Electron power absorption dynamics in magnetized capacitively coupled radio frequency oxygen discharges

Abstract

The influence of a uniform magnetic field parallel to the electrodes on radio frequency capacitively coupled oxygen discharges driven at 13.56 MHz at a pressure of 100 mTorr is investigated by one-dimensional particle-in-cell/Monte Carlo collision (1D PIC/MCC) simulations. Increasing the magnetic field from 0 to 200 G is found to result in a drastic enhancement of the electron and the ion density due to the enhanced confinement of electrons by the magnetic field. The time and space averaged O− ion density, however, is found to remain almost constant, since both the dissociative electron attachment (production channel of O−) and the associative electron detachment rate due to the collisions of negative ions with oxygen metastables (main loss channel of O−) are enhanced simultaneously. This is understood based on a detailed analysis of the spatio-temporal electron dynamics. The nearly constant O− density in conjunction with the increased electron density causes a significant reduction of the electronegativity and a pronounced change of the electron power absorption dynamics as a function of the externally applied magnetic field. While at low magnetic fields the discharge is operated in the electronegative drift-ambipolar mode, a transition to the electropositive α-mode is induced by increasing the magnetic field. Meanwhile, a strong electric field reversal is generated near each electrode during the local sheath collapse at high magnetic fields, which locally enhances the electron power absorption. A model of the electric field generation reveals that the reversed electric field is caused by the reduction of the electron flux to the electrodes due to their trapping by the magnetic field. The consequent changes of the plasma properties are expected to affect the applications of such discharges in etching, deposition and other semiconductor processing technologies.