Gold-Aluminum Bond Failure Mechanism

The degradation of Au/Al contacts on semiconductor devices as a function of process parameters and environment has been studied. Test samples were bonded under variable conditions of temperature and pressure and tested in atmospheres of N2, O2, Air, H2O at 200°C and 300°C. Bond strengths and bond degradation were evaluated by bond pull tests and metallographic and SEM techniques. The two major failure modes are found to be ball lift-off at the pad and wire break immediately above the ball. The effect of process parameters, especially temperature and pressure are evident after exposure to 200°C and 300°C bakes. The effects of environment can be qualitatively correlated with the bond process. To define the failure mechanism precisely is difficult. The role oxygen may play in enhancing aluminum-gold bond degradation is argumentative at best. The presumably simple oxidation of a metal such as iron or aluminum is actually not that simple. It consists of several steps with intermediate stages as metal to metal bonds are disrupted and the oxygen molecule itself is disassociated. It may well be that during this oxidation sequence when aluminum-aluminum bonds are disrupted, intermetallic formation with gold is accelerated. If such an accelerating mechanism does exist, a nitrogen ambient would not show the sane effect. However, it is quite probable that smrall amounts of water vapor would act similarly to oxygen.