Tomáš Kozák

Splitting of CO2 by vibrational excitation in non-equilibrium plasmas: a reaction kinetics model

We present a zero-dimensional kinetic model of CO2 splitting in non-equilibrium plasmas. The model includes a description of the CO2 vibrational kinetics (25 vibrational levels up to the dissociation limit of the molecule), taking into account state-specific VT and VV relaxation reactions and the effect of vibrational excitation on other chemical reactions. The model is applied to study the reaction kinetics of CO2 splitting in an atmospheric-pressure dielectric barrier discharge (DBD) and in a moderate-pressure microwave discharge. The model results are in qualitative agreement with published experimental works. We show that the CO2 conversion and its energy efficiency are very different in these two types of discharges, which reflects the important dissociation mechanisms involved. In the microwave discharge, excitation of the vibrational levels promotes efficient dissociation when the specific energy input is higher than a critical value (2.0 eV/molecule under the conditions examined). The calculated energy efficiency of the process has a maximum of 23%. In the DBD, vibrationally excited levels do not contribute significantly to the dissociation of CO2 and the calculated energy efficiency of the process is much lower (5%).

Evaluation of the energy efficiency of CO2 conversion in microwave discharges using a reaction kinetics model

We use a zero-dimensional reaction kinetics model to simulate CO2 conversion in microwave discharges where the excitation of the vibrational levels plays a significant role in the dissociation kinetics. The model includes a description of the CO2 vibrational kinetics, taking into account state-specific VT and VV relaxation reactions and the effect of vibrational excitation on other chemical reactions. The model is used to simulate a general tubular microwave reactor, where a stream of CO2 flows through a plasma column generated by microwave radiation. We study the effects of the internal plasma parameters, namely the reduced electric field, electron density and the total specific energy input, on the CO2 conversion and its energy efficiency. We report the highest energy efficiency (up to 30%) for a specific energy input in the range 0.4–1.0 eV/molecule and a reduced electric field in the range 50–100 Td and for high values of the electron density (an ionization degree greater than 10−5). The energy efficiency is mainly limited by the VT relaxation which contributes dominantly to the vibrational energy losses and also contributes significantly to the heating of the reacting gas. The model analysis provides useful insight into the potential and limitations of CO2 conversion in microwave discharges.

A parametric model for reactive high-power impulse magnetron sputtering of films

We present a time-dependent parametric model for reactive HiPIMS deposition of films. Specific features of HiPIMS discharges and a possible increase in the density of the reactive gas in front of the reactive gas inlets placed between the target and the substrate are considered in the model. The model makes it possible to calculate the compound fractions in two target layers and in one substrate layer, and the deposition rate of films at fixed partial pressures of the reactive and inert gas. A simplified relation for the deposition rate of films prepared using a reactive HiPIMS is presented. We used the model to simulate controlled reactive HiPIMS depositions of stoichiometric films, which were recently carried out in our laboratories with two different configurations of the inlets in front of the sputtered target. The repetition frequency was 500 Hz at the deposition-averaged target power densities of 5 Wcm?2and 50 Wcm?2 with a pulse-averaged target power density up to 2 kWcm?2. The pulse durations were 50 ?s and 200 ?s. Our model calculations show that the to-substrate inlet provides systematically lower compound fractions in the target surface layer and higher compound fractions in the substrate surface layer, compared with the to-target inlet. The low compound fractions in the target surface layer (being approximately 10% at the deposition-averaged target power density of 50 Wcm?2 and the pulse duration of 200 ?s) result in high deposition rates of the films produced, which are in agreement with experimental values.

Transport and ionization of sputtered atoms in high-power impulse magnetron sputtering discharges

We use a non-stationary two-zone model to verify predictions of a steady-state phenomenological model (Vl?ek and Burcalov?) under the conditions in typical high-power impulse magnetron sputtering discharges (copper target of 50?mm diameter, argon pressure of 1?Pa, rectangular voltage pulses of 200??s length with amplitudes from 400 to 1000?V and repetition frequency of 100?Hz). It is shown that the steady-state phenomenological model provides a reliable description of fundamental deposition parameters characterizing efficiency of magnetron sputtering and the transfer of target material ions to the substrate in these discharges with relatively long steady-state discharge regimes established during pulses. Based on the results, we recommend to lower the magnetic field strength in a magnetron system at a fixed average target power density in a pulse and thereby use a higher magnetron voltage in order to enhance the deposition rate and keep or even increase the ionized fraction of sputtered target material atoms in the flux onto the substrate.

Effect of voltage pulse characteristics on high-power impulse magnetron sputtering of copper

We present a model analysis of high-power impulse magnetron sputtering of copper. A non-stationary two-zone model is used to calculate the deposition rate and the ionized fraction of sputtered copper atoms in the flux onto the substrate for various pulse lengths (20?400??s), pulse shapes (a fixed target voltage of 900?V and stepwise ascending (800?1000?V) or descending (1000?800?V) target voltages), repetition frequencies (100?2000?Hz) and magnetic field strengths in front of the target (350?450?G) at an argon pressure of 1?Pa. We show that sufficiently long pulses (at least 100??s), which allow for the build-up of a high-density plasma in front of the target (diameter of 50?mm), are necessary to achieve high target power densities in a pulse and, consequently, high degrees of ionization of the sputtered atoms. However, the high degree of ionization of the sputtered copper atoms leads necessarily to lower deposition rates when compared with dc magnetron sputtering at the same target power density. This is mainly due to the return of ionized sputtered atoms onto the target. The model results show that the average target power density in a pulse is, in addition to the target voltage, a fundamental quantity which determines the average ionized fraction of sputtered atoms in the flux onto the substrate and the deposition rate per average target power density in a period applied to the discharge. Such model calculations can be beneficial for determining the optimum working conditions for specific deposition applications.

Relationships between the distribution of O atoms on partially oxidized metal (Al, Ag, Cu, Ti, Zr, Hf) surfaces and the adsorption energy: A density-functional theory study

We investigate the oxidation of selected metal (Al, Ag, Cu, Ti, Zr, and Hf) surfaces by the density functional theory. We go through a wide range of (233 per metal) distributions of O atoms on a partially oxidized metal surface. First, we focus on the qualitative information whether the preferred distribution of O atoms is heterogeneous (stoichiometric oxide + metal) or homogeneous (substoichiometric oxide). We find that the former is energetically preferred, e.g., for Al, while the latter is energetically preferred, e.g., for Ti, Zr, and Hf. Second, we provide the quantitative values of adsorption energies corresponding to the energetically preferred O atom distributions for various partial coverages of various metals by O. Third, we discuss and show an example of implications of the aforementioned findings for the understanding and simulations of sputtering.

Dynamics of processes during the deposition of ZrO2 films by controlled reactive high-power impulse magnetron sputtering: A modelling study

A time-dependent parametric model was applied to controlled reactive high-power impulse magnetron sputtering (HiPIMS) depositions of stoichiometric ZrO2 films, carried out in our laboratories, (i) to clarify the complicated dynamics of the processes on the target and substrate surfaces during voltage pulses, and (ii) to corroborate the importance of the O2 inlet configuration (position and direction) which strongly affects the O2 dissociation in the discharge and the chemisorption flux of oxygen atoms and molecules onto the substrate. The repetition frequency was 500 Hz at the deposition-averaged target power densities of 25 Wcm−2, being close to a target power density applicable in industrial HiPIMS systems, and 50 Wcm−2 with a pulse-averaged target power density up to 2 kWcm−2. The pulse duration was 50 μs. For the experimental conditions with the to-substrate O2 inlets, the deposition-averaged target power density of 50 Wcm−2, and the oxygen partial pressure of 0.05 Pa (being close to the mean value during controlled depositions), our model predicts a low compound fraction, changing between 8% and 12%, in the target surface layer at an almost constant high compound fraction, changing between 92% and 93%, in the substrate surface layer during the pulse period (2000 μs). The calculated deposition rate of 89 nm/min for these films is in good agreement with the measured value of 80 nm/min achieved for optically transparent stoichiometric ZrO2 films prepared under these conditions.A time-dependent parametric model was applied to controlled reactive high-power impulse magnetron sputtering (HiPIMS) depositions of stoichiometric ZrO2 films, carried out in our laboratories, (i) to clarify the complicated dynamics of the processes on the target and substrate surfaces during voltage pulses, and (ii) to corroborate the importance of the O2 inlet configuration (position and direction) which strongly affects the O2 dissociation in the discharge and the chemisorption flux of oxygen atoms and molecules onto the substrate. The repetition frequency was 500 Hz at the deposition-averaged target power densities of 25 Wcm−2, being close to a target power density applicable in industrial HiPIMS systems, and 50 Wcm−2 with a pulse-averaged target power density up to 2 kWcm−2. The pulse duration was 50 μs. For the experimental conditions with the to-substrate O2 inlets, the deposition-averaged target power density of 50 Wcm−2, and the oxygen partial pressure of 0.05 Pa (being close to the mean value du...