A series of quaternary ammonium compounds with different hydrophobic groups as spacers were synthesized. The influence of the hydrophobic groups on the additives for copper electrodeposition was investigated using a galvanostatic measurement on a RDE electrode. The increase of the length of the alkyl chain within the compounds enhances the suppressing capability. The introduction of a hydroxyl group or a phenyl ring into the hydrophobic groups in the molecular structures reduces the suppressing capacity. The results obtained from SEM measurements indicate that the presence of the quaternary ammonium compounds in the electrolytes may produce regular and relative leveling morphologies of copper deposits. The via-filling experiments without SPS exhibit that an almost complete filling was achieved for compound 1d (C-6 alkyl chain as a hydrophobic group), whereas a bad filling was obtained for compound 1a (C-3-OH chain as a hydrophobic group). With SPS, the vias were perfectly filled both for compound 1a and 1d. However, the initial increment of growth was still conformal for the compound 1a. These results indicate the influence of the hydrophobic groups of the quaternary ammonium compounds on the filling behaviors and they are in agreement with the galvanostatic measurements at different electrode rotation speeds. (c) 2014 The Electrochemical Society. All rights reserved.