The Study of Initial Mechanism for Al–Au Solid Phase Diffusion Flip-chip Bonding

The authors have been developing a thermo-compression flip-chip bonding technique in which an LSI chip is electrically and mechanically connected to a glass substrate through Au bumps formed on the LSI chip and aluminum (Al) electrodes on the glass substrate. In this interconnection, Au bumps and Al electrodes are connected by aluminum–gold (Al–Au) solid phase diffusion. The Al thin film deposited by sputtering at room temperature has a columnar microcrystal structure. In this paper, the study of initial bonding mechanism of the Au bump to the Al thin film with the columnar microcrystals is presented. Initial diffused states were observed. The bonding mechanism is proposed as follows: the Au bump bonding breaks not only the surface oxide layer of Al thin film but also the Al columnar microcrystals, and a part of the Au bump intrudes into the grain boundary. This enables Au atoms to meet a new Al surface of the deformed columnar grain. Thus, the Al–Au solid phase diffusion occurs immediately in the region whose Al columnar microcrystal is deformed.