We have developed a new technique for direct patterning and formation of cobalt silicide structures using focused ion beam implantation. This mask-free fabrication technique takes advantage of the influence on the kinetics of ion-beam mixing and properties of thin barrier oxides during silicide line formation. Silicide structures with dimensions of the order of 170 nm were produced on (100) silicon substrates. The process involves the ion implantation of 200 keV As++ through a thin cobalt film on SiO2/Si structure. A selective reaction barrier at the Si/Co interface comprising of a thin (similar to2 nm) oxide (SiO2), prevents unwanted reactions with silicon. Ion-beam mixing was instrumental in fracturing of the oxide layer, thereby allowing the migration of metal atoms across the Si/Co boundary for the silicidation reaction to proceed during subsequent rapid thermal anneal treatments. Diffusion controlled reactions advanced rapidly in the implanted areas, requiring a two-step anneal sequence to inhibit reaction elsewhere. A threshold dose of 3 X 10(15) cm(-2) was required for process initiation. Four-terminal resistance test structures were formed for electrical measurements. Resistivity obtained ranged on the order of 12 to 23 muOhm-cm. Application of this method can facilitate a wide variety of silicide structures.