Porous silicon/polysilicon for improved sensitivity pressure sensors

This paper describes the technique of improving the sensitivity of bulk micromachined silicon pressure sensors with judicious use of porous silicon (PS) and porous polysilicon (PPS). Since PS has a much lower Young's modulus than Si, pressure sensors with Si/PS composite membranes show higher sensitivity. The sensitivity also increases as the porosity of the PS layer increases. Further improvement in sensitivity can be achieved by replacing the conventional polysilicon piezoresistors with porous polysilicon ones. Though the composite membranes exhibit higher sensitivity, they suffer from higher offset voltage due to the stress developed by the formation of porous silicon. Pressure sensors with silicon membrane and porous polysilicon piezoresistors yield modest improvement in sensitivity without increase in offset voltage. A combination of porous polysilicon piezoresistors and composite membranes with microporous silicon gave the highest sensitivity value as well as the highest offset. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim