Phase Resolved Optical Emission Research Articles

Experimental investigation of mode transitions in asymmetric capacitively coupled radio-frequency Ne and CF4 plasmas

The dependence of the electron density and the emission intensity on external parameters during the transitions of the electron power absorption mode is experimentally studied in asymmetric electropositive (neon) and electronegative (CF4) capacitively coupled radio-frequency plasmas. The spatio-temporal distribution of the emission intensity is measured with phase resolved optical emission spectroscopy and the electron density at the discharge center is measured by utilizing a floating hairpin probe. In neon discharge, the emission intensity increases almost linearly with the rf voltage at all driving frequencies covered here, while the variation of the electron density with the rf voltage behaves differently at different driving frequencies. In particular, the electron density increases linearly with the rf voltage at high driving frequencies, while at low driving frequencies the electron density increases slowly at the low-voltage side and, however, grows rapidly, when the rf voltage is higher than a certain value, indicating a transition from α to γ mode. The rf voltage, at which the mode transition occurs, increases with the decrease of the driving frequency/the working pressure. By contrast, in CF4 discharge, three different electron power absorption modes can be observed and the electron density and emission intensity do not exhibit a simple dependence on the rf voltage. In particular, the electron density exhibits a minimum at a certain rf voltage when the electron power absorption mode is switching from drift-ambipolar to the α/γ mode. A minimum can also be found in the emission intensity at a higher rf voltage when a discharge is switching into the γ mode.

Fundamental study of an industrial reactive HPPMS (Cr,Al)N process

In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the plasma is nearly constant for the frequency variation. In contrast, the metal to gas ion flux ratio is changed from JM/JG = 0.51 to JM/JG = 0.10 for increasing frequency. Regarding the coating properties, a structure refinement as well as lower residual stresses, higher universal hardness, and a changing crystal orientation from (111) to (200) were observed at higher frequencies. By correlating the plasma and coating properties, it can be concluded that the change in the gas ion to metal ion flux ratio results in a competitive crystal growth of the film, which results in changing coating properties.

Striations in electronegative capacitively coupled radio-frequency plasmas: Effects of the pressure, voltage, and electrode gap

Capacitively coupled radio-frequency (CCRF) CF4 plasmas have been found to exhibit a self-organized striated structure at operating conditions, where the plasma is strongly electronegative and the ion-ion plasma in the bulk region (largely composed of CF3+ and F– ions) resonates with the excitation frequency. In this work, we explore the effects of the gas pressure, the RF voltage, and the electrode gap on this striated structure by phase resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collisions simulations. The measured electronic excitation patterns at different external parameters show a good general agreement with the spatio-temporal plots of the ionization rate obtained from the simulations. For a fixed driving frequency, the minima of the CF3+ or F– ion densities (between the density peaks in the bulk) are comparable and independent of other external parameters. However, the ion density maxima generally increase as a function of the pressure or RF voltage, leading to the enhanced spatial modulation of plasma parameters. The striation gap (defined as the distance between two ion density peaks) is approximately inversely proportional to the pressure, while it exhibits a weak dependence on the RF voltage and the electrode gap. A transition between the striated and non-striated modes can be observed by changing either the pressure or the RF voltage; for 13.56 and 18 MHz driving frequencies, we present a phase diagram as a function of the pressure and voltage amplitude parameters.

Transient propagation dynamics of flowing plasmas accelerated by radio-frequency electric fields

Flowing plasmas are of significant interest due to their role in astrophysical phenomena and potential applications in magnetic-confined fusion and spacecraft propulsion. The acceleration of a charge-neutral plasma beam using the radio-frequency self-bias concept could be particularly useful for the development of neutralizer-free propulsion sources. However, the mechanisms that lead to space-charge compensation of the exhaust beam are unclear. Here, we spatially and temporally resolve the propagation of electrons in an accelerated plasma beam that is generated using the self-bias concept with phase-resolved optical emission spectroscopy. When combined with measurements of the extraction-grid voltage, ion and electron currents, and plasma potential, the pulsed-periodic propagation of electrons during the interval of sheath collapse at the grids is found to enable the compensation of space charge.

Striations in electronegative capacitively coupled radio-frequency plasmas: analysis of the pattern formation and the effect of the driving frequency

Self-organized striated structures of the plasma emission have recently been observed in capacitive radio-frequency CF4 plasmas by phase resolved optical emission spectroscopy (PROES) and their formation was analyzed and understood by particle in cell/Monte Carlo collision (PIC/MCC) simulations (Liu et al 2016 Phys. Rev. Lett. 116 255002). The striations were found to result from the periodic generation of double layers due to the modulation of the densities of positive and negative ions responding to the externally applied RF potential. In this work, an in-depth analysis of the formation of striations is given, as well as the effect of the driving frequency on the plasma parameters, such as the spatially modulated charged species densities, the electric field, and the electron power absorption is studied by PROES measurements, PIC/MCC simulations, and an ion–ion plasma model. The measured spatio-temporal electronic excitation patterns at different driving frequencies show a high degree of consistency with the simulation results. The striation gap (i.e., the distance between two ion density maxima) is found to be inversely proportional to the driving frequency. In the presence of striations the minimum ( ) ion densities in the bulk region exhibit an approximately quadratic increase with the driving frequency. For these densities, the eigenfrequency of the ion–ion plasma is near the driving frequency, indicating that a resonance occurs between the positive and negative ions and the oscillating electric field inside the plasma bulk. The maximum ion densities in the plasma bulk are found not to exhibit a simple dependence on the driving frequency, since these ion densities are abnormally enhanced within a certain frequency range due to the ions being focused into the ‘striations’ by the spatially modulated electric field inside the bulk region.

Power coupling mode transitions induced by tailored voltage waveforms in capacitive oxygen discharges

Low-pressure capacitively coupled radio frequency discharges operated in O2 and driven by tailored voltage waveforms are investigated experimentally and by means of kinetic simulations. Pulse-type (peaks/valleys) and sawtooth-type voltage waveforms that consist of up to four consecutive harmonics of the fundamental frequency are used to study the amplitude asymmetry effect as well as the slope asymmetry effect at different fundamental frequencies (5, 10, and 15 MHz) and at different pressures (50–700 mTorr). Values of the DC self-bias determined experimentally and spatio-temporal excitation rates derived from phase resolved optical emission spectroscopy measurements are compared with particle-in-cell/Monte Carlo collisions simulations. The spatio-temporal distributions of the excitation rate obtained from experiments are well reproduced by the simulations. Transitions of the discharge electron heating mode from the drift-ambipolar mode to the α-mode are induced by changing the number of consecutive harmonics included in the driving voltage waveform or by changing the gas pressure. Changing the number of harmonics in the waveform has a strong effect on the electronegativity of the discharge, on the generation of the DC self-bias and on the control of ion properties at the electrodes, both for pulse-type, as well as sawtooth-type driving voltage waveforms The effect of the surface quenching rate of oxygen singlet delta metastable molecules on the spatio-temporal excitation patterns is also investigated.

Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: excitation dynamics and ion flux asymmetry

Parallel plate capacitively coupled plasmas in hydrogen at relatively high pressure (~1 Torr) are excited with tailored voltage waveforms containing up to five frequencies. Predictions of a hybrid model combining a particle-in-cell simulation with Monte Carlo collisions and a fluid model are compared to phase resolved optical emission spectroscopy measurements, yielding information on the dynamics of the excitation rate in these discharges. When the discharge is excited with amplitude asymmetric waveforms, the discharge becomes electrically asymmetric, with different ion energies at each of the two electrodes. Unexpectedly, large differences in the fluxes to each of the two electrodes are caused by the different energies. When the discharge is excited with slope asymmetric waveforms, only weak electrical asymmetry of the discharge is observed. In this case, electron power absorption due to fast sheath expansion at one electrode is balanced by electron power absorption at the opposite electrode due to a strong electric field reversal.

A computationally assisted spectroscopic technique to measure secondary electron emission coefficients in radio frequency plasmas

A computationally assisted spectroscopic technique to measure secondary electron emission coefficients (γ-CAST) in capacitively-coupled radio-frequency plasmas is proposed. This non-intrusive, sensitive diagnostic is based on a combination of phase resolved optical emission spectroscopy and particle-based kinetic simulations. In such plasmas (under most conditions in electropositive gases) the spatio-temporally resolved electron-impact excitation/ionization rate features two distinct maxima adjacent to each electrode at different times within each RF period. While one maximum is the consequence of the energy gain of electrons due to sheath expansion, the second maximum is produced by secondary electrons accelerated towards the plasma bulk by the sheath electric field at the time of maximum voltage drop across the adjacent sheath. Due to these different excitation/ionization mechanisms, the ratio of the intensities of these maxima is very sensitive to the secondary electron emission coefficient γ. This sensitvity, in turn, allows γ to be determined by comparing experimental excitation profiles and simulation data obtained with various γ-coefficients. The diagnostic, tested here in a geometrically symmetric argon discharge, yields an effective secondary electron emission coefficient of for stainless steel electrodes.

Experimental investigations of electron heating dynamics and ion energy distributions in capacitive discharges driven by customized voltage waveforms

Capacitively coupled radio frequency plasmas driven by customized voltage waveforms provide enhanced opportunities to control process-relevant energy distributions of different particle species. Here, we present an experimental investigation of the spatio-temporal electron heating dynamics probed by Phase-Resolved Optical Emission Spectroscopy (PROES) in an argon discharge driven by up to three consecutive harmonics of 13.56 MHz with individually adjustable harmonics' amplitudes and phases. PROES and voltage measurements are performed at fixed total voltage amplitudes as a function of the number of driving harmonics, their relative phases, and pressure to study the effects of changing the applied voltage waveform on the heating dynamics in collisionless and collisional regimes. Additionally, the ion energy distribution function (IEDF) is measured at low pressure. In this collisionless regime, the discharge is operated in the α-mode. The velocity of energetic electron beams generated by the expanding sheaths is found to be affected by the number of driving harmonics and their relative phases. This is understood based on the sheath dynamics obtained from a model that determines sheath voltage waveforms. The formation of the measured IEDFs is understood and found to be directly affected by the observed changes in the electron heating dynamics. It is demonstrated that the mean ion energy can be controlled by adjusting the harmonics' phases. In the collisional regime at higher pressures changing the number of harmonics and their phases at fixed voltage is found to induce heating mode transitions from the α- to the γ-mode. Finally, a method to use PROES as a non-invasive diagnostic to monitor and detect changes of the ion flux to the electrodes is developed.

Phase resolved analysis of the homogeneity of a diffuse dielectric barrier discharge

Cold atmospheric pressure plasmas have already proven their ability of supporting the healing process of chronic wounds. Especially simple configurations like a dielectric barrier discharge (DBD), comprising of one driven electrode which is coated with a dielectric layer, are of interest, because they are cost-effective and easy to handle. The homogeneity of such plasmas during treatment is necessary since the whole wound should be treated evenly. In this investigation phase resolved optical emission spectroscopy is used to investigate the homogeneity of a DBD. Electron densities and reduced electric field distributions are determined with temporal and spatial resolution and the differences for applied positive and negative voltage pulses are studied.

Electron dynamics and frequency coupling in a radio-frequency capacitively biased planar coil inductively coupled plasma system

The electron dynamics in a planar coil inductively coupled plasma (ICP) system with a capacitively biased electrode is investigated using space and phase resolved optical emission spectroscopy. The two power source frequencies are exact multiple of each other and phase-locked. In this configuration, the system is investigated when the coil is operated in both E-mode and H-mode. The results show that in a phase synchronized RF biased ICP, the electrode bias power couples with the capacitive contribution of the coil, in both E-mode and H-modes, similar to dual-frequency capacitively coupled plasmas (2f-CCPs). It is also demonstrated that in H-mode, the phase between the electrode bias frequency and the ICP coil frequency influences the electron heating, similar to the electrical asymmetry effect in 2f-CCPs.

Influence of a phase-locked RF substrate bias on the E- to H-mode transition in an inductively coupled plasma

The effect of a capacitive radio frequency (RF) substrate bias on the E- to H-mode transition and electron-heating dynamics in a low-pressure inductively coupled plasma (ICP) operated in hydrogen is investigated by phase-resolved optical emission spectroscopy (PROES) and Langmuir probe measurements. The inductive and capacitive power sources are driven at the same frequency and operated in a phase-locked mode with fixed but adjustable phase between them, as well as without a phase lock. For both operations, when the discharge is in the E-mode, the plasma density is significantly influenced by the choice of capacitive power. This directly affects the mode transition power: already low values of bias power can substantially reduce the threshold for the E- to H-mode transition. This coupling between both power sources is strongly dependent on the adjustable phase between them and is attributed to a phase-sensitive confinement mechanism for the highly energetic electrons produced by the expanding sheaths at the substrate and at the ICP coil. At higher pressures the beam electrons do not interact with the opposing sheath and, consequently, the effect diminishes. Using phase-unlocked operation reduces the overall beam confinement and also results in less pronounced coupling effects. In contrast, by using electrodes with ring-shaped trenches the initial energy of the beam electrons is enhanced, increasing the influence of the RF bias on the operation of the ICP discharge.

Influence of surface conditions on plasma dynamics and electron heating in a radio-frequency driven capacitively coupled oxygen plasma

The impact of changing surface condition on plasma dynamics and electron heating is investigated by means of numerical simulations, based on a semi-kinetic fluid model approach, and compared with measurements of the nanosecond electron dynamics in the plasma-surface interface region using phase resolved optical emission spectroscopy (PROES). The simulations are conducted in a one-dimensional domain and account for a geometrical asymmetry comparable to the experimental setup of a radio-frequency driven capacitively coupled plasma in a gaseous electronics conference reference cell. A simple reaction scheme is considered, including electrons, positive ions, negative ions and metastable singlet delta oxygen (SDO) as individual species. The role of surface loss and effective lifetime of SDO is discussed. To simulate different surface conditions, the SDO surface loss probability and the secondary electron emission coefficient were varied in the model. It is found that a change in surface condition significantly influences the metastable concentration, electronegativity, spatial particle distributions and densities as well as the ionization and electron heating dynamics. The excitation dynamics obtained from simulations are compared with PROES measurements. This allows to determine experimentally relevant SDO surface loss probabilities and secondary electron emission coefficient values in-situ and is demonstrated for two different surface materials, namely aluminum and Teflon.

Electron heating during E-H transition in inductively coupled RF plasmas

A planar inductively coupled RF discharge (13.56 MHz) in argon and oxygen was exemplarily studied using space and phase resolved optical emission spectroscopy. The characteristic excitation rate pattern due to the electron heating during the sheath expansion was found for both gases in the E-mode. Furthermore, an intensive pattern in oxygen appears during the sheath collapse. This is associated with the electron heating caused by electric field reversal due to the strong electronegativity. The transition from the E- to the H-mode may be stepwise or continuous, depending on the gas type and total gas pressure. In the H-mode, significant differences in the excitation rate patterns exist. A broad and weakly modulated pattern is found over the RF cycle in argon, whereas in oxygen two separated patterns appear representing the electron heating for each half cycle. The reason may be the different excitation processes of the investigated resonant states and the influence of metastable argon atoms as well as attachment/detachment processes and dissociative recombination in oxygen. The E-H transition in oxygen at 5 Pa develops continuously and was studied in detail through the excitation rate. During the transition, the E- and H-mode are present and a hybrid mode was observed.

Propagation mechanisms of guided streamers in plasma jets: the influence of electronegativity of the surrounding gas

Atmospheric pressure plasma jets for biomedical applications are often sustained in He with small amounts of, for example, O2 impurities and typically propagate into ambient air. The resulting poorly controlled generation of reactive species has motivated the use of gas shields to control the interaction of the plasma plume with the ambient gas. The use of different gases in the shield yields different behavior in the plasma plume. In this paper, we discuss results from experimental and computational investigations of He plasma jets having attaching and non-attaching gas shields. We found that negative ion formation in the He-air mixing region significantly affects the ionization wave dynamics and promotes the propagation of negative guided streamers through an electrostatic focusing mechanism. Results from standard and phase resolved optical emission spectroscopy ratios of emission from states of N2 and He imply different electric fields in the plasma plume depending on the composition of the shielding gas. These effects are attributed to the conductivity in the transition region between the plasma plume and the shield gas, and the immobile charge represented by negative ions. The lower conductivity in the attaching mixtures enables more extended penetration of the electric field whereas the negative ions aid in focusing the electrons towards the axis.

Heating mode transition in capacitively coupled CF4 discharges: comparison of experiments with simulations

The electron heating mode transitions in capacitively coupled CF4 discharges were studied by synergistically using two diagnostic methods in combination with Particle-in-Cell/Monte Carlo collision (PIC/MCC) simulations. Based on the method of phase resolved optical emission spectroscopy of trace rare gas, the spatiotemporal evolutions of energetic electrons were presented. The time-average electron density at the discharge center was measured by using a hairpin probe. All the experimental results were compared with those obtained from PIC/MCC simulations. Two different electron heating modes were observed depending on the discharge conditions: (1) the α mode (or electropositive mode), in which the electron heating maximum occurs near the sheath boundary, dominated by the sheath electric field during its expansion phase, (2) the drift-ambipolar (DA) mode (or electronegative mode), in which the electron heating maxima occur inside the entire bulk plasma and near the collapsing sheath edge, dominated by the drift field inside the bulk and the ambipolar fields near the collapsing sheath edge, respectively. The transitions between the two modes were presented when changing the rf power, working pressure and driving frequency.By increasing the power, the heating mode experiences a transition from DA to α mode. This is ascribed to the fact that at high powers, the sheath heating is enhanced, leading to a drastic decrease in the electronegativity, and consequently the DA electric field is significantly reduced. By increasing the pressure, a heating mode transition from a pure α mode, then a combination of α and DA modes, finally into a DA mode is induced. We found that the mode transition is much more sensitive to the change of working pressure than that of rf power. When increasing the pressure, there is an evident enhancement in the electron attachment, which can generate the negative ions and deplete the electrons, resulting in a higher electronegativity as well as a higher DA field, and therefore the excitation and ionization in the bulk are enhanced. The driving frequency is found to significantly affect the electronegativity, i.e. as the driving frequency increases, the discharge becomes more electropositive, and the sheath heating (α mode) dominates. Furthermore, we conclude that as the driving frequency is increased, the pressure, at which the mode transition occurs, is increased, while the power, at which the mode transition occurs, is decreased.

Spatial phase-resolved optical emission spectroscopy for understanding plasma etching uniformity

Plasma chemistry of an oxygen-argon discharge in an electron cyclotron resonance (ECR) plasma etcher with a SiO2 wafer is observed. The study involves: Phase-resolved optical emission spectroscopy (PROES) of neutral atomic argon (Ar I) and oxygen (O I) spectral lines, spectroscopic ellipsometry of the wafer and a magnetic-field measurement of the ECR etcher's electro-magnet. Spatial PROES results together with the ellipsometry and magnetic-field measurements are used to assess the plasma etching uniformity of the SiO2 wafer. To evaluate the cross-dependences of the measured outputs for a wide range of process parameters, a design-of-experiment approach is taken. Spatial PROES of the oxygen atom shows a different spectral radiation pattern for the oxygen from the gas phase and those from the solid phase due to surface etching.

Instabilities in a capacitively coupled oxygen plasma

Periodic fluctuations in the frequency range from 0.3 to 3 kHz were experimentally investigated in capacitively coupled radio frequency (13.56 MHz) oxygen plasma. The Gaussian beam microwave interferometry directly provides the line integrated electron density fluctuations. A system of two Langmuir probes measured the floating potential spatially (axial, radial) and temporally resolved. Hence, the floating potential fluctuation development is mapped within the discharge volume and provides a kind of discharge breathing and no wave propagation. Finally, it was measured the optical emission pattern of atomic oxygen during the fluctuation as well as the RF phase resolved optical emission intensity at selected phase position of the fluctuation by an intensified charge-coupled device camera. The deduced excitation rate pattern reveals the RF sheath dynamics and electron heating mechanisms, which is changing between low and high electronegativity during a fluctuation cycle. A perturbation calculation was taken into account using a global model with 15 elementary collision processes in the balance equations for the charged plasma species (O2+, e, O−, O2−) and a harmonic perturbation. The calculated frequencies agree with the experimentally observed frequencies. Whereby, the electron attachment/detachment processes are important for the generation of this instability.

Strong ionization asymmetry in a geometrically symmetric radio frequency capacitively coupled plasma induced by sawtooth voltage waveforms.

The ionization dynamics in geometrically symmetric parallel plate capacitively coupled plasmas driven by radio frequency tailored voltage waveforms is investigated using phase resolved optical emission spectroscopy (PROES) and particle-in-cell (PIC) simulations. Temporally asymmetric waveforms induce spatial asymmetries and offer control of the spatiotemporal dynamics of electron heating and associated ionization structures. Sawtooth waveforms with different rise and fall rates are employed using truncated Fourier series approximations of an ideal sawtooth. Experimental PROES results obtained in argon plasmas are compared with PIC simulations, showing excellent agreement. With waveforms comprising a fast voltage drop followed by a slower rise, the faster sheath expansion in front of the powered electrode causes strongly enhanced ionization in this region. The complementary waveform causes an analogous effect in front of the grounded electrode.

Mode transition in a planar-coil inductively driven discharge caused by an external magnetic field

A hydrogen discharge inductively driven by a planar coil is studied by employing the phase resolved optical emission spectroscopy method, which permits observations not only on the stationary discharge structure but also of its time evolution over the cycle of the rf signal producing the discharge. Since the discharge is considered as a single element of a matrix source of negative hydrogen ions, it is equipped with an extraction device forming an additional grounded metal wall on the side opposite to that where the coil is positioned. Regarding use of a magnetic filter (a localized external magnetic field), the modifications in the discharge caused by the magnetic field have been studied. The results show: (i) transition of the discharge from a capacitive mode to an inductive one with the shift of the magnetic filter from the extraction device towards the coil, (ii) asymmetry both of the stationary and time-varying discharge structure of the inductive mode caused, respectively, by a diamagnetic drift and an -drift in the rf field, (iii) formation in the capacitive mode of the discharge of two electron beams, starting from the position of the magnetic filter, in addition to the beams well known as electron acceleration at the wall sheath expansion and (iv) asymmetry in the structure of the capacitive mode due to -drifts in the dc and rf fields.