ICMCTF2017 Session B7: Plasma Diagnostics and Growth Processes

Reliable correlations between a set of plasma parameters and corresponding surface phenomena on the atomistic scale are scarce. The main reason being the complex physics of the two different states of matter, where at least one of which - the plasma - is far from an equilibrium state. In this contribution, we provide an initial step towards a consistent plasma surface interaction model, which covers the combination of generic materials as well as plasma discharges. As a proof of concept, we present a model which couples a direct current magnetron sputtering discharge with the resulting deposition of a titanium nitride thin film. In particular, the nucleation of cubic titanium nitride is studied at various substrate temperatures and fluxes starting from the amorphous state. Initially, the desired plasma parameter range is found by the extrapolation of classical molecular dynamics findings, validated by density functional theory calculations. This a priori knowledge is employed as a starting point for test particle simulations taking consistently into account the plasma and its global parameters. The resulting heavy particle fluxes and corresponding distribution functions are then again input to molecular dynamics simulations. Finally, the outcome of these simulations are compared to experimental results for the respective plasma discharge. In conclusion, it is argued that the proposed plasma surface interaction model is applicable to various plasma and material systems as well as surface modification phenomena.

Financial support provided be the German Research Foundation (DFG) in the frame of the collaborative research centre SFB-TR 87 is gratefully acknowledged.

High power impulse magnetron sputtering (HiPIMS) discharges demonstrate plasma self-organization, in which distinct ionization zones (often called spokes) can be seen to rotate in the ExB direction. Recently, a phenomena of spoke splitting and merging was observed in HiPIMS plasma using an array of phase correlated azimuthally arranged Langmuir probes around the discharge perimeter.

In our experiments, to gain more information on the temporal and spatial behaviour of self-organized spoke structures in HiPIMS plasmas, a correlation between the broadband 2-D optical image of an individual spoke and the current it delivers to the target has been made for a wide range of magnetron operating conditions. As a spoke passes over a set of embedded and isolated strip probes in the niobium cathode target, a distinct modulation in the local current density is observed, typically up to twice the average value associated with the spoke, as it delivers more current that the background plasma between spokes. It matching very well the radially integrated optical emission intensities obtained using 200 nanosecond time-resolved remote ICCD camera imaging. This allows us to relate the shape of spokes seen optically to their "electrical" footprint on the target.

The dual diagnostic system allows us to study the merging and splitting of a set of rotating spokes. It is found that during the merging process the trailing spoke retains its velocity. However, it is unclear whether during the merging process the leading spoke has decreased its velocity or the merging spokes have increased their azimuthal lengths. In the merged spoke both the plasma emission intensity and current collected by the embedded probes is redistributed to have their maximum at a trailing edge. In the spoke splitting process, the total charge collected by an embedded probe is conserved. Merged or split spokes are not always stable in time. Often the spoke system reverses to its former state a short time later.

A simple phenomenological model is developed to describe a stable spoke configuration. Assuming that argon presence is essential for spoke sustainment and based on spoke dimensions, spoke velocity and background gas atom velocity a stable amount of spokes can be predicted. The model provides realistic estimates for particular spoke and plasma conditions.

Alumina-zirconia nanostructured layers were coated on 7075 Al alloy through a Plasma Electrolytic based technique in DC potentiostatic mode in the range of 540- 600 V. The layers were coated in an electrolyte containing nano ZrO₂ powder as the zirconia source. The microstructure and phase composition of the coatings were studied along with their tribological properties as a function of processing voltage.

The results showed formation of alumina- zirconia composites with different properties for the layers treated at different voltages. It was found that at higher processing voltage, the composite layers consist of high temperature phases (tetragonal zirconia and a-alumina) in addition to monoclinic zirconia. Also higher voltages introduced larger amounts of zirconia to the coated layers due to higher applied energies to the nanoparticles in electrolyte. The layers coated at 540 V and 580 V showed the lowest friction coefficient (0.14) compared to untreated Al alloy (0.69) in addition to better wear resistance (about 100 times higher in comparison to bare Al alloy). These improvements can be attributed to the formation of hard phases at high processing voltage. Furthermore, formation of tetragonal zirconia with better toughness and mechanical properties can result in improved wear resistance. The results revealed a hybrid of plasma electrolytic oxidation (PEO) and electrophoretic process as an effective coating method. In this regard, PEO was responsible for the formation of transformed tetragonal zirconia, while deposition of unchanged monoclinic zirconia from the electrolyte resulted from the electrophoretic process.

We have investigated the influences of incident particle fluxes during film growth on the growth and properties of gallium-doped zinc oxide (GZO) films on glass substrates (@200 °C) deposited by ion-plating with dc arc discharge. The Ga₂O₃ contents in the pellets was 4.0 wt.%. Deposition conditions were as follows: the oxygen (O₂) gas flow rates (OFRs) and discharge current (I_D) were varied from 0 to 20 sccm and from 100 to 140 A, respectively.

We measured the incident particle flux of the neutral atoms and ions for each species quantitatively at the substrate level using a mass-energy analyzer (Hiden, EQP300), a Langumuir probe and a diaphragm gauge during the deposition. To clarify the factors limiting the growth rate, carrier density (N), hall mobility (μ_H) and optical mobility (μ_opt) of GZO films, we investigated the relationship between the growth rates, N, μ_H, μ_opt and incident flux (IF) properties of Zn species such as neutral Zn atoms and Zn⁺ ions and O species such as neutral O atoms, O⁺ and O₂⁺ ions.

Investigations over the last few years have shown that plasma in continuous and pulsed magnetron discharges is not azimuthally uniform rather it is organized in dense structures called ionization zones or spokes [1-2]. In this work we present measurements of the plasma potential of moving ionization zone in a direct current magnetron sputtering [3]. Measurements were recorded in a space and time resolved manner using movable emissive and floating probes. This allowed us to make a three-dimensional representations of the plasma potential and derive the related electric field, space charge and electron heating distributions. The data reveal the existence of strong electric fields parallel and perpendicular to the target surface. The largest E-fields result from a double layer structure at the leading edge of the ionization zone. Measurements imply that the double layer plays a crucial role in the energization of electrons since electrons can gain several tens of electronvolts from azimuthal E-field. As electrons drift over the magnetron there is a sustained coupling between the potential structure, electron heating, and ionization processes. The ionization zone moves in the –E_z×B direction from which the to-be-heated electrons arrive and into which the heating region expands. The motion of the zone is dictated by the force of the local electric field on the ions at the leading edge of the ionization zone. We postulate that electron heating caused by the potential jump and physical processes associated with the double layer also apply to high power impulse magnetron sputtering.

[1] A. Anders et al., J. Appl. Phys.111 (2012) 053304

[2] M. Panjan et al., Plasma Sources Sci. Technol.24 (2015) 065010