Capacitive RF Discharge Research Articles

Optical and electrical properties of capacitive coupled radio frequency Ar-H2 mixture discharge at the low pressure

In this study, the electrical and optical properties and plasma's parameters of capacitive radio frequency (RF) discharge for the argon and hydrogen mixture at low pressure are investigated using optical emission spectroscopy (OES) and a Langmuir probe. The wavelengths of spectral lines obtained from OES are between 400 and 950 nm. By using the spectral lines and probe data, the electron temperature for every different RF power, mixture rate and flow rate are determined by the modified Boltzmann curve method. The Langmuir probe gives the electron temperature and other plasma properties for the same discharge parameters.

Performance of a Gyratory Square-Shaped Capacitive Radio Frequency Discharge Plasma Sputtering Source for Materials Processing

We proposed a gyratory square-shaped capacitive radio-frequency discharge plasma sputtering source for materials processing and functional film preparation, composed of magnet arrangements consisting of eight neodymium bar magnets of dimensions 30 mm × 5 mm × 3 mm. In order to evaluate its performance, two square-shaped magnetic arrangements were investigated: case (a) without iron shielding and case (b) with iron shielding of dimensions 5 mm × 3 mm × 1 mm. The magnetic field simulation is analyzed, while the plasma discharge characteristics and the film properties are measured. The film thickness and the resistivity profiles of case (b) are more uniform than their corresponding profiles in case (a). The lowest electrical resistivity of the film is 4.33 × 10−8 Ω m at r = 30 mm for case (b), which is of the same order as the bulk resistivity of the copper. The roughnesses of the film thickness profile for cases (a) and (b) are ±24.4 and ±7.2%, respectively. Using atomic force microscopy analysis, the film surface for case (b) was observed to show an improved smooth surface with reduced needle-shaped grain size and a lower surface roughness than that of case (a). The surface roughness of the films is approximately 3.73 and 2.49 nm for case (a) and case (b), respectively. From the X-ray diffraction patterns, the film texture, the relative intensity ratios of the (111) peak to the (200) [I(111)/ I(200)] were found to be 13.76 and 4.08 for the cases (a) and (b), respectively.

Electrical characteristics for capacitively coupled radio frequency discharges of helium and neon

In this study, a symmetric radio frequency (RF) (13.56 MHz) electrode discharge system of simple geometry has been designed and made. The electrical properties of capacitive RF discharge of pure neon and pure helium have been obtained from current and voltage waveforms using different reactor designs. Calculations are done, in detail, according to the homogeneous discharge model of capacitively coupled RF. Electrical properties of bulk plasma and sheath capacitance are also investigated at low pressure using this model.

Fluid modeling of plasma dynamics in pulsed RF capacitive glow discharges in low pressure argon

Based on the drift-diffusive approximation, one-dimensional fluid modeling is carried out for the pulsed RF capacitive glow discharges in low pressure argon. Investigated are the effects of various discharge parameters, such as the duty cycle ratio and frequency of the pulsed modulation, and the material properties of the electrode, on the plasma characteristics such as the electron recombination rate, during both the initial and the steady state phases of the discharge. The modeling results show that, after switching off the applied voltage during the pulsed modulation of the RF discharge, the electron density increases first and then decreases. This phenomenon is particularly pronounced before the discharge reaches steady state. Meanwhile, independent of whether the discharge has reached steady state or not, right after the applied voltage is switched on during each modulation period, the electron and ion densities and the metastable argon atom density, as well as their generation rate, experience a time delay (phase lag) with respect to the applied voltage. The results also show that, at the initial phase of the pulsed modulation, during the steady state discharge, the electron temperature in the center of the bulk plasma is almost not affected by the applied voltage, or by the material properties of the electrode such as the secondary electron emission rate. The electron density, however, does increase with these parameters, resulting in increased power density dissipation of the plasma. On the other hand, at fixed applied voltage, the central electron temperature of the bulk plasma is reduced by increasing several parameters, including the modulation duty ratio, the distance between two electrodes, and the modulation frequency, as well as the electron recombination rate due to different choices of the electrode material. This eventually leads to a reduction of the dissipated power density in the plasma. In particular, with the increase of the modulation duty ratio, the distance between electrodes, or the RF modulation frequency, the electron temperature decays slower after switching off the applied voltage within the modulation period. All the above studies are also accompanied by a systematic investigation of the temporal and spatial distributions of the electron energy conversion during the pulsed RF discharge.

Rotation of a single vortex in dusty plasma**Project supported by the National Natural Science Foundation of China (Grant Nos. 11205044 and 11405042), the Program for Young Principal Investigators of Hebei Province, China, the Natural Science Foundation of Hebei University, China (Grant No. 2011JQ04), the NaturalScience Fund for Excellent Young Scholars of Hebei Province, China(Grant No. A2017201099), and the Midwest Universities

A single vortex is obtained in radio-frequency capacitive discharge in argon gas. The dust subsystem is confined in the horizontal plane with an asymmetrical saw structure placed on the lower electrode. The vortex rotates as a whole along the long side of the saw-teeth. Asymmetry of the saw structure plays an important role in the rotation of the vortex. Nonzero curl of the total force resulting from the local ion flow and the electric field in the plasma sheath could be attributed to the persistent rotation of vortex.

Radio-frequency capacitive discharge with flowing liquid electrodes at reduced gas pressures

Results are presented from experimental studies of the electrophysical and spectral characteristics of the low-temperature plasma of a radio-frequency capacitive discharge excited between two flowing liquid electrodes at gas pressures of 103−105 Pa. The plasma composition, the electron density, and the vibrational and rotational temperatures of gas molecules are estimated. The types and shapes of discharge are described, and the thermal and gas-hydrodynamic processes in the discharge zone are analyzed.

Radio-frequency discharge in a molecular gas flow at a frequency of 1.76 MHz

Results are presented from experiments on the initiation of a homogeneous stable capacitive RF discharge at a frequency of 1.76 MHz in a high-speed molecular gas flow between metal electrodes at a pressure of 5 Torr or dielectric-coated electrodes at a pressure of up to 50 Torr. A mechanism of the current closure in the electrode sheaths related to the primary photoelectric current generated alternately from different electrodes is proposed. The average electron energy increases via second-kind (superelastic) collisions, and fast electrons with energies corresponding to the amplitude value of the RF voltage appear in the electron energy spectrum. As a result, primary emission arises due to the excitation of emitting states from metastable molecular levels. The primary photoelectric current initiates electron avalanches in the electrode sheaths due to secondary photoelectron emission. According to calculations, the current−voltage characteristic of the sheath in this type of discharge is ascending and the field strengths in the electrode sheath and positive column are lower than those in a self-sustained dc discharge. The calculated results are compared with the experimental data.

Ionization instability induced striations in atmospheric pressure He/H2O RF and DC discharges

One-dimensional particle-in-cell (PIC) simulations of a 1 mm gap atmospheric pressure He/2% rf capacitive discharge showed standing striations in the bulk (Kawamura et al 2016 Plasma Sources Sci. Technol. 25 054009). We found that these striations were consistent with an ionization instability induced by non-local electron kinetics. We developed a theoretical instability criterion in good agreement with the numerical results which showed that discharges with larger bulk recombination rates tend to be more unstable. We also determined a critical wavelength such that shorter wavelengths are suppressed by diffusion while longer wavelengths may be restricted by the gap width. In this paper, we extend the gap size of the atmospheric pressure He/2% discharges in the PIC simulations to 2 and 4 mm and drive them by either dc or rf current sources. We compare the results to the 1 mm gap rf simulations and theoretical model in Kawamura et al (2016 Plasma Sources Sci. Technol. 25 054009). We find that wider gap discharges tend to be more unstable as they can accommodate a wider range of wavelengths. Furthermore, the mixture of the various excited modes in the wider gaps can lead to distinctly non-sinusoidal spatial oscillations.

Spectroscopic study and numerical simulation of low-pressure radio-frequency capacitive discharge with argon downstream

In this study, the characteristic properties and plasma parameters of capacitive radio frequency (RF) argon (Ar) discharge and supplementary discharge at low pressure are investigated with optical emission spectroscopy (OES). The wavelengths of spectral lines from OES are obtained between 650–900 nm. Using OES lines and related experimental data, the electron temperatures for different RF power, flow, and measurement periods are determined. Eventually, the properties of plasma including the electron temperature are identified. In addition, the numerical modelling of the discharge parameters is carried out by using the “extended” fluid model for plasma, and consistency between numerical and experimental results is discussed.

Modification of carbon fabrics by radio-frequency capacitive discharge at low pressure to regulate mechanical properties of carbon fiber reinforced plastics based on it

To increase the values of mechanical properties of carbon fiber (CF) composite materials used in sports equipment production the method of radio-frequency capacitive (RFC) low-pressure plasma treatment in air was proposed. Previously it was found that this type of modification allows to effectively regulate the surface properties of fibers of different nature. This treatment method differs from the traditional ones by efficiency and environmental friendliness as it does not require the use of aggressive, environmentally hazardous chemicals. In this paper it was established that RFC low-pressure air plasma treatment of carbon fabrics enhances the interlaminar shear strength (ILSS) of carbon fiber reinforced plastic (CFRP). As a result of experimental studies of CF by Fourier Transform Infrared (FTIR) spectroscopy method it was proved that after radio-frequency capacitive plasma treatment at low pressure in air the oxygen-containing functional groups is grafted on the surface. These groups improve adhesion at the interface “matrix-fiber”.

Determination of plasma density from data on the ion current to cylindrical and planar probes

To improve probe methods of plasma diagnostics, special probe measurements were performed and numerical models describing ion transport to a probe with allowance for collisions were developed. The current–voltage characteristics of cylindrical and planar probes were measured in an RF capacitive discharge in argon at a frequency of 81 MHz and plasma densities of 1010–1011 cm–3, typical of modern RF reactors. 1D and 2D numerical models based on the particle-in-cell method with Monte Carlo collisions for simulating ion motion and the Boltzmann equilibrium for electrons are developed to describe current collection by a probe. The models were used to find the plasma density from the ion part of the current–voltage characteristic, study the effect of ion collisions, and verify simplified approaches to determining the plasma density. A 1D hydrodynamic model of the ion current to a cylindrical probe with allowance for ion collisions is proposed. For a planar probe, a method to determine the plasma density from the averaged numerical results is developed. A comparative analysis of different approaches to calculating the plasma density from the ion current to a probe is performed.

Mathematical simulation of a low-pressure capacitive RF discharge in an external radial magnetic field using the KARAT code

A high-frequency capacitive discharge is simulated in the geometry of a plasma accelerator with closed electron drift. It has been shown that, in such a discharge, as in a dc discharge, an azimuthal electron drift takes place and a potential drop is formed at the discharge channel exit, which leads to the emergence of an accelerated ion beam from the channel.

Hybrid global model of water cluster ions in atmospheric pressure Ar/ H2O RF capacitive discharges

Water is a trace gas of strong interest for plasma-based medical applications. We use a hybrid global model to simulate a chemically complex Ar/ atmospheric pressure, radio frequency capacitive discharge, including 47 species with positive ion clusters up to . For a discharge gap of 1.5 mm driven at 27.12 MHz, we determine the discharge properties over a range of rf currents (150–500 A m−2) and initial concentrations (0.25–2%). An isothermal plug-flow model is used with a gas residence time of 0.2 s for most calculations, with the gas temperature calculated self-consistently from the input power. The cluster density distributions are determined, and we find that the higher mass cluster densities decrease rapidly with increasing gas temperature. A simplified cluster dynamics analytic model is developed and solved to determine the cluster density distributions, which is in good agreement with the hybrid simulation results.

Standing striations due to ionization instability in atmospheric pressure He/H2O radio frequency capacitive discharges

One-dimensional particle-in-cell (PIC) simulations of a narrow gap atmospheric pressure He/2% radio frequency capacitive discharge showed standing striations in the bulk plasma region while previously conducted PIC simulations of a narrow gap atmospheric pressure He/0.1% discharges [] showed no such instabilities. We successively modified the base He/ chemistry to make it more similar to the He/ chemistry in order to determine the cause of the striations. Setting the e– scattering, attachment, vibrational and rotational excitation rates to zero did not suppress the striations. However, a systematic reduction of the e–ion recombination cross section resulted in a transition to a stable state with no striations. The results are interpreted in terms of a model in which the balance between bulk direct ionization and bulk recombination loss determines the bulk plasma equilibrium. Perturbing the equilibrium, we find that the striations are consistent with an ionization instability induced by non-local electron kinetics that form a spatially-varying high energy tail of the electron energy distribution, causing the ionization rate coefficient to decrease with increasing electron temperature Te and root-mean-square electric field E in the instability regime.

Electron power absorption dynamics in capacitive radio frequency discharges driven by tailored voltage waveforms in CF4

The power absorption dynamics of electrons and the electrical asymmetry effect in capacitive radio-frequency plasmas operated in CF4 and driven by tailored voltage waveforms are investigated experimentally in combination with kinetic simulations. The driving voltage waveforms are generated as a superposition of multiple consecutive harmonics of the fundamental frequency of 13.56 MHz. Peaks/valleys and sawtooth waveforms are used to study the effects of amplitude and slope asymmetries of the driving voltage waveform on the electron dynamics and the generation of a DC self-bias in an electronegative plasma at different pressures. Compared to electropositive discharges, we observe strongly different effects and unique power absorption dynamics. At high pressures and high electronegativities, the discharge is found to operate in the drift-ambipolar (DA) heating mode. A dominant excitation/ionization maximum is observed during sheath collapse at the edge of the sheath which collapses fastest. High negative-ion densities are observed inside this sheath region, while electrons are confined for part of the RF period in a potential well formed by the ambipolar electric field at this sheath edge and the collapsed (floating potential) sheath at the electrode. For specific driving voltage waveforms, the plasma becomes divided spatially into two different halves of strongly different electronegativity. This asymmetry can be reversed electrically by inverting the driving waveform. For sawtooth waveforms, the discharge asymmetry and the sign of the DC self-bias are found to reverse as the pressure is increased, due to a transition of the electron heating mode from the α-mode to the DA-mode. These effects are interpreted with the aid of the simulation results.

Preparation of Silicon Thin Films of Different Phase Composition from Monochlorosilane as a Precursor by RF Capacitive Plasma Discharge

Monochlorosilane/argon/hydrogen (SiH3Cl-Ar-H2) mixture of different ratios was investigated from the point of PECVD application. RF capacitive plasma discharge of 40.68 MHz frequency was used. The process of deposition was studied by optical emission spectroscopy. The silicon thin films of different phase composition were obtained. The thin films were characterized by Raman-spectroscopy, atomic force microscopy, and secondary ion mass spectrometry. The exhaust gas mixture was analyzed by IR-spectroscopy in outlet of the reactor during PECVD process. The chemical mechanism for the deposition process was also proposed.

Capacitive radio frequency discharges with a single ring-shaped narrow trench of various depths to enhance the plasma density and lateral uniformity

Spatial structures of the electron density and temperature in ring-shaped hollow cathode capacitive rf plasma with a single narrow trench of 2 mm width have been investigated at various trench depths of D = 5, 8, 10, 12, and 15 mm. It is found that the plasma density is increased in the presence of the trench and that the radial profile of the plasma density has a peak around the narrow hollow trench near the cathode. The density becomes uniform further away from the cathode at all trench depths, whereas the electron temperature distribution remains almost uniform. The measured radial profiles of the plasma density are in good agreement with a theoretical diffusion model for all the trench depths, which explains the local density increase by a local enhancement of the electron heating. Under the conditions investigated, the trench of 10 mm depth is found to result in the highest plasma density at various axial and radial positions. The results show that the radial uniformity of the plasma density at various axial positions can be improved by using structured electrodes of distinct depths rather than planar electrodes.

Radial profile of the electron energy distribution function in RF capacitive gas-discharge plasma

This paper reports experimental results on low-pressure argon capacitive RF discharge (parallel-plate capacitively-coupled plasma - CCP) under different conditions, namely, gas pressure in the range 3 -r- 30 Pa and RF power in the range 10 - 100 W. The IV characteristics measured were processed by two different second-derivative probe techniques for determination of the plasma parameters and the electron energy distribution function. The radial profiles of the main plasma parameters are presented.

The influence of electron reflection/sticking coefficients at the electrodes on plasma parameters in particle-in-cell simulations of capacitive radio-frequency plasmas

Capacitive radio-frequency discharges are frequently used to process different materials. In these systems, plasma-surface interactions are known to affect the discharge by particle emission and reflection. In simulations, the corresponding surface coefficients are input parameters, which are often unknown and are, therefore, roughly estimated or ignored. Electron reflection at boundary surfaces is typically either neglected completely or the reflection coefficient, , where S is the sticking coefficient, is assumed to be small, independently of the surface material and its conditions, although it is known to cover a wide range depending on the material. Here, we systematically investigate the effect of changing ρ in particle-in-cell simulations on plasma parameters such as the plasma density, electric field, and ionization rates, in geometrically symmetric single- and dual-frequency discharges operated in argon at a fundamental frequency of 13.56 MHz and at pressures of 5 Pa–20 Pa. We find that the plasma density strongly depends on the reflection coefficient. High coefficients cause electric field reversals during sheath collapse at the electrodes and an enhanced generation of energetic electron beams during sheath expansion, which lead to additional ionization and higher plasma densities. Different reflection coefficients at both electrodes are found to induce a discharge asymmetry that leads to the generation of a DC self-bias and different mean ion energies at the electrodes. In dual-frequency discharges, the electrical generation of the DC self-bias as a function of the phase between two consecutive driving harmonics via the electrical asymmetry effect can be significantly enhanced by choosing electrode materials with different reflection coefficients. In this way the electrical control range of the mean ion energy via phase control is shifted to different energies.

The effects of low-temperature plasma treatment on the capillary properties of inorganic fibers

Solving the problem of achieving high adhesion between the components in the polymeric composite material (PCM) based on carbon fibers (CF) and basalt fibers (BF) is proposed to use the radio-frequency (RF) plasma under lower pressure by virtue of efficiency, environmental friendliness and rationality of the method. The paper gives the results of studies of the properties of CF and BF after RF capacitive discharge plasma treatment. The plasma modification modes of carbon and basalt fiber were investigated. The efficiency of treatment tool in surface properties modification of carbon and basalt fibers was found, namely capillary properties of CF and BF were researched. The optimal treatment modes were selected. It was found that the method of plasma modification in the radio-frequency capacitive discharge under the lower pressure contributes enhancing the capillary properties of inorganic fibers, in particular carbon and basalt ones. It shows the tendency to increase of the adhesive properties in PCM, and, consequently, the increase of the physical and mechanical properties of the products.