The electrochemical etching of tungsten STM tips

The achievement of high resolution in scanning tunnelling microscopy (STM) relies on the use of sharp, high-aspect ratio tips which are usually produced from electrochemical etching of metals such as tungsten or platinum/platinum–iridium alloys under dc or ac conditions. A few general reviews on tip fabrication methods have appeared in the literature 1, 2, 3, 4 and 5, but many STM publications omit crucial details on the tip preparation method for their studies. Where they are reported, very little attention is given to optimizing the methods used to produce tips of high aspect ratio and small radii of curvature. Recently, much effort has focused on the use of two-step fabrication techniques; the first step to produce a fine point using electrochemical etching, followed by a second process, usually ion-milling, to obtain the sharp tip apex [6]. For many applications, including those where atomic resolution is not required, two-step fabrication methods are often unnecessary and electrochemical etching alone will produce sufficiently sharp tips. This short communication reports the findings of a systematic study to address the relative importance of various factors affecting one-step electrochemical etching of tungsten wire to produce sharp tips under steady-state dc conditions. The aim was to define conditions where the etching rate was high while achieving acceptably sharp tips.