|
SCANNING ELECTRON MICROSCOPY (SEM)
A more recent and extremely useful investigative tool is the scanning electron microscope (SEM). The surface of the specimen to be examined is scanned with an electron beam, and the
reflected (or back-scattered) beam of electrons is collected, then displayed at the same scanning rate on a cathode ray tube (similar to a TV screen). The image on the screen, which may be photographed, represents
the surface features of the specimen. The surface may or may not be polished and etched, but it must be electrically conductive; a very thin metallic surface coating must be applied to nonconductive materials.
Magnifications ranging from 10 to in excess of 50,000
diameters are possible, as are also very great depth-of-field. Accessory equipment permits qualitative and semiquantitative analysis of the elemental composition of very localized surface areas.
The figure bellow schematically illustrates its principles of operation. Basically, an electron gun produces an electron beam in an evacuated column which is focused and directed so that it
impinges on a small spot on the target. Scanning coils allow the beam to scan a small area of surface of the sample. Low-angle backscattered electrons interact with the protuberances of the surface and generate
secondary backscattered electrons to produce an electronic signal, which in tern produces an image having a depth of field.
Instructions for the operation of the JEOL SEM
Model JSM-5310LV
1. Make sure that the detector is filled with liquid N2.
2. When the HT (high-tension) key is ready (means that the vacuum LV control is fine), push the VENT key and wait for a short time.
3. Open the sample chamber and insert your sample stub (wearing gloves) into the brass cup, and gently tighten the set screw.
4. Close the door to the sample chamber and lock the clasp.
5. Push EVAC and wait for a few minutes.
6. When HT is ready and the VA is 100 %, push HT by which the image will display.
7. Depending on the sample, set up the accelerating voltage on 10 kV, Working Distance (WD) on 17 mm (normally 20 mm).
8. Set up the Spot Size (SS) on 15 mm (it should be less than 17 mm for image).
9. Magnifications ranging from X35 to in excess of X100,000 diameters are possible.
10. Focus the image using the focus coarse and fine knobs.
11. Adjust the brightness and contrast controls to your liking.
12. Push the key
13. Now is the time to operate the Noran EDS Voyager by double clicking on the image display on the desktop.
14. Click on Tools, then click on Spectrum Attributes and type values of Acc. Voltage, Magnification, Working Distance, Spot Size and Beam Current. Then close the window.
15. To quantify your sample elements, click on Analyze, then on Quant Periodic Table.
16. Now click on Spectral Display to get the spectrum.
TO SAVE SPECTRUM, go to File, then save as. Choose a name for your file, click on OK and close the window.
17. Click on Tools, then on Image Display by which you'll get the image.
TO SAVE IMAGE, go to File, then save as. Choose a name for your image, click on OK, and close the window.
Home page Back to topic ........ Previous page ......... Next page
|
