Influence of Material Composition on Copper-Aluminum Wire Bonding Reliability

In the semiconductor industry, copper (Cu) wire has replaced gold (Au) wire in most of the market applications, due to its economic advantages and better electrical or thermal performances. When Cu wire is coupled with Al-based bondpads, the reliability of the resulting intermetallic joint is a key-aspect requiring an adequate choice of the materials to maximize the lifetime. This study examines workability and reliability performances of Cu-Al Inter-Metallic compound (IMC) joints using three Cu wire types and varying aluminum (Al) bond pad compositions and thicknesses. The test samples have been prepared exploiting laser ablation for bond-pad patterning, and then characterized in terms of initial conformity to in-process quality requirements. High Temperature Storage (HTS) trials have been conducted in highly accelerated conditions in order to investigate the effect of the above materials characteristics on the IMC joints evolution. Wire pull and ball shear tests have been used to characterize the mechanical response to the thermal ageing. Then cross-section techniques combined with SEM and TEM inspections allowed to establish correlation between mechanical behavior and changes in the IMC morphology and stoichiometry.