Thermal discharge plasma

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

Hot RF and - DC plasma, are discharge, plasma jets Oxy-acetylene flames Low pressure microwave plasma, holt filament. Low pressure DC or RF glow discharge Thermal decomposition... 

In addition to the thermal CVD reactions described above, a glow discharge plasma at 480-650°C has been used to deposit HB2 from the mixed chlorides,... 

T0032 Alzeta Corporation, EDGE Thermal Processing Units T0370 High Mesa Technologies, L.L.C., Silent Discharge Plasma... 

The second type of system is the group of polymers produced by radiation, thermal, or plasma discharge treatment. The structure of such products is, as a rule, not known due to the high heterogeneity. 

In DC discharge plasmas, the sudden decrease of contact angle after 15 min treatment has been assigned by S. Okasaki et al. to a structural change of the material surface from crystallised graphite to an amorphous state [93]. It has been shown that the fluorination of PAN-based carbon fibers is more effective in the case of CF4-He plasmas than in 5% F2—He plasmas [94]. The study of the thermal stability of these F-treated fibers has shown however that a 70% loss of fluorine occurred when the samples were heated at 293°C for 10 min. 

Basically two alternative activation techniques have been reported to deposit tungsten other than by thermal activation. These are activation by aid of a gas discharge (plasma enhanced or PECVD) or by optical activation (photo -mostly laser- enhanced or LCVD). The advantage of these techniques is that the substrate temperature can be relatively low which might be of importance for future developments. In the next two sections we will discuss both PECVD and LCVD. 

A glow discharge plasma is not in local thermodynamic equilibrium (LTE), as the number of collisions is too low to thermally stabilize the plasma. Thus the electron temperatures are high (5000 K for the electrons involved in recombination and >10000 K for the high-energy electrons responsible for excitation through electron impact) but the gas temperatures (below 1000 K) are low. 

Monomers Not Polymerizable by Plasma Initiation. When styrene and a-methy1styrene were subjected to plasma treatment, the monomers became yellowish and only trace amounts of insoluble films were formed. The discoloration was intensified and extensive formation of dark films were observed if carbon tetrachloride was added as the solvent. No post-polymerization was detectable for these monomers. Generally styrene and a-methylstyrene readily undergo thermal polymerization. However, no thermal polymerization was possible for these monomers after having been subjected to plasma treatment for one minute or less. It has been demonstrated from the emission spectra of glow discharge plasma of benzene (6) and its derivatives (7 ) that most of the reaction intermediates are phenyl or benzyl radicals which subsequently form a variety of compounds such as acetylene, methylacetylene, allene, fulvene, biphenyl, poly(p-phenylenes) and so forth. It is possible that styrene and a-methylstyrene also behave similarly, so that species from the monomer plasma are poor initiators for polymerization. 

The EEDF strongly depends on electric field and gas composition in plasma (especially in non-thermal discharges) and often can be very far from the equilibrium distribution. 

Photo-ionization takes place in collisions of nentrals with photons, which result in the formation of an electron-ion pair. Photo-ionization is mostly important in thermal plasmas and in some mechanisms of propagation of non-thermal discharges. 

Different mechanisms of destmction of negative ions releasing an electron are discnssed in special books by Massey (1976), McDaniel (1964), and Smirnov (1982). We are going to consider three detachment mechanisms most important in plasma-chemical systems. The first one, which is especially important in non-thermal discharges, is associative detachment ... 

Relaxation of electronically excited atoms and molecules is due to different mechanisms. Superelastic collisions (energy transfer back to plasma electrons) and radiation are essential mostly in thermal plasma. Relaxation in collision with other heavy particles dominates in non-thermal discharges. Relaxation of electronic excitation into translational degrees... 

Stepwise Ionization by Electron Impact. Estimate a stepwise ionization rate coefficient in Ar at electron temperatures of 0.5 and 1 eV, assuming quasi-equilibrium between plasma electrons and electronic excitation of atoms. Comment on why the stepwise ionization is a preferential one in thermal plasmas and less important in non-thermal discharges. 

The Saha formula is commonly used for quasi-equihbrium thermal plasma. It can also be applied, however, for estimations of non-thermal discharges assuming the temperature in (3-14) is that of electrons, 7),. One should take into account in this case that the Saha equation describes ionization equilibrium A+ + e A, which corresponds to a detailed balance of ionizationby electron impact and three-body recombination e - - e + e + A+. As was... 

The first example is related to thermal discharges with electron temperature deviating from the temperature of heavy particles, which can take place, in particular, in boundary layers separating plasma from electrodes and walls. In this case, the two-temperature statisties and thermodynamies can be developed (Boulos et al., 1994). These models assume that partition functions depend on two temperatures. Electron temperature determines the partition functions related to ionization processes, whereas chemical processes are determined by the temperature of heavy particles. The partition functions can then be applied to calculate thermodynamic functions, composition, and properties. An example of such a calculation of composition in two-temperature Ar plasma is given in Fig. 3-2. 

Electron energy distribution functions (EEDFs) in non-thermal discharges can be very sophisticated and quite different from the quasi-equilibrium statistical Boltzmann distribution discussed earlier, and are more relevant for thermal plasma conditions. EEDFs are usually strongly exponential and significantly influence plasma-chemical reaction rates. 

Plasma conductivity (3-67) is determined by electron density (the contribution of ions will be discussed next) and the frequency of electron-neutral collisions, Ven. The electron density can be found using the Saha equation (3-14) for quasi-equilibrium thermal discharges and from the balance of charged particles in non-equilibrium non-thermal discharges. The frequency of electron-neutral collisions, Ven, is proportional to pressure and can be found numerically for some specific gases from Table 3-1. Relations (3-67) and (3-68) determine the power transferred from the electric field to plasma electrons. This power, calculated per unit volume, is referred to as Joule heating ... 

The energy efficiency of the qtrasi-eqtrilibritrm plasma-chemical systems performed in thermal discharges is nstrally relatively low (less than 10-20%), which is dne to two major effects ... 

Discharge plasma parameter Thermal arc discharge Non-completely-thermal arc... 

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