Onion Layer Structure of Luminous Gas Phase

Very important factors in LCVD are (1) the location of the critically important layer, i.e., the dissociation glow, in a glow discharge, and (2) the location of the substrate with respect to the onion layer structure, i.e., in which layer of an onion structure the substrate is placed. The location of the critical layer depends on what kind of discharge system is employed to create a luminous gas phase. In a strict sense, it is impossible to uniformly coat a substrate placed in a fixed position in a reactor, and the relative motion of a substrate to the onion layer structure of luminous gas phase is a mandatory requirement if high uniformity of coating is required. 

Adaptability of an LCVD process in an industrial scale operation greatly depends on the nature of the onion structure of the luminous gas phase that could be accommodated in the operation. The change of reactor size inevitably changes the basic onion layer structure of the luminous gas phase, which constitutes the main (often insurmountable) difficulty in the scale-up attempt by increasing the size of reactor. (The scale-up principle is discussed in Chapter 19.) 

For the ease of comparison, let us term the most critical layer in a luminous gas phase as core. The meaning of the core depends on what kind of process with respect to the objective is considered. The reactor parameters, such as the distance 

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