A Cu-Mn alloy film was deposited on SiO2 to self-form a diffusion barrier layer of MnOx at their interface. Transmission electron microscopy revealed that the barrier layer was formed not on the Cu alloy side but on the SiO2 side. Capacitance-voltage measurement revealed the decrease in the dielectric constant of the barrier layer from 11.4 to 5.1 with increasing annealing temperature. The obtained results indicated that the self-forming barrier layer with Cu-Mn is an excellent barrier layer without posing any negative influences on the interconnect line resistance and on the insulator capacitance. (C) 2009 American Vacuum Society. [DOI:10.1116/1.3224884]