Fabrication of thick Si resonators with a frontside-release etch-diffusion process

Abstract

A frontside-release etch-diffusion process has been developed to create released single-crystalline Si microstructures without the need for wafer bonding. This frontside-release process is simple and requires only a single mask. A deep dry etch in an electron cyclotron resonance source is used to define the structures, followed by a short boron diffusion to convert them to p/sup ++/ Si. A short etch in ethylenediamine pyrocatechol (EDP) is then used to undercut and release the structures from the frontside of the Si wafer. The structures are isolated from the substrate using a reverse-biased p/sup ++//n junction. Since the structures have a high aspect ratio, beams longer than 1 mm can be released without sticking to the substrate, and thick resonators are flat with no bending due to stresses. Resonant microstructures with thicknesses ranging from 10 to 55 /spl mu/m thick have been fabricated using this process and their resonant frequency has been measured. For typical clamped-clamped beam resonators that were 24 /spl mu/m thick, 5 /spl mu/m wide, and 400 /spl mu/m long, with 2-/spl mu/m comb gaps, a resonant frequency of 90.6 kHz and a quality factor of 362 were measured in air.