Patent Publication Number: US-2010108525-A1

Title: Light-induced plating

Description:
FIELD OF THE INVENTION 
     The invention relates to an apparatus and a method for the light-supported precipitation of a metal from an electrolyte on a semiconductor component. The invention also relates to a semiconductor component. 
     BACKGROUND OF THE INVENTION 
     Light-induced and light-supported galvanic processes are suitable for the production of highly efficient solar cells. Generally, the light source for such methods is located opposite the front side of the solar cell, the electrolyte being arranged between the solar cell and the light source. What is disadvantageous here is that the electrolyte absorbs more or less of the light of the light source. Moreover, a light source arranged directly in front of the surface to be coated adversely affects, on the one hand, convection directly in front of the electrode and, on the other hand, stream line distribution, which can have a very adverse effect on the galvanic process. Finally, if the light source is arranged in the galvanic bath, the constructional and safety effort is considerable. 
     SUMMARY OF THE INVENTION 
     The invention is therefore based on the object of improving an apparatus and a method for the light-supported precipitation of a metal from an electrolyte on a semiconductor component. The invention is also based on the object of creating an improved semiconductor component. 
     Said objects are achieved by an apparatus for the light-supported precipitation of a metal from an electrolyte on a semiconductor component comprising a plating bath with an electrolyte, a first electrode arranged in the plating bath and a second electrode arranged outside the plating bath, a holding device for the semiconductor component and an irradiation device for irradiating the semiconductor component with electromagnetic radiation, the irradiation device being arranged outside the plating bath and the electrolyte forming a layer with a layer thickness of a maximum 10 mm between the semiconductor component and the irradiation device. Furthermore, said objects are achieved by a method for producing a semiconductor component comprising the steps of providing a plating bath with an electrolyte, a first electrode arranged in the plating bath and a second electrode arranged outside the plating bath, providing an irradiation device for generating electromagnetic radiation, providing a semiconductor substrate of a planar design with a first side and a second side lying opposite thereto, immersing at least the second side of the semiconductor substrate into the electrolyte, producing an electrical contact between the first side of the semiconductor substrate and the second electrode, and irradiating at least the first side of the semiconductor substrate by means of the irradiation device. Said objects are further achieved by a method according to the invention, wherein the first side of the semiconductor substrate is its front side. The core of the invention consists in arranging the irradiation device for the irradiation of the semiconductor component outside the plating bath. 
     The irradiation device is preferably arranged on the semiconductor component side not to be coated, i.e. the semiconductor component is illuminated from the side not to be coated. 
     As a light source there is advantageously considered an arrangement of light-emitting diodes and/or one or a plurality of halogen lamps. The electromagnetic radiation generated by the irradiation device preferably exhibits an intensity maximum in the red to near-infrared range. 
     The method according to the invention is characterised in that the semiconductor substrate is immersed into the electrolyte with at least one first side, while it is irradiated by the irradiation device on the second side lying opposite to the first side. 
     The semiconductor substrate is preferably immersed into the electrolyte only so far that the second side remains dry. 
     Alternatively, full immersion of the semiconductor substrate in the electrolyte is possible such that the second side is covered with only a few milli-metres of electrolyte. 
     Features and details of the invention result from the description of several embodiments based on the drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  a schematic representation of the apparatus for the light-supported precipitation of an electrolyte on a semiconductor component according to a first embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a first embodiment of the invention is described with reference to  FIG. 1 . An apparatus for the light-supported precipitation of an electrolyte on a semiconductor component  1  comprises a plating bath  2  with an electrolyte  3 , a first electrode  4  arranged in the plating bath  2  and a second electrode  5  arranged outside the plating bath  2 . The electrolyte  3  contains at least some cobalt and/or nickel and/or silver and/or copper and/or tin and/or a compound of said metals. The electrodes  4 ,  5  are electrically conductively connected to a voltage source  14 . Instead of the voltage source  14  there can also be envisaged only an electrical contact with the electrically conductive connection of the electrodes  4  and  5 . 
     The semiconductor component  1  is especially a solar cell. The semiconductor component  1  comprises a semiconductor substrate  6  of a planar design with a first side  7 , a second side  8  lying opposite thereto and a thickness D in a direction vertically to the sides  7 ,  8 . According to the first embodiment, the first side  7  of the semiconductor substrate  6  is its rear side. The semiconductor substrate  6  consists at least partly of silicon. Other semiconductor materials are, however, also conceivable. 
     The apparatus also comprises a holding device  9  for holding the semiconductor component  1 . The holding device  9  comprises at least three, especially a plurality of supports  10 , by means of which the position of the semiconductor component  1  is fixable in the apparatus. The supports  10  are arranged in the plating bath  2 . They are especially attached to a floor  11  of the plating bath  2 . The supports  10  are preferably adjustable sideways, i.e. in the direction parallel to the floor  11  and thus parallel to the sides  7 ,  8  of the semiconductor substrate  6 . This way it is possible to ensure that they are positioned, with respect to the semiconductor component  1 , at predeterminable points, e.g. in the area of a busbar and/or lying opposite thereto. Advantageously, the supports  10  are designed to be height-adjustable. For height adjustment, there is preferably envisaged an adjustment device  12  indicated only schematically in  FIG. 1 . 
     Furthermore, the apparatus comprises contact elements  13  for electrically contacting the semiconductor component  1 . The contact elements  13  are preferably designed as spring load mounted contact pins. They are electrically conductively connected to the second electrode  5 . They are also electrically conductively connectable to the first side  7  of the semiconductor substrate  6 . The contact elements  13  can be designed to be part of the holding device  9 . Advantageously, the contact elements  13  are each arranged in extension of one of the supports  10 . Hence, they each stand opposite one of the supports  10  with respect to the semiconductor substrate  6 . This prevents bending of the semiconductor substrate  6 . The contact elements  13 , especially their electrical connection to the second electrode  5 , are preferably arranged outside the plating bath  2 . 
     Finally, the apparatus comprises an irradiation device  15  for irradiating the semiconductor component  1  with electromagnetic radiation. The irradiation device  15  is advantageously arranged outside the plating bath  2 . It is thus arranged on the side  7  of the semiconductor component  1  facing away from the side  8  to be immersed into the plating bath  2 . The irradiation device  15  comprises at least one light source  16  designed as a light-emitting diode (LED). The light source  16  comprises especially a plurality of LEDs. The LEDs are arranged in a raster of planar design. 
     The electromagnetic radiation generatable by means of the irradiation device  15  exhibits at least a portion, especially an intensity maximum, in the wavelength range of 650 nm to 1200 nm, especially in the range of 840 nm to 1050 nm, especially in the range of 940 nm to 970 nm. It is thus electromagnetic radiation with a portion in the red to near-infrared range. The output radiatable by the irradiation device  15  is at least 100 mW, especially at least 1 W. The irradiation device  15  is controllable by means of a control device  18  illustrated only schematically in  FIG. 1 . 
     According to the first embodiment, the first side  7  of the semiconductor substrate  6  is its rear side. Decisive for the method according to the invention is that the first side  7  facing the irradiation device  15  is designed such that it is, at least in some areas, at least partly, especially at least 50% permeable for the electromagnetic radiation from the irradiation device  15 . 
     The semiconductor substrate  6  is thus designed such that it is at least 50% permeable, at least in some areas, in at least one direction for electromagnetic radiation from the irradiation device  15  up to a penetration depth of at least 50%, especially at least 75%, especially at least 90% of the thickness D. 
     To this end it exhibits a metalisation  19  on the first side  7  of the semiconductor substrate  6 , which is designed to be at least partly transparent. The metalisation  19  is preferably designed as a grid. Alternatively, the metalisation  19  can also be designed as a transparent semiconductor or as a metal layer that is only a few nanometres thin and thus transparent. Finally, the semiconductor component  1  can also exhibit on the first side  7  of the semiconductor substrate  6  a metalisation of planar design with laser-fired contacts, at which the metalisation  19  is selectively opened and thus permeable for the electromagnetic radiation generated by the irradiation device  15 . 
     The second side  8  of the semiconductor substrate  6 , which side is to be immersed into the electrolyte  3  and faces away from the irradiation device  15 , exhibits predetermined precipitation areas  20 , on which the galvanic precipitation of the electrolyte  3  occurs. The precipitation areas  20  are preferably designed as a seed layer applied onto the semiconductor substrate  6  by means of fine line printing. Alternatively, the precipitation areas  20  may be designed as apertures in an anti-reflection layer on the second side  8  of the semiconductor substrate  6 . 
     The following describes the function of the apparatus on the basis of a method, according to the present invention, for producing a semiconductor component  1 . According to the method according to the present invention, there is first provided the plating bath  2  with the electrolyte  3  and the electrodes  4 ,  5  as well as the irradiation device  15 . Next, the semiconductor substrate  6  is arranged in the holding device  9  with the second side  8  of the semiconductor substrate  6  facing the electrolyte  3 . Then, the semiconductor substrate  6  is immersed, with its second side  8 , into the electrolyte  3  by adjusting the supports  10  by means of the adjustment device  12 . The semiconductor substrate  6  is preferably immersed into the electrolyte  3  only in part, especially only so far that the first side  7  remains dry. To this end, the semiconductor substrate  6 , after immersion in the electrolyte  3 , is lifted back out of the electrolyte  3  by pushing out the supports  10  so far that a surface meniscus  17  forms between the second side  8  of the semiconductor substrate  6  and the electrolyte  3 . The second side  8  is thus at least largely, especially fully in direct contact with the electrolyte  3 . The first side  7  of the semiconductor substrate  6 , however, is above the electrolyte  3  in the plating bath  2 . 
     Alternatively, the semiconductor substrate  6  is immersed fully into the electrolyte  3 , however, only so far that the first side  7  is covered by the electrolyte  3  by a layer with a depth of a maximum 10 mm, especially a maximum 5 mm, especially a maximum 2 mm. The electrolyte  3  thus forms between the semiconductor component  1  and the irradiation device  15  a layer with a layer thickness of a maximum 10 mm, especially a maximum 5 mm, especially a maximum 2 mm. The layer thickness is preferably 0 mm, i.e. there is no electrolyte  3  between the semiconductor component  1  and the irradiation device  15 . 
     The first side  7  of the semiconductor substrate  6  is electrically conductively connected to the second electrode  5  arranged outside the plating bath  2 . For the light-induced or light-supported galvanic precipitation of a metal from the electrolyte  3  on the second side  8 , the first side  7  of the semiconductor substrate  6 , which side  7  lies opposite the second side  8  to be coated, is irradiated by means of the irradiation device  15 . Owing to the appropriately selected spectral range of the irradiation device  15 , especially in the red to near-infrared range, the electromagnetic radiation generated by means of the irradiation device  15  exhibits a depth penetration into the semiconductor substrate  6  of at least 100 μm, especially at least 150 μm, especially at least 180 μm. Thus, free charge carriers are generated in the semiconductor substrate  6  near the PN junction of the semiconductor substrate  6  by the electromagnetic radiation produced by means of the irradiation device  15 . Said free charge carriers lead to a current flow in the circuit formed by the electrodes  4 ,  5  and the semiconductor component  1  and thus to a galvanic precipitation of the electrolyte  3  in predeterminable areas on the second side  8  of the semiconductor substrate  6 . 
     In a second embodiment the irradiation device  15  comprises, instead of the LEDs, at least one halogen lamp as a light source  16 . Said lamp has a broad spectrum with a large portion in the long-wave range. This way it is ensured that a sufficient portion of the electromagnetic radiation emitted by the irradiation device  15  exhibits a penetration depth of at least 50%, especially at least 75%, especially at least 90% of the thickness D of the semiconductor substrate  6 , and that in this way the free charge carriers produced by the irradiation device  15  in the semiconductor substrate  6  reach the PN junction of the semiconductor substrate  6 . 
     According to a third embodiment of the invention, the second side  8  of the semiconductor substrate  6  is the rear side of a solar cell. The first side  7  thus forms the light incidence side of the solar cell. On this embodiment, the second side  8  is preferably designed as an opened anti-reflection layer, but it may also be coated across its entire surface. On this embodiment, the contact elements  13  are preferably arranged on the conductor paths formed by the metalisation  19  designed as a grid and/or the busbars of the semiconductor component  1 . 
     According to a fourth embodiment of the invention, the supports  10  serve to produce an electrical contact between the second side  8  and the second electrode  5 . To this end, the supports  10  are electrically conductively connected to the second electrode  5 . They are designed to be insulated from the electrolyte  3 . This embodiment is particularly suited for galvanic thickening of the contacts on the rear side of an emitter-wrap-through solar cell. 
     According to a fifth embodiment of the invention, the galvanic bath  2  is designed as a so-called cup plater for the application of fountain plating. Such a cup plater is e.g. known from DE 10 2007 020 449 A1. Here, the plating bath  2  comprises a preferably hollow cylinder-shaped insert, which, in the area of the floor  11 , exhibits an aperture from which electrolyte  3  continuously flows into its interior space. To this end, the electrolyte  3  is pumped through the aperture on the floor  11  into the interior space of the insert by means of a pump. The electrolyte  3  pumped into the interior space flows over an upper rim of said insert. The insert protrudes clearly from the surface of the electrolyte  3  in the plating bath  2 . The insert has, especially in the area of its upper rim, a cross-section that is adapted to the size and shape of the semiconductor substrate  6 . During operation of the apparatus, the semiconductor substrate  6  is arranged such on the rim of the insert, which serves as supports  10 , that the second side  8  faces the interior space of the insert. The electrolyte  3  flowing through the aperture into interior space of the insert thus flows past the second side  8  of the semiconductor substrate  6 . As a result of the continuous flow of the electrolyte  3  generated by the pump, there is ensured on the one hand good convection in the immediate electrode vicinity, and on the other hand the semiconductor substrate  6  is pushed against the rim serving as supports  10  and is, as a result, fixed to prevent displacement. With this arrangement, especially the first side  7  of semiconductor substrate  6  is kept dry. 
     The apparatus can, of course, also be designed as an in-line plant, with the semiconductor component  1  being transportable parallel to the sides  7 ,  8  of the semiconductor substrate  6  through the apparatus by means of a transportation device not shown in  FIG. 1 , and the supports  10  being designed as rollers on which the semiconductor substrate  6  is movable through the plant, and especially the contact elements  13  also being designed as rollers. 
     In a vertical design the semiconductor substrate  6  is attached to a steel strap, by which it is at the same time contacted and guided vertically through the plating bath  2 . During this, the semiconductor substrate  6  side  7 ,  8  to be irradiated is guided as closely as possible past a transparently designed wall of the plating bath  2 , on the rear side of which, i.e. opposite, relative to the wall, of the side  7 ,  8  to be irradiated, the irradiation device  15  is arranged. The side  7 ,  8  to be irradiated is preferably the rear side of the semiconductor component  1 .