The correlations of ionization potential with secondary ion yields are not perfect. The ion yields are relative to silicon in a silicon matrix with oxygen sputtering. For example, the following figure shows the logarithm of positive ion yields plotted as a function of ionization potential. The most obvious influences on ion yield are ionization potential for positive ions and electron affinity for negative ions. Ion yields vary over many orders of magnitude for the various elements. The SIMS ionization efficiency is called ion yield, defined as the fraction of sputtered atoms that become ionized. The best SIMS reference is Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications, and Trends, by A. Only dynamic SIMS will be treated in this surface analysis computer aided instruction package because only dynamic SIMS yields quantitative information.īombardment of a sample surface with a primary ion beam followed by mass spectrometry of the emitted secondary ions constitutes secondary ion mass spectrometry (SIMS). The resulting ion fragmentation patterns contain information useful for identifying molecular species. Shallow sputtering minimizes the damage done to organic substances present on the sample surface. This slow sputtering mode is called static SIMS in contrast to dynamic SIMS used for depth profiles. When the sputtering rate is extremely slow, the entire analysis can be performed while consuming less than a tenth of an atomic monolayer. Continuous analysis while sputtering produces information as a function of depth, called a depth profile. During SIMS analysis, the sample surface is slowly sputtered away. Controlling where the primary ion beam strikes the sample surface provides for microanalysis, the measurement of the lateral distribution of elements on a microscopic scale. The SIMS primary ion beam can be focused to less than 1 um in diameter. The SIMS ion source is one of only a few to produce ions from solid samples without prior vaporization.
Today, SIMS is widely used for analysis of trace elements in solid materials, especially semiconductors and thin films. This SIMS theory tutorial includes the uses of SIMS, with explanations of Ion Beam Sputtering and other effects.
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