Chemical conversion of C₂ F₆ and uniformity of etching SiO₂ in a radial flow plasma reactor

Abstract

Plasma etching of SiO₂} with C₂}F₆} in a radial flow reactor was investigated to determine relationships between chemical aspects of the discharge, operating parameters such as power and flow rates, and uniformity of etching over a large area. The chemical conversion of C₂}F₆} in the discharge was monitored by infrared spectrometry of the exhaust gases, as a function of discharge power and gas flow rate. The input gas was found to be transformed mainly to CF₂} and a polymeric material, and at sufficiently long residence times (∼1 sec) a "steady state " was achieved. At the steady state condition C₂}F₆} was found to be ∼60% converted. The amount of conversion was not influenced by the presence of SiO₂} being etched, however, the production of CF₄} was reduced by the presence of SiO₂}. For a known flow rate and reactor dimensions the concentrations of species as a function of radial position in the reactor were calculated. It was demonstrated that the etch rate of SiO₂} was dependent on this radial concentration profile. Uniform etching was obtained if the etching zone lay entirely within the "steady state" region of the discharge. Reduced etch rates or polymerization on the substrates was observed if large concentration gradients (rich in C₂}F₆}existed in the etching zone. It was concluded that the etch rate of SiO₂} is dependent then on the local gas composition in the discharge, and can be manipulated by means of adjusting power and gas flow rates.