In order to develop a reliable interconnect integration scheme by using Cu in ultra large scale integrated devices (ULSI), the evolutions of the concentration profile of copper ions in SiO2 was simulated under bias temperature stress (BTS) test. Diffusion equation was solved numerically in two electric field modes. One is constant electric field mode where copper drift was simulated with the assumption that electric field is constant within SiO2 film. In variable electric field mode, simulation was carried out considering the variation of electric field in SiO2 due to copper ions. The diffusion of copper ions in variable electric field mode is slower than that in constant electric field mode, because copper ions in SiO2 reduce electric field near the interface between Cu and SiO2. Flatband voltage shift ) (ΔVFB increases parabolically as BTS time increases in constant electric field mode. However, it has linear relation with BTS time in variable electric field mode, which is typically observed in experiments.