Session JW1: RF and Microwave-Driven Plasmas

Simulation study of collisionless heating in single-frequency capacitively coupled discharges

Stochastic heating is an important phenomenon in low-pressure rf capacitive discharge. Recent theoretical work on this problem using several different approaches has produced results that are broadly in agreement insofar as scaling with the discharge parameters is concerned, but there remains some disagreement in detail concerning the absolute size of the effect. Here we report a simulation study that has two main aims. One is to investigate the limitations of the scaling law, especially in the case of high frequency where resonant circuit effects occur, and where plasma wave emission may be observed at the sheath edge. This relatively extensive set of simulation data may be used to validate theories over a wide range of parameters. The second aim is to study wave emission from the sheath with a frequency near $\omega _{pe}$. This is the result of a progressive failure of quasi-neutrality near the electron sheath edge. There is a possibility for this wave energy to be transferred to electron thermal energy by Landau damping or a related process, and this mechanism may contribute significantly to electron heating. The emission of waves is associated with a field reversal during the expanding phase of the sheath. Trapping of electrons near this reverse field region is observed.   

Fluid simulation of the phase-shift effect in hydrogen capacitively coupled plasmas

A 2D self-consistent fluid model coupled with the full set of Maxwell equations is established to investigate the phase shift effect on the radial uniformity of plasma characteristics in a hydrogen capacitively-coupled plasma. The study was carried out at various frequencies in the range of 13.56-200 MHz. At 13.56 MHz, the plasma density is off-axis peaked at $\varphi =0$. When the frequency rises to 60 MHz, the profiles shift smoothly from edge-peaked over uniform to center-peaked as the phase difference increases. At 100 MHz, a similar behavior is observed, except that the maximum moves again towards the radial edge at $\varphi =\pi $. When the frequency is 200 MHz, a better uniformity is again obtained at $\varphi =\pi $. Moreover, the phase shift effect on the transient behavior of electrodynamics has also been examined at 13.56 MHz and 100 MHz.   

Electron Heating and Plasma Generation at Electrode Edge - Fluid and Kinetic Modeling

Low pressure ($<$ 10 Pa) low electron temperature ($<$ 5 eV) plasmas are widely used for thin film processing in the semiconductor industry. With uniformity requirements become more stringent, it is important to understand the sources of plasma non-uniformity and how these influence the characteristics of plasma species (e.g., ions, neutrals) at the substrate. The electrode edge in a capacitively coupled plasma presents unique challenges in this regard. Electric fields are expected to be higher at sharp corners. Conventional fluid theory therefore predicts that more electron heating occurs near the electrode edge with higher concentration of most plasma species. These theoretical predictions agree with measurements at moderate pressures [e.g., McMillin and Zachariah, J. Appl. Phys. {\bf 77}, 5538 (1995)]. As the gas pressure decreases, electron heating and plasma production no longer remain local. The fate of the plasma at the electrode edge is unclear. Do edge effects still enhance the plasma near the electrode edge or does the plasma get weaker due to lack of electrons coming from beyond the electrode edge? We investigate these questions using 2-dimensional fluid and particle-in-cell models over a range of pressures (0.75--75 Pa) and source frequencies (13.56--60 MHz).   

Experimental characterization of dual-frequency capacitively coupled plasmas

Dual-frequency (DF) capacitively coupled plasma (CCP) is of great interest in various industry applications due to its capability of controlling the ion flux and ion energy to the electrode surface independently. A GEC-like reactor was built in the Plasma Simulations and Experiments Group recently, to character the effects of controllable parameters on the plasma properties in DF-CCP. The driving frequency is 60 MHz and 0.8-3 MHz, respectively, the electrode spacing is adjustable within 1-6 cm and a four-way gas feed allows gas mixtures of Ar, O2, CF4 and N2. We have adopted a newly developed complete floating double probe, an optical emission spectrometer and quadruple mass spectrometer to character the DF-CCP. We have performed spatially resolved measurements of the ion density, the electron temperature and the light emission in DF-CCP discharges, and investigated the influence of discharge parameters in various working gases on the ion energy distributions. In particular, our recent measurements confirmed for the first time the existence of the so-called collisionless electron bounce-resonance heating.   

STUDENT AWARD FINALIST: Role of additional inductive power on the electron energy distribution in Ar/O2 capacitively coupled plasma

Changes in the electron energy distributions (EEDs) by the additional inductive power and related electron heating mechanisms were studied in Ar/O$_{2}$ capacitively coupled plasma (CCP) [1-3]. In low pressure Ar CCP, collisionless heating of low energy electrons was observed when a little inductive power ($<$20 W) was applied to the CCP. This indicates that collisionless heating in the skin layer is an important electron heating mechanism of low pressure ICP even in E mode. We also studied effects of the additional inductive power in Ar/O$_{2}$ mixture CCP. As the O$_{2}$ mixing ratio was increased in low pressure Ar/O$_{2}$ CCP, smaller additional inductive power (a few Watts) was needed for the evolutions of the EEDs from the bi-Maxwellian to the Maxwellian distribution. This abrupt change in the EEDs with a very small inductive power appears to be attributed to a combined effect of collisionless heatings by capacitive and induced electric fields in electro-negative plasma. We also performed study on the controls of the EED and the electron temperature by the additional inductive power in the CCP and found that the EED and the electron temperature were dramatically controlled without changes in the plasma density. [1] Lee et al., Appl. Phys. Letts. \textbf{93}, 231503 (2008). [2] Lee et al., Phys. Rev. E. \textbf{81}, 046402 (2010). [3] Lee et al., Phys. Plasmas. \textbf{17}, 013501 (2010).   

RF capacitively coupled plasma with multi-hole multi electrode

In the photovoltaic industry, it is desired to make plasma discharge of high electron density for the deposition of microcrystalline silicon layer, which is a bottle-neck process in the fabrication of thin film solar cell. So multi-hole electrode instead of plane electrode is used to make capacitively coupled discharge and the deposition rate could be increased because of the plasma density increases by the increased ionization by the energetic secondary electron surrounded by sheath region. To further increase the productivity of the process, high frequency and large electrode area is demanded, however the uniformity of the process is degraded by the change. To solve the matter, the concept of dividing a multi-hole electrode into multiple multi-hole electrode is introduced in the presentation. By dividing electrode into several region and differentiating the hole configuration of each region, local hollow cathode effect can be controlled to make more uniform discharge. To verify the feasibility of the concept, an electrode of RF capacitively coupled plasma is divided and the hole configuration of each electrode. And with 13.56MHz power applied to the electrode, the spatial plasma distribution of the discharge is measured.   

The Interaction of the Capacitive and Inductive Coupling Mechanisms with RF Substrate Bias

The interaction of coil power with RF substrate bias at E and H modes is investigated with the Hybrid Plasma Equipment Model. The different electron density changes with RF bias at E and H mode are predicted. In the simulation, the density in E mode increases with increasing RF bias, while in H mode it first increases and then decreases with RF bias. The reason for the difference between the model prediction in H mode and the experiment, where the density keeps decreasing in the considered RF bias range, is explored. It is found that the self-bias plays an important role in the electron behavior. When the self-bias is more negative, the range of low RF bias during which the density increases with RF bias is reduced. Besides, the electron$^{ }$temperature, plasma potential and EEDF are also examined. Lastly, when the RF bias is fixed, the evolution of plasma properties with the coil power is also studied and compared with experiments. This last work is aimed at predicting the collision-less electron heating mechanism by the inductive field, which has been experimentally measured.   

Atomic chlorine and electron densities in inductively-coupled plasmas in pure Cl2 : Comparison of numerical simulations with experiments

Two-photon laser-induced fluorescence (TALIF) at 233.2nm was used to measure the absolute density of Cl atoms in a 13.56MHz Inductively-coupled plasma in pure chlorine. The variation of the absolute Cl density at the reactor centre was measured as a function of pressure and RF power in the range 3-90 mTorr and 20-500W. We also used the TALIF technique to determine the recombination coefficient of atomic chlorine at the reactor walls from the rate of decay of the Cl density in the afterglow of a pulsed discharge. Finally, the electron density was measured by an hairpin microwave resonator in the same conditions. The electron density radial profiles were also recorded. This large set of experimental results was compared to the HPEM hybrid numerical simulations developed by Mark Kushner at the University of Michigan. The experimental recombination coefficient of atomic chlorine at the reactor walls was used as an input of the simulations. A fairly good agreement between simulations and experiments was found (both on electron density and Cl atom density) at the lowest pressure investigated (3-5 mTorr). However, the agreement was not satisfactory at higher pressure. The possible reasons for this discrepancy will be discussed during the presentation.   

Influence of capacitive bias on electron dynamics in an inductively coupled plasma

We investigate the electron dynamics in a planar inductively coupled plasma (ICP) with an RF capacitive bias on the counter electrode. Various sinusoidal frequencies are applied to the bias electrode. Diagnostics used are current and voltage probe along with phase and space resolved optical emission spectroscopy (PROES). The measurements reveal that the RF bias has significant influence on the dynamics of the electrons in the plasma. In addition to inductive heating twice in each RF cycle, energetic electrons originate in front of the ICP antenna causing excitation once in each RF cycle. Fourier analysis of the time resolved excitation distinguishes the contribution of the different excitation mechanisms and their spatial structures. By tuning the phase between the bias and the ICP waveforms, these excitation contributions can be enhanced or suppressed. This may offer possibilities for improving radial uniformity.   

UV Radiation from Hydrogen Containing Microwave Plasmas

Extreme ultraviolet (EUV) light sources are of great importance in many applications, ranging from photochemistry to astrophysics. In this work, EUV emissions from He and H$_{2}$ surface wave produced plasmas operating at low-pressures (0.1--2 mbar) are investigated. This surface wave source is created using a waveguide surfatron-based setup, the microwave power being provided by a 2.45 GHz generator. The discharge takes place inside a quartz tube with internal/external radii of 3/5mm, under laminar flow conditions (20 to 100 sccm). The EUV end-on emission has been detected by a Horiba Jobin-Yvon Plane Grating Monochromator working in the 8 - 125 nm range. The variations of the ultraviolet spectrum in the range 10 - 120 nm with changes in pressure and power delivered to the launcher have been investigated. The results are interpreted in the framework of a theoretical model based on a self-consistent treatment of particle kinetics, gas dynamics and wave electrodynamics.