RF compensated probes for high-density discharges

Abstract

A Langmuir probe for the study of high-density RF discharges has been developed and tested in a helicon discharge in which the RF potential ( approximately=100 V) is much larger than the electron temperature ( approximately=4 eV). Carbon probe tips are used to minimize erosion by ion sputtering. Miniature RF chokes located close to the probe tip present RF impedances of 150 k Omega at the operating frequency of 27.12 MHz and 300 k Omega at the first harmonic at 54.24 MHz. It is further necessary to drive the probe tip to follow the RF fluctuations by coupling it to a larger floating electrode. We have been able to measure T_e values as low as 3.4 eV in argon plasmas of 10¹³ cm^-3 density; these temperatures are 1.6 eV lower than ones measured by probes with chokes alone, and 2.3 eV lower than measured by uncompensated probes.