100-nm-thick Cu layers were grown on MgO(001) substrates by ultra-high vacuum magnetron sputter deposition at substrate temperatures T-S ranging from 40 to 300 degrees C. X-ray diffraction omega-20 scans, omega-rocking curves, and pole figures show that layers grown at T-s=40 and 100 degrees C are complete single crystals with a cube-on-cube epitaxial relationship with the substrate: (001)(Cu)parallel to(001)(MgO) with [100](Cu)parallel to[100](MgO). In contrast, T-s >= 200 degrees C leads to polycrystalline Cu layers with 001, 203, and 175-oriented grains. The transition from a single- to a polycrystalline microstructure with increasing T, is attributed to temperature-induced mass transport that allows Cu nuclei to sample a larger orientational space and find lower energy (and/or lower lattice mismatch) configurations. The large Cu-to-MgO lattice mismatch of 14% is relieved by 7 x 7 Cu unit cells occupying 6x6 MgO cells. In addition, for T-s >= 200 degrees C, the 001-oriented grains relax by tilting by 4 degrees or 15 degrees about 110 or 100 axes, respectively, while the 203 and 175-oriented grains exhibit complex epitaxial relationships with the substrate: (203)(Cu)parallel to(001)(MgO) with [010](Cu) parallel to[110](MgO) and [302](Cu)parallel to[110](MgO); and (175)(Cu)parallel to(001)(MgO) with [211](Cu)parallel to[100](MgO) and [435](Cu)parallel to[101](MgO). The surface roughness, as determined by X-ray reflectivity, increases with growth temperature. The smoothest layers are grown at 40 degrees C and exhibit an rms surface and buried interface roughness of 0.7 and 1.4 nm, respectively. (c) 2006 Elsevier B.V All rights reserved.