Abstract:
The invention as described in the following relates to an apparatus for the electrochemical deposition of a metal on a substrate, which apparatus is capable of refreshing an electrolyte used for the deposition in a continuous way. Furthermore, the invention as described relates to a method of refreshing an electrolyte for the electrochemical deposition of a metal on a substrate.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Application 11174683.0-1227 filed Jul. 20, 2011, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention as described in the following relates to an apparatus for the electrochemical deposition of a metal on a substrate, which apparatus is capable of refreshing an electrolyte used for the deposition in a continuous way. Furthermore, the invention as described relates to a method of refreshing an electrolyte for the electrochemical deposition of a metal on a substrate. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electrochemical deposition of a metal, also referred to as electroplating, is a well-know process used, for example, to deposit a metal coating on a substrate surface or work-piece. Electroplating is versatile used in many industrial fields, like e.g. automotive industry, aerospace industry, fitting industry, machine building, and electric/electronic industry. 
         [0004]    In general, for the electrochemical deposition of a metal on a substrate surface, the surface on which the metal is intended to be deposited is brought into contact with a conductive electrolyte comprising ions of the metal to be deposited. While the substrate surface is brought into contact with the electrolyte an electric current between the substrate surface and a counter-electrode is applied. By the applied electric current the metals ions are reduced to the metal and deposited on the substrate surface according to the general formula: 
         [0000]      M n+   +n e   − →M 0 ↓,
 
         [0005]    A special field of electric/electronic industry in this concern is solar industry producing solar cells for transforming sunlight into electric energy. Due to boost on renewable energy, especially the demand on solar cells has increased. A modern type of solar cell is the so called CIGS thin film solar cell (copper, indium, gallium, sulfur and selene thin film solar cell), which is only about 3 μm thick and can also be flexible. 
         [0006]    For the production of such CIGS thin film solar cells, for example a (Cu(InGa)Se 2 -layer is deposited on a substrate surface, like e.g. a glass substrate, a polymer substrate, or a metal foil. Other combinations of layers are possible, too. For example, CIS, CuGaSe 2 , or CuInGaSeS layer combinations are also known in the state of the art, as well as bypass cells. The quality of the metal deposition is quite sensitive to the concentration of the metal to be deposited in the electrolyte used for the electrochemical deposition process, especially with respect to the thickness distribution of the deposited metal layer. 
         [0007]    US 20090315148 A1 discloses an electrochemical deposition method to form uniform and continuous Group IIIA material rich thin films with repeatability. Such thin films are used in fabrication of semiconductor and electronic devices such as thin film solar cells. In one embodiment, the Group IIIA material rich thin film is deposited on an interlayer that includes 20-90 molar percent of at least one of In and Ga and at least 10 molar percent of an additive material including one of Cu, Se, Te, Ag and S. The thickness of the interlayer is adapted to be less than or equal to about 20% of the thickness of the Group IIIA material rich thin film. 
         [0008]    US 20090173634 A1 discloses gallium (Ga) electroplating methods and chemistries to deposit uniform, defect free and smooth Ga films with high plating efficiency and repeatability. Such layers may be used in fabrication of electronic devices such as thin film solar cells. In one embodiment, the present invention provides a solution for application on a conductor that includes a Ga salt, a complexing agent, a solvent, and a Ga-film having submicron thickness is facilitated upon electrodeposition of the solution on the conductor. The solution may further include one or both of a Cu salt and an In salt. 
         [0009]    US 20100140098 A1 discloses a selenium containing electrodeposition solutions used to manufacture solar cell absorber layers. In one aspect is described an electrodeposition solution to electrodeposit a Group IB-Group VIA thin film that includes a solvent; a Group IB material source; a Group VIA material source; and at least one complexing that forms a complex ion of the Group IB material. Also described are methods of electroplating using electrodeposition solutions. 
         [0010]    US 20100059385 A1 discloses a method for fabricating CIGS thin film solar cells using a roll-to-roll system. The invention discloses method to fabricate semiconductor thin film Cu(InGa (SeS).sub.2 by sequentially electroplating a stack comprising of copper, indium, gallium, and selenium elements or their alloys followed by selenization at a temperature between 450 C and 700 C. 
         [0011]    US 20040206390 A1 discloses a photovoltaic cell exhibiting an overall conversion efficiency of at least 9.0% is prepared from a copper-indium-gallium-diselenide thin film. The thin film is prepared by simultaneously electroplating copper, indium, gallium, and selenium onto a substrate using a buffered electro-deposition bath. The electrodeposition is followed by adding indium to adjust the final stoichiometry of the thin film. 
         [0012]    US 20100317129 A1 discloses a methods and apparatus for providing composition control to thin compound semiconductor films for radiation detector and photovoltaic applications. In one aspect of the invention, there is provided a method in which the molar ratio of the elements in a plurality of layers are detected so that tuning of the multi-element layer can occur to obtain the multi-element layer that has a predetermined molar ratio range. In another aspect of the invention, there is provided a method in which the thickness of a sub-layer and layers there over of Cu, In and/or Ga are detected and tuned in order to provide tuned thicknesses that are substantially the same as pre-determined thicknesses. 
       SUMMARY OF THE INVENTION 
       [0013]    Among other aspects it is an object of the invention to provide an improved apparatus and method the deposition of metal layers on a substrate surface. Furthermore, it is an aspect of the invention to provide an improved apparatus and method for the deposition of a metal layer as used in the production of thin film solar cells. 
         [0014]    Surprisingly, it was found that the object of the invention with respect to the apparatus is solved by an apparatus according to independent claim  1 . 
         [0015]    By independent claim  1 , an apparatus for the electrochemical deposition of a metal on a substrate is provided, the apparatus comprising a plating tank holding a plating electrolyte and a refreshing tank in fluidic connection to the plating tank for refreshing the plating electrolyte, wherein the plating tank comprises an inert electrode electrically connected to the substrate by a rectifier, and wherein the refreshing tank comprises a first compartment and a second compartment, the compartments are separated from each other by a semi permeable separator, wherein the first compartment comprises at least one soluble anode of a metal to be deposited on the substrate and the second compartment comprises at least one inert electrode, the inert electrode and the soluble anode are electrically connected to a rectifier. 
         [0016]    The apparatus comprises a plating tank holding a plating electrolyte for the electrochemical deposition of a metal on a substrate surface. The electrolyte comprises ions of at least one metal to be deposited. The ions of the metal to be deposited are dissolved in a solvent, like e.g. water. The ions of the metal do be deposited are comprised in the electrolyte in an amount suitable to allow a metal deposition on the substrate surface by applying an electrical current between the substrate surface and a counter electrode, e.g. an inert anode electrically connected to the substrate by e.g. a rectifier. 
         [0017]    The use of an inert anode in the plating tank is advantageous, because the inert anode is dimensionally stable and allows a constant anodic current density. Furthermore, an inert anode allows a higher degree of freedom in the design of the cell. 
         [0018]    Additionally, the electrolyte may comprise additive influencing/supporting the deposition of the metal and/or stabilizing the electrolyte. Further additives may be comprised in the electrolyte. Due to the appliance of the electrical current on the substrate surface the ions of the metal the metal to be deposited are reduced to the metal, thereby depositing a metal layer on the substrate surface. Due to the reduction of the metal ions to the metal and the deposition of the metal layer on the substrate surface, the concentration of the metal ions within the electrolyte decreases. However, the plating result depends significantly on the concentration of the ions of the metal to be plated within the electrolyte. It is therefore mandatory to maintain the concentration of the ions of the metal to be deposited in the electrolyte. For maintenance of the metal ion concentration, a salt of the respective metal can be added to the electrolyte. However, while the added salt will dissolve in the electrolyte and the metal ions will be reduced to the metal and will be deposited on the substrate surface, the anions of the metal salt will remain in the electrolyte which subsequently influences the features of the electrolyte with respect to, e.g. the density, or the pH-value. However, the plating process is sensitive also to these features, so that the plating result, especially the thickness distribution of the deposited metal layer, may change over the aging of the electrolyte. According to the invention, a refreshing tank in fluidic connection to the plating tank for refreshing the plating electrolyte is provided. The refreshing tank comprises a first compartment and a second compartment, the compartments are separated from each other by a semi permeable separator, wherein the first compartment comprises at least one soluble anode of a metal to be deposited on the substrate and the second compartment comprises at least one inert electrode, the inert electrode and the soluble anode of the metal to be deposited are electrically connected to a rectifier. Soluble anode in this concern means, that metal ions of the metal to be deposited are dissolved from the soluble anode by applying a current between the soluble anode and the inert electrode in the refreshing tank. Accordingly, the concentration of the ions of the metal to be deposited in the refreshing tank can be controlled by the current density and/or voltage of the current applied to the electrodes in the refreshing tank. Since the refreshing tank and the plating tank are in fluidic connection, the concentration in the plating electrolyte of the ions of the metal to be deposited can be maintained by dissolving metal ions from the soluble anode. Advantageously, this allows maintaining the ion concentration of the metals to be deposited within a narrow range without influencing the other features of the plating electrolyte, like e.g. pH-value, or density. 
         [0019]    According to an embodiment of the invention, the semi permeable separator separating the at least two compartments of the refreshing tank from each other is at least one of a membrane, a diaphragm, and a micro-porous wall. 
         [0020]    According to another embodiment of the invention, the second compartment of the refreshing tank in which the inert electrode is located holds an electrolyte which is different from said plating electrolyte. The electrolyte comprised in said second compartment may be a conductive aqueous solution of a conductive salt, like e.g. sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium chloride, potassium chloride, lithium chloride, sodium sulfate, potassium sulfate, lithium sulfate, sodium hydrogen sulfate, potassium hydrogen sulfate, sodium hydrogen carbonate, potassium hydrogen carbonate, and the like. Additionally, acids like e.g. sulfuric acid, hydrochloric acid can be used. 
         [0021]    According to an embodiment of the invention, the electrical conductivity between the electrolyte in the plating tank and the electrolyte in the refreshing tank is ≦1*10 −4 S, preferably ≦1*10 −5 S. By this, an interdependency between the current applied in the plating tank to enable deposition of a metal layer on the substrate and the current applied in the refreshing tank for dissolving metal-ions from the soluble anode can be omitted. 
         [0022]    According to another embodiment of the invention, the low electrical conductivity between the electrolytes in plating tank and refreshing tank is realized by a galvanic isolation of the electrolytes. According to a preferred embodiment of the invention, the apparatus comprises a drop section within the fluidic connection between the plating tank and the refreshing tank. In the drop section, the fluidic stream of electrolyte between the refreshing tank and the plating tank is dissipated into separated droplets falling from an upper drop section to a lower drop section. While the upper drop section is in electrical contact with the one of the plating tank or refreshing tank, the lower drop section is in electrical contact with the other tank. Due to the separate droplets of electrolyte, the two tanks are electrically/galvanically separated from each other. 
         [0023]    According to another embodiment of the invention, the fluidic connection between the plating tank and the refreshing tank is long enough to enable an electrical conductivity of ≦1*10 −4 S between the two tanks. Depending on the relative conductivity of the electrolyte, the diameter and the length of the fluidic connection between the refreshing tank and the plating tank is to be chosen to generate an electrical resistance which is high enough to assure an electrical conductivity of ≦1*10 −4 S. 
         [0024]    According to another embodiment of the invention, the refreshing tank comprises at least two soluble anodes of two different metals to be deposit on the substrate, each of the soluble anodes is connected to a separate rectifier. By providing two soluble anodes in the refreshing tank, also electrolytes used for co-deposition of metals on a substrate surface can be maintained by the apparatus according to the invention. For example, in an electrolyte used for the deposition of indium and gallium on a substrate in the process of producing a CIGS thin film solar cell, the concentration of the indium within the plating electrolyte as well as the concentration of the gallium within the plating electrolyte can be maintained. Due to the separate rectifiers used in the refreshing tank, the dissolution of the metal from the soluble anodes can be controlled separately. This enables an accurate control and maintenance of the metal ion concentration of the metals to be deposited. According to another embodiment of the invention, the refreshing tank comprises a separate inert electrode per rectifier. By this, the accuracy of the maintenance is further enhanced. According to another preferred embodiment, the separate inert electrodes are located in separated compartments, each separated from the compartment holding the electrolyte to be refreshed by a semi permeable separator. 
         [0025]    According to another embodiment of the invention, at least on soluble anode is at least during the operation of the apparatus in liquid state. Liquid state in this concern should be understood as that the soluble anode is not dimensionally stable, e.g. is fused. In a way of example, gallium has a melting point of 29.76° C. When a plating electrolyte is operated at a temperature of about 30° C. to 40° C., a soluble gallium electrode in the refreshing tank will be fused. According to an embodiment of the invention, the refreshing tank comprises a kettle for holding the fused soluble anode. Said kettle comprises an electrical contact for contacting the fused soluble anode to the rectifier. Preferably, the kettle is located at the bottom of the refreshing tank. 
         [0026]    According to another embodiment of the invention, the electrode in the plating tank comprises at least an electrode base body and a screen, said screen reducing the exchange of liquid in the direct environment of the electrode base body. Advantageously, due to the reduced exchange of liquid in the direct environment of the electrode, reactions of components of the electrolyte at the surface of the electrode, like e.g. decomposition of organic compounds, are reduced. Said screen can be e.g. a fabric screen or an inert metal mesh. In a further embodiment, the screen is an inert metal mesh which is isolated from said electrode base body and a current is applied to the mesh. By appliance of a current, an electrostatic barrier is formed which further reduces the exchange of liquid in the direct environment of the electrode base body. This further increases the stability of the electrolyte due to the reduced decomposition of electrolyte compounds. 
         [0027]    In another aspect, the invention relates to a method of refreshing an electrolyte for electrochemical deposition of a metal on a substrate, the method comprising the steps of:
       separating at least a partial stream of the electrolyte from a plating tank and feeding said stream to a refreshing tank, said refreshing tank comprising a first compartment and a second compartment, the compartments are separated from each other by a semi permeable separator, wherein the first compartment comprising at least one soluble anode of a metal to be deposited on the substrate and the second compartment comprises at least one inert electrode;   connecting the inert electrode and the soluble anode electrically to a rectifier;   setting the current and/or voltage of the rectifier to a range suitable to electrochemically dissolve metal from the soluble anode;   re-feeding at least a partial stream of the electrolyte in the refreshing tank to the plating tank.       
 
         [0032]    Advantageously, by the inventive method maintenance of the plating electrolyte with respect to the concentration of the metal ion of the metal to be deposited is enabled with substantially not influencing other features of the plating electrolyte, like e.g. pH-Value or density. 
         [0033]    According to an embodiment of the inventive method, the concentration in the electrolyte of the ions of the metal to be deposited is analyzed and the current and/or voltage of the rectifier or rectifiers in the refreshing tank is set to maintain a concentration of the metal to be deposited within a variation limit of ≦3% by weight, preferably ≦2% by weight, most preferred ≦1% by weight. Appropriate methods for analyzing the concentration of the respective metal ions are, e.g. IR-spectroscopy, AAS (atom absorption spectroscopy), UV-VIS analysis, or titrimetric analysis. 
         [0034]    In another aspect the invention relates to the use of an apparatus as described above for the electrochemical deposition of a metal form an alkaline electrolyte. Preferably, the invention relates to the use of an apparatus as described above for the electrochemical deposition of a metal of the group consisting of gallium, indium, and thallium. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0035]    In the following the invention is described in terms of figures and examples, while the inventive concept is not limited the examples as described. 
           [0036]      FIG. 1  shows a schematic view of an apparatus according to the invention; 
           [0037]      FIG. 2  shows a schematic view of an apparatus according to the invention having a screen shielded electrode; 
           [0038]      FIG. 3  shows a drop section as it can be comprised in an apparatus according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]      FIG. 1  shows a schematic view of an apparatus  100  according to the invention. The apparatus  100  for the electrochemical deposition of a metal on a substrate  900  comprises a plating tank  110  holding a plating electrolyte  500  and a refreshing tank  400 . The refreshing tank  400  is in fluidic connection  200 / 210  to the plating tank  110  for refreshing the plating electrolyte  500 . The plating tank  110  comprises an inert electrode  120  electrically connected to the substrate  900  by a rectifier  190 . For the deposition of a metal layer on the substrate  900  a current is applied by the rectifier  190 . The refreshing tank  400  comprises a first compartment  410  and a second compartment  420 , wherein the compartments  410 / 420  are separated from each other by a semi permeable separator  430 . An appropriate separator  430 , e.g. a membrane, a diaphragm, and a micro-porous wall. The first compartment  410  comprises at least one soluble anode  440  of a metal to be deposited on the substrate  900 . The second compartment comprises at least one inert electrode  450 . The inert electrode  450  and the soluble anode  440  are electrically connected to a rectifier  490 . By appliance of a current between the soluble anode  440  and the inert electrode  450  metal ions are dissolved from the soluble anode  440 . Due to the fluidic connection  200 / 210  between the plating tank  110  and the refreshing tank  400 , the metal ion concentration of the metal to be deposited can be maintained by the metal ions dissolved from the soluble anode  440 . Since the metal of the soluble anode  440  may not maintain dimensional stability during operation of the inventive apparatus (it may fuse due to the operating temperature), the refreshing tank may comprise a kettle  445  to hold the fused soluble anode. The kettle comprises an electrical contact  446  for contacting the fused soluble anode to the rectifier  490 . 
         [0040]      FIG. 2  shows an embodiment of the inventive apparatus  100  in which the electrode  120  on the plating tank  110  is covered by a screen  150 . The screen  150  may be a fabric reducing the liquid exchange in the near environment of the electrode  150 . Due to the reduced liquid exchange, less electrolyte  500  is brought into direct contact with the surface of the electrode  120 . Since decomposition of other components of the electrolyte  500  take place on the surface of the electrode  120 , the reduced liquid exchange results in less decomposition of electrolyte components, like e.g. organic additives. This further increases the stability of the electrolyte and elongates the electrolytes life time. 
         [0041]      FIG. 3  shows a drop section  800  as it may be comprised in the fluidic connection  200  and/or  210  connecting the plating tank  110  to the refreshing tank  400 . The drop section  800  is capable to electrically isolate the plating tank  110  from the refreshing tank  400 . The drop section  800  comprises an inlet connected to the fluidic connection  200  or  210 . The electrolyte entering the drop section  800  by the inlet  810  is guided to a strainer  820  separating the electrolyte into droplets  830 . The droplets  830  are collected in a receiving section  840 , to which receiving section  840  an outlet  850  is connected. The outlet is connected to the fluidic connection  210  or  200 , respectively. In an embodiment of the invention the apparatus comprises two drop sections  800 , one in each of the fluidic connections  200  and  210 . 
         [0042]    When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0043]    As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. The scope of invention is defined by the appended claims and modifications to the embodiments above may be made that do not depart from the scope of the invention.