Dual-frequency Discharges Research Articles

Experimental and numerical study on atmospheric‐pressure air dielectric barrier discharge via 50 Hz/5000 Hz dual‐frequency excitation

AbstractA dual‐frequency (DF) dielectric barrier discharge, excited by the superposition of a 50 Hz low frequency (LF) and a 5000 Hz intermediate frequency (IF), is proposed to enhance discharge. The effect of the LF voltage component on the breakdown behaviour of the DF discharge during different periods has been studied both experimentally and numerically. The number of high‐current pulses rises as the LF voltage increases. The statistical analysis shows that the number of the current pulse amplitude above 80 mA in the DF discharge reaches nearly 6 times that in the IF discharge. Additionally, the total discharge energy in the DF discharge is significantly higher than that in the IF discharge. The simulation reproduces the temporal variation of the breakdown behaviours in the DF discharge. The simulated results reveal that the maximal electric field strength of the breakdown process is greater in the DF discharge compared to the IF discharge during a half‐period of DF. Finally, the comparison between the IF and DF discharges exhibits that the LF voltage regulates the accumulation of residual charged species on the dielectric surface after the breakdown by modulating the residual voltage between the air gap.

Application of similarity laws to dual-frequency capacitively coupled radio frequency plasmas with the electrical asymmetry effect

Similarity laws (SLs) are useful for correlation and prediction of plasma parameters at different scales, which have been verified for geometrically symmetric capacitive radio-frequency (rf) discharges in nonlocal kinetic regimes. In this work, we demonstrate the applicability of SLs to dual-frequency rf discharges and confirm that similarity relations still hold considering the electrical asymmetry effect (EAE). By simultaneously tuning the control parameters (the gas pressure p, discharge gap d, and driving frequency f), we examine the similarity relations in rf plasmas via fully kinetic particle-in-cell simulations with the external circuits coupled and solved self-consistently. The validity of the SL scalings in dual-frequency rf plasmas with the EAE is confirmed for parameters such as the electron/ion density, ion flux, dc self-bias, ion energy distribution function, and power absorption. Although adjusted by the EAE, the dc self-bias and ion energy distribution functions are identified as similarity invariants under similar discharge conditions. Furthermore, the plasma series resonance phenomenon, filamentation of power depositions of electrons and ions in bulk plasma, and electric field reversal are observed in dual-frequency discharges with the EAE, which can also be exactly replicated under similar discharge conditions. The results further extend the application of SL scaling to dual-frequency rf plasmas, providing a more comprehensive understanding of the scaling characteristics in rf plasmas.

A resonator-based dual-frequency driven atmospheric pressure plasma jet

An atmospheric-pressure argon plasma jet featuring a novel integrated resonator-based multi-frequency impedance matching is presented and briefly characterized. Two narrow RF frequency bands can be chosen for operation or used simultaneously. This includes a mode with the higher frequency value being exactly five times the lower one. Phase-resolved optical emission spectroscopy measurements show a distinct influence of the input frequency combination on the discharge dynamics. Measurements of the dissipated electrical power and the emission spectrum for each operating mode complete the basic characterization of the device. Although it is constructively much simpler and more compact than dual-frequency discharges using a conventional impedance matching system, the presented device shows an excellent performance in dual-frequency operation.

Experimental diagnosis of electron density and temperature in capacitively coupled argon plasmas: Triple-frequency discharges and two-dimensional spatial distributions

An approach combining optical emission spectroscopy with a collisional radiative model (OES-CRM) has been used to diagnose electron density and electron temperature in low pressure capacitively coupled argon plasmas. The electron density and electron temperature obtained by the OES-CRM show reasonable agreement with the results measured by a Langmuir probe. The method was first applied to diagnosis in triple-frequency (2, 13.56, and 27.12 MHz) plasmas. Compared with dual-frequency (2 and 27.12 MHz) discharges, it was found that in triple-frequency discharges, the high frequency source controls electron density more independently and with less influence on electron temperature as the intermediate frequency power increases. Next, the method was extended to a two-dimensional diagnosis based on the use of a charge coupled device camera and optical bandpass interference filters. The results showed that the axial and radial distributions of the electron density are more uniform at a lower radio frequency (RF) power. The axial uniformity of the electron density is better at a lower discharge frequency while the radial profiles of the electron temperature are flatter at a higher frequency. In all the cases, the electron temperature is highly uniform within the bulk plasma. Moreover, a mode transition from an α to a γ mode is observed at 13.56 MHz with the increasing RF power, and this is accompanied by a significant enhancement in electron density and a sharp reduction in electron temperature.

Investigations of the sheath in a dual-frequency capacitively coupled rf discharge by optically trapped microparticles

The sheath of a dual-frequency capacitively coupled rf discharge has been investigated with microparticles as probes based on two different methods: by the first method, intensity profiles of the emitted light of the discharge and the scattered light of the microparticles are used to study the effects of phase variation between the fundamental frequency (13.56 MHz) and its first harmonic (27.12 MHz) or a change in the dc self-bias, respectively, on the expansion of the sheath and the levitation height of microparticles in the sheath. These measurements show that the sheath width as well as the levitation height of the microparticles correlates with the dc self-bias as a function of the phase between the applied voltage harmonics. By a second method, profiles of the force acting on an optically trapped microparticle in an optical tweezer in the sheath were measured. On the basis of these force profiles, the strength of the electric field force as a function of pressure and the dependency on the distance of the probe to the rf electrode as well as the extent of the sheath are determined in both a single- and a dual-frequency discharge. From the results, it can be deduced that the sheath thickness and the electric field force in a dual-frequency discharge are influenced by both excitation frequencies and, thus, differ from a single-frequency discharge.

Study of titanium nitride film growth by plasma enhanced pulsed laser deposition at different experimental conditions

In this work, we have combined a conventional pulsed laser deposition (PLD) system with a dual radio frequency (RF) discharge to create a plasma-enhanced PLD system, known as (PE-PLD). This system was used to grow titanium nitride (TiN) thin films in order to study the feasibility of obtaining control over the nitrogen concentration and the deposition rate. The characterization of the RF plasma was performed using an RF compensated Langmuir probe, while atomic force microscopy (AFM), optical profilometry, Raman spectroscopy, and x-ray diffraction (XRD) were used to analyze the deposited thin films. We observed a significantly enhanced nitrogen concentration and deposition rate as a function of the RF plasma parameters, substrate temperature, and deposition time. Furthermore, we have observed that, using a dual RF configuration, it becomes possible to modify independently the flow and the bombardment energy of the nitrogen ions that impact the substrate. These means allow the control of the nitrogen concentration and deposition rate.

Optimization of discharge antenna turn and modulation of EEPFs and plasma parameters in dual-frequency cylindrical ICP source at different gas pressures

This paper deals with the optimization of the number of discharge antenna turns N, mainly including the effect of N on the plasma parameters [plasma density ne, effective electron temperature Teff, plasma potential Vp, power transfer efficiency η, and electron energy probability function (EEPF)] in cylindrical inductively coupled plasma (ICP) source, and compares axial and radial distribution of ne with different antenna arrangement topologies, mainly referring to the non-interleaved and interleaved antenna arrangement in cylindrical dual-frequency (DF) ICPs. It is found that the cylindrical DF-ICPs with interleaved antenna arrangement can produce a higher ne due to the superposition effect of the high-frequency (HF) and low-frequency (LF) discharge region. Furthermore, based on the above optimized parameters, an optimized cylindrical DF-ICP with an interleaved antenna arrangement is employed, and the modulations of EEPFs and plasma parameters (ne, Teff, and Vp) in the case of (1) fixed HF power and varying additional LF power and (2) fixed LF power and varying additional HF power correspondingly are investigated and compared at different gas pressures of 1, 15, and 60 mTorr systematically. With the wide gas pressure range from 1 to 60 mTorr considered in a combination with two discharge frequencies of 2 and 13.56 MHz, the DF discharge covers discharge maintenance under different conditions with respect to electron heating mechanisms, e.g., collision-less and collision heating. This is a relatively comprehensive work about cylindrical DF-ICPs, which will provide some insights into the design and plasma parameters modulation of cylindrical DF-ICPs.

Charged particle dynamics and distribution functions in low pressure dual-frequency capacitively coupled plasmas operated at low frequencies and high voltages

In high aspect ratio (HAR) dielectric plasma etching, dual-frequency capacitively coupled radio-frequency plasmas operated at low pressures of 1 Pa or less are used. Such plasma sources are often driven by a voltage waveform that includes a low-frequency component in the range of hundreds of kHz with a voltage amplitude of 10 kV and more to generate highly energetic vertical ion bombardment at the wafer. In such discharges, the energetic positive ions can overcome the repelling potential created by positive wall charges inside the etch features, which allows high aspect ratios to be reached. In order to increase the plasma density a high-frequency driving component at several 10 MHz is typically applied simultaneously. Under such discharge conditions, the boundary surfaces are bombarded by extremely energetic particles, of which the consequences are poorly understood. We investigate the charged particle dynamics and distribution functions in this strongly non-local regime in argon discharges by particle-in-cell simulations. By including a complex implementation of plasma-surface interactions, electron induced secondary electron emission (δ-electrons) is found to have a strong effect on the ionization dynamics and the plasma density. Due to the high ion energies at the electrodes, very high yields of the ion induced secondary electron emission (γ-electrons) are found. However, unlike in classical capacitive plasmas, these γ-electrons do not cause significant ionization directly, since upon acceleration in the high voltage sheaths, these electrons are too energetic to ionize the neutral gas efficiently. These γ- and δ-electrons as well as electrons created in the plasma bulk and accelerated towards the electrodes to high energies by reversed electric fields during the local sheath collapse are found to induce the emission of a high number of δ-electrons, when they hit boundary surfaces. This regime is understood fundamentally based on the following approach: first, dual-frequency discharges with identical electrode materials are studied at different pressures and high-frequency driving voltages. Second, the effects of using electrodes made of different materials and characterized by different secondary electron emission coefficients are studied. The electron dynamics and charged particle distribution functions at boundary surfaces are determined including discharge asymmetries generated by using different materials at the powered and grounded electrodes.

Atmospheric pressure dual RF-LF frequency discharge: influence of LF voltage amplitude on the RF discharge behavior

This work is a contribution to a better understanding of dual frequency discharge at atmospheric pressure. Based on experiments and numerical modeling, it is focused on radio frequency (5 MHz)—low frequency (50 kHz) plane/plane dielectric barrier discharge in a Penning mixture (Ar-NH3). The discharge is in the α-RF mode, biased by an LF voltage with an amplitude ranging from 0 to 1300 V. When the LF amplitude increases, there is a threshold (around 600 V for a 2 mm gap) from which the light intensity (experiment) and the ionization level (modelling) drastically increase. In this work the physics of the RF-LF DBD below and above this threshold is studied. Depending on the respective RF and LF polarity, the net voltage applied to the gas is alternatively enhanced or reduced which induces an increase or a decrease in the ionization level. In all cases the ion drift to the cathode due to the LF voltage results in an ion loss and a production of secondary electrons. For an LF voltage amplitude lower than 600 V, the ion loss to the cathode is higher than the ion creation related to the secondary electrons. The consequence is a decrease in the plasma density. This density oscillates at a frequency equal to 2LF: it is at maximum each time the LF voltage amplitude is equal to 0 and at minimum when the LF voltage amplitude is at maximum. For an LF voltage amplitude higher than 600 V, when the LF and RF polarity are the same, the secondary electrons emission is high enough to counterbalance the ion loss, to enhance the bulk ionization and the discharge becomes a γ-RF. The gas voltage is controlled by the dielectric charge like a low frequency DBD. Around the gas voltage maximum, on each RF cycle, the discharge is alternatively an α-RF and a γ-RF discharge. When the discharge is in the γ mode, the ions flux at the cathode is increased by a factor of 40.

Effects of secondary electron emission on plasma characteristics in dual-frequency atmospheric pressure helium discharge by fluid modeling**Project supported by the National Natural Science Foundation of China (Grant No. 11505089).

A one-dimensional (1D) fluid simulation of dual frequency discharge in helium gas at atmospheric pressure is carried out to investigate the role of the secondary electron emission on the surfaces of the electrodes. In the simulation, electrons, ions of He+ and , metastable atoms of He* and metastable molecules of are included. It is found that the secondary electron emission coefficient significantly influences plasma density and electric field as well as electron heating mechanisms and ionization rate. The particle densities increase with increasing SEE coefficient from 0 to 0.3 as well as the sheathʼs electric field and electron source. Moreover, the SEE coefficient also influences the electron heating mechanism and electron power dissipation in the plasma and both of them increase with increasing SEE coefficient within the range from 0 to 0.3 as a result of increasing of electron density.

Electron Information in Single- and Dual-Frequency Capacitive Discharges at Atmospheric Pressure

Determining the electron properties of weakly ionized gases, particularly in a high electron-neutral collisional condition, is a nontrivial task; thus, the mechanisms underlying the electron characteristics and electron heating structure in radio-frequency (rf) collisional discharges remain unclear. Here, we report the electrical characteristics and electron information in single-frequency (4.52 MHz and 13.56 MHz) and dual-frequency (a combination of 4.52 MHz and 13.56 MHz) capacitive discharges within the abnormal α-mode regime at atmospheric pressure. A continuum radiation-based electron diagnostic method is employed to estimate the electron density (ne) and temperature (Te). Our experimental observations reveal that time-averaged ne (7.7–14 × 1011 cm−3) and Te (1.75–2.5 eV) can be independently controlled in dual-frequency discharge, whereas such control is nontrivial in single-frequency discharge, which shows a linear increase in ne and little to no change in Te with increases in the rf input power. Furthermore, the two-dimensional spatiotemporal evolution of neutral bremsstrahlung and associated electron heating structures is demonstrated. These results reveal that a symmetric structure in electron heating becomes asymmetric (via a local suppression of electron temperature) as two-frequency power is simultaneously introduced.

Atmospheric-pressure diffuse dielectric barrier discharges in Ar/O2 gas mixture using 200 kHz/13.56 MHz dual frequency excitation

Atmospheric-pressure diffuse dielectric barrier discharges (DBDs) were obtained in Ar/O2 gas mixture using dual-frequency (DF) excitation at 200 kHz low frequency (LF) and 13.56 MHz radio frequency (RF). The excitation dynamics and the plasma generation mechanism were studied by means of electrical characterization and phase resolved optical emission spectroscopy (PROES). The DF excitation results in a time-varying electric field which is determined by the total LF and RF gas voltage and the spatial ion distribution which only responds to the LF component. By tuning the amplitude ratio of the superimposed LF and RF signals, the effect of each frequency component on the DF discharge mechanism was analysed. The LF excitation results in a transient plasma with the formation of an electrode sheath and therefore a pronounced excitation near the substrate. The RF oscillation allows the electron trapping in the gas gap and helps to improve the plasma uniformity by contributing to the pre-ionization and by controlling the discharge development. The possibility of temporally modifying the electric field and thus the plasma generation mechanism in the DF discharge exhibits potential applications in plasma-assisted surface processing and plasma-assisted gas phase chemical conversion.

Effect of phase angle on plasma characteristics in electrically asymmetric capacitive discharge

In addition to the separate control of the ion energy and ion flux, the so-called electrical asymmetry effect (EAE) also plays an important role in improving the plasma radial uniformity. In this work, a two-dimensional fluid model combined with a full set of Maxwell equations is used to investigate the plasma characteristics in an electrically asymmetric capacitive discharge sustained by multiple consecutive harmonics. The effects of the phase angle θn on the dc self-bias (Vdc) and on the plasma radial uniformity for different numbers of consecutive harmonics k are discussed. The simulation results indicate that the phase angles of different harmonics θn have different influences on the dc self-bias Vdc. For instance, Vdc varies almost linearly with θ1 with a period π in dual frequency discharge, and the period is 2π for other discharge conditions. Besides, the modulation of Vdc becomes less obvious by changing the phase angle of the highest harmonic θk, especially for k>3. In addition, both the axial component of the power density Pz and the radial component of the power density Pr vary with θn, thus the plasma radial uniformity can be adjusted. When the total power density at the radial edge becomes comparable to that in the discharge center, the plasma distribution becomes uniform. For instance, when k=2, the plasma radial uniformity is the best at the phase angle θ1=π/2 and θ2=π. However, for k=3, the best radial uniformity is observed at θ1=3π/2, and the nonuniformity degree α is only 0.41% under this condition. It is worth noting that at k=8, the maximum of α is seven times higher than the minimum by changing the phase angles θ1 and θ2, which means that the plasma radial uniformity can be adjusted effectively. However, the modulation induced by θk(k>3) becomes less obvious, especially for k=8. Indeed, the electron density shows an edge-high profile, and the radial uniformity is always bad for all θ8 investigated. The results obtained in this work can help us to gain an insight into the optimization the plasma process by utilizing the EAE.

Kinetic investigation of the ion angular distribution in capacitive radio-frequency plasmas

One of the key parameters in the context of plasma assisted processing in semiconductor fabrication using capacitive radio-frequency plasmas is the ion flux distribution at the substrate. Whereas the ion energy distribution function determines the etching rate and selectivity, the ion angular distribution controls the etching profile. In this contribution, we reveal the effect of the ion flux and the sheath potential on the ion angular distribution and the direct ion heat flux at the bottom of etching profiles in geometrically symmetric plasma reactors. The ion angular distribution and the direct ion heat flux are calculated as a function of the sheath potential, the driving frequency, and the phase shift between the two distinct harmonics of the driving voltage of dual frequency discharges. For this task, self-consistent particle-in-cell simulations subject to Monte Carlo collision are carried out. The results from particle-in-cell simulations which are computationally very expensive are compared and verified with those from the novel ensemble-in-spacetime model. It is confirmed that increasing the voltage of the high-frequency component, the high-frequency component, and/or make a phase shift of π/2 between the dual frequency, narrow the ion angular distribution and increase the direct ion heat flux to the etching profile bottom. In all simulation cases, a correlation between the narrowing of the ion angular distribution and the increase of the sheath potential and the sheath ion flux is found.

The effect of realistic heavy particle induced secondary electron emission coefficients on the electron power absorption dynamics in single- and dual-frequency capacitively coupled plasmas

In particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations of capacitively coupled plasmas (CCPs), the plasma-surface interaction is generally described by a simple model in which a constant secondary electron emission coefficient (SEEC) is assumed for ions bombarding the electrodes. In most PIC/MCC studies of CCPs, this coefficient is set to γ = 0.1, independent of the energy of the incident particle, the electrode material, and the surface conditions. Here, the effects of implementing energy-dependent secondary electron yields for ions, fast neutrals, and taking surface conditions into account in PIC/MCC simulations is investigated. Simulations are performed using self-consistently calculated effective SEECs, , for ‘clean’ (e.g., heavily sputtered) and ‘dirty’ (e.g., oxidized) metal surfaces in single- and dual-frequency discharges in argon and the results are compared to those obtained by assuming a constant secondary electron yield of for ions. In single-frequency (13.56 MHz) discharges operated under conditions of low heavy particle energies at the electrodes, the pressure and voltage at which the transition between the α- and γ-mode electron power absorption occurs are found to strongly depend on the surface conditions. For ‘dirty’ surfaces, the discharge operates in α-mode for all conditions investigated due to a low effective SEEC. In classical dual-frequency (1.937 MHz + 27.12 MHz) discharges significantly increases with increasing low-frequency voltage amplitude, , for dirty surfaces. This is due to the effect of on the heavy particle energies at the electrodes, which negatively influences the quality of the separate control of ion properties at the electrodes. The new results on the separate control of ion properties in such discharges indicate significant differences compared to previous results obtained with different constant values of γ.

On singlet metastable states, ion flux and ion energy in single and dual frequency capacitively coupled oxygen discharges

We apply particle-in-cell simulations with Monte Carlo collisions to study the influence of the singlet metastable states on the ion energy distribution in single and dual frequency capacitively coupled oxygen discharges. For this purpose, the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is used, in which the discharge model includes the following nine species: electrons, the neutrals O(3P) and O), the negative ions O−, the positive ions O+ and O, and the metastables O(1D), O and O2(b). Earlier, we have explored the effects of adding the species O) and O2(b), and an energy-dependent secondary electron emission yield for oxygen ions and neutrals, to the discharge model. We found that including the two molecular singlet metastable states decreases the ohmic heating and the effective electron temperature in the bulk region (the electronegative core). Here we explore how these metastable states influence dual frequency discharges consisting of a fundamental frequency and the lowest even harmonics. Including or excluding the detachment reactions of the metastables O) and O2(b) can shift the peak electron temperature from the grounded to the powered electrode or vice versa, depending on the phase difference of the two applied frequencies. These metastable states can furthermore significantly influence the peak of the ion energy distribution for O-ions bombarding the powered electrode, and hence the average ion energy upon bombardment of the electrode, and lower the ion flux.

Electromagnetic attractive forming of sheet metals by means of a dual-frequency discharge current: design and implementation

The ability to generate an attractive force for electromagnetic forming is an interesting and challenging issue, compared with conventional electromagnetic repulsion processes. This work presents a discharge system with two sets of power supplies and a timing control system for the production of a dual-frequency discharge current in a single coil. The discharge current can be employed to generate an attractive force between the coil and the workpiece in the forming process. The effectiveness of the system was verified both by numerical simulations and by a series of experiments of sheet metal forming. Our results show that an AA 1060 aluminum alloy sheet with a thickness of 1 mm, at a distance of 9 mm from the coil bottom, can be attracted towards the coil with a maximum deformation of about 4.7 mm. We also demonstrate that there is an optimum value for deformation depth, which is related to the initial discharge voltage of the fast discharge system. The presented method and results can be helpful in designing electromagnetic forming systems and widening their applications.

Electron heating mode transition induced by mixing radio frequency and ultrahigh frequency dual frequency powers in capacitive discharges

Electron heating mode transitions induced by mixing the low- and high-frequency power in dual-frequency nitrogen discharges at 400 mTorr pressure are presented. As the low-frequency (13.56 MHz) power decreases and high-frequency (320 MHz) power increases for the fixed power of 200 W, there is a transition of electron energy distribution function (EEDF) from Druyvesteyn to bi-Maxwellian type characterized by a distinguished warm electron population. It is shown that this EEDF evolution is attributed to the transition from collisional to collisionless stochastic heating of the low-energy electrons.

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

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

Dual-frequency glow discharges in atmospheric helium

In this paper, the dual-frequency (DF) glow discharges in atmospheric helium were experimented by electrical and optical measurements in terms of current voltage characteristics and optical emission intensity. It is shown that the waveforms of applied voltages or discharge currents are the results of low frequency (LF) waveforms added to high frequency (HF) waveforms. The HF mainly influences discharge currents, and the LF mainly influences applied voltages. The gas temperatures of DF discharges are mainly affected by HF power rather than LF power.