Redeposition of etch products on sidewalls during SiO2 etching in a fluorocarbon plasma. V. Effects of C∕F ratio in plasma gases

Abstract

The effects of C∕F ratio in fluorocarbon gases, used in the plasma etching of SiO2, on the properties of sidewalls of etched trenches, as affected particularly by the redeposition of particles emitted from the bottom, were investigated using three different plasma gases, CF4, CHF3, and C4F8. The use of a Faraday cage and step-shaped substrates permitted an effective analysis of the chemical and physical properties of the sidewall surface in a scale larger than that for micro-patterns. The step-shaped substrates consisted of one bottom and two sidewalls: One of the sidewalls was affected by bottom-emitted particles, designated as sidewall (A), and the other was unaffected by the particles, designated as sidewall (B). Comparison of the surface properties of the two sidewalls allowed us to observe the redeposition effect, independently from other complicated phenomena involved in the plasma etching. For all cases of CF4, CHF3, and C4F8, the rate of film deposition on sidewall (A) was larger than that on sidewall (B), which indicated that the formation of a passivation layer on the sidewall was accelerated by the redeposition of bottom-emitted particles. The contribution of redeposition to the formation of a sidewall passivation layer rapidly decreased with an increase in C∕F ratio, in the order CF4>CHF3>C4F8 approaching zero for C4F8. The change in the surface roughness of sidewall (A) with C∕F ratio was correlated with the contribution of redeposition particles, instead of the thickness and carbon content of the polymer film formed on the sidewall. The chemical composition of carbon-containing polymer layer of sidewall (A) and the extent of chemical etching of the SiO2 layer beneath the polymer layer were distinctly different for the three plasma gases.