Dirk Luggenhölscher

Plasma diagnostics by optical emission spectroscopy on argon and comparison with Thomson scattering

A novel optical emission spectroscopy (OES) technique for the determination of electron temperatures and densities in low-pressure argon discharges is compared with Thomson scattering (TS). The emission spectroscopy technique is based on the measurement of certain line ratios in argon and a collisional‐radiative model (CRM) including metastable transport. The investigations are carried out in a planar inductively coupled neutral loop discharge (NLD) over a wide range of pressures, p = 0.05Pa‐5Pa. This discharge is a weakly magnetized novel radio-frequency (rf) plasma source, proposed for plasma etching. The NLD is operated in pure argon at a frequency of f = 13.56MHz and powers varied between P = 1kW and 2kW. Both diagnostics, OES and TS, are applied in parallel. The electron energy distribution functions obtained by TS are clearly Maxwellian at low pressures but exhibit a certain enhancement of the energetic tail at higher pressures. Electron densities and temperatures obtained by both diagnostic techniques are compared. Further, absolute numbers of the metastable densities derived from the measurement by the CRM are compared with earlier measurements under similar conditions. Excellent agreement is found throughout if depletion of the neutral gas density by increasing gas temperature and electron pressure is included in the CRM. Electron pressure is the dominant depletion mechanism at gas pressures p 0.1Pa and rf powers P> 1kW. There, the electron pressure exceeds more than 3 times the neutral pressure and the ionization degree approaches 7% while at pressures p> 1Pa the degree of ionization is relatively low (<10 −3 ) and neutral gas depletion is dominated by gas heating. (Some figures in this article are in colour only in the electronic version)

Stochastic heating in asymmetric capacitively coupled RF discharges

Electron dynamics in a strongly asymmetric capacitively coupled radio-frequency (RF) discharge at low pressures is investigated by a combination of various diagnostics, analytical models and simulations. Electric fields in the sheath are measured phase and space resolved using fluorescence dip spectroscopy in krypton. The results are compared with a fluid sheath model. Experimentally obtained input parameters are used for the model. The excitation caused by beam-like highly energetic electrons is measured by phase resolved optical emission spectroscopy (PROES) and compared with the results of a hybrid Monte Carlo model based on the electric field resulting from the sheath model. The plasma itself is characterized by Langmuir probe measurements in terms of electron density, electron mean energy and electron energy distribution function (EEDF). The RF voltage and the current to the chamber wall are measured in parallel. At low pressures the plasma series resonance (PSR) effect is observed. It leads to high frequency oscillations of the current (non-sinusoidal RF current waveforms) and, consequently, to a faster sheath expansion. The measured current is compared with an analytical PSR model. Another analytical model using experimentally obtained input parameters determines the influence of beams of highly energetic electrons on the time averaged isotropic EEDF as measured by Langmuir probes. The main result is the observation of beams of highly energetic electrons during the sheath expansion phase, that are enhanced by the PSR effect. The paper shows that the nature of stochastic heating is closely related to electron beams and the PSR effect.

Experimental and modeling analysis of fast ionization wave discharge propagation in a rectangular geometry

Fast ionization wave (FIW), nanosecond pulse discharge propagation in nitrogen and helium in a rectangular geometry channel/waveguide is studied experimentally using calibrated capacitive probe measurements. The repetitive nanosecond pulse discharge in the channel was generated using a custom designed pulsed plasma generator (peak voltage 10–40 kV, pulse duration 30–100 ns, and voltage rise time ∼1 kV/ns), generating a sequence of alternating polarity high-voltage pulses at a pulse repetition rate of 20 Hz. Both negative polarity and positive polarity ionization waves have been studied. Ionization wave speed, as well as time-resolved potential distributions and axial electric field distributions in the propagating discharge are inferred from the capacitive probe data. ICCD images show that at the present conditions the FIW discharge in helium is diffuse and volume-filling, while in nitrogen the discharge propagates along the walls of the channel. FIW discharge propagation has been analyzed numerically usi...

Electron beams in asymmetric capacitively coupled radio frequency discharges at low pressures

The generation of directed energetic electrons by the expanding sheath is observed in asymmetric capacitively coupled radio frequency discharges at low pressures (≤ 1 Pa) in different gases. The phenomenon of such electron beams is investigated by a combination of experimental diagnostics, an analytical model and simulations. At sufficiently low pressures multiple reflections of electron beams at the plasma boundaries are observed. An analytical model shows how these beams lead to an enhanced high energy tail of the electron energy distribution function. Thus, stochastic heating is closely related to electron beams.

Space and time resolved electric field measurements in helium and hydrogen RF-discharges

The sheath dynamics of helium and hydrogen RF-discharges at 13.56 MHz in the GEC reference cell are studied by laser spectroscopic electric field measurements. In case of helium, the Stark splitting of the n = 11 Rydberg state is measured by LIF spectroscopy applied to metastable helium atoms in the 2s1S0 state. A novel laser spectroscopic technique in atomic hydrogen allows the sensitive measurements of electric fields down to about 10 V cm-1. Two-dimensional space and time resolved results are presented. Sheath voltages, sheath ion and net charge densities and displacement current densities are directly derived from the measured electric fields. Under certain assumptions, also the electron conduction and ion current densities, the power dissipated in the discharge and the ion energy distribution function can be inferred. The experimental results show that the common step-model for the spatial electron distribution in the sheath is a reasonable approximation in the case of helium but is less appropriate in hydrogen. In the hydrogen RF-discharge, field reversal during the anodic phase of the applied voltage is observed and investigated in detail. A simple analytic model for the field reversal effect is developed that describes quantitatively the experimental results.

Different modes of electron heating in dual-frequency capacitively coupled radio frequency discharges

A systematic study of different modes of electron heating in dual-frequency capacitively coupled radio frequency (CCRF) discharges is performed using a particle-in-cell simulation. Spatio-temporal distributions of the total excitation/ionization rates under variation of gas pressure, applied frequencies and gas species are discussed. Some results are compared qualitatively with an experiment (phase resolved optical emission spectroscopy) operated under conditions similar to a parameter set used in the simulation. Different modes of electron heating are identified and compared with α- and γ-mode operation of single-frequency CCRF discharges. In this context the frequency coupling and its relation to the ion density profile in the sheath are discussed and quantified. In light gases the ion density in the sheath is time modulated. This temporal modulation is well described by an analytical model and is found to affect the excitation dynamics via the frequency coupling. It is shown that the frequency coupling strongly affects the generation of beams of highly energetic electrons by the expanding sheath and field reversals caused by the collapsing sheath. The role of secondary electrons at intermediate and high pressures is clarified and the transition from α- to γ-mode operation is discussed. Depending on the gas and the corresponding cross sections for excitation/ionization the excitation does not generally probe the ionization as is usually assumed.

Self-excited nonlinear plasma series resonance oscillations in geometrically symmetric capacitively coupled radio frequency discharges

At low pressures, nonlinear self-excited plasma series resonance (PSR) oscillations are known to drastically enhance electron heating in geometrically asymmetric capacitively coupled radio frequency discharges by nonlinear electron resonance heating (NERH). Here we demonstrate via particle-in-cell simulations that high-frequency PSR oscillations can also be excited in geometrically symmetric discharges if the driving voltage waveform makes the discharge electrically asymmetric. This can be achieved by a dual-frequency (f+2f) excitation, when PSR oscillations and NERH are turned on and off depending on the electrical discharge asymmetry, controlled by the phase difference of the driving frequencies.

Phase resolved optical emission spectroscopy: a non-intrusive diagnostic to study electron dynamics in capacitive radio frequency discharges

Various types of capacitively coupled radio frequency (CCRF) discharges are frequently used for different applications ranging from chip and solar cell manufacturing to the creation of biocompatible surfaces. In many of these discharges electron heating and electron dynamics are not fully understood. A powerful diagnostic to study electron dynamics in CCRF discharges is phase resolved optical emission spectroscopy (PROES). It is non-intrusive and provides access to the dynamics of highly energetic electrons, which sustain the discharge via ionization, with high spatial and temporal resolution within the RF period. Based on a time dependent model of the excitation dynamics of specifically chosen rare gas levels PROES provides access to plasma parameters such as the electron temperature, electron density and electron energy distribution function (EEDF). In this work the method of PROES is reviewed and some examples of its application are discussed. First, the generation of highly energetic electron beams by the expanding sheath in geometrically symmetric as well as asymmetric discharges and their effect on the EEDF are investigated. Second, the physical nature of the frequency coupling in dual frequency discharges operated at substantially different frequencies is discussed. Third, the generation of electric field reversals during sheath collapse in single and dual frequency discharges is analysed. Then excitation dynamics in an electrically asymmetric novel type of dual frequency discharge is studied. Finally, limitations of PROES are discussed.

Electric field reversals in the sheath region of capacitively coupled radio frequency discharges at different pressures

Electric field reversals in single and dual-frequency capacitively coupled radio frequency discharges are investigated in the collisionless (1Pa) and the collisonal (65Pa) regimes. Phase resolved optical emission spectroscopy is used to measure the excitation of the neutral background gas caused by the field reversal during sheath collapse. The collisionless regime is investigated experimentally in asymmetric neon and hydrogen single frequency discharges operated at 13.56MHz in a GEC reference cell. The collisional regime is investigated experimentally in a symmetric industrial dual-frequency discharge operated at 1.937 and 27.118MHz. The resulting spatio-temporal excitation profiles are compared with the results of a fluid sheath model in the single frequency case and a particle-in-cell/Monte Carlo simulation in the dual-frequency case. The results show that field reversals occur in both regimes. An analytical model gives an insight into the mechanisms causing the reversal of the electric field. In the dual-frequency case a qualitative comparison between the electric fields resulting from the PIC simulation and from the analytical model is performed. The field reversal seems to be caused by different mechanisms in the respective regimes. In the collisionless case it is caused by electron inertia, whereas in the collisional regime it is caused by a combination of the low mobility of electrons due to collisions and electron inertia. Finally, the field reversal during the sheath collapse seems to be a general source for energy gain of electrons in both single and dual-frequency discharges. (Some figures in this article are in colour only in the electronic version)

Electric field measurement in an atmospheric or higher pressure gas by coherent Raman scattering of nitrogen

The feasibility of electric field measurement based on field-induced coherent Raman scattering is demonstrated for the first time in a nitrogen containing gas at atmospheric or higher pressure, including open air. The technique is especially useful for the determination of temporal and spatial profiles of the electric field in air-based microdischarges, where nitrogen is abundant. In our current experimental setup, the minimum detectable field strength in open air is about 100?V?mm?1, which is sufficiently small compared with the average field present in typical microdischarges. No further knowledge of other gas/plasma parameters such as the nitrogen density is required.