The effect of finish layer on the interfacial cracking failure of AuSi bonding. (September 2020) - Gao, Li-Yin
Highlights: Failure analysis was conducted on the Au-Si solder joints between CPC substrate and chip. Two commercial CPC substrates with novel NiCo barrier layer were used. The interfacial IMCs were characterized as bulk-like NiSi2 and (Ni, Co)2 Si layer. Cracks initiated at the interface between the (Ni, Co)2 Si layer and NiCo layer. The failed samples had higher Ni content within NiCo layer and smaller size of Au grains. Abstract: The interfacial reliability of Au-Si bonding between Cu/MoCu/Cu (named as CPC) substrate and silicon chip is analyzed. The results of optical microscopy (OM) and conformal laser scanning microscopy (CLSM) revealed that the Au layer of the cracked sample has small roughness without any patterns on it. Furthermore, the results of scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and electron probe micro-analysis (EPMA) demonstrated that the void or crack initiated at the interface between a thin intermetallic compound (IMC) layer and the NiCo layer. It showed that the cracked sample had higher Ni content within NiCo layer and smaller size of Au layer. Moreover, the IMCs were characterized using transmission electron microscopy (TEM), which were identified as bulk-like dispersed NiSi2 and a thin continuous (Ni, Co)2 Si layer. Based on the experimental results, the failure mechanism of Au-Si bonding is concluded coming from the unbalanced diffusion rate between Ni and Si during interfacial reaction. Higher Ni content of NiCo layer and small grain size of Au and (Ni, Co)2 Si layer accelerate the diffusion of Ni, which also promotes the growth of interfacial Ni-contained IMC. As a result, the vacancy flows diffuse inversely towards the interface and form voids along the interface, which leads to the final crack failure.