Experimental study of field emission from ultrasharp silicon, diamond, GaN, and tungsten tips in close proximity to the counter electrode

Abstract

The patterning process in field-emission scanning probe lithography (FE-SPL), a high-resolution and cost-effective method for nanofabrication, is based on the field emission of electrons from ultrasharp tips in close proximity to a sample (distances below 100 nm). Thereby, the emitted electrons expose directly an ultrathin resist film. The field enhancement at the tip apex is crucial for the field emission current, which follows the Fowler–Nordheim theory. Despite the success of FE-SPL in nanofabrication, systematic experimental studies of the field-emission process, including the determination of the tip radius and tip-to-sample distance during the measurement, for these small tip-to-sample distances and different tip materials are lacking. To resolve this issue, experimental measurements of the field-emission current for tip–sample proximity distances below 100 nm were performed. For this purpose, the developed AFM in SEM system was modified,1,2 which enables one to monitor the tip–sample distance with a high accuracy using SEM while simultaneously recording the field-emission current. The authors present experimental results of the dependence of the field-emission current on the tip shape, tip material, applied voltage, and tip–sample distance. Therefore, the emission characteristics of silicon, diamond, GaN, and tungsten tips are shown. The knowledge about the field-emission process for small tip-to-sample distances will help to understand and improve the current FE-SPL, regarding also the choice of tip material. Furthermore, these measurements enable the detailed comparison with current FE models beyond state-of-the-art since all necessary parameters (voltage, current, tip diameter, and tip-to-sample distance) could be measured and controlled during the FE experiment due to the unique experimental system.