Electron-impact El Secondary Neutral Mass Spectrometry SNMS

Electron-impact (El) Secondary Neutral Mass Spectrometry (SNMS) 

The limitations of SIMS - some inherent in secondary ion formation, some because of the physics of ion beams, and some because of the nature of sputtering - have been mentioned in Sect. 3.1. Sputtering produces predominantly neutral atoms for most of the elements in the periodic table the typical secondary ion yield is between 10 and 10 . This leads to a serious sensitivity limitation when extremely small volumes must be probed, or when high lateral and depth resolution analyses are needed. Another problem arises because the secondary ion yield can vary by many orders of magnitude as a function of surface contamination and matrix composition this hampers quantification. Quantification can also be hampered by interferences from molecules, molecular fragments, and isotopes of other elements with the same mass as the analyte. Very high mass-resolution can reject such interferences but only at the expense of detection sensitivity. 

Compared with the other methods of surface and thin film analysis, the main advantages of SNMS are  

Compared with XPS and AES sputter depth profiling After achieving sputter equilibrium, and until a layer with different sputtering behavior is reached [3.59], the SN flux represents stoichiometry and not altered layer concentrations evolving because of preferential sputtering effects. 

Compared with GD-OES (and -MS, if used for depth profiling) SNMS provides somewhat better depth resolution (1 nm range) HE-plasma SNMS hardly suffers from molecule formation in the plasma gas (Ar), as do the CD techniques, in which ar-gides are formed because of the comparatively high pressure. 

---