Capacitively Coupled Radio Frequency Discharges Research Articles

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.

Ion dynamics in capacitively coupled argon–xenon discharges

An argon–xenon (Ar/Xe) plasma is used as a model system for complex plasmas. Based on this system, symmetric low-pressure capacitively coupled radiofrequency discharges are examined utilizing particle-in-cell/Monte Carlo collisions simulations. In addition to the simulation, an analytical energy balance model fed with the simulation data is applied to analyze the findings further. This work focuses on investigating the ion dynamics in a plasma with two ion species and a gas mixture as background. By varying the gas composition and driving voltage of the single-frequency discharge, fundamental mechanics of the discharge, such as the evolution of the plasma density and the energy dispersion, are discussed. Thereby, close attention is paid to these measures’ influence on the ion energy distribution functions at the electrode surfaces. The results show that both the gas composition and the driving voltage can significantly impact the ion dynamics. The mixing ratio of argon to xenon allows for shifting the distribution function for one ion species from collisionless to collision dominated. The mixing ratio serves as a control parameter for the ion flux and the impingement energy of ions at the surfaces. Additionally, a synergy effect between the ionization of argon and the ionization of xenon is found and discussed.

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.

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.

Disparity between current and voltage driven capacitively coupled radio frequency discharges

In simulation as well as analytical modeling studies of low-pressure capacitively coupled radio frequency (CCRF) discharges, the assumption of both a driving voltage source or a driving current source is commonly used. It is unclear, however, how and to what extent the choice of the mode of driving, that prescribes either a sinusoidal discharge voltage or a sinusoidal discharge current, itself defines the discharge dynamics that results from these studies. To address this issue, 1d3v cylindrical particle-in-cell/Monte Carlo collisions simulations of asymmetric CCRF discharges are performed in the low pressure regime (p < 2 Pa). We study the nonlocal and nonlinear dynamics of these discharges on a nanosecond timescale. We find that the excitation of the plasma series resonance in the voltage driven case strongly enhances the nonlinear electron power dissipation. However, this resonance is suppressed when a current source is used, because the excitation of harmonics in the RF current is not allowed. Consequently, significant differences between both driving sources are observed in the plasma density as well as in the electron and the power coupling dynamics. We conclude that caution is advised in comparisons between simulations and experiments, as in the former the discharge dynamics is partly defined by the method of driving of the plasma source, while in the latter the addressed resonance phenomena are inherently present at low pressures, since experiments are typically voltage driven.

Impact of ion energies in Ar/H2 capacitively coupled radio frequency discharges on PEALD processes of titanium films

A plasma enhanced atomic layer deposition (PEALD) process combined with ion energy control of capacitively couples radio frequency (CCRF) discharges in Ar/H2 gas mixture for fabricating Ti films was performed, and the impact of the ion energies on the film characteristics was evaluated. Titanium tetrachloride (TiCl4) was adsorbed on a silicon wafer and Ti films were synthesized via the reduction of Cl atoms from TiCl4 via the CCRF discharges. The growth per cycle of the films clearly increases with increasing the mean ion energy, ⟨εi⟩, suggesting that increase in the ion energies of the discharges promotes the reduction process of Cl atoms. Also, the film stress shows a clear dependence on ⟨εi⟩; it changes towards compressive with increasing ⟨εi⟩. On the other hand, the resistivity and the film density are almost constant independent of ⟨εi⟩, which indicates that the reduction reaction is saturated through the sequential PEALD cycles independent of ⟨εi⟩. In this paper, a simulation model based on one-dimensional (1D) Particle-in-Cell/Monte Carlo Collision (PIC/MCC) has been utilized for CCRF discharges in Ar/H2 gas mixture.

Effect of ion energies on the film properties of titanium dioxides synthesized via plasma enhanced atomic layer deposition

A plasma enhanced atomic layer deposition process for synthesizing titanium dioxide (TiO2) films, which allows the film properties to be modified by tuning the ion energies of the discharges, was performed. The films were deposited via the oxidation of titanium tetrachloride in a typical capacitively coupled radio frequency (CCRF) discharge in argon/oxygen mixtures at a low temperature of 100 °C, resulting in all the films synthesized being amorphous. The energy distributions of ions hitting on the lower electrode (wafer surface) in the CCRF discharges were varied by tuning the impedance of the lower electrode, aimed at controlling the film properties. The wet etching rate of TiO2 films shows a clear correlation with the mean ion energy, 〈εi〉, i.e., a higher 〈εi〉 realizes a higher value of the wet etching rate. The variation of the film properties is explained by a change in the balance between the ion bombardment and the oxidation; the former is greatly affected by 〈εi〉, whereas the latter is mainly determined by radio frequency (RF) power. In a high 〈εi〉 condition, the bombardment of high energetic ions is pronounced, resulting in the formation of fine pores in the films, which was confirmed by physical analyses such as positron annihilation spectroscopy. In this study, the dependence of CCRF discharges as well as the film properties on RF power is also discussed.

Saturation spectroscopy of an optically opaque argon plasma

A pure argon (Ar) plasma formed by a capacitively coupled radio-frequency discharge was analyzed using Doppler-free saturation spectroscopy. The expected line shape was a characteristic of sub-Doppler spectra in the presence of velocity-changing collisions, a narrow Lorentzian centered on a Doppler pedestal, but the observed line shapes contain a multi-peak structure, attributed to opacity of the medium. Laser absorption and inter-modulated fluorescence spectroscopy measurements were made to validate opacity as a driving factor of the observed line shapes. Spectral line shapes are further complicated by the spatial dependence of the pump laser, probe laser and of the absorbing medium, as well as the large absorbance of the transition under investigation. A numerical line shape was derived by accounting for the spatial variation of the pump and probe with a saturated line shape obtained from the rate equations for an equivalent two-level system. This simulated line shape shows good qualitative agreement with the trends observed in the data.

Transitions between various diffuse discharge modes in atmospheric-pressure helium in the medium-frequency range

In this paper, we investigate DBDs in the medium frequency range (MF, 0.3–3 MHz). More precisely, for a 2 inter-dielectric gap in helium at atmospheric pressure, the frequency is varied from 1.0 to 2.7 MHz. The generated discharge shows similarities with both the low-frequency atmospheric-pressure glow discharge (APGD) and the atmospheric pressure capacitively coupled radio-frequency (CCRF) discharge. In the frequency range under investigation, two diffuse discharge modes can be observed depending on the voltage applied between the electrodes. At low applied voltage, the discharge emissions are barely visible and are concentrated in the center of the gas gap similarly to CCRF discharges in the mode where the electron density is concentrated in the bulk. Ohmic heating is the main power transfer mechanism. At higher applied voltage, the discharge emissions are 10 times more intense and are closer to the dielectric surfaces similarly to the more common radio-frequency α mode. These two discharge modes can be observed in the same experimental conditions with the amplitude of the applied voltage as sole control parameter. The gas temperature obtained from N2 impurities rotational spectrum increases from room temperature to about 500 K while the power density rises from 10−1 to 101 W cm−3 when the applied voltage is increased. In addition, when the discharge transits back and forth from the to the α mode, a hysteresis is observed. The transition from the to the α mode occurs abruptly with a large RMS current increase while the transition from the α to the mode is rather smooth with no significant discontinuity in the RMS current.

A Calculation of the Electron Temperature of Complex Plasma of Noble Gases Mixture in CCRF Discharge

AbstractIn this paper the results of studying of the electron temperature of buffer and complex plasmas in mixtures of noble gases (helium + argon) in capacitively coupled radiofrequency (CCRF) discharge are presented. The optical properties of dusty plasma in argon, helium and their mixtures have been studied using optical diagnostic methods. Based on spectral lines of plasma forming gases, the dependence of the electron temperature on gas pressure and discharge power has been determined. The axial distribution of electron temperature in the interelectrode gap has been measured. Measurements have been made using an RF compensated electric probe. The comparison of the experimental results shows that admixture of a small amount of argon to helium leads to a decrease in the electron temperature of buffer plasma. The presence of dust particles in the plasma causes an increase in the electron temperature. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ion Distribution Functions in Electrically Asymmetric Capacitively Coupled Radio-Frequency Discharges in Hydrogen

Electrically asymmetric capacitively coupled radiofrequency discharges are known to give control over the ion distribution functions and the mean ion energy at the electrodes. So far, distribution functions of heavy ions such as argon have been measured and calculated. Hydrogen ions are able to follow the time-dependent electric field to a greater extent due to their low mass. This leads to unique structures in the distribution functions in case of collisional sheaths. Images of these ion distribution functions are presented.

Influence of finite geometrical asymmetry of the electrodes in capacitively coupled radio frequency plasma

Capacitively coupled radio frequency (CCRF) plasmas are widely studied in last decades due to the versatile applicability of energetic ions, chemically active species, radicals, and also energetic neutral species in many material processing fields including microelectronics, aerospace, and biology. A dc self-bias is known to generate naturally in geometrically asymmetric CCRF plasma because of the difference in electrode sizes known as geometrical asymmetry of the electrodes in order to compensate electron and ion flux to each electrode within one rf period. The plasma series resonance effect is also come into play due to the geometrical asymmetry and excited several harmonics of the fundamental in low pressure CCRF plasma. In this work, a 13.56 MHz CCRF plasma is studied on the based on the nonlinear global model of asymmetric CCRF discharge to understand the influences of finite geometrical asymmetry of the electrodes in terms of generation of dc self-bias and plasma heating. The nonlinear global model on asymmetric discharge has been modified by considering the sheath at the grounded electrode to taking account the finite geometrical asymmetry of the electrodes. The ion density inside both the sheaths has been taken into account by incorporating the steady-state fluid equations for ions considering that the applied rf frequency is higher than the typical ion plasma frequency. Details results on the influences of geometrical asymmetry on the generation of dc self-bias and plasma heating are discussed.

Effect of Addition of Nitrogen to a Capacitively Radio-Frequency Hydrogen Discharge

A hybrid PIC/MC model is developed in this work for H2-xN2 capacitively coupled radio-frequency (CCRF) discharges in which we take into account 43 kinds of collisions reaction processes between charged particles (e−, H+3, H+2, H+, N+2, N+) and ground-state molecules (H2, N2). In addition, the mean energies and densities of electrons and ions (H+3, H+2, H+), and electric field distributions in the H2-N2 CCRF discharge are simulated by this model. Furthermore, the effects of addition of a variable percentage of nitrogen (0–30%) into the H2 discharge on the plasma processes and discharge characteristics are studied. It is shown that by increasing the percentage of nitrogen added to the system, the RF sheath thickness will narrow, the sheath electric field will be enhanced, and the mean energy of hydrogen ions impacting the electrodes will be increased. Because the electron impact ionization and dissociative ionization rates increase when N2 is added to the system, the electron mean density will increase while the electron mean energy and hydrogen ion density near the electrodes will decrease. This work aims to provide a theoretical basis for experimental studies and technological developments with regard to H2-N2 CCRF plasmas.

Control of radial density profile of nano-particles produced in reactive plasma by amplitude modulation of radio frequency discharge voltage

We investigate uniformity of radial density profile of nano-particles in capacitively-coupled radio frequency discharge with an amplitude modulation (AM) of rf discharge voltage, by a two-dimensional laser light scattering method. Their densities at the center and that at the edge of the electrode increase by 3 and 24 times when the amplitude modulation level increases from 0 (without AM) to 30%. The uniformity of radial density profile of nano-particles is significantly improved by using the AM method.

Plasma immersion ion implantation into cylindrical bore using internal inductively-coupled radio-frequency discharge

Plasma immersion ion implantation (PIII) is a potentially excellent interior surface treatment technique due to no line-of-sight restriction. However, some problems have been encountered due to the low ion energy and ion fluence non-uniformity especially for treatment of the interior wall of a thin tube. In this paper, a new method for inner surface PIII using internal inductively-coupled radio-frequency (RF) discharge is described. A cylindrical inductive coil inserted inside the tube serves as both the plasma source and grounded electrode to avoid overlapping of the plasma sheath fronts propagating from opposite sides. The effects of the gas species, gas pressure, RF power, and number of coil turns are investigated. Our results demonstrate the feasibility of this novel inner surface treatment method and the number of turns in the coil has a critical influence on the discharge behavior. If the number of turns is little, the plasma density is low and non-uniform inside the tube due to the relatively intense capacitively-coupled RF discharge at the two ends. In contrast, the plasma density and uniformity are evidently improved by using more turns in the coil.

Plasma inhomogeneities near the electrodes of a capacitively-coupled radio-frequency discharge containing dust particles

Small plasma spheroids are evidenced and analyzed in front of the electrodes of a capacitively-coupled radio-frequency discharge in which dust particles are growing. These regions are characterized by a spherical shape, a slightly enhanced luminosity and are related to instabilities induced by the presence of dust particles. Several types of behaviors are identified and particularly their chaotic appearance or disappearance and their rotational motion along the electrode periphery. Correlations with the unstable behavior of the global plasma glow are performed. These analyses are obtained thanks to high-speed imaging which is the only diagnostics able to evidence these plasma spheroids.

Electrical time resolved metrology of dust particles growing in low pressure cold plasmas

The electrical parameters of a capacitively coupled radiofrequency (CCRF) discharge change significantly when dust arises in the discharge. This work demonstrates the ability to follow in real time the evolution of the size and of the concentration of dust particles forming in a CCRF discharge from the variation of the electron density and of the self-bias voltage of the active electrode. According to experimental findings, it appears that the variation of this self-bias voltage depends on the surface of the dust particles. This trend is confirmed by an analytical modelling considering the low frequency behaviour of the phenomenon.

The electrical asymmetry effect in capacitively coupled radio-frequency discharges

We present an analytical model to describe capacitively coupled radio-frequency (CCRF) discharges and the electrical asymmetry effect (EAE) based on the non-linearity of the boundary sheaths. The model describes various discharge types, e.g. single and multi-frequency as well as geometrically symmetric and asymmetric discharges. It yields simple analytical expressions for important plasma parameters such as the dc self-bias, the uncompensated charge in both sheaths, the discharge current and the power dissipated to electrons. Based on the model results the EAE is understood. This effect allows control of the symmetry of CCRF discharges driven by multiple consecutive harmonics of a fundamental frequency electrically by tuning the individual phase shifts between the driving frequencies. This novel class of capacitive radio-frequency (RF) discharges has various advantages: (i) A variable dc self-bias can be generated as a function of the phase shifts between the driving frequencies. In this way, the symmetry of the sheaths in geometrically symmetric discharges can be broken and controlled for the first time. (ii) Almost ideal separate control of ion energy and flux at the electrodes can be realized in contrast to classical dual-frequency discharges driven by two substantially different frequencies. (iii) Non-linear self-excited plasma series resonance oscillations of the RF current can be switched on and off electrically even in geometrically symmetric discharges. Here, the basics of the EAE are introduced and its main applications are discussed based on experimental, simulation, and modeling results.

Numerical simulations of electrical asymmetry effect on electronegative plasmas in capacitively coupled rf discharge

Recently a so-called electrical asymmetry effect (EAE), which could achieve high-degree separate control of ion flux and energy in dual-frequency capacitively coupled radio-frequency (CCRF) discharges, was discovered theoretically by Heil et al. [J. Phys. D: Appl. Phys. 41, 165202 (2008)] and was confirmed by experiments and theory/numerical simulations later on for electropositive argon discharges. In this work simulations based on particle-in-cell/Monte Carlo collision are performed to study the EAE on electronegative oxygen plasmas in geometrically symmetric CCRF discharges. Dual frequency discharges operating at 13.56 and 27.12 MHz are simulated for different pressures and the results are compared with those of electropositive argon discharges at the same conditions. It is found that in general the EAE on oxygen discharges has similar behavior as on argon discharge: The self-bias voltage η increases monotonically and almost linearly with the increase in the phase angle θ between the two driving voltages in the range 0&amp;lt;θ&amp;lt;90°, and the maximum ion energy varies by a factor of 3 by adjusting θ. However, the ion flux varies with θ by ±12% for low pressure and by ±15% for higher pressure, due primarily to an enhanced plasma series resonance, which then leads to dramatic changes in plasma density, power absorption and consequently the electronegativity. This may place a limitation for achieving separate control of ion energy and flux for electronegative plasma via the EAE.