Nanometer‐scale Si structures have been fabricated by anodic oxidation with an atomic force microscope (AFM) and dry etching using an electron cyclotron resonance (ECR) source. The AFM is used to anodically oxidize a thin surface layer on a H‐passivated (100) Si surface. This oxide is used as a mask for etching in a Cl2plasma generated by the ECR source. An etch selectivity ≳20 was obtained by adding 20% O2to the Cl2plasma. The AFM‐defined mask withstands a 70 nm deep etch, and linewidths∼10 nm have been obtained with a 30 nm etch depth. ©1995 American Institute of Physics.