LCVD process

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. 

Above all of these requirements, SAIE must produce products that are superior to the conventional products. In other words, low-pressure plasma SAIE is not an alternative process it should be a new approach to create superior composite materials that could not be obtained by other means, which is of utmost importance with respect to the use of LCVD. It is often mentioned that plasma polymerization was successfully used in the surface modification but that a conventional, more economical, wet chemical process later replaced it. Such an attempt to use LCVD process based only on the laboratory curiosity is an absolutely wrong approach. This aspect is explored in Chapter 12. 

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.)... 

The photon-emitting species are vitally important in luminous chemical vapor deposition (LCVD) process, and the location of the luminous gas phase indicates where actions occur within the interelectrode space. On the other hand, whether any particular photon-emitting species is primarily responsible for LCVD is a dilferent issue. As is described in Chapter 5 for the growth and deposition mechanisms, many chemically reactive species, such as various forms of free radicals that do not emit photons, are major reactive species that carry the growth reactions. No single species could be identified as the precursor or chemically reactive species for the process. 

In any case, dissociation of organic molecules is the main route to create chemically reactive or polymerizable species in LCVD processes. The dissociation of monomer by the luminous gases occurs based on the principle of the energy... 

Describing a sequential LCVD process, the first process is followed by the second process separated by / sign. For example, LCVD of CH4 is followed by glow discharge treatment of O2, and the sequential process is expressed by CH4/O2. If O2 is mixed with CH4, it is expressed by LCVD of (CH4-1-02) by placing the gas mixture in parentheses. 

The product of deposition rate and deposition time determines the film thickness. Hence, W FM)t is an important practical parameter to control the thickness of the deposition. In many practical applications in which the actual thickness of deposition is extremely difficult to measure, the overall functional character of LCVD process itself can be controlled by this parameter. 

Some oligomers formed in LCVD process of F-containing gas, which adhere to some surfaces (metal oxides) by chemisorption, cannot be pumped out under vacuum, even under the high vacuum used for XPS. Consequently, XPS analysis depicts plasma oligomers as indistinguishable from plasma polymers unless detailed analysis of the interface is performed. 

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. 

The large-scale operation should be conceptually built first, considering all requirements necessary to produce products in an industrial operation. The shape and nature of the substrate, e.g., continuous sheet of film or fibers, large or small disks, etc., dictate what kind of operation could be feasible in the industrial scale operation. From the conceptual operation, the key factors of LCVD process should be extracted, and then a laboratory scale reactor should be designed and constructed. In other words, a specific laboratory reactor should be built for a specific industrial scale operation. When this approach is followed, the scale-up of a successful laboratory operation is actually the scale-back to the original conceptual operation. 

The relationship between the mass density of excited species of carrier gas and energy input to the cascade generator is described in Eq. (16.5), and its validity is confirmed by Figure 16.7. The argon emission intensity showed a linear dependence on the combined parameter, [W FM). The total energy applied to the monomers in the CAT-LCVD process can be expressed numerically with this combined experimental parameter, [W FM). ... 

Surface configuration can be fixed by chemical reactions. This is the foundation for the creation of an imperturbable surface. LCVD processing is extremely useful in this particular aspect. 

The vacuum LCVD process is probably an ultimate green process because minimal material is used and virtually no effluent yields from the processing. The economical advantage of such a green process could be estimated in two typical cases. One is the case in which LCVD replaces polluting and/or hazardous processes. 

The largest cost that can be eliminated is the chromate conversion coating itself and the cost for treatment of spent solution and rinse water. In comparison to these costs, the cost for trimethylsilane (LCVD gas) to be used in the closed system LCVD mode is almost negligible. Therefore, the addition of LCVD process is economically favored, if one considers the cost for overall processing for corrosion protection of IVD-processed metallic objects. 

The applied physics community uses the low pressure LCVD process as a time saving device first to prototype the reaction rates and deposition kinetics in this relatively small system and then to apply the results to the large scale surface deposition of films in a conventional CVD process [1], More recently, this process was used to fabricate a wide range of microstructures directly from the vapor phase. The resulting products include low diameter carbon, boron and... 

Figure 3. Schematic drawing of the iow-pressure LCVD process. Redrawn from F. T. Wallenberger, P. C. Nordine and M. Boman, inorganic fibers and microstructures directiy from the vapor phase. Composites Science and Technoiogy, 5,193-222 (1994).

The use of low reaction chamber pressures (<1 bar) in the generic LCVD process yields large (>200 pm) diameter carbon fibers [8j with low growth rates (<10 pm/s) when a CO laser is used. Small (<20 pm) diameter carbon, boron and silicon fibers [1-2j [4j [7] are produced with equally low growth rates at <400 mbar when an efficient Ar laser is used. In summary, the low growth rates are due to the low pressure regime, and the fiber diameters are a function of the individual laser capability. 

The need for new structural and new sensor fibers is well known, but they cannot be made without extensive process and product research. In fact, each fiber would require its own lengthy process development before it could be evaluated in a minimum adequate composite or sensor application. Thus, the staggering cost of research precludes the development of new fibers. In this context, the HP-LCVD process has become an ideal tool for the rapid fabrication, without extensive process research, of test specimens of a variety of continuous length fiber candidates. These specimens can then be rapidly evaluated against the needs of a given end use long before a decision is needed if a new fiber should be commercialized by the LCVD process itself or by an adaptation of another fiber process. 

The HP-LCVD process appears to be a valuable rapid fabrication tool to design and explore new fibers in the field of ultra high temperature sensor systems before first developing a costly conventional commercial process. Using HP-LCVD as such a tool would facilitate the rapid evaluation of new and potentially useful compositions for emerging sensor applications where current industrial and aerospace design specifications exceed the capabilities of commercial sapphire and YAG fibers. 

Potentially useful single crystal HP-LCVD fibers include hafnium boride and tantalum carbide and have projected service temperatures ranging from 2170 to 2715 C. Presently envisioned applications include the potential use of these fibers as consumable sensors to monitor rocket exhaust temperatures. Other HP-LCVD sensor fibers, including Si, Ge and ZnSe, (Figure 15), promise to offer high value in premium automotive and medical sensor systems. Single crystal HP-LCVD germanium [20] and silicon carbide [21] fibers can now also become available for exploration. In summary, the HP-LCVD process is an ideally suited tool for the rapid fabrication and evaluation, without extensive process research, of test samples of potentially new fiber candidates for structural and sensor uses. 

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