Uwe Czarnetzki

The 2012 Plasma Roadmap

Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap.

Control of plasma properties in capacitively coupled oxygen discharges via the electrical asymmetry effect

Using a combined experimental, numerical and analytical approach, we investigate the control of plasma properties via the electrical asymmetry effect (EAE) in a capacitively coupled oxygen discharge. In particular, we present the first experimental investigation of the EAE in electronegative discharges. A dual-frequency voltage source of 13.56 MHz and 27.12 MHz is applied to the powered electrode and the discharge symmetry is controlled by adjusting the phase angle θ between the two harmonics. It is found that the bulk position and density profiles of positive ions, negative ions, and electrons have a clear dependence on θ, while the peak densities and the electronegativity stay rather constant, largely due to the fact that the time-averaged power absorption by electrons is almost independent of θ. This indicates that the ion flux towards the powered electrode remains almost constant. Meanwhile, the dc self-bias and, consequently, the sheath widths and potential profile can be effectively tuned by varying θ. This enables a flexible control of the ion bombarding energy at the electrode. Therefore, our work proves the effectiveness of the EAE to realize separate control of ion flux and ion energy in electronegative discharges. At low pressure, the strength of resonance oscillations, which are found in the current of asymmetric discharges, can be controlled with θ.

PIC simulations of the separate control of ion flux and energy in CCRF discharges via the electrical asymmetry effect

Recently a novel approach for achieving separate control of ion flux and energy in capacitively coupled radio frequency (CCRF) discharges based on the electrical asymmetry effect (EAE) was proposed (Heil et al 2008 J. Phys. D: Appl. Phys. 41 165202). If the applied, temporally symmetric voltage waveform contains an even harmonic of the fundamental frequency, the sheaths in front of the two electrodes are necessarily asymmetric. A dc self-bias develops and is a function of the phase angle between the driving voltages. By tuning the phase, precise and convenient control of the ion energy can be achieved while the ion flux stays constant. This effect works even in geometrically symmetric discharges and the role of the two electrodes can be reversed electrically. In this work the EAE is verified using a particle in cell simulation of a geometrically symmetric dual-frequency CCRF discharge operated at 13.56 and 27.12MHz. The self-bias is a nearly linear function of the phase angle. It is shown explicitly that the ion flux stays constant within ±5%, while the self-bias reaches values of up to 80% of the applied voltage amplitude and the maximum ion energy is changed by a factor of 3 for a set of low pressure discharge conditions investigated. The EAE is investigated at different pressures and electrode gaps. As geometrically symmetric discharges can be made electrically asymmetric via the EAE, the plasma series resonance effect is observed for the first time in simulations of a geometrically symmetric discharge. (Some figures in this article are in colour only in the electronic version)

Frequency coupling in dual frequency capacitively coupled radio-frequency plasmas

An industrial, confined, dual frequency, capacitively coupled, radio-frequency plasma etch reactor (Exelan®, Lam Research) has been modified for spatially resolved optical measurements. Space and phase resolved optical emission spectroscopy yields insight into the dynamics of the discharge. A strong coupling of the two frequencies is observed in the emission profiles. Consequently, the ionization dynamics, probed through excitation, is determined by both frequencies. The control of plasma density by the high frequency is, therefore, also influenced by the low frequency. Hence, separate control of plasma density and ion energy is rather complex.

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)

The electrical asymmetry effect in capacitively coupled radio frequency discharges – measurements of dc self bias, ion energy and ion flux

The recently theoretically predicted electrical asymmetry effect (EAE) (Heil et al 2008 IEEE Trans. Plasma Sci. 36 1404, Heil et al 2008 J. Phys. D: Appl. Phys. 41 165202, Czarnetzki et al 2009 J. Phys.: Conf. Ser. at press) in capacitively coupled radio frequency (CCRF) discharges and the related separate control of ion energy and flux via the EAE (Czarnetzki et al 2009 J. Phys.: Conf. Ser. at press, Donk´ o et al 2008 J. Phys. D: Appl. Phys. 42 025205) are tested experimentally for the first time. A geometrically symmetric CCRF discharge (equal electrode surface areas) operated at 13.56 and 27.12MHz with variable phase angle between the harmonics is operated in argon at different pressures. The dc self bias, the energy as well as the flux of ions at the grounded electrode, and the space and phase resolved optical emission are measured. The results verify the predictions of models and simulations: via the EAE a dc self bias is generated as an almost linear function of the phase. This variable dc self bias allows separate control of ion energy and flux in an almost ideal way under various discharge conditions. (Some figures in this article are in colour only in the electronic version)

The electrical asymmetry effect in multi-frequency capacitively coupled radio frequency discharges

The electrical asymmetry effect (EAE) in geometrically symmetric capacitively coupled radio frequency discharges operated at multiple consecutive harmonics is investigated by a particle-in-cell (PIC) simulation and an analytical model. The model is based on the original EAE model, which is extended by taking into account the floating potentials, the voltage drop across the plasma bulk, and the symmetry parameter resulting from the PIC simulation. Compared with electrically asymmetric dual-frequency discharges we find that (i) a significantly stronger dc self-bias can be generated electrically and that (ii) the mean ion energies at the electrodes can be controlled separately from the ion flux over a broader range by tuning the phase shifts between the individual voltage harmonics. A recipe for the optimization of the applied voltage waveform to generate the strongest possible dc self-bias electrically and to obtain maximum control of the ion energy via the EAE is presented.

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.