Patent Publication Number: US-2012028395-A1

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

Description:
FIELD OF THE INVENTION 
     The subject matter disclosed herein relates generally to methods and systems for depositing thin films during manufacture of cadmium telluride photovoltaic devices. More particularly, the subject matter disclosed herein relates generally to integrated systems for the deposition of a cadmium telluride layer and subsequent cadmium chloride treatment during manufacture of cadmium telluride photovoltaic devices, and their methods of use. 
     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 to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy 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 cloudy conditions as compared to other conventional materials. 
     The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current. 
     During the production of CdTe PV modules, the surface of the CdTe PV module is typically cooled, transported to a subsequent treatment apparatus for cadmium chloride treatment (e.g., a cadmium chloride wash), and then subsequently annealed. This process of heating, cooling, and re-heating is inefficient in both energy consumption and cost. Additionally, the cadmium telluride layer is exposed to the environment during transport to the subsequent treatment apparatus. Such exposure can result in the introduction of additional atmospheric materials into the cadmium telluride layer, which can lead to the introduction of impurities in the CdTe PV module. Additionally, the room atmosphere naturally varies over time, adding a variable to a large-scale manufacturing process of the CdTe PV modules. Such impurities and additional variables can lead to inconsistent CdTe PV modules from the same manufacturing line and process. 
     Thus, a need exists for methods and systems for reducing the introduction of impurities and additional variables into a large-scale manufacturing process of making the CdTe PV modules, as well as increasing the energy efficiency of the process. 
     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. 
     An integrated apparatus is generally provided for sequential vapor deposition of a sublimated source material as a thin film on a photovoltaic (PV) module substrate and vapor treatment of the thin film. The apparatus can include a load vacuum chamber, a first vapor deposition chamber; and a second vapor deposition chamber that are integrally connected such that substrates being transported through the apparatus are kept at a system pressure less than about 760 Torr. The load vacuum chamber can be connected to a load vacuum pump configured to reduce the pressure within the load vacuum chamber to an initial load pressure. A conveyor system can be operably disposed within the apparatus and configured for transporting substrates in a serial arrangement into and through load vacuum chamber, into and through the first vapor deposition chamber, and into and through the second vapor deposition chamber at a controlled speed. 
     Processes are also provided for manufacturing a thin film cadmium telluride thin film photovoltaic device. The substrate can be first transferred into a load vacuum chamber connected to a load vacuum pump, and a vacuum drawn in the load vacuum chamber using the load vacuum pump until an initial load pressure is reached in the load vacuum chamber. The substrate can then be transported from the load vacuum chamber into a first vapor deposition chamber. The first vapor deposition chamber comprises a source material (e.g., cadmium telluride), and a cadmium telluride layer can be deposited on the substrate by heating the source material to produce source vapors that deposit onto the substrate. The substrate can then be transported from the first vapor deposition chamber into a second vapor deposition chamber. The second vapor deposition chamber comprises a treatment material (e.g., cadmium chloride), and the cadmium telluride layer can be treated by heating the treatment material to produce treatment vapors that deposit onto the substrate. In the process, the substrate is transported through the first vapor deposition chamber and the second vapor deposition chamber at a system pressure that is less than about 760 Torr. 
     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 the embodiment of  FIG. 2  in cooperation with a substrate conveyor; 
         FIG. 5  is a top view of the receptacle component within the embodiment of  FIG. 2 ; and, 
         FIG. 6  represents a diagram of an exemplary process according to one embodiment of the present invention. 
     
    
    
     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. 
     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 at least two vapor deposition apparatus  100  ( FIGS. 2 through 5 ), sequentially positioned within the system 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”) and subsequent treatment. The thin film may be, for example, a film layer of cadmium telluride (CdTe), and the subsequent treatment may be, for instance, cadmium chloride treatment to the cadmium telluride film layer. It should be appreciated that the present system  10  is not limited to the vapor deposition apparatus  100  illustrated in  FIGS. 2-5 . Other vapor deposition apparatus may be used in the system  10  for vapor deposition of a thin film layer onto a PV module substrate  14 . 
     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  12  that includes a load vacuum chamber  28  and a load buffer chamber  30 . A “rough” (i.e., initial) vacuum pump  32  is configured with the load vacuum chamber  28  to drawn an initial load pressure, and a “fine” (i.e., final) vacuum pump  38  is configured with the load buffer chamber  30  to increase the vacuum (i.e. decrease the initial load pressure) in the load buffer chamber  30  to reduce the vacuum pressure within the entry vacuum lock station  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 vacuum chamber  28  and the load buffer chamber  30 , and between the load vacuum chamber  30  and the heating station  13 . 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 station  12  in a step-wise manner without affecting the vacuum within the subsequent heating station  13 . 
     In operation of the system  10 , an operational vacuum is maintained in the system  10  by way of any combination of rough and/or fine vacuum pumps  40 . In order to introduce a substrate  14  into the load vacuum station  12 , the load vacuum chamber  28  and load buffer chamber  30  are initially vented (with the valve  34  between the two modules in the open position). The valve  34  between the load buffer chamber  30  and the first heater module  16  is closed. The valve  34  between the load vacuum chamber  28  and load conveyor  26  is opened and a substrate  14  is moved into the load vacuum chamber  28 . At this point, the first valve  34  is shut and the rough vacuum pump  32  then draws an initial vacuum in the load vacuum chamber  28  and load buffer chamber  30 . The substrate  14  is then conveyed into the load buffer chamber  30 , and the valve  34  between the load vacuum chamber  28  and load buffer chamber  30  is closed. The fine vacuum pump  38  then increases the vacuum in the load buffer chamber  30  to approximately the same vacuum in the heating station  13 . At this point, the valve  34  between the load buffer chamber  30  and heating station  13  is opened and the substrate  14  is conveyed into the first heater module  16 . 
     Thus, the substrates  14  are transported into the exemplary system  10  first through the load vacuum chamber  28  that draws a vacuum in the load vacuum chamber  12  to an initial load pressure. For example, the initial load pressure can be less than about 250 mTorr, such as about 1 mTorr to about 100 mTorr. Optionally, a load buffer chamber can reduce the pressure to about 1×10 −7  Torr to about 1×10 −4  Torr, and then backfilled with an inert gas (e.g., argon) in a subsequent chamber within the system  10  (e.g., within the sputtering deposition chamber  112 ) to a deposition pressure (e.g., about 10 mTorr to about 100 mTorr). 
     The substrates  14  can then be transported into and through a heating station  13  including heating chambers  16 . The plurality of heating chambers  16  define a pre-heat section  13  of the system  10  through which the substrates  14  are conveyed and heated to a first deposition temperature before being conveyed into the vapor deposition chamber  19 . Each of the heating chambers  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 heating chambers  16  can heat the substrates  14  to a deposition temperature, such as about 350° C. to about 600° C. Although shown with three heating chambers  16 , any suitable number of heating chambers  16  can be utilized. 
     The substrates  14  can then be transferred into and through the first vapor deposition chamber  19  for deposition of a thin film onto the substrates  14 , such as a cadmium telluride thin film. The first vapor deposition chamber  19  can include the deposition apparatus  100 , such as shown in  FIGS. 2-5  and discussed in greater detail below. As diagrammatically illustrated in  FIG. 1 , a first feed device  24  is configured with the vapor deposition apparatus  100  to supply source material, such as granular cadmium telluride. The 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 . 
     After deposition of the thin film in the first vapor deposition chamber  19 , the substrates  14  can be transported into and through a second vapor deposition chamber  21  for subsequent vapor treatment of the thin film. The second vapor deposition chamber  21  can also include the deposition apparatus  100 , such as shown in  FIGS. 2-5 . As diagrammatically illustrated in  FIG. 1 , a second feed device  25  is configured with the vapor deposition apparatus  100  to supply treatment material, such as cadmium chloride. The second 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 . Thus, the cadmium telluride thin film on the substrates  14  can be treated with cadmium chloride within the system  10  without prior exposure to the outside environment. 
     Between the first vapor deposition chamber  19  and the second vapor deposition chamber  21  a heating chamber, the substrates  14  can be transported into and through a post-heat chamber  22  and first cooling chamber  23 . In the illustrated embodiment of system  10 , at least one post-heat chamber  22  is located immediately downstream of the vapor deposition apparatus  100  and upstream of the second vapor deposition chamber  21  in a conveyance direction of the substrates  14 . The post-heat chamber  22  maintains a controlled heating profile of the substrate  14  until the entire substrate is moved out of the first vapor deposition chamber  19  to prevent damage to the substrate  14 , 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. 
     Then, the substrates  14  can be cooled in the first cooling chamber  23  to a vapor treatment temperature prior to entering the second vapor deposition chamber  21 . For example, first cooling chamber  23  can subsequently cool the substrates to a vapor treatment temperature that is less than the deposition temperature prior to entering the second vapor deposition chamber  21 . The treatment temperature can be, for instance, about 20° C. to about the anneal temperature discussed below. 
     The substrates  14  can be transported from the second vapor deposition chamber  21  into the anneal chamber  27  heated by heater  18 . The substrates  14  can be annealed in the anneal chamber  27  by heating to an anneal temperature of 350° C. to about 500° C. after treatment of the cadmium telluride layer with the cadmium chloride vapors, such as about 375° C. to about 450° C. or about 390° C. to about 420° C. 
     A cool-down chamber  20  is positioned downstream of the first vapor deposition chamber  19  and the second deposition chamber  21 . The cool-down chamber  20  allow the substrates  14  having the treated thin film are conveyed and cooled at a controlled cool-down rate prior to the substrates  14  being removed from the system  10 . The cool down chamber  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 chamber  20 . In other embodiments, a plurality of cool down chambers  20  can be utilized in the system  10 . 
     An exit vacuum lock station  15  is configured downstream of the cool-down chamber  20 , and operates essentially in reverse of the entry vacuum lock station  12  described above. For example, the exit vacuum lock station  15  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 system  10  in a step-wise fashion without loss of vacuum condition within the system  10 . 
     System  10  also includes a conveyor system configured to move the substrates  14  into, through, and out of each of load vacuum station  12 , the pre-heating station  12 , the first vapor deposition chamber  19 , the post-heat chamber  22 , the first cooling chamber  23 , the second vapor deposition chamber  21 , the annealing chamber  27 , and the second cooling chamber  20 . 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 , that can be utilized in either or both of the first vapor deposition chamber  19  and/or the second vapor deposition chamber  21 . 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 source material (not shown) or treatment material. As mentioned, the source material or treatment material may be supplied by a feed device or system  24 ,  25 , respectively, via a feed tube  148  ( FIG. 4 ). The feed tube  148  is connected to a distributor  144  disposed in an opening in a top wall  114  of the deposition head  110 . The distributor  144  includes a plurality of discharge ports  146  that are configured to evenly distribute the granular source 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 embodiment, 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  FIG. 4 , 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, and subsequent vapor treatment. 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. 
     For example,  FIG. 6  shows an exemplary diagram of a process  600  where the substrate can be subjected to a load vacuum at  602 , and heated to a deposition temperature at  604 . Cadmium telluride can then be deposited onto the substrate (e.g., onto a cadmium sulfide layer on the substrate) to form a cadmium telluride layer at  606 . The substrate can then be heated or cooled and subjected to a buffer vacuum in steps  608 ,  610 , and  612 . For example, the buffer vacuum of  610  can separate the cadmium telluride source material from intermixing with the cadmium chloride treatment. The cadmium telluride layer can be treated with cadmium chloride at  614 , and subsequently annealed at  616 . Finally, the substrate can be cooled at  618  and then exited from the system  620 . 
     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.