Quantitative Depth Profile Analysis by Secondary Neutral Mass Spectrometry (SNMS)

Abstract

Concentration depth profiles at or below a solid surface are mostly determined by combining a surface sensitive analytical technique with controlled sputter removal of the sample. When electron spectroscopic techniques as AES or XPS are employed, the analytical signals refer to the mostly drastically changed surface stoichiometry during the sputtering attack [1], Mass spectrometric techniques as Secondary Ion or Secondary Neutral Mass Spectrometry, SIMS or SNMS, monitoring the ejected particle flux, are less sensitive to such misleading influences, at least for quasi-stationary conditions, i.e. for slowly varying concentration profiles. With SIMS, the true concentration profiles are often veiled by matrix effects. Such effects, as well as the high selectivity of SIMS, are circumvented by SNMS where, contrary to the secondary ion formation, the emission and the ionization process of an analyzed particle are decoupled. Thus, the ionization probability ${\rm{\alpha }}_{\rm{X}}^{\rm{O}}$ for an ejected neutral species X becomes a constant of the SNMS apparatus [2,3] and is no longer a function of the chemical environment at the emission site.