ZnO nanowires along with ZnO thin films were obtained on copper-metallized silicon substrates using an radio frequency-reactive sputter-deposition technique. Residual tensile stresses were found in both the copper layer and the ZnO layer. The ZnO nanowires were observed exclusively at the grain boundaries of the ZnO thin films. The average diameter of ZnO nanowires varies only slightly with the ZnO deposition time, while the average length increases linearly with the ZnO deposition time. Based on the observations a growth model involving stress-assisted diffusion of copper and reaction-controlled catalytic growth of ZnO nanowires is suggested.