This paper reports the study of the thermal stability of the Cu(150 nm)/Ta(25 nm)/ultra low-k (ULK) porous polymer(600 nm)/SiO2/Si structures for multilevel interconnects of the new generation integrated circuits (IC). It was found that the sheet resistance of the structure increased slowly with the annealing time, at 400 degreesC, for up to 120 min, and it increased abruptly after being annealed for 150 min. The results of the secondary ion mass spectrometry revealed that the significant increase of the sheet resistance is due to the diffusion of Ta and 0 to the Cu film. It was also revealed that the annealing at 400 degreesC for more than 120 min also caused diffusion of Ta and 0 and even Cu into the ultra low-k porous polymer (ULKPP), and the Ta and 0 diffused faster in the dielectric than in the Cu film. The diffusion coefficient of Ta in the ultra low-k porous polymer at 400 degreesC is about 0.051 nm(2)/S. In addition, the out-diffusion of the C and H in the dielectric towards Ta and Cu layers was also observed and H was found to diffuse faster than C. 2004 Elsevier B.V. All rights reserved.