RF HARMONIC SUPPRESSION IN LANGMUIR PROBE MEASUREMENTS IN RF DISCHARGES

RF currents at frequencies well above the ion plasma frequency omega(p i) can be easily suppressed in the space charge sheath of a Langmuir p robe by using a suitably chosen low capacitance series resistor R. Suc h a resistor was used to eliminate the influence of RF currents on the probe characteristics: for blocking harmonics in RF discharges with d riving frequencies of the order of omega(pi), such as the commonly use d 13.56 MHz discharges and also for blocking the fundamental in discha rges at higher frequencies (27 MHz and above). The influence of the re sistor R on the probe characteristics was eliminated either numericall y or automatically by including the series resistor into the negative feedback loop of a current-voltage converter circuit made from a low f requency operational amplifier. Its output was directly used as the pr obe 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 res ult in probe characteristics which are unperturbed even around the spa ce potential.