Stress stability and thermo-mechanical properties of reactively sputtered alumina films

The stability of residual stress inherent on deposition in reactively sputtered alumina films is studied during thermal cycling and annealing, simulating temperature excursions experienced by the films during device fabrication and subsequent operation. Increasing the magnitude of substrate bias applied during deposition acts to reduce the amount of argon incorporated in the films; more incorporated argon corresponds to smaller values of modulus and hardness and a larger coefficient of thermal expansion (CTE). Large, irreversible changes in film stress develop on heating, acting to decrease the compressive residual stress of films deposited on silicon substrates to a smaller, equilibrium value, whereas films deposited on Al₂O₃-TiC substrates behave differently. Thermal cycling and annealing have little effect on the modulus and CTE, but the hardness increases significantly and the threshold load for indentation crack initiation decreases precipitously during heat treatment. Possible mechanisms of irreversible stress development and mechanical property modifications are discussed.