Silicon etching mechanisms in a CF₄/H₂ glow discharge

Abstract

Fluorocarbon film deposition onto Si and its influence on the measured Si etch rate in CF4/H2 reactive ion etching in a symmetric two electrode reactor has been studied as a function of CF4/H 2 feed gas composition, total gas flow, and applied rf power. For reactive ion etching, the fluorocarbon film thickness on Si increases as the percentage x of H2 in CF4/x% H2 is increased. The fluorocarbon film thickness depends on the total gas flow and is greater for greater gas flows. The observed Si etch rate is controlled by the fluorocarbon film. The Si etch rate is directly proportional to the inverse of the F,C-film thickness for fluorocarbon films thicker than ∼10 Å, which may indicate a diffusion-limited mechanism. Both in-diffusion of fluorine and out-diffusion of SiF4 etch product through the fluorocarbon film are consistent with the decrease of the Si etch rate. The relative importance of the lowering of the atomic F concentration in the gas phase on the Si etch rate, e.g., by the H scavenging mechanism, has also been studied. This reaction can be important for conditions where either the inner walls of the etching apparatus can be maintained free of C,F film throughout the etching experiment, and/or a low hydrogen concentration (≤20%) in the CF4/H2 feed gas is used. For a fixed gas composition of CF4/40% H2, the rf-power dependence of the C,F-film thickness and of the Si etch rate was studied. In reactive ion etching, i.e., if rf power is supplied to the bottom (substrate) electrode, at first a monotonic rise in deposited fluorocarbon film thickness with increasing rf power is observed; at high rf-power levels a dramatic decrease in the C,F-layer thickness occurs, which is concomitant with a greater intensity of near surface lattice disorder (from ion channeling studies) and Si etching. Silicon etching is not observed for lower rf powers. In cases where rf power was supplied to the top electrode only, C,F-film deposition has been observed and no Si etching. These data are consistent with a recombinant model of etch anisotropy.