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Chemical-Vapor-

Deposited Materials for High Thermal Conductivity Applications

Jitendra S. Goela, Nathaniel E. Brese, Michael A. Pickering, and John E. Graebner Introduction Chemical vapor deposition (CVD) is an attractive method for producing bulk and thin-film materials for a variety of applications. In this method, gaseous reagents condense onto a substrate and then react to produce solid materials.1 The materials produced by CVD are theoretically dense, highly pure, and have other superior properties.2–5 By varying the process parameters, the CVD process can produce materials in a variety of forms such as single-crystal, polycrystalline, or amorphous. This process also has the potential to produce near-net-shape and precisionreplicated components that do not require post-deposition fabrication.6–8 The CVD process is scalable and reproducible. Monolithic sheets up to 2 m long, 1.5 m wide, and 10 cm thick have been successfully produced.2 Due to these attractive features, the use of CVD technology has expanded significantly in the last several years. Some examples of materials produced by CVD are diamond,9–11 SiC,3–5 Si,6,12–15 Si3N4 ,16–18 pyrolytic graphite and BN,19–20 ZnS,2,4,7,21 ZnSe,2,4,22 TiB2,23,24 and B4C.3 To illustrate the advantages of the CVD method, we take the example of SiC. Four forms of SiC are commercially available: CVD SiC, single-crystal,25,26 reaction-bonded—which includes a second phase of Si (10–40%)—and hotpressed.27–29 Their thermal conductivity values are respectively 300–374 W m1 K1, 490 W m1 K1, 120–170 W m1 K1, and 50–120 W m1 K1. We see that thermal

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conductivity is the highest for singlecrystal material. However, this material is only available as wafers of, at most, a few inches in diameter. The next best candidate is CVD SiC, which is a fine-grained polycrystalline material. This material has significantly better thermal conductivity and polishability than hot-pressed and reaction-bonded SiC. In this article, we present CVD materials for high thermal conductivity applications. Due to limited space, the scope of this article has been restricted primarily to diamond and SiC.

CVD Diamond At temperatures above approximately 50 K, diamond has the highest thermal conductivity of any known material. At room temperature, gem-quality diamond exhibits values of thermal conductivity  in the range of 2000–2500 W m1 K1, which is 5–6 times higher than that of copper.30,31 Diamond of lower optical quality has correspondingly lower thermal conductivity. In the last 10 years, a great deal of progress has been made in the synthetic production of diamond films by CVD.32–35 These films are polycrystalline, but under proper growth conditions, they can be optically clear and have thermal conductivity values36–37 of more than 2000 W m1 K1. The synthesis of diamond is usually difficult because graphite, rather than diamond, is the thermodynamically stable form of carbon at room temperature and

normal pressures. The use of simultaneous high pressure (50 kbar) and high temperature