Examination of fluorocarbon-based plasmas used in the selective and uniform etching of silicon dioxide by response-surface methodology: Effects of helium addition

Abstract

The process parameter space for fluorocarbon plasmas (C2F6/CHF3 and CF4/CHF3, with and without He) which are used to etch SiO2 has been examined with a Lam Research Autoetch 580 parallel plate, single‐wafer etching system by response‐surface methodology. The response or behavior of the plasmas differs very significantly with reactor pressure, etch gas mixture, and He addition. In particular, substantially higher SiO2 etch rates are obtained with C2F6/CHF3 plasmas than with CF4/CHF3 plasmas at an equivalent radio frequency power density for similar gas flow rates at significantly different reactor pressures: ∼2500 Å/min at ∼400 mTorr with C2F6/CHF3 mixtures and ∼750 Å/min at ∼1500 mTorr with CF4/CHF3 mixtures. Upon addition of He to the plasmas, the SiO2 etch rate maximum for the C2F6‐based plasma shifts to ∼800 mTorr and declines to ∼2000 Å/min, while that for the CF4‐based plasma remains near 1400 mTorr and increases to ∼1000 Å/min. Whereas the addition of He sharply degrades the etching uniformity with C2F6/CHF3 mixtures, it has no apparent effect on the etching uniformity with CF4/CHF3 mixtures, which is high with or without the addition of He over the parameter space which was explored. For the C2F6/CHF3 plasma, the etch selectivity to silicon is highest at 700–1100 mTorr, while for the CF4/CHF3 plasma the selectivity is highest at 1100–1500 mTorr. Although addition of He substantially improves the selectivity with the CF4/CHF3 plasmas, it has no effect on selectivity with the C2F6/CHF3 plasma. These detailed characterizations of process parameter space have led to a uniform and selective SiO2 sidewall spacer etching process which may be readily extended to other SiO2 etching processes with this equipment and these gas mixtures.