Wurtzite structure ZnO films doped with similar to 2 at% Cu were deposited at substrate temperatures (T-s) from 350 to 600 degrees C by helicon magnetton sputtering. All the films exhibited room-temperature (RT) ferromagnetism (FM) and the maximum saturation magnetization (M-s) was 1.2 emu/cm(3) (similar to 0.15 mu(B)/Cu). Cu ions were mainly in a divalent state as identified by X-ray photoelectron spectroscopy. FM tended to increase with decreasing T-s, and vacuum annealing enhanced the M-s. These results suggested that oxygen vacancies and/or zinc interstitials might contribute to the ferromagnetic performance. Thus, the observed FM was explained in terms of the defect related mechanism. (C) 2009 Elsevier B.V. All rights reserved.