Hot wire chemical vapor deposition

Device quality a-Si H made by HWCVD (as they termed it) was first reported by Mahan et al. [19, 527], They obtained n-Si H with hydrogen concentrations as low as 1%. Deposition rates as high as 5 nm/s [528] and 7 nm/s [529] have been achieved for n-Si H of high quality. In order to obtain device quality material it was shown by Doyle et al. [525] that the radicals that are generated at the filament (atomic Si and atomic H) must react in the gas phase to yield a precursor with high surface mobility. Hence, the mean free path of silane molecules should be smaller than the distance between filament and substrate, d(s- Too many reactions between radicals and silane molecules, however, result in worse material. In fact, optimal film properties are found for values of pdf of about 0.06 mbar-cm [530, 531]. 

Doping is achieved by adding dopant gases such as phosphine, diborane, or trimethylboron to the silane [545-547]. The doped material is of electronic quality comparable to PECVD-doped material. 

Cross-sectional view of a HWCVD deposition reactor. (From K. F. Feenstra. Ph D. Thesis, Universiteit Utrecht. Utrecht, the Netherlands. 1998. with permission.) 

A shutter can be placed in front of the substrate to prevent deposition during filament preheating. The reaction gases are injected on the lower-left side of the reactor, and directed towards the reactor wall in order to achieve more homogeneous gas flows. The (unreacted) gas is pumped out at the right side, so the gas flow is perpendicular to the filaments. 

The substrate is radiatively heated by heaters that are placed outside the vacuum. A backing plate ensures a laterally homogenous temperature profile. In the same chamber also PECVD can be carried out. The backing plate then is the grounded electrode, and the RF voltage is applied to the counter electrode. 

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Lau, K. K. S. (2001), Hot-wire chemical vapor deposition (HWCVD) of fluorocarbon and organosilicon thin films, Thin Solid Films, 395(1-2), 288-291. 

Another way of plasmachemical preparation involves the use of electron cyclotron resonance for plasma assisted chemical vapor deposition (ER-CVD) [7]. An alternative method to prepare undoped microcrystalline (pc) SiC H alloy films is developed recently as so-called hot wire chemical vapor deposition (HWCVD) technique [8]. 

Hot Wire Chemical Vapor Deposition (HWCVD-technique) yields devicequality material of Si C H-aUoy-films from pure methane and silane... 

HFPO is a useful source of difluorocarbene. When heated above 160°, HFPO decomposes to difluorocarbene and trifluoroacetyl fluoride (Eq 13.12). This reaction has been utilized to prepare thin perfluorinated conformal coatings on a wide variety of substrates using a hot-wire chemical vapor deposition process commercialized by GVD Corporation. 

K. E.H. Gilbert, Continuous hot wire chemical vapor deposition of high-density carbon multiwall nanotubes. Nano Lett., 3, 1425-1429 (2003). 

Table 7.2 summarizes various subtypes of CVD polymerization processes. These methods differ in the means by which the CVD chemistry is driven (plasma, thermal, or UV). For hot wire chemical vapor deposition (HWCVD) and initiated chemical vapor deposition (iCVD), no plasma excitation or UV exposure is utilized during the polymerization, eliminating the possibility for forming defects in the films via these... 

Yokomichi, H., Sakai, F., Ichihara, M., Kishimoto, N. - Attempt to synthesize carbon nanotubes by hot wire chemical vapor deposition . Thin Solid Films 395 (2001) 253-256... 

Dillon, A., Mahan, A., Alleman, 1., Heben, M., Barilla, P., Jones, K. - Hot wire chemical vapor deposition of carbon nanotubes . Thin Solid Films 430 (2003) 292-295... 

Boron, silicon carbide, diamond and other materials can be deposited by chemical vapor deposition on the surface of hot wires or hot fibers. If a minimal vapor deposit is applied, the process will modify only the surface of the fiber and produce a coating, while leaving its core functionality unchanged. If, however, a thick vapor deposit is applied, the process will create a new and very large diameter fiber that has the functionality of the sheath and a sacrificial core. 

Preparation of uranium metal. As discussed previously, some nuclear power plant reactors such as the UNGG type have required in the past a nonenriched uranium metal as nuclear fuel. Hence, such reactors were the major consumer of pure uranium metal. Uranium metal can be prepared using several reduction processes. First, it can be obtained by direct reduction of uranium halides (e.g., uranium tetrafluoride) by molten alkali metals (e.g., Na, K) or alkali-earth metals (e.g.. Mg, Ca). For instance, in the Ames process, uranium tetrafluoride, UF, is directly reduced by molten calcium or magnesium at yoO C in a steel bomb. Another process consists in reducing uranium oxides with calcium, aluminum (i.e., thermite or aluminothermic process), or carbon. Third, the pure metal can also be recovered by molten-salt electrolysis of a fused bath made of a molten mixture of CaCl and NaCl, with a solute of KUFj or UF. However, like hafnium or zirconium, high-purity uranium can be prepared according to the Van Arkel-deBoer process, i.e., by the hot-wire process, which consists of thermal decomposition of uranium halides on a hot tungsten filament (similar in that way to chemical vapor deposition, CVD). 

Silicon carbide (SiC) monofilaments are usually made by chemical vapor deposition (CVD) by decomposing a silane such as methyltrichlorosilane (CHjSiCy in a hydrogen atmosphere onto a hot and fast-moving tungsten wire or pyrolitic carbon monofilament at a temperature of 1300°C. The equipment and process is the same as that used for making boron fibers (see Section 18.4.2). The chemical reaction occurring at the surface of the hot substrate is ... 

Deposition of a-Si H and related materials is most commonly performed by chemical vapor deposition enhanced with a plasma or hot wire. Sputtering is also used. 

In hot-wire CVD (HWCVD), hot wires are used to initiate the reaction and the substrate is kept in lower temperature. In this case the thermal activation occurs in a spatially separated location, and the substrate is the deposition surface. In such a process the chemical activation of vapor and the deposition of materials are spatially separated, whereas in the ordinary CVD both processes occur in the same place. 

The principal methods of gas activation are thermal and electrical much less common are chemical and photochemical activation. In the most commonly used thermal activation technique - the hot filament technique - a W or Ta wire is arranged in the immediate vicinity of the substrate to be coated by diamond (Fig. 1). The wire is heated until it reaches the temperature when H2 molecules dissociate readily. The gas phase is a mixture of a carbon-containing gas (e.g. methane, acetone or methanol vapor), at a concentration of a few per cent, and hydrogen. Upon the contact of the gas with the activator surface, excited carbon-containing molecules and radicals are produced, in addition to the hydrogen atoms. They are transferred to the substrate surface, where deposition occurs. Table 2 gives an indication of the hot-filament deposition process parameters. 

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