High-Speed Ultrasmooth Etching of Fused Silica Substrates in SF6, NF3, and H2O-Based Inductively Coupled Plasma Process

This paper presents a new paradigm for high aspect ratio etching of fused silica substrates using a modified inductively coupled plasma (ICP) etch chamber. In particular, we have incorporated a stainless steel gas diffuser ring on the mechanical substrate clamping plate of the etcher to introduce NF3 and H2O gases right above the wafer, whereas SF6 is introduced through the ICP source. This configuration of plasma etching allows for incomplete breakdown of NF3 + H2O gas mixture, thereby creating a high local density of F, NFx, and HF (Hydrogen Fluoride) while achieving large flux of SFx+ ion bombardment. Using this configuration, source power of 2500 W, substrate power of 400 W, and SF6/NF3/H2O flow rates of 60/100/50 sccm, we were able to achieve a surface roughness of similar to 5 angstrom at an etch rate of similar to 1 mu m/min. In situ residual gas analysis of the plasma conditions show high concentrations of F, HF, and SFx, along with a large concentration of NFx species. The highest etch rate was also found to be a function of the ion flux. The anisotropy of the etch was enhanced by the formation of an inert nickel fluoride/oxide skin layer on the sidewalls of the etched features.