The relationship between deposition conditions, the beta to alpha phase transformation, and stress relaxation in tantalum thin films

Abstract

We demonstrate that the high temperature polymorphic tantalum phase transition from the tetragonal beta phase to the cubic alpha phase causes a large decrease in the resistance of thin films and a complete stress relaxation in films that were intrinsically compressively stressed. 100 nm beta tantalum thin films with intrinsic stresses of 2.0×1010 dynes/cm 2 (tensile) to -2.3×1010 dynes/cm2 (compressive) were deposited onto thermally oxidized (100) silicon wafers by evaporation or dc magnetron sputtering with argon. In situ stress and resistance at temperature were measured at 10°C/min up to 850°C in purified helium. Upon heating, the main stress mechanisms were elastic deformation at low temperature, plastic deformation at moderate temperatures and stress relief because of the beta-to-alpha phase transition at high temperatures. The temperature ranges over which the elastic and plastic deformation and the beta-to-alpha phase transition occurred varied with deposition pressure and substrate biasing. Incomplete compressive stress relaxation at high temperatures was observed if the film was initially deposited in the alpha phase or if the beta phase did not completely transform into alpha by 800°C due to substrate biasing during the deposition. We conclude that the main stress relief mechanism for tantalum films with intrinsic compressive stresses to completely relax their stress is the beta-to-alpha phase transition, while for intrinsically tensile films, this transformation has a much smaller effect on the stress.