We have investigated multilayer and bilayer Ni/Si thin films by nano-differential scanning calorimetry (DSC) at ultra rapid scan rates, in a temperature-time regime not accessible with conventional apparatus. DSC experiments were completed at slower scan rates as well, where it was possible to conduct parallel rapid thermal annealing (RTA) experiments for comparison. Post-experimental characterization was accomplished by x-ray diffraction (XRD), and by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) of thin cross sections prepared by focused ion beam (FIB) milling. We found that rate of heating has a profound effect on the DSC signal, as well as on the resulting microstructure. After heating to 560 °C at 105 °C/s, the general microstructure of the multilayer was preserved, in spite of extensive interdiffusion of Ni and Si. By contrast, after heating to 560 °C at 13,500 °C/s, the multilayer films were completely homogeneous with no evidence of the original multilayer microstructure. For the slower scan rates, we interpret the results as indicating a solid state diffusion-nucleation-growth process. At the higher scan rates, the temperature increased so rapidly that a metastable liquid was first formed, resulting in complete intermixing of the multilayer, followed by crystallization to form solid phases. The integrated DSC enthalpies for both multilayer and bilayer films are consistent with this interpretation, which is further supported by thermodynamic predictions of metastable Ni/Si melting and solid state Ni/Si interdiffusion. Our results suggest that use of heating rates >10,000 °C/s may open new avenues for intermetallic mico- and nano-fabrication, at temperatures well below those prevailing during explosive silicidation.
Citation: Journal of Applied Physics
Pub Type: Journals
calorimeter, metastable, multilayer, nanocalorimeter, nickel, phase diagram, silicide, silicon