CuInSe2 thin films were formed from the selenization of co-sputtered Cu-In alloy layers. These layers consisted of only two phases, CuIn2 and Cu11In9, over broad Cu-In composition ratio. The concentration of Cu,,In, phase increased by varying the composition from In-rich to Cu-rich. The composition of co-sputtered Cu-In alloy layers was linearly dependent on the sputtering power of Cu and In targets. The metallic layers were selenized either at a low pressure of 10 mTorr or at 1 atm Ar. A small number of Cu-Se and In-Se compounds were observed during the early stage of selenization and single-phase CuInSe2 was more easily formed in vacuum than at 1 atm Ar. Therefore, CuInSe2 films selenized in vacuum showed smoother surface and denser microstructure than those selenized at 1 atm. The results showed that CuInSe2 films selenized in vacuum had good properties suitable for a solar cell. (C) 2000