An electron microscope is a type of microscope that produces an electronically-
magnified image of a specimen for detailed observation. The electron microscope
(EM) uses a particle beam of electrons to illuminate the specimen and create a
magnified image of it. The microscope has a greater resolving power than a light-
powered optical microscope, because it uses electrons that have wavelengths about
100,000 times shorter than visible light (photons), and can achieve magnifications of
up to 1,000,000x, whereas light microscopes are limited to 2000x magnification.
The electron microscope uses electrostatic and electromagnetic "lenses" to control the
electron beam and focus it to form an image. These lenses are analogous to, but
different from the glass lenses of an optical microscope that form a magnified image
by focusing light on or through the specimen.
Electron microscopes are used to observe a wide range of biological and inorganic
specimens including microorganisms, cells, large molecules, biopsy samples, metals,
and crystals. Industrially, the electron microscope is primarily used for quality control
and failure analysis in semiconductor device fabrication.
Transmission electron microscope (TEM)
Main article: Transmission electron microscope
The original form of electron microscope, the transmission electron microscope
(TEM) uses a high voltage electron beam to create an image. The electrons are
emitted by an electron gun, commonly fitted with a tungsten filament cathode as the
electron source. The electron beam is accelerated by an anode typically at +100 keV
(40 to 400 keV) with respect to the cathode, focused by electrostatic and
electromagnetic lenses, and transmitted through the specimen that is in part
transparent to electrons and in part scatters them out of the beam. When it emerges
from the specimen, the electron beam carries information about the structure of the
specimen that is magnified by the objective lens system of the microscope. The spatial
variation in this information (the "image") is viewed by projecting the magnified
electron image onto a fluorescent viewing screen coated with a phosphor or
scintillator material such as zinc sulfide. The image can be photographically recorded
by exposing a photographic film or plate directly to the electron beam, or a high-
resolution phosphor may be coupled by means of a lens optical system or a fibre optic
light-guide to the sensor of a CCD (charge-coupled device) camera. The image
detected by the CCD may be displayed on a monitor or computer.
Resolution of the TEM is limited primarily by spherical aberration, but a new
generation of aberration correctors have been able to partially overcome spherical
aberration to increase resolution. Hardware correction of spherical aberration for the
High Resolution TEM (HRTEM) has allowed the production of images with
resolution below 0.5 Ångström (50 picometres) at magnifications above 50 million
times. The ability to determine the positions of atoms within materials has made the
HRTEM an important tool for nano-technologies research and development.
Scanning electron microscope (SEM)
Main article: Scanning electron microscope (SEM)
Unlike the TEM, where electrons of the high voltage beam carry the image of the
specimen, the electron beam of the Scanning Electron Microscope (SEM) does not at
any time carry a complete image of the specimen. The SEM produces images by
probing the specimen with a focused electron beam that is scanned across a
rectangular area of the specimen (raster scanning). At each point on the specimen the
incident electron beam loses some energy, and that lost energy is converted into other
forms, such as heat, emission of low-energy secondary electrons, light emission
(cathodoluminescence) or x-ray emission. The display of the SEM maps the varying
intensity of any of these signals into the image in a position corresponding to the
position of the beam on the specimen when the signal was generated. In the SEM
image of an ant shown at right, the image was constructed from signals produced by a
secondary electron detector, the normal or conventional imaging mode in most SEMs.
Generally, the image resolution of an SEM is about an order of magnitude poorer than
that of a TEM. However, because the SEM image relies on surface processes rather
than transmission, it is able to image bulk samples up to many centimetres in size and
(depending on instrument design and settings) has a great depth of field, and so can
produce images that are good representations of the three-dimensional shape of the
sample.
