Zinc oxide (ZnO) films have been deposited on 1 mu m SiO2/Si (100) substrates by rf magnetron sputtering. Using a sputtering gas of pure oxygen, a pressure regime is found in which the ZnO films grow on room temperature substrates with a single (0001) orientation, small grains (crystallite sizes similar to 10-15 nm), and high intrinsic biaxial compressive stress (similar to 6 GPa). The effects of post-deposition annealing these films in air was investigated over a range of temperatures (200-1000 degrees C) and durations (2-2000 min). Annealing resulted in lower biaxial compressive stresses and increased average crystallite sizes in all films. Additional ZnO grain orientations were detected only after annealing above 500 degrees C for longer than 90 min, and the results are interpreted in terms of film recrystallization. Consequently, a relatively rapid thermal anneal at 1000 degrees C for 5 min caused grain recovery without recrystallization, resulting in maximum stress reduction (90%-100% of stress was relieved and average crystallized size tripled) while maintaining the original film orientation. The film surface area - measured by atomic force microscopy-decreased by up to 25% during annealing. X-ray photoelectron spectroscopy results indicate that although the surfaces of as-deposited films have a slight excess of oxygen, annealing as low as 200 degrees C results in a stoichiometric ZnO surface. High values of electrical resistivity (similar to 10(5) Omega cm) measured across the thickness of unannealed oriented films indicate low levels of elemental zinc clusters in the film bulk.