Automated process metrology in solar cell manufacturing

Optimizing a solar cell manufacturing line must take into account a variety of issues. Wafers used for solar cells are typically thinner than those used in semiconductor IC manufacturing. This makes the solar cell wafers susceptible to surface and edge defects such as deep scratches and cracks. The wafer slicing operation can induce thickness non-uniformity as well as surface roughness variation. Wafer texturing (typically via etching) must result in an optimal pyramid height in the case of monocrystalline wafers and an optical grain size in the case of polycrystalline wafers. Silicon Nitride grown on the wafer can induce stress and eventual breakage of the wafer. An uneven nitride film can cause a drop in the overall efficiency of the wafer. The metal contact lines account for a significant cost in the production of a solar cell wafer. The process engineer must pay attention to the contact line height and width while minimizing the total amount of metal used. Based on all these requirements, an optical profiler, the Zeta-200, was developed to provide rapid and meaningful feedback to the process line. In this paper we present results from various process points in solar cell manufacturing, such as bare wafer roughness, silicon nitride film thickness and contact line dimensions.