To improve the via fill capability of electroless (EL) Cu, we explored the influence of the Pd activation process prior to EL Cu deposition. We found that EL Cu roughness is closely related to the Pd nuclei size range and density, which are functions of I'd activation time. With activation time, Pd deposition goes through the sequential stages of growth, secondary nucleation, and ripening. The smoothest EL Cu film can be achieved with a Pd nucleation time that corresponds to the beginning of the ripening stage. A Pd activation process that leads to the smoothest EL Cu deposit also yields the most conformal via filling. (c) 2006 Elsevier B.V. All rights reserved.