Deposition from silane

In many cases, the deposited material can retain some of the original chemical constituents, such as hydrogen in siUcon from the deposition from silane, or chlorine in tungsten from the deposition from WCl. This can be beneficial or detrimental. For example, the retention of hydrogen in siUcon allows the deposition of amorphous siUcon, a-Si H, which is used in solar cells, but the retention of chlorine in tungsten is detrimental to subsequent fusion welding of the tungsten. 

Deposition from silane (SiH4) at low pressure (1 to 15 mTorr) at 750-800°C and by plasma deposition with a temperature range of 600-800°C.[ " ]l l... 

Plasma-enhanced deposition from silane (see below). 

So-called clustering reactions of SiH + ions (n = 0-3) with silanes and other small molecules have attracted considerable interest. In particular, the reaction sequences which might lead to the formation of large-sized clusters have received attention, as these processes are thought to be involved in undesirable dust-formation which occurs during silicon-film depositions from silane plasmas and vapors97-99. Thus, Mandich and Reents... 

Anderson and Spear (1977) have determined the change in the optical gap energy Eopt in a-Si C iH with composition. The value of increases monotonically with decreasing x, from 1.6 eV at x = 1.0 to a maximum of 2.8 eV at x = 0.3 and decreases down to 2.4 eV at x = 0, for the films GD-deposited from silane-ethylene mixtures at about 200°C. Figure 3 shows the (ahv)1/2 versus hv plot, where a is the absorption coefficient and hv is photon energy, for several samples prepared by GD decomposition of silane-tetramethylsilane mixtures at the same substrate temperature. The... 

Fig. 2.3. Examples of the coltunnar microstnicture of a-Si H films seen in the fracture suiface (( ) and (J>)) and in the plane of growth ((c) and (d)). Film (a) is deposited from pure silane at low rf power and film (t) is deposited from silane dilut to S % in argon and at high rf power (Knights and Lujan 1979).

Fig. 2.4. Diagram showing the deposition conditions for columnar and non-columnar a-Si H films deposited from silane/argon mixtures at...

Fig. 2.5. Diagram showing the typical deposition conditions for microcrystalline silicon and a-Si H films deposited from silane/hydrogen mixtures at different rf power.

C. Pavelescu, C. Cobianu, L. Condriuc, and E. Segal, Etch rate behaviour of S1O2 films chemically vapour deposited from silane, oxygen and nitrogen gas mixtures at low temperatures. Thin Solid Films 114, 291, 1984. 

Ramalingam, S., Maroudas, D., and Aydil. E. S.. Atomistic simulation of SiH interactions with silicon surfaces during deposition from silane containing plasmas. Appl. Phys. Lett. 72, 578-580 (1998a). 

In an attempt to infer the nature of silane coatings on the glass surface, our study characterized polymerized silane films, deposited from silane solutions of different concentrations. Since methyltrimethoxysilane (MS) has the simplest chemical composition among the silane compounds used as coupling agents, polymerized silane films of the MS were chosen as a model system. The nature of other silane coatings on the glass surface is inferred on the basis of this model system. 

Viswanathan R, Thompson D L and Raff L M 1984 Theoretical investigations of elementary processes in the chemical vapor deposition of silicon from silane. Unimolecular decomposition of SiH J. Chem. Phys. 80 4230 0... 

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

The thermal decomposition of silanes in the presence of hydrogen into siUcon for production of ultrapure, semiconductor-grade siUcon has become an important art, known as the Siemens process (13). A variety of process parameters, which usually include the introduction of hydrogen, have been studied. Silane can be used to deposit siUcon at temperatures below 1000°C (14). Dichlorosilane deposits siUcon at 1000—1150°C (15,16). Ttichlorosilane has been reported as a source for siUcon deposition at >1150° C (17). Tribromosilane is ordinarily a source for siUcon deposition at 600—800°C (18). Thin-film deposition of siUcon metal from silane and disilane takes place at temperatures as low as 640°C, but results in amorphous hydrogenated siUcon (19). 

Tantalum disilicide, TaSi2, is very refractory and chemically resistant. It is deposited from the reaction of the chloride, TaCl5, with silane (SiH4), or dichlorosilane (SiH2Cl2), the latter precursor being preferred. The reaction takes place in a plasma as follows ... 

Some of the most significant developments in the CVD of Si02 include experiments in plasma CVD at 350°C via electron cyclotron resonance (ECR) to gain improved control of the deposition rate and obtain a quality equivalent to that of the thermally grown oxide (see Ch. 5). Deposition from diacetoxyditertiarybutoxy silane at 450°C has also been shown to significantly improve the Si02 film properties. " ]... 

To illustrate some typical results, consider the deposition of silicon from silane. The gas-phase reaction mechanism consists of 27 elementary reaction... 

FIG. 33. The depletion of silane and the corresponding production of hydrogen for several process conditions, covering both the a- and the ) -regime. The solid line represents the case where all the consumed silane is converted into a-Si H() and 1.95H2. The dashed line represents the case where 30% of the consumed silane is converted into disilane instead of being deposited. (From E. A. G. Hamers, Ph.D. Thesis. Universiteit Utrecht. Utrecht, the Netherlands, 1998. with permission.)... 

In the early 1970s, Spear and coworkers (Spear, 1974 Le Comber et al., 1974), although unaware of the presence of hydrogen, demonstrated a substantial reduction in the density of gap states (with a corresponding improvement in the electronic transport properties) in amorphous silicon films that were deposited from the decomposition of silane (SiH4) in an rf glow discharge. 

Chemical vapor deposition (CVD) reactions in electronics materials processing provide further examples. One is the deposition of Si from silane ... 

Alternatives to activated tungsten wire emitters are also known, but less widespread in use. Cobalt and nickel [44,47] as well as silver [48] can be electrochemi-cally deposited on wires to produce activated FD emitters. Mechanically strong and efficient emitters can be made by growing fine silicon whiskers from silane gas on gold-coated tungsten or tantalum wires of 60 pm diameter. [45] Finally, on the fracture-surface of graphite rods fine microcrystallites are exposed, the sharpness of which provides field strengths sufficient for ionization. [49]... 

The flux summary within the boxes in Fig. 17.2 shows the mass flux (g/cm2-s) of both the silane and silylene to the surface, resulting in deposition of silicon and release of volatile hydrogen. At low temperature, the film growth is primarily from silane, although it is quite low. By Ts = 925 K, there is sufficient silane decomposition that the surface fluxes of the two species are becoming comparable. At Ts = 1300 K, the silylene flux is dominant, carrying most of the silicon to the surface. 

Films produced by the deposition of silanes from aqueous solutions were investigated and the effects of thickness were explored using specular reflectance. 

Alkaline-cleaned substrate. After alkaline cleaning, the steel substrate was completely devoid of aliphatic fatty acids, as was verified by TOFSIMS. The freshly cleaned substrate was immediately immersed in the 1% silane solution and analyzed. The wettability of the cleaned steel by the solution was considerably improved by the alkaline cleaning procedure. The spectra obtained with a film deposited from a pH 10.5 solution are shown in Fig. 7. A rigorous peak identification procedure was applied here. The masses of all major peaks could be... 

To demonstrate the main features of the flow in horizontal CVD reactors, the deposition of silicon from silane is used as an example (87). The conditions are as follows an 8-cm-wide reactor with either adiabatic side walls or side walls cooled to the top wall temperature of 300 K, a 1323 K hot susceptor (bottom wall), a total pressure of 101 kPa, and an initial partial pressure of silane in H2 of 101 Pa. The growth rate of silicon is strongly influenced by mass transfer under these conditions. Figure 8 shows fluid-particle trajectories and spatially varied growth rates for three characteristic cases. 

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