Holger Kersten

The energy balance at substrate surfaces during plasma processing

Abstract A summary is given of different elementary processes influencing the thermal balance and energetic conditions of substrate surfaces during plasma processing. The discussed mechanisms include heat radiation, kinetic and potential energy of charged particles and neutrals as well as enthalpy of involved chemical surface reactions. The energy and momentum of particles originating from the plasma or electrodes, respectively, influence via energy flux density (energetic aspect) and substrate temperature (thermal aspect) the surface properties of the treated substrates. The various contributions to the energy balance are given in a modular mathematical framework form and examples for an estimation of heat fluxes and numerical values of relevant coefficients for energy transfer, etc. are given. For a few examples as titanium film deposition by hollow cathode arc evaporation, silicon etching in CF 4 glow discharge, plasma cleaning of contaminated metal surfaces, and magnetron sputtering of aluminum the energetic balance of substrates during plasma processing will be presented. Furthermore, the influence of the resulting substrate temperature on characteristic quantities as etching or deposition rates, layer density, microstructure, etc. will be illustrated for some examples, too.

A calorimetric probe for plasma diagnostics

A calorimetric probe for plasma diagnostics is presented, which allows measurements of the power taken by a test substrate. The substrate can be biased and used as an electric probe in order to obtain information about the composition of the total heating power. A new calibration technique for calorimetric probes, which uses monoenergetic electrons at low pressure, has been developed for an improved accuracy. The use of the probe is exemplified with an experiment where both energetic neutral atoms and ions heat the test substrate.

Electrocatalysts for Oxygen Reduction Prepared by Plasma Treatment of Carbon-Supported Cobalt Tetramethoxyphenylporphyrin

during the pyrolysis of CoTMPP. Additionally, solid decomposition products of the oxalate metal and oxides form a framework embedded within the pyrolysis product which is removed by a subsequent acid treatment. Finally, a highly porous carbon matrix with embedded centers is obtained. With this procedure materials with high electrochemical activities toward the oxygen reduction have been achieved in rotating disk electrode RDE measurements in 0.5 M H2SO4 at 0.7 V normal hydrogen electrode NHE close to that of commercial 20% Pt/C E-TEK. However, even with this advanced technique scanning electron microscopy SEM images reveal particles of several micrometer dimension composed of that highly porous material. In technical applications e.g., gas diffusion electrodes in fuel cells this presumably leads to a lower efficiency due to long diffusion pathways for protons and gas molecules. Therefore, alternative synthesis techniques are desired which result in smaller particle sizes 50‐100 nm as it is state of the art for carbon supports which are utilized for commercial platinum catalysts Vulcan, Black Pearls ca. 20 nm. In the last decades plasma treatment of organic material was intensively investigated and is considered as a promising nanotechnological approach. Our contribution shows that the transfer of CoTMPP into highly electrochemical active CoN4 centers embedded in a carbon matrix can be reached by plasma treatment instead of conservative heat-treatment. In order to find optimal operating parameters CoTMPP on Black Pearls was processed by plasma treatment as well as by classical heat-treatment. The obtained products were compared in terms of structure and catalytic activity.

On the temperature dependence of the deposition rate of amorphous, hydrogenated carbon films

The temperature behavior of the deposition rate of amorphous, hydrogenated carbon films is analyzed both experimentally and theoretically. A reactor based on the supersonic expansion of an arc plasma is used. The film thickness is measured using in situ He–Ne ellipsometry. The surface temperature is measured with thermocouples. Comparison of the presented model with the experimental results suggests that the deposited atoms and radicals diffuse over the surface in a weakly bound, adsorbed layer before they are incorporated in the film. Direct incorporation upon chemisorption is improbable.

Low-temperature plasmas in carbon nanostructure synthesis

Plasma-based techniques offer many unique possibilities for the synthesis of various nanostructures both on the surface and in the plasma bulk. In contrast to the conventional chemical vapor deposition and some other techniques, plasma-based processes ensure high level of controllability, good quality of the produced nanomaterials, and reduced environmental risk. In this work, the authors briefly review the unique features of the plasma-enhanced chemical vapor deposition approaches, namely, the techniques based on inductively coupled, microwave, and arc discharges. Specifically, the authors consider the plasmas with the ion/electron density ranging from 10^10 to 10^14 cm−3, electron energy in the discharge up to ∼10 eV, and the operating pressure ranging from 1 to 10^4 Pa (up to 105 Pa for the atmospheric-pressure arc discharges). The operating frequencies of the discharges considered range from 460 kHz for the inductively coupled plasmas, and up to 2.45 GHz for the microwave plasmas. The features of the direct-current arc discharges are also examined. The authors also discuss the principles of operation of these systems, as well as the effects of the key plasma parameters on the conditions of nucleation and growth of the carbon nanostructures, mainly carbon nanotubes and graphene. Advantages and disadvantages of these plasma systems are considered. Future trends in the development of these plasma-based systems are also discussed.

On the heating of nano- and microparticles in process plasmas

Determination and understanding of energy fluxes to nano- or microparticles, which are confined in process plasmas, is highly desirable because the energy balance results in an equilibrium particle temperature which may even initiate the crystallization of nanoparticles. A simple balance model has been used to estimate the energy fluxes between plasma and immersed particles on the basis of measured plasma parameters. Addition of molecular hydrogen to the argon plasma results in additional heating of the particles due to molecule recombination. The measured particle temperature is discussed with respect to appearing plasma–particle interactions which contribute to the particles energy balance.

Energy flux measurements in high power impulse magnetron sputtering

The total energy flux in a high power impulse magnetron sputtering (HiPIMS) plasma has been measured using thermal probes. Radial flux (parallel to the magnetron surface) as well as axial flux (perpendicular to the magnetron surface) were measured at different positions, and resulting energy flux profiles for the region between the magnetron and the substrate are presented. It was found that the substrate heating is reduced in the HiPIMS process compared with conventional direct current magnetron sputtering (DCMS) at the same average power. On the other hand, the energy flux per deposited particle is higher for HiPIMS compared with DCMS, when taking into account the lower deposition rate for pulsed sputtering. This is most likely due to the highly energetic species present in the HiPIMS plasma. Furthermore, the heating due to the radial energy flux reached as much as 60% of the axial energy flux, which is likely a result of the anomalous transport of charged species present in the HiPIMS discharge. Finally, the experimental results were compared with theoretical calculations on energy flux of charged species and were found to be in good agreement.

Particles as probes for complex plasmas in front of biased surfaces

An interesting aspect in the research of complex (dusty) plasmas is the experimental study of the interaction of micro-particles with the surrounding plasma for diagnostic purposes. Local electric fields can be determined from the behaviour of particles in the plasma, e.g. particles may serve as electrostatic probes. Since in many cases of applications in plasma technology it is of great interest to describe the electric field conditions in front of floating or biased surfaces, the confinement and behaviour of test particles is studied in front of floating walls inserted into a plasma as well as in front of additionally biased surfaces. For the latter case, the behaviour of particles in front of an adaptive electrode, which allows for an efficient confinement and manipulation of the grains, has been experimentally studied in terms of the dependence on the discharge parameters and on different bias conditions of the electrode. The effect of the partially biased surface (dc and rf) on the charged micro-particles has been investigated by particle falling experiments. In addition to the experiments, we also investigate the particle behaviour numerically by molecular dynamics, in combination with a fluid and particle-in-cell description of the plasma.

Hydrogen in plasma-nanofabrication: Selective control of nanostructure heating and passivation

The possibility of independent control of the surface fluxes of energy and hydrogen-containing radicals, thus enabling selective control of the nanostructure heating and passivation, is demonstrated. In situ energy flux measurements reveal that even a small addition of H2 to low-pressure Ar plasmas leads to a dramatic increase in the energy deposition through H recombination on the surface. The heat release is quenched by a sequential addition of a hydrocarbon precursor while the surface passivation remains effective. Such selective control offers an effective mechanism for deterministic control of the growth shape, crystallinity, and density of nanostructures in plasma-aided nanofabrication.

Investigation of the energy transfer to the substrate during titanium deposition in a hollow cathode arc

During titanium layer deposition with a hollow cathode arc discharge the integral energy influx to the substrate has been monitored by measuring temperature gradients. Discharge power and substrate voltage have been varied. Simultaneously, the plasma parameters in front of the substrate were determined by means of Langmuir‐probe measurements. From the integral energy influx and the plasma parameters the contributions of the charge carriers and the heat radiation to the substrate heating were calculated. The contribution of the titanium condensation heat could be estimated by means of Rutherford backscattering and ellipsometrical measurements. It was found that heat radiation and charge carriers mainly contribute to the integral substrate heating. The contribution of condensation can be neglected.