Test methodology of Cu wire bond on aluminum metallization to secure good reliability

There is growing interest in Cu wire bonding for IC (Integrated Circuit) interconnection due to lowered the cost, better the electrical performance and mechanical properties to against the mold flow. However, the growth of Cu/Al intermetallic compounds (IMC) at the wire/pad interfaces is poorly understood and it is difficult to interpret the IMC neither by a optical microscopy inspection nor normal Scanning Electron Microscopy (SEM). To study the Cu/Al IMC growth in Cu ball bonds, high temperature aging at 250°C for 30 mins has been used to accelerate the aging process of the bonds. Back-Scatter Electron (BSE) in a Scanning Electron Microscopy (SEM) system were then chosen to verified the IMC coverage among various test methodologies. The formation of Cu-Al IMC with voids and cracks have a significant effect on the strength and reliability of bonds. The tendency of oxidation and corrosion of Cu plays the risk in long-term reliability in a humid environment. The reliability for Cu wire bonding under high temperature and humidity was investigated in an high accelerated stress test (HAST) to testify the bond integrity. In our study, we report the interfacial morphology of copper bonds respect to IMC coverage and establish a pre-qualification procedure (IMC buy-off, fast corrosion…) prior to real reliability stress tests.