Silicon-dioxide

Silicon dioxide (Si02), also known as silica, is a major industrial material with many applications particularly in the semiconductor industry in the form of coatings, which are produced mostly by CVD. It is an excellent electrical insulator with very low thermal expansion and good resistance to thermal shock. Its characteristics and properties are summarized in Table 11.4. 

Chemical Resistance. Silica is resistant to oxidation and is not attacked by most chemical reagents at room temperature. 

Composition Si02 Molecular Weight 60.09 g/mol Color clear 

Melting Point 1610 °C (molten Si02 has high viscosity) 

Note Test temperature is 20°C imless otherwise stated 

Silicon dioxide. Red atoms are oxygen and yellow atoms are silicon. PUBLISHERS RESOURCE GROUP 

An especially interesting form of silicon dioxide is silica gel, a powdery form of amorphous silicon dioxide that is highly adsorbent. An adsorbent material (in contrast to an absorbent material) is one that is capable of removing a material, such as water, ammonia, alcohol, or other gases, out of the air. The second material bonds weakly to the outer surface of silica gel particles. Silica gel is able to adsorb anywhere from 30 to 50 percent of its own weight in water 

Stardust, a U.S. National Aeronautics and Space Administration (NASA) spacecraft, used silica gel to collect particles of debris from the tail of comet Wild-2. 

Although silica gel has been known since the midseventeenth century, practical applications for the material were not discov- 

Although methods are available for synthesizing silicon dioxide, there is no practical reason for doing so. The abundant quantities of silicon dioxide found in the earth s crust are sufficient to satisfy all industrial needs. Among the minerals and earths that contain silicon dioxide in an uncombined form are quartz, flint, diatomite, stishovite, agate, amethyst, chalcedony, cristobalite, and tridymite. 

Silicon dioxide (Si02) is a least expensive material with good thermal and electrical insulation properties. Its resistivity is 10 (Q-cm) and thermal conductivity is about 0.014 (W/cm- C). It is used as a low-cost material for masks in microfabrication process such as etching and deposition. It is also used as a sacrificial material in surface micromachining. 

Silicon dioxide is grown using thermal oxidation procedure. Thermal oxidation can be categorized as both dry oxidation and wet oxidation. Silicon reacts with dry oxygen at high temperature (800-1200 °C) during dry oxidation process with the following reaction  

In wet oxidation process, water vapor reacts with silicon at high temperature, and the corresponding reaction is 

The growth rate of thermal oxidation decreases with increase in the thickness of oxide layer, because it relies on the diffusion of oxygen. The chemical vapor deposition (CVD) process can make thicker silicon dioxide layer and does not require silicon substrate. Silane is a toxic extremely flammable chemical compound with chemical formula SiH4. The silane CVD process using silane gas is based on the following reaction  

The plasma-enhanced CVD (PECVD) process uses the reaction  

BP Colloidal anhydrous silica PhEur Silica colloidalis anhydrica USPNF Colloidal silicon dioxide 

Aerosil Cab-O-Sil Cab-O-Sil MSP-, colloidal silica fumed silica light anhydrous silicic acid silicic anhydride silicon dioxide fumed Wacker HDK. 

Colloidal silicon dioxide is widely used in pharmaceuticals, cosmetics, and food products see Table I. Its small particle size and large specific surface area give it desirable flow characteristics that are exploited to improve the flow properties of dry powders in a number of processes such as tableting.  

In aerosols, other than those for inhalation, colloidal silicon dioxide is used to promote particulate suspension, eliminate hard settling, and minimize the clogging of spray nozzles. Colloidal silicon dioxide is also used as a tablet disintegrant and as an adsorbent dispersing agent for liquids in powders. Colloidal silicon dioxide is frequently added to suppository formulations containing lipophilic excipients to increase 

Colloidal silicon dioxide is a submicroscopic fumed silica with a particle size of about 15 nm. It is a light, loose, bluish-white-colored, odorless, tasteless, nongritty amorphous powder. 

Silicon Solid-state Chemistry. Following the pattern set in the previous volume, this aspect of silicon chemistry will be subdivided into four sections in which the literature published on silicon dioxide, silicates, aluminosilicates, and zeolites is described separately. In all four sections, emphasis will be laid on the inorganic chemistry of these materials, and papers describing solely their 

A comprehensive review of the chemical composition of the lunar surface, as ascertained by analysis of samples obtained on the Surveyor and subsequent Apollo and Luna missions, has been collated by Turkevich. The lunar surface is made up of silicate rocks, the principal minerals being calcium-rich feldspars and pyroxenes. In many respects the chemical composition of the maria analysed are similar to those of the terrestrial basalts. The terra regions analysed are distinctly different from the maria in having considerably smaller amounts of iron and titanium and larger amounts of calcium and aluminium. Apollo missions have shown that the lunar mare material is very dry and was produced under relatively reducing conditions. 

The participation of silicon 3d-type functions in the high-energy oxygen 2p non-bonding orbitals was found to be very small. 

The Raman spectra of a- and -crystobalite and of /5-quartz have been measured. The spectra of the crystobalite samples were determined as a 

A Knudsen effusion study with i3-crystobalite on a thermomicrobalance has been carried out over the temperature range 1823—1983 K. Interpretation of the effusion data, based on the reactions (93) and (94), gives rise to the 

Chapter 3. Surprisingly few papers have been published, during the period of this Report, which describe aspects of the chemistry of silicon dioxide indeed, the majority of the reported data deal with surface properties and the chemistry of the species adsorbed thereon. Interest in the silicates has been maintained, with a marked increase in the number of communications relating to the chemistry of the interlamellar complexes formed by the layered silicates, particularly montmorillo-nite. 

A normal-co-ordinate analysis of the vibrations of Mg2Si04 has been carried out to investigate the group behaviour of the SiO ion. An i.r. and Raman study of a number of silicates with SiaO ring structures has been undertaken to show that the essential spectroscopic features of these rings are modified under the influence of structure change with or without modification of the local symmetry of the ring. 

Thermodynamic properties of fused silicates have been calculated on the basis of a polymeric model assuming ideal ionic solutions satisfactory agreement with experimental data was achieved. 

Silicon Dioxide. The nature of the a-j8 transition in quartz has been considered it occurs in a temperature interval of 0.05 K and is characterized by temperature hysteresis (1.4 0.3K). The nature of the defects produced in non-crystalline Si02 and o-quartz single crystals by fast neutron irradiation has 

2 X 10 ° neutrons cm , the quartz sample was transformed into a non-crystalline material. At 9x 10 neutrons cm , however, remnant crystallites of the original a-quartz remained. On annealing ( 1000 °C), the structures of the irradiated samples transformed into that of unirradiated SiOa- The Fe e.p.r. spectrum in irradiated quartz has also been investigated.  

Silicon Solid-state Chemistry.—Contrary to the pattern adopted in previous volumes, the chemistry of aluminosilicates and zeolites will not be discussed here the data published during the period of this Report associated with these materials are considered in detail in Chapter 3. In this section, silicon dioxide and the silicates will be described separately emphasis will be laid on the inorganic chemistry of these compounds, and papers describing solely their catalytic, adsorption, diffusion, and other similar properties will not be considered. 

Silicon Dioxide. Although most authors in this field are interested in the chemistry of species adsorbed on the surface of Si02, a limited number of papers have been published describing the physical and chemical properties 

The nature of the impurity water in synthetic quartz289 and of the O- hole centre in natural quartz290 has been examined by i.r. and e.s.r. spectroscopic techniques, respectively. 

Frieser292 has shown that hydrophobic, hydrophilic, and organophilic Si02 surfaces may be distinguished easily by two independent contact-angle measurements with different liquids. Interest in Si02 surface chemistry has centered, however, on the characterization (principally by means of i.r. and e.s.r. spectroscopy) of the functional groups present on surfaces subjected to 

Senemaud, M. T. Costa Lima, J. A. Roger, and A. Cachard, Chem. Phys. Letters, 1974, 26, 

In the present chapter, we will turn our attention to films deposited by thermal CVD that are either dielectrics or semiconductors. There are, as one would expect, many films that can be deposited by this technique. In addition, there are many gaseous reactants that one can use to create each film, the choice depending on the film characteristics desired. Rather then attempt to catalogue all of the possible films and reactants, we will choose instead to focus on silicon dioxide, silicon nitride, polysilicon, and epitaxial silicon as the films of interest. At the same time, we will only look at those reactant gases that have been used for integrated circuit manufacture. An excellent survey of the film types that can be deposited by CVD and the many reactants that have been used to obtain them has been given by Kern.1 

The silica particles obtained by the above process were by and large spherical in nature except when only Rokafenol N-9 was used. The formulation-dependent size ranges also varied with the mode of stirring -820-1560 nm (rapid stirring motor) - 620-810 nm (ultrasonic bath) -870-1100 nm (homogenizer). In a similar way, the bulk density of the particles could also be tailored in the range 90-215 g/dm  

Yamashita and others [188] synthesized microporous silica gel particles by 

The primary particle size varied in the range 8-24 nm, and the surface area was significantly high, i.e. in the range 65 -400 mVg. 

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The size of the exciton is approximately 50 A in a material like silicon, whereas for an insulator the size would be much smaller for example, using our numbers above for silicon dioxide, one would obtain a radius of only 3 A or less. For excitons of this size, it becomes problematic to incorporate a static dielectric constant based on macroscopic crystalline values. 

For some materials, the most notable being silicon, heating alone sufiBces to clean the surface. Commercial Si wafers are produced with a thin layer of silicon dioxide covering the surface. This native oxide is inert to reaction with the atmosphere, and therefore keeps the underlying Si material clean. The native oxide layer is desorbed, i.e. removed into the gas phase, by heating the wafer in UHV to a temperature above approximately 1100 °C. This procedure directly fonus a clean, well ordered Si surface. 

Xu X and Goodman D W 1992 New approach to the preparation of ultrathin silicon dioxide films at low temperature Appl. Phys. Lett. 61 774... 

Figure Bl.26.15. The Del/Psi trajectory for silicon dioxide on silicon with angle of mcidence ( )j = 70° and wavelength X = 6328 A (Tompkins FI G 1993 A Users Guide to Ellipsometry (San Diego, CA Academic)).

Silicon, like carbon, is unaffected by dilute acids. Powdered silicon dissolves incompletely in concentrated nitric acid to give insoluble silicon dioxide, SiOj ... 

When silica (silicon dioxide) and silicon are heated in vacuo to 1700 K,... 

Laughing gas, see Nitrogen(I) oxide Lautarite, see Calcium iodate Lawrencite, see Iron(II) chloride Lechatelierite, see Silicon dioxide Lime, see Calcium oxide Litharge, see Lead(II) oxide... 

Oldhamite, see Calcium sulfide Opal, see Silicon dioxide Orpiment, see Arsenic trisulfide Oxygen powder, see Sodium peroxide... 

Rochelle salt, see Potassium sodium tartrate 4-water Rock crystal, see Silicon dioxide Rutile, see Titanium(IV) oxide... 

Sellaite, see Magnesium fluoride Senarmontite, see Antimony(III) oxide Siderite, see Iron(II) carbonate Siderotil, see Iron(II) sulfate 5-water Silica, see Silicon dioxide Silicotungstic acid, see Silicon oxide—tungsten oxide—water (1/12/26)... 

Thenardite, see Sodium sulfate Thionyl, see Sulflnyl Thorianite, see Thorium dioxide Topaz, see Aluminum hexafluorosilicate Tridymite, see Silicon dioxide Troilite, see Iron(II) sulflde... 

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal, which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively, the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used in place of carbon.

Elastomeric shield materials (ESM) have been developed as low density flexible ablators for low shear appHcations (49). General Electric s RTV 560 is a foamed silicone elastomer loaded with silicon dioxide [7631-86-9] and iron oxide [1317-61 -9] particles, which decomposes to a similar foam of Si02, SiC, and EeSiO. Silicone resins are relatively resistant to thermal decomposition and the silicon dioxide forms a viscous Hquid when molten (50) (see... 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

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. 

Amorphous silica, ie, silicon dioxide [7631-86-9] Si02, does not have a crystalline stmcture as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quart2 at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

As an example of the use of AES to obtain chemical, as well as elemental, information, the depth profiling of a nitrided silicon dioxide layer on a silicon substrate is shown in Figure 6. Using the linearized secondary electron cascade background subtraction technique and peak fitting of chemical line shape standards, the chemistry in the depth profile of the nitrided silicon dioxide layer was determined and is shown in Figure 6. This profile includes information on the percentage of the Si atoms that are bound in each of the chemistries present as a function of the depth in the film. 

Metallic iron is most often extracted from hematite ore, which consists of iron(III) oxide mixed with impurities such as silicon dioxide, Si02. 

Sand consists mainly of silicon dioxide. When sand is heated with an excess of coke (carbon), pure silicon and carbon monoxide are produced. 

Finally, the strongly basic metal oxides react with silicon dioxide to form a glassy product ... 

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