Diffusion barrier property of Co-W layer compared with Ni layer in fine pitch micro-bumps during high temperature storage

Co-W alloy has the potential to replace Ni barrier layer in Cu/Sn solder joints of advanced electronic packaging due to its excellent electromigration resistance and diffusion barrier property. Nevertheless, little attention has been paid to the fine pitch micro-bumps on the diffusion barrier consumption. In this work, Sn/Co-9at. %W/Cu and Sn/Ni/Cu micro-bumps with a diameter of 12 mu m were fabricated by electrodeposition and the consumption of the barrier was investigated during solid stage aging at 150 degrees C and 200 degrees C. The consumption of the Co-W barrier was accompanied by the growth of the layered amorphous Co-W-Sn. The depletion of Ni barrier was at least 4 and 2 times faster than the Co-W barrier at 150 degrees C and 200 degrees C, respectively. Compared with Sn/Co-W/ Cu planar diffusion couple, the consumption rate of Co-W barrier layer in micro-bumps did not increase. A theory of amorphous Co-W-Sn layer without grain boundary and high melting point W reducing the diffusion coefficient was proposed to explain the eliminated scaling effect.