HWCVD deposition

FIG. 68. Cross-sectional view of a HWCVD deposition reactor. (From K. F. Feenstra. Ph D. Thesis, Universiteit Utrecht. Utrecht, the Netherlands. 1998. with permission.)... 

The HWCVD deposition process is more or less the same as for PECVD, and was described in Section 1.7. Important differences between the two is the absence of ions, and the limited number of different species present in the gas phase, in the former. At low pressure atomic Si is the main precursor. This yields void-rich material with a high microstructure factor. Increasing the pressure allows gas phase reactions with Si and H to create more mobile deposition precursors (SiH3), which improves the material quality. A further increase leads to the formation of higher silanes, and consequently to a less dense film. 

In further sections extensions or adaptations of the PECVD method will be presented, such as VHF PECVD [16], the chemical annealing or layer-by-layer technique [17], and modulation of the RF excitation frequency [18]. The HWCVD method [19] (the plasmaless method) will be described and compared with the PECVD methods. The last deposition method that is treated is expanding thermal plasma CVD (ETP CVD) [20, 21]. Other methods of deposition, such as remote-plasma CVD, and in particular electron cyclotron resonance CVD (ECR CVD), are not treated here, as to date these methods are difficult to scale up for industrial purposes. Details of these methods can be found in, e.g., Luft and Tsuo [6]. 

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]. 

The deposition mechanism in HWCVD of a-Si H can be divided into three spatially separated processes. First, silane is decomposed at the tungsten filament. Second, during the diffusion of the generated radicals (Si, H) from the filament to the substrate, these radicals react with other gas molecules and radicals, and new species will be formed. Third, these species arrive at the substrate and contribute to the deposition of a-Si H. 

Using threshold ionization mass spectrometry and in situ ellipsometry, Schroder and Bauer [555] have shown that the Si2H4 radical may well be the species responsible for deposition, rather than SiH3 as in PECVD. This larger and less mobile precursor is thought to be the cause of the observed differences in the deposition conditions required in HWCVD and PECVD to obtain device quality material. 

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. 

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 spatially decoupled activation and deactivation can be also seen in a mode of PP known as low-pressure cascade arc torch (LPCAT) polymerization), which is described in Chapter 16. The activation of a carrier gas (e.g., argon) occurs in a cascade arc generator, and the chemical activation of a monomer or a treatment gas takes place near the injection point of the argon torch in the deposition chamber. The material deposition (deactivation) occurs in the deposition chamber. This is the same situation as the HWCVD, except that the mode of activation is different. 

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... 

Chemical vapor deposition (CVD). In CVD, the material s components come from the decomposition of one or more volatile chemical precursors that decompose and/or react on the substrate. Depending on deposition conditions (temperature and pressure) and the precursor nature, different terminologies are used to define CVD processes. Eor example, MOCVD is metal organic CVD, LPCVD is low-pressure CVD, ALCVD is atomic layer CVD, HWCVD is hotwire CVD, and PECVD is plasma-enhanced CVD. These processes can lead to single crystal, amorphous, or polycrystaUine films. 

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... 

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