Matthias Wolter

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.

Aluminium atom density and temperature in a dc magnetron discharge determined by means of blue diode laser absorption spectroscopy

Diode laser absorption studies of aluminium atoms produced in a direct current (dc) magnetron discharge with argon as well as argon/nitrogen and argon/oxygen mixtures as working gas are reported. The measured Al atom density increases with increasing discharge power. The observed temperature being in the range of 340–420 K rises with increasing power but shows little pressure dependence. A small admixture of oxygen gas leads to a complete disappearance of the absorption signal, a result which is not yet fully understood.

Force measurements in dusty plasmas under microgravity by means of laser manipulation

Experiments in a dusty plasma under the microgravity conditions of parabolic flights are presented. Under microgravity, extended dust structures and a central dust-free region (“void”) are formed. Here, the forces and the force balance at the void boundary are studied by means of laser manipulation of the dust particles: A focused laser beam is moved in a controlled way to drive particles in the extended dust cloud and at the void boundary. From the observed particle motion, the forces on the particles in the dust cloud and at the void boundary are derived. Together with Langmuir probe measurements, a quantitative description of the force balance has been achieved. Special attention has been paid to the ion drag force, which is crucial in understanding the void formation. The results are compared to prevalent ion drag models.

On the measurement of energy fluxes in plasmas using a calorimetric probe and a thermopile sensor

Two different diagnostics for the determination of the energy influx in plasma processes were used to characterize an ion beam source and an asymmetric RF discharge. The related energy fluxes were measured in dependence on the ion energy and on the RF power, respectively. The first sensor, called HFM (Heat Flux Microsensor) is a thermopile which allows for direct energy flux measurements. With the second sensor, a calorimetric probe, the energy influx has been calculated from the temporal temperature evolution preliminarily registered. Although the working principle of both sensors is different, the obtained results are in good agreement. In the ion beam (<1.5 keV)) rather high energy influxes are achieved (up to 700 mW cm−2), whereas the values measured in the asymmetric RF discharge were lower than 50 mW cm−2 for discharge powers in the range 10–100 W. The performances and limitations of both sensors are compared and discussed.

Disentangling fluxes of energy and matter in plasma-surface interactions: Effect of process parameters

The possibility to discriminate between the relative importance of the fluxes of energy and matter in plasma-surface interaction is demonstrated by the energy flux measurements in low-temperature plasmas ignited by the radio frequency discharge (power and pressure ranges 50-250 W and 8-11.5 Pa) in Ar, Ar+ H2, and Ar+ H2 + CH4 gas mixtures typically used in nanoscale synthesis and processing of silicon- and carbon-based nanostructures. It is shown that by varying the gas composition and pressure, the discharge power, and the surface bias one can effectively control the surface temperature and the matter supply rates. The experimental findings are explained in terms of the plasma-specific reactions in the plasma bulk and on the surface.

Microparticles in a RF plasma under hyper gravity conditions

Summary form only given: For diagnostic purposes micrometer-sized particles can be used as floating electrostatic probes. Once injected into a complex rf plasma, these particles will become negatively charged and can be trapped in the plasma sheath due to an equilibrium of several forces working on them, e.g. the electrostatic force, gravity, drag forces and thermophoresis. Measuring for example the position of the particles in the plasma sheath and the interparticle distance while varying plasma parameters (power, pressure, temperature, gas etc.) gives important information about plasma properties like the ion flux and the sheath potential. We experimentally investigated the behavior of micrometer sized particles inserted and trapped in a rf plasma under varying gravity conditions in a centrifuge. Here we present first results of those measurements. The experiments were carried out in a Perspex box containing a parallel plate capacitively coupled rf argon plasma at pressures between 20 and 115 Pa. The typical forward power applied to the bottom electrode was ~10 Watt. The squared electrodes are separated 5 cm from each other and both contain centered holes in order to trap the particles in the created potential well. This also gains possibilities to observe particle behavior from below. The monodisperse particles which are made of melamineformaldehyde and have sizes ranging from 5 up to 12 ?m are illuminated by an expanded 532 nm laser beam. The height of the particles on which the forces are in equilibrium is measured from pictures collected with an onboard CCD camera. This whole setup is mounted on a centrifuge originally developed to study high pressure metal halide lamps under hyper gravity conditions. Results show that under these condition particles can be trapped in the plasma sheath when the gravitational force is 2.6g or less. When larger acceleration forces are applied the particles are lost from the discharge. Due to the increased apparent gravity of the particles in the centrifuge the height of the cloud above the powered bottom electrode decrease with ~2 mm when the acceleration force is increased from 1g up to 2.6g.

Micro‐Particles as Electrostatic Probes for Plasma Sheath Diagnostic

An interesting aspect in the research of complex (dusty) plasmas is the experimental study of the interaction of micro‐particles of different sizes with the surrounding plasma for diagnostic purpose. In the plasma micro‐disperse particles are negatively charged and confined in the sheath. The particles are trapped by an equilibrium of gravity, electric field force and ion drag force. From the behavior, local electric fields can be determined, e.g. particles are used as electrostatic probes. In combination with additional measurements of the plasma parameters with Langmuir probes and thermal probes as well as by comparison with an analytical sheath model, the structure of the sheath can be described. In the present work we focus on the behavior of micro‐particles of different sizes and several plasma parameters e.g. the gas pressure and the rf‐power.

Electric field measurements in the sheath of an argon rf discharge by probing with microparticles under varying gravity conditions

Summary form only given. The electric field profile in the plasma sheath of an argon RFplasma has been determined by measuring the equilibrium height and the resonance frequency of plasma-confined microparticles. In order to determine the electric field structure at any position in the plasma sheath without the discharge being changed or disturbed an additional, non-electric, force is introduced which does not alter the plasma conditions, but which does allow for manipulation of the particle position through the sheath: (hyper-)gravity, induced by a centrifuge. Consequently, the electric field and the particle charge can be determined as function of the position in the sheath, using one and the same particle for measurements at several positions throughout the sheath. Particle charges between 6000 and 7000 times the electron charge are determined. Closer to the electrode, an increase of the particle charge is observed. Over the largest part of the sheath the electric field is linear, while close to the sheath edge its behaviour appears to be non-linear. Absolute values of the electric field at the electrode (-25.000 V/m) are consistent with literature.

Complex (Dusty) Plasmas: Application in Material Processing and Tools for Plasma Diagnostics

Formation, trapping, and modification of powder particles in laboratory received growing interest during the last decade in research and technology with novel and unique properties. Applications of complex (dusty) plasmas are numerous, most of them emerging in modern material science. For the optimizations of these processes, a detailed knowledge of plasma–particle interaction is needed. On the other hand, due to the interaction between small (test) particles and the surrounding plasma information on the electric field in front of surfaces and the energy fluxes in the plasma can be obtained. Dust particles can be used as a diagnostic tool, for example, by observing position and motion of the particles in dependence on the discharge parameters. In a certain sense, one can state that “complex (dusty) plasmas” are a rapidly expanding field of research at the border between plasma physics, material processing, and diagnostics, for example, for synthesis and modification of powder particles.

Plasma Treatment of Polyethylene Powder Particles in Hollow Cathode Glow Discharge

Polyethylen (PE) is widely used in the production of foils, insulators, packaging materials, plastic bottles etc. Untreated PE is hydrophobic due to its unpolar surface. Therefore, it is hard to print or glue PE and the surface has to be modified before converting.In the present experiments a hollow cathode glow discharge is used as plasma source which is mounted in a spiral conveyor in order to ensure a combines transport of PE powder particles. With this set‐up a homogeneous surface treatment of the powder is possible while passing the glow discharge. The plasma treatment causes a remarkable enhancement of the hydrophilicity of the PE powder which can be verified by contact angle measurements and X‐ray photoelectron spectroscopy.