To improve the achievable uncertainty of high resolution microscopes (Scanning Probe or Scanning Electron) attempts have been made to manufacture calibration standard on the basis of a new principle. It has been intended to couple nanostructures produced by a STM that reaches atomic resolution to the regular lattice of a crystal by counting the atoms between the deposition positions. Then their lateral dimensions and spacings can be given as multiples of the lattice parameter. To archive small uncertainties the reproducibility of the nanostructuring method used must not exceed a few nanometers. Therefore nanostructuring by means of applying an additional voltage impulse (amplitude UP , duration TP ) between tip and sample in order to deposit tip material on the sample surface directly were chosen and carried out under ultrahigh-vacuum conditions. Using electrochemical etched tungsten and gold tips and Si(111)-7x7 samples of different doping levels the dependencies of the nanostructures' size on the impulse parameter (UP, TP) and the tunneling current I in the set point were investigated systematically. In addition, the signal of tunneling current I and of the z-piezo Urz were recorded. The atomic resolution of the tip and the structure of the surface have been preserved as required for orientation to the lattice of the crystal surface. By increasing the impulse amplitude UP from 3 to10V the diameter of the deposits increased linearly from 3 to 25 nm for gold tips and from 2 to 10nm for tungsten tips. The sample doping has only an insignificant influence on the structure size. The dependence of the structure diameter on the tunneling current I is logarithmic. Together with the occurrence of a minimum pulse amplitude, this points to a field-induced deposition mechanism. Furthermore, the evaluation showed that the structure volume, which is proportional to the number of atoms deposited, depends radically on the impulse duration Tp and the time between the deposition processes. These dependencies, which had not been observed before, allowed the diffusion-assisted field desorption of tip adsorbates to be identified as the deposition mechanism.
Proceedings Title: Technical Program for the 4th IASPM Conference
Pub Type: Conferences
nanostructuring, silicon, UHV-STM