A sidewall image transfer process for forming sub-lithographic structures employs a layer of sacrificial polymer containing silicon that is deposited over a gate conductor layer and covered by a cover layer. The sacrificial polymer layer is patterned with conventional resist and etched to form a sacrificial mandrel. The edges of the mandrel are oxidized or nitrided in a plasma at low temperature, after which the polymer and the cover layer are stripped, leaving sublithographic sidewalls. The sidewalls are used as hardmasks to etch sublithographic gate structures in the gate conductor layer.