Plasma polymer deposition

Primary interest was in the barrier properties obtained from plasma organo-silicones and from inorganic "SIN" coatings. Spectral grade HMDSO was used in the former case, while mixtures of SiH and NH were used to produce the SIN structures. The substrate in much or the work was DuPont Kapton type H polylmide film, 51 pm thick. Substrate temperatures extended to 450 C, as described earlier (6). The thickness of plasma-polymer deposits was about 0.5 pm. Moisture permeation was evaluated by the routine of ASTME-96-53 T (water vapor transmission of materials in sheet form). Additional, more precise data, were obtained with both a Dohrmann Envirotech Polymer Permeation Analyser, modified as previously described (6), and a Mocon "Permatran W" moisture permeation apparatus. 

X-ray photoelectron spectroscopy (XPS) was used for elemental analysis of plasma-deposited polymer films. The photoelectron spectrometer (Physical Electronics, Model 548) was used with an X-ray source of Mg Ka (1253.6 eV). Fourier transform infrared (FTIR) spectra of plasma polymers deposited on the steel substrate were recorded on a Perkin-Elmer Model 1750 spectrophotometer using the attenuated total reflection (ATR) technique. The silane plasma-deposited steel sample was cut to match precisely the surface of the reflection element, which was a high refractive index KRS-5 crystal. 

Figure 4. FTIR spectrum of trimethylsilane plasma polymer. Deposition conditions are the same as those in Fig. 2.

For the cyclic corrosion test, a layer of acrylosilane polymer coating (10-25 fim thick) was dip-coated onto the plasma-deposited substrates. The coated samples were then subjected to 25 scab cycles. The test results are plotted in Fig. 7. Corrosion performance (as described by the length of scribe creep) was correlated to the wattage used for plasma film deposition. As discussed in the previous section, the chemical structure and properties correlated with the deposition conditions, especially the power level applied. Therefore, atomic compositions for plasma polymers deposited at different power levels were also plotted in Fig. 7.A... 

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). 

The circulation of a temperature-controlled liquid controlled the temperature of the crystal surface on which the plasma polymer deposits. In order to measure the substrate temperature accurately, two thermocouples are placed in the fluid-circulating tubes (inlet and outlet) just outside of the plasma reactor. The substrate temperature is estimated from the average of the thermocouple readings. 

The temperature dependence of plasma polymer deposition is generally negative. Some monomers show very little dependence, but it seems that no plasma polymerization system that has positive temperature dependence exists. Consequently, polymer deposition can be prevented if the temperature of the substrate is raised above the ceiling temperature of deposition, which is far above the steady-state ambient temperature of the plasma. 

Figure 10.1 Schematic diagram of the Competitive Ablation and Polymerization (CAP) principle (1) dissociation (ablation) of monomer to form reactive species, (2) deposition of plasma polymer and ablation of solid including plasma polymer deposition, (3) deposition to and ablation from nonsubstrate surfaces, and (4) removal of stable molecules from the system.

Figure 10.4 XPS cross-sectional profile of TMS plasma polymers deposited in a flow system and in a closed system.

Figure 10.5 XPS cross-sectional profiles of plasma polymers deposited on an aluminum surface, and that adhering to primer film peeled off from the aluminum substrate.

If the adhesion of plasma polymer to the substrate is good, cracking of the plasma polymer results, as evidenced by many studies of transport characteristics of plasma polymers as a function of the thickness of plasma polymer (see Chapter 34). If the adhesion is poor, buckling of the plasma polymer deposited onto a rigid substrate such as a glass plate occurs. 

From Figure 13.4 it can be seen that, with the increase of anode spacing from 60 mm to 160 mm, the deposition rate on cathode (substrate) showed an increasing trend. The deposition on the cathode (substrate) surface seemed to reach the maximum when the anodes were removed from the plasma system, i.e., no anode assembly was present and the grounded reactor wall functioned as anode. In contrast, it is noted that, from Figure 13.5, the deposition on the anode surface decreased with the increase of anode spacing. These results clearly indicated that the too-close anode spacing not only reduced the preferred plasma polymer deposition on substrate (cathode) but also induced more undesired deposition on the anode surface. In other words, DC cathodic polymerization without anode assembly seems to be a more efficient and realistic approach in its practical applications. 

The advantages of magnetron for plasma polymer deposition can be summarized as follows ... 

When an LDPE film is immersed in a salt solution (0.9% NaCl), the AC resistivity decreases as a function of the immersion time, as shown in Figure 24.11. These figures include the effect of a nanofilm of plasma polymer deposited on the surface of LDPE. With hydrophobic plasma polymer (HFE + H2), the decrease of AC resistivity was not observed. These figures indicate that the surface state breakdown occurs when the salt intrusion takes place. The salt intrusion can be prevented by the application of a plasma polymer, which is an amorphous network (one phase and no weak boundary). The extent of protection seems to be dependent on the hydrophobicity of the network. 

The same plasma polymer deposited in a closed-system reactor has a graded elemental composition with a carbon-rich top surface, and the oligomer content is much lower [10], both of which increase the level of adhesion. The adhesion of the same water-borne primer is excellent and survives 8 h immersion in boiling water. When this surface is treated with O2 plasma, the adhesion does not survive 1 h of boiling, while the dry tape test still remains at the level of 5. The water-sensitivity of adhesion depends on the chemical nature of the top surface as depicted by XPS data shown in Figure 28.12. Water-insensitive tenacious adhesion, coupled with good transport barrier characteristics, provides excellent corrosion protection, as supported by experimental data [1-4], and constitutes the basic principle for the barrier-adhesion approach. 

According to this scheme of plasma polymerization of TMS in a closed system, it is anticipated that the atomic composition of the plasma polymer should continuously change with the plasma polymerization time. Figure 13.21 depicts comparison of XPS cross-section profile of C/Si ratios for plasma polymers deposited in a flow system reactor and that in a closed system reactor. The results clearly show that a closed system plasma polymerization of TMS indeed produces a... 

The TMS plasma polymer deposited on the oxygen-pretreated steel surface has Si-O-Si or Si-O-alkyl chain structure, which is similar to the film deposited from a mixture of TMS and O2. Oxygen was always found in the plasma film when the steel surface was pretreated with oxygen plasma. The source of oxygen is very likely from the oxide layer. During TMS plasma deposition, oxygen redeposited with the TMS to form the final film. These results were obtained during an in situ experiment, and the treated surface was not exposed to ambient environment before the deposition of plasma polymer of TMS. Therefore, the influence of oxides on the chemical structure of plasma polymer of TMS is quite evident. 

Tables 34.1 and 34.2 show the gas permeability results pertaining to plasma polymers prepared from various nitrile-type monomers that were deposited onto 1-mil thick silicone and silicone-carbonate copolymer sheets [7]. Shown also are the results obtained from the uncoated polymer sheets for comparisons. Both silicone and silicone-carbonate polymer films prior to the plasma polymer deposition show H2/CH4 permeability ratios of 0.79 and 0.97, respectively. After being coated with a...

A capacitively coupled reactor designed to permit continuous coating of a moving substrate with plasma polymer has been described [ 1 ]. In this paper the results of a study of the plasma polymerization of tetrafluoroethylene in such a reactor presented. Plasma polymer has been deposited on aluminum electrodes as well as on an aluminum foil substrate placed midway between electrodes. The study particularly explores conditions in which deposition is minimized on the electrode. For this reason the chemical nature of the polymer formed in a low flow rate (F = 2 cm (S.T.P.)/min) and low pressure (p = 60 mlllltorr) plasma has been analyzed by the use of ESCA (electron spectroscopy for chemical analysis) and deposition rate determinations. This method combined with the unusual characteristics of TFE plasma polymerization (described below) has yielded Information concerning the distribution of power in the inter-electrode gap. The effects of frequency (13.56 MHz, 10 KHz and 60 Hz), power and magnetic field have been elucidated. The properties of the TFE plasma polymer prepared in this apparatus are compared to those of the plasma polymer deposited in an inductively coupled apparatus [2,3]. 

Figure 1. C (Is) spectrum of plasma polymer deposited on the electrode 4 cm from the electrode center with a flow rate of 2 (STP)mL/min, Pu = 60 mtorr, 19 mA AF current and no magnets...

Next, reaction of the maleic anhydride plasma polymer deposited films with aUylamine (Aldrich, 99-i-%) was carried out under vacuum without exposure to air. At this stage, timing of the surface functionaHzation reaction commenced. Upon termination of exposure, the allylamine reservoir was isolated, and the... 

Inagaki, N., Tasaka, S., Takami, Y, 1990. Durable and hydrophobic surface modification by plasma polymers deposited from acetone/hexafluoroacetone, ethylene/hexafluoroacetone, and ethane/hexafluoroacetone mixture. J. Appl. Polym. Sci. 41, 965-973. 

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