The effect of substrate temperature and reactant gas pressure on the chemical vapor deposition of tungsten films

Abstract

The growth kinetics, crystal morphology and mechanical behavior of chemically vapor deposited tungsten films are strongly affected by substrate temperature. Lower substrate temperatures results in vastly stronger and harder materials. For example, a temperature of 500°C resulted in material with a higher yield strength than did a temperature of 700 °C. The difference amounted to 3000 psi∗∗psi 7.04 × 10−4 = kg/mm2. at four test temperatures between 200 °C and 800 °C. Deposition pressure had little effect on film strength or appearance. Pressures of 300 and 500 torr were evaluated for deposits made at 500 °C. The main structural difference observed between films produced at the two substrate temperatures was that at 700 °C the columnar grains grew straight through the film but at 500 °C there was a “fanning out” or a distribution in growth directions through the film thickness. Scanning electron microscopy reveals that the individual tungsten crystals advance with four sided pyramids at the interface, similar in appearance to those found in the electrodeposition of copper single crystals. We find that these pyramids become steeper and more irregular with increasing substrate temperature. The pyramids have {111} surfaces, and propagate by the advance of ledges up the {111} facets. An activation energy of 16 kcal/mole was determined for film growth, indicative of control by surface diffusion, adsorption and desorption processes.