Stress and surface morphology of TiNiCu thin films: effect of annealing temperature

TiNi-based films sputtered at room temperature are amorphous; thus, postsputtering annealing is a must because shape memory effect only occurs in their crystalline form. It is suggested that the lowest possible annealing temperature be used in a bid to conserve thermal processing budgets, and to minimize thermal stresses and possible interfacial reactions between film and its substrate. In this paper, Ti49.5Ni47.5Cu3 (at.%) films with a thickness of 3.5 μm were deposited on Si substrate by cosputtering of TiNi and Cu targets at room temperature, then annealed at different temperatures from 430 to 650 °C. Phase transformation behaviors, crystalline structure, residual stress and stress evolution of the films were systematically studied. At the gas pressure of 0.8 mTorr, the residual stress in the as-deposited films was 260 MPa, compressive. A minimum annealing temperature (450 °C) was necessary for film crystallization; thus, large thermal stress could be released significantly due to martensitic transformation. With increase of annealing temperature, crystallite and martensite plate sizes in the film increased; thus, both recovery stress and stress-increase rate increased, while the transformation temperatures shifted to higher values. The surface roughness increased drastically with increase of annealing temperature in correlation to martensitic transformation.