A UNIFORMITY DEGRADATION MECHANISM IN RAPID THERMAL CHEMICAL VAPOR-DEPOSITION

In this letter, a new physical mechanism that can significantly degrade the thickness uniformity in rapid thermal chemical vapor deposition (RTCVD) has been identified and experimentally verified using polycrystalline silicon (polysilicon) deposition on silicon dioxide. The mechanism is initiated by small temperature variations across the wafer typically caused by edge cooling or large area patterns. The amount of light absorbed in a silicon wafer is determined by the wafer absorptivity weighted by the emission spectrum of the heat lamp. The absorptivity is a strong function of the thickness and optical properties of the layers on the wafer surface. Consequently, during RTCVD, once a nonuniform deposition pattern is established, the absorptivity and hence, the temperature uniformity become strongly dependent upon the thickness uniformity. This causes the nonuniformity to increase with process time and the thickness of the deposited layer.