An Al/SiO2interfacial structure is investigated by cross‐sectional transmission electron microscopy (TEM) and x‐ray photoelectron spectroscopy (XPS) before and after post‐metallization annealing. An interfacial reacted layer with uniform thickness of 2.5 nm is observed by TEM for samples annealed at 450 °C for 1 h. Further reaction is suppressed at higher annealing temperatures up to the eutectic point (577 °C) of an Al‐Si system. XPS analysis of the interfacial structure is performed by removing the unreacted metallic Al layer selectively from the surface. The XPS spectra of the reaction products show that the interface has a layered structure of Al/Al2O3/Si/SiO2. This is discussed in relation to thermodynamic stability. ©1995 American Institute of Physics.