Long Time Reliability of Cu Wire on Substrates with Al Based Metallisations

The story of copper wire has existed for decades, and it always looked to be a very promising material. However, despite all of its positive properties copper bonding wire never managed to find such a wide use as did gold wire. In recent years many semiconductor manufacturers made great efforts to replace gold with copper wire. Driving forces are the high gold price and some design benefits due to the higher electrical and thermal conductivity of copper, as well as the higher mechanical strength of copper, in particular for fine pitch applications. On the other hand, certain disadvantages of copper still hinder its wide use. The bonding window of copper wire is smaller and its shelf life is shorter. The price benefits can be easily depleted by lower yield, caused by the narrow bonding window. Nowadays, new real chances are arising for copper bonding wire on the strength of newer bonding machines and growing experience with substrate metallisation finishing. Predominant chip metallisation remains aluminium based. The available data base for the long term behaviour of copper on aluminium substrates shows some gaps in the investigations, especially for different metallisation types and thickness. In this paper the experimental results for bondability and reliability of fine copper bonding wire on different aluminium substrates wire will be presented. Diffusion behaviour and possible failure mechanisms will be discussed as well as the influence of thickness and composition of aluminium metallisation on the reliability of interconnection.