Discussion of single-carbon production on the sur- sur-face during glow and afterglow

4.2.6.1 Wall production of CF

Figure 4.34: CF and CF2 densities in a chemical simulation during glow in a pulsed discharge (beginning of pulse sequence). Notice that onlyCF2is produced at the walls, and the effect on theCFdensity profile.

In the literature, CF production at the electrodes is occasionally ob-served [4, 23, 124–126]. In the present work, no concave density profiles and therefore, no indications for such processes were found. Furthermore, especially in hydrogen containing plasmas, care must be taken whether con-caveCFdensity profiles are not induced by theCF2profiles in combination with high-rate H-atom induced abstraction of fluorine (reaction channel 96 in Table 3.8). This can be generalized to cases whereCF2densities are or-ders of magnitude larger than theCFdensity, as also other processes result in fluorine abstraction. In Figure 4.34, a simulated diffusion-flow profile of CF2 is shown together with the resulting CFprofile. Only slight, but significant differences are observed at the electrodes (z = 0,45), where the pseudo-diffusion profile ofCFshows a typical flattening. Such deviations are easily overlooked when conventional cross-beam LIF techniques are applied.

Often, the height-resolution is poor or acquisition times are too long such

that heating of the electrodes and other time-dependent effects cause large scattering and prevent to identify the curvature of the density distributions.

4.2.6.2 Production of CF₂ at the surface

TheCF₂ densities show a height distribution dominated by diffusion fluxes from the electrodes towards the reactor center. The diffusion fluxes depend on the plasma conditions as there are power, duty cycle, pressure, gas flows, and the chemical state and polymer coverage of the walls as shown before in Section 4.2.2.2.

The phenomenon of CF₂ surface fluxes is under discussion for a long time. It was observed under many different experimental conditions (differ-ent monomer gases, plasma excitation, wall materials, and others, see Table 4.3). Profiles in these different plasmas were acquired by LIF.

• ΓCF₂ is sometimes accompanied by significant volume losses which were e.g. attributed to ionization [126] and oligomerization [50].

• Production at the electrodes can result from ion neutralization, ion sputtering, electron-induced fragmentation [36] as well as dissociation of oligomer [127] or other chemical reactions [126, 128].

• Additional to that, an electron-collision induced transition from sin-glet ¹A¹CF₂ to triplet ³B¹CF₂ was suggested by Booth and Corr [129]. Accordingly, the singlet-triplet transition would reduce the cen-ter¹A¹CF2density, followed by a triplet-singlet transition at the elec-trodes and thus, reappearance of the singlet at the elecelec-trodes. Such, a concave profile would result.

• Temperature gradients can generally affect density profiles ofCFand CF2 [4, 130].

• The possibility of plasma-sheath processes by e.g. ion-neutral collisions was pointed out in [115, 131].

The monomer used also has a large effect, and a major criterion for a Γ_CF2 abundance is the fluorine-to-carbon ratio [29, 125, 132]. It is found that under fluorine-rich conditions like inCF4 plasmas, ionic processes are presumably leading to CF2 formation as neutralized fragments. For the further kinetics it must be noticed that in these plasmas, no film is deposited.

Such, in the afterglow, molecules are observed to stick to the surface, which results in an opposed diffusion.

Table 4.3:CF2 wall production in the literature source gas geom. wall density

profile power supply [126] CF4 tube quartz concave, radial rf, hw, p/cw

[23] C4F8 tube quartz concave, radial* rf, hw, p [50] (a) CF4 pp Al > at powered el. rf, ccp, p [50] (b) C₂F₆ pp Al concave, axial rf, ccp, p

[29] CF4 pp Al > at powered el. rf, RIE, p

[125] CF₄+ H₂ tube quartz concave, radial rf, ccp, cw [127] CHF3+ Ar pp Si concave, axial rf, icp, cw

p: pulsed excitation, cw: continuous wave excitation, *:afterglow

Contrary to that, at a lower fluorine-to-carbon ratio,CF2 densities are larger (see comparative studies on CF4 and C4F8 in [132]), which results from a strong wall production. A cyclic model of oligomerization, neutral or ion deposition, and subsequent (chemical) formation and desorption of CF₂ was suggested.

The influence of the fluorine-to-carbon ratio on the surface fluxes in a CF4+ H2 plasma was examined by Sasakiet al. [125]. ΓCF₂ turned from negative values (−0.5×10¹⁵cm⁻²s⁻¹) in pureCF₄ to 4.5×10¹⁵cm⁻²s⁻¹ at 50% hydrogen, and same time, the CF₂ density increased by a factor of about 20. Especially the almost linear relation between theCF2 density andΓCF₂supports the previously mentioned model of cyclic oligomerization, deposition, andCF2 desorption in [132].

4.2.6.3 Surface production processes under the examined condi-tions

During the experiments performed, no wall production ofCFwas observed.

Concerning CF₂, several of the above mentioned mechanisms can be ex-cluded:

• Volume losses are not responsible as shown by evaluation of the net rate in Section 4.2.4.2.

• Neutralization of CFx ions at the electrodes can be discarded as ion current densities are too low by two orders of magnitude (Figure 4.35) and, due to the gas mixture, consist of about 80-93% argon ions.

Figure 4.35: Ion currents (top) andΓCF₂ (bottom) versus plasma power.

• Concerning singlet-triplet transitions, it is known from literature that triplet-triplet annihilation is taking place in the gas phase as well [133].

At pressures as used in the experiments, the annihilation should be well pronounced in the gas phase. In addition, the emission from the spin-triplet³CF₂at 540.5, 560.6 and 582.3 nm [134] was not visible in OES. The conversion of ¹CF₂ to ³CF₂ is therefore unlikely to occur at rates high enough to explain the observed profiles.

• Temperature gradients can be excluded as well as absorption spectra showed that the rotational and vibrational temperatures are around room temperature.

• The same holds for ion-neutral processes in the sheath, because the ion-neutral cross sections commonly decrease upon increasing energy, and such, ion-neutral reaction rates would be higher in the plasma bulk than in the sheath region [29].

Instead, the dependence of the CF₂ density on the surface condition (Figure 4.24) shows increase and saturation when a homogeneous, closed film is obtained (see Section 4.4.1). This is in agreement with the model in [132]. Such, the kinetics of the wall production could be (a) sputtering by ions and/or electron-induced detachment, or (b) chemical reactions.

To (a): The ion current densities as obtained from the discharge sim-ulation in Section 3.2 are depicted in Figure 4.35. A comparison toΓ_CF2 shows that the ion current densities are by two orders of magnitude lower thanΓCF₂. Therefore, to obtain the surface flux, a sputtering yield of 100 molecules per incident ion would be at least necessary. This yield is too large compared to known sputtering yields at these ion energies. However, the energy input by impinging ions or electrons may be import to initiate chemical reactions.

To (b): Chemical reactions are consequently the major production source of difluorocarbene. The reaction kinetics, especially at the ignition stage, strongly depend on the plasma power and the duty cycle (Section 3.3). This is seen from the dependence of oligomer formation in the gas phase on the plasma conditions (Section 4.3):

• Upon increasing the plasma power, more oligomer is formed.

• Upon increasing the duty cycle, more oligomer is formed.

This both affects the maximum in the diffusion flux, Γ_CF2. The faster increase observed at higher duty cycle could then be attributed to more

residual oligomers in the gas phase after a shorter pause (due to less pump-out). In this case, oligomer would be faster available for reactions on the surface, or even still be physisorbed to the surface. During the plasma glow, reactions leading to cross linking of the films are initiated. These reactions depend on energy supply by the plasma. Indeed, formation of such active sites inside polymers, resulting from plasma treatments or irradiation has been probed by ESR in [135, 136] and [137], respectively. This leads to restructuring of the material.

Electromagnetic radiation (emission of excited atoms or molecules) has not been taken into consideration so far. In the literature, the possi-ble effect of radiation on polymerization and/or decomposition of polymer in fluorocarbon plasmas has yet not been mentioned, although it is well known that CF₂ can be e.g. obtained from flash-photolysis of several mo-lecular substances [21, 138–140], amongst them several pure fluorocarbons [17, 21, 48, 141]. Bond breaks and excitations can be induced in polymers as well. The electronic states ofPTFE for example reveal several possibil-ities for excitations of C−Cand C−Fbonds, mostly in the vacuum-UV region [142]. These processes can be used for surface processing. Such way, polishing by electronic excitation viaF2-laser irradiation was recently pub-lished [143]. A significant weight-loss of 1-2 mg was found after irradiation of a PTFE substrate. In this example, the loss of mass would correspond to about3.3×10¹⁸CF2molecules cm⁻², the total energy input to1.6×10¹⁹ photons (λ = 157nm). Both numbers are comparable, showing the high efficiency of this process. The ejection of molecules from a PTFE sur-face would consequently require a similar number of radiative events in the plasma bulk. Depending on the emitting atom/molecule, the delay com-pared to a plasma pulse start could result from formation and excitation processes of the emitter, furthermore on the availability of oligomer as well as the reaction velocity within the film. The production of CF₂ in the af-terglow is mainly attributed to persistent reactions inside the polymer, but also the post-discharge emission due to metastables could have an influence.

Concluding, strong indications are found thatCF2is mainly chemically formed in the layer by reactions which are promoted by energy supply from the plasma. However, besides the proposed photonic model, no detailed chemical research on this topic was carried out yet, and it is not clear from chemical point of view, what kind of process can result in carbene formation

at such high rates. Candidates could be processes like cheletropic reactions⁶ which result in ejection of CF2 from ionized molecules [144]. But these details require future investigations, otherwise, statements and conclusions on this special topic will remain speculative.