SIMS Instrumentation: Secondary Ion Extraction and Transfer

Secondary ions are extracted from the sample as they are produced. If large mass spectrometer components are held at ground potential, the sample must be held at high voltage, the accelerating potential. The secondary ions accelerate toward the ground plate of an electrostatic lens. This first lens is called the immersion or ion extraction lens. The second (transfer lens) focuses the ion beam onto the mass spectrometer entrance slits or aperture. This two lens system constitutes an ion microscope. The secondary ions could be projected onto an image detector for viewing the sample surface. Different transfer lenses produce different magnifications.

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 field aperture is located approximately at the point where the ion beam image comes into focus. The entrance aperture is sometimes called a contrast diaphragm. Smaller aperture diameters intercept ions with off-axis energy components. This reduces image aberrations but unfortunately it also reduces secondary ion intensity.

Ions that arise from off the secondary ion optical axis contribute to lower mass resolution. These off-axis ions arise because the primary beam raster pattern sputters an area rather than the single point where the axis intercepts the sample. The dynamic emittance deflectors adjust the secondary ion beam back on-axis. The deflectors operate in synchrony with the primary beam raster generator to provide continuous adjustment.

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