Abstract:
A roll to roll system for depositing a material on a workpiece is provided. In one embodiment, the system includes a drum, which rotates about an axis that is transverse to a process direction, and a number of PVD deposition units. The drum further includes a peripheral surface that includes a groove having a recessed workpiece contact surface that is parallel to the axis and disposed between a first side wall and a second side wall. A portion of the recessed workpiece contact surface supports a section of the workpiece and the first and second side walls maintain the section of the workpiece on the portion of the recessed workpiece contact surface as the workpiece is moved along the process direction. The PVD deposition units are disposed across from some of the portion of the peripheral surface and continuously deposit the material across a width of some of the section of the workpiece.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Application No. 61/109,144 filed Oct. 28, 2008 entitled “IMPROVED DRUM DESIGN FOR WEB PROCESSING”, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to deposition methods and, more particularly, to methods for physical vapor deposition of metallic thin films on a conductive surface for manufacturing solar cells. 
         [0004]    2. Description of the Related Art 
         [0005]    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. 
         [0006]    Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. 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, in summary, 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. 
         [0007]    The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te) 2  thin film solar cell is shown in  FIG. 1 . The device  10  is fabricated on a substrate  11 , such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film  12 , which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te) 2 , is grown over a conductive layer  13  or contact layer, which is previously deposited on the substrate  11  and which acts as the electrical contact to the device. The absorber film  12  is formed by depositing a precursor layer including Group IB and Group IIIA elements on the contact layer and then reacting this precursor stack film with one of Se and S to form the absorber layer. The substrate  11  and the conductive layer  13  form a base  20  on which the absorber film  12  is formed. Various conductive layers comprising Mo, Ta, W, Ti, and their nitrides have been used in the solar cell structure of  FIG. 1 . If the substrate itself is a properly selected conductive material, it is possible not to use a contact layer  13 , since the substrate  11  may then be used as the ohmic contact to the device. After the absorber film  12  is grown, a transparent layer  14  such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation  15  enters the device through the transparent layer  14 . Metallic grids (not shown) may also be deposited over the transparent layer  14  to reduce the effective series resistance of the device. 
         [0008]    A variety of materials, deposited by a variety of methods such as evaporation, electroplating and sputter deposition, can be used to provide the various layers of the device shown in  FIG. 1 . Sputtering and evaporation techniques, which are also known as physical vapor deposition (PVD) techniques, are the preferred methods to deposit contact layers and transparent layers, although they may be used to deposit the components of the precursor films also. Such layers can be deposited on a continuous flexible substrate using a well known roll to roll process tool in which the flexible substrate is fed from a supply roll into a process chamber and after receiving deposition the flexible substrate is taken up from the process chamber and wrapped around a receiving roll. The process chamber can have for example one or more sputter deposition units or cathodes to deposit a desired material onto the continuous flexible substrate from the targets mounted on the cathodes. 
         [0009]    In general, the process chambers are equipped with a support apparatus to support the continuous flexible substrate during the deposition.  FIG. 2  shows in perspective view of an exemplary cylindrical support apparatus  50  or drum, supporting a continuous flexible workpiece  52  or web. Drums with various sizes are used to control the tension of the web and to transfer the heat out of the web. The drums can be using oil, water, or gas based cooling mechanisms to transfer heat from the web that gets heated by the sputtering cathodes. Top surface  54  of the web  52  is exposed to the depositing material (depicted as arrows “M”) originating from the target materials mounted on the cathodes. During the process, the web  52  is advanced while in contact with the curved surface  56  of the drum  50  which can rotate as the workpiece moves. The quality of the deposited film depends upon the physical contact between the web and the drum surface. As shown in  FIG. 2 , the curved surface  56  is a perfectly cylindrical surface. 
         [0010]    During the process, small distortions in the web may disturb the physical contact between the web and the curved surface which may cause the web to move non-uniformly such as side ways, and/or up and down on the curved surface. Distortions in the web can be caused by the process temperature. Such distortions in turn affect the quality of the deposited layer and cause contamination of an edge area  58  of the curved surface  56 , which further deteriorate the physical contact between the web and the curved surface as the web edge contacts this contaminated edge of the curved surface. As a result, an improved drum design is needed to address the above described issues so that more optimal process results may be obtained. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a method an apparatus for the confinement of the web in a specific section of a drum, a better web contact with cooled surfaces, and depositing on the full width of the web on the drum. 
         [0012]    In a first embodiment, the drum has a groove that guides the web. This allows the web to be confined to specific section of the drum that is kept free of deposits. With this approach, full width of the web can be deposited. Since the web is confined to groove, deposition on the drum takes place on the sides of the web. Since these areas are not traveled by the web, deposits can be removed with known methods without impacting the interaction between the web and the drum. 
         [0013]    In a second embodiment, a buffer material in the form of a buffer belt or a buffer layer is placed between the drum and the web. The buffer material can be highly conductive yet flexible material. The width of the buffer material can be wide enough to capture all deposition flux. Once significant deposition is made either the buffer material can be cleaned or replaced with a new one. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic view of a prior art solar cell structure; 
           [0015]      FIG. 2  is a perspective view of a prior art; 
           [0016]      FIG. 3A  is a schematic view of a roll to roll deposition system of the present invention having a drum to support a continuous substrate, wherein the drum surface includes a groove to guide the continuous substrate; 
           [0017]      FIG. 3B  is schematic side view of an embodiment of the drum shown in  FIG. 3A ; 
           [0018]      FIG. 4A  is a schematic view of another embodiment of a roll to roll deposition system of the present invention having a drum to support a continuous substrate, wherein the drum includes a smooth surface, and wherein a buffer belt has been disposed between the drum surface and the continuous substrate; 
           [0019]      FIG. 4B  is schematic side view of the drum shown in  FIG. 4A ; 
           [0020]      FIG. 4C  is a schematic side view of the drum shown in  FIG. 3B , wherein a buffer belt has been disposed between the drum surface and the continuous substrate; 
           [0021]      FIG. 5A  is schematic view of the roll to roll system shown in  FIG. 4A , wherein the buffer belt has been replaced with a buffer layer that is coated on the drum surface; 
           [0022]      FIG. 5B  is schematic side view of the drum shown in  FIG. 5A ; and 
           [0023]      FIG. 5C  is a schematic side view of the drum shown in  FIG. 3B , wherein a buffer layer has been disposed between the drum surface and the continuous substrate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present invention provides a system for depositing thin films on a continuous substrate or web which is supported by a curved surface of a support base of the system during the deposition. In one embodiment the support base may have a cylindrical shape having a curved surface with a groove region that a continuous substrate is supported during the deposition process. The groove region prevents the substrate from slipping sideways and controls the movement of the substrate. In another embodiment, a flexible buffer material is disposed between the substrate and the curved surface of the support base. The flexible buffer material increases the friction between the substrate and the surface of the drum by making a better contact with the substrate and reduces the distortions or quilting caused by the excessive heat. The flexible buffer material can accommodate the small distortion on the substrate and make contact with the full substrate surface. This significantly enhances the heat transfer from the distorted areas of the continuous substrate. A roll to roll system of the present invention may be used to manufacture Group IBIIIAVIA thin film solar cells. 
         [0025]      FIG. 3A  shows a roll to roll system  100  with a deposition station  102 . The deposition station  102  may be in a chamber or enclosure (not shown). The chamber may or may not be under vacuum. The deposition station includes a support base or drum  104  to support a workpiece  108  during a deposition process. One or more deposition units  106  for a PVD process, such as sputter deposition units, is generally positioned across from the lower half of the drum  104 . The workpiece  108  contacts a cylindrical peripheral surface  110  of the drum as it is extended between a supply spool  112 A and a receiving spool  112 B. A number of auxiliary rollers  114  are symmetrically positioned at both sides of the drum to enable workpiece  108  to contact to at least a lower half of the cylindrical peripheral surface  110  as the workpiece is fed from the supply spool  112 A and wrapped around the receiving spool  112 B after the process. As the workpiece  108  is advanced along a process direction ‘A’ by a moving mechanism (not shown), it is tensioned on the surface  110  of the drum  104  and a front surface  116 A of the workpiece  108  receives depositing material from the deposition units  106  while a back surface  116 B of the workpiece  108  is in physical contact with the surface  110  of the drum  104 . Both long edges of the workpiece  108  are substantially parallel to the process direction ‘A’. The material from the deposition units  106  deposits onto a deposition path on the front surface  116 A of the workpiece  108  as the workpiece is advanced in front of the units  106 . The deposition units may include sputter deposition apparatus to sputter-deposit a material onto the front surface  116 A of the workpiece. The deposition path may have a width which is equal to or less than the width of the workpiece. 
         [0026]    The drum  104 , in all of the embodiments, is made from a heat conducting material, preferably a metallic material such as stainless steel, though other heat conducting materials can be used. Conventional known methods can be used to make the drums. Modified process steps are required for making the grooves as described above, and additional process steps are used when adding additional materials such as the flexible buffer layer described below. It is noted that the dimension of a typical drum  102  can vary, though in many implementations a diameter of 3-10 ft is typical. A web width of around 2-6 ft is also typical in manufacturing environments. 
         [0027]      FIG. 3B  shows the side cross sectional view of the drum  104 . In this embodiment the peripheral surface  110  of the drum  104  includes a groove  118  having a peripheral recessed surface  120 A and side walls  120 B (a first side wall and a second side wall). As the workpiece moved during the process, the back surface  116 B of the workpiece  108  contacts the peripheral recessed surface  120 A, also referred to as a workpiece contact surface, and the side walls  120 B confine the workpiece  108  within the groove  118 . The groove has an unvarying depth across the cylindrical surface  110 , which is in the range of the workpiece thickness or greater. The groove  118  enables workpiece to stay in the deposition path and move in the process direction ‘A’. If any contamination happens, the contamination stays on the side walls  120 B, and since the workpiece  108  cannot move laterally, no substantial contamination gets underneath the workpiece  108 . Contaminated areas of the side walls  120 B can be cleaned at process intervals. In this design workpiece  108  will be guided to the groove and the same area will always be kept clean ensuring near constant interaction between the workpiece  108  and the drum  104 . Since the workpiece  108  is confined into the groove  118  and moves only in the process direction ‘A’, the material from the deposition units  106  may be deposited onto the full front surface of the workpiece in an edge to edge manner covering the full width without concerning about any unwanted deposition over the side walls  120 B because the side walls are not contacted by the workpiece  108 . 
         [0028]    In the above embodiment, the groove region of the surface of the drum prevents the workpiece from slipping sideways and controls the movement of the workpiece. The movement of workpiece may also be controlled by a flexible buffer material such as a silicon based polymer material that is disposed between the workpiece and the surface of the drum. The flexible buffer material increases the friction between the workpiece and the drum surface by making a better contact with the back of the workpiece, thereby reducing the distortions or quilting caused by the excessive heat. The buffer material may be used with the drums having grooves as described above as well as with a regular drum with a smooth surface which does not include any groove. 
         [0029]      FIG. 4A  shows a system  200  which is similar to the system  100  except the system  200  uses another drum embodiment and an associated buffer belt assembly. The system  200  is constructed with replacing the drum  104  of the system  100  in  FIGS. 3A-3B  with a drum  204  without a groove and also including a buffer belt assembly  201  to provide buffer material. As shown in  FIG. 4B , in this embodiment a cylindrical surface  210  of the drum  204  is a smooth surface without a groove. A buffer belt  202  is positioned between the surface  210  of the drum and the back surface  116 B of the workpiece  108 . The buffer belt  202  is tensioned by a belt roller  203  which may move vertically. The width of the buffer belt  202  may preferably be equal to the width of the surface  210  of the drum  204 . The width of the buffer belt may be equal to or greater than the width of the workpiece  108 . If the width of the buffer belt  202  is greater than the width of the workpiece, sides  205  of the buffer belt  202  may be exposed, not covered by the workpiece  108 . The exposed sides  205  collect the unwanted deposited material and keep edge surfaces  206  of the drum  204  free from contaminants or excess deposited material. Surfaces of the sides  205  of the buffer belt may be made rough while a surface section of the buffer belt  202  that goes under the workpiece  108  may have a smooth surface for better heat transfer. A smooth surface in the application may have a surface roughness (peak to valley) of 50-250 nm. A roughened surface that collects the excess deposited material or contaminants may have a surface roughness in the range of tens of micrometers up to a millimeter. Such rough surfaces are typically obtained by plasma spraying a material such as aluminum on the surface to be roughened. This way the rough surfaces of the exposed sides  205  may help to collect a greater amount of contaminants before they are cleaned and thereby reduce the number of process interruptions for cleaning. 
         [0030]    The buffer belt  202  may comprise a material that is flexible yet thermally very efficient conductor such as silicones filled with high thermal conductivity materials. A flexible belt will make a better contact with the workpiece  108  and reduce the distortions or quilting caused by excessive heat. The buffer belt  202  may accommodate the small distortion on the workpiece  108  and make contact with full back surface  116 B of the workpiece. This buffer belt  202  will significantly enhance the heat transfer from distorted areas of the workpiece  108  compared to solid surfaces in the prior art. Furthermore, the buffer belt  202  can be driven by a motorized roll or be driven by the drum; the tension on the belt can be controlled by the belt roller  203 , for example the buffer belt  202  can have a constant tension setting with spring such that it can move close or away from the drum  210  freely to keep the constant tension; the buffer belt  210  can have an edge guide to control its precise position on the drum; and the belt can be cleaned or replaced once exposed sides receive significant deposits. In another embodiment, the buffer belt  202  may be replaced with a pair of cleaning belts (not shown) which may only touch and cover the edge surfaces  206  of the drum  204  but not extend under the workpiece  108  so that the back surface  116 B of the workpiece touch and cover the surface area between the edge surfaces  206 . The surface of the cleaning belts may be rough to collect the contaminants. Cleaning belts may be cleaned at intervals or replaced with the clean ones. 
         [0031]    As shown in  FIG. 4C , the system  200  may also use the drum  104 , which is described in the previous embodiment, in combination with the buffer belt  202  described above. In this embodiment, the buffer belt  202  is between the recessed surface  120 A and the back surface  116 B of the workpiece  108 . The side walls  120 B confines the workpiece  108  and the buffer belt  200  within the groove  118 . 
         [0032]    As shown in  FIGS. 5A-5B , the buffer belt  202  shown in  FIG. 4A  may be replaced with a buffer material layer  300  that is coated on the entire cylindrical surface  210  that touches the workpiece  108  for the same effect. The width of the buffer layer  300  may preferably be equal to the width of the surface  210  of the drum  204 . The width of the buffer layer  300  may be equal to or greater than the width of the workpiece  108 . If the width of the buffer layer  300  is greater than the width of the workpiece, sides of the buffer layer  300  may be exposed, not covered by the workpiece  108 . 
         [0033]    As shown in  FIG. 5C , the buffer layer may also be employed on the surface  110  of the drum  104  shown in  FIG. 3B . In this embodiment, the buffer layer  300  is between the recessed surface  120 A and the back surface  116 B of the workpiece  108 . The side walls  120 B and the buffer layer  300  confine the workpiece  108  within the groove  118 . In this embodiment, the buffer layer  300  functions the same way as the buffer belt  202  shown in  FIG. 4C . 
         [0034]    Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.