Biomedical Engineering Reference
In-Depth Information
The cold-cathode gun consists of an extremely acute monocrystalline tungsten
tip. It is not heated (pure field effect), but rather a significant electrical field is
applied (2-7 keV). These guns require an extremely high vacuum in the microscope
(10
-7
-10
-8
Pa) in order to prevent tip contamination. This particular requirement
makes FEGs very expensive and delicate machines. The emitted electrons have a
very low energetic dispersion (see energy window in Table
3.1)
. The beam is highly
coherent.
Another type of gun is composed of a semi-conducting tip (Schottky effect).
These guns have very high brightness. These types of guns are used in SEMs and
TEMs to produce very bright nanometric-sized probes.
3.2 Illumination Modes and Detection Limits
By controlling the stability of the high voltage and through improvements in
objective lens quality, detection limits are related mainly to beam coherence and
brightness. In particular, a gun's brightness increases from a source with a tungsten
filament to a field emission gun ranging from 10
5
to 10
8
A/cm
2
/sr. This increase
in brightness enables a reduced spot size, i.e., it reduces the spatial resolution of
the analyses. For example, the smallest analyzable size is from 50 to 0.025
m
in an SEM, from 2 to 1 nm in a TEM (from tungsten guns to FEGs) and from 1 to
0,1 nm in a STEM. Additionally, beam coherence is well adapted for high-resolution
microscopy conditions (HRTEM).
Lens manufacturing defects must be considered in addition to the type of detec-
tion limit described above. These lens defects create an imperfect lens, and as a
result they do not have rotational symmetry. Thus, they introduce an aberration
for which the microscopist must compensate. The most significant aberration, “the
spherical aberration” (
C
s
), affects the objective lens. This aberration has a defined
value for a given objective lens. The spherical aberration astigmatism correction is
the determining limitation for the HRTEM imaging illumination mode. Its effect
may be reduced by the introduction of an objective aperture or it can be eliminated
using
C
s
correctors. These types of correctors, composed of additional magnetic
lenses, are part of the latest generation of microscopes.
µ
3.3 Microscope Resolutions and Analysis
3.3.1 Resolution Limit of the TEM
A microscope's resolution corresponds to the shortest distance between two details
that can be distinguished within the image. For a transmission electron microscope,
two resolutions are defined: one in two directions, called “point” resolution, which
is necessary for atomic resolution, and the other in one single direction, or “line”
resolution of the interreticular planes, which is found in the lattice-fringe technique.
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