Mechanical stress in atomic-layer deposition (ALD)-Al2O3 films was investigated at room temperature and during thermal cycling up to 870 degrees C. The films were generally under tensile stress. Thicker films (25-60 nm) showed a sharp stress increase at about 780-790 degrees C. X-ray diffraction (XRD)-, X-ray reflectance (XRR)- and X-ray photoelectron spectroscopy (XPS)-measurements indicate an irreversible phase transition from amorphous AlO(OH) to a mixture of different crystalline Al2O3-phases. Annealing at higher temperatures leads to a stress reduction as a result of diffusion and recovery processes. The stress behaviour of thinner films (< 20 nm) during thermal cycling is quite different. Tensile stress increases with increasing temperature and decreases to nearly the same value during cooling down. The process is continuous and reversible. (c) 2005 Elsevier B.V. All rights reserved.