A process has been developed for growing ultrathin oxide on a poly-silicon layer at low temperature by using electron cyclotron resonance (ECR) N2O plasma. Sub-4nm thick polyoxides on n(+) and p(+) polysilicon layers were grown and characterised. The oxides have large breakdown voltage and small, charge trapping. QBD values of up to 7C/cm(2) for polyoxide on p(+) polysilicon and up to 5C/cm(2) for polyoxide on n(+) polysilicon were obtained.