In this paper, we analyze in detail the etching parameters that are used for
manufacturing gold tips for tip-enhanced near-field optical microscopy and
spectroscopy. From the current variation versus time, we first demonstrate
that the sharpest tips are obtained if the etching is stopped at a specific
time where the curve presents an inflexion point. We then analyze the
influence of the concentration of the etching solution and the applied
voltage on the roughness and the tip apex. As a result, we propose a set of
parameters allowing for reproducible production of gold tips with 20 nm
radius of curvature.