Metal wires, electrochemical etching

A needle source consists of a hairpin filament (M80 pm diameter), usuafly of a refractory metal such as tungsten, with a short length of smaller diameter (M25 pm) wire spot-welded to it, Figure lb. The tip of the latter wire, the emitter, is electrochemically etched to a point with a radius of curvature at the apex of 2-5 pm the etching technique for tungsten has been described in detail by others (7,29). As quickly as possible after the assembly is thermally cleaned under vacuum (n<10 " Pa), the emitter is dipped into a molten pool of liquid metal and then withdrawn. If done correctly, the junction formed by the bend in the filament and the emitter wire will hold a small bead of metal, and the emitter will appear shiny from the thin film of metal on its surface. 

Reduction of the size of SECM probes is important, since the resolution of SECM depends strongly on the size of tip used. In order to achieve this goal, one has to thin down the diameter of commercially available metal wires by electrochemical etching or by pulling in glass capillary as described below. Further coating and exposing steps are then necessary, as in the case of unetched wires, to expose only the electrode surface. One method for the... 

Homemade AFM probes fabricated from metal microwires [79-81] (such as W, Ni, Pt, or Pt/Ir) where the end of the wire has been etched to form a sharp tip and then bent to form the cantilever, have also found much use as probes of the electrical, frictional, and topographical properties of sample substrates, in air or under vacuum. In many cases, the microwires are electrochemically etched using dissolution procedures borrowed from STM methodology for the production of sharp STM tips [10]. In some cases, the bent wire is flattened to form a beam- (or even V-)shaped cantilever [65, 82]. For C-AFM imaging measurements, solid metal microwire AFM probes are an attractive alternative to the metal-coated conventional AFM tips, as loss of probe conductivity due to wear, no longer poses a problem. 

Fine tips can be manufactured by electrochemical etching of tungsten wire. Typically, tip radii of < 1 pm can be obtained with no difficulty at all. Application of 100 V leads to an electric field strength at the tip of 1 MV/cm. This is the order of magnitude where field emission from metals becomes noticeable. In the initiation of electric breakdown, this process is of paramount importance. 

For measurements of optical frequencies, ultrafast metal-insulator-metal (MIM) diodes have been developed [4.113], which can be operated up to 88 THz (A, = 3.39 jtm). In these diodes, a 25-(jtm diameter tungsten wire with its end electrochemically etched to a point less than 200 nm in radius serves as the point contact element, while the optically polished surface of a nickel plate with a thin oxide layer forms the base element of the diode (Fig. 4.97). 

The electrochemical etching procedures usually involve the anodic dissolution of the metals. There are two ways in which this can be done an ac etch or a dc etch according to the applied potential. Each procedure gives a different tip shape the ac-etched tips have a conical shape and much larger cone angles than the dc-etched tips. The dc-etched tips, on the other hand, have the shape of a hyperboloid and are much sharper than ac-etched tips. The etching procedures used for several metal wires are described as following. 

The first documented SECM-AFM probe is described in [6], fabricated from a 50 pm diameter Pt wire. In the arena of nanoscopic electrode fabrication [7] and the production of tips for electrochemical STM [8], procedures are well established for the electrochemical etching and insulation of microwires, to produce sharpened and insulated, except at the very apex of the tip, nanoelectrodes. In the AFM field, solid metal AFM probes, for the measurement of the electrical, frictional, and topographical properties of a substrate, have been fabricated previously, by hand, from metal microwires [9]. Thus, by combining the two methodologies together, it was possible to fabricate SECM-AFM probes. 

The processability of certain CEPs has been utilized in the construction of microsystems, particularly miniature sensor systems. For example, simply dip-coating connecting platinum wires with a polyaniline formulation produces a useful humidity sensor.133 CEPs can also be screen-printed or ink-jet-printed to produce the complex shapes needed for various devices. Electrodeposition of CEPs is also a popular processing method, and this technique is compatible with conventional MEMS fabrication, where lithography and etching can be used to prepattern metal electrodes. Subsequent deposition of CEP by electrochemical polymerization produces the CEP microdevice.129... 

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