Fabrication and characterization of piezoresistive strain sensors for high temperature applications

In this work, we studied the fabrication and characterization of strain sensors based on semiconductor materials for high temperature applications: non-stoichometric amorphous silicon carbide (a-Si_xC_y) thin film and SOI (Silicon-On-Insulator) substrates. a-Si_xC_y were deposited onto thermally oxidized (100) Si wafers by plasma enhanced chemical vapor deposition (PECVD) technique using silane (SiH₄) and methane (CH₄) as precursor gases. The SOI wafer used had a sandwich structure of a 0.2 μm thick top p-type Si layer, 0.5 μm thick buried oxide (BOX) layer and 250 μm Si substrate. The piezoresistive strain sensitivity element in the sensors is a a-Si_xC_y thin-film resistor or a Si p-type resistor formed on SOI substrate. Gauge factor (GF) measurements were done using the beam-bending method. One resistor of each type was bonded near the clamped edge of a stainless steel cantilever beam and on the free edge were applied different forces. The electrical resistance of each resistor was measured without applied load on the beam and during subsequent tensile load. The temperature coefficient of resistance (TCR) also was investigated from room temperature up to 250°C. The results indicate that the a-Si_xC_y thin-film resistor has a GF of 48 and a TCR of 35 ppm/°C whereas p-type Si on SOI substrate has gauge factor of 22 and TCR of 140 ppm/°C.