Interactions of Adhesion Materials and Annealing Environment on Resistance and Stability of MEMS Platinum Heaters and Temperature Sensors
Feng Yi, William A. Osborn, Jordan F. Betz, David A. LaVan
We evaluate the microstructural and electrical stability of Pt thin films with Ti or Ta as the adhesion layer after annealing in three different environments at 750 °C. Test devices were made with Pt and adhesion layers of 100 nm and 10 nm, respectively. After annealing, the resistivity anomalously increased for samples annealed in Ultra High Purity N2 (UHP, 99.999 %) while the resistivity decreased, as expected, for samples annealed in 99.95 % N2 or air. The Ta/Pt film stack shows better microstructural and electrical stability compared to Ti/Pt. XPS data indicate that diffusion of the Ti and Ta adhesion layers through the Pt film occurs in samples annealed in UHP N2, which is responsible for the remarkable increase of resistivity. For samples annealed in air, the oxidation of Ti/Ta suppresses the diffusion process and expected grain growth occurs in the Pt, thus decreasing the resistivity. Furthermore, X-ray photoelectron spectroscopy (XPS) elemental mapping and atomic force microscope (AFM) imaging sheds light on void formation / dewetting seen under certain conditions.