SiH4 surface reactions silanes

Silicon cluster reactions are an example of a newly emerging field of research which is very amenable to study with electronic structure methods. This exercise will examine the potential surface for silicon cation reacting with silane (SiH4). Such reactions are central to the growth of large silicon clusters, which occurs by sequential additions of -SiHj ... 

In silicon based MEMS processing, common CVD films include polysilicon, silicon oxide, and silicon nitride. For polysilicon films (usually the structural layer), an LPCVD pyrolysis method is generally used with silane (SiH4) as the source gas [see Eq. (1)]. To obtain a imiform film across the wafer, the process is carried out at low pressure to ensure that the deposition is surface reaction controlled and not diffusion limited. Typical process temperatures are in the range of 580-650°C, and pressures between 0.1 and 0.4Torr. 

The reactivity of as-formed porous Si is dominated by the chemistries of silicon-hydrogen (Si-H) and silicon-silicon (Si-Si) bonds. Both of these species are competent reducing agents (Eqs. 1 and 2), and both are able to reduce water to hydrogen (Eqs. 3 and 4). The surface of porous Si contains a mixture of Si-H, SiH2, and SiHs species (Gupta et al. 1991), which are represented notionally in the reactions of Eqs. 2 and 4 by the molecular species silane, SiH4. 

Two other significant conclusions can be made from this study i) the observed reaction between SiH4 and UF5 was most likely surface catalyzed and ii) UF and HF show fundamentally different reactivity towards partially fluorinated silane derivatives. Evidence for the first conclusion was obtained from reactions involving either SiH4 or UFg and reactor walls preconditioned with UF5 or SiH4, respectively, which proceeded in a very similar fashion to the bulk reactions between the two species. The latter reactions proceeded at or below ambient temperature whether the two species were mixed directly in the gas phase or first condensed together at -196 before warming up to ambient temperature, which supports the apparently active role of the reactor walls in the system s chemistry. 

The co-depositing material for reactive deposition can be from a second sputtering target. However, it is often in the form of a chemical vapor precursor, which is decomposed in a plasma and on the surface. Chemical vapor precursors are such materials as acetylene (C2H2) or methane (CH4) for carbon, silane (SiH4) for silicon, and diborane (B2H6) for boron. This technique is thus a combination of sputter deposition and PECVD and is used to deposit materials such as carbides, borides, and silicides. It should be noted that co-deposition does not necessarily mean reaction. For example, carbon can be deposited with titanium to give a mixture of Ti + C but the deposit may contain little TIC. 

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