LCVD reactor

These characteristics of glow in an LCVD reactor cast some serious questions regarding the nature of glow and the domain of plasma in a reactor. It is certain that one cannot intuitively assume that the luminous gas phase (glow) in glow discharge is plasma, while plasma has characteristic glow. 

Figure 3.7 Distribution of electron temperature, electron density and Debye length in DC discharge of Ar in an LCVD reactor A Debye length, cm x 10 B electron temperature, eV C electron density, cm x 10. ...

Because solid materials must maintain the vacuum, the reactor wall always exists in an LCVD reactor. The plasma also interacts with wall materials as well as any other materials that exist in the plasma, such as substrate and support. Therefore, polymer-forming intermediates and gaseous by-products may also originate from solid materials with which plasma interacts by virtue of the ablation caused by the luminous gas. In this sense, any material that interacts with plasma becomes a source of monomer for plasma polymerization. 

The measurement of pg requires a pressure transducer system that is not influenced by the electric power used for the plasma polymerization, particularly when a high-frequency radio frequeny power is employed. Some pressure transducers that give pressure readouts independent of the nature of a gas are ideally suited for plasma polymerization. Some electronic gauges the readout of which depends on the nature of the gas (e.g., thermal conductivity) do not provide accurate readings of Pg because in most cases the composition the gas mixture in the LCVD reactor is unknown and there is no way to calibrate the meter for an unknown gas mixture. 

The deposition in a luminous chemical vapor deposition (LCVD) system occurs on surfaces that are either in contact with luminous gas phase (glow) or in the vicinity of glow. The amount of polymer deposition is influenced by three important geometrical factors of LCVD reactor. These are the relative position of polymer deposition with respect to (1) the location of electric energy input, (2) the monomer flow, and (3) the position within a reactor. The system pressure, which determines the mean free path of gaseous species, has a great influence on the distribution of polymer deposition. In general, the lower the system pressure, the wider or more even is the distribution [1,2]. 

In order to examine the elfect of flow pattern in a reactor, which is a crucially important design factor of an LCVD reactor, it is necessary to examine the profile of deposition in a simple reactor first. A tubular reactor with an external radio frequency power coupling is ideally suited to the study of the distribution of polymer deposition. In such a reactor, 100% of the monomer passes through the luminous gas phase in the reactor, and the situation is very close to the case in which no bypass of monomer occurs. The experimental setup used for... 

The first method is a sure way to expose the surface of powder uniformly if one pass is sufficient to achieve the surface modification, but it is not easy to recycle the substrate in the luminous gas phase in vacuum. Therefore, the main issue in this approach is how to repeat the interaction of surface with the luminous gas phase efficiently, which entirely depends on the flow dynamics of powders. Multiple-step operation requires multiple discharge systems or repeated operation. The generation of discharge is more or less the same as the conventional modes used in LCVD reactors. External radio frequency electrodes or coil with glass tube is the most... 

The initial cost of an LCVD reactor, which is substantial, is an extremely important factor in the consideration of a new manufacturing operation. Consequently, the... 

The ACTIS process described above is a typical example of low-pressure plasma polymerization or LCVD, which is an ultimate green process with no effluent in the practical sense. Microwave plasma is used for plasma polymerization of acetylene. ACTIS process, as an example of LCVD, has an ideal combination of unique advantages in (1) very high reaction yield (monomer to coating), (2) no effluent from the process, (3) no reactor wall contamination because the reactor wall is the substrate surface, and (4) very short reaction time. However, whether such an ideal LCVD process is an industrially viable practice is a totally different issue. 

When an organic vapor rather than an inert gas is used in the same discharge reactor, a nearly completely different phenomenon occurs, in which deposition of material is an aspect. Deposition of material constitutes the foundation of LCVD. In an LCVD environment, the composition of the gas phase changes continuously as deposition proceeds. This difference could be further illustrated by examples for glow discharge of argon and of acetylene. 

In contrast to this situation, the glow discharge of acetylene in a closed system extinguishes in a few seconds to few minutes depending on the size of the tube and the system pressure. This is because acetylene forms deposit and coats the wall of the reactor. In this process of LCVD (plasma polymerization) of acetylene, very little hydrogen or any gaseous species is created, and the LCVD of acetylene acts as a vacuum pump. When the system pressure decreases beyond a certain threshold value, the discharge cannot be maintained. 

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.)... 

When plasma polymerization is carried out in a flow system, in which glow covers the entire cross-section of reactor with respect to the direction of monomer flow, the monomer molecules coming into the reactor first encounter the luminous gas phase. It is very unlikely that the molecules pass through the luminous gas phase without interacting with it and reach the relatively narrow zone in which IG or DG, located near the electrode surface, occurs. Therefore, the mode of activation that occurs in LPCAT without the influence of ionization is important in terms of the creation of chemically reactive species in LCVD. The creation of reactive species by the luminous gas is the mechanism considered here. 

Plasma polymerization is initiated via the dissociation of molecules caused by varieties of energetic species in the luminous gas phase as described in Chapter 4. It is important to recognize that the reactive species created in the luminous gas phase are not initiators of plasma polymerization. Some species, e.g., free radicals, could be initiators of some monomers that have specific functional groups under special conditions, e.g., in the off period of pulsed glow discharge and in the nonglow zone of a reactor (remote plasma). In most cases, the reactive species created in luminous gas phase are reactive building blocks of LCVD. 

When the substrate polyethylene (PE) is placed on the cathode, unlike Al, it will not act as a part of the cathode, and the film produced is identical to that prepared by the 40-kHz discharge where the substrate is floating in the luminous gas. For comparison, both the substrates Al foils and PE fibers were placed in the reactor and plasma coated at the same time. Results showed that signals from TMS LCVD on PE are very different from those of TMS LCVD on Al as shown in Figure 6.12. Unlike the broad ESR line observed when Al was used as the substrate, hyperfine structures were observed with use of the substrate PE. 

The unit of normalized deposition rate is m, and normalized deposition rate decreases with the total surface on which deposition occurs. This aspect can be conceived as loading factor of luminous chemical vapor deposition (LCVD). Mass balance in a reactor (flow system) can be established as... 

Thus, the material formation in the luminous gas phase (deposition G), which is given in the form of normalized deposition rate (D.R./F Af), can be controlled by the composite parameter WjFM (normalized energy input parameter), which represents the energy per unit mass of gas, J/kg. Because of the system-dependent nature of LCVD, WjFM is not an absolute parameter and varies depending on the design factor of the reactor. The value of WjFM in a reactor might not be reproduced in a different reactor however, the dependency remains the same for all deposition G. 

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. 

If growing species stray from the active LCVD environment, they will deposit on the reactor walls, which is out of the luminous gas phase, as oligomers (not polymers). The presence of oligomer deposition on the reactor walls could cause serious interference with subsequent plasma polymerization, which is carried out in a contaminated reactor. 

However, when noncontinuous substrates are employed, the process could be run either by continuous operation or by batch operation, probably equally well, and the choice heavily depends on factors other than LCVD. The requirement for a continuous operation is that the total numbers of products must be large enough to justify a continuous operation for a year, for instance, and there is no sense in building a continuous reactor for processing only a few items. If the number, size, and shape of substrate to be handled changes, e.g., parts and components of machine or aircraft, the butch operation is mandatory choice. 

In batch processing of LCVD or LCVT, substrates are placed in a reactor, and LCVD or LCVT is carried out as a unit operation. Repeating the same operation... 

The continuous operation of noncontinuous substrates, e.g., contact lenses, video disks, microsensors, etc., is performed by placing a certain number of substrate in an evacuation/transfer chamber, in which the evacuation is carried out and samples are transferred to the adjacent sample holding chamber in vacuum. The evacuated sample holders are placed on a conveyer one by one and pass through glow discharge zones. The coated substrates follow the reverse process at the downstream end of a reactor to be taken out in the ambient environment. Thus, the substrate charge is done in butch mode, but the LCVD process is done continuously. 

Another important factor to be considered in conjunction with the flow rate of monomer is that LCVD occurs predominantly in the glow region. Therefore, the true reaction volume is close to the volume of glow, Fg. However, the volume of glow is not always the same as the volume of the reactor, V, and in certain cases... 

It is important to recognize that all surfaces that contact with the luminous gas phase participate and influence LCVD operation. Therefore, in principle, in a batch operation, the first run with clean reactor wall could not be replicated in the second run with contaminated reactor wall. Thus, it is necessary to include the step for cleaning the reactor. If only hydrocarbons were used in an LCVD, the cleaning could be done by O2 discharge prior to the normal LCVD operation. (The influence of wall contamination was described in Chapter 10.) In this respect, the effort to minimize the deposition on nonsubstrate surfaces is important even in batch operation of LCVD. Magnetron discharge is quite effective in this respect, as described in Chapter 14. 

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