On the electrochemical etching of tips for scanning tunneling microscopy

The sharpness of tips used in scanning tunneling microscopy (STM) is one factor which affects the resolution of the STM image. In this paper, we report on a direct-current (dc) drop-off electrochemical etching procedure used to sharpen tips for STM. The shape of the tip is dependent on the meniscus which surrounds the wire at the air–electrolyte interface. The sharpness of the tip is related to the tensile strength of the wire and how quickly the electrochemical reaction can be stopped once the wire breaks. We have found that the cutoff time of the etch circuit has a significant effect on the radius of curvature and cone angle of the etched tip; i.e., the faster the cutoff time, the sharper the tip. We have constructed an etching circuit with a minimum cut-off time of 500 ns which uses two fast metal–oxide semiconductor field effect transistors (MOSFET) and a high-speed comparator. The radius of curvature of the tips can be varied from approximately 20 to greater than 300 nm by increasing the cutoff time of the circuit.