SiH4 reduction

To address the issue under (a) it is more common to let the hydrogen step be preceded by a short SiH4 reduction step (SiH4/WF6) which has been proven to reduce the nucleation time sufficiently in most cases. 

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. 

The deposition of Si by the reduction of SiH4 is a convenient example of a CVD process that clearly displays the reaction steps just listed. SiH4... 

It might be thought that reductive methods would be suitable for the preparation of hydrido-complexes. In fact, most such compounds are prepared by substitution reactions, using a source of H such as NaBH4 the formation of, e.g., NaBCl4 as a product assists the thermodynamics (cf. the use of LiAlH4 in the preparation of SiH4 in Section 10.3 above). 

An extreme example is given by the reduction of hexachlorodisilane (10a). Whereas disilane, Si2H6, can be obtained in 87% yield when 15% excess of a solution of lithium aluminum hydride in ether is added to the chloro-disilane, the product is exclusively monosilane, SiH4, when a solution of the chlorodisilane is added to a solution of lithium aluminum hydride so that an excess of the latter is always present. 

The reactions that have been used to create epi silicon films commercially involve the H2 reduction of the chlorosilanes. As we learned earlier in Chapter 1 in studying the equilibrium behavior of the H-CI-Si system, we can deposit solid silicon from SiCI4 + H2, SiCI3H, or SiCI2H2. Also, H2 can be added to the latter two, if desired. Obviously, silicon will also deposit from SiH4. The deposition rates of Si as a function of temperature, at atmospheric pressure, from the CVD of the above source gases are shown in Figure 16. 

No reaction is observed between rra s-(PnN)Pt(Cl)Me (Cl trans to P) and HSiR3 (4th reaction in Scheme 1). DFT calculations show that oxidative addition of the silane would be endothermic by 16.4 kcal/mol, and - more important - no feasible reductive elimination pathway was found. However, it was shown that the enei etically less favored isomer ds-(PnN)Pt(Cl)Me (Cl cis to P) is the active species in catalytic C-Cl/Si-H exchange reactions. The energy difference between the cis and trans isomer is 9.0 kcal/mol. The initial step of the catalytic cycle is the formation of (PnN)Pt(Me)H from SiH4 and ds-(PnN)Pt(Cl)Me (3rd reaction in Scheme 1) [3, 6]. 

When the SiH4/WF6 chemistry is used, the demands on the chemical compatibility of the adhesion layer are relaxed since this chemistry is so much milder than the hydrogen chemistry [Ellwanger et al.14]. In this way Al and Ti become acceptable adhesion layers. Unfortunately,the step coverage of the SiH4/WF6 chemistry is very poor (see section 2.3.2) and is therefore not suitable for contact fill applications. The silane reduction is, however, still applied to start the tungsten deposition especially atop of TiN (see 2.2.1) followed by the tungsten deposition based on the H2/WF6 chemistry. 

Silanes. Monosilane, SiH4, is best prepared13 on a small scale by heating Si02 and LiAlH4 at 150-170°. On a larger scale the reduction of SiOz or... 

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