Electro-Chemical Corrosion of Polysilicon in MEMS

Surface-micromachined microelectromechanical systems (MEMS) are produced by the sequential deposition and patterning of materials such as polysilicon, silicon oxide and silicon nitride on a silicon wafer and then etching away one of these materials. In MUMPs(TM) (Multi-User MEMs Process)[1], phsophorus-doped polysilicon is used as the structural layers and phosphorus-doped silicon oxide (or phosphorus doped silica glass, PSG) is used as the sacrificial layer. PSG is selectively etched with hydrofluoric acid (HF) and the mechanism and kinetics of this process has been thoroughly studied. Chemical etching of sacrificial doped SiO2 layers is an almost universal post-processing step for polysilicon surface-micromachined MEMS.