Patent Publication Number: US-9433070-B2

Title: Plasma cell with floating flange

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/916,048, filed Dec. 13, 2013, entitled FLOATING FLANGE CELL DESIGN, naming Ilya Bezel, Anatoly Shchemelinin and Amir Torkaman as inventors, which is incorporated herein by reference in the entirety. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to plasma based light sources, and, more particularly, to a plasma cell equipped with one or more floating flanges. 
     BACKGROUND 
     As the demand for integrated circuits having ever-small device features continues to increase, the need for improved illumination sources used for inspection of these ever-shrinking devices continues to grow. One such illumination source includes a laser-sustained plasma source. Laser-sustained plasma light sources are capable of producing high-power broadband light. Laser-sustained light sources operate by focusing laser radiation into a gas volume in order to excite the gas, such as argon or xenon, into a plasma state, which is capable of emitting light. This effect is typically referred to as “pumping” the plasma. Typical plasma cell designs fail to provide adequate resistance to high temperature and high pressure environments, compromising the integrity of the seals, the body of the plasma cell and the quality of the atmosphere inside of the plasma cell. Therefore, it would be desirable to provide a system and method for curing defects such as those of the identified above. 
     SUMMARY 
     A system for forming light-sustained plasma is disclosed, in accordance with an illustrative embodiment of the present invention. In one illustrative embodiment, the system includes an illumination source configured to generate illumination. In another illustrative embodiment, the system includes a plasma cell. In one illustrative embodiment the plasma cell includes a transmission element having one or more openings and configured to contain a volume of gas; one or more terminal flanges disposed at or near the one or more openings of the transmission element; and one or more floating flanges disposed between at least one of the one or more terminal flanges and the transmission element. In another illustrative embodiment, the one or more floating flanges are movable to compensate for thermal expansion of the transmission element. In another illustrative embodiment, the system includes a collector element arranged to focus the illumination from the illumination source into the volume of gas in order to generate a plasma within the volume of gas contained within the plasma cell. In another illustrative embodiment, the plasma emits broadband radiation. In another illustrative embodiment, the transmission element of the plasma cell is at least partially transparent to at least a portion of the illumination generated by the illumination source and at least a portion of the broadband radiation emitted by the plasma. 
     A plasma cell for forming a light-sustained plasma is disclosed, in accordance with an illustrative embodiment of the present invention. In one illustrative embodiment, the plasma cell includes a transmission element having one or more openings and configured to contain a volume of gas. In another illustrative embodiment, the plasma cell includes a first terminal flange disposed at or near the one or more openings of the transmission element. In another illustrative embodiment, the plasma cell includes a second terminal flange disposed at or near the one or more openings of the transmission element. In another illustrative embodiment, the plasma cell includes at least one floating flange disposed between at least one the first terminal flange or the second terminal flange and the transmission element. In another illustrative embodiment, the at least one floating flange is movable to compensate for thermal expansion of the transmission element. In another illustrative embodiment, the at least one floating flange is configured to enclose the internal volume of the transmission element in order to contain a volume of gas within the transmission element. In another illustrative embodiment, the transmission element is configured to receive illumination from an illumination source in order to generate a plasma within the volume of gas. In another illustrative embodiment, the plasma emits broadband radiation. In another illustrative embodiment, the transmission element is at least partially transparent to at least a portion of the illumination generated by the illumination source and at least a portion of the broadband radiation emitted by the plasma. 
     A plasma cell for forming a light-sustained plasma is disclosed, in accordance with an illustrative embodiment of the present invention. In one illustrative embodiment, the plasma cell includes a transmission element having one or more openings and configured to contain a volume of gas. In another illustrative embodiment, the plasma cell includes one or more terminal flanges disposed at or near the one or more openings of the transmission element. In another illustrative embodiment, the plasma cell includes one or more floating flanges disposed between at least one of the one or more terminal flanges and the transmission element, wherein the one or more floating flanges are movable to compensate for thermal expansion of the transmission element. In another illustrative embodiment, the transmission element is configured to receive illumination from an illumination source in order to generate a plasma within the volume of gas. In another illustrative embodiment, the plasma emits broadband radiation. In another illustrative embodiment, the transmission element is at least partially transparent to at least a portion of the illumination generated by the illumination source and at least a portion of the broadband radiation emitted by the plasma. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1A  is a high level schematic view of a system for forming a light-sustained plasma, in accordance with one embodiment of the present invention. 
         FIG. 1B  is a high level schematic view of a plasma cell equipped with connecting rods, in accordance with one embodiment of the present invention. 
         FIG. 1C  is a high level schematic view of a plasma cell equipped with fins, in accordance with one embodiment of the present invention. 
         FIG. 1D  is an end-on schematic view of a plasma cell equipped with fins, in accordance with one embodiment of the present invention. 
         FIG. 1E  is a high level schematic view of a plasma cell having one or more coolant transport connecting rods, in accordance with one embodiment of the present invention. 
         FIG. 1F  is a high level schematic view of a plasma cell having one or more heat conduction connecting rods, in accordance with one embodiment of the present invention. 
         FIG. 1G  is a high level schematic view of a plasma cell equipped with one or more radiation shielding elements, in accordance with one embodiment of the present invention. 
         FIG. 1H  is a high level schematic view of a plasma cell equipped with one or more radiation shielding elements, in accordance with one embodiment of the present invention. 
         FIG. 1I  is a high level schematic view of a plasma cell equipped with one or more plume control elements, in accordance with one embodiment of the present invention. 
         FIG. 1J  is a high level schematic view of a plasma cell mounted within the collector/reflector, in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. 
     Referring generally to  FIGS. 1A through 1J , a system for generating a light-sustained plasma is described in accordance with the present disclosure. Embodiments of the present invention are directed to the generation of broadband light with a light-sustained plasma light source. Embodiments of the present invention provide a plasma cell equipped with a transmission element that is transparent to both the pumping light (e.g., light from a laser source) used to sustain a plasma within the plasma cell and broadband light emitted by the plasma. Embodiments of the present invention may provide an intermediate floating flange and/or a compressive sealing element disposed between the transmission element and a terminal flange of the plasma cell. The intermediate floating flange and/or compressive sealing element provide for the compensation of thermal expansion of various components of the plasma cell, such as the transmission element and connecting rods. The connecting rods of the plasma cell of the present disclosure may serve to apply a preload to the various seals of the plasma cell. Embodiments of the present invention may also provide various control elements (e.g., temperature control, convective control and the like) and/or protective elements (e.g., radiation shield and the like) that are coupled to, or integrated with, one or more portions of the plasma cell, such as one or more flanges (e.g., metal flanges or ceramic flanges) and/or caps, which serve to terminate openings of the transmission element of the plasma cell. 
     It is noted herein that the expansion compensation features provided by the floating flange and compressive sealing element of the plasma cell of the present disclosure allow for the use of many types of materials in the connecting rods, transmission element, and flanges irrespective of thermal expansion coefficients of the given materials. Further, these features also provide for the use of the plasma cell of the present disclosure in an expanded range of temperatures, thermal gradients and internal pressures. The plasma cell of the present disclosure reduces the need to match thermal expansion coefficients for the connecting rods and the transmission element of the plasma cell. It is noted herein that the plasma cell of the present disclosure reduces contact stress on the transmission element from the various seals to a level necessary to avoid damaging the transmission element, while maintaining adequate contact stress for maintaining pressure within the transmission element. Such a configuration allows the plasma cell to operate in a larger range of temperatures and internal pressures. 
       FIGS. 1A-1J  illustrate a system  100  for forming a light-sustained plasma, in accordance with embodiment of the present invention. The generation of plasma within inert gas species is generally described in U.S. patent application Ser. No. 11/695,348, filed on Apr. 2, 2007; and U.S. patent application Ser. No. 11/395,523, filed on Mar. 31, 2006, which are incorporated herein in their entirety. Various plasma cell designs and plasma control mechanisms are described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated herein by reference in the entirety. The generation of plasma is also generally described in U.S. patent application Ser. No. 14/224,945, filed on Mar. 25, 2014, which is incorporated by reference herein in the entirety. Plasma cell and control mechanisms are also described in U.S. patent application Ser. No. 14/231,196, filed on Mar. 31, 2014, which is incorporated by reference herein in the entirety. Plasma cell and control mechanisms are also described in U.S. patent application Ser. No. 14/288,092, filed on May 27, 2014, which is incorporated by reference herein in the entirety. Plasma cell and control mechanisms are also described in U.S. patent application Ser. No. 13/741,566, filed on Jan. 15, 2013, which is incorporated by reference herein in the entirety. 
     Referring to  FIG. 1A , in one embodiment, the system  100  includes an illumination source  111  (e.g., one or more lasers) configured to generate illumination of a selected wavelength, or wavelength range, such as, but not limited to, infrared radiation or visible radiation. In another embodiment, the system  100  includes a plasma cell  102  for generating, or maintaining, a plasma  104 . In another embodiment, the system  100  includes a collector/reflector element  105  (e.g., an ellipsoid-shaped collector element) configured to focus illumination emanating from the illumination source  111  into a volume of gas  103  contained within the plasma cell  102 . 
     In another embodiment, the plasma cell  102  includes a transmission element  108 . In another embodiment, as shown in  FIGS. 1B-1H , the transmission element  108  may have one or more openings  109   a ,  109   b  (e.g., top opening  109   a  and bottom opening  109   b ). In one embodiment, the one or more openings  109   a ,  109   b  may be located at one or more end portions of the transmission element  108 . In another embodiment, the first opening  109   a  and the second opening  109   b  are in fluidic communication with one another such that the internal volume of the transmission element  108  is continuous from the first opening  109   a  to the second opening  109   b  For example, as shown in  FIGS. 1B-1H , a first opening  109   a  may be located at a first end portion of the transmission element  108 , while a second opening  109   b  may be located at a second end portion, opposite of the first end portion, of the transmission element  108 . 
     In another embodiment, as shown in  FIGS. 1B-1H , the plasma cell  102  includes one or more terminal flanges  110 ,  112 . In one embodiment, the one or more terminal flanges  110 ,  112  are disposed at or near the one or more openings  109   a ,  109   b  of the transmission element  108 . For example, the plasma cell  102  may include, but is not limited to, a first terminal flange  110  (e.g., top flange) and a second terminal flange (e.g., bottom flange). 
     In another embodiment, the plasma cell  102  includes one or more floating flanges  113 . For example, a floating flange  113  may be disposed between a terminal flange, such as terminal flange  112 , and the transmission element  108 . In one embodiment, the one or more floating flanges  113  are movable. In this regard, the movement of the one or more floating flanges  113  provides for the compensation of the thermal expansion of one or more components of the plasma cell  102 , such as, but not limited to, the transmission element  108 . In this regard, the floating flange  113  may be thought of as an intermediate flange located between a terminal flange (e.g., flange  110 ,  112 ) and the transmission element  108  of the plasma cell  102 . 
     In one embodiment, the transmission element  108  is configured to contain a volume of gas  103 . In one embodiment, the first terminal flange  110  (or the second terminal flange  112 ) and the floating flange  113  are configured to enclose the internal volume of the transmission element  108  so as to contain a volume of gas  103  within the body of the transmission element  108 . In this regard, the first terminal flange  110  and the floating flange  113  may be closed so as to create a closed volume when the flanges are in contact with the transmission element  108 . It is noted herein that the closed volume of the plasma cell  102  may also be formed with one or more caps, such as caps  134  and  136  depicted in  FIG. 1J , described further herein. In one embodiment, the plasma cell includes a first cap  134  couplable to the first terminal flange  110  via mounting screws  138 . In another embodiment, the plasma cell  102  includes a second cap  136  couplable to the second flange  112  via mounting screws  140 . In one embodiment, the first cap  134  and the second cap  136  are configured to enclose the internal volume of the transmission element  108  so as to contain a volume of gas  103  within the body of the transmission element  108 . In this regard, the first terminal flange  110  and the floating flange  113  may be open so as to create a closed volume when the caps  134 ,  136  are in contact with the first terminal flange  110  and the second terminal flange  112   
     In another embodiment, the plasma cell  102  includes a compressive sealing element  122  disposed within a gap between the one or more floating flanges  113  and the one or more terminal flanges  110 ,  112 . In one embodiment, the compressive sealing element  122  includes an incompletely compressed seal. For example, the compressive sealing element  122  includes, but is not limited to, an incompletely compressed C-ring seal (e.g., metal C-ring seal), an E-ring seal (e.g., metal e-ring seal) or O-ring seal (e.g., metal O-ring seal). By way of another example, the compressive sealing element  122  includes, but is not limited to, a bellows. 
     It is noted herein that the compressive sealing element  122  may provide a seal between the transmission element  108  and the floating flange  113 , while also allowing for thermal expansion of the various components (e.g., transmission element  108 ) of the plasma cell  102 . For example, thermal expansion of the transmission element  108  may cause the displacement of the floating flange  113  (e.g., displacement along vertical direction in  FIG. 1B-1H ), which in turn, compresses the compression sealing element  122 . Such a configuration provides for minimal, or at least reduced, compressive stress, thereby allowing for an increased range in operating temperatures and tolerable thermal gradients in one or more components (e.g., transmission element  108 , connecting rods  118  and the like) of the plasma cell  102  without breaking the seal between the transmission element  108  and the floating flange  113 . 
     In another embodiment, as shown in  FIGS. 1B-1H , the plasma cell  102  includes one or more seals  114 . In one embodiment, the seals  114  are configured to provide a seal between the body of the transmission element  108  and the one or more terminal flanges, such as terminal flange  110 , and the floating flange  113 . The seals  114  of the plasma cell  102  may include any seals known in the art. For example, the seals  114  may include, but are not limited to, a brazing, an elastic seal, an O-ring, a C-ring, and E-ring and the like. In one embodiment, the seals  114  may include one or more metals or metal allows. For example, the seals  114  may include a soft metal alloy, such as, but not limited to, an indium-based alloy. In another embodiment, the seals  114  may include an indium-coated C-ring. 
     In another embodiment, one or more of the first terminal flange  110 , the second terminal flange  112  or the floating flange  113  includes one or more coolant channels  116 . For example, the coolant channels  116  may be configured to circulate a gas or liquid in order to cool the given flange. For instance, the coolant channels  116  may circulate water, air or any other suitable heat exchange fluid. In one embodiment, the coolant channels  116  of a given flange may be fluidically coupled to an external coolant source, along with other coolant system components. 
     It is noted herein that thermal management of the transmission element  108  and the flanges is required for high-power cell operation. For example, low temperature of the seal areas may be required if indium is used as the seal material, which has a melting temperature of 156.6° C. It is noted that operating operation conditions of glass bulbs without the thermal management of the present disclosure may reach many hundreds of degrees Celsius. Thermal management of the top and bottom flanges  110 ,  112  can be achieved through thermal coupling of the flanges with cooled end caps  132 ,  134  (e.g., water cooled end caps). It is further noted that the floating flange  113  may require separate cooling (e.g., water cooling), since thermal conductivity through the compressive sealing element  122  (e.g., C-ring) may not be adequate for the given application. It is further noted that thermal management of the transmission element  108  can be achieved via a conductive cooling pathway across the compressive sealing element  122  to the cooled (e.g., water cooled) components. 
     It is noted herein that the terminal flanges  110 ,  112  and/or the floating flanges  113  may be formed from any suitable material known in the art. For example, the terminal flanges  110 ,  112  and/or the floating flanges  113  may be formed from at least one of a metal or ceramic material. 
     In another embodiment, as shown in  FIG. 1B , the plasma cell  102  includes one or more connecting rods  118 . In one embodiment, the one or more connecting rods  118  of the plasma cell  102  may serve to secure the one or more terminal flanges  110 ,  112  at or near the openings  109   a ,  109   b . In one embodiment, the one or more connecting rods  118  may secure the one or more terminal flanges  110 ,  112  with mounting screws  127 ,  129 . In another embodiment, the floating flange  113  includes one or more pass-through holes  115 , allowing the one or more connecting rods  118  to mechanically couple the terminal flanges  110  and  112  to each other, as shown in  FIG. 1B . In another embodiment, the one or more pass-through holes  115  of the floating flange  113  and the one or more connecting rods  118  are sized to allow movement (e.g., movement along vertical direction in  FIG. 1B ) of the floating flange  113  upon thermal expansion (or contraction) of the transmission element  108 . For example, in the case of a cylindrical transmission element  108 , the connecting rods  118  may be coupled to a first flange  109   a  and a second flange  109   b  positioned on the opposite end of the transmission element  108  from the first flange  109   a . In this regard, the connecting rods  118  serve to provide a mechanical force tending to secure the top flange  110  to the top end of the transmission element  108  and the floating flange  113  (and the connected bottom flange  112 ) to the bottom end of the transmission element  108 . 
     In another embodiment, as shown in  FIG. 1B , the one or more connecting rods  118  of  FIG. 1B  are configured to provide a preload on the seals  114  and/or the compressive sealing element  122 . In this regard, the one or more connecting rods  118  serve to provide a compressive stress to the transmission element  108 , allowing sealing of the transmission element  108 . It is noted that this compressive stress on the seals  114  and the transmission element  108  allows for maintaining the seals at high operating pressure inside the volume  103  of the plasma cell  102 . 
     The small amount of elasticity of the compressive sealing element  122  allows for compensation of thermal expansion of the transmission element  108  and connecting rods  118 , which hold the terminating flanges  110 ,  112  together. Further, the compressive sealing element  122  may provide for compensation of an elongation of the connecting rods caused by the internal gas pressure of the gas within the internal volume  103  of the plasma cell  102 . It is noted that the combination of the compressive sealing element  122  and the connecting rods  118  (or fins  124 ) allows for the large area seal provided by the compressive sealing element  122  to remain compressively stressed, while keeping the magnitude of the stress relatively constant as a function of internal gas pressure of the plasma cell  102  and temperature of the transmission element  108  and connecting rods  118  (or fins  124 ). 
     It is further noted that the use of a large area of contact for the seals  114  allows for even distribution of the preload stress across the end transmission element  108  and allows for the use of brittle materials, such as, but not limited to, CaF 2 . In addition, the use of a large contact area of the seals  114  to both the flanges  110 ,  112 ,  113  and transmission element  108  allows good thermal contact between the flanges  110 ,  112 ,  113  and the transmission element  108 . Such a configuration allows for improved thermal management of the transmission element via conductive cooling through the abutting seals  114 . 
     It is further noted that, in the case where the diameter of the compressive sealing element  122  is larger than the diameter of the seals  114  for the transmission element  108 , extra compressive pressure may be applied on the transmission element  108  once internal cell pressure is increased. Such additional pressure may serve to compensate for the loss of compressive pressure on the transmission element  108  due to flexure of connecting rods  118  (or fins  124 ). Further, the compensating pressure may aid in maintaining the preload on the seals  114  of the transmission element  108  for a larger range of operating pressures. 
     In another embodiment, as shown in  FIGS. 1C and 1D , the plasma cell  102  includes one or more fins  124 . In one embodiment, the one or more fins  124  (e.g., three fins or four fins) of the plasma cell  102  may serve to secure the one or more terminal flanges  110 ,  112  at or near the openings  109   a ,  109   b  in a manner similar to the connecting rods  118  described previously herein. In one embodiment, the one or more fins  124  may secure the one or more terminal flanges  110 ,  112  with mounting screws  127 ,  129 . In another embodiment, a rod portion of the one or more fins  124  may pass through pass-through holes  115  and serve to mechanically couple the terminal flanges  110  and  112 , as shown in  FIG. 1C . In this regard, the fins  124 , like the connecting rods  118 , serve to provide a mechanical force tending to secure the top flange  110  to the top end of the transmission element  108  and the floating flange  113  (and the connected bottom flange  112 ) to the bottom end of the transmission element  108 . It is further recognized that the fins may be made suitably thin (and/or wedged) in order to limit obscuration between the illumination source  111  and the transmission element  108  and/or the transmission element  108  and the collection element  105 . In another embodiment, the fins  124  are configured to cool the plasma cell  102  by transferring thermal energy from one or more portions of the plasma cell  102  to an ambient atmosphere (e.g., surrounding air). 
     In another embodiment, as shown in  FIG. 1C , the one or more fins  124  of  FIG. 1C  are configured to provide a preload on the seals  114  and/or the compressive sealing element  122 . In this regard, the one or more fins  124  serve to provide a contact stress to the transmission element  108 , allowing sealing of the transmission element  108 . It is again noted that this compressive stress on the seals  114  and the transmission element  108  provided by the fins  124  allows for maintaining the seals at high operating pressure inside the volume  103  of the plasma cell  102 . 
     In one embodiment, the transmission element  108  may contain any selected gas (e.g., argon, xenon, mercury or the like) known in the art suitable for generating plasma upon absorption of suitable illumination. In one embodiment, focusing illumination  113  from the illumination source  111  into the volume of gas  103  causes energy to be absorbed through one or more selected absorption lines of the gas or plasma within the transmission element  108 , thereby “pumping” the gas species in order to generate or sustain a plasma. In another embodiment, although not shown, the plasma cell  102  may include a set of electrodes for initiating the plasma  104  within the internal volume  103  of the transmission element  108 , whereby the illumination source  113  from the illumination source  111  maintains the plasma  104  after ignition by the electrodes. 
     In another embodiment, the plasma  104  generated, or maintained, within the volume  103  of the transmission element  108  emits broadband radiation. In one embodiment, the broadband illumination  115  emitted by the plasma  104  includes at least vacuum ultraviolet (VUV) radiation. In another embodiment, the broadband illumination  115  emitted by the plasma  104  includes deep ultraviolet (DUV) radiation. In another embodiment, the broadband illumination  115  emitted by the plasma  104  includes ultraviolet (UV) radiation. In another embodiment, the broadband illumination  115  emitted by the plasma  104  includes visible radiation. For example, the plasma  104  may emit short-wavelength radiation in the range of 120 to 200 nm. In this regard, the transmission element  108  allows the plasma cell  102  of system  100  to serve as a VUV radiation source. In another embodiment, the plasma  104  may emit short-wavelength radiation having a wavelength below 120 nm. In another embodiment, the plasma  104  may emit radiation having a wavelength larger than 200 nm. 
     The transmission element  108  of system  100  may be formed from any material known in the art that is at least partially transparent to radiation generated by plasma  104 . In one embodiment, the transmission element  108  of system  100  may be formed from any material known in the art that is at least partially transparent to VUV radiation generated by plasma  104 . In another embodiment, the transmission element  108  of system  100  may be formed from any material known in the art that is at least partially transparent to DUV radiation generated by plasma  104 . In another embodiment, the transmission element  108  of system  100  may be formed from any material known in the art that is transparent to UV light generated by plasma  104 . In another embodiment, the transmission element  108  of system  100  may be formed from any material known in the art transparent to visible light generated by plasma  104 . 
     In another embodiment, the transmission element  108  may be formed from any material known in the art transparent to radiation  113  (e.g., IR radiation) from the illumination source  111 . 
     In another embodiment, the transmission element  108  may be formed from any material known in the art transparent to both radiation from the illumination source  111  (e.g., IR source) and radiation (e.g., VUV radiation, DUV radiation, UV radiation and visible radiation) emitted by the plasma  104  contained within the volume  103  of the transmission element  108 . 
     For example, the transmission element  108  may include, but is not limited to, calcium fluoride (CaF 2 ), magnesium fluoride (MgF 2 ), crystalline quartz and sapphire, which are capable of transmitting radiation (from the plasma  104 ) and laser radiation (e.g., infrared radiation) from the illumination source  111 . It is noted herein that materials such as, but not limited to, CaF 2 , MgF 2 , crystalline quartz and sapphire provide transparency to radiation with wavelengths shorter than 190 nm. For instance, CaF 2  is transparent to radiation having a wavelength as short as approximately 120 nm. Further, these materials are resistant to rapid degradation when exposed to short-wavelength radiation, such as VUV radiation. By way of another example, in some instances, fused silica may be utilized to form the transmission element  108 . It is noted herein that fused silica does provide some transparency to radiation having wavelength shorter than 190 nm, showing useful transparency to wavelengths as short as 170 nm. 
     The transmission element  108  may take on any shape known in the art. In one embodiment, the transmission element  108  may have a cylindrical shape, as shown in  FIGS. 1A-1H . In another embodiment, although not shown, the transmission element  108  may have a spherical shape. In another embodiment, although not shown, the transmission element  108  may have a composite shape. For example, the shape of the transmission element  108  may consist of a combination of two or more shapes. For instance, the shape of the transmission element  108  may consist of a spherical center portion, arranged to contain the plasma  104 , and one or more cylindrical portions extending above and/or below the spherical center portion, whereby the one or more cylindrical portions are coupled to a terminal flange  110 ,  112  and the floating flange  113 . 
     In the case where the transmission element  108  is cylindrically shaped, the one or more openings  109   a ,  109   b  may be located at one or more end portions of the cylindrically shaped transmission element  108 . In this regard, the transmission element  108  takes the form of a hollow cylinder, whereby a channel extends from the first opening  109   a  to the second opening  109   b . In another embodiment, the flange  110  (or  112 ) and the floating flange  113  together with the wall(s) of the transmission element  108  serve to contain the volume of gas  103  within the channel of the transmission element  108 . It is recognized herein that this arrangement may be extended to a variety of transmission element  108  shapes, as described previously herein. 
       FIGS. 1E and 1F  illustrate the plasma cell equipped with one or more active connection rods, in accordance with one or more embodiments of the present invention. It is noted herein that since the plasma cell  102  of the present disclosure does not required the matching of thermal expansion of all structures the connecting rods/fins of the plasma cell  102  can be used to carry out auxiliary functions (e.g., cooling functions). 
     In one embodiment, as shown in  FIG. 1E , the plasma cell is equipped with one or more coolant transport connection rods  126 ,  128 . For example, the coolant transport connection rods  126 ,  128  may mechanically couple the first terminal flange  110  and the second terminal flange  112 . In another embodiment, the coolant transport connection rods  126 ,  128  are configured to transfer heat from a first flange to a second flange. For example, the coolant transport connection rods  126 ,  128  may, but are not required to, contain and circulate a coolant such that heat is carried from the bottom terminal flange  112  to the top terminal flange  110 . By way of another example, the coolant transport connection rods  126 ,  128  may, but are not required to, contain and circulate a coolant such that heat is carried from the top terminal flange  110  to the bottom terminal flange  112 . 
     In another embodiment, as shown in  FIG. 1F , the plasma cell  102  is equipped with one or more heat conduction rods  130 . For example, the heat conduction rods  130  may mechanically couple the first terminal flange  110  and the second terminal flange  112 . In another embodiment, the heat conduction rods  130  are configured to transfer heat from a first flange to a second flange. For example, the heat conduction rods  130  may, but are not required to, conduct heat from the bottom terminal flange  112  to the top terminal flange  110 . By way of another example, the heat conduction rods  130  may, but are not required to, conduct heat from the top terminal flange  110  to the bottom terminal flange  112 . 
       FIGS. 1G and 1H  illustrate the plasma cell  102  equipped with one or more radiation shield elements  132 ,  134 , in accordance with one or more embodiments of the present disclosure. In one embodiment, the one or more radiation shielding elements  132  and/or  134  may include a radiation shield proximate to the one or more openings of the transmission element configured to block radiation from at least one of the illumination source  111  and the radiation generated by the plasma  104  from reaching one or more seals  114  of the plasma cell  102 . 
     In one embodiment, the radiation shielding elements  132  and/or  134  may include a structure suitable for shielding one or more portions of the plasma cell  102  from radiation from the plasma  104  or from the illumination from the light source  111  (e.g., radiation from laser). For example, as shown in  FIG. 1G , the one or more radiation shielding elements  132  may be disposed on or near the external surface of the transmission element  108 . By way of another example, as shown in  FIG. 1H , the one or more radiation shielding elements  134  may be disposed on or near the internal surface of the transmission element  108 . 
     In another embodiment, the one or more radiation shielding elements  132 ,  134  include a coating material applied to one or more inside or outside portions of the transmission element  108  in order to block radiation from the plasma  104  from one or more selected portions of the plasma cell  102 . In another embodiment, the plasma cell  102  may include a coating layer proximate to the one or more openings of the transmission element configured to block at least a portion of the radiation generated by the plasma from reaching one or more seals of the plasma cell. For example, a coating material (e.g., metal material) may be applied to one or more inside or outside end portions of a cylindrical transmission element  108  in order to block radiation (e.g., UV radiation) from the plasma  104  from damaging (or at least limit damage) the seals  114 . In another embodiment, an anti-reflective coating material may be applied to one or more inside or outside portions of the transmission element  108  in order to block radiation from the plasma  104  from one or more selected portions of the plasma cell  102 . The utilization of radiation shields and radiation blocking coating layers is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of radiation shields and radiation blocking coating layers is generally described in U.S. patent application Ser. No. 14/231,196, filed on Mar. 31, 2014, which is incorporated previously herein by reference in the entirety. 
     In another embodiment, the plasma cell  102  may include one or more control elements coupled to one or more of the flanges  110 ,  112 ,  113 . In one embodiment, plasma cell  102  may include one or more control elements for controlling one or more characteristics of the plasma cell  102 , the transmission element  108 , the gas within volume  103 , the plasma  104  and/or a plume from the plasma. 
     In one embodiment, the one or more control elements coupled to the one or more flanges  110 ,  112 ,  113  may include an internal control element. For example, the one or more control elements of the one or more flanges  110 ,  112 ,  113  may include an internal control element located within the internal volume of the transmission element  108 . In one embodiment, the one or more control elements of the one or more flanges  110 ,  112 ,  113  may include an external control element. For example, the one or more control elements of the one or more flanges  110 ,  112 ,  113  may include an external control element mounted to a surface of the one or more flanges  110 ,  112 ,  113  that is external to the internal volume of the transmission element  108 . 
     In one embodiment, the one or more flanges  110 ,  112 ,  113  may include a temperature control element. For example, the temperature control element may be disposed inside or outside of the transmission element  108  of the plasma cell  102 . The temperature control element may include any temperature control element known in the art used to control the temperature of the plasma cell  102 , the plasma  104 , the gas, the transmission element  108 , the one or more flanges  110 , 112 ,  113  and/or the plasma plume (not shown). 
     In one embodiment, the temperature control element may be utilized to cool the plasma cell  102 , transmission element  108 , the plasma  104 , the flanges  110 ,  112 ,  113  and/or the plume of the plasma by transferring thermal energy to a medium external to the transmission element  108 . In one embodiment, the temperature control element may include, but is not limited to, a cooling element for cooling plasma cell  102 , transmission element  108 , the plasma  104 , the gas, the flanges  110 , 112 ,  113  and/or the plume of the plasma. For example, as shown in  FIGS. 1B-1J , the one or more flanges  110 ,  112 ,  113  may include one or more cooling elements  116  (e.g., water cooling elements), as noted previously herein. 
     In another embodiment, the one or more flanges  110 ,  112 ,  113  may include one or more passive heat transfer elements coupled to one or more portions of the one or more flanges  110 ,  112 ,  113 . For example, the one or more passive heat transfer elements may include, but are not limited to, baffles, chevrons or fins arranged to transfer thermal energy from the hot plasma  104  to a portion of the plasma cell  102  (e.g., top electrode), the one or more flanges  110 ,  112 ,  113  or the transmission element  108  to facilitate heat transfer out of the transmission element  108 . 
     The utilization of heat transfer elements is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of heat transfer elements is also generally described in U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which is incorporated by reference herein in the entirety. The utilization of heat transfer elements is also generally described in U.S. patent application Ser. No. 14/224,945, filed on Mar. 25, 2014, which is incorporated by reference above in the entirety. The utilization of heat transfer elements is also generally described in U.S. patent application Ser. No. 14/231,196, filed on Mar. 31, 2014, which is incorporated by reference above in the entirety. 
     In another embodiment, the one or more flanges  110 ,  112 ,  113  include one or more convection control elements. For example, a convection control element may be disposed inside or outside of the transmission element  108  of the plasma cell  102 . The convection control element may include any convection control device known in the art used to control convection in the transmission element  102 . For example, the convection control element may include one or more devices (e.g., structures mechanically coupled to one or more flanges  110 , 112 ,  113  and positioned inside transmission element  108 ) suitable for controlling convection currents within the transmission element  108  of plasma cell  102 . For instance, the one or more structures for controlling convection currents may be arranged within the transmission element  108  in a manner to impact the flow of hot gas from the hot plasma region  104  of the plasma cell  102  to the cooler inner surfaces of the transmission element  108 . In this regard, the one or more structures may be configured in a manner to direct convective flow to regions within the transmission element  108  that minimize or at least reduce damage to the wall of the transmission element  108  caused by the high temperature gas. 
     In another embodiment, the cooling elements described previously herein (e.g., water cooling elements  116 ) may provide convection control, allowing the system  100  to capture, direct and/or dissipate the plasma plume. 
     The utilization of convection control devices is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of convection control devices are also generally described in U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which is incorporated by reference above in the entirety. The utilization of convection control devices is also generally described in U.S. patent application Ser. No. 14/224,945, filed on Mar. 25, 2014, which is incorporated by reference above in the entirety. The utilization of convection control devices is also generally described in U.S. patent application Ser. No. 14/231,196, filed on Mar. 31, 2014, which is incorporated by reference above in the entirety. 
     In another embodiment, as shown in  FIG. 1I , the one or more flanges  110 ,  112 ,  113  may include one or more plume control devices  135 . For example, the plume control device  135  may include a plume capture or redirection device coupled to the one or more flanges  110 ,  112 ,  113  and positioned disposed inside of the transmission element  108  of plasma cell  102 , as shown in  FIG. 1I . The plume control element may include any plume control device known in the art used to capture or redirect the plume of plasma  104  within the transmission element  108 . For example, the plume control element may include one or more devices having a concave portion suitable for capturing and redirecting a convection plume emanating from the plasma region  104  within the transmission element  108  of the plasma cell  102 . For instance, the plume control element may include one or more electrodes (e.g., top electrode) coupled to the internal surface of one or more flanges  110 ,  112 ,  113  and positioned within the transmission element  108  of plasma cell  102  having a concave portion or a hollow portion suitable for capturing and/or redirecting a convection plume emanating from the plasma region  104  within the transmission element of the plasma cell  102 . The utilization of plume control devices is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of plume control devices is also generally described in U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which is incorporated by reference above in the entirety. The utilization of plume control devices is also generally described in U.S. patent application Ser. No. 14/224,945, filed on Mar. 25, 2014, which is incorporated by reference above in the entirety. The utilization of plume control devices is also generally described in U.S. patent application Ser. No. 14/231,196, filed on Mar. 31, 2014, which is incorporated by reference above in the entirety. 
     In another embodiment, one or more flanges  110 ,  112 ,  113  may include one or more plasma ignition elements. For example, one or more electrodes may be mounted on the internal surface of one or more flanges  110 ,  112 ,  113  and positioned within the internal volume of the transmission element  108 . The utilization of various electrode configurations is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of various electrode configurations is generally described in U.S. patent application Ser. No. 14/231,196, filed on Mar. 31, 2014, which is incorporated by reference above in the entirety. 
     In another embodiment, one or more flanges  110 ,  112 ,  113  may include one or more sensors (not shown) configured to measure one or more characteristics (e.g., thermal characteristics, pressure characteristics, radiation characteristics and the like) of the plasma cell  102 , the transmission element  108 , the plasma  104 , the gas, the plume of the plasma and the like. In one embodiment, the one or more sensors may include a sensor disposed on the outside or inside surface of one or more flanges  110 ,  112 ,  113 . For example, the one or more sensors may include, but are not limited to, a temperature sensor, a pressure sensor, a radiation sensor and the like. 
       FIG. 1J  illustrates a simplified schematic diagram of the plasma cell  102  coupled to the collector  105 , in accordance with one or more embodiments of the present invention. In one embodiment, plasma cell  102  is mechanically coupled to the collector via mounting screws  142  or any other suitable mounting device. 
     In another embodiment, the plasma cell  102  includes one or more gas control elements  132 . In one embodiment, a gas control element  132  may be coupled to one or more of the caps  138 , 140  of the plasma cell. For example, the gas control element  132  may include a feedthrough  132 . For instance, the gas control element  132  includes a gas pipe or tube serving to fluidically couple a gas source and the transmission element  108 . In another embodiment, the system  100  may include a gas valve positioned along the gas line (between the gas source and the transmission element  108 ), allowing a user to control the amount and type of gas contained within the transmission element  108 . In another embodiment, the gas control element  132  may be coupled to one or more of the flanges  110 ,  112 ,  113 . The utilization of gas fill devices is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of gas fill devices is generally described in U.S. patent application Ser. No. 14/231,196, filed on Mar. 31, 2014, which is incorporated by reference above in the entirety. 
     It is noted herein that the feedthrough  132  depicted in  FIG. 1J  is not limited to a gas feedthrough. It is recognized herein that the plasma cell  102  of the present invention may include any number of feedthroughs. For example, the plasma cell  102  may include, but is not limited to, a gas feedthrough, a cooling feedthrough or an electrical feedthrough. In this regard, any one of the terminal flanges  110 ,  112 , the floating flange  113  or the caps  134 ,  136  may include feedthroughs allowing gas, coolant or electrical wiring to pass from the outside of the plasma cell  102  to some interior portion of the plasma cell  102 . 
     Referring again to  FIG. 1A , the collector element  105  may take on any physical configuration known in the art suitable for focusing illumination emanating from the illumination source  111  into the volume of gas  103  contained within the transmission element  108  of the plasma cell  102 . In one embodiment, as shown in  FIG. 1A , the collector element  105  may include a concave region with a reflective internal surface suitable for receiving illumination  113  from the illumination source  111  and focusing the illumination  113  into the volume of gas  103  contained within the transmission element  108 . For example, the collector element  105  may include an ellipsoid-shaped collector element  105  having a reflective internal surface, as shown in  FIG. 1A . 
     In another embodiment, the collector element  105  is arranged to collect broadband illumination (e.g., VUV radiation, DUV radiation, UV radiation and/or visible radiation) emitted by plasma  104  and direct the broadband illumination to one or more additional optical elements (e.g., filter  123 , homogenizer  125  and the like). For example, the collector element  102  may collect at least VUV broadband illumination emitted by plasma  104  and direct the broadband illumination to one or more downstream optical elements. By way of another example, the collector element  105  may collect DUV broadband illumination emitted by plasma  104  and direct the broadband illumination to one or more downstream optical elements. By way of another example, the collector element  105  may collect UV broadband illumination emitted by plasma  104  and direct the broadband illumination to one or more downstream optical elements. By way of another example, the collector element  105  may collect visible broadband illumination emitted by plasma  104  and direct the broadband illumination to one or more downstream optical elements. In this regard, the plasma cell  102  may deliver VUV radiation, UV radiation and/or visible radiation to downstream optical elements of any optical characterization system known in the art, such as, but not limited to, an inspection tool or a metrology tool. It is noted herein the plasma cell  102  of system  100  may emit useful radiation in a variety of spectral ranges including, but not limited to, DUV radiation, VUV radiation, UV radiation, and visible radiation. Further, it is noted herein that the system  100  may utilize any of these radiation bands, while mitigating damage caused to the transmission region  108  by the VUV radiation. In this regard, the transmission element  108  may be formed from a material that is resistant to VUV light, even in cases where the primary purpose of the system  100  does not include the utilization of the VUV light. 
     In one embodiment, system  100  may include various additional optical elements. In one embodiment, the set of additional optics may include collection optics configured to collect broadband light emanating from the plasma  104 . For instance, the system  100  may include a cold mirror  121  arranged to direct illumination from the collector element  105  to downstream optics, such as, but not limited to, a homogenizer  125 . 
     In another embodiment, the set of optics may include one or more additional lenses (e.g., lens  117 ) placed along either the illumination pathway or the collection pathway of system  100 . The one or more lenses may be utilized to focus illumination from the illumination source  111  into the volume of gas  103 . Alternatively, the one or more additional lenses may be utilized to focus broadband light emanating from the plasma  104  onto a selected target (not shown). 
     In another embodiment, the set of optics may include a turning mirror  119 . In one embodiment, the turning mirror  119  may be arranged to receive illumination  113  from the illumination source  111  and direct the illumination to the volume of gas  103  contained within the transmission element  108  of the plasma cell  102  via collection element  105 . In another embodiment, the collection element  105  is arranged to receive illumination from mirror  119  and focus the illumination to the focal point of the collection element  105  (e.g., ellipsoid-shaped collection element), where the transmission element  108  of the plasma cell  102  is located. 
     In another embodiment, the set of optics may include one or more filters  123  placed along either the illumination pathway or the collection pathway in order to filter illumination prior to light entering the transmission element  108  or to filter illumination following emission of the light from the plasma  104 . It is noted herein that the set of optics of system  100  as described above and illustrated in  FIG. 1A  are provided merely for illustration and should not be interpreted as limiting. It is anticipated that a number of equivalent optical configurations may be utilized within the scope of the present invention. 
     It is contemplated herein that the system  100  may be utilized to sustain a plasma in a variety of gas environments. In one embodiment, the gas used to initiate and/or maintain plasma  104  may include an inert gas (e.g., noble gas or non-noble gas) or a non-inert gas (e.g., mercury). In another embodiment, the gas used to initiate and/or maintain a plasma  104  may include a mixture of gases (e.g., mixture of inert gases, mixture of inert gas with non-inert gas or a mixture of non-inert gases). For example, it is anticipated herein that the volume of gas used to generate a plasma  104  may include argon. For instance, the gas  103  may include a substantially pure argon gas held at pressure in excess of 5 atm (e.g., 20-50 atm). In another instance, the gas may include a substantially pure krypton gas held at pressure in excess of 5 atm (e.g., 20-50 atm). In another instance, the gas  103  may include a mixture of argon gas with an additional gas. 
     It is further noted that the present invention may be extended to a number of gases. For example, gases suitable for implementation in the present invention may include, but are not limited, to Xe, Ar, Ne, Kr, He, N 2 , H 2 O, O 2 , H 2 , D 2 , F 2 , CH 4 , one or more metal halides, a halogen, Hg, Cd, Zn, Sn, Ga, Fe, Li, Na, Ar:Xe, ArHg, KrHg, XeHg, and the like. In a general sense, the present invention should be interpreted to extend to any light pump plasma generating system and should further be interpreted to extend to any type of gas suitable for sustaining a plasma within a plasma cell. 
     In another embodiment, the illumination source  111  of system  100  may include one or more lasers. In a general sense, the illumination source  111  may include any laser system known in the art. For instance, the illumination source  111  may include any laser system known in the art capable of emitting radiation in the infrared, visible or ultraviolet portions of the electromagnetic spectrum. In one embodiment, the illumination source  111  may include a laser system configured to emit continuous wave (CW) laser radiation. For example, the illumination source  111  may include one or more CW infrared laser sources. For example, in settings where the gas of the volume  103  is or includes argon, the illumination source  111  may include a CW laser (e.g., fiber laser or disc Yb laser) configured to emit radiation at 1069 nm. It is noted that this wavelength fits to a 1068 nm absorption line in argon and as such is particularly useful for pumping argon gas. It is noted herein that the above description of a CW laser is not limiting and any laser known in the art may be implemented in the context of the present invention. 
     In another embodiment, the illumination source  111  may include one or more diode lasers. For example, the illumination source  111  may include one or more diode laser emitting radiation at a wavelength corresponding with any one or more absorption lines of the species of the gas contained within volume  103 . In a general sense, a diode laser of the illumination source  111  may be selected for implementation such that the wavelength of the diode laser is tuned to any absorption line of any plasma (e.g., ionic transition line) or any absorption line of the plasma-producing gas (e.g., highly excited neutral transition line) known in the art. As such, the choice of a given diode laser (or set of diode lasers) will depend on the type of gas contained within the plasma cell  10   d   2  of system  100 . 
     In another embodiment, the illumination source  111  may include an ion laser. For example, the illumination source  111  may include any noble gas ion laser known in the art. For instance, in the case of an argon-based plasma, the illumination source  111  used to pump argon ions may include an Ar+ laser. 
     In another embodiment, the illumination source  111  may include one or more frequency converted laser systems. For example, the illumination source  111  may include a Nd:YAG or Nd:YLF laser having a power level exceeding 100 Watts. In another embodiment, the illumination source  111  may include a broadband laser. In another embodiment, the illumination source may include a laser system configured to emit modulated laser radiation or pulsed laser radiation. 
     In another embodiment, the illumination source  111  may include one or more non-laser sources. In a general sense, the illumination source  111  may include any non-laser light source known in the art. For instance, the illumination source  111  may include any non-laser system known in the art capable of emitting radiation discretely or continuously in the infrared, visible or ultraviolet portions of the electromagnetic spectrum. 
     In another embodiment, the illumination source  111  may include two or more light sources. In one embodiment, the illumination source  111  may include or more lasers. For example, the illumination source  111  (or illumination sources) may include multiple diode lasers. By way of another example, the illumination source  111  may include multiple CW lasers. In a further embodiment, each of the two or more lasers may emit laser radiation tuned to a different absorption line of the gas or plasma within the plasma cell  102  of system  100 . 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “connected”, or “coupled”, to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “couplable”, to each other to achieve the desired functionality. Specific examples of couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components. 
     It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes. Furthermore, it is to be understood that the invention is defined by the appended claims.