The reduction of Au—Al intermetallic formation and electromigration in hydrogen environments

A series of experiments to determine the effect of the environment on gold-aluminum intermetallic compound formation have been carried out. The results of these experiments led to an investigation of the effect of the environment on electromigration in aluminum. Bond pull tests as well as SEM examination were done on commercial IC packages. After removing the lid, the packages were heated (350 or 400°C) in hydrogen or argon (0.5 atm) for 1 to 150 h. The results of the bond pull tests (150 bonds per test) show a drastic deterioration in bond strength for the specimens heat-treated in argon. Failure for the hydrogen-treated specimens occurred almost entirely at the wire span. The SEM examination showed the formation of Au-Al intermetallics in the specimens treated in argon. The specimens treated in hydrogen showed little or no compound formation. In order to confirm the results of these experiments substrates of silicon, thermally oxidized silicon, nitrided silicon, and sapphire were coated with aluminum films (4200 Å thick) across one-half the surface. Gold films (8200 Å) were deposited on these same substrates such that there was a 2-mm gold overlap on the aluminum. These specimens were then heated (350 or 400°C) for 1-3 h, in half atmospheres of H
2
, Ar, O
2
, N
2
, He, or air and examined in an SEM. If we ignore the formation of aluminum oxide in the air and oxygen environments, the same general results were found; namely, that the Au-Al compounds readily formed in all environments except hydrogen. In the worst case for hydrogen the compounds formed only at the gold-aluminum contact line. Since hydrogen does not interact with gold and since the compounds form via a diffusion mechanism, it seemed reasonable to assume that the observed effects were due to a reduction in the aluminum flux due to an adsorption of hydrogen on aluminum grain boundaries and vacancies. If this assumption is correct then a hydrogen environment should reduce electromigration in aluminum. A series of electromigration experiments was conducted on 1.15 × 10
^-2
-cm wide by 4200-Å thick aluminum stripes in hydrogen and argon environments at current densities of 0.7-1.1 × 10
⁶
A/cm
²
. The stripes tested in argon showed hillock formation at the positive electrode after approximately 12 h and generally failed by voiding after approximately 24 h. The stripes tested in hydrogen did not show any hillock formation after 24 h and none failed in that same period. The maximum resistance change observed for the hydrogen tests was ∼ 20 Ω after 60 h.