Polysilicon thin-film transistors (poly-Si TFT's) with thin-gate oxide grown by electron cyclotron resonance (ECR) nitrous oxide (N2O)-plasma oxidation is presented, ECR N2O-plasma oxidation successfully incorporates nitrogen atoms at the SiO2/poly-Si interface, consequently forms a nitrogen-rich layer with Si = N bonds at a binding energy of 397.8 eV, ECR N2O-plasma oxide grown on poly-Si films shows higher breakdown fields than thermal oxide. The fabricated poly-Si TFT's with N2O-plasma oxide show better performance than those with ECR O-2-plasma oxide, which results not only front the smooth interface but also oxygen- and nitrogen-plasma passivation.