The wave propagation and absorption in planar microwave discharges without
magnetic fields are calculated with nonuniformity in plasma density taken i
nto account. When the wave is incident in the direction oblique to the dens
ity gradient, the field amplitude is strongly peaked at a position where th
e wave frequency coincides with the plasma frequency. The absorption coeffi
cient of the wave for the oblique incidence is calculated to be much larger
than for the parallel incidence. A comparative experiment of the plasma pr
oduction is performed to find that the power coupling efficiency is larger
for the oblique incidence antenna than for the parallel incidence antenna,
especially in low collisionality regimes. The comparison between the calcul
ation and the experimental result reveals that the mechanism of the efficie
nt production of overdense plasmas in planar microwave discharges is due to
the enhanced electric field near the plasma resonance and subsequent accel
eration of electrons in the case of oblique incidence. (C) 2000 American In
stitute of Physics. [S1070-664X(00)02205-9].