An investigation into the fundamental phase and orientation properties of sputtered tungsten and tantalum thin films

Tungsten (W) is widely used in a variety of applications. The refractory properties of W such as a high melting point, low vapor pressure, and good mechanical strength at high temperature make tungsten one of the most suitable and widely used refractory metals. Sputtering is commonly selected to deposit thin tungsten films. In this study, an interesting phenomenon was observed with a chromium (Cr) underlayer. It was found that the tungsten films deposited with chromium as an interlayer between the tungsten layer and the substrate show an α phase body center cubic (b.c.c.) structure. The quality of the tungsten films can be significantly improved with this technique due to the formation of the α phase. In contrast, tungsten films deposited directly on a silicon dioxide layer show a β phase (an A-15) structure. This Cr-assisted phenomenon is first observed and reported in this dissertation. This dissertation focused on understanding this interesting and potentially useful phenomenon. An engineering and applied approach was taken to investigate and address this interesting phenomenon. A detailed study of W on some commonly used thin metal films was conducted. In addition, a similar phase dependence on the interlayer metal was also observed in tantalum (Ta) films. Some hypotheses were suggested to explain the cause of such phase transformation. A systematic study of other metals, deposited as an interlayer prior to the tungsten deposition in the sputtering chamber, was investigated to understand the relationship between the crystal structure of the sputter-deposited tungsten and the interlayer metal deposited underneath it. The results were used to evaluate the hypotheses. A lattice match and a simulation based on the lattice match hypothesis were presented which explains the experimental results. It is believed that whether the metastable phase or the stable phase is dominant in W or Ta films is a result of which crystal structure requires less energy to deform and fit onto the lateral atomic arrangement of the interlayer metal.