Influence of Atmosphere on Ultrasonic Bonding of Silicon to Metal

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Ultrasonic bonding was carried out with some kinds of foil (Au, Ag, Al, Cu, Ni, Fe). In the case of similar metal bonding, good bonding was carried out. However, in the case of dissimilar metal bonding, only a few combination of metal foil enabled good bonding. The bonding of metal foil with Ag foil was very poor. Next, the bonding ability of metal foil to Si wafer was examined and good bonding was obtained with Au foil and Al foil. In the case of bonding of Si to Al foil, there was large scatter in bonding strength (shear strength). Since large scatter in strength of Si/Al foil bonding might be caused by oxide layer on the bonding face, Si and Al foil were immersed in acid and alkali to remove oxide layer. Namely, Si was immersed in HF water solution for 24 h, and Al foil was immersed in NaOH solution of different content (0 - 10 kmol/m_3) for different times (0 - 10 min). Immersion of Si in HF solution gave no influence on bonding character of Si to Al foil, but after immersion of Al … More foil in NaOH solution, minimum bonding strength increased at each bonding condition and scatter of bonding strength decreased. There was no clear influence of NaOH content in solution and immersion time on bonding character. Since many small holes were observed on the oxide layer of Al foil during immersion in NaOH solution through SEM, it was thought that the increase in bonding strength was caused by easy detachment of oxide layer from the surface of metal foil during ultrasonic vibration. Then, the ultrasonic bonding of Si to Al foil was done under a pressure of 2 Pa, but no increase in the strength and no decrease in scatter of the strength was obtained. Au film was formed on Si surface by spattering, and then the bonding of Au film to Al foil was done under air atmosphere. The bonding strength increased with increasing thickness of Au film. The fracture occurred at an interface between Au film and Si substrate, and the bonding strength was dependent on the strength of Au foil and the strength of Si/Al interface. From these results, it was expected that the bonding strength of Si/metal foil bonding increases through surface treatment of Si and metal surface. Less