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, NF_x, and HF (Hydrogen Fluoride) while achieving large flux of SF_x⁺ 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 \sim 5 Å at an etch rate of \sim 1 \mu m/min. In situ residual gas analysis of the plasma conditions show high concentrations of F, HF, and SF_x, along with a large concentration of NF_x 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.