Sub-10 nm structures were fabricated by lift-off and by etching following electron-beam exposure of poly(methylmethacrylate) (PMMA) resist on solid semiconductor substrates. Electron beam lithography at 80 kV with a beam diameter smaller than 5 nm was used to expose PMMA resist on either Si or GaAs substrates. The exposed resist was developed with a 3:7 cellosolve:methanol mixture in an ultrasonic bath for 5 s followed by rinsing in IPA and blown dry with pure nitrogen. Ultrasonic agitation during development was found to be essential for forming sub-10 nm structures in PMMA. The patterned PMMA resist was used either as a lift-off mask or an etching mask and successful transfer of the pattern to the substrates was achieved. For lift-off an ionized beam deposition method, which gives smaller grain size and better adhesion of the metal film to the substrate, was used to deposit a layer of AuPd. Metal dots with sub-10 nm diam and metal structures with sub-10 nm gaps were fabricated. For sub-10 nm etched structures reactive ion etching was used to transfer either the PMMA pattern or the lift-off metal pattern to either Si or GaAs substrates. Etched lines and pillars with dimensions smaller than 10 nm were obtained.