Verlag des Forschungszentrums Jülich
JUEL-3562
Leukens, Armin
Untersuchung des plasmainduzierten Aufbaus großer Moleküle in einer Niederdruck-Hohlkathodenanordnung
119 S., 1998
DC-Glow-Discharges in organic gases are widely used for the carbonization of surfaces. It is known, that these plasmas are suitable to constitute agglomers, which can be applied systematically for the sythesis of nanoparticles, but can be problematic with respect to the plasma induced thin film deposition. Since the beginning of the 90th the mechanisms leading to the generation of these large particles are intensively investigated, but in many cases not well understood. This experimental work is dealing with an examination of responsible processes, which enhance the generation of large molecules in a Hollow-Cathode-Discharge. Discharges in rare gases, methane, ethane, ethene, ethine and mixtures of them are employed in a pressure regime between 0.1Pa and 5Pa. The experimental apparatus is equipped with a plasma process monitor (Balzers PPM 421 Spec.) as the central diagnostic for the energy-dispersive Plasma-Ion-Mass-Spectrometry (PIMS). This diagnostic has been especially des!
igned for the the demands of this work. A retarding field analyzer is used to calibrate the plasma monitor. Therefore quantitative data from the measured ion-fluxes can be obtained. In addition, the experiment is fitted with a conventional quadrupole-mass-spectrometer (Balzers QME 112) for the rest gas analysis and with electrical probes for the characterization of elementary plasma parameters. Special efforts have been spended to find negative ions. This has lead to the development of a modulation technic, which is suitable to extract negatively charged particles from the plasma bulk. Due to the time-correlated switching of the discharge and the detector sensitivity, the influence of the active discharge periode to the formation of negative ions can be investigated.
The experimental results add to the assumption, that the mechanisms leading to the formation of these particles do not exclusively occur in the afterglow of the plasma. All during this work applied organic plasmas (discharges in methane, ethane, ethene and ethine) tend to form large molecular ions up to 600amu. The experimental results of the plasma induced formation of heavy ions show, that the observed polymerization can be attributed to plasma stimulated volume processes. This is demonstrated by a stationary rate equation model based on ambipolar diffusion. In this case the cationic polymerization cannot precede unlimited, since the according confinement times are restricted.
On this occasion the detection of negative ions becomes very important. The measured anion-fluxes provide an easy methode to estimate the total anion density in reactive plasmas. It is shown, that this density can reach values of approximately 107cm-3, just two orders of magnitude below the cation density. The negative charge carriers influence the ambipolar transport and reduce the diffusion speed drastically. The simulation calculation of the methane plasma gives evidance to an increase of the cation confinement time up to 37% compared with a corresponding electropositive plasma. Since the confinement time of the reactands is proportional to the production rate of larger molecules, one can expect higher polymerization degrees in a plasma as a consequence of the lowered transport. The magnitude of the anion density in the investigated organic plasmas is surprising. Especially, if methane is used as working gas, a lower anion density will be expected due to the small reaction !
coefficient, which describles the dissociative electron attachment to the CH4-molecule as the dominant mechanism for the anion formation. Instead the experimental results give rise to the presumption, that the anions are predominately generated by electronic attachment to neutrals found in the C2Hx-group (1<=x<=6), which are already present in high concentration by neutral reactions. It is carried out, that the actual electrode configuration fulfills special confinement criteria for the negative charge carriers and therefore supports the plasma polymerization based on volume processes.
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