Capacitively Coupled Radio Frequency Research Articles

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.

Second-harmonic currents in rf-biased, inductively coupled discharges

Capacitively-coupled plasmas generate strong current or voltage signals at harmonics of their driving frequencies. Inductively coupled plasma (icp) systems generally do not, unless they are equipped with capacitively-coupled rf bias, which generates strong signals at harmonics of its driving frequency. Recently, however, at an asymmetric, rf-biased electrode, a current component was detected at the second harmonic of the inductive source frequency, not the rf-bias frequency. The origin of this current is here investigated (in argon discharges at 1.3 Pa) by comparison with measurements made at a symmetric electrode and predictions made by two numerical models. The first simulates the sheath at the rf-biased electrode; the second models the plasma. Because capacitive coupling from the inductive source was minimized by a Faraday shield, the nonlinearity of the sheath contributes negligible second-harmonic current. Modulation of the photon flux in the plasma, however, produces a second-harmonic current photoemitted from the rf-biased electrode. The external circuitry and nonlinear inductive coupling produce a second-harmonic sheath voltage, which in turn generates second-harmonic current both directly and through a transit-time effect. The second model simulates how electrons emitted from the electrode—and then reflected at the quartz dielectric window of the inductive source—are deflected by the electric and magnetic fields in the plasma. It also gives predictions for the transit-time effect. Magnetic deflections and the transit-time effect usually dominate the electric deflection. Together these three mechanisms produce a second-harmonic current that has a Fourier amplitude approximately half the current that is elastically reflected at the icp window. These results suggest it may be possible to use the second-harmonic current to determine the elastic reflection coefficient at the window.

Parametric Studies of CCRF in Ar on 1D Model: Effect of Pressure and Dielectric Layers

A detailed study on the capacitively coupled radio-frequency (CCRF) argon discharge with metastable atom density and the effect pressure has been undertaken. Specifically, dielectric layers of borosilicate glass and alumina were introduced to the discharge to examine their influence on the discharge characteristics and to study the properties of the plasma. A model for the argon plasma based on the three moments of the Boltzmann equation together with the Poisson equation was used. The properties of the argon plasma presented are based on the averaged cycle, especially the electron temperature, the electric potential, and the metastable atom density. The electron temperature and the metastable atom density decreased in the presence of the dielectric layers, whereas the electric potential had a high value. Sensitivity analysis showed that the surface charge concentrations on the borosilicate glass and alumina dielectrics and the gap voltage increased with the increase of the relative permittivity; the plasma parameters also increased with increasing gap voltage.

Study of the Neon Dielectric Barrier Discharge on a Capacitively Coupled Radio Frequency at a Low Pressure with Metastable Atom Density: Effect of the Pressure

We study the neon dielectric barrier discharge with metastable atom density on a capacitively coupled radio frequency at a pressure of about 4–12 Torr. The transport parameters of neon are dependent on the electron energy, and their range is about 0.04–50 eV. A one-dimensional fluid model and the drift-diffusion theory are used to describe the neon dielectric barrier discharge. The effect of the gas pressure on the properties of neon dielectric barrier discharge is presented for the cycle-averaged regime. It is shown that the particle densities, electric potential, and metastable atom density increase with the pressure. In addition, the surface charge concentration and the gap voltage increase as well.

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

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

Continuum modelling of an asymmetric CCRF argon plasma reactor: Influence of higher excited states and sensitivity to model parameters

AbstractPrecise control of capacitively coupled radiofrequency (CCRF) plasma reactors is required to achieve desired outcomes in surface functionalisation and material synthesis processes. This necessitates detailed mapping of the large process parameter space and a thorough understanding of spatial and temporal variations of the plasma throughout the reactor. These goals can only feasibly be achieved with accurate numerical modelling. Previous numerical studies of CCRF discharges have implemented a range of simplifying assumptions to improve numerical tractability, such as small electrode spacing, radial uniformity, fewer active species and simplified boundary conditions, while neglecting self‐bias formation. Although this approach is useful in developing the methodology for continuum plasma modelling, it poses challenges for direct comparison with experimental data and for understanding the behaviour of plasma processes employed in the surface treatment of large, complex objects, or the synthesis of nanoparticles. Here we report the development of a two‐dimensional axisymmetric continuum model for a CCRF reactor with a pure argon 13.56‐MHz discharge using the finite element method. The large electrode spacing and reactor design result in two distinct discharge regions and the formation of a strong DC self‐bias on the powered electrode. The plasma discharge is studied as the pressure is varied from 0.1 to 0.3 Torr, over the radiofrequency input power range of 25–100 W, which leads to consistent enhancements of the electron density and self‐bias. The impact of the electron energy distribution function (EEDF) on the discharge is assessed, with the assumption of a Druyvesteyn EEDF resulting in a bulk electron density and temperature of 3.4 × 1015 m−3 and 3.3 eV, respectively, compared with 8.1 × 1015 m−3 and 1.9 eV in the Maxwellian case. The asymmetric power distribution throughout the reactor is quantified to build a reduced domain model with a lower computational cost. The effect of an electrically floating parallel plate electrode is assessed, resulting in a 42% higher bulk plasma potential as compared with the grounded case. The inclusion of resonant and 2p excited states of argon is shown to have a major impact on the discharge dynamics, leading to an order of magnitude reduction in bulk electron density. This study proposes a robust numerical model of a CCRF argon plasma discharge to facilitate future simulations of more complex discharges with important implications in plasma surface engineering and synthesis of materials.

Dim and bright void regimes in capacitively-coupled RF complex plasmas

We demonstrate experimentally that the void in capacitively-coupled RF complex plasmas can exist in two qualitatively different regimes. The ‘bright’ void is characterized by bright plasma emission associated with the void, whereas the ‘dim’ void possesses no detectable emission feature. The transition from the dim to the bright regime occurs with an increase of the discharge power and has a discontinuous character. The discontinuity is manifested by a kink in the void size power dependencies. We reproduce the bright void (mechanically stabilized due to the balance of ion drag and electrostatic forces) by a simplified time-averaged 1D fluid model. To reproduce the dim void, we artificially include the radial ion diffusion into the continuity equation for ions, which allows to mechanically stabilize the void boundary due to very weak electrostatic forces. The electric field at the void boundary occurs to be so small that it, in accordance with the experimental observation, causes no void-related emission feature.

Mode Transition in Capillary Capacitively Coupled Radio Frequency Thruster

A miniature capacitively coupled radio frequency thruster with a capillary tube (C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> RFT) is being developed and tested at the Technion. The thruster utilizes a nonsymmetric electrode arrangement and a dedicated radio frequency generator. The small diameter of the dielectric tube allows operation of the device in very low flow rates, <; 8 SCCM in Ar. The measured thrust time response and wall temperature of 400 °C confirm an electrothermal mechanism of thrust generation. At constant input power, the thrust is shown to increase linearly with the flow rate. At an input power of 14 W and a flow rate of 8 SCCM, the thrust increases significantly compared to the cold flow case, for a total thrust of ≈160 μN and specific impulse ≈69 s in Ar. When powering the larger inlet “back” electrode, discharge is observed to diverge from classical theory of asymmetric capacitive coupled plasma, leading to mode transition. This phenomenon is marked by the appearance of bright plasma exhaust and reduction in input power from ≈15.17 to ≈9 W. The thrust to power ratio increased by 25% after the transition. A simple sheath breakdown model is proposed to explain the transition.

Deposition of organosilicon coatings from trimethylsilyl acetate and oxygen gases in capacitively coupled RF glow discharge

The aim of the present work was to prepare SiOxCyHz coatings in capacitively-coupled RF glow discharge from gaseous mixture of trimethylsilyl acetate (TMSAc) monomer and oxygen. The properties of deposited coatings have been investigated using several characterization methods. The presented study proves that properties of TMSAc-based thin films are significantly dependent on the monomer ratio in TMSAc/O2 mixture which varied from 25.0% to 92.3%. Using a low TMSAc ratio (high content of oxygen gas) resulted in organosilicon materials similar to SiO2 in terms of chemical composition and mechanical properties. Increasing the fraction of TMSAc in gaseous mixture led to the formation of more organic polymeric structures enriched with carbon atoms bonded in CHx chains. Values of Martens hardness decreased from 3.97 to 0.76 GPa with increasing TMSAc fraction from 25.0% to 92.3%. The degree of hydrophobicity of prepared coatings evolved in accordance with observed structural changes corresponding to applied monomer ratios. The water contact angle gradually increased from 75.0° to 95.3° with increasing TMSAc fraction. The prepared TMSAc plasma polymers became more hydrophobic with more intensive integration of hydrocarbon species into the surface structure.

N2–H2 capacitively coupled radio-frequency discharges at low pressure. Part I. Experimental results: effect of the H2 amount on electrons, positive ions and ammonia formation

The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3–1 mbar) and low coupled power (3–13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 × 109 cm−3, corresponding to ionization degrees from 2 × 10−8 to 4 × 10−7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 × 1013 cm−3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical–neutral collisions (ex: N2H+, NH4+, H3+), while an increase of the coupled power favours ions formed by direct ionization (ex: N2+, NH3+, H2+).

Electron dynamics in low pressure capacitively coupled radio frequency discharges

In low temperature plasmas, the interaction of the electrons with the electric field is an important current research topic that is relevant for many applications. Particularly, in the low pressure regime (≤10 Pa), electrons can traverse a distance that may be comparable to the reactor dimensions without any collisions. This causes “nonlocal,” dynamics which results in a complicated space- and time-dependence and a strong anisotropy of the distribution function. Capacitively coupled radio frequency (CCRF) discharges, which operate in this regime, exhibit extremely complex electron dynamics. This is because the electrons interact with the space- and time-dependent electric field, which arises in the plasma boundary sheaths and oscillates at the applied radio frequency. In this tutorial paper, the fundamental physics of electron dynamics in a low pressure electropositive argon discharge is investigated by means of particle-in-cell/Monte Carlo collisions simulations. The interplay between the fundamental plasma parameters (densities, fields, currents, and temperatures) is explained by analysis (aided by animations) with respect to the spatial and temporal dynamics. Finally, the rendered picture provides an overview of how electrons gain and lose their energy in CCRF discharges.

Electron dynamics during the reignition of pulsed capacitively-coupled radio-frequency discharges

The authors report on phase resolved optical emission spectroscopy (PROES) measurements of pulsed capacitive coupled plasmas (CCPs) through argon. The PROES results indicate that under some conditions, the electron heating mechanism can be changed substantially from that dominant in continuous CCPs. The normally dominant α heating mode of electropositive plasmas can be aided by a drift-ambipolar (DA) heating mode during the early portion of the reignition. The DA heating mode is ordinarily only found in electronegative discharges. The authors found that Ar discharges pulsed at 10 kHz only exhibited the α heating mode throughout the reignition process, while those pulsed at 0.1 kHz exhibited a mixed α and DA heating mode during the reignition. The differences in the two heating modes cause substantial differences in the spatial pattern of the light emission from the plasma in addition to an overshoot in the light emission intensity.

An Inductively-Coupled Plasma Electrothermal Radiofrequency Thruster

The “cubesat” form factor has been adopted as the defacto standard for a cost effective and modular, nano-satellite platform. Many commercial options exist for nearly all components re- quired to build such satellite; however, there is a limited range of thruster options that suit the power and size restrictions of a cubesat. Based on the prior work on the “Pocket Rocket” electro- thermal capacitively-coupled radiofrequency (RF) plasma thruster operating at 13.56 MHz, a new design is proposed which is based on an inductively-coupled radiofrequency plasma system operat- ing at 40.68 MHz. The new thruster design, which includes a compact and efficient radiofrequency matching network adjacent to the plasma cavity is presented, together with the first direct thrust measurements using argon as the propellant with the thruster immersed in vacuum and attached to a calibrated thrust balance. These initial results of the unoptimised first prototype indicate an up to 40% instantaneous thrust gain from the plasma compared to the cold gas thrust: typical total thrust at 100 SCCM of argon and 50 W RF power is ∼ 1.1 mN.

Comparison of Three Radio-Frequency Discharge Modes on the Treatment of Breast Cancer Cells in Vitro

Nonthermal plasmas (NTPs) are known for their ability to induce thermal-free cytotoxic effects on cancer cells. However, as the variety of NTP devices increases, the comparison of their cytotoxic effect becomes increasingly essential. In this article, we compare the cytotoxicity of three different radio-frequency (RF) NTPs. MDA-MB-231 triple-negative breast cancer cells are treated in suspension in DMEM culture medium by the effluents of a single RF discharge device operating in three modes, namely the $\Omega $ and $\gamma $ modes of the capacitively coupled RF (CCRF) discharge and an RF plasma jet mode. All three discharge modes reduce the proliferative capacity of MDA-MB-231 cells, but the treatment time required to reach the same efficacy is more than ten times longer using the $\Omega $ and the $\gamma $ modes than using the jet mode. In all cases, using the appropriate treatment time, cells exhibit an impaired proliferation and eventually start to show signs of cell death (about 48 h after treatment). The three discharge modes also induce nuclear DNA damages. Plasma-produced H2O2 was not found to contribute to the cytotoxicity of the treatment. Furthermore, short-lived reactive species (liquid phase species with a lifetime below 1 s) are expected to play an important role in the anti-cancer effect of all three discharge modes.

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

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

Obtaining of carbon nanowalls in the plasma of radio-frequency discharge

In this work a synthesis of carbon nanowalls (CNWs) in the plasma of argon and methane gases of capacitively coupled radio-frequency (CCRF) discharge is considered. The carbon nanowalls were synthesized by plasma enhanced chemical vapor deposition method at relatively lower values of discharge power and gas flow rate for low-cost CNWs production. The growth process of carbon nanowalls was studied at different synthesis parameters. The morphology and structure quality of obtained samples were investigated by Raman spectroscopy, scanning electron and atomic force microscopies. It was found, that at a lower value of RF power the formation of CNWs goes better with increasing methane flow rate and further increasing RF power leads to agglomeration of nanowalls and formation of nanoclusters, which then transfer into multilayered graphene. Thus, on the basis of obtained results, a process map with different zones of deposited carbon nanostructures is developed.

Plasma cleaning of steam ingressed ITER first mirrors

Abstract In ITER, the first mirrors (FMs) are vulnerable to an in-vessel coolant leak which could severely diminish their optical properties. To understand the scope of this potential impact, several FM samples were exposed to a steam and humidity test simulating the event in ITER. Both rhodium and molybdenum mirrors, observed a loss in specular reflectivity as a result (the loss being greater for the Mo mirror). Their surfaces were tarnished with the development a thin Rh oxide and a thick Mo oxide (120–170 nm). This study focusses on capacitively coupled radio frequency (CCRF) plasma cleaning of steam ingressed (SI) FM samples and follow their optical recovery. Plasma cleaning experiments were performed with 13.56 MHz CCRF plasma using argon and/or hydrogen as process gas (with 230 eV ion energy). Initial and final reflectivity measurements, chemical surface analysis using in vaccuo X-ray photoelectron spectroscopy, scanning electron microscopy, focused ion beam and roughness measurements, were carried out for each sample to evaluate the cleaning efficiency. Using the plasma cleaning technique, it was possible to remove the SI induced contamination from the mirror surfaces and recover their optical properties to the pristine levels. Several ‘voids/inclusions’ were seen to arise along the grain boundaries as a result of the SI procedure. The concentration of these ‘voids/inclusions’ was observed to increase till a certain point followed by a decrease with increasing cleaning time.

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

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

RF plasma cleaning of water-cooled mirror equipped with notch filter based on shorted λ/4 line

Physical and engineering aspects of RF plasma cleaning developed for actively cooled first mirror serving as grounded DC-coupled electrode are under consideration. Feasibility of the water-cooling lines implemented as the coaxial notch filter for ITER in-vessel diagnostic mirrors was demonstrated for Capacitively Coupled Radio Frequency (CCRF) discharge in neon at 1–10 Pa. The impinging ions energy as a function of absorbed RF power was measured for both the DC-coupled and -decoupled schemes. Sputtering yield variation over rectangular surface of stainless steel mirror was studied experimentally. The measured sputtering yield profiles were found to be in qualitative agreement with the RF plasma numerical simulations. It was demonstrated that Al/Al2O3 used as Be/BeO proxy can be removed from the water cooled mirror surface by 100 eV neon ions with the use of the notch filter. The 14 h plasma exposure didn’t affect the mirror reflectivity.

Influence of magnetic field strength on nanoparticle growth in a capacitively-coupled radio-frequency Ar/C2H2 discharge

The growth of nanoparticles in a magnetised chemically active discharge (Ar/C2H2) is investigated. The influence of the strength of the magnetic field on dust particle growth dynamics is explored. The structure of the grown nanoparticles is studied ex situ. It is revealed that the strength of the magnetic field (up to 2.5 T) has a major impact on discharge parameters (such as the self-bias of the powered electrode) as well as on the growth and morphology of the nanoparticles. At high magnetic field, the dust cloud is confined in the sheath above the grounded electrode while without magnetic field the dust cloud occupies most of the interelectrode space. Moreover, at high magnetic field, large porous spherical agglomerates were grown. The modification of the self-bias is explained by the influence of the magnetic field on the diffusion of charged species resulting in a confinement of the plasma under the powered electrode. Complementary particle-in-cell simulations confirm that the electric field and plasma distributions are strongly affected by the magnetic field explaining the experimentally observed dust cloud localisation. The large porous spherical agglomerates are most probably due to an enhanced agglomeration caused by the modified confinement owing to the magnetic field.