Secondary neutral MS (SNMS) is an interesting variation on secondary ion MS (SIMS). Both techniques provide sensitive analysis of the chemical composition of the sample surface, as well as the ability to probe layers beneath the surface in destructive depth-profiling applications.

Despite the similarities, SNMS provides certain advantages that may be important for particular applications. SIMS and SNMS work by directing a beam at the surface and analyzing the results of the interaction between the beam and the surface. In both techniques, the probing beam is an ion beam generated by an ion gun, generally under condition of a high vacuum. As the beam of high-energy ions impacts the surface, atoms, molecules and molecular fragments are liberated (sputtered) from the surface and reside in the vacuum. In the sputtering process, some of the sputtered species are ionized. In SIMS, these secondary ions are directed to a MS.

However, the sputtering process actually produces many more neutral particles than charged ions. SNMS is carried out by analyzing the neutral species sputtered from the sample. Since the neutrals are uncharged, they are not yet suitable for analysis. The novel addition to an SNMS system is a secondary method for ionizing the secondary neutral particles. This is commonly accomplished by use of a plasma source or laser source to provide the necessary ionization energy. Then, the formerly neutral analytes are directed to the MS.

The primary advantage of SNMS is not that more neutrals are created by sputtering, providing at least the potential for a strong signal; rather, it is that the SNMS is less affected by matrix effects than SIMS. Matrix effects result from chemical interactions among the surface atoms, resulting in some elements being preferentially liberated, skewing the analysis. Since it is relatively unaffected by these preferential sputtering effects, SNMS provides better quantitative analysis.

Because of the close relationship between SNMS and SIMS, SNMS systems can perform in both modes. Indeed, there are few commercial instruments that are specifically designed for SNMS; many SNMS instruments are modified versions of SIMS systems. The primary changes needed are a screening voltage to eliminate the ions normally analyzed by SIMS, followed by a secondary mechanism to ionize the sputtered neutrals.

There are only two dedicated commercial SNMS instruments. SPECS markets the INA-X, a magnetic sector SNMS product stemming from its 1992 acquisition of the surface analysis line of Leybold. Hiden Analytical offers the MAXIM SIMS/SNMS Workstation, which has a quadrupole mass analyzer. SRI International has developed a SNMS system with a laser ionization system, but is not selling the system. Since some SNMS systems are modified SIMS systems, SIMS suppliers like Cameca and Ulvac-PHI are represented indirectly in the SNMS market, which was valued at about $4 million for 2010.

SNMS at a Glance:

Leading Suppliers

• SPECS GmbH

• Hiden Analytical

Largest Markets

• Semiconductors

• Magnetic Media

• Thin Films

Instrument Cost

• $400,000–$500,000