The interaction and reactivity of Ti and Co with SiO2 and Si3N4 have been investigated. In the case of Ti salicide, SiO2 sidewall spacer showed no lateral silicide overgrowth and low leakage current between gate and source/drain up to silicidation temperature of 750 1C. However, Si3N4 sidewall spacer showed dopant dependence of the lateral silicide growth and leakage current. This discrepancy between SiO2 and Si3N4 and dopant dependence is closely related to the reactivity. For Co, lateral silicide overgrowth is greatly reduced. Instead, Co films on SiO2 and Si3N4 layer were agglomerated by annealing. An annealing at 1050°C caused not only agglomeration of Co film but penetration of Co agglomerates through the layers. Interestingly, the CoSi2 spike of B type epitaxial and twinned orientation was formed in the Si substrate by the penetrated Co source.