Piezoresistance Gauge Factors in Heavily Boron-Doped Polysilicon from Infrared Piezoreflectance

Abstract

AbstractHeavily boron-doped (4×1019 < [B] >1021 cm-3) polycrystalline silicon thin films on oxidized silicon cantilevers were studied by FTIR spectroscopy. The experimental reflectance spectra (400–7000 cm-l) were compared to those expected for a poly-Si/SiO2/c-Si/SiO2 multilayer where the doped layer is described by a simple Drude model. The resulting free carrier concentration and conductivity were checked against electrical measurements.The optical response of the polycrystalline layer in the medium infrared range (400–3000 cmn−1) was shown to be affected by a static uniaxial stress applied along the film plane by a four point bending mechanical device. The resulting microscopic piezoresistivity longitudinal gauge factors in the 15 - 25 range are in close agreement with those deduced from conventional electrical piezoresistivity measurements on sub-millimeter size patterns. The effective range of applicability of this contactless method and the potential application of such layers to temperature-independent miniature pressure sensors are discussed