The manufacturing of the sensory thin-film system
In most cases they are manufactured out of hardened steel and in shape they are reminiscent of washer geometries, which can easily be used to monitor threaded connections. For this, initially with the aid of a PACVD process, a DiaForce® film that was specially developed at the Fraunhofer ST, is deposited. Then individual circular electrode structures are manufactured out of chromium, and these structures form the load measuring sensor surfaces (see adjacent figure). Then on a subsequent, electrically-isolating SiCON® intermediate layer, a hydrocarbon coating modified with silicon and oxygen, which likewise is deposited in the PACVD process, conductive traces are structured to contacting points, as well as temperature-measuring meander structures out of chromium. These structures are protected against wear with a second and final SiCON® layer (see adjacent figure of the coating system).
In test stands of the Fraunhofer IST the temperature-dependent and load-dependent characteristic curves of each individual sensor structure are measured. In the case of piezo-resistive sensors these are characteristic linear resistance dependencies on the load. In this process a full-bridge or half-bridge circuit is established and a constant voltage of 5 V is applied. The thermo-resistive meander structures likewise show linear resistance dependencies. They are structured in a so-called four-wire arrangement; via the outer conductors a constant current of 10 mA, for example, is applied and the voltage change is measured via the inner conductors. Due to the fact that the piezo-resistive DiaForce® sensor film, as an amorphous carbohydrate film, is a semiconductor it has an exponential resistance dependency on temperature. This effect can be compensated through the additional integration of temperature-compensating structures in the contacting area. Sample characteristic curves of a force sensor structure and of a temperature meander are presented in the adjacent diagram.