Secondary Ion Mass Spectroscopy (SIMS)   



Secondary Ion Mass Spectroscopy (SIMS) is a failure analysis technique used in the compositional analysis of a sample.  SIMS operates on the principle that bombardment of a material with a beam of ions with high energy (1-30 keV) results in the ejection or sputtering of atoms from the material. A small percentage of these ejected atoms leave as either positively or negatively charged ions, which are referred to as 'secondary ions.'  


The collection of these sputtered secondary ions and their analysis by mass-to-charge spectrometry gives information on the composition of the sample, with the elements present identified through their atomic mass values.  Counting the number of secondary ions collected can also give quantitative data on the sample's composition. Thus, SIMS works by analyzing material removed from the sample by sputtering, and is therefore a locally destructive technique.


The yield of secondary ion sputtering, which affects SIMS sensitivity, depends on the specimen's material, the specimen's crystallographic orientation,  and the nature, energy, and incidence angle of the primary beam of ions.


The proper choice of primary ion beam is therefore important in enhancing the sensitivity of SIMS.  Oxygen atoms are usually used for sputtering electropositive elements or those with low ionization potentials such as Na, B, and Al.  Cesium atoms, on the other hand, are better at sputtering negative ions from electronegative elements such as C, O, and As.  The detection limit of SIMS is severely reduced with improper selection of the ion beam.  Liquid metal ion sources are used for high-resolution work, since these can provide smaller beam diameters.


Since the sputtered ions escape from shallow depths, the sputtering of the sample has to be prolonged in order to extend the analytical zone of the sample into deeper regions of the bulk material.  Monitoring secondary ion emission in relation to sputtering time therefore allows depth profiling of the sample's composition. Layers of up to 10,000 angstroms thick can be depth-profiled using SIMS.  Using SIMS as a depth-profiling tool is the dynamic mode of SIMS operation.


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Figure 1.  Example of a SIMS Spectrum


SIMS offers several advantages over other composition analysis techniques, namely: 1) the ability to identify all elements, including H and He; 2)  the ability to identify elements present in very low concentration levels, such as dopants in semiconductors.


Of course, SIMS also has its own drawbacks as an analysis technique. Its range of beam diameter (1-200 microns) is limited, with the sensitivity of the technique suffering as the beam diameter is reduced, since less ions for analysis are sputtered from the material.   It also suffers from secondary mass interference problems and, as mentioned earlier, is a locally destructive technique. 


See Also:  Failure AnalysisAll FA TechniquesLIMSEDX/WDX Analysis;

Auger AnalysisESCA or XPSFTIR SpectroscopyChromatography;

FA Lab EquipmentBasic FA Flows Package FailuresDie Failures    





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