The Transmission Electron Microscope
There are four main components to a
transmission electron microscope (TEM): an electron optical column, a vacuum
system, the necessary electronics (lens supplies for focusing and deflecting the
beam and the high voltage generator for the electron source), and control software.
A modern TEM typically comprises an operating console surmounted by a vertical column
and containing the vacuum system, and control panels conveniently placed for the
operator. The microscope may be fully enclosed to reduce interference from environmental
sources, and operated remotely.
The electron column includes elements analogous to those of a light microscope. The light source of the light microscope
is replaced by an electron gun, which is built into the column. The glass lenses
are replaced by electromagnetic lenses. Unlike glass lenses, the power (focal length)
of magnetic lenses can be changed by changing the current through the lens coil.
The eyepiece or ocular is replaced by a fluorescent screen and/or a digital camera.
The electron beam emerges from the electron gun, and passes through a thin specimen,
transmitting electrons which are collected, focused, and projected onto the viewing
device at the bottom of the column. The entire electron path from gun to camera
must be under vacuum.
Aberration-corrected transmission electron microscopy
The recent development of a dedicated commercial aberration-corrected TEM has enabled
major advances in TEM capability. Without correction, TEM resolution is limited primarily by spherical aberration, resulting
in a general blurring of the image, but also in a phenomenon called delocalization,
in which periodic structures appear to extend beyond their actual physical boundaries.
The ability to correct spherical aberration leaves the reduction or correction of
the effects of chromatic aberration as the next major challenge in improving TEM
performance. Chromatic aberration correctors have been successfully incorporated
into the FEI Titan™
TEM platform, but their design and operation are substantially more complex
than spherical aberration correctors.
Environmental transmission electron microscopy
An environmental TEM (ETEM), such as FEI's
Titan ETEM, uses a specially designed vacuum system to allow researchers
to observe specimens in a range of conditions approaching more "natural" environments,
with gas pressures in the sample chamber as high as a few percent of atmospheric
pressure. This is important for observing interactions between the sample and the
environment. ETEM relies on pressure-limiting apertures and differential vacuum
pumping to permit less restrictive vacuum conditions in the vicinity of the sample
while maintaining high vacuum in the rest of the electron column.
Next: The Scanning Electron Microscope (SEM)