Schematic views of (a) the detailed anodic film flow during electrochemical etching of tungsten wires, (b) static electrochemical etching, and (c) dynamic electrochemical etching. The dynamic electrochemical etching is a combination of static etching with the lifting-up process, in which lifting-up speed and distance define the final shape of the fabricated probes.
The experimental apparatus for electrochemical etching with controllable tip profile and characteristic parameters. (a) Schematic view of the apparatus, (b) apparatus photo during the electrochemical etching.
Images of the fabricated exponential shape probe under different etching conditions: (a) etching time 950 s, voltage 5.0 V, electrolyte concentration 2.5 mol./L, immersed depth 1.0 mm, aspect ratio 20:1; (b) etching time 1200 s, voltage 4.5 V, electrolyte concentration 2.0 mol./L, immersed depth 800 μm, aspect ratio 130:1; (c) etching time 1620 s, voltage 3.0 V, electrolyte concentration 1.0 mol./L, immersed depth 600 μm, aspect ratio 450:1.
Scanning electron microscope (SEM) images of the fabricated probe under the optimized experimental condition which is suitable for high aspect ratio probe of voltage 3.0 V, electrolyte concentration 1.0 mol./L, immersed depth of 600 μm. (a) the finally patterned probe with high aspect ratio nanometric tip; (b) the detailed characteristics of the tip. The inset shows its radius and conical angle, ×50 000.
Probes fabricated by the combination of static and dynamic electrochemical etching. Initially, the static etching was applied, and after 5, 6, and 7 min respectively, dynamic etching was used to achieve long and conical shape probes. The tungsten wire was lifted up at the velocities of (1) 3.0 μm/s, (2) 3.5 μm/s, and (3) 4.0 μm/s.
The image of multidiameters shape probe that fabricated by four lifting-up procedures. The tungsten wire was lifted up at the velocity of 10.0 μm/s.
(a) The relationship of aspect ratio as well as tip apex radius with applied etching voltage during the electrochemical etching under the experimental condition of electrolyte concentration of 1.5 mol./L, immersed depth of 600 μm. (b) The relationship of aspect ratio as well as tip apex radius with electrolyte concentration during the electrochemical etching under the experimental condition of applied etching voltage of 3.3 V, immersed depth of 600 μm. (c) The relationship of aspect ratio as well as tip apex radius with immersion depth during the electrochemical etching under the experimental condition of applied etching voltage 3.3 V, electrolyte concentration of 1.5 mol./L.
The effect of the cathode (platinum ring) position and its contribution to the final shape of probe (a) cathode position is at the same level of air–solution interface; (b) cathode position is 1.0 mm below the air–solution interface, and (c) cathode position is 2.0 mm below the air–solution interface.
By controlling the anodic flow, the multitimes neck-in can be realized and calabashlike shape probe can be customized. The inset shows the SEM image of the detailed tip profile and functional characteristics of the calabashlike shape probe, ×50 000.
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