The effect of nitrogen on the stability of the <i>β</i> phase in W thin films

Abstract

The metastable β phase (A15 structure) of tungsten can be made by atom-by-atom deposition in the presence of impurities and has unique properties of interest for use in microelectronic and other devices. In this work, we investigated the effect of nitrogen on the formation and stability of β-W thin films. We fabricated W thin films with nitrogen contents from 2.2 to 6.7 at. % using DC magnetron sputtering under different nitrogen flow rates and developed an x-ray diffraction rocking curve analysis that accounts for film texture to quantify the volume fractions of α and β phases. Films with nitrogen contents of 3.9 at. % or greater were found to be 100% β phase. Samples heated in vacuum transformed to α-W between 300 and 700 °C. Increasing the nitrogen content enhanced the thermal stability of β-W. We built a model for the β–α phase transformation based on a modified nonisothermal Johnson–Mehl–Avrami–Kolmogorov analysis and used it to show that the energy barrier to the β–α phase transition increases both during phase transition and with nitrogen content. We propose an explanation in which nitrogen impedes the phase transformation by solute drag. This model showed excellent agreement with the experimental data and provides fundamental insights into the formation and stability of β-W that may be useful in device fabrication and application.