Radial Etch Rate Nonuniformity in Reactive Ion Etching

A study was performed to determine some of the causes of the edge‐to‐center “bullseye” clearing pattern, in which the etch rate decreases monotonically from the wafer periphery to its center, observed when certain films are etched in a parallel‐plate reactive ion etching system. It was found that gradients in local reactant concentrations, with a higher number density present near the edge of the substrate than over the center, are largely responsible for the observed nonuniformity. The concentration gradients appear to come about as a consequence of two reactant generation phenomena and one loss mechanism. The reactant generation phenomena are: ( i ) the “hot spot,” which the edge of the wafer may represent to the plasma, causing enhanced reactant generation there, and ( ii ) differences in capacitive coupling from the RF generator to the plasma between the wafer's location on the cathode and the rest of the cathode. Stronger coupling surrounding the wafer leads to increased production of reactant around the wafer compared to the area directly above it. The reactant loss mechanism is controlled by the relative etch rates of the wafer vs. the cathode material, the using cathodes which etch at an appreciable rate in the plasma was found to promote improved uniformity. Ion bombardment was found to be uniform across the surface of the wafer, and, consequently, etch processes which are strongly bombardment dependent were found to be uniform.