Planar magnetron glow discharge plasma as an atom and ion source for atomic spectroscopy.

Abstract

A planar magnetron glow discharge plasma is evaluated and developed. In this unique device, a coaxial magnet pair aids in trapping plasma electrons in closed loops parallel to the cathode surface and increasing Ar$\sp+$ ion formation. Stable operation of the plasma can be obtained at all pressures from 0.0004 Torr to over 2.5 Torr, with dc voltage below 500 V and a current range from less than 25 mA to over 400 mA. A ring-shaped plasma is obtained. Sputtering of the cathode surface occurs only in a 2-mm wide, 24-mm diameter track. Because mean free paths are greater at lower pressure, Ar$\sp+$ ion kinetic energy increases and backscattering of sputtered materials is reduced. This results in increased sample material sputter rates. Maximum linear etch rates for Al, Cu and Au are 31.7, 52.0 and 75 0 kA/min, respectively, at 0.0025 Torr and 300 mA. A scanning electron microscope is used to characterize the sample surfaces sputtered at different conditions. A much cleaner environment for chemical analysis is obtained. Extensive atomic emission measurements are made to study the excitation efficiencies. Minor and trace elements, Al, Mg, Ni, and Mn in the Zn-base NIST Standard Reference Materials are analyzed. Detection limits for Mn and Ni are 0.0060% and 0.00055%, respectively. High ion currents of Al, Zn, Mn, and Mg are found by using a quadrupole mass spectrometer. Sampling cones with different orifice diameter, 0.5, 1, 2, and up to 5 mm, are used in the study. Al and Zn ion distributions in the glow discharge are plotted in the three dimensional maps. Reactive gases, CF$\sb4$ and CF$\sb2$Cl$\sb2$, are introduced into the glow discharge. Comparison of mass loss rates in pure Ar and 1/1 Ar/CF$\sb4$ at low pressure shows enhancement for Al and Ag, but reduction for Cu, brass, Zn and Au in the mixture. Addition of reactive gases results in significant quenching of optical and mass spectral features at both high and low pressures.