19-10-2012, 05:10 PM
Electron Microscopy: The Basics
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Abstract
Since its invention, electron microscope has been a valuable tool in the
development of scientific theory and it contributed greatly to biology, medicine
and material sciences. This wide spread use of electron microscopes is based on
the fact that they permit the observation and characterization of materials on a
nanometer (nm) to micrometer (μm) scale. This paper presents the basic theory
for electron microscopy, focusing on the two basic types of Ems; SEM, TEM.
Introduction
Electron Microscopes are scientific instruments that use a beam of highly
energetic electrons to examine objects on a very fine scale. This examination can
yield information about the topography (surface features of an object),
morphology (shape and size of the particles making up the object), composition
(the elements and compounds that the object is composed of and the relative
amounts of them) and crystallographic information (how the atoms are arranged
in the object).
Electron Microscopes were developed due to the limitations of Light Microscopes
which are limited by the physics of light to 500x or 1000x magnification and a
resolution of 0.2 micrometers. In the early 1930's this theoretical limit had been
reached and there was a scientific desire to see the fine details of the interior
structures of organic cells (nucleus, mitochondria...etc.). This required 10,000x
plus magnification which was just not possible using Light Microscopes.
The Transmission Electron Microscope (TEM) was the first type of Electron
Microscope to be developed and is patterned exactly on the Light Transmission
Microscope except that a focused beam of electrons is used instead of light to
"see through" the specimen. It was developed by Max Knoll and Ernst Ruska in
Germany in 1931.
Electron Gun
The first and basic part of the microscopes is the source of electrons. It is usually
a V-shaped filament made of LaB6 or W (tungsten) that is wreathed with Wehnelt
electrode (Wehnelt Cap). Due to negative potential of the electrode, the electrons
are emitted from a small area of the filament (point source). A point source is
important because it emits monochromatic electrons (with similar energy). The
two usual types of electron guns are the conventional electron guns and the field
emission guns (FEG). Figure 1 illustrates the geometry of an electron gun.
In conventional electron guns, a positive electrical potential is applied to the
anode, and the filament (cathode) is heated until a stream of electrons is
produced. The electrons are accelerated by the positive potential down the
column, and because of the negative potential of cap, all electrons are repelled
toward the optic axis. A collection of electrons occurs in the space between the
filament tip and Cap, which is called a space charge. Those electrons at the
bottom of the space charge (nearest to the anode) can exit the gun area through
the small (<1 mm) hole in the Whenelt Cap and then move down the column to
be later used in imaging.
A field emission gun consists of a sharply pointed tungsten tip held at several
kilovolts negative potential relative to a nearby electrode, so that there is a very
high potential gradient at the surface of the tungsten tip. The result of this is that
the potential energy of an electron as a function of distance from the metal
surface has a sharp peak (from the work function), then drops off quickly (due to
electron charge traveling through an electric field). Because electrons are
quantum particles and have a probability distribution to their location, a certain
number of electrons that are nominally at the metal surface will find themselves
at some distance from the surface, such that they can reduce their energy by
moving further away from the surface. This transport-via-delocalization is called
'tunneling', and is the basis for the field emission effect. FEGs produce much
higher source brightness than in conventional guns (electron current > 1000
times), better monochromaticity, but requires a very good vacuum (~10-7 Pa).
Electron-specimen interactions
When an electron beam interacts with the atoms in a sample, individual incident
electrons undergo two types of scattering - elastic and inelastic (Figure 2). In the
former, only the trajectory changes and the kinetic energy and velocity remain
constant. In the case of inelastic scattering, some incident electrons will actually
collide with and displace electrons from their orbits (shells) around nuclei of
atoms comprising the sample. This interaction places the atom in an excited
(unstable) state. Specimen interaction is what makes Electron Microscopy
possible. The interactions (inelastic) noted on the top side of the diagram are
utilized when examining thick or bulk specimens (Scanning Electron Microscopy,
SEM) while on the bottom side are those examined in thin or foil specimens
(Transmission Electron Microscopy, TEM).
Reactions Exploited In SEM
Secondary Electrons
When a sample is bombarded with electrons, the strongest region of the electron
energy spectrum is due to secondary electrons. The secondary electron yield
depends on many factors, and is generally higher for high atomic number targets,
and at higher angles of incidence. Secondary electrons are produced when an
incident electron excites an electron in the sample and loses most of its energy in
the process. The excited electron moves towards the surface of the sample
undergoing elastic and inelastic collisions until it reaches the surface, where it can
escape if it still has sufficient energy.
Production of secondary electrons is very topography related. Due to their low
energy (5eV) 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 efficiencies. Collection of these electrons is aided by using a "collector"
in conjunction with the secondary electron detector.Figure 3 presents two
secondary electron images from SEM.