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Microscopy Society of America

| Theory || Qualitative analysis || Quantitative analysis || Back-scattered and secondary electron imaging || Cathodoluminescence imaging |

The sample chamber houses the back-scattered electron (BE) and secondary electron (SE) detectors, the EDS detector, and the sample stage. The BE detector is mounted above the sample, whereas, the SE and EDS detectors are mounted on the side. The SE detector provides a topographic image accentuating the fractures, ridges and holes. Below is an SE image of a cross-section of a tree stump:

The BE detector is most useful in compositional imaging (A+B mode), but it can be used in a differential mode (A-B mode) to obtain topographic images also. Below are BE images in A+B mode (left) and A-B mode (right) of a Zn-Sn composite:

Sample preparation:

If only a qualitative analysis is desired, the sample may be mounted with double-stick conductive carbon tape on the sample holder. For precise quantitative analysis, however, careful sample preparation is essential to ensure that the geometrical requirements for wavelength dispersive spectrometry are met. For good results, the surface at the spot being analyzed must be horizontal and the electron beam must be vertical. Hence, the surface must be well polished first with coarse SiC-paper and then with fine diamond paste or corundum (Al2O3) grit slurry. The final grit size should be at least 0.25 micron and preferably 0.06 micron. For samples that dissolve in water, dry polishing with fine polishing paper is recommended. Except for water-soluble samples, the polished sample is then washed with clean water in an ultrasonic cleaner and dried in air and with blow-duster.

Electrically insulating samples are carbon-coated to ensure conduction of the beam electrons away from the sample. Carbon coating is carried out by carbon evaporation under vacuum. A polished brass block is used to monitor the carbon-coat thickness on the sample. As the thickness of coat increases on brass, it's color changes from orange (150 Å) to indigo red (200 Å), blue (250 Å) and bluish green (300 Å). A thickness of about 225 Å is recommended for microprobe analysis at 15-20 kV accelerating voltage and 10-20 nA beam current.

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(Last Revised 11/29/2007 by Nilanjan Chatterjee)