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Accueil > Séminaires et conférences > Séminaires, soutenances de thèses et HDR précédents > 2017 > Soutenances de thèses et HDR 2017 > Le lundi 20 novembre à 14h

Le lundi 20 novembre à 14h

Soutenance de thèse de Yue Liu, "Two dimensional PIC/MCC simulations of RF CCPs with a dielectric side wall",
Ecole Polytechnique, salle CPHT, aile 0, le 20/11/2017 à 14h.

Abstract
Capacitively coupled plasmas (CCPs) are widely used in the microelectronics industry for etching and deposition of thin films, and have been the subject of many academic studies in the past few decades. Among these studies, two issues are in great concern : how to produce uniform high density plasmas, and how to achieve independent control of the ion flux and the bombarding ion energy and angular distribution functions. Previously plasma non-uniformities in CCPs were thought to be caused by either an enhanced electric field caused by the proximity of powered and grounded electrodes, or by electromagnetic effects at a very high exciting frequency, such that the reactor size is not negligible compared to the radiofrequency wavelength. In this work, we develop a Particle-In-Cell/Monte Carlo Collision (PIC/MCC) code in a 2D Cartesian framework with a dielectric wall at the reactor edge, aiming to investigate the plasma uniformity when the enhanced electric field at the electrode junction is shielded by a thick dielectric. However, even with this configuration we still observe that the time-averaged electron density, electron power deposition and ionization rate all are non-uniform, with maxima adjacent to the dielectric edge. The axially integrated electron power deposition peaks closer to this edge than the electron density. Electrons are predominantly heated by the axial electric field, which has strong RF oscillations. By numerically deriving the different components of electron heating from the PIC/MCC simulations and using an analytical model, we find that this enhanced electron heating results from increased Ohmic heating as electron density decreases towards the x edge. In analytical models, the use of the expression of the collision term −𝑚𝑛0𝜈𝑚𝑢 including a time-averaged momentum transfer frequency 𝑣𝑚 can cause large errors since it neglects both the phase and amplitude information in the collision term. Furthermore, the relationship 𝐽𝑇̃=(𝜎𝑃+𝑗𝜔𝜀0)𝐸𝑧̃ must be used with caution, particularly close to the x edge, where the pressure gradient term becomes significant but is ignored in this relationship. By changing the pressure, voltage amplitude, and driving frequency, we find that pressure has little effect on the electron density uniformity and the electron power uniformity. However, increasing either the voltage amplitude or the driving frequency can improve the uniformity of electron density, while the axially integrated electron power deposition uniformity becomes more enhanced close to the x edge. At higher frequency, significant electron heating in the x direction (𝑃𝑒𝑥) is observed close to the corners, which is caused by a significant oscillating RF field in the x direction near the corners despite the presence of the dielectric wall. Separate control of the ion flux and the mean ion bombardment energy to the grounded electrode by the use of tailored voltage waveforms (TVWs) is demonstrated in our simulations. However, both increasing the number of harmonics, and shifting the phase have little effect on the uniformity of electron density. Using a peak pulsed
waveform, the ion angular distribution (IAD) becomes narrower at the powered electrode as the number of harmonics is increased, but remains unchanged at the grounded electrode.


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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Exploitant du site : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Pascal Chabert (Directeur)