Nanocrystalline piezoresistive polysilicon film obtained by aluminum induced crystallization for pressure sensing applications

The work presented here is a low temperature method for obtaining polysilicon piezoresistive thin films using aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) film. We have obtained nanocrystalline polysilicon films with average grain sizes of 45-55 nm at temperatures ranging from 400°C to 500°C with annealing time of 60 minutes, and an average grain size of 50 nm at 500°C for a shorter annealing time of 30 minutes. An additional advantage of this process is that the polysilicon films are simultaneously doped p-type, thereby eliminating any additional doping step. By varying the aluminum (Al) and a-Si layer thicknesses, annealing temperature and duration, the growth of polysilicon grains ranging from few tens of nanometers to tens of microns in diameter can be effectively obtained. Exploring the piezoresistive properties of the above mentioned low temperature nanocrystalline polysilicon thin films deposited on plastic substrates for pressure sensing applications was an additional focus of this research. To test the piezoresistive properties of the AIC obtained polysilicon films, prototype MEMS based pressure sensors were fabricated on flexible polyimide substrate. The sensors comprised of a surface-micromachined silicon nitride diaphragms with polysilicon resistors as pressure sensing elements connected in a half-Wheatstone bridge configuration. The polysilicon resistors showed linear IV characteristics with typical resistance values between 15 - 30 kΩ. Atomic Force Microscope was used in contact mode to study the response of the pressure sensor with applied pressure in the 2 kPa to 19 kPa range. For the higher range of 450 kPa to 2 MPa, a load-cell with a nanopositioner was utilized. The pressure sensor sensitivity was measured to be 41.12 mV/MPa and 5.02 mV/MPa, respectively, for these ranges, when the Wheatstone bridge was biased at 1 V.