Etching, electrochemical

Au is an excellent electrode material. It is inert in most electrochemical environments, and its surface chemistry is moderately well understood. It is not, however, the substrate of choice for the epitaxial formation of most compounds. One major problem with Au is that it is not well lattice matched with the compounds being deposited. There are cases where fortuitous lattice matches are found, such as with CdSe on Au(lll), where the Vs times the lattice constant of CdSe match up with three times the Au (Fig. 63B) [115,125]. However, there is still a 0.6% mismatch. A second problem has to do with formation of a compound on an elemental substrate (Fig. 65) [384-387]. Two types of problems are depicted in Fig. 65. In Fig. 65A the first element incompletely covers the surface, so that when an atomic layer of the second element is deposited, antiphase boundaries result on the surface between the domains. These boundaries may then propagate as the deposit grows. In Fig. 65b the presence of an atomically high step in the substrate is seen to also promote the formation of antiphase boundaries. The first atomic layer is seen to be complete in this case, but when an atomic layer of the second element is deposited on top, a boundary forms at the step edge. Both of the scenarios in Fig. 65 are avoided by use of a compound substrate. 

For deposition of II-VI compounds, three scenarios for lattice-matched compound substrates are readily available (1) deposition on the compound itself, homoepitaxy (2) deposition on a second II-VI compound that happens to be lattice matched, such as HgTe on CdTe or (3) deposition on a corresponding III-V compound. There are two problems with homoepitaxy—one is why Most of the time there is no need to grow a layer of a compound on a substrate of the same compound. There are cases where the substrate is grown by a method that produces an inexpensive but inferior material, and a high-quality epi layer of the same compound is grown 

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Antiphase domain formation in polar on nonpolar epitaxy (a) incomplete prelayer coverage, (b) odd step height. (From Ref. 387.) 

In the manufacture of printed circuit boards the plastic board is initially covered with a complete copper layer (typically 35 pm thick). Selected areas are then protected by photoresist or other insulation and the rest of the copper is etched 

The mechanism for storage of the charge on the carbon particles is not yet proven and in principle could be a mixture of double layer charging, reversible oxidation of surface redox systems or intercalation of sulphate species into the graphite lattice. The first is thought to be the major method because of the low potential used at the anode ( 1 V vs NHE). Even so it is possible to store 100 C per gram of carbon on the high-surface-area material used. 

It is found that the etching rate increases with the conductivity and storage capacity of the carbon, particle surface area and concentration, spraying pressure and temperature, and the process is also catalysed by the presence in solution of a soluble redox species. Hence the process solution generally contains 30 g 1 of ferric sulphate. 

Canning Handbook on Electroplating, 22nd edn, E. and F. N. Spon, London (1978). 

) ( 91 ) Industrial Electrochemical Processes, Elsevier, London. 

The deliberate (selective) and controlled removal of an electrically conducting (or semiconducting) material in an electrolyte may be practised for several important purposes, including the following  

Selective attack of active zones and phases on a metal surface (e.g. grain boundaries) to provide suitable contrast for microscopic techniques the principle of metallographic etching. 

Metal samples may be thinned prior to transmission electron microscopy. 

Controlled surface roughening in order to promote adhesion of a coating via more direct chemical bonding, as in the mild etching of metals prior to electroplating or anodizing (Chapter 8). 

Aluminium alloys may be electrograined in order to enhance their capacity for receiving and retaining the image of a printing ink (section 9.3.3). 

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AMPS-sulfuric acid paste, electrochemical etching 11.6 7.1... 

Fumed silica/sulfuric acid paste, electrochemical etch 21.7 10.7... 

In situ resolution of the crystalline order has been achieved by Villegas et al.594 on Cu(100) electrodes purposely disordered by oxidation or ion bombardment. Ordering was achieved by chemical and electrochemical etching and confirmed by LEED, SEM, and STM. 

Kois J, Bereznev S, Volobujeva O, MeUikov E (2007) Electrochemical etching of copper indium diselenide surface. Thin Solid Films 515 5871-5875... 

In a subsequent work [182], it was shown that the photoelectrochemical performance of InSe can be considerably improved by means of selective (photo)electrochemical etching. Interestingly, whereas the cleavage vdW plane showed little improvement, the photocurrent in the face parallel to the c-axis was doubled. Note that, in contrast to InSe crystals cleaved in the plane perpendicular to the c-axis that are almost defect free, the crystals cut in the plane parallel to the c-axis contain a high density of defects on their surface which leads to a high rate of electron-hole recombinations and inferior quantum efficiency. The asymmetry in the role of electrons and holes, as manifested, e.g., in the fact that surface holes carry out the selective corrosion of the semiconductor surface in both cleavage orientations, was discussed. 

Picardi et al. introduced a method to fabricate a sharp Au tip for STM by electrochemical etching [31]. The efficiency of TERS for a thin BCB dye layer using the etched sharp tip was then compared with that using an Au-coated AFM tip. 

Ren, B., Picardi, G. and Pettinger, B. (2004) Preparation of gold tips suitable for tip-enhanced Raman spectroscopy and light emission by electrochemical etching. Rev. Sci. Instrum., 75, 837-841. 

By far the most common method of STM tip preparation is electrochemical etching, which, in its simplest application, involves suspending a 0.5-1 mm diameter wire in a low-concentration electrolyte (0.3-3 m) such as NaOH or KOH and applying 1-10 V AC or DC between it and a counter electrode (Figure 3.13). Overall the etching reaction corresponds to... 

All the STM results from our group presented in this chapter employed the variable temperature STM, with tips made by electrochemical etching of tungsten wire. For noncontact AFM (NC-AFM), we employ commercial conducting silicon cantilevers with force constants of approximately 2-14 rn 1 and resonant frequencies of approximately 60-350kHz (Nanosensors and Mikromasch). The NC-AFM images we present here were recorded in collaboration with Professor Onishi at Kobe University and employed a UHV JEOL (JSPM-4500A) microscope. 

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

Electrochemical etching (i.e. etching using an electrochemical potential for the control and stopping of the etching process). 

Electrochemical etching is one way of controlling the etch rate and determine a clear etch stop layer when bulk micromachining Silicon. In this case, the wafer is used as anode in an HF-Electrolyte. Sufficiently high currents lead to oxidation of the silicon. The resulting oxide which is dissolved by the HF-solution. Since lowly doped silicon material is not exhibiting a notable etch rate, it can be used as an etch stop. 

Figure 6. Scanning electron micrograph (SEM) of n-GaAs surface electrochemically etched with a scanning electrochemical and tunneling microscope (SETM). Etching was accomplished in Aq. 5 mU NaOH, 1 mM EDTA. Photoelectric current - 0.7 /iA, Scan rate - 0.1 /tm/sec, bias voltage — 4 V. Tip was moved in an "L" pattern. Reproduced with permission of Ref. 89. Copyright 1987 The Electrochemical Society Inc.

Electrochemical efficiency, batteries, 3 414 Electrochemical etching, in membrane preparation, 15 813t Electrochemical ethylene oxidation,... 

If the relevant literature is surveyed for the keywords etch stops and silicon, a confusing multiplicity of methods is found using different electrolytes, different bias and differently doped silicon substrates. This section does not aim to be a comprehensive review of all these techniques [Co2], but an introduction to the basic principles of electrochemical etch stops, which will be illustrated by a few typical examples. 

All electrochemical etch stops of silicon are based on the dissolution behavior discussed above and a method to have different parts of the electrode interface under different potentials. 

The easiest way to have different parts of the electrode surface under different bias is to disconnect them by an insulator. This method is elucidated by an experiment in which an electrochemical etch-stop technique has been used to localize defects in an array of trench capacitors. In a perfect capacitor the polysilicon in the trench is insulated from the substrate whereas it is connected in a defect capacitor, as shown in Fig. 4.15 a. If an anodic bias is applied the bulk silicon and the polysilicon in the defect trench will be etched, while the other trenches are not etched if an aqueous HF electrolyte is used, as shown in Fig. 4.15b. The reverse is true for a KOH electrolyte, because the only polysilicon electrode in the defect trench is passivated by an anodic oxide, as shown in Fig. 4.15 c. 

Small leakage currents or a transistor-like action of the junction are sufficient to generate a small current that may cause undesired passivation. This can be circumvented by application of an additional potential to the etching layer, shown by the broken line in Fig. 4.16 a. This electrochemical etch-stop technique is favorable compared to the conventional chemical p+ etch stop in alkaline solutions, because it does not require high doping densities. This etch stop has mainly been apphed for manufacturing thin silicon membranes [Ge5, Pa7, Kll] used for example in pressure sensors [Hil]. 

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