Electron microscope

A device operating on the basis of electron-optical processes

The most renowned electron microscope is the transmission electron microscope by which very fast electrons are transmitted through an object in a vacuum system and deflected by means of electrical or magnetic fields similar to a beam of light transmitted through lenses.

The overall structure of an electron microscope is analog to that of an optical microscope. At the upper end of the column of an electron microscope the radiation voltage, which commonly ranges between 40,000 and 100,000 V and can be set to a required value in some microscopes, is introduced into the cathode element through a shielded cable. Within the element the nearly parallel beam of fast electrodes required for transmission through an object emerges from the glow cathode. The condensor (1 or 2 magnetic lenses, i.e. encapsulated magnet coils) positioned underneath the cathode element makes it possible to not only irradiate large object surfaces but also object sections with diameters of a few µm. For viewing by means of the electron microscope, the object must be “inserted” into the vacuum of the column through an “object airlock” since the movement of electrons only takes place without collisions or deflections in a vacuum. In conventional transmission electron microscopes, the object thickness must be between 50 and 100 nm (= 0.05 to 0.1 µm).

The limit of resolution in state-of-the-art electron microscopes is at 0.1-0.2 nm (1 nm = 1 millionth of a mm). The (useful) magnification rate obtained by an electron microscope is generally chosen at such a value to ensure the separate and individual mapping of the detail resolution obtained through the electron microscope on the image material. For example, with a resolution of fine-grain material of 50 µm, the useful magnification for high-performance electron microscopes is 500,000-fold. However, to make the resolved details visible for the human eye, further optical magnification is required.

An advanced development is the scanning electron microscope that provides magnified images of any kinds of surfaces, regardless of their thickness or material and is nowadays used for the most application cases.

Additional references:

Electron microscopy
Electron probe micro analysis
Environmental scanning electron microscope
Scanning force microscope
Scanning tunneling microscope
Transmission electron microscope

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