Device contamination induced by O2 plasma ashing of a photoresist was investigated using a capacitor structure that was fabricated by typical MOS processing. Emphasis was put on the process of ashing the ion-implanted resist and on the subsequent annealing process for the purpose of activating the ion-implanted atoms. It was found that annealing after the ashing process led to contamination of oxides under polysilicon gate films because of the rapid thermal diffusion of the impurities induced in the oxide by ashing. A contamination process model was proposed, and ion microprobe mass analysis showed that the impurities contaminating the gate oxide were sodium and potassium.