Enhanced Mass Transport in Ultrarapidly Heated Ni/Si Thin-Film Multilayers
NATIONAL INST OF STANDARDS AND TECHNOLOGY GAITHERSBURG MD
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We investigated multilayer and bilayer NiSi thin films by nanodifferential scanning calorimetry nano-DSC at ultrarapid 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 experiments for comparison. Post experimental characterization was accomplished by x-ray diffraction, and by transmission electron microscopy TEM and energy-filtered TEM of thin cross sections prepared by focused ion beam milling. We found that rate of heating has a profound effect on the resulting microstructure, as well as on the DSC signal. After heating to 560 C at 120 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 16 000 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, we suggest that 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 NiSi melting and solid state NiSi interdiffusion. Our results suggest that use of heating rates 10 000 C s may open new avenues for intermetallic micro- and nanofabrication, at temperatures well below those prevailing during explosive silicidation.
- Inorganic Chemistry
- Surface Transportation and Equipment
- Test Facilities, Equipment and Methods