Physical Principles of Electron Microscopy: An Introduction to TEM ...

The Scanning Electron Microscope 139
mounted just below the objective lens. Impurity atoms (arsenic, phosphorus)
are added to the silicon to make it electrically conducting. The diode consists
of an n-type layer, in which conduction is by electrons, and a p-type layer, in
which conduction is by holes (absence of electrons in an otherwise full
valence band). At the interface between the p- and n-layers lies a transition
region, in which current carriers (electrons and holes) are absent because
they have diffused across the interface. A voltage applied between the n- and
p-regions (via metal surface electrodes) will therefore create a high internal
electric field across this high-resistivity transition region. If a backscattered
electron arrives at the detector and penetrates to the transition region, its
remaining kinetic energy is used to excite electrons from the valence to the
conduction band, creating mobile electrons and holes. These free carriers
move under the influence of the internal field, causing a current pulse to
flow between the electrodes and in an external circuit. BSE arrival can
therefore be measured by counting current pulses or by measuring the
average current, which is proportional to the number of backscattered
electrons arriving per second. Because secondary electrons do not have
enough energy to reach the transition region, they do not contribute to the
signal provided by the solid-state detector.
Because the Robinson and solid-state detectors are mounted directly
above the specimen, their BSE signal contains little topographic contrast but
does show “material contrast” due to differences in local atomic number in
the near-surface region of the specimen. The orientation of crystal planes
(relative to the incident beam) also affects the electron penetration into the
specimen, through diffraction effects, which gives rise to some “orientation
contrast” between the different grains in a polycrystalline specimen.
5.5 Other SEM Imaging Modes
Although SE and BSE images suffice for most SEM applications, some
specimens benefit from the ability to use other types of signal to modulate
the image intensity, as we now illustrate with several examples.
A specimen-current image is obtained by using a specimen holder that
is insulated from ground and connected to the input terminal of a sensitive
current amplifier. Conservation of charge implies that the specimen current
Is flowing to ground (through the amplifier) must be equal to the primarybeam
current Ip minus the rate of loss of electrons from secondary emission
and backscattering:
Is = Ip � IBSE � ISE = Ip (1 ����) (5.4)