Plasma-chemical reaction mechanism

Like the literature of plasma-assisted etching, the literature on the PECVD of specific materials is considerable. Because film properties are ultimately determined by chemical reaction mechanisms, reactor design, and film structure (Figure 5), the determination of the exact relationships between properties and processing is difficult. At present, the fundamental understanding of such relationships is limited, and thus, empirical efforts have been the norm. In this chapter, the more widely studied film materials deposited by PECVD will be briefly discussed. More extensive information on these and other films can be found in a number of review articles (9-14, 32, 50, 200-203) and references therein. 

The firm theoretical understanding of the spectra of model molecules has been a major impetus to apply the techniques to complex, unknown solid-state structures of technological importance. Characterization of atomic composition, structure, and bonding in the surface and subsurface of practical specimens is the first step. Most studies interpret changes in the core-level spectra in terms of surface-chemical mechanisms involved in, for example, processing conditions, exposure to inert or reactive gas plasmas, chemical reactions, or aggressive service environments. 

Plasma-chemical reaction rates depend on the probability of relevant elementary processes from a fixed quantum-mechaiucal state with fixed energy, which was considered in the previons chapter, and the nnmber density of particles with this energy and in this particular qnantnm-mechanical state, which is to be considered in this chapter. A straightforward determination of the particles distribution in plasma over different energies and different quantum-mechaitical states is related to detailed physical kinetics (see Fridman Keimedy,... 

This mechanism can provide the highest energy efficiency of endothermic plasma-chemical reactions in non-equilibrium conditions because of the following four factors ... 

Mechanisms of stimulation of plasma-chemical reactions by electronic excitation were discussed in Section 2.5.5. No one of the four kinetic factors mentioned in Section 3.6.2 can be apphed in this case therefore, energy efficiency is relatively low, usually below 20-30%. Plasma-chemical processes through electronic excitation can be energy effective if they initiate chain reactiorrs. Such a situation takes place, for example, in NO synthesis, where the Zeldovich mechanism can be effectively initiated by dissociation of molecular oxygen through electronic excitation (see Section 6.1.2). 

L.I. Slovetskii, Mekhanizmy Khimicheskikh Reaktsii v Neravno-vesnoi Plazme (Mechanisms of Chemical Reactions in Nonequilibrium Plasma), Moscow Nauka, 1980, 310p. 

Free radicals are short-lived, highly-reactive transient species that have one or more unpaired electrons. Free radicals are common in a wide range of reactive chemical environments, such as combustion, plasmas, atmosphere, and interstellar environment, and they play important roles in these chemistries. For example, complex atmospheric and combustion chemistries are composed of, and governed by, many elementary processes involving free radicals. Studies of these elementary processes are pivotal to assessing reaction mechanisms in atmospheric and combustion chemistry, and to probing potential energy surfaces (PESs) and chemical reactivity. 

Silicon Dioxide. Si02 layers produced by PECVD are useful for intermetal dielectric layers and mechanical or chemical protection and as diffusion masks and gate oxides on compound-semiconductor devices. The films are generally formed by the plasma-enhanced reaction of SiH4 at 200-300 °C with nitrous oxide (N20), but CO, C02, or 02 have also been used (238-241). Other silicon sources including tetramethoxysilane, methyl dimethoxysilane, and tetramethylsilane have also been investigated (202). Diborane or phosphine can be added to the deposition atmosphere to form doped oxide layers. 

Plasma chemistry is at present mainly an empirical technique. Little is known about relationships between the properties of the plasma and the reaction mechanism or the product distribution. Experimental conditions have to be optimized for every new reaction. Once more experimental material and improved methods of diagnosing plasma become available it may become routine to adjust plasmas for each specific chemical problem. 

The mechanism of action of DFO is the formation of a stable complex with iron. It prevents the iron from entering into further chemical reactions, It is important that DFO chelates iron from hemosiderin and ferritin, but not from transferrin, It does not bind with the iron from hemoglobin and cytochromes. It is theorized that DFO is metabolized by plasma enzymes. The chelate is soluble in water and passed easily through the kidney (reddish color of urine). 

Ion-assisted gas-surface chemistry mechanism is probably the best terminology to describe the plasma processing of a surface. A remarkable illustration of the effect of ion bombardment is reported in Fig. 16 [66]. The reaction rate of XeF2 with Si increases drastically upon the simultaneous combination of chemical species (XeF ) and ions (Ar+) on the surface. Obviously chemical reaction and some sputtering processes are expected to occur and to be responsible of the ablation of the material, but the combined effect of active neutral species and ion bombardment is more efficient than the sum of the individual processes. 

FIGURE 20.15 Schematic showing the principal elements in the complex diamond CVD process flow or reactants into the reactor, activation of the reactants hy the thermal and plasma processes, reaction and transport of the species to the growing snrface, and surface chemical processes depositing diamond and other forms of carhon. (From Pehrsson, P.E., Cehi, F.G., and Butler, J.E., in Chemical Mechanisms of Diamond CVD, Davis, R.E., Ed., Noyes Puhhcations, New Jersey, 1993.)... 

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