Metal alloys with piezo-resistive property have been used for strain gauge applications. To fabricate the strain gauge, the metal foils of a few micrometers in thickness were usually employed. Commercially readily available strain gauges are fabricated by pattering the metal alloy foils, which are laminated on to the insulating polymer films. In manufacturing of foil strain gauges, however the yield decrease and the mass production is impossible because of the complexity in processing by manual work and the automation problem. Accordingly the developments of strain gauges using thin films have been required for process automation and cost-down.
In recent years, many metallic thin films have been studied for the fabrication of the thin film strain gauges. For reliable and stable thin film resistors, the metal film must have a very low temperature coefficient of resistance (TCR) and a large and stable sheet resistance. NiCrFe alloys have the several advantages that are a good linearity in a wide strain range, easy temperature compensation and relatively low TCR.
In this study, a DC magnetron sputtering method has been adopted to fabricate NiCrFe thin film strain gauges for electronic load cell applications. Thin films of NiCrFe were deposited by varying sputtering pressure and film thickness. Electrical properties and structural changes of the films were analyzed by various methods. The electrical resistivity and TCR were characterized as a function of the deposition conditions. As Ar pressure increased, the electrical resistivity increased and TCR decreased. As the film thickness increased, the electrical resistivity decreased and TCR increased.
To analyze the contributions of thickness, roughness and grain size contribution on resistivity, Sambles equation(3-8) were employed. Resistivity variation was plotted a function of film thickness with roughness and grain size parameters measured in present experiments. The Sambles equation fitted well with the experimental results when bulk mean free path of 50 to 100 Å and grain boundary reflection coefficient of 0.03 to 0.07 were assumed. TCR equation was derived from Sambles equation considering film thickness, roughness, grain size and thermal strain and fitted with the experimental results.
