Marc Böke

High power impulse magnetron sputtering discharges: Instabilities and plasma self-organization

We report on instabilities in high power impulse magnetron sputtering plasmas which are likely to be of the generalized drift wave type. They are characterized by well defined regions of high and low plasma emissivity along the racetrack of the magnetron and cause periodic shifts in floating potential. The azimuthal mode number m depends on plasma current, plasma density, and gas pressure. The structures rotate in E→×B→ direction at velocities of ∼10 km s−1 and frequencies up to 200 kHz. Collisions with residual gas atoms slow down the rotating wave, whereas increasing ionization degree of the gas and plasma conductivity speeds it up.

Modeling and simulation of the plasma absorption probe

The plasma absorption probe (PAP) was invented as an economical and robust diagnostic device to determine the electron density distribution in technical plasmas. It consists of a small antenna enclosed by a dielectric tube which is immersed in the plasma. A network analyzer feeds a rf signal to the antenna and displays the frequency dependence of the power absorption. From the absorption spectrum the value of the electron density is calculated. The original evaluation formula was based on the dispersion relation of plasma surface waves propagating along an infinite dielectric cylinder. In this letter the authors present the analysis of a less idealized configuration. The calculated spectra are in good qualitative agreement with their experimental counterparts, but differ considerably from those predicted by the surface wave ansatz. An evaluation scheme which takes our findings into account will improve the performance of the PAP technique further.

Spatial dynamics of helium metastables in sheath or bulk dominated rf micro-plasma jets

Space resolved concentrations of helium He metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. The spatial profile of metastable atoms in the volume between the electrodes was deduced for various electrode gap distances. Density profiles reveal the sheath structure and reflect the plasma excitation distribution, as well as the dominance of the α-mode discharge. Gap width variations show the transition from a normal glow plasma to a pure sheath discharge. In order to analyse and verify the experimentally observed profiles of the metastable atoms, a two-dimensional simulation model was set up. Applying an appropriate He/N2/O2 chemistry model, the correlation between the metastable profiles and the underlying excitation mechanisms was obtained.

The characteristic shape of emission profiles of plasma spokes in HiPIMS: the role of secondary electrons

A time-resolved analysis of the emission of high power impulse magnetron sputtering (HiPIMS) plasmas reveals inhomogeneities in the form of rotating spokes. The shape of these spokes is very characteristic depending on the target material. The localized enhanced light emission has been correlated with the ion production. Based on these data, the peculiar shape of the emission profiles can be explained by the localized generation of secondary electrons, resulting in an enhanced outward diffusion. This general picture is able to explain the observed emission profile for different target materials including gas rarefaction and second ionization potential of the sputtered elements.

Terahertz time-domain spectroscopy as a new tool for the characterization of dust forming plasmas

We report the application of terahertz time-domain spectroscopy as a new tool for plasma diagnostics. The short broadband THz pulses were radiated from a low temperature grown GaAs emitter by free charge carriers which were generated by focusing a 20 femtosecond TiSa-laser pulse onto the emitter. For sensitive signal recording a coherent detection scheme was applied. This allowed the measurement of the amplitude and sign of the electromagnetic field of the THz pulse after passing the plasma chamber. Fourier transformation allowed us to obtain the full spectrum in the frequency domain.We compared the transmitted THz intensities of a pure argon (Ar) and an acetylene (C2H2)/argon plasma. The presence of the ethynyl–radical (CCH) and cyclopropenylidene (c-C3H2) in the (C2H2)/argon plasma could be confirmed by the observations of rotational transitions in the region from 8 to 16 cm−1 corresponding to 0.3–0.5 THz.

Impurity intrusion in radio-frequency micro-plasma jets operated in ambient air

Space and time resolved concentrations of helium He (3S1) metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Spatial profiles and lifetime measurements show significant influences of air entering the discharge from the front nozzle and of impurities originating from the gas supply system. Quenching of metastables was used to deduce quantitative concentrations of intruding impurities. The impurity profile along the jet axis was determined from optical emission spectroscopy as well as their dependence on the feed gas flow through the jet.

Surface pre-treatment for barrier coatings on polyethylene terephthalate

Polymers have favourable properties such as light weight, flexibility and transparency. Consequently, this makes them suitable for food packaging, organic light-emitting diodes and flexible solar cells. Nonetheless, raw plastics do not possess sufficient barrier functionality against oxygen and water vapour, which is of paramount importance for most applications. A widespread solution is to deposit thin silicon oxide layers using plasma processes. However, silicon oxide layers do not always fulfil the requirements concerning adhesion and barrier performance when deposited on films. Thus, plasma pre-treatment is often necessary. To analyse the influence of a plasma-based pre-treatment on barrier performance, different plasma pre-treatments on three reactor setups were applied to a very smooth polyethylene terephthalate film before depositing a silicon oxide barrier layer. In this paper, the influence of oxygen and argon plasma pre-treatments towards the barrier performance is discussed examining the chemical and topological change of the film.It was observed that a short one-to-ten-second plasma treatment can reduce the oxygen transmission rate by a factor of five. The surface chemistry and the surface topography change significantly for these short treatment times, leading to an increased surface energy. The surface roughness rises slowly due to the development of small spots in the nanometre range. For very long treatment times, surface roughness of the order of the barrier layers thickness results in a complete loss of barrier properties. During plasma pre-treatment, the trade-off between surface activation and roughening of the surface has to be carefully considered.

Temporal evolution of the radial plasma emissivity profile in HIPIMS plasma discharges

High power impulse magnetron sputtering (HIPIMS) is a plasma vapour deposition technique used for deposition of dense coatings. In order to contribute to a better understanding of the dynamics of a HIPIMS plasma discharge, time- and wavelength-resolved measurements of the light emitted from the plasma were performed. Lateral images of the HIPIMS plasma were recorded using an ICCD camera with a gate width of 1??s. For each picture, Abel inversion was performed to compute the radial emissivity profile of the plasma. Band-pass interference filters were used to isolate the desired wavelength in order to observe lines of ions and neutrals of metal and argon (Ar) in HIPIMS plasma discharges with an aluminium (Al), titanium (Ti) or chromium (Cr) target in Ar atmosphere. The result is the temporal evolution of the radial emissivity profile of metal and gas neutrals and singly charged ions.

Practical implementation of a two-hemisphere plasma absorption probe

The plasma absorption probe is a recently developed tool for efficient determination of electron densities of low temperature plasmas. The occurrence of multiple absorption signals was a serious drawback for interpretation of the probe data. To remedy this drawback, a spherically symmetric design of an absorption probe is proposed. A spherical probe is tested in experiment and simulation and the suppression of the multiple absorption signals is demonstrated. The proof of principle for the concept is given.

Spoke transitions in HiPIMS discharges

Spokes, localised light emission patterns appearing during the HiPIMS discharge, are investigated using optical emission diagnostic and electrical probes. Both, temporal and spatial aspects of the spoke phenomenon have been investigated. The evolution from stochastic to self-organised patterns consisting of a finite number of spokes along the racetrack (= spoke mode number), the transition between two spoke modes, and the stability of a spoke mode is presented. For a constant discharge current, spokes steadily rotate above the magnetron racetrack. The transition between two spoke modes is described by a model which links the spoke mode number decrease with increasing discharge current. It is based on the interaction between the current in the spoke, the Ar rarefaction and Ar replenishment times that prevail over electrostatic repulsion and charged particle diffusion. Two spokes merge by an acceleration of the trailing spoke with respect to the direction of rotation.