Biomedical Engineering Reference
In-Depth Information
4 The Different Types of Microscopes: SEM, TEM, and STEM
Modern microscopes are increasingly enabling conventional transmission elec-
tron microscopy (CTEM), scanning electron microscopy (SEM), and the scanning
electron mode (TEM/STEM) in transmission electron microscopes (TEM).
4.1 Scanning Electron Microscope (SEM)
In the scanning electron microscope (in reflection), the narrow incident electron
beam scans the specimen surface in convergent mode by means of the sequential
displacement of the electronic probe on the sample (by using the deflection coils).
This displacement is combined with the simultaneous synchronized transmission
of the signal coming from the sample onto a display screen (as in a television).
Each point on the screen corresponds to a point of the probe on the sample. The
signal is collected by detectors located above the sample. This signal can be quite
varied (e.g., secondary, backscattered, and scattered electrons by the surface, X-rays
emitted, the sample current) Therefore, for a chosen signal, the image is a map of
the sample's response to the electron probe. The advantage of certain newer SEMs
is the possibility of working at controlled pressure near atmospheric pressure or in
either humid or reactive atmospheres. SEMs with field emission guns have enabled
the use of very small probes and especially low voltages (less than 500 eV) for the
direct observation of insulating samples.
4.2 Conventional Transmission Electron Microscope (CTEM)
The column of a transmission electron microscope is composed of a set of electro-
magnetic condenser lenses that illuminate the sample, according to different modes
(parallel or convergent beam), an objective lens to form the image and diffraction of
the object, and magnification lenses (diffraction, intermediate, and projector lenses)
placed after the objective lens to magnify either the sample's image or the diffrac-
tion pattern. The sample is placed in the air gap of the objective lens in a sample
holder that can be tilted in one or two directions and possibly rotated. The space
available for the sample in the specimen holder is small, on the order of the magni-
tude of the objective lens's focal length. This lens will determine the type of stage,
which can be either high resolution or analytical, enabling tilt angles of
10
◦
±
to
70
◦
for the latter. The microscope's vacuum is
about 10
-5
-10
-6
Pa, depending on its various pumping systems.
Figure
3.4a
shows a cross section of a conventional transmission electron micro-
scope with a symmetrical objective lens that is used to produce a parallel or
convergent beam with very small probe sizes.
The second type of microscope (Fig.
3.4b
) contains a Köhler illumination system
that enables a completely parallel observation mode for all magnifications and an
Omega electron energy filter in the column.
25
◦
30
◦
±
for the former and
±
to
±
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