Thermal Stability of RuO2 Thin Films and Effects of Annealing Ambient on Their Reduction Process

RuO2 films prepared by reactive sputtering were annealed in air and vacuum and the changes of their crystal structure, chemical binding state and resistivity were studied. In air, the RuO2 films maintain a rutile structure below 800°C. Crystal grain growth was found above 600°C and the minimum resistivity of 46 µΩcm was obtained at 800°C. The vacuum annealing was conducted with two types of annealing systems, one using an oil diffusion pump and the other using a turbomolecular pump as the main pump. The RuO2 films annealed in the system using the turbomolecular pump were not reduced below 500°C, however, the surface of the films was reduced as low as 200°C in the system using the oil diffusion pump. The difference in the reduction processes was examined on the basis of the thermodynamics of RuO2 and the influence of reducing residual gases in vacuum.