Deposition boron nitrides

Boron nitride exists in many different structures due to the special bonding behaviors of boron and nitrogen. Although the most well-defined crystallographic structures are hexagonal BN (h-BN), cubic BN (c-BN), and wurtzitic BN (w-BN), other crystalline structures, such as explosion boron nitride (e-BN) and ion beam-deposited boron nitride (i-BN) [124—135] and amorphous BN (a-BN) [136,137] also exist. 

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]. 

Li, P. C., Capriulo, A. J., and Lepie, M. P., Chemically Vapor Deposited Boron Nitride, Proc. OSU-RTD Symp. on Electro-Magnetic Windows, Vol. 1 (June 1964)... 

Uses. Hot-pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, especially in inert or reducing atmospheres, and for special materials such as IITV semiconductors (qv). Its low thermal expansion makes it resistant to thermal shock. The powder can be used as a mold release agent or as thermal insulation. Boron nitride is also available in fiber form (19). BN deposited pyrolyticaHy on refractory substrates at 1200—1800°C has a turbostratic stmcture and low porosity it has greater chemical resistance and is impervious to helium. 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

Nakamura, K., Preparation and Properties of Boron Nitride Films by Metal Organic Chemical Vapor Deposition, /. Electochem. Soc., 133-6 120-1123 (1986)... 

The organic deposition sources are made of a variety of materials including ceramics (e.g., boron nitride, aluminum oxide, and quartz) or metallic boats (e.g., tantalum or molybdenum). Deposition is carried out in high vacuum at a base pressure of around 10-7 torr. The vacuum conditions under which OLEDs are fabricated are extremely important [41] and evaporation rates, monitored using quartz oscillators, are typically in the range 0.01 0.5 nm/s in research and development tools. In manufacturing, higher rates or multiple sources may be used to reduce tact times. 

The multilayer nanocomposite films containing layers of quasi-spherical Fe nanoparticles (d — 5.8 nm) separated by dielectric layers from boron nitride (BN) are synthesized by the repeated alternating deposition of BN and Fe onto a silicon substrate [54]. In this work the authors managed to realize the correlation in the arrangement of Fe nanoparticles between the layers the thin BN layer deposited on the Fe layer has a wave-like relief, on which the disposition of Fe nanoparticles is imprinted as a result, the next Fe layer deposited onto BN reproduces the structure of the previous Fe layer. Thus, a three-dimensional ordered system of the nanoparticles has been formed on the basis of the initial ordered Fe nanoparticle layer deposited on silicon substrate [54]. The analogous three-dimensional structure composed of the Co nanoparticles layers, which alternate the layers of amorphous A1203, has been obtained by the PVD method [55]. 

Bando and co-workers271 have prepared BN nanolubes by the reaction of MgO, FeO and B in the presence of NH, at 1400 °C. Reaction of boric acid or B20, with N2 or NH, at high temperatures in the presence of carbon or catalytic metal particles has been employed in the preparation of BN nanotubes.2 2 Boron nitride nanotubes can be grown directly on substrates at 873 K by a plasma-enhanced laser-deposition technique.172 Recently, GaN nanotube brushes have been prepared using amorphous carbon nanotubes templates obtained using AAO membranes.274... 

Finally, an important form of boron nitride should be mentioned, pyrolytic boron nitride. It is manufactured by reacting ammonia and a boron halogenide at about 2000°C and depositing the BN vapor on a graphite substrate or mandrel. The characteristic feature of pyrolytic boron nitride is the high degree of crystal orientation with the hexagonal basal plane parallel to the mold surface and the c-direction perpendicular to the substrate. 

Yokoyama H., Okamoto M. et al.. Effects of a negative self-bias on the growth of cubic boron nitride prepared by plasma chemical vapor deposition, Jpn. J. Appl. P/2yj., 30 (1991) pp. 344-348. 

Ikeda T., Cubic boron nitride films synthesized by low-energy ion-beam-enhanced vapor deposition, Appl. Phys. Lett., 61 (1992) pp. 786-788. 

Tanabe N., Haysshi T., Iwaki M., Deposition of cubic boron nitride thin films by ion-beam-enhanced deposition, DiamondRelat. Mater., 1 (1992) pp. 883-890. 

Friedmann T. A., Mirkarimi P. B. et al.. Ion-assisted pulsed laser deposition of cubic boron nitride films, J. Appl. Phys., 76 (1994) pp. 3088-3101. 

McCarty K. F. et al., On the low-temperature threshold for cubic boron nitride formation in energetic film deposition. Diamond Relat. Mater., 5 (1996) pp. 1519-1526. 

Rednction of boron trihaUdes to elemental boron can be accomplished by heating with alkali metals, alkaline earth metals, or hydrogen. Under the proper conditions, rednctions of this type can also yield diborane and, under selected conditions, boron subhalides (see below). Metal hydrides also react with boron trihalides to give diborane. Boron nitride and boron carbide have been prepared by the high-temperature reductions of boron trihalides with ammonia and methane, respectively, and deposited on metal substrates by CVD. 

While plasma-enhanced methods are very usefiil to lower the substrate temperature, the as-deposited films are typically less conformal and often contain more surface impurities than competing methods. In this method, reactive radicals, ions, and atoms/molecules are formed in the gas phase that interact with the relatively low-temperature substrate to generate a film. Some of the more recent applications for plasma CVD include growth of cubic boron nitride (c-BN) thin films. 

Chemical vapor infiltration (CVI) is widely used in advanced composites manufacturing to deposit carbon, silicon carbide, boron nitride and other refractory materials within porous fiber preforms. " Because vapor phase reactants are deposited on solid fiber surfaces, CVI is clearly a special case of chemical vapor deposition (CVD). The distinguishing feature of CVI is that reactant gases are intended to infiltrate a permeable medium, in part at least, prior to... 

The results of this analysis are presented in a dimensionless form readily applicable to a range of preform thicknesses, fiber diameters, fiber volume fractions, and deposition chemistries. To illustrate the application of these results to a practical problem, the optimum process conditions are determined for a sample problem in which a boron nitride coating is deposited from boron trichloride and ammonia. 

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