Studies On Fabrication Of WTi Thin Film Strain Gauge

Resistive thin film strain gauge based on strain resistance effect or piezoresistive effect is used to measure strain on objects.It is widely used in construction,medical and electronics fields.In this thesis,author chose tungsten-titanium alloy as the research object.Tungsten-titanium thin film was prepared on the amorphous substrate by DC magnetron sputtering.The film thickness was controlled by deposition time to improve commercial products'poor gauge factor.In order to explore the growth mechanism of tungsten-titanium film,author used glass substrate to study the relationship between deposition time and microstructures.The scanning electron microscopy and X-ray diffraction results showed that the tungsten-titanium thin film was amorphous-like structure at 2.5 min~5 min and gradually changed from amorphous-like to crystal structure from 5 min to 12.5 min.The(001)direction of the body-centered cubic crystal structure preferentially grew along the normal direction of the substrate to form a columnar grain structure.In order to further investigate the influence of microstructures on the electrical properties of the film,the relationship between deposition time and resistivity and temperature coefficient of resistance was studied.The results showed that as the deposition time increased,the resistivity of the film decreased while the temperature coefficient of resistance changed from negative to positive.In order to reveal the influence mechanism,an electron scattering model was introduced to theoretically analyze the experimental results.The results showed that the large increase in resistivity was determined by amorphous scattering in the amorphous-like structure and the grain boundary scattering in the crystal structure.When the film thickness is less than 100 nm,amorphous scattering plays a dominant role,and the temperature coefficient of resistance is negative.When the film thickness is greater than 205 nm,grain boundary scattering plays a dominant role,and the temperature coefficient of resistance is positive.To ensure that the tungsten-titanium film can work in-55 ~oC~125 ~oC,tungsten-titanium thin films were annealing at 150°C in air.The results showed that the heat treatment caused the temperature coefficient of resistance increased after heat treatment.The difference of tungsten-titanium thin film on these two substrates was studied.The X-ray diffraction results show that the tungsten-titanium films have similar microstructure on these two substrates,but at the same deposition time,the amorphous phase content on polyimide substrate was greater and the deposition rate of the film on the polyimide substrate was faster.The results of resistivity and temperature coefficient of resistance showed that the tungsten-titanium film has similar electrical properties on the two substrates.The film on the polyimide film substrate has a larger resistivity and a more negative temperature coefficient of resistance.Based on the above experimental results,the strain gauge was designed and fabricated on polyimide substrate.The obtained strain gauge had good resistance uniformity.The adhesion of the tungsten-titanium film was studied by the cross-cut method and the results showed that the adhesion grade was greater than level 2.In order to optimize the performance of the strain gauge,the relationship between deposition time,150 ~oC heat treatment and gauge factor was studied by cantilever method.When the deposition time was 7.5 min,strain gauge had the best performance after annealing.The gauge factor was 4.43 while the ratio of resistance temperature coefficient to gauge factor(absolute value)was 9 ppm/~oC.The linearity and hysteresis error were 0.68%and 1.55%,respectively.The overall performance is better than the current commercial nickel-chromium thin film strain gauge.