The CVD of Silicon Nitride

Silicon nitride (Si3N4) is a major industrial material which is produced extensively by CVD for electronic and structural applications. It is an excellent electrical insulator and diffusion barrier (to sodium and water vapor) and has replaced CVD oxides in many semiconductor devices.l l Silicon nitride coatings are produced by the reaction of silicon tetrachloride (SiCl4) with ammonia  

The optimum deposition temperature is 850. Pressure may be up to 1 atm.. A hydrogen or nitrogen atmosphere is used with a high ratio of N2 to reactants. 

Another reaction uses dichlorosilane (SiH2Cl2), also with ammonia  

The range of deposition temperature is 755-810 C with a high dilution of nitrcgen.I When a high-frequency plasma (13.56 MHz) is used, the deposition temperature is lower (400-600 C).f l 

Another common deposition reaction combines ammonia with silane as the silicon source  

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This approach was successfully used in modeling the CVD of silicon nitride (Si3N4) films [18, 19, 22, 23]. Alternatively, molecular dynamics (MD) simulations can be used instead of or in combination with the MC approach to simulate kinetic steps of film evolution during the growth process (see, for example, a study of Zr02 deposition on the Si(100) surface [24]). Finally, the results of these simulations (overall reaction constants and film characteristics) can be used in the subsequent reactor modeling and the detailed calculations of film structure and properties, including defects and impurities. 

Silicon oxynitride (SiO cN),) films exhibit properties that fall somewhere between those of Si02 and those of Si3N4 films and have diverse applications in microelectronics. The oxynitride layers can be obtained if nitrogen oxide (N2O) is involved in the reaction of silane with ammonia [108, 109, 117-119], when silicon nitride is deposited onto an oxidized silicon substrate (silicon dioxide nitrification is incomplete at 800°C [100, 107]), or upon addition of gaseous oxygen during the CVD of silicon nitride [64, 100, 104, 120]. 

The thermal nitration of a H-terminated Si surface and the CVD of silicon nitride were studied in situ by FTIR-IRRAS [160, 161]. The adsorption and thermal decomposition of phosphine (PH3) on a Si surface was also studied by IR absorption depending on the coverage and the exposure to the flux, PH3 adsorbs both nondissociatively and dissociatively, and the IR absorption peaks... 

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. 

The last class of CVD reaction is what we will call co-deposition. This indicates deposition from a mixture of precursors, where atoms from several species contribute to the deposited film. This approach is generally used for the deposition of compound materials, where the desired film is composed of several elements. Examples of this kind of CVD system include the deposition of gallium arsenide from trimethylgallium (TMG) and arsine Ga(CH3)3 + ASH3 GaAs + 3 CH, as well as the deposition of silicon nitride from silicon... 

The formation of silicon nitride whiskers was observed in several different reactions, including vapor deposition, CVD, and growth from a melt. However, only the following techniques are considered to have commercial significance nitriding of metallic silicon or silicon-silica mixture, carbothermal reduction of silica with simultaneous nitridation, and thermal decomposition of silicon halides. 

If a plasma is used to generate ions or radicals that recombine to give the desired film, the process is plasma-enhanced CVD (PEC VD). In PECVD it is possible to use much lower substrate temperatures because the plasma provides energy for the reaction to proceed. A major commercial application of PECVD is the formation of silicon nitride films for passivation and encapsulation of semiconductor devices. At this stage of the fabrication process the device caimot tolerate temperatures much above 300°C. High temperatures would still be required if crystalline or epitactic films were required. Many nitrides have been prepared in thin-film form by PECVD, including AIN, GaN, TiN, and BN. A more complete list of films deposited by PECVD is given in Table 28.3. 

A CVD-plasma reactor is shown schematically in Fig. 15.2 and several variations are used on a large scale for the deposition of silicon nitride for semiconductor devices. The reactor generally operates at 450 kHz or 113.56 MHz. Typical deposition conditions are 360 C and 260 Pa.l65]... 

Grannen, K. J., Xiong, F., and Chang, R., The Growth of Silicon-Nitride Crystalline Films using Microwave Plasma-Enhanced CVD, J. Mater. Res., 9(9)12341-2348 (1994)... 

Dielectric Deposition Systems. The most common techniques used for dielectric deposition include chemical vapor deposition (CVD), sputtering, and spin-on films. In a CVD system thermal or plasma energy is used to decompose source molecules on the semiconductor surface (189). In plasma-enhanced CVD (PECVD), typical source gases include silane, SiH4, and nitrous oxide, N20, for deposition of silicon nitride. The most common CVD films used are silicon dioxide, silicon nitride, and silicon oxynitrides. 

A second type of silicon nitride, called stoichiometric silicon nitride, is deposited at much higher temperatures using CVD or LPCVD in the form of Si3N4. Stoichiometric silicon nitride can be used as a mask for the selective oxidation of silicon. Here the silicon nitride is patterned over a silicon substrate, and the exposed silicon is oxidized. The silicon nitride oxidizes very slowly compared to the silicon. 

Plasma-enhanced (PE) CVD deposition of silicon nitride is typically performed at temperatures between 250 °C and 400 °C from silane (SiH4) and ammonia (NH3) gases. The plasma enhances the deposition rates. The deposition rate strongly depends on the deposition temperature and RF power, typical rates being around 400 nmmin4 [17]. Because of the low deposition temperature,... 

CVD of boron nitride films on silicon or germanium or on printed circuit boards is now a common practice in the electronic industry [154 to 162]. The high thermal conductivity combined with the excellent electrical insulation properties are most valuable for these applications [163] see additional references in Section 4.1.1.10.8, p. 129. The use of a-BN layers is of particular importance in the manufacture of electrophotographic photoreceptors (such as solar cells) and of X-ray lithographic masks (see Section 4.1.1.10.8, p. 129). In the last mentioned application, structural aspects of the deposited films are of importance. In films still containing hydrogen, (N)H moieties are depleted by annealing at about 600°C, while (B)H moieties are depleted above 1000°C [164]. Also, elastic stiffness and thermal expansion of boron nitride films have to be viewed in connection with the temperature-dependent stress of CVD-deposited boron nitride films [165]. Reviews of properties and electronic applications of boron nitride layers have appeared in Polish [166] and Japanese [167]. 

IR data (vSiN 820-833 cm ) were used to characterise SiN films formed by a variety of CVD techniques. The IR spectra of amorphous SiOxN3 thin films included vSiO and vSiN features at 1105 cm", 865 cm respectively Two other reports have been made of IR spectra of CVD-produced silicon nitride and oxynitride films " " The FTIR spectrum of an SisN4 powder shows a feature at 1200 cm corresponding to the presence of Si-O units ... 

Ho has measured the dielectric properties of a wide variety of silicon nitride ceramics (CVD, sintered, hot pressed, differing composition and sintering aids) and of polycrystalline alumina versus single crystal sapphire. His results have shown that the temperature dependence of the dielectric constant is intrinsic to the crystalline lattice properties of the... 

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. 

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). 

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