Silicon sources

Taylor, P. A., Silicon Source Gases for Chemical Vapor Deposition f Solid State Technology, pp. 143-138 (May 1989)... 

A common deposition reaction, used widely in semiconductor processing, combines ammonia with silane as the silicon source ... 

A similar reaction uses dichlorosilane as the silicon source ... 

Deposition temperature is 800°C and either atmospheric pressure or low pressure is used. This reaction can also be carried out in a plasma at very low pressure and at much lower temperature (450°C). P ] A silicon substrate, such as the silicon wafer itself or a thin predeposited layer of silicon, may be used as the silicon source with the... 

Tungsten disilicide (WSi2) is refractory and stable with low resistivity. As with other silicides, a common deposition system uses silane as the silicon source with tungsten hexafluoride as follows ... 

Reaction (1) is being replaced by a reaction where dichlorosilane is the silicon source. This provides better conformity and less cracking and peeling P lP l... 

Another deposition reaction uses disilane as the silicon source at atmospheric pressure and at a deposition temperature of290-300°C, with nitrogen and hydrogen dilution as follows ... 

In zeolite synthesis (ref. 2) an aqueous mixture containing a silicon source, an aluminum source, an alkali source (usually NaOH) is autoclaved and subjected to hydrothermal treatment. Hydrated Na-ions are then filling the pore system in the as-synthesized zeolite. In the case of relatively high Si/Al zeolites an organic template is required which is usually a tetraalkylammonium compound, applied as the bromide or the hydroxide. 

In order to illustrate exactly what we are discussing, the following diagram of a CMOS design used in 1997 is presented on the following diagram. That is, ap-type silicon source is formed on an n-type well, and vice-versa. 

After the calcination step, experimental data (XRD, 29 i maSNMR) show that a zeolite with the silicalite structure has been formed. 29 i MASNMR indicates for the zeolite material a Si/Al ratio depending on the sample prepared it has been observed that both the natures of the silicon source and of the alumina supports may originate these fluctuations. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

Silicon presents an attractive option among eledrophilic activating and dehydrating agents of hemiacetals because of the wide commercial availability of eledrophilic silicon sources. The two main classes of silicon electrophiles used, namely silyl halides and silyl sulfonates, have been demonstrated to promote a variety of glycosylations including some examples of oligosaccharide synthesis. 

The mechanism proposed for this transformation is outlined in Scheme 24 (235). The slow step of this reaction is silyl transfer from the copper alkoxide 353. This step may occur through the intermediacy of an external silicon source (intermolecular) or by internal transfer of the silyl group (intramolecular). To probe this issue, these workers conducted a double-crossover experiment involving two distinct nucleophiles with different silyl groups, 342a and 359, and examined the products prior to desilylation. The results show conclusively that silicon transfer has a significant intermolecular component, and is somewhat sensitive to the solvent, Eq. 199. 

Typically, for production of zeolites (Figure 4.8), a silicon source such as sodium silicate and an aluminum source such as sodium aluminate, are prepared in solutions containing sodium and water contents as required for the formation of the respective zeolite [6], These solutions are mixed in a reactor and reacted at temperatures typically in the range between 80 and 200°C. The reaction time may vary from hours to days, and for reactions at temperatures > 100°C the reactions have to... 

In Situ Characterization. As a result of this study, it is clear that Co or other metals may play a dual catal)hic role catalyzing both the growth of nanowires and the production of airborne Si species, such as silane, which acts as a silicon source. An in situ characterization method will eventually be needed. 

Reymond, S. Brunei, J. M. Buono, G. (2000) New development in the enantioselective ring opening of meso-epoxides by various ion silicon sources catalyzed by an o-methoxyaryldiazaphosphonamide Lewis base., Tetrahedron Asymmetry, 11 4441-4445. 

The presence of silicon on the deactivated additive raised several questions about the silicon source and how it gets to the additive. The effect of steam was examined first. 

Synthesis of MCM-41 with Additives. The hydrothermal crystallization procedure as described earlier [10] was modified by adding additional salts like tetraalkylammonium (TAA+) bromide or alkali bromides to the synthesis gel [11]. Sodium silicate solution ( 14% NaOH, 27% Si02) was used as the silicon source. Cetyltrimethylammonium (CTA) bromide was used as the surfactant (Cl6). Other surfactants like octadecylltrimethylammonium (ODA) bromide (C,8), myristyltrimethylammonium (MTA) bromide (C,4) were also used to get MCM-41 structures with different pore diameter. Different tetralkylammonium or alkali halide salts were dissolved in little water and added to the gel before addition of the silica source. The final gel mixture was stirred for 2 h at room temperature and then transferred into polypropylene bottles and statically heated at 100°C for 4 days under autogeneous pressure. The final solid material obtained was washed with plenty of water, dried and calcined (heating rate l°C/min) at 560°C for 6 h. 

A difference in the relationship between UCV and degree of substitution x has recently been reported in connection with the use of tetramethyl orthosilicate (TMOS) as the silicon source instead of TEOS. The Ti content can be increased up to a value of 0.05 without evidence for extra-framework titanium species in the UV-visible spectra. However the UCV increases do not follow the relationship of Eq. (5) and the maximum value obtained, 5390 A3 at x = 0.05, is the... 

This reaction is not a simple one. There are a number of intermediate chlorosilanes generated by competing reactions (10). The process is sensitive both to the thermodynamics and kinetics of the chemical reactions, and to the fluid mechanics (qv) of the gas flow in the reactor. The overall procedure involves purging the reactor with hydrogen gas, raising the temperature of the reactor, cleaning the wafers with a brief HC1 etch, and replacing the HQ with the silicon source gas. A complete process cycle can take up to an hour. 

Silicon Dioxide. Si02 layers produced by PECVD are useful for intermetal dielectric layers and mechanical or chemical protection and as diffusion masks and gate oxides on compound-semiconductor devices. The films are generally formed by the plasma-enhanced reaction of SiH4 at 200-300 °C with nitrous oxide (N20), but CO, C02, or 02 have also been used (238-241). Other silicon sources including tetramethoxysilane, methyl dimethoxysilane, and tetramethylsilane have also been investigated (202). Diborane or phosphine can be added to the deposition atmosphere to form doped oxide layers. 

Figure 9 Channel structure of a phase separator generated by ASE deep etching of silicon. (Source IMM.)...

The SiC coating is processed based on the reaction of SiO vapor and carbon materials. Commercial SiO powders (99.9% pure) are provided as the silicon source. The carbon materials are placed on the SiO powder bed via a carbon felt as illustrated in Fig. 10.1. This assembly is covered with carbon sheets in an alumina crucible to keep the SiO gas pressure in the crucible, and heated in a vacuum furnace at various temperatures from 1150 to 1550°C in vacuum (about 0.03 Pa) for periods of time between 1 and 90 minutes. It is necessary to heat at a temperature greater than 1150°C for the vaporization of solid SiO. 

Instead of introducing a degree of mesoporosity into a microporous catalyst, the problem can also be approached from the opposite direction. Kloetstra et al. reported the introduction of crystalline microporous domains inside mesoporous MCM-41 by the partial recrystalhzation of the pore walls.[81] The mesoporous host can be regarded as the aluminium and silicon source for the zeolite crystallization. 

Figure 3 The CVI apparatus with SiAc4 as a silicon source. Abbreviations identical to the ones used in Fig-...

Effect of the Structure of Silicon Sources. Hoebbel et al. used silicic acid sols, silicic acid gels, or Aerosil as a silica source of tetramethylammonium silicate aqueous solutions (9). In the solutions at the conditions that a N/Si ratio is 1.0 and Si02 concentration is ca. 1.4 mol dm-, the distributions of silicate anions are almost the same, and the cubic octamer is a dominant species, although the degradation rates of the silica sources are different. This suggests that the cubic octamer is formed in the tetramethylammonium silicate aqueous solution, regardless of the type of silica source with tetra-functionality. Tetraalkoxysilanes (Si(0R), R denotes an alkyl group) can be used as a silica source as well (4,12,14). 

Mesoporous phosphomolybdic acid has been hydrotheimally synthesized with the surfaetant template cetyltrimethylammonium bromide, phosphomolybdic acid and tetraethyl orthosilicate as the silicon source. The parent materials are extracted in a solvent to remove the oi anic structuredirecting agent. The materials have been characterized by chemical analysis. X-ray diffraction, TEM, Infrared spectra and nitrogen BET surface measurement. After extraction of the silica-modified salts with HCI/EtOH, the surface area of the mesoporous material is up to 140 m /g, and the pore diameter is 30 40 nm. 

The chemistry of these hydrolysis and condensation reactions is very complicated. Both reactions are pH sensitive. Acids and bases catalyze the hydrolysis reactions, although each to a different extent (i). The condensation reaction is also highly pH dependent. Water is consumed in the hydrolysis reaction but liberated in the condensation reaction. To fully activate all of the silanols initially requires twice as much water as is consumed in the net reaction. To use less water means the reactions will be closely coupled. The reactions are usually conducted in a solvent, because the alkoxides are immiscible with the water required for hydrolysis. The usual solvent is the alcohol corresponding to the alkoxide group of the silicon source, which is also a byproduct of the reactions. 

Especially rather diluted synthesis mixtures at pH 13.5-14.0 were applied. Aerosil 200 (Degussa) was used as the silicon source and the presence of alkah metal ions was kept as low as possible. The TPA to Si02 ratio was relatively high. After ageing the synthesis mixtures while stirring for 5 hr at ambient conditions, the hydrothermal syntheses were performed at 180°C for 45-50 hr. 

There are several structurally different types or polymers that are suitable precursors for ternary Si-C-N ceramics. By far the most investigated precursors are polysilazanes of the general type [Si(R )(R°)N(R°)] (R, R°, R° = H, alkyl, aryl, alkenyl, etc.). In contrast to the limited number of starting compounds, H SiCl(4 ) (x = 0-3) as the silicon source and NH3 or H2N-NH2 as the nitrogen source for synthesis of polysilazanes as precursors for binary Si-N ceramics, the chemistry of polycarbosilazanes, that is, carbon-containing or modified polysilazanes, is very multifaceted. The attachment of various organic groups to the silicon atoms allows adjustment of their physicochemical properties, to control their thermolysis chemistry, and also to influence materials properties. The first... 

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