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.

Space and phase resolved plasma parameters in an industrial dual-frequency capacitively coupled radio-frequency discharge

The dynamics of high energetic electrons (>= 11.7 eV) in a modified industrial confined dual-frequency capacitively coupled RF discharge (Exelan, Lam Research Inc.), operated at 1.937 MHz and 27.118 MHz, is investigated by means of phase resolved optical emission spectroscopy. Operating in a He-O-2. plasma with small rare gas admixtures the emission is measured, with one-dimensional spatial resolution along the discharge axis. Both the low and high frequency RF cycle are resolved. The diagnostic is based on time dependent measurements of the population densities of specifically chosen excited rare gas states. A time dependent model, based on rate equations, describes the dynamics of the population densities of these levels. Based on this model and the comparison of the excitation of various rare gas states, with different excitation thresholds, time and space resolved electron temperature, propagation velocity and qualitative electron density as well as electron energy distribution functions are determined. This information leads to a better understanding of the dual-frequency sheath dynamics and shows, that separate control of ion energy and electron density is limited.

The role of the relative voltage and phase for frequency coupling in a dual-frequency capacitively coupled plasma

Frequency coupling in multifrequency discharges is a complex nonlinear interaction of the different frequency components. An alpha-mode low pressure rf capacitively coupled plasma operated simultaneously with two frequencies is investigated and the coupling of the two frequencies is observed to greatly influence the excitation and ionization within the discharge. Through this, plasma production and sustainment are dictated by the corresponding electron dynamics and can be manipulated through the dual-frequency sheath. These mechanisms are influenced by the relative voltage and also the relative phase of the two frequencies.

Concentric split ring element for dual frequency reflectarray antennas

The reflection phase response of a two-layer array of orthogonally oriented concentric split rings is presented. Splitting the ring elements suppresses the interlayer coupling and produces polarisation sensitive scattering. Simulated and measured results at X-band demonstrate that these proper-ties enable the reflection phase coefficients of a reflect-array to be independently optimised at two different frequencies.

The influence of the relative phase between the driving voltages on electron heating in asymmetric dual frequency capacitive discharges

The influence of the relative phase between the driving voltages on electron heating in asymmetric phase-locked dual frequency capacitively coupled radio frequency plasmas operated at 2 and 14 MHz is investigated. The basis of the analysis is a nonlinear global model with the option to implement a relative phase between the two driving voltages. In recent publications it has been reported that nonlinear electron resonance heating can drastically enhance the power dissipation to electrons at moments of sheath collapse due to the self-excitation of nonlinear plasma series resonance (PSR) oscillations of the radio frequency current. This work shows that depending on the relative phase of the driving voltages, the total number and exact moments of sheath collapse can be influenced. In the case of two consecutive sheath collapses a substantial increase in dissipated power compared with the known increase due to a single PSR excitation event per period is observed. Phase resolved optical emission spectroscopy (PROES) provides access to the excitation dynamics in front of the driven electrode. Via PROES the propagation of beam-like energetic electrons immediately after the sheath collapse is observed. In this work we demonstrate that there is a close relation between moments of sheath collapse, and thus excitation of the PSR, and beam-like electron propagation. A comparison of simulation results to experiments in a single and dual frequency discharge shows good agreement. In particular the observed influence of the relative phase on the dynamics of a dual frequency discharge is described by means of the presented model. Additionally, the analysis demonstrates that the observed gain in dissipation is not accompanied by an increase in the electrode’s dc-bias voltage which directly addresses the issue of separate control of ion flux and ion energy in dual frequency capacitively coupled radio frequency plasmas.

Cold atmospheric pressure plasma jets:Interaction with plasmid DNA and tailored electron heating using dual-frequency excitation

Recent progress in plasma science and technology has enabled the development of a new generation of stable cold non-equilibrium plasmas operating at ambient atmospheric pressure. This opens horizons for new plasma technologies, in particular in the emerging field of plasma medicine. These non-equilibrium plasmas are very efficient sources for energy transport through reactive neutral particles (radicals and metastables), charged particles (ions and electrons), UV radiation, and electro-magnetic fields. The effect of a cold radio frequency-driven atmospheric pressure plasma jet on plasmid DNA has been investigated. The formation of double strand breaks correlates well with the atomic oxygen density. Taken with other measurements, this indicates that neutral components in the jet are effective in inducing double strand breaks. Plasma manipulation techniques for controlled energy delivery are highly desirable. Numerical simulations are employed for detailed investigations of the electron dynamics, which determines the generation of reactive species. New concepts based on nonlinear power dissipation promise superior strategies to control energy transport for tailored technological exploitations. © 2012 American Institute of Physics.

The challenge of revealing and tailoring the dynamics of radio-frequency plasmas

The complex dynamics of ionization and excitation mechanisms in capacitively coupled radio-frequency plasmas is discussed for single- and dual-frequency operations in low-pressure and atmospheric pressure plasmas. Electrons are energized through the dynamics of electric fields in the vicinity of the plasma boundary sheaths. Distinctly different power dissipation mechanisms can either co-exist or initiate mode transitions exhibiting characteristic spatio-temporal ionization structures. Phase resolved optical emission spectroscopy, in combination with adequate modelling of the population dynamics of excited states, and numerical simulations reveal dissipation associated with sheath expansion, sheath collapse, transient electron avalanches and wave–particle interactions. In dual-frequency systems the relative phase between the two frequency components provides additional strategies to tailor the plasma dynamics.

Electron dynamics and frequency coupling in a radio-frequency capacitively biased planar coil inductively coupled plasma system

The electron dynamics in a planar coil inductively coupled plasma (ICP) system with a capacitively biased electrode is investigated using space and phase resolved optical emission spectroscopy. The two power source frequencies are exact multiple of each other and phase-locked. In this configuration, the system is investigated when the coil is operated in both E-mode and H-mode. The results show that in a phase synchronized RF biased ICP, the electrode bias power couples with the capacitive contribution of the coil, in both E-mode and H-modes, similar to dual-frequency capacitively coupled plasmas (2f-CCPs). It is also demonstrated that in H-mode, the phase between the electrode bias frequency and the ICP coil frequency influences the electron heating, similar to the electrical asymmetry effect in 2f-CCPs.

Monodomain strained ferroelectric PbTiO(3) thin films: Phase transition and critical thickness study

This work demonstrates that instead of paraelectric PbTiO(3), completely c-oriented ferroelectric PbTiO(3) thin films were directly grown on (001)-SrTiO(3) substrates by pulsed-laser deposition with thickness up to 340 nm at a temperature well above the Curie temperature of bulk PbTiO(3). The influence of laser-pulse frequency, substrate-surface termination on growth, and functional properties were studied using x-ray diffraction, transmission electron microscopy, and piezoresponse force microscopy. At low growth rates (frequency 8 Hz) a domains were formed for film thickness above 20-100 nm. Due to coherency strains the Curie temperature (T(c)) of the monodomain films was increased approximately by 350 degrees C with respect to the T(c) of bulk PbTiO(3) even for 280-nm-thick films. Nonetheless, up to now this type of growth mode has been considered unlikely to occur since the Matthews-Blakeslee (MB) model already predicts strain relaxation for films having a thickness of only similar to 10 nm. However, the present work disputes the applicability of the MB model. It clarifies the physical reasons for the large increase in T(c) for thick films, and it is shown that the experimental results are in good agreement with the predictions based on the monodomain model of Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)].

Geometrical dependence in photoionization of H-2(+) in high-intensity, high-frequency, ultrashort laser pulses

The ionization dynamics of H2 + exposed to high-intensity, high-frequency, ultrashort laser pulses is investigated with two theoretical approaches. The time-dependent Schrödinger equation is solved by a direct numerical method, and a simple two-center interference-diffraction model is studied. The energy and angular distributions of the photoelectron for various internuclear distances and relative orientations between the internuclear axis of the molecule and the polarization of the field are calculated. The main features of the photoelectron spectrum pattern are described well by the interference-diffraction model, and excellent quantitative agreement between the two methods is found. The effect of quantal vibration on the photoelectron spectrum is also calculated. We find that vibrational average produces some broadening of the main features, but that the patterns remain clearly distinguishable.

High-frequency oscillations in a solar active region observed with the RAPID DUAL IMAGER

High-cadence, synchronized, multiwavelength optical observations of a solar active region (NOAA 10794) are presented. The data were obtained with the Dunn Solar Telescope at the National Solar Observatory/Sacramento Peak using a newly developed camera system: the rapid dual imager. Wavelet analysis is undertaken to search for intensity related oscillatory signatures, and periodicities ranging from 20 to 370 s are found with significance levels exceeding 95%. Observations in the H-α blue wing show more penumbral oscillatory phenomena when compared to simultaneous G-band observations. The H-α oscillations are interpreted as the signatures of plasma motions with a mean velocity of 20 km s-1. The strong oscillatory power over H-α blue-wing and G-band penumbral bright grains is an indication of the Evershed flow with frequencies higher than previously reported.