Nanostructures with lateral dimensions below 10 nm are fabricated on Si(111)7X7 surfaces by scanning tunneling microscope induced dissociation of the precursor gases SiH4 and SiH2Cl2. The molecules are activated between the tunnel tip and the surface, where highly reactive species are produced by the interaction of molecules with free electrons in the tunnel gap. The molecular fragments accumulate directly below the tip and build up the nanostructure. Information on the deposition mechanism that is effective at the onset of deposition is obtained by careful evaluation of the deposition voltage threshold regime.