Microfabrication and reliability study of sapphire based Ti/Pt electrodes for thin-film gas sensor applications

The paper describes design, fabrication and characterisation of a Ti/Pt electrode system (an interdigital electrode with an aligned heater) on a sapphire substrate. It serves as substrates for subsequent deposition of gas sensitive films. The fabrication has been accomplished on both sides of a sapphire wafer. Wet chemical etching and the lift-off method have respectively been used for structuring the electrode and the heater patterns. The mechanical stability and the electrical conductivity of the resultant Ti/Pt films are greatly affected by the subsequent heat treatment procedures. The diffusion of titanium ions into platinum has been examined using SIMS depth profiles. After annealing at 600 degrees C for 5 hours, the temperature coefficient of the Ti/Pt film stabilised on the value 0.0033 K-1, which approaches the pure platinum bulk value of 0.0039 K-1. With a good mechanical and thermodynamical stability at high temperatures and under oxidising and reducing conditions, the sapphire based Ti/Pt electrode system is well suited for gas sensor fabrication. As an example of its application, we report a highly sensitive O-3 gas sensor developed by evaporating a WO3 thin-film onto this electrode.