Large-area high-density plasma excitation using standing pure and hybrid surface waves

Plasma processing of large flat surfaces requires low pressure high density (n(e) = 10(11) - 10(12) cm(-3)) plasmas with uniform plasma density distribution near to the processed surface. Microwave discharges may provide a valuable alternative to the inductively coupled plasmas applied widely now for this purpose. In a recent article [Jpn. J. Appl. Phys., Part 1 35, L341 (1996)] we proposed a plasma source in which the plasma is sustained by a standing surface wave propagating radially and azimuthally along the interface between the plasma and a dielectric plate located at the top wall of a large-diameter cylindrical metal chamber, the wave being launched by a pair of slot antennas cut in the top chamber wall above the dielectric plate. Here we present new experimental results at lower pressures (down to 3 mTorr) and in a non-noble reactive gas (CF4) demonstrating the applicability of the new source for dry etching. The electron density was about one order of magnitude lower than the one observed by previous experiments at 0.2-1 Torr in Ar. We present an eigenmode analysis suggesting that this lower density should cause the appearance of hybrid surface TMmns eigenmodes with an axial mode number s = 1, which, in contrast to the case of s = 0 of the pure surface modes observed previously at higher electron densities, are evanescent along the chamber axis only in the plasma but not in the dielectric region. Electron density and microwave filed distribution measurements in the plasma confirmed the presence of the hybrid surface modes at low pressures (3-15 mTorr). (C) 1998 American Vacuum Society.