Introduction: The optical nature of a charged particle beam 11
Figure 1.5: Types of electron microscopes, schematic.
tral axis. Actually, all object points in the specimen S are imaged
simultaneously.
A scanning transmission electron microscope (STEM) is shown
in (b). An aperture A is illuminated from above. The transmitted
current is focused by a lens L onto a transparent specimen S, and is
scanned sequentially over the specimen in a raster pattern. Again,
some electrons are elastically scattered by the specimen, and some
remain unscattered. Some fraction of the scattered current is measured on the annular dark field detector D, and the resulting signal
sent to a display which is scanned synchronously with the beam of
the microscope. Areas of the specimen which scatter strongly thus
appear bright on the display, and areas which scatter weakly appear dark. This is not an image in the sense of Figure 1.4, because
the intensity of each pixel is determined sequentially. However,
it does produce an intensity map of the specimen which is just
as useful as an optically formed image. The current which passes
through the annular dark field detector D is measured on a bright
field detector B. This signal can alternatively be displayed, with
strongly scattering regions appearing dark, and weakly scattering regions appearing bright. The ultimate resolution of the TEM
Figure 1.5: Types of electron microscopes, schematic.
tral axis. Actually, all object points in the specimen S are imaged
simultaneously.
A scanning transmission electron microscope (STEM) is shown
in (b). An aperture A is illuminated from above. The transmitted
current is focused by a lens L onto a transparent specimen S, and is
scanned sequentially over the specimen in a raster pattern. Again,
some electrons are elastically scattered by the specimen, and some
remain unscattered. Some fraction of the scattered current is measured on the annular dark field detector D, and the resulting signal
sent to a display which is scanned synchronously with the beam of
the microscope. Areas of the specimen which scatter strongly thus
appear bright on the display, and areas which scatter weakly appear dark. This is not an image in the sense of Figure 1.4, because
the intensity of each pixel is determined sequentially. However,
it does produce an intensity map of the specimen which is just
as useful as an optically formed image. The current which passes
through the annular dark field detector D is measured on a bright
field detector B. This signal can alternatively be displayed, with
strongly scattering regions appearing dark, and weakly scattering regions appearing bright. The ultimate resolution of the TEM