Stress and Stability of Sputter Deposited A-15 and BCC Crystal Structure Tungsten Thin Films

Abstract

Magnetron sputter deposition was used to fabricate body centered cubic (bec) and A-15 crystal structure W thin films. Previous work demonstrated that the as-deposited crystal structure of the films was dependent on the deposition parameters and that the formation of a metastable A-15 structure was favored over the thermodynamically stable bec phase when the films contained a few atomic percent oxygen. However, the A-15 phase was shown to irreversibly transform into the bec phase between 500°C and 650°C and that a significant decrease in the resistivity of the metallic films was measured after the transformation. The current investigation of 150 nm thick, sputter deposited A-15 and bec tungsten thin films on silicon wafers consisted of a series of experiments in which the stress, resistivity and crystal structure of the films was measured as a function of temperature cycles in a Flexus 2900 thin film stress measurement system. The as-deposited film stress was found to be a function of the sputtering pressure and presputter time; under conditions in which the as-deposited stress of the film was ∼ 1.5 GPa compressive delamination of the W film from the substrate was observed. Data from the thermal studies indicated that bec film stress was not affected by annealing but transformation of the A-15 structure resulted in a large tensile increase in the stress of the film, regardless of the as-deposited stress of the film. In several instances, complete transformation of the A-15 structure into the bec phase resulted in ≥ 1 GPa tensile increase in film stress.