Characteristics and Cleaning of Apertures
Platinum Melting Point: 1768°C (3214.4°F)
Molybdenum Melting Point: 2610°C (4730°F)
Tungsten Melting Point: 3422°C (6191.6°F)
Aperture contamination occurs when it is exposed to the electron beam, in which case regular
cleaning or replacement is required. The smaller the hole, the more quickly contamination
builds up and astigmatism increases. Dirty aperture systems make alignment difficult due to
beam deflection caused by dirt charging up.
Platinum/Iridium apertures may be cleaned by flaming them and applying temperature where the
aperture does not exceed a color of cherry-red (not white-heat). Bunsen burner or small flame
devices (alcohol, butane) have been used for this procedure. About 10-15 seconds should be
sufficient for this procedure. A "Flamer" for this purpose is described below. The
entire surface will reach the same color when completely clean.
Molybdenum apertures must be heated in a vacuum below 10-4 mbar. After proper vacuum is reached
and current applied, a color of cherry-red is achieved. Use a tungsten boat. This will require
about 20-30 seconds. Molybdenum apertures can be successfully cleaned more frequently and used
again, than Platinum/Iridium.
Platinum/Iridium disc apertures can be flamed in our platinum gauze "Aperture Flamer"
(No. 60001) for cleaning purposes.
The cleaned aperture(s) should be inspected on a clean glass slide at 30-40X. The hole should be
clean and round. Misshapen apertures should be rejected.
Bils RF, 1992. Electron Microscopy Laboratory Manual and Handbook, 2nd ed,
Alpha Editions, 329p.
Chapman SK, 1986. Maintaining and Monitoring the Transmission Electron Microscope, Royal
Microscopy Society, Microscopy Handbooks 08, Oxford University Press.
|