Capacitively Coupled RF Discharge Research Articles

ITER-Relevant First Mirrors Unit With Integrated Mirror Cleaning System Utilizing Capacitive Coupled RF Discharge

ITER-Relevant First Mirrors Unit With Integrated Mirror Cleaning System Utilizing Capacitive Coupled RF Discharge

Electrical and Optical Characteristics of Wide Pressure-Range Capacitive Coupled RF Discharge

Electrical and Optical Characteristics of Wide Pressure-Range Capacitive Coupled RF Discharge

Time evolution of neutral and charged species in Ar/C2H2 capacitively-coupled RF discharges

The first stages of polymerization in plasmas of Ar/C2H2 have been studied both experimentally and using a kinetic model, by following the time evolution of the main neutral and ionic species in the discharge. The abundances of the different neutrals and ions have been measured by quadrupole mass spectrometry in conditions specifically chosen to avoid dust formation in the plasma. Maxima occurring at different times from the ignition of the discharge were detected for the density of the neutral species sampled, generally consistent with a stepwise polymerization mechanism. Ions were found to fundamentally follow the trend of their neutral counterparts, with the time evolution for negative ions stressing the difference in the chemical mechanisms responsible for the formation of the two primitive anions, C2H− and H2CC−. Positive and negative ion distributions were measured in the steady state of the discharge. Detection of anions with up to 22 carbon atoms was possible in mixtures with high acetylene content. A volume averaged kinetic model with a simple description of the Ar/C2H2 chemistry was used to identify the main reaction pathways and was able to satisfactory reproduce the experimental results.

Measurement of gas temperatures in dust-free and dusty argon discharges

Gas temperatures in an argon low-power capacitively-coupled rf discharge were measured by means of tunable diode laser absorption spectroscopy on 1s5 metastable states. It was shown that the uncertainty of the measurements in dust-free plasma could be minimized to 1 K. In the presence of dust, due to instabilities in the dust subsystem (e.g. dust density waves), the uncertainty of the temperature measurements increased several times. Nevertheless, an evidence of the gas temperature increase in the presence of dust was detected. This result suggests that the microparticle suspensions, immersed in the low-pressure plasmas, significantly modify the temperature fields of the neutral gas.

Capacitively coupled radio-frequency discharge in alpha-mode as a variable capacitor

In many cases, the capacitance of a plasma discharge is determined by the sheath capacitance, and therefore controlling the sheath thickness can result in a novel plasma-tunable capacitor. In this work, experiments were conducted with capacitively-coupled RF discharge in the alpha regime using parallel-plate electrode geometry in air at 1 Torr with an RF amplifier operating at a constant voltage in a wide range of frequencies, 20–120 MHz. The impedance characteristics and the sheath and plasma parameters were inferred from the current and voltage measurements supplemented with optical imaging. The experimental data show that the sheath thickness is approximately inversely proportional to the excitation RF frequency, so that the capacitance is proportional to the frequency, in agreement with a simple theory. In applications, tuning the sheath thickness by varying the excitation RF frequency could be used to control the impedance variation in a different range of probing RF frequencies, including control of the transition from capacitive to inductive behaviour.

Capacitively coupled rf discharge with a large amount of microparticles: Spatiotemporal emission pattern and microparticle arrangement.

The effect of micron-sized particles on a low-pressure capacitively coupled rf discharge is studied both experimentally and using numerical simulations. In the laboratory experiments, microparticle clouds occupying a considerable fraction of the discharge volume are supported against gravity with the help of the thermophoretic force. The spatiotemporally resolved optical emission measurements are performed with different arrangements of microparticles. The numerical simulations are carried out on the basis of a one-dimensional hybrid (fluid-kinetic) discharge model describing the interaction between plasma and microparticles in a self-consistent way. The study is focused on the role of microparticle arrangement in interpreting the spatiotemporal emission measurements. We show that it is not possible to reproduce simultaneously the observed microparticle arrangement and emission pattern in the framework of the considered one-dimensional model. This disagreement can be attributed to the two-dimensional effects (e.g., radial diffusion of the plasma components) or to the lack of the proper description of the sharp void boundary in the frame of fluid approach.

Numerical solution of the model problem of CCRF-discharge at atmospheric pressure

This work describes a 1D mathematical model of capacitive coupled RF discharge between symmetrical electrodes in argon at atmospheric pressure in a local approximation. Electrons, atomic and molecular ions, metastable atoms and argon dimmers as well as ground-state atoms are considered. A simplified diagram of argon excited states when two metastable and two resonance states are replaced with the uniform level. Such diagram is frequently used to simulate argon plasma due to efficient mixing of these layers at electron impacts. Velocity factors of electron impact processes were calculated using Boltzmann equation with a glance to electron-electron collisions. This work describes numerical algorithm of mathematical model implementation, which is based on finite-dimensional approximation of the problem using difference schemes together with iteration process. The software was developed to implement iterative processes using MatLab. Characteristics of atmospheric pressure capacitive coupled RF discharge at interelectrod distance 20 mm are calculated.

Capacitive coupled RF discharge: modelling at the local statement of the problem

In this paper a mathematical model of capacitively coupled RF discharge at atmospheric pressure is constructed, a method of numerical realization of the model is developed, and the numerical calculations are carried out. Comparison of the results of the numerical experiments with the data of other authors, in particular, with the experimental data, is demonstrated as well a model adequacy as effectiveness of the numerical method. A results of calculations of the model problem at pressure of 760 Torr, frequency of generator of 13.76 MHz and interelectrode distance of 20mm, in local approximation are presented.

Capacitive coupled RF discharge: modelling at the local and not local statement of the problem

The models provided in the research paper describe a capacitive coupled radiofrequency discharge in argon between two parallel plate electrodes, one of which is grounded, and the other is connected to the high-frequency capacitive generator. Herein we review various approaches to simulate a high-frequency capacitive discharge depending on modelled pressure rates. The model of a high-frequency capacitive discharge under low pressure is simulated in non-local approximation, and under high pressure is simulated in local approximation and is sensitive to dimers and molecular ions. We provide calculation data with respect to different pressures and make comparative analysis of data provided by other authors in particular, analysis of data obtained with real experiment.

A novel plasma-assisted atomistic surface finishing on freeform surfaces of fused silica

A novel non-contact type plasma-assisted atomistic surface finishing of freeform surface on a fused silica is conceived. The developed process is targeted to obtain ultrafine surface finish while eliminating the surface and subsurface damages. Stable plasma with He and Ne processing gases and O2 and SF6 reactive gases up to 30 mbar pressure is achieved by using dielectric barrier capacitive coupled RF discharge. Atomic emission spectroscopy is used to identify the relative density of the excited species in the plasma to study the atomistic material removal mechanism. On machined fused silica surface, improvement in surface roughness is up to 68% with He-O2 plasma while there is 85% improvement in surface waviness. Further surface finish improvement is achieved with He-SF6-O2 gas mixture plasma with 0.008 mm³/min material removal rate. Atomistic material removal has also enabled reduction in surface residual stresses and surface cracks thereby enhancing the surface integrity.

RF discharge for in situ mirror surface recovery in ITER

Almost all optical diagnostic systems in ITER will require the implementation of mirror recovery and protection systems. Plasma cleaning is considered to be the most promising technique for the removal of metal deposits from optical surfaces. The engineering and physical aspects of RF discharge application for continuous or periodic plasma treatment are discussed with a focus on implementation under ITER conditions. The ion flux parameters obtained in capacitively coupled (CC) RF discharge were measured in the mock-up of a plasma cleaning system. The uniformity of sputtering in CC RF discharge with and without a magnetic field was studied experimentally for the cylindrical discharge reactor geometry and compared with numerical simulations. The sharp increase in the sputtering rate resulting from the non-uniform radial distribution of the ion flux was observed near the electrode edges. The longitudinal magnetic field improves sputtering uniformity. It was demonstrated that Al/Al2O3 deposits can be removed in the Ne and D2 plasma of CC RF discharge but long-term exposition results in the degradation of the polycrystalline molybdenum mirror surface. The efficiency of Al sputtering in the atmosphere containing O2 and N2 fractions was studied in the D2/O2 and D2/N2 plasma of glow discharge. The addition of 2% of oxygen or nitrogen increases the sputtering yield by 3–4 times as compared with that in a nominally pure D2 discharge. The impact of metal deposits on the performance of diagnostic mirrors is discussed. It was shown that an ultrathin metallic film with a thickness as low as a few nm may cause a significant degradation of diagnostic mirrors with a transparent coating.

Application of low temperature plasmas for restoration/conservation of archaeological objects

The low-temperature low-pressure hydrogen based plasmas were used to study the influence of processes and discharge conditions on corrosion removal. The capacitive coupled RF discharge in the continuous or pulsed regime was used at operating pressure of 100-200 Pa. Plasma treatment was monitored by optical emission spectroscopy. To be able to study influence of various process parameters, the model corroded samples with and without sandy incrustation were prepared. The SEM-EDX analyzes were carried out to verify corrosion removal efficiency. Experimental conditions were optimized for the selected most frequent materials of original metallic archaeological objects (iron, bronze, copper, and brass). Chlorides removal is based on hydrogen ion reactions while oxides are removed mainly by neutral species interactions. A special focus was kept for the samples temperature because it was necessary to avoid any metallographic changes in the material structure. The application of higher power pulsed regime with low duty cycle seems be the best treatment regime. The low pressure hydrogen plasma is not applicable for objects with a very broken structure or for nonmetallic objects due to the non-uniform heat stress. Due to this fact, the new developed plasmas generated in liquids were applied on selected original archaeological glass materials.

Plasmachemical removal of corrosion layers from Iron in pulsed RF discharge

Plasmachemical treatment of archaeological artefacts by RF hydrogen low pressure plasma is a relatively new technique developed during the 1980s. The corrosion removal process is very complex and its mechanism is not fully understood up to now and also optimal conditions are not known well. To study the process condition influence, the model corroded samples were prepared on iron with defined roughness using HCl, HNO3 and H2SO4 corrosion agents. The corrosion layers before and after plasma treatment were analyzed by electron microscopy including the study of corrosion layer profile. The experiment was done in Quartz reactor with outer electrodes creating capacitive coupled RF discharge in pure hydrogen at pressure of 170 Pa. The duty cycle was varied from continuous regime up to 1:25. The plasma treatment duration was 40–120 minutes depending on the duty cycle. The results showed that the application of pulsed discharge regime could maintain the corrosion removal process at lower mean power as well and thus the treated samples were affected by lower heating stress. On the other hand, the process was significantly longer.

Rapid transport of nano-particles having a fractional elementary charge on average in capacitively-coupled rf discharges by amplitude-modulating discharge voltage

We have observed transport of nano-particles having, on average, a fractional elementary charge in single pulse and double pulse capacitively-coupled rf discharges both without and with an Amplitude Modulation (AM) of the discharge voltage, using a two-dimensional laser-light scattering method. Rapid transport of nano-particles towards the grounded electrode is realized using rf discharges with AM. Two important parameters for the rapid transport of nano-particles are the discharge voltage and the period of AM. An important key of the rapid transport is fast redistribution of ion current over the whole discharge region; that is, fast change of spatial distribution of forces exerted on nano-particles. The longer period of the modulation is needed for rapid transport for the larger nano-particles. The higher discharge voltage of the modulation is needed for rapid transport of nano-particles having a smaller mean charge. Local perturbation of electric potential using a probe does not bring about global rapid transport of nano-particles, whereas it leads to their local transport near the probe.

Diagnostics of the plasma series resonance effect in radio-frequency discharges

The intention of the paper is to give an example on how different plasma diagnostics can be combined in a synergistic way in order to investigate new physics. The link between the individual diagnostics has to be provided by theoretical concepts that predict certain relations between the different plasma parameters. The example chosen here is the effect of self-excited plasma series resonances in asymmetric capacitively coupled RF discharges. These resonance oscillations lead to high frequency current oscillations and are caused by a series resonance between the capacitive sheath and the effective inductance of the bulk which results from electron inertia. The non-linearity of the sheath is essential for the self-excitation of these oscillations. Laser spectroscopic electric field measurements, phase and space resolved optical emission spectroscopy, current, voltage, and Langmuir probe measurements are combined. The synergistic effect of these diagnostics in combination with a simple analytical model for the modification of the electron energy distribution function by electron beams yields information on cause and effect of electron heating and a better understanding of these fundamental phenomena.

Dust as fine electrostatic probes for plasma diagnostic

Here we present a novel use of fine dust for diagnostic measurements in the sheath of a planar rf discharge and in the inertial electrostatic confinement (IEC) plasma. Since dust charge is a function of a number of plasma parameters, and the dust charge adjusts itself to changes in plasma conditions instantaneously, dust particles may be used as an ideal diagnostic tool. A major advantage of such a diagnostic approach is its simplicity as the only measurement of a position of a dust particle or its motion after a perturbation is necessary. This technique only requires access to a discharge chamber, dust particles, a laser to illuminate them and a camera to capture the motion of the dust. The dependences of the sheath profiles on the discharge pressure and power have been determined. The radial potential profiles in a cylindrically-symmetrical capacitively-coupled rf discharge and in an IEC plasma were obtained. The direction of the ion flux in the radial IEC discharge was found.

Prospects of Phase Resolved Optical Emission Spectroscopy as a Powerful Diagnostic Tool for RF‐Discharges

AbstractPhase resolved optical emission spectroscopy (PROES) bears considerable potential for diagnostics of RF discharges that give detailed insight of spatial and temporal variations of excitation processes. Based on phase and space resolved measurements of the population dynamics of excited states several diagnostic techniques have been developed. Results for a hydrogen capacitively coupled RF (CCRF) discharge are discussed as an example. The gas temperature, the degree of dissociation and the temporally and spatially resolved electron energy distribution function (EEDF) of energetic electrons (>12eV) are measured. Furthermore, the pulsed electron impact excitation during the field reversal phase, typical for hydrogen CCRF discharges, is exploited for measurements of atomic and molecular data like lifetimes of excited states, coefficients for radiationless collisional de‐excitation (quenching coefficients), and cascading processes from higher electronic states. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polymer-Like C:H Thin Film Coating of Nanopowders in Capacitively Coupled RF Discharge

Nanopowders of amorphous SiO2, with typical particle sizes of 30–80 nm, were treated under the non-equilibrium plasma conditions, created by a capacitively coupled (CC) RF discharge formed in pure methane or ethane. The plasma gas flow rate varied between 0.02 and 0.06 slpm, with reactor pressures between 1000 and 5000 Pa, and applied RF power inputs between 700 and 1500 W. The plasma properties were monitored through measurements of the rotational temperature, as derived from the C2 5160 A Swan band and N2 second positive 3577 A band, and the atomic hydrogen excitation temperature, from the Hβ Hγ and Hδ lines during the powder treatment process. The compositions of the gases that passed through the plasma were analyzed by mass spectrometry. In spite of the evidence for the presence of CnH2n+2 and CnH2n (n=1–3) species and acetylene in the discharge, the homogeneous formation of soot was not observed. At the same time, the introduced nanoparticles were observed to act as centers for the inception and growth of C:H thin coatings in the form of a polymer-like hydrocarbon layers, of thickness between <5 and 30 nm. The results of TEM, IR spectroscopy, thermo-gravimetric and precision calorimetric analysis performed on the treated powders provide evidence to the formation of an amorphous, high density C:H matrix on the particles' surfaces.

Ion flux's pressure dependence in an asymmetric capacitively coupled rf discharge in NF3

Abstract Starting from an analytical macroscopic/phenomenological model yielding the self-bias voltage as a function of the absorbed radio-frequency (rf) power of an asymmetric capacitively coupled discharge in NF3 this paper studies the dependence of the ion flux onto the powered electrode on the gas pressure. An essential feature of the model is the assumption that the ions' drift velocity in the sheath near the powered electrode is proportional to E α, where E=−ΔU (U being the self-bias potential), and α is a coefficient depending on the gas pressure and cross section of elastic ion-neutral collisions. The model also considers the role of γ-electrons, stochastic heating as well as the contribution of the active electron current to the global discharge power balance. Numerically solving the model's basic equations one can extract the magnitude of the ion flux (at three different gas pressures) in a technological etching device (Alcatel GIR 220) by using easily measurable quantities, notably the self-bias voltage and absorbed rf power.

Electron plasma parameters and ion energy measurement at the grounded electrode in an rf discharge

Investigations on the electron plasma parameters and ion energy distributions (IEDs) at the grounded electrode (anode) of an asymmetrical capacitively coupled rf discharge in argon are presented. These are compared with studies from previous papers related to the sheath formed in front of the powered electrode.The I–V characteristics provided by a self-compensated Langmuir probe show the existence of a bi-Maxwellian distribution with a bulk cold electron group and a hot electron group. From mass spectrometric investigations, the IEDs exhibit a double peak shape. The theoretical model for transport of the ions and electrons in the sheath of a capacitively coupled, asymmetric rf discharge was considered. Combination of two effects, the rf modulation of the sheath potential and charge exchange as a dominant inelastic interaction process in the sheath, leads to pronounced and characteristic additional structures in the IEDs.