Large area plasma-enhanced chemical vapor deposition of thin films such as silicon nitride or amorphous silicon is widely used for thin film transistor fabrication in the flat panel display industry. A numerical three-dimensional model to calculate the deposition uniformity over the whole electrode surface for rf rectangular showerhead reactors powered at 13.56 MHz is presented. The simulation tool is a commercially available finite-volume software (CDF-ACE (R)) which solves the multispecies, multireaction chemistry in capacitively coupled rf plasma. In order to simplify the three-dimensional geometry,the injected gas flow distribution across the showerhead is calculated separately and introduced as volumetric source terms for the gas flow and species continuity equations. The model is applied to the particular case of silicon nitride deposition and the results are compared with uniformity profiles obtained in an industrial plasma enhanced chemical vapor deposition reactor. Perturbations due to reactor edges together with nonuniform distribution of voltage due to standing wave effect are investigated as possible sources of the inhomogeneity of the thin film. (c) 2005 American Vacuum Society.