Additives-assisted "bottom-up" fill during copper metallization of semiconductor interconnects is analyzed experimentally and by numerical modeling. The rate of the bottom-up growth is measured as a function of the "suppressor" additive concentration on specifically designed test structures that provide a highly nonuniform suppressor concentration distribution. These test structures include (i) constant space-varying critical dimension (CD) structures that provide suppressor transport preferentially to the trenches at the periphery of trench clusters and (ii) constant CD-varying space structures that provide substantially enhanced suppressor transport to isolated trenches in comparison to trench clusters. Partial-fill experiments on these test structures indicate that the rate of bottom-up fill is diminished at locations such as isolated trenches or trenches located at the periphery of dense-feature clusters. Such locations are characterized by a relatively low surface area per unit volume for suppressor adsorption, and therefore, a localized zone with only a small suppressor concentration depletion is developed in their vicinity. Dense-feature clusters that develop a significantly large suppression depletion zone in their vicinity show accelerated bottom-up growth. The pattern-density-dependent modulation of the local suppressor concentration field and its subsequent impact on the bottom-up fill rate is illustrated by simple numerical modeling. (c) 2007 The Electrochemical Society.