The nanoscale modification of H-terminated Si(100) surfaces with an in situ electrochemical scanning tunneling microscope operating in an aqueous hydrofluoric acid (HF) solution is demonstrated. The modification was carried out in diluted HF solution by applying a train of positive potential pulses on the silicon surface while the feedback loop of the STM is interrupted in order to keep the tip-sample distance constant. Apparently depressed squares of 10 nm width and lines of 20 nm x 200 nm spaced at 100 nm were reliably made. The modifications produced are assumed to be silicon oxide induced by an electrochemical oxidation under the electric field between tip and surface.