Melting dynamics ofNiSi2/Si under pulsed laser irradiation

Abstract

Thermally grown ${\mathrm{NiSi}}_2$ layers 220 nm thick on 〈111〉 Si substrates have been irradiated by 40-ns Nd laser pulses with energy densities in the range 0.3–2.1 J/${\mathrm{cm}}^2$. Time-resolved reflectivity measurements using a He-Ne laser probe have been performed during the irradiations. Samples have been subsequently analyzed by 2.0-MeV ${\mathrm{He}}^{+}$ Rutherford backscattering spectrometry in combination with the channeling effect. Three different energy-density thresholds for three phenomena have been found. At 0.37 J/${\mathrm{cm}}^2$ the sharp increase detected in the reflectivity signal indicates the melting of the ${\mathrm{NiSi}}_2$ surface layer, at 0.92 J/${\mathrm{cm}}^2$ channeling measurements show the transition in the silicide layer from the (A+B)-type crystalline mixture to a mainly B-type single-crystal structure, and finally at 1.1 J/${\mathrm{cm}}^2$ the underlying silicon starts to mix with the silicide layer, modifying its stoichiometry. This behavior is discussed in terms of the phase transitions predicted from the Ni-Si phase diagram, and quantitatively is compared with kinetic calculations and the heat-flow model. Melting starts at the ${\mathrm{NiSi}}_2$ free surface at $T_0$=1276 K and propagates toward the inside, while the silicon atoms at the ${\mathrm{NiSi}}_2$/Si interface dissolve into the liquid solution at the ${\mathrm{NiSi}}_2$ liquidus temperature (1400 K), much lower than the pure Si melting point (1685 K).