 ,  702 , and  703  for a surface inspection apparatus (wherein its illumination system, e.g. similar to that of illumination system  601 , is not shown for simplicity). 
     U.S. Pat. Nos. 5,108,176, 5,377,001, 5,377,002, 5,189,481, 5,712,701, 6,118,525, 6,201,601, 6,271,916, 6,608,676, 7,088,443, 7,492,451, 7,525,649, and 7,957,066 as well as U.S. Published Applications 2009/0180176, and 2011/0073982 (all incorporated by reference herein) describe other inspection systems embodiments that can include laser system  100 . Laser system  100  can also be combined with one or more of the pulse stretching and pulse rate multiplication schemes disclosed in U.S. Provisional Patent Application 61/496,446, filed Jun. 13, 2011, which is incorporated by reference herein. 
       FIG. 8  illustrates an exemplary pulse multiplier  800  configured to generate pulse trains from each input pulse  801  output by laser system  100  ( FIG. 1 ). Input pulse  801  impinges on a polarizing beam splitter  802 , which because of the input polarization of input pulse  801 , transmits all of its light to a lens  806 . Thus, the transmitted polarization is parallel to the input polarization of input pulse  801 . Lens  806  focuses and directs the light of input pulse  801  to a half-wave plate  805 . In general, a wave plate can shift the phases between perpendicular polarization components of a light wave and hence change the polarization state of the light passing through it. In a wave plate, one polarization component propagates slightly slower than the perpendicular component. Half-wave plate  805  is fabricated such that for light exiting, one polarization component is substantially half of a wavelength delayed (180 degrees) relative to the other polarization component. In some preferred embodiments, half-wave plate  805  is oriented such that its optical axis is at substantially 27.4° to the plane of the input polarization. Thus, when light with the input polarization strikes the half-wave plate  805 , its polarization is rotated by substantially 54.8° in these preferred embodiments. 
     The light exiting half-wave plate  805  is reflected by mirrors  804  and  803  back to polarizing beam splitter  802 . Thus, polarizing beam splitter  802 , lens  806 , half-wave plate  805 , and mirrors  804  and  803  form a ring cavity configuration. The light impinging on polarizing beam splitter  802  after traversing the ring cavity is polarized at angle relative to the input polarization (an angle of substantially 54.8° in some preferred embodiments) because of half-wave plate  805 . Therefore, polarizing beam splitter  802  transmits some light and reflects other light, as indicated by arrows  809 . Specifically, polarizing beam splitter  802  transmits the light from mirror  803  having the same polarization as input pulse  801 . This transmitted light exits pulse multiplier  800  as output pulses  807 . The reflected light, which has a polarization perpendicular to that of input pulse  801 , is re-introduced into the ring cavity (pulses not shown for simplicity). In those preferred embodiments with the half-wave plate  805  axis oriented at substantially 27.4° to the input polarization, substantially two thirds of the energy of a single incident pulse will be reflected back into the ring cavity and substantially one third of the incident energy single pulse will be transmitted. 
     Notably, these re-introduced pulses can traverse the ring in the manner described above with further polarization rotation by half-wave plate  805  and then light splitting by polarizing beam splitter  802 . Thus, in general, the above-described ring cavity is configured to allow some light to exit and the rest of the light (with some minimal losses) to continue around the ring. During each traversal of the ring if no additional input pulses were introduced, the energy of the total light would decrease due to the light exiting the ring as output pulses  807 . 
     Periodically, a new input pulse  801  is provided to pulse multiplier  800 . The size of the ring, and thus the time delay of the ring, can be adjusted by moving mirror  804  along the axis indicated by arrows  808 . If the time delay of the ring is adjusted to be approximately half the repetition time between pulses from the laser  100 , then half the output pulses will be approximately halfway between input pulses and the other half of the pulses from the ring will approximately coincide with input pulses. 
     In one exemplary embodiment, the laser might have a repetition rate of 125 MHz and a pulse width of approximately 10 to 20 ps, and the cavity could have a delay time of 4.05 ns, so that a pulse that has made one trip around the cavity arrives approximately half way between two input pulses and a second pulse that has made two round trips arrives approximately 100 ps after the next input laser pulse. So a steady stream of narrow input pulses at 125 MHz in this example embodiment would be turned into a stream of much broader pulses at a repetition rate of 250 MHz (broadened because the cavity time has deliberately been set to be close to, but not exactly, half the time interval between input pulses). In the preferred embodiment where the optical axis of the half-wave plate is oriented at substantially 27.4° to the polarization of the input pulse, the output pulses will be substantially equal to one another in total energy. 
     The various embodiments of the structures and methods of this invention that are described above are illustrative only of the principles of this invention and are not intended to limit the scope of the invention to the particular embodiments described. For example, the above-described inspection systems can be incorporated into either dark-field inspection tools or bright-field inspection tools. Thus, the invention is limited only by the following claims and their equivalents.