Material removal rate anodic reactions

If the surface of a single crystal n-type semiconductor is damaged by abrasion or by high energy particle bombardment, the generation (recombination) velocity for holes in the surface layer is increased to the extent that holes are readily available to carry out anodic reactions at a rapid rate. This condition lasts until most of the damaged surface material is removed. Then the anode current is limited as previously described due to hole depletion. 

Chemical micromachining (CMM) involves one or more chemical reactions by which a workpiece substrate is oxidized to produce reaction products, which are carried away Ifom the surface by the medium. In general, oxidation-reduction or complexation-type reactions are involved in CMM. Figure 1.16 shows the basic arrangements of CMM. Although no external current is supplied, anodic and cathodic sites are present on the reactive surface such that the rate of material removal (oxidation) is balanced by the rate of reduction of the etchant species. The metal removal reaction typically... 

During EMM, oxide layer is formed on the workpiece due to anodic reaction. For some material this oxide layer is passive in nature which minimize the rate of current flow and thus lower the machining rate. Laser beam can also minimize the formation of oxide layer by increasing the temperature of the upper surface of the workpiece which restricts the chemical reaction responsible for the formation of passive metal oxide layer especially in the case of passivating electrolytes. Hence, the combination of laser beam with EMM will prove to be beneficial for higher material removal. 

The rate of evolution of the gas generated at the cathode can be similarly determined. In principle, therefore, we have an alternative way of removing material from an anodic workpiece. Thus if a nickel anode were used, it would dissolve, yielding nickel hydroxide. If brass or steel, for example, is substituted for copper as the cathode, the reaction at that electrode is still hydrogen gas generation. 

Under diffusion-controlled dissolution conditions (in the anodic direction) the crystal orientation has no influence on the reaction rate as only the mass transport conditions in the solution detennine the process. In other words, the material is removed unifonnly and electropolishing of the surface takes place. 

The rate of the reaction of electro-oxidation is slow, and there are two methods to overcome this obstacle. One method is to choose noble metal electrocatalysts as the electrode materials, and the other method is to use suitable redox mediators for indirect oxidation of SO2. These redox mediators can be generated or regenerated at the anode, such as a Ce(IV)-assisted process for the removal of SO2, as shown in Eqn (14.9) ... 

Etching of the material may be carried out either chemically under open-circuit conditions (i.e. controlled corrosion (Chapter 10)) or it may be electrochemically driven by applying a potential. The former case is more common. It requires no power supply or auxiliary electrodes the electrolyte conditions are chosen such that the species to be removed is dissolved at a reasonable rate, courtesy of a simultaneous cathodic process. Taking the case of the dissolution of a metal M, the anodic process in reaction (9.1) is supported by a suitable electroreduction ... 

However, reactions (12.47) to (12.49) above still represent a very simplified reaction scheme because the cracking process is usually highly complex. In any case, the anode material in SOFGs must be able to oxidize the cracking products, and especially the formed carbon at sufficient rates. The formed carbon (coke) can also be removed by steam or carbon dioxide ... 

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