Plasma polymerization deposition rate

An electrode in an AC discharge is the cathode for half of the deposition time and the anode for the other half of the time. Comparing Eq. (8.2) and Eq. (8.7), the contribution of the cathodic polymerization can be estimated by examining the system pressure dependence of the deposition rate (at a fixed flow rate). If plasma polymerization (deposition G) is the dominant factor, it is anticipated that the deposition rate would be independent of the system pressure. If cathodic polymerization (deposition E) is the dominant factor, the deposition rate onto an electrode is dependent on the system pressure, and the value of deposition rate is expected to be one-half of that for DC cathodic polymerization. 

The number of attempts to model the kinetics of plasma-polymerization has been limited thus far. Nevertheless, these efforts have been useful in demonstrating the role of different processes in initiating polymerization and the manner in which the physical characteristics of the plasma affect the polymerization rate. It is anticipated that future modeling efforts will provide more detailed descriptions of the polymer deposition kinetics and thereby aid the development of a better understanding of the interactions between the physical characteristics of a plasma and the chemistry associated with polymer formation. 

The hydrodynamic factors that influence the plasma polymerization process pose a complicated problem and are of importance in the application of plasma for thin film coatings. When two reaction chambers with different shapes or sizes are used and when plasma polymerization of the same monomer is operated under the same operational conditions of RF power, monomer flow rate, pressure in the reaction chamber etc., the two plasma polymers formed in the two reaction chambers are never identical because of the differences in the hydrodynamic factors. In this sense, plasma polymerization is a reactor-dependent process. Yasuda and Hirotsu [22] systematically investigated the effects of hydrodynamic factors on the plasma polymerization process. They studied the effect of the monomer flow pattern on the polymer deposition rate in a tubular reactor. The polymer deposition rate is a function of the location in the chamber. The distribution of the polymer deposition rate is mainly determined by the distance from the plasma zone and the... 

Figure 2. XPS spectra of a plasma-polymerized trimethylsilane film on steel. Plasma deposition conditions flow rate 2 seem pressure 50 mTorr, power 2 W, deposition time 2 min.

Plasma polymer layers were deposited in the same reactor as described before. However, in this case, the pulsed plasma mode was applied. The duty cycle of pulsing was adjusted generally to 0.1 and the pulse frequency to 103Hz. The power input was varied between P 100 ()() V. Mass flow controllers for gases and vapours, a heated gas/vapour distribution in the chamber, and control of pressure and monomer flow by vaiying the speed of the turbomolecular pump were used. The gas flow was adjusted to 75-125 seem and the pressure was varied between 10 to 26 Pa depending on the respective polymerization or copolymerization process. The deposition rate was measured by a quartz microbalance. 

There are two important questions which may readily be addressed by ESCA. Firstly, what is the rate of deposition of the polymer film at a given site in a plasma reactor and secondly how does the structure depend on the site of deposition To illustrate the great power of the technique in answering these questions we consider here a recent detailed investigation of the inductively coupled plasma polymerization of pentafluorobenzene (13). 

When the deposition rate and the system pressure shown on the recorder are confirmed to be steady, the deposition rate reading and the crystal temperature were recorded. Then changing the thermostat control of the circulating bath, while the plasma polymerization is kept at the steady state, lowered the temperature of the crystal. The deposition rate at the next temperature is read and recorded after steady-state readings are obtained at the new temperature. In this way, the relationship between deposition rate and substrate temperature can be obtained at a set of flow rates and power. A similar procedure is repeated for another set of flow rates and power. 

Figure 5.6 depicts the temperature dependence of deposition rate for the plasma polymerization of PFBTHF shown as plots of k versus T. The XPS C Is spectra of polymers deposited at different temperatures under different energy input levels are shown in Figure 5.7. Table 5.3 depicts the details of XPS C Is spectra shown in Figure 5.7. The important aspects of the results are as follows ...

The deposition rate of plasma polymerization depends on many experimental factors of glow discharge. A large number of attempts have been made to correlate the polymer deposition rate with such operational variables as flow rate, discharge power, current density, and system pressure. Although reasonable agreement is found... 

As already pointed out, the temperature dependence of polymer deposition is not related to the conditions of plasma polymerization (i.e., the flow rate and... 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

The material formation in the luminous gas phase (plasma polymerization) is less specific to the chemical structure of molecules. Benzene, which is a nonpolymer-izable solvent in the free radical chain growth polymerization, polymerizes readily in the luminous gas phase. Benzene not only polymerizes, but its rate of deposition is nearly equivalent to that of acetylene, i.e., a benzene molecule is equivalent to three molecules of acetylene in the luminous gas phase. 

The normalized deposition rate is the only form of deposition rate that can be used to compare deposition characteristics of different monomers with different chemical structures and molecular weights under different discharge conditions (flow rate, system pressure, and discharge power). Similarly, WjFM can be considered as the normalized power input. When only one monomer is employed, D.R. can be used to establish the dependency of deposition rate on operational parameters. Even in such a simple case, D.R. cannot be expressed by a simple function of W or F, and its relationship to those parameters varies depending on the domain of plasma polymerization. 

As the power input is increased (at a given flow rate), the domain of plasma polymerization approaches the monomer-deficient one, which can be recognized by the asymptotical approach of D.R. value to a horizontal line as the power input increases. In the monomer-deficient domain, the deposition rate (plateau value) increases as the flow rate is increased and shows a linear dependence on the monomer feed-in rate at a given discharge power and the system pressure (Fig. 8.2), i.e.,... 

When the equation for plasma polymerization [Eq. (8.2)] is applied to express the thickness growth rate of the material that deposits on the cathode cathodic polymerization), it becomes quite clear that the deposition kinetics for the cathodic polymerization is quite different. There is a clear dependence of the deposition rate on WjFM, but no universal curve could be obtained. In other words, the relationship given by Eq. (8.2) does not apply to cathodic polymerization. The best universal dependency for cathodic polymerization was found between D.R./M (not D.R./F M) and the current density IjS), where / is the discharge current and S is the area of cathode surface [5]. Figure 8.7 depicts this relationship for all cathodic polymerization data, which were obtained in the same study, covering experimental parameters such as flow rate, size of cathode, and mass of hydrocarbon monomers but at a fixed system pressure. The details of DC discharge polymerization are described in Chapter 13. 

Figure 8.8 The system pressure dependence of deposition rate of TMS on Si wafer with electrical contact to the powered electrode in DC (cathode), 40-kHz, and 13.56-MHz plasma polymerization processes discharge conditions are 1 seem TMS, 5-W power input.

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. 

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