RF harmonic suppression in Langmuir probe measurements in RF discharges

RF currents at frequencies well above the ion plasma frequency omega(pi) can be easily suppressed in the space charge sheath of a Langmuir probe by using a suitably chosen low capacitance series resistor R. Such a resistor was used to eliminate the influence of RF currents on the probe characteristics: for blocking harmonics in RF discharges with driving frequencies of the order of omega(pi), such as the commonly used 13.56 MHz discharges and also for blocking the fundamental in discharges at higher frequencies (27 MHz and above). The influence of the resistor R on the probe characteristics was eliminated either numerically or automatically by including the series resistor into the negative feedback loop of a current-voltage converter circuit made from a low frequency operational amplifier. Its output was directly used as the probe current signal. RF blocking by a combination of a resistor and two resonant coils for 13.5 MHz and 9.5 MHz respectively, is shown to result in probe characteristics which are unperturbed even around the space potential.