Maskless fabrication of nanogap electrodes by using Ga-focused ion beam etching

Abstract

We develop maskless fabrication methods using sputter etching with Ga-focused ion beams (FIBs) to obtain nanogap electrodes with high reproducibility. This method is based on in situ monitoring of etching steps by measuring current through patterned electrode films. The etching steps are terminated electrically at a predetermined current level. In the present experiment, 30-keV Ga FIBs with beam size of ~12 nm is irradiated, and the effect of film structures and monitoring current is investigated to obtain reliable fabrication methods. We find that electrode gaps much narrower than the beam size can be reproducibly fabricated. The controllability of the fabrication steps is significantly improved by using triple-layered films consisting of a thin Ti top, an Au electrode, and a bottom Ti adhesion layer. The minimum gap width achieved is ~3 nm, and the fabrication yield reached ~90% for ~3- to 6-nm-wide gaps. Most of the fabricated nanogap electrodes show high insulating resistance ranging from 1 G to 1 T.