Abstract:
A laser-sustained plasma illuminator system includes at least one laser light source to provide light. At least one reflector focuses the light from the laser light source at a focal point of the reflector. An enclosure substantially filled with a gas is positioned at or near the focal point of the reflector. The light from the laser light source at least partially sustains a plasma contained in the enclosure. The enclosure has at least one wall with at least one property that is varied to compensate for optical aberrations in the system.

Description:
PRIORITY CLAIM 
     This patent is a continuation-in-part of U.S. patent application Ser. No. 14/183,134 entitled “GAS REFRACTION COMPENSATION FOR LASER-SUSTAINED PLASMA BULBS” filed Feb. 18, 2014, which claims priority to U.S. Provisional Patent Application No. 61/767,917 filed Feb. 22, 2013, which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to laser-sustained plasma illuminator systems. More particularly, the invention relates to systems and methods for compensating for optical aberrations to optimize plasma performance and UV light collection. 
     2. Description of Related Art 
     Plasmas sustained by lasers have shapes defined by the laser light intensity distribution near the laser focus. The laser light intensity distribution may be a function of optical aberrations (e.g., how well the light is focused in the plasma cell). Many optical aberrations present in typical laser-sustained plasma illuminator systems are aberrations introduced by an enclosure (e.g., a bulb) used to contain the gas and the plasma. Such bulb-introduced aberrations may be significant optical aberrations, especially for plasmas sustained by lasers operating in the near IR range (wavelengths of about 1000 nm). These significant optical aberrations may result in large size plasmas, the inability to control the bulb envelope, and/or irreproducible plasma shapes. 
       FIG. 1  depicts different plasma shapes resulting from various optical abberrations of a pump beam in different bulbs. Shape  100  results from a pump beam with significant aberrations. These significant aberrations produce a conventional shaped plasma for shape  100 . Shape  102  results from a pump beam with less aberrations. These fewer aberrations may produce a spherical shaped plasma for shape  102 . Shape  104  results from a pump beam with the fewest aberrations. Shape  104  may be the smallest and brightest plasma shape of the three shapes depicted in  FIG. 1  because of the fewest aberrations (e.g., shape  104  may be a “compensated” plasma shape or plasma shape that results after compensating for aberrations). 
     Aberrations may become particularly large when a high NA (numerical aperture) is used for pumping the plasma. Large pump laser NAs are used as light sources in many current laser-sustained plasma illuminator systems. U.S. Pat. No. 7,705,331 to Kirk et al., which is incorporated by reference as if fully set forth herein, describes an example of a high NA system.  FIG. 2  depicts an example of a laser-sustained light source with a high NA. Light source  200  may include laser  202 , turn mirror  204 , cold mirror  206 , homogenizer  208 , filters  210 , ellipse  212 , and enclosure  214 . Enclosure  214  may be, for example, a bulb. Ignition cable  216  may be coupled to enclosure  214 . Plasma  217  may be generated inside enclosure  214  at or near a focal point of ellipse  212 . As shown in  FIG. 2 , light from laser  202  (e.g., light  218 ) may be reflected off ellipse  212  and focused in the middle of enclosure  214  at plasma  217 . Broad-band UV light (e.g., light  220 ) from homogenizer  208  may be reflected by cold mirror  206 , reflected off ellipse  212 , and focused in the middle of enclosure  214  at plasma  217 . Light passing through enclosure  214  may be used to excite and/or sustain plasma  217  inside the enclosure. Plasma  217  inside enclosure  214  may provide light for illumination of a specimen for a process performed on the specimen (e.g., an inspection process performed on the specimen). As shown in  FIG. 2 , light passing through enclosure  214  may have a high NA. 
     In addition to the aberrations introduced by the enclosure itself, the refractive index of the gas inside the enclosure is another source of aberrations in the system. Gas related aberrations may be especially significant in high-pressure enclosures.  FIG. 3  depicts images taken of a bulb at different pressures of Xe (xenon) in the bulb. As shown in  FIG. 3 , aberrations seen from the bulb increase with increasing pressure. 
     U.S. Pub. Pat. Appl. Nos. 2007/0228288 and 2007/0228300 to Smith, each of which is incorporated by reference as if fully set forth herein, disclose one method of compensating for aberrations introduced by the refractive index of the walls of the enclosure by modifying the shape of the reflector (e.g., a reflective ellipse). Modifying the shape of the reflector, however, can only account for aberrations from reproducible enclosure shapes. Modifying reflector shapes for each individual enclosure shape and/or different fill pressures is difficult to impractical to implement for most laser-sustained plasma illuminator systems. 
     SUMMARY 
     In certain embodiments, a laser-sustained plasma illuminator system includes at least one laser light source to provide light. At least one reflector focuses the light from the laser light source at a focal point of the reflector. An enclosure substantially filled with a gas is positioned at or near the focal point of the reflector. The light from the laser light source at least partially sustains a plasma contained in the enclosure. The enclosure has at least one wall with at least one property that is varied. The at least one property of the wall may be varied to compensate for optical aberrations in the system. In some embodiments, a thickness of the wall is varied. In some embodiments, a refractive index of the wall is varied. 
     In certain embodiments, a method for compensating for optical aberrations in a laser-sustained plasma illuminator system includes providing an enclosure for containing a plasma to the laser-sustained plasma illuminator system. The enclosure may have at least one wall with at least one property that is varied to compensate for optical aberrations in the system. 
     In certain embodiments, a laser-sustained plasma illuminator system includes at least one laser light source to provide light. At least one reflector focuses the light from the laser light source at a focal point of the reflector. An enclosure may include two or more different materials and be substantially filled with a gas positioned at or near the focal point of the reflector. The light from the at least one laser light source at least partially sustains a plasma contained in the enclosure. The enclosure has at least one wall formed from the two or more different materials with at least one property of the wall being varied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of the methods and apparatus of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts different plasma shapes resulting from various optical abberrations of a pump beam in different bulbs. 
         FIG. 2  depicts an example of a laser-sustained light source with a high numerical aperture (NA). 
         FIG. 3  depicts images taken of a bulb at different pressures of Xe (xenon) in the bulb. 
         FIG. 4A  depicts an embodiment of an ideal enclosure with no compensation needed. 
         FIG. 4B  depicts an embodiment of an enclosure with shape induced aberrations and no compensation. 
         FIG. 4C  depicts an embodiment of an enclosure with walls having varying thickness to compensate for enclosure shape aberrations. 
         FIG. 4D  depicts an embodiment of an enclosure with walls having varying refractive index to compensate for enclosure shape aberrations. 
         FIG. 4E  depicts an embodiment of an enclosure made of two different materials. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     One or more properties of a wall of an enclosure (e.g., a bulb) may be varied (e.g. adjusted) to compensate for optical aberrations such as shape aberrations in the enclosure and/or aberrations induced by the gas refractive index (e.g., fill pressure aberrations). In certain embodiments, the wall thickness of the enclosure is adjusted to compensate for optical aberrations.  FIG. 4A  depicts an embodiment of an ideal enclosure with no compensation needed. Enclosure  400 A has no aberrations in shape and no gas induced aberrations. Thus, all light from pump laser  402  is focused at plasma  404 .  FIG. 4B  depicts an embodiment of an enclosure with shape induced aberrations and no compensation. Enclosure  400 B has shape aberrations that, without compensation, cause some light from pump laser  402  to not be focused at plasma  404  (e.g., light  406 ). 
       FIG. 4C  depicts an embodiment of an enclosure with walls having varying thickness to compensate for enclosure shape aberrations. Enclosure  400 C has walls  408  with varying thickness. The varying thickness of walls  408  compensates for any enclosure shape aberrations and/or fill pressure aberrations to focus light from pump laser  402  at plasma  404 . For example, as shown in  FIG. 4C , light  406  is now focused at plasma  404 . 
     In certain embodiments, enclosure  400 C is a bulb. The bulb may be, for example, a lamp made of glass (fused silica) using a bulb-specific manufacturing process. In some embodiments, enclosure  400 C is any other type of enclosure, vessel, or container that encloses/contains gas and has walls made of a transparent material. Enclosure  400 C may be an enclosure made of glass, quartz, sapphire, CaF 2 , MgF 2 , or similar materials with proper sealing to enclose/contain a gas. For example, enclosure  400 C may be a tube or cell made of glass with sealing to enclose a gas. 
     In certain embodiments, the thickness variation in walls  408  (e.g., the shape of the walls as defined by changes in the wall thickness along a section of the wall) is defined based on the shape of the envelope of enclosure  400 C and/or the gas fill pressure of the enclosure. Varying the thickness of the walls of enclosures (e.g., walls  408  of enclosure  400 C) to compensate for aberrations in the enclosures (e.g., enclosure wall thickness compensation) allows a single uncompensated reflector to be used for all types of enclosures with varying shapes and/or fill pressures. Thus, a laser-sustained plasma illuminator system using enclosures with enclosure wall thickness compensation may have improved performance and/or improved cost efficiency compared to typical current laser-sustained plasma illuminator systems (e.g., systems using modified reflector shapes for aberration compensation). 
     In some embodiments, enclosure wall thickness compensation is used to compensate for aberrations in the collected light path (e.g., the path of light before the light enters the enclosure or the path of light from the light source (laser) through focusing optics (such as mirrors and/or reflectors). In some embodiments, enclosure wall thickness compensation is used to introduce a controlled amount of aberration into a laser-sustained plasma illuminator system. For example, wall thickness may be varied to provide a controlled amount of aberration to optimize plasma performance in the laser-sustained plasma illuminator system. 
     In some embodiments, enclosure wall thickness compensation is used in combination with other compensation methods. Combining enclosure wall thickness compensation with other compensation methods may provide higher levels of control of aberrations in a laser-sustained plasma illuminator system. For example, in one embodiment, enclosure wall thickness may be varied in combination with the shape of the enclosure. In some embodiments, enclosure wall thickness compensation is combined with compensation using modified reflector shapes to provide greater control of the shape of the plasma. 
     In certain embodiments, the refractive index of the enclosure is adjusted to compensate for optical aberrations.  FIG. 4D  depicts an embodiment of an enclosure with walls having varying refractive index to compensate for enclosure shape aberrations. Enclosure  400 D may be a bulb or any other type of enclosure, vessel, or container that encloses/contains gas and has walls made of a transparent material as described above. Enclosure  400 D may be an enclosure made of glass, quartz, sapphire, CaF 2 , MgF 2 , or similar materials with proper sealing to enclose/contain a gas. 
     In certain embodiments, enclosure  400 D includes walls  408 ′ with varying refractive index. Varying the refractive index of walls  408 ′ compensates for any enclosure shape aberrations and/or fill pressure aberrations to focus light from pump laser  402  at plasma  404 . For example, as shown in  FIG. 4D , light  406  is focused at plasma  404 . 
     In some embodiments, the refractive index of walls  408 ′ of enclosure  400 D is varied by varying (e.g. altering) the chemical content of materials used in the walls. For example, one or more materials used in walls  408 ′ may be doped to alter the chemical content (or composition) of the walls. The dopant(s) concentration in walls  408 ′ may be varied to provide a tailored or controlled refractive index profile in the walls. For example, the dopant concentration may provide one or more abrupt transitions (changes) in refractive index in walls  408 ′ or the dopant concentration may provide a gradual change in refractive index in the walls. In some embodiments, the refractive index of walls  408 ′ of enclosure  400 D is varied by varying (e.g. altering) a structure (e.g., physical and/or chemical structure) of the walls. For example, the structure of walls  408 ′ may be changed (altered) to be more or less porous to vary the refractive index of the walls. 
     In some embodiments, the refractive index of walls  408 ′ of enclosure  400 D is varied by varying a temperature along the walls. For example, differences in temperature along walls  408 ′ may provide different refractive indices along the walls depending on the material used for the walls. In some embodiments, walls  408 ′ have selected (e.g., patterned) absorption along the walls to vary the temperature along the walls. In some embodiments, walls  408 ′ have selected (e.g., patterned) cooling flow along the walls to vary the temperature along the walls. 
     In certain embodiments, an enclosure (such as enclosure  400 C or enclosure  400 D described above) is formed by combining two or more different materials. The combination of two or more different materials may be used to form an enclosure with varying wall thickness (e.g., enclosure  400 C) or an enclosure with varying refractive index (e.g., enclosure  400 D). For example, the refractive index of walls  408 ′ of enclosure  400 D may be varied by combining two or more different refractive index materials to form the walls of the enclosure. 
       FIG. 4E  depicts an embodiment of enclosure  400 D′ made of two different materials. Enclosure  400 D′ includes walls  408 ″. Walls  408 ″ may include two different materials  410 A,  410 B. In certain embodiments, material  410 A has a different refractive index from material  410 B. The different refractive indices of materials  410 A,  410 B may provide a varying refractive index in walls  408 ″ of enclosure  400 D′. In certain embodiments, enclosure  400 D′ is formed by coupling, connecting, or attaching together two or more enclosures made of the different materials (e.g., materials  410 A,  410 B) to form the enclosure. For example, a first enclosure may include material  410 A and a second enclosure may include material  410 B and enclosure  400 D′ is formed by coupling together the first enclosure and the second enclosure. In some embodiments, the enclosures of the different materials are concentric enclosures such as concentric cylindrical enclosures. 
     It is to be understood the invention is not limited to particular systems described which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification, the singular forms “a”, “an” and “the” include plural referents unless the content clearly indicates otherwise. Thus, for example, reference to “a wall” includes a combination of two or more walls and reference to “a gas” includes mixtures of gases. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.