Selective reactive ion etching of silicon nitride over silicon using CHF3 with N2 addition

In this article the effects of process parameters of CHF3+N2 plasma etching chemistry (rf power 50–70 W, pressure 22.5–52.5 mTorr, and N2 content 0%–95%) and mask materials (photoresist, aluminum, and silicon nitride) on the etching selectivity of silicon nitride over polysilicon are investigated. It was found that the selectivity increased with the N2 content in the range of 0%–85% and then decreased, leading to the maximum selectivity (16) at 7.5 sccm CHF3+42.5 sccm N2 (85% N2) 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 with resist masks was found to be higher than with either aluminum or nitride masks. Removal of N atoms is suggested to be one of the major rate-limiting factors in the nitride etching. It is argued that the addition of N2 in CHF3 dilutes the polymer-forming radicals and generates abundant N atoms, which diffuse to the substrate and combine with the N atoms adsorbed there due to etching. Both of these effects enhance nitride etch rate.