Theory of aluminum metallization corrosion in microelectronics

We present a time-dependent numerical model for corrosion in microelectronics, focusing on aluminum bondpads, which can be very beneficial for the design as well as the interpretation of reliability data of microelectronics. The model includes charge transport through the polymer microelectronics encapsulant as well as the formation of layers of space charge at all polymer interfaces, which strongly influences the electrochemical charge transfer rate at the polymer¿metal interfaces via the generalized Frumkin¿Butler¿Volmer equation. The system we consider consists of two parallel gold bondwires that are each electrically connected to two aluminum bondpads, which are assumed to contain weak spots in the protective native oxide layer, i.e. pits. This system is encapsulated in an epoxy molding compound, which is the usual low-conductive, and slightly hydrophilic, microelectronic encapsulant. We assume that a cathodic reaction takes place at the gold wires, and an anodic reaction at the weak spots of the aluminum bondpads. Furthermore,weassume the presence of a large excess of inert supporting salt compared to the reactive hydroxyl ions. Numerical calculations were made in a two-dimensional geometry as function of the applied voltage difference between the two wires, the concentration of absorbed moisture in the encapsulation, and the ambient temperature. We show that the model results predict trends similar to the empirical industrial standards for failure of microelectronic products due to corrosion.