Revised electrochemical etching system for a reproducible fabrication of ultra-sharp tungsten tips

This article proposes an improved version of the drop-off electrochemical etching technique, developed for a reproducible production of ultra-sharp metal tips. Firstly, a comprehensive survey of other mechanical and physicochemical procedures is made in order to individuate three issues commonly neglected: (i) the influence of external perturbations, (ii) the positions of the electrode (a W wire) and counter-electrode (a Pt ring) and (iii) the counter-electrode shape. These issues need to be carefully fixed, in an enclosed and geometrically well-defined configuration, to better control the opening angle, radius of curvature and aspect ratio. The quality of the tips produced is evaluated comparing their performances, in imaging a reference sample with scanning tunnelling microscopy, to those of commercial tips. Using established procedures and data reported in the literature, an optimal set of the parameters can be identified obtaining high-quality tips with an opening angle < 10° and a radius of curvature ~ 30 nm. Furthermore, achieving a higher reproducibility allows us to investigate the electrochemical process monitoring the time dependence of the current from the etching parameters, such as the wire immersion, electrolyte concentration and applied bias voltage. Thus, the current versus time curves can be analysed in terms of linear scan voltammetry and electrochemical impedance spectroscopy, in order to investigate the phenomena occurring during the wire etching. In particular, at the beginning of the electrochemical process, the replacement of the native W oxide with a thicker layer of WO3(H2O)x oxide is observed, in agreement with previous observations.