Christoph Hollenstein

Influences of a high excitation frequency (70 MHz) in the glow discharge technique on the process plasma and the properties of hydrogenated amorphous silicon

Hydrogenated amorphous silicon has been prepared at a plasma excitation frequency in the very‐high‐frequency band at 70 MHz with the glow discharge technique at substrate temperatures between 280 and 50 °C. The structural properties have been studied using hydrogen evolution, elastic recoil detection analysis, and infrared spectroscopy. The films were further characterized by dark and photoconductivity and by photothermal deflection spectroscopy. With respect to films prepared at the conventional frequency of 13.56 MHz considerable differences concerning the electronic and structural properties are observed as the substrate temperature is decreased from 280 to 50 °C. Down to a substrate temperature of 150 °C the electronic film properties change only a little and the total hydrogen content cH and the degree of microstructure that can be directly correlated to cH increase only moderately. Below 150 °C the electronic properties deteriorate in the usual manner but still the total hydrogen content does not ex...

The physics of plasma-enhanced chemical vapour deposition for large-area coating: industrial application to flat panel displays and solar cells

Designing plasma-enhanced chemical vapour deposition (PECVD) reactors to coat large-area glass plates (similar to1 m(2)) for flat panel display or solar cell manufacturing raises challenging issues in physics and chemistry as well as mechanical, thermal, and electrical engineering, and material science. In such reactive glow discharge plasma slabs, excited at RF frequency (from 13.56 MHz up to similar to 100 MHz), the thin-film deposition uniformity is determined by the gas flow distribution, as well as the RF voltage distribution along the electrodes, and by local plasma perturbations at the reactor boundaries. All these aspects can be approached by analytical and numerical modelling. Moreover, the film properties are largely determined by the plasma chemistry involving the neutral radicals contributing to film growth, the effect of ion bombardment, and the formation and trapping of dust triggered by homogeneous nucleation. This paper will review progress in this field, with particular emphasis on modelling developments.

The Role of Metastable Atoms in Argon-Diluted Silane Radiofrequency Plasmas

The evolution of the argon metastable states density has been studied by absorption spectroscopy in power-modulated plasmas of argon and a mixture of 4% silane in argon. A small concentration of silane suppresses the argon metastable states density by molecular quenching. This molecular quenching adds to the electronic collisional dissociation to increase the silane dissociation rate as compared with pure silane plasmas. Using time-resolved emission spectroscopy, the role of metastable states in excitation to the argon 2P2 state has been determined in comparison with production from the ground state. In silane plasmas, emission from SiH* is due essentially to electron impact dissociation of silane, whereas in 4% silane-in-argon plasmas, emission from SiH* seems to be due to electron impact excitation of the SiH ground state. These studies demonstrate that argon is not simply a buffer gas but has an influence on the dissociation rate in plasma-assisted deposition of amorphous silicon using argon-diluted silane plasmas.

Time-resolved imaging of anodic arc root behavior during fluctuations of a DC plasma spraying torch

The fluctuating behavior of a Sulzer Metco F4 DC plasma gun has been investigated by simultaneous measurement of the time dependencies of the are voltage and of images from the nozzle interior. An end-on imaging arrangement using a mirror and a mask in the optical path from the are to the camera allows visualization of the anodic arc attachment by strongly attenuating the bright emission from the are column. With the torch operating in the restrike mode, sequences of images have been acquired in synchronization with several typical features of the are voltage fluctuations showing that the attachment nature changes during a restrike cycle. Multiple attachments which coexist at least during the 1 /spl mu/s exposure time of the camera have been evidenced and are interpreted as a continuous process of creation/vanishing of successive arc roots with a smooth transfer of the current from one to the other. The anode wear is shown to have a strong effect on the root position over the anode periphery, with a preference for attachment in eroded regions. The effects of operation parameters such as current, gas flow and injector type on the attachment nature and position are also presented.

Optical emission spectroscopy of electrical discharge machining plasma

Plasma created during EDM is systematically investigated with optical emission spectroscopy. Typical spectra show a strong H,,, and continuum radiation, with many lines emitted by impurities coming from electrode and workpiece materials. The dielectric molecules are cracked by the discharge. Time-resolved spectroscopy shows that the electron density is extreme at the beginning of the discharge. This causes merging of lines, strong Stark broadening and shift of the H,, line. The electron temperature is low and remains roughly constant around 0.7 eV The low temperature, the high density and other spectroscopic evidences prove that the EDM plasma is non-ideal

Experimental Investigation of Pulsed Dielectric Barrier Discharge Actuators in Sub- and Transonic Flow

Experiments were conducted in order to investigate the ability of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators to control flow on airfoils in subsonic and transonic flow (up to Ma1 = 0.75, Re = 1.35 • 10^6). A NACA 0015 profile equipped with a leading edge mounted ns-DBD actuator was investigated up to Re = 2.3 • 10^5 (u1 = 24 m/s). Measurements of the surface pressure distribution clearly confirm the actuator’s potential to delay leading edge separation. In the following the impact on the control authority of different voltage pulse parameters, such as voltage amplitude, actuation frequency and rise/fall time of the pulse were investigated. The experiments in transonic flow were conducted on a NACA 3506 compressor blade profile. A ns-DBD actuator was placed at x/c = 0.33 where the foot of the shock-wave and boundary-layer separation was observed. Schlieren flow visualization showed the shock-wave boundary-layer interaction and was used to investigate the actuator’s effect on the shock position and shape. A high-speed camera allowed to acquire schlieren images at high acquisition rates and investigate as well the movement of the shock in the frequency domain. These results were verified with measurements of the static pressure at the side wall using unsteady pressure transducers.

Effects of High Voltage Pulsed DBD on the Aerodynamic Performances in Subsonic and Transonic Conditions

The paper presents the results of a research activity that aims to investigate the control effect of fast rising pulse Dielectric Barrier Discharge actuators (DBD) in high speed flow conditions. At this aim, several experimental tests have been performed on a 2D wind tunnel model in subsonic and transonic conditions to collect information concerning effects of DBD actuators on the main aerodynamic aerofoil performance. The Mach number was varied between 0.4 and 0.85 at angles of attack ranging between -2° and 8° and Reynolds numbers between Re=1.7∙10 6 and Re=2.5∙10 6 . The aerofoil geometry selected for these tests was the supercritical BAC3-11 profile with 11% of maximum thickness. During the experiments, quantitative measurements have been made through steady and instantaneous pressure sensors. In particular, 45 pressure taps and 10 high frequency pressure transducers were installed on the surface of the model. The experimental data were used to develop and validate numerical tools, able to predict the plasma behaviour in presence of convective fields and, therefore, to support the design and setting of more effectiveness DBD actuator. A Computational Fluid Dynamics (CFD) solver developed at CIRA has been used for numerical simulations. A theoretical model for dielectric barrier discharge (DBD) via bodyforce and power density terms has been implemented in order to support the experimental test campaign.

Global visualization of powder trapping in capacitive RF plasmas by two-dimensional laser scattering

Understanding particle dynamics requires comparison of spatially resolved measurements with two-dimensional (2-D) simulations. In this work, a cross section of a capacitive discharge is illuminated with an expanded laser beam, and global scattered light is recorded by camera. Plasma equipotentials are visualized using the scattered intensity from a small amount of particles trapped in an argon plasma. At low RF voltages, good agreement is found with 2-D fluid simulations, whereas displacement of the power clouds toward the sheaths is observed at high voltage.

Low ion energy RF reactor using an array of plasmas through a grounded grid

A reactor using localized remote plasma in a grid electrode is presented in this study. The aim is to reduce the ion bombardment energy inherent in RF capacitively coupled parallel plate reactors used to deposit large area thin film silicon solar cells. High ion bombardment energy could cause defects in silicon layers and deteriorate electrical interfaces, therefore, by reducing the ion bombardment energy, lower defect density might be obtained. In this study, the low ion bombardment energy results from the reactor design. By inserting a grounded grid close to the RF electrode of a parallel plate reactor, the electrode area asymmetry is increased while retaining the lateral uniformity required for large area deposition. This asymmetry causes a strong negative self-bias voltage, which reduces the time-averaged plasma potential and thus lowers the ion bombardment energy. In addition to the self-bias, the time evolution of plasma light emission and plasma potential RF waveform are also affected by the grid, thereby further reducing the time-averaged plasma potential and ion bombardment energy. Finally, a good correlation between the measured time-averaged plasma potential and measured low ion bombardment energy is found in a broad range of RF voltages.

Radio frequency breakdown between structured parallel plate electrodes with a millimetric gap in low pressure gases

This paper presents an investigation into radio frequency (rf) breakdown for electrodes with holes or protrusions, approximating the situation in real reactors and providing a benchmark for fluid simulations. rf breakdown curves (voltage versus pressure) generally show a steep left-hand branch at low pressures and a flatter right-hand branch at higher pressures. Introducing protrusions or holes in parallel plate electrodes will lower the breakdown voltage in certain conditions. Yet experiments show that the breakdown curves are not perceptibly influenced by the increased electric field at sharp edges or ridges. Instead, both experiments and simulation show that breakdown at high pressure will occur at the protrusion providing the smallest gap, while breakdown at low pressure will occur in the aperture providing the largest gap. This holds true as long as the feature in question is wide enough. Features that are too narrow will lose too many electrons due to diffusion, either to the walls of the apertures or to the surroundings of the protrusion, which negates the effect on the breakdown voltage. The simulation developed presents a tool to aid the design of complex rf parts for dark-space shielding.