Patent Publication Number: US-2012024695-A1

Title: Systems and methods for high-rate deposition of thin film layers on photovoltaic module substrates

Description:
FIELD OF THE INVENTION 
     The subject matter disclosed herein relates generally to systems and methods for deposition of thin films on a substrate, and more particularly to a high throughput system for deposition of multiple thin film layers on photovoltaic module substrates. 
     BACKGROUND OF THE INVENTION 
     Thin film photovoltaic (PV) modules (also referred to as “solar panels” or “solar modules”) are gaining wide acceptance and interest in the industry, particularly modules based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy (sunlight) to electricity. For example, CdTe has an energy bandgap of 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap (1.1 eV) semiconductor materials historically used in solar cell applications. Also, CdTe converts energy more efficiently in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in low-light (e.g., cloudy) conditions as compared to other conventional materials. 
     Typically, CdTe PV modules include multiple film layers deposited on a glass substrate before deposition of the CdTe layer. For example, a transparent conductive oxide (TCO) layer is first deposited onto the surface of the glass substrate, and a resistive transparent buffer (RTB) layer is then applied on the TCO layer. The RTB layer may be a zinc-tin oxide (ZTO) layer and may be referred to as a “ZTO layer.” A cadmium sulfide (CdS) layer is applied on the RTB layer. These various layers may be applied in a conventional sputtering deposition process that involves ejecting material from a target (i.e., the material source), and depositing the ejected material onto the substrate to form the film. 
     Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner. The capital costs associated with production of PV modules, particularly the machinery and time needed for deposition of the multiple thin film layers discussed above, is a primary commercial consideration. 
     Accordingly, there exists an ongoing need in the industry for an improved system for economically feasible and efficient large scale production of PV modules, particularly CdTe based modules. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     An apparatus is generally provided for sequential sputtering deposition of a target source material as a thin film on a photovoltaic module substrate. The apparatus includes a load vacuum chamber, a first sputtering deposition chamber, and a second sputtering deposition chamber. The load vacuum chamber is connected to a load vacuum pump configured to reduce the pressure within the load vacuum chamber to an initial load pressure. The first sputtering deposition chamber includes a first target, which can be configured to deposit a first thin film layer on a substrate. The second sputtering deposition chamber includes a second target, which can be configured to deposit a second thin film layer on a substrate. A conveyor system is operably disposed within the apparatus and configured for transporting substrates in a serial arrangement into and through load vacuum chamber, into and through the first sputtering deposition chamber, and into and through the second sputtering deposition chamber at a controlled speed. The first sputtering deposition chamber and the second sputtering deposition chamber are integrally connected such that the substrates being transported through the apparatus are kept at a system pressure that is less than about 760 Torr. 
     A process is also generally provided for manufacturing a thin film cadmium telluride thin film photovoltaic device. A substrate is transported into a load vacuum chamber connected to a load vacuum pump, and a vacuum is drawn in the load vacuum chamber using the load vacuum pump until an initial load pressure is reached in the load vacuum chamber. The substrate is then transferred from the load vacuum chamber into a first sputtering deposition chamber including a first target source material, and the first target source material is sputtered to form a first thin film layer on the substrate. The substrate is then transferred from the first sputtering deposition chamber into a second sputtering deposition chamber including a second target source material, and the second target source material is sputtered to form a second thin film layer on the first thin film layer. The substrate is transported through first sputtering deposition chamber and the second sputtering deposition chamber at a system pressure that is less than about 760 Torr. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  is a cross-sectional view of a CdTe photovoltaic module; 
         FIG. 2  shows a top plan view of an exemplary system in accordance with one embodiment of the present invention; 
         FIG. 3  is a perspective view of an embodiment of a substrate carrier configuration; 
         FIG. 4  is a perspective view of an alternative embodiment of a substrate carrier configuration; 
         FIG. 5  is diagrammatic view of an embodiment of a sputtering chamber for deposition of a thin film on a substrate; and, 
         FIG. 6  is a diagrammatic view of an alternative embodiment of a sputtering chamber. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     In the present disclosure, when a layer is described as “on” or “over” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers, unless otherwise expressly stated. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer. Additionally, although the invention is not limited to any particular film thickness, the term “thin” describing any film layers of the photovoltaic device generally refers to the film layer having a thickness less than about 10 micrometers (“microns” or “μm”). 
     It is to be understood that the ranges and limits mentioned herein include all ranges located within the prescribed limits (i.e., subranges). For instance, a range from about 100 to about 200 also includes ranges from 110 to 150, 170 to 190, 153 to 162, and 145.3 to 149.6. Further, a limit of up to about 7 also includes a limit of up to about 5, up to 3, and up to about 4.5, as well as ranges within the limit, such as from about 1 to about 5, and from about 3.2 to about 6.5. 
     Generally speaking, methods and systems are presently disclosed for increasing the efficiency and/or consistency of in-line manufacturing of cadmium telluride thin film photovoltaic devices. Specifically, a first sputtering deposition chamber and a second sputtering deposition chamber, separated by at least one buffer vacuum chamber, are present in the system  100 . The first sputtering deposition chamber, the vacuum buffer chamber(s), and the second sputtering deposition chamber are integrally interconnected such that substrates passing through and between these chambers are not exposed to the outside atmosphere. For example, the first sputtering deposition chamber and the second sputtering deposition chamber can be integrally connected such that the substrates being transported through the apparatus are kept at a system pressure that is less than about 760 Torr (e.g., less than about 250 mTorr, such as about 1 mTorr to about 100 mTorr). 
     In one particular embodiment, integrated systems and methods for thin film deposition of the resistive transparent buffer (RTB) layer and the cadmium sulfide layer on the substrate are generally disclosed. For example, the integrated systems and methods can be utilized to first deposit the RTB layer on the substrate. For instance, the RTB layer can be sputtered from a RTB target (e.g., including a zinc tin oxide (ZTO) target) onto a conductive transparent oxide layer on the substrate. The substrate can then be transferred from the first sputtering chamber to a vacuum buffer chamber to remove any particles from the substrate and/or chamber atmosphere before depositing subsequent layers (e.g., any excess particles in the first sputtering atmosphere). Then, the cadmium sulfide layer can be deposited on the RTB layer, such as by sputtering a sputtering target including cadmium sulfide. 
     As mentioned, the present system and method have particular usefulness for deposition of multiple thin film layers in the manufacture of PV modules, especially CdTe modules.  FIG. 1  represents an exemplary CdTe module  10  that can be made at least in part according to system and method embodiment described herein. The module  10  includes a top sheet of glass as the substrate  12 , which may be a high-transmission glass (e.g., high transmission borosilicate glass), low-iron float glass, or other highly transparent glass material. The glass is generally thick enough to provide support for the subsequent film layers (e.g., from about 0.5 mm to about 10 mm thick), and is substantially flat to provide a good surface for forming the subsequent film layers. 
     A transparent conductive oxide (TCO) layer  14  is shown on the substrate  12  of the module  10  in  FIG. 1 . The TCO layer  14  allows light to pass through with minimal absorption while also allowing electric current produced by the module  10  to travel sideways to opaque metal conductors (not shown). The TCO layer  14  can have a thickness between about 0.1 μm and about 1 μm, for example from about 0.1 μm to about 0.5 μm, such as from about 0.25 μm to about 0.35 μm. 
     A resistive transparent buffer (RTB) layer  16  is shown on the TCO layer  14 . This RTB layer  16  is generally more resistive than the TCO layer  14  and can help protect the module  10  from chemical interactions between the TCO layer  14  and the additional layers subsequently deposited during processing of the module  10 . In certain embodiments, the RTB layer  16  can have a thickness between about 0.075 μm and about 1 μm, for example from about 0.1 μm to about 0.5 μm. In particular embodiments, the RTB layer  16  can have a thickness between about 0.08 μm and about 0.2 μm, for example from about 0.1 μm to about 0.15 μm. In particular embodiments, the RTB layer  16  can include, for instance, a combination of zinc oxide (ZnO) and tin oxide (SnO 2 ), and is referred to as a zinc-tin oxide (“ZTO”) layer  16 . 
     The CdS layer  18  is shown on ZTO layer  16  of the module  10  of  FIG. 1 . The CdS layer  18  is a n-type layer that generally includes cadmium sulfide (CdS) but may also include other materials, such as zinc sulfide, cadmium zinc sulfide, etc., and mixtures thereof, as well as dopants and other impurities. The CdS layer  18  may include oxygen up to about 25% by atomic percentage, for example from about 5% to about 20% by atomic percentage. The CdS layer  18  can have a wide band gap (e.g., from about 2.25 eV to about 2.5 eV, such as about 2.4 eV) in order to allow most radiation energy (e.g., solar radiation) to pass. As such, the cadmium sulfide layer  18  is considered a transparent layer on the device  10 . 
     The CdTe layer  20  is shown on the cadmium sulfide layer  18  in the exemplary module  10  of  FIG. 1 . The CdTe layer  20  is a p-type layer that generally includes cadmium telluride (CdTe), but may also include other materials. As the p-type layer of the module  10 , the CdTe layer  20  is the photovoltaic layer that interacts with the CdS layer  18  (i.e., the n-type layer) to produce current from the absorption of radiation energy by absorbing the majority of the radiation energy passing into the module  10  due to its high absorption coefficient and creating electron-hole pairs. The CdTe layer  20  can have a bandgap tailored to absorb radiation energy (e.g., from about 1.4 eV to about 1.5 eV, such as about 1.45 eV) to create the maximum number of electron-hole pairs with the highest electrical potential (voltage) upon absorption of the radiation energy. Electrons may travel from the p-type side (i.e., the CdTe layer  20 ) across the junction to the n-type side (i.e., the CdS layer  18 ) and, conversely, holes may pass from the n-type side to the p-type side. Thus, the p-n junction formed between the CdS layer  18  and the CdTe layer  20  forms a diode in which the charge imbalance leads to the creation of an electric field spanning the p-n junction. Conventional current is allowed to flow in only one direction and separates the light induced electron-hole pairs. 
     The cadmium telluride layer  20  can be formed by any known process, such as vapor transport deposition, chemical vapor deposition (CVD), spray pyrolysis, electro-deposition, sputtering, close-space sublimation (CSS), etc. In particular embodiments, the CdTe layer  20  can have a thickness between about 0.1 μm and about 10 μm, such as from about 1 μm and about 5 μm. 
     A series of post-forming treatments can be applied to the exposed surface of the CdTe layer  20 . These treatments can tailor the functionality of the CdTe layer  20  and prepare its surface for subsequent adhesion to the back contact layer(s)  22 . For example, the cadmium telluride layer  20  can be annealed at elevated temperatures (e.g., from about 350° C. to about 500° C., such as from about 375° C. to about 424° C.) for a sufficient time (e.g., from about 1 to about 10 minutes) to create a quality p-type layer of cadmium telluride. Without wishing to be bound by theory, it is believed that annealing the cadmium telluride layer  20  (and the module  10 ) converts the normally lightly p-type doped, or even n-type doped CdTe layer  20  to a more strongly p-type layer having a relatively low resistivity. Additionally, the CdTe layer  20  can recrystallize and undergo grain growth during annealing. 
     Additionally, copper can be added to the CdTe layer  20 . Along with a suitable etch, the addition of copper to the CdTe layer  20  can form a surface of copper telluride (Cu 2 Te) on the CdTe layer  20  in order to obtain a low-resistance electrical contact between the cadmium telluride layer  20  (i.e., the p-type layer) and a back contact layer(s)  22 . 
     The back contact layer  22  generally serves as the back electrical contact, in relation to the opposite, TCO layer  14  serving as the front electrical contact. The back contact layer  22  can be formed on, and in one embodiment is in direct contact with, the CdTe layer  20 . The back contact layer  22  is suitably made from one or more highly conductive materials, such as elemental nickel, chromium, copper, tin, aluminum, gold, silver, technetium or alloys or mixtures thereof. Additionally, the back contact layer  22  can be a single layer or can be a plurality of layers. In one particular embodiment, the back contact layer  22  can include graphite, such as a layer of carbon deposited on the p-layer followed by one or more layers of metal, such as the metals described above. The back contact layer  22 , if made or comprised of one or more metals, is suitably applied by a technique such as sputtering or metal evaporation. If it is made from a graphite and polymer blend, or from a carbon paste, the blend or paste is applied to the semiconductor device by any suitable method for spreading the blend or paste, such as screen printing, spraying or by a “doctor” blade. After the application of the graphite blend or carbon paste, the device can be heated to convert the blend or paste into the conductive back contact layer. A carbon layer, if used, can be from about 0.1 μm to about 10 μm in thickness, for example from about 1 μm to about 5 μm. A metal layer of the back contact, if used for or as part of the back contact layer  22 , can be from about 0.1 μm to about 1.5 μm in thickness. 
     In the embodiment of  FIG. 1 , an encapsulating glass  24  is shown on the back contact layer  22 . Other components (not shown) can be included in the exemplary module  10 , such as bus bars, external wiring, laser etches, etc. The module  10  may be divided into a plurality of individual cells that, in general, are connected in series in order to achieve a desired voltage, such as through an electrical wiring connection. Each end of the series connected cells can be attached to a suitable conductor, such as a wire or bus bar, to direct the photovoltaically generated current to convenient locations for connection to a device or other system using the generated electric. A convenient means for achieving the series connected cells is to laser scribe the module  10  to divide the device into a series of cells connected by interconnects. Also, electrical wires can be connected to positive and negative terminals of the PV module  10  to provide lead wires to harness electrical current produced by the PV module  10 . 
       FIG. 2  represents an exemplary integrated deposition system  100  in accordance with aspects of the invention for deposition of multiple thin film layers on PV module substrates  12  ( FIGS. 3 and 4 ) that are conveyed through the system  100 . It should be noted that the system  100  is not limited by any particular type of thin film or thin film deposition process, as described in greater detail herein. In one embodiment, the system  100  can be utilized to sequentially deposit, via sputtering deposition, the RTB layer  16  over the TCO layer  14  and then the CdS layer  18  over the RTB layer  16 . 
     The integrated deposition system  100  shown in  FIG. 2  includes a load vacuum chamber  106 , a first sputtering chamber  112 , a vacuum buffer chamber  120 , and a second sputtering chamber  128 . Each of the chambers is integrally interconnected together such that the substrates  12  passing through the system  100  are substantially protected from the outside environment within the integrated vacuum  101 . In other words, the chambers  112 ,  120 , and  128  of the system  100  are directly integrated together such that a substrate  12  exiting one chamber immediately enters the adjacent section directly, without exposure to the room atmosphere. Thus, the substrates  12  can be protected from outside contaminants being introduced into the thin films, resulting in more uniform and efficient devices. Of course, other intermediary chambers may be included within the system  100 , as long as the system remains integrally interconnected to the other chambers of the system  100 . 
     Through the integration of these deposition chambers into a single system, the energy consumption required for the deposition of the sputtered layers (e.g., a RTB layer and a CdS layer) can be reduced, when compared from separated deposition systems, during the manufacturing of a cadmium telluride thin film device. For instance, once the load vacuum is drawn in the load vacuum chamber  106 , no need for an additional load vacuum chambers exists, since the system pressure can remain below atmospheric pressure (i.e., about 760 Torr) through the first sputtering chamber  112 , the vacuum buffer chamber  120 , and the second sputtering chamber  128 . For example, in certain embodiments, the system pressure can remain below 250 Ton, such as about 3 mTorr to about 100 Ton. In one particular embodiment, the system pressure can remain below the initial load vacuum pressure (e.g., less than about 250 mTorr). For example, in one embodiment, the system pressure can be substantially constant through the first sputtering chamber  112 , the vacuum buffer chamber  120 , and the second sputtering chamber  128  (and any chambers positioned therebetween). 
     The illustrated system  100  includes a loading system  171  wherein substrates  12  are loaded onto carriers  122  and then conveyed into the load vacuum chamber  106 . The substrates  12  may be loaded into the carriers  122  in a load station  152  by automated machinery  153  from the supply conveyor  155 . For example, robots or other automated machinery may be used for this process. In an alternative embodiment, the substrates  12  may be manually loaded onto carriers  122 . 
     As shown in  FIG. 2 , the individual substrates  12  first enter the load vacuum chamber  106  through the entry slot  102 . The first entry slot  102  defines a flap  103  that can close to separate the internal atmosphere within the load vacuum chamber  106  from the outside environment. The load vacuum chamber  106  is connected to a load vacuum pump  108  configured to draw a load pressure within the load vacuum chamber  106 . Specifically, the load vacuum pump  108  can reduce the pressure within the load vacuum chamber  106  to an initial load pressure of about 1 mTorr to about 250 mTorr. 
     The substrates  12  can then pass from the load vacuum chamber  106  into the fine vacuum chamber  110  connected to the fine vacuum pump  111  that can reduce the pressure to an increased vacuum. For instance, the fine vacuum chamber(s)  110  can reduce the pressure to about 1×10 −7  Torr to about 1×10 −4  Ton, and then be backfilled with an inert gas (e.g., argon) in a subsequent chamber within the system  100  (e.g., within the sputtering deposition chamber  112 ) to a deposition pressure (e.g., about 10 mTorr to about 100 mTorr). 
     In the embodiment shown, the individual carriers  122  associated with the adjacently disposed vertical substrates  12  are controlled so as to convey the substrates  12  through the system at a controlled, constant linear speed to ensure an even deposition of the thin film onto the surface of the substrates  12 . On the other hand, the carriers  122  and substrates  12  are introduced in a step-wise manner into and out of system  100 . In this regard, the load vacuum chamber  106  and the fine vacuum chamber  110  are configured with vacuum lock valves  154  with associated controllers  156 . Additional, non-vacuum modules at the entry for loading the carriers  122  into the system  100 , and buffering the carriers  122  relative to the outside atmosphere may also be included. 
     For example, referring to  FIG. 2 , the system  100  includes a plurality of adjacently disposed vertical processing modules. A first one of these modules (i.e., the load vacuum chamber  106 ) defines an entry vacuum valve  103 , which may be, for example, a gate-type slit valve or rotary-flapper valve that is actuated by an associated actuator  156 . The initial valve  103  is open and a carrier  122  is conveyed into the load vacuum chamber  106  from the load module  152 . The entry valve  103  is then closed. At this point, the “rough” vacuum pump  108  pumps from atmosphere to an initial “rough” vacuum in the millitorr range. The rough vacuum pump  162  may be, for example, a claw-type mechanical pump with a roots-type blower. Upon pumping to a defined crossover pressure, the valve  154  between the load vacuum chamber  106  and an adjacent fine vacuum chamber  110  is opened and the carrier  122  is transferred into the fine vacuum chamber  110 . The valve  154  between the chambers  106  and  110  is then closed, the load vacuum chamber  106  is vented, and the initial valve  103  is opened for receipt of the next carrier  122  into the module. A “high” or “fine” vacuum pump  111  draws an increased vacuum in the fine vacuum chamber  110 , and the fine vacuum chamber  110  may be backfilled with process gas to match the conditions in the downstream processing chambers. The fine vacuum pump  111  may be, for example, a combination of cryopumps or turbo molecular pumps configured for pumping down the module to about less than or equal to 9×10 −5  torr. Finally, the valve  154  between the fine vacuum chamber  110  and the integrated chamber  101  is opened and the carrier  122  is transferred into the first module of the integrated chamber  101  (e.g., an optional heating chamber  124  or the first sputtering chamber  119 ). 
     The substrates  12  are then transferred from the load vacuum chamber  106  and fine vacuum chamber  110  to the first sputtering deposition chamber  112  and second sputtering chamber  128 . Between the first sputtering deposition chamber  112  and second sputtering chamber  128  is a buffer vacuum chamber  120  connected to the buffer vacuum pump  123  configured to remove any residual particles from the atmosphere and/or substrates  12  passing therethrough. As such, the buffer vacuum chamber  120  can inhibit cross-contamination between the first sputtering deposition chamber  112  and the second sputtering chamber  128 . In one embodiment, a backfill gas port configured to provide an inert gas to the vapor deposition temperature can be included within the vacuum buffer pump  122 . In one particular embodiment, the buffer vacuum chamber  120  can include slit valves on its entry slit and/or its exit slit to further inhibit cross-contamination between the first sputtering deposition chamber  112  and second sputtering chamber  128 . 
     Sputtering deposition generally involves ejecting material from a target, which is the material source, and depositing the ejected material onto the substrate to form the film. DC sputtering generally involves applying a direct current to a metal target (i.e., the cathode) positioned near the substrate (i.e., the anode) within a sputtering chamber to form a direct-current discharge. The sputtering chamber can have a reactive atmosphere (e.g., including sulfur in addition to oxygen, nitrogen, etc.) that forms a plasma field between the metal target and the substrate. Other inert gases (e.g., argon, etc.) may also be present. The pressure of the reactive atmosphere can be between about 1 mTorr and about 20 mTorr for magnetron sputtering. The pressure can be even higher for diode sputtering (e.g., from about 25 mTorr to about 100 mTorr). When metal atoms are released from the target upon application of the voltage, the metal atoms deposit onto the surface of the substrate. For example, when the atmosphere contains oxygen, the metal atoms released from the metal target can form a metallic oxide layer on the substrate. The current applied to the source material can vary depending on the size of the source material, size of the sputtering chamber, amount of surface area of substrate, and other variables. In some embodiments, the current applied can be from about 2 amps to about 20 amps. Conversely, RF sputtering involves exciting a capacitive discharge by applying an alternating-current (AC) or radio-frequency (RF) signal between the target (e.g., a ceramic source material) and the substrate. The sputtering chamber can have an inert atmosphere (e.g., an argon atmosphere) which may or may not contain reactive species (e.g., oxygen, nitrogen, etc.) having a pressure between about 1 mTorr and about 20 mTorr for magnetron sputtering. Again, the pressure can be even higher for diode sputtering (e.g., from about 25 mTorr to about 100 mTorr). 
     As shown, the each of the first sputtering deposition chamber  112  and the second sputtering chamber  128  generally includes a target  114  connected to a power source  116  (e.g., a DC or RF power source) via wires  117 . The power source  116  is configured to control and supply power (e.g., DC, RF, or pulsed DC power) to the sputtering deposition chamber  112 . As shown in  FIGS. 5 and 6 , the power source  116  applies a voltage to the target  114  (acting as the cathode) to create a voltage potential between the target  114  and an anode formed by the shields  115  and the chamber walls  117 , such that the substrates  12  is within the magnetic fields formed therebetween. Although only a single power source  116  is shown for each target  114 , the voltage potential can be realized through the use of multiple power sources coupled together. 
     The substrates  12  are generally positioned within the sputtering deposition chamber  112  such that a thin film layer (e.g., a RTB layer or a CdS layer) is formed on the surface of the substrates  12  facing the target  114 . A plasma field  118  is created once the sputtering atmosphere is ignited, and is sustained in response to the voltage potential between the target  114  and the chamber walls  110  acting as an anode. The voltage potential causes the plasma ions within the plasma field  118  to accelerate toward the target  114 , causing atoms from the target  114  to be ejected toward the surface on the substrate  12 . As such, the target  114  (also can be referred to as the cathode) acts as the source material for the formation of the thin film layer on the surface of the substrate  12  facing the target  114 . 
     A sputtering atmosphere control system  119  can control the sputtering atmosphere within the sputtering deposition chamber  112 , such as reducing to the sputtering pressure (e.g., about 10 to about 25 mTorr). Generally, the sputtering atmosphere control system  119  can provide an inert gas (e.g., argon) to the sputtering deposition chamber  112 . Optionally, the sputtering atmosphere can also include oxygen, allowing oxygen particles of the plasma field  118  to react with the ejected target atoms to form a thin film layer that includes oxygen. A sputtering vacuum  121  can also be included to control the pressure in the sputtering chamber  112 . 
     For example, the sputtering deposition chamber  112  can be utilized to form a cadmium sulfide layer on the substrate. In this embodiment, the target  114  can be a ceramic target, such as of cadmium sulfide. Additionally, in some embodiments, a plurality of targets  114  can be utilized. A plurality of targets  114  can be particularly useful to form a layer including several types of materials (e.g., co-sputtering). 
     Optionally, the substrates  12  can be transferred into and through a heating chamber  124  positioned prior to either of the first sputtering deposition chamber  112  and the second sputtering chamber  128 , such as shown in  FIG. 1 . The heating chamber  124  can include a heating element  126  configured to heat the substrates  12  to a sputtering temperature prior to entering the sputtering chamber  112  and/or  128 , such as about 50° C. to about 250° C., depending on the parameters of the sputtering deposition. In an alternative embodiment, the sputtering chamber  112  and/or  128  can optionally include heaters  127  configured to heat the substrates  12  within the sputtering chamber  112  and/or  128  (as shown in  FIG. 5 ) instead of, or in addition to, the heating chamber  124 . 
     To exit the system  100 , the substrates  12  can pass through an optional exit buffer vacuum chamber  140  connected to a buffer vacuum  121 . The substrates  12  can then pass through a series of exit valves  154  controlled by independent motors  156  to exit the system  100  while maintaining the vacuum within the integrated chamber  101 . As such, the carriers  122  and substrates  12  can pass through the valve  154  between the exit buffer vacuum chamber  140  and into a first exit lock chamber  142  connected to a first exit lock pressure system  143 . The valve  154  can then be closed, and the first exit lock chamber  142  vented to a “rough” exit pressure. Then, the valve  154  between the first exit lock chamber  142  and the second exit lock chamber  144  can be opened and the substrates  12  conveyed therethrough. The valve  154  between the first exit lock chamber  142  and the second exit lock chamber  144  can then be closed and the second exit lock chamber  144  vented to atmospheric pressure. The exit valve  146  can then be opened, and the carriers  122  removed from the system  100  through the exit slot  147 . The substrates  12  can be removed from the carrier  122 , and placed on the post-processing conveyor  150  for further processing via machine arm  153 . The carriers  122  can then be returned to the start of the system  100  via return conveyor  160 . 
     Carriers  122  can have one or more substrates loaded thereon are introduced into the system  100 . In the embodiment shown in  FIGS. 2 and 6 , the carriers  122  can be configured for simultaneous deposition of substrates  12  positioned back-to-back. 
     Each of the chambers may include an independently driven and controlled conveyor system  162  for moving the substrate carriers  122  in a controlled manner through the respective chambers. In particular embodiments, the conveyors  162  may be roller-type conveyors, belt conveyors, and the like. The conveyors  162  for each of the respective chambers may be provided with an independent drive (not illustrated in the figure). 
     The various substrates  12  can be vertically oriented in that the carriers  122  to convey the substrates  12  in a vertical orientation through the system  100 . Referring to  FIG. 4 , an exemplary carrier  122  is illustrated as a frame-type of structure made from frame members  170 . The frame members  170  define receipt positions for substrates  12  such that the substrates  12  are horizontally or vertically received (relative to their longitudinal axis) within the carrier  122 . It should be appreciated that the carrier  122  may be defined by any manner of frame structure or members so as to carry one or more of the substrates  12  in a vertical orientation through the processing sides. In the embodiment of  FIG. 4 , the carrier  122  is configured for receipt of two substrates  12  in a horizontal position. It should be readily appreciated that the multiple substrates  12  could also be disposed such that the longitudinal axis of the respective substrates is in a vertical position. Any orientation of the substrates  12  within the carrier  122  is contemplated within the scope and spirit of the invention. The frame members  124  may define an open-type of frame wherein the substrates  12  are essentially received within a “window opening” defined by the carrier  122 . In an alternative embodiment, the carrier  122  may define a back panel against which the substrates  12  are disposed. 
     The embodiment of the carrier  122  illustrated in  FIG. 5  is configured for receipt of four substrates  112 , wherein pairs of the substrates  12  are in a back-to-back relationship. For example, a pair of the substrates  12  is disposed in the upper frame portion of the carrier  112 , and a second pair of the substrates  12  is disposed in the lower frame portion of the carrier  112 . The configuration of  FIG. 5  may be used when four or more of the substrates  12  are simultaneously processed in the system  100 , as described in greater detail below with respect to the deposition apparatus illustrated in  FIG. 7 . 
     Referring again to  FIG. 2 , the system  100  may be particularly configured with at least two vertical sputtering chambers for subsequent deposition of a zinc-tin oxide (ZTO) layer on the substrates conveyed therethrough and then a cadmium sulfide (CDS) layer on the ZTO layer. Operation of vacuum sputtering chambers is well known to those skilled in the art and need not be described in detail herein. 
       FIG. 5  shows a general schematic cross-sectional view of an exemplary vertical deposition chamber  119 . A power source  116  is configured to control and supply DC or RF power to the chamber  119 . In the case of a DC chamber  119 , the power source  116  applies a voltage to the cathode  114  to create a voltage potential between the cathode  114  and an anode. In the illustrated embodiment, the anode is defined by the shield  115  and the chamber wall  117 . The glass substrates  12  are held by the carrier  122  so as to be generally opposite from the cathode  114  (which is also the target source material). A plasma field  118  is created once the sputtering atmosphere is ignited and is sustained in response to the voltage potential between the cathode  114  and the anode. The voltage potential causes the plasma ions within the plasma field  118  to accelerate towards the cathode  114 , causing atoms from the cathode  114  to be ejected towards the surface of the substrates  12 . As such, the cathode  114  is the “target” and is defined by the source material for formation of the particular type of thin film desired on the surface of the substrates  12 . For example, the cathode  114  can be a metal alloy target, such as elemental tin, elemental zinc, or mixtures of different metal alloys. Oxygen in the chamber  166  reacts with the ejected target atoms to form an oxide layer on the substrates  12 , such as a ZTO layer. 
     A cadmium sulfide (CdS) thin film layer may be formed in an RF sputtering chamber  119  ( FIG. 5 ) by applying an alternating-current (AC) or radial-frequency (RF) signal between a ceramic target source material and the substrates  12  in an essentially inert atmosphere. 
     Although single power sources are illustrated in  FIGS. 5 and 6 , it is generally understood that multiple power sources may be coupled together with a respective target source for generating the desired sputtering conditions within the chamber  166 . 
       FIG. 5  illustrates a heater element  127  within the chamber  119 . Any manner or configuration of heater elements may be configured within the chamber  119  to maintain a desired deposition temperature and atmosphere within the chamber. 
     In the embodiment of  FIG. 5 , the vertical deposition module  128  is configured for deposition of a thin film layer on the side of the substrates  12  oriented towards the target source material  114 .  FIG. 6  illustrates an embodiment wherein the chamber  119  includes dual sputtering systems for applying a thin film onto the outwardly facing surfaces of the back-to-back substrates  12  secured in the carriers  122 , such as the carrier  122  configuration illustrated and described above with respect to  FIG. 4 . Thus, with the vertical deposition module  119  illustrated in  FIG. 6 , four substrates are simultaneously processed for deposition of a particular thin film layer thereon. 
     The system  100  in  FIG. 2  is defined by a plurality of interconnected chambers, as discussed above, with each of the chambers serving a particular function. The respective conveyors configured with the individual modules are also appropriately controlled for various functions, as well as the valves  154  and associated actuators  156 . For control purposes, each of the individual chambers may have an associated controller  166  configured therewith to control the individual functions of the respective module. 
     It should be readily appreciated that, although the deposition chambers  119  are described herein in particular embodiments as sputtering deposition modules, the invention is not limited to this particular deposition process. The vertical deposition chambers  119  may be configured as any other suitable type of processing chamber, such as a chemical vapor deposition chamber, thermal evaporation chamber, physical vapor deposition chamber, and so forth. In the particular embodiments described herein, the first deposition chamber may be configured for deposition of a ZTO layer and the second deposition chamber may be configured for deposition of a CdS layer on the ZTO layer. Each chamber  119  may be configured with four DC water-cooled magnetrons. As mentioned above, each chamber  119  may also include one or more vacuum pumps mounted on the back chambers between each cathode pair. 
     The present invention also encompasses various process embodiments for deposition of multiple thin film layers on a photovoltaic (PV) module substrate. The processes may be practiced with the various system embodiments described above or by any other configuration of suitable system components. It should thus be appreciated that the process embodiments according to the invention are not limited to the system configuration described herein. 
     In a particular embodiment, the process includes transporting the substrates into a load vacuum chamber connected to a load vacuum pump to draw a vacuum in the load vacuum chamber using the load vacuum pump until an initial load pressure is reached in the load vacuum chamber. Optionally, the substrate can be transported into and through a buffer vacuum chamber and/or a heating chamber as discussed above with respect to  FIG. 2 . The substrate can then be transferred from the load vacuum chamber into a first sputtering deposition chamber including a first target source material (e.g., including zinc and tin or a zinc/tin oxide), where the first target source material can be sputtered to form a first thin film layer (e.g., a resistive transparent buffer layer) on the substrate. The substrate can then transferred from the first sputtering deposition chamber into a second sputtering deposition chamber including a second target source material (e.g., cadmium sulfide), where the second target source material can be sputtered to form a second thin film layer (e.g., a CdS layer) on the first thin film layer. The substrate can be transported through the load vacuum chamber, the first sputtering deposition chamber, and the second sputtering deposition chamber at a system pressure that is less than about 760 Torr. Optionally, the substrates can be transported into and through a buffer vacuum chamber as discussed above with respect to  FIG. 2 . 
     The process may include moving the carriers and attached substrates into and out of vacuum chambers in a step-wise manner, for example through a series of vacuum locks, yet conveying the carriers and attached substrates through the vacuum chambers at a continuous linear speed during the deposition process. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.