secondary ionisation mass spectrometry abbr., SIMS (rus. масс-спектрометрия вторичных ионов abbr., МСВИ; ВИМС) — a destructive method for chemical analysis of surfaces based on ionisation of molecular or atomic monolayers through bombardment of the substance with a focused ion beam (Xe+, Cs+, Ga+, etc.) with the energy up to several keV.

Description

The bombardment of a surface with a beam of ions (called “primary ions”) with kinetic energy of several keV causes desorption of neutral charged particles (neutrals) as well as their charged particles (ions) (see Fig.). The primary ions bombarding the surface are usually ions of inert gases (Ar+, Xe+), oxygen (O2+, O-), or metals (Ga+, In+, Bi+, Cs+, etc.). The ions formed during desorption are called “secondary ions”; their registered mass spectrum gives information about chemical composition of the sample surface.

The yield of secondary ions measured by the mass spectrometer depends on several factors, such as energy of primary ions, the angle of incidence of the beam, the acceptance angle for the ejected ions and the surface composition of the material. To ensure the maximum ionisation of the ejected neutrals the sample is exposed to a beam of low-energy ions moving in a cylindrically symmetric magnetic field. Quantitative analysis is performed using calibration curves showing the dependence of the secondary ion yield coefficient on the chemical composition of the surface layer. For example, under identical conditions, the yield of copper ions from an aluminium alloy containing 2% copper exceeds the copper ion yield from pure copper.

There are three types of SIMS:

- static (elemental analysis of the surface monolayer);

- dynamic (determining elemental composition of several layers as a function of depth);

- SIMS image (a combination of the two previous approaches, establishing relationships between the chemical composition and morphology of the surface). Modern methods of SIMS using a system of focusing and Ga+ ion beam make it possible to achieve a resolution of 50-60 nm.

Secondary ionisation mass spectrometry is characterised by a much higher sensitivity than other methods of chemical analysis of the surface (X-ray microanalysis, Auger spectroscopy); it can detect impurities or dopants, when their concentration is in the order of several per mille.

On the other hand, this method has disadvantages: first of all, the method is destructive, as atomic layers are successively removed from the surface; in addition, there is a problem in the calibration of mass spectrometer, since the yield of specific ions significantly depends on the concentration of other elements. Moreover, knowledge of isotopic composition and charge of the generated ions are required for interpretation of the spectrum.

As a method of surface analysis SIMS is applied to polymeric materials (determining composition of oligomers and structure of monomer units, identification of additives), metals, glass, paper, semiconductor materials, pharmaceuticals, biomaterials, coatings and paints on various substrates.

Illustrations

The concept of SIMS technique.
The concept of SIMS technique.

Authors

  • Goldt Ilya V.
  • Streletskiy Alexey V.

Source

  1. D. Brandon, W. Kaplan. Microstructural Characterization of Materials . — Wiley, 2008. — 550p.

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