Oxygen impurity effects on the formation of thin titanium silicide films by rapid thermal annealing

Abstract

The behaviour of oxygen impurities during rapid thermal processing of Ti/Si diffusion couples has been studied between 480 and 800 degrees C. Samples were prepared by sputter deposition of 40 nm of titanium of (001) silicon wafers which were then annealed in ambient argon. The investigation techniques included depth profiling using Auger electron spectroscopy, transmission electron microscopy, and electrical sheet resistance measurements. The oxygen impurity effects are mainly controlled by kinetic parameters, i.e. the silicide reaction rate and the oxygen diffusivity in titanium. At low temperatures, the oxygen contamination originating from the annealing furnace is the cause of major impurity effects. Below 550 degrees C the oxygen that spreads into the metal film blocks the silicide reaction while above 650 degrees C, silicide formation dominates over oxygen diffusion and contamination-free silicide is produced. From 550 to 600 degrees C, the competition between silicide reaction and oxygen diffusion results in the formation of a silicon-deficient, oxygen-rich Ti-Si-O sublayer on top of the silicide. This layer appears to be composed of TiOx and TiSi2. In some cases, all Ti is reacted but oxygen remains in the layer and the morphology of the silicide is strongly affected. In any case, the only crystalline silicide phase that forms is TiSi2, and not TiSi or Ti5Si3, as confirmed by electron diffraction and high-resolution electron microscopy.