STM tips fabrication for critical dimension measurement

In this contribution a method is described for sharpening Tungsten (W) tips through a two-step electrochemical etching. In the first step, under strong reaction conditions, we obtain a long hyperbolic cone, while in the second with a micropositioner we bring only the apex of the first cone in contact with a thin film of etchant. In this way, controlling the meniscus height with an optical microscope, only the very end of the tip is etched. Some processing parameters such as the rate of the electrochemical reaction of erosion of the W wire, related to the electrolyte concentration and to the applied voltage, the length of the wire immersed in the solution and the shape of the meniscus have been investigated. Both direct current (DC) and alternating current (AC) were tested, observing two different ways of W wire erosion. The fabrication process provides very sharp tips, tips with radius of curvature below 10nm and cone angle aperture within 30° have been obtained in some cases. Further improvements are in progress, namely to extend the tip-shape repeatability as given by the two-step process. Some promising STM images of diffraction gratings have been obtained using tips fabricated with this process.