Intermediate-Pressure O 2 RFCCP:Different Power and pressure
Résumé
Radio frequency (RF) capacitively coupled plasma (CCP) sources are of great relevance due to their numerous industrial applications[1]. However, previous research has focused on pressures below 1 Torr, due to the difficulty of common diagnostics at higher pressures. We have measured oxygen atom densities and temperatures by cavity ringdown spectroscopy (CRDS) at 630nm[2], combined with electrical measurements of the plasma impedance and ion fluxes in O2 plasmas over the pressure range 1-6 Torr and 100-650W RF power at 13.56 MHz. Measurements were made in a highly symmetric RF CCP chamber made of untreated aluminium, 50 cm diameter, 2.5 cm gap. The RF voltage, power and phase are measured by an Impedans Octiv probe installed between the matchbox and the chamber, and the true electrode voltage is measured directly with a Tektronix P6100 probe. A 9-probe ion flux array is installed on the upper (ground) electrode. The CRDS mirrors are installed on bellows outside the chamber. At 4 Torr and above, the 0 atom density increases monotonically with RF power. However, at 1 and 2 Torr the O density passes through a maximum with power. The steady-state O density is determined by the balance of production (by O2 dissociation) and loss (principally surface recombination). At higher pressures, the observed increase in O density corresponds to increasing electron density (and thus the O 2 dissociation rate) with power, while the surface reaction probability appears to remain constant. However, at lower pressure where the sheath becomes less collisional, the ion energy at the electrodes increases with the applied power. Therefore, the decrease in O density at low pressure and high power suggests that the surface recombination probability at the electrodes is increased by the onset of higher-energy ion bombardment.
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