Fabrication of Ultra-thin Si-Based Strain Gauges for MEMS Pressure Sensor

Ultra-thin (20 ㎛) silicon strain gauges were fabricated with a silicon-on-insulator (SOI) wafer by a wet etching process. A buffered oxide etchant (BOE, NH₄F: HF =6:1) solution was used for the wet etching process in which the operating temperature was 50℃. Photoresist was deposited on the upper side of the SOI wafer as a passivation layer to minimize strain gauge damage by the chemical etchants. Small amounts of octylamine and 1-octanol were added to BOE solution to improve surface wettability and SiO₂/Si selectivity. The fabricated strain gauges were attached to the pressure diaphragm and the performance of the strain gauges was investigated by measuring with the hydraulic pressure system. The resistance changed linearly with tensile and compressive strains. Maximum values of non-linearity, hysteresis, thermal coefficient of resistance (TCR) and sensitivity were -0.341%, 0.909%, 4,128 ppm/℃ and 34.22 mV/V, respectively. The fabricated strain gauges might be well applicable to hydrogen pressure sensors detectable on a high pressure range.