Biomedical Engineering Reference
In-Depth Information
1.5.2 Beam-specimen interactions in the SEM
When the electron beam strikes the sample many types of interactions can
occur in the penetration teardrop. Specifi c signals are generated at specifi c
depths with the interaction volume or penetration teardrop. The various
signals escape, are collected and produce images of the sample. The overall
size of the interaction volume depends on several factors:
Atomic number
- higher atomic number materials absorb or stop more
electrons and produce smaller interaction volumes.
Accelerating voltage (KV)
- higher KV penetrate further into the sam-
ple and generate larger interaction volumes.
Tilt angle
- the more the specimen is tilted, beam angle of incidence
further from normal, the smaller the interaction volume.
The main imaging signals utilized in the SEM are:
Backscattered electrons -
caused by an incident electron colliding with
an atom in the specimen which is nearly normal to the incident's
path. The incident electron is then scattered 'backward' 180˚.
Utilization
- the production of backscattered electrons varies directly
with the specimen's atomic number. These differing production
rates cause areas in the specimen containing higher atomic num-
ber elements to appear brighter than those areas containing lower
atomic number elements. This signal is utilized to differentiate
parts of the specimen that have different average atomic number.
Secondary electrons -
caused by an incident electron passing 'near' an
atom in the specimen to impart some of its energy to a lower energy
electron (usually in the K-shell). This causes a slight energy loss and
path change in the incident electron and the ionization of the electron
in the specimen atom. This ionized electron then leaves the atom with a
very small kinetic energy (5 eV) and is then termed a 'secondary elec-
tron'. Each incident electron can produce several secondary electrons.
Utilization -
production of secondary electrons is very topography related.
Due to their low energy, 5 eV, only secondaries that are very near the
surface (<10 nm) can exit the sample and be examined. Any changes
in topography in the sample that are larger than this sampling depth
will change the yield of secondaries due to collection effi ciencies.
Characteristic X-rays -
caused by the de-energization of the specimen
atom after a secondary electron is produced. Since a lower (usually
K-shell) electron was emitted from the atom during the secondary
electron process an inner (lower energy) shell now has a vacancy. A
higher energy electron can 'fall' into the lower energy shell, fi lling
