Formation of CoSi2 in the conversion sequence of pure Co, CoSi, and CoSi2 was characterized using x-ray photoelectron spectroscopy (XPS). With pure Co deposited on single crystalline Si wafers and capped by Ti thin film, the wafers were rapid thermal annealed (RTA) at 470 degrees C for 60 s so as to convert Co to CoSi. These wafers were then stripped with APM (NH4OH/H2O2/H2O) and SPM (H2SO4/H2O2), followed by RTA at 700 degrees C for 60 s to obtain final CoSi2. XPS was used to determine the chemical composition of these thin films and the Co Auger parameter was continuously monitored along with ion sputtering to provide chemical state depth profile. After the RTA at 470 degrees C, the XPS depth profile shows that a mixed Ti/Co/Si region exists between the CoSi film and Ti cap. After the strip, the Ti cap and Ti/Co/Si region were removed. The uniformed CoSi, film was present after the RTA at 700 degrees C. The Co Auger parameters of pure Co, CoSi, and CoSi2 are 1552.1, 1551.6, and 1551.4 eV, respectively. Monitoring the Co Anger parameter along with the ion sputtering depth profile indicates that CoSi2 has the most stable cobalt chemical state after RTA at 700 degrees C. The interface region with film composition of Co/Si ratio less than 1:2 is a mixture of CoSi2 and pure Si. For the interface regions with film composition of Co/Si ratio less than 1:2 of the as-deposited Co wafer and the wafer after RTA at 470 degrees C, the Co Auger parameters are 1551.2 eV, which suggests that a possible distinctive cobalt chemical state exists.