Mg-Si thin films are deposited by ion beam sputtering from the target composed of Mg and Si with 50%:50% area ratios. Intermetallic compound magnesium silicide (Mg2Si) films are obtained at room temperature on glass substrates. Observation of the thin film by cross sectional transmission electron microscopes indicates that films of 1 mum thickness show a microstructure composed of a crystalline upper layer Of Mg2Si with columnar structure 300 nm thick and an amorphous bottom layer with uniform structure 700 nm in thickness. Energy dispersive x-ray measurements show that magnesium concentration in the crystalline Mg2Si upper layer are larger than in the amorphous bottom layer. Moreover, the magnesium concentration at grain boundaries is larger than that in grains in the crystalline Mg2Si upper layer. It is also observed that magnesium segregates on the film surface. The film formation is explained in terms of magnesium migration-tosurface and magnesium evaporation from film surface. (C) 2004 American Vacuum Society.