A tunable passive circuit is introduced for radio frequency (rf) filtering
of Langmuir probes used to measure plasma properties. The circuit produces
a high impedance between the probe tip and ground so that the probe tip fol
lows potential fluctuations in the plasma so that the probe bias voltage wi
th respect to the plasma is constant on the time scale of rf fluctuations.
Filtering is implemented at the fundamental frequency (13.56 MHz in this ca
se) and the second and third harmonics. Representative probe traces and ele
ctron energy distribution functions from an inductively coupled plasma are
presented to demonstrate filter performance. (C) 2001 American Institute of
Physics.