Abstract:
A substrate processing system particularly suitable for fabricating solar cells. The system has a front end module transporting cassettes, each cassette holding a preset number of substrates therein; a loading module coupled to the front end module and having mechanism for loading substrates from the cassettes onto carriers; and a plurality of processing chambers coupled to each other in series, each having tracks for transporting the carriers directly from one chamber to the next; wherein selected chambers of the plurality of processing chambers comprise at least one combination source having a sputtering module and an evaporation module arranged linearly in the direction of travel of the carriers.

Description:
RELATED APPLICATIONS 
     This application claims priority benefit from U.S. provisional application Ser. No. 61/084,600, filed on Jul. 29, 2008, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     The subject invention relates to processing of substrates and, more specifically, for systems for forming thin films over substrates to produce devices, such as solar cells. 
     2. Related Art 
     Vacuum processing systems are used to fabricate hard-drive disks, semiconductor computer chips, solar panels, and the like, from substrates made of materials such as semiconductor wafers, glass, stainless steel, etc. Typically, the vacuum processing systems include several substrate chambers that perform various processes that modify the substrate by performing deposition, cleaning, etching, heating/cooling, etc., on the substrate. Deposition of films is generally accomplished using, e.g., physical vapor deposition (PVD) or chemical vapor deposition (CVD). PVD can be performed using, e.g., sputtering or evaporation systems. Sputtering process can be controlled relatively well and thin films formed using sputtering sources can be of high quality and uniformity. However, sputtering sources are relatively expensive and target utilization is relatively low. On the other hand, evaporation systems are relatively of low cost and high utilization, albeit they are more difficult to control to form films of precise thickness and uniformity. 
     Fabrication of solar cells is a recent emerging field which utilizes thin film technologies. There are several basic forms of solar cells, including c-Si, a-Si:H, n-Si:H, CIS/CIGS/CIGS-S, CdTe, GaAs and Organic or Dye Sensitized devices. There are many layer combinations that comprise modern cells, many of which may be fabricated using thin film fabrication techniques. For example, absorber layers, low resistivity rear electrodes, high resistivity intermediate or buffer layers and high optical transmission moderate resistivity window layers are essential components in the fabrication of solar cells. In order to tailor such layers to achieve requisite results on specified figures of merit, such as Voc, Isc, Fill Factor, conversion efficiency and numerous other parameters, precise atomic concentrations of materials must be deposited. 
     While precise formation of the various layers is paramount to achieving high performing solar cells, the speed of fabrication required in the market of solar cells is rather high. For example, traditional semiconductor fabrication equipment provides order of magnitudes slower throughput than required by the solar industry and is, therefore, inadequate. Consequently, fabricators of solar cells are in constant search for manufacturing equipment that can provide the required precision, but at exceedingly high throughput. 
     SUMMARY 
     The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
     The subject invention aims to solve the problems present in the prior art. Embodiment of the subject invention utilize novel deposition technology to produce precise layers at very high throughput. Systems made according to embodiments of the invention can be beneficially utilized to form thin films, for example, bi- or multi-layer, films of single or several nanometer-thick, which can be utilized for formation of, e.g., enhanced absorber and window layers of solar cells. 
     According to aspects of the invention, sputter deposition sources and evaporation effusion cells are utilized in situ in a continuous deposition process to form the thin films. 
     According to an aspect of the invention, a substrate processing system is provided, which is particularly suitable for fabricating solar cells. The system has a front end module transporting cassettes, each cassette holding a preset number of substrates therein; a loading module coupled to the front end module and having mechanism for loading substrates from the cassettes onto carriers; and a plurality of processing chambers coupled to each other in series, each having tracks for transporting the carriers directly from one chamber to the next; wherein selected chambers comprise combination source having a sputtering module and an evaporation module arranged linearly in the direction of travel of the carriers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
         FIG. 1  illustrates a system according to an embodiment of the invention; 
         FIG. 2  illustrates a cross section of one of chambers  140 ; 
         FIG. 3  is a simplified schematic illustrating a combination source according to an embodiment of the invention, while  FIG. 3A  illustrates a source with multiple thin sputtering sources and  FIG. 3B  is a simplified schematic illustrating a multiple evaporation source  372  according to an embodiment of the invention. 
         FIG. 4  is a simplified schematic showing a cross section of a processing chamber having two sources, one on each side, for simultaneous fabrication of both sides of a substrate. 
         FIG. 5  is a simplified schematic showing a cross section of a processing chamber having two sources, one on each side, for simultaneous fabrication of two substrates. 
         FIG. 6  illustrates a heater according to an embodiment of the invention, which is structured to have similar fittings onto the chamber as the deposition sources. 
         FIG. 7  is a simplified cross-section schematic of an evaporator according to an embodiment of the invention. 
         FIG. 8  illustrates a process carrier, which may be used for processing two substrates simultaneously, according to an embodiment of the invention. 
         FIG. 9  illustrates a system structured according to an embodiment of the invention for fabricating CIGS solar cells. 
         FIG. 10  illustrates yet another embodiment of a mixed sputtering and evaporation source according to the invention. 
         FIG. 11  illustrates yet another embodiment of a mixed sputtering and evaporation source according to the invention. 
         FIG. 12  illustrates a processing chamber according to an embodiment of the invention. 
         FIG. 13  illustrates another evaporator according to an embodiment of the invention. 
         FIG. 14  illustrates another processing chamber according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description will now be given of solar cells processing system according to embodiments of the invention.  FIG. 1  illustrates a system for high capacity sequential processing of substrates, which employs unique sputter and evaporation combined deposition sources. The system can be used for solar cells production. The system is generally constructed of several identical processing chambers  140  connected in a linear fashion, such that substrates can be transferred directly from one chamber to the next. While in the embodiment of  FIG. 1  two rows of chambers are stacked one on top of the other, this is not necessary, but it provides a reduced footprint. 
     A front end module  160  includes tracks  164  for transporting cassettes  162  containing a given number of substrates  166 . The front end unit  160  maintains therein a clean atmospheric environment. A robotic arm  168  removes substrates  166 , one by one, from the cassette  162  and transfers them into a loading module  170 . Loading module  170  loads each substrate  166  onto a substrate carrier  156 , and moves the substrate  166  and carrier  156  into a vacuum environment. In the embodiment of  FIG. 1 , each carrier is shown to hold a single substrate, but other embodiments can utilize carriers that hold two substrates, either in tandem or back to back. Thereafter the carriers  156  and substrates  166  traverse the processing chambers  140 , each of which operates in vacuum and is isolated from other processing chambers by gate valves  142  during processing. The motion of the carrier  156  is shown by the broken-line arrows. Once processing is completed, the substrate  166  is removed from the carrier  156  and is moved to an atmospheric environment and placed in the cassette  162  by robot arm  168 . 
       FIG. 2  illustrates a cross section of one of chambers  140 . Substrate  266  is shown mounted vertically onto carrier  256 . Carrier  256  has wheels  221 , which ride on tracks  224 . The wheels  221  may be magnetic, in which case the tracks  224  may be made of paramagnetic material. In this embodiment the carrier is moved by linear motor  226 , although other motive forces and/or arrangements may be used. Depositions source  272  is shown mounted onto one side of the chamber  240 . The carrier passes by deposition source  272 , such that deposition is performed on the surface of the substrate as the substrate is moved passed the source. 
       FIG. 3  is a simplified schematic illustrating a combination source  372  according to an embodiment of the invention. In this embodiment, combined source  372  includes two sputtering sources  382  and one evaporation source  384 . In this embodiment sputtering sources  382  have narrow-elongated sputtering targets  383  and evaporation source  384  includes nozzles  386 . In this manner, as the carrier moves the substrate passed the source  272 , three layers are deposited over the substrate: first a sputtered layer, then an evaporated layer, and then another sputtered layer. Of course, the order and number of these sources can be changed and be different for each successive chamber. For example, a first chamber may have three sputtering sources for depositing molybdenum layer, while a following chamber may have a first sputtering source, followed by two evaporation sources, wherein the evaporation sources are used to deposit p-type semiconductor, such as copper-indium-gallium-diselenide (CIGS) or copper-indium-diselenide (CIS). The movement and deposition control afforded by lining up the multiple sources  272  and outfitting them with different precursor materials allows precise tailoring of deposited layer to less than 1 nm thickness. The layers may form oxide, semiconductor, conductor, or combinations thereof. Preferred crystallographic orientation in a quasi-epitaxial fashion can be achieved with minimal deposition time in a cost effective manner. 
       FIG. 3A  is a simplified schematic illustrating a multiple (here triple) sputtering source  372  according to an embodiment of the invention. In this embodiment, source  372  includes three thin sputtering sources  382 , each having a narrow sputtering target  383 . In this embodiment the three sputtering sources may have target  383  of same or different material. In this manner, as the carrier moves the substrate passed the source  272 , three layers are deposited over the substrate in sequence. The use of three thin sources enables sequential sputtering of thin layers of different materials or sequential sputtering of three layers of same material with highly accurate control over the thickness of the resulting layer. For example, the three sources can be controlled individually such that each can be turned on/off independently, such that the source may sputter from one, two or all three targets  383 . Also, when only one or two sources needs to be turned on, different ones can be turned on at each substrate pass so as to average the use of the targets and prolonged the time between service of the targets  383 . 
       FIG. 3B  is a simplified schematic illustrating a multiple (here triple) evaporation source  372  according to an embodiment of the invention. In this embodiment, source  372  includes three evaporation sources  384 . In this embodiment the three evaporation sources may have crucibles containing same or different solutions. In this manner the layer formed on the substrate can be precisely controlled. For example, in some solar cells structures, such as CIGS, the layers may have gradients of each of the materials. The use of three evaporation sources in a single chamber enables sequential deposition of thin layers of different materials, to thereby generate the required gradient. For example, the three evaporation sources can be controlled individually such that each can be turned on/off independently to tightly control the amount of each material evaporated onto the substrate. 
       FIG. 4  illustrates a cross section of a chamber having two sources,  472 A and  472 B, one on each side, for simultaneous processing on both sides of substrate  466 . The sources may be the same or different. For example, one source can be a sputtering source for depositing the back contact, while the other source may be a combination source for depositing the junction layers.  FIG. 5  illustrates another embodiment of the invention, wherein the chamber has two sources,  572 A and  572 B, one on each side of the chamber, but each one used for processing on one surface of one substrate. That is, carrier  556  is structured such that it can support two substrates  566 A and  566 B, back to back, such that only one surface of each substrate is exposed for processing. In this manner, two substrates can be processed in each cycle, thereby doubling the throughput of the system without or with minimal change to its overall footprint. 
     Some steps in solar cell fabrication require heat treatment, such as annealing. In order to make the processing flow without interruption, a heater is designed to fit into the same provisions made in the chambers for the deposition source.  FIG. 6  illustrates a heater  673 , which is structured to have similar fittings onto the chamber as the deposition sources. The heater  673  employs resistive or ceramic heater element  683 . Heater  673  further employs a Al 2 O 3  or Ti cover  685  (shown in broken line) which covers the heater element  683  to prevent particle deposition on the heater element  683 . 
       FIG. 7  is a simplified cross-section schematic of an evaporator according to an embodiment of the invention. According to this embodiment, each evaporation source, such as source  384  of  FIG. 3 , consists of several inserts, each forming an evaporator that may be energized independently of other inserts in the same source. This enables increased control of formation of the deposited layers. Each insert includes a crucible  700  containing the liquid to be evaporated, heater  705 , e.g., resistive heater, injector  710 , and valve  715 . The valve  715  can be used to control the amount of deposition material or to completely shut off deposition. In this embodiment the valve is embedded in the crucible body, so that its temperature is the same as that of the crucible, thereby avoiding condensation on the valve and adverse temperature effects on the deposition stream. 
       FIG. 8  illustrates a process carrier, which may be used for processing two substrates simultaneously. The carrier  852  has a base  810  having wheels  830  and magnets  850 . The substrates  866 A and  866 B are supported by arc  840 , which has clips  842  to hold the substrates at the periphery only. In this manner, the entire surface of each substrate is exposed for processing. As illustrated in  FIG. 8 , two substrates,  866 A and  866 B are held by clips  842 , facing back to back. In this manner, when the carrier enters a processing chamber, the front surfaces of both substrates are processed simultaneously. 
       FIG. 9  illustrates a system structured according to an embodiment of the invention for fabricating CIGS solar cells. The process begins by depositing the back contact layers starting with molybdenum in chambers  1  and  2 , which utilized, e.g., single or multiple sputtering sources. Then a chromium/molybdenum layer is deposited using, e.g., a multiple sputtering source having mixed targets, e.g., first target molybdenum and second and third targets chromium. The fourth chamber houses a heater for high temperature heat treating the deposited contact layers. Then in chambers  5 - 13 , the CIGS layers are being deposited using, e.g., combination sources which provide sputtering and evaporation sources. Chamber  14  is used for cooling the substrate before it enters chamber  15  for deposition of n-doped n-ZnO layer, which can be done using a multiple sputtering or a combination source. Chamber  16  is again used for cooling before another n-ZnO layer is deposited. This is followed by a layer of intrinsic i-ZnO layer in chamber  18 , and then two deposition chambers for the ITO, which forms the top transparent contact layer. 
       FIG. 10  illustrates yet another embodiment of a mixed sputtering and evaporation source according to the invention. In the embodiment of  FIG. 10 , the source includes one sputtering source  1082  having a narrow-elongated sputtering target  1083 , and two evaporation sources  1084 , each having nozzles  386 . The number and arrangement of these mixed sources can be varied, for example the source may include one evaporation and one sputtering source, two sputtering and one evaporation source, etc. In this embodiment, the evaporation sources are controlled so as to create a vapor pressure enabling mixing and/or interaction of the evaporated material, e.g., selenium, with the sputtered material. That is, this source is operated in a single chamber so as to deposit only one layer consisting of evaporated and sputtered material, rather than distinct layers of evaporated and sputtered material. 
       FIG. 11  illustrates yet another embodiment of a mixed sputtering and evaporation source according to the invention. In the embodiment of  FIG. 11 , the source includes one sputtering source  1082  having a narrow-elongated sputtering target  1083 , and a plurality of evaporation nozzles  1086 . In this embodiment the evaporator nozzles are arranged so as to evaporate material, e.g., selenium, so as to mix and interact with the sputtered material to form a layer on the substrate, wherein the layer includes sputtered and evaporated materials. The number and arrangement of the evaporation sources can be varied, for example the evaporation sources can be provided around the sputtering target, to one side of the target, etc. In this embodiment, the evaporation sources are controlled so as to create a vapor pressure enabling mixing and/or interaction of the evaporated material, e.g., selenium, with the sputtered material. 
       FIG. 12  illustrates a processing chamber according to an embodiment of the invention. In this embodiment, chamber  1240  includes two sputtering sources  1272 A and  1272 B for sputtering material either on both surfaces of substrate  1266 , or each onto one surface of a substrate, similar to what is shown in  FIG. 5 . Additionally, evaporation sources  1286 A and  1286 B are provided to evaporate material into the processing chamber. The evaporators are controlled so as to create a vapor pressure enabling mixing and/or interaction of the evaporated material, e.g., selenium, with the sputtered material. 
       FIG. 13  illustrates another evaporator according to an embodiment of the invention. The evaporator of  FIG. 13  has a heated chamber of molten metal  1305 , which evaporates vapor into vapor chamber  1315 . The vapor from chamber  1315  travels through orifices in diffusion plate  1325  into a second vapor chamber  1325 . The vapor than travels through orifices in the second diffusion plate  1345 . The second diffusion plate  1345  has more orifices than the first diffusion plate  1325 . Also, while only two vapor chambers and two diffusion plates are shown, this is just an example and more diffusion plates can be provided with more vapor chambers. The entire evaporator is heated by a heater that is schematically indicated as circles  1335 . The evaporator of  FIG. 13  can be used in any embodiment shown above instead of or in addition to the injector-based evaporators. Also,  FIG. 14  illustrates an evaporator processing chamber using two evaporators  1472 A and  1472 B as shown in  FIG. 13 . 
     It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations will be suitable for practicing the present invention. 
     The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.