TY - BOOK

T1 - Towards Knowledge-based Design of Multi-element Cathodes for Cathodic Arc Deposition Processes

AU - Golizadeh Najafabadi, Mehran

N1 - embargoed until null

PY - 2021

Y1 - 2021

N2 - The increasing demand for multi-element thin films and coatings for multifunctional purposes has pushed the cathode material industry to produce multi-element products, e.g., composite and alloy targets. The cathodes are exposed to the discharge plasma during the deposition process that alters their surface properties. The surface modifications are particularly severe for cathodic arc deposition processes since the cathode spots impose countless melting-solidification cycles on the cathode material near the surface, leading to the formation a modified layer. The formation mechanisms and properties of the modified layer influence the plasma properties and, hence, the film growth conditions. The aim of this thesis is to contribute to the understanding of fundamental physical processes occurring on the surface of cathodes during cathodic arc processes in order to enable the design of multi-element cathodes that yield optimal plasma parameters and hence the desired coating properties. In a first step, a novel multilayer cathode with individual sublayer thicknesses of 500 nm was developed in this work in order to distinguish short-scale diffusion processes in the heat-affected zone of the craters. This multilayer design also enabled studying the progress of the crater formation and the corresponding induced material intermixing processes. It was found that the temperature gradient below a single crater is very sharp and no considerable solid-state diffusion was activated. The material intermixing was revealed to occur mainly in liquid state in very short periods of time. The repetitive formation of the µm-sized craters leads to the formation of the modified layer whose homogeneity increases with increasing number of melting-solidification cycles it undergoes. In a second step, composite Al-Cr cathodes with varying grain size were selected to further investigate the influence of cathode microstructure design on the formation mechanisms as well as the phase composition of the modified layer. It was revealed that the modified layers mainly comprise of metastable phases irrespective of the grain size. An average cooling rate of 10E6 K/s was estimated for the solidification of the modified layer. The cooling rate estimation is of significant importance for the cathode design since it enables to predict the ultimate phases in the modified layer whose cohesive energies influence the plasma parameters. In a final step, the knowledge obtained in the previous steps was employed to study the surface modifications on multi-element cathodes during reactive cathodic arc deposition. It was found that the reactive plasmas cause surface nitridation and oxidation in the respective discharge. The oxide or nitride layers trigger repetitive ignition of type 1 cathode spots along with type 2 ones. In N2 atmosphere the modified layer formation process is, in principle, similar to the one in inert Ar gas (previous steps) but the craters are shallower and the material intermixing process seems to be slower. In O2 atmosphere, deeper craters are formed but their number is less than in N2. Additionally, the surface oxidation causes significantly more type 1 spot ignitions whose craters contribute to the material intermixing process. The obtained results contribute to a better understanding of macroparticle generation during crater formation and arc plasma properties in reactive cathodic arc deposition processes.

AB - The increasing demand for multi-element thin films and coatings for multifunctional purposes has pushed the cathode material industry to produce multi-element products, e.g., composite and alloy targets. The cathodes are exposed to the discharge plasma during the deposition process that alters their surface properties. The surface modifications are particularly severe for cathodic arc deposition processes since the cathode spots impose countless melting-solidification cycles on the cathode material near the surface, leading to the formation a modified layer. The formation mechanisms and properties of the modified layer influence the plasma properties and, hence, the film growth conditions. The aim of this thesis is to contribute to the understanding of fundamental physical processes occurring on the surface of cathodes during cathodic arc processes in order to enable the design of multi-element cathodes that yield optimal plasma parameters and hence the desired coating properties. In a first step, a novel multilayer cathode with individual sublayer thicknesses of 500 nm was developed in this work in order to distinguish short-scale diffusion processes in the heat-affected zone of the craters. This multilayer design also enabled studying the progress of the crater formation and the corresponding induced material intermixing processes. It was found that the temperature gradient below a single crater is very sharp and no considerable solid-state diffusion was activated. The material intermixing was revealed to occur mainly in liquid state in very short periods of time. The repetitive formation of the µm-sized craters leads to the formation of the modified layer whose homogeneity increases with increasing number of melting-solidification cycles it undergoes. In a second step, composite Al-Cr cathodes with varying grain size were selected to further investigate the influence of cathode microstructure design on the formation mechanisms as well as the phase composition of the modified layer. It was revealed that the modified layers mainly comprise of metastable phases irrespective of the grain size. An average cooling rate of 10E6 K/s was estimated for the solidification of the modified layer. The cooling rate estimation is of significant importance for the cathode design since it enables to predict the ultimate phases in the modified layer whose cohesive energies influence the plasma parameters. In a final step, the knowledge obtained in the previous steps was employed to study the surface modifications on multi-element cathodes during reactive cathodic arc deposition. It was found that the reactive plasmas cause surface nitridation and oxidation in the respective discharge. The oxide or nitride layers trigger repetitive ignition of type 1 cathode spots along with type 2 ones. In N2 atmosphere the modified layer formation process is, in principle, similar to the one in inert Ar gas (previous steps) but the craters are shallower and the material intermixing process seems to be slower. In O2 atmosphere, deeper craters are formed but their number is less than in N2. Additionally, the surface oxidation causes significantly more type 1 spot ignitions whose craters contribute to the material intermixing process. The obtained results contribute to a better understanding of macroparticle generation during crater formation and arc plasma properties in reactive cathodic arc deposition processes.

KW - cathodic arc deposition

KW - multi-element cathode

KW - cathode spot

KW - electron microscopy

KW - microstructure

KW - arc plasma

KW - phase transformation

KW - kathodische Lichtbogenverdampfung

KW - Multielementkathode

KW - Kathodenspot

KW - Elektronenmikroskopie

KW - Mikrostruktur

KW - Lichtbogenplasma

KW - Phasentransformation

M3 - Doctoral Thesis

ER -