Characterization of Over Pad Metallization (OPM) for High Temperature Reliability

Abstract

Temperature is a key accelerating factor for failures in semiconductor devices which brings reliability challenges for electronic packaging process because of the thermo-mechanical, metallurgical and chemical properties of the materials used in packaging. Failures at high temperatures (≥ 175 °C) often originate from aluminum – gold wirebonding systems because of the formation of Au-Al intermetallic phases and associated Kirkendall voiding which degrade the interface. A stack of Nickel/Palladium /Gold (Over Pad Metallization or OPM) formed on aluminum or copper bond pads are reported to be reliable pad finishing for high temperature applications. In this study OPM stack is formed by electroless plating on aluminum bond pads separated by 10 μm or more and the process is optimized to achieve high process yields. Al bond pad contamination is shown to be an important factor for achieving good plating quality and yield. Various plating defects which can lead to electrical failures are characterized. It is shown that defective rough plating can lead to wirebonding failures such as non-stick on pads (NSOP). Interface of gold ball bond and OPM after thermal aging at 225 °C for 168 hours is characterized by High Resolution Transmission Electron Microscopy (TEM) and Focused ion beam (FIB) cross-section analysis. Excellent thermal reliability with no degradation of ball shear or wire pull strengths achieved with non-defective OPM pads. High temperature (175 °C) package reliability with OPM is demonstrated for gold and copper wires.