Abstract:
The present invention provides deposition sources that can efficiently and controllably provide vaporized material for deposition of thin film materials. Deposition sources described herein can be used to deposit any desired material and are particularly useful for depositing high melting point materials at high evaporation rates. An exemplary application for deposition sources of the present invention is deposition of copper, indium, and gallium in the manufacture of copper indium gallium diselenide based photovoltaic devices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/188,671 filed Aug. 11, 2008 entitled HIGH TEMPERATURE DEPOSITION SOURCES AND METHODS, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to vacuum deposition sources. In particular, the present invention is directed to vacuum deposition sources having heated effusion orifices that help to prevent deposition material from accumulating near such effusion orifices and restricting flow of deposition material. A preferred exemplary deposition source in accordance with the present invention comprises a rear-loading crucible and a conical heat shield assembly comprising a conical cover positioned relative to effusion orifice end of the crucible. Vacuum deposition sources in accordance with the present invention can be used in vacuum environments in the millitorr range as well as vacuum environments suitable for ultra-high vacuum applications such molecular beam epitaxy. 
     BACKGROUND 
     Compounds of copper indium diselenide (CIS) with gallium substituted for all or part of the indium (copper indium gallium diselenide or CIGS) are used in photovoltaic devices. For example, CIGS provides absorber layers in thin-film solar cells. CIGS semiconductor materials have a direct band gap that permits strong absorption of solar radiation in the visible range. CIGS cells have demonstrated high efficiencies and good stability as compared to other absorber layer compounds such as cadmium telluride (CdTe) or amorphous silicon (a-Si). 
     Solar cell devices typically include a substrate, a barrier layer, a back contact layer, a semiconductor layer, alkali materials, an n-type junction buffer layer, an intrinsic transparent oxide layer, and a conducting transparent oxide layer. In a device that utilizes CIGS, the semiconductor layer includes copper, indium, gallium, and selenium. The CIGS layers used for photovoltaic conversion need to have a p-type semiconductor character and good charge transport properties. These charge transport properties are favored by good crystallinity. The CIGS thus need to be at least partially crystallized in order to have sufficient photovoltaic properties for use in the fabrication of solar cells. Crystallized CIGS compounds have a crystallographic structure corresponding to the chalcopyrite or sphalerite systems, generally depending on the deposition temperature. 
     CIGS thin films can be deposited by various techniques, typically vacuum based. One technique involves the use of precursors. In this technique, intermediate compounds are used and have physicochemical properties that are distinct from those of CIGS and make them incapable of photovoltaic conversion. The precursors are initially deposited in a thin film form, and the thin film is subsequently processed to form the intended CIGS layer. When precursor materials are deposited at a low temperature, the resulting CIGS thin films are weakly crystallized or amorphous. These thin films need to be annealed by supplying heat to improve the crystallization of the CIGS and provide satisfactory charge transport properties. At the temperatures that allow at least partial crystallization of the CIGS, however, one of the constituent elements of the CIGS (selenium) is more volatile than the other elements. It is therefore difficult to obtain crystallized CIGS with the intended composition and stoichiometry without adding selenium during annealing of the precursor layer. Time consuming annealing of the precursor deposits in the presence of selenium excess in the vapor phase is thus needed to form suitable material. 
     Another technique for depositing CIGS thin films involves vacuum evaporation. Devices formed by this technique often have high photovoltaic conversion efficiencies compared to techniques that use precursor materials. Typically, co-evaporation of the copper, indium, gallium, and selenium is performed in the presence of a substrate. This co-evaporation technique has an advantage in that the content of gallium in the thin film light-absorbing layer can be regulated to achieve the optimum bandgap. Evaporation is a technique that can be difficult to use on the industrial scale, however, particularly because of non-uniformity problems with the thin film deposits over large surface areas and a low efficiency of using the primary materials. 
     There are additional challenges that arise when using vacuum deposition techniques for depositing CIGS thin films. For example, selenium reacts aggressively with many materials that are typically used in the manufacture of vacuum deposition sources especially at elevated temperatures. Accordingly, the materials and the mechanical design of deposition sources used in a selenium environment are carefully considered. 
     Additionally, undesirable accumulation of deposition material in the vicinity of the effusion orifice of a vacuum deposition sources can occur under certain deposition conditions. Typically, such deposition conditions include one or more of high deposition temperatures and high deposition rates such as those used for deposition of high temperature metals or semiconductors materials, such as copper, for example. Continued accumulation of deposition material can reduce the area of the effusion orifice and thereby reduce the deposition rate. Ultimately, continued accumulation of deposition material can effectively close the effusion orifice so the deposition rate is unacceptably low or non-existent. 
     SUMMARY 
     The present invention provides deposition sources that can efficiently and controllably provide vaporized material for deposition of thin film materials without the above-described problem related to accumulation of deposition material at the effusion orifice. Moreover, deposition sources in accordance with the present invention are particularly useful for use in a corrosive high temperature environment such as in the presence of selenium vapor. Deposition sources described herein can be used to deposit any desired materials, however, and are particularly useful for depositing materials at high evaporation rates (in excess of 30 grams per hour, for example) and at high temperatures (up to 1500° C., for example). An exemplary application for deposition sources of the present invention is deposition of copper, indium, and gallium in the manufacture of copper indium gallium diselenide based photovoltaic devices. 
     Deposition sources in accordance with the present invention preferably use a heater made from layers of pyrolytic boron nitride and pyrolytic graphite wherein the pyrolytic graphite functions as the resistive element and is sandwiched between layers of pyrolytic boron nitride. Because the resistive element is effectively encapsulated in pyrolytic boron nitride the resistive element is protected from the surrounding environment. Moreover, the heater is preferably designed so the heater is closely coupled with the crucible, which can help to keep the region near the effusion orifice of the crucible hot enough to prevent undesirable condensation of deposition material near the effusion orifice. Deposition sources in accordance with the present invention also preferably include a conical cover that prevents any material that falls back to the source from creating a seed that could cause deposition material to accumulate near the effusion orifice of the deposition source. 
     In an exemplary aspect of the present invention a vacuum deposition source is provided. The vacuum deposition source preferably comprises: a base flange configured to mount the vacuum deposition source to a vacuum chamber; a crucible operatively supported relative to the base flange and configured to hold vacuum deposition material, the crucible comprising a cylindrical body portion, a conical portion, and an effusion orifice; and a heater operatively supported relative to the base flange and at least partially surrounding the crucible, the heater comprising a cylindrical body portion configured and positioned to provide thermal radiation to at least a portion of the cylindrical body portion of the crucible and a conical portion configured and positioned to provide thermal radiation to at least a portion of the conical portion of the crucible, the heater comprising a layered structure comprising a pyrolytic graphite electrically resistive layer positioned between pyrolytic boron nitride electrically insulative layers. 
     In another exemplary aspect of the present invention a vacuum deposition source is provided. The vacuum deposition source preferably comprises: a base flange configured to mount the vacuum deposition source to a vacuum chamber; a crucible operatively supported relative to the base flange and configured to hold vacuum deposition material, the crucible comprising a cylindrical body portion, a conical portion, and an effusion orifice; a heater operatively supported relative to the base flange and at least partially surrounding the crucible, the heater comprising a cylindrical body portion configured and positioned to provide thermal radiation to at least a portion of the cylindrical body portion of the crucible and a conical portion configured and positioned to provide thermal radiation to at least a portion of the conical portion of the crucible, the heater comprising a layered structure comprising a pyrolytic graphite electrically resistive layer positioned between pyrolytic boron nitride electrically insulative layers; a liquid cooling enclosure operatively attached to the base flange at a first end of the liquid cooling enclosure and at least partially surrounding the crucible; and a conical cover positioned at a second end of the liquid cooling enclosure opposite the first end of the cooling enclosure, the conical cover comprising an opening positioned relative to the effusion orifice of the crucible. 
     In yet another exemplary aspect of the present invention a vacuum deposition source is provided. The vacuum deposition source preferably comprises: a base flange configured to mount the vacuum deposition source to a vacuum chamber; a crucible support assembly comprising a support flange removably mounted to the base flange and a crucible support cup supported relative to the support flange; a crucible operatively supported by the support cup of the crucible support assembly and configured to hold vacuum deposition material, the crucible comprising a cylindrical body portion, a conical portion, and an effusion orifice; and a heater operatively supported relative to the base flange and at least partially surrounding the crucible, the heater comprising a cylindrical body portion configured and positioned to provide thermal radiation to at least a portion of the cylindrical body portion of the crucible and a conical portion configured and positioned to provide thermal radiation to at least a portion of the conical portion of the crucible, the heater comprising a layered structure comprising a pyrolytic graphite electrically resistive layer positioned between pyrolytic boron nitride electrically insulative layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate several aspects of the present invention and together with description of the exemplary embodiments serve to explain the principles of the present invention. A brief description of the drawings is as follows: 
         FIG. 1  is a perspective view of an exemplary deposition source in accordance with the present invention and showing in particular a conical heat shield assembly comprising a conical cover. 
         FIG. 2  is a cross-sectional view of the deposition source shown in  FIG. 1  and showing in particular an exemplary liquid cooling enclosure in accordance with the present invention. 
         FIG. 3  is a partial exploded view of the deposition source shown in  FIG. 1  and showing in particular an exemplary removable crucible support assembly in accordance with the present invention. 
         FIG. 4  is a perspective view of the removable crucible support assembly shown in  FIG. 3  in accordance with the present invention. 
         FIG. 5  is a partial exploded view of the deposition source shown in  FIG. 1  and showing in particular an exemplary heater support assembly with an exemplary heater and an exemplary tubular heat shield assembly in accordance with the present invention. 
         FIG. 6  is a partial exploded view of the heater support assembly and heater shown in  FIG. 5  in accordance with the present invention. 
         FIG. 7  is an exploded view of the tubular heat shield assembly shown in  FIG. 5  in accordance with the present invention. 
         FIG. 8  is an exploded view of the liquid cooling enclosure shown in  FIG. 2  in accordance with the present invention. 
         FIG. 9  is a partial cross-sectional view of the deposition source shown in  FIG. 1  in accordance with the present invention. 
         FIG. 10  is a partial cross-sectional perspective view of the deposition source shown in  FIG. 1  in accordance with the present invention. 
         FIG. 11  is an exploded view of the conical heat shield assembly shown in  FIG. 1  and showing in particular plural layers of conical heat shielding and the conical cover in accordance with the present invention. 
         FIG. 12  is a partial side view of the deposition source shown in  FIG. 1  and showing in particular exemplary electrical contacts in accordance with the present invention. 
         FIG. 13  is a partial exploded view of the deposition source shown in  FIG. 1  and further showing electrical contacts in accordance with the present invention. 
         FIG. 14  is a partial exploded view of the deposition source shown in  FIG. 1  and further showing electrical contacts in accordance with the present invention. 
         FIG. 15  is a partial cross-sectional perspective view of the deposition source shown in  FIG. 1  and further showing electrical contacts in accordance with the present invention. 
         FIG. 16  is a partial cross-sectional perspective view of the deposition source shown in  FIG. 1  and further showing electrical contacts in accordance with the present invention. 
         FIG. 17  is a partial top down cross-sectional view of the deposition source shown in  FIG. 1  and further showing electrical contacts in accordance with the present invention. 
         FIG. 18  is a partial perspective view of another exemplary deposition source in accordance with the present invention and showing in particular electrical contacts in accordance with another exemplary embodiment of the present invention. 
         FIG. 19  is a partial cross-sectional perspective view of the deposition source shown in  FIG. 18 . 
         FIG. 20  is a schematic top down cross-sectional view of another exemplary deposition source in accordance with the present invention and showing in particular electrical contacts in accordance with another exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The exemplary embodiments of the present invention described herein are not intended to be exhaustive or to limit the present invention to the precise forms disclosed in the following detailed description. Rather the exemplary embodiments described herein are chosen and described so those skilled in the art can appreciate and understand the principles and practices of the present invention. Structural aspects of the present invention that are illustrated and described are exemplary and alternative structures that provide the desired functionality will be apparent to those of skill in the art and can be used in accordance with the present invention. 
     An exemplary vacuum deposition source  10  in accordance with the present invention is illustrated in  FIGS. 1 through 17 . In an exemplary application vacuum deposition source  10  is used to provide efficient deposition of copper, indium, and gallium for forming CIGS based photovoltaic devices such as those used in solar cells. In such application, one or more deposition sources  10  of the present invention are used with one or more selenium deposition sources in a vacuum deposition system (not shown) for deposition of such CIGS based materials. Preferably, when deposition sources in accordance with the present invention are used in a selenium environment materials used for construction of such deposition sources are selected accordingly. In particular, materials that are known to corrode when in the presence of selenium and high temperatures are preferably avoided when possible. 
     Deposition sources in accordance with the present invention are particularly useful in harsh vacuum environments such as those where corrosive materials such as selenium are used. It is contemplated, however, that deposition sources in accordance with the present invention can be used for deposition of any desired material in any desired vacuum environment including but not limited to metals, ceramics, semiconductors, and elemental materials, for example. Vacuum deposition sources in accordance with the present invention are also particularly useful in vacuum environments having a background pressure less than about 1 millitorr. Vacuum deposition sources in accordance with the present invention can also be used in vacuum environments having a background pressure in the high vacuum and ultrahigh vacuum regime such as those used in conventional thermal evaporation and molecular beam epitaxy, for example. When deposition sources in accordance with the present invention are used in an environment free from corrosive materials such as selenium, materials used for construction of such deposition sources are preferably selected in view of a particular operating environment in which a deposition source is to be used. When appropriate, conventional materials for construction of vacuum equipment are preferably used such as stainless steel, refractory metals, and pyrolytic boron nitride, for example. 
     Deposition sources in accordance with the present invention can be used for deposition on any desired substrates such as glass, semiconductor materials, and/or plastic materials, for example. 
     Referring initially to  FIG. 1  exemplary vacuum deposition source  10  in accordance with the present invention is illustrated. Vacuum deposition source  10  includes base flange  12 , deposition source body  14 , and effusion orifice  16 . Base flange  12  supports deposition body  14  and functions to attach deposition source  10  to a vacuum deposition system (not shown). Base flange  12  also preferably includes optional handles  13  as shown in the exemplary illustrated embodiment. As is common with vacuum deposition equipment, base flange  12  preferably comprises stainless steel. 
     As illustrated, deposition source  10  includes optional nipple  18  having flange  20  attached to base flange  12 . Nipple  18  also include optional handle  19  as shown in the exemplary illustrated embodiment. Nipple  18  is typically used to adapt deposition source  10  to a particular vacuum deposition system (not shown). When nipple  18  is used, vacuum deposition source  10  is attached to a vacuum deposition system (not shown) with flange  22  of nipple  18 . Stainless steel construction is preferably used for nipple  18 . 
     Referring to  FIG. 2  generally, exemplary deposition source  10  preferably includes crucible  24 , crucible support assembly  26 , heater  28 , heater support assembly  30 , tubular heat shield assembly  32 , liquid cooling enclosure  34 , conical heat shield assembly  36 , and conical cover  38 . 
     Crucible  24  is removably and adjustably positioned relative to base flange  12  with crucible support assembly  26 , which can be seen in more detail in  FIGS. 3 and 4 . Advantageously, crucible support assembly  26  allows crucible  24  to be removed from deposition source  10  through base flange  12  of deposition source  10 . This rear loading capability allows for easier reloading of deposition material into crucible  24  because deposition source  10  does not need to be completely removed from the vacuum deposition system (not shown) as is needed for top loading deposition sources. Moreover, the rear loading design of deposition source  10  of the present invention allows greater flexibility in designing the effusion end of a deposition source because the crucible does not need to be removed from the effusion end of the deposition source and can be removed from the opposite end. 
     Crucible  24  preferably comprises a monolithic restricted-orifice vessel capable of holding a desired deposition material. As can be seen in  FIGS. 2 and 3  crucible  24  comprises spherical end  25 , cylindrical body portion  27 , first conical portion  29 , and second conical portion  31 . An exemplary preferred crucible material is pyrolytic boron nitride, however, other materials can be used for crucible  24  as would be apparent to those of skill in the art. Pyrolytic boron nitride is a preferred material for vacuum deposition crucibles as well as for components used for vacuum deposition equipment. Pyrolytic boron nitride is generally inert, can withstand high temperatures, is generally clean and does not contribute undesirable impurities to the vacuum environment, is generally transparent to certain wavelengths of infrared radiation, and can be fabricated into complex shapes, for example. 
     Crucible material is preferably selected based on parameters such as material compatibility, operating temperature, thermal conductivity, and electrical conductivity, for example. Alternatives crucible materials that can be used include graphite, ceramics, and refractory metals, for example. Crucible  24  does not need to be monolithic and does not need to utilize the restricted-orifice design of exemplary crucibles  24 . Crucible material and geometry is preferably selected based on factors such as the deposition material to be used in the environment in which the crucible will be located. Exemplary crucibles are described in U.S. Pat. No. 5,820,681, U.S. Pat. No. 5,827,371, and U.S. Pat. No. 5,932,294, the entire disclosures of which are incorporated by reference herein for all purposes. Crucibles that can be used with deposition sources in accordance with the present invention are available from Veeco Instruments, Inc. of St. Paul, Minn. and Momentive Performance Materials of Strongville, Ohio. 
     Referring to the perspective view of  FIGS. 3 and 4  and the cross-sectional view of  FIG. 2  crucible support assembly  26  includes support flange  40  that removably and sealingly mates to base flange  12  of deposition source  10 . Support flange  40  preferably comprises stainless steel as is conventionally used for vacuum equipment. Preferably, as can be seen in  FIG. 2  an o-ring seal  42  is used between support flange  40  and base flange  12 . O-ring seals are preferred over metal gaskets when used in certain corrosive environments such as in the presence of selenium vapor, for example. Alternatively, depending on the application, a Conflat® style seal can be used, for example, which seal comprises flanges having knife-edges that embed into a soft metal seal gasket such as a copper or nickel gasket or the like. Appropriate sealing techniques for various vacuum applications are well known to those skilled in the art of vacuum equipment. 
     Further referring to  FIGS. 2 ,  3 , and  4 , crucible support assembly  26  includes vacuum feed-through  44  that provides linear motion through support flange  40  from the ambient atmosphere side of support flange  40  to the vacuum side of support flange  40 . Vacuum feed-through  44  preferably includes shaft  46  coupled to shaft assembly  48  and crucible support cup  50  attached to shaft assembly  48 . 
     Shaft assembly  48  and crucible support cup  50  are preferably made from graphite while shaft  46  preferably comprises a dissimilar material such as titanium or stainless steel to provide a thermal break to help prevent heat from damaging feed-through  44 , which preferably comprises an o-ring seal as noted below. Graphite is a preferred material because graphite provides a readily machinable material resistant to reaction with corrosive materials such as selenium and the like as well as deposition materials such as copper, indium, and gallium and is tolerant to the necessary process temperatures. Such processing temperatures, for example, can be as high as 1500° C. for some applications. Graphite also has thermal expansion properties compatible with pyrolytic boron nitride. Graphite is also relatively soft and thus provides a suitable support for crucibles made from pyrolytic boron nitride or other fragile or otherwise delicate materials. Graphite material is available from Poco Graphite, Inc. of Decatur, Tex., for example. A preferred graphite material is referred to as fine grain isostatically molded graphite. 
     Vacuum feed-through  44  also preferably includes adjustment knob  52  and lock nut  54 . Rotation of adjustment knob  52  causes shaft  46 , shaft assembly  48 , and crucible support cup  50  to linearly translate along the direction indicated by reference numeral  56 . Vacuum feed-through  44  preferably comprises a stainless steel shaft (shaft  46 ), threaded connection to provide linear motion, and an o-ring based vacuum seal. Vacuum feed-through  44  is exemplary and any device or mechanism that can provide linear motion of crucible support  50  along the direction indicated by reference to a  56  can be used. Such linear motion feed-through devices are well known to those skilled in the art of vacuum equipment. 
     Crucible support assembly  26  also includes liquid cooling enclosure  58  and heat shield assembly  60 . Liquid cooling enclosure  58  and heat shield assembly  60  are preferably designed to shield vacuum feed-through  44  from direct radiant heat and help prevent o-ring seal of linear feed-through  44  from excess heat. Liquid cooling enclosure  58  and heat shield assembly  60  illustrate exemplary structure for providing such shielding and any desired structure that functions to help provide the desired cooling and heat shielding functionality can be used. As can be seen in  FIG. 2 , liquid cooling enclosure  58  preferably comprises inside wall  60  and outside wall  62  spaced apart from inside wall  60 . Liquid cooling enclosure  58  is preferably welded to support flange  40  but other connection techniques such as those including use of removable fasteners can be used to attach liquid cooling enclosure  58  to support flange  40 . Liquid cooling enclosure  58  preferably comprises stainless steel. 
     Heat shield assembly  64  preferably comprises plural layers of refractory metal sheets such as those made from tantalum, tungsten, niobium, and molybdenum, for example. Such refractory metal sheets may be flat, knurled, dimpled, or otherwise embossed to help space apart adjacent sheets to provide thermal breaks between adjacent sheets. Preferably, plural dimpled sheets are used. In another exemplary embodiment a combination of alternating flat and dimpled sheets is used. Alternatives materials that can be used include ceramics and graphite, for example Refractory metal sheets are available from Plansee LLC of Franklin, Mass., for example. 
     Heat shield assembly  64  is preferably attached to tube assembly  58  and includes opening  70  that allows shaft assembly  48  to translate along linear direction  56  discussed above. An exemplary attachment technique is illustrated in  FIG. 4  and includes using wire  66  to attach heat shield assembly  64  to anchor  68  of outside wall  62  of tube assembly  58  at plural locations. Use of wire  66  and anchor  68  to attach heat shield assembly  64  to outside wall  62  of tube assembly  58  is exemplary and other attachment techniques that achieve the same result can be used such as those including use of fasteners or spot welding or the like. 
     Vacuum deposition source  10  preferably comprises an optional alignment system to aid in positioning crucible support assembly  26  relative to base flange  12  during assembly as can be seen with reference to  FIGS. 2 and 3 . Base flange  12  includes alignment rods  72  extending from base flange  12 . Crucible support assembly  26  includes alignment tubes  74  that extend from support flange  40  and correspond with openings  76  provided in support flange  40 . During assembly, alignment rods  72  are aligned with and inserted into openings  76  and are subsequently linearly guided by alignment tubes  74  as crucible support assembly  26  is moved into position to be attached to base flange  12 . 
     As can be seen in  FIG. 2 , crucible support assembly  26  also preferably includes thermocouple  78  that functions to measure temperature and provide control feedback. Thermocouple  78  is removably attached to port  80  of support flange  40 . Thermocouple  78  includes junction  82  that is preferably positioned within heater  28  adjacent to crucible support cup  50 . Preferably, thermocouple  78  is encapsulated in pyrolytic boron nitride, which helps to protect thermocouple  78  from deposition materials and also helps to provide mechanical support for thermocouple  78 . An appropriate thermocouple can be selected based on the particular temperature range to be measured as well as the vacuum environment in which the thermocouple will be used. For example, a Type-C thermocouple can be used. Any desired temperature measurement device can be used, however, as such devices are well known to those skilled in the art of vacuum equipment. Temperature measurement devices including thermocouples are available from Omega Engineering, Inc. of Stamford, Conn., for example. 
     Referring to  FIGS. 5 and 6  in particular, heater  28  is preferably removably and adjustably positioned relative to base flange  12  by heater support assembly  30 . Heater support assembly  30  preferably comprises support base  84 , heat shield assembly  86 , and height adjustment legs  88 . Support base  84  comprises a generally cylindrical ring-like structure having recessed region  90  that receives end  92  of heater  28 . Preferably, recessed region  90  and end  92  are designed to have a close sliding fit. Threaded openings  94  of support base  84  along with openings  96  of heater  28  when end  92  of heater  28  is positioned in recessed region  90  of support base  84 . Fasteners  98  comprise a threaded portion  100  and a non-threaded portion  102 . When positioned in openings  94  of support base  84  non-threaded portions  102  of fasteners  98  engage with openings  96  of heater  28  to secure heater  28  in place relative to heater support base  84 . Heater support base  84  and fasteners  98  preferably comprises graphite. Graphite material is available from Poco Graphite, Inc. of Decatur, Tex., for example. A preferred graphite material is referred to as fine grain isostatically molded graphite. 
     Heater support assembly  30  further preferably comprises plural rotatable height adjustment legs  88  that rotatably engage with respective fixed support legs  104  of base flange  12 . Adjustment legs  88  each comprise threaded portion  106  that threads into each respective threaded bore  108  of heater support base  84 . Adjustment legs  88  also each include bore  110  that rotatably engages with a shaft portion (not shown) of each respective fixed support leg  104  of base flange  12 . Height adjustment legs  88  preferably comprise graphite. Graphite material is available from Poco Graphite, Inc. of Decatur, Tex., for example. A preferred graphite material is referred to as fine grain isostatically molded graphite. Fixed support legs  104  preferably comprise stainless steel and are preferably welded or otherwise secured to base flange  12 . 
     In use, rotation of height adjustment legs  88  varies the height of support base  84  relative to base flange  12  as well as the height of orifice  16  of crucible  24  relative to base flange  12 . Such height adjustment is described in further detail below. Height adjustment legs  88  also each include opening  112  that can be aligned with a corresponding opening (not shown) of each fixed leg  104 . A suitable wire or the like (not shown) is preferably positioned in opening  112  and the corresponding opening of fixed leg  104  to prevent rotation of height adjustment  88  relative to fixed leg  104 . 
     Continuing to refer to  FIGS. 5 and 6 , heater support assembly  30  also comprises heat shield assembly  86  as noted above. Heat shield assembly  86  preferably comprises support plate  114  and plural layers of refractory metal sheets  116  such as those made from tantalum, tungsten, niobium, and molybdenum, for example. With reference to  FIG. 6 , in particular, heat shielding assembly  86  includes openings  118  that receive height adjustment legs  88 . Height adjustment legs  88  each comprise shoulder  120 , which supports heat shield assembly  86  when assembled as shown in  FIG. 5 . Height adjustment legs  88  also each comprise channel  122  spaced apart from shoulder  120  and which can receive a wire or retaining clip (not shown) to secure heat shield assembly  86  relative to height adjustment legs  88 . Support plate  114  preferably comprises pyrolytic boron nitride. Alternative materials that can be used include ceramics and graphite, for example Refractory metal sheets  116  may be flat, knurled, dimpled, or otherwise embossed to help space apart adjacent sheets to provide thermal breaks between adjacent sheets. Preferably, plural and knurled (or dimpled or the like) sheets are used. In another exemplary embodiment a combination of alternating flat and dimpled sheets is used. 
     Vacuum deposition source  10  also preferably includes tubular heat shield assembly  32  as noted above. Heat shield assembly  32  functions to help prevent radiant heat from escaping from vacuum deposition source  10 , which helps to improve efficiency and controllably of vacuum deposition source  10  during operation. Heat shield assembly  32  also functions to help position and support heater  28  as explained in more detail below. 
     Referring to  FIG. 7 , exemplary tubular heat shield assembly  32  comprises support rings  124 , first heat shield  126 , and second heat shield  128 . Support rings  124  preferably comprise plural arcuate sections  130 , Support rings  124  may, however, comprise a single monolithic ring structure as plural sections are not required but are used in a preferred embodiment to provide efficient use of material. Although three support rings  124  are illustrated in exemplary tubular heat shield assembly  32 , it is contemplated that any desired number of support rings can be used. Preferably, support rings  124  comprise pyrolytic boron nitride but other materials can be used. 
     Preferably, arcuate sections  130  are interconnected using refractory metal wire that passes through openings in overlapping ends of adjacent arcuate sections  130 . Any desired attachment technique can be used, however, such as by using fasteners or the like, for example. As illustrated, support rings  124  also optionally comprise openings  132  that function to provide conductance for pumping. In one preferred embodiment, at least one of support rings  124  provides a close fit with either  28 . Preferably, such support ring includes an identifying mark or the like such as by using square openings in contrast with round openings  132 , for example. 
     Support rings  124  additionally include plural tabs  134  spaced apart around the circumference of the outside diameter of support rings  124 . When assembled, tabs  134  mate with slots  136  provided in first and second heat shields  126  and  128 , respectively. Wires  125  are preferably wrapped around first and second heat shields  126  and  128  and join together such as by twisting respective ends together. Preferably, wires  125  are also engaged with tabs  134 . Edges of first and second heat shields  126  and  128  may be overlapped or may be butted together. 
     First and second heat shields  126  and  128 , respectively, preferably comprise arcuate refractory metal sheets such as those made from tantalum, tungsten, niobium, and molybdenum, for example. While two heat shields are illustrated in the exemplary heat shield assembly  32  it is contemplated that any number of arcuate heat shield portions having plural seams can be used to form heat shield assembly  32  including use of a single sheet of refractory material that is rolled to form a cylindrical structure having a single seam. First and second heat shields  126  and  128 , respectively, each comprise a single layer, as shown. It is contemplated, however, that plural layers of refractory metal material can be used. For example, plural layers of alternating flat and knurled (or dimpled or the like) refractory metal sheets can be used. 
     Referring back to  FIGS. 5 and 6 , support base  84  comprises outside surface  140  that preferably removably and slidingly couples with inside surface  142  of heat shield assembly  32 . When assembled, end  144  of heat shield assembly  32  preferably rests against shoulder  146  of support base  84 . Preferably, outside surface  140  of support base  84  and inside surface  142  of heat shield assembly  32  are designed to have a sliding fit. Such a sliding fit is exemplary and it is contemplated that other coupling techniques can be used to couple heat shield assembly  32  with support base  84  such as by using one or more of fasteners, pins, and mechanical devices, for example. 
     With reference to the cross-sectional view of  FIG. 2  and the exploded view of  FIG. 8 , liquid cooling enclosure  34  preferably comprises inside and outside spaced apart walls,  146  and  148 , respectively, end ring  149 , and mounting flange  150 . Preferably, walls  146  and  148 , end ring  149 , and mounting flange  150  comprise stainless steel. Inside and outside spaced apart walls,  146  and  148 , respectively, at least partially define fluid channel  152 . Fluid channel  152  is in fluid communication with tubes  154  that allow a desired cooling fluid, such as water, for example, to be circulated through fluid channel  152 . Liquid cooling enclosure  34  functions to help prevent radiant heat from escaping from vacuum deposition source  10 , which helps to improve efficiency and controllably of vacuum deposition source  10  during operation. 
     When assembled, mounting flange  150  is attached to base flange  12  of deposition source  10  using conventional threaded fasteners or the like. Tubes  154  pass through base flange  12  via sealing feed-throughs (not visible). Such feed-throughs are conventional and well-known and typically include an o-ring and ferrule that provide an appropriate vacuum seal suitable for the desired vacuum level and operating temperatures. Suitable feed-throughs are available from Swagelok, Fluid System Technologies of Solon, Ohio, for example. 
     Referring to the exploded view of  FIG. 8  and the detail views of  FIGS. 9 and 10 , in particular, liquid cooling enclosure  34  preferably comprises heat shield sheets  156 , heat shield sheets  158 , and support ring  162 . When assembled, heat shield sheets  156  and  158  preferably form a cylindrical layered assembly  160 . Heat shield sheets  156  and  158  are preferably attached to support ring  162  and can be attached to each other, if desired, such as by using one or more of refractory metal wire, fasteners, clips, and spot welding. 
     As illustrated, heat shield sheets  156  and  158  are provided in pairs and are preferably arranged so an overlapping layer covers seams between ends of sheets. Heat shield sheets  156  and  158  do not need to be provided as pairs, however, and can be provided as any number of arcuate sheet portions comprising plural seams or as a single sheet comprising a single seam. Although heat shield sheets  156  and  158  are illustrated as providing three layers of heat shielding, any number of layers can be used to achieve a desired heat shielding function. For example, plural layers of alternating flat and knurled (or dimpled or the like) refractory metal sheets can be used. 
     Cylindrical layered assembly  160  is preferably positioned inside liquid cooling enclosure  34  as can be seen in the cross-sectional detail views of  FIGS. 9 and 10 . As shown, heat shield sheets  156  and  158  are in contact with support ring  162 . Support ring  162  engages with end ring  149  of liquid cooling enclosure  34 , as shown, preferably to secure cylindrical layer assembly  160  relative to liquid cooling enclosure  34 . Preferably, as can be seen in  FIG. 8 , cylindrical layered assembly  160  is also held in place within liquid cooling enclosure  34  using plural holding clips  164  that are preferably spot welded to the inside wall  146  of liquid cooling enclosure  34 . Engagement between support ring  162  and end ring  149  and use of holding clips  164  as illustrated is exemplary and any suitable structure can be used to provide appropriate heat shielding for vacuum deposition source  10 . 
     As shown, cylindrical layered assembly  160  extends along a portion of the length of liquid cooling enclosure  34  less than the overall length of liquid cooling enclosure  34 . Cylindrical layered assembly  160  can, however, be designed to extend along any desired portion of inside wall  146  of liquid cooling enclosure  34 . Also, it is contemplated that cylindrical layer assembly  160  can comprise plural sections that are assembled or positioned relative to each other to form a structure having a desired length. 
     Heat shield sheets  156  and  158  as well as support ring  162  preferably comprise refractory metal sheets such as those made from tantalum, tungsten, niobium, and molybdenum, for example. In one exemplary preferred embodiment heat shield sheets  156  comprise molybdenum and heat shield sheets  158  comprise tungsten and support ring  162  comprises tantalum. In such exemplary preferred embodiment, heat shield sheets  158  form the innermost layer of cylindrical layered assembly  160 . That is, heat shield sheets  156  preferably surround heat shield sheets  158 . Such arrangement is exemplary and heat shield sheets  156  and  158  can be arranged in any desired order, comprise any desired material, and comprise any desired knurling, dimpling, or embossing to achieve a desired heat shielding function. 
     As noted above, vacuum deposition source  10  includes conical heat shield assembly  36  and conical cover  38 . Conical heat shield assembly  36  and conical cover  38  can be seen in cross-section in  FIGS. 9 and 10  and can be seen as an exploded perspective view in  FIG. 11 . Conical heat shield assembly  36  preferably comprises plural conical heat shield sheets  166  and conical base  168 . Preferably, conical heat shield sheets  166  and conical base  168  are made from refractory metals such as tantalum, tungsten, niobium, and molybdenum, for example. In an exemplary preferred embodiment, conical heat shield sheets  166  comprise molybdenum and conical base  168  comprises tantalum. Also, in an exemplary preferred embodiment, conical heat shield sheets  166  comprise one or more of knurling, dimpling, and embossing. Although conical heat shield sheets  166  are illustrated as providing three layers of heat shielding, any number of layers can be used to achieve a desired heat shielding function. Also, plural layers of alternating flat and knurled (or dimpled or the like) refractory metal sheets can be used. The illustrated embodiment of  FIG. 11  is exemplary and heat shield sheets  166  can be arranged in any desired order, comprise any desired material, and comprise any desired knurling, dimpling, or embossing to achieve a desired heat shielding function. 
     Conical base  168 , as can be seen in  FIGS. 9 ,  10 , and  11 , comprises annular lip  170 , opening  172 , and end  174 . When assembled as shown in  FIGS. 9 and 10 , end  174  rests on support ring  162  and opening  172  is positioned over heater  28 . Conical heat shield sheets  166  preferably rest on and are supported by conical base  168 , as illustrated in the exemplary embodiment. Annular lip  170  helps to retain and position conical heat shield sheets  166  relative to conical base  160 . Preferably, conical base  168  and conical heat shield sheets  166  are held in place by gravity and by conical cover  38  as noted below. If desired, however, fasteners, holding devices, retaining devices, and the like may be used to secure any of conical base  168  and conical heat shield sheets  166  relative to one or more of each other and end ring  149 . 
     Continuing to refer to  FIGS. 9 ,  10 , and  11 , conical cover  38  preferably comprises opening  172 , conical portion  174 , and cylindrical portion  176 . When assembled as shown in  FIGS. 9 and 10 , cylindrical portion  174  mates with recessed portion  178  of end ring  149  of liquid cooling enclosure  34 . Opening  172  is positioned over heater  28  and preferably rests on annular lip  170  of conical base  168 . Assembled as such, conical cover  38  helps to trap conical heat shield sheets  166  between support ring  162  and conical cover  38 . Preferably, cylindrical portion  176  and recessed portion  178  of end ring  149  are designed to provide a close friction fit when assembled together. That is, preferably friction between cylindrical portion  176  and recessed portion  178  functions to hold conical cover  38  removably in place on end ring  149  without the use of additional fasteners or holding devices. It is contemplated, however, that any desired fasteners, holding devices, retaining devices, and the like may be used to secure conical cover  38  to end ring  149 . 
     In an exemplary embodiment, conical cover  38  comprises pyrolytic boron nitride. It is contemplated, however, that conical cover  38  may comprise any desired material depending on the particular application of deposition source  10 . Exemplary materials that can be used to form conical cover  38  include refractory metals and ceramics, for example. Pyrolytic boron nitride is a preferred material for construction of components used for vacuum deposition. Pyrolytic boron nitride is generally inert, can withstand high temperatures, is generally clean and does not contribute undesirable impurities to the vacuum environment, is generally transparent to certain wavelengths of infrared radiation, and can be fabricated into complex shapes, for example. 
     As noted above in the Background section, undesirable accumulation of deposition material in the vicinity of the effusion orifice of vacuum deposition sources can occur under certain deposition conditions. Typically, such deposition conditions include one or more of high deposition temperatures and high deposition rates such as those used for deposition of metals or semiconductors materials, such as copper, indium, and gallium, for example. For purposes of the present invention, high deposition temperatures refer to the operating temperature of the region near the effusion orifice of a crucible. Continued accumulation of deposition material can reduce the area of the effusion orifice and thereby reduce the deposition rate. Ultimately, continued accumulation of deposition material can effectively close the effusion orifice so the deposition rate is unacceptably low or non-existent. 
     Deposition sources in accordance with the present invention such as exemplary deposition source  10  can advantageously provide high deposition rates at high operating temperatures of materials without the above-described problem related to accumulation of deposition material at the effusion orifice. For example, as can be seen in  FIGS. 9 and 10 , heater  28  includes cylindrical portion  180  and conical portion  182 . Conical portion  182  of heater  28  is preferably designed to provide uniform radiant heat to first and second conical portions,  29  and  31 , respectively of crucible  24 . Such uniform heating can be accomplished by designing heater  28  to be closely fit or otherwise slidingly engaged with crucible  24 , provide heating to substantially all surfaces of crucible  24 , and by designing heat shielding that helps to keep radiant heat within a predetermined region of the deposition source  10 . When designed as such, cold surfaces of crucible  24  are minimized and undesirable condensation of deposition material on the inside surface of crucible  24  is accordingly minimized or eliminated. 
     Another aspect of the present invention that is believed to help to minimize or eliminate the above-described problem related to accumulation of deposition material at the effusion orifice relates to conical cover  38 . Because of the conical shape of conical cover  38 , any particles that might be ejected from effusion opening  16  and land on conical cover  38  will tend to slide off of conical cover  38 . Preferably, the slope of conical cover  38  is selected based on factors such as a particular deposition material to be used and the angle at which deposition source  10  is positioned in a vacuum deposition system. In a preferred exemplary embodiment, the angle of deposition source  10  is less than the angle of conical cover  38 . In a preferred exemplary embodiment the angle of deposition source  10  as measured with respect to vertical is less than 40° and more preferably less than 30°. Determination of the geometry of conical cover  38  and the angle of deposition source  10  can be determined empirically for the particular deposition material and vacuum environment. 
     Yet another aspect of the present invention that is believed to help to minimize or eliminate the above-described problem related to accumulation of deposition material at the effusion orifice relates to the relative positions of conical cover  38  and heater  28 . Referring to  FIG. 9 , edge  184  of heater  28  preferably extends past edge  186  of conical cover  38 . In an exemplary embodiment, edge  184  of heater  28  preferably extends past edge  186  of conical cover  38  by approximately 0.0-5.0 millimeters. In another exemplary embodiment, edge  184  of heater  28  preferably extends past edge  186  of conical cover  38  by approximately 2.0-5.0 millimeters. In yet another exemplary embodiment, edge  184  of heater  28  preferably extends past edge  186  of conical cover  38  by approximately 2.0-2.4 millimeters. The ranges indicated above are exemplary and the distance by which edge  184  of heater  28  extends past edge  186  of conical cover  38  can also be determined empirically for the particular deposition material and vacuum environment. 
     Adjustment of edge  184  of heater  28  with respect to edge  186  of conical cover  38  can be performed using height adjustment legs  88  of heater support assembly  36 . Referring back to  FIG. 5 , height adjustment legs  88  can be rotated with respect to fixed legs  104  to translate heater support base  84  and heater  28  relative to the base flange  12 . Preferably, a fixture (not shown) is used to set the height of edge  184  of heater  28  with respect to a known surface such as a surface of base flange  12 . 
     Yet another aspect of the present invention that is believed to help to minimize or eliminate the above-described problem related to accumulation of deposition material at the effusion orifice relates to the relative positions of heater  28  and crucible  24 . Referring to  FIG. 9 , edge  188  of crucible  24  preferably extends past edge  184  of heater  28 . In an exemplary embodiment edge  188  of crucible  24  preferably extends past edge  184  of heater  28  by approximately 0.0-2.0 millimeters. In another exemplary embodiment edge  188  of crucible  24  preferably extends past edge  184  of heater  28  by approximately 0.0-2.0 millimeters. In another exemplary embodiment edge  188  of crucible  24  preferably extends past edge  184  of heater  28  by approximately 0.2-1.0 millimeters. In yet another exemplary embodiment edge  188  of crucible  24  preferably extends past edge  184  of heater  28  by approximately 0.2-0.5 millimeters. The ranges indicated above are exemplary and the distance by which edge  188  of crucible  24  extends past edge  184  of heater  28  can also be determined empirically for the particular deposition material and vacuum environment. 
     Adjustment of edge  188  of crucible  24  with respect to edge  184  of heater  28  can be performed using adjustment knob  52  of crucible support assembly  26 . Referring to  FIG. 2  rotation of knob  52  translates crucible support cup  50  and crucible  24  relative to base flange  12 . When a desired position for crucible  24  is obtained lock not  54  is engaged to lock the position of crucible  24 . 
     Heater  28  preferably comprises a monolithic heating device comprising pyrolytic graphite conductive material sandwiched between insulating pyrolytic boron nitride. Such heaters are available from Momentive Performance Materials of Strongsville, Ohio. Preferably, heater  28  includes two distinct serpentine resistive elements that provide two distinct heating zones that can be controlled independently from each other. One heating zone is preferably used to heat cylindrical portion  180  and the second heating zone is preferably used to heat conical portion  182 . Advantageously, conical portion  182  can be operated at a higher temperature than cylindrical portion  180 , which can help prevent condensation of deposition material near the effusion orifice  16  of crucible  24 . It is contemplated that heater  28  may comprise any desired number of resistive elements including a single resistive element. 
     Referring now to  FIGS. 12-17  generally, and  FIGS. 12-13  in particular, support base  84  further preferably comprises cutout regions  190  that align with electrical contacts  192  of heater  28  when end  92  of heater  28  is positioned in recessed region  90  of support base  84 . As illustrated, heater  28  comprises four electrical contacts  192  that provide power to two distinct resistive elements. Power is provided to electrical contacts  192  via power feed-throughs  194  that are preferably removably coupled with base flange  12 . Power feed-throughs  194  each comprise power conductor  196 , which typically comprises a molybdenum post and an o-ring vacuum seal. 
     Electrical contacts  192 , as can be seen in  FIG. 6 , each comprise an exposed region of the associated graphite resistive element. Because end  92  of heater  28  is generally cylindrical, electrical contacts  192  comprise a cylindrical curvature having a known radius. Accordingly, structure used for electrical connection to cylindrically curving electrical contacts  192  is preferably designed to maximize contact area with electrical contacts  192  and provide consistent pressure to electrical contacts  192  throughout the operating temperature range of deposition source  10 . 
     Power feed-throughs  194  are electrically removably connected to power straps  198  by flexible power cables  200 . Preferably, power straps  198  are cylindrically curved to correspond with the radius of end  92  of heater  28  and taking into consideration the thickness of conductive washer  226  (described below). Cable connectors  202  receive flexible power cables  200  and power conductors  196  and function to clamp flexible power cables  200  to power conductors  196 . Insulating power strap isolators  204  are preferably positioned on power conductors  196  below cable connectors  202 , as illustrated. Insulating power strap isolators  204  each include curved slot  206  that receives end  208  of each power strap  198  and helps to hold each power strap  198  in place. In a preferred embodiment, insulating power strap isolators  204  comprise pyrolytic boron nitride although it is contemplated that other insulating materials can be used. Alternative structures for positioning ends  208  of power straps  198  can also be used. Use of cable connectors  202  and flexible power cables  200  illustrates an exemplary technique to removably electrically connect power conductors  196  to power straps  198  and those of skill in the art will recognize that other suitable techniques can be used to make such connection such as the use of one or more of alternative clamping structures, fasteners, connectors, and spot welding, for example. 
     Flexible power cables  200  are connected to power straps  198  with cable clamps  210 . As illustrated, cable clamps  210  comprise clamping plates  212  and fasteners  214  that function to compressively clamp flexible power cables  200  to power straps  198 . Cable clamps  210  illustrates an exemplary technique to removably electrically connect flexible power cables  200  to power straps  198  and those of skill in the art will recognize that other suitable techniques can be used to make such connection such as the use of one or more of alternative clamping structures, fasteners, connectors, and spot welding, for example. Preferably, a vacuum deposition source  10  is used in the presence of a corrosive vapor such as selenium, cable clamps  210  preferably comprise molybdenum with a stainless steel screw. Power straps  198  preferably comprise tungsten. Power cables  200  preferably comprise multi-stranded molybdenum wire. 
     In each connection, power strap  198  is preferably electrically removably connected to electrical contact  192  of heater  28  as illustrated by the exemplary connection technique shown in  FIGS. 13-17 . Power strap  198  preferably passes through notch  216  provided in support base  84 . Electrical connection to electrical contact  192  is preferably maintained using spring  218  and loading pins  220  provided at each end of spring  218 . Spring  218  applies spring force to pressure pin  222 , which applies pressure to contact washer  224 . As can be seen best in the cross-sectional view of  FIG. 15 , power strap  198  is preferably sandwiched between contact washer  224  and conductive washer  226 . 
     As illustrated in the exemplary embodiment, loading pins  220  each preferably comprise cylindrical shoulder  236  rotatably positioned in bore  230  of support base  84 . Referring to the cross-sectional view of  FIG. 16 , cylindrical shoulder  228  of each spring loading pin  220  includes opening  232  that receives retaining wire  234  that helps to hold spring loading pin  220  in place. It is contemplated that other retaining structure could be used such as use of a retaining clip and groove or the like. Referring back to  FIG. 14 , each spring loading pin  220  also preferably comprises flat portion  236  that engages with surface  238  of spring  218  when assembled, such as is illustrated in  FIGS. 15 and 16 . Varying the location of flat portion  236  can be used for adjustment of spring force. 
     Referring now to  FIGS. 14 and 15 , each pressure pin  222  preferably comprises head portion  240  and post portion  242 . Post portion  242  of each pressure pin  222  preferably passes through opening  244  of contact washer  224 , opening  246  of conductive washer  226 , and opening  248  of heater  28  associated with each electrical contact  192 . Preferably, surface  243  of head portion  240  comprises a spherically curving surface and is in contact with surface  250  of spring  218 . A spherically curving surface preferred because a spherically curving is not orientation dependent when contacting surface  250 . Preferably, the radius of spherically curving surface  243  comprises the radius that is less than or equal to than the distance that spring  218  needs to flex. 
     Surface  252  of head portion  240  preferably comprises a flat surface that mates with flat surface  253  of contact washer  244 . Surface  257  of contact washer  224  preferably comprises a cylindrically curving surface and preferably has a radius determined by considering the radius of electrical contact  192 , thickness of conductive washer  226 , and thickness of power strap  198 . Power strap  198  also preferably at a cylindrically curving shape that corresponds with the radius of electrical contact  192 . 
     Springs  218  preferably comprise a resilient material that can maintain its ability to apply consistent pressure throughout the operating temperature range of deposition source  10 . An exemplary preferred material comprises pyrolytic boron nitride because pyrolytic boron nitride is vacuum compatible, insulating, and can maintain a spring force at high temperatures. Other materials that can be used include insulating materials having suitable elastic properties, for example. 
     As shown in the exemplary illustrated embodiment, springs  218  preferably comprise a generally rectangular plate. The dimensions, geometry, and thickness of springs  218  are preferably designed to provide the desired spring force. Suitable characteristics for springs  218  can be determined empirically. Preferably, in an exemplary embodiment, a load of between about 3 to 8 pounds, as applied to electrical contacts  192 , is used. Springs  218  may have any desired geometry, however such as that including serpentine structures or the like. Also, plural layers of material can be used to form springs  218  such as to provide a leaf spring structure, for example. 
     Loading pins  220 , pressure pin  222 , and contact washer  224  preferably comprise graphite. Other suitable materials can be used, however, for loading pins  220 , pressure pin  222 , and contact washer  224 . Conductive washer  226  preferably comprises graphite. Other suitable materials can be used, however, for conductive washer  226 . Graphite material is available from GrafTech Advanced Energy Technology, Inc. of Lakewood, Ohio. One preferred graphite material is referred to as nuclear grade GTA material. 
     In  FIGS. 18 and 19 , another exemplary electrical contact assembly that can be used in accordance with the present invention is illustrated. In the embodiment illustrated in  FIGS. 18 and 19 , a flat power strap  254  is used instead of the arcuate power strap  198  described above. As shown, loading pins  220  and spring  218  apply force to pressure pin  222  in a similar manner as described above. Flat power strap  254  is preferably sandwiched between pressure pin  222  and conductive washer  256 . Conductive washer  256  is in contact with contact washer  258 , which is in contact with conductive washer  226 . Conductive washer  226  contacts electrical contact  192  (not visible in  FIGS. 18 and 19 ) of heater  28 . 
     In  FIG. 20 , another exemplary electrical contact assembly that can be used in accordance with the present invention is illustrated. In the embodiment illustrated in  FIG. 20 , spring  260  is positioned inside heater  262 . Pressure pin  264  applies pressure to contact washer  266  and arcuate power strap  268  is sandwiched between contact washer  266  and conductive washer  270 . Conductive washer  270  is in contact with an electrical contact (not shown) of heater  262 . Pressure pin  264  includes post portion  272  that passes through contact washer  266 , arcuate power strap  268 , conductive washer  270 , heater  262 , and spring  260 . End  274  of post portion  272  includes retaining clip  276  and loading tube  278  that function to maintain pressure of conductive washer  270  with the electrical contact (not shown) of heater  262  by the spring force provided by spring  260 . Spring  260  may comprise a pyrolytic boron nitride spring as described above. 
     Vacuum deposition sources that can use electrical contacts described in the present invention are described in Applicant&#39;s co-pending US Patent Application entitled Electrical Contacts For U With Vacuum Deposition Sources, filed on Aug. 11, 2009 and having Ser. No. 12/539,458, the entire disclosure of which is incorporated by reference herein for all purposes. 
     The present invention has now been described with reference to several exemplary embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference for all purposes. The foregoing disclosure has been provided for clarity of understanding by those skilled in the art of vacuum deposition. No unnecessary limitations should be taken from the foregoing disclosure. It will be apparent to those skilled in the art that changes can be made in the exemplary embodiments described herein without departing from the scope of the present invention. Thus, the scope of the present invention should not be limited to the exemplary structures and methods described herein, but only by the structures and methods described by the language of the claims and the equivalents of those claimed structures and methods.