John M. DeLucca

Corrosion of the Cu/Al interface in Cu-Wire-bonded integrated circuits

A model for corrosion induced failure in wire bond devices made with either Cu or Au wire was developed. The model is based on detailed analysis of the chemical composition, crystallography, and micro structure of the corrosion induced failure sites. The detailed analysis was enabled by both a scanning electron microscope (SEM) and a transmission electron microscope (TEM) equipped with the appropriate analytical detectors. The combined characterization results were used to develop a detailed failure mechanism model which explains not only the overall chemical reactions but also the resultant two phase microstructures of the corrosion product observed in both metal systems. In addition the model helps to explain why Cu wire bonded devices are more susceptible to corrosion than Au wire bonded devices. In both systems corrosion occurs in the IMC not the end components pure metals. It is proposed this is due to the oxide on the surface of the IMCs being less resistant to pitting corrosion than that for Al. Once IMC passivity is broken down, corrosion of the IMC proceeds via selective oxidation of Al. This leads to the formation of a corroded region which is composed of a two phase microstructure, crystalline γ-Al2O3 with embedded crystalline Au and Cu metal particles. The resulting oxidized interface is highly susceptible to fracture, which is the ultimate reason for device failure. Although similar there are differences in the Cu/Al and Au/Al systems. There is a notable difference in the size and distribution of Cu particles in the aluminum oxide corrosion product of the Cu-Al system as compared to the Au particles in the aluminum oxide corrosion product of the Au-Al system. In particular, the Cu particles appear to be more uniformly distributed as compared to their Au counterparts. This difference is likely related to the crystal structure of the IMC from which the corrosion product was formed. It is proposed that this difference is related to the presence and probability of Al to Al bonding in the IMC phase/s. Furthermore, the IMC structures are also suspected to be responsible for the better immunity to contamination (specifically Halide) induced corrosion of Au relative to Cu.

Observations of IMC Formation for Au Wire Bonds to Al Pads

A materials investigation of Au wire bonds to Al pads revealed the evolution of a multiphase system whose terminal phases depended on the composition of the Au wire. Scanning transmission electron microscopy/energy-dispersive spectroscopy and electron diffraction data are presented for Au/Al wire bonds using both Pd-doped, 99% pure Au wire (2N) and 99.99% pure Au wire (4N) in the as-formed state, upon completion of overmold operations, and after reflow and aging. The reacted interfaces of both the 2N and 4N bonds were found to take on a bilayer intermetallic compound (IMC) microstructure that persisted with aging and phase changes; it is the interface of this bilayer that is believed to be susceptible to mechanical degradation. Pd was found to accumulate in the IMC near the Au/IMC interface for 2N wire bonds and appears to lead to a phase evolution different from that for 4N wire that may be responsible for enhanced reliability of the 2N wire bond with high-temperature aging.

Thermal degradation of sputtered Al/Ni(V)/Cu-UBM in Pb-free flip chip solder joints connected to substrate with different surface finishes

The thermal stability of flip chip solder joints made with Al/Ni(V)/Cu-UBM and SAC-405 solder bumps on substrates with either electroless Ni(P)-immersion gold (ENIG) or Cu surface finish (Cu-SOP) was determined at 170C. On ENIG, the resistance changed by more than one order of magnitude after 400 hours of high temperature storage, whereas on Cu-SOP, no change in resistance was observed up to 2400 hours of aging. The detailed electrical characterization of the solder joints was supplemented with time dependent physical characterization (microstructure and materials composition) of the joint using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), focused ion beam microscopy (FIB), and transmission electron microscopy (TEM). The microstructural characterization clearly showed that electrical degradation of the ENIG devices was a direct result of the conversion and interfacial oxidation of the as deposited Ni(V)-barrier UBM layer into a porous layer that contained a web like fine grain V3Sn IMC. This conversion was driven by the Au from the ENIG surface finish. The results were similar to that reported by the authors for eutectic PbSn solder bumps, the exception being that for ENIG surface finish the resistance stability of SAC solder was 25x greater than eutectic that for PnSn solder.