Fine pitch Cu wire bonding — As good as gold

Fine pitch gold wire bonding is the pre-eminent technology for die interconnection and has been advanced to very fine wire diameters. Copper wire bonding has been used for many years as well but was relegated to large wire diameters in automotive and power applications. The surge in gold commodity prices to more than 1,000 USD has fueled great interest in converting fine wire packaging from gold to copper. For course wire bonding, the challenges of harder and stiffer wire metallurgy, the propensity of oxidation and corrosion have been managed successfully. Fundamental studies had determined that intermetallic growth was an order of magnitude less than for gold and that the analysis for intermetallic coverage was considerably more difficult than in the case of gold. Additional challenges have arisen by the advancement of wafer nodes which have employed ever more brittle dielectrics and complex metal stack structures. Here, the development of high volume, fine pitch copper wire bonding will be described. Many of the coarse wire approaches can be adapted to fine pitch copper bonding with success. A rigorous evaluation and qualification methodology has been adopted to ensure highly efficient processes with yields equivalent to gold bonding and superior bonder machine efficiencies. At present devices of the 65 nm nodes are in production and 40/45 nm nodes are in qualification. Extended JEDEC type reliability testing has been performed to demonstrate the long term reliability of copper wire bonding.