GEOMETRICAL FIELD EFFECTS IN VOLTAGE PULSE FABRICATION OF NANOSTRUCTURES USING SCANNING-TUNNELING-MICROSCOPY

Voltage pulsed modification of surfaces in air with a scanning tunneli ng microscope has been studied with a view to understanding the physic al processes involved. Incremented negative pulses have been applied t o a tungsten tip to determine the threshold for feature writing on gol d. The primary event observed with virgin tips is pit formation, which is interpreted as due to the transfer of gold cations from sample to tip. Subsequent pulsing establishes a threshold for a secondary proces s in which hillocks form on the gold surface and which are thought to be the result of gold anion retransfer from tip to sample. The thresho lds are 4.16 and 3.92 V, respectively, for the two processes. For one particular tip, which gave rise to a Y-shaped pit, a three-dimensional profile was subsequently obtained by crashing it at a remote site on the surface and imaging the crater formed. The geometrical parameters of the tip, so elaborated, allowed a model of the electrostatic potent ial between tip and sample during pulsing to be numerically evaluated by solution of Laplace's equation and the field pattern over the surfa ce to be determined. For the blunt pyramid involved, there is substant ial anisotropy though modest field reduction at positions some nm from the projection of the tip on the surface. It is then a straightforwar d matter to understand the Y shape of the pit formed on the surface by the initial pulse. Field evaporation is thereby confirmed as the oper ative process. Reference experiments using gold tips showed no thresho ld difference between primary and secondary modification, a result con sistent with the field evaporation mechanism. (C) 1998 American Instit ute of Physics.