The characterization of n + ‐polysilicon etching in discharges was undertaken using a Box‐Behnken experimental design. Etching rate, ion flux, Cl concentration, and selectivity with respect to the underlying oxide were measured as a function of three variables: power, pressure, and He fraction over ranges of 0.4–1.2 W/cm 2 , 150–300 mtorr, and He percentage between 20 and 50, respectively. The process is anisotropic with a small linewidth loss (0.1–0.2 μm), which is almost independent of the process parameters over the ranges explored and does not seem to vary with overetching. The selectivity varies between 10 and 21. Kinetic modeling of the process successfully predicts the atomic Cl concentration and indicates that dissociates in the discharge substantially (possibly more than 45%); the extent of dissociation depends on the power and the residence time. The primary etchant is assumed to be atomic Cl, which has an etching rate that is enhanced by ion bombardment. The experimental data were successfully modeled assuming that Cl competes with carbonaceous species for sorption on the polysilicon surface, thus causing the etching rate to be inversely proportional to the total concentration.
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Gogolides et al. (1989) studied this question.