The formation mechanism of self-aligned MgO layers obtained from Cu(Mg) alloys has been investigated. Self-aligned surface and interfacial MgO layers were formed upon annealing Cu(Mg)/SiO2/Si multilayer films in an oxygen ambient, resulting in a structure of MgO/Cu/MgO/SiO2/Si. Upon annealing at 300 degreesC or higher in an oxygen ambient, Mg segregates preferentially to the Cu surface until a dense, uniform MgO layer is formed. A growth limited thickness of the surface MgO was found to be about 150 Angstrom, Providing substantial passivation of the exposed Cu surface. After a dense MgO layer forms, substantial Mg segregation to the SiO2 surface takes place. However, the formation of the interfacial MgO caused a sudden increase in resistivity after annealing for about 20 min, which can be due to the release of free Si being diffused into the Cu film by the reaction of Mg with Si in the SiO2. The optimum Mg contents in Cu(Mg) alloy films with various thickness were thus estimated to obtain the Cu(Mg) alloy multilayer film with substantially lower resistivity while retaining the beneficial properties of Cu passivation in an oxygen ambient.