Silicon, CVD

Giling LJ, De Moor HHC, Jacobs WPJH, Saaman AA (1986) J Crystal Growth 78 303 A study on silicon CVD using SiH4 dealing with adsorption processes related to supersaturation of the gas phase is reported. 

H. Moffat and K.F. Jensen. Three-Dimensional Flow Effects in Silicon CVD in Horizontal Reactors. J. Electrochem. Soc., 135(2) 459-471,1988. 

Some of the specifics of the epi silicon CVD process will be covered in the balance of this chapter. 

MAD was synthesized according to Collioud et a/.8 Silicon wafers with a naturally grown oxide layer (referenced as silicon), CVD deposited Si3N4 on silicon wafers (silicon nitride) and hot filament CVD (HFCVD) deposited... 

The CVD method is very versatile and can work at low or atmospheric pressure and at relatively low temperatures. Amorphous, polycrystalline, epitaxial, and uniaxially oriented polycrystalline layers can be deposited with a high degree of purity, control, and economy. CVD is used extensively in the semiconductor industry and has played an important role in past transistor miniaturization by making it possible to deposit very thin films of silicon. CVD also constitutes the principal building technique in surface micromachining (see below). 

This method can be used for the synthesis of various cyclic tri-and tetrasiloxanes. These cyclic siloxanes can be applied as precursor for functional silicones, CVD precursor for insulation layer of semiconductors, etc. 

CVD gaseous reactants (precursors) delivered to a heated substrate in a flow reactor undergo tliennal reaction to deposit solid films at atmospheric or reduced pressure, and volatile side products are pumped away. CVD is used for conductors, insulators and dielectrics, elemental semiconductors and compound semiconductors and is a workliorse in tire silicon microelectronics industry. 

Silicon Epitaxy. A critical step ia IC fabricatioa is the epitaxial depositioa of sdicoa oa an iategrated circuit. Epitaxy is defined as a process whereby a thin crystalline film is grown on a crystalline substrate. Silicon epitaxy is used ia bipolar ICs to create a high resistivity layer oa a low resistivity substrate. Most epitaxial depositioas are doae either by chemical vapor depositioa (CVD) or by molecular beam epitaxy (MBE) (see Thin films). CVD is the mainstream process. 

Molecular beam epitaxy is a non-CVD epitaxial process that deposits silicon through evaporation. MBE is becoming more common as commercial equipment becomes available. In essence, silicon is heated to moderate temperature by an electron beam in a high vacuum... 

Purification of Silicon. Chemical purity plays an equally important role in the bulk of materials as on the surface. To approach the goal of absolute stmctural perfection and chemical purity, semiconductor Si is purified by the distillation of trichlorosilane [10025-78-2] SiHCl, followed by chemical vapor deposition (CVD) of hulk polycrystalline siUcon. 

Also noted is the rapid expansion of a number of materials produced by CVD, which include copper, tungsten, diamond, silicon carbide, silicon nitride, titanium nitride, and others. The coverage of the chemistry and deposition techniques of these materials has been greatly expanded. 

Chemical vapor deposition (C VD) is a versatile process suitable for the manufacturing of coatings, powders, fibers, and monolithic components. With CVD, it is possible to produce most metals, many nonmetallic elements such as carbon and silicon as well as a large number of compounds including carbides, nitrides, oxides, intermetallics, and many others. This technology is now an essential factor in the manufacture of semiconductors and other electronic components, in the coating of tools, bearings, and other wear-resistant parts and in many optical, optoelectronic and corrosion applications. The market for CVD products in the U.S. and abroad is expected to reach several billions dollars by the end of the century. 

Generally, epitaxial films have superior properties and, whenever possible, epitaxial growth should be promoted. The epitaxial CVD of silicon and III-V and E-VI compounds is now a major process in the semiconductor industry and is expected to play an increasingly important part in improving the performance of semiconductor and optoelectronic designs (see Chs. 13-15). 

Fujimo, K., Nishimoto, Y., Tokumasu, N., andMaeda, K., Doped Silicon Oxide Deposition by Atmospheric Pressure and Low Temperature CVD using Tetraethoxysilane and Ozone, J. Electrochem. Soc., 138(10) (Oct. 1991)... 

The term metallo-organic is used somewhat loosely in CVD parlance, since it includes compounds of elements, such as silicon, phosphorus, arsenic, selenium, and tellurium, that are not considered metallic. To conform to what appears to be a well-established tradition, such nonmetal compounds will be included here as metallo-organics. 

Hoffman, D. M., et al., Plasma-Enhanced CVD of Silicon Nitride Films from a Metallo-Organic Precursor, J. Mater. Res., 9(12) 3019-3021 (1994)... 

Ultra-High Vacuum Reactors. CVD reactions at extremely low pressures (i.e., 10 Torr) are being developed for the deposition of semiconductor materials, such as silicon-germanium and some optoelectronic materials. Advantages appear to be better control of the deposit structure and reduction of impurities. 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

Plasma CVD was first developed in the 1960s for semiconductor applications, notably for the deposition of silicon nitride. The number and variety of applications have expanded greatly ever since and it is now a major process on par with thermal CVD. 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

In addition to the thermal CVD reactions listed above, tungsten can be deposited by plasma CVD using Reaction(l)at350°C.[ ll P At this temperature, a metastable alpha structure (aW) is formed instead of the stable be.c. Tungsten is also deposited by an excimer laser by Reaction (1) at < 1 Torr to produce stripes on silicon substrate.P l... 

Tsao,K.Y.,andBusta,H.H, Low Pressure CVD of Tungsten on Poly cry stalline and Single-Crystal Silicon viathe SiliconReduction, J. Electrochem. Soc., 131(ll) 2702-2708 (Nov. 1984)... 

This chapter is a review of the CVD of non-metallic elements and covers boron, silicon, and germanium. Silicon and germanium are borderline elements with metalloid characteristics. Both are important semiconductor materials, particularly silicon, which forms the backbone of the largest business in the world the electronic industry. All three materials are deposited by CVD on an industrial scale and a wide variety of CVD reactions are available. 

Silicon has the crystal structure of diamond and its properties are influenced by the crystal orientation. ] CVD silicon can be... 

Several precursors and CVD reactions are available to deposit silicon. The deposit can be either single crystal (epitaxial), polycrystalline, or amorphous. 

Amorphous Silicon. Amorphous silicon is generally deposited by Reaction (4) at a deposition temperature of 560°C and at low pressure (ca. 1 Torr).P l Helium RF plasma CVD is also commonly used, especially in the production of solar photovoltaic devices. 

Many applications of silicon are found in integrated circuits and other semiconductor devices and include the following (see Chs. 13-16 on applications of CVD). 

Reif, R., Low Temperature Silicon Epitaxy by Plasma Enhanced CVD, Proc. 5th European Conf. on CVD, (J. Carlsson and J. Lindstrom, eds.), pp. 13-19, Univ. ofUppsala, Sweden (1985)... 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

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