Dependence of Electromigration Lifetime for Via Chains on Slope Angles of Via Holes

Abstract

Dependence of electromigration (EM) lifetime for via chains with conventional sputtered AI-Si-Cu films on slope angles of via holes has been investigated. EM testing for tapered (slope angle of 55 ~ and vertical (slope angle of 85 ~ via chains with 11 ~m long first metal lines and 12 ~m long second metal lines was performed under the same stress condition. Mean time to failure (MTF) of the EIK for the tapered via chains is one order of magnitude shorter than that for the vertical via chains, although the step coverage of the Ai-Si=Cu films in the vias is improved by controlling the slope angle of the via hole for the tapered via. It was found that the MTF for the via chains degrades with decreasing the slope angle of the via hole. In this case, thicknesses of the local passivation layer in the via hole increase with decreasing the slope angle of the via hole because of improvement in the step coverage of the passivation layer. The MTF for the tapered via chains with a SiN/PSG dual passivation layer was i0 h, while that with a PSG single passivation layer was at least over 2000 h. It is inferred from these results that the thermal tensile stress induced in the A1 film due to the thicker passivation layer increases the diffusivity of Al atoms in the vicinity of the via interface, which results in degradation of the EM resistance for the tapered via chains.