Electrochemical tip etching

The art of making sharp tips using electrochemical etching was developed in the 1950s for preparing samples for field ion microscopy (FIM) and field electron spectroscopy (FES). A description of various tip-etching procedures can be found in Section 3.1.2 in the book of Tsong (1990). 

The preferred method for preparing STM tips is the dc dropoff method. The basic setup is shown in Fig. 13.1. It consists of a beaker containing an electrolyte, typically 2M aqueous solution of NaOH. A piece of W wire, mounted on a micrometer, is placed near the center of the beaker. The height of the W wire relative to the surface of the electrolyte can then be adjusted. The cathode, or counterelectrode, is a piece of stainless steel or platinum placed in the beaker. The shape and location of the cathode has very little effect on the etching process, which can be chosen for convenience. A positive voltage, 4 V to 12 V, is applied to the wire, which is the anode. Etching occurs at the air-electrolyte interface. The overall electrochemical reaction is (Ibe et al., 1990)  

The most important parameter that affects the final shape of the tip end is the time for the etching current to cut off after the lower part drops off. The shorter the cutoff time, the etching current, the sharper the tip end, as shown in Fig. 13.2. To shorten the cutoff time, a simple electronic circuit is helpful. When the lower part of the wire drops off, the etching current suddenly drops. The electronic circuit senses the drop of the etching current and turns off the current completely through an electronic switch. An example of such a circuit is given in detail by Ibe et al. (1990). 

Various improved methods for making STM tips linked with SEM studies have been discussed in the literature, for example, Lemke (1990), Melmed (1991), and references therein. However, since the STM resolution does not have a direct correlation with the look of the tip under SEM, the simple dc dropoff method, as described here, is usually sufficient. From the experience of the author, two simple improvements can be helpful. The first is to install an insulating piece between the cathode and anode across the liquid surface to prevent the hydrogen bubbling on the cathode from perturbing the meniscus near the anode. The second is to save the dropped piece as the tip, which might be better than the upper one. 

The exposure of the tip to the electrolyte and air results in the formation of a surface oxide (WO-j). This oxide coating has to be removed before 

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Directional walk of atoms 42 DNA 341 Dominant pole 265 Eddy-current damper 248 Elasticity theory 365—376 Electrochemical tip etching 282—285 Electrochemistry 323... 

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