Anodic oxidation of silicon

Anodic oxidation is a very common process in the electrochemical industry, used for example in the manufacture of aluminum and tantalum capacitors. The anodic oxidation of silicon is not of comparable importance, because the electrical properties of anodic oxides are inferior to those of thermal oxides. 

A wide variety of electrolyte compositions used for anodic oxidation of silicon can be found in the literature. The electrolytes can be categorized in inorganic or organic solutions. In the latter case electrolytes like ethylene glycol [Ja2, Me6, Ma5, Mel3], methanol [Ma2] or tetrahydrofuryl alcohol [Be3] are used, with salts such as KN03 added in order to improve the conductivity. Studies with pure water [Ga2, Mo3, Hu3] as an electrolyte were performed, as well as with additions... 

TABLE 5.7. Rate-Limiting Steps Involved in the Anodic Oxidation of Silicon"... 

G. Mende, Detection of mobile ion during the anodic oxidation of silicon, J. Electrochem. Soc. 127, 2085, 1980. 

E. Guerrero, G. Tobolka, and A. Baghai, Calibration for the anodic oxidation of silicon. Thin Solid Films 76, 237, 1981. 

R. Nannoni and M. J. Musselin, Anodic oxidation of silicon effects of water on oxide properties, Thin Solid Films 6, 397, 1970. 

M. Croset and D. Dieumegard, Anodic oxidation of silicon in organic baths containing fluorine, J. Electrochem. Soc. 120(4), 526, 1973. 

E. F. Duffek, E. A. Benjamini, and C. Mylroie, The anodic oxidation of silicon in ethylene glycol solutions, Electrochem. Tech. 3(3-4), 75, 1965. 

The last reaction is particularly convenient for the passivation of device surfaces because of the low temperature required. Low-temperature layers can also be formed by the vacuum evaporation of Si02 powder or by ion-plasma sputtering of Si in an argon-oxygen mixture. The anodic oxidation of silicon in an ethylene glycol solution of KNO3 and various chemical oxidation reactions of Si (e.g., in H2O2) can also be used. 

The anodic oxidation of silicon in pure water has been reported to give highly insulating oxide films. The electrical strength of these films was quoted as 5 to 16 x 10 V/cm. 

C. Albonetti, J. Martinez, N. S. Losilla, P. Greco, M. Cavallini, F. Borgattl, M. Montecchi, L. Pasquali, R. Garcia, and E Biscarini, Parallel-local anodic oxidation of silicon surfaces by soft stamps. Nanotechnology, 19, 435303-435303 [2008],... 

Daumengrofes Labor aus Aluminium-Folie, Blick durch die Wirtschafi, June 1997 Heterogeneous gas-phase micro reactor micro-fabrication of this device anodic oxidation of aluminum to porous catalyst support vision of complete small laboratory numbering-up development of new silicon device [225]. 

Suda and coworkers described the anodic oxidation of 2-silyl-l,3-dithianes which have two sulfur atoms on the carbon adjacent to silicon [42], In this case, however, the C Si bond is not cleaved, but the C-S bonds are cleaved to give the corresponding acylsilanes (Scheme 12). Although the detailed mechanism has not been clarified as yet, the difference in the anode material seems to be responsible for the different pathway of the reaction. In fact, a platinum plate anode is used in this reaction, although a carbon anode is usually used for the oxidative cleavage of the C-Si bond. In the anodic oxidation of 2-silyl-l,3-dithianes the use of a carbon anode results in a significant decrease in the yield of acylsilanes. The effects of the nature of the solvent and the supporting electrolyte may also be important for the fate of the initially formed cation radical intermediate. Since various 2-alkyl-2-silyl-l,3-dithianes can be readily synthesized, this reaction provides a convenient route to acylsilanes. 

For anodic oxidation of a silicon electrode in fluoride-free electrolytes, the reaction of step 2 is only expected during the first seconds of anodization, until all Si-H surface groups are replaced by Si-OH. 

To understand the electrochemical behavior of silicon, however, the formation and the properties of anodic oxides are important The formation of an anodic oxide on silicon electrodes in HF and HF-free electrolytes will therefore be discussed in detail in this chapter. The formation of native and chemical oxides is closely related to the electrochemical formation process and will be reviewed briefly. The anodic oxidation of porous silicon layers is closely related to the morphology and the luminescent properties of this material and is therefore discussed in Section 7.6. 

Typical anodization curves of silicon electrodes in aqueous electrolytes are shown in Fig. 5.1 [Pa9]. The oxidation can be performed under potential control or under current control. For the potentiostatic case the current density in the first few seconds of anodization is only limited by the electrolyte conductivity [Ba2]. In this respect the oxide formation in this time interval is not truly under potentiostatic control, which may cause irreproducible results [Ba7]. In aqueous electrolytes of low resistivity the potentiostatic characteristic shows a sharp current peak when the potential is switched to a positive value at t=0. After this first current peak a second broader one is observed for potentials of 16 V and higher, as shown in Fig. 5.1a. The first sharp peak due to anodic oxidation is also observed in low concentrated HF, as shown in Fig. 4.14. In order to avoid the initial current peak, the oxidation can be performed under potentiodynamic conditions (V/f =const), as shown in Fig. 5.1b. In this case the current increases slowly near t=0, but shows a pronounced first maximum at a constant bias of about 19 V, independently of scan rate. The charge consumed between t=0 and this first maximum is in the order of 0.2 mAs cnT2. After this first maximum several other maxima at different bias are observed. 

Carbon-carbon bond formation has also been achieved using electroauxiliaries. The method developed by Yoshida and coworkers uses an auxiliary (silicon, tin, sulfur), which when added to a molecule, decreases the oxidation potential of the starting compound. Thus the chance of overoxidation can be avoided. The anodic oxidation of compounds having a... 

Again it seems not necessary to discuss the considerations of the chemical versus electrochemical reaction mechanism. It is clear from the extremely negative standard potential of silicon, from Eqs. (2) and (6), that the Si electrode is in all aqueous solutions a dual redox system, characterized by its OCP, which is the resultant of an anodic Si dissolution current and a simultaneous reduction of oxidizing species in solution. The oxidation of silicon gives four electrons that are consumed in the reduction reaction. Experimental results show clearly that the steady value of the OCP is narrowly dependent on the redox potential of the solution components. In solutions containing only HF, alternatively alkaline species, the oxidizing component is simply the proton H+ or the H2O molecule respectively. 

Silicon dioxide layers can be formed using any of several techniques, including thermal oxidation of silicon, wet anodization, CVD, or plasma oxidation. Thermal oxidation is the dominant procedure used in IC fabrication. The oxidation process selected depends on the thickness and properties of the desired oxide layer. Thin oxides are formed in dry oxygen, whereas thick (>0.5 fin1) oxide layers are formed in a water vapor atmosphere (13). 

Anodic oxidation of aliphatic aldehydes and ketones is generally difficult because their oxidation potentials are very high (>2.5 V). However, silylation at the carbonyl carbon causes a marked decrease in the oxidation potential as shown in Table 933,40. This silicon effect is much smaller in the case of aromatic carbonyl analogues40. The silicon effect is attributed to the rise of the HOMO level by the interaction between the C—Si a orbital and the nonbonding p orbital of the carbonyl oxygen, which in turn favors the electron transfer. 

Partially functionalized cyclopolysilanes recently attracted attention as model substances for siloxene and luminescent silicon. The yellow luminescent silicon is formed by the anodic oxidation of elemental silicon in HF-containing solutions and may be used for the development of silicon-based materials for light-emitting structures which could be integrated into optoelectronic devices77. Because the visible photoluminescence of... 

The high electropositivity of silicon means that the carbon-silicon bond is readily oxidized. Yoshida has carried out an extensive study of the anodic oxidation of benzyl and allyl silanes95. Oxidation converts the silane to a carbocation, which then reacts with a nucleophilic component of the medium (Scheme 18). Yoshida has shown that the reaction... 

Porous anodic alumina films were formed by a two-step anodic oxidation of aluminum foil (99.99% purity) (thickness 100 jum) or of thin aluminum film sputtered onto silicon substrate. First step was performed under lOmA/cm constant current density in 40 g/1 aqueous solution of (COOH)2 during 60 min. After first anodization the formed anodic oxide was removed in the aqueous solution of 0.35 M H3PO4 and 0.2 M CrOs at 90°C. The second anodization was performed in the same regimes as the first one. The formed oxide was removed from the specimen after the first anodization. Nanostructured aluminum samples were rinsed in deionized water and dried in an argon flow. 

There are few data on the effect of substrate on the formation of anodic oxide. In 2M KOH, the formation characteristics of anodic oxides on (100) and (111) surfaces in the potential range from 6 to 15V are the same. Ion implantation of substrate silicon has been found to greatly affect the anodization behavior of silicon. An increase in the anodization rate occurs after implantation of C, N", P, As", Ar, ... 

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