Abstract:
A containment vessel for evaporating materials for use in applying film coatings to a substrate includes a body fabricated from a refractory material. In one embodiment the body includes end portions capable of being connected to other bodies in an end to end fashion. In another embodiment, the body includes an integral patterned conductor incorporated into the outer surface portion of the body to facilitate association with an electrical power source for heating.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an evaporation source used, for example, for the evaporation of materials in the production of thin films. 
       BACKGROUND OF THE ART 
       [0002]    Containment vessels are used for the evaporation of materials, for example, to deposit thin films on substrates. Typically, such containment vessels are crucibles capable of withstanding high temperatures for the vaporization of the contents of the crucible. In the photovoltaic industry, for example, materials such as copper, aluminum, indium, gallium or selenium are vaporized and deposited as a thin film coating on a substrate. 
         [0003]    The crucibles are heated by conventional means such as electrical resistance heating coils, induction heating and the like to provide an evaporation system. The crucibles can be arranged in an array below the substrate, which is coated by the vaporized material us it is passed over the top of the evaporation system. The system is operated under a sufficiently low vacuum to enable evaporation of the metals. Many of the techniques are similar to those used for molecular beam epitaxi (“MBE”) which has a sufficiently low vacuum to provide a molecular beam flow from the crucible to the substrate. 
         [0004]    Ideally, the crucibles will have several desirable features. They should be resistant to the corrosive action of the molten materials which they hold, and to the metal vapors. They should be stable up to about 1,800° C., and in vacuum. They should be easy to charge with a sufficient volume of film-forming materials, easy to heat, and they should have a top opening geometry engineered to control the pattern of vapor flux flowing from the crucible(s) to the substrate. This is true for MBE and is well documented in the literature, and, by logical extension, is needed in the photovoltaic industry as well. 
         [0005]    Typically the crucibles are made from such materials as hot pressed boron nitride (hpBN), pyrolytic boron nitride (pBN), and graphite (particularly, graphite coated with pBN). 
         [0006]    Crucibles can take various forms, but are generally cylindrical or conical, on the order of 10 mm in diameter and 20 mm in length up to about 100 mm in diameter and 400 mm in. length. Larger size crucibles and heaters are advantageous for photovoltaic needs, with standard size production panels typically at 1,200 mm×600 mm. 
         [0007]    Historically, MBE crucibles have been open on the top, with substantially straight cylindrical sidewalls, or very open in a conical form. In part, this is to help control the “beam” flow. The shape of the top of the crucible impacts the deposition profile, and impacts the stability of the source material. A conical exit cone seems to be preferred in the MBE industry. 
         [0008]    MBE cells are often used at an angle of about 45°, so an open, conical shape may not hold sufficient volume of material. When tipped at an angle, an open cone will pour out its contents. A one-piece, integral crucible, with a large body and a narrow orifice is known in the art. This design is available under the designation SUMO® from Veeco Instruments Inc. 
         [0009]    For many metal sources, the material of choice for the crucible is a pyrolytic boron nitride (PBN). PBN is a material produced from chemical vapor deposition (CVD) on a graphite mandrel. To make a narrow orifice crucible part, the narrow orifice is machined into a graphite mandrel, the mandrel is then CVD coated with pBN, then the mandrel is oxidized out of the body of the crucible. 
         [0010]    U.S. Pat. No. 4,812,326 discloses an evaporation source having a two-piece design. The evaporation material is vaporized and jetted through a nozzle having a flared opening to control the size of the atom clusters of the vapor jet. 
         [0011]    WO96/35091 and WO98/08780 disclose unibody monolithic, one-piece negative draft crucibles for use in MBE effusion cells. 
         [0012]    There yet remains need for improvements in crucible construction to accommodate the MBE process and other thermal evaporation processes as used in photovoltaic production. 
       SUMMARY OF THE INVENTION 
       [0013]    A containment vessel for evaporating materials for use in applying a thin film coating to a substrate is provided herein. In one embodiment the containment vessel includes at least one single piece monolithic body fabricated from a refractory ceramic material, the body including a reservoir for containing material to be evaporated, and having two opposite ends with means associated at said ends to permit mechanical joining of at least two of said bodies in an end to end fashion such that the reservoirs of each of said bodies are joined to provide a continuous interior space; and means to control emission of vapor from the containment vessel. 
         [0014]    In another embodiment the containment vessel includes at least one body fabricated from a refractory ceramic material, the body having a reservoir to contain material to be evaporated. The body further includes heating means incorporated into an outer exterior surface of the body. The containment vessel further includes means for controlling the emission of vapor therefrom. 
         [0015]    In another embodiment the containment vessel includes two pieces which engage each other to form an hermetic seal between them without the necessity of overcoating. 
         [0016]    The containment vessel advantageously provides flexibility and adaptability for the arrangement of a system for thin film coating of substrates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Various embodiments are described below with reference to the drawings wherein: 
           [0018]      FIG. 1  illustrates a tubular first embodiment of the crucible of the invention; 
           [0019]      FIG. 1A  is a sectional view of another tubular embodiment of the crucible; 
           [0020]      FIG. 2  illustrates a second embodiment of the crucible of the invention; 
           [0021]      FIG. 2A  illustrates the assembly of crucibles of the embodiment of  FIG. 2  with an intermediate gasket member; 
           [0022]      FIG. 3  illustrates a cover plate for the embodiment of  FIG. 2 ; 
           [0023]      FIG. 4  is a partially cut away perspective view illustrating an embodiment of the invention having a generally cylindrical shape with an hour glass shaped upper portion; 
           [0024]      FIG. 5  illustrates an elongated embodiment of the crucible; 
           [0025]      FIG. 6  illustrates yet another embodiment of the crucible of the invention; 
           [0026]      FIG. 7  illustrates an embodiment with an engineered power distribution system; 
           [0027]      FIG. 8  is an exploded partly sectional view of another embodiment of the invention including a two part crucible; and 
           [0028]      FIG. 9  is yet another embodiment of the two part crucible. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise value specified in some cases. 
         [0030]    As used herein, the term “crucible” may be used interchangeably with “vessel,” or a “container”. The term “multi-piece” may be used interchangeably with “multi-part” or “multiple part” to refer to the multiple pieces that make up the unibody (or one piece, a single body) crucible of the invention. 
         [0031]    Referring now to  FIG. 1 , in one embodiment the containment vessel  100  of the invention includes a cylindrical body  110  having a central axial bore  111  defined by interior surface  115 . The bore  111  provides a reservoir for containing material to be evaporated. A plurality of spaced apertures  116  is disposed lengthwise along the body in at least one linear row. The apertures  116  extend from the exterior surface  113  of the middle portion  117  of the body  110  to the bore  111  so as to permit vapors from the bore to exit therethrough. 
         [0032]    The body  110  includes a middle portion  117  and opposite end portions  112  and  114 . In one embodiment, the exterior surface  113   a  of the end portion  112  and the interior surface  112   a  of the end portion  112  are tapered inward to provide a gradually smaller exterior diameter. End portion  114  is flared outward so as to provide a gradually increasing bore diameter. Optionally, both the annular exterior surface  114   a  and the interior surface of the end portion  114  can be flared outward. However, in containment vessel embodiment  100   a,  as shown in  FIG. 1A , the exterior surface  114   a  of the end portion  114  can be of the same diameter as the exterior surface  113  of the central portion  117  of the body  110 , with only the inner surface  114   b  of the body at the end portion  114  flaring outward so as to provide a gradually increasing bore diameter. The inner surface of the end portion  112  can be of the same diameter as that of inner surface  115  with only the outer surface  113  a tapered inward. Preferably, the inclination of tapered outer surface  113  of end portion  112  conforms to the outward inclination of interior flared surface  114   b  of end portion  114  so as to facilitate a fitted engagement of the end surfaces of respective bodies of containment vessel  100   a.  The purpose of the configuration of the end portions  112  and  114  is to provide means for assembling two or more vessel bodies  110  together in an end to end fashion so as to provide a containment vessel assembly with a length which can be varied by joining of the appropriate number of bodies, wherein the reservoirs, i.e. bores  111  of the individual bodies are joined to provide a continuous interior space. The flared surface  114   b  is flared outwardly and the tapered surface  113   a  is angled inwardly at an angle of from about 3 degrees to about 10 degrees, and is typically such that the friction fitting of the pressed joint provides a stable structure. 
         [0033]    Each individual body  110  is preferably of single piece monolithic construction and is fabricated from a nitride, carbide, carbonitride, boride or oxynitride of elements selected from a group consisting of B, Al, Si, Ga, refractory hard metals, transition metals, and rare earth metals, or complexes and/or combinations thereof. In one embodiment, the crucible body is fabricated from a refractory ceramic such as aluminum nitride (AlN), titanium diboride, and preferably pyrolytic boron nitride (pBN) or pBN-coated graphite. Methods for fabricating such vessels are known in the art. 
         [0034]    Referring now to  FIGS. 2 and 3 , in a second embodiment the containment vessel  200  includes a generally longitudinally extending body  210  having an arcuate inner surface  211  and a generally U-shaped lateral cross section. The arcuate inner surface  211  defines a reservoir for containing the material to be evaporated (e.g., a metal such as Cu, Al, In, Ga, Se, and the like). The body  210  is preferably of single piece monolithic construction and is preferably fabricated from pBN or pBN-coated graphite. The body  210  includes lengthwise extending linear flange portions  214  and  215 , and terminal arcuate flange portions  212  and  213 . Terminal flange portions  212  and  213  include one or more apertures  219  and  218 , respectively. Linear flange portions  214  and  215  include one or more apertures  216  and  217 , respectively. Apertures  218  and  219  are adapted to receive fasteners such as screws, bolts (not shown) and the like, and are configured to align with each other so as to facilitate the end-to-end removable assembly of individual bodies  210  to form a containment vessel assembly with a length which can be varied as desired, wherein the reservoirs defined by the inner surfaces  211  of the individual bodies  210  are joined to provide a continuous inner space. Optionally, one end of the body  210  can include a longitudinally extending arcuate flange  205  and the other end can include an arcuate recessed portion  206  configured and dimensioned to mate with said flange  205  for the reasons stated below. A cover plate  220  ( FIG. 3 ) is configured and dimensioned so as to be mounted to the top of body  210 . Cover plate includes one or more apertures  226  configured to align with apertures  216  of the linear flange portion  214 , and one or more apertures  227  configured to align with apertures  217  of linear flange portion  215  of the body  210 , so as to receive fasteners (screws, bolts, etc.) to permit removable mounting of plate  220  to body  210 . Plate  220  includes a plurality of apertures  222  to permit emission of vapor therethrough. 
         [0035]      FIG. 2A  illustrates an assembly of bodies  210   a  and  210   b  joined end to end with a gasket  230  disposed therebetween. The gasket is preferably fabricated from graphite, such as a Grafoil type gasket, and is crescent shaped to match the terminal arcuate flanges  212  and  213 . Preferably, the gasket  230  is protected from contact with the molten metal coating material by the longitudinally extending arcuate flange  205  which overhangs the gasket. The gasket also inhibits “welding” of the connected individual bodies at the terminal flanges  212  and  213  which might otherwise be caused by depositing of evaporated metal at the joint between the bodies. 
         [0036]    Referring now to  FIG. 4 , in another embodiment the containment vessel  300  includes a crucible body  310  having an interior reservoir  313  for containing the material to be evaporated. Body  310  has a cylindrical portion  311  and an upper hourglass shaped portion  312 , which tapers to a narrow portion  316  and expands thereafter to circular top opening  314 . The narrow portion  316  is configured and dimensioned so as to provide a cross section adapted to control the quantity and direction of vapor flow therethrough. The body  310  is preferably made from a non-electrically conductive material such as pBN, but includes an integral patterned refractory electrically conductor  315  of pyrolytic or bulk graphite which wraps around the exterior of the cylindrical portion  311  of the body  310 . A method of making the integral heater is described in U.S. Pat. No. 7,259,358, which is herein incorporated by reference. 
         [0037]    Preferably, the conductor  315  is overcoated with an additional layer of pBN. In one embodiment, an electrical power source  320  is connected to terminal points  315   a  and  315   b  of conductive line  315  by means of wires  321   a  and  321   b,  respectively. During operation, electrical power applied to conductive line  315  causes it to heat body  310  to a temperature sufficiently high to cause vaporization of at least part of the contents within the interior reservoir  313  of the containment vessel. Heating can be by resistance heating or induction heating. 
         [0038]    Referring now to  FIG. 5 , in an alternative embodiment  400  the container vessel includes a body  410  having a lower portion  411  with an interior reservoir adapted to contain a material to be vaporized. Unlike cylindrical body  310  of the previously described embodiment, body  410  of embodiment  400  has a rectangular box-like configuration with a linear lengthwise extension along axis A. Upper portion  412  tapers to a narrow portion  416  and expands laterally to a rectangular top opening  414 . The narrow portion  416  is configured and dimensioned so as to provide a cross sectional outlet area adapted to control the quantity and direction of the vapor flow therethrough. The body  410  is preferably fabricated from pBN, but also includes an integral patterned conductor  415  of pyrolytic or bulk graphite which extends along one or both of lengthwise sides  413   a  and/or  413   b.  Preferably, the conductor  415  is overcoated with a layer of pBN. Typically, body  410  will have a length L of from 100 mm to 600 mm to cover the width of a standard photovoltaic (“PV”) panel, or up to 1,200 mm to cover the length of a standard PV panel, and a width W of from about 50 mm to about 100 mm or larger as necessary to contain sufficient volume of material for operational efficiency. In use, an electrical power source  420  is connected to terminal points  415   a  and  415   b  of the conductive line  415  by means of wires  421   a  and  421   b,  respectively. During operation, electrical power applied to conductive line  415  causes it to heat body  410  to a temperature sufficiently high to cause vaporization of at least part of the contents of the containment vessel  400 . Heating can be accomplished by resistance heating or induction heating. 
         [0039]    In yet another embodiment, referring now to  FIG. 6 , containment vessel  500  includes a body  510  having a rectangular box-like structure defined by wall  511 , which encloses an interior reservoir  513  for containing material to be vaporized. Opening  514  extends across the top of body  514 . Optionally, the body  510  includes apertures  518  along the top surface of wall  511  to receive fasteners by which a plate such as that shown in  FIG. 3  and described above can be attached to regulate the emission of vapor from containment vessel  500 . 
         [0040]    Body  510  is preferably fabricated with pBN, but includes an integral patterned conductor  515  of pyrolytic or bulk graphite, which extends along at least one or both of lengthwise sides  511   a  and  511   b.  Conductor  515  is preferably overcoated with a layer of pBN. The length and width dimensions of body  510  can be similar to those of body  410  describe above. In use, an electrical power source  520  is connected to terminal points  515   a  and  515   b  by means of wires  521   a  and  521   b,  respectively. During operation, as described above, electrical power is applied to conductive line  515  to heat body  510 . Heating can be accomplished by resistance heating. Alternatively, induction heating can be employed. 
         [0041]    Optionally, the embodiments  100  and  200  of the invention illustrated in  FIGS. 1 and 2  can include the heating means (e.g.  315 ,  415  or  515 ) of embodiments  300 ,  400  or  500 . 
         [0042]    Referring now to  FIG. 7 , an embodiment  600  of the containment vessel is shown including a body  610  having a lower portion  611  with an interior reservoir adapted to contain material to be vaporized. Upper neck portion  612  tapers to a narrow portion  616  and expands laterally to a rectangular top opening  614 . The narrow portion  616  is configured and dimensioned to provide a cross sectional outlet area adapted to control the quantity and direction of the vapor flow therethrough. The body  610  is preferably fabricated from a non-conductive refractory ceramic such as pBN, but includes an integral patterned electrode conductor  641  of pyrolytic or bulk graphite which extends along one or both of the lengthwise sides  613   a  and  613   b.  Preferably the conductor  641  is overcoated with a layer of pBN. In use a power supply system  640  including conductive leads  643  and  644 , and a power source  642  is connected to the conductor  641  to heat the containment vessel  600  to a temperature sufficient to vaporize the material in the interior of the body  610 . 
         [0043]    Condensation of the material is a problem which often occurs in such coating processes as described herein. For example, some material evaporated from the lower portion  611  of the containment vessel can condense in the hour-glass shaped upper portion  612  and deposit at relatively cooler portions on the interior wall. This deposited material can obstruct the flow of vapor and cause non-uniformities of the coating process. 
         [0044]    To alleviate this problem the present invention comprises an engineered power distribution system which includes a tailored electrode. That is, the integral patterned conductor incorporated into the wall of the containments vessel is positioned and configured to selectively distribute more or less power to the containment vessel to compensate for the differential temperatures and areas of condensation of the containment vessel. Thus, cold spots can be overcome by applying more power to the affected areas. 
         [0045]    Referring to  FIG. 7 , a conductor  631 , i.e., a tailored electrode, is incorporated into the surface of the body  610  at the upper portion  612 . Conductive line  631  can be patterned, for example, by closer spacing or by other suitable configuration to apply heat differentially to cooler spots of the containment vessel  600  so as to provide more uniform heating. The conductor  631  can be connected to the same power supply  640  as conductor  641  or it can be separate from or integral with conductor  641 . In another embodiment, conductor  631  can be connected to a separate power supply  630  which includes electrical lines  633  and  634 , and a power source  632 . Power levels can be adjusted to meet the needs of the local heating requirements to compensate for, and eliminate, cold spots at which unwanted condensation of the material can occur on the inner surface of the containment vessel  600 . 
         [0046]    Referring now to  FIGS. 8 and 9 , a two part “Sumo-style” crucible is illustrated. As stated previously to make a single piece pBN crucible with a narrow orifice the pBN is typically deposited by, for example, chemical vapor deposition onto a graphite mandrel of the appropriate shape to form the body of the crucible. However, the graphite mandrel must then be oxidized to remove it as it cannot be mechanically withdrawn from the crucible. To alleviate this problem multi-part crucibles have been made, as disclosed for example, in U.S. Pub. No. 2007/0289526. The graphite mandrels can be physically separated from the respective crucible parts. These parts can then be assembled. Yet the method described in this reference nevertheless requires an additional step, i.e., overcoating the assembled crucible parts at least at the joint so as to provide an hermetic seal. As used herein the term “hermetic seal” means that there is no visual leak/failure in the joint of the parts of the crucible after the crucible is exposed to a molten metal for a continuous period of at least 8 hours. We have discovered a method to provide and assemble a two-piece crucible which obviates the need for an additional overcoating. 
         [0047]    The individual pieces are fabricated on separate graphite mandrels by CVD deposition of pBN or other material from which the crucibles can be made. The graphite mandrels are then physically separated from the crucible body parts. A significant feature of the crucible herein is the tapered ends which engage each other and fit snugly so as to provide a tight seal and obviate the need for any subsequent overcoating. This represents considerable savings of time, labor and expense in the fabrication process. 
         [0048]    Referring more particularly now to  FIG. 8 , two-piece crucible  700  includes an upper piece  710 , and a lower piece  720  for containing the material to be vaporized. The upper piece  710  includes an hour-glass shaped body  711  defining a vapor flow path  712  having a narrow orifice portion  713  for controlling the vapor flow therethrough. The wide bottom part  714  of the upper piece terminates with a tapered annular portion  715 . 
         [0049]    The tapered outer facing surface of annular portion  715  is adapted to snugly engage a correspondingly tapered inner facing annular surface  723  at the upper portion of the lower piece  720 . Lower piece  720  includes a generally cylindrical shaped body  721  having an open upper end  722  and a closed bottom end  724 . The thickness of the body  721  in the vicinity of the upper end decreases to provide the tapered surface  723 . However the outer diameter of the body  721  remains uniform throughout the length of the body. The upper piece  710  and lower piece are jointed by inserting the bottom part  714  of the upper piece into the open upper end  722  of the lower piece so as to mate the tapered surfaces  715  and  723  in a snug fitting engagement. The assembled crucible  700  can then be used in evaporative coating processes without requiring an overcoating to seal the joint area. Alternatively the angle of taper on the top and bottom sections can be reversed. 
         [0050]    Referring now to  FIG. 9 , portion of an alternative embodiment of the two piece crucible is illustrated wherein the wall  716  of the upper piece terminates in an annular portion  717  with a tapered outer surface  718  provided by, for example, a reduction in the thickness of the wall  716 . 
         [0051]    The bottom piece  726  includes an annular upper outwardly flared portion  727  to receive the tapered portion  717  of the upper piece. The flared portion  727  is flared outwardly from vertical orientation V along an angled orientation A wherein angle α between V and A ranges from about 3 degrees to about 10 degrees and is typically such that the friction fitting of the pressed joint provides a stable structure. 
         [0052]    The insertion depth between the upper edge  729  of the bottom piece  726  and the lower edge  719  of the wall  716  of the upper piece can range from a minimum insertion depth D min  of from about 1 to 4 mm to a maximum insertion depth D max  of from abut 9 mm to about 11 mm, to insure a snug fit between the upper and lower pieces to provide a mechanically robust tight seal without the need for overcoating the joint, and provide sufficient sealing between the two components to prevent excessive leakage of the source material. In an embodiment of the invention the insertion depth ranges from 3 mm (D min ) to 10 mm (D max ). 
         [0053]    While the above description contains specifics, these specifics should not be construed as limitations of the invention, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision many other embodiments within the scope and spirit of the invention as defined by the claims appended hereto.