Experimental observation of standing wave effect in low-pressure 200 MHz capacitive discharges

Summary form only given. Radial uniformity measurements of plasma density were carried out by using a floating double probe in a cylindrical (21 cm in diameter) low-pressure capacitive discharge driven over a wide range of frequencies (27~220 MHz). Unlike previous experimental observations, at low rf power a secondary density peak caused by standing wave effect was seen at 130 MHz, and it became pronounced and shift toward the discharge axis while the density peak at the axis become steep as the driving frequency further to increase. With increasing rf power, the secondary density peak shift toward the radial edge, namely the standing-wave wavelength was increased. These are quite consistent with previous results obtained by a transmission line theory
¹
and by a self-consistent fluid simulation coupled with full Maxwell's equation
²
. With increasing pressure, the standing wave effect seems to weaken. At higher pressures, i.e., 30 Pa, the plasma density peaks at the edge of the electrode. This indicates that the rf power was primarily deposited at the periphery of the electrode, due to the fact that the electromagnetic wave was strongly damped as they propagated from the discharge edge into the center when the driving frequency falls within the stop band
²
.