Scanning Electron Microscopy


There are a number of cases where scanning Auger microscopy can be used to determine the near-surface composition of minerals with extremely high lateral resolution. This involves collecting Auger spectra with reasonable signal to noise ratios without encountering significant beam induced charging or surface degradation, even if the beam is impinging on a grain less than 1μm in diameter. We typically use a 3 keV beam with less than 10 nA beam current on a sample surface that is tilted (to increase backscattered and secondary electron emission efficiency) and relatively flat. To further minimize surface degradation, vacuum levels are kept high and the beam is rastered or defocused whenever possible. The Auger spectra of minerals can be used to study modification of surfaces due to geochemical influences or to identify submicron grains if the near-surface composition is representative of the bulk composition. Also, high lateral resolution step scans can be performed across sharp interfaces between two grains, allowing for short-range studies of solid-solid interactions in rocks at grain boundaries. We also report on preliminary attempts to chemically quantify Auger peak intensities for silicate minerals. Measurements of peak-to-peak heights for oxygen and silicon lines for eight silicate minerals of well-known composition indicate that Auger sensitivity factors can vary significantly with O/Si ratio.

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