A radio frequency compensated emissive probe

We discuss a technique for using an emissive probe to measure accurately the time-invariant plasma potential in a rf generated plasma, usually used in plasma processing. With an emissive probe, a short length of filament, heated to thermionic emission, is immersed in the plasma. We have introduced into each lead of the filament two LC circuits, one resonant at 13.6 MHz, the fundamental rf frequency, and the other at 27.2 MHz, the first harmonic. With the LC circuits in place a single peak of di/dv versus v occurs with a maximum at the time-invariant plasma potential. The inflection point method of measuring this potential described by Smith, Hershkowitz, and Coakley [Rev. Sci. Instrum. 50, 210 (1979)] in a dc plasma is now operative. The estimated error in measuring this potential is ±3.5%.