The use of a high-pressure scanning tunneling microscope as a lithography tool for modifications of amorphous hydrogenated carbon films

Abstract

The use of a modified scanning tunneling microscope (STM) working in controlled oxygen atmospheres to create structures on the surface of a steel sample covered with 10 nm amorphous hydrogenated carbon (a-C:H) is presented. The STM lithography experiments showed the capability of producing either μm sized hills or holes depending on the oxygen pressure and the other parameters such as feedback-settings, bias-voltage, and setpoint-current. It will be shown that the hills were hollow and consisted of a locally delaminated a-C:H film induced by a dielectric breakdown between tip and sample. This explains the observed mobility and elasticity of the hills. Assuming total relaxation of the a-C:H film within the hills yielded the ratio h/d=0.07–0.1. The measured geometries of the hills were in good agreement with the estimated ratios. The resistance of a hill formed by the local delamination of a-C:H was measured, yielding a semiconducting behavior. The gap voltage and the total resistance decreased irreversibly, which was interpreted as a transformation of the a-C:H film within the hill to a more graphitic state. Holes were only created when all of the following conditions were fulfilled: (a) O2 pressure greater than 1 bar, (b) low energy density dissipated in the a-C:H surface (no dielectric breakdown), (c) positive bias voltage ⩾13 V between the sample and the tip. The Fowler–Nordheim plots log(I/U2) vs 1/U for the lithography experiments that led to holes were characteristic for field emission of electrons from the tip. From this it was concluded that the process responsible for hole formation is reactive oxygen etching. Other possible processes have been excluded experimentally and theoretically, respectively. For lithography experiments performed with the feedback system on, it was possible to find lithography parameters leading to a reproducible creation of holes.