Patent Publication Number: US-7220321-B2

Title: Apparatus and processes for the mass production of photovoltaic modules

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of application Ser. No. 10/200,265 filed on Jul. 22, 2002 now abandoned, which is in turn a divisional application of prior application Ser. No. 09/583,381 filed on May 30, 2000, now issued as U.S. Pat. No. 6,423,565. 

   GOVERNMENT SUPPORT  
   This invention was made with Government support under grants awarded by the National Science Foundation and the Department of Energy. The Government has certain rights in this invention. 

   FIELD OF THE INVENTION  
   The present invention relates to apparatus and processes for the mass production of low cost photovoltaic modules and, more specifically to an inline continuous vacuum apparatus and process for fabricating the critical semiconductor layers, which together with attendant non-vacuum processes, are all accomplished at high throughput. 
   BACKGROUND OF THE INVENTION  
   Photovoltaic (PV) modules are used to generate electricity from sunlight by the photovoltaic effect. It has been recognized for decades that if these modules could be mass produced at low cost, they could be used to meet a considerable portion of the world&#39;s energy needs. Major companies, such as Royal Dutch/Shell and BP-Amoco, have stated that PV modules have the potential to become a major energy source and that their use has significant benefits to the global environment. However, for these benefits to be realized, PV modules must be produced at many times the current volume and at costs below $100/m 2 , as discussed by Bonnet et. al. in “Cadmium-telluride material for thin film solar cells”, J. Mater. Res., Vol. 13, No. 10 (1998). Currently, PV modules are manufactured in small quantities at costs of about $500/m 2 . About one hundred times the current yearly production is required to sustain a PV module manufacturing capacity that can contribute just 5% of the current electricity generated. Consequently, the manufacturing volume of PV modules needs to be greatly increased and costs significantly reduced. 
   To realize the required increases in production volume and decreases in manufacturing costs, PV modules must be produced as a commodity. Commodity level manufacturing requires innovation to develop highly automated production processes and equipment, which are designed to specifically fabricate the commodity product. Commodity manufacturing necessitates high production speeds (high throughput), minimal labor costs, and a continuous process flow. Low capital costs and ease of expanding production capacity also facilitate commodity manufacturing. There are a variety of known PV devices, but only the cadmium telluride (CdTe) thin film PV device has the potential to satisfy the requirements for commodity manufacturing. 
   Since 1974, there have been many industrial efforts to create technologies for CdTe PV module manufacturing. Most of these industrial efforts, as exemplified by the teachings of U.S. Pat. Nos. 4,319,069, 4,734,381, and 5,501,744, have been terminated because of fundamental inadequacies in their manufacturing technologies. To date, no technology suitable for commodity level manufacturing of CdTe PV modules has been developed, thus demonstrating the need for innovation in this area. 
   The most common CdTe PV cells are thin film polycrystalline devices, in which the CdTe layer is paired with a cadmium sulfide (CdS) layer to form a heterojunction. The thin films of a CdS/CdTe PV device can be produced through a variety of vacuum and non-vacuum processes. Of the many types of thin film deposition methods, sublimation in vacuum is most amenable to commodity manufacturing. This is because vacuum sublimation of CdS/CdTe PV modules exhibits deposition rates 10 to 100 times higher than any other PV module deposition method. Vacuum sublimation of the semiconductor layers for CdS/CdTe PV modules can also be performed in modest vacuum levels and does not require costly high vacuum equipment. Vacuum deposition methods for other thin film PV devices require costly, complex high vacuum equipment and results in low throughput. 
   Due to the high rate of deposition and low capital cost, the CdS/CdTe thin film cell fabricated by vacuum sublimation is the most suitable for commodity level manufacturing of PV modules. However, cadmium is a Group B carcinogen. According to U.S. government regulations, the quantity of this material which can be lawfully released into the environment or into an occupational setting is extremely small. The known prior art in CdS/CdTe vacuum sublimation requires process and hardware innovations to achieve occupational and environmental safety as required by federal regulations, as well as commodity scale manufacturing. 
   One known configuration for a CdTe device is the back wall configuration, in which the thin films are deposited onto a glass superstrate, hereinafter referred to as a substrate. The CdTe device is most often fabricated on a glass substrate coated with a transparent conductive oxide (TCO) film onto which other film layers are deposited in the following order: a) a CdS film, b) a CdTe film, c) an ohmic contact layer, and d) a metal film. Along with the deposition of these films, many heat treatments are also needed to enhance the device properties. The TCO and the metal films form the front and back electrodes, respectively. The CdS layer (n-type) and the CdTe layer (p-type) form the p/n junction of the device. The cells are deployed with the substrate facing the sun. Photons travel through the glass and TCO film before reaching the p/n junction of the device. A module is formed by interconnecting individual cells in series to produce a useful voltage. 
   Thus, a process for manufacturing CdS/CdTe modules includes the following steps: 1) cleaning the TCO coated glass substrates, 2) heating the substrates, 3) depositing an n-type CdS layer, 4) depositing a p-type CdTe layer, 5) performing a CdCl 2  treatment to improve CdTe grain structure and electrical properties, 6) forming a p+ ohmic low resistance contact layer to improve current collection from the CdTe, 7) depositing a metal layer (metallization) to form the back electrode, 8) scribing the film layers into individual cells, 9) interconnecting the cells in series and providing a means of electrical connection to the module, and 10) encapsulating the finished module. 
   All of the prior art methods for the production of CdTe modules have limitations that render them unsuitable for commodity level manufacturing. For example, prior art methods of CdCl 2  treatment are disconnected, low throughput batch operations, rather than continuous flow processes. These batch type processes are inefficient and involve extremely high costs in order to increase throughput to the commodity manufacturing level. Most of the known methods of CdCl 2  treatment also require rinsing, which generates liquid wastes that contain cadmium. Known methods of ohmic contact formation are also batch type processes that exhibit low throughput rates. Prior art metallization steps also exhibit low throughput and require costly process equipment. It is necessary to improve the current methods of CdCl 2  treatment, ohmic contact formation, and metallization in order to achieve high throughput continuous processes. 
   Prior art methods for scribing the layers to form a module include laser scribing, mechanical scribing, and abrasive blasting. Known laser scribing methods used in the PV industry are associated with low production speed and high capital cost. Laser scribing was abandoned recently in one industrial setting due to laser equipment failure as discussed by Borg in, “Commercial Production of Thin-Film CdTe Photovoltaic Modules”, NREL/SR-520-23733, October 1997. Known mechanical and abrasive blast scribing methods have only been shown on a small scale as typified by U.S. Pat. No. 5,501,744 to Albright and require innovation and improvement to be suitable for commodity level manufacturing. 
   Specific examples of prior art relating to CdS deposition and CdTe deposition performed by vacuum sublimation are described in detail below. The other prior art steps that are necessary to form a complete CdTe PV module are also discussed below. 
   One known vacuum method of producing CdTe solar cells by vacuum sublimation is taught in U.S. Pat. No. 5,536,333 to Foote et. al. This method is further described by Sasala et. al. in “Technology Support for Initiation of High-Throughput Processing of Thin-Film CdTe PV Modules”, NREL/SR-520-23542, pp. 1–2, (1997). These references discuss a technique known as vapor transport deposition (VTD), which involves heating of the semiconductor materials in a contained vessel in order to create vapor. An inert carrier gas, such as nitrogen, transports the vapor of the semiconductor to the substrate through heated conduits. The substrate is held horizontally in a heated environment and supported from beneath by ceramic rollers in the heated environment. The deposition of the semiconductor is made onto the top surface of the substrate. In accordance with this prior art method, the ceramic rollers prevent the glass substrate from sagging under its own weight due to the elevated temperatures involved. 
   The entire VTD method is very complex and costly. It is possible to deposit a complete CdTe solar cell in a very short time and at sufficiently low substrate temperatures to eliminate glass sagging completely or reduce it to a very small acceptable value. Thus, the expensive ceramic rollers of the VTD method are not needed. Reloading starting material may also be performed in a much simple manner than as shown in this prior art. Since the films are thin, only small amounts of material are required to form them. Consequently, only very small volumes of starting material are needed for many days of operation, thus eliminating the need for this complex reloading arrangement. The heated vessels of this method contain toxic vapors, which pose significant occupational safety problems when they are opened for reloading during processing. In the VTD method, vapors are transported through long distances in a carrier gas, an arrangement which will likely lead to the formation of very small nano-particles through condensation of the vapors. These nano-particles degrade the film qualities and lead to occupational hazards when the system is serviced. Furthermore, in the VTD method, the continuous flow of carrier gas has to be maintained along the substrate. Any CdS or CdTe vapors that are carried past the substrate will be wasted. Any deposits of waste material on the inner surface of the vacuum chamber, pumps, exhaust, etc. must be cleaned, thereby exposing maintenance workers to toxic materials and raising occupational safety issues. In order to prevent unwanted condensation of CdS and CdTe vapors, the VTD method also requires continued heating of large portions of the equipment, including the vaporization vessel itself, the conduits, the deposition chamber, etc. This wastes energy and increases the capital costs. The VTD method is only used for depositing the p/n junction layers. Other processing steps, such as the CdCl 2  treatment, ohmic contact formation, and metallization are inherently low throughput batch processes. Scribing is taught to be either laser scribing or photolithography, both of which are slow and costly processes. 
   In another prior art reference entitled “The CdTe Thin Film Solar Cell,” International Journal of Solar Energy, vol. 12, 1992, Bonnet proposes an inline production method for fabricating CdS/CdTe layers using a close-spaced sublimation (CSS) type deposition process. This prior art reference describes inline deposition within one vacuum boundary for only the steps of substrate heating, CdS deposition, and CdTe deposition. The other steps of CdCl 2  treatment, ohmic contact formation, and metallization are not shown as part of a continuous inline vacuum process, and are presumably performed by previously known methods. As described above, known methods for performing these steps have limitations. The Bonnet method does allow multiple substrate processing of more than one film. However, it is not clear if long term operation and uniformity of deposition over time and across large substrates are achieved. Furthermore, the Bonnet method relies on CSS, which by definition, requires a space of 2–3 mm between the source and the substrate. This gap will allow a vapor leak at the edge of the substrate. As the source charge sublimes away over time the gap will increase. It is the present inventors&#39; experience that this gap and the associated vapor leak causes non-uniform deposition on the substrates and also results in condensation of toxic materials on unwanted areas on the inner surfaces of the process chamber. This leak will be reduced if the background chamber pressure is held high enough to decrease the mean free path between gas molecules in the process chamber. However, higher pressures lead to lower deposition rates and greater nano-particle formation. An operating pressure of 750 millitorr is specified. At these pressures, nano-particles will be formed, since the vapor will homogeneously condense in the ambient gas near the edge of the deposition space. These very small particles degrade film quality and pose a health hazard to workers during routine maintenance inside the vacuum chamber. 
   Each of the individual prior art process steps required to produce a CdTe PV module exhibits limitations, as outlined above. In addition, nothing in the prior art describes an overall process to perform the series of steps of substrate heating, depositing an n-type CdS layer, depositing a p-type CdTe layer, performing a CdCl 2  treatment, and forming an ohmic contact inline, continuously, and in one vacuum boundary. In particular, the steps of CdCl 2  treatment and ohmic contact formation require significant innovation before they can be included in a continuous inline vacuum process. Such a continuous inline vacuum process would have significant advantages for commodity manufacturing of CdTe PV modules. 
   Any vacuum process for manufacturing CdTe PV modules would also require an apparatus to transport substrates through the process steps within vacuum and to transport the substrates into and out of vacuum rapidly. This apparatus should be robust, simple, and low cost. The apparatuses described in the prior art simply do not meet these requirements. 
   BRIEF SUMMARY OF THE INVENTION  
   The present invention is directed to commodity scale manufacturing of CdTe PV modules and involves innovations in both manufacturing processes and hardware. 
   One aspect of the present invention involves providing all of the processing steps for the critical semiconductor layers of a CdTe PV device inline, with all of those steps being completely performed within one vacuum boundary, at high throughput. They include rapid substrate heating, deposition of CdS, deposition of CdTe, CdCl 2  treatment, and ohmic contact formation. These steps are performed at modest vacuum pressures without requiring costly high vacuum equipment. By utilizing inline vacuum processing to form all of the critical layers without breaking vacuum, an improvement in process throughput, film quality, device efficiency, and device stability is realized, while at the same time avoiding pinhole formation. All inline vacuum processing is also advantageous in that it limits the production of toxic waste and environmental and occupational exposure to toxic compounds. 
   Another aspect of the present invention is a novel CdCl 2  treatment step, which may be performed in the same vacuum boundary and inline with the other fabrication steps. Our CdCl 2  treatment process has the advantage of a high throughput rate while producing stable, high efficiency devices and, at the same time, limiting environmental and occupational exposure to toxic compounds. 
   Another aspect of the present invention is a novel vacuum process to produce a p+ ohmic contact region by subliming a metal salt onto a CdTe layer. Our ohmic contact formation process has the advantage of producing, at high throughput rates, a low resistance ohmic contact that is stable over time. 
   Another aspect of the present invention is a unique high throughput, low cost spray process to form the back electrode. This process has the advantage of producing a durable, high conductivity back electrode using known low cost industrial spray methods. 
   Another aspect of the present invention is to provide novel high throughput, low cost processes to perform module scribing using abrasive blasting or mechanical brushing through a mask. These novel scribing processes have the advantage of selectively scribing the semiconductor layers without scribing the TCO layer. In addition, the scribe process may be adjusted so that the TCO layer may be removed as well. 
   Another aspect of the present invention is a novel vacuum process station to selectively heat substrates and films, to expose substrates and films to vapor, to deposit thin films on a substrate, and to strip thin films off of a substrate. This vacuum process station allows substrates to be transported into and out of vacuum. When used as a deposition source, our vacuum process station has the advantage of very minimal vapor leakage, which significantly reduces occupational exposure to toxic materials. This unique deposition source deposits very uniform layers and is suitable for long term continuous operation. 
   Another aspect of the present invention is a novel substrate transport apparatus, which is employed to move substrates within vacuum or, in combination with a unique opening, to transport substrates rapidly into and out of vacuum. This substrate transport apparatus is robust, simple, and low cost and has the additional advantage of not collecting any coatings as it moves substrates through the thin film deposition processes. 
   Yet another aspect of the present invention is to provide substrate cleaning in a clean mini-environment at the entrance to the inline continuous vacuum process, which has the advantage of greatly reduced cost resulting from the elimination of a clean room to contain the entire process line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic top plan view of a system for making photovoltaic modules in accordance with the present invention. 
       FIG. 2A  is a partially broken away schematic elevation view in longitudinal section along the line  50 — 50  of  FIG. 1 , illustrating a substrate transport apparatus and a plurality of vacuum processing stations. 
       FIG. 2B  is a partially broken away schematic elevation view in longitudinal section, illustrating an alternate embodiment of the substrate transport apparatus of  FIG. 2A . 
       FIG. 3  is a schematic elevation view in cross section along the line  60 — 60  of  FIG. 2A  illustrating the sealing arrangement by which the introduction of outside air into the vacuum chamber is minimized. 
       FIG. 4  is a partial schematic top plan view in longitudinal section along the line  70 — 70  of  FIG. 2A , further illustrating the sealing arrangement of  FIG. 3 . 
       FIG. 5  is a schematic elevation view along the line  100 — 100  of  FIG. 4 , further illustrating the substrate transport apparatus of  FIG. 2A . 
       FIGS. 6A–B  are schematic sectional elevation views illustrating enclosed and unenclosed substrates as they are positioned in prior art apparatuses. 
       FIG. 7A  is a schematic elevation view in cross section along the line  80 — 80  in  FIG. 2A , illustrating details of a vacuum process station of the substrate transport apparatus of  FIG. 2A . 
       FIG. 7B  is a schematic elevation view in cross section to illustrate an alternative embodiment of the vacuum process station of  FIG. 7A . 
       FIG. 8  is a schematic elevation view in cross section to illustrate a second alternate embodiment of the vacuum process station of  FIG. 7A . 
       FIG. 9A  is a schematic perspective view along the line  90 — 90  of  FIG. 1 , illustrating the way in which photovoltaic modules are scribed. 
       FIG. 9B  is a schematic perspective view along the line  90 — 90  of  FIG. 1 , illustrating an alternative way to scribe photovoltaic modules. 
       FIG. 10A  is a flow chart illustrating the process steps of the present invention. 
       FIG. 10B  is a flow chart of some of the alternate embodiments of the process steps of  FIG. 10A . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
   Referring now to  FIG. 1 , there is shown an overview of the preferred embodiment of an apparatus  1000  for producing photovoltaic modules in accordance with the present invention. A substrate cleaning station  55  cleans the commercially available substrates that typically comprise low cost soda lime glass coated with transparent conducting oxide (TCO). After cleaning, a pair of metal strip belts  1  transport the substrates through the next portion of the apparatus  1000 . The belts may be moved bidirectionally by means of a known stepper motor  7 . Precise indexing of the position of the pair of belts  1  is accomplished by the use of a commercially available stepper motor controller  8 . The pair of metal strip belts  1  pass through a pair of openings  2  and a vacuum chamber  3 . A plurality of process stations for the processing of the critical semiconductor layers are contained within the vacuum chamber  3 , and are described in detail below. Another series of process stations are linked by a plurality of conveyor belts  51 . These process stations do not require vacuum. A first spray process station  52   a  is followed by a first film scribing process station  53   a . These process stations are followed by a second spray process station  52   b  and a second film scribing process station  53   b . The final process station in the apparatus  1000  is an encapsulation station  56 . The description of the apparatus and processes of the instant invention is presented in the following order: the substrate transport apparatus and openings, the vacuum process stations, the metallization method, the film scribing method, and the process steps to produce a photovoltaic module. Following the description of the preferred embodiment is a description of alternate embodiments. 
     FIGS. 2A ,  3 ,  4 , and  5  in combination illustrate the complete assembly of the preferred embodiment of a pair of air-to-vacuum-to-air (AVA) openings and the substrate transport system of the present invention. The pair of AVA openings  2  facilitate the rapid transport of substrates from air into vacuum and then back into air.  FIG. 2A  illustrates vacuum chamber  3 , which is pumped to a suitable vacuum by known techniques. A pair of AVA openings  2  is shown on either side of the vacuum chamber  3 . A pair of continuous metal strip belts  1  pass through both of the AVA openings  2  and also through the entire length of the vacuum chamber  3 . A plurality of substrates  10 , which are carried on the twin metal belts  1 , are also shown. The AVA openings  2  are shown with a plurality of pockets  14  and a plurality of vacuum pump ports  15  for vacuum pumping. A pair of injection ports  11  are shown for the injection of process gas. A plurality of pulleys  4  are provided for moving the belts  1 . A belt tensioner  9  maintains the belts  1  at a predetermined tension. 
     FIG. 3  shows the details of the AVA openings  2  in cross section. A top channel  2   a  and a bottom plate  2   b  are split by a belt bearing plate  20 . In total, parts  2   a ,  2   b , and  20  form the cross section of the opening  2  of  FIGS. 2A ,  3 , and  4 . A clearance distance  21  on the order of 0.005 in. ±0.002 in. is formed around the substrate  10 . This clearance distance  21  is formed between the substrate  10  and the top channel  2   a  and between the substrate  10  and the belt bearing plate  20 . The clearance distance  21  is large enough so that the substrate  10  may move freely, but is small enough to provide resistance to fluid flow through the clearance distance  21 . This resistance to fluid flow allows the vacuum chamber  3  to be maintained at a desired vacuum level. The pair of continuous metal strip belts  1  provide a substrate carrier to convey the plurality of substrates  10 . 
     FIG. 5  is a detailed view of a pair of tabs  6  which are attached to the metal strip belt  1 . The tabs  6 , a plurality of which are placed periodically along the belt  1 , provide a means of locating the substrate  10 . 
   The pair of continuous metal strip belts  1  provide a substrate carrier that will not be coated by the vacuum deposition processes. The elimination of coating on the belt  1  is a significant improvement over the prior art. As exemplified by Charschan in U.S. Pat. No. 3,294,670 (1966), the prior art utilized substrate carriers which enclosed the perimeter of the substrate in a “picture frame” type arrangement. The picture frame type substrate carriers are necessarily coated during deposition. These coatings lead to jamming in the close tolerance seals during motion, present occupational safety issues due to particle generation, and result in cross contamination between the processes. In the present invention, these problems have been addressed by exposing only the substrates  10  to coating during a vacuum deposition. In addition, unlike all of the prior art, the clearance distance  21  is not formed between the substrate carrier and the openings. Rather, the clearance distance  21  is formed between the precisely cut substrate  10  and the AVA openings  2 . The clearance  21  is formed specifically between the substrate  10  and the top channel  2   a  and between the substrate  10  and the linear bearing plate  20 . 
   The use of the pair of metal strip belts  1  as the substrate carrier in accordance with the present invention has many other advantages. The fact that the belts  1  have a low mass will create less wear on any bearing surface used. The low mass will also greatly decrease the inertia of the substrate carrier, thus allowing the substrate to be moved quickly between processing stations. 
   Continuous metal strip belts  1  also have advantages in situations calling for a series of vacuum processes to be carried out in one vacuum chamber  3 . Continuous metal strip belts  1  of 200 feet in length can be fabricated by known methods. This allows a large number of process steps to be carried out in one vacuum chamber  3 . 
   Another advantage is the cost of fabrication of this type of substrate carrier, which is considerably less than the cost of machining close tolerance picture frame type substrate carriers of the prior art. The present substrate carriers also facilitate scaling up the process. For larger substrates  10 , the openings  2  must be increased in size. However, the strip belts  1  may be simply spaced further apart. 
   The continuous metal strip belts  1  are preferably formed of metal alloys that have high resistance to corrosion and that retain high strength at elevated temperatures. These belts may be coated with other materials to alter the surface properties further, if needed. Many different attachments or tabs  6  for retaining the substrate  10  may be welded or fixed to the metal strip belts  1  at relatively low cost. 
   A cut tolerance of ±0.003 inch is required for the glass substrates  10  which are carried on the continuous metal strip belts  1 . High throughput, low cost equipment is commercially available for cutting the substrates  10  to the required tolerances. The precision glass cutting required for this invention is not an additional processing step, since the substrates  10  must be cut to size in any method. 
   The continuous metal strip belts  1  are also relatively small in cross section. The small cross section decreases the thermal expansion as the belts pass through high temperature vacuum processes, which will decrease any tendency to jam in the AVA openings  2  due to thermal expansion. 
   The relatively low mass of the metal strip belts  1  also allows the substrate carrier to cool and heat more rapidly than the prior art picture frame type substrate carrier. The low mass of the metal strip belts  1  allows the belt temperature to match the glass temperature during processing, thereby reducing thermal gradients in the glass substrates  10  which could otherwise lead to stresses that may crack the glass. 
   Referring now to  FIG. 2A , the vacuum chamber  3  and a plurality of vacuum processing stations, generally indicated by  200  and  300 , are illustrated in a partial schematic elevation view in section. The metal strip belts  1  carry the plurality of substrates  10  through the plurality of vacuum processing stations  200  and  300 . The process stations  300  provide for cooling of substrates  10  by radiation to a pair of water cooled plates  34 . 
   Each of the process stations  200  of  FIG. 2A  contain a pair of heated pockets  33 . These heated pockets  33  provide a flux of heat or vapor and are suitably shaped blocks of a material with a high thermal conductivity. In  FIG. 7A  the pair of heated pockets  33  are shown with details of the pair of continuous metal strip belts  1  in cross section. Both upper and lower heated pockets  33  have a machined pocket  29 . A deposition material  35  is shown in the pocket  29  of the lower heated pocket  33 . Only the substrate  10  spans the pocket  29 . No part of the belts  1  crosses the pocket  29 , thus avoiding deposition anywhere but on the substrate  10 . The upper heated pocket  33  serves as a heater for the substrate  10 . 
   Heated pocket  33  may be fabricated using any material which has an acceptable level of thermal conductivity. Further, that material should have a low level of porosity to prevent adsorption of air and water vapor and a low level of impurities. One material which has proven useful is purified pyrolitic grade graphite. However, metals and metals coated with ceramics, as well as other suitable materials, may be chosen. 
   In the preferred embodiment of the invention, the heated pockets  33  are heated by a plurality of quartz halogen lamps  38 , which are not damaged when used in vacuum at high temperatures. The temperature control of the heated pocket  33  may be accomplished by placing a thermocouple in the wall of the heated pocket  33  and using well known proportional with integral and derivative (PID) temperature control of the power to the lamps  38 . Lamps  38  are contained in boxes  39  for protection. Boxes  39  also serve as support structure for the lamps  38  and the heated pocket  33 . The boxes  39  also provide electrical isolation for safety, prevent arcing from the lamps  38 , and also serve as radiation shields to reduce the loss of energy. Other means of heating the heated pocket  33 , including resistive heating and inductive heating, for example, may be used as well. 
   Each of the process stations  200  may perform a variety of processes on a substrate  10 . These processes include: i) rapidly heating a substrate, ii) annealing a substrate and films, iii) thermally stripping a film from a substrate, iv) exposing a substrate and films to a vapor treatment, or v) depositing a thin film on a substrate. The chosen processes performed by station  200  depend on three parameters: a) the temperature of the lower heated pocket  33 , b) the temperature of the upper heated pocket  33 , and c) the presence or absence of the deposition material  35 . For processes i) through iii) described above, no deposition material  35  is loaded into the pocket  29 . For process i) the upper and lower heated pockets  33  are maintained at temperatures such that the rapid heating of glass substrates  10  may be done in a short cycle time, on the order of one minute. The uniformity of heating prevents cracking of the substrates  10 . For process ii), the upper and lower heated pocket  33  and substrate  10  are maintained at a temperature which will not resublime thin films off of the substrate  10 . For process iii) the upper and lower heated pocket  33  and substrate  10  are maintained at a temperature that is high enough to cause films on the substrate  10  to sublime away. 
   For processes iv) and v) described above, deposition material  35  is loaded into the pocket  29 . For process iv), the upper heated pocket  33  and substrate  10  are maintained at a higher temperature than the lower heated pocket  33 . In this manner, the deposition material  35  will sublime in the pocket  29  of the lower heated pocket  33  and expose the lower surface of the substrate  10  to vapor, but the vapor will not be deposited as a film on the substrate  10 . For process v), the lower heated pocket  33  is maintained at a relatively higher temperature than the substrate  10  and the upper heated pocket  33 . In this manner, the deposition material  35  will sublime in the pocket  29  of the lower heated pocket  33 , and the vapor will condense and be deposited as a film on the lower surface of the substrate  10 . All of these processes are robust, may be performed at high throughput, are occupationally and environmentally safe, and involve low capital cost. 
   In accordance with the present invention, the pair of metal belts  1  transports the substrates  10  through the plurality of process stations  200  or  300 . The metal belts  1  and the substrates  10  may be moved continuously. However, the preferred embodiment of the present invention provides for intermittent motion of the metal belt  1  and the substrates  10 . This motion is accomplished using a conventional stepper motor  7  and a stepper motor motion controller  8 . The substrates  10  are placed at equal pitch distances on the belts  1 , and the process stations  200  or  300  are placed one pitch distance apart. The intermittent motion consists of pauses in the motion of the belts  1  for processing and brief periods of motion of the belts  1  to advance the plurality of substrates  10  through the series of process steps. In this manner, an individual substrate  10  will pause at each process station  200  or  300  for an equal amount of time. A useful period of time, hereinafter defined as the cycle time, consists of the total period of time for one pause and one movement of the belts  1 . One substrate  10  will complete the series of inline vacuum process steps for each period of cycle time. 
   Since the process stations  200  or  300  are modular, a plurality of one type of process may be placed in series. If a given process step requires a processing time which exceeds the cycle time, then the given process step may be carried out over a series of similar processing stations  200  or  300 . For example, an annealing step, which requires a longer period of time to complete than the cycle time, may be carried out over a series of annealing process stations  200 . 
   The deposition of thin films onto substrates  10  is a critical processing step of the present invention. A detailed description of the deposition apparatus is given below. 
   The intermittent motion of the belts  1  allows a pause for deposition. During the pause, as shown in  FIG. 2A , the individual substrates  10 , which are spaced at regular pitch intervals on the metal strip belts  1 , will be in a sealing relationship with individual heated pockets  33 . During this pause, the individual heated pockets  33  will deposit a film onto the substrates  10 . For a given pause time, the thickness of the films can be varied by varying the temperature of each individual heated pocket  33 . Closed loop control of film thickness may be achieved by measuring film thickness with known thickness monitors and adjusting the heated pocket  33  temperature as needed. Very precise control may be achieved since the deposition rate is a function of the temperature of the heated pocket  33 . 
   The present invention is specifically designed to produce very uniform film deposition across the surface of the substrate  10  at high throughput so that in a mass manufacturing environment the reproducibility of the film uniformity from one substrate  10  to another substrate  10  is tightly controlled over long periods of time. In order to accomplish this desired aspect of the present invention, the substrate  10  is held in a novel sealing relationship with the pocket  29 . 
   The novel sealing relationship is unique and is not shown or suggested in the prior art.  FIG. 6A  and  FIG. 6B  show an elevation view of two prior art devices used for film deposition by vacuum sublimation. In  FIG. 6A , the known method of Bozler in U.S. Pat. No. 3,636,919 (1972) shows the source deposition material  35   a  and substrate  10   a  in an enclosed space. The enclosed space is formed by an upper heater  36  U and a lower heater  36  L. Because the substrate  10   a  is sealed in the enclosed space, the substrate  10   a  could not be moved and only one film could be deposited for each pump down of the vacuum chamber.  FIG. 6B  details another known method according to Bonnet in U.S. Pat. No. 5,304,499 (1994) which is commonly known as close spaced sublimation (CSS). In this known method, the deposition material  35   a  is placed on a flat heated plate  36  L and the substrate is held at a typical distance of 2 to 3 mm from the deposition material  35   a . The disadvantage of this known method is that the clearance between the deposition material  35   a  and the substrate  10   a  forms a leak path for the vapor. This leak will lead to non-uniform deposition and the formation of toxic nano particles in the process space. 
   The heated pocket deposition apparatus of the present invention overcomes the limitations of the prior art. With reference to  FIG. 7A , there is a clearance distance  37  on the order of 0.001 in. to 0.018 in. between the substrate  10  and the top surface of the heated pocket  33  block. Clearance distance  37  is formed on the two sides of the substrate  10  where the substrate  10  spans the belts  1 . The clearance distance  37  is necessary to prevent the substrates  10  from touching the heated pocket  33  and damaging the deposited films. The clearance distance  37  is formed in such a manner as to provide a close tolerance slip fit seal between the top of the heated pocket  33  and the bottom of each of the substrates  10 . Any vapor leak through the clearance distance  37  will be in the molecular flow regime and will be very small. This effectively eliminates vapor leaks through the clearance distance  37  and allows the substrates  10  to act as a shutter across the source pocket  29 . 
   Furthermore, the walls of the pocket  29  will collimate the vapor flux from the subliming material  35 . Since the clearance distance  37  is at the top of the pocket  29  and at a right angle to the collimated vapor flux, nearly all of the vapor flux will pass the entry of the clearance distance  37  without directly entering it. Any vapor which does enter the clearance distance  37  due to gas scattering will be readily deposited on the surface of the substrate  10 . The design of the clearance distance  37  allows for movement of multiple substrates  10  while maintaining a vapor seal. This design has advantages over the prior art since it will maintain the deposition uniformity across the substrate  10 . In the present invention, the use of the substrates  10  to act as a shutter virtually eliminates cross contamination between heated pockets  33 . The elimination of cross contamination allows the use of one vacuum boundary for the multiple processing steps and reduces the capital cost of the system. In most vacuum processes for processing PV devices, load locks or load locks in combination with intermediate chambers are used to prevent cross contamination between processing steps. 
   As shown in  FIG. 7A , the deposition material  35  must be distributed in an evenly spaced pattern across the floor of the pocket  29 . The deposition material  35  may be in the form of powder, pellets pressed from powder, or random chunks. All of these forms of material for the fabrication of CdTe PV are commercially available. Also, the distance between the deposition material  35  and the lower surface of the substrate  10 , which is provided by the depth of the pocket  29  in the lower heated pocket  33 , must be sufficient to allow for gas scattering of the sublimed species. 
   Gas scattering is the result of collisions among the sublimed species or between the sublimed species and the molecules of the ambient background gas. These collisions deflect and scatter the sublimed species from following a straight line path from the deposition material  35  to the substrate  10 . This scattering of the sublimed species results in a uniform deposition on the substrate  10  The Knudsen number is a known dimensionless parameter which is used to quantify the amount of gas scattering present. The Knudsen number is the ratio of the mean free path in the pocket  29 , at a given temperature and pressure, divided by the distance between the deposition material  35  and the substrate  10 . If the Knudsen number is less than 0.01, then the species within the pocket  29  are in viscous flow and gas scattering will be significant In this viscous flow regime, the gas scattering may lead to such a loss of energy from the sublimed species that they condense to form nano-particles. For Knudsen numbers greater than 1, the species will be in a molecular flow regime with very little gas scattering. In this molecular flow regime, the sublimed species will travel in straight lines to the substrate. This line of sight deposition causes non-uniform film thickness across the substrate  10 . For Knudsen numbers between 0.01 and 1, the species will be in transition flow with some gas scattering. In the transition regime, the vapor flux is randomized by gas scattering. However, the number of collisions are few enough that the sublimed species retain most of their energy and therefore do not condense into nano-particles before striking the substrate  10 . In the present invention, deposition has been done with Knudsen numbers in the transition regime from 0.07 to 0.44, which has led to uniform film thickness across the substrate  10 . 
   An additional advantage of the present invention is the heating of the process gas in the pocket  29 . Since the substrates  10  act as a shutter for the heated pocket  33 , the process gas in the source pocket  29  comes to a uniform high temperature. The high temperature of the process gas in the heated pocket  33  is another factor which prevents the formation of nano-particles due to the gas scattering collisions described above. 
   The preferred embodiment of the present invention provides a means of forming a back electrode layer at high throughput and using equipment that is low capital cost. This layer is adherent to the other layers in the film stack and has a very low electrical resistivity. As shown in  FIG. 1 , the back electrode is fabricated by a novel spray process at atmospheric pressure in process stations  52   a  and  52   b . A layer of conductive graphite coating followed by a layer of conductive Ni coating are applied by known industrial spray methods to form the back electrode for carrying current. Sprays containing other metals besides Ni are known and may also be used. A metal conductive coating layer may also be applied by spray directly to the ohmic contact layer without an intervening carbon layer. In this description, carbon is not considered a metal. To limit the inclusion of oxygen and water vapor into the metallization layer, the gas used as a propellant in the spray process may be a dry inert gas such as N 2  or Ar, and the process may be performed in a controlled environment. 
   The thick film of the back electrode contains a polymer binder, which provides a level of encapsulation and protection for the completed device. The spray is performed at room temperatures and does not harm or introduce defects into the previously fabricated semiconductor layers. The spray method has the advantage of low capital cost. Other known methods of metallization for PV devices, such as sputtering, require expensive high vacuum equipment. 
   The preferred embodiment of the present invention also provides a means of scribing that exhibits high throughput and low cost. As shown in  FIG. 1 , a scribing step may be performed at the film scribing station  53   a  after the graphite layer is formed by the spray process at the process station  52   a . The scribing is performed after the application of the graphite layer to prevent any damage to the semiconductor layers by handling. 
     FIG. 9A  illustrates the details of the preferred embodiment of the film scribing method. A rotating wire brush  42  is brought into contact through openings  45  in a mask  41  to remove portions of the film  40  from the substrate  10 . The openings  45  in the mask  41  are tapered in cross section and are narrower near the contact of the mask  41  and the film layers  40 . This facilitates the entry of the rotating brush  42  into the openings  45 . The mask  41  may be coated with a hard coating, such as titanium nitride, to reduce wear. 
   The preferred embodiment of the film scribing method shown in  FIG. 9A  does not require a precisely defined rotating brush  42  since the openings  45  in the mask  41  define the area of the film  40  which will be removed. The rotating wire brush  42  is passed axially along the openings  45  in the mask  41  over the substrate  10  to perform a scribe. A plurality of rotating metal brushes  42  may also be provided so that one pass along the axis of the openings  45  in the mask  41  will complete the plurality of scribes on an entire substrate  10 . This film scribing method has the ability to scribe layers selectively. By using an abrasive powder with the rotating brush  42 , the TCO layer may be scribed. By using the rotating brush  42  alone, all of the layers except the TCO layer may be removed. 
   The process steps of the preferred embodiment of the present invention are shown in the flow chart of  FIG. 10A . The overall system  1000  required to perform the process is shown in  FIG. 1 . The apparatus required to perform the vacuum portion of the process, from step  2 ) of  FIG. 10A  through step  12 ) of  FIG. 10A , is illustrated in  FIG. 2A . With reference to  FIG. 2A , the substrates  10  are transported through the vacuum process stations  200  and  300  on the metal strip belts  1 . As shown in  FIG. 10A , these process steps may be performed by a single one or a series of the vacuum process stations  200  or  300  illustrated in  FIG. 2A . The total time required for processing at each process station  200  or  300  and the transport of the substrate  10  to the next process station  200  or  300  is a unit of cycle time as defined above. The cycle time in the process description below is in the range of 30 seconds to 2 minutes. 
   With reference to  FIG. 10A , a description of the process steps performed on each individual substrate  10  as the plurality of substrates  10  pass through the process is given below. 
   In step ( 1 ) the substrate  10 , which may have a TCO layer on one surface, is ultrasonically cleaned, rinsed, dipped in isopropyl alcohol to remove water from the surface, and dried in a clean room type mini-environment. 
   In step ( 2 ) the substrate  10  is transported into the vacuum chamber  3  through the AVA opening  2  using the metal strip belts  1 . 
   In step ( 3 ) the substrate  10  is heated to a temperature in the range of 500° C. to 560° C. and transported to the next process. 
   In step ( 4 ) with the temperature of the substrate  10  in the range of 500° C. to 560° C., a CdS film is deposited onto the TCO layer on the substrate  10  and the substrate  10  is transported to the next process. 
   In step ( 5 ) with the temperature of the substrate  10  in the range of 500° C. to 560° C., a CdTe film is deposited onto the CdS layer on the substrate  10  and the substrate  10  is transported to the next process. 
   In step ( 6 ) and step ( 7 ) a CdCl 2  treatment is performed on the CdS/CdTe layers. In step ( 6 ) with the temperature of the substrate  10  in the range of 300° C. to 500° C., the CdS/CdTe layers on the substrate  10  are exposed to CdCl 2  and the substrate  10  is transported to the next process. The CdCl 2  exposure of the CdS/CdTe layers on the substrate  10  may be to a CdCl 2  vapor or a CdCl 2  film may be deposited on the CdTe layer. Either method of CdCl 2  exposure will produce high efficiency CdTe PV devices. For this process step, a series of CdCl 2  treatment stations may be required. 
   In step ( 7 ) with the temperature of the substrate  10  in the range of 400° C. to 450° C., the CdCl 2  treated layers on the substrate  10  are annealed, any CdCl 2  film is removed, and the substrate  10  is transported to the next process. For this process step, a series of annealing stations may be required. In combination, step ( 6 ) and step ( 7 ) provide the CdCl 2  treatment of the CdS/CdTe layers. The CdCl 2  treatment of CdTe PV devices in vacuum is known. However, no prior art shows the CdCl 2  treatment step performed inline between the deposition of the CdTe layer and the formation of the ohmic contact layer and without the substrate leaving vacuum. Also, no prior art shows directly transporting substrates from CdTe deposition to CdCl 2  treatment. 
   In step ( 8 ) the substrate  10  and films are cooled to the required temperature in the range of 25° C. to 100° C. and the substrate  10  is transported to the next process. For this process step, a series of cooling stations may be required. 
   In step ( 9 ) and step ( 10 ) an ohmic low resistance contact is formed on the CdTe layer. In step ( 9 ) with the substrate  10  temperature in a range of 150° C. to 300° C., a metal salt is deposited onto the CdTe layer on the substrate  10 , and the substrate  10  is transported to the next process. One metal salt which has been used for this process step is CuCl. In step ( 10 ) with the temperature of the substrate  10  in a range of 150° C. to 250° C., the CdS/CdTe/metal salt layers on the substrate  10  are annealed, and the substrate  10  is transported to the next process. For this process step a series of annealing stations may be required. In combination, step ( 9 ) and step ( 10 ) produce a reaction between the metal salt and the surface of the CdTe layer and this reaction produces a thin p+ semiconductor layer on the CdTe surface to form the ohmic contact. In the case of a Cu salt, a copper telluride such as CuTe X  or Cu doped CdTe:Cu or both may be formed. The thin p+ layer provides a low resistance ohmic contact on the surface of the CdTe layer and is stable. The prior art does not show the formation of an ohmic contact layer on CdTe by deposition of a metal salt onto the CdTe layer in vacuum. 
   In step ( 11 ) the substrate  10  and films are cooled to the required temperature in the range of 25° C. to 100° C. and the substrate  10  is transported to the next process. For this process step, a series of cooling stations may be required. 
   In step ( 12 ) the substrate  10  is transported out of the vacuum chamber  3  through the AVA opening  2  on the metal strip belts  1 . 
   In step ( 13 ) the ohmic contact layer is exposed to air for an optimum time in the range of 4 to 16 hours before a layer of conductive coating containing carbon is applied onto the ohmic contact layer by a spray process. The air exposure has led to PV devices with increased long term stability. 
   As described in the prior art, a series of scribes in selected film layers on the substrate  10  are required to isolate individual PV cells on the substrate and to interconnect the individual cells on the substrate  10  to form a completed module. 
   In step ( 14 ) a plurality of first scribes through the all of the film layers on the substrate  10  including the transparent conductive oxide are performed. A second plurality of scribes through the carbon and the p/n layers without removing the TCO are performed parallel to the first set of scribes. Both of these sets of scribes are performed by the novel film scribing method described above with reference to  FIG. 9A . 
   In step ( 15 ) a layer of conductive coating containing Ni is applied by spray onto the layer of conductive coating containing carbon. Since the Ni metalization layer is formed after the second set of scribes, the Ni layer will fill the cuts in the semiconductor layers from the second scribe. This will cause an electrical connection of the back electrode of one cell to the front electrode of another cell. This step in combination with the following step of the final scribe of the metalization layer will complete the interconnection of the PV cells in series to form the PV module. 
   In step ( 16 ) a plurality of third scribes through only the metallization layers are performed. This third set of scribes is performed by the novel film scribing method described above with reference to  FIG. 9A . 
   In step ( 17 ) electrical connections are made and the finished module is encapsulated. 
   In any of the film deposition steps, including the steps  4 ),  5 ),  6 ), or  9 ) of  FIG. 10A , more than one layer of a particular material may be deposited. These multiple layers may be deposited by a series of heated pocket deposition apparatuses. If one layer of material is deposited in a given cycle time, to a given thickness, then this single layer may be replaced by many thinner layers built up to the thickness of the single layer of material by using many depositions at shorter cycle times. Multiple depositions are advantageous since for every decrease in cycle time there is a corresponding increase in production rate. Multiple processing stations can also be used to decrease the cycle time for the other steps including annealing, vapor treatment, cooling, etc. 
   It would also be possible to create a multijunction solar cell in accordance with the teachings of the present invention. In this case, a monolithic multijunction structure could be fabricated on the substrate. In this structure, two or more solar cells would be stacked on one substrate in such a way that solar radiation passes through the larger band gap material first, and residual radiation passes through the stack to a smaller band gap material. The band gap of the materials needed could be tailored by using semiconductors formed by the combination of any of the elements Zn, Cd, Hg, S, Se, or Te. These elements are from group IIB and group VIB of the periodic table. These semiconductors may be in the form of alloys containing three or more elements. The multiple layers may be deposited by a series of heated pocket deposition apparatuses. Also, by varying the composition of the deposition material in the multiple heated pockets, a graded band gap photovoltaic device may be fabricated. 
   Many devices were fabricated by following the process steps ( 1 ) through ( 13 ) and step ( 15 ) of  FIG. 10A  described above. CuCl was used as the metal salt in step ( 9 ). Individual PV devices with an area of 0.3 sq. cm. were formed on many substrates  10 . These devices were defined by masking certain areas and removing the rest of the films on the substrates  10  with an abrasive blast. The best device had a conversion efficiency of 11.8% and was produced on a commercially available low cost SnO X :F coated soda lime glass substrate  10 . Such high efficiency devices have also proven to be stable. Accelerated stress testing by light soaking at 1000 W/m 2  and 65° C. at open circuit condition was performed on many devices. Over a time period of hundreds of hours, measured device efficiencies were at least 98% of the original efficiencies. 
   A description of the preferred embodiment of the present invention was given above. Other alternate embodiments of the present invention are described below. These specific alternate embodiments are best described with reference to  FIG. 10A , which is a flow chart of the process steps of the preferred embodiment of the invention. 
   In alternate embodiments of step  9 ) in  FIG. 10A , the deposition of the metal compound may include compounds of Cu, Ag, Au, Hg, Sn, Sb, and Pb. These compounds may be metal salts as described in the preferred embodiment or organometallic compounds of Cu, Ag, Au, Hg, Sn, Sb, and Pb may also be used. These compounds may be deposited onto CdTe in vacuum to form an ohmic low resistance contact layer by reaction with the CdTe layer. This ohmic low resistance contact may be formed by tellurides of Cu, Ag, Au, Hg, Sn, Sb, and Pb, or the ohmic contact may be highly doped CdTe. An alternative embodiment to step  10 ) of  FIG. 10A  is to anneal the ohmic contact layer in air, inert gas or other atmospheres outside the vacuum chamber. 
   In other embodiments of the process steps  4 ) and  5 ) of  FIG. 10A  other semiconductors may be used instead of CdS and CdTe. These semiconductors are formed by the combination of any of the elements Zn, Cd, Hg, S, Se or, Te. These elements are from group IIB and group VIB of the periodic table. These semiconductors may also be in the form of alloys containing three or more elements. These compound semiconductors are known to be useful in the formation of PV devices, are readily sublimable, and may be deposited in vacuum by the present invention. A series of stations may be used to deposit the n-type and p-type semiconductors. Thus, more than one layer may be used to form the n-type and p-type regions of the device This allows a faster cycle time while still maintaining adequate semiconductor film thickness. Also, the different stations may deposit different IIB-VIB compound semiconductors made up of the elements Zn, Cd, Hg, S, Se or, Te. 
   In alternate embodiments of step  6 ) of  FIG. 10A , halogen containing substances such as HCl or Cl 2  gas may be used in place of or in addition to CdCl 2 . In this case, a controlled amount of the gas can be introduced into the heated pocket  33 . Other halogen containing substances which are known to have effects similar to CdCl 2  may also be used. Some known examples of these compounds are CdBr 2  and CdI 2 . 
   In another alternate embodiment of the process steps of  FIGS. 10A , a CdTe PV device may be fabricated without the deposition of CdS shown in step  4 ) of  FIG. 10A . In this embodiment, the p/n junction is formed between the n-type TCO and the p-type CdTe or other IIB-VIB compounds. 
   In another embodiment of the process steps of  FIG. 10A , a CdTe device may be fabricated by depositing the CdS layer on the substrate  10  outside of the vacuum chamber  3  by known methods, including chemical bath deposition. In this embodiment, the substrate  10  with the CdS layer would be brought into the vacuum chamber  3  and steps  3 ) through step  11 ) of  FIG. 10A  would be performed in vacuum. 
   In another embodiment of the process steps of  FIG. 10A , a CdCl 2  treatment may be performed after step  4 ) CdS deposition and before step  5 ) CdTe deposition. This CdCl 2  treatment is in addition to the step  6 ) CdCl 2  treatment as shown in  FIG. 10A . The additional CdCl 2  treatment is known to further increase the device performance. 
   In other alternative embodiments of  FIG. 10A , the HPD deposition heated pocket  33  may be used to apply other film layers which would improve the performance of a CdS/CdTe PV device. One such layer is an anti-reflection (AR) coating which would be deposited on the glass substrate  10  on the opposite side of the substrate  10  from the TCO. The AR coating would face the sun and reduce the amount of incoming sunlight which is reflected off of the glass surface. This would increase the current that the device could produce. One such AR coating is a thin film of MgF 2 . Since MgF 2  is sublimable, this film may be applied with the heated pocket  33  deposition. The AR coating can be done at a suitable location in the vacuum chamber  3 . Another layer which is known to increase the efficiency of a CdS/CdTe device is a layer of high resistivity intrinsic tin oxide (i-SnO X ). This layer would be applied between the TCO layer and the CdS layer or a layer of SnO X  of the desired resistivity could be deposited directly on the glass substrate  10 . This intrinsic layer has a much higher electrical resistivity than the TCO layer and has been shown to increase device efficiency. This resistive layer would allow the CdS layer to be thinner. The thinner CdS layer would allow more light to pass into the CdTe layer and increase the current the device would produce. The heated pocket  33  could be used to sublime i-SnO X . The i-SnO X  deposition would be performed before the CdS deposition of step  4 ) of  FIG. 10A . 
   An alternate embodiment to step  15 ) of  FIG. 10A  is that the graphite and Ni may be sprayed through a mask to form a patterned deposition. This patterned back electrode would eliminate the need for the second and third set of scribes shown in steps  14 ) and  16 ) of  FIG. 10A . In another embodiment, a spray through a mask may also be used to fill the cut of the first scribe with an insulating compound before the spraying of the Ni conductive coating. The insulating compound eliminates electrical shunts between the Ni layer and the TCO. 
   Other alternate embodiments of the present invention are described below. The figures which show these alternative embodiments are  FIGS. 2B ,  7 B,  8 ,  9 B, and  10 B. These alternative embodiments are described with reference to these figures. 
     FIG. 2B  shows an alternate embodiment of the present invention in which the pair of continuous metal strip belts  1  are entirely contained within the vacuum chamber  3 . A pair of known load locks  5  on either side of the chamber  3  provide a means of transporting the substrates  10  into and out of vacuum. 
     FIG. 7B  illustrates an alternate embodiment of the vacuum process station of  FIG. 7A  involving a different arrangement for the upper heated pocket  33 . This alternate embodiment involves the addition of a baffle having a plurality of holes  28 . The deposition material  35  is placed above the baffle in a confined space. The confined space has a lid  27 , which may be removed to reload the deposition material  35 . As the upper heated pocket  33  is heated, the deposition material  35  will sublime, and vapor will pass through the holes  28  into the source pocket  29  of the upper heated pocket  33 . This alternative embodiment of the vacuum process station  200  may be used as the heated pocket  33  to deposit films on the upper surface of the substrate  10 . When the deposition is on the upper surface of the substrates  10 , the substrates  10  can be transported by arrangements such as rollers, robotic arms, etc., which are well known. This would be especially useful for the deposition of the AR coating. 
     FIG. 8  illustrates an alternative to the heated pocket  33 . This embodiment can be used to generate plasma in the pocket  29 . This arrangement is called plasma enhanced heated pocket deposition (PEHPD). The alternate embodiment of the heated pocket  33  provides a high voltage pin  30  for the generation of plasma, the pin  30  may be made from graphite. The pin  30  is electrically isolated by insulation  31  which may be a quartz tube. The high voltage from the DC power supply  32  generates the plasma. The heated pocket-to-substrate distance in the PEHPD heated pocket must be large enough to produce ions within the pocket  29 . If the pocket  29  is too shallow it will not produce a glow discharge at desirable pressures, when the substrate  10  is sealing the pocket  29  during deposition. In alternate embodiments of the present invention, both the CdS and CdTe heated pocket may optionally be of the PEHPD type so that the advantages of plasma enhanced deposition may be incorporated. These advantages of plasma enhanced deposition, which serve to improve the device efficiency, include: (i) doping of CdTe with nitrogen, (ii) passivation of the defects in CdTe, (iii) alteration of the morphology of CdS, (iv) doping of CdS, and (v) mixing of the CdS/CdTe interface. In addition, ZnTe may be doped with nitrogen by PEHPD to form ZnTe:N, a p+ semiconductor layer which may be used as an ohmic contact to CdTe. 
     FIG. 10B  is a flow chart of the process steps of some of the alternate embodiments of the invention. The processing steps as shown schematically in that figure include ( 1 ) cleaning the substrate by known means in a clean room type mini-environment, ( 2 ) transporting the substrate into the vacuum chamber using AVA or load lock transport, ( 3 ) heating the substrate, ( 4 ) depositing a CdS film on the substrate using heated pockets with or without plasma, ( 5 ) depositing a CdTe film onto the CdS film using heated pockets with or without plasma, ( 6 ) performing a treatment, with a halogen containing substance, on the CdS/CdTe films using a heated pocket, ( 7 ) annealing the substrate and films, ( 8 ) forming an ohmic contact on the CdTe layer by heated pocket deposition of a Te layer on the CdTe layer or by heated pocket deposition with plasma of a ZnTe:N layer on the CdTe layer, ( 9 ) depositing a metallization layer onto the ohmic contact layer by heated pocket deposition of a sublimable metal layer such as Zn or Cd, ( 10 ) annealing the substrate and films, ( 11 ) cooling the substrate and films, ( 12 ) transporting the substrate and films from the vacuum chamber using AVA or load lock transport, ( 13 ) performing a first scribe through the entire film stack including the metallization and the transparent conductive oxide, ( 14 ) screen printing the electrical connection of the back electrode of one cell to the front electrode of the next cell by known screen printing methods, and ( 15 ) encapsulating the furnished module. Step ( 13 ) involves a first scribe that may be performed in accordance with the teachings of the present invention or by one of several known means including mechanical or laser scribing and a second scribe through the carbon and the p/n layers without removing the TCO. 
   Step  8 ) in  FIG. 10B  illustrates another alternative embodiment of the present invention in which Te is used as an ohmic contact layer Te is easily sublimed and is known to form an ohmic contact material for CdTe devices. After the CdCl 2  treatment and annealing steps, Te could be deposited inline by heated pocket deposition with or without a cooling step. A metal back electrode can be applied to the Te ohmic contact by either a vacuum deposition step or by the spray process of the present invention. 
   Step  9 ) of  FIG. 10B  illustrates an alternate embodiment of the present invention in which the back electrode metallization may be accomplished inline in vacuum. In this embodiment, the back electrode metallization may be applied by a heated pocket deposition of metals including, but not limited to, Zn or Cd, which are readily sublimable and electrically conductive. 
     FIG. 9B  illustrates an alternative embodiment of the scribing method of the present invention involving the use of an abrasive blast  43  which is provided by known methods. The abrasive blast  43  is performed through openings  45  in the mask  41  to remove portions of the film  40  from the substrate  10 . This embodiment of the scribing method does not require a precisely defined abrasive blast  43  since the mask  41  defines the area of the film  40  which will be removed. The abrasive blast  43  is passed over the substrate  10  to perform a scribe. Each abrasive blast  43  may enter more than one opening  45  in the mask  41  so that one abrasive blast  43  may perform more than one scribe for each pass over the substrate  10 . A plurality of abrasive blasts  43  may be provided so that one pass along the axis of the openings  45  in the mask  41  will complete the plurality of scribes of an entire substrate  10 . Different abrasive media with different hardness and size may be provided so that various layers in the film stack  40  may be scribed selectively. This allows the scribing method of the present invention to:  1 ) scribe all the way through the film stack including the relatively hard TCO layer, and  2 ) selectively scribe through all of the layers above the TCO layer without removing the TCO.