Interaction between low-energetic plasmas and hydrogenated amorphous carbon

The interaction between plasmas and surfaces play an important role in both low-temperature (industrial) plasma processing and experimental nuclear fusion reactors. Exerting control over the plasma-surface interaction leads to tunability of the growth rate of thin films and their material properties such as density or surface roughness. Furthermore, these interaction processes are of particular interest in high-temperature fusion plasmas since they are considered a significant obstacle in the development of nuclear fusion as a sustainable energy source for the future. A fundamental understanding of plasma-surface interactions is therefore indispensable in constructing the next generation experimental nuclear fusion reactors. An important nuclear and industrial material with a variety of amorphous and crystalline structures is carbon. It is used as wall material in experimental nuclear fusion reactors and as a protective coating in daily applications such as razor blades and hard drives. To improve the material quality or lifetime of a reactor wall, the plasma deposition and erosion of carbon has been extensively investigated over the last 20 years. Hydrogenated amorphous carbon (a-C:H) was used as a material model in this thesis to expand upon those plasma-surface interaction studies. The goal of this thesis was the investigation of the fundamental erosion mechanisms of a-C:H for ion kinetic energies between thermal energies (0:03 eV) and 10 eV. This energy range is expected to play a key role in ITER, the next generation fusion reactor. Moreover, the bond energy (3 – 5 eV) of the amorphous carbon network falls within this relatively unexplored energy range. Expanding thermal plasmas with an ion kinetic energy below 2 eV, without substrate biasing, were used to obtain that goal. Despite these very low ion energies, this thesis established a synergistic effect in the etch rate between hydrogen radicals and argon ions. Hydrogen ions with an ion energy up to 10 eV were likewise shown to play a role in the erosion of a-C:H thin films, even though the fraction of hydrogen ions in the total flux was below 0:02%. These results indicate that ion-assisted chemical erosion is the dominant erosion mechanism. Characterization of the a-C:H thin films during plasma erosion — and also during plasma deposition—occurred via in situ spectroscopic ellipsometry (SE). Analysis of the SE data obtained was successfully carried out with B-splines; a first for thin carbon layers. B-splines are a purely mathematical approach and provide a fast and reliable method to determine thickness, roughness and optical constants of thin films. Their use not only simplifies the data analysis, but can also be applied to data sets with a limited wavelength range. This approach was furthermore instrumental in establishing a correlation between the SE surface roughness and the etch rate. For low etch rates, as in the case of pure argon and pure hydrogen plasmas, the surface roughness increases strongly from a few nanometers to tens of nanometers. For high etch rates, as in the case of mixed argon - hydrogen plasmas, the surface roughness fluctuates on the order of a few nanometers. Carbon erosion inevitably releases etch products, such as methane and acetylene, into the plasma expansion. Fragmentation of these etch products occurs via charge transfer with the plasma ions, followed in rapid succession by dissociative recombination. It is known from previous deposition studies that polymerization of such hydrocarbon fragments occurs when the initial hydrocarbon density exceeds the ion density. Typical hydrocarbon densities during carbon erosion experiments, as carried out in this work, were, however, lower than the ion density. Nevertheless, the presence of H2 — either from the plasma directly or as dissociation products from the hydrocarbons themselves — was shown in this thesis to have a negative impact on the ion density, thereby paving the way for the formation of higher hydrocarbons. So far, the expanding thermal plasma has been used to gain insight into the carbon erosion mechanisms and the hydrocarbon plasma chemistry. The converse is possible as well: the carbon erosion process was used to investigate the gas flow dynamics of the plasma expansion. The distribution in electronically excited hydrocarbon species, produced via charge transfer and dissociative recombination reactions, is due to the convolution of the ion and hydrocarbon density throughout the reactor. Moreover, it was found that both the placement of a substrate and the movement of a shutter inside the plasma expansion affect the ion density and the rotational temperature of electronically excited CH radicals via a reorganization of the recirculation cells. This thesis is a study of the fundamental erosion mechanisms of hydrogenated amorphous carbon by low-energetic plasmas and, therein, it has been concluded that the etch processes are dominated by ion-assisted chemical erosion.