Initial sequence and kinetics of silicide formation in cobalt/amorphous-silicon multilayer thin films

Abstract

The sequence of cobalt silicide phase formation in cobalt/amorphous-silicon multilayer thin films has been investigated using differential scanning calorimetry, cross-sectional transmission electron microscopy, thin film x-ray diffraction, and energy dispersive x-ray analysis in a scanning transmission electron microscope. Multilayer thin films with various overall atomic concentration ratios (2Co:1Si, 1Co:1Si, 1Co:2Si) and various bilayer thicknesses were used in this study. It was found that an amorphous intermixed layer about 10 nm thick preexisted at the as-deposited cobalt/amorphous-silicon interface. Crystalline CoSi was always the first phase to nucleate in the interdiffused layer. The CoSi nucleates and coalesces into a continuous layer at temperatures as low as about 260 °C, but does not thicken until the next phase forms. Which phase forms second is determined by whether Co and Si remain after formation of the CoSi layer. The activation energy for the initial nucleation and growth of the CoSi layer was found to be 1.6±0.1 eV. When excess cobalt is present, Co2Si is the next phase to form at about 350 °C. Co2Si grows with the coexisting CoSi layer remaining approximately a constant thickness of about 5 nm. The activation energy for growth of the Co2Si was 2.0±0.1 eV. After the cobalt is fully consumed, Co2Si reacts with excess silicon (if there is any) to form more CoSi at about 400 °C. This process has an activation energy of 1.9±0.1 eV. If excess silicon still exists after all of the Co2Si has transformed to CoSi, CoSi2 formation follows at about 500 °C with an activation energy of 2.5±0.1 eV. The activation energies of the above phase transformations were determined by analyzing calorimetric data.