The formation of disilicides from bilayers of Ni/Co and Co/Ni on silicon: Phase separation and solid solution

• Published in: Thin solid films Vol. 135; no. 2; pp. 229 - 243
• Main Authors: Finstad, T.G., Anfiteatro, D.D., Deline, V.R., d'Heurle, F.M., Gas, P., Moruzzi, V.L., Schwarz, K., Tersoff, J.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 15.01.1986; Elsevier Science
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Metals, semimetals and alloys; Metals. Metallurgy; Other nonelectronic physical properties; Physical properties of thin films, nonelectronic; Physics; Solid surfaces and solid-solid interfaces; Specific materials; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Scanning electron microscopy; Annealing; Support; Solid solid interface; Transition metal; Experimental study; Bimetallic layer; Diffusion pair; Thin film; Substrate; Rutherford backscattering; Nickel Silicides; Vapour deposition; Cobalt Silicides; Silicon; Chemical diffusion; Diffusion; Auger electron spectrometry
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/0040-6090(86)90130-6

Bilayers of Ni/Co and Co/Ni were deposited onto silicon by means of electron beam evaporation. The annealing behavior was investigated as a function of time in the temperature range 475–550 °C by Rutherford backscattering spectrometry, Auger electron spectroscopy, scanning electron microscopy and X-ray analysis. In this temperature range it is observed that the disilicide (solid solution) nucleates and grows from a film containing both the monosilicide of cobalt and that of nickel. These nearly insoluble monosilicides are separated into layers; the disilicide grows from the interface between the NiSi and the CoSi phase. If there are equal amounts of the two metals this interface is located approximately midway between the upper surface and the silicon substrate. The disilicide grows simultaneously both toward the free surface and toward the silicon substrate. The growth kinetics of the disilicide have been analyzed and are found to be complex. The general behavior of the disilicide formation is found to be similar to that found for Co-Ni alloy films and as for the latter case indicates the primacy of nucleation over diffusion as the controlling mechanism in the disilicide formation. The site of nucleation of the disilicide at the NiSi-CoSi interface is explained by a model based on (a) the phase separation of the two monosilicides and (b) the high entropy of mixing of the mixed disilicide. In agreement with this model the nucleation of the mixed disilicides is shown to occur at temperatures much lower than those for either CoSi 2 or NiSi 2. Given the common crystal structure and the almost identical lattice parameters, the mutual solubilities of CoSi 2 and NiSi 2 follow from elementary concepts of alloy phase theory. However, the behavior of the monosilicides is more unexpected. The limited solubilities of the monosilicides, CoSi and NiSi, in each other are shown to result from specific features of the electronic band structure of these two compounds, features which are not found for example in the isomorphous disilicides CoSi 2 and NiSi 2.