Silicide formation in cobalt amorphous silicon, amorphous Co-Si and bias-induced Co-Si films

The silicide formation in cobalt/amorphous silicon multilayer films, amorphous cobalt-silicon films, and bias-induced cobalt-silicon films has been examined by differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. For amorphous cobalt-silicon and bias-induced cobalt-silicon films, Co2Si forms as a first silicide phase, followed by the formation of CoSi and CoSi2. For a Co/a-Si multilayer film with the atomic concentration ratio of the cobalt to silicon layer being 1:2, CoSi is found to be formed as the first silicide phase. It is confirmed that CoSi, Co2Si, CoSi, and CoSi2 form sequentially as the scanning temperature increases. The observed phase sequence is analyzed by the effective heat of formation. A structure factor in addition to the effective heat of formation is used to explain the difference in the formation of the first phase between cobalt/amorphous silicon multilayer films, amorphous cobalt-silicon alloy films, and bias-induced cobalt-silicon films. For the case of bias-induced cobalt-silicon films prepared at various substrate temperatures and bias conditions, the phase sequence and crystallinity of cobalt silicide have a stronger dependence on the substrate bias voltage than on the substrate temperature due to the effects of collisional cascade mixing, in-situ cleaning, and an increase in the number of nucleation sites by ion bombardment an the growing surface. Also, bias-induced epitaxial CoSi2 layer is grown at 200 degrees C, a much lower temperature than molecular beam epitaxy. In order to quantitatively explain low-temperature epitaxial growth of the CoSi2 layer, the Ar ion energy transferred to Co and Si atoms and the resputtering yield as a function of substrate bias voltage are calculated.