Comparison of SiO x and polyimide as a dielectric layer on stainless steel in thin-film pressure sensor manufacture

Abstract

The pressure-measurement technique of using a strain gauge bonded to the mechanical substrate has been and still is extensively employed, but is going to become inadequate for some new user requirements. The deposition of the strain gauge directly on the mechanical support of the pressure transducer by using the thin-film technique, instead of gluing on it a strain gauge laminated on polymeric foil, leads to a great improvement in sensor performance, especially when the temperature exceeds 120 °C. The major features of the application of thin- film technology to pressure sensor manufacture are: elimination of glue between the gauge and mechanical substrate; elimination of the polymeric foil; elimination of assembly problems as the strain gauge is directly deposited on the mechanical support; and an enhanced opportunity to test a great variety of new materials. In manufacturing thin-film pressure sensors for industrial applications, one of the most important items is the setting up of the insulating barrier between the stainless steel membrane and the Wheatstone bridge of the strain-gauge circuit. We have experimentally investigated two different dielectric layers: polyimide and SiO x . The former is spun on the stainless steel diaphragm and cured in an oven. The latter is obtained by plasma-enhanced chemical vapour deposition (PECVD) on the same substrate. This paper describes the processes used to get the insulating layers of polyimide and SiO x and compares the properties of the two layers. Data on their behaviour during fatigue tests, temperature cycle tests and long-term tests obtained on the complete thin-film pressure sensors are reported. The evaluation also includes measurements of linearity, repeatibility, hysteresis, creep and drift (zero and span) versus temperature, made on complete thin-film pressure sensor devices.