Yield and efficiency optimization through improved control of surface oxide during string ribbon growth

To minimize production costs and maximize yields, Evergreen Solar has developed a "no-etch" process sequence in which wafers move directly from string ribbon growth to diffusion with no intermediate processing. In this process sequence, the surface oxide thickness d/sub ox/ can affect the diffusion process if it becomes too large. To characterize the process window, we have measured the dependence of sheet resistance and phosphorus dose on d/sub ox/ for Evergreen's standard diffusion process. We find that the sheet resistance has good within-wafer uniformity and is relatively insensitive to oxide thickness for d/sub ox/ < 80 /spl Aring/, yielding a broad process window. Lastly, we report that significant advances in Evergreen's crystal growth furnace design have enabled much tighter control over surface oxide thickness than was previously possible. We demonstrate excellent control of oxide thickness in the range 10-70 /spl Aring/ in a pilot line of dual-ribbon production furnaces over a period of two months.