The thermal stability of Cu interconnects fabricated by dry etching with simultaneous formation of a self-aligned passivation film has been investigated. The passivation film on the sidewall of the interconnects is composed of SiON free from impurities such as Cl. This film acts as a barrier layer to prevent Cu from corroding and oxidizing during any subsequent process, such as formation of a dielectric overcoating. Using this etching process, the Cu interconnect of a TiN/Cu/TiN multilayered structure is formed. The resistivity of the Cu interconnects is about 1.7-2.2 mu Omega cm in the width range 0.2-10 mu m and remains unchanged on annealing up to 700 degrees C. Diffusion of Cu into the substrate is not observed up to 800 degrees C annealing. These results demonstrate that the sidewall film prevents diffusion of Cu. Therefore the Cu interconnect covered with TiN and the thick sidewall film is suitable for the ultralarge-scale integration process.