METASTABLE PHASE FORMATION IN TUNGSTEN SILICON THIN-FILMS

The thermal stability of CVD amorphous WSi2.4 thin films from room temperature to 800°C has been studied by 'in-situ' sheet resistance measurements. X-ray diffraction patterns show that two consecutive phase transformations were observed. The first which occurs at 420°C is the crystallization of the amorphous film to a metastable hexagonal phase WSi2, the second which occurs at 600°C is the transformation of the hexagonal to the thermodynamically stable tetragonal phase WSi2. The crystallization was investigated by differential scanning calorimetry (DSC) and elecrical sheet resistance measurements (ERM) while the transformation was investigated only by ERM. The consecutive phase transformations were observed to occur via nucleation and growth processes. The activation energy for the hexagonal to tetragonal transformation (3.5 ev) was found to be higher than that for the crystallization (2.2 ev). The transformation mode parameter, n, was found to be equal to 3 and 2 for the hexagonal and tetragonal phase formations, respectively. Moreover, from DSC an enthalpy of crystallization of 0.06 ev/atom was estimated.