Abstract

This talk is about the design of sensors for embedded systems. Porous silicon, prepared by controlled anodic etching of monocrystalline silicon in acidic bath, has a very large surface to volume ratio desirable for fabricating a high sensitivity vapor or chemical sensor. Its pore structure can be varied widely from nanostructure to microstructure to mesostructure by controlling the formation current density, concentration of acidic bath and doping density of the base wafer. It is compatible to silicon IC processing like MEMS and can be integrated on the same chip with its signal-processing unit. The active diaphragm of a MEMS cantilever or inertial mass can be converted to porous silicon for improved sensitivity and better performance. When used as a vapor or humidity sensor, the dielectric constant of porous silicon changes significantly through adsorption, diffusion and condensation of vapor molecules in its pores leading to a significant change of the phase difference, which can be detected by a conventional, phase detection signal processing unit. Porous silicon can also be used as a pressure sensor because of its piezoresistive nature. Further, it can be represented as a distributed R-C network with R as piezoresistive. This leads to the possibility of phase detection electronics even with piezoresistive change leading to better noise immunity and reduced temperature sensitivity. In order to develop the ASIC design for such a phase detecting signal-conditioning unit and to perform CAD simulations and verify our design we have developed a model for the porous silicon transducer. In this talk we shall discuss a model for the porous silicon transducer that can be incorporated as a SPICE model file while simulation. The SPICE model file incorporates as model parameters different process and design parameters.