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 NF
₃
and H
₂
O gases right above the wafer, whereas SF
₆
is introduced through the ICP source. This configuration of plasma etching allows for incomplete breakdown of NF
₃
+ H
₂
O 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 SF
₆
/NF
₃
/H
₂
O flow rates of 60/100/50 sccm, we were able to achieve a surface roughness of ~5 Å at an etch rate of ~1 μm/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.