Quantitative analysis in general aims at determining the value of a selected physical or chemical property of a given sample. With SIMS, in particular, the parameters to be determined are the concentration of a given element and also its distribution, both in depth and along the surface. Every measurement is affected by a given overall uncertainty due to statistical fluctuations in the measured signal and to systematic errors related to the experimental conditions. A discussion is devoted to the experimental conditions which influence the reproducibility of the measured signal and give rise to systematic errors, e.g. charging effects, matrix effects, selective sputtering and mass-dependent transmission, etc. The various methods used to derive the desired elemental concentration and its distribution in the sample from the raw measured data are evaluated, namely the use of calibration curves, relative sensitivity factors, fitting parameter methods and first-principle methods. Finally, the question is considered of how representative is the value determined from the (usually small) volume with respect to the composition of the bulk of the sample (sampling error).
The generation of charge build-up, caused by bombardment of insulating samples with energetic particles, and its role in altering the relative secondary-ion currents and reducing their absolute values, sometimes even to zero, are discussed. Proposed methods for charge reduction by bombardment with negative ions or with neutral particles, or by use of an auxiliary electron beam or spray gun, are shown to be not useful in every experimental situation. A further method involving introduction of an auxiliary conducting electrode when using negative primary ions is considered mechanistically, and tested by placing tantalum diaphragms onto the surface of yttrium iron garnet (YIG). The final charging values and mass spectra thus obtained are compared to those measured when using bare and metallic grid-covered surfaces of the same YIG sample. The satisfactory results achieved with the diaphragm show that SIMS analyses of insulators can be performed in a simple, quick, and inexpensive manner.
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