Optimization of electrochemical etching procedure to produce atomically sharp tungsten scanning tunneling microscopy tips.

Scanning tunneling microscopy (STM) utilizes quantum tunneling to produce topographic images of a sample’s surface with atomic resolution by scanning a tip across the surface of the sample. To produce an image with atomic resolution, STM must use tips with symmetric geometry and a single-atomic apex . Here, we study the optimization of an electrochemical etching procedure to reliably and efficiently produce tungsten STM tips with a controlled geometry and sharp apex (<50 nm). The set up is dynamic with a step-motor to precisely move the tip before and during etching. The tip is etched within the meniscus of sodium hydroxide suspended in a single ring. Insulation is used on the wire to preserve clean etching boundaries. The dynamic etching method allows vertical displacement and a controlled feed rate to be tracked as etching parameters during the process. Using systematic etching trials and measurements using scanning electron microscopy, we studied the relationship between etching parameters and the resultant tip shape. We observed that the ratio between the initial displacement of the wire in the etching process and the feed rate applied to the displacement of the tip while etching is directly tied to the length of the tip produced. This optimized etching procedure allows a controlled method for producing short, symmetric tips while retaining a near atomically sharp apex that is suitable for STM.