LPCAT

The spatially decoupled activation and deactivation can be also seen in a mode of PP known as low-pressure cascade arc torch (LPCAT) polymerization), which is described in Chapter 16. The activation of a carrier gas (e.g., argon) occurs in a cascade arc generator, and the chemical activation of a monomer or a treatment gas takes place near the injection point of the argon torch in the deposition chamber. The material deposition (deactivation) occurs in the deposition chamber. This is the same situation as the HWCVD, except that the mode of activation is different. 

LPCAT is a luminous CVD process with decoupled ionization process, i.e., the chemically reactive species are created by the neutral species-impact dissociation of molecules, but not by ion-impact or electron-impact dissociations. Because of this aspect, LPCAT provides important information pertinent to the nature of the creation of chemically reactive species in LCVD and will be discussed in some detail in the following chapters. 

LPCAT polymerization Vapor phase in a separate chamber by plasma Substrate in a separate chamber Ts < Tv AE<0... 

The luminous gas phase created by a special mode of DC discharge recognized as the low-pressure cascade arc torch (LPCAT) provides an especially important case for understanding the fundamental aspects of the luminous gas phase. The luminous gas phase in form of luminous gas jet stream or torch are created by blowing out DC discharge into an expansion chamber in vacuum. The luminous gas jet of Ar mainly consists of photon-emitting excited neutral species of Ar, which is certainly not the plasma of classical definition. The core of LPCAT is the tip of injection nozzle however, it is not the core of electrical discharge. 

The luminous gas phase that can be characterized in the LPCAT very likely exists as remnant of electrical discharge plasma. The luminous gas phase in LPCAT provides an important foundation to elucidate the creation of chemically reactive species in LCVD, which is presented in Chapter 4. The details of LPCAT are given in Chapter 16. LPCAT provides an example that can distinguish the luminous gas phase from plasma, i.e., the luminous gas jet created by LPCAT is not in the state of plasma. In this book, the luminous gas phase is emphasized without being bound by the concept or definition of plasma. 

The existence of the dissociation glow in DC discharge strongly suggests that the creation of chemically reactive species in LCVD involves different mechanism than those in the electron impact ionization. However, in DC discharge, electron impact and ion impact reactions cannot be eliminated. Low-pressure cascade arc torch (LPCAT) provides a unique opportunity to investigate the formation of chemically reactive species with minimal influence of ions and electrons. That is, the creation of chemically reactive species from an organic molecule by the luminous... 

When plasma polymerization is carried out in a flow system, in which glow covers the entire cross-section of reactor with respect to the direction of monomer flow, the monomer molecules coming into the reactor first encounter the luminous gas phase. It is very unlikely that the molecules pass through the luminous gas phase without interacting with it and reach the relatively narrow zone in which IG or DG, located near the electrode surface, occurs. Therefore, the mode of activation that occurs in LPCAT without the influence of ionization is important in terms of the creation of chemically reactive species in LCVD. The creation of reactive species by the luminous gas is the mechanism considered here. 

In cascade arc plasma polymerization, a monomer (or monomers) is introduced in the expansion chamber. Because of an extremely high velocity of gas injected from a small nozzle (e.g., 3 mm in diameter), the second gas injected into the expansion chamber in vacuum cannot migrate into the cascade arc generator. Thus, the activation of Ar in the cascade arc generator and deactivation of the excited neutral species of Ar in the expansion chamber, which activate the monomer introduced in the expansion chamber, are totally decoupled. LPCAT plasma polymerization occurs under such a spatially and temporally decoupled activation/ deactivation system. 

Figure 4.13 Electron density (cm ) as a function of axial position and methane flow rate in low pressure cascade arc torch (LPCAT) 8.00 A, 2000 seem argon, and 560mtorr (75 Pa).

In low-pressure cascade arc torch (LPCAT), the electrical power is applied in the cascade arc generator, in which only carrier gas, generally Ar, is activated to create luminous gas. The luminous gas created in the cascade arc generator is blown into the second expansion chamber, in which the monomer is introduced. Thus, the luminous gas of Ar neutrals primarily creates polymerizable species, and following these two steps should treat the deposition kinetics. Principles described in this chapter apply to each of the two steps. Details of deposition kinetics in LPCAT are described in Chapter 16. 

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 LPCAT LCVD are compared as follows ... 

INTERNAL STRESS IN PLASMA POLYMERS PREPARED BY LPCAT... 

First, internal stress is plotted against arbitrarily selected single parameters just as the data in glow discharge polymers were presented. All films prepared by LPCAT showed the same kind of curling force, from which internal stress can be calculated. Figure 11.6 shows the influence of LPCAT operation parameters on... 

Figure 11.7b shows the internal stress in LPCAT films of cyclic siloxanes 1,3,5,7-tetramethylcyclotetrasiloxane (TMTSO) and 2,4,6,8-tetravinyl-2,4,6,8-tetra-methylcyclotetrasiloxane (TVTMTSO). The large siloxane ring structure in these two monomers did not provide any decrease of internal stress in resultant plasma polymer films, compared with simple siloxane monomers, i.e., TMTSO, HMDSO, and VpMDO. 

In general plasma polymerization processes it has been established that the deposition rate and properties of a plasma polymer primarily depend on the value of the normalized energy input parameter WjFM, as described in Chapter 8. In LPCAT polymerization processes, as described in Chapter 16, the deposition rate of a plasma polymer primarily depends on the value of the normalized energy input parameter, which is given by W FM)J FM). In this composite parameter, W is the power input applied to arc column, FM) is the mass flow rate of carrier gas (argon), and FM) is the mass flow rate of monomer that is injected into the cascade arc torch. The quantity of W FM)J FM) can be considered as the energy, which is transported by carrier gas plasma, applied to per mass unit of monomers. 

A single monoatomic gas, e.g., argon or helium, is used as the carrier gas of the cascade arc discharge. When the luminous gas is injected into an expansion chamber under low pressure, e.g., 1 torr or less, the flame extends a significant length (e.g., 1 m), which depends on the fiow rate, input power, diameter of the nozzle, and pressure of the expansion chamber. This mode of cascade arc torch is termed low-pressure cascade arc torch (LPCAT), which is useful in the surface modification by means of low-pressure cascade arc torch treatment and low-pressure cascade arc torch polymerization. 

As seen in Figure 16.3, the LPCAT fiame is relatively narrow, implying that a uniform diffused luminous gas phase is not created in the expansion chamber. Consequently, the treatment that can be achieved by an LPCAT is governed by the... 

Another potential mode of LPCAT processing is that the integrated cascade arc generator, such as that shown in Figure 16.2, is placed in a vacuum chamber held by a robot arm. In this mode, LPCAT jet could scan over a complex shaped substrate by the robotic operation. 

LPCAT can be utilized in the following three modes in the surface modification of materials ... 

In the LPCAT process, only an inert gas such as Ar exists in the cascade arc generator, and DC voltage is applied between the cathode and the anode. Therefore, it is a DC discharge of Ar, but it occurs under much higher pressure than in most low-pressure DC discharges, and the gas travels very fast in one direction in the generator. The basic process of ionization of Ar takes place in the cascade arc generator, which can be depicted as follows. 

The argon emission intensity showed a linear dependence on the combined parameter, W FM), i.e., the total energy applied to the carrier gas in the LPCAT process, as described by Eq. (16.5), can be expressed by this combined experimental parameter, W FM)c ... 

Figure 16.12 The emission from excited oxygen atom (a) oxygen in Ar LPCAT and (b) oxygen in helium LPCAT.

Figure 16.13 The presence of oxygen ion and the quenching of He LPCAT (a) O2 added to He LPCAT, (b) He LPCAT (without O2).

The creation of chemically reactive species from polymer-forming gas (monomer) in plasma jet of LPCAT follows the same principle described for non-polymer-forming gases, but the major reaction is molecular dissociation of monomer by energy transfer mechanism. Upon addition of monomers to the argon luminous gas jet, the emissions of argon luminous gas are highly quenched. The dominant features... 

In comparison with conventional electrical discharge processes, LPCAT is a very different process in that its activation of carrier gas and the creation of polymerizable species by the activated carrier gas are temporally and spatially separated. When discharge power is applied to the cascade arc generator, the plasma of carrier gas (usually argon) is produced in the cascade arc column and the luminous gas phase is blown into a vacuum chamber where monomers are introduced. The deactivation of the reactive species, some of which lead to the creation of polymerizable species in the luminous gas phase, occurs within the relatively narrow beam of an argon luminous gas jet. The higher the flow rate of Ar, the narrower is the beam and the longer the luminous gas flame. 

Normalized Energy Input Parameter of LPCAT LCVD... 

The plotting deposition rates of different monomers with different feed rates in Normalized DR) versus W FM)cI FM)t coordinates would provide the most objective comparison of their tendencies with regard to deposition in CAT LCVD. In LCVD, it is very important that the polymerization (material formation) is atomic rather than molecular processes, implying that the depositing entities are fragmented species of the original monomer molecule. Therefore, the deposition rate in LPCAT polymerization is determined largely by the type of atoms contained in the monomer structure rather than by molecular structures. 

---