SIMS Instrumentation: SIMS Primary Ion Column

Primary ions are extracted from the sources and passed to the sample through the primary ion column. The column usually contains a primary beam mass filter that transmits only ions with a specified mass-to-charge (m/z) ratio.

This mass filter eliminates impurity species in the beam. For example, Cr, Fe, and Ni ions sputter from stainless steel surfaces within a duoplasmatron. Without a primary beam mass filter, these metal contaminants deposit onto the sample surface, raising the detection limits for stainless steel elements.

In the figure above, the electromagnetically active components are shown in blue. The ion beam trajectories (indicated in red) are greatly exaggerated in the lateral directions.

The electrostatic lenses and the apertures control the intensity and width of the primary ion beam. Several aperture diameters are usually available at each aperture location. The primary beam intensity can be reduced by defocusing the ion beam onto the back of the first aperture (nearest the magnet). A narrow beam (at the sample) results from defocusing the ion beam (with the middle lens) onto the back of the second aperture, and then adjusting the last lens to transfer the image of the cross-over from behind the aperture onto the sample.

Electrostatic deflectors steer the primary beam in a raster pattern onto the sample. A finely focused and rastered primary ion beam delivers uniform primary beam intensity to an area on the sample. This leads to flat bottom sputter craters. The best depth resolution in a depth profile results when the secondary ions are sampled from the flat bottom of such a crater without contributions from the crater edges. Other deflectors (not shown) are located near the apertures. They help tune the primary beam through the middle of the electrostatic lenses.

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