Shape Evolution of Electrodeposited Copper Bumps with High Peclet Numbers

Abstract

We report the shape evolution of initial copper bumps at Peclet numbers higher than a hundred. The role of vortices and of penetration flow within the cavity was discussed with numerical fluid dynamics computation to obtain a bump with a single hump at the center. The current distributions, or flux profiles, were calculated at the diffusion controlled overpotential and were compared with the electrodeposited bump shapes. For the 100 μm cavity width, the vortices increase at the upstream corners with Peclet numbers 1410 and 7311. The vortices are the local resistance of mass transfer to the cathode. These vortices cause the hollows in flux profiles at the upstream corner with these Peclet numbers. The penetration flow collides with the photoresist sidewall and the vortices decrease at downstream corners. These decreased vortices cause the increase in flux profile at downstream corners. For a 30 μm cavity width a single large vortex forms for the higher Peclet number 44,500 and a single hump in flux is achieved.