Capacitive Argon Discharge Research Articles

The effects of magnetic field and negative DC voltage on the capacitive argon discharge

A one-dimensional implicit particle-in-cell/Monte Carlo collision simulation was employed to study the effects of a uniform weak magnetic field and negative direct-current (DC) voltage on a radio frequency capacitively coupled plasma (CCP). The simulation results indicate that the application of a magnetic field to RF/DC hybrid power-driven CCP discharge can increase the plasma density and cause it to exhibit an asymmetric distribution. When the magnetic field strength increases, pronounced striations can be observed within the DC sheath in the spatiotemporal plots of an electron heating rate and an ionization rate. This is attributed to the generation of a large number of secondary electrons by the DC electrode. These secondary electrons are accelerated by the sheath voltage and undergo E × B drift motion. When the energy of these electrons reaches the ionization threshold of an argon gas, ionization occurs. At this point, the electrons are still situated within the DC sheath, and hence, they repeatedly undergo this process until they exit the DC sheath. Additionally, the electron energy distribution function reveals that an increase in a magnetic field can cause a transition from stochastic heating to ohmic heating. The simulation results of magnetized CCP discharge under the influence of negative DC voltage show that increasing negative DC voltage can effectively improve plasma density. The application of negative DC voltage and magnetic field strength has similar effects on the heating stripe phenomenon. As the negative DC voltage increases, the striation phenomenon becomes more pronounced.

Influence of magnetic field gradient on the capacitive argon discharge at 8 MHz and 40 MHz

A one-dimensional implicit particle-in-cell/Monte Carlo collision model is used to study the effects of magnetic field gradients on the capacitively coupled argon plasma at 8 MHz and 40 MHz. The magnetic field strength at the powered electrode is fixed at 10 G, while varies from 30 to 100 G at the grounded electrode. The simulations show that the magnetic field with variable gradient can produce controllable asymmetry in the plasma density and ion flux profiles to each electrode. Increasing the magnetic field gradient will generate a significant dc self-bias, which results in a large ion bombardment energy at the powered electrode. The magnetic field gradients have been demonstrated to be an approach to create the dc self-bias and also effectively improve the plasma density. It is also found that at a higher frequency of 40 MHz, the dc self-bias voltage decreases, due to the fact that high collision rate of electrons with background gas will disturb the cyclotron motion of electrons, so the effect of the magnetic field is weakened. As a result, the ability to independently control ion energy and flux is weakened.

Plasma parameters and tungsten sputter rates in a high-frequency CCP

Monitoring and investigating the fusion plasma in ITER will be crucial to pave the way to a fusion power plant. However, the harsh conditions in the vacuum vessel are detrimental for the optical diagnostics systems. Replacing the element with a direct line of sight to the fusion plasma by a metallic mirror shifts the problem to this component. The flux of impurities onto these mirrors accumulate to deposits, which degrade their optical properties over time. It has been proposed to address this issue by igniting discharges in front of the mirrors during the maintenance phases allowing the deposited material to be sputtered away and recover the mirror properties. To further the knowledge for such an option, in this work, plasma parameters and sputter rates in a high-frequency (60 MHz) capacitive discharge in argon at pressures below 10 Pa are studied. The powered electrode consists of tungsten as a cheap rhodium proxy—the material of the metallic mirrors in ITER—and to simulate tungsten deposition. Its size is equivalent to a mirror for charge-exchange recombination spectroscopy at ITER (8.5 cm × 18 cm). The discharge is studied using and interpreting voltage measurements, microwave interferometry, electrical probe measurements, and optical emission spectroscopy. These investigations provide the opportunity to identify the optimal conditions for the process based on various requirements, such as damage threshold of the mirror crystal and severity and type of contamination.

Nonlinear transmission line (NTL) model study of electromagnetic effects in high-frequency asymmetrically driven capacitive discharges

In this work, we generalize a nonlinear transmission line (NTL) model introduced in a previous work [Liu et al. Plasma Sources Sci. Technol. 30, 045017 (2021)] to achieve more comprehensive simulations. The improved NTL model allows a variable-sized (instead of a one grid cell) vacuum spacer between the powered and grounded electrodes and takes into account the (previously neglected) electron-neutral elastic collision frequency term in the plasma dielectric constant, and the radial variation of the plasma density. Using this model, we study the effects of the spatial and series resonances, associated with the (axially) z-antisymmetric and z-symmetric radially propagating surface wave modes, and the nonlinear harmonic excitations on the plasma uniformity. We conduct simulations in which we increase the driving frequency f from 30 to 120 MHz for an asymmetric capacitive argon discharge at a fixed pressure and electron power of pg=20 mTorr and Pe=40 W, respectively. The first antisymmetric mode resonance frequency fa1 occurs between 80 and 90 MHz, and the first symmetric mode resonance frequency fs1 occurs at about 100 MHz. The powered electrode sheath becomes smaller than the grounded electrode sheath for f≈fa1, and the source voltage reaches a minimum for f≈fs1. The radially varying electron power density shows a narrow center peak due to the finite mode wavelengths and the nonlinearly excited harmonics, which can be further enhanced by spatial or series resonances. A second peak appears above the vacuum spacer edge as f exceeds fs1 at 120 MHz because the shorter wavelengths at higher f allow secondary maxima of the surface waves to form within the reactor.

Surface effects in a capacitive argon discharge in the intermediate pressure regime

One-dimensional particle-in-cell/Monte Carlo collisional simulations are performed on a capacitive 2.54 cm gap, 1.6 Torr argon discharge driven by a sinusoidal rf current density amplitude of 50 A m−2 at 13.56 MHz. The excited argon states (metastable levels, resonance levels, and the 4p manifold) are modeled self-consistently with the particle dynamics as space- and time-varying fluids. Four cases are examined, including and neglecting excited states, and using either a fixed or energy-dependent secondary electron emission yield due to ion and/or neutral impact on the electrodes. The results for all cases show that most of the ionization occurs near the plasma-sheath interfaces, with little ionization within the plasma bulk region. Without excited states, secondary electrons emitted from the electrodes are found to play a strong role in the ionization process. When the excited states, and secondary electron emission due to neutral and ion impact on the electrodes are included in the discharge model, the discharge operation transitions from α-mode to γ-mode, in which nearly all the ionization is due to secondary electrons. Secondary electron production due to the bombardment of excited argon atoms was approximately14.7 times greater than that due to ion bombardment. Electron impact of ground state argon atoms by secondary electrons contributes about 76% of the total ionization; primary electrons, about 11%; metastable Penning ionization, about 13%; and multi-step ionization, about 0.3%.

Simulation of nonlinear standing wave excitation in very-high-frequency asymmetric capacitive discharges: roles of radial plasma density profile and rf power

It is recognized that in large-area, very-high-frequency capacitively coupled plasma (VHF CCP) reactors, the higher harmonics generated by nonlinear sheath motion can lead to enhanced standing wave excitation. In this work, a self-consistent electromagnetic model, which couples a one-dimensional, radial nonlinear transmission line model with a bulk plasma fluid model, is employed to investigate the nonlinear standing wave excitation in a VHF driven, geometrically asymmetric capacitive argon discharge operated at low pressure. By considering a radially non-uniform plasma density profile (case I) calculated self-consistently by the nonlinear electromagnetic model and the corresponding radially-averaged, uniform plasma density profile (case II), we first examine the effect of the plasma density non-uniformity on the propagation of electromagnetic surface waves in a 3 Pa argon discharge driven at 100 MHz and 90 W. Compared to case II, the higher plasma density at the radial center in case I determines a higher plasma series resonance frequency, yielding stronger high-order harmonic excitations and more significant central peak in the harmonic current density J z,n and the harmonic electron power absorption p n profiles. Therefore, under the assumption of the radially uniform plasma density in a CCP discharge, the self-excitation of higher harmonics at the radial center should be underestimated. Second, using the self-consistent electromagnetic model, the effect of the rf power on the excitation of nonlinear standing waves is investigated in a 3 Pa argon discharge driven at 100 MHz. At a low power of 30 W, the discharge is dominated by the first two harmonics. The higher harmonic excitations and the nonlinear standing waves are observed to be enhanced with increasing the rf power, resulting in a more pronounced central peak in the radial profiles of the total electron power absorption density p e, the electron temperature T e, and the electron density n e. For all rf powers, the calculated radial profiles of n e show good agreement with the experimental data obtained by a floating double probe.

Benchmarked and upgraded particle-in-cell simulations of a capacitive argon discharge at intermediate pressure: the role of metastable atoms

The capacitive argon discharge operated in the intermediate pressure regime is studied by performing one-dimensional particle-in-cell Monte Carlo collision simulations. First, the basic object-oriented plasma device code (oopd1-v1) is strictly benchmarked against the well-established xpdp1 code over a wide range of pressure (0.05–15 Torr) and varying blocking capacitor of the external circuit (5–105 nF), and excellent agreement is obtained. The oopd1-v1 is upgraded to oopd1-v2 and oopd1-v3, by adding excited atoms modeled as time- and space-evolving fluid species. The metastable Arm, the radiative Arr, and the Ar(4p) manifold, and their roles in discharge equilibrium are explored. It is found that the presence of the metastable Arm enhances the plasma density by a factor of 3 at 1.6 Torr and even higher at pressures up to 5 Torr. At low pressure (0.05 Torr), electron impact ionization from the ground state atom dominates the ionization over the whole discharge region, while metastable pooling and step-wise ionization has small contribution. The proportion of metastable pooling ionization and step-wise ionization increases with increasing pressure and becomes the dominant ionization source at 5–15 Torr.

Experimental investigation of the electromagnetic effect and improvement of the plasma radial uniformity in a large-area, very-high frequency capacitive argon discharge

We performed an experimental investigation on the electromagnetic effect and the plasma radial uniformity in a larger-area, cylindrical capacitively coupled plasma reactor. By utilizing a floating hairpin probe, dependences of the plasma radial density on the driving frequency and the radio-frequency power over a wide pressure range of 5–40 Pa were presented. At a relatively low frequency (LF, e.g. 27 MHz), an evident peak generally appears near the electrode edge for all pressures investigated here due to the edge field effect, while at a very high frequency (VHF, e.g. 60 or 100 MHz), the plasma density shows a sharp peak at the discharge center at lower pressures, indicating a strong standing wave effect. As the RF power increases, the center-peak structure of plasma density becomes more evident. With increasing the pressure, the standing wave effect is gradually overwhelmed by the ‘stop band’ effect, resulting in a transition in the plasma density profile from a central peak to an edge peak. To improve the plasma radial uniformity, a LF source is introduced into the VHF plasma by balancing the standing wave effect with the edge effect. A much better plasma uniformity can be obtained if one chooses appropriate LF powers, pressures and other corresponding discharge parameters.

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.

Determination of the excited argon states densities in high-frequency capacitive discharge

An experimental measurement of the density of excited argon states 1s5, 1s4, 1s3, and 1s2 was carried out for an rf capacitive discharge in argon at a frequency of 13.56 MHz and 81 MHz for two pressures of 50 and 100 mTorr in a wide range of discharge applied power. A collision-radiative model was developed to calculate the densities of 1s states from the known electron density and the electron energy distribution function. The electron energy distribution function is obtained from a self-consistent discharge simulation based on the Particle in cell with Monte Carlo collisions model. It is shown that the calculated densities of the metastable states of argon are in good agreement with the experimentally measured ones. The calculated resonance state densities are underestimated.

Passive inference of collision frequency in magnetized capacitive argon discharge

A non-invasive method of determining the collision frequency νm by measuring the net plasma impendence in a magnetized, capacitive-coupled, radio-frequency (rf) discharge circuit is developed. The collision frequency has been analytically expressed in terms of bulk plasma reactance, wherein standard sheath models have been used to estimate the reactance offered due to the capacitive rf sheaths at the discharge plates. The experimental observations suggest that in the un-magnetized case, νm remains constant over a range of rf current but steadily increases as the background pressure reduces. In the magnetized case, the collision frequency has been observed to decay with the increase in rf current while it remains unaffected by the background pressure. A qualitative discussion has been presented to explain these characteristics.

Mathematical simulation of the low-temperature plazma at the interaction with oil products

Self-consistent mathematical models of a non-equilibrium low-temperature plasma of a radio frequency capacitive discharge in argon in different pressure ranges have been created. These mathematical models allow us to calculate the internal structure and discharge parameters for controlling parameters of the plasma torches at solving the target problems in order to optimize their functioning. The methods for numerical realization of the proposed models are developed. The results of numerical calculations for test problems are analyzed.

Electron energy probability function modulation with external electron beam in capacitive coupled radio frequency discharges

We investigate the effect of external electron beam injection on plasma kinetics of a radio‐frequency (rf) capacitive argon discharge at a pressure of 100 mTorr. The rf voltage is applied to the one electrode. Monoenergetic electron beam is injected from the other electrode, with a laboratory accessible injection current and energy. The electron beam injected electrode is grounded. A direct‐implicit particle‐in‐cell/Monte‐Carlo method is used to self‐consistently simulate the plasma density, electron temperature, plasma potential, electron energy probability function (EEPF), and the electron and ion fluxes. The presence of the electron beam increases the plasma density, while decreases the electron temperature and the plasma potential. The plasma density can increase by a factor of 5, at the same time the electron temperature can decrease from 3 to 0.5 eV. The reason is that, with the electron injection, there are more low‐energy electrons occupation in the EEPF. The EEPF is sensitive to the beam modulation: even a small beam current of 0.01A can modify the EEPF significantly. This may have crucial importance for plasma processing of polymers and graphene, where low energy treatment is desired. In addition, electron and ion fluxes to the ground electrode, are also significantly modified with increasing flux and decreasing bombardment energy.

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.

Self-consistent charge-up simulation for the microscopic feature of SiO2 layer in rf capacitive discharge

The charge-up simulation of the microscopic feature with SiO2 layer was investigated in various conditions of rf capacitive discharge by using the three-dimensional (3-D) particle-in-cell (PIC) charge-up simulation coupled with the one-dimensional (1-D) particle-in-cell Monte Carlo collision (PIC-MCC) simulation of rf capacitive argon discharge. The result showed that the charge-up effect on the micro-trench was greatly influenced by the conditions of the gas pressure and the discharge voltage in rf capacitive discharge. Based on the analysis of the distributions of electrons and ions arriving at the substrate in various plasma conditions, the charge-up effect and its reduction mechanisms on the micro-trench of capacitive discharge were discussed. This article is expected to provide qualitative and quantitative insight for the understanding of charging and its reduction mechanism on many plasma processes performed by the rf capacitive discharge.

Measurement of the electron-impact excitation rate coefficients from argon 1s states to 3p states

The apparent rate coefficients for the electron-impact excitation from argon 1s to 3p states are determined in the electron temperature (Te) range of 0.8 to 1.2 eV, in the afterglow of a capacitive argon discharge by using the temporal behavior of different state densities. The measured parameters include the densities of the 3p states (by optical emission), the densities of the four argon 1s states (by diode laser absorption) and Te (by a Langmuir probe and a line-ratio technique). Among the 24 rate coefficients obtained in this work, the results for excitation from the two resonance states (1s2 and 1s4) are reported here for the first time. As for the excitation from the two metastable states (1s3 and 1s5), the measured rate coefficients are compared with the available values from the distorted wave method and the electron beam experiment.

Measurements of the populations of metastable and resonance levels in the plasma of an RF capacitive discharge in argon

The behavior of the populations of two metastable and two lower resonance levels of argon atoms in the plasma of an RF capacitive discharge was studied. The populations were measured by two methods: the method of emission self-absorption and the method based on measurements of the intensity ratios of spectral lines. It is shown that the populations of resonance levels increase with increasing power deposited in the discharge, whereas the populations of metastable levels is independent of the RF power. The distribution of the populations over energy levels is not equilibrium under these conditions. The population kinetics of argon atomic levels in the discharge plasma is simulated numerically. The distribution function of plasma electrons recovered from the measured populations of atomic levels and numerical simulations is found to be non-Maxwellian.

Study of the effects of a transverse magnetic field on radio frequency argon discharges by two-dimensional particle-in-cell-Monte-Carlo collision simulations

The influence of an applied magnetic field on plasma-related devices has a wide range of applications. Its effects on a plasma have been studied for years; however, there are still many issues that are not understood well. This paper reports a detailed kinetic study with the two-dimension-in-space and three-dimension-in-velocity particle-in-cell plus Monte Carlo collision method on the role of E×B drift in a capacitive argon discharge, similar to the experiment of You et al. [Thin Solid Films 519, 6981 (2011)]. The parameters chosen in the present study for the external magnetic field are in a range common to many applications. Two basic configurations of the magnetic field are analyzed in detail: the magnetic field direction parallel to the electrode with or without a gradient. With an extensive parametric study, we give detailed influences of the drift on the collective behaviors of the plasma along a two-dimensional domain, which cannot be represented by a 1 spatial and 3 velocity dimensions model. By analyzing the results of the simulations, the occurring collisionless heating mechanism is explained well.

Diagnostics of the atmospheric-pressure plasma parameters using the method of near-field microwave sounding

A method of resonant near-field microwave probing is developed for contactless diagnostics of a high-pressure plasma. The efficiency of this method in measuring the parameters of the plasma of an rf capacitive discharge in argon under atmospheric pressure is demonstrated. The experimental results are compared with the data obtained using the independent method, the microwave radiation “cutoff,” and with theoretical estimates.

Making a geometrically asymmetric capacitive rf discharge electrically symmetric

The electrical asymmetry effect in a spherical, geometrically asymmetric capacitive argon discharge driven by two consecutive harmonics is investigated using particle in cell simulations and analytical modeling. We find that the discharge asymmetry can be reduced electrically by tuning the phase shift between the driving frequencies, i.e., the absolute value of the dc self-bias voltage can be completely reduced and the mean ion energies at both electrodes can be adapted.