CHARACTERIZATION OF HELIUM ARGON WORKING GAS SYSTEMS IN A RADIOFREQUENCY GLOW-DISCHARGE ATOMIC-EMISSION SOURCE - PART I - OPTICAL-EMISSION,SPUTTERING AND ELECTRICAL CHARACTERISTICS/

Studies are performed to determine the influence of discharge gas comp osition (helium/argon working gas mixtures) on the analyte emission si gnal intensities, sputtering rates, and DC-bias characteristics of an analytical radiofrequency glow discharge atomic emission spectroscopy (RF-GD-AES) source. As the partial pressure of He is increased from 0 to 15 ton, increased emission intensity is observed for a range of bul k and trace elements in NIST 1250 SRM (low alloy steel), regardless of the base pressure of Ar in the source (5 and 9 ton). In contrast to i ncreases in analyte emission intensity of up to 300%, counterindicativ e decreases in the sputtering rates on the order of about 30-50% are o bserved. The magnitude of these effects depends on both the partial pr essure of helium introduced to the source and the total pressure of th e He and Ar gases. Use of relative emission yield (REY) to normalize c hanges in emission intensity to sputtering rates indicates that excita tion efficiencies increase under these conditions. Increases in averag e electron energy and temperature appear to control this response. Dec reases in both analyte emission intensities and sputter rates occur wi th increasing He partial pressure when the total pressure in the cell remains fixed (11 ton in these studies). Emission yields for the fixed pressure, mixed gas plasmas decrease as the partial pressure of He(He / Ar ratio) in the RF-GD source increases. In this case, decreases in electron number densities appear to dictate the lower REYs. Measuremen t of DC-bias values at the sample surface provide understanding with r espect to the observed changes in sputtering rates as well as suggest the origins of changes in plasma electron energetics. Use of a diamond stylus profilometer provides both the quantitative sputter rate infor mation as well as qualitative insights into the use of mixed gas plasm as for enhanced depth profiling capabilities. The analyte emission cha racteristics of these mixed gas plasmas can be further understood by e xamining fundamental Langmuir probe data for pure He and mixed Ar/He p lasmas, which are presented in the following companion paper. (C) 1998 Elsevier Science B.V. All rights reserved