Piezoresistive Properties of Boron-Doped PECVD Micro- and Polycrystalline Silicon films

The electrical and piezoresistive properties of in-situ doped PECVD silicon films deposited on oxided silicon wafers have been investigated. One series of films was deposited in the so-called microcrystalline state at 450°C. The other set of samples was deposited in the amorphous state at 320°C and subjected to rapid thermal annealing. Structural properties (grain size, texture, residual stress) were evaluated experimentally through TEM and grazing angle X ray diffraction and related to the measured gauge factor. A maximum longitudinal gauge factor of 28 is measured in the case of advantageously textured microcrystalline material, the magnitude of the gauge factor decreasing sharply for randomly oriented material. For the amorphous deposited and subsequently annealed material, the longitudinal gauge factor is in the range 22–27 depending on dopant concentration. These experimental features are compared to the results of a theoretical approach of piezoresistance in polysilicon. We derive various expressions of the gauge factor according to the assumptions of either constant stress or constant strain within the aggregate. In the case of untextured films, analytical Voigt-Reuss-Hill averages for the elements of piezoresistive and elastoresistive tensors lead to greatly simplified expressions. Theoretical estimates are shown to be in reasonable agreement with the experimental measurements. This confirms the great potential of PECVD microcrystalline and polycrystalline silicon for strain gauges.