Cathodic LCVD

A luminous chemical vapor deposition (LCVD) layer could be used as transition phase between metal and polymer (coatings or films). The deposition of LCVD layer on metals can be done by the cathodic LCVD, in which the metal to be incorporated is used as the cathode of DC discharge. This method provides an excellent adhesion of LCVD nanofilm to the metal as well as the adhesion of coatings that will be applied on the surface of the LCVD layer [1,2]. This method is described in some detail in Part IV. 

Developing a reactor for a luminous chemical vapor deposition (LCVD) operation to deal with large numbers of materials to be coated, probably the most difficult problem encountered is how to hold the substrates, place them in appropriate position, and move them in the luminous gas phase. In order to coat large number of substrate uniformly, the movement of substrate within the luminous gas phase is a mandatory requirement, with the exception of direct current (DC) cathodic LCVD. The difficulty progressively increases as the size of the substrate decreases, and it becomes virtually impossible to hold as the size reaches millimeters or less. For instance, the surface of small particulate matters cannot be treated or coated by the conventional modes of LCVD in which substrates are held by some kind of holder. 

CATHODIC LCVD WITH ANODE MAGNETRON 2.1. IVD/Plasma Polymer/E-coat Systems... 

CATHODIC LCVD WITHOUT ANODE ASSEMBLY 3.1. Nature of Anode... 

A nanofilm of plasma polymer (up to about 100 nm) has sufficient electrical conductance as evidenced by the fact that an LCVD-coated metal plate can be coated by the electrolytic deposition of paint (E coating), i.e., plasma polymer-coated metals can be used as the cathode of the electrolytic deposition of paint (see Chapter 31). Thus, the plasma polymer layer remains in the same electrical potential of the cathode (within a limited thickness) and the work function for the secondary electron emission does not increase significantly. When the thickness of plasma polymer deposition increases beyond a certain value, the coated metal becomes eventually insulated, and DC discharge cannot be sustained. DC cathodic polymerization is primarily aimed to lay down a nanofilm (10-100 nm) on the metal surface that is used as the cathode (see Chapter 13). 

In DC discharge for LCVD, the main core is the DG adhering to the cathode surface, and the anode is out of the onion structure in most cases. In DC discharge of Ar for glow discharge treatment or sputter deposition of the cathode material, the core is the IG, which does not touch the cathode surface. 

In alternating current discharge for LCVD, up to about 100 kHz, the DG adhering to the electrode surface (in the cathodic cycle) is the core. The discharge system has two cores, and the interelectrode space is filled with two onion structures overlapping in part, which approach each other when the value of Wjpd increases. 

The ionization of an organic molecule is the basic principle of mass spectroscopy however, it should be recognized that mass spectroscopy of a simple molecule such as ethane generally shows multiple ions, covering the fragmented species to partially polymerized species, which indicates that the dissociation of the molecule and some extent of polymerization of fragmented species occurred in the mass spectrometer. The presence of the DG (cathode glow) in DC cathodic polymerization implies that the formation of chemically reactive species via ionization, as depicted by Eq. (4.1), is a very unlikely primary event under the conditions 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 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. 

The characteristics of the deposition on the cathode surface (deposition E) and the deposition on the electrically floating surface placed in gas phase (deposition G) in 40-kHz discharge LCVD are compared as follows ... 

SIMULTANEOUS SPUTTER COATING AND LCVD 3.1. RF Discharge with Metai Cathode... 

The rate of sputtering of aluminum from the electrode used in a magnetron plasma polymerization system is dependent on the plasma energy density, which can be stipulated by the external parameter Vjp, which is the acceleration potential in the vicinity of the cathode, for Ar discharge, while the deposition of CH4 is dependent on WjFM in joules per kilogram of CH4 for LCVD as described in Chapter 8. 

Considering the fact that the refractive index continues to increase after most of the polymerizable species are exhausted in the gas phase, DC LCVD of TMS in a closed system contains the aspect of LCVT of once-deposited plasma polymer coating by hydrogen luminous gas phase. In the later stage of closed-system LCVD, oligomeric moieties loosely attached to a three-dimensional network are converted to a more stable form, and significantly improved corrosion protection characteristics (compared to the counterpart in flow system polymerization of TMS) were found, details of which are presented in Part IV. Thus, the merit of closed-system cathodic polymerization is well established. 

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. 

Cathodic plasma polymerization or LCVD of trimethylsilane (TMS) applied to an appropriately prepared aluminum alloy surface yields a roughly 50-nm-thick layer of amorphous Si C H network, which is covalently bonded to aluminum oxide at the interface [7], The XPS cross-sectional profiles given in Figure 28.10 show the conspicuous shifts in O Is and Si 2p at the interface that indicate the changes of chemical bonds. 

The hybrid process of IVD/cathodic luminous chemical vapor deposition (LCVD) showed great advantage of providing improved corrosion protection by environmentally benign process [3-5]. Table 32.1 shows the comparison of typical... 

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