The art of electrochemical etching for preparing tungsten probes with controllable tip profile and characteristic parameters.

Using custom made experimental apparatus, the art of electrochemical etching was systematically studied for fabricating micro/nano tungsten probes with controllable tip profiles of exponential, conical, multidiameter, and calabashlike shapes. The characteristic parameters of probe including length, aspect ratio, and tip apex radius could also be well defined. By combining of static and dynamic etching, the conical-shape probe with length up to several millimeters, controllable tip apex radius, and cone angle could be fabricated. In addition, by continuously lifting the tungsten wire up during the electrochemical etching with different speeds and distances, the multidiameter shape probe could be fabricated. Finally by controlling the anodic flow, the multiple "neck-in" could be realized creating a calabashlike probe. The aspect ratio of probes depends on (i) the effective contact time between the surrounding electrolyte and the wire, (ii) the neck-in position of immersed tungsten wire. Under the optimized etching parameters, tungsten probes with a controllable aspect ratio from 20:1 to 450:1, apex radius less than 20 nm, and cone angle smaller than 3° could be achieved. The technique is well suited for the tungsten probe fabrication with a stabilized stylus contour, ultra-sharp apex radius, and high production reproducibility. The art for preparing microprobes will facilitate the application of such microprobes in diverse fields such as dip-pen nanolithography, scanning probe microscopy, micromachining, and biological cellular studies.