The effect of nitrogen on the thermomechanical behavior of β-W thin films

Abstract

The effect of nitrogen on the stress-temperature behavior of β‑tungsten thin films on silicon substrates was investigated. β-W thin films containing 2.2 to 6.8 at.% N were deposited using DC magnetron sputtering. Film stress was characterized in-situ during temperature cycles in vacuum using substrate curvature measurements. Two large irreversible tensile stress changes were seen in all films. Analysis of stress generation mechanisms revealed their origins. One, of nearly constant magnitude ~2.6 GPa, occurred at temperatures that increased with N content from 223 to 564 °C and was attributed to the density change of the β-α phase transformation indicating that higher nitrogen content enhances the thermal stability of β-W. The other occurred over a constant temperature range of 750–900 °C, increased in magnitude from 351 to 863 MPa with N content, and was attributed to nitrogen outgassing through grain boundary diffusion. Although grain growth was significant, contributions to stress were relatively minor. Knowledge of stress generation mechanisms facilitated construction of β-α phase transformation curves from the stress-temperature data. Analysis of these curves using a non-isothermal Johnson-Mehl-Avrami-Kolmagorov model indicates that the activation energy barrier for the phase transformation increases with the α phase volume fraction as well as with the initial nitrogen concentration, consistent with solute drag due to N at α/β phase boundaries. These thermomechanically-derived results are remarkably similar to prior results obtained using x-ray diffraction methods. These methods are compared, and implications for devices based on β-W are discussed.