SNMS - Secondary Neutral Mass Spectrometry

    This technique is similar to SIMS and is also known as 'Sputtered Neutral Mass Spectrometry'.

     In this method the neutral atoms are sputtered from a sample surface by the help of the Ar, Kr, Ne, etc. plasma and detected after a post-ionization. The post-ionization can be performed by a laser light or Electron Cyclotron Wave Resonance (ECWR) plasma. The main part of sputtered particles is in neutral state (near 99 %), so the quantitative estimation by SNMS is much better than by SIMS.

   Secondary Neutral Mass Spectrometry (SNMS) is a suitable technique to measure the chemical composition of almost any sample, because the flux of atoms sputtered from the sample is representative of the stoichiometry of the top-most layer. This is in contrast to X-ray Photoelectron Spectroscopy (XPS) systems combined with an ion gun, where preferential sputtering makes analysis more difficult.

plasma ne

Ne plasma

    Further advantage of SNMS is its applicability to insulators, which causes serious problems in methods, which rely on electron emission or excitation by a primary beam of electrons, (such as Auger Electron Spectroscopy, AES or XPS).

   In SNMS, the sample is bombarded with rare gas ions with energy in the range of 0.5 to 5.0 keV. This leads to the sputtering of atoms and molecules from the sample, which leave the surface. 

plasma kr

Kr plasma

    The flux of sputtered particles consists of ions and neutral atoms. The neutral atoms are detected by post-ionizing the atoms that are ejected from the surface. The probability to sputter ionized atoms from a surface can vary between 10-5 to 10-1 and depends strongly on the surface composition (matrix effect). The majorities of sputtered particles are neutral and thus only vary between 90% and 99.9999%. This smaller sensitivity to the surface composition reduces matrix effect and allows a much better quantitative estimate of the stoichiometry.