Electroplated Cu is being used by semiconductor manufacturers as an interconnect material because it offers lower line resistance and better electromigration performance compared to conventional Al metallization. The Cu electrodeposition process yields void and seam free filling of high aspect ratio trenches as a result of accelerated electrodeposition near the trench base, often known as "bottom-up'' fill. In order to evaluate the mechanism of bottom-up filling, the voltage-time and current-time characteristics of the Cu electroplating process during trench filling were studied. Depolarization (i.e., decrease of cathodic potential at fixed applied current! is observed during Cu electroplating into trenches using baths containing a mixture of current suppressing and accelerating organic additives. This depolarization behavior is explained by accumulation of current accelerating additives in the trenches, and correlates to the bottom-up fill observed during copper electrodeposition from electrolytes containing current accelerating and suppressing additives. A fill mechanism involving accumulation of strongly adsorbing current accelerating additives, which displace initially adsorbed current suppressing additives as surface area decreases within trenches, is consistent with all polarization data collected. (C) 2002 The Electrochemical Society.