Langmuir probe measurements of weakly collisional electropositive RF discharge plasmas

We report on Langmuir probe measurements of low-pressure (0.1-20 Pa) electr opositive plasmas in an RF discharge at 13.56 MHz. From the probe I-V chara cteristic it is found that the electron density inferred from the ion curre nt in the ion saturation region using radial motion (Allen, Boyd and Reynol ds, ABR) theory can be up to one-half that obtained directly from the elect ron current at the plasma potential. The reduction in the ion current is at tributed to orbital motion (OM) of the ions and also to a small number of i on-neutral collisions in the presheath. We show that if a sufficiently larg e probe is chosen so as to minimize the OM effects then the collisional the ory developed by Shih and Levi (1971) can be used to give an appropriate co rrection factor over a narrow pressure range. The corrected electron densit y is found to agree with the knee current value to typically 10% for Ar, N- 2 and Kr plasmas.