Modern microelectronic semiconductor devices use layered thin films on Si wafers. Interdiffusion and reaction between two neighboring thin-film layers has been a technological issue from the point of view of yield and reliability of the devices. On a piece of Si chip the size of a fingernail, there are now more than several hundred millions of FETs, each of them with source, drain, and gate contacts. These contacts are typically made of silicides, which are IMCs of metal and Si, and each of the contacts must be the same or have the same electrical properties. Hence, in manufacturing VLSI circuits on a Si chip, the formation of silicide contacts and gates has been a critical processing step. So the controlled formation of silicide by depositing and reacting a thin metal film on a Si substrate has been a very active area of study [1–4]. What is unique in the thin-film interfacial reaction is the requirement of “single-phase formation” . This means that we need to form a specific single-silicide phase in all the contacts and gates. Why it is unique is because of the difference between the reaction in bulk diffusion couples and that in thin-film couples. Multiple phases are formed simultaneously in the bulk couples, but multiple phases are formed sequentially one-by-one in thin films, so we can have single-phase instead of multiple-phase formation in thin-film reactions.