Early stages of silicide formation on W, Ni, and Pt surfaces, an atom probe, and field ion microscope study

Abstract

The atomic structures and composition of silicide layers grown on W, Ni, and Pt surfaces have been studied in the atom probe and the field ion microscope. Four stages of silicide growth on W surfaces have been identified. On the W{112} plane, the atomic image structure of the top surface layer in the first stage, and the first three atomic layers in the second stage of silicide formation do not correlate well with the atomic image structures of WSi2. The W{110} plane does not react with Si atoms during the first two stages. In the third stage, the growth of thin WSi2 films proceeds nearly epitaxially from the W{001} planes to cover the entire W substrate, forming mismatches of silicide lattices along the [110] zone lines of the W. In the final stage, thick layers of polycrystalline silicides are formed. Their orientations are no longer well correlated to the W substrate. Atom-probe analyses show the stoichiometry of these atomically perfect polycrystallines to be WSi2. Identifications of crystal planes of WSi2 can be best done by comparing the image structures with the atomic arrangements of W in the silicide surface planes since the majority of Si atoms are not imaged in the field ion microscope. A pure Si atomic layer on top of the WSi2 (112) plane gives a faint image of ∼1% the regular image intensity of a W layer. FIM images show the W–WSi2 phase boundary to be very sharp. Thick layers of silicide of platinum and nickel have also been analyzed in the atom probe. The platinum silicide layers near the Pt interface have the composition of Pt2Si. Beyond the relatively sharp interface, a few Si atoms can still be found tens of atomic layers inside the platinum matrix. The composition of nickel silicide varies from one region to another. No sharp boundaries seem to exist between different phases. We also find the thick tungsten silicide films to be resistive to oxidation.