Oxidation of SiC

Thermal oxidation of the two most common forms of single-crystal silicon carbide with potential for semiconductor electronics applications is discussed 3C-SiC formed by heteroepitaxial growth by chemical vapour deposition on silicon, and 6H-SiC wafers grown in bulk by vacuum sublimation or the Lely method. SiC is also an important ceramic ana abrasive that exists in many different forms. Its oxidation has been studied under a wide variety of conditions. Thermal oxidation of SiC for semiconductor electronic applications is discussed in the following section. Insulating layers on SiC, other than thermal oxide, are discussed in Section C, and the electrical properties of the thermal oxide and metal-oxide-semiconductor capacitors formed on SiC are discussed in Section D. 

For semiconductor electronic applications, thermal oxides on SiC are employed as a masking material for ion implantation and dry etching, as a gate insulator for field-effect devices, and as a surface passivation. Oxidation can also be used to etch the surface of SiC, as well as for polarity determination and for the delineation of defects and boundaries in SiC [1]. The slow oxidation rate of deposited SiC has been used for local oxidation inhibition of silicon [2]. 

Each of SiC s crystalline polytypes has a distinct oxidation rate under the same oxidation conditions [1,3,4]. For the various SiC polytypes, the oxidation rate on the (0001) Si faces increases with the decrease in the percentage of hexagonality of the SiC polytype, while the growth rate on the (0001) C faces does not depend dramatically on polytype [3]. The dramatic difference in oxidation rates between opposite faces of the polar SiC crystal has long been known. Intermediate faces have intermediate oxidation rates [3]. As with other semiconductors, conduction type, dopant density, surface roughness and crystalline quality should also be expected to have an effect on the oxidation rate [5-7]. Selective oxidation at antiphase boundaries has been reported for wet oxidation of 3C-SiC heteroepitaxial layers, but not for dry oxidation [8-10]. 

The oxidation rates of the cubic (3C) [1,3,9,11-15] and hexagonal (6H or 4H) [3,4,16-23] polytypes have been studied. SiC has been oxidised with procedures very similar to those used in Si integrated circuit fabrication. Dry and wet oxidation have been employed in the typical open-ended, resistance-heated quartz tubes, at temperatures ranging from 850 to 1250°C. Since, in general, SiC oxidises at a much slower rate than Si, temperatures in excess of 1000°C are more practical. 

1 (Editor s note at the time of going to press, June 1995, the area of knowledge reviewed here had not changed substantially since the cut-off date, October 1992.) 

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However, during long exposures to medium-temperature operating conditions, e.g. 1000°C, spinel formation is certainly expected. Wang etal.60 demonstrated this for the Ni-alumina system, showing the diffusion of Ni atoms to the free surface of the nanocomposite, followed by the formation of a nickel spinel surface coating which then limits the kinetics of subsequent oxidation. In this case the formation of a spinel surface layer may be beneficial to mechanical properties, since the reaction results in a volume increase, and the formation of compressive residual stresses. An analogous behavior was reported for ceramic particle nanocomposites, where oxidation of SiC particles results in an increase in volume and compressive residual stresses.61... 

Nagano, T., Ishikawa, Y., and Shibata, N. Effects of surface oxides of SiC on carbon nanotube formation by surface decomposition. Jpn. J. Appl. Phys. 42, 2003 1380-1385. 

When exposed to oxygen-containing environments at temperatures typically higher than 1250°C, SiC undergoes an oxidation reaction whose primary product is silica glass. A number of independent studies have shown that the oxidation of SiC whiskers initially proceeds according to the reaction 54,55... 

TEM observations of the oxidized scale have revealed mullite grains with transgranular cracks, a phenomenon that is not surprising when one considers that the oxidation of SiC produces a volume expansion of 100%. When the reaction product contains a solid as well as a liquid product, as in the present situation, the volume expansion can be accommodated by squeezing out the liquid phase, resulting in a liquid cap on top of the solid reaction products. This has been observed by Luthra and Park,13 and is apparent in the micrograph shown in Fig. 8.5. 

In this section we will discuss the kinetics of thermal oxidation of SiC substrates with porous layers formed on both carbon and silicon terminated faces. 

To reduce interface traps, the oxidation of SiC should be terminated in such a manner that no oxidation can take place during the time that the SiC is cooling to room... 

Procedures for thermal oxidation of SiC have been developed and shown to produce oxide layers useful in the fabrication of planar SiC microelectronic devices. The SiC oxidation rate has been studied under conditions commonly used in integrated circuit fabrication. The oxidation rate constants derived in these studies are useful for predicting the oxide thickness formed on SiC under similar conditions. The metal-oxide-semiconductor capacitors formed by thermal oxide layers on both 3C- and 6H-SiC have been shown to have low interface charge densities, suitable for transistor applications. 

Devitrification of binder at the pore surfaces has been frequently reported for similar and first generation clay bonded hot gas filters after the use in real combustion conditions. In the current study similar devitrification was qualitatively found to increase as function of time and the amount of water vapour in water vapour environment. Further, oxidation of SiC occurred in both materials after 500 h exposure at 850°C atmospheric water vapour. The results of the current study are in accordance with the results of similar filters operated up to 1500 h in pilot plant in varying combustion environments. Our earlier evaluations of the filter material performance have indicated severe oxidation of SiC and this combined with the crystallization of the oxidation layer and cracking due to thermal stresses and mismatch due the phase transformation of Si02 has been pointed as a mechanism to degrade the strength and operation lifetime of SiC-based clay bonded hot gas filters, ... 

The presence of small amounts of aluminium or alkalis in the oxidation layer has been found to increase the oxidation rate of SiC significantly both alkalis and aluminium are present in the binder phase. It is also well known that oxidation of SiC is more severe in water vapour than in oxygen due to the higher solubility of water than oxygen in silicaAgainst this background the oxidation rate found in this study is less than expected and the binder surrounding the SiC seems to be protective,... 

Crystallization of amorphous binder and oxidation of SiC were found but the rate of oxidation was less than expected. Furthermore there was a clear difference in the resistance to crystallization and oxidation of the two materials although there was only small difference in the composition of the amorphous phase of the materials as-received. The two materials also showed different trends of apparent density as function of time and amount of water vapour. 

Surface moisture is a problem of concern in ceramic powders, and IR has been used to characterize the surface groups of -OH and -H [58,63,64]. IR was also applied to characterize chemically bound hydrogen in chemical vapor-deposited silicon nitride at various ammonia-silane ratios [65]. Surface silicon dioxide on SiC powders was determined by photoacoustic IR and diffuse reflectance IR spectroscopy [66,67]. IR spectroscopy was also used to study the surface oxidation of SiC and SisN4 [68,69]. 

Surface contamination Chemical Oxidation of SiC can passivate the surface... 

Figure 11. Transition temperatures and oxygen pressures for active and passive modes of oxidation of SiC after [15].

It should be noted that the oxygen pressures for the transition in Fig. 11 should not be confused with those obtained by CO (-equilibria. Despite low oxygen partial pressures of, for example CO in equilibrium with carbon, we have passive oxidation of SiC up to w 1400°C in CO, because CO is a reactive and oxidi2dng gas species for SiC [52,85,86] Consequently active corrosion in CO -environments is measured only at very high temperatures [87]. 

The active oxidation of SiC is distinguished from the passive oxidation reaction described above ... 

Internal oxidation of SiC/RBSN is severe between 800 to 1100°C, and the depth of the oxidation damage zone is directly related to the pore size the smaller the pore size, the lower is the oxidation induced damage. In addition, internal oxidation is also found to generate tensile residual stresses which affect strength properties of SiC/RBSN. 

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