A SELF-COMPENSATING LANGMUIR PROBE FOR USE IN RF (13.56 MHZ) PLASMA SYSTEMS

The operation of Langmuir probes in rf plasmas typical of those employed in semiconductor processing is considered, with particular attention being paid to the rf potential difference which appears across the sheath of a dc probe inserted in an rf plasma. To overcome the problem, the probe described incorporates a subsidiary electrode, placed close to the measurement region, which senses the plasma potential variations and feeds them to the probe tip. The design of the self-compensating probe is described and test data obtained for 13.56 MHz discharges in argon at input powers of 10, 50 and 100 W are discussed. The results demonstrate that the probe design is capable of giving reproducible data for parameters of interest in the monitoring of processing plasmas. In particular the floating and plasma potentials can be continuously monitored with good accuracy. Other data, such as the electron concentration and mean energy, can be deduced with an accuracy limited by the validity of the usual assumption of a Maxwellian energy distribution.