Asymmetric Capacitively Coupled Plasma Sources Having Clean and Contaminated Electrodes

Summary form only given. Energetic ion bombardment of substrates in capacitively coupled plasma (CCP) sources and the resulting ionization from secondary electrons are critical to sustaining the plasma. During operation of CCPs for plasma materials processing, the surface condition of substrates often changes due to etching and deposition phenomena. As a result, the secondary electron emission coefficient (SEEC) due to ion bombardment may also change. The SEEC for contaminated metal surfaces is a sensitive function of ion energy, while that for clean electrodes (<800-1000 eV) is rather weak. In this paper, we discuss results from a computational investigation of the consequences of contamination of electrodes, and the resulting change in SEEC, on the performance of asymmetric CCPs sustained in Ar and Ar/O₂ mixtures for pressures of les100 mtorr. The study was performed using particle-in-cell/Monte Carlo and hybrid Monte Carlo-fluid simulations. In asymmetric CCPs where the powered electrode is smaller than the opposing grounded electrode, the DC self-bias on the powered electrode is negative. This results in the ion energies bombarding the powered electrode being significantly larger than for the grounded electrode. For clean electrodes, the difference in ion energies bombarding the two electrodes does not make a significant change in the SEEC; and so differences in the total ionization due to secondary electrons result only from their energies. When the electrodes are contaminated because of the plasma chemistry of the process, the SEEC for the powered electrode, with its higher ion energy bombardment, can be an order of magnitude larger than for the grounded electrode. As a result, there are even larger disparities in the total ionization due to secondary electrons. These disparities continue to increase as the process of contamination of the electrodes increases. This effect is sufficiently severe that for low pressures (<30 mtorr) the dischar- e may extinguish as the electrodes become contaminated