Akitoshi Ishizaka1, Seiichi Iwata1 and Hiroshi Yamamoto1
1Central Research Laboratory, Hitachi Ltd., Kokubunji
The mechanism of the thermo-compression bonding of Au wire to Al thin film electrodes on Si semiconductor devices was studied. Because of the importance of breaking the oxide films on Al for obtaining reliable bonds, this investigation was carried out to clarify how the deformation of Au and Al at the bond interface broke the oxide film on Al and made the reaction, or the bond, between Au and Al possible.
The results obtained are as follows:
(1) Even though the plastic strain of Au wire exceeded 40% in thermo-compression bonding, very little macroscopic metal flow was observed on Al at the bond interface. This was explained by the facts that the apparent hardness of Al thin film (1 μ m) was affected by the presence of the hard substrate (glass and Si) and that the Al thin film was harder than the bulk Al. This behavior of Al at the bond interface is quite different from that in the ordinary compression bonding of the bulk metals.
(2) The microscopic observation (by SEM) revealed the formation of Au-Al intermetallic compounds along the steps formed on Al by the compression of the Au wire with the traces of slip lines on its surface.
(3) The area of the exposed clean surface, where the oxide films on Al were apparently broken, was directly related to the plastic strain of the Au wire.
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