Platinum-zirconium composite thin film electrodes for high-temperature micro-chemical sensor applications

Stable metal interconnect thin films are critical in the development of various micro-machined devices that may operate continuously at elevated temperatures. The main objective of this work was to investigate the microstructural and electrical stability of a functionally gradient platinum (Pt)-zirconium (Zr) composite thin film electrode designed for resistive-type chemical sensors. Thin film electrodes were fabricated using a DC magnetron sputtering process. Zirconium was used as both the conventional adhesion promoter and the Pt grain modifier within the bulk electrode microstructure. The thin film deposition was completed on highly polished alumina substrates at 200 °C. The various composite Pt thin films were further annealed at 1200 °C after deposition for 1-24 h for rapid evaluation of the microstructure stability. This temperature was chosen since the electrodes are expected to operate beyond 1000 °C for high-temperature MEMS applications. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were conducted to characterize the alterations in chemistry, microstructure and distribution of the constituent elements through the film thickness. The electrical resistivity of the as-deposited and thermally processed Pt thin films was measured by utilizing a van der Pauw's four-point probe technique. The work identified a Zr/Zr + Pt/Pt composite thin film with the 525 nm total film thickness that demonstrated resistivity <5.08 × 10−7 Ω m after being processed to 1200 °C for 15 h. A lift-off technique was finally used to produce a micro-electrode patterns with the optimized film structure for high-temperature applications.