Application of glow discharge mass spectrometry and sputtered neutral mass spectrometry to materials characterization

Postsputtering ionization using an inert gas plasma to decouple the sputtering and ionization processes minimizes matrix effects commonly associated with conventional secondary ion mass spectrometry. Glow discharge mass spectrometry (GDMS) utilizes ions generated in a dc inert gas plasma to sputter atoms into the ambient plasma from the surface of a cathode composed of the material to be studied. The sputtered atoms, predominantly neutral, are then ionized in the plasma by Penning and electron impact ionization. GDMS provides excellent sensitivity and signal stability, and has found wide uses for characterization of bulk materials such as metals and GaAs. The sensitivity and large sampling volume also make it ideal for determining nonuniform trace contaminants in materials, e.g., U and Th in metals. Sputtered neutral mass spectrometry uses either low-energy plasma ions (direct bombardment mode or DBM) or an independent focused ion beam (separate bombardment mode or SBM) to sputter atoms into a low-pressure high-frequency plasma for electron impact ionization. DBM offers better depth resolution than conventional surface analytical techniques and is ideal for thin-film studies. The plasma electrons compensate for sample charging in SBM and make this method ideal for the analysis of insulating material such as phosphosilicate glass.