CoSi2 formation was studied in the presence of Ti, Ta and W, both for bilayers and as-deposited alloys. Phase formation and preferential orientation of the COSi2 phase are strongly dependent on the interlayer thickness. For thin interlayers and dilute Co alloys, COSi2 nucleation is delayed to higher temperature and CoSi2(220)//Si(400) nuclei are formed preferentially. The presence of Ti, Ta or W in the grain boundaries of the CoSi layer influences CoSi2 nucleation by enhancing the CoSi grain boundary cohesion and/or reducing grain boundary diffusion. For thicker interlayers (typically >1 nm), the interlayer acts as a diffusion mediating medium, slowing down Co diffusion and thus promoting the formation of a COSi2 layer that is epitaxially aligned with the Si substrate. (C) 2004 Elsevier B.V. All rights reserved.