The recently theoretically predicted electrical asymmetry effect (EAE)

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.

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, E i , and the ion flux, i , at the substrate surface: (i) classical discharges operated at substantially different frequencies, where the low-and high-frequency voltage amplitudes, φ lf and φ hf , are used to control E i and i , 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 i and θ is used to control E i. 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 E i on i for different secondary yields, γ. We find a dramatic effect of tuning φ lf in classical DF discharges, which is caused by a transition from α-to γ-mode induced by changing φ lf. In EA discharges we find that no such mode transition is induced by changing θ within the parameter range studied here and, consequently, i 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.

We present an analytical model to describe capacitively coupled radio-frequency (CCRF) discharges and the electrical asymmetry effect (EAE) based on the non-linearity of the boundary sheaths. The model describes various discharge types, e.g. single and multi-frequency as well as geometrically symmetric and asymmetric discharges. It yields simple analytical expressions for important plasma parameters such as the dc self-bias, the uncompensated charge in both sheaths, the discharge current and the power dissipated to electrons. Based on the model results the EAE is understood. This effect allows control of the symmetry of CCRF discharges driven by multiple consecutive harmonics of a fundamental frequency electrically by tuning the individual phase shifts between the driving frequencies. This novel class of capacitive radio-frequency (RF) discharges has various advantages: (i) A variable dc self-bias can be generated as a function of the phase shifts between the driving frequencies. In this way, the symmetry of the sheaths in geometrically symmetric discharges can be broken and controlled for the first time. (ii) Almost ideal separate control of ion energy and flux at the electrodes can be realized in contrast to classical dual-frequency discharges driven by two substantially different frequencies. (iii) Non-linear self-excited plasma series resonance oscillations of the RF current can be switched on and off electrically even in geometrically symmetric discharges. Here, the basics of the EAE are introduced and its main applications are discussed based on experimental, simulation, and modeling results.

In most PIC/MCC simulations of radio frequency capacitively coupled plasmas (CCPs) several simpli cations are commonly made: (i) fast neutrals are not traced, (ii) heavy particle induced excitation and ionization are neglected, (iii) secondary electron emission from boundary surfaces due to neutral particle impact is not taken into account, and (iv) the secondary electron emission coef cient is assumed to be constant, i.e. independent of the incident particle energy and the surface conditions. Here, we examine the validity of these simpli cations under conditions typical for plasma processing applications. We study the effects of including fast neutrals and using realistic energy-dependent secondary electron emission coef cients for ions and fast neutrals in simulations of CCPs operated in argon at 13.56 MHz and at neutral gas pressures between 5 Pa and 100 Pa. We nd an increase of the plasma density and the ion ux to the electrodes under most conditions when heavy particles are included realistically in the simulation. The sheath widths are found to be smaller and the simulations are found to diverge at high pressures for high voltage amplitudes in qualitative agreement with experimental ndings. By switching individual processes on and off in the simulations we identify their individual effects on the ionization dynamics and plasma parameters. While the gas-phase effects of heavy particle processes are found to be moderate at most conditions, the self-consistent calculation of the effective secondary electron yield proves to be important in simulations of CCPs in order to yield realistic results.

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.

Dual-frequency capacitive discharges are used to separately control the mean ion energy, ε¯ion, and flux, Γion, at the electrodes. We study the effect of secondary electrons on this separate control in argon discharges driven at 2+27 MHz at different pressures using Particle in Cell simulations. For secondary yield γ≈0, Γion decreases as a function of the low frequency voltage amplitude due to the frequency coupling, while it increases at high γ due to the effective multiplication of secondary electrons inside the sheaths. Therefore, separate control is strongly limited. ε¯ion increases with γ, which might allow an in situ determination of γ-coefficients.

To cite this version:J Schulze, E Schüngel, Z Donkó, D Luggenhölscher, U Czarnetzki. Phase resolved optical emission spectroscopy: a non-intrusive diagnostic to study electron dynamics in capacitive radio frequency discharges. Journal of Physics D: Applied Physics, IOP Publishing, 2010, 43 (12) Abstract. 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, that 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 analyzed. Then excitation dynamics in an electrically asymmetric novel type of dual frequency discharge is studied. Finally, limitations of PROES are discussed.