The effect of reflected and secondary electrons on lithography with the scanning tunneling microscope

Abstract

The scanning tunneling microscope offers a unique tool for very high resolution, low voltage electron beam lithography. In the STM, most resist systems will use voltages in the range of 10 to 100 eV for exposure. At these voltages the instrument is operating in the field emission regime rather than the tunneling regime, and when the electrons strike the surface, they can either be reflected back from the surface, or generate secondary electrons that can also escape from the surface. The secondary and reflected electrons are subject to the high fields in gap between the tip and the sample, which causes most of them to return to the sample. These electrons are capable of causing additional resist exposure some distance away from the primary beam. We have modeled the current density as a function of position using a Monte Carlo program to follow the trajectories of random electrons. The results show that these electrons cause a background “fog” exposure of about 10% of the primary beam with a radius several times that of the primary beam.