A fabrication technology of the vertical ultrafine SiO2 wall masks using oxidation of sidewalls of a polycrystalline silicon (poly-Si) layer and Si nanowires utilizing the SiO2 wall masks have been developed. To obtain the vertical SiO2 wall mask, an optimum electron cyclotron resonance (ECR) plasma etching and a suitable wet etching of the poly-Si layer after the sidewall oxidation has been achieved. The vertical ultrafine SiO2 wall masks 10 nm wide and 90 nm high with 33 nm in space have been successfully fabricated for the first time. The dimensions of width and space become essentially smaller than the size of an electron beam resist pattern. The height is sufficient for ECR plasma etching. Si nanowires 10 nm wide and 18 nm thick have been precisely obtained by the ECR plasma etching of the thinned silicon-on-insulator layer using the vertical ultrafine SiO2 wall masks. The fabrication technology using the vertical ultrafine SiO2 wall masks has remarkable merits of fineness, scalability, and wide applicability. It can be utilized for fabricating various designed nanostructures. © 1999 American Vacuum Society.