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The current work is focused on the study of two surface modification plasma processes, (i) the active screen plasma nitriding (ASPN) and nitrocarburizing (ASPNC) for the hardening of ferrous surfaces and (ii) the microwave plasma assisted chemical vapor deposition (MW-PACVD) for the synthesis of single crystal and doped diamond. Conventional and active screen plasma nitriding processes have been investigated in a cylindrical, industrial scale ASPN reactor with a volume of about 1 m3, using low-pressure pulsed dc H2-N2 plasmas with admixtures of CH4 or CO2. The experiments were carried out (i) with the plasma at an internal model probe, (ii) with the plasma at the active screen (floated model probe) and (iii) with the plasma at the active screen and an additional plasma at the biased model probe. For deeper insights in ASPN and ASPNC processes, a laboratory scale plasma nitriding monitoring reactor, PLANIMOR, has been constructed. The main feature of this reactor is the linear configuration of the electrode setup combined with a tubular glass vessel, overcoming the experimental disadvantages of cylindrical laboratory scale ASPN reactors. With the help of infrared laser absorption spectroscopy (IRLAS) the rotational temperature of the stable molecules in the gas phase and the concentrations of the precursor, CH4, and the reaction products (NH3, HCN, C2H2, C2H4, CO, CH3) could be determined in both reactors, depending on the plasma power, the gas mixture, the plasma at the model probe and the admixture of CH4. Furthermore, the admixture of CO2 as the carbon containing precursor has been studied in the ASPN reactor leading to an additional reaction product H2O. The concentration of the molecular species has been found being in a range of 1012 to 1016 molecules cm-3. Also optical emission spectroscopy (OES) has been applied during the studies for analyzing the emission of the plasmas in the nitriding and nitrocarburizing processes. A similar behavior of the plasma chemistry in PLANIMOR comparing to that in the ASPN reactor has been found. Beside the plasma chemical investigations, both reactors have been used for the treatment of C15 steel samples. These samples have been analyzed with the help of GDOES resulting in the elements profile of the treated surfaces. It has been found that samples treated in PLANIMOR reach comparable nitriding results as samples treated in the ASPN reactor. Another focus of interest during the investigations about plasma nitrocarburizing has been the application of a carbon containing screen electrode as carbon source. For this purpose the carbon containing precursor and the steel screen have been substituted by a meshed carbon electrode, acting as the active screen. This change of the setup leads to a decrease of the NH3 production by a factor of 2.5 and an increase of the concentrations of HCN by a factor of 30 and of C2H2 by a factor of 70. The investigations of MW-PACVD processes used for diamond layer deposition have been carried out in a jacketed stainless steel reactor (JR), dedicated to the deposition of single crystalline diamond under high pressure and plasma power conditions. Using H2-plasmas with admixtures of CH4 and B2H6, the experiments were carried out in order to analyze the dependence of the plasma chemistry on several parameters, such as plasma power, pressure and gas mixture, in a wide pressure (p = 25…270 mbar) and power range (P = 0.6…4 kW). Using IRLAS the concentrations of six molecular species (B2H6, CH4, C2H2, C2H4, C2H6, CH3) have been monitored. With the help of OES the concentration of atomic boron could be determined. The concentrations of the detected molecular and atomic species were found to be in a range of 1010 to 1017 cm-3. With the help of the line-ratio-method the rotational temperature of the stable molecules has been determined. The temperature increased with pressure and power from 340 to 425 K. Using the Doppler broadening of the absorption line of CH3 at ν = 612,413 cm-1, the gas temperature has found to be Tg = (2000 ± 200) K under lower pressure and power conditions. For the H2-CH4 gas mixture, the experimental obtained molecular densities have been compared to those of a 1D-radial thermochemical model. The calculated radial densities have been integrated axially. For the same range the chemical processes in JR have been compared with those in a bell-jar (BJ) reactor. The hydrocarbon chemistry in JR has found to be similar to that in a BJ reactor.
In this work, various aspects of fundamental physics and chemistry of molecular gas discharges are presented with emphasis on the interaction between species, activated by low-pressure plasmas, and surfaces. As already known, synergistic effects of multiple plasma-generated species are responsible for surface modification. However, due to the large number of internal parameters of a discharge and the complex plasma processes the identification of correlations between plasma characteristics and their effects on surfaces are complicated. Therefore, the aim of this thesis is to improve the understanding of several phenomena associated with plasma–surface interactions by measuring or calculating fundamental kinetic, transport or spectroscopic data needed to interpret measurements and hereby, to support some future applications of plasmas.
Tunable Diode Laser Absorption Spectroscopy in the mid InfraRed spectral range (IR-TDLAS) has been applied to investigate the behaviour of CF, CF2 and C2F4 species produced in pulsed CF4/H2 capacitively coupled radio frequency plasmas (13.56 MHz CCP). This experimental technique was shown to be suitable for temporally resolved measurements of the absolute number density of the target molecules in the studied fluorocarbon discharges. The temporal resolution of about 20…40 ms typically achieved in the standard data acquisition mode (“stream mode”) was sufficient for the real-time measurements of CF2 and C2F4, but not of CF whose kinetics was observed to be much faster. Therefore, a more sophisticated approach (“burst mode”) providing a temporal resolution of 0.94 ms was established and successfully applied to CF density measurements. In order to enable the TDLAS measurements of the target species, preliminary investigations on their spectroscopic data had been carried out. In particular, pure C2F4 has been produced in laboratory by means of vacuum thermal decomposition (pyrolysis) of polytetrafluoroethylene and used as a reference gas. Therefore, an absorption structure consisting of several overlapping C2F4 lines around 1337.11 cm-1 was selected and carefully calibrated, which provided the first absolute measurements of the species by means of the applied experimental technique. The absolute number density traces measured for CF, CF2 and C2F4 in the studied pulsed plasmas were then analysed, in which two differential balance equations were proposed for each of the species to describe their behaviour during both “plasma on” and “plasma off” phases. Analytical solutions of the balance equations were used to fit the experimental data and hence to deduce important information on the kinetics of the studied molecules. In particular, during the “plasma off” phase, the self-recombination of CF2 (CF2 + CF2 (+M) → C2F4 (+M)) was found to be dominant in the kinetics of the radical, but of minor importance for C2F4 production. A rapid consumption of CF observed within 7…25 ms after switching off the plasma was explained mainly by volume reaction with other species (most likely with CF3), whereas diffusion of the radical towards the reactor walls followed by sticking on the surfaces was found to contribute only at relatively low pressures (<10 Pa). Under certain discharge conditions, measured CF density traces exhibited significant overshoots in 50…150 ms after the plasma ignition, which had not been known from literature before. The electron impact fragmentation of C2F4 was shown to be essential for CF production at the beginning of the “plasma on” phase and therefore for formation of the observed CF density overshoots. Finally, the broad band FTIR spectroscopy was applied in order to better characterize the gas phase composition of the studied plasmas. Thus, absorption bands of CF4, C2F4, C2F6, C3F8, CHF3 and HF stable molecules were detected in the FTIR spectra recorded between 400 and 4000 cm-1. The spectra were then successfully deconvolved and the absolute concentration of the detected species was estimated. In particular, the absolute number density of C2F4 obtained from the FTIR measurements was in a good agreement with that achieved by means of the IR-TDLAS technique. The work was supported by the German Research Foundation (DFG) within the framework of the Collaborative Research Centre Transregio 24 “Fundamentals of Complex Plasmas” (SFB/TRR24, project section B5).