Anodic oxide layers

Raja KS, Misra M, Paramguru K (2005) Formation of self-ordered nano-tubular structure of anodic oxide layer of titanium. Electrochim Acta 51 154—165... 

Figure 27. Types of anion concentration profiles in anodic oxide layers on aluminum, from the O/S interface (O) inwards to the M/O interface (L). LA front of anion penetration.

For the case of Si02 etching, HF, (HF)2 and HF2- are assumed to be the active species [Vel, Jul]. If HC1 is added to the solution the concentration of the HF2-ion becomes negligible, which leaves HF and its polymers to be the active species [Ve3]. Because for high current densities the electrochemical dissolution of silicon occurs via a thin anodic oxide layer it can be concluded that, at least for this regime, the same species are active. This is supported by the observation that F- is... 

Anodic oxide formation suggests itself as a passivating mechanism in aqueous electrolytes, as shown in Fig. 6.1a. However, pore formation in silicon electrodes is only observed in electrolytes that contain HF, which is known to readily dissolve Si02. For current densities in excess of JPS a thin anodic oxide layer covers the Si electrode in aqueous HF, however this oxide is not passivating, but an intermediate of the rapid dissolution reaction that leads to electropolishing, as described in Section 5.6. In addition, pore formation is only observed for current densities below JPS. Anodic oxides can therefore be excluded as a possible cause of pore wall passivation in PS layers. Early models of pore formation proposed a... 

The HF tester is a commercial safety tool for sensing whether an unidentified liquid contains HF [2], It shows in an exemplary way how the electrochemical properties of a silicon electrode, namely its I-V curve in HF, can be applied for sensing. The ability to dissolve an anodic oxide layer formed on silicon electrodes in aqueous electrolytes under anodic bias is a unique property of HF. HF is therefore the only electrolyte in which considerable, steady-state anodic currents are observed, as shown schematically in Fig. 3.1. This effect has been exploited to realize a simple but effective safety sensor, which allows us to check within seconds if a liquid contains HF. This is useful for safety applications, because HF constitutes a major health hazard in semiconductor manufacturing, as discussed in Section 1.2. 

The oxide layer of a metal such as copper may be seen as a semiconductor with a band gap, which may be measured by absorption spectroscopy or photocurrent spectroscopy and photopotential measurements. Valuable additional data are obtained by Schottky Mott plots, i.e. the C 2 E evaluation of the potential dependence of the differential capacity C. For thin anodic oxide layers usually electronic equilibrium is assumed with the same position of the Fermi level within the metal and the oxide layer. The energetic position of the Fermi level relative to the valence band (VB) or conduction band (CB) depends on the p- or n-type doping. Anodic CU2O is a p-type semiconductor with cathodic photocurrents, whereas most passive layers have n-character. 

The photoelectrochemical and UPS results yield the energy diagram of Fig. 44 for a Cu electrode covered with the anodic oxides. For these diagrams an electronic equilibrium is assumed that leads to the same energy position of the Fermi level for Cu and its two anodic oxide layers. This situation defines an energetic difference of the upper valence band edge of CU2O and the Fermi level of 0.8 eV. 

The surface of the oxidized silicon layer is terminated by OH in KOH but may be terminated by both OH and F in HF. In HF on the surface covered by an anodic oxide layer, adsorption of OH is required for the growth of the oxide, while adsorption of F is required for the dissolution of oxide. The Si—O—Si bonds are rather stable in KOH such that the dissolution rate in the passive region is very low. On the other hand, the Si—O—Si bonds are not stable in HF... 

Owing to the finite electrolyte volume ofthe nanoliter droplet, the diffusion boundary layer reaches the dimensions of the droplet within seconds, assuming a diffusion coefficient of for example D = 110-5 cm2 s 1. A constant diffusion gradient can not be established because the bulk concentration is reduced. Thus, measurements of diffusion limited currents are not stable over time. The electron transfer reactions discussed below were carried out on anodic oxide layers with maximum current densities 3 to 4 orders of magnitude lower than the diffusion limited current densities, thus ensuring a stable support of consumables over the time of the measurement. In order to prevent evaporation of electrolyte, the ambient was saturated with water vapor. 

In the following sections the electrochemical reactivity of single grains of polycrystalline Ti is explored by using the nl-droplet method. The results from electrochemical measurements and the optical laser techniques from the previous section are combined to yield a band structure model for anodically grown anodic oxide layers. Other applications of this method to study laser induced corrosion, texture dependent photocurrent and corrosion of anodic oxide films are described in Refs. [89,90 and 91]. 

The anodizing process consists of 1) trim forming, 2) cleaning, 3) anodic electrolysis usually in sulfuric acid and, 4) sealing in an aqueous bath. Each of these steps are important, for example, the anodized oxide layer must be controlled to provide an optimum thickness. Sealing the anodized layer is also very important and must be complete. Corrosion problems associated with anodized aluminum trim include etching of the oxide film (known as "blush and bloom ) and pitting of the aluminum. 

Lohrengel M M 1993 Thin anodic oxide layers on aluminum and other valve metals high field regime Mater. Sci. Eng. R... 

Stoichiometric coefficient of the anodic oxide layer as a function of oxidation potential lead electrode in H2SO4 solution [116],... 

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