J P Booth

Electron heating in capacitively coupled plasmas revisited

We revisit the problem of electron heating in capacitively coupled plasmas (CCPs), and propose a method for quantifying the level of collisionless and collisional heating in plasma simulations. The proposed procedure, based on the electron mechanical energy conservation equation, is demonstrated with particle-in-cell simulations of a number of single and multi-frequency CCPs operated in regimes of research and industrial interest. In almost all cases tested, the total electron heating is comprised of collisional (ohmic) and pressure heating parts. This latter collisionless component is in qualitative agreement with the mechanism of electron heating predicted from the recent re-evaluation of theoretical models. Finally, in very electrically asymmetric plasmas produced in multi-frequency discharges, we observe an additional collisionless heating mechanism associated with electron inertia.

Atomic hydrogen densities in capacitively coupled very high-frequency plasmas in H2 : Effect of excitation frequency

Absolute hydrogen atom densities in pure hydrogen capacitive discharges were measured as a function of excitation frequency (13.56, 27.12, and 40.68 MHz), nominal electrical power, and gas pressure (between 0.1 and 1 Torr). Quantitative measurements were made using two-photon absorption laser-induced fluorescence (TALIF), put on an absolute scale by comparison with the TALIF signal from a known density of krypton gas, as proposed by Niemi, Schultz von Gathen, and Dobele [J. Phys. D 34, 2330 (2001)]. The H atom density increases with gas pressure and electrical power, and at a given power and pressure it increases significantly with excitation frequency. The latter can be attributed in part to increased electron density. However, time-resolved TALIF measurements in the afterglow showed that the H atom surface loss probabilities are not constant, becoming somewhat smaller when the sheath voltage is lowered, as is the case when the excitation frequency is increased, contributing to the increase in H density.

CF and CF2 radical kinetics and transport in a pulsed CF4 ICP

The kinetics of CF and CF2 radicals (in their electronic ground states) was investigated using space and time resolved laser induced fluorescence (LIF) in an inductively coupled discharge in CF4 gas at 33 mTorr, both in the steady state and in pulsed plasmas. The gas temperature was determined from the rotationally resolved LIF excitation spectra of the CF radical, reaching about 800 K in the centre of the chamber. The densities were put on an absolute scale with the aid of broad-band UV absorption spectroscopy. The density profiles were used to deduce the radical diffusive transport, and therefore the localized (gas-phase and surface) net production or destruction rates. However, when gas heating is significant, the radical transport becomes more complex due to gas density gradients and thermo-diffusion. In the afterglow, gas convection induced by gas cooling and contraction can play a significant role. The CF and CF2 axial fluxes estimated from measured density and temperature profiles indicate that these species are emitted from the top and bottom surfaces, and are destroyed in the gas phase at high rates (≈900 s−1 and 1500 s−1, respectively): for CF2 this process appears to be electron-impact induced. In the afterglow, the CF2 density at the reactor centre increases abruptly, reaching four times the steady-state value after 3 ms, followed by a slow decay (100 s−1). This phenomenon is the combined result of the return of CF2-rich gas as the central region cools and contracts, along with gas-phase reactions producing CF2. The central CF density shows only a slight initial increase in the afterglow, followed by a rapid decay (600 s−1), due to gas-phase chemical reactions, e.g. with F2 or CF3, converting it to CF2, processes which are also likely to be important in the steady state.

Ion flux asymmetry in radiofrequency capacitively-coupled plasmas excited by sawtooth-like waveforms

Using particle-in-cell simulations, we predict that it is possible to obtain a significant difference between the ion flux to the powered electrode and that to the grounded electrode—with about 50% higher ion flux on one electrode—in a geometrically symmetric, radiofrequency capacitively-coupled plasma reactor by applying a non-sinusoidal, ‘Tailored’ voltage waveform. This sawtooth-like waveform presents different rising and falling slopes over one cycle. We show that this effect is due to differing plasma sheath motion in front of each electrode, which induces a higher ionization rate in front of the electrode which has the fastest positive rising voltage. Together with the higher ion flux comes a lower voltage drop across the sheath, and therefore a reduced maximum ion bombardment energy; a result in contrast to typical process control mechanisms.

Direct observation of ozone formation on SiO 2 surfaces in O 2 discharges

Ozone production is studied in a pulsed O2 discharge at pressures in the range 1.3–6.7 mbar. Time-resolved absolute concentrations of O3 and O are measured in the post-discharge using UV absorption spectroscopy and two-photon absorption laser-induced fluorescence. In a bare silica discharge tube ozone is formed mainly by three-body gas-phase recombination. When the tube surface is covered by a high specific surface silica catalyst heterogeneous formation becomes the main source of ozone. The efficiency of this surface process increases with O2 pressure and is favoured by the presence of OH groups and adsorbed H2O on the surface. At p = 6.7 mbar ozone production accounts for up to 25% of the atomic oxygen losses on the surface. (Some figures may appear in colour only in the online journal) Ozone is a strong oxidizing agent which is widely used for treatment of water and decontamination of various surfaces. Ozone formation from surface recombination of atomic oxygen with oxygen molecules

Modes and the alpha-gamma transition in rf capacitive discharges in N2O at different rf frequencies

This paper reports current-voltage characteristics and pressure-voltage transition curves from the weak-current α-mode to the strong-current γ-mode for rf capacitive discharges in N2O at frequencies of 2MHz, 13.56MHz, and 27.12MHz. At 2MHz the rf discharge is mostly resistive whereas at 13.56MHz and 27.12MHz it is mostly capacitive. The weak-current α-mode was found to exist only above a certain minimum gas pressure for all frequencies studied. N. Yatsenko [Sov. Phys. Tech. Phys. 26, 678 (1981)] previously proposed that the α−γ transition corresponds to breakdown of the sheaths. However, we show that this is the case only for sufficiently high gas pressures. At lower pressure there is a smooth transition from the weak-current α-mode to a strong-current γ-mode, in which the sheaths produce fast electrons but the sheath has not undergone breakdown.

Global (volume-averaged) model of inductively coupled chlorine plasma : influence of Cl wall recombination and external heating on continuous and pulse-modulated plasmas

An inductively coupled radio-frequency plasma in chlorine is investigated via a global (volume-averaged) model, both in continuous and square wave modulated power input modes. After the power is switched off (in a pulsed mode) an ion–ion plasma appears. In order to model this phenomenon, a novel quasi-neutrality implementation is proposed. Several distinct Cl wall recombination probability measurements exist in the literature, and their effect on the simulation data is compared. We also investigated the effect of the gas temperature that was imposed over the range 300–1500 K, not calculated self-consistently. Comparison with published experimental data from several sources for both continuous and pulsed modes shows good agreement with the simulation results.

Tailored voltage waveform capacitively coupled plasmas in electronegative gases: frequency dependence of asymmetry effects

Capacitively coupled radio frequency plasmas operated in an electronegative gas (CF4) and driven by voltage waveforms composed of four consecutive harmonics are investigated for different fundamental driving frequencies using PIC/MCC simulations and an analytical model. As has been observed previously for electropositive gases, the application of peak- shaped waveforms (that are characterized by a strong amplitude asymmetry) results in the development of a DC self-bias due to the electrical asymmetry effect (EAE), which increases the energy of ions arriving at the powered electrode. In contrast to the electropositive case (Korolov et al 2012 J. Phys. D: Appl. Phys. 45 465202) the absolute value of the DC self- bias is found to increase as the fundamental frequency is reduced in this electronegative discharge, providing an increased range over which the DC self-bias can be controlled. The analytical model reveals that this increased DC self-bias is caused by changes in the spatial pro le and the mean value of the net charge density in the grounded electrode sheath. The spatio-temporally resolved simulation data show that as the frequency is reduced the grounded electrode sheath region becomes electronegative. The presence of negative ions in this sheath leads to very different dynamics of the power absorption of electrons, which in turn enhances the local electronegativity and plasma density via ionization and attachment processes. The ion ux to the grounded electrode (where the ion energy is lowest) can be up to twice that to the powered electrode. At the same time, while the mean ion energies at both electrodes are quite different, their ratio remains approximately constant for all base frequencies studied here.

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 (n1 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 H2+ fluxes to each of the two electrodes are caused by the different H3+ 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.

Gas temperature measurements in oxygen plasmas by high-resolution Two-Photon Absorption Laser-induced Fluorescence

One of the most important, and difficult to measure, parameters of laboratory discharges in molecular gases is the gas translational temperature. We propose a novel technique to measure directly, with excellent spatial and temporal resolution, the velocity distribution of ground-state atoms (oxygen atoms in this case) in plasmas from the Doppler broadening of their laser excitation spectra. The method is based on the well-known Two-Photon Laser-induced Fluorescence (TALIF) technique, but uses a specially-built pulsed tunable ultraviolet laser with very narrow bandwidth which allows the Doppler profiles to be measured with high precision. This laser consists of a pulsed Nd:YAG-pumped Ti:Sapphire ring cavity which is injection-seeded by a single-mode cw Ti:sapphire laser. The single-mode infrared output pulses are frequency quadrupled by two non-linear crystals to reach the necessary UV wavelength (226 nm, 0.2 mJ) for TALIF excitation. This technique should be applicable to a wide range of discharges, ranging from low-pressure RF plasmas for surface processing to atmospheric pressure plasmas. Results of preliminary tests on low-pressure O2 DC discharges are presented.