Hot-wire CVD

Figure 4.40. Schematic of a hot-wire CVD system. Reprinted from Povolny, H. S. Deng, X. Thin Solid Films 2003, 430, 125. Copyright 2003, with permission from Elsevier.

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. 

IR and Raman spectroscopy were used to characterise hydrogenated silicon thin films formed by magnetron r.f. sputtering, or by hot-wire CVD. An IR study (vSiH) has been made of the photoinduced changes in the structure of undoped hydrogenated amorphous silicon, a-Si H.379 IR bands due to Si-H-Si units were reported for silicon-isotopically enriched silicon crystals.380... 

Other variations on the CVD method, such as hot-wire CVD (HWCVD) [66], rapid thermal CVD (RTCVD) [67], microwave plasma CVD (MPCVD) [68], and the floating catalyst technique [69,70] have led to improvements in product controllability, yield, and cost. 

HouweUng ZS, Geus JW, Schropp REI. Synthesis of WO3 nanogranular thin films by hot-wire CVD. Chem Vap Decomp 2010 16(4-6) 179-84. 

Thermal (resistively heated wires/filaments) No Hot Wire CVD (HWCVD) or hot filament (HFCVD) or catalytic-CVD (cat-CVD)... 

A single narrow peak due to vSiD was seen in the IR spectrum of D Si(l 11)-(1 X 1). For the analogue containing H rather than D, both vSiH and 5SiH were observed vSiH bands were assigned from the IR spectrum of hydrogenated microcrystalline silicon The Raman spectra of polycrystalline sihcon films prepared by hot-wire CVD show vSiH at 2000 cm only. The IR spectra show an additional band at about 2100 cm for specimens produced at a wire temperature of 1900°C. ... 

Deshpande R, Dillon AC, Mahan AH, Alleman J, Mitra S (2006) Hydrogen adsorption in single-walled and multi-walled carbon nanotubes grown in a hot-wire CVD (Cat-CVD) reactor. Thin Solid Films 501(l-2) 224-226... 

Plasma polymerization is a vapor phase-activated (by plasma) A/D-coupled CVD. Parylene polymerization is a vapor phase-activated (by thermal process) A/D-decoupled CVD. Cascade arc torch (CAT) polymerization is a vapor phase-activated (by plasma) A/D-decoupled CVD. General CVD is a surface-activated (by thermal process) A/D-coupled CVD. HWCVD is a hot wire activated A/D-decoupled CVD. Coupled and decoupled refer only to the spatial separation. 

In traditional polymer CVD, the precursor stream flowing with a carrier gas is fed over a heated tungsten wire (hot wire) or through a microwave cavity (plasma) in a reduced atmosphere chamber. The activated monomers then condense into a polymer film on nearby surfaces. This technique can be used to create high quality insulators [50] as well as doped organic conducting polymer materials [51] by proper selection of the source materials and the use of suitable catalysts. 

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]. 

The hot fiber (wire) CVD process has been commercially used for 30 years to produce continuous sheath/core bicomponent boron/tungsten and silicon carbide/carbon fibers. Since they are continuous fibers, they are discussed in Chapter 3.3. More recently, this process was used to produce discontinuous, i.e., short, experimental sheath/core diamond/carbon fibers by depositing a thick diamond sheath on short pieces of a potentially carbon fiber. 

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). 

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... 

Diamond deposition by the hot filament method consists of a carbon containing gas and hydrogen, which undergo dissociation by passing through a hot filament usually made of tungsten wire. The dissociated molecules then deposit on a substrate (at approximately 900°C) where a carbon matrix grows in the form of diamond. Deposition dynamics is described by the CVD process. 

PVD of a-Si is achieved by electrically heating an Si wire (0.4-2 mm cross-Section, 25 mm length) at 10 mbar in front of the substrate surface at a distance of 4 mm. The substrate is moderately heated by radiation from the hot Si wire only. Typically, the Si wire can be used for 3 hours until thirming and breakage due to Si evaporation. CVD of a-Si can be achieved with the same set-up by pyrolyzing 0.5... 

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 ... 

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