Abstract:
The present inventions provide method and apparatus that employ constituents from one or more constituent supply source or sources to form one or more films of a precursor layer formed on a surface of a continuous flexible workpiece. Of particular significance is the implementation of PVD systems that operate upon a horizontally disposed portion of a continuous flexible workpiece and a vertically disposed portion of a continuous flexible workpiece, preferably in conjunction with a short free-span zone of the portion of a continuous flexible workpiece.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/342,648 filed Jan. 3, 2012, which is a continuation of U.S. patent application Ser. No. 12/769,321 filed Apr. 28, 2010, now U.S. Pat. No. 8,088,224, which claims priority and is related to U.S. Provisional Patent Application Ser. No. 61/295,567 filed Jan. 15, 2010, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE RELATED ART 
       [0002]    Described are methods and apparatus for preparing thin films of semiconductor films for radiation detector and photovoltaic applications. 
       BACKGROUND 
       [0003]    Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970&#39;s there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. 
         [0004]    Group IBIIIAVIA compound semiconductors are excellent absorber materials for thin film solar cell structures. Group IBIIIAVIA compound semiconductors includes some of: the Group IB elements of the periodic table such as copper (Cu), silver (Ag), and gold (Au); the Group IIIA elements of the periodic table such as boron (B), aluminum (Al), gallium (Ga), indium (In), and (Tl); and, the Group VIA elements of the periodic table such as oxygen ( 0 ), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se) 2  or CuIn 1-x Ga x (S y Se 1-y ) k , where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications. Alkali metals of Group IA, such as K, Na and Li are often included in the CIGS(S) absorbers as dopants to improve their photovoltaic properties. 
         [0005]    The structure of a conventional Group IBIIIAVIA compound photovoltaic cell  10  such as a Cu(In,Ga,Al)(S,Se,Te) 2  thin film solar cell is shown in  FIG. 1 . The photovoltaic cell  10  includes a base  11  having a substrate  12  and a conductive layer  13  formed on the substrate. The substrate  12  can be a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. An absorber thin film  14 , which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te) 2 , is formed on the conductive layer  13 . The conductive layer can be a Mo, Ta, W, or Ti layer, and functions as an ohmic contact to the photovoltaic cell. However, if the substrate  12  is a properly selected conductive material such as Ta foil or Mo foil, it is also possible not to use a conductive layer, since the substrate  12  can be used as an ohmic contact to the photovoltaic cell. After the absorber film  14  is formed, a transparent layer  15 , for example, a CdS, ZnO or CdS/ZnO film stack is formed on the absorber film. Light  16  enters the photovoltaic cell  10  through the transparent layer  15 . Metallic grids (not shown) are formed over the transparent layer  15  to reduce the effective series resistance of the device. The preferred electrical type of the absorber film  14  is p-type, and the preferred electrical type of the transparent layer  15  is n-type. However, an n-type absorber and a p-type window layer can also be formed. The device structure shown  FIG. 1  is called a substrate-type structure. A so called superstrate-type structure can also be formed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te) 2  absorber film, and finally forming an ohmic contact to the device by a conductive layer. In the superstrate structure light enters the device from the transparent superstrate side. 
         [0006]    One technique for growing Cu(In,Ga)(S,Se) 2  type absorber thin films for solar cell applications is a two-stage process where metallic components or constituents of the Cu(In,Ga)(S,Se) 2  material, i.e. Cu, In and Ga, are first deposited onto a substrate, and then reacted with the non-metallic constituents (or semi-metallic constituents), i.e. S and/or Se, in a high temperature annealing process. Alternatively, Group VIA material layers can be also included in the precursor. For example, Se and/or S can be deposited over a stack of Cu, In and/or Ga films, and this precursor stack is annealed at elevated temperatures (400-600° C.) to initiate reaction between the metallic elements and the Group VIA material(s) to form the Cu(In,Ga)(S,Se) 2  compound. During the annealing, additional Se and/or S sources, such as Se and S vapors, or Se and S containing gases can be also delivered into the reactor. Selenium vapor migration to adjacent stations and equipment is an important problem in deposition systems using Se evaporators. In such systems, during the deposition, selenium vapor migrates to adjacent deposition stations or end stations and the mechanisms such as winding mechanisms in the end stations or various rollers supporting the web during the deposition. One prior art solution is to increase free-span distance of the web to move adjacent stations and winding mechanisms further away from selenium evaporators. However there are drawbacks with this solution, because a longer span causes the web to droop with a catenary shape, which results in degradation of uniformity in the depositing layer due to uneven tension. Such large free span also increases system footprint. The absorber layer  14  shown in  FIG. 1  may contain dopant elements, such as Na to enhance cell performance, in addition to the primary elements (Cu, In, Ga, Se and/or S) required to form the absorber layer. Prior research on possible dopants for Group IBIIIAVIA absorber layers has shown that alkali metals, such as Na, K, and Li, affect the structural and electrical properties of such absorber layers. Especially, inclusion of Na in CIGS layers was shown to be beneficial for their structural and electrical properties and for increasing the conversion efficiencies of solar cells fabricated on such layers provided that its concentration is well controlled. 
         [0007]    Design of a system to carry out Group VIA material and/or dopant material deposition is critical for the quality of the resulting absorber film, the efficiency of the solar cells, throughput, material utilization and cost of the process. The deposition flux from thermal sources tends to vary considerably during the deposition processes. Having the ability to measure and control deposition flux is critical for process stability. Therefore, there is need for new processes and tools to deposit such layers efficiently to form high quality, low cost CIGS type absorber layers for solar cells. 
       SUMMARY 
       [0008]    Described are methods and apparatus that employ constituents deposited using a PVD process from one or more constituent supply source or sources to form one or more films of a precursor layer formed on a surface of a continuous flexible workpiece. 
         [0009]    Of particular significance is the implementation of PVD deposition systems that operate upon a horizontally disposed portion of a continuous flexible workpiece and a vertically disposed portion of a continuous flexible workpiece, preferably in conjunction with a short free-span zone of the portion of a continuous flexible workpiece. 
         [0010]    In one aspect is provided A roll-to-roll PVD deposition system for depositing a plurality of films of Group IA and Group VIA materials on a front surface of a continuous sheet shaped workpiece that is advanced in a process direction, comprising: a process housing through which the continuous sheet shaped workpiece is advanced between an entrance opening and an exit opening of the process housing, the process housing including a first process section located by a horizontal peripheral wall of the process housing and a second process section located by a vertical peripheral wall of the process housing, wherein the first process section is associated with a horizontally disposed portion of the continuous sheet shaped workpiece and the second process section is associated with a vertically disposed portion of the continuous sheet shaped workpiece; a workpiece tensioning and drive assembly for advancing the continuous sheet shaped workpiece in the process direction between the entrance opening and the exit opening of the process housing; a first PVD unit disposed at the first process section to continuously deposit a first film onto the horizontally disposed portion of the continuous sheet shaped workpiece by vertically directing a first material toward the first process section as the continuous sheet shaped workpiece is advanced through the at least first PVD unit; and a second PVD unit disposed at the second process section to continuously deposit a second film that is different from the first film onto the vertically disposed portion of the continuous sheet shaped workpiece by horizontally directing a second material toward the second process section as the continuous sheet shaped workpiece is advanced through the at least second PVD unit; wherein one of the first and second PVD units deposits one of the first and second respective materials directly onto a front surface portion of the continuous sheet shaped workpiece to obtain a new front surface portion, wherein the workpiece tensioning and drive assembly includes a plurality of rollers, wherein different portions of only a backside of the continuous sheet shaped workpiece rests on the plurality of rollers, and wherein each of the first and second PVD units continuously vapor deposit toward a frontside of the continuous sheet shaped workpiece and the other of the first and second PVD units deposits the other of the first and second respective material onto the new front surface portion. 
         [0011]    In a preferable aspect to the above, the first PVD station deposits directly onto the front surface portion of the continuous sheet shaped workpiece to obtain the new front surface portion and the second PVD station deposits onto the new front surface portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other aspects and features will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures, wherein: 
           [0013]      FIG. 1  is a cross-sectional schematic illustration of an exemplary solar cell structure of the prior art; 
           [0014]      FIG. 2A  is a cross-sectional schematic illustrations of a continuous flexible workpiece having a partial precursor layer; 
           [0015]      FIG. 2B  is a cross-sectional schematic illustrations of the continuous flexible workpiece of  FIG. 2A  having a precursor layer formed in a PVD system; 
           [0016]      FIG. 3A  is a side view schematic illustration of an embodiment of a PVD system; 
           [0017]      FIG. 3B  is a schematic illustrations of free span positions of the PVD system shown in  FIG. 3A ; and 
           [0018]      FIGS. 4A-4B  are schematic side and front illustrations of a sealable gate as the flexible workpiece is advanced through an opening of the sealable gate in a free span position. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    The preferred embodiments as describe herein provide systems to deposit multiple material layers using physical vapor deposition (PVD) techniques such as evaporation and sputtering, and methods that use the systems. Specifically are discussed methods and apparatus to deposit Group VIA and Group IA materials to form precursor layers for CIGS(S) type absorbers of solar cells or photovoltaic cells; in an in-line manner, preferably in a roll-to-roll or reel to reel manner. In-line processing where a precursor or a portion of a precursor is formed on a workpiece while the workpiece is moved continuously through a deposition system is attractive for manufacturing. Roll-to-roll processing technology increases throughput and minimizes substrate handling. 
         [0020]    One embodiment comprises: a first deposition station to deposit a first material, for example, a Group material IA material, or a dopant material, on a surface of a continuous flexible workpiece, such as one of Na, K and Li; and a second deposition station to deposit a second material, for example, a Group VIA material such as Se over the Group IA material deposited in the first deposition station. The first and second deposition stations may preferably be PVD stations, such as sputter deposition that deposit material as atoms or evaporation deposition stations that deposit material as vapor. Both PVD stations may be sputter deposition stations or one may be a sputter deposition station and the other may be an evaporation deposition station. The thickness of the films deposited in the first and second deposition stations in this embodiment may depend on the thickness of the precursor layers and resultant absorber layer thickness, and may be within the range of 10 to 50 nm and 1 to 4 μm, respectively. The continuous flexible workpiece may include a base having a flexible substrate, such as a stainless steel or aluminum foil substrate or web, and a contact layer such as a Mo, W, Ru, Os and Ir layer, or their multilayer stacks including two or more layers, or other materials used as solar cell contact layers. The workpiece may be a stainless steel web of thickness between 25 and 100 μm and width between 300 and 1000 mm or wider. The continuous flexible workpiece also includes a first portion of a precursor layer, comprising at least some of the precursor materials to form an absorber layer, formed over the contact layer. As will be described more fully below, in this embodiment, the system is used to form a second portion of the precursor layer on the first portion to complete the precursor structure before an annealing and reaction step described above in the background section. The second portion of the precursor layer includes, in a particular embodiment, the first material, e.g., a Group IA material, and the second material, e.g., a Group VIA material. The precursor materials of the first portion of the precursor layer may comprise the constituents of a CIGS(S) type absorber layer such as Cu, In and Ga, and optionally Se. The first portion of the precursor layer may be formed as a stack including films of the constituent materials, or films including their alloys, deposited on top of each other in various orders, such as Cu/In/Ga, Cu/Ga/Cu/In, Cu/In/Ga/Se, Cu/Ga/Cu/In/Se or any other order combination. 
         [0021]    During the deposition process, the continuous flexible workpiece may be supplied from a supply roll; advanced through the first and the second deposition stations along a process direction to form the precursor layer; and picked up and wound as a receiving roll. In the system, the first deposition station includes a first deposition chamber to deposit the first material onto the workpiece and the second deposition station includes a second deposition chamber to deposit the second material onto the first material. The first and second deposition chambers are isolated from one another so that the material produced in one of the chamber does not migrate to the other chambers or outside of them, preferably both. The first and the second deposition chambers are preferably elongated chambers that extend along a first process axis and a second process axis, respectively. The first process axis may be a horizontal axis and the second process axis may be a vertical axis that is perpendicular to the first process axis. In this configuration, as the workpiece is fed from the supply roll into the first deposition chamber in the process direction, the workpiece travels parallel to the first process axis while the first material is deposited onto the workpiece surface. The workpiece with the first material leaves the first deposition chamber and enters the second deposition chamber where the workpiece travels vertically parallel to the second process axis while the second material deposits onto the first material, thereby forming the precursor layer. The workpiece with the precursor layer leaves the second deposition chamber and is wound around the receiving roll. 
         [0022]      FIGS. 2A  show an exemplary unprocessed portion  100 A of a continuous flexible workpiece  100  (shown in  FIG. 3 ) to process using the system  200  (shown in  FIG. 3 ). The continuous flexible workpiece  100  is also referred to as workpiece herein. The unprocessed portion  100 A includes a base  102  having a substrate  104 , such as a stainless steel foil, and a contact layer  106  such as a Mo, W, Ru, Os or Ir layer, formed over the substrate  104 . The contact layer may also be a Mo layer deposited onto the substrate and a Ru layer deposited on the Mo layer. A thin Cu layer with a thickness between 10 and 100 nm may be deposited on the Ru layer. A first precursor layer portion  108 A including Cu, In, and Ga, and optionally Se is formed over the contact layer  106 . The first precursor layer portion  108 A may be formed using any deposition methods such as electroplating, evaporation, sputtering, nano particle coating and the like. 
         [0023]      FIG. 2B  shows a processed portion  100 B of the workpiece  100  (shown in  FIG. 3 ) including a precursor layer  110  formed by depositing a second precursor layer portion  108 B onto the first precursor layer portion  108 A using the system  200 . The first and second precursor layer portions form the precursor layer  110 . The second precursor layer portion  108 B may be formed using two deposition steps which preferably employ a PVD process. In a first step of the process, a first film  112  of a first material including a dopant material, such as Na, may be deposited onto the first precursor layer portion  108 A of the unprocessed portion  100 A of the workpiece  100 . In a second step of the process, a second film  114  of a second material including a Group VIA material, such as Se, may be deposited onto the first film  112  to complete the formation of the precursor layer  110 . The second film  114  forms top of the processed portion  100 B of the workpiece  100 . 
         [0024]      FIG. 3A  shows in side view an embodiment of the roll to roll PVD system  200  processing the workpiece  100 . The PVD system  200  includes a process housing  201 , a first PVD station  202  to deposit the first material including a Group IA material (dopant material), such as Na, Li or K, to form the first film  112  ( FIG. 2B ) and a second PVDstation  204  to deposit the second material including a Group VIA material such as Se to form the second film  114  ( FIG. 2B ). As described above, the first material and the second material form the second precursor portion  108 B (see  FIG. 2B ) including a dopant material and Se which completes the formation of the precursor layer  110  on the workpiece  100 . 
         [0025]    The process housing  201 extends between a loading station  205 A and an unloading station  205 B of the PVD system  200 . The process housing  201  preferably includes a first section  201 A, a second section  201 B and optionally a third section  201 C, such that associated with each section is a deposition station as described herein. The first PVD station  202  is located within the first section  201 A such that a horizontal portion of the continuous flexible workpiece  100  is advanced from the loading station  205 A through the first PVD station  202 , and within the first PVD station  202  a horizontal process gap is maintained between a front surface of that portion of the continuous flexible workpiece therein and a first PVD unit, described further hereinafter, that is associated with the first PVD station  202 . The second PVD station  204  is located within the second section  201 B such that a vertical portion of the continuous flexible workpiece  100  is advanced through the second PVD station  204 , and within the second PVD station  204  a vertical process gap is maintained between a front surface of that portion of the continuous flexible workpiece therein and a second PVD unit, described further hereinafter, that is associated with the second PVD station  204 . The third section  201 C is located in the process direction between the second section  201 B and the unloading station  205 B and provides a path for the continuous flexible workpiece  100  to the unloading station  205 B. The third section  201 C is a cooling zone, may have an active cooling unit disposed therein, and as illustrated is in a preferred embodiment is parallel to the first section  201 A. The first section  201 A is defined by a first peripheral wall  220  including a first wall  220 A, a second wall  220 B and side walls (not shown). The first and the second side walls of the first section  201 A are preferably parallel to one another, and the distance between the first wall  220 A and the second wall  220 B becomes the gap height of the first section  201 A. The second section  201 B is defined by a second peripheral wall  230  including a first wall  230 A, a second wall  230 B and side walls (not shown). The first and the second side walls of the second section  201  B are preferably parallel to one another and the distance between the first wall  220 A and the second wall  220 B becomes the gap height of the second section  201 B. In both sections, the gap height is in the range of 1 cm to 20 cm, preferably 1 to 5 cm. During the process, the unprocessed portion  100 A of the workpiece  100  is unwound from a supply roll  206 A located in the loading station  205 A; advanced in a process direction ‘P’ while being processed in the first PVD station  202  and the second PVD station  204 ; and the processed portion  100 B of the workpiece  100  is picked up and wound as a receiving roll  206 B located in the unloading station  205 B. The unloading station  205 B may also include an interleaf roll  209  to provide a protective interleaf sheet  213  onto the front surface  101 B of the workpiece as it is wound. 
         [0026]    When moved in the system  200  by a moving mechanism (not shown), a back surface  101 A of the workpiece  100  is supported by a number of auxiliary rollers, such as primary rollers  208 A- 208 E, and secondary rollers  218 A and  218 B while a front surface  101 B of workpiece is left exposed for the aforementioned deposition processes without being physically touched by any system component, i.e., rollers or the like. The auxiliary rollers  208 A- 208 E,  218 A and  218 B are utilized to support, tension and change the direction of motion of the workpiece or the angle of the direction of motion. As will be described more fully below, the workpiece  100  is advanced from the loading station  205 A though a first sealable gate  211 A of the process housing  201 . After traveling through the first section  201 A, the second section  201 B and the third section  201 C, the workpiece  100  enters into unloading station  205 B through the second sealable gate  211 B of the process housing  201 , in its tensioned state. The primary rollers  208 A and  208 D are placed within the loading and unloading stations in very close proximity of the sealable gates  211 A and  211 B respectively. 
         [0027]    The workpiece  100  also preferably passes through a third sealable gate  211 C placed after the first section  201 A, adjacent and before the primary roller  208 B. Also, optionally, a fourth sealable gate  211 D placed before the second section  201 B and after the primary roller  208 B may be included. Also, optionally a fifth sealable gate  211 E placed after the second section  201 B and before the primary roller  208 C can be used. Also, and optionally, a sixth sealable gate  211 F placed before the third section  201 C and after the primary roller  208 C can be used. In this respect, the primary rollers  208 B and  208 C are positioned at the corners of the system at roller positions  231  and  232 , and are sealed by the sealable gates  211 C,  211 D and  211 E,  211 F respectively. 
         [0028]    With usage of the sealable gates  211 , this also allows the control of the different chambers, such that one deposition chamber can be being used for service (deposition or other processing occurring within) while the others are under vacuum (without deposition or other processing occurring). 
         [0029]    The primary roller  208 B changes the orientation of the workpiece from horizontal to vertical, and the primary roller  208 C again changes the orientation, this time from vertical to horizontal. The secondary rollers  218 A and  218 B further tension the workpiece by causing a wrap angle of about 15° at the primary rollers  208 A and  208 B respectively. The sealable gates  211 A- 211 F may preferably be rectangular narrow slits which are dimensioned very close to the width and thickness of the work piece  100 . The mechanics of moving the workpiece within the process housing  201  and through the sealable gates will be described below in connection with  FIGS. 3B-4B . The sealable gates  211 A- 211 F block any deposition material migration into adjacent sections and the loading and unloading chambers and allow independent servicing of the PVD stations while maintaining vacuum in adjacent stations. 
         [0030]    Referring to  FIGS. 2A ,  2 B and  3 A, it will be appreciated that, although it is referred to as the front surface  101 B for clarity, the front surface  101 B of the workpiece has different material films, which are described above, at various stages of the process performed in the PVD system  200 . For example, before entering the first PVD station  202 , the front surface  101 B includes the first precursor portion  108 A; before entering the second PVD station  204 , the front surface  101 B includes the first film  112  deposited onto the first precursor portion  108 A; and, after the second PVD station  204 , the front surface  101 B includes the second film  114 . 
         [0031]    Referring back to  FIG. 3A , the first PVD station  202  includes a first PVD unit  203  with a first PVD chamber  210  and a first PVD apparatus  212 , to provide the first deposition material, e.g., Na vapor, to form the first film  112  on the front surface  101 B while the workpiece  100  is advanced in a horizontal direction through the first PVD chamber  210  of the first PVD station  202 . The first PVD apparatus may be either a sputter deposition apparatus or an evaporation deposition apparatus. In the preferred embodiment the first PVD apparatus is a sputter deposition apparatus. The first PVD apparatus  212  is located across from the front surface  101 B of the workpiece within the first deposition chamber  210 , which chamber  210  is also referred to herein as the horizontal process gap, which is in certain embodiments a subset area of the first section  201 A, as explained more fully below. The first PVD apparatus  212  is preferably mounted so that material therefrom is provided through an opening in a peripheral wall of the first section  201 A to an area within the first section  201 A where deposition occurs, and which area is thus referred as the first PVD chamber  210 . The first PVD chamber  210  will preferably occupy a portion of the first section  201 A, for example the portion between the points ‘A’ and ‘B’. The horizontal direction of travel of the workpiece  100  through the first PVD chamber is parallel to an X-axis shown in  FIG. 3 . Although in this embodiment, the first PVD station  202  has only one deposition unit, it may include a plurality of other deposition units to deposit other materials, and this aspect is within the intended scope herein. 
         [0032]    The second deposition station  204  includes a second PVD unit  207  with a second PVD chamber  214  and a second PVD apparatus  216 , to provide the second material, i.e., Se, to form the second film  114  on the vertically disposed front surface  101 B while the workpiece  100  is advanced vertically up and through the second PVD chamber  214  of the second PVD station  204 . The second PVD apparatus  216  is located across from the front surface  101 B of the workpiece within the second PVD chamber and is capable of delivering the depositing material to a vertically disposed workpiece, which chamber  214  is also referred to herein as the vertical process gap, which is in certain embodiments a subset area of the first section  201 B, as explained more fully below. The second PVD apparatus  216  is preferably mounted so that depositing material therefrom is provided through an opening in a peripheral wall of the second section  201 B to an area within the second section  201 B where deposition occurs, and which area is thus referred as the second PVD chamber  214 . The second PVD chamber  214  will preferably occupy a portion of the second section  201 B, for example the portion between the points ‘C’ and ‘D’. Although in this embodiment, the second PVD station  204  has only one deposition unit, it may include a plurality of other deposition units to deposits other materials. 
         [0033]    The vertical orientation of the second PVD chamber  214  is parallel to a Y-axis shown in  FIG. 3  so that the workpiece  100  is advanced vertically up in the second deposition chamber  214 . As shown in  FIG. 3A , during the deposition of Se, since the portion of the workpiece being operated upon is in vertical orientation, there will not be a need to apply high tension to flatten the workpiece; as a result, the Se layer deposits in a uniform manner. Further, the deposition of Se happens in a so called free span zone where no roller or other moving component of the system touches the workpiece  100 . This advantageously prevents excess Se build up on such components and thereby reduces system downtime for clean-ups and the associated cost. In this embodiment, the first peripheral wall  220  of the first section  201 A and the second peripheral wall  230  of the second section  201 B are shielded by replaceable shield layers (not shown) or plates made of a metal or ceramic. The shield layers may be partially or fully cooled by cooling systems to collect excess material, whether vapors or atoms, on the shield layers so that such material does not deposit onto other system components or the peripheral walls of the sections and limit migration of Se into adjacent zones. Shield layers with excess material deposits are replaced in process intervals. The vertical configuration of the second PVD station  204  also effectively reduces system foot-print and provides a compact system. The vertical configuration of the second PVD station  204 , along with the horizontal configuration of the first PVD station  202 , also results in a line-of-sight of the material depositing of the second PVD station not being within the line-of-sight of the first PVD station, and likewise the line-of-sight of the material depositing of the first PVD station not being within the line-of-sight of the second PVD station. 
         [0034]    Various sputtering stations can be integrated into the system. These sputtering stations can be set to deposit materials that can include oxides, metals, ceramics etc. The sputtering stations can employ RF (radio frequency), DC (direct current), or pulsed DC sputtering. 
         [0035]    In the deposition system  200 , each deposition step is performed when the workpiece  100  is in a free span zone. This aspect will now be further described with help of  FIG. 3B  which is a simplified illustration of  FIG. 3A  to explain mechanics of free-span configurations in the system  200 . Accordingly, in the system  200  as shown, the workpiece  100  has three sequential free-span zones as it travels in the process direction, namely a first free span zone  250 A or a first horizontal free-span zone, a second free span zone  250 B or a vertical free span zone, and a third free-span zone  250 C or a second horizontal free-span zone. The first free-span zone  250 A occurs while the workpiece is tensioned between the primary roller  208 A and  208 B to deposit the dopant material (depicted by arrows, though occurring within the deposition unit) onto the front surface  101 B. As mentioned above in the background section, in the prior art, long free span zones causes workpiece to droop with a catenary shape resulting in changes in the uniformity of the depositing layer due to uneven tension. 
         [0036]    For example, a 1 m wide, 50 μm thick stainless steel web or substrate at a tension of 900 Newton (N) will deflect by nearly 1.4 cm at the center of a 5 m free-span. By reducing the free-span to 2.5 m, the deflection of the same web at the center will be only 0.3 cm. Moreover, for wide substrates especially, it is difficult to evenly tension across the width due to non-flat web shape and imperfect web path alignment. In this case, the tension applied to the web may be concentrated at one of the two edges, both edges, or somewhere between both edges. The web may further develop tramlines, diagonal ripples that travel across the web in the free span. The portion of the web under lower tension will deflect more than the portion at higher tension and thereby degrade the deposition uniformity since the distance from the deposition source to the substrate will vary and, generally, deposition flux varies inversely proportional to the square of the distance. As in the example above, increasing the free-span of the web will exacerbate the deflection and consequent degradation of deposition uniformity. For example, if the nominal distance from a deposition source to web is 15 cm and a free span of 5 m, a deflection of 1.4 cm in the center of the web would reduce the distance to the deposition source to 13.6 cm and increase the deposition rate by over 20%. In contrast, for a 2.5 m free-span, with a deflection of 0.3 cm, the deposition rate would increase by only 4%. By separating the primary rollers  208 A and  208 B so that there is a minimum spacing between them while still allowing the PVD unit  203  to exist therebetween will minimize the free span zone length. Since the workpiece  100  is also made substantially horizontal and flat between the primary rollers  208 A and  208 B, this allows deposition of layers with uniform thickness. As will be described below, when the workpiece  100  is horizontal and flat in the first free-span zone, sealable gates  211 A and  211 B may be advantageously made as very narrow slits. Such sealable gates with narrow slits, when open during deposition, provide a better seal against the escape of undesired material, whether vapors or atoms, from the deposition units  203  and  207  produced during the PVD process. 
         [0037]    Due to the vertical orientation of that portion of the workpiece  100  being operated upon in the second PVD station  204 , the second free-span zone  250 B is not susceptible to the drawbacks of the horizontal free span zones, as the second free span zone is less susceptible to bowing in the middle between horizontally disposed rollers. Rather, in the second free-span zone  250 B, due to the vertical position of the workpiece  100 , flatness of the workpiece is inherently achieved and advantageously established even if the second free-span zone is made longer than the first free span-zone  250 A, since the second free-span zone  250 B is established by and is located between the primary rollers  208 B and  208 C and the vertically disposed workpiece portion therebetween. As such, when depositing the selenium (depicted by arrows) onto the portion of the front surface  101 B of the workpiece  100 , the portion of the workpiece  100  that is vertically disposed essentially hangs from the top roller  208 C, and so even without tension the weight of the vertical workpiece portion will result in the desired flatness, and under slight tension between primary rollers  208 B and  208 C, flatness is achieved. Because of the flatness of the vertical workpiece portion in the second free-span zone  250 B, the slits of the sealable gates  211 D and  211 F that are disposed vertically on opposite sides of the second PVD station  204  may be made very narrow to better prevent migration of the selenium material, whether vapors or atoms, to adjacent deposition zones. 
         [0038]    Similar to the first free-span zone  250 A, the third free-span zone  250 C also benefits a shorter free span zone and results in better sealing ability of the sealable gates  208 C and  208 D. In another embodiment, by adding driven rollers between the deposition zones, tension can be controlled independently. In the vertical deposition zone, the tension can be lower than in the horizontal sections, without sacrificing deposition uniformity. The tension required in each zone may depend on the web material, thickness, and width of the web, and may be within the range of 200 to 4000 N. 
         [0039]      FIG. 4A  shows another portion of the workpiece  100  in the first free-span zone  250 A in detail.  FIG. 4B  shows the work piece  100  in front cross sectional view as it passes through one of the sealable gates such as the first sealable gate  211 A. In this embodiment, the sealable gate  211 A is a rectangular opening between an upper seal member  240 A and a lower seal member  240 B. Due to the flatness of the workpiece, the height ‘H’ and width ‘W’ of the opening may be made very close to the thickness and width of the workpiece  100 . For instance, the width needs only to be 2-4 mm wider, preferably 2 mm wider, than the web to allow for some web misalignment. Since the deflection is minimized, the height of the opening can be reduced to 2-10 mm, preferably 2-5 mm, without causing mechanical interference. 
         [0040]    The PVD deposition system  200  may also include a metrology station (not shown) including an XRF analyzer located for example in the unloading station  205 B of the system. The XRF analyzer measures the thickness of the deposited Se layer and provides feedback for a deposition control system. The XRF is positioned over a roller to ensure constant measurement height and measurement accuracy. Further, heating and cooling stations located before the deposition chambers anneal the workpiece at a controlled temperature. 
         [0041]    Although the embodiments have been particularly described with reference to embodiments thereof, it should be readily apparent to those of ordinary skill in the art that various changes, modifications and substitutes are intended within the form and details thereof, without departing from the spirit and scope thereof. Accordingly, it will be appreciated that in numerous instances some features will be employed without a corresponding use of other features. Further, those skilled in the art will understand that variations can be made in the number and arrangement of components illustrated in the above figures.