Patent Publication Number: US-2011062028-A1

Title: Process and apparatus for electroplating substrates

Description:
CROSS-REFERENCE 
     The subject matter described and claimed herein below is also described in German Patent Application No. 10 2009 029 551.8, filed on Sep. 17, 2009 in Germany. This German Patent Application provides the basis for a claim of priority of invention for the invention described and claimed herein below under 35 U.S.C. 119 (a)-(d). 
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to a process for electroplating at least one surface of at least one substrate, especially solar cells, in which the substrate is moved through an electrolyte coating liquid, is irradiated with light and an electroplating current is applied to the substrate by means of an electrolytic cell rectifier circuit. 
     It also relates to an apparatus for performing the process, comprising an electrochemical coating bath, which comprises a coating tank filled with an electrochemical coating liquid, a conveying device for transporting the substrate through the coating bath, a light source circuit with at least one light source for irradiating the substrate and an electrolytic cell rectifier circuit for the substrate with an anode. 
     2. The Description of the Related Art 
     DE 10 2007 038 120 A1 describes a type of coating apparatus, in which solar cells are conveyed through a coating tank, which contains a coating bath, by means of conveying rollers. Different types of light sources, e.g. LEDs, whose wavelengths are adjusted or set according to the respective coating liquid, are arranged on the underside of the tank. 
     The coating can be exclusively light induced, but also can be current assisted. The irradiation with the light sources generates a voltage in the cell, which induces a current flow. This current flow initiates a deposition of metal from the coating bath on the front side of the solar cell. The solar cell is the cathode in the electroplating current circuit, which is characterized as an electrolytic cell rectifier circuit. However the combined light- and current-generated coating process produces no significant increase of throughput. 
     DE 42 25 961 A1 teaches that operating with constant direct voltage in an electroplating circuit is harmful. The object to be coated is located in substantially the same electric field on its path through the coating bath. The plating speed, i.e. the speed with which the deposited metal layer is built up on the substrate, is however comparatively small. That means that the length of the electrochemical plating apparatus for a given speed of movement of the object must be very great. 
     In order to achieve a greater plating speed the electrochemical circuit, i.e. the electrolytic cell rectifier circuit, is operated with a pulsed current, as proposed in DE 42 25 961 A1. It has been said that the plating speed can then be increased many times with these measures. The current-free time intervals between the current pulses are compensated for by increasing the current applied during the current pulse. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to increase the throughput of the substrate through the coating apparatus as well as the speed of the coating process. 
     According to a first aspect of the invention the apparatus for electroplating a substrate according to the invention comprises an electrochemical coating bath, which has a coating tank filled with the electrochemical coating liquid; a conveying device for transporting the substrate through the coating bath; a light source circuit with at least one light source for irradiating the substrate; an electrolytic cell rectifier circuit for the substrate with an anode and means for producing synchronous current pulses and light pulses, so that during a time interval between the current pulses the irradiating is interrupted. 
     According to a second aspect of the invention the process for electroplating the substrate according to the invention comprises:
         a) moving the substrate through an electrolytic coating liquid;
           b) during the moving of the substrate irradiating the substrate with light;   c) during the irradiating passing an electroplating cu rent through the substrate by means of an electrolytic cell rectifier circuit; and   d) pulsing the electrical current and the light synchronously, so that during a time interval between current pulses the irradiating is interrupted.   
               

     The apparatus for the invention is particularly characterized by means for producing synchronous current pulses and light pulses, so that the irradiation is interrupted between the current pulses. 
     It has been shown that the throughput can be increased when the process not only uses current pulses but also light pulses synchronized with the current pulses. Synchronous pulses means that the light pulse is always applied to the substrate when the current pulses is applied. The pulse interval between the current pulses always coincides with the pulse interval between the light pulses. In order to interrupt the irradiation of the substrate surface to be coated, for example the light source can be shut off. Also it is possible to mask the light source during the pause interval. LEDs are the preferred light sources. 
     The plating speed could be still further increased many times with the synchronous light pulses in comparison to the constant irradiation according to DE 10 2007 038 120 A1. 
     Preferably the means for synchronous production of current pulses and light pulses comprises at least one pulse generator in the electrolytic cell rectifier circuit and/or in the light source circuit. 
     The electrolytic cell rectifier circuit and/or the light source circuit are preferably connected to the pulse generator. 
     A pulse generator can be provided to which both circuits are connected. Also an individual pulse generator can be provided in each circuit. The individual pulse generators in each circuit would be connected together for production of synchronous pulses. 
     Preferably the light source circuit is coupled by means of an optocoupler with the electrolytic cell rectifier circuit. Because of the galvanic separation of the input and the output the coupling provided by the optocoupler has the advantage that interfering effects from one circuit will not be transmitted to the other circuit. The coupling of the circuits by means of an optocoupler thus provides the advantage of a greater stability and uniformity of the generated pulses. 
     According to another embodiment of the apparatus according to the invention the light source circuit and/or the electrolytic cell rectifier circuit have at least one control device, which can control the light intensity of the light source and/or the strength of or current applied during the current pulses according to the widths of the current pulses and/or the light pulses. The morphology of the deposited metal layers can be influenced by means of the at least one control device via the respective heights of the light pulses and the current pulses. Graduated metal layers with optimized properties can be deposited. Another possibility consists of simultaneous variation of the heights of the light pulses and the current pulses, whereby the height of the current pulses is decreased and the height of the light pulses is simultaneously increased during the electrochemical plating process. 
     In order to further increase the plating speed and thus the throughput, means for producing a coating liquid flow are provided. Since the at least one substrate is moved through the resting coating liquid, a flow is already present at the at least one surface of the at least one substrate. When the means for producing a coating liquid flow is operated, an additional flow over the at least one surface of the at least one subtrate in addition to the flow due to the substrate motion is thus provided. 
     Indeed when the current density is increased, e.g. by increasing the height of the current pulses, the throughput is indeed increased, because the coating thickness to be attained is reached sooner. However the current density cannot be arbitrarily increased, because the so-called limiting current density must be considered. The term “limiting current density” means the current density at which the free metal ion concentration at the cathode surface to be coated, approaches zero. 
     When the limiting current density is exceeded, gaeous hydrogen is produced because of depletion of the metal ions in the electrolyte, which produces, among other things, pores in the prevoiusly deposited metal layer, with the consequence that the morphology of the metal layer is impaired, which can lead to pulverization of the metal layer. 
     It has been shown that the higher the flow speed of the coating liquid at the surface of the cathode area of the at least one substrate, the higher the limiting current density. 
     According to the first embodiment the means for producing the flow of the coating liquid comprises at least one nozzle for conducting the coating liquid. Because of the use of the at least one nozzle the coating liquid flow is advantageously a turbulent flow, which is preferably so large that it prevails or reinforces the flow caused by the transport of the substrate through the coating liquid at the surface to be coated. 
     Preferably the at least one nozzle is arranged in the coating bath opposite to the surface to be coated. 
     The at least one nozzle is preferably oriented perpendicular to the at least one surface of the at least one substrate to be coated. Because of that a flow of coating liquid is directed to the at least one surface of the at least one substrate so that whirls or eddies in the flow are produced at the substrate. These eddies or whirls at the surface of the substrate should have a speed which is greater than the relative speed of the coating liquid caused by the transport of the substrate. 
     Preferably the nozzles are arranged between the anodes of the electrolytic cell rectifier circuit. The liquid fed from the nozzles can then flow unhindered to the surface to be coated and the flow is not impaired by the presence of the anodes. 
     The nozzles are preferably Venturi nozzles, with which a high outlet speed of the coating liquid can be attained. 
     According to a second embodiment the means for producing the flow speed of the coating liquid is a circulating device. 
     This circulating device is preferably a countercurrent flow device. The countercurrent device produces a flow which is in a direction that is opposite to the transport direction of the substrate in the coating bath. The inlet of the coating tank is preferably arranged at the outlet of the transport device and the outlet of the coating tank is preferably arranged at the inlet of the transport device. The coating liquid is preferably pumped in a direction that is opposite to the transport direction of the substrate. Preferably the inlet and outlet of the counter-current flow device are at the same height as the transport device, i.e. arranged at the height of the substrate to be transported, so that the counter flow is not hindered by the other structures in the coating bath. 
     The device with the nozzles can similarly be used alone as the circulating device. It is however preferred to provide both devices in combination with each other. 
     The process for electroplating at least one surface of the at least one substrate is characterized in that the electroplating current and the light are synchronously pulsed, wherein the irradiation with the light is interrupted in the time interval between the current pulses. 
     The respective widths of the light pulses and the current pulses advantageously amount to from 0.1 ms to 10000 ms. Preferred pulse widths are 1 ms to 1000 ms, particularly 1 ms to 100 ms. 
     The pulse widths are preferably selected to be equal to the time interval between the pulses. 
     The current pulses and/or the light pulses are preferaly rectangular pulses. 
     The heights and the widths of the light pulses and/or the current pulses can be varied. The variation of the pulses permits deposition of e.g. graded metal layers with optimized properties. In this way individual plating programs may be realized by means of the control device or control devices. 
     It has been shown that it is preferable to reduce the pulse heights of the light pulses when the pulse widths are increased. Thus for example if the light intensity is 10% of the maximum light intensity when the pulse width is 100 ms, the light intensity can be 20% of the maximum with a pulse width 1 ms and 80% with a pulse width of 0.5 ms. 
     To improve the throughput a direct current can be applied in the interval between the current pulses, whose strength I 1  is less than or equal to 0.5×I 2  wherein I 2  is the pulse height of or current applied during the current pulses. However the value I 1  of the current during the time interval between the current pulses is preferably low enough so that the required regeneration of the electrolytic coating liquid and/or the metal concentration at the cathode surface to be coated is not impaired. 
     The coating liquid is preferably put into a counter flow opposite to the feed direction of the at least one substrate in the vicinity of the at least one surface of the at least one substrate to be coated. 
     In addition to this counter flow or even independently of this counter flow a turbulent flow of this coating liquid can be produced at least in the vicinity of the at least one surface of the at least one substrate to be coated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiments, with reference to the accompanying figures in which: 
         FIG. 1  is a vertical cross-sectional view through a coating tank according to the invention; 
         FIG. 2  is a circuit diagram for and a diagrammatic view of one embodiment of an arrangement of the light sources; 
         FIG. 3  is a circuit diagram for and a diagrammatic view of another embodiment of an arrangement of the light sources; and 
         FIGS. 4 and 5  are respective graphical illustrations of the correlation between current and light intensity pulses in the process according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An apparatus  10  for coating substrates  1  is illustrated in  FIG. 1 . Solar cells  1   a  are used as substrates in the present example. The apparatus  10  includes a coating tank  12 , which comprises a coating bath  13 , which contains an electrolytic coating liquid  14 . 
     A conveying device  15 , which comprises upper conveying rollers  16  and lower conveying rollers  18 , is provided in the upper part of the coating tank  12 . The solar cells  1   a  are held between the upper and lower conveying rollers  16 ,  18  and conveyed in the direction of the arrow  5 . Since the coating tank  12  is filled with the coating liquid  14 , the solar cells  1   a  are completely within the coating bath  13 . 
     A circulation device  30  for producing a flow in the coating bath  13 , which has a first liquid conductor  36 , is provided. The coating tank  12  has a first outlet  22  below the conveying device  15 , to which the first liquid conductor  36  is connected. The coating liquid  14  is pumped from the coating tank  12  via the first outlet  22  and fed back into the coating tank  12  through a first inlet  20 , which is arranged in the bottom region of the coating tank  12 , by means of a first pump  32  arranged in this first liquid conductor  36 . 
     An inlet flow system  40  with an inlet pipe  42  in the interior of the coating tank  12  is connected at one end to the first liquid conductor  36 . At the other end of the inlet pipe  42  a horizontally oriented distributor pipe  44  is connected, in which a plurality of Venturi nozzles  46  are provided. These Venturi nozzles  46  are oriented vertically and thus arranged perpendicular to the solar cells  1   a.    
     The input coating liquid flows with a high speed from these Venturi nozzles  46  and thus impinges substantially perpendicularly on the facing or front side  3  of the solar cells  1   a  to be coated from below (see  FIG. 2 ). 
     So that this flow of coating liquid is not disturbed by parts or components of the apparatus, these Venturi nozzles  46  are arranged between the anodes  54  of the direct current circuit  50  (see  FIG. 2 ). Light sources  64 , which are LED light strips, are arranged above the anodes  54 . The arrangement of light strips is only illustrated schematically. Especially this leads to turbulent flow on the surface  3  of the solar cells  1   a , so that the coating liquid  14  can regenerate rapidly in the vicinity of the surface  3  to be coated during the interval between the pulses. 
     In order to produce a flow opposite to the conveying direction  5  within the coating bath the coating tank  12  has a second outlet  23  in its left lower region. A second liquid conductor  38  is connected to this second outlet  23 . A second pump  34  is provided in the second liquid conductor  38 . This conductor  38  opens into a second inlet  21  in the upper right region of the coating tank  12 . The second inlet  21  is located in the vicinity of the solar cells  1   a , so that a horizontal flow in a direction opposite to the conveying direction  5  is produced between the second inlet  21  and the first outlet  22 . 
     The apparatus  10  for example can be equipped without the second liquid conductor  38 . Another embodiment of the apparatus can be designed without the inlet flow system  40  with and without the second liquid conductor  38 . In the embodiment without the second liquid conductor  38  the first liquid conductor  36  is preferably connected to the second inlet  21  to the coating tank  12 . 
     A solar cell  1   a  is illustrated in the detailed cross-sectional view that is part of  FIG. 2 . It has a metallization on its back side  2  and strips  4  comprising a suitable paste for formation of contact fingers on its front side  3 . Screen printing paste is advantageously used for formation of the electrode structures. Metal is deposited from the electrochemical coating liquid only in the vicinity of this screen printing paste during the electrochemical deposition. 
     The upper conveying rollers  16  contact the metallized rear side  2  of the solar cells  1   a  and can be used for application of an electrochemical current. For this purpose an electrolytic cell rectifier circuit  50  is provided, which connects the upper transport rollers  16  with the anodes  54 , which are preferably silver anodes. A first voltage source  52  and a pulse generator  53  are provided in the electrolytic cell rectifier circuit  50 . Current pulses are applied to the solar cells  1   a  and the anodes  54  by means of the pulse generator  53 . 
     Furthermore a light source  64 , e.g. a LED, is shown in  FIGS. 2 and 3  and represents a plurality of light sources for irradiation of the surface  3  of the substrate to be coated. This light source  64  is connected in a light source circuit  60 , which has a second voltage source  62 . 
     Both circuits  50  and  60  are coupled with each other by means of an optocoupler  56 . The input  57  of the optocoupler  56  is connected to the electrolytic cell rectifier circuit  50  and the outlet  58  of the optocoupler is connected to the light source circuit  60 . 
     The optocoupler  56  is switched in such way that the light source  64  is turned on at the same time that a current pulse is generated, so that the light pulse is produced simultaneously with the current pulse. During the time interval between the current pulses the light source  64  is turned off. 
     In  FIG. 3  an additional embodiment of the apparatus according to the invention is illustrated, in which no optocoupler  56  is provided. Instead of the optocoupler  56  the light source circuit  50  is connected directly to the pulse generator  53 , which thus produces the pulse for both circuits  50  and  60 . 
     Optionally a second control device  66  can be provided in the light source circuit  60 . The control device  66  can control the light pulse strength for example according to the length of the pulse, i.e. the pulse width. 
     Similarly a control device  59  can be provided, with which current pulses having different pulse widths and height can be generated. A completely automatic electroplating program with individual coating steps may be provided by means of both control devices  59 ,  60 . This is of special advantage in order to form graduated layers on which specific electrode structures are formed by the paste  4 . 
     Two diagrams of the current pulses and the light pulses are illustrated in  FIG. 4 . The light pulses and the current pulses are of equal length. Respective time intervals are provided between the current pulses and the light pulses, which are of the same length as the pulse widths. The light pulses and the current pulses are completely synchronized. The light intensity is zero between the pulses in the time interval. In order to achieve this the light source can be turned off or masked during the time interval. 
     Another embodiment of the process according to the invention is illustrated in  FIG. 5 , in which a direct current, with a value I 1 , is applied during the time interval between the current pulses. The current, I 1 , amounts to 50% of the current I 2 , which is applied to the substrate during the current pulses. The current pulse in the embodiment of  FIG. 5  is the same height as that shown in  FIG. 4 . 
     PARTS LIST 
     
         
         
           
               1  substrate 
               1   a  solar cell 
               2  rear side 
               3  front side 
               4  paste 
               5  conveying direction 
               10  apparatus 
               12  coating tank 
               14  electrolytic coating liquid 
               15  conveying device 
               16  upper conveying rollers 
               18  lower conveying rollers 
               20  first inlet 
               21  second inlet 
               22  first outlet 
               23  second outlet 
               30  circulating device 
               32  first pump 
               34  second pump 
               36  first liquid conductor 
               38  second liquid conductor 
               40  inlet flow system 
               42  inlet pipe 
               44  distributor pipe 
               46  Venturi nozzle 
               50  electrolytic cell rectifier circuit 
               52  first voltage source 
               53  pulse generator 
               54  anode 
               56  optocoupler 
               57  input of the optocoupler 
               58  output of the optocoupler 
               59  first control device 
               60  light source circuit 
               62  second voltage source 
               64  light source, LED 
               66  second control device 
           
         
       
    
     While the invention has been illustrated and described as embodied in a process and apparatus for electroplating substrates, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 
     What is claimed is new and is set forth in the following appended claims.