Characterization of Intermetallic Growth for Gold Bonding and Copper Bonding on Aluminum Metallization in Power Transistors

Gold wire bonding to aluminum pad is a widely applied interconnection method in microelectronics industry for years. Copper wire bonding has gained popularity due to its economic advantage and superior electrical performance compared to gold wire bonding. It has been known that at elevated temperatures, interdiffusion between ball bond and aluminum pad leads to intermetallics growth and voids formation at the gold-aluminum interface. Our study reveals that this would influence the drain-to-source on-resistance R_DS(on), which is an important parameter for power MOSFET as it determines how much current the device can carry for low to medium frequency applications. We observe that both gold bonds and copper bonds demonstrating very stable R_DS(on) at 150degC thermal aging up to a few thousands hours. Nevertheless, at much higher thermal aging temperature of 200degC, gold bonds start to form Kirkendall voids at early aging stage. The voiding causes the gold ball to be isolated away from the aluminum pad, resulting in undesired drastic rise of R_DS(on) value. In contrast, copper bonds render much more stable R_DS(on) at 200degC thermal aging over time owing to the relatively small interdiffusion rate of Cu-Al phases. As a result, copper is more suitable to be utilized as bonding method in MOSFET devices. In Fairchild Semiconductor, we progressively replace gold bonding with copper bonding in our power transistor products since 2006 so that the customers can have more reliable and longer life products. In addition, our research findings will further top-up the customer confidence to use our copper wire bonding devices in high temperature field applications based on the strong point that, there is no problem associated with the intermetallic growth defects for copper bonds at elevated temperatures.