Phase Resolved Optical Emission Spectroscopy Research Articles

An experimental and computational investigation of discharge mode transitions in a partially magnetized radio frequency capacitively coupled oxygen discharge

AbstractA magnetized capacitively coupled oxygen plasma was studied synergistically using phase resolved optical emission spectroscopy and particle‐based kinetic simulation. Discharge mode transitions from ambipolar mode into drift mode and finally into α mode induced by increasing the magnetic field were observed at different driving frequencies and different electrode gaps. The simulation results demonstrate that the discharge operating in the same mode exhibits a similar degree of electronegativity. By increasing driving frequency or electrode gap, the same discharge mode transition tends to occur at a lower magnetic field, and, meanwhile, the high electric field and electron power absorption shift from the bulk region to the sheath edge.

Deposition of hydrogenated amorphous carbon films by CH4/Ar capacitively coupled plasma using tailored voltage waveform discharges

We investigated the effects of tailored voltage waveform (TVW) discharges on the deposition of hydrogenated amorphous carbon (a-C:H) films in CH4/Ar capacitively coupled plasma. TVW discharges employ two driving radio frequencies (13.56 MHz and 27.12 MHz) and control their phase shifts to independently regulate ion bombardment energy (IBE) and ion flux. In this study, a-C:H films were deposited by changing DC-self bias with phase shift and constant applied voltage peak-to-peak. Additionally, we investigated phase-resolved optical emission spectroscopy (PROES) for plasma characterization. As a result, plasma-enhanced CVD (PECVD) for a-C:H films using TVW discharges realize control of film properties such as mass density, sp3 fraction, and H content, while keeping the deposition rate constant. Thus, it is suggested that TVW discharges realize the independent control of IBE and ion flux with high accuracy, highlighting its utility in a-C:H film depositions.

Effect of a negative DC bias on a capacitively coupled Ar plasma operated at different radiofrequency voltages and gas pressures

The effect of a negative DC bias, |V dc|, on the electrical parameters and discharge mode is investigated experimentally in a radiofrequency (RF) capacitively coupled Ar plasma operated at different RF voltage amplitudes and gas pressures. The electron density is measured using a hairpin probe and the spatio-temporal distribution of the electron-impact excitation rate is determined by phase-resolved optical emission spectroscopy. The electrical parameters are obtained based on the waveforms of the electrode voltage and plasma current measured by a voltage probe and a current probe. It was found that at a low |V dc|, i.e. in α-mode, the electron density and RF current decline with increasing |V dc|; meanwhile, the plasma impedance becomes more capacitive due to a widened sheath. Therefore, RF power deposition is suppressed. When |V dc| exceeds a certain value, the plasma changes to α–γ hybrid mode (or the discharge becomes dominated by the γ-mode), manifesting a drastically growing electron density and a moderately increasing RF current. Meanwhile, the plasma impedance becomes more resistive, so RF power deposition is enhanced with |V dc|. We also found that the electrical parameters show similar dependence on |V dc| at different RF voltages, and α–γ mode transition occurs at a lower |V dc| at a higher RF voltage. By increasing the pressure, plasma impedance becomes more resistive, so RF power deposition and electron density are enhanced. In particular, the α–γ mode transition tends to occur at a lower |V dc| with increase in pressure.

DYNAMICS OF PLASMA IN A DUAL-FREQUENCY CAPACITIVE DISCHARGE IN AN AR/XE MIXTURE

The spatiotemporal dynamics of the plasma of a symmetrical dual-frequency 81 MHz/1.76 MHz capacitive discharge under the in uence of a low-frequency eld of 1.76 MHz has been studied.Using the method of phase-resolved optical emission spectroscopy, the dynamics of the radiation intensity of argon and xenon in plasma was obtained.The electron energy distribution function at the center of the discharge was measured using a Langmuir probe and the electron density was measured using a microwave probe. The main result is the dynamics of the intensity ratios of the selected argon and xenon lines depending on di erent conditions: at pressures of 40, 200 and 400 mTorr, LF voltage amplitude of 100, 200 and 400 V, input power at 81 MHz of 3 W and 15 W. The dynamics of high-energy electrons is studied based on a two-temperature approximation of the electron energy distribution function.

An experimental and computational study on the ignition process of a pulse modulated dual-RF capacitively coupled plasma operated at various low-frequency voltage amplitudes

The ignition process of a pulse modulated capacitively coupled argon discharge driven simultaneously by two radio frequency voltages [12.5 MHz (high frequency) and 2.5 MHz (low frequency, LF)] is investigated by multifold experimental diagnostics and particle in cell / Monte Carlo collision simulations. In particular, (i) the effects of the LF voltage amplitude measured at the end of the pulse-on period, VL,end , on the spatiotemporal distribution of the electron impact excitation rate determined by phase resolved optical emission spectroscopy, and (ii) the electrical parameters acquired by analyzing the measured waveforms of the plasma current and voltage, are studied. Computed electrical parameters and spatio-temporal excitation maps show a good qualitative agreement with the experimental results. Especially, various breakdown mechanisms are found at different VL,end . At low values of VL,end , the ‘RF-avalanche’ mode (volume process) dominates the electron multiplication process. By increasing VL,end , the ionization caused by the volume electrons is suppressed and the electron loss at the electrodes is enhanced, leading to a delayed ignition. At higher values of VL,end , the avalanche ionization is significantly enhanced by ion-induced secondary electron emission at the electrodes, so that the ignition is significantly advanced.

Experimental Investigation on the Effect of “Linear-Rise” Amplitude Modulation on a Pulsed Capacitively Coupled Radio-frequency Argon Discharge

Objective: This work focused on the effect of “linear-rise” amplitude modulated waveform on the time evolution of some typical electrical and plasma parameters in a pulsed radio-frequency capacitively coupled argon discharge. Methods: The pulse-on phase of a square wave pulse is divided into two distinct stages, namely, the “linear rise” phase (T1 phase) and “constant amplitude” phase (T2 phase). The study focused on exploring the impact of varying T1 durations on the temporal evolutions of these parameters. In the experiment, the electron density was measured time-resolved by a hairpin probe. Phase-resolved optical emission spectroscopy was employed to determine the spatial-temporal distribution of the electron impact excitation dynamics. Additionally, we determined the amplitudes and the relative phase of the discharge voltage and current, as well as the power deposition by analyzing the waveforms obtained from a voltage and a current probe. Results: It was found that with the increase of T1, the critical voltage value required for the plasma ignition becomes lower, and the RF power and the light intensity overshoot less significantly. Upon the overshoot, the plasma when the light intensity exhibits the maximum is dominated by the “overshoot” mode. After the pulse is turned off, the electron density under different T1 durations first decreases with the same rate, and then the decay rate is decreased with the increase of T1. This is because for a longer T1, the neutral gas temperature gets lower, leading to a higher density of neutral gas, so the electron diffusion loss is suppressed. Conclusion: Compared to the square pulse modulated radio frequency capacitively coupled plasmas, the “linear rise” amplitude modulation approach introduces external control knobs to modulate the plasma parameters, which benefits practical material processing. This paper is an experimental measurement implemented in an electropositive gas Ar discharge, and is expected to be extended to the plasmas operated in complex electronegative gases in the future.

Experimental validation of particle-in-cell/Monte Carlo collisions simulations in low-pressure neon capacitively coupled plasmas

Plasma simulations are powerful tools for understanding fundamental plasma science phenomena and for process optimisation in applications. To ensure their quantitative accuracy, they must be validated against experiments. In this work, such an experimental validation is performed for a one dimensional in space and three dimensional in velocity space particle-in-cell simulation complemented with the Monte Carlo treatment of collision processes of a capacitively coupled radio frequency plasma driven at 13.56 MHz and operated in neon gas. In a geometrically symmetric reactor the electron density in the discharge centre and the spatio-temporal distribution of the electron impact excitation rate from the ground into the Ne 2p1 level are measured by a microwave cutoff probe and phase resolved optical emission spectroscopy, respectively. The measurements are conducted for electrode gaps between 50 mm and 90 mm, neutral gas pressures between 20 mTorr and 50 mTorr, and peak-to-peak values of the driving voltage waveform between 250 V and 650 V. Simulations are performed under identical discharge conditions. In the simulations, various combinations of surface coefficients characterising the interactions of electrons and heavy particles with the anodised aluminium electrode surfaces are adopted. We find, that the simulations using a constant effective heavy particle induced secondary electron (SE) emission coefficient of 0.3 and a realistic electron–surface interaction model (which considers energy-dependent and material specific elastic and inelastic electron reflection, as well as the emission of true SEs from the surface) yield results which are in good quantitative agreement with the experimental data.

High-speed plasma diagnostics based on the floating harmonic method in inductively coupled plasma and pulsed plasma

Abstract The floating harmonic method is a diagnostic technique for obtaining plasma parameters, such as ion density and electron temperature, by applying a sinusoidal voltage to a floating probe. The typically applied frequency is in the kilohertz range. This method has been widely used in plasma diagnostics of semiconductor processes due to its robustness to RF fluctuations and fast measurement speed. However, recently, pulsed plasma has become common in semiconductor processes. As the plasma sheath is analyzed with a high-time-resolution diagnostic method such as phase-resolved optical emission spectroscopy, the development of high-speed plasma diagnostic techniques has become increasingly important. In this study, we investigated high-speed plasma diagnostic measurements based on the floating harmonic method. When the frequency of the voltage applied to the floating probe increases up to 1 MHz, the electron temperature can be underestimated due to the currents flowing through the capacitive sheath and the ceramic sleeve of the probe. We found that the displacement current of the probe sheath increases rapidly compared to the conduction current as the plasma density and electron temperature decrease. We also removed the additional harmonic currents flowing through the ceramic sleeve via two approaches. The plasma parameters obtained using the proposed method are in good agreement with the measurements performed using the floating harmonic method in the kilohertz range. Moreover, the electron temperature of the pulsed plasma was measured.

Local enhancement of electron heating and neutral species generation in radio-frequency micro-atmospheric pressure plasma jets: the effects of structured electrode topologies

The effects of structured electrode topologies on He/O2 radio frequency micro-atmospheric pressure plasma jets driven at 13.56 MHz are investigated by a combination of 2D fluid simulations and experiments. Good qualitative agreement is found between the computational and experimental results for the 2D spatio-temporally resolved dynamics of energetic electrons measured by phase resolved optical emission spectroscopy, 2D spatially resolved helium metastable densities measured by tunable diode laser absorption spectroscopy and 2D spatially resolved atomic oxygen densities measured by two photon absorption laser induced fluorescence. The presence of rectangular trenches of specific dimensions inside the electrodes is found to cause a local increase of the electron power absorption inside and above/below these surface structures. This method of controlling the electron energy distribution function via tailored surface topologies leads to a local increase of the metastable and atomic oxygen densities. A linear combination of trenches along the direction of the gas flow is found to result in an increase of the atomic oxygen density in the effluent, depending linearly on the number of trenches. These findings are explained by an enhanced Ohmic electric field inside each trench, originating from (a) the low electron density, and, consequently, the low plasma conductivity inside the trenches, and (b) the presence of a current focusing effect as a result of the electrode topology.

Propagation dynamics and interaction of multiple streamers at and above adjacent dielectric pellets in a packed bed plasma reactor

The propagation and interaction between surface streamers propagating over dielectric pellets in a packed bed plasma reactor operated in Helium are studied using phase and space resolved optical emission spectroscopy and simulations. Such a discharge is known to generate cathode directed positive streamers in the gas phase at the positions of minimum electrode gap followed by surface streamers that propagate along the dielectric surface. By systematically varying the gap between neighboring dielectric pellets, we observe that a larger gap between adjacent dielectric pellets enhances plasma emission near the contact points of the dielectric structures. In agreement with the experiment, the simulation results reveal that the gap influences the attraction of streamers towards adjacent dielectric pellets via polarization of the surface material and the repulsion induced by nearby streamers. For a smaller gap, the streamer propagation changes from along the surface to propagation through the volume and back to surface propagation due to a combination of repulsion between adjacent streamers, polarization of adjacent dielectric surfaces, as well as acceleration of electrons from the volume towards the streamer head. For a wider gap, the streamer propagates along the surface, but repulsion by neighboring streamers increases the offset between the streamers. The streamer achieves a higher speed near the contact point earlier in the absence of an adjacent streamer, which indicates the role of mutual streamer interaction via repulsion.

Experimental and simulation study of a capacitively coupled radiofrequency plasma with a structured electrode

A low-pressure capacitively coupled radiofrequency (RF) helium discharge with a structured electrode is investigated experimentally and via kinetic simulations. In the experiment, phase resolved optical emission spectroscopy provides information about the excitation dynamics by high energy electrons, with high spatial and nanosecond temporal resolution within the RF (13.56 MHz) period. The numerical studies are based on a newly developed 2d3v particle-in-cell/Monte Carlo collisions code carried out on graphics processing units. The two approaches give consistent results for the penetration of the plasma into the trench situated in one of the electrodes and the particular electron dynamics resulting from the presence of the structured electrode. In addition, the fluxes of He+ ions and vacuum ultraviolet photons incident on the different surfaces in and around the trench structure are studied. These are discussed with respect to the homogeneous treatment of complex structures, relevant for advanced surface modification and disinfection processes.

Electron power absorption in capacitively coupled neon–oxygen plasmas: a comparison of experimental and computational results

Phase resolved optical emission spectroscopy (PROES) measurements combined with 1d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are used to study the electron power absorption and excitation/ionization dynamics in capacitively coupled plasmas (CCPs) in mixtures of neon and oxygen gases. The study is performed for a geometrically symmetric CCP reactor with a gap length of 2.5 cm at a driving frequency of 10 MHz and a peak-to-peak voltage of 350 V. The pressure of the gas mixture is varied between 15 Pa and 500 Pa, while the neon/oxygen concentration is tuned between 10% and 90%. For all discharge conditions, the spatio-temporal distributions of the electron-impact excitation rate from the Ne ground state into the Ne 2p53p0 state measured by PROES and obtained from PIC/MCC simulations show good qualitative agreement. Based on the emission/excitation patterns, multiple operation regimes are identified. Localized bright emission features at the bulk boundaries, caused by local maxima in the electronegativity are found at high pressures and high O2 concentrations. The relative contributions of the ambipolar and the Ohmic electron power absorption are found to vary strongly with the discharge parameters: the Ohmic power absorption is enhanced by both the high collisionality at high pressures and the high electronegativity at low pressures. In the wide parameter regime covered in this study, the PROES measurements are found to accurately represent the ionization dynamics, i.e. the discharge operation mode. This work represents also a successful experimental validation of the discharge model developed for neon–oxygen CCPs.

Striations in dual-low-frequency (2/10 MHz) driven capacitively coupled CF4 plasmas

In electronegative radiofrequency plasmas, striations (STRs) can appear if the bulk plasma is dominated by positive and negative ions that can react to the driving frequency. Here, we investigate such self-organized structures in dual-frequency (2/10 MHz) capacitively coupled CF4 plasmas by phase-resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collision simulations. This choice of the frequencies is made to ensure that the ions can react to both the lower (2 MHz, ‘low frequency’, LF) and the higher (10 MHz, ‘high frequency’, HF) components of the excitation waveform. A strong interplay of the two excitation components is revealed. As the STRs appear in the plasma bulk, their number depends on the length of this region. By increasing the LF voltage, ϕ LF, the sheath widths at both electrodes increase, the bulk is compressed and the number of STRs decreases. The maximum ion density decreases slightly as a function of ϕ LF, too, due to the compressed plasma bulk, while the minimum of the ion density remains almost constant. The spatio-temporal distributions of the excitation and ionization rates are modulated both by the LF and HF with maxima that occur at the first HF period that follows the complete sheath collapse at a given electrode. These maxima are caused by a high local ambipolar electric field. At a given phase within a HF period the current density is different at different phases within the LF period because of frequency coupling. The LF components of the F− ion velocity and of the electric field are much lower than the respective HF components due to the lower LF component of the displacement current in the sheaths. The LF component of the total current is dominated by the ion current at low values of ϕ LF but by the electron current at high values. The HF component of the total current is dominated by the electron current and decreases slightly as a function of ϕ LF.

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 study on the ignition process of a pulsed capacitively coupled RF discharge: Effects of gas pressure and voltage amplitude

The effects of gas pressure and voltage amplitude on the ignition process of a pulse capacitively coupled RF argon discharge are experimentally investigated. The electron density is measured by a hairpin probe, the spatiotemporal distribution of the electron impact excitation dynamics is determined by phase resolved optical emission spectroscopy, and the electrical parameters are obtained by analyzing the measured current and voltage waveforms. In this work, the pulse plasma is ignited with few initial electrons, so the ignition process behaves like gas breakdown. Based on the measured RF breakdown curve, the gas pressures and voltage amplitudes are selected, and then different characteristics of ignition processes are compared and discussed in detail. Particularly, the spatiotemporal pattern of the electron impact excitation rate obtained within the selected pressure range, as well as other results, aid the intuitive understanding of a typical “V-shaped” RF breakdown curve. At lower pressures, the excitation pattern exhibit shorter and tilted regions, ending at electrodes during the early ignition stage, implying a substantial electron energy loss, while at relatively high pressures, the excitation pattern becomes wider and less tilted, and the proportion of electron energy consumed by excitation processes increases. In addition, by increasing the voltage amplitude, the ignition is advanced and becomes more significant, manifesting a faster increase in discharge current and a stronger overshoot of RF power deposition. Meanwhile, at high voltage amplitude, the excitation pattern exhibits complex spatiotemporal distribution due to enhanced local electric field when the plasma emission intensity overshoots.

Wave-like emission propagation and fine structures at the contact points of adjacent dielectric pellets in packed bed plasma reactors (PBPRs) operated in helium

Packed bed plasma reactors (PBPRs) inherently have complex geometries where the volume between the electrodes is filled with dielectric/catalytic pellets to form a large array of voids. While the dimension of the plasma region can be several centimeters, the size of a single void at the edges and pores of dielectrics/pellets can reach micrometer dimensions. The understanding of plasma propagation on these diverse length scales is essential for optimizing and controlling plasma processes performed in such discharges. It is known that plasmas are generated in PBPRs as multiple pulses due to cathode-directed positive streamers in the volume, surface ionization waves, or surface streamers over the dielectric surface and stationary microdischarges at the contact points of adjacent dielectrics. In this work, we have investigated the discharge formation and propagation as a function of applied voltage in simplified PBPRs with a single layer of hexagonally arranged hemispherical pellets, operated in helium, using phase and space resolved optical emission spectroscopy. Despite similar discharge conditions at multiple positions, the emission intensity during each pulse spreads like a wave from the center to the edges in the whole discharge cell. The emission due to surface ionization waves is significantly reduced compared to earlier works. These observations could be explained by possible interactions between adjacent microdischarges, already known in other arrays of microdischarges or adjacent streamers. Higher resolution images of the contact points show that the discharge has fine structures with stronger emission at the edges of the contact points; this effect is enhanced as a function of the driving voltage amplitude. This is possibly the consequence of non-uniform electric field distribution at the contact points due to the polarization of dielectrics.

Comprehensive understanding of the ignition process of a pulsed capacitively coupled radio frequency discharge: the effect of power-off duration

The effect of the pulse-off duration on the time evolution of the plasma and electrical parameters during the ignition phase in a pulsed capacitively coupled radio frequency argon discharge operated at 450 mTorr and 12.5 MHz is investigated synergistically by multifold experimental diagnostics, particle-in-cell/Monte Carlo collision simulations and an analytical model. In the experiment, the electron density is measured time-resolved by a hairpin probe, the spatio-temporal distribution of the electron impact excitation dynamics is studied by phase resolved optical emission spectroscopy, and the amplitudes and the relative phase, φ vi, of the discharge voltage and current are determined based on the waveforms measured by a voltage and a current probe. The experimental results show that the plasma and electrical parameters during the ignition process depend strongly on the duration of the afterglow period, T off, primarily because of the dependence of the remaining charge density on this parameter. Computed values of φ vi show a similar time-dependence compared to the experiment, if the simulations are initialized with specific initial charged particle densities, n ini. This allows us to further understand the time evolution of φ vi for different values of T off based on the simulation results together with an analytical model. In particular, the optical emission intensity is found to change with time in the same fashion as the power deposition into the system at T off ⩾ 100 μs, suggesting that the power is primarily absorbed by the electrons, which dissipate their energy via inelastic collisions. The system goes through different mode transitions of electron power absorption during the ignition phase depending on T off. Specifically, for short T off (high n ini), the α mode dominates during the entire ignition process, as the electric field is largely shielded by the abundant charge located in the interelectrode space. For intermediate values of T off (moderate n ini), another excitation pattern caused by an enhanced drift electric field at the center of the gap is observed, since a large fraction of the externally applied potential can penetrate into the central region in the absence of high charged particle densities. For longer T off (very low n ini), the ignition of the pulsed plasma behaves like a gas breakdown.

2D spatially resolved O atom density profiles in an atmospheric pressure plasma jet: from the active plasma volume to the effluent

Two-dimensional spatially resolved absolute atomic oxygen densities are measured within an atmospheric pressure micro plasma jet and in its effluent. The plasma is operated in helium with an admixture of 0.5% of oxygen at 13.56 MHz and with a power of 1 W. Absolute atomic oxygen densities are obtained using two photon absorption laser induced fluorescence spectroscopy. The results are interpreted based on measurements of the electron dynamics by phase resolved optical emission spectroscopy in combination with a simple model that balances the production of atomic oxygen with its losses due to chemical reactions and diffusion. Within the discharge, the atomic oxygen density builds up with a rise time of 600 µs along the gas flow and reaches a plateau of 8 × 1015 cm−3. In the effluent, the density decays exponentially with a decay time of 180 µs (corresponding to a decay length of 3 mm at a gas flow of 1.0 slm). It is found that both, the species formation behavior and the maximum distance between the jet nozzle and substrates for possible oxygen treatments of surfaces can be controlled by adjusting the gas flow.

The effect of a negative direct-current voltage on striated structures and electrical parameters in a capacitively coupled rf discharge in CF4

The effects of a negative direct-current (dc) voltage on the striated structures and the electrical parameters have been studied by multi-fold experimental diagnostics and particle-in-cell/Monte Carlo collision (PIC/MCC) simulations in a dc superposed rf (radio-frequency, 8 MHz) capacitive discharge in CF4. The electron density, the spatio-temporal distribution of electron-impact excitation rate and the electrical parameters measured by a hairpin probe, phase resolved optical emission spectroscopy and a voltage and a current probe are compared with the corresponding simulation results. With the increase of the magnitude of the dc voltage, , all the experimental observations are well reproduced and analyzed by the simulations. It was found that the plasma bulk is compressed and the electron density decreases slightly at low , while the plasma bulk broadens at high when the ionization at the edge of the completely expanded sheath adjacent to the powered electrode is significantly enhanced due to the secondary electron emission (SEE). By increasing , the region of the strong ionization/excitation shrinks towards the edge of the collapsing sheath adjacent to the grounded electrode. And the ionization inside the bulk region is significantly suppressed during the collapsing phase of the sheath adjacent to the powered electrode. This is mainly attributed to the fact that the dc component of the bulk electric field and the spatial gradient of the electron density are simultaneously enhanced during the transition from the ‘striation’ mode to ‘striation-γ’ hybrid mode when increasing . Besides, we found that increasing can somewhat suppress the rf power deposition at low , and enhance the rf power deposition at high . The magnitude of the dc current exhibits a complex dependence on , due to nonlinear change of the dc resistance of the plasma.

Optical emission intensity overshoot and electron heating mechanisms during the re-ignition of pulsed capacitively coupled Ar plasmas

Phase resolved optical emission spectroscopy (PROES) measurements were combined with measurements of the optical emission intensity (OEI) and electrical characteristics (RF current and voltage, power, and DC bias voltage) as a function of time during the re-ignition of Ar plasmas pulsed at 100 Hz and 10 kHz. The OEI exhibits a large overshoot at the 100 Hz pulsing rate even though no such overshoot is present in any of the electrical characteristics. The OEI overshoot occurs at a point in time when the RF power, voltage, DC bias voltage, and electron density are all smaller than they become later in the glow. PROES measurements in combination with the time resolved electrical characteristics indicate that the heating mechanism for the electrons changes during the time of the overshoot in the OEI from stochastic heating to a combination of stochastic and ohmic heating. This combination appears to enable a more efficient transfer of the electrical energy into the electrons.