Deep Trenches in Silicon Structure using DRIE Method with Aluminum as an Etching Mask

In this paper, we describe our experience with deep reactive ion etching (DRIE) of silicon to depths ranging from 10 mum to more than 250 mum for MEMS applications, including MEMS microphone. The DRIE was produced in oxygen-added sulfur hexafluoride (SF₆) plasma, with sample cooling to cryogenic temperature. We used an inductively coupled plasma reactive ion etcher and Al mask. A 90 min etching experiments using etching gas SF₆ of 60 standard cubic centimeters per minutes (seem) with oxygen (13 seem) were performed by supplying RF power of 5 W to an ICP of 600 watts. For the Al mask, an etch rate of 5.44times10^-3 nm/min was achieved. By controlling the major parameters for plasma etch, anisotropic etch profiles and smooth etched surfaces, an etch rate of 2.85 microns per minute and a high selectivity of 5.24times10⁵ to the Al etch mask have been obtained. An etch depth of 257 mum was demonstrated. Our experiments show that Al is a best mask material for very deep trenches etch.