In situ scanning tunneling microscopy (STM) was used to study copper deposition at vacancy defects on a copper thin film under potentiostatic conditions at -0.20 V (vs. Ag/AgCl) in a formula containing sulfuric acid, chloride, polyethylene glycol (PEG), and bis-3-sodiumsulfopropyl-disulfide (SPS) - the widely used mixture to facilitate Cu superfilling at recessed features in semiconductor processing. The vacancy island measuring ~70 nm wide and 12 nm deep sat in the middle of a facetted surface structure at the beginning. Cu deposit nucleated mainly at the rim of the vacancy and grew into stacked Cu(111) facets. These local pyramidal Cu stacks could restructure into wider Cu(111) terraces by transferring Cu atoms rapidly from higher to lower planes. Voltammetric results showed that Cu deposition was suppressed in a plating bath containing 1 mM KCl + 88 μM PEG8000 + 10-7 M SPS. Steps with sharp edges bunched in the course of Cu deposition. The vacancy island was filled with Cu deposit assuming smooth terraces with sharp step edges aligned mainly in the 121 directions of the Pt(111) electrode, suggesting crystalline packing in the Cu deposit. Atomic-resolution STM imaging revealed a hexagonal array presumed to be the (v3 × v3)R30? - Cl- adlattice.