A simple method for estimating charge buildup on gate oxide during plasma processing is proposed. This method is based on the observation that metal oxidation rate at the SiO2–metal interface is accelerated by the electric field formed by electrons diffused from the SiO2surface to the metal surface during plasma processing. The thickness of the metal oxide caused by electron‐induced charge buildup can be estimated by ellipsometry analysis. We applied the method to biased electron cyclotron resonance (ECR) plasma deposition of SiO2, compared the metal oxidation enhancement for biased ECR plasma deposition with that for exposing electron beams directly and also compared with the injected charge densityQinjfor gate–oxide breakdown during biased ECR plasma deposition evaluated by time‐dependent dielectrics breakdown measurements. In this way, we clarify that (i) the oxidation enhancement takes place at an electron‐beam exposure of over 10 C/cm2, and (ii) theQinjof over 10 C/cm2is closely related to gate–oxide breakdown. These results confirm that the metal–oxide thickness is strongly associated with the degradation in gate–oxide yield related to the injected electrons and that the charge buildup damage is caused by electrons diffused during biased ECR plasma deposition.