Comparative Reliability Study of Au Wire Bond Contacts on Al Metallization vs. over Pad Metallization

Wire bonding is still the dominating technology for realizing the first level contact of semiconductors. In the last decades miniaturization and increasing connection density resulted in new challenges in terms of reliability and long term stability. But now additionally the movement towards harsher application environments leads to new problems due to aggressive media like oily or salty atmosphere. The usage of a mono-metallic contact system is one possibility to improve the reliability and long-time stability of the bond interfaces. The paper focusses on systematic comparative investigations of gold wire contacts bonded on common aluminum pad metallization respectively bonded on an over pad metallization system of nickel-palladium-gold (OPM). The interface micro structure was analyzed in the initial state as well as after different reliability tests like high temperature storage, 85°C/85%-rh and temperature annealing in aggressive environment. Focused ion beam (FIB) preparation with subsequently scanning electron microscopy (SEM) and selective transmission electron microscopy (TEM) provided the reasons for the different aging behavior. The gold-aluminum contacts degraded due to diffusion induced Kirkendall voiding during temperature annealing and corrosion affected intermetallic layer in harsh environment. In opposite to that the mono-metallic system of the gold contacts on OPM gives no degradation potential because no intermetallic diffusion occurs. So it is shown that for high reliability and/ or harsh environment applications the usage of an over pad metallization can improve the wire bond reliability significantly.