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Accueil > A propos du LPP > Communication > Actualités archivées > 2015 > A new method to control plasma excitation localisation using sawtooth radiofrequency waveforms

A new method to control plasma excitation localisation using sawtooth radiofrequency waveforms

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Space and time resolved excitation : comparison of model (PIC) to experiment (PROES).
[up ]With classical sinusoidal excitation weak excitation occurs at both sides (but shifted in phase).
[down] With sawtooth excitation a single bright excitation event occurs close to the powered (right) electrode during sheath expansion.

Capacitively-coupled radiofrequency plasmas are widely used in industry for etching and deposition of thin films, processes which are essential, for instance, in CMOS integrated circuit manufacture. Researchers at the Low-temperature Plasmas team at LPP, in collaboration with colleagues at LPICM(Laboratory of Physics of Interfaces and Thin films, also at Ecole Polytechnique) have been working since 2010 on the effect of using different radiofrequency waveforms and how they can give increased control of the flux and energy of ions impinging on a processed substrate. Until recently work has focussed on so-called “peaks” and “valleys” waveforms comprising positive or negative voltage spikes on a flat background. Now the team has investigated sawtooth-type waveforms, which have a sharp fall followed by a slow rise (or vice-versa). Particle-in-cell simulations predicted that strongly-enhanced heating of high-energy electrons (those above about 15 eV which are responsible for ionisation and excitation of the feedstock gas) will occur in front of the electrode to which a rapid voltage drop is applied, due to so-called sheath expansion heating, and cause localised ionisation and increased ion flux to that electrode. Phase-resolved optical emission spectroscopy (PROES) measurements carried out in collaboration with York Plasma Institute were in excellent agreement with the PIC simulations, confirming the effect. These results, which open interesting perspectives for industrial applications, have recently been published in Physical Review Letters.

Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms
B. Bruneau, T. Gans, D. O’Connell, A. Greb, E V. Johnson and J.-P. Booth
Phys. Rev. Lett. 114, 125002 (2015)

Contacts :
Jean-Paul Booth : jean-paul.booth lpp.polytechnique.fr
Bastien Bruneau : bastien.bruneau polytechnique.edu
Erik Johnson : erik.johnson polytechnique.edu


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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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