Electrode Samples

The substrate used in the study was a highly doped Si wafer which has a thermally grown silicon dioxide (SiO2) layer with a thickness of ~200 or ~300 nm on the surface. Double-layered metal (Au/Ti) and triple-layered metal (Ti/Au/Ti) films were deposited on the SiO2 surfaces using Ar sputtering. The Au film was used as an electrode. The bottom Ti film was necessary to obtain good adhesion of the Au film to the underlying SiO2 surface. The top Ti film in the Ti/Au/Ti structure was prepared to prevent excess etching of the underlying Au film by FIB tails during the FIB etching steps. This was effective because Ti has a much lower sputter rate than Au. The thicknesses of the top and bottom Ti films were ~6 nm and 1–2 nm, respectively. The thickness of the Au film was varied from ~10 to ~30 nm to examine the influence of the electrode thickness on the nanogap fabrication. As shown in Fig. 1, ~5–7-μm wide and ~3-μm long patterns with ~100-μm wide leads and contact pads were defined in the layered metal films by using photolithography and Ar sputter etching.

Fabrication Processes

A schematic diagram of the nanogap electrode fabrication setup is shown in Fig. 2. This setup consists of an FIB system (SII Nano Technology Inc., JFIB-2300) with an auxiliary beam blanking circuit, voltage source, and ammeter. The minimum spot size (full width at half maximum ([FWHM]) of the FIB generated from a 30-keV Ga ion source was ~12 nm. A current density of ~1 A/cm2 was used. A constant DC voltage ranging from 50 to 200 μV was applied to the sample to monitor the gap formation etch step.
Nanogap fabrication using FIB sputter etching involves two steps. In the first step, the FIB was irradiated in U-shaped pattern #1 (Fig. 1) to form nanowires with a width of a few tens of nanometers and a length of 100–200 nm in the multilayered metal films. The etching step was monitored in situ by measuring the current in the sample, which decreased with the progress of the etching step. To correct the beam drift during the etching, the beam position was corrected periodically according to a spot marker, which was formed by FIB etching just before the first etching step. In the first etching step, the etching was terminated manually after the formation of the nanowire. In the second step, the FIB was irradiated in pattern #2 (Fig. 1) to form a nanogap in the nanowire with a single line scan. The nanogap formation step was terminated by blanking the FIB automatically. The sample current or etching depth at which the FIB was blanked was preset by a reference voltage, Vref. By the in situ monitoring of the etching steps, the gap width can be reliably controlled, and the etching in the underlying substrate can be minimized.
The fabricated nanogap electrodes were observed using a scanning electron microscope (SEM) (JEOL Ltd., JSM-6700F). The width and electrical resistance of nanogaps were estimated from SEM images and DC measurements, respectively.