LCVD film

The control of the deposition thickness is crucially important in the LCVD operation, regardless of the modes of operation. However, it is very difficult to measure the actual thickness of ultrathin film deposited on polymeric substrates. If an LCVD film is deposited on polymeric substrates, such as films, fibers, and molded articles, it is nearly impossible to determine the thickness by a simple nondestructive method that can be done quickly enough to monitor the LCVD operation. In order to circumvent this problem, the use of special substrates added or attached to the normal substrate is found to be satisfactory. A small piece of Si wafer is a typical case of this approach. Ellipsometer can measure the deposition on the Si wafer easily and quickly, which provides thickness and refractive index values. 

The significance of LCVD is in the unique aspect of creating a new surface state that is bonded to the substrate material particularly polymeric material. The new surface state can be tailored to be surface dynamically stable. However, caution should be made that not all LCVD films fit in this category. Appropriately executed LCVD to lay down a type A plasma polymer layer creates surface dynamically stable surface state. In the domain, in which surface dynamic instability is a serious concern in the use of materials, a nanofilm by LCVD is quite effective in providing a surface dynamic stability, and other methods do not fare well in comparison to LCVD. 

Because of high packing density of type A plasma polymer (LCVD film), the segmental mobility discussed above section is minimal, and the surface of type A LCVD film is anticipated to be imperturbable. However, there are some observations... 

The role of the second plasma treatment by HFE or Ar seems to be the removal of type B plasma polymer of TMS from the top surface region or possibly converting the type B plasma polymer to type A plasma polymer. Electron spin resonance (ESR) data (described in Chapter 6) indicate that the number of Si-based dangling bonds decreases by these second plasma treatments. The weight loss observed with some plasma polymers and the ESR data for TMS film suggest that type B plasma polymer in the top surface region of an LCVD film could be up to nearly 30% of the... 

It is clear from the study on pure iron that oxides participate in LCVD of TMS, and characteristics of plasma polymer films differ depending on the extent of oxides present on the surface when LCVD is applied. Oxides on the surface of pure iron are more stable than those on steel and hence more difficult to remove, but this can be effected by plasma pretreatment with (Ar + H2) mixture. SAIL by LCVD involving removal of oxides provides excellent corrosion protection of pure iron. The key factor of SAIL by LCVD for corrosion protection of metals in general is the handling of oxides, which depends on the characteristic nature of the metal oxide to be handled. Once strong chemical bonds were formed between nanofilm of plasma polymer, either through oxides or direct bonding to the substrate metal, the LCVD film acts as the barrier to corrosive species. 

The water transport mechanism changes from the flow mechanism in porous membrane to the diffusive transport in nonporous homogeneous membrane due to the deposition of a homogeneous LCVD layer that fills the pore, i.e., water transport changes from bulk flow to diffusive flow when pores are covered by LCVD film. 

The three unique and important features of type A LCVD nanofilm—imperturbable surface (Chapter 29), nanoscale molecular sieve (Chapter 34), and new surface state of material (Chapter 24) make LCVD coating an ideal tool in preparation of biomaterials. It should be reiterated that these three features of LCVD films are limited to type A plasma polymers described in Chapter 8, and type B plasma polymers should be excluded in LCVD coatings for biomaterials based on the concept of imperturbable surface. The particularly important aspect is that the LCVD nanofilm becomes the new surface state of the substrate material, i.e., it is not just a coating placed on the surface. The first and second features describe the nature of the new surface state. 

These techniques have crossovers in the sense that, for example, LCVD can be done with metal-organic precursors. Cheon and Zink (1997) deposited thin films of ZnS, CdS, and Zn,Cdi S onto quartz substrates. The source of the metal and the sulfur was the organometallic compound diethyl dithiocarbamate zinc or cadmium M(S2CNEt2)2- In this case a single precursor was used to produce a binary compound as a thin film. 

In order to find the domain of LCVD, it is necessary to compare various vacuum deposition processes chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma chemical vapor deposition (PCVD), plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), and plasma polymerization (PP). All of these terms refer to methods or processes that yield the deposition of materials in a thin-film form in vacuum. There is no clear definition for these terms that can be used to separate processes that are represented by these terminologies. All involve the starting material in vapor phase and the product in the solid state. 

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. 

Although numerous kinds of reactions could occur in the luminous gas phase, as far as the dissipation of vapor phase molecules (LCVD deposition) is concerned, one benzene molecule behaves as three acetylene molecules. Consequently, the final polymers formed (from acetylene and benzene) under the condition of relatively high WjFM are very similar. The transport characteristics of ultrathin films of plasma polymers and copolymers (with N2 and/or H2O) of acetylene and benzene are nearly identical. 

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. 

Powder formation in an LCVD system is a reflection of the polymer deposition mechanism. The size and number of particles may be taken as a measure of the polymerization-deposition mechanism or the status of an LCVD system. At one extreme is exclusive powder formation, as reported by Liepins and Sakaoku [7] at the opposite extreme is the formation of a continuous film in which no visible particles can be found. Even in the latter case, however, the work of Havens et al. [12] involving the use of small-angle X-ray scattering indicates that detectable domains... 

When a very thin film (e.g., thicknesses of less than 1 pm) of a polymer is applied to a smooth surface of platinum, most polymers peel off within minutes upon immersion in liquid Lf20. This is also true for most plasma polymers applied to platinum surfaces. However, when an ultrathin film of CH4 LCVD was deposited under the conditions that provide plasma energy density sufficiently high to sputter aluminum from the electrodes, tenacious adhesion that survived over 10 h of boiling in saline solution was obtained, probably due to the incorporation of electrode metal at the interface. 

This film system was seen to outperform others not incorporating the adhesion-promoting HFE film when a primer is applied to the LCVD coated alloy surface. The alloy panels were always treated with O2 plasma to remove any organic contaminants from the alloy surface prior to film deposition. The entire steps involved in the plasma coating process are ... 

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. 

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