Evaluation of an orifice probe for plasma diagnostics

Abstract

An orifice probe is described which permits determination of bulk plasma parameters, including temperatures, number densities and space potential, and which also permits simultaneous use of a mass spectrometer sampling through the same orifice. This probe combines a small, sharp-edged orifice located in a conducting wall with a planar internal geometry which is amenable to calculation. A detailed current-collection theory for a collisionless plasma is described. Experimental orifice probe characteristics are analysed for low-pressure (< 10−2 Torr) RF plasmas in argon operating at 90 MHz with power levels of less than 60 W. The effects of secondary electron emission, electron reflection, field penetration, plasma inhomogeneities, and collisions in the sheath are examined. The electron and ion saturation regions (isat=10−8−10−9 A) and the electron-retarding region of the characteristics are found to indicate the properties of the bulk plasma. Values are obtained for the electron temperature kTe (<5 eV) and number density ne (108−1010 cm−3), together with an independent estimate of ion number density ni=O(ne). Space potential (15-25 V) can be determined within 0·5 V. An upper bound for kTi can be found, together with an indication of the degree of homogeneity of the plasma being sampled.