Optimum reactive ion etching of x-cut quartz using SF6 and Ar

In this paper we investigate the effect of Ar concentration and shadow mask material on the etch rate, surface roughness, and micromask pit density and depth for reactive ion etching (RIE) of x-cut alpha quartz. The ratio of SF6 to Ar is varied at constant power and pressure to find an optimum Ar concentration for thinning x-cut quartz while maintaining good surface quality. The recipe is used with an STS320 RIE system to produce smooth, through-hole-free, insulating quartz membranes of 7 µm thickness in selective locations on 100 µm thick x-cut quartz wafers. We demonstrate that surface quality is much improved by replacing at least 75% of the SF6 by volume with Ar, and that for non-inductively coupled plasma RIE the standard mask material nickel is detrimental to surface quality for long etch times.