Abstract
Characteristics of 40-cm-sized planar plasmas excited at 915 MHz using various types of slot antennas in Ar and O2 gases are investigated. Axial profiles of the wave electric field and the electron density clearly reveal that high-density surface-wave plasmas are easily produced even at relatively low powers: the electron density increases linearly from 5×1010 cm-3 (the critical density for pure surface-wave mode excitation) to 6×1011 cm-3 with the net incident power from 100 W to 1 kW at 80 mTorr in Ar. Thus, the 915 MHz discharge enables one to control the plasma, even at low densities where a conventional 2.45 GHz surface-wave discharge suffers from unstable density jumps. In addition, the present results indicate that almost linear plasma production characteristics are obtained in three types of slot antennas, that is, one inclined slot, a pair of transverse slots and a combination of two transverse slots and two longitudinal slots. These advantages of the 915 MHz excitation are due to the critical density being lower than that of the 2.45 GHz excitation by a factor of seven. A large-area uniform plasma with a homogeneity of ±5% is obtained for both Ar and O2 discharges over a horizontal length of about 20 cm at an axial position of z=23 cm away from the top quartz plate just below the slot antennas.