Instrumentation

 

Secondary Ion Mass Spectrometry is a powerful technique used for elemental surface analysis and depth profiling in a wide range of materials.

Detection limits are typically in the ppm-ppb range, and all elements, including H, can be analysed in minimal amounts (the sample consuption during analysis is generally at ng level). In addition, SIMS is capable of characterising the spatial distribution and concentration of elements in a sample.

Secondary ion images provide spatially resolved two-dimensional elemental information, and they can be produced either by direct ion imaging with an ion microscope, or by the scanning ion microprobe technique.

The instrument that employs SIMS methods is the ion microprobe. It combines the concepts of the mass spectrometer and the secondary ion emission microscope.

The ion probe installed at CNR-IGG (Pavia) is a Cameca ims 4f (no. 116 of the commercial series) (Fig. 1).

 

How it works:

Local microanalysis is possible by a primary ion column, which combines a hollow cathode duoplasmatron (mainly for O- and O2+) and a Cs+ ion source with an ion gun of three lenses. This system can vary the diameter of a static primary ion beam from less than 3 mm to greater than 200 mm, while maintaining a high current density.

The energetic ion bombardment of the surface of a material causes atoms to be ejected by the sputtering process. Some of the sputtered material is ejected as positive or negative ions. Each point of the surface therefore becomes a source of secondary ions which are characteristic of the elements or isotopes found in the near surface region.

This emission is used to obtain a local analysis in the following way:

  • Electrostatic optics are employed to extract and transport the secondary ions from the sample while maintaining their spatial relationship.
    The electrostatic optics consists of an accelerating and collection stage, and a focusing system called the "transfer optics", which optimise the sensitivity of the spectrometer as a function of the dimensions of the surface analysed for a given mass resolution.

  • The mass spectrometer is made up by an electrostatic sector and a magnetic prism. Its focal properties are the following:

- Double focusing, in the radial plane and in the transverse section (axial plane) 

- Energy focusing in the plane of the exit slits (optimising the spectrometer transmission) and in the image plane (allowing for the acquisition of an achromatic image).

This triple focusing permits the simultaneous acquisition of an image of  the distribution of the elements in the sample, with the necessary mass resolution and high sensitivity.

  • The mass-resolved ion image is then magnified and transformed to an equivalent electron image, which then can be displayed on a fluorescent screen. This image can also be observed through binoculars and photographed. Thus one can obtain a distribution map of elements and isotopes from the surface of the sample. Ion-intensity measurements can be made on the mass separated ion beam using a Faraday cup or an electron multiplier.

The ion optical system of the instrument is viewed in Fig. 2 (courtesy from Cameca, France).

So far, our instrument is set up for the quantitative analysis of trace (and minor) elements in silicate matrixes, phosphates, natural and synthetic glasses microscope, or by the scanning ion microprobe technique.