Radio-frequency Discharge In Argon Research Articles

Experimental validation of the analytic model for the temporal decay of the density auto-correlation function in a strongly coupled dusty plasma

In a recent theoretical work [Dhaka et al. Sci. Rep. 12, 21883 (2022)], the method of determining the transport coefficients of a system from the time dynamics of the density auto-correlation function (DAF) was extended to complex plasma systems using the framework of a generalized hydrodynamics model. An exact analytical form of the density auto-correlation function of the thermal level spontaneous fluctuations of a Yukawa system was obtained. In the present work, we provide the first experimental validation of this analytical model for a strongly coupled dusty plasma system. The dusty plasma is produced by introducing micron-sized melamine formaldehyde particles in radio frequency argon discharges, and the DAF of the spontaneous dust density fluctuations is determined by optically tracking the trajectories of the dust particles. The experimentally obtained DAF is found to show a trend that is consistent with our earlier theoretical and numerical predictions. It is further used to determine the microscopic rate of heat diffusion for various values of the fluctuation wave-number k and obtain an extrapolated value of the macroscopic heat diffusion rate in the limit k →0. The experimental validation lends strong support to our generalized theoretical model, which can be usefully employed now in a variety of strongly coupled systems.

Numerical study on dust particle charging and dynamics in continuous and pulsed radio frequency argon discharges

AbstractCharging and dynamics of a spherical dust grain injected into a continuous and pulsed radio frequency (RF) discharge have been studied using a one‐dimensional fluid model. First, the plasma characteristics of the two types of discharges are computed and compared. In the pulsed discharge, it is found that the central electron density exhibits a periodic variation while the averaged electron density is lower compared to that in the continuous discharge due to the decrease in the total ionization rate. Further, the dust charge is computed using the plasma characteristics. It is found that the dust charge negatively increases as the duty cycle ratio increases. Also, the charge in the pulsed discharge is lower in comparison to the continuous discharge due to the shorter duration of the pulsed RF discharge limiting the amount of energy transferred to electrons. On the other hand, the dust particle remains in the powered sheath region exhibiting a damped oscillation in the two discharges with higher oscillation frequency in the continuous discharge.

Experimental Investigation on the Effect of “Linear-Rise” Amplitude Modulation on a Pulsed Capacitively Coupled Radio-frequency Argon Discharge

Objective: This work focused on the effect of “linear-rise” amplitude modulated waveform on the time evolution of some typical electrical and plasma parameters in a pulsed radio-frequency capacitively coupled argon discharge. Methods: The pulse-on phase of a square wave pulse is divided into two distinct stages, namely, the “linear rise” phase (T1 phase) and “constant amplitude” phase (T2 phase). The study focused on exploring the impact of varying T1 durations on the temporal evolutions of these parameters. In the experiment, the electron density was measured time-resolved by a hairpin probe. Phase-resolved optical emission spectroscopy was employed to determine the spatial-temporal distribution of the electron impact excitation dynamics. Additionally, we determined the amplitudes and the relative phase of the discharge voltage and current, as well as the power deposition by analyzing the waveforms obtained from a voltage and a current probe. Results: It was found that with the increase of T1, the critical voltage value required for the plasma ignition becomes lower, and the RF power and the light intensity overshoot less significantly. Upon the overshoot, the plasma when the light intensity exhibits the maximum is dominated by the “overshoot” mode. After the pulse is turned off, the electron density under different T1 durations first decreases with the same rate, and then the decay rate is decreased with the increase of T1. This is because for a longer T1, the neutral gas temperature gets lower, leading to a higher density of neutral gas, so the electron diffusion loss is suppressed. Conclusion: Compared to the square pulse modulated radio frequency capacitively coupled plasmas, the “linear rise” amplitude modulation approach introduces external control knobs to modulate the plasma parameters, which benefits practical material processing. This paper is an experimental measurement implemented in an electropositive gas Ar discharge, and is expected to be extended to the plasmas operated in complex electronegative gases in the future.

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.

Experimental investigation of the radially-dependent ignition process in a pulsed capacitively coupled RF discharge: Effects of pressure, voltage and afterglow duration

The effects of the gas pressure, the voltage amplitude and the afterglow duration on the ignition process over 300 mm-diameter in a pulse-modulated capacitively coupled radio-frequency argon discharge are experimentally investigated. A home-made optical probe is used to measure the optical emission intensity (OEI) as a function of time since the beginning of each pulse at various radial positions. A voltage and a current probe are used to measure the voltage and current waveforms at the power feeding point and then the time-dependent power deposition is also obtained. It was found that the radial profile of the OEI exhibits complex evolution when changing these external conditions. At lower pressures, the ignition occurs earlier, and the radial-integral OEI and the power deposition overshoot more slightly during the ignition. By increasing the pressure, these two quantities overshoot more significantly, and the OEI gradually evolves from an edge-peaked radial profile to a center-high radial profile for a given time when the OEI increases rapidly during the ignition. When increasing the voltage amplitude, the ignition tends to occur earlier, featuring a more significant overshoot of the OEI and power deposition at a higher voltage. Compared to that at high voltage, the OEI exhibits a significant center-high radial profile at low voltage amplitude during the ignition. By increasing the afterglow duration, T off, the ignition is delayed and the overshoot becomes more significant, due to a lower initial electron density when each pulse is turned on. During the phase when the growth rate of the OEI versus time is maximum, the OEI exhibits different radial distributions, i.e., it appears a center-high profile at short T off, an edge-peaked profile at intermediate T off, and a center-high profile at relatively long T off.

Two-dimensional complex (dusty) plasma with active Janus particles

A two-dimensional complex plasma containing active Janus particles was experimentally studied. A single layer of micrometer-sized plastic microspheres was suspended in the plasma sheath of a radio frequency discharge in argon at low pressure. The particle sample used was a mixture of regular particles and Janus particles, which were coated on one side with a thin layer of platinum. Unlike a suspension consisting of regular particles only, the suspension with the inclusion of Janus particles did not form an ordered lattice in the experimental conditions used. Instead, the particles moved around with high kinetic energy in a disordered suspension. Unexpectedly, the mean kinetic energy of the particles declined as the illumination laser power was increased. This is explained by the competition of two driving forces: the photophoretic force and the oppositely directed ion drag force. The mean-squared displacement of the particles scaled as tα with α = 2 at small times t indicating ballistic motion and α=0.56 ± 0.27 at longer times due to the combined effect of the Janus particle propensity to move in circular trajectories and external confinement.

Stability of two-dimensional complex plasma monolayers in asymmetric capacitively coupled radio-frequency discharges.

In this article, the stability of a complex plasma monolayer levitating in the sheath of the powered electrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Compared to earlier studies, a better integration of the experimental results and theory is achieved by operating with actual experimental control parameters such as the gas pressure and the discharge power. It is shown that for a given microparticle monolayer at a fixed discharge power there exist two threshold pressures: (i) above a specific pressure p_{cryst}, the monolayer always crystallizes; (ii) below a specific pressure p_{MCI}, the crystalline monolayer undergoes the mode-coupling instability and the two-dimensional complex plasma crystal melts. In between p_{MCI} and p_{cryst}, the microparticle monolayer can be either in the fluid phase or the crystal phase: when increasing the pressure from below p_{MCI}, the monolayer remains in the fluid phase until it reaches p_{cryst} at which it recrystallizes; when decreasing the pressure from above p_{cryst}, the monolayer remains in the crystalline phase until it reaches p_{MCI} at which the mode-coupling instability is triggered and the crystal melts. A simple self-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges, and the microparticle resonance frequency as a function of power and background argon pressure. Combined with calculation of the lattice modes the main trends of p_{MCI} as a function of power and background argon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phase is dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane lattice mode induced by the change of the sheath profile. Ion wakes are shown to have a significant effect too.

Fluid modeling on effects of discharge parameters on ionization in capacitive radio frequency argon discharges at low pressure

Based on plasma fluid theory, a one-dimensional fluid model on capacitive radio frequency argon discharges at low pressure is established to study effects of the discharge parameters on the ionization in the discharges. The model can include the ground state excitation and is solved numerically by a finite difference method for the cases of including and without ground state excitation. The numerical results are compared and indicate that the ground state excitation is significant and can not be ignored in the electron energy equation in the model. Furthermore, for the steady state of the discharges, some characteristics in the case of including the ground state excitation for different discharge parameters are discussed. The analysis shows that, when the gap between the parallel electrodes increases from 2.1 cm to 2.9 cm, the peak value of the ionization rate decreases slightly; with the neutral gas pressure increasing from 60 mTorr to 140 mTorr, the peak value of the ionization rate increases; the peak value of the ionization rate increases as the peak voltage increasing from 60 V to 140 V. The conclusion can be commendably to understand the ionization dynamics in the discharges.

Comprehensive understanding of the ignition process of a pulsed capacitively coupled radio frequency discharge: the effect of power-off duration

The effect of the pulse-off duration on the time evolution of the plasma and electrical parameters during the ignition phase in a pulsed capacitively coupled radio frequency argon discharge operated at 450 mTorr and 12.5 MHz is investigated synergistically by multifold experimental diagnostics, particle-in-cell/Monte Carlo collision simulations and an analytical model. In the experiment, the electron density is measured time-resolved by a hairpin probe, the spatio-temporal distribution of the electron impact excitation dynamics is studied by phase resolved optical emission spectroscopy, and the amplitudes and the relative phase, φ vi, of the discharge voltage and current are determined based on the waveforms measured by a voltage and a current probe. The experimental results show that the plasma and electrical parameters during the ignition process depend strongly on the duration of the afterglow period, T off, primarily because of the dependence of the remaining charge density on this parameter. Computed values of φ vi show a similar time-dependence compared to the experiment, if the simulations are initialized with specific initial charged particle densities, n ini. This allows us to further understand the time evolution of φ vi for different values of T off based on the simulation results together with an analytical model. In particular, the optical emission intensity is found to change with time in the same fashion as the power deposition into the system at T off ⩾ 100 μs, suggesting that the power is primarily absorbed by the electrons, which dissipate their energy via inelastic collisions. The system goes through different mode transitions of electron power absorption during the ignition phase depending on T off. Specifically, for short T off (high n ini), the α mode dominates during the entire ignition process, as the electric field is largely shielded by the abundant charge located in the interelectrode space. For intermediate values of T off (moderate n ini), another excitation pattern caused by an enhanced drift electric field at the center of the gap is observed, since a large fraction of the externally applied potential can penetrate into the central region in the absence of high charged particle densities. For longer T off (very low n ini), the ignition of the pulsed plasma behaves like a gas breakdown.

Effect of driving frequency on dust particle dynamics in radiofrequency capacitive argon discharge

This paper investigates the influence of the driving frequency on the dynamics of a single dust particle in argon radiofrequency discharge. A one-dimensional fluid model is presented and solved in the entire inter-electrode domain using the finite difference method. In order to solve the particle equation of motion, the coefficients describing the amplification and the damping of the dust particle oscillations are analytically calculated around the equilibrium position, these coefficients allow us to find the relation between the plasma and dust parameters. The results obtained cover the discharge characteristics, the charge and the dynamics of the dust particle. It has been found that the driving frequency has a significant effect on not only the discharge properties but also on the damped oscillatory motion and the equilibrium position of the dust particle. Hence, these oscillations become closer to the electrodes with increasing driving frequency whereas the dust equilibrium position becomes relatively farther from the powered electrode when the dust size decreases.

Active Janus particles in a complex plasma

Active Janus particles suspended in a plasma were studied experimentally. The Janus particles were micron-size plastic microspheres, one half of which was coated with a thin layer of platinum. They were suspended in the plasma sheath of a radio-frequency discharge in argon at low pressure. The Janus particles moved in characteristic looped trajectories suggesting a combination of spinning and circling motion; their interactions led to the emergence of rich dynamics characterized by non-Maxwellian velocity distribution. The particle propulsion mechanism is discussed, the main force driving the particle motion is identified as photophoretic force.

Observation of dominant Ohmic electron power absorption in capacitively coupled radio frequency argon discharges at low pressure

We present a spatio-temporally resolved analysis of electron power absorption in capacitively coupled argon plasmas at low pressures (1–10 Pa), based on the 1D momentum balance equation embedded into 1d3v particle-in-cell/Monte Carlo collisions simulations. In contrast to the predictions of theoretical models we find ‘Ohmic heating’ to be the dominant electron power absorption mechanism on time average at the lowest pressures, and not ‘stochastic’ or ‘Pressure heating’. The cause for this is identified to be the attenuation of electron power absorption due to electron acceleration by the ‘ambipolar’ electric field on time average at low pressure, which is a consequence of the collisionless transit of energetic beam electrons generated during sheath expansion at one electrode to the opposite electrode. At such conditions, these energetic electrons arrive during the local sheath collapse and can be lost to the surface, thereby reducing the plasma density and creating a temporally more symmetric electron temperature within the radio frequency (RF) period compared to that in discharges operated at higher pressures. The more symmetric temperature profile causes a reduction of ‘Pressure heating’ on time average. The latter is reduced further, even to negative values, by the attenuation of the ‘ambipolar’ electric field at each electrode during the local sheath collapse, which is a consequence of the temporal modulation of the electron density profile within the RF period, observed at the lowest pressures studied.

Influence of metastable atoms in low pressure magnetized radio-frequency argon discharges

One-dimensional particle-in-cell simulations with Monte Carlo collisions are used to investigate the influence of metastable atoms in low pressure radio-frequency argon discharges with magnetic fields ranging from 0 G to 60 G. Two metastable levels of argon species are included and tracked as particles, enabling multistep ionization and metastable pooling. At low magnetic fields, the metastable argon atoms have little influence on the discharge. At higher magnetic fields, the electron density increases and the electron temperature decreases at the center of the discharge with the inclusion of metastable atoms. The reduction in electron temperature is attributed to the depletion of energetic electrons due to low-energy-threshold reactions related to the metastable atoms. The reduced electron temperature leads to a reduction in the total ionization rate, albeit the contribution of multistep ionization increases with the magnetic field. The suppression of plasma diffusion at low electron temperatures plays a greater role than the reduction in ionization rate, resulting in a higher electron density. A transformation in metastable density profiles from parabolic to saddle type is observed with the increase in the magnetic field. Metastable atoms may play an important role in modulating the electron temperature in low pressure magnetized discharges.

Electrical Characteristics of Capacitive Coupled Radio Frequency Discharges in Argon and Hydrogen

A symmetric radio frequency (RF) (13.56 MHz) electrode discharge system with different geometry has been investigated at low pressure. All electrical properties of symmetric capacitive RF discharge in pure argon (Ar) and pure hydrogen (H2) have been obtained from the current and voltage waveforms. Here, the changes of voltage and current were investigated according to external electrical discharge parameters. The electron densities of these discharges were measured with a single Langmuir probe. A homogeneous model of capacitive coupled RF (CCRF) discharge was used to determine the electrical properties of a parallel-plate RF discharge system and their dependences on the input RF power and gas pressure .

Simple experiment on the sputtering rate of solids in gas discharges

We present a very simple and sensitive method to measure the sputtering rate of solid materials in stationary low-pressure gas discharges. The method is based on the balance of the centrifugal force and the confinement electric force acting on a single electrically charged dust particle in a rotating environment. We demonstrate the use and sensitivity of this method in a capacitively coupled radio frequency argon discharge. We were able to detect a reduction of 10 nm in the diameter of a single dust particle.

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.

Mode transition and hysteresis in inductively coupled radio frequency argon discharge

This contribution presents experimental results about the mode transition of an inductively coupled radio frequency (RF) (13.56 MHz) argon discharge at different total gas pressures. In particular, the positive ion saturation current and the line integrated electron density are measured by Langmuir probe and 160 GHz microwave interferometer, respectively. The mode transition strongly depends on the total gas pressure and can appear stepwise or continuously. The space resolved positive ion saturation current is separately shown for the E- and H-mode at different total gas pressures. Therewith, the pressure dependency of the RF sheath thickness indicates a collisional sheath. The hysteresis phenomenon during the E-H and the inverse H-E transition is discussed within the framework of the matching situation for different total gas pressures. The hysteresis width is analyzed using the absorbed power as well as the coil voltage and current. As a result, the width strongly increases with pressure regarding the power and the coil voltage in the E-mode and remains constant in the H-mode. In addition, the phase shift between the coil voltage and current shows a hysteresis effect, too.

Synthesis of amorphous hydrogenated carbon thin films by magnetized radio-frequency discharge in argon–acetylene mixture at very low gas pressure

This paper presents a characterization study of carbon thin films grown on crystalline silicon substrates using a magnetized high-frequency discharge in argon–acetylene mixture at very low pressure. Thin films with different morphological structure are obtained. Depending on the gas pressure, acetylene percentage and process time, it is possible to categorize the surface film morphology into three categories: smooth, cracked or microstructured. Moreover, when applying external magnetic field, it has been observed that depending on the substrate direction (perpendicular or parallel) to the reactor axis, it is possible to obtain different film morphologies. For specific conditions, energetic argon ions are formed which lead to film surface damaging and to their inclusion when they impinge the film surface. Chemical pathways and most likely isomers that contribute to the growth are also identified and discussed. Synthesizing materials in such low pressure conditions is of particular interest for better understanding the complex phenomena taking place in the plasma such as instabilities induced by particles and infra-red spectra of carbonaceous interstellar dust.

Langmuir Probe Diagnostics of Low-pressure Inductively Coupled Argon Plasmas in a Magnetic Field

Plasma parameters of an inductively coupled radio frequency discharge in argon in an external axial magnetic field have been investigated at working pressures of 5×10−4–1×10−2 mbar and magnetic field of 0–80 G. Axial distributions of plasma density and electron temperature have been derived from Langmuir probe measurements. Plasma density monotonically increased with the increase in magnetic field at the highest pressure (1×10−2 mbar), whereas at lower pressures it reached its maximum (∼ 3×1011 cm−3) at a B-field ∼ 19 G. Plasma density was almost independent of the working pressure under low pressure regimes, which means that the degree of plasma ionization increased with the decrease in pressure reaching several percent at 5×10−4 mbar.

Stability and size of particle pairs in complex plasmas

Particle pairing in a complex plasma was experimentally studied with the emphasis on pair spatial extent and stability. Micron-size particles were suspended in the (pre)sheath area above the lower electrode in a capacitively coupled radio-frequency discharge in argon. They formed vertical pairs due to the ion wakes created by the flow of ions past particles. We discuss the confinement mechanism for the lower particle, resulting from a combination of the wake field and the field of non-uniform sheath. A model of particle pairs is proposed, which provides good description for the dependence of pair size and stability on experimental parameters.