The Coming of Copper UBM

Several developments in the past few years have solved the numerous problems associated with using copper metal in place of aluminum as the IC interconnect metal. Copper is about three times more conductive than aluminum, and allows higher frequencies to be used with smaller line widths. Many fabricators are converting to copper not just for speed, but also for cost reduction. Thinner conductors allow closer spacing and smaller chips. The switch to copper allows many times more dies per wafer, and this is where the savings come from.
Since copper is much more compatible with bump metals than aluminum, the transition to copper is expected to boost flip chip technology, possibly by eliminating the UBM step. It remains to be seen what the final finish will be for copper ICs. The industry will continue to use wire bonding as the main interconnect method, so the final pad must be compatible with gold wire bonding.
IBM has indicated that its copper chips will have aluminum as the final pad layer to accommodate wire bonding, and this may become standard practice. However, aluminum can be removed easily, without affecting the copper underneath. Aluminum is an amphoteric metal that can be dissolved in both acid and base. Dilute caustic (sodium hydroxide) quickly removes aluminum. Many other reagents also can be used.
Thus, even if new copper chips come with an aluminum finish, a simple aqueous washing step will unveil the desired copper layer. In fact, the aluminum over copper would serve to protect the copper from oxidation. Gold over nickel could also be used on copper pads similar to PWB common finishes. This too would be a very good surface for most bumps.
Conductive adhesives would receive a real boost in the switch to copper because none are compatible with aluminum. Some of the conductive adhesives form reasonably stable junctions with bare copper, especially those using an oxide-penetrating mechanism. Even for those adhesives which are not suitable for bare copper, simple UBM methods could be used. Silver and other finishes can be applied to copper by electroless, maskless plating.
The advent of copper-based chip interconnection metallurgy undoubtedly will simplify FC fabrication in the near future. It will probably be possible to directly bond the copper pads with conductive adhesives. This simple processing ability would have a great impact on cost and infrastructure issues by eliminating UBM and maybe the bumping step. Assemblers could run the entire FC preconditioning and bonding process.