Deposition of Si Thin Films by Reactive CVD
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(n=0, 1,2,3), hydrogen and their ions, via the electronic excitation caused by the inelastic collision with accelerated electrons in the plasma.3' This process is characterized in a chemical view as follows: it is a kind of self-decomposition of silane induced by the plasma, so that its fragmentation, i.e., the generation of precursors (SiHn n=O, 1,2,3) which can cause film growth, does not need any further interaction with chemical species in the plasma: it is a homogeneous chemical process, involving only silicon and hydrogen containing molecules, radicals and ions: the reaction space for the generation of the precursors is confined to the plasma region between two electrodes for a capacitive coupling of rf-power, because the life time of exited silane is very short compared with its residence time in the reactor. The silyl radical, i.e., SiH3, which is considered to be the major precursor in this film growth,4 • is a monovalent radical and has a relatively long life time in the silane plasma. This radical is reproduced by its most possible secondary reaction with other silane molecules as it diffuses in the plasma.
Moreover, its reactivity is rather small both in the plasma and on the surface, compared with the other related radicals such as SiH2, SiH and Si. Therefore the silyl radicals are quite beneficial for reducing the defects density of film, because these help reduce the contribution of polymeric species to the film growth and the density of active sites on the growing surface. All of these properties provide the basis for the reliable film growth in terms of film quality and uniformity. However, these concurrently determine a limitation of this process. In fact, chemical processes of the film growth can be modified only through plasma parameters indirectly, such as the plasma temperature and their density. Even so, what can be done is to balance concentrations of the precursors and atomic hydrogen that is simultaneously formed by the fragmentation of silane, in addition to controlling the contribution of ionic species to the surface. Therefore it would be expected that any film deposition technique which enables us to manipulate the chemical processes of the film growth directly may reduce these problems, where the possibility of heterogeneous processes could play important roles for the formation of Si-network structure. In this paper, the principal of low-temperature CVD is generally discussed and then the new concept of Reactive CVD is proposed and discussed its general features in comparison with the plasma CVD in terms of both characteristics of chemical process and film growth.
Moreover, the current status of "Spontaneous Chemical Deposition",
5,6,7,8,9)
which is a real
example of the reactive CVD for a-Si and poly-Si thin films is outlined. In addition, a recent Thermal CVD is extension of the present concept to the thermal CVD, i.e., Reactive 10 demonstrated in the low temperature growth of poly-SiGe thin films.° •1,12) PRINCIPAL OF LOW-TEMPERATURE CVD The chemical process of film growth in
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