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SIMS Theory: Ion Imaging
Ion images show secondary ion intensities as a function of location on sample surfaces. Image dimensions vary from 500 um to less than 10 um. Ion images can be acquired in two operating modes, called ion microscope or stigmatic imaging, and ion microbeam imaging or raster scanning. Ion microscopy requires a combination ion microscope/mass spectrometer capable of transmitting a mass selected ion beam from the sample to the detector without loss of lateral position information. Image detectors indicate the position of the arriving ions. Ion microscope images are usually round because the ion detectors are round. Lateral resolutions of 1 um are possible. A SIMS analyst selects images with higher lateral resolution at the expense of signal intensity and higher mass resolution at the expense of image field diameter.
For ion microbeam imaging, a finely focused primary ion beam sweeps the sample in a raster pattern and software saves secondary ion intensities as a function of beam position. Microbeam imaging uses standard electron multipliers and image shape follows raster pattern shape, usually square. Lateral resolution depends on microbeam diameter and extends down to 20 nm for liquid metal ion guns. Some instruments simultaneously produce high mass resolution and high lateral resolution. However, the SIMS analyst must trade high sensitivity for high lateral resolution because focusing the primary beam to smaller diameters also reduces beam intensity.
The example (microbeam) images show a pyrite (FeS2) grain from a sample of gold ore with gold located in the rims of the pyrite grains. The image on the right is 34S and the left is 197Au. The numerical scales and the associated colors represent different ranges of secondary ion intensities per pixel.
Three-dimensional analyses are possible by acquiring images as a function of sputtering time (image depth profiles). Microscope sputtering rates exceed microbeam rates, often by several orders of magnitude. Thus microscope imaging produces depth scales more compatible with the scale of the lateral images. Microbeam imaging usually provides a better combination of image features, except when faster sputtering is required for three-dimensional analysis or for removing an overlayer before image acquisition.
