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

Here we report an optimized etching procedure for scanning probe microscope tips with: (1) minimized apical radius (for atomic resolution imaging); (2) low aspect ratio (for mechanical stability); (3) high symmetry (for accurate tip-sample capacitance measurements). Going beyond the standard focus on tip apex, we use finite element analysis to determine the optimal mm-scale aspect ratio for high mechanical resonance frequency. We dynamically etch tungsten (W) wires using an insulating barrier to preserve etch boundaries, and a stepper motor to precisely move the tip within a suspended droplet of sodium hydroxide (NaOH) solution. To achieve sharp apices, we implement a “pinch off” method where we abruptly increase the current in the final few seconds of the etch. We image the etched tips using both scanning electron microscopy (SEM) to quantify the nanoscale apical radius, and optical microscopy to quantify the macroscopic aspect ratio and symmetry. We observe a positive correlation between the final tip aspect ratio, and length of wire within the droplet at the end of etching. Our etching procedure enables efficient, reliable production of short, symmetric tips with apical radius < 50 nm, suitable for scanning tunneling microscopy (STM).