Structural tuning of nanogaps using field-emission-induced electromigration with bipolar biasing

We report a new approach for the fabrication of nanogaps using electromigration method induced by a field emission current. The method is so-called “activation” and is demonstrated here by a current source with alternately reversing polarities. The activation procedure with alternating current bias, in which the polarity of the current source alternates between positive and negative bias, is performed with planar nanogaps of Ni defined on SiO₂/Si substrates at room temperature. During negative biasing, a Fowler-Nordheim (F-N) field emission current flows from source to drain electrode. Therefore, Ni atoms at the tip of the drain electrode are activated and then are migrated across the gap from drain to source electrode. On the other hand, in the positive case, the field emission current moves the activated atoms from source to drain electrode. These two procedures were repeated until the tunnel resistance of the nanogaps was successively decreased from 100 TΩ to 48 kΩ. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) indicated that the separation of the gap became narrower from approximately 95 nm to less than a few nm, which is due to the Ni atoms accumulated at the tip of both source and drain electrodes. These results suggest that the alternately biased activation, which is a newly proposed atom transfer technique, can successfully control the tunnel resistance of nanogaps and is suitable for the formation of ultrasmall nanostructures of interest.