Dissociation glow

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. 

CHANGE OF DISSOCIATION GLOW AND IONIZATION GLOW WITH REACTION TIME... 

The faint negative glow observed with the primary dissociation glow of TMS shown in Figure 3.11 intensifies with discharge time, particularly in a closed system but to... 

The direct evidence to show that reactive species are created in the dissociation glow rather than in the ionization glow was found in the in situ Optical Emission Spectroscopy (OES) analysis aimed specifically at the dissociation glow and at the ionization glow of TMS DC discharge in a closed system [4]. Figures 4.4 and 4.5... 

Figure 4.4 Optical emission spectra (OES) measured from (A) dissociation glow and (B) negative glow in DC glow discharge of trimethylsilane (TMS) flow system, 1 seem TMS, 50 mtorr, DC power 5 W.

Figure 4.5 The time dependence of emission intensity of various photo-emitting species detected by OES in TMS DC glow discharge in a closed reactor (a) polymerizable species in dissociation glow, (b) Hoc emission line at 656 nm 50 mtorr TMS, DC power 5 W.

Figure 4.6 Dissociation glow and ionization glow in DC glow discharge of TMS and respective OES species material forming species are mainly in the dissociation glow and the ionization glow consists of mainly hydrogen species.

Polymerizable species that do not emit photons are created by the molecular dissociation reaction occurring in the dissociation glow, i.e. A and/or B in Eq. (4.2) should be replaced by A and B, in this case. 

The existence of the dissociation glow in DC discharge strongly suggests that the creation of chemically reactive species in LCVD involves different mechanism than those in the electron impact ionization. However, in DC discharge, electron impact and ion impact reactions cannot be eliminated. Low-pressure cascade arc torch (LPCAT) provides a unique opportunity to investigate the formation of chemically reactive species with minimal influence of ions and electrons. That is, the creation of chemically reactive species from an organic molecule by the luminous... 

The material formation in LCVD generally requires the production of gaseous by-products, which do not form deposition, in order to create new chemical bonds for the material formation. For instance, LCVD of saturated hydrocarbons requires hydrogen abstraction in the dissociation glow. In presence of double and triple bonds, the hydrogen production becomes very small. 

Since the dissociation glow can be considered to be the major medium in which polymerizable species are created, the location of the dissociation glow, i.e., whether on the electrode surface or in the gas phase, has the most significant influence on where most of the LCVD occurs. The deposition of plasma polymer could be divided into the following major categories (1) the deposition that occurs to the substrate placed in the luminous gas phase (deposition G) and (2) the deposition onto the electrode surface (deposition E). The partition between deposition G and deposition E is an important factor in practical use of LCVD that depends on the mode of operation. 

The equation indicates that cathodic polymerization is controlled by the conditions of the local environment near the cathode. The normalized deposition rate in DC (deposition E) is D.R./[M], not D.R./[FM], and the normalized power input parameter is Wc/S, not WjFM. In DC discharge, the dissociation glow virtually adheres to the cathode surface. Therefore, the equation proves that the dissociation glow controls the deposition rate on the cathode surface. 

The material formation in LCVD is caused by the dissociation glow (DG) and the ionization glow (IG). In DC discharge, the material formed in the cathode glow deposits nearly exclusively on the cathode surface due to the adherence of DG to the cathode, but some of them could deposit on surfaces in the reactor. The situation with the material formed in the negative glow is the same, i.e., it could deposit on the cathode, the anode, and surfaces placed in the reactor. Distribution of the deposition (to the cathode and the anode) is dependent on the distance between the cathode and the anode. Consequently, the total deposition on the cathode is also dependent on the distance. 

Comparing the deposition rate dependence on operational parameters for the deposition in the diffused luminous gas phase and for the cathodic deposition [Eqs. (8.2) and (8.7)], the contribution of the cathodic polymerization can be estimated by examining the system pressure dependence of the deposition rate (at a fixed flow rate). If the material formation in the diffused luminous gas phase is the dominant factor, it is anticipated that the deposition rate would be independent of the system pressure. If the material formation in the molecular dissociation glow is... 

If one considers that the overall DC plasma polymerization is a mixture of cathodic polymerization (material formation in the dissociation glow that is adhering to the cathode surface) and plasma polymerization (material formation in the diffused luminous gas phase), the deposition on the cathode is primarily cathodic polymerization. With an insulating layer between the substrate and the cathode surface, there is no cathode glow, and hence no cathodic polymerization that deposits polymer on the substrate. The substrate on the cathode surface without electrical contact or any noncathode surface receives the products of plasma polymerization in negative glow. 

In plasma polymerization, the character of the dissociation glow that occurs near the surface of the cathode is more important than ion bombardment in determining the deposition rate and its distribution described in previous chapters. The edge effect seems to be less pronounced in LCVD than in the sputtering of the cathode material. The anode magnetrons seem to overcompensate the edge effect in LCVD. 

The electrode surface is considered the energy input plane. In radio frequency discharge, the molecular dissociation glow, in which the major creation of chemically reactive species occurs, does not adhere to the electrode surface but is very close to the electrode surface. Therefore, in this case the major factor that determines the distribution of polymer deposition is the dififusional transport of monomer from the periphery of the electrode to the body of luminous gas phase that occupies the interelectrode space. 

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