We demonstrate a new and simple process to fabricate tungsten tips with good control of the tip profile. In this process, we use a commercial function generator without any electronic cutoff circuit or complex mechanical setup. The tip length can be varied from 160 μm to 10 mm, corresponding to an aspect ratio of 1.6–100. The radius of curvature of the tip apex can be controlled to a size <10 nm. Surface roughness and the taper angle can be controlled independently. Through control of the etching parameters, the tip length, the radius of curvature, surface roughness, and the taper angle can be controlled to suit different requirements of various applications. The possible etching mechanisms are also discussed.

Method of electrochemical etching of tungsten tips with controllable profiles

Advances in polishing of optical freeform surfaces: A review

Ultraprecision micro-structured functional surfaces on hard and brittle materials, e.g. ceramic and glass, are gaining increasing application in a range of areas such as engineering optics and semiconductor and biomedical products. However, due to their tendency of being damaged in brittle fracture in machining, it is challenging to achieve both a high surface finish and complex surface shapes. In this paper, ultraprecision machining of micro-structured functional surfaces on brittle materials by fast tool servo diamond turning is studied. A machining model has been developed to ensure ductile regime machining of the brittle material, in which the material is removed by both plastic deformation and brittle fracture, but the cracks produced are prevented from being extended into the finished surface. Based on the model, an iterative numerical method has been proposed to predict the maximum feed rate for producing crack-free micro-structured surfaces. Machining experiments on typical micro-structured functional surfaces have been carried out to validate the effectiveness of the proposed method for producing ultraprecision micro-structured functional surfaces.

http://iopscience.iop.org/article/10.1088/0960-1317/21/9/095011/pdf

Ultraprecision machining of micro-structured functional surfaces on brittle materials

Dynamic electrochemical etching technique is optimized to produce tungsten tips with controllable shape and radius of curvature of less than 10 nm. Nascent features such as “dynamic electrochemical etching” and reverse biasing after “drop-off” are utilized, and “two-step dynamic electrochemical etching” is introduced to produce extremely sharp tips with controllable aspect ratio. Electronic current shut-off time for conventional dc “drop-off” technique is reduced to ∼36 ns using high speed analog electronics. Undesirable variability in tip shape, which is innate to static dc electrochemical etching, is mitigated with novel “dynamic electrochemical etching.” Overall, we present a facile and robust approach, whereby using a novel etchant level adjustment mechanism, 30° variability in cone angle and 1.5 mm controllability in cone length were achieved, while routinely producing ultra-sharp probes.

Two-step controllable electrochemical etching of tungsten scanning probe microscopy tips.

This paper describes an electrotactile display with micro-needle electrodes. The electrotactile display can produce tactile sensations by stimulating tactile receptors using an electric current. Micro-needle electrodes can drastically decrease the threshold voltage required to stimulate tactile receptors by penetrating the stratum corneum, which has a higher impedance than the dermis. In addition, the optimized length of the needle allows us to stimulate tactile receptors painlessly. In the present study, we developed a process for fabricating a micro-needle array in which the length and tip radius can be controlled using electrochemical etching. A micro-needle array was successfully fabricated to form an electrotactile display. In addition, we experimentally determined the suitable shape of the micro-needle electrodes for electrotactile display applications. When the tip radius of the needle is too small, the impedance between the finger and micro-needles becomes large due to the small contact area. On the other hand, when the tip radius is too large, the needle cannot penetrate the skin surface and the impedance is not sufficiently small. The experiments verified the superiority of needle electrode devices to flat electrode devices with respect to the threshold voltage at each frequency.

http://iopscience.iop.org/article/10.1088/0960-1317/25/2/025016/pdf

Electrotactile display using microfabricated micro-needle array

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 ...

Yaozheng Ge

Papers

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

A reproducible electropolishing technique to customize tungsten SPM probe: From mathematical modeling to realization