Abstract:
An enclosure that maintains the environment of one or more optical crystals and allows efficient frequency conversion for light at wavelengths at or below 400 nm with minimal stress being placed on the crystals in the presence of varying temperatures. Efficient conversion may include multiple crystals of the same or different materials. Multiple frequency conversion steps may also be employed within a single enclosure. Materials that have been processed specifically to provide increased lifetimes, stability, and damage thresholds over designs previously available are employed. The enclosure allows pre-exposure processing of the crystal(s) such as baking at high temperatures and allowing real time measurement of crystal properties.

Description:
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/008,260, entitled “Enclosure for Controlling the Environment of Optical Crystals,” inventor J. Joseph Armstrong, filed Dec. 18, 2007, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to illuminators used in conjunction with inspection systems, such as semiconductor wafer inspection systems and photomask inspection systems, and more particularly to a frequency converted light source for use with such inspection systems. 
     2. Description of the Related Art 
     The demands of the semiconductor industry for wafer and photomask inspection systems exhibiting high throughput and improvements in resolution are ongoing. Successive generations of such inspection systems tend to achieve higher resolution by illuminating the wafer or reticle using light energy having shorter wavelengths. 
     Certain practical advantages may be achieved when illuminating the wafer or reticle with light with wavelengths at or below 400 nm. Providing suitable lasers for high quality wafer and photomask inspection systems is particularly challenging. Conventional lasers generating light energy in the deep ultraviolet (DUV) range are typically large, expensive devices with relatively short lifetimes and low average power. Semiconductor wafer and photomask inspection systems generally require a laser generally having a high average power, low peak power, and relatively short wavelength in order to provide for inspection having sufficient throughput and adequate defect signal-to-noise ratio (SNR). 
     The primary method to provide adequate DUV power entails generating shorter wavelength light from longer wavelength light. This process of changing wavelengths is commonly called frequency conversion. Frequency conversion in this context requires high peak power light energy production in order to produce a nonlinear response in an optical crystal. To increase the efficiency of this process the longer wavelength light may have high average powers, short optical pulses, and may be focused into the optical crystal. The original light is typically called fundamental light. 
     High efficiency is important for a DUV laser. High efficiency allows a lower power fundamental laser source that is more reliable, smaller, and produces less heat. A low power fundamental laser will produce less spectral broadening if a fiber laser is used. Higher efficiency also tends to lead to lower cost and better stability. For these reasons, efficient frequency conversion to the DUV is relatively important. 
     Generating light at wavelengths below 400 nm, and especially below 300 nm can be very challenging. Light sources used for semiconductor inspection require relatively high powers, long lifetimes, and stable performance. Light sources meeting these requirements for advanced inspection techniques are nonexistent. The lifetime, power, and stability of current DUV frequency converted lasers is generally limited by the frequency conversion crystals and conversion schemes, especially those exposed to DUV wavelengths like 355, 266, 213, and 193 nm. 
     Relatively few nonlinear crystals are capable of efficiently frequency converting light to UV/DUV wavelengths. Most crystals that have traditionally been employed have low damage thresholds if not properly prepared and the operating environment maintained. Thus the crystal has typically been contained within an enclosure to maintain the environment. In order to frequency convert an infrared laser to the DUV, more than one crystal can be employed. When multiple crystals are employed, it can be an advantage to place them all within the enclosure. Crystal alignment complications can result, and it can be difficult to collect and focus light in such an enclosure. 
     It would therefore be desirable to offer an enclosure that maintains the environment of the optical crystal and allows efficient frequency conversion at wavelengths at or below 400 nm. This efficient conversion may include multiple crystals of the same or different materials. Multiple frequency conversion steps may also be employed within a single enclosure. It is also important that any enclosure use materials that can provide increased lifetimes, stability, and/or damage thresholds as compared with designs previously available. In addition, it is desirable for an enclosure to allow pre-exposure processing of the crystal such as baking at high temperatures and allowing real time measurement of crystal properties. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present design, there is provided an environmentally controlled enclosure comprising a crystal. Multiple crystals may be provided in certain embodiments. The enclosure comprises securing hardware configured to secure the crystal within the enclosure such that temperature changes within the enclosure produce negligible stress on the crystal. The enclosure further includes a window configured to permit light to enter the enclosure and contact the crystal and may include a seal formed between the window and the enclosure. 
     In certain embodiments, a frame is provided for the enclosure, and an outlet configured to purge gas from the enclosure many be provided. Heating or cooling elements may be provided to control the temperature of the enclosure and the crystal or crystals provided therein, and a temperature reading element may be provided that controls temperature using feedback. The window or windows may be provided at Brewster&#39;s angle. 
     These and other advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which: 
         FIG. 1  illustrates an enclosure with built in angle adjustment hinge; 
         FIG. 2  is a frame with built in angle adjustment flexure; 
         FIG. 3  shows a built in angle adjustment bottom pin; 
         FIG. 4  illustrates a built in angle adjustment top pin; 
         FIG. 5  is a crystal holding using a spring with resultant force along the diagonal of the crystal; 
         FIG. 6  shows a glass to metal seal; 
         FIG. 7  illustrates a glass to metal seal primary seal with secondary ring seal; 
         FIG. 8  is a glass to metal primary seal with o-ring secondary seal and cooling fins; 
         FIG. 9  shows multiple crystals in one enclosure without independent adjustment; 
         FIG. 10  illustrates multiple crystals in one enclosure with independent adjustment; 
         FIG. 11  illustrates two crystals with lens relay; 
         FIG. 12  represents multiple crystals within a lens relay; 
         FIG. 13  is two crystals with a mirror relay; 
         FIG. 14  shows multiple crystals with a mirror relay; 
         FIG. 15  represents an image relay inside an enclosure; 
         FIG. 16  illustrates multiple crystals in one enclosure with extended output; 
         FIG. 17  shows multiple crystals in one enclosure with external optics and Brewster windows; and 
         FIG. 18  represents crystal enclosures for heat processing only. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present design, an enclosure for one or more optical crystals that maintains a desirable environment is provided. The enclosure design allows stable, long lifetime, high power frequency conversion of light to UV/DUV wavelengths. In addition, the same enclosure may be employed to preprocess the crystal(s) before exposing the crystal(s) to frequency conversion light. 
     Frequency conversion in this design uses at least one optical crystal within an enclosure, but may also utilize more than one crystal. In the case of multiple crystals, the crystals may be made of the same materials or different materials. The multiple crystals may be used to generate multiple wavelengths or increase the frequency conversion efficiency of a single wavelength. 
     Further, the present design may provide an advanced light source having a novel method for producing light energy. The present design may use nonlinear optical crystals within an enclosure, where the enclosure processes the crystal(s) before use and maintains the environment of the processed crystal during frequency conversion. The enclosure also includes optics for focusing light into crystals and collecting light from crystals. These optics may be external to the enclosure or included within the enclosure. 
     The present design allows for one or more cells to be used in the current embodiments and each cell may contain one or more crystals. In addition these cells may be translated to focus light through different parts of the crystal. This is typically done to increase the lifetime of a single crystal. The diameter of the light beam focused into the crystal is typically much smaller than the dimension of the crystal cross section. When a particular position in the crystal is damaged, the crystal may be translated to an undamaged region and continue to be used. 
     A particular aspect of the present design is the ability for the enclosure and crystal to enable heating or cooling of the environment and crystal without placing significant stress or by placing negligible stress on the crystal. 
     Other embodiments include an enclosure for preexposure processing only. Using one enclosure to preprocess the crystal and another for frequency conversion reduces the risk of crystal contamination from the preexposure process and allows for a simplified frequency conversion enclosure. 
     Enclosures for Optical Crystals 
     An enclosure for an optical crystal includes several parts, such as the enclosure frame, hardware to secure the crystal within the frame, windows to allow light in and frequency converted light out, hardware to secure the window to the frame, hardware to seal the window and the frame, and an inlet and outlet for purge gas. Also, it is often desirable to include a heating or cooling element in contact with the enclosure. This heating or cooling element is used to provide a stable temperature for the crystal, i.e. a temperature above or below ambient. The heating or cooling element can also be used to adjust the crystal temperature for preexposure processing. In addition, the enclosure can include hardware enabling adjusting of phase matching angles of the crystal to optimize frequency conversion. 
       FIG. 1  illustrates an enclosure for an optical crystal. The primary portion of this enclosure is the frame  101 . This frame is manufactured using materials and techniques that minimize the impact of photocontamination. Two possible materials that can be used for the frame are aluminum and stainless steel. It is often desirable to coat the material with a layer of nickel to inhibit contaminants within the metal from escaping. It is also desirable to have the frame electropolished and cleaned in order to minimize any remaining surface contamination. It may also be desirable to integrate an angle adjustment mechanism into the frame to allow adjustment of the phase matching angles. 
     The metal frame  101  may be formed from a variety of metals or materials including but not limited to stainless steel, aluminum, beryllium copper, copper, brass, and/or nickel. The metal frame  101  may be coated with nickel and/or electropolished. 
     In  FIG. 1 , a hinge plate  102  is included in the frame, and a hinge rotation pin  103  and adjuster screws  104  are also used to adjust the angle. The angle adjustment works by keeping hinge plate  102  in a fixed position. Fixing bolts  105  are loosened to allow rotation about fixing bolt  103 . Rotation is accomplished by turning the push pull adjuster screws  104 . When in the proper position, Fixing bolts  105  are locked.  FIG. 1  also includes a window within a window holding plate  106 . This window may be antireflection coated or may be oriented at Brewster&#39;s angle. Both of these techniques may be employed to improve the efficiency of the light transmission. 
     Providing a window at Brewster&#39;s angle requires the window to be mounted in proximity of Brewster&#39;s angle. One method to achieve this requires the frame  102  to be machined to support a window at this angle. The window holding plate  106  then fixes the window to the frame. A second method is to add an extension between the frame  101  and the holding plate  106 . This extension can mount to the frame  101  at one end and hold the window at Brewster&#39;s angle at the other. The windows should be placed far enough away from the crystal so they are not damaged by the light focused into the crystal or the light exiting the crystal. This distance can be calculated based on the light wavelengths, the focusing conditions into the crystal and housing, the crystal type, the window material, the window orientation, and any coatings that may be on the crystal or windows. 
     A seal  107  that effectively separates the external environment from the internal environment is provided. This seal can be a direct contact between the polished glass and a polished metal surface, or can be a ring of compressive material. This material should withstand high temperatures without significant photocontamination. Two possible compression materials that withstand increased temperatures with minimal outgassing are Viton and Kalrez. 
     A ring made from a soft metal or a metal ring coated with a soft metal such as silver may be employed. The cross section of the ring can be a circle or a C shape. The seal can also be made in two stages, where a glass to metal seal is used as the primary or inner seal and a ring of compressive material is used as a secondary seal. The design of  FIG. 1  also allows for a purge gas in order to maintain the interior environment. This helps remove residual material outgassing and any low level leaking of the external environment into the enclosure. 
     In certain embodiments, the design may also incorporate a heating or cooling element. This element (not shown) can be attached to surface  109 , in proximity to the optical crystal within the enclosure. Possible heating elements that can be easily used are flat elements or cylindrical elements. These elements can be easily attached to the enclosure. A cooling device may also be attached to surface  109 . One possible type of cooler is a thermoelectric device which produces a cool surface on one side of the device and a hot surface on the other side when a voltage is applied. Thus this device can be either used to heat or cool. Alternately, a resistive or ceramic heating element may be employed. Heating and cooling can also be accomplished using standard heat exchanger techniques that run hot or cold liquid through a heat exchanging plate in contact with the frame  101 . Cooling may also be employed by using, for example, a heat tube or venturi. 
     An alternate design shown in  FIG. 2  provides a flexure hinge mechanism for adjusting the optical crystal phase matching angle. In this embodiment, frame  201 , adjuster plate  202  and flexure hinge  203  can be fabricated from a single piece of metal. Angle adjustment operation is similar to the embodiment shown in  FIG. 1 . Frame  201  also includes holes  204  for angle adjustment screws, and a recessed portion  205  that is inside the frame and in contact with the optical crystal. Holder  206  is used for the seal between the frame and the window. Holes  207  are used to secure the window and seal (not shown) to the frame  201 . Holes  208  are used to attach heating or cooling element (not shown). Holes  209  are used for fixing bolts (not shown) and holes  210  are used for purge gas entry and exit. The holes are located such that entering purge gas must flow across the faces of the crystal before exiting the enclosure. 
       FIG. 3  illustrates an alternate angle adjustment technique. From  FIG. 3 , a hole is placed in housing  301  to allow insertion of a rotation pin  303  in the bottom of the frame. The center axis of the pin  303  is generally in proximity to the center of the crystal  305  so the crystal will rotate about its center. Angle adjustment is made using a technique similar to the embodiment in  FIG. 1  where the hinge is replaced by the rotation pin  303 . In  FIG. 3 , light may enter through window  302  and exit through window  304 . Alternately the light may enter through window  304  and exit through window  302 . 
       FIG. 4  illustrates an alternate angle adjustment technique. From  FIG. 4 , a hole is placed in housing  401  to allow insertion of a rotation pin  403  in the top of the frame. The center axis of the rotation pin  403  is in proximity to the center of the crystal  405  so the crystal will rotate about its center. Angle adjustment is made using a technique similar to the embodiment in  FIG. 1  where the hinge is replaced by rotation pin  405 . In  FIG. 4 , light may enter through a window placed at  402  and exit through a window placed at  404 . Alternately the light may enter through a window placed at  404  and exit through a window placed at  402 . 
       FIG. 5  illustrates holding the crystal in place using a spring. This method holds the crystal by producing a resultant force in proximity to the diagonal of the face of the crystal. Maintaining the crystal in this manner is accomplished by providing two surfaces  502  at a 90 degree angle to each other within the frame  501 . Two sides of the crystal  506  are in contact with surfaces  502 . A cap  505  also has two surfaces at a 90 degree angle to each other. These two surfaces are in contact with the opposing surfaces of crystal  506 . Space is allowed so the cap will not come in contact with the surfaces  502  or frame  501 . One surface of the cap  505  is beveled at 45 degrees to the sides of the crystal  506  and is generally aligned parallel to surface  503  of frame  501 . In addition, one surface of the frame  501  is parallel or nearly parallel to the bevel on cap  505 . A low force spring  504  can be used to hold the optical crystal in place. This spring  504  is made from stainless steel and may be positioned by placing a recess in the bevel of the cap. This geometry maximizes the contact area of crystal  506  with frame  501  to aid in the efficiency and uniformity of heating and cooling. It also allows for the frame  501  and optical crystal  506  to be heated or cooled over a large temperature range without increasing stress on the optical crystal  506 . Exterior frame edge  508  and interior  507  of frame  501  are also shown. 
       FIG. 6  shows a cross section of a window using only a glass to metal seal. In this case window  601  is held against frame  603  using retaining plate  602 . This type of seal is suited for extreme high temperature applications because all materials can withstand temperatures in excess of 500 degrees C. The choice of window materials depends mostly on the optical damage threshold. When an enclosure is used to generate light in the UV and DUV spectral region and the light is focused into the crystal, either fused silica or calcium fluoride can be employed. Calcium fluoride windows also allow for efficient transmission in the infra red region of the light spectrum. This can be advantageous when making measurements of the properties of the optical crystal while the crystal is in the enclosure. In practice it is difficult to get a perfect seal between the metal and the glass unless the metal is polished to an optical quality surface. However, in many cases, the positive pressure from the purge gas can be sufficient to prevent the external environment from contaminating the inside of the enclosure. 
       FIG. 7  shows a cross section of a window using a primary glass-to-metal seal and a secondary ring seal. In this case a window  701  is held against frame  704  using a retaining plate  703 . In addition, a ring seal  702  is placed between the edge of window  701  and frame  704 . This type of seal is very good at preventing external contaminants from entering the enclosure. This design also minimizes any photocontamination from the ring seal because outgased material would have to leak through the primary glass-to-metal seal to enter the enclosure. 
     The choice of ring seal material is important and depends on the temperatures to which the enclosure will be raised. For applications where the ring seal will experience temperatures of less than 150 deg C., it is sufficient to use Viton or Kalrez material. For applications where the ring seal will experience between 150 deg C. and 250 deg C., high temperature Kalrez is typically used. For applications where the ring seal will experience greater than 250 deg C., a metal ring seal is likely required. This metal ring is a soft metal material so it provides an adequate seal against the glass. If the metal material is not soft, too much pressure can be required to compress the ring and the window will crack. Many different types of metal ring seal are available. Hollow circular cross sections and hollow C cross sections are available. Some material choices for the metal ring seals are copper, brass, nickel, stainless steel, silver, or gold, but other metals may be employed. Silver is a good choice because it is a soft metal and has good sealing properties. In addition it is also possible to use a silver coating on other metal ring materials to improve the seal. Other metals offer different benefits. This type of seal is well suited for high temperature applications. 
     Optionally, the ring seal  702  can be placed between the window and the frame  704 . This is a more traditional configuration that uses a ring seal as the primary seal. 
     An additional embodiment for window mounting and seal is shown in  FIG. 8 .  FIG. 8  shows a cross section of a window using a primary glass to metal seal and a secondary ring seal. In this case window  802  is held against frame  804  using retaining plate  801 . In addition a ring seal  803  is placed between the edge of window  802  and frame  804 . This design has additional cooling fins  805  in proximity to ring seal  803 . This design approach helps reduce the temperature of the frame in the proximity of the ring seal allowing the use of lower temperature materials for the ring seal. The ring seal typically satisfies the same conditions/constraints as described above with respect to the design of  FIG. 7 . 
       FIG. 9  shows an embodiment of an enclosure containing two optical crystals  905  and  906 . This enclosure can employ all the features mentioned herein. The cross section drawing illustrates the components of the enclosure. Windows  904  and  910  are mounted to frame  901  using mounting plates  902  and  909 . Windows  904  and  910  may be sealed using a primary glass to metal seal together with a secondary ring seal  903  and  908 . Other seal types disclosed with respect to the previous embodiments may also be employed. Brewster windows can also be employed as described in previous embodiments. In addition, the windows should in most cases be placed far enough away from the crystal so they are not damaged by the light focused into the crystal or the light exiting the crystal as previously described. 
     The two crystals  905  and  906  are supported within the cell by recessed portion or support element  907 . Both crystals can be held in place using a similar technique as is described above with respect to  FIG. 5 . The crystals can be of the same type or different types. When crystals of the same type are used, it can increase the frequency conversion efficiency or improve the beam shape of the light energy beam provided to the design. 
     Two typical methods of enhancing the beam are walkoff compensation (WOC) and distributed delta k (DDK) compensation. For WOC, the system rotates the second crystal to produce walkoff in the opposite direction to the first crystal. This can improve both efficiency and beam shape. In general, walkoff represents the situation where the intensity distribution of the beam in the crystal drifts away from the direction of the light wave vector. Thus walkoff compensation tends to decrease the walkoff for the first crystal in the two crystal arrangement. For DDK, the crystal angles are optimized for slightly different angles to optimize the conversion efficiency. 
     If the crystals are different types it is often to generate more than one wavelength within the same enclosure. For example, light at a wavelength of 1064 can enter the enclosure. A portion of the light is converted by a first crystal to 532 nm. Then the 532 nm light and the residual 1064 nm light can be mixed in a second crystal to produce 355 nm light. It is also possible to use more than two crystals by placing them in series. 
       FIG. 10  shows an embodiment of an enclosure containing two optical crystals with the added ability of being able to independently align one of the crystals in the phase matching direction. This enclosure can employ all the construction features mentioned above. The cross sectional drawing illustrates the components of the enclosure. Windows  1004  and  1010  are mounted to frame  1001  using mounting plates  1002  and  1009 . Windows are sealed using a primary glass to metal seal together with a secondary ring seal  1003  and  1011 . Other seal types disclosed herein may alternatively be employed. Again, Brewster windows can also be employed as previously described. In addition, the windows should generally be placed far enough away from the crystal so they are not damaged by the light focused into the crystal or the light exiting the crystal as previously described. 
     Crystal  1005  is supported within the cell by recessed portion  1007 . Crystal  1006  is supported on a pedestal  1013  that is inserted into the frame  1001 . This pedestal  1013  can be sealed using a ring seal  1012 . This ring seal can be similar material to the window seals in previous embodiments. In addition, because the seal here is a metal to metal interface, other types of sealing can be used, including but not limited to metal crush washers used for vacuum seals. Pedestal  1008  can be rotated with respect to crystal  1005  for appropriate alignment, and then fastened in place using external fasteners  1010 . Both crystals can be held in place using a similar technique described with respect to  FIG. 5 . 
       FIG. 11  shows an example of an optical arrangement using multiple crystals. In some cases, placement of two crystals in proximity to each other is not efficient. In a situation where two crystals must be located close to one another, an optical relay collects light from a region proximate one crystal and focuses light to a region proximate a following crystal. In practice this can be accomplished using several optical schemes. One such arrangement is shown in  FIG. 11 . Light from first crystal  1101  is collimated by first lens  1102 . A waveplate  1103  can be used to modify the polarization of the light before the light is focused using lens  1104  into second crystal  1105 . In many cases it is not necessary to modify the polarization of the light from the first crystal, and thus waveplate  1103  is optional. It is also possible to use a single lens to collect light from crystal  1101  and focus it into crystal  1105 . 
       FIG. 12  shows an alternate example of an optical arrangement for using multiple crystals. This example extends the example of  FIG. 11  to three crystals. In practice, additional crystals can be added in a similar fashion, and three are provided here as an example and are not intended to be limiting. Light from a first crystal  1201  is collected by a first lens  1102 . A waveplate  1103  can be used to modify the polarization of the light before the light is focused using lens  1204  into second crystal  1205 . Lens  1206  then collects light from crystal  1205 . A waveplate  1207  can be used to modify the polarization of the light before the light is focused by lens  1208  into crystal  1209 . 
     As for the embodiment in  FIG. 11 , it is often not necessary to modify the polarization of the light one or more of the crystals. If polarization is not modified, waveplates  1203  or  1207  are not necessary. It is also possible to use a single lens to collect light from crystal  1201  and focus the light into crystal  1205 . It is also possible to use a single lens to collect light from crystal  1205  and focus light into crystal  1209 . 
       FIG. 13  shows another example of an optical arrangement for using multiple crystals. In this case the optical relay uses mirrors instead of lenses to collect light from a region proximate to one crystal and focus the light in proximity to a following crystal. This can be beneficial in certain circumstances, such as when using high power light and UV/DUV light. When small diameter beams of high power light transmit through lens elements, absorption can change the properties of the lens. This includes local lens heating that results in changes in the focus position of the light. This phenomenon is called thermal lensing and can cause changes in the focus position of the light. This focus change can reduce the frequency conversion efficiency and change the beam profile. In addition, when high power UV/DUV light transmits through a lens, high power light can cause long term damage including compaction, scattering, color center formation, and eventually catastrophic damage. Use of mirrors can reduce these problems because there is no bulk material for the light to transmit through. Mirrors can also exhibit high damage thresholds. In addition, using a mirror geometry can make the frequency conversion system more compact and use fewer optical components. Mirrors can also be used with dichroic coatings which reflect one wavelength and transmit another. This enables the frequency converted light to be separated from residual unconverted light. 
     The embodiment in  FIG. 13  includes a lens  1301  to focus light into a first crystal  1302 . This lens could also be replaced with a mirror. Light from crystal  1302  is then collected by mirror  1303 . Light from mirror  1303  then passes through waveplate  1304  and is collected by mirror  1305 . Waveplate  1304  can be used to modify the polarization of the light. However, in some frequency conversion schemes such polarization modification is not necessary and waveplate  1304  is not needed. Mirror  1305  then focuses light into second crystal  1306 . 
     The design in  FIG. 14  shows an extension of the arrangement in  FIG. 13  to more than two crystals. The embodiment in  FIG. 14  includes a lens  1401  to focus light into a first crystal  1402 . This lens could also be replaced with a mirror. Light from crystal  1402  is then collected by mirror  1403 . Light from mirror  1403  then passes through waveplate  1404  and is collected by mirror  1405 . Waveplate  1404  can be used to modify the polarization of the light, and again, in some frequency conversion schemes, polarization modification is not necessary and waveplate  1404  is optional. Mirror  1405  then focuses light into second crystal  1406 . In a similar manner to the light from crystal  1402 , light from crystal  1406  is then collected by mirror  1407 , and light from mirror  1407  passes through waveplate  1408  and is collected by mirror  1409 . Waveplate  1408  can be used to modify the polarization of the light, but is optional and not needed when polarization modification is not required. Mirror  1409  then focuses light into third crystal  1410 . Dichroic mirror coatings may also be used as described previously. 
       FIG. 15  shows an example of an enclosure that contains at least one optical crystal and optics for collecting and focusing light. The enclosure may contain optical and crystal systems similar to those shown in  FIGS. 11-14 . In the system of  FIG. 15 , light enters the enclosure through window  1501 . Window  1501  is sealed to frame  1503  as described in previous embodiments. Window  1501  should be made from a material with a high enough damage threshold to endure light at expected wavelengths passing through the window. In the UV/DUV light range, fused silica and calcium fluoride are very good materials for this window application. 
     Light is then focused by lens  1502  into optical crystal  1504 . Light from crystal  1504  is then collected by mirror  1505 , and light then passes through waveplate  1506  and is collected by mirror  1507 . Waveplate  1506  can be used to modify the polarization of the light if desired. Mirror  1507  then focuses light into second crystal  1508 , and light exits the enclosure through window  1509 . Brewster windows can also be employed as described previously, and the windows should generally be placed far enough away from the crystal so they are not damaged by the light focused into the crystal or the light exiting the crystal. 
       FIG. 16  shows an alternate example of an enclosure that contains at least one optical crystal and optics for collecting and focusing light. This enclosure places the exit window a further distance from the second crystal than the design of  FIG. 15 , allowing the beam to diverge and reduce the light intensity on the window. This is especially important when UV/DUV wavelengths transmit through the window and can dramatically increase the lifetime of the window. The enclosure may contain optical and crystal systems similar to those shown in  FIGS. 11-14 . 
     In the system of  FIG. 16 , light enters the enclosure through window  1601 . Window  1601  is sealed to frame  1603  as described previously. Window  1601  is made from a material with suitable high damage threshold for light at the wavelengths traveling therethrough. In the UV/DUV light range, fused silica and calcium fluoride are very good materials for this application. Light is then focused by lens  1602  into optical crystal  1604 . Light from crystal  1604  is then collected by mirror  1605 , passes through waveplate  1606 , and is collected by mirror  1607 . Waveplate  1606  can be used to modify the polarization of the light if desired. Mirror  1607  then focuses light into second crystal  1608  and light exits the enclosure through window  1610 . Window  1610  is placed a distance away from crystal  1608  to allow the beam to expand and reduce the intensity of the beam using extension  1609 . Extension  1609  can be a separate piece from frame  1603  or can be an integral part of frame  1603 . In the case where extension  1609  is a separate piece from frame  1603 , sealing can occur using the ring seal techniques described herein. Brewster windows can also be employed as described and windows should generally be placed far enough away from the crystal so they are not damaged by the light focused into the crystal or the light exiting the crystal. 
       FIG. 17  shows an alternate embodiment of a crystal enclosure that contains the optical crystals inside the enclosure while the collection and focusing optics are provided outside the enclosure. This embodiment may employ optical and crystal arrangements similar to those shown in  FIGS. 11-14 . The typical method for using multiple crystals would be to use a single enclosure for one crystal and another crystal without an enclosure. These would be used with multiple collection and focusing lenses, but as a result, cost and size can be issues. Using more than one crystal usually requires separate crystal support, enclosures, and translation systems. This in combination with the associated collection and focusing lenses can produce a system that is quite large and expensive. Also, use of many frequency conversion steps can produce many wavelengths and polarization combinations. This typically requires many optics with different coatings. 
     The embodiment in  FIG. 17  has infrared P-polarized light coming into the enclosure through window  1702 . This window is sealed to the frame using techniques described herein. The window is oriented at Brewster&#39;s angle so there is no reflection loss of the P-polarized light. The entering light is focused into optical crystal  1703 . A typical crystal that can be used in this location is LBO (Lithium Triborate). For example, crystal  1703  can be an LBO crystal that is noncritically phase matched to frequency double the infrared light to a visible wavelength. It can be beneficial to have one of the two crystals be noncritically phase matched so the design is less alignment sensitive. In this case, crystal  1703  uses Type I phase matching to produce visible light polarized orthogonally to the IR light. Window  1704  is oriented near Brewster&#39;s angle for the frequency converted visible light. Mirror  1705  collects the visible light from crystal  1703 . Mirrors  1705  and  1707  can have dielectric coatings that are highly reflective for the visible wavelength and highly transmissive for the infrared wavelength. This allows only the visible light to reflect from mirror  1705  and residual infrared light to pass through. 
     Visible light then transmits through Brewster window  1704  again, through frame  1701  and through Brewster window  1706  before being collected by mirror  1707 . Mirror  1707  then focuses visible light back through Brewster window  1706  and into crystal  1708 . In this embodiment crystal  1708  may be a CLBO (Cesium Lithium Borate) crystal or a BBO (Barium Borate) crystal. In this embodiment crystal  1708  uses Type I phase matching to produce DUV light polarized orthogonally to the visible light. DUV light and residual visible light then pass through window  1709  oriented at Brewster&#39;s angle for the DUV wavelength. External dichroic mirrors (not shown) can be used to separate the DUV light from the visible light. 
     In this embodiment, other types of phase matching may be used in crystal  1703  or crystal  1708 . This can produce other polarizations and require rotation of the Brewster windows by 90 degrees. Alternately, the windows can be oriented near zero degrees angle of incidence and antireflection coated. Such modifications are possible by those skilled in the art and can be considered as part of the present design. 
       FIG. 18  shows a crystal enclosure that is only for crystal preparation before use. In many cases the crystal must be baked for an extended period of time at high temperatures. In this case it can be advantageous to have a simplified crystal enclosure optimized for high temperature processing. In this arrangement it is often not necessary to have windows to get light into and out of the crystal. This lack of a window requirement removes the complexity of making a high temperature seal between glass and metal that does not outgas and cause photocontamination. 
     The enclosure in  FIG. 18  includes a metal frame  1801  and a pedestal  1802  for supporting the optical crystal  1806 . Pedestal  1802  is sealed to frame  1802  using ring seal  1803 . This ring seal is positioned between two metal surfaces and can use any of the types of seals or sealing methods described herein. In addition, a metal crush washer can be employed, since the ring seal is a seal between two metal surfaces and a large amount of clamping force can be applied. As described in previous embodiments, crystal  1806  is held in place using spring  1804  and cap  1805 . This allows the crystal to be processed over an extended temperature range without producing stress that could crack the crystal. In addition, inlets and outlets are provided for purge gas to maintain the environment inside the enclosure. 
     Thus, in summary, an environmentally controlled enclosure comprising a crystal is provided. Multiple crystals may be provided in certain embodiments. The enclosure comprises securing hardware configured to secure the crystal within the enclosure such that temperature changes within the enclosure produce negligible stress on the crystal. The enclosure further includes a window configured to permit light to enter the enclosure and contact the crystal and may include a seal formed between the window and the enclosure. 
     In certain embodiments, a frame is provided for the enclosure, and an outlet configured to purge gas from the enclosure many be provided. Heating or cooling elements may be provided to control the temperature of the enclosure and the crystal or crystals provided therein, and a temperature reading element may be provided that controls temperature using feedback. The window or windows may be provided at Brewster&#39;s angle. 
     The crystal may be secured within the enclosure comprises a low force spring. An exit window may be provided, and windows may be fabricated from fused silica or calcium fluoride. The enclosure may be configured for built in angle adjustment along a primary phase matching axis. Angle adjustment may be provided using a goniometer with a rotation axis in proximity to a center of the crystal. The crystal can be a nonlinear crystal configured to perform frequency mixing formed from CLBO, LBO, BBO, KBBF, CBO, KDP, KTP, KD*P, or BIBO. 
     A seal between the window and the enclosure or frame may be provided, in some cases a primary seal and a secondary seal, configured to seal the entry window to the enclosure. The primary seal may be a ring formed from a high temperature low outgassing material. The secondary seal can comprise a ring formed from a metal comprising at least one from a group comprising silver, stainless steel, aluminum, and nickel. 
     Optics may be provided inside or outside the enclosure, configured to collect light from a first crystal and refocus the light in proximity of a second crystal. When multiple crystals are employed, the crystals may be oriented to perform at least one from a group comprising walkoff compensation and distributed delta k compensation. 
     The design presented herein and the specific aspects illustrated are meant not to be limiting, but may include alternate components while still incorporating the teachings and benefits of the invention. While the invention has thus been described in connection with specific embodiments thereof, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within known and customary practice within the art to which the invention pertains.