BONDING AT CU-AL INTERFACES - RELEVANCE TO MICROCIRCUIT PACKAGING

The nature of bonding interactions at Cu-Al interfaces is investigated both theoretically and experimentally. Extended Huckel-type tight binding calculations were performed on a model for Al binding to a Cu (111) surface. Binding at a three-fold bridge site was most preferred, followed by binding at a two-fold bridge site, followed by binding directly atop a surface Cu atom. Cu-Al interactions at all sites are strongly attractive. These results suggest that the metals should form chemical bonds, even though the interface may be very well defined. Thus, any bonding technique which cleans the surfaces of oxides and allows intimate contact between the metals should produce strong, electrically conductive bonds. Interfacial bonding was probed experimentally by sectioning Cu samples to which Al wire had been ultrasonically bonded. After bonding, the sample was baked in vacuo to accelerate formation of intermetallic phases. The intermetallic phases were observed after only 2 h of annealing at 300-degrees-C.