Patent Publication Number: US-2012028393-A1

Title: Vapor deposition apparatus and process for continuous deposition of a doped thin film layer on a substrate

Description:
FIELD OF THE INVENTION 
     The subject matter disclosed herein relates generally to the field of thin film deposition processes wherein a doped thin film layer, such as a semiconductor material layer, is deposited on a substrate. More particularly, the subject matter is related to a vapor deposition apparatus and associated process for depositing a doped thin film layer of a photo-reactive material on a glass substrate in the formation of photovoltaic (PV) modules. 
     BACKGROUND OF THE INVENTION 
     Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy (sunlight) to electricity. For example, CdTe has an energy bandgap of 1.45 eV, which enables it to convert more energy from the solar spectrum (sunlight) as compared to lower bandgap (1.1 eV) semiconductor materials historically used in solar cell applications. Also, CdTe converts light more efficiently in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in low-light (i.e., cloudy) conditions as compared to other conventional materials. 
     Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner. 
     Certain factors greatly affect the efficiency of CdTe PV modules in terms of cost and power generation capacity. For example, CdTe is relatively expensive and, thus, efficient utilization (i.e., minimal waste) of the material is a primary cost factor. In addition, the energy conversion efficiency of the module is a factor of certain characteristics of the deposited CdTe film layer. Non-uniformity or defects in the film layer can significantly decrease the output of the module, thereby adding to the cost per unit of power. Also, the ability to process relatively large substrates on an economically sensible commercial scale is a crucial consideration. 
     CSS (Close Space Sublimation) is a known commercial vapor deposition process for production of CdTe modules. Reference is made, for example, to U.S. Pat. No. 6,444,043 and U.S. Pat. No. 6,423,565. Within the vapor deposition chamber in a CSS system, the substrate is brought to an opposed position at a relatively small distance (i.e., about 2-3 mm) opposite to a CdTe source. The CdTe material sublimes and deposits onto the surface of the substrate. In the CSS system of U.S. Pat. No. 6,444,043 cited above, the CdTe material is in granular form and is held in a heated receptacle within the vapor deposition chamber. The sublimated material moves through holes in a cover placed over the receptacle and deposits onto the stationary glass surface, which is held at the smallest possible distance (1-2 mm) above the cover frame. The cover is heated to a temperature greater than the receptacle. 
     While there are advantages to the CSS process, the related system is inherently a batch process wherein the glass substrate is indexed into a vapor deposition chamber, held in the chamber for a finite period of time in which the film layer is formed, and subsequently indexed out of the chamber. The system is more suited for batch processing of relatively small surface area substrates. The process must be periodically interrupted in order to replenish the CdTe source, which is detrimental to a large scale production process. In addition, the deposition process cannot readily be stopped and restarted in a controlled manner, resulting in significant non-utilization (i.e., waste) of the CdTe material during the indexing of the substrates into and out of the chamber, and during any steps needed to position the substrate within the chamber. 
     Accordingly, there exists an ongoing need in the industry for an improved vapor deposition apparatus and process for economically feasible large scale production of efficient PV modules, particularly CdTe modules. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In accordance with an embodiment of the invention, an apparatus is provided for vapor deposition of a sublimated source material, such as CdTe, as a thin film on a photovoltaic (PV) module substrate. Although the invention is not limited to any particular film thickness, a “thin” film layer is generally recognized in the art as less than 10 microns (μm). The apparatus includes a deposition head and a receptacle disposed therein. A first feed tube and a second feed tube are configured to supply a source material into the deposition head, and a heated distribution manifold is configured to heat said receptacle. A distribution plate is disposed below said receptacle and at a defined distance above a horizontal conveyance plane of an upper surface of a substrate conveyed through said apparatus, said distribution plate comprising a pattern of passages therethrough. In one embodiment, the heated distribution manifold can be disposed below the receptacle, and can include a plurality of passages defined therethrough. 
     Variations and modifications to the embodiments of the vapor deposition apparatus discussed above are within the scope and spirit of the invention and may be further described herein. 
     In still another aspect, the invention encompasses a process for vapor deposition of a sublimated source material, such as CdTe, as a thin film on a photovoltaic (PV) module substrate. The process includes supplying source material to a receptacle within a deposition head, and supplying a dopant material into the deposition head in a solid state. The receptacle can be indirectly heated with a heat source member to sublimate the source material. Individual substrates can be conveyed below the receptacle, such that the sublimated source material is deposited onto an upper surface of the substrates. The substrates may be conveyed at a constant linear rate through the apparatus, with the sublimated source material being directed from the receptacle primarily as transversely extending leading and trailing curtains relative to the conveyance direction of the substrates. 
     Variations and modifications to the embodiment of the vapor deposition process discussed above are within the scope and spirit of the invention and may be further described herein. 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims, or may be obvious from the description or claims, or may be learned through practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, is set forth in the specification, which makes reference to the appended drawings, in which: 
         FIG. 1  is a plan view of a system that may incorporate embodiments of a vapor deposition apparatus of the present invention; 
         FIG. 2  is a cross-sectional view of an embodiment of a vapor deposition apparatus according to aspects of the invention in a first operational configuration; 
         FIG. 3  is a cross-sectional view of the embodiment of  FIG. 2  in a second operational configuration; 
         FIG. 4  is a cross-sectional view of one embodiment of  FIG. 2  in cooperation with a substrate conveyor; and, 
         FIG. 5  is a top view of the receptacle component within the embodiment of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention encompass such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Chemical elements are discussed in the present disclosure using their common chemical abbreviation, such as commonly found on a periodic table of elements. For example, hydrogen is represented by its common chemical abbreviation H; helium is represented by its common chemical abbreviation He; and so forth. 
     In the present disclosure, when a layer is being described as “on” or “over” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer. 
     Additionally, although the invention is not limited to any particular film thickness, the term “thin” describing any film layers of the photovoltaic device generally refers to the film layer having a thickness less than about 10 micrometers (“microns” or “μm”). 
       FIG. 1  illustrates an embodiment of a system  10  that may incorporate a vapor deposition apparatus  100  ( FIGS. 2 through 5 ) in accordance with embodiments of the invention configured for deposition of a thin film layer on a photovoltaic (PV) module substrate  14  (referred to hereafter as a “substrate”). The thin film may be, for example, a film layer of cadmium telluride (CdTe). As mentioned, it is generally recognized in the art that a “thin” film layer on a PV module substrate is generally less than about 10 microns (μm). It should be appreciated that the present vapor deposition apparatus  100  is not limited to use in the system  10  illustrated in  FIG. 1 , but may be incorporated into any suitable processing line configured for vapor deposition of a thin film layer onto a PV module substrate  14 . 
     In addition to the source material for the thin film, a dopant or mixture of dopants (collectively referred to as “dopant(s)”) can be co-deposited on the substrate within the vapor deposition apparatus  100 . As used herein, a “dopant” is an impurity element that is included within the thin film (in very low concentrations) in order to alter the electrical properties and/or optical properties of the thin film. For instance, the atoms of the dopant can take the place of elements that were in or would have been in the crystal lattice of the thin film. For example, using the proper types and amounts of dopant(s) in thin film semiconductors can produce p-type semiconductors and n-type semiconductors. In certain embodiments, the dopant(s) can be included in the thin film in trace concentrations, such as about 0.1 atomic parts per million (at ppm) to about 1,000 at ppm (e.g., about 1 at ppm to about 750 at ppm). 
     When the thin film is deposited from a source material of cadmium telluride (i.e., a cadmium telluride thin film layer) in the manufacture of a cadmium telluride thin film PV device, suitable dopants can include, but are not limited to, B, Al, Ga, In, Sc, Y, Cu, Au, N, As, P, Sb, Bi, Cl, F, Br, Li, Na, K, compound containing those elements, and mixtures thereof. In one particular embodiment, the cadmium telluride layer can include a p-type dopant(s), such as Cu, Au, N, As, P, Sb, Bi, Cl, F, Br, Li, Na, K, compound containing those elements, or mixtures thereof. According to one particular embodiment, the dopant can be supplied as a solid composition at room temperature and pressure (i.e., at about 20° C. and about 760 Torr) to a vapor deposition apparatus for inclusion within the vapor deposition apparatus. As such, the dopant elements can be supplied as a compound that is a solid (e.g., Cl can be included in CdCl 2 ). Particularly suitable compounds include, but are not limited to, CuP 3 , Cd 3 P 2 , Cd 2 As 2 , Sb 2 Te 3 , Bi 2 Te 3 , or mixtures thereof. 
     If the amount of dopant material is too small to be directly mixed in with the CdTe, a carrier material could be mixed with the dopant to facilitate transport of the dopant in smaller concentration. For example, the carrier material could be coated with a thin layer of the dopant, and then dispensed into the deposition apparatus. Suitable carrier materials can be essentially inert at the deposition conditions, such as silica (SiO 2 ), alumina (Al 2 O 3 ), etc. The carrier material can be in any shape (e.g., beads) for coating with the dopant material. 
     For reference and an understanding of an environment in which the vapor deposition apparatus  100  may be used, the system  10  of  FIG. 1  is described below, followed by a detailed description of the apparatus  100 . 
     Referring to  FIG. 1 , the exemplary system  10  includes a vacuum chamber  12  defined by a plurality of interconnected modules. Any combination of rough and fine vacuum pumps  40  may be configured with the modules to draw and maintain a vacuum within the chamber  12 . The vacuum chamber  12  includes a plurality of heater modules  16  that define a pre-heat section of the vacuum chamber through which the substrates  14  are conveyed and heated to a desired temperature before being conveyed into the vapor deposition apparatus  100 . Each of the modules  16  may include a plurality of independently controlled heaters  18 , with the heaters defining a plurality of different heat zones. A particular heat zone may include more than one heater  18 . 
     The vacuum chamber  12  also includes a plurality of interconnected cool-down modules  20  downstream of the vapor deposition apparatus  100 . The cool-down modules  20  define a cool-down section within the vacuum chamber  12  through which the substrates  14  having the thin film of sublimated source material deposited thereon are conveyed and cooled at a controlled cool-down rate prior to the substrates  14  being removed from the system  10 . Each of the modules  20  may include a forced cooling system wherein a cooling medium, such as chilled water, refrigerant, gas, or other medium, is pumped through cooling coils (not illustrated) configured with the modules  20 . 
     In the illustrated embodiment of system  10 , at least one post-heat module  22  is located immediately downstream of the vapor deposition apparatus  100  and upstream of the cool-down modules  20  in a conveyance direction of the substrates. The post-heat module  22  maintains a controlled heating profile of the substrate  14  until the entire substrate is moved out of the vapor deposition apparatus  100  to prevent damage to the substrate, such as warping or breaking caused by uncontrolled or drastic thermal stresses. If the leading section of the substrate  14  were allowed to cool at an excessive rate as it exited the apparatus  100 , a potentially damaging temperature gradient would be generated longitudinally along the substrate  14 . This condition could result in breaking, cracking, or warping of the substrate from thermal stress. 
     As diagrammatically illustrated in  FIG. 1 , a first feed device  24  is configured with the vapor deposition apparatus  100  to supply source material for depositing the thin film on the substrate  14 , such as granular CdTe. The first feed device  24  may take on various configurations within the scope and spirit of the invention, and functions to supply the source material without interrupting the continuous vapor deposition process within the apparatus  100  or conveyance of the substrates  14  through the apparatus  100 . 
     In addition, a second feed device  25  is configured with the vapor deposition apparatus  100  to supply dopant(s) material for including within the thin film on the substrate  14 . The second feed device  25  may take on various configurations within the scope and spirit of the invention, and functions to supply the dopant(s) material without interrupting the continuous vapor deposition process within the apparatus  100  or conveyance of the substrates  14  through the apparatus  100 . 
     Still referring to  FIG. 1 , the individual substrates  14  are initially placed onto a load conveyor  26 , and are subsequently moved into an entry vacuum lock station that includes a load module  28  and a buffer module  30 . A “rough” (i.e., initial) vacuum pump  32  is configured with the load module  28  to drawn an initial vacuum, and a “fine” (i.e., final) vacuum pump  38  is configured with the buffer module  30  to increase the vacuum in the buffer module  30  to essentially the vacuum pressure within the vacuum chamber  12 . Valves  34  (e.g., gate-type slit valves or rotary-type flapper valves) are operably disposed between the load conveyor  26  and the load module  28 , between the load module  28  and the buffer module  30 , and between the buffer module  30  and the vacuum chamber  12 . These valves  34  are sequentially actuated by a motor or other type of actuating mechanism  36  in order to introduce the substrates  14  into the vacuum chamber  12  in a step-wise manner without affecting the vacuum within the chamber  12 . 
     In operation of the system  10 , an operational vacuum is maintained in the vacuum chamber  12  by way of any combination of rough and/or fine vacuum pumps  40 . In order to introduce a substrate  14  into the vacuum chamber  12 , the load module  28  and buffer module  30  are initially vented (with the valve  34  between the two modules in the open position). The valve  34  between the buffer module  30  and the first heater module  16  is closed. The valve  34  between the load module  28  and load conveyor  26  is opened and a substrate  14  is moved into the load module  28 . At this point, the first valve  34  is shut and the rough vacuum pump  32  then draws an initial vacuum in the load module  28  and buffer module  30 . The substrate  14  is then conveyed into the buffer module  30 , and the valve  34  between the load module  28  and buffer module  30  is closed. The fine vacuum pump  38  then increases the vacuum in the buffer module  30  to approximately the same vacuum in the vacuum chamber  12 . At this point, the valve  34  between the buffer module  30  and vacuum chamber  12  is opened and the substrate  14  is conveyed into the first heater module  16 . 
     An exit vacuum lock station is configured downstream of the last cool-down module  20 , and operates essentially in reverse of the entry vacuum lock station described above. For example, the exit vacuum lock station may include an exit buffer module  42  and a downstream exit lock module  44 . Sequentially operated valves  34  are disposed between the buffer module  42  and the last one of the cool-down modules  20 , between the buffer module  42  and the exit lock module  44 , and between the exit lock module  44  and an exit conveyor  46 . A fine vacuum pump  38  is configured with the exit buffer module  42 , and a rough vacuum pump  32  is configured with the exit lock module  44 . The pumps  32 ,  38  and valves  34  are sequentially operated to move the substrates  14  out of the vacuum chamber  12  in a step-wise fashion without loss of vacuum condition within the vacuum chamber  12 . 
     System  10  also includes a conveyor system configured to move the substrates  14  into, through, and out of the vacuum chamber  12 . In the illustrated embodiment, this conveyor system includes a plurality of individually controlled conveyors  48 , with each of the various modules including a respective one of the conveyors  48 . It should be appreciated that the type or configuration of the conveyors  48  may vary. In the illustrated embodiment, the conveyors  48  are roller conveyors having rotatably driven rollers that are controlled so as to achieve a desired conveyance rate of the substrates  14  through the respective module and the system  10  overall. 
     As described, each of the various modules and respective conveyors in the system  10  are independently controlled to perform a particular function. For such control, each of the individual modules may have an associated independent controller  50  configured therewith to control the individual functions of the respective module. The plurality of controllers  50  may, in turn, be in communication with a central system controller  52 , as diagrammatically illustrated in  FIG. 1 . The central system controller  52  can monitor and control (via the independent controllers  50 ) the functions of any one of the modules so as to achieve an overall desired heat-up rate, deposition rate, cool-down rate, conveyance rate, and so forth, in processing of the substrates  14  through the system  10 . 
     Referring to  FIG. 1 , for independent control of the individual respective conveyors  48 , each of the modules may include any manner of active or passive sensors  54  that detects the presence of the substrates  14  as they are conveyed through the module. The sensors  54  are in communication with the respective module controller  50 , which is in turn in communication with the central controller  52 . In this manner, the individual respective conveyor  48  may be controlled to ensure that a proper spacing between the substrates  14  is maintained and that the substrates  14  are conveyed at the desired conveyance rate through the vacuum chamber  12 . 
       FIGS. 2 through 5  relate to a particular embodiment of the vapor deposition apparatus  100 . Referring to  FIGS. 2 and 3  in particular, the apparatus  100  includes a deposition head  110  defining an interior space in which a receptacle  116  is configured for receipt of a granular source material (not shown) and dopant material. As mentioned, the granular source material may be supplied by a first feed device or system  24  ( FIG. 1 ) via a first feed tube  148  ( FIG. 4 ). Additionally, the dopant material may be supplied by a second feed device of system  25  via a second feed tube  149 . The first feed device  24  and second feed device  25  can be configured to control the supply rate of the source material and the dopant material, respectively, to the apparatus  100 . As shown, the first feed tube  148  and second feed tube  149  is connected to a distributor  144  disposed in an opening in a top wall  114  of the deposition head  110 . However, in an alternative embodiment, the first feed tube  148  and second feed tube could individually be connected to separate distributors (not shown). 
     Such a second feed tube  149  is particularly useful to supply the dopant material in a solid state when supplied to the receptacle  116 . The distributor  144  includes a plurality of discharge ports  146  that are configured to evenly distribute the granular source material and dopant material into the receptacle  116 . The receptacle  116  has an open top and may include any configuration of internal ribs  120  or other structural elements. 
     In the illustrated embodiments, at least one thermocouple  122  is operationally disposed through the top wall  114  of the deposition head  110  to monitor temperature within the deposition head  110  adjacent to or in the receptacle  116 . 
     The deposition head  110  also includes longitudinal end walls  112  and side walls  113  ( FIG. 5 ). Referring to  FIG. 5  in particular, the receptacle  116  has a shape and configuration such that the transversely extending end walls  118  of the receptacle  116  are spaced from the end walls  112  of the head chamber  110 . The longitudinally extending side walls  117  of the receptacle  116  lie adjacent to and in close proximation to the side walls  113  of the deposition head so that very little clearance exists between the respective walls, as depicted in  FIG. 5 . With this configuration, sublimated source material will flow out of the open top of the receptacle  116  and downwardly over the transverse end walls  118  as leading and trailing curtains of vapor  119  over, as depicted by the flow lines in  FIGS. 2 ,  3 , and  5 . Very little of the sublimated source material will flow over the side walls  117  of the receptacle  116 . The curtains of vapor  119  are “transversely” oriented in that they extend across the transverse dimension of the deposition head  110 , which is generally perpendicular to the conveyance direction of the substrates through the system. 
     A heated distribution manifold  124  is disposed below the receptacle  116 . This distribution manifold  124  may take on various configurations within the scope and spirit of the invention, and serves to indirectly heat the receptacle  116 , as well as to distribute the sublimated source material that flows from the receptacle  116 . In the illustrated embodiment, the heated distribution manifold  124  has a clam-shell configuration that includes an upper shell member  130  and a lower shell member  132 . Each of the shell members  130 ,  132  includes recesses therein that define cavities  134  when the shell members are mated together as depicted in  FIGS. 2 and 3 . Heater elements  128  are disposed within the cavities  134  and serve to heat the distribution manifold  124  to a degree sufficient for indirectly heating the source material within the receptacle  116  to cause sublimation of the source material. The heater elements  128  may be made of a material that reacts with the source material vapor and, in this regard, the shell members  130 ,  132  also serve to isolate the heater elements  128  from contact with the source material vapor. The heat generated by the distribution manifold  124  is also sufficient to prevent the sublimated source material from plating out onto components of the head chamber  110 . Desirably, the coolest component in the head chamber  110  is the upper surface of the substrates  14  conveyed therethrough so as to ensure that the sublimated source material plates onto the substrate, and not onto components of the head chamber  110 . 
     Still referring to  FIGS. 2 and 3 , the heated distribution manifold  124  includes a plurality of passages  126  defined therethrough. These passages have a shape and configuration so as to uniformly distribute the sublimated source material towards the underlying substrates  14  ( FIG. 4 ). 
     In the illustrated embodiment, a distribution plate  152  is disposed below the distribution manifold  124  at a defined distance above a horizontal plane of the upper surface of an underlying substrate  14 , as depicted in  FIG. 4 . This distance may be, for example, between about 0.3 cm to about 4.0 cm. In a particular embodiment, the distance is about 1.0 cm. The conveyance rate of the substrates below the distribution plate  152  may be in the range of, for example, about 10 mm/sec to about 40 mm/sec. In a particular embodiment, this rate may be, for example, about 20 mm/sec. The thickness of the CdTe film layer that plates onto the upper surface of the substrate  14  can vary within the scope and spirit of the invention, and may be, for example, between about 1 micron to about 5 microns. In a particular embodiment, the film thickness may be about 3 microns. 
     The distribution plate  152  includes a pattern of passages, such as holes, slits, and the like, therethrough that further distribute the sublimated source material passing through the distribution manifold  124  such that the source material vapors are uninterrupted in the transverse direction. In other words, the pattern of passages are shaped and staggered or otherwise positioned to ensure that the sublimated source material is deposited completely over the substrate in the transverse direction so that longitudinal streaks or stripes of “un-coated” regions on the substrate are avoided. 
     As previously mentioned, a significant portion of the sublimated source material will flow out of the receptacle  116  as leading and trailing curtains of vapor, as depicted in  FIG. 5 . Although these curtains of vapor will diffuse to some extent in the longitudinal direction prior to passing through the distribution plate  152 , it should be appreciated that it is unlikely that a uniform distribution of the sublimated source material in the longitudinal direction will be achieved. In other words, more of the sublimated source material will be distributed through the longitudinal end sections of the distribution plate  152  as compared to the middle portion of the distribution plate. However, as discussed above, because the system  10  conveys the substrates  14  through the vapor deposition apparatus  100  at a constant (non-stop) linear speed, the upper surfaces of the substrates  14  will be exposed to the same deposition environment regardless of any non-uniformity of the vapor distribution along the longitudinal aspect of the apparatus  100 . The passages  126  in the distribution manifold  124  and the holes in the distribution plate  152  ensure a relatively uniform distribution of the sublimated source material in the transverse aspect of the vapor deposition apparatus  100 . So long as the uniform transverse aspect of the vapor is maintained, a relatively uniform thin film layer is deposited onto the upper surface of the substrates  14  regardless of any non-uniformity in the vapor deposition along the longitudinal aspect of the apparatus  100 . 
     As illustrated in the figures, it may be desired to include a debris shield  150  between the receptacle  116  and the distribution manifold  124 . This shield  150  includes holes defined therethrough (which may be larger or smaller than the size of the holes of the distribution plate  152 ) and primarily serves to retain any granular or particulate source material from passing through and potentially interfering with operation of the movable components of the distribution manifold  124 , as discussed in greater detail below. In other words, the debris shield  150  can be configured to act as a breathable screen that inhibits the passage of particles without substantially interfering with vapors flowing through the shield  150 . 
     Referring to  FIGS. 2 through 4  in particular, apparatus  100  desirably includes transversely extending seals  154  at each longitudinal end of the head chamber  110 . In the illustrated embodiment, the seals define an entry slot  156  and an exit slot  158  at the longitudinal ends of the head chamber  110 . These seals  154  are disposed at a distance above the upper surface of the substrates  14  that is less than the distance between the surface of the substrates  14  and the distribution plate  152 , as is depicted in  FIG. 4 . The seals  154  help to maintain the sublimated source material in the deposition area above the substrates. In other words, the seals  154  prevent the sublimated source material from “leaking out” through the longitudinal ends of the apparatus  100 . It should be appreciated that the seals  154  may be defined by any suitable structure. In the illustrated embodiment, the seals  154  are actually defined by components of the lower shell member  132  of the heated distribution manifold  124 . It should also be appreciated that the seals  154  may cooperate with other structure of the vapor deposition apparatus  100  to provide the sealing function. For example, the seals may engage against structure of the underlying conveyor assembly in the deposition area. 
     Any manner of longitudinally extending seal structure  155  may also be configured with the apparatus  100  to provide a seal along the longitudinal sides thereof. Referring to  FIGS. 2 and 3 , this seal structure  155  may include a longitudinally extending side member that is disposed generally as close as reasonably possible to the upper surface of the underlying convey surface so as to inhibit outward flow of the sublimated source material without frictionally engaging against the conveyor. 
     Referring to  FIGS. 2 and 3 , the illustrated embodiment includes a movable shutter plate  136  disposed above the distribution manifold  124 . This shutter plate  136  includes a plurality of passages  138  defined therethrough that align with the passages  126  in the distribution manifold  124  in a first operational position of the shutter plate  136  as depicted in  FIG. 3 . As can be readily appreciated from  FIG. 3 , in this operational position of the shutter plate  136 , the sublimated source material is free to flow through the shutter plate  136  and through the passages  126  in the distribution manifold  124  for subsequent distribution through the plate  152 . Referring to  FIG. 2 , the shutter plate  136  is movable to a second operational position relative to the upper surface of the distribution manifold  124  wherein the passages  138  in the shutter plate  136  are misaligned with the passages  126  in the distribution manifold  124 . In this configuration, the sublimated source material is blocked from passing through the distribution manifold  124 , and is essentially contained within the interior volume of the head chamber  110 . Any suitable actuation mechanism, generally  140 , may be configured for moving the shutter plate  136  between the first and second operational positions. In the illustrated embodiment, the actuation mechanism  140  includes a rod  142  and any manner of suitable linkage that connects the rod  142  to the shutter plate  136 . The rod  142  is rotated by any manner of mechanism located externally of the head chamber  110 . 
     The shutter plate  136  configuration illustrated in  FIGS. 2 and 3  is particularly beneficial in that, for whatever reason, the sublimated source material can be quickly and easily contained within the head chamber  110  and prevented from passing through to the deposition area above the conveying unit. This may be desired, for example, during start up of the system  10  while the concentration of vapors within the head chamber builds to a sufficient degree to start the deposition process. Likewise, during shutdown of the system, it may be desired to maintain the sublimated source material within the head chamber  110  to prevent the material from condensing on the conveyor or other components of the apparatus  100 . 
     Referring to  FIGS. 4 and 6 , the vapor deposition apparatus  100  may further comprise a conveyor  160  disposed below the head chamber  110 . This conveyor  160  may be uniquely configured for the deposition process as compared to the conveyors  48  discussed above with respect to the system  10  of  FIG. 1 . For example, the conveyor  160  may be a self-contained conveying unit that includes a continuous loop conveyor on which the substrates  14  are supported below the distribution plate  152 . In the illustrated embodiment, the conveyor  160  is defined by a plurality of slats  162  that provide a flat, unbroken (i.e., no gaps between the slats) support surface for the substrates  14 . The slat conveyor is driven in an endless loop around sprockets  164 . It should be appreciated, however, that the invention is not limited to any particular type of conveyor  160  for moving the substrates  14  through the vapor deposition apparatus  100 . 
     The present invention also encompasses various process embodiments for vapor deposition of a sublimated source material to form a thin film on a PV module substrate. The various processes may be practiced with the system embodiments described above or by any other configuration of suitable system components. It should thus be appreciated that the process embodiments according to the invention are not limited to the system configuration described herein. 
     In a particular embodiment, the vapor deposition process includes supplying source material to a receptacle within a deposition head, and indirectly heating the receptacle with a heat source member to sublimate the source material. The sublimated source material is directed out of the receptacle and downwardly within the deposition head through the heat source member. Individual substrates are conveyed below the heat source member. The sublimated source material that passes through the heat source is distributed onto an upper surface of the substrates such that leading and trailing sections of the substrates in the direction of conveyance thereof are exposed to the same vapor deposition conditions so as to achieve a desired uniform thickness of the thin film layer on the upper surface of the substrates. 
     In a unique process embodiment, the sublimated source material is directed from the receptacle primarily as transversely extending leading and trailing curtains relative to the conveyance direction of the substrates. The curtains of sublimated source material are directed downwardly through the heat source member towards the upper surface of the substrates. These leading and trailing curtains of sublimated source material may be longitudinally distributed to some extent relative to the conveyance direction of the substrates after passing through the heat source member. 
     In yet another unique process embodiment, the passages for the sublimated source material through the heat source may be blocked with an externally actuated blocking mechanism, as discussed above. 
     Desirably, the process embodiments include continuously conveying the substrates at a constant linear speed during the vapor deposition process. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.