Thin Polysilicon Gauge for Strain Measurement of Structural Elements

In this paper, we propose a thin polysilicon strain gauge for the measurement of strain in structural elements. Metal-foil strain gauges are commonly used for such measurements even though polysilicon strain gauges have better sensitivity. However, the proposed polysilicon strain gauge can be applied to structural elements because the strain gauge element is separated from the silicon wafer, due to the small size and low thickness of the proposed gauge. A microelectromechanical system fabrication process of the poly-silicon strain gauge and an inorganic bonding process with a metal substrate have been established. An inorganic bonder-glass frit-was used since the commonly used organic bonders exhibit plastic behavior. Subsequently, the performance of polysilicon strain gauges glass-frit-bonded onto metal cantilever beams was evaluated. The results demonstrate that the resistance increases linearly with tensile stress while it decreases with compressive stress. The gauge factor, which represents the sensitivity of the strain gauges, was 34.0. The resistance decreases linearly with temperature with a temperature coefficient of resistance (TCR) of -328 ppm/°C. The TCR is influenced by two factors-differences in the thermal expansion of the metal cantilever beam and silicon strain gauge and the thermionic emission of the carriers. In this study, the resistance change due to thermionic emission was more effective than that by thermal expansion. The nonlinearity and hysteresis values were 0.21% FS and 0.17 % FS, respectively; this is lower than those of conventional metal-foil strain gauges. Hence, our proposed strain gauge is useful for the measurement of the strain of structural elements.