Piezoresistive sensors on plastic substrates using doped microcrystalline silicon

The piezoresistive behavior of n-type and p-type microcrystalline silicon films deposited on polyethylene terephthalate plastic substrates by hot-wire, and radio-frequency, plasma-enhanced chemical vapor deposition, at a substrate temperature of 100°C, is studied. The crystallite size was 10 nm for hot-wire films and 6.5 nm for radio-frequency films and the crystalline fraction varied between 50 to 80%. A four-point bending jig allowed the application of positive and negative strains in the films. Repeated measurements of the relative changes in the resistance of the samples during the strained condition showed reversible behavior, with p-type microcrystal line films having positive gauge factor in the range from 25 to 30 and n-type μc-Si:H films having negative values of gauge factor from -40 to -10. The induced strain in the films varied in the interval between 0 and ±0.3%. The films were used in the as-deposited size (50 mm × 10 mm) as sensors, utilizing their piezoresistive properties to map the contour of an acrylic model with the shape of an Archimedes´ spiral. Micron-sized devices were patterned and used to map the shape of the same model.