Etching kinetics

Another more complex case is that of the etching of GaAs in CrOj-HF solutions, studied both in the dark and under illumination by Kelly et al. [82, 83]. The etching kinetics depend upon the Cr03/HF concentration ratio at a low ratio, the etching rate is limited by diffusion of Cr(VI) from solution, while at high values, the etching is kinetically controlled and depends only on the HF concentration. II-... 

In order to discuss the correlation between etching kinetics and etching morphology, let us first reconsider the current-potential characteristics of III-V semiconductors, as depicted in Fig. 1. For the sake of clarity, the behavior in alkaline solutions (Fig. 1 (b)) is treated first. As announced, the discussion is mainly based on experimental results obtained from GaP single crystals. 

The many etch rate equations described above are empirical in nature even though mechanistic arguments are made in each specific case. One important omission in these quantitative formulations on the etching kinetics is the lack of consideration of the effect of the structure of silicon oxides. As shown in Fig. 4.40, etch rate can vary over more than three orders of magnitude for different types of oxides. It increases with increasing disorder of the oxide structure with the most ordered oxide, that is, quartz, having the lowest etch rate. The structural disorder of the silicon oxide can be due to impurities, partial oxidation of the silicon atoms, and degree of crystallinity. 

The details of this reaction are discussed in more detail in Chapter 5. As far as the etching kinetics is concerned, according to Seidel et the dependence of etch rate on KOH concentration in the concentration range of 10 to 60% can be best fitted by... 

K. Osseo-Asare, Etching kinetics of silicon dioxide in aqueous fluoride solutions A surface com-plexation model, J. Electrochem. Soc. 143, 1339, 1996. 

Relations (8-38), (8-39), (8-41), and (8 2) for the neutral etchant density and flux can be effectively used in surface etching kinetics, particularly, in equations (8-20)-(8-22). 

The etching kinetics of germanium and P B compounds in acidic solutions of iron (III) chloride agree with ideas on the structure of these compounds and the nature of the chemical bonds in them. 

Woo TK, Kim SE, Ahn HS (2012b) Experimentally derived catalytic etching kinetics for defect-utilized dual-porous silicon formation. J Phys Chem C 116 7040-7049... 

J.V. Macpherson, C.J. Slevin, P.R. Unwin. Probing the oxidative etching kinetics of metals with the feedback mode of the scanning electrochemical microscope. J Chem Soc Faraday Trans. 92 3799 (1996). W.J. Albery, M.L. Hitchman. Ring Disc Electrodes. Clarendon Press, Oxford, U.K. (1971). 

Thus the surface coverage due to the adsorbed film is important in etching kinetics, and the injection of holes from Cr and dissolution of GaAs via nucleophilic attack are possible only at the free surface. 

Etching kinetics of semiconductors may be diffusion-controlled in two ways (24). In the first case, the reduction reaction is diffusion limited and controls the etching kinetics. Under these conditions a well defined crystallographic facet is obtained. This behavior is observed at low pH. In the second case of high pH, the rate of anodic dissolution of the semiconductor wafer depends on the mass transport of OH ions to the electrode. Electroless etching based on this limitation shows rounded profiles typical of diffusion-controlled dissolution. 

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