Impact of process parameters on plasma processes

Effect of plasma frequency: Effects of activating frequency (rf- or mw-plasma) has not been intensively investigated yet therefore not completely understood. In general however higher deposition rate using the same monomer at the same activating energy was observed by mw-polymerisation. Higher cross-linking of deposited films was on the other hand observed while operating at low frequencies (e.g. rf-plasma) [Wertheimer 1985, Claude 1987].

Fig. 2.5. Relation between basic plasma parameters and plasma process parameters [Kay 1988]

Effect of plasma power and monomer flow rate: As described earlier the composition of a plasma polymer film is mainly dependent upon the fragmentation of the monomer by the plasma. This depends further on the electric energy coupled into the plasma, the density and

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residence time of monomers, and the location of plasma monomer interaction in the reactor. One of the most frequently used parameters describing plasma polymerisation processes is the power to flow-rate ratio W/FM, where W is the plasma power, F the monomer flow rate and M the molecular weight of the monomer. It is assumed that deposition rate and physical, chemical properties of a plasma polymer remain constant when other parameters are the same (pressure reactor, activating frequency etc.); so the magnitude of W/FM is considered to be

proportional to the concentration of activated species in the plasma. Yasuda et al. identified two regions of plasma polymerisation [Yashuda 1983]. In the monomer-sufficient or energy-deficient region (low W/FM) activated species have a far lower concentration than the monomer, thus monomer molecules are subjected to less fragmentation, and plasma polymers with less rearrangement and small loss of functional groups are formed. In the monomer-deficient region however molecules are subjected to an extensive fragmentation and plasma polymers with much rearrangement and large loss of functional groups are formed (fig. 2.6).

Figure 2.6. Deposition domain for plasma polymerisation from Yashuda (1985).

Effect of processes time: Thickness of deposited plasma polymer films can be controlled by the duration of polymerisation processes. It should be noted however that after reaching a certain thickness the accumulation of intrinsic stress in the layer result in cracking and delamination of the film [Yashuda 1976].

Effect of pressure: Pressure influences the plasma treatment processes in several ways. The density of gas molecules in the plasma is directly proportional to the pressure. The average electron energy ε is proportional to E/p, where E is the activating energy. So if the activating energy is constant at high pressure (many molecules) the average electron energy is lower, and less activated species can be formed. On the other hand at higher pressure the free pathway of molecules^∗ (λ) is smaller, more collision occurs and more activated species can be formed if the plasma energy is high enough. It was indicated that the appearance of deposited

∗λ =¼πr²N, where r is the molecular radius, and N the gas density

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polymers depends on the pressure [Tibbit 1977, Molinari 1974]. Operating at higher pressure, at constant flow rate and power, Molinari observed the formation of oily films of ethylene plasma polymer, whereas on the other hand powder-like polymers were deposited at low pressures.

Effect of substrate position: Depending on the sample position in the reactor we can distinguish direct and remote plasma treatments. In former case sample is placed directly in the plasma zone and interacts with all species (ions electrons and radicals) of the plasma. As a result degradation occurs more likely, and a higher variety of functional groups might be created. In remote position however the concentration of radicals – having a longer recombination time than ions - is higher, therefore functionalisation could be more effective and ion bombardment could be avoided or minimised [Goldman 1983, Inagaki 1996]. Plasma deposited films are more cross-linked and a more intensive rearrangement of monomer structure can be observed when substrates are placed directly in the plasma region.

Effect of substrate temperature: To reach a high deposition rate, products should have a low vapour pressure at room temperature, or substrates have to be cooled down. Otherwise the excited monomer escapes in the by-product gas stream. Both these methods lead to a rapid adsorption therefore plasma induced polymerisation mechanism is favoured. Products absorb namely fast on the surface, so the residence time in the active zone of the plasma is relatively short. Deposited polymers are weakly cross-linked, and the monomer structure is more or less retained. With the increase of temperature the deposition rate decreases logarithmically highly cross-linked surfaces are obtained and a loss or reorganisation of functional groups is observed also [Yashuda 1995].

Continuous wave and pulse plasma operation: Changing from continuous operation (cw-plasma) to pulse plasma, the fragmentation of monomer could be minimised. Pulse plasma operation means that the input power is switched off and on periodically over the treatment time. Pulse processes can be characterised by the pulse frequency (ν), and duty cycle (Tg).

ν where tpulse means the plasma “on time”. The average power input (Pav) can be expressed by the input power (Ppulse) and the duty cycle, as follows:

g

As a result of pulse plasma operation the time dependency of electron temperature and density differs in the ignition and in relaxation phase [Liebermann 1996]. In the ignition period the electron temperature increases rapidly and approaches through the pulse period almost the value of cw-processes, and relaxes rapidly when the power is switched off. The electron density however reacts significantly slower for the alteration of plasma power. In general the electron density is higher by pulse plasma than by cw-processes with the same average power input.

It has to be stressed again that considerations of impacts of process variables discussed above can only be guidelines to optimise plasma reactions, and are in many cases controversial.

Depending on the application for what the substrate is going to be used, plasma parameters can be tailored. In many cases it is desirable that the monomer structure is preserved in the resulted polymer, or in others physically or chemically resistant surfaces with high cross-link density are preferred. If the objective is to deposit polymers in which the monomer structure is preserved the degree of interaction between reactive species and monomer should be limited. This can be realised by avoiding ionic bombardment (remote plasma), limiting the residence time of the monomer in the plasma, limiting the power input, and working at relatively high pressures (decreasing the average electron energy) [Mas 1997].

Applications of plasma polymerisation are ranging from surface functionalisation [Hochart 2000, Inagaki 1998], permselective membrane preparation [Ulbricht 1996, Hyun 2001], over protective coatings [Ramachandran 1997, Bogaerts 2002], film transistors [Liebermann 1999], to optical coatings [Morosoff 1990, Bogaerts 2002] as described in the cited review articles.