Capacitive RF Discharge Research Articles

Numerical Simulation of the Dynamics of RF Capacitive Discharge in Carbon Dioxide

In this research, the one-dimensional fluid code SIGLO-rf was used to study the internal parameters of RF capacitive discharge in carbon dioxide, focusing mainly on time-averaged and spatio-temporal distributions of discharge parameters. With the help of this code, in the range of distances between electrodes d = 0.04 – 8 cm, RF frequencies f = 3.89 – 67.8 MHz, and values of carbon dioxide pressure p = 0.1 – 9.9 Torr, averaged over the RF period axial profiles of the density of electrons, positive and negative ions were calculated as well as potential and electric field strength. It is shown that the discharge plasma in CO2 contains electrons, positive ions, as well as negative ions. The negative ions of atomic oxygen are formed by the dissociative attachment of electrons to CO2 molecules. Studies of the spatio-temporal dynamics of plasma parameters (electron density, potential and electric field strength, as well as ionization and attachment rates) in RF capacitive discharge in CO2 showed that during half of the RF period, 1 to 3 ionization bursts are usually observed. They correspond to stochastic heating in the near-electrode sheath and the formation of passive and active double layers near the sheath boundaries. The passive double layer appears in the cathode phase and maintains the discharge plasma. The active layer is formed in the anodic phase and ensures a balance of positive and negative charges escaping to the electrode during the RF period. It was found that when the conditions pd = 2 Torr cm and fd = 27.12 MHz cm are met simultaneously, during half of the RF period, 4 intense ionization peaks are observed: resulting from stochastic heating, passive, active, and additional (auxiliary) double layers. The auxiliary double layer helps bring electrons to the surface of the temporary anode and occurs near its surface inside the near-electrode sheath. Using the similarity law, the conditions for the existence of these 4 ionization peaks in a wide range of RF frequencies, carbon dioxide pressures, and distances between electrodes were verified.

Effect of Multi-Frequency Excitation on Dust Particle Charging in Weakly Collisional Radio-Frequency Capacitive Discharge

Effect of Multi-Frequency Excitation on Dust Particle Charging in Weakly Collisional Radio-Frequency Capacitive Discharge

Characteristics of Capacitive RF Discharge in a Magnetic Field with a Predominant Radial Component

Characteristics of Capacitive RF Discharge in a Magnetic Field with a Predominant Radial Component

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

Transition in radio frequency gas breakdown with a transverse magnetic field

This paper presents the quantification of gas breakdown voltages in low-pressure argon capacitive radio frequency (rf) discharges in the presence of an external transverse magnetic field via fully kinetic particle-in-cell/Monte Carlo collision simulations. It is found that as the magnetic field increases, the left branch of the breakdown curve, a double-valued rf breakdown voltage regime, shifts toward a lower-pressure region. A split of the breakdown curve into lower and upper branches occurs when the external magnetic field rises to a critical value. The observed abrupt transition of rf breakdown behaviors can be understood based on the electron kinetics behaviors, with the consideration of the enhanced ionization and electron confinement induced by drift motions. An improved analytical model is developed, which can estimate the critical magnetic field required for the occurrence of the transition.

Similarity-based scaling networks for capacitive radio frequency discharge plasmas

We demonstrate similarity-based scaling networks for capacitive radio frequency (RF) plasmas, which extensively correlate discharge characteristics under varied conditions, incorporating the transition from original to similarity states. Based on fully kinetic particle-in-cell simulations, similar RF discharges in argon are demonstrated with three external control parameters (gas pressure, gap distance, and driving frequency) simultaneously tuned. A complete set of scaling pathways regarding fundamental discharge parameters is obtained, from which each plasma state finds its neighboring node with only one control parameter tuned. The results from this study provide a promising strategy for plasma multi-parameter mapping, enabling effective cross-comparisons, prediction, and manipulation of RF discharge plasmas.

Physical Properties of a Low-Power Helicon Source Operating on a High-Frequency Discharge with a Capacitive Component

The results of an experimental study of a low-power RF plasma source placed in a longitudinal magnetic field (helicon thruster), when it operates on a capacitive RF discharge and inductive RF discharges with a capacitive component, are presented. A significant dependence of the characteristics of the ion and electron fluxes of the source on the induction of a constant magnetic field is shown. The fundamental applicability of capacitive RF discharge as a working process in the studied plasma source is demonstrated. It is shown that the increase in the average energy of ions in the flow at the outlet of the source with the appearance of the capacitive component of the discharge is slight.

Spatial distributions of the ion flux in a capacitive hydrogen RF discharge using a hollow cathode with double toroidal grooves enclosed by magnets

A hydrogen high-density capacitively coupled plasma has been developed using a hollow cathode with double toroidal grooves enclosed by magnets and without an iron yoke disk. It is found that this plasma source allows generating higher plasma densities compared to the conventional RF magnetron plasma sources. Spatial distributions of the ion flux have been measured at various H2 gas pressures, p, of 1–20 Pa. It is found that the hybrid combination of a hollow cathode effect and magnetic confinement of electrons is attained for p ≥ 5 Pa, while for p ≤ 3 Pa, a conventional capacitive discharge is generated. The radial profile of the ion flux becomes uniform with increasing axial distance from the target for the hollow cathode discharge. The characteristic decay length of the roughness of the ion flux profile increases with increasing the gas pressure.

On the importance of excited state species in low pressure capacitively coupled plasma argon discharges

In the past three decades, first principles-based fully kinetic particle-in-cell Monte Carlo collision (PIC/MCC) simulations have been proven to be an important tool for the understanding of the physics of low pressure capacitive discharges. However, there is a long-standing issue that the plasma density determined by PIC/MCC simulations shows quantitative deviations from experimental measurements, even in argon discharges, indicating that certain physics may be missing in previous modeling of the low pressure radio frequency (rf) driven capacitive discharges. In this work, we report that the energetic electron-induced secondary electron emission (SEE) and excited state atoms play an important role in low pressure rf capacitive argon plasma discharges. The ion-induced secondary electrons are accelerated by the high sheath field to strike the opposite electrode and produce a considerable number of secondary electrons that lead to additional ionizing impacts and further increase of the plasma density. Importantly, the presence of excited state species even further enhances the plasma density via excited state neutral and resonant state photon-induced SEE on the electrode surface. The PIC/MCC simulation results show good agreement with the recent experimental measurements in the low pressure range (1–10 Pa) that is commonly used for etching in the semiconductor industry. At the highest pressure (20 Pa) and driving voltage amplitudes 250 and 350 V explored here, the plasma densities from PIC/MCC simulations considering excited state neutrals and resonant photon-induced SEE are quantitatively higher than observed in the experiments, requiring further investigation on high pressure discharges.

Effects of a radial variation of surface coefficients on plasma uniformity in capacitive RF discharges

With the increasing demands toward large area plasma etching and deposition, the radial uniformity of capacitively coupled plasmas (CCPs) becomes one of the key factors that determine process performance in industrial applications. However, there is a variety of parasitic effects, e.g. electromagnetic and electrostatic edge effects, that typically lead to the formation of nonuniform radial plasma density profiles at various discharge conditions with a density peak appearing either at the center or near the edges of the electrodes. Moreover, in commercial CCPs different surface materials are in contact with the plasma at various positions as parts of boundary surfaces such as focus rings, masks, showerhead electrodes, wall and/or target materials. Via complex material specific plasma-surface interactions, the presence of such different surface materials affects plasma uniformity in a way that is typically not understood and, thus, not controlled. In this work, aided by 2d3v graphics processing unit accelerated particle-in-cell/Monte Carlo collision simulations, we study the effects of radial variations of electrode materials on the plasma via their different ion and electron induced secondary electron emission as well as electron reflection coefficients on the discharge characteristics. Based on such fundamental understanding we tailor the radial variation of boundary surface materials to improve plasma uniformity in low pressure CCPs. Such investigations are performed at different neutral gas pressures, where both center and edge high radial plasma density profiles form in the presence of radially uniform surface coefficients that resemble the presence of a single electrode material. It is demonstrated that by radially varying the surface coefficients at the grounded electrode, the radial plasma density profile can be finely adjusted and the plasma uniformity above the wafer placed at the powered electrode can be improved in both cases.

Electron dynamics in planar radio frequency magnetron plasmas: III. Comparison of experimental investigations of power absorption dynamics to simulation results

In magnetized capacitively coupled radio-frequency (RF) discharges operated at low pressure the influence of the magnetic flux density on discharge properties has been studied recently both by experimental investigations and in simulations. It was found that the magnetic asymmetry effect allows for a control of the DC self-bias and the ion energy distribution by tuning the magnetic field strength. In this study, we focus on experimental investigations of the electron power absorption dynamics in the presence of a magnetron-like magnetic field configuration in a low pressure capacitive RF discharge operated in argon. Phase resolved optical emission spectroscopy measurements provide insights into the electron dynamics on a nanosecond-timescale. The magnetic flux density and the neutral gas pressure are found to strongly alter these dynamics. For specific conditions energetic electrons are efficiently trapped by the magnetic field in a region close to the powered electrode, serving as the target surface. Depending on the magnetic field strength an electric field reversal is observed that leads to a further acceleration of electrons during the sheath collapse. These findings are supported by two-dimensional particle in cell simulations that yield deeper insights into the discharge dynamics.

Electron drift velocity in acetylene and carbon dioxide determined from rf breakdown curves

In this work, we measured the breakdown curves of radio-frequency (13.56 MHz) capacitive discharge in acetylene and in carbon dioxide. The electron drift velocity values were determined in the reduced electric field range 319 Td ≤ E/N ≤ 3409 Td (1 Townsend = 10−17 Vcm2) in acetylene and 272 Td ≤ E/N ≤ 6240 Td in carbon dioxide from turning points on the measured breakdown curves. Treating the electron motion equations and the gas breakdown criterion in the rf electric field we have demonstrated that the method of electron drift velocity determination we employed is universal for arbitrary gasses, and a possible dependence of the electron mobility and diffusion on the reduced electric field strength E/N does not affect the drift velocity values obtained with it. We have demonstrated that in the range of E/N values we have studied the ionization rate exceeds the attachment rate considerably therefore one may neglect the effect the attachment of free electrons to gas molecules plays on electron drift velocity. We have explained the reason why the values of the electron drift velocity we have obtained differ from those obtained by other authors.

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

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

Synthesis of Nanoparticles and Theoretical Model of Their Retention in Plasma of RF Capacitive Discharge with Vertically Arranged Electrodes in Acetylene.

In the present research, experiments on the formation and retention of nanoparticles (NPs) in the plasma of radio frequency (RF) capacitive discharge in acetylene were carried out with vertically positioned internal electrodes. It has been shown via SEM and TEM techniques that NPs found on the horizontal tube wall after the discharge operation have a spherical shape with a predominant diameter of approximately 400-600 nm. HRTEM analysis reveals their amorphous structure. At the same time, such NPs were not found on vertical electrodes, only a polymer film was deposited. To elucidate the possibility of NPs leaving the plasma in the direction of vertical electrodes, a model of NP retention in the near-electrode sheath of an RF capacitive discharge was elaborated. The model has shown that nanometer- and even micrometer-sized particles formed in the plasma cannot cross the near-electrode sheath and reach the electrode surface. For the plasma consisting of three charged components (positive ions, electrons, and NPs), an analytical model of ambipolar diffusion was developed. Applying this model, it has been shown that the ambipolar electric field can keep the micrometer-sized NPs in the plasma if their concentration is low. However, in the case of a high concentration of NPs, they can be retained with a diameter of no more than a few hundred nanometers due to a significant decrease in the ambipolar electric field. The calculation results are in agreement with our experimental data.

One-dimensional particle-in-cell/Monte Carlo collision simulation for investigation of amplitude modulation effects in RF capacitive discharges

One-dimensional particle-in-cell/Monte Carlo collision simulation for investigation of amplitude modulation effects in RF capacitive discharges

Plasma-chemical deposition of diamond-like coatings in capacitively coupled RF plasma in toluene

Experimental study of the rates of plasma-chemical deposition of diamond-like coatings in toluene vapour in capacitive RF discharge was carried out. It is shown that in the RF power range of 100-300 W, the deposition rate is mainly affected by the toluene pressure in the vacuum chamber. The maximum deposition rate obtained was 4.15 μm/h.

On the validity of the classical plasma conductivity in capacitive RF discharges

The plasma conductivity is an important input parameter for various plasma models. It is typically obtained from a simplified version of the electron momentum balance equation, where only a single inertia term and a simplified description of the collisional momentum transfer are included. The electric field is assumed to be a harmonic function of the driving frequency, higher harmonics of the current and spatial variations are neglected. Through particle-in-cell/Monte Carlo collision (PIC/MCC) simulations and analysis of the electric field generation based on velocity moments of the Boltzmann equation, the validity of this classical model is studied in capacitively coupled plasmas (CCPs). We find that these assumptions/simplifications result in significant inaccuracies of the conductivity in many cases. In single frequency CCPs, a deviation of more than an order of magnitude from the effective PIC-conductivity obtained from the simulations is found at low pressures in the discharge center and at the maximum sheath edge. In the center, this deviation is caused by neglecting the temperature gradient term in the momentum balance equation and adopting an approximation of the Ohmic term in the classical model, while at the maximum sheath edge it is induced by neglecting the density gradient term that accounts for the effect of the ambipolar electric field. The inaccuracy in the discharge center is reduced at higher pressures where the Ohmic term dominates and the approximations made in the classical model are more applicable. Better performance of the classical model is also found under conditions at which the inertia term included in the model plays an important role. Generally, neglecting higher harmonics of the current and spatial variations of plasma parameters is found to cause strong inaccuracies. Thus, the classical model can result in an inaccurate calculation of the power absorbed by electrons. Our results indicate that its applicability must be evaluated for a given set of conditions before using it to avoid introducing errors to plasma models.

PRODUCTION OF ELECTRICALLY CONDUCTIVE TEXTILE MATERIALS LAMINATED WITH POLYMER FILMS

The article presents the results of a study of the complex properties of metallised textile materials laminated with polymer films by thermal pressing. The application of a copper metal coating to the surface of a textile material was carried out by magnetron sputtering. To increase the adhesion of synthetic textile materials to the metal coating, pretreatment in plasma of a radio-frequency capacitive (RFC) discharge of reduced pressure was used, carried out immediately before spraying the metal within one technological cycle. It has been established that the metallised textile materials obtained have a shielding coefficient of up to 50 dB, and the use of perforated films for lamination makes it possible to obtain materials with air and vapour permeability. The use of plasma modification in the process makes it possible to increase the adhesion of polymer materials to metal coatings, which contributes to an increase in the wear resistance of the resulting textile materials with shielding properties.

Collisionless magnetized sheath resonance heating induced by a transverse magnetic field in low-pressure capacitive rf discharges

Abstract The mechanism of resonance heating between the gyrating electrons and the oscillating sheath induced by a small transverse magnetic field in low-pressure capacitively coupled plasmas is investigated. The gyrating electrons will coherently collide with the expanding sheath if the electron gyro-frequency coincides with half the driving frequency. These electrons will gain substantial energy from collision-less heating, which strongly enhances the electron power absorption and the plasma density at a constant driving voltage. The electron kinetics is revealed at resonant conditions by particle simulations. Our numerical results demonstrate that the relation between the magnetic field and the driving frequency determines this resonance effect. Besides, it is found that the operating pressure, electrode gap, and driving voltage all strongly affect this electron resonance mechanism. The resonance effect is more pronounced at the conditions of low pressure, large gap, and high voltage.

Similarity properties in capacitive radio frequency plasmas with nonlinear collision processes

Similarity laws (SL) are essential for correlating the characteristics of plasmas at different dimensional scales, which have been validated for radio frequency (rf) discharges at low pressure but under limited conditions. In this work, we evaluate the effects of nonlinear collisions (e.g. stepwise ionization) on the similarity properties in capacitive rf discharges in argon across a wide range of pressure regimes via fully kinetic particle-in-cell simulations. The SL scalings of fundamental discharge parameters, e.g. the electron power absorption and electron energy probability function, are examined with and without nonlinear collisions, respectively. Without the nonlinear collisions, the similarity scalings are found to be rigorously valid. When the nonlinear collisions are considered, the similarity properties in rf discharges still exist approximately, which indicates that the violations caused by the nonlinear collisions are still minor in the studied cases. The reasons for the effectiveness of SL scalings with nonlinear collision processes are also discussed. The results from this study confirmed the validity of similarity transformations with more complex reaction kinetics, which may promote the applicability of the SL scalings for the design and fabrications of plasma devices.