Plasma studies on the Leybold-Heraeus INA3 secondary neutral mass spectrometry system

Abstract

Electron gas secondary neutral mass spectrometry (SNMS) has been introduced recently by Leybold-Heraeus (LH) as a commercially available method for quantitative surface and bulk analysis of solids. Being the central part of electron gas SNMS, the employed special low-pressure rf discharge has to be characterized carefully in terms of electron temperature Teplasma (or electron) density ne and plasma potential in order to ensure reproducible sensitivity factors for different elements. In the present study the fundamental dependences of Teand ne on adjustable experimental parameters have been established on the first INA3 combined SNMS-secondary ion mass spectrometer system. The results could be explained semiquantitatively by means of the charge carrier and power balances of the plasma. The INA3 system features two measurable parameters for plasma characterization: The floating potential Upof an insulated Langmuir probe and the current Ipmeasured on a second negatively biased probe. Unique relations could be determined between UpIp and Te ne✓Te respectively, as predicted by plasma theory. Furthermore, the variation of Teand ne as a function of discharge burning time was measured in order to investigate warm-up effects which were found to affect the residual gas suppression efficiency of the system. A detailed study of residual gas influence by adsorption and resputtering from the sample surface showed evidence for a significant reduction of the detection sensitivity of reactive elements like C, N, and O, which may occur on samples with high sticking probabilities for residual gas components. © 1988, American Vacuum Society. All rights reserved.