The effects of substrate heating on the stoichiometry and the electrical properties of pulsed dc reactively sputtered tantalum oxide films over a range of film thickness (0.14 to 5.4 μ m) are discussed. The film stoichiometry, and hence the electrical properties, of tantalum oxide films; e.g., breakdown field, leakage current density, dielectric constant, and dielectric loss are compared for two different cases: (a) when no intentional substrate/film cooling is provided, and (b) when the substrate is water cooled during deposition. All other operating conditions are the same, and the film thickness is directly related to deposition time. The tantalum oxide films deposited on the water-cooled substrates are stoichiometric, and exhibit excellent electrical properties over the entire range of film thickness. "Noncooled" tantalum oxide films are stoichiometric up to ∼ 1 μ m film thickness, beyond that the deposited oxide is increasingly nonstoichiometric. The presence of partially oxidized Ta in thicker (> ∼ 1 μ m) noncooled tantalum oxide films causes a lower breakdown field, higher leakage current density, higher apparent dielectric constant, and dielectric loss. The growth of nonstoichiometric tantalum oxide in thicker noncooled films is attributed to decreased surface oxygen concentration due to oxygen recombination and desorption at higher film temperatures (>∼ 100 ° C). The quantitative results presented reflect experience with a specific piece of equipment; however, the procedures presented can be used to characterize deposition processes in which film stoichiometry can change. © 2005 American Vacuum Society.