Selective etching of SiO2 over polycrystalline silicon using CHF3 in an inductively coupled plasma reactor

Abstract

Selective etching of SiO2 over polycrystalline silicon has been studied using CHF3 in an inductively coupled plasma reactor (ICP). Inductive powers between 200 and 1400 W, as well as pressures of 6, 10, and 20 mTorr were used in this study of the etch rate and selectivity behaviors for silicon dioxide, silicon, and passively deposited fluorocarbon films. Using in situ ellipsometry, the etch rates for all three of these materials were obtained for a self-bias voltage of −85 V, as well as passive deposition rates of fluorocarbon films. X-ray photoelectron spectroscopy has been used to examine the composition of steady-state fluorocarbon films present on the surfaces of polycrystalline silicon, and silicon dioxide during etching at high and low inductive powers. The dependence of the silicon etching behavior is shown to be clearly linked to the fluorocarbon polymerization and etching behavior. Thus, the polymerization and etching behavior of the fluorocarbon is the overwhelming parameter that governs the etch selectivity process within the ICP. Selectivities of oxide to silicon are determined to increase with the inductive power, and are found to be the highest at the intermediate pressure of 10 mTorr. While the stoichiometry of the fluorocarbon films are critical factors in determining the overall etch rate behavior, the fluorocarbon film thickness on the polycrystalline and crystalline silicon is the dominant factor in determining the SiO2 over silicon etch selectivity. The mechanisms involved in attaining high selectivity are dominated by a defluorination of the fluorocarbon steady-state film on polycrystalline silicon, while maintaining a high ion current to the wafer.