Design and fabrication of a small-scale thin film strain gauge

The miniaturization of force or pressure sensors requires miniaturization of thin film strain gauge by the application of microfabrication techniques. A miniature strain gauge was designed, fabricated by microlithographic lift-off technique and tested.; Nichrome (Ni-Cr 80/20 wt%) was chosen as the material for the strain gauge. The electromechanical properties of nichrome films were evaluated. Based on these properties, miniature strain gauges were designed. The design value of the width of the resistance path, i.e., gridline of the strain gauge is 20 mum and the overall size of the grid is 100 mum x 140 mum. The ratio of longitudinal resistance to total resistance of the gauge is 0.727. The gauges were designed for two different thicknesses, i.e. 25 and 50 nm.; The strain gauges were first fabricated on glass substrates and tested. After initial success on glass substrates, the gauges were deposited on steel substrates coated with .5 mum thick alumina films and tested. The gauges were found to work well.; The widths of the resistance paths were generally larger than the design value of 20 mum, typically 22--28 mum, because of limitations of the fabrication technique. This caused a reduction of the resistance of the deposited gauges to 70--82% of the design values. The design of the measurement circuit reduced the ratio of longitudinal to total resistance and thus reduced the gauge factor of the deposited gauges to 86--87% of the design values. After adjusting for the design values of the width and the ratio of longitudinal to total resistance, the results agreed closely with the respective design values.