SELECTIVE REACTIVE ION ETCHING OF SILICON-NITRIDE OVER SILICON USING CHF3 WITH N-2 ADDITION

In this article the effects of process parameters of CHF3+N-2 plasma e tching chemistry (rf power 50-70 W, pressure 22.5-52.5 mTorr, and N-2 content 0%-95%) and mask materials (photoresist, aluminum, and silicon nitride) on the etching selectivity of silicon nitride over polysilic on are investigated. It was found that the selectivity increased with the N-2 content in the range of 0%-85% and then decreased, leading to the maximum selectivity (16) at 7.5 seem CHF3+42.5 seem N-2 (85% N-2) at 60 W and 37.5 mTorr. The selectivity increased linearly with power and decreased with pressure. The higher the Si/N ratio of the nitride, the faster the nitride is etched. No influence of the residual stress level on the etch rate of the nitride was observed. The selectivity w ith resist masks was found to be higher than with either aluminum or n itride masks. Removal of N atoms is suggested to be one of the major r ate-limiting factors in the nitride etching. It is argued that the add ition of N-2 in CHF3 dilutes the polymer-forming radicals and generate s abundant N atoms, which diffuse to the substrate and combine with th e N atoms adsorbed there due to etching. Both of these effects enhance nitride etch rate. (C) 1995 American Vacuum Society.