Relationships between cluster secondary ion mass intensities generated by different cluster primary ions

Abstract

Measurements are described to evaluate the constitution of secondary ion mass spectra for both monatomic and cluster primary ions. Previous work shows that spectra for different primary ions may be accurately described as the product of three material-dependent component spectra, two being raised to increasing powers as the cluster size increases. That work was for an organic material and, here, this is extended to (SiO 2 ) t OH Ϫ clusters from silicon oxide sputtered by 25 keV Bi n ϩ cluster primary ions for n ϭ 1, 3, and 5 and 1 Յ t Յ 15. These results are described to a standard deviation of 2.4% over 6 decades of intensity by the product of a constant with a spectrum, H SiOH ‫ء‬ , and a power law spectrum in t. This evaluation is extended, using published data for Si t ϩ sputtered from Si by 9 and 18 keV Au Ϫ and Au 3 Ϫ , with confirmation that the spectra are closely described by the product of a constant with a spectrum, H Si ‫ء‬ , and a simple spectrum that is an exponential dependence on t, both being raised to appropriate powers. This is confirmed with further published data for 6, 9, 12, and 18 keV Al Ϫ and Al 2 Ϫ primary cluster ions. In all cases, the major effect of intensity is then related to the deposited energy of the primary ion at the surface. The constitution of SIMS spectra, for monatomic and cluster primary ion sources, is shown, in all cases, to be consistent with the product of a constant with two component spectra raised to given powers. (J Am Soc Mass Spectrom 2010, 21, 370 -377) © 2010 American Society for Mass Spectrometry T he analysis of complex molecules in SIMS has been enhanced in recent years by the application of primary cluster ions of the type Au n Ϯ , Bi n ϩ , C 60 nϩ , etc. These provide significantly higher yields of the larger fragments that are important in the analysis of larger molecules. Over the years, there has not been a great focus on the relative intensities of the many secondary ions in the spectrum and their changes with the change in primary ion cluster size and energy. Analysts have usually focused on the enhancement of intensity of a particular characteristic ion obtained when using larger cluster primary ions. However, the fragment ions in the spectrum can be used to construct the structure of the molecules or the arrangement of atoms at a surface as well as provide important data as a function of depth in depth profiles [1][2][3][4][5][6][7][8]. Studies of the whole secondary ion spectrum and its evolution from one primary ion source to another are an important part of the infrastructure to the whole field of secondary ion mass spectrometry.Much historical data and data in spectral libraries [9 -11] are for inert gas primary ions. Over the years, the primary ion sources used have changed and changed again. Different spectrometer manufacturers fit different primary ion sources. It is important, therefore, if analysts are to make the best use of their instruments and of the published literature and data sources, that the relationships for the secondary ion ...

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