Experimental measurements of the RF sheath thickness with a cylindrical Langmuir probe

The small layer oscillating in front of a radio frequency (rf) biased electrode in an asymmetric rf plasma discharge without a magnetic field is diagnosed using an rf compensated cylindrical probe. Thanks to this probe (0.15mm in diameter), the floating potential is measured in this area. Radio frequency plasmas and sheath properties are then derived from the I-V characteristics measured by the probe at different rf power levels in both capacitive and direct couplings. In direct coupling, the plasma biasing is, as expected, nearly equal to the applied rf potential except at high power levels for which the current collected by the electrode saturates and the sheath potential gap is reversed. In capacitive coupling, the self-biasing of the electrode is strongly negative due to the matching box used. From the difference between the plasma potential and the floating potential, we found a sheath thickness of about 3 lambda(De). Within the rf power scan performed, the sheath thicknesses deduced from the potential and density profiles are 3 times higher than those from the Child-Langmuir law both in direct and capacitive coupling in a low collisional helium plasma.