Effect of SF6 flow rate on the etched surface profile and bottom grass formation in deep reactive ion etching process

Abstract

While deep reactive ion etching (DRIE) has proven to be a boon for silicon micromachining applications, certain factors still exist which affect the satisfactory performance of DRIE. Some of the process limitations include bottom grass formation, RIE lag, loading and notching effects and aspect ratio dependent etching. This paper presents the effect of SF6 flow rate and etching/passivation cycle time on the etched shape profile and bottom grass formation. Rectangular trenches of varying widths are etched using Alcatel etching system. Critical DRIE process parameters, such as SF6 flow rate and ratio of etching and passivation cycle time, are varied to explore the dependence of etched shape profile on these parameters. It is found that low SF6 flow rate, i.e. 250 sccm, results in relatively smooth bottom surface. As SF6 flow rate is increased, bottom surface roughness increases and grass forms on the bottom of etched trenches. Shape of etched surface profile was found to be changed from positive profile to negative profile, when the SF6 flow rate was increased. Ratio of etching/passivation cycle was also found to be critical for prevention of bottom grass formation. DRIE process parameters were optimised to get smooth and vertical sidewalls.