Interactions of Adhesion Materials and Annealing Environment on Resistance and Stability of MEMS Platinum Heaters and Temperature Sensors

We evaluate the microstructural and electrical stability of Pt thin films with Ti or Ta as the adhesion layer after furnace annealing and rapid thermal annealing up to 750°C in three different environments. Test devices were made with 100 nm of Pt with a 10-nm adhesion layer. After annealing, the resistance anomalously increased for samples annealed in ultrahigh purity N₂ (UHP, 99.999%), while the resistance decreased, as expected, for samples annealed in 99.95% N₂ or air. The Ta/Pt film stack shows better microstructural and electrical stability compared with Ti/Pt. X-ray photoelectron spectroscopy (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 resistance. 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 resistance. Furthermore, XPS elemental mapping and atomic force microscope imaging shed light on void formation/dewetting seen under certain conditions.