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_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_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_ox < 80 Å, 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 Å in a pilot line of dual-ribbon production furnaces over a period of two months.