Contamination of Apertures
- Practical Electron Microscopy and Database -
- An Online Book -  

This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.


In conventional EM systems, the real objective aperture locates near the poorer vacuum of the specimen chamber, so it is easy to contaminate the aperture. However, in many modern SEMs and STEMs the beam-limiting aperture is located after the last condenser and before the objective lens, called as "virtual objective aperture" (VOA).

Contamination of aperture surfaces is normally originated from deposition of organic compounds. The contamination sources are:
        i) Vacuum pump oil used in the pumping system.
        ii) Gaskets.
        iii) O-rings used to seal the vacuum system.
        iv) Greases used to lubricate O-rings.
        v) Specimen itself.
Due to the exposure of the deposited organic molecules to the electron beam, a hydrocarbon film is coated on the aperture surfaces. With the contamination accumulation, the contaminated apertures need to be either replaced or cleaned. The cleaning methods are:
        i) The apertures are heated with a propane torch or within a vacuum evaporator. The drawback of the heating methods is temperature gradients are created and thus the aperture strip can be deformed or even the welds in the apertures can be destroyed.
        ii) Plasma cleaning.




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