Table of ContentsIntroductionSIMS and SNMSConclusionAbout Hiden Analytical
Sputtered Neutral Mass Spectrometry is perfect for analyzing thin metal films where composition, thickness and interface condition can be determined. The example presented here, shows a magnetic film stack comprising Cr, Ni, Cu and Fe. Of the different methods of data storage, magnetic hard discs still offer highly economic means for high density rapid access. The continual move towards more and more compact read/write heads and more closely spaced data tracks has lead to a considerable development of magnetic materials and structures for this demanding application. Analysis of the metallic layer structures used in magnetic data storage is vital for both development and quality management with secondary ion mass spectrometry (SIMS) and sputtered neutral mass spectrometry (SNMS) offering information on minor and major element composition respectively.
SIMS and SNMS
Both SIMS and SNMS use a concentrated, mono- energetic, chemically pure ion beam of typically 1-10 keV to sputter erode the surface under study. A small fraction of the sputtered material gets ionized due to the sputtering process itself and in SIMS, it is these ions that offer the sensitive information for which the technique is known. Being a mass spectrometry technique, all elements and isotopes may be detected, and in favorable conditions the detection limit can be in the low ppb region. However, because the ionization mechanism for SIMS takes place at the sample surface, it is very much dependent on the local chemistry and the ionized fraction can vary by several orders of magnitude. This makes SIMS ideal for trace analysis in materials of known matrix but quantification in materials of changing matrix can be complicated. SNMS overcomes the “matrix effect” by separating the sputtering and ionization events.
Even in high ion yielding situations the fraction of ions rarely exceeds 1% of the sputtered material, so the neutral flux is more representative of the sample composition. Ionization for SNMS takes place in an electron bombardment cell at the front of the analyzer, which means that the ionization probability is a constant and does not depend upon the sample chemistry. To quantify SIMS it is important that the reference material be as similar to the unknown as possible and must certainly be of the same matrix material. For SNMS, this matrix matching of reference materials is unnecessary, as calibration factors do not change with matrix therefore, the required sensitivity factors can be determined from easily available metal and ceramic samples of published composition.
Further, SNMS is ideal for the analysis of insulators, as the neutral species are unaffected by sample charging, however, charge compensation is still advisable in order to maintain consistent primary beam conditions.
The ionized secondary particles are studied and detected in the mass spectrometer. At very low ion beam currents analysis is confined to the top few monolayers, excellent for detection of surface contamination. As the ion beam dose is increased and sputtering becomes more aggressive, subsequently deeper layers are exposed and concentration as function of depth can be identified. Using a focused ion beam, both SIMS and SNMS become spatially resolving and elemental images can be recorded.
The analysis shown here was made using the Hiden SIMS workstation, a complete and highly flexible quadrupole SIMS/SNMS instrument equipped with the IG20 gas ion gun and MAXIM SIMS/SNMS analyzer.
About Hiden Analytical
Hiden Analytical is a leading manufacturer of quadrupole mass spectrometers for both research and for process engineering. Their products Our products address a diverse range of applications including:
- Precision gas analysis
- Plasma diagnostics by direct measurement of plasma ions and ion energies
- SIMS probes for UHV surface science
- Catalysis performance quantification
- Thermo-gravimetric studies
These analytical instruments are designed to work over a pressure range extending from 30 bar processes down to UHV/XHV.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
For more information on this source, please visit Hiden Analytical.