Abstract:
Embodiments of the inventions provide methods and apparatus to electroplate films of tellurides such as CdTe, or its alloys on multiple large area workpieces. In one embodiment a method of forming a solar cell absorber film on multiple work pieces uses a self adjusting mechanism taking advantage of the high resistivity of the solar cell absorber film. Larger deposits of the plating material onto one workpiece, due for example, to non-uniformity of solution flow, results in larger resistance thus decreasing the current flowing through that workpiece. The decreased current then deposits less material over that workpiece. In another embodiment multiple workpieces can be electroplated using a single power supply in a single plating bath.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of U.S. Provisional Application No. 61/401,632 filed on Aug. 17, 2010, the contents of which are hereby incorporated by reference in their entirety for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present inventions relate to methods and apparatus for preparing thin films of Group IIB-VIA compound semiconductor films, specifically CdTe films, for radiation detector and photovoltaic applications. Inventions are applicable to large scale deposition of thin films on large substrates for manufacturing thin film solar module. 
       BACKGROUND OF THE INVENTION 
       [0003]    Solar cells and modules are photovoltaic (PV) devices that convert sunlight into electrical power. The most common solar cell material is silicon (Si). However, lower cost PV cells may be fabricated using thin film growth techniques that can deposit solar-cell-quality polycrystalline compound absorber materials on large area substrates using low-cost methods. 
         [0004]    Group IIB-VIA compound semiconductors comprising some of the Group IIB (Cd, Zn, Hg) and Group VIA (O, S, Se, Te, Po) materials of the periodic table are excellent absorber materials for thin film solar cell structures. Especially CdTe has proved to be a material that can be used in manufacturing high efficiency solar panels at a cost below $1/W. 
         [0005]      FIGS. 1A and 1B  show the two different device structures employed in CdTe based solar cells.  FIG. 1A  shows a “super-strate” structure, wherein light enters the device through a transparent sheet  11  that it is fabricated on. The transparent sheet  11  serves as the support on which the active layers are deposited.  FIG. 1B  depicts a “sub-strate” structure, wherein the light enters the device through a transparent conductive layer  12  deposited over a CdTe absorber film  14  which is grown over a sheet substrate  16 . 
         [0006]    In fabricating the “super-strate” structure  10  of  FIG. 1A , the transparent conductive layer (TCL)  12  is first deposited on the transparent sheet  11 . Then a junction partner layer  13  is deposited over the TCL  12 . The CdTe absorber film  14  is next formed on the junction partner layer  13 . Then an ohmic contact layer  15  is deposited on the CdTe absorber film  14 , completing the solar cell. As shown by arrows  18  in  FIG. 1A , light enters this device through the transparent sheet  11 . In the “super-strate” structure  10  of  FIG. 1A , the transparent sheet  11  may be glass or a material (e.g. a high temperature polymer such as polyimide) that has high optical transmission (such as higher than 80%) in the visible spectra of the sun light. The TCL  12  is usually a transparent conductive oxide (TCO) layer comprising any one of; tin-oxide, cadmium-tin-oxide, indium-tin-oxide, indium-zinc-oxide, and zinc-oxide, which may be doped to increase their conductivity. Multi layers of these TCO materials as well as their alloys or mixtures may also be utilized in the TCL  12 . The junction partner layer  13  is typically a CdS layer but may alternately be a CdZnS layer or a selenide layer comprising Zn. The ohmic contact  15  is made of a highly conductive metal such as Mo, Ni, Cr, or a doped transparent conductive oxide such as the TCOs mentioned above. Nitrides of refractory metals (such as Mo, Ta, W and Cr) and Al may also be used in multilayer ohmic contact structures. The rectifying junction, which is the heart of this device, is located near an interface  19  between the CdTe absorber film  14  and the junction partner layer  13 . 
         [0007]    In the “sub-strate” structure  17  of  FIG. 1B , the ohmic contact layer  15  is first deposited on the sheet substrate  16 , and then the CdTe absorber film  14  is formed on the ohmic contact layer  15 . This is then followed by the deposition of the junction partner layer  13  and the transparent conductive layer (TCL)  12  over the CdTe absorber film  14 . As shown by arrows  18  in  FIG. 1B , light enters this device through the TCL  12 . There may also be finger patterns (not shown) on the TCL  12  to lower the series resistance of the solar cell. The sheet substrate  16  does not have to be transparent in this case. Therefore, the sheet substrate  16  may comprise a sheet or foil of metal, a sheet of glass or a foil of a high temperature polymeric material such as polyimide. 
         [0008]    The CdTe absorber film  14  of  FIGS. 1A and 1B  may be formed using a variety of methods. For example, U.S. Pat. No. 4,388,483 granted to B. M. Basol et al., describes the fabrication of a CdS/CdTe solar cell wherein the thin CdTe film is obtained by a cathodic compound electrodeposition technique at low electrolyte temperatures, and then the as-deposited n-type CdTe film is type-converted to p-type through a high temperature annealing step to form the rectifying junction with the underlying CdS layer. The compound electrodeposition or electroplating technique typically uses acidic aqueous electrolytes and forms high quality rectifying junctions after the type-conversion step yielding high quality solar cells and modules with conversion efficiencies exceeding 10% (D. Cunningham et al, “CdTe PV module manufacturing at BP solar”, Progress in Photovoltaics, vol. 10, p. 159 (2002)). However, this electroplating technique is slow, yielding 1-2 micron thick CdTe layers in 2-5 hours depending upon the plating conditions and the size of the substrate. As explained in a review titled “Electrodeposition of Semiconductors” (D. Lincot, Thin Solid Films, vol. 487, p. 40 (2005)), CdTe can be cathodically electrodeposited out of an acidic electrolyte containing Cd and Te, at a potential which is more negative than the Te plating potential but more positive than the Cd plating potential. The reason for this is the fact that the free energy formation of CdTe, which is −98.8 kJ/mol, drives the reaction of the Cd species in the solution with Te, forming CdTe, once Te is electrodeposited on the cathode. Kinetics of this reaction is slow and the Te concentration in the acidic solution is low. Other prior art information regarding CdTe electrodeposition may be found in U.S. Pat. No. 4,261,802 by Fulop et al., U.S. Pat. No. 4,425,194 by Kroger et al., U.S. Pat. No. 4,400,244 by Kroger et al., and U.S. Pat. No. 4,345,107 by Fulop et al. There is no in-depth teaching on depositing CdTe films on large area workpieces in these prior art documents. Depositing CdTe films on large workpieces has its challenges both in terms of cost and technology. 
         [0009]    While electrodepositing a CdTe film over a large workpiece, such as a large transparent sheet (such as a glass substrate with dimensions of 60 cm×60 cm or larger) comprising a transparent conductive layer and a junction partner layer, there are voltage drops that need to be taken into consideration. During deposition, the electroplating current flows; i) through an electrical contact made to the transparent conductive layer, ii) through the transparent conductive layer (such as a transparent conductive oxide layer) flowing parallel to its surface, iii) through the junction partner layer (such as a CdS layer) flowing perpendicular to its surface, and iv) through an already deposited portion of the CdTe film, flowing perpendicular to its surface. This is schematically shown in  FIG. 2 , which depicts a prior art electrodeposition system  100 . 
         [0010]    The electrodeposition system  100  comprises a container  101  holding a plating solution  102  comprising Cd and Te species. The stack comprising the transparent sheet  11 , the transparent conductive layer  12 , and the junction partner layer  13  is used as the cathode  107 , which at the same time is the workpiece. There is a conductive anode  103  across from the cathode  107 . Both the anode  103  and the cathode  107  are immersed in the plating solution  102 . A voltage “V” is applied between the anode  103  and the cathode  107  through an electrical contact  105  made to the transparent conductive layer  12 , using a power supply  104 , such that the cathode  107  is made more negative with respect to the anode  103 . The electrical contact  105  is protected from the plating solution  102  by a protective seal  106  which may be made of a non-conductive polymeric material. Upon application of the voltage “V” a plating current “I” starts to flow through the conductive wires connecting the power supply  104  to the anode  103  and the cathode  107 . The plating current “I” flows from the anode  103  to the cathode  107  through the plating solution  102  while depositing a CdTe film over the junction partner layer  13 . The plating current “I” flows through the conductive portions of the cathode  107  in a distributed manner. For example, at or around a location “A” on the cathode  107 , the plating current flows through an already deposited portion  14 A of the CdTe film, and then it flows through the junction partner layer  13  and into the transparent conductive layer  12 . Then it flows horizontally to the electrical contact  105  as shown by arrow  108 . On the other hand, at or around a location “B”, which may be farther away from the electrical contact  105 , the current flow path within the transparent conductive layer  12  between the location “B” and the electrical contact  105  is much longer corresponding to a larger voltage drop. As a result, the surface potential of the CdTe film  14 A at the location “B” is expected to be different (more positive) than it is at the location “A” if the plating current distribution over the surface of the cathode  107  is uniform. It should be noted that the sheet resistance of a typical transparent conductive oxide used in solar cell structure may be in the range of 5-20 ohms per square. 
         [0011]    In experimental deposition systems employing small area substrates (such as 10 cm×10 cm substrates) the voltage drops across the substrate do not constitute a big problem and the plating voltage and current values are closely controlled by a power supply, typically employing a reference electrode, assuring that the deposition conditions fall within the region where stoichiometric CdTe is deposited over the whole substrate. In a manufacturing environment, however, where hundreds of large area substrates need to be processed, it may become very expensive to build hundreds of plating systems such as the one shown in  FIG. 2 , each plating system processing one large area substrate at a time. It also may become expensive to use one power supply per substrate because processing of hundreds of substrates at the same time, in batch mode, requires use of hundreds of power supplies. 
         [0012]    As the above review demonstrates, there is a need to develop methods and apparatus to deposit high quality stoichiometric CdTe layers on multiple large substrates in a cost effective way. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1A  is a cross-sectional view of a prior-art CdTe solar cell with a “super-strate structure”. 
           [0014]      FIG. 1B  is a cross-sectional view of a prior-art CdTe solar cell with a “sub-strate structure”. 
           [0015]      FIG. 2  is a prior-art plating system. 
           [0016]      FIG. 3  shows the different process zones yielding films with different Te/Cd molar ratios. 
           [0017]      FIG. 4A  shows a workpiece over which a CdTe film would be electrodeposited. 
           [0018]      FIG. 4B  shows a cross sectional view of the workpiece depicted in  FIG. 4A . 
           [0019]      FIG. 5A  is a top view of an electroplating tool processing multiple workpieces. 
           [0020]      FIG. 5B  is a side view of the multi-workpiece electroplating tool shown in  FIG. 5A . 
           [0021]      FIG. 6  is a top view of another multi-workpiece electroplating tool. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The chemical composition of an electrodeposited CdTe film is a function of its surface potential during the electrodeposition period. Potentials close to or more negative than the deposition potential of Cd yield Cd-rich deposits, whereas potentials close to the deposition potential of Te yield Te-rich compositions. What is required for high efficiency solar cell fabrication is a stoichiometric CdTe layer with a Cd/Te molar ratio of near 1.0, which can be obtained only within a specific voltage range between the deposition potentials of Te and Cd. 
         [0023]    In the example of  FIG. 2 , because of the different voltage drops between the location “A” and the location “B”, the CdTe layer deposited at or around location “B” may be more Te-rich than the CdTe layer deposited at or around location “A”. Composition of the CdTe film also depends on current density and the solution flow. These are challenges for large area deposition of CdTe films. 
         [0024]    In general, the present invention forms high quality CdTe thin films on multiple large area substrates using one plating system and one power supply. This is achieved by defining the geometry of the electroplating tool and by carefully selecting the process conditions as will be described below. 
         [0025]      FIG. 3  schematically shows a general relationship between the plating potential (E), plating current density (J), and the plated species when a film is electrodeposited on a conductive cathode out of an acidic plating bath containing Cd and Te species. As can be seen from this figure, there are four different zones or regions in this process. For process conditions that fall within Zone I, the compound CdTe is electroplated on the cathode surface in its stoichiometric form, i.e. in a form where Cd/Te molar ratio is near 1.0. In Zone II, a deposit comprising the compound CdTe or CdTex (where x&gt;1) may be electrodeposited with or without excess elemental Te. Alternately, it is also possible to have the compound CdTex in the form of CdTe2 in the deposited film in zone II. Zone III corresponds to electrodeposition potentials that are very close to or more negative than the deposition potential of Cd, which is shown as “V 1 ” in  FIG. 3 . That means elemental Cd can be electroplated in zone III possibly along with some CdTe phase. Zone IV is close to the Te deposition potential, shown as V 2 , and therefore elemental Te may be electroplated in region IV, possibly along with a Te-rich CdTex phase. It should be noted that the values of the deposition potentials V 1  and V 2  may be about −1.1 V and −0.7V, respectively, with respect to a Mercurous Sulfate Electrode, respectively. 
         [0026]      FIG. 3  demonstrates the fact that the region or the process window to electrodeposit stoichiometric CdTe compound with a Te/Cd molar ratio of 1.0 (Zone I) is relatively small and it gets narrower as the current density increases. For example, while at a current density value of “JA” stoichiometric CdTe compound can be electroplated at a deposition voltage range between “VA” and “V 1 ”, it can only be deposited at a voltage range between “VB” and “V 1 ” once the current density is increased to “JB”. As an example, “JA” may be in the range of 0.05-0.5 mA/cm2 while “JB” may be in the range of 0.7-5.0 mA/cm2. This means that for electrodepositing on large area workpieces, the voltage drop present on the surface of the large workpiece between areas near the electrical contacts and areas away from the contacts would not allow use of high current densities if the goal is to electrodeposit a stoichiometric CdTe compound over the whole surface of the workpiece. Therefore, to achieve uniformly stoichiometric CdTe over large area, low current densities would be used reducing the throughput of the process. With lower throughput, one needs to process many substrates at a time, e.g. hundreds of substrates at a time. As discussed before, if each substrate has its own power supply and its own plating system, processing hundreds of substrates in a batch mode may be expensive and complex. 
         [0027]    Unlike electrodeposited metals, electrodeposited Group IIB-VIA compound layers, such as CdTe layers, have high electrical resistivities. In their as deposited forms CdTe layers may have resistivity values in a range of 104-107 ohm-cm, compared to metal resistivities, which may be in the range of 10-4-10-6 ohm-cm. This means that as a CdTe layer is electrodeposited on a junction partner layer, such as a CdS layer, the voltage drop across the deposited CdTe film increases as the thickness of the CdTe layer increases. Present inventions utilize this fact in a beneficial way to deposit CdTe layers over many workpieces in a single electrodeposition tool. 
         [0028]    The workpieces over which CdTe films are electrodeposited have a first dimension which is larger than 50 cm. A second dimension of the workpieces may be larger than or equal to the first dimension, preferably larger than the first dimension. A sketch of an exemplary workpiece  39  comprising a transparent sheet  40 , a transparent conductive layer  45  and a junction partner layer  48  is shown in  FIG. 4A .  FIG. 4B  shows a cross sectional view of the workpiece  39  taken along the Z-Z direction. The transparent sheet  40  has two short edges  41 A and  41 B with a first dimension, and two long edges  42 A and  42 B or contacting edges, with a second dimension. A first contact strip  43  and a second contact strip  44  are placed along the two long edges  42 A and  42 B, respectively, such that these contact strips, which are conductive, make good physical and electrical contact to the transparent conductive layer  45  disposed on the transparent sheet  40 . The contact strips  43 ,  44  are protected from the plating solution by a non-conductive protective layer  46 . The first contact strip  43  and the second contact strip  44  are electrically shorted to each other by wire  49 , which in turn is connected to a terminal wire  49 A. The junction partner layer  48  (such as a CdS layer) is disposed over the transparent conductive layer  45 . The workpiece  39  has a front surface  39 A and a back surface  39 B. The CdTe film is electrodeposited over the front surface  39 A, which comprises the junction partner layer  48 , during the electrodeposition process. 
         [0029]      FIGS. 5A and 5B  schematically show a top view and a side view, respectively, of an exemplary plating tool  300  that processes  20  workpieces at the same time. Such tools may be used to process at least 20, preferably at least 40 and most preferably more than 50 large size workpieces at the same time. Each workpiece may be similar to the workpiece  39  shown in  FIGS. 4A and 4B . The smallest dimension of the workpieces is preferably more than 50 cm, most preferably at least 60 cm. Electrical contact is made to each workpiece using two contact strips disposed over the two long edges of the workpiece as shown in  FIG. 4A . The plating tool  300  has a container  301  that holds a plating solution  302  comprising Cd and Te species. Each workpiece  304  has a front surface  305 A and a back surface  305 B, which are similar to the surfaces  39 A and  39 B described in relation with  FIG. 4B . Accordingly, CdTe electrodeposition is carried out over the front surface  305 A of each workpiece  304  at the same time. Multiple anodes  303  are placed in the plating solution  302  and each anode  303  faces two front surfaces  305 A of two workpieces  304 . All the anodes are electrically shorted to an anode busbar  309  through wires  308 . All the workpieces  304  are also electrically connected to a single cathode busbar  307  through wires  306 . It should be noted that each wire  306  is equivalent to the terminal wire  49 A of  FIG. 4A  and therefore they have electrical connection to contact strips (not shown in  FIGS. 5A and 5B ) employed for each workpiece  304 . 
         [0030]    During process, power is applied between the anode busbar  309  and the cathode busbar  307 , initiating cathodic CdTe deposition over the front surfaces of all workpieces at the same time. Power may be applied through application of a controlled voltage or controlled current by a single power supply because the deposition process is self correcting. If for example, the current density increases for a specific workpiece in the group for any reason (such as non-uniformity of solution flow), the thickness of the CdTe film deposited over that workpiece would also increase. As indicated above, the resistivity for deposited CdTe layers is at least about 104 ohm-cm, and typically falls in the range of about 104-107 ohm-cm. Increased CdTe thickness would increase the resistance of the electrical circuit for that specific workpiece. Increased resistance would, in turn, lower the deposition current, therefore self adjusting the process back to normal where all the workpieces receive substantially the same current density, and therefore substantially the same stoichiometric CdTe layer (e.g., same thickness and same compositional constituents). The above mentioned self adjustment mechanism allows use of a single power supply and eliminates the need for employing one power supply for each workpiece and continually monitoring the voltage-current values. It should be noted that if the plated material was a low resistivity metallic film, a thickness change of the deposited film over one specific substrate would not introduce any significant change in resistance and therefore, the self adjustment mechanism would not work. 
         [0031]      FIG. 6  schematically shows a top view of another exemplary electroplating tool  600  that processes eight workpieces at the same time. The electroplating tool  600  has a near cylindrical container  601  that holds a plating solution  602  comprising Cd and Te species. Each workpiece  605  has a front surface  605 A and a back surface  605 B, which are similar to the surfaces described in relation with  FIG. 4B . Accordingly, CdTe deposition is carried out over the front surface  605 A of each workpiece  605 . A single anode  603  is placed in the plating solution  602  facing the front surface  605 A of the workpieces  605 . All the workpieces  605  are electrically connected to a single cathode busbar (not shown). During process, power is applied between the anode  603  and the cathode busbar, initiating cathodic CdTe deposition on all of the workpieces at the same time. 
         [0032]    The preferred conditions of running the process of the present inventions are as follows: If the number of large area workpieces is given by “N”, the length of the short edges of each workpiece ( 41 A and  41 B in  FIG. 4A ) is “W”, the length of the long edges of each workpiece ( 42 A and  42 B in  FIG. 4A ) is “L”, then the preferred plating current applied to the plating system is preferably less than or equal to a value given by the formula, I=4NL/5W, where I is in Amperes. For example, if 50 workpieces of 60 cm by 120 cm size are processed at the same time, the current applied would be (4×50×120)/(5×60)=80 amperes. This corresponds to a current density of 80000/(50×60×120)=0.22 mA/cm2 on each workpiece. By selecting such a current density value, it is assured that the voltage drop across the transparent conductive layer of each workpiece is within an acceptable range to yield compositionally uniform CdTe layers. 
         [0033]    In general, electrodepositing CdTe films over multiple large size workpieces, each with a short edge dimension of “W”, a plating current density of less than about 1000/W2 is preferred, wherein W is given in units of centimeters and the current density is given in the units of milliamps per square centimeter (mA/cm2). However, the plating current density should be more than about 300/W2 to avoid excessive time loss during deposition. Accordingly, the preferred current density for processing 20 workpieces, each with a short edge dimension of 80 cm would be less than 1000/6400=0.16 mA/cm2, and more than about 0.05 mA/cm2. The total current applied would be less than (0.16×20×80×L), where L is the dimension of the longer edge of each workpiece. 
         [0034]    The techniques described above are also applicable to the formation of Group IIB-VIA absorber layers that include other elements, such as films comprising alloys of CdTe with materials such as Zn, Hg, Mn and Mg. The technique are also applicable for absorber layers containing Te wherein the electroplating solution comprises Te.
       Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.