12 Introduction: The optical nature of a charged particle beam
and STEM is the same, but the contrast differs in the two cases,
depending on the accelerating voltage and numerical aperture chosen. In both cases, the numerical aperture is equal to the beam
semi-angle subtended by the aperture A at the specimen S.
A scanning electron microscope (SEM) is shown in (c). An aperture A is illuminated from above. The transmitted current is focused by a lens L onto an opaque bulk specimen S, and is scanned
sequentially over the specimen in a raster pattern. Low energy secondary electrons are excited by the beam in the interaction volume
depicted by the darker area of the specimen S. These secondary
electrons are accelerated to a collector C, and the current thus
detected is used to form a signal which is sent to the display.
The ultimate resolution of the SEM is roughly equal to the size
of the interaction volume, which is typically on the order of a few
nanometers. One nanometer is one-billionth of a meter, and will
be abbreviated 1 nm throughout the text. This is a very good resolution, compared with a typical light microscope, for which the
resolution is typically a few hundred nm. In addition, an SEM
has superior depth of focus. This means that one need not focus
precisely, in order to obtain a sharp image, allowing a seemingly
three-dimensional depiction of a bulk sample. This is shown in Figure 1.6, courtesy of L.T. Varghese and L. Fan, Purdue University
[90].
The schematic depiction in Figure 1.5(c) applies equally well to
a scanning ion microscope (SIM), where the beam consists of ions
rather than electrons. A bright source of helium ions can be formed
using a sharp tip in a low pressure helium gas. The tip is elevated
to a potential of a few tens of kilovolts relative to the surrounding
chamber, causing a high electric field around the tip. Helium gas
atoms are polarized in the field gradient, and attracted to the tip,
where they dissociate to form positive helium ions. The ions are
accelerated away from the tip by the electric field to form the ion
beam.
and STEM is the same, but the contrast differs in the two cases,
depending on the accelerating voltage and numerical aperture chosen. In both cases, the numerical aperture is equal to the beam
semi-angle subtended by the aperture A at the specimen S.
A scanning electron microscope (SEM) is shown in (c). An aperture A is illuminated from above. The transmitted current is focused by a lens L onto an opaque bulk specimen S, and is scanned
sequentially over the specimen in a raster pattern. Low energy secondary electrons are excited by the beam in the interaction volume
depicted by the darker area of the specimen S. These secondary
electrons are accelerated to a collector C, and the current thus
detected is used to form a signal which is sent to the display.
The ultimate resolution of the SEM is roughly equal to the size
of the interaction volume, which is typically on the order of a few
nanometers. One nanometer is one-billionth of a meter, and will
be abbreviated 1 nm throughout the text. This is a very good resolution, compared with a typical light microscope, for which the
resolution is typically a few hundred nm. In addition, an SEM
has superior depth of focus. This means that one need not focus
precisely, in order to obtain a sharp image, allowing a seemingly
three-dimensional depiction of a bulk sample. This is shown in Figure 1.6, courtesy of L.T. Varghese and L. Fan, Purdue University
[90].
The schematic depiction in Figure 1.5(c) applies equally well to
a scanning ion microscope (SIM), where the beam consists of ions
rather than electrons. A bright source of helium ions can be formed
using a sharp tip in a low pressure helium gas. The tip is elevated
to a potential of a few tens of kilovolts relative to the surrounding
chamber, causing a high electric field around the tip. Helium gas
atoms are polarized in the field gradient, and attracted to the tip,
where they dissociate to form positive helium ions. The ions are
accelerated away from the tip by the electric field to form the ion
beam.