Capacitive RF Discharge Research Articles

On certain properties of filamentary RF discharge

Results of studying the plasma-formation dynamics of the capacitive rf discharge in inert gases and in the air at atmospheric pressure are presented. Measurements have been conducted using an original technique for determining the sizes of luminous objects with an emission-line spectrum. The conditions for a transition from a torch discharge to a filamentary one are found. A qualitative model of the formation of a filament discharge taking into account its inherent electromagnetic field is offered.

The Effect of the Earthed Electrode Size on the Ignition Voltage of Low-Pressure RF Capacitive Discharge in Argon

The effect of the grounded electrode diameter on the ignition voltage using 13.56 MHz in argon gas is studied experimentally. The results indicate a systematic decrease of the breakdown voltage with increasing electrode area for the same pd value. No multi-valued breakdown voltages are observed. The Paschen minimum is not affected by the electrode diameter as long as the parallel plane approximation is valid. A modified Paschen equation which takes into account indirect discharge via the chamber walls at high pd values gives reasonable fits to the experimental data.

Study on Cu2S mineral surface modification by low temperature Ar/O2 plasmas

The oxidation of mineral and synthetic chalcocite (Cu2S), using low temperature plasmas, has been investigated and compared to thermodynamic calculations. The main aim of this work is to understand the fundamental interaction between mineral surfaces and low temperature plasmas, with a view to improving froth flotation by mineral pretreatments. Capacitive radio frequency (RF) discharge Ar/O2 plasmas, operating at different external parameters, have been used to treat powder samples, which resulted in surface modifications. These were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). In situ mass spectrometry (MS) was used to determine oxidation reaction rates. The energy flux density from the plasma on the sample surface was determined by active thermal probe measurements, and the density of atomic oxygen produced within the plasma zone was obtained by optical emission spectroscopy (OES). Sulfur dioxide (SO2), one reaction product of mineral Cu2S plasma oxidation, was emitted with a certain delay, which depends on energy flux density and atomic oxygen density. In contrast, no delay was found as synthetic Cu2S was treated. This indicates that the contamination by pyrite (FeS2), found in mineral samples, plays an important role, significantly influencing the mechanisms of plasma surface interaction. Comparisons of mass spectrometry (MS)-, XPS- and XRD-measurement results with thermodynamic calculations give evidence for a stepwise plasma processing, whereas the transition of sulfur atoms from FeS2 to Cu2S could be identified as a first step in forming cupric sulfate ( CuSO4). This effect might be used to develop selective plasma surface pretreatments for mineral mixtures in order to improve their separation efficiency of froth flotation.

Simulation study of wave phenomena from the sheath region in single frequency capacitively coupled plasma discharges; field reversals and ion reflection

Capacitively coupled radio-frequency (RF) discharges have great significance for industrial applications. Collisionless electron heating in such discharges is important, and sometimes is the dominant mechanism. This heating is usually understood to originate in a stochastic interaction between electrons and the electric fields. However, other mechanisms may also be important. There is evidence of wave emission with a frequency near the electron plasma frequency, i.e., ωpe, from the sheath region in collisionless capacitive RF discharges. This is the result of a progressive breakdown of quasi-neutrality close to the electron sheath edge. These waves are damped in a few centimeters during their propagation from the sheath towards the bulk plasma. The damping occurs because of the Landau damping or some related mechanism. This research work reports that the emission of waves is associated with a field reversal during the expanding phase of the sheath. Trapping of electrons near to this field reversal region is observed. The amplitude of the wave increases with increasing RF current density amplitude J̃0 until some maximum is reached, beyond which the wave diminishes and a new regime appears. In this new regime, the density of the bulk plasma suddenly increases because of ion reflection, which occurs due to the presence of strong field reversal near sheath region. Our calculation shows that these waves are electron plasma waves. These phenomena occur under extreme conditions (i.e., higher J̃0 than in typical experiments) for sinusoidal current waveforms, but similar effects may occur with non-sinusoidal pulsed waveforms for conditions of experimental interest, because the rate of change of current is a relevant parameter. The effect of electron elastic collisions on plasma waves is also investigated.

Critical evaluation of analytical models for stochastic heating in dual-frequency capacitive discharges

Dual-frequency capacitive discharges are widespread in the semiconductor industry and are used, for example, in etching of semiconductor materials to manufacture microchips. In low-pressure dual radio-frequency capacitive discharges, stochastic heating is an important phenomenon. Recent theoretical work on this problem using several different approaches has produced results that are broadly in agreement insofar as scaling with the discharge parameters is concerned, but there remains some disagreement in detail concerning the absolute size of the effect for the case of dual-frequency capacitive discharges. In this work, we investigate the dependence of stochastic heating on various discharge parameters with the help of particle-in-cell (PIC) simulation. The dual-frequency analytical models are in fair agreement with PIC results for values of the low-frequency current density amplitude Jlf (or dimensionless control parameter Hlf ∼ 5) typical of many modern experiments. However, for higher values of Jlf (or higher Hlf), new physical phenomena (like field reversal, reflection of ions, etc) appear and the simulation results deviate from existing dual-frequency analytical models. On the other hand, for lower Jlf (or lower Hlf) again the simulation results deviate from analytical models. So this research work produces a relatively extensive set of simulation data that may be used to validate theories over a wide range of parameters.

Simulation study of stochastic heating in single-frequency capacitively coupled discharges with critical evaluation of analytical models

Stochastic heating is an important phenomenon in low-pressure radio-frequency capacitive discharges. Recent theoretical work on this problem using several different approaches has produced results that are broadly in agreement insofar as scaling with the discharge parameters is concerned, but there remains some disagreement in detail concerning the absolute size of the effect. In this paper we investigate the dependence of stochastic heating on various discharge parameters by scaling these parameters with the help of particle-in-cell (PIC) simulation. The analytical models are satisfactory for intermediate current density amplitude (or control parameter H) and in agreement with PIC results. However, for higher (or higher H), new physical effects appear (such as field reversal, electron trapping, reflection of ions, etc) and the simulation results deviate from existing analytical models. In most experiments, but not all, H ∼ 5 and these effects are not observed there. On the other hand, for lower (or lower H) again the simulation results deviate from analytical models. So we have produced a relatively extensive set of simulation data that may be used to validate theories over a wide range of parameters. The dependence of stochastic heating on applied frequency is also investigated here with the help of the self-consistent PIC method.

Normal regime of the weak-current mode of an rf capacitive discharge

This paper studies the normal and abnormal regimes of a weak-current rf discharge in ammonia, nitrogen, hydrogen and N2O for the rf electric field frequencies of 13.56 and 27.12 MHz. We reveal that only the abnormal regime of burning is observed at low pressures when the current growth is accompanied by an rf voltage increase while the surface of the electrodes is completely covered with the discharge. The normal regime occurs at higher gas pressures when the current growth is due to the increase in the surface area occupied by the discharge on the electrodes. The discharge burns in the abnormal mode after the surface of the electrodes is completely covered with the discharge. We demonstrate that the normal current density is directly proportional to the gas pressure and it depends approximately on the square of the rf electric field frequency. We present an analytical model for two limiting cases: constant free path length and constant mobility of positive ions furnishing a satisfactory description of the experimental data.

Fundamental investigations of capacitive radio frequency plasmas: simulations and experiments

Capacitive radio frequency (RF) discharge plasmas have been serving hi-tech industry (e.g. chip and solar cell manufacturing, realization of biocompatible surfaces) for several years. Nonetheless, their complex modes of operation are not fully understood and represent topics of high interest. The understanding of these phenomena is aided by modern diagnostic techniques and computer simulations. From the industrial point of view the control of ion properties is of particular interest; possibilities of independent control of the ion flux and the ion energy have been utilized via excitation of the discharges with multiple frequencies. ‘Classical’ dual-frequency (DF) discharges (where two significantly different driving frequencies are used), as well as discharges driven by a base frequency and its higher harmonic(s) have been analyzed thoroughly. It has been recognized that the second solution results in an electrically induced asymmetry (electrical asymmetry effect), which provides the basis for the control of the mean ion energy. This paper reviews recent advances on studies of the different electron heating mechanisms, on the possibilities of the separate control of ion energy and ion flux in DF discharges, on the effects of secondary electrons, as well as on the non-linear behavior (self-generated resonant current oscillations) of capacitive RF plasmas. The work is based on a synergistic approach of theoretical modeling, experiments and kinetic simulations based on the particle-in-cell approach.

Comparison of a hybrid model to a global model of atmospheric pressure radio-frequency capacitive discharges

A one-dimensional hybrid analytical–numerical global model of atmospheric pressure radio-frequency (rf) driven capacitive discharges, previously developed, is compared with a basic global model. A helium feed gas with small admixtures of oxygen is studied. For the hybrid model, the electrical characteristics are calculated analytically as a current-driven homogeneous discharge. The electron power balance is solved analytically to determine a time-varying Maxwellian electron temperature, which oscillates on the rf timescale. Averaging over the rf period yields effective rate coefficients for gas phase activated processes. For the basic global model, the electron temperature is constant in time and the sheath physics is neglected. For both models, the particle balance relations for all species are integrated numerically to determine the equilibrium discharge parameters. Variations of discharge parameters with composition and rf power are determined and compared. The rate coefficients for electron-activated processes are strongly temperature dependent, leading to significantly larger neutral and charged particle densities for the hybrid model. For small devices, finite sheath widths limit the operating regimes to low O2 fractions. This is captured by the hybrid model but cannot be predicted from the basic global model.

The electrical asymmetry effect in geometrically asymmetric capacitive radio frequency plasmas

The electrical asymmetry effect (EAE) allows an almost ideal separate control of the mean ion energy, 〈Ei〉, and flux, Γi, at the electrodes in capacitive radio frequency discharges with identical electrode areas driven at two consecutive harmonics with adjustable phase shift, θ. In such geometrically symmetric discharges, a DC self bias is generated as a function of θ. Consequently, 〈Ei〉 can be controlled separately from Γi by adjusting the phase shift. Here, we systematically study the EAE in low pressure dual-frequency discharges with different electrode areas operated in argon at 13.56 MHz and 27.12 MHz by experiments, kinetic simulations, and analytical modeling. We find that the functional dependence of the DC self bias on θ is similar, but its absolute value is strongly affected by the electrode area ratio. Consequently, the ion energy distributions change and 〈Ei〉 can be controlled by adjusting θ, but its control range is different at both electrodes and determined by the area ratio. Under distinct conditions, the geometric asymmetry can be compensated electrically. In contrast to geometrically symmetric discharges, we find the ratio of the maximum sheath voltages to remain constant as a function of θ at low pressures and Γi to depend on θ at the smaller electrode. These observations are understood by the model. Finally, we study the self-excitation of non-linear plasma series resonance oscillations and its effect on the electron heating.

Study of an Asymmetric Capacitive Discharge in Oxygen with Global Model and Langmuir Probe

In this work an oxygen asymmetric capacitive radio frequency discharge was investigated with numerical global model and Langmuir Probe to evaluate the behavior of the electron density (ne) and the electron temperature (Te) as a function of input power and gas pressure. The experimental Langmuir probe measurements show that Te increases for pressures below 30 mTorr, which corresponds to low ne, as usual for Reactive Ion Etching type reactor. Moreover, the experimental and simulated electron temperature are in good agreement for gas pressure values above 25 mTorr, because in this case the electron energy distribution function (EEDF) is Maxwellian, according to assumption made in the global model, for the rate coefficient calculation. For electron density the discrepancy is higher for all pressure range, probably because the effect of secondary electron emission that is not considered in our global model simulations.

Gas molecule dissociation effect on rf discharge burning in low pressure ammonia

This Letter outlines the experimental study of molecule dissociation effect on rf capacitive discharge burning. We show that for each ammonia pressure value there exists some threshold rf voltage value below which the dissociation degree does not exceed 3%, but at higher rf voltage it grows to 30%. Increasing NH3 dissociation degree accelerates the discharge current growth against rf voltage. The rf discharge remains in a weak-current α-mode at low as well as at high dissociation degree because all ammonia dissociation products possess ionization potentials exceeding that for NH3.

Hybrid simulation of a dc-enhanced radio-frequency capacitive discharge in hydrogen

A PIC-MCC/fluid hybrid model was employed to study a parallel-plate capacitively coupled radio-frequency discharge in hydrogen, under the application of a dc bias voltage. When a negative dc voltage was applied to one of the electrodes of a continuous wave (cw) plasma, a ‘beam’ of secondary electrons was formed that struck the substrate counter-electrode at nearly normal incidence. The energy distribution of the electrons striking the substrate extended all the way to VRF + |Vdc|, the sum of the peak RF voltage and the absolute value of the applied dc bias. Such directional, energetic electrons may be useful for ameliorating charging damage in etching of high aspect ratio nano-features. The vibrational distribution function of molecular hydrogen was calculated self-consistently, and was found to have a characteristic plateau for intermediate values of the vibrational quantum number, v. When a positive dc bias voltage was applied synchronously during a specified time window in the afterglow of a pulsed plasma, the ion energy distributions (IEDs) of positive ions acquired an extra peak at an energy equivalent of the applied dc voltage. The electron energy distribution function was slightly and temporarily heated during the application of the dc bias pulse. The calculated IEDs of and ions in a cw plasma without dc bias were found to be in good agreement with published experimental data.

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.

Particle simulation of the nonlinear oscillation of electrons induced by a nanosecond pulse in rf capacitive hydrogen discharges

A particle-in-cell simulation was employed to investigate the nature and physical cause of the nonlinear oscillation of electrons induced by a nanosecond pulse in rf capacitive hydrogen discharges. It was found that the applied nanosecond pulse converted the plasma quickly from the bi-Maxwellian equilibrium formed in the rf capacitive discharge into another temporal bi-Maxwellian equilibrium. When the applied electric field collapses within a few nanoseconds, the electric field arising from the space charge serves as a restoring force to generate a swift oscillation of the electrons. The energy stored in the plasma is converted gradually into the chemical energy during the electron periodic movement. It is also found that the rise-, plateau-, and fall-times of the applied pulse affect the evolution of the electron energy distribution. The collective electron oscillation has a repetition frequency approximately equal to the electron plasma frequency, independent of pulse rise-, plateau-, and fall-times. This oscillation of electrons induced by a nanosecond pulse can be used to generate highly excited vibrational states of hydrogen molecules, which are a necessary precursor for negative hydrogen ions.

Mode coexistence in radio frequency capacitive discharge of flowing argon at atmospheric pressure with bare perforated electrodes

Three different modes (α, γ and their coexistence) of radio frequency (RF) capacitive discharge are achieved in flowing argon gap at atmospheric pressure with parallel bare metal electrodes perforated with honeycomb arrays of holes. The holes in the electrodes permit active species produced in discharges exiting out for applications such as surface modification and coating deposition. In this paper, pure argon is used as working gas for only discharge characterization. The gas is fed through via the electrode holes. With electrical measurements of discharge voltage, current and their phase angles, α and γ modes are identified and characterized. However, the measurements fail to distinguish the coexistence mode from γ mode since the I–V slopes of the coexistence mode and γ mode are analogous to each other, while the discharge photography can reveal all the three modes. Therefore, the OES (optical emission spectroscopy) of the discharges is measured not only to evaluate the discharge gas temperatures but also to distinguish the three modes. The 2nd positive bands of N2 and the violet bands of OH are simulated to determine the rotational temperature of N2 and OH, which is believe strongly correlated with gas temperature. The temperature trends vs discharge power can discriminate the three modes better than electrical measurement. By the way, the rotational temperature of OH is well consistent with that of nitrogen, which means that the neutral species are in thermal equilibrium for the three discharge modes.

Structure of RF capacitive discharge in swirl airflow at atmospheric pressure

Conditions for the onset of transitions between corona and pinch types of one-electrode radiofrequency capacitive discharges in swirl airflow at atmospheric pressure have been analyzed. It is established that the transitions observed at various values of the gas flow rate and swirl parameter are related primarily to the gasdynamic structure of airflow. Calculated data obtained in the framework of a proposed theoretical model are qualitatively consistent with the available experimental data.

Verification of collisionless sheath model of capacitive rf discharges by particle-in-cell simulations

A global model for high voltage rf argon capacitive discharges in the collisionless sheath regime is verified by particle-in-cell simulations, for both current- and voltage-driven sources. The ion energy distributions (IEDs) and the IED widths are investigated and show good agreement with a theoretical model, with proper adjustment of the dc bias voltage. The sensitivities of IEDs to sources (current or voltage driven) are described. It is found that for the same variations of rf source amplitudes, larger voltage shifts are expected in the IEDs for the current-driven than the voltage-driven cases. The effects of rf frequencies on IEDs are determined for a fixed rf voltage-driven source amplitude. The IEDs show a surprising independence of the rf frequencies, which can be understood reasonably well by the combined scalings of the global discharge model and IED theoretical model.

Secondary electrons in dual-frequency capacitive radio frequency discharges

Two fundamentally different types of dual-frequency (DF) capacitively coupled radio frequency discharges can be used for plasma processing applications to realize separate control of the ion mean energy, ⟨Ei⟩, and the ion flux, Γi, at the substrate surface: (i) classical discharges operated at substantially different frequencies, where the low- and high-frequency voltage amplitudes, ϕlf and ϕhf, are used to control ⟨Ei⟩ and Γi, respectively; (ii) electrically asymmetric (EA) discharges operated at a fundamental frequency and its second harmonic with fixed, but adjustable phase shift between the driving frequencies, θ. In EA discharges the voltage amplitudes are used to control Γi and θ is used to control ⟨Ei⟩. Here, we report our systematic simulation studies of the effect of secondary electrons on the ionization dynamics and the quality of this separate control in both discharge types in argon at different gas pressures. We focus on the effect of the control parameter for ⟨Ei⟩ on Γi for different secondary yields, γ. We find a dramatic effect of tuning ϕlf in classical DF discharges, which is caused by a transition from α- to γ-mode induced by changing ϕlf. In EA discharges we find that no such mode transition is induced by changing θ within the parameter range studied here and, consequently, Γi remains nearly constant as a function of θ. Thus, despite some limitations at high values of γ the quality of the separate control of ion energy and flux is generally better in EA discharges compared with classical DF discharges.

Tensile Properties and Morphology of Carbon Fibers Stabilized by Plasma Treatment

Commercial PAN fibers were thermally stabilized at 220 or 240 C for 30 min. Those fibers were further stabilized using radio-frequency (RF) capacitive plasma discharge during 5 or 15 min. From Fourier transform infrared spectroscopy results, it was observed that an additional plasma treatment led to further stabilization of PAN fibers. After stabilization, carbonization was performed to investigate the final tensile properties of the fabricated carbon fibers (CFs). The results revealed that a combination of thermal and plasma treatment is a possible stabilization process for manufacturing CFs. Morphology of CFs was investigated using scanning electron microscopy. The morphology shows that the plasma stabilization performed by the RF large gap plasma discharge may damage the surface of the CF, so it is necessary to select a proper process condition to minimize the damage.