Abstract
Low-pressure capacitively coupled radio frequency discharges operated in O2 and driven by tailored voltage waveforms are investigated experimentally and by means of kinetic simulations. Pulse-type (peaks/valleys) and sawtooth-type voltage waveforms that consist of up to four consecutive harmonics of the fundamental frequency are used to study the amplitude asymmetry effect as well as the slope asymmetry effect at different fundamental frequencies (5, 10, and 15 MHz) and at different pressures (50–700 mTorr). Values of the DC self-bias determined experimentally and spatio-temporal excitation rates derived from phase resolved optical emission spectroscopy measurements are compared with particle-in-cell/Monte Carlo collisions simulations. The spatio-temporal distributions of the excitation rate obtained from experiments are well reproduced by the simulations. Transitions of the discharge electron heating mode from the drift-ambipolar mode to the α-mode are induced by changing the number of consecutive harmonics included in the driving voltage waveform or by changing the gas pressure. Changing the number of harmonics in the waveform has a strong effect on the electronegativity of the discharge, on the generation of the DC self-bias and on the control of ion properties at the electrodes, both for pulse-type, as well as sawtooth-type driving voltage waveforms The effect of the surface quenching rate of oxygen singlet delta metastable molecules on the spatio-temporal excitation patterns is also investigated.
Highlights
The application of tailored voltage waveforms to excite capacitively coupled radio frequency (RF) discharges offers a promising new direction for advanced control of particle fluxenergy distribution functions in technological plasmas [1].Such waveforms, called customized voltage waveforms, have a non-sinusoidal shape and can be generated as the sum of signals with a fundamental frequency and a number of its higher harmonics with defined phase shifts between them
Technol. 26 (2017) 034002 excitation of capacitively coupled plasmas (CCPs) with tailored voltage waveforms affects the division of sheath voltages and the energy of ions arriving at the two electrode surfaces via modification of the shape of the applied voltage waveform
This method is a generalization of the electrical asymmetry effect (EAE): in [2] it was shown that exciting a RF plasma by two consecutive harmonics, such as 13.56 and 27.12 MHz, a DC self-bias voltage is induced that can be changed by varying the phase between the two components
Summary
The application of tailored voltage waveforms to excite capacitively coupled radio frequency (RF) discharges offers a promising new direction for advanced control of particle fluxenergy distribution functions in technological plasmas [1].Such waveforms, called customized voltage waveforms, have a non-sinusoidal shape and can be generated as the sum of signals with a fundamental frequency and a number of its higher harmonics with defined phase shifts between them. 26 (2017) 034002 excitation of capacitively coupled plasmas (CCPs) with tailored voltage waveforms affects the division of sheath voltages and the energy of ions arriving at the two electrode surfaces via modification of the shape of the applied voltage waveform This method is a generalization of the electrical asymmetry effect (EAE): in [2] it was shown that exciting a RF plasma by two consecutive harmonics, such as 13.56 and 27.12 MHz, a DC self-bias voltage is induced (in a geometrically symmetric discharge configuration) that can be changed by varying the phase between the two components. This technique significantly improved the separation of control of the ion properties at the two electrodes, such as the ion flux and mean ion energy, compared to the case of classical dual-frequency excitation by using significantly different frequencies in CCPs [3,4,5,6,7,8]
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