Released Si microstructures fabricated by deep etching and shallow diffusion

A novel etch-diffusion process is developed for fabricating high-aspect-ratio Si structures for microsensors. This is accomplished by first dry etching narrow gap Si microstructures using an electron cyclotron resonance (ECR) source, followed by a shallow B diffusion to fully convert the etched microstructures to p/sup ++/ layer. Microstructures up to 40 /spl mu/m deep with 2-/spl mu/m-wide gaps were etched with a Cl/sub 2/ plasma generated using the ECR source. Vertical profile and smooth morphology were obtained at low pressure. A shallow B diffusion at 1175/spl deg/C for 5.5 h. was then carried out to convert the 40-/spl mu/m-thick resonant elements to p/sup ++/ layer. A second dry etching step was used to remove the thin p/sup ++/ layer around the bottom of the resonant elements, followed by bonding to glass and selective wet etch. Released high-aspect-ratio Si microsensors with thicknesses of 35 /spl mu/m have been demonstrated. At atmospheric pressure, only 5 V/sub dc/ driving voltage is needed for 2.5 /spl mu/m vibration amplitude, which is less than the 10 V/sub dc/ required to drive 12-/spl mu/m-thick resonators fabricated by conventional dissolved wafer process.