The heterogeneous nucleation and subsequent growth of chemical vapor deposited copper using hexafluoroacetylacetonate-Cu(I)-trimethylvinylsilane on physical vapor deposited tantalum and chemical vapor deposited titanium nitride was studied by means of electron microscopy and atomic force microscopy. It was found that the nucleation densities are about two orders of magnitude higher on TiN than on Ta. This leads to an increased roughness of films deposited on Ta compared to those produced on TiN. Moreover, the Cu films on the Ta substrate show a large number of voids, whereas no such voids were observable in the Cu films deposited on top of TiN. A simplified model for the influence of gas-surface reaction and surface self-diffusion on the shape of the Cu grains was developed. This model, which is supported by the experimental data, shows that if the grain shape changes from spherical to nonspherical before coalescence with neighboring grains, voids occur. A critical grain size and nucleation density of about 150 nm and 5 × 1013m-2, respectively, were calculated for the deposition conditions used in this work.