Transition of electron heating mode in a planar microwave discharge at low pressures

Spatial distributions of electron density ne, electron temperature Te, and wave field intensity in a planar microwave discharge at low pressures (10–100 mTorr) are measured with the discharge power as a parameter. Two different modes of electron heating are found: a bulk heating mode characterized with high Te(∼10 eV) in underdense plasma, and a surface heating mode with low Te(∼3 eV) in overdense plasma with a hot layer near the plasma-dielectric boundary. Abrupt transition between the two modes takes place near the cutoff density nc where the electron plasma frequency coincides with the discharge frequency (2.45 GHz). In the underdense case (ne nc), the evanescent wave (surface wave) near the microwave window strongly heats the electrons in the boundary layer, which are transported to the bulk region and ionize the neutral particles ...