Patent Publication Number: US-2012034734-A1

Title: System and method for fabricating thin-film photovoltaic devices

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 13/173,100, titled “System and Method for Fabricating Thin-Film Photovoltaic Devices” and filed Jun. 30, 2011, which is a continuation-in-part of U.S. patent application Ser. No. 13/101,538, titled “System and Method for Fabricating Thin-Film Photovoltaic Devices” and filed May 5, 2011, which is a continuation-in-part application of U.S. patent application Ser. No. 12/850,939, titled “System and Method for Fabricating Thin-Film Photovoltaic Devices” and filed Aug. 5, 2010. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the manufacture of electronic devices. More particularly, the invention relates to a method and a system for forming photovoltaic light absorbing Chalcopyrite compound layers of copper indium gallium diselenide (CIGS) on substrates for fabrication of thin film solar cells and modules. 
     BACKGROUND OF THE INVENTION 
     Thin film solar cells have attracted significant attention and investment in recent years due to the potential for lowering the manufacturing costs of photovoltaic solar panels. Most solar panels are fabricated from crystalline silicon and polycrystalline silicon. While silicon-based technology enables fabrication of high efficiency solar cells (up to 20% efficiency), material costs are high due the embodied energy to refine and grow the bulk silicon ingots of silicon from silicon dioxide. In addition, sawing these ingots into wafers results in approximately 50% of the material being wasted. These solar cells are the primary component of the majority of solar panels made and sold today. Presently, silicon solar cells are approximately 90 μm thick. In contrast, thin film solar cells include layers that are approximately 1 μm to 3 μm thick and are deposited directly onto low cost substrates. Among the most popular materials used are amorphous silicon, copper indium diselenide and its alloys with gallium or aluminum (CIS, CIGS, CIAS) and cadmium telluride (CdTe). 
     Typically, amorphous silicon has the lowest manufacturing costs in terms of cost per unit of power produced, but the efficiencies of the solar cells are generally less than 10% which is low relative to the efficiencies of other materials. CIGS and CdTe cells have higher efficiencies and in the lab have achieved efficiencies approaching and sometime exceeding the efficiencies of silicon-based cells. Small area laboratory-scale cells have demonstrated efficiencies in excess of 20% and 18% for CIGS and CdTe, respectively; however, the transition to volume manufacturing and larger substrates is difficult and substantially lower efficiencies are realized. 
     Recently, CIGS solar cells have been produced in the laboratory and in production using a three phase co-evaporation process. In this process effusion sources of copper (Cu), indium (In) and gallium (Ga) evaporate at the same time in the presence of a selenium source. In this manner, deposition and selenization occur in a single step as long as the substrate temperature is maintained between about 400° C. and 600° C. Typically, higher temperatures result in higher efficiencies; however, not all substrates are compatible with higher temperatures. Sodium is often added to the mixture of sources and has been shown to enhance minority carriers and to improve voltage. Sodium may also passivate surfaces and grain boundaries. The deposition is repeated three times. For each deposition, the relative concentrations of copper, indium and gallium are changed, thus producing a graded compositional structure that can be more effective at absorbing and converting incident light into electrical power. 
     Scaling the three phase co-evaporation process to production levels is complicated due to a number of fundamental difficulties. First, effusion sources require high power consumption at production scale because the sources need to be maintained at temperatures as high as 1,500° C. At these high temperatures many materials are extremely reactive. Longevity of system components is decreased and process control and maintenance are difficult. Thus costs associated with production systems are high and downtime can be significant. 
     The substrate temperature is high during the selenization process. Consequently, the selenium residence time on the substrate surface is small and the selenium utilization efficiency is low. Selenium utilization and unwanted accumulation in various regions of the process chamber make the co-evaporation process difficult to manage in a production environment. 
     A number of groups have fabricated solar cells using the co-evaporation process while other groups have adopted production-compatible alternatives. One common alternative approach is based on a two-step process that typically includes depositing the metals (copper, indium and gallium) on a substantially cold substrate, that is, a substrate near or at ambient temperature. The deposited metals are then selenized in a hydrogen selenide (H 2 Se) gas or in a selenium vapor from a solid source. An ambient temperature is maintained between about 250° C. and 600° C. 
     The metals are typically deposited by electroplating, sputter deposition or printing. The metal deposition step is often followed by a cold deposition of selenium prior to the substrate entering a selenization furnace. The selenium deposition thickness is in the range of approximately 1 μm to 2 μm. By creating a sacrificial selenium layer on top of the CIG layer, indium is prevented from diffusing out of the metal layer in the form of a volatile indium selenide during the ramping of the furnace temperature. The temperature ramp can be of long duration, especially for thick glass substrates; however, for thin flexible foils, rapid temperature ramps (e.g., 10° C./s) are possible and are significant in reducing the problem of indium depletion. This two-step process is more controllable and easier to implement in system equipment in comparison to the co-evaporation technique; however, the resulting efficiencies generally are lower than those obtained by co-evaporation by 2% to 4%. The lower efficiencies are due to non-ideal grain formation and to the segregation of gallium and indium during the selenization step. Typically, gallium diffuses toward the back electrode to form a CuGaSe compound, while indium diffuses toward the barrier layer to form an indium rich compound near the front surface of the cell. Sulfur is sometimes added to the selenium in the furnace to compensate for this diffusion problem by increasing the bandgap of the material at the surface; however, the resulting absorbing layer is not a true CuInGaSe 2  compound and the known advantages of adding gallium to CIS are moderated. 
     A hybrid technique has been used to implement a co-sputtering/selenization; however, selenium poisoning of the sputtering targets can occur and the hot substrate results in poor selenium utilization. Thus this technique is generally more difficult to control than the co-evaporation process. 
     SUMMARY 
     In one aspect, the invention features a method of depositing a copper indium gallium diselenide film on a substrate in which a layer of indium is deposited on a substrate and a layer of copper gallium is deposited on the layer of indium. The layers of copper and indium are selenized, and the steps of depositing a layer of indium, a layer of copper gallium and selenizing are repeated a plurality of times. 
     In another aspect, the invention features a method of depositing a copper indium gallium diselenide film on a substrate in which a first layer of indium is deposited on a substrate and a first layer of copper gallium is deposited on the first layer of indium. The first layers of indium and copper gallium are selenized. A second layer of indium is deposited on the selenized first layers of indium and copper gallium and a second layer of copper gallium is deposited on the second layer of indium. The second layer of indium has an increased indium content relative to the first layer of indium and the second layer of copper gallium has a decreased gallium content relative to the first layer of copper gallium. The second layers of indium and copper gallium are selenized. 
     In yet another aspect, the invention features a method of depositing a copper indium gallium diselenide film on a substrate. A first layer of copper gallium is deposited on a substrate and a first layer of indium is deposited on the first layer of copper gallium. A second layer of indium is deposited on the first layer of indium and a second layer of copper gallium is deposited on the second layer of indium. The first and second layers of indium and copper gallium are selenized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in the various figures. For clarity, not every element may be labeled in every figure. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is an illustration of an embodiment of an apparatus for depositing a copper indium gallium diselenide film on a web according to the invention. 
         FIG. 2  is a flowchart representation of an embodiment of a method of depositing a copper indium gallium diselenide film on a web according to the invention. 
         FIG. 3  illustrates a selenization furnace for the apparatus of  FIG. 1  that includes three independently controlled heating zones according to an embodiment of the invention. 
         FIG. 4A  is a schematic illustration of a selenium trap for the apparatus of  FIG. 1  according to an embodiment of the invention. 
         FIG. 4B  is a cross-sectional side view illustration of pair of selenium traps and a selenization oven according to an embodiment of the invention. 
         FIG. 4C  is a top view of an inner module of one of the selenium traps of  FIG. 4B . 
         FIG. 4D  is an end view of one of the selenium traps of  FIG. 4B . 
         FIG. 5  is a block diagram of an embodiment of a system for deposition of a thin film on a substrate according to the invention. 
         FIG. 6A  and  FIG. 6B  are a perspective view and a top view, respectively, of an embodiment of a system for deposition of a thin film on a substrate according to the invention. 
         FIG. 7  is a flowchart representation of an embodiment of a method of depositing a thin film on a substrate according to the invention. 
         FIG. 8  is a block diagram of another embodiment of a system for deposition of a thin film on a substrate according to the invention. 
         FIG. 9  is a flowchart representation of an embodiment of a method of depositing a copper indium gallium diselenide film on a substrate according to the invention. 
         FIG. 10  is an illustration of the structure of a CIGS film during processing according to the method of  FIG. 9 . 
         FIG. 11  is a flowchart representation of another embodiment of a method of depositing a copper indium gallium diselenide film on a substrate according to the invention. 
         FIG. 12  is an illustration of the structure of a CIGS film during processing according to the method of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     The systems and methods of the present invention may include any of the described embodiments or combinations of the described embodiments in an operable manner. In brief overview, the systems and methods of the invention enable the deposition of a CIGS thin film by sputtering deposition on metal and plastic thin foils and discrete substrates. As used herein, a discrete substrate means an individual component such as a glass plate, a glass panel or a wafer. The flexibility and bandgap engineering advantages of co-evaporation techniques are realized without the production scaling problems of prior art co-evaporation systems. CIGS devices having high conversion efficiencies are manufactured using a multistep process that includes sputtering and selenization sequences. First, a substantially thin metal layer of CuInGa (e.g., approximately 0.15 μm thickness) is deposited onto a cold web substrate or a discrete substrate. For example, the substrate temperature in the sputtering region is preferably as low as practical (e.g., ambient temperature) but may be up to 300° C. due to operation of the sputtering equipment. Subsequently, selenization occurs in a selenization furnace which is in-line with the sputtering system. The process is repeated a number of times until a desired thickness of the absorber layer is attained (e.g., approximately 2.5 μm). The composition of each incremental thin metal layer can be varied throughout the full deposition process to achieve desired bandgap gradients and other film properties. Segregation of gallium and indium is substantially reduced or eliminated because each incremental layer is selenized before the next incremental layer is deposited. This epitaxial growth process (or layer-by-layer method) by a co-sputtering/selenization process eliminates the problems associated with the presence of selenium in the sputtering chamber. The process can be implemented in a roll-to-roll production system to deposit CIGS films on metal and plastic foils. Alternatively, the process can be implemented in a discrete substrate production system to deposit CIGS films on discrete substrates such as glass substrates and wafers. 
     Referring to  FIG. 1 , an embodiment of an apparatus  10  for deposition of a copper indium gallium diselenide film on a web includes a payout zone  14 , a first sputtering zone  18 A, a selenization zone  22 , a second sputtering zone  18 B and a take-up zone  26 . As used herein, the term zone means one or more chambers that can be operated to perform a specific process. The sputtering zones  18  and selenization zone  22  are coupled to respective pump systems (not shown) so that the vacuum level for the zones can be independently controlled. Low conductance slits  28  between the zones achieves a high degree of vacuum isolation between neighboring zones. 
     The payout zone  14  includes a payout roll  30  of web material  34 , such as a thin plastic or metal foil, that is dispensed and transported through the other zones. The payout zone  14  also includes an idler roll  38 A, a load cell  42  to maintain web tension and a cooling roll  46 A that has a substantially larger diameter than the other rolls. The take-up zone  26  includes a take-up roll  50  to receive the web  34  after passage through the other zones. The take-up zone also includes rolls  38 B,  42 B and  46 B that function as counterparts to rolls in the payout zone  14 . At least one of the payout roll  30  and the take-up roll  50  is coupled to a web transport mechanism as is known in the art that enables the web  34  to pass through the intervening zones. The operation of the payout roll  30  and the take-up roll  50  can be reversed, that is, the payout roll  30  can also perform as a take-up roll and the take-up roll  50  can perform as a payout roll when the web is transported in a reverse direction (right to left) as described below with respect to  FIG. 2 . 
     The first sputtering zone  18 A is a chamber having a plurality of sputtering magnetrons  54 . The magnetrons  54  can be planar magnetrons or rotating cylindrical magnetrons as are known in the art. Target material composition for each magnetron  54  can vary relative to the materials of the targets for the other magnetrons  54  to achieve a graded composition structure in the resulting film. 
     The selenization zone  22  includes two cooling rolls  58  that surround two differentially pumped selenium traps  62  and a selenization furnace  66  having a selenium source  70 . A multiple zone resistive heater comprising heating components  74  enables the furnace temperature along the web path through the selenization furnace  66  to vary. 
       FIG. 2  shows a flowchart representation of an embodiment of a method  100  of depositing a copper indium gallium diselenide film on a web according to the invention. Referring to  FIG. 1  and  FIG. 2 , the web  34  is transported (step  102 ) from the payout zone  14  into the first sputtering zone  18 A where the pressure is maintained below 0.01 Torr. During passage through the sputtering zone  18 A, a deposition (step  104 ) of an incremental layer of copper, indium and gallium occurs. The targets of each magnetron  54  can have a variety of compositions. For example, each target material can be copper, indium, or alloys of each as with gallium or aluminum. The thickness of the incremental layer deposited on the web  34  during passage through the sputtering zone  18 A varies according to different process parameters such as the web transport speed. By way of example, the thickness of the deposited incremental layer can be between 100 Å and 2000 Å. 
     After the first incremental layer is deposited, the web  34  enters the selenization zone  22 . The web  34  first passes over a cooling roll  58 A to cool (step  106 ) the web  34  before it enters a multistage differentially pumped selenium trap  62 A. The trap  62 A prevents selenium that may escape from the selenization furnace  66  from entering the sputtering zone  18 A. The web  34  is pre-coated (step  108 ) with a thin layer (e.g., approximately 0.5 μm) of selenium in the trap  62 A before entering the furnace  66 . The relatively cold web temperature (e.g., less than 150° C.) allows selenium to condense on the web  34  as it moves through the trap  62 . The web  34  then moves through the furnace  66  where selenization occurs (step  110 ) at a pressure that is substantially higher than the sputter pressure and at a temperature between 250° C. and 600° C. For example, the selenization can occur at a pressure in a range between 0.0001 Torr and 10 Torr. The pre-coating of selenium is advantageous in preventing indium depletion when the web temperature increases rapidly inside the furnace  66 . 
     After exiting the furnace  66 , the web  34  is cooled (step  112 ) to a lower temperature (e.g., less than 100° C.) by a second cooling roll  58 B. The web  34  then passes through the second sputtering zone  18 A where a second incremental layer of copper indium gallium of varying composition is deposited (step  114 ). 
     Once most of the web material from the payout roll  30  has been processed by transport in the forward direction, that is, dispensed from the payout roll  30  through the intervening zones and accumulated onto the payout roll  50 , the deposition method  100  continues by transporting the web  34  in the reverse direction (step  116 ). While the web  34  moves back through the intervening zones, the original payout zone  14  functions as a take-up zone and the original take-up zone  26  functions as a payout zone. The web  34  passes through the sputtering and selenization zones  18  and  22  in reverse order to execute a sequence of steps (steps  118  to  128 ) that is reversed to the sequence of steps used during the forward transport. Thus a third incremental layer of copper indium gallium is deposited (step  118 ) on top of the second incremental layer in the second sputtering zone  18 B before the second selenium pre-deposition occurs (step  122 ). Selenization is performed (step  124 ) during passage through the furnace  66  before a fourth incremental layer of copper indium gallium (step  128 ) is deposited onto the web  34 . 
     Except for the first pass of the web  34  through the first sputtering zone  18 A, it can be seen that selenization is performed after two consecutive passes of the web  34  through the same sputtering zone  18 A or  18 B. Thus two incremental layers are formed on the web  34  before selenization is performed. Advantageously, in some embodiments the power densities for the sputtering magnetrons can be reduced relative to the power densities for a single pass deposition of an incremental layer prior to selenization. In addition, because the power densities can be changed between passes, the composition of each layer can be changed without the need to change targets. 
     Forward and reverse transport processing are repeated a number of times until a CuInGaSe 2  film of a desired total thickness is deposited onto the web  34  (as determined at step  130 ). It should be noted that at the end of the process, the magnetrons in the sputtering zone  18 A or  18 B used after the last passage through the selenization furnace  66  are disabled (step  132 ) and the web  34  is cooled before a final rewind (step  134 ). 
     The iterative selenization implemented throughout the process reduces or eliminates the gallium and indium segregation problem that is common to two-step CIG processes because the first incremental layer and the pairs of consecutive incremental layers from round-trip passage through a sputtering zone  18  are selenized before the next pair of incremental layers is deposited. Moreover, because the layers to be selenized are thin, the time required for the web  34  to pass through the selenization furnace  66  can be short. Consequently, the web transport speed can be high. The multiple pass forward and reverse process and high web transport speed permit efficient construction of a multilayer structure having a varying composition and bandgap. 
     Although the apparatus  10  and method  100  described above relate primarily to a configuration having a single selenization furnace  66  and a pair of sputtering zones  18 , it should be recognized that other configurations are contemplated according to principles of the invention. For example, multiple selenization furnaces and additional sputtering zones can be employed to enable multiple layers to be deposited and subsequently selenized while the web is transported in a single direction. 
     In some embodiments the selenization furnace  66  has multiple heating zones.  FIG. 3  shows a selenization furnace  78  having three independently controlled heating zones. For example, ZONE  1  has a higher power density than ZONE  2  and ZONE  3  when the web  34  is transported from left to right in the figure. Conversely, ZONE  3  has a higher power density than the other zones when the web  34  moves in the opposite direction, that is, from right to left. By varying the temperature of the zones in this manner, a more rapid heating of the web  34  occurs as it enters the furnace  78 . In some embodiments, the set temperature for the furnace  78  varies for each pass. 
     Various types of selenium traps can be used. For example, different schemes based on differential pumping to gradually transition from a higher pressure region to a lower pressure region as are known in the art can be used. 
       FIG. 4A  is a schematic representation of an embodiment of a selenium trap  82  according to the invention. The trap  82  includes alternating plenums  86  and narrow gaps  90  of low conductance. The plenums  86  are maintained at a low temperature, for example, at a temperature between 0° C. and 20° C., while the gaps  90  are maintained at a substantially higher temperature, for example, 200° C. or greater. During operation, selenium does not accumulate on the hot surfaces of the gaps  90  but does accumulate on the cold surfaces of the plenums  86 . In a preferred embodiment, the selenium pressure is reduced by a factor between approximately 5 and 10 for each gap  90  and neighboring plenum  86  with increasing distance from the selenization furnace  66 . The numbers of gaps  90  and plenums  86  are preferentially determined by the desired pressure differential. 
       FIG. 4B  is a cross-sectional side view illustration of a selenization oven  300  between a pair of selenium traps  304 A and  304 B according to an embodiment of the invention. Advantageously, each selenium trap  304  allows the consumption of selenium to be reduced by recapturing selenium and permitting the accumulated selenium to be recycled. In addition, the selenium remains localized and therefore does not contaminate other regions of the deposition system. Thus maintenance requirements are reduced. 
     The traps  304  enable various other system modules to operate under high vacuum conditions while maintaining a high selenium partial pressure in the oven  300 . For example, the selenium partial pressure can be between 0.050 Torr and 10 Torr. In alternative applications, one or more selenium traps  304  can be used in systems in which various system modules operate near or at atmospheric pressure. 
     Each selenium trap  304  includes an inner module  308  and an outer module  312  that together function to recapture selenium that escapes through the oven apertures  316 A and  316 B. In a preferred embodiment, the inner module  308  is fabricated from graphite. Graphite is a suitable choice of material due to its relatively light weight and corrosive resistance. The inner module  308  includes a transport channel  320  to pass a web substrate  34  or discrete substrate. The transport channel  320  extends between a first trap aperture  328 A at one end of the module  308  and a second trap aperture  328 B at the opposite end of the module  308 . Preferably the trap apertures  328  are shaped as slits. The trap apertures  328  and cross-section of the transport channel  320  are sized to pass the web substrate  34  (or discrete substrates) with sufficient clearance while limiting selenium vapor conductance from the selenization oven  300 . By way of a numerical example, the slits can have a height of 5 mm and a width that is several millimeters greater than the width of the web substrate  34 . A thin rectangular shape is also preferred for a discrete substrate system where the trap apertures  328  have a vertical dimension that is not substantially greater than the thickness of the discrete substrates. 
     Reference is also made to  FIG. 4C  and  FIG. 4D  which show a top view of the inner module  308  and an end view of the selenium trap  304 , respectively. A number of plenums  332  extend from the transport channel  320  to an outer surface  336  of the inner module  308 . The outer module  312  includes three body sections  312 A,  312 B and  312 C bolted together or otherwise secured to each other. Similarly, the inner module includes two body sections  308 A and  308 B. The body of the outer module  312  substantially surrounds the body of the inner module  308  while leaving the ends with the entrance and exit apertures  328  accessible. Preferably, the gap between the inner module  308  and outer module  312  is small (e.g., less than 0.25 in.). In a preferred embodiment, the body sections of the outer module  312  are nickel-plated aluminum and the two sections of the inner module  308  are secured together using a stainless steel plate. 
     The outer module  312  includes a number of collection surfaces, preferably in the form of recessed regions or “pockets”  340  ( FIG. 4B ), that effectively terminate the plenums  332  across the gap and opposite the outer surface  336  of the inner module  308 . Preferably, the depths of the pockets  340  decrease with increasing distance from the selenization oven  300  to accommodate the decreasing vapor condensation in each plenum  332 . By way of a specific numerical example, in one embodiment the depth of the pocket  340  closest to the selenization oven is 0.25 in. 
     In some embodiments, the inner module  308  includes one or more heaters, such as an electrical cartridge heater, to ensure that the inner module  308  remains above the condensation temperature of the selenium vapor (approximately 200° C.). In other embodiments, heat conducted due to a direct coupling of the inner module  308  to the selenization oven  300  (e.g., by attachment) is sufficient to maintain the inner module temperature above the selenium condensation temperature. The outer module  312  is maintained at a temperature substantially below the selenization condensation temperature by a cooling system. In the illustrated embodiment, the cooling system includes coolant channels  344  that are arranged vertically and horizontally and that receive a coolant, such as water, from a coolant pump or other coolant source. 
     The inner and outer modules  308 ,  312  can be fabricated as compact units that enable the selenium traps  304  to be easily mounted along the transport path of the substrate at both sides of the selenization oven  300 . By way of a numerical example, the length of the traps  304  can be between 10 cm and 30 cm and the width of the traps  304  is determined primarily according to the width of the substrate. 
     During operation of the illustrated embodiment as shown in  FIG. 4B , the selenization oven  300  is maintained at a temperature typically in excess of 400° C. with a selenium partial pressure in excess of 0.050 Torr. The web substrate  34  (or discrete substrate) passes through the transport channel  320  of the first selenium trap  304 A, through the selenization oven  300  and then through the transport channel  320  of the second selenium trap  304 B. Selenium vapor that escapes from the oven  300  into a trap  304  does not condense onto surfaces of the inner module  308  which are at temperatures well above the selenium condensation temperature. Instead, the selenium vapor passes into the plenums  332  and selenium condenses on the relatively cold surfaces of the pockets  340  of the outer module  312 . 
     The cold pocket surfaces allow efficient operation of the selenium pump  304 . The arrangement of plenums  332  and pockets  340  act as a multi-stage differential pumping apparatus. For example, the selenium pressure is reduced by approximately a factor of ten for each stage progressing away from the selenization oven  300 . 
     The trap  304  is configured to allow selenium that accumulates during system operation to be reclaimed. As described above, the density of the vapor in the plenums  332  decreases as the distance to the selenization oven  300  decreases, therefore the depth of a pocket  340  is preferably selected to accommodate the corresponding selenium accumulation rate for that pocket  340 . Maintenance personnel can open the outer module  312 , for example, by unbolting the body sections  312 A,  312 B and  312 C to obtain access to the pockets  340  and to permit reclamation of the selenium deposits. After removal of the selenium, the body components of the outer module  312  are secured together about the inner module  308  so that the trap  304  can be reused. The reclaimed selenium can be reused in subsequent system operations. 
     It will be appreciated that the selenium trap can be adapted for a variety of other systems and applications, and that various changes to the structural features are contemplated. For example, in other embodiments the trap is a vapor trap used to restrict the location of other types of vapors for a variety of purposes, such as preventing contamination of surfaces or system components located away from a region of high vapor concentration and reclamation of other types of deposits from vapor condensation in the trap. Various features of the vapor trap, such as the number of plenums and the shapes and cross-sectional areas of the plenums and transport channel, can vary according to a particular application without departing from the principles of the invention. Moreover, the temperatures of the inner and outer modules for trapping various types of vapors are generally determined according to the condensation temperatures of the vapors. 
       FIG. 5  is a functional block diagram of an embodiment of a system  150  for deposition of a thin film on a substrate. By way of example, the system  150  can be used to deposit a copper indium gallium diselenide film on a discrete substrate. The system  150  includes a metal deposition zone  152 , a selenization zone  154  and a return cooling chamber  156 . The system  150  also includes a substrate transport system (not shown) that transports a number of discrete substrates along a closed path  158  that passes through the zones  152 ,  154  and the return cooling chamber  156 . The metal deposition zone  152  is configured to deposit a layer of a composite metal onto the discrete substrates as they pass through the zone. As used herein, a closed path means a path which has no beginning and no end. For example, a closed path can be a rectangular path or circular path along which the substrates are transported. 
     The metal deposition zone  152  can be a sputtering zone as is known in the art. The selenization zone  154  receives the discrete substrates after they pass through the metal deposition zone  152 . Except for the final pass through the system  150 , the return cooling chamber  156  receives the discrete substrates after they exit the selenization zone  154 . The return cooling chamber  156  cools the discrete substrates before the substrates arrive at the metal deposition zone  152  for deposition of the next incremental layer. 
       FIG. 6A  and  FIG. 6B  are a perspective view and a top view, respectively, of an embodiment of a system  160  for deposition of a thin film on a substrate. The system  160  includes the system components shown in the functional block diagram of  FIG. 5  in the form of a sputtering chamber  162 , a selenization furnace  164  and selenium traps  172 A and  172 B, and a cooling chamber  166 . For convenience, a portion of the top and side of the cooling chamber  166  are removed from  FIG. 6A  and  FIG. 6B  so that the substrate transport system  180  inside the cooling chamber  166  is visible. In various embodiments, the substrate transport system  180  includes one or more belt or roller type conveyance mechanisms to move the discrete substrates along the closed loop path  158 . 
     The deposition system  160  also includes two load locks  168  and  174 , and buffer stations  170 A and  170 B. In the illustrated embodiment, a load mechanism  176  (e.g., a robotic load station) retrieves discrete substrates from a supply of discrete substrates and places them onto a substrate transport system. Once the final pass through the sputtering chamber  162  and selenization furnace  164  is completed, an unload mechanism  178  (e.g., a robotic unload station) removes the discrete substrates from the substrate transport system  180  after the discrete substrates emerge from the exit load lock  174 . 
     The sputtering chamber  162  includes a plurality of sputtering magnetrons  54 , such as planar magnetrons or rotating cylindrical magnetrons. In some embodiments in which a copper indium gallium diselenide film is deposited, the targets are composed of copper, indium, or alloys of each with gallium or aluminum. In various embodiments, the target material composition for each magnetron  54  varies with respect to the target material composition for the other magnetrons  54  so that a graded composition structure is achieved in the deposited film. 
     In various embodiments, the selenization furnace  164  operates in a temperature range of about 250° C. to 600° C. Optionally, the selenization furnace  164  can include a multiple zone resistive heater so that the temperature along the closed path  158  within the furnace varies. The two selenium traps  172  on each side of the selenization furnace  164  preferably are differentially pumped multistage traps. The selenium traps  172  prevent selenium that may escape the furnace  164  from entering the sputtering chamber  162  or adversely affecting other system components. 
     The sputtering chamber  162  and selenization furnace  164  are coupled to separate pump systems (not shown) to permit the vacuum levels for each of these zones to be independently controlled. Low conductance apertures, or substrate passages, at locations between system components and selenium traps  172  results in a high degree of vacuum isolation and enables more efficient vacuum control. 
     The cooling chamber  166  operates at atmospheric pressure is configured to reduce the temperature of the discrete substrates prior to a subsequent pass through the sputtering chamber  162  and selenization furnace  164 . Various forms of coolers may be employed. In one embodiment, a cold plate extending at least along a portion of the length of the cooling chamber  166  is mounted above the substrate path such that discrete substrates passing underneath are cooled by atmospheric conduction. 
       FIG. 7  is a flowchart representation of an embodiment of a method  200  of depositing a thin film, for example, a copper indium gallium diselenide film, on a substrate according to the invention. Referring to  FIGS. 6A ,  6 B and  7 , discrete substrates are loaded (step  202 ) on or into the substrate transport system  180  which transports the substrates into the load lock  168 . After the substrate environment is reduced to the appropriate vacuum level, the discrete substrates exit the load lock  168 , pass through the first buffer station  170 A and pass (step  204 ) through the sputtering chamber  162  where a layer of composite metal is deposited. The discrete substrates continue along the closed path and are transported (step  206 ) through the first selenium trap  172 A, the selenization furnace  164  and the second selenium trap  172 B. Subsequently, the discrete substrates pass through the second buffer station  170 B before entering the exit load lock  174  where the substrate environment is returned to atmospheric pressure. If it is determined (step  208 ) that further incremental deposition layers are to be deposited, the discrete substrates that leave the exit load lock  174  are transported (step  210 ) through the cooling chamber  166  before subsequent deposition and selenization occur (steps  204  and  206 ). If it is determined (step  208 ) that the last incremental layer has been deposited, the discrete substrates exit the exit load lock  174  and are unloaded (step  212 ) or removed from the substrate transport system  180 . The number of passes that the discrete substrates make along the closed path can be based on a variety of parameters, for example, the desired structure and thickness of the deposited films and the transport speed. 
     Although the embodiments of a system for discrete substrates described above relate to transporting the discrete substrates along a closed path, in alternative embodiments the system transports discrete substrates along an open path, that is, a path that includes two ends: a load end and an unload end.  FIG. 8  is a functional block diagram of an embodiment of one such system  182  where each discrete substrate passes through a group of system components that includes a metal deposition zone  152 , a selenization zone  154  and a cooling chamber  156 . Unlike the system  150  of  FIG. 5 , each additional incremental layer is deposited by a single pass through a subsequent group of system components that includes a metal deposition zone  152 , selenization zone  54  and cooling chamber  156 . By way of example, each group of system components can include a cooling chamber  166  and the various components between the load locks  168  and  174 , inclusive, as illustrated in  FIGS. 6A and 6B . Although the embodiment illustrated in  FIG. 8  shows three groups of system components, any number of groups that is greater than or equal to two can be used. It should be understood that the number of incremental layers that can be deposited on the discrete substrate is equal to the number of groups of system components. In still other embodiments, a system can include a combination of one or more closed paths and one or more open paths with each path having at least one group of system components. 
     Various embodiments of methods for depositing a copper indium gallium diselenide film on a web or discrete substrate are described above. Variations on these methods are possible and can be used to achieve desired properties. For example, it may be desirable to generate a CIGS film that where the content of gallium and indium vary along the thickness of the film. In certain embodiments, such a film increases in gallium content with decreasing distance to the substrate and increases in indium content with increasing distance from the substrate. 
     Conventional processes for creating a CIGS film with a gallium and indium gradient include first depositing copper, indium and gallium by a technique such as vacuum evaporation, sputtering, electroplating or inkjet printing and then performing a selenization step. The deposition step is performed in a manner to achieve the desired gallium and indium gradients; however, during the subsequent selenization, indium in contact with the selenium at temperatures of approximately 200° C. to 400° C. results in formation of indium selenide, a volatile compound that depletes the indium in the CIGS layer. In an alternative known procedure, a thin layer (e.g., 2 μm) of selenide is deposited onto the cold CIGS layer and then the substrate and deposited layers are subjected to a thermal process that rapidly increases the temperature to a value that is greater than the critical selenization temperature to achieve full selenization. This alternative procedure is difficult to control and may not completely prevent indium depletion. 
       FIG. 9  is a flowchart representation of an embodiment of a method  300  of depositing a CIGS film on a substrate that reduces or eliminates the problem of indium depletion during selenization.  FIG. 10  illustrates a CIGS film  250  formed of incremental bi-layers that are deposited and selenized when performing the method  300 . It should be recognized that one or more layers of material, such as a molybdenum layer for a back electrical contact, may be formed on the substrate prior to initiation of the method  300 . Initially, an incremental layer of indium  252 A is deposited (step  302 ) followed by deposition (step  304 ) of an incremental layer of copper gallium  252 B to create a first bi-layer  252  that is subsequently selenized (step  306 ). By “capping” the indium layer  252 A with the copper gallium layer  252 B, there is no direct contact of the indium with the selenium during the selenization process and therefore depletion of the indium through the creation of indium selenide is avoided. The next bi-layer  254  is formed first by depositing (step  308 ) an incremental indium layer  254 A that has increased indium content relative to the preceding incremental indium layer  252 A and then by depositing (step  310 ) a copper gallium layer  254 B that has decreased gallium content relative to the preceding incremental copper gallium layer  252 B. The second bi-layer  254  is then selenized (step  312 ). If another bi-layer is to be created (step  314 ), the method  300  returns to step  308  and continues through step  312  until the desired number of additional bi-layers (256, . . . , 258) are formed and selenized. In this manner, a full CIGS film  250  is formed with the desired gallium and indium content gradients. In preferred embodiments, the thickness of each incremental layer of indium or copper gallium is in a range of about 300 Å to about 1,500 Å. Although the embodiment of the method  300  includes increasing indium content and decreasing gallium content as each incremental indium layer and each incremental copper gallium layer is deposited, respectively, alternative embodiments can have opposite content gradients. Furthermore, the content gradients are not limited to constant values, that is, the indium content and gallium content along the thickness of the CIGS film  250  can vary in any desired manner. 
     The method  300  can be performed with a deposition system that can deposit the incremental layers of each bi-layer and then selenize each bi-layer. By way of examples, the method can be performed using the system  150  of  FIG. 5 , the system  160  of  FIGS. 6A and 6B , or the system  182  of  FIG. 8  to create the CIGS film on a discrete substrate. In another example, the apparatus  10  of  FIG. 1  can be adapted using the method  300  to create the CIGS film on a web substrate. 
       FIG. 11  is a flowchart representation of an alternative embodiment of a method  400  of depositing a CIGS film on a substrate and  FIG. 12  is an illustration of the structure of a CIGS film  260  formed of incremental layers that are formed according to the method  400 . The method  400  is effective for reducing or eliminating the depletion of indium during selenization processing. Reference is also made to the roll-to-roll deposition apparatus  10  shown in  FIG. 1  although it will be recognized that other deposition systems capable of depositing the desired CIGS film structure  260  of incremental layers onto a web substrate or a discrete substrate, and performing the appropriate selenization of layers can be used. 
     According to the illustrated embodiment of the method  400 , a web  34  is transported (step  402 ) in a forward direction. The web may include one or more layers of intervening material, such as a molybdenum layer, that are formed prior to initiation of the method  400 . The web  34  passes through the first sputtering zone  18 A where an incremental layer of indium  262 A is deposited (step  404 ) followed by deposition (step  406 ) of an incremental layer of copper gallium  262 B. The proper order of incremental layers is achieved by configuring the order of sputtering targets within the sputtering zone  18 A. After exiting the first sputtering zone  18 A, the web  34  is cooled (step  408 ) by cooling roll  58 A before entering the selenization furnace  66  to selenize (step  410 ) the bi-layer  262 . The incremental copper gallium layer  262 B “caps” the incremental indium layer  262 A, therefore there is no direct exposure of the incremental indium layer  262 A with selenium during the selenization process. 
     The web  34  is cooled (step  412 ) after exiting the selenization furnace  66  and then enters the second sputtering zone  18 B where a first incremental layer of copper gallium  264 A is deposited (step  414 ) and then a first incremental layer of indium  264 B is deposited (step  416 ). Preferably, the web  34  is cooled before wound on the take-up roll  50 . The web  34  is then transported (step  418 ) in the reverse direction so that it passes again through the second sputtering zone  18 B where a second incremental layer of indium  264 C and then a second incremental layer of copper gallium  264 D are deposited (steps  420  and  422 , respectively). The quad-layer  264  includes incremental indium layers  264 B and  264 C that have increased indium content relative to the preceding incremental indium layer  262 A. The quad-layer  264  includes incremental copper gallium layers  264 A and  264 D that have decreased gallium content relative to the preceding incremental copper gallium layer  262 B. The desired order of the incremental layers within the quad-layer  264  is achieved by appropriate arrangement of sputtering targets within the second sputtering zone  18 B. The last two incremental layers  264 C and  264 D can have different indium and gallium content than their counterparts in the first two incremental layers  264 B and  264 A, respectively, in order to continue the development of the desired gradients however, this is not a requirement. For example, gradients within the quad-layer  264  can be achieved by adjusting operating parameters and targets of the second sputtering zone  18 B between the forward and reverse passes. 
     After exiting the second sputtering zone  18 B in the reverse direction, the web  34  is cooled (step  424 ) by cooling roll  58 A before entering the selenization furnace  66  to selenize (step  426 ) the quad-layer  264 . The second incremental copper gallium layer  264 D “caps” the incremental indium layers  264 B and  264 C, and reduces indium depletion during the selenization of the quad-layer  264 . The web  34  is cooled (step  428 ) after exiting the selenization furnace  66 . If the CIGS film is not complete (step  430 ), the method  400  returns through steps  414  to  428  to generate and selenize the next quad-layer  266  with modifications to the indium and gallium content of the respective incremental layers to achieve the desired compositional gradients. The method  400  continues until it is determined (step  430 ) that the complete CIGS film  260  comprised of bi-layer  262  and all quad-layers  264 ,  266 , . . . ,  268  with the desired indium and gallium content gradients is formed. Subsequently, the magnetrons  54  of the sputtering chambers  18  are disabled (step  432 ) and the web  34  is cooled (step  434 ) for a final rewind. 
     In the embodiments of the methods  300  and  400  described above, the incremental layers, bi-layers and quad-layers are sufficiently thin so that the local distribution of indium and gallium has a negligible affect on the macro distribution of indium and gallium in the CIGS film. Moreover, there is a diffusion of the incremental layers into adjacent layers after selenization such that the discrete nature of each incremental layer is less apparent. Advantageously, by using copper gallium as a cap layer for each bi-layer or quad-layer, the generation of indium selenide during selenization and the corresponding depletion of indium from the CIGS film are prevented. Moreover, the methods  300  and  400  enable the desired indium and gallium content gradients to be formed in the CIGS film. 
     While the invention has been shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as recited in the accompanying claims. For example, in one such embodiment a thin layer of selenium is deposited onto each cap layer of copper gallium of the cooled substrate prior to the selenization of the bi-layer or quad-layer. The deposition of each selenium layer further improves the reduction in indium depletion during the selenization steps. In other variations, the number of incremental layers that are deposited and subsequently selenized is different from the bi-layer and quad-layer structures as long as the last deposited incremental layer is a cap layer of copper gallium.