Corrosion of Aluminum Interconnect Passivated with Polymide or Silicon Nitride.

To determine if polyimide is an appropriate over-metal passivation dielectric for non-hermetically sealed high voltage devices, the corrosion rate of sputtered aluminum test patterns, with either polyimide or the standard plasma-deposited silicon nitride passivation, has been measured at 85C, 85% RH, 100V. No cover coat was used over the test devices. The time dependence of failures can be explained by wear-out processes. For the silicon nitride passivated devices, the wear-out occurred mainly by anodic corrosion of bond pads due to unintentional chloride contamination. Even under these aggressive conditions, essentially no corrosion of the silicon nitride passivated aluminum was observed. For the polyimide devices, the wear-out occurred by wide-area aluminum corrosion, probably preceded by loss of polyimide adhesion. Auger spectroscopy revealed that the loss of adhesion occurred at the polyimide-coupling agent interface. While the presence of the coupling agent has been shown to improve the polyimide to substrate adhesion, we conclude that the adhesion is insufficient in the presence of water to ensure reliable over-metal passivation of non-hermetic devices.