Oxidation of silicon

Wang L S, Nicholas J B, Dupuis M, Wu FI and Colson S D 1997 SijO (x = 1-6) models for oxidation of silicon surfaces and defect sites in bulk oxide materials Phys. Rev. Lett. 78 4450... 

Oxidation of Silicon. Silicon dioxide [7631-86-9] Si02, is a basic component of IC fabrication. Si02 layers are commonly used as selective masks against the implantation or diffusion of dopants into silicon. Si02 is also used to isolate one device from another. It is a component of MOS devices, and provides electrical isolation of multilevel metalliza tion stmctures (12). A comparison of Si and Si02 properties is shown in Table 1. 

Coesite. Coesite, the second most dense (3.01 g/cm ) phase of silica, was first prepared ia the laboratory by heating a mixture of sodium metasibcate and diammonium hydrogen phosphate or another mineraliser at 500—800°C and 1.5—3.5 GPa (14,800—34,540 atm). Coesite has also been prepared by oxidation of silicon with silver carbonate under pressure (67). The stmcture is monoclinic = 717 pm, Cg = 1.238 pm, and 7 = 120°. 

Figure 1 shows a segment of the FTIR absorbance spectrum of a thin film of the oxide of silicon deposited by chemical vapor deposition techniques. In this film, sil-... 

Both our original prediction about the effect of ionization energy on acid-base behavior and the trend which we have observed in the first three elements lead us to expect that the hydroxide or oxide of silicon should not be basic, but perhaps should be weakly acidic. This is in fact observed. Silicon dioxide, Si02, can exist as a hydrated solid containing variable amounts of water,... 

Many of the materials used in the most advanced technologies are made from one of the oldest known materials, common clay. Most clays used commercially are oxides of silicon, aluminum, and magnesium. China clay contains primarily kaoli-nite, a form of aluminum aluminosilicate that can be obtained reasonably free of the iron impurities that make many clays look reddish brown, and so it is white. However, other clays contain the iron oxides that cause the orange color of terra cotta tiles and flower pots. 

Scheme 23 Possible mechanism for the oxidation of silicon surfaces by molecular oxygen.

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth s crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust the excess oxygen occurs mostly as the volatiles CO2 and H2O. The CO2 now resides primarily in carbonate rocks whereas the H2O is almost all in the oceans. 

Zeolites are prepared by the linking of basic structural units around a template molecule. The structural units are typically based on oxides of silicon and aluminium, and the templates are usually individual small molecules. Under the right conditions, the silicon and aluminium oxide precursors will link up around the template to form a crystalline three-dimensional matrix containing the template molecules. The template... 

The principal oxide of silicon is silica. The empirical formula of silica is Si02, but the compound consists of a continuous network of Si—O bonds rather than individual Si02 molecules. Figure 9-17 shows part of this network. Each silicon atom is at the center of a regular tetrahedron, bonded to four oxygen atoms. As in water... 

Chemically, all clays are composed of oxides of silicon, aluminum, and hydrogen - namely, silicon dioxide, aluminum trioxide, and water in a weight proportion that can be expressed by the following general formula (see Fig. 50) ... 

Direct measurement of the change in interfacial potential difference at the oxide-electrolyte interface with change in pH of solution can be measured with semiconductor or semiconductor-oxide electrodes. These measurements have shown d V g/d log a + approaching 59 mV for TiC (36, 37). These values are inconsistent with the highly sub-Nernstian values predicted from the models with small values of K. (Similar studies 138.391 have been performed with other oxides of geochemical interest. Oxides of aluminum have yielded a value of d t)>q/A log aH+ greater than 50 mV, while some oxides of silicon have yielded lower values.)... 

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... 

Whatever the initial step of formation of surface silyl radicals, the mechanism for the oxidation of silicon surfaces by O2 is expected to be similar to the proposed Scheme 8.10. This proposal is also in agreement with the various spectroscopic measurements that provided evidence for a peroxyl radical species on the surface of silicon [53] during thermal oxidation (see also references cited in [50]). The reaction being a surface radical chain oxidation, it is obvious that temperature, efficiency of radical initiation, surface precursor and oxygen concentration will play important roles in the acceleration of the surface oxidation and outcome of oxidation. 

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. 

Perfection especially is required on the silicon surface. A 100 surface of silicon contains 6.8 x 1014 atoms/cm2. Surface defect densities must be less than one part in 105—105 defects/cm2 for satisfactory MOSFET operation. In fact, the discovery of the original point contact transistor was only possible because the native oxide on single-crystal germanium has surface defect densities less than one part in 104. Good silicon devices required the discovery (10) that the thermal oxidation of silicon could produce an excellent Si—Si02 interface. 

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