Diffusion Furnaces Employing Ultra Low Mass Transport Systems and Methods of Wafer Rapid Diffusion Processing

A method of continuous diffusion or co-diffusion layer firing of Photo-Voltaic (PV) solar cell wafers having a bottom surface and a top surface, comprising the steps of:

• a) applying at least one dopant composition to at least one of said wafer bottom surface and said wafer top surface to produce a plurality of wafers doped on at least one of said top and bottom surfaces with a layer of dopant composition;
• b) transporting said plurality of said co-doped wafers sequentially through a plurality of heating and cooling zones from a furnace inlet to a furnace outlet, said zones being disposed in abutting relationship to define a continuous longitudinal processing conveyor path oriented in a generally horizontal plane and said wafers being oriented with said top surface facing upward;
• c) supporting said wafers during said transporting step on a low-mass conveyor system comprising small diameter non-rotating refractory tubes supporting said wafers, which refractory tubes are suspended on wires spanning across the wafer processing path from a first longitudinal outer side of said furnace to a second longitudinal outer side of said furnace, and which wafers are continuously advanced through said zones at a selected rate;
• and
• d) heating said wafers directly by at least one of IR lamp radiation and thermal resistance radiation or re-radiation directed onto said top and bottom surfaces in said heating zones for a time sufficient to promote diffusion of said dopant from said coating layer into said wafer substrate material to complete development of at least one of a p-n junction top surface layer and a back contact bottom surface layer.

An apparatus for thermal processing of Si wafers, having a top surface and a bottom surface, in steps of diffusion and metallization comprising in operative combination:

• a) at least one doper module for applying a doping composition selected from at least one of a Boron dopant composition and a Phosphorous dopant composition;
• b) a continuous conveyor IR lamp-heated diffusion furnace receiving doped Si wafers from said doper module, said diffusion furnace having an elongated heating zone that is split horizontally at the level of the conveyor into an upper portion and a lower portion, and which includes lifting devices that permit relative motion of the upper and lower portions to provide access to the interior of said heating zone, said furnace being adapted for diffusion firing of doped Si wafers;
• c) an anti-reflectance coating module disposed downstream of said diffusion furnace to receive diffusion fired Si wafers from said diffusion furnace and including means for applying an anti-reflectance coating (ARC) to at least a top face of said Si wafers;
• d) a printer/dryer module disposed downstream of said anti-reflectance coating module to receive said ARC-coated Si wafers, said printer/dryer module including means for application of back contact paste to said bottom surface and printing fine collector lines on said top surface to produce printed wafers;
• e) a dryer module for receiving and heating said printed wafers in an IR lamp-heated zone to up to about 650° C. to burn off volatile organic binders from said paste and lines on said top and bottom surfaces of said printed Si wafers;
• and
• f) a metallization furnace having an elongated heating zone that is split horizontally at the level of the conveyor into an upper portion and a lower portion, and which includes lifting devices that permit relative motion of the upper and lower portions to provide access to the interior of said heating zone, said furnace being adapted for IR lamp-heated metallization firing of printed Si wafers to within the range of from about 750° C. to about 1100° C.;
• thereby to produce Si wafers ready for processing, including cleaning, testing and lamination, into solar panel arrays.