RF harmonic suppression in Langmuir probe measurements in RF discharges

Abstract

RF currents at frequencies well above the ion plasma frequency 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 , 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.