Fabrication of sub-5nm gaps between metallic electrodes using conventional lithographic techniques

Abstract

The reproducible fabrication of nanoscale gaps below 5nm between metallic electrodes is key to the study of the electronic characteristics of individual molecules, but is hampered by the resolution limit and mechanical instabilities of commonly used electron-sensitive resists. We describe a fabrication process for the creation of nanoscale gaps between metallic electrodes based on conventional lithographic techniques. The process involves the patterning of a lithographic gap of 5–∼20nm between metallic electrodes on an oxidized silicon substrate. The SiO2 not covered by the electrodes is undercut and another metal film is thermally evaporated onto the substrate. Due to the slow buildup of material at the edges of the patterned electrode, the gap size can be reduced in a controllable way, and the final gap size is determined by the thickness of the evaporated metal film. This batch fabrication process is suitable for high-density fabrication of nanoscale gaps with the attractive feature that a self-aligned gate can be formed underneath the gap. We have investigated the effect of annealing samples for a short period at 125°C in air. Scanning electron microscopy data of a batch of identical gaps is presented which illustrates the variation in gap size and morphology after annealing. Gaps down to 1–∼2nm can be resolved directly using a scanning electron microscope. For gaps below 1nm, the separation between the two metallic electrodes cannot be resolved. To determine whether a tunnel gap is present, electrical measurements are required. Use of the Simmons tunnel model to fit an analytical curve to the measured IV characteristics of a gap gives a separation of 1.2±0.2nm and also verifies the consistency of parameters such as the effective barrier height in air indicating the presence of contaminants on the electrodes.