SEM/EDX History |
The earliest
concept involving the theory of scanning electron microscopy
was first introduced in Germany (1935) by M. Knoll. The
standard concept of the modern SEM was constructed by von
Ardenne in 1938 who added scan coils to a transmission
electron microscope. The SEM design was modified
considerably by Zworykin et al. in 1942 while working for
RCA Laboratories in the United States. The design was again
re engineered by C. W. Oatley in 1948 while a professor at
Cambridge University. Since then there have been many other
significant contributions that has greatly enhanced and
optimized the modern day scanning electron microscope.

The principal of
a scanning electron microscope or SEM functions by scanning
a finely focused beam of electrons onto a sample. The
impinging electrons interact with the samples molecular
composition. The energy of the impinging electrons onto a
sample is directly in proportion to the type of electron
interaction that is generated from the sample. A series of
measurable electron energies can be produced which are
analyzed by a sophisticated microprocessor that creates a
pseudo three-dimensional image or spectrum of the unique
elements that exist in the sample analyzed. It is this
series of electrons which are deflected by collisions with
the samples electrons. |
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Block diagram of a typical SEM
(Redrawn from J. W. S. HEARLE, J. T. SPARROW, P. M. CROSS,
1972)
Before exploring
the types of electrons produced by a typical SEM, a basic
understanding of the theory surrounding classified elements
of the periodic table needs to be mentioned.
Throughout history many physicists, mathematicians, and
chemist studied many of the earth's elements. To name a few:
Georg Bauer a
sixteenth century metallurgist.
-
Robert
Boyle (1627-1691) who carefully measured the
relationship between the pressure and volume of
gases.
-
George
Stahl (1660-1734) who postulated that a substance
burning in a closed container eventually stopped
burning, a term he called "phlogiston".
-
Joseph
Priestly (1733-1804 was the first to discover oxygen
or as he called it "dephlogisticated air".
-
Antoine
Lavoisier (1743-1794) who discovered the law of
conservation of mass.
-
Joseph
Proust (1754-1826) who showed that a given compound
always contains exactly the same proportions of
elements by weight, this is also known as the Law of
Definite Proportion.
-
It was
the work of Proust that inspired John Dalton
(1766-1844) to develop his hypothesis into the "Law
of multiple proportions": When two elements form a
series of compounds, the ratios of the masses of the
second element that combine with 1 gram of the first
element can always be reduced to small whole
numbers.
There needed to
be a system for relating this information in an easily
understood manner. A Russian chemist named Dmitri Mendelèev
(1834-1907) arranged the 63 known elements into a table
based on their atomic mass. This arrangement of the elements
eventually transformed into the modern periodic table of the
elements used throughout the world.
Through the hard work of these men and a host of others, an
enormous amount of information was compiled and tested to
establish the basic underlying principles used today in the
development of the modern scanning electron microscope. |
Atomic Weight |
The mass of a fixed number of
atoms of an element. The standard scientific unit for
dealing with atoms in macroscopic quantities is the mole
(mol), which is defined arbitrarily as the amount of a
substance with as many atoms or other units as there are in
12 grams of the carbon isotope 12C. The number of atoms in a
mole is called Avogadro's number, the value of which is
approximately 6 x 1023. The atomic mass of an element
expressed in daltons, or more commonly atomic mass units (amu's),
is the number of grams in one mole of the element. The amu
is convenient because atomic masses are nearly equal to
atomic mass numbers and therefore are close to integer
values. (Encyclopedia Britannica). |
The Dual Nature of Electrons |

Before exploring further into
the theory and functionality of a SEM/EDX microscope, it is
worth mentioning the duality of electrons and x-rays. Early
experiments with electrons and its physical characteristics
have led scientist to modify their basic understandings of
physics. Before the advancement of modern day technologies,
many scientists had to explain physical behaviors based on
chemical interactions that were seen with the naked eye.
When the principles of electro magnetic radiation was first
introduced, the concept was easiest explained as waves that
travel in specific lengths and frequencies. Later
experiments showed that light and x-rays are actual
particles that can be detected. In short, x-rays are
considered as both waves and particles. More specifically,
they are small packets of electromagnetic waves called
quanta or particles called photons. Each element is
comprised of a nucleus and a series of orbiting electrons
(Figure 1). The nucleus or Z value is the combined weight of
the positively charged particles (protons) and uncharged
particles (neutrons). The negatively charged electrons, each
only 0.05% as heavy as a proton, orbit the nucleus. The
electrons orbit the nucleus of an atom in a series of
levels. Each level or shell is designated K, L, M etc.
depending on its relationship to the nucleus of the atom.
The K shell is closer to the nucleus and each additional
shell is farther away. The number of electrons that orbit an
atom depends on the molecular weight of that atom. For
example, helium (Z=2) has only two electrons in the K shell;
Gold (Z=79) has two K electrons, 8 L electrons, 18 M
electrons, 32 N electrons, 18 O electrons and 1 P electron.
Each electron has several
quantum numbers that uniquely define it and specify the
shell it may occupy. The quantum numbers are:
Number |
Name |
Defines |
Notes |
n |
principal quantum number |
electron shell (1=K,
2=L, 3= M...) |
Principle binding
energy. |
l |
azimuthal quantum number |
electron cloud shape
(0=sphere, 1=dumbbell...) |
Orbital angular momentum. Chemists follow the
conventions of optical spectroscopy using letters
rather than numbers to indicate the value of l:
sharp (l = 0), principal,
(l = 1), diffuse (l =
2), and fundamental (l = 3). |
m |
magnetic quantum number |
electron shell
orientation when in a magnetic field |
In
the absence of an external magnetic field, the
magnetic number has no meaning and will be ignored
in the subsequent discussion. |
s |
spin quantum number |
electron spin direction |
Clockwise or counterclockwise. |
j |
inner precession |
total angular momentum |
As
noted under permitted values, for s orbitals (with
l = 0), j can only be +½ (a
vector sum is always positive). The value of
j is important for determining what
transitions are possible between |
|
The electrical charge for each element is defined as: |
Electric charge:
The normal atom is electrically neutral, meaning that it
carries a net electric charge of zero. Some atoms, however,
have lost or gained electrons in chemical reactions or in
collisions with other particles. Atoms with a net charge,
from either the gain or loss of electrons, are called ions.
If a neutral atom loses an electron, it becomes a positive
ion; if it gains an electron, it becomes a negative ion.
The charge on any particle is a whole multiple of the
electron's charge, either positive or negative. The quarks
are an exception to this rule. They have charges of +2/3e
and -1/3e. However, they exist only in groups, and each
group as a whole has an integral multiple of the electron's
charge. The amount of charge in this fundamental unit is
equal to 1.6 x 10-19 coulomb. This means that in a current
of one ampere-roughly what a 100-watt light bulb uses in the
ordinary 110-volt household circuit-about 6 x 1018 electrons
pass through the wire every second. (Encyclopedia
Britannica)
Depending on the type of information an analyst is
interested in depends on the type of electrons one studies.
Every electron that is generated from the primary electron
beam produced by the SEM when impinging a given sample
produces an electron of specific energy that can be
measured. The types of electrons generated for any given
sample needs to first be explored. The electrons produced
from a samples molecular composition are classified as
either elastic and inelastic electrons.
Inelastic electrons are low energy electrons deflected from
the sample. Most are absorbed by the specimen, but those
that escape are near the surface. These electrons are
referred as secondary electrons, that is emergent electron
energies of less than 50eV; 90% of secondary electrons have
energies less than 10 eV, most are from 2 to 5 eV. Secondary
electrons give information of the surface topography and a
black and white, three-dimensional image of the sample. This
is the most common image most people associate with the SEM.
Elastic electrons are any electrons that interact with the
primary electron beam to produce a specific energy from the
collision and retain most of its energy. These electrons are
categorized as:
- Backscattered electrons-
yielding topological, compositional and
crystallographical surface information.
- Absorbed current- which
enables the study of the internal structure of
semi-conductors or (EBIC).
- Cathodluminescence-
shows the distribution and energy levels in phosphors.
- Auger electrons-
contains elemental and chemical information of the
surface layers.
- Characteristic X-ray
Radiation- yields microanalysis and distribution of
elements of a given sample.
A typical SEM has the ability
to analyze a particular sample utilizing any of the above
mentioned methods. Unfortunately, each type of analysis
considered is an additional peripheral accessory for the
SEM. The most common accessory equipped with a SEM is the
energy dispersive x-ray detector or EDX (sometimes reffered
to as EDS). This type of detector allows a user to analyze a
samples molecular composition.
The first known detection of x-rays was discovered
accidentally by the German physicist Wilhelm Conrad
Roeentgen in 1895 while studying cathode rays in a
high-voltage, gaseous discharge tube (It was known that when
the cathode of an electric circuit was heated in a vacuum
with a large potential difference applied between that
cathode and the anode, a beam appeared to travel between the
two electrodes. Originally this was thought to be an
electromagnetic wave, and so they were called cathode rays,
J.J. Thompson (1856-1940) created the cathode ray tube-CRT
the basis for modern-day computer monitors and televisions
). Since the exact reason for the phenomena was not know at
the time, Wilhelm Conrad Roeentgen coined the term
"x-radiation". The electromagnetic wavelength of x-rays are
about 0.01 to 100 angstroms (An Angstrom (abbreviated Å) is
one ten-billionth (1/10,000,000,000) of a meter. A hydrogen
atom measures about 1 Å across). |

In an SEM,
x-rays are produced by accelerating the primary electron
beam with enough current to pass through the sample thereby
interacting with the elements inner core electrons. When
enough high-velocity electron bombardment contacts the inner
most electron shell of an atom, it forces the orbiting
electron to be kicked out. Subsequently, this results in the
neighboring outer electrons to move into the vacant inner
electron shell. The release of energy from the escaping
electrons from the inner most orbiting shell or core
electrons are analyzed and measured based on their
classification type. The two types of escaping electrons are
classified as either being of high energy or low energy
electrons.
The first type of escaping electrons to be discussed are the
low energy electrons known as the Auger effect first
observed in 1925 by the French Physicist Pierre-Victor
Auger. This phenomenon occurs when an electron is released
from one of the inner orbiting shells thereby creating two
electron vacancies of the residual atom and is repeated as
the new vacancies are filled or x-rays are emitted. It
should be noted that the detection of Auger electrons or
specific Auger yield for a particular element decreases with
atomic number. For example, the emission of x-rays and Auger
electrons of Zinc (atomic number 30) is about equal. This
type of analysis was developed in the late1960's and called
Auger Spectroscopy or AES. The technique is useful in
studying the qualitative and quantitative surface layer
composition of compounds, elements or subatomic particles
known as muons.

Characteristic
x-rays are escaping high energy electrons produced from the
bombardment of energetic electrons on the orbiting inner
most electron shell of an atom, thereby leaving a vacancy.
An electron from the outer orbiting shell then jumps into
the empty electron vacant shell. The emitting energy, called
a photon or minute energy packet of electromagnetic
radiation is specific for each element in the periodic
table. The deceleration of the beam electrons when hitting a
sample or passing through the field of atomic nuclei is
measured and is known as bremsstralung or braking radiation.
The energy loss is continuous and dependent on the incident
electron voltage and angle of incidence. The exact
calculations for the specific values of energies are
calculated using the Hartree Theory and Bethe process. The
data is extrapolated from mathematical formulas via micro
processing into a readable spectrum for analysis.
Bremsstrahlung is the main source of X-rays produced by
diagnostic X-ray tubes.

The above figure
is a an illustration of the classical models showing the
production of bremsstrahlung, characteristic X-rays, and
Auger electrons. (left) Electrons are scattered elastically
and inelastically by the positively charged nucleus. The
inelastically scattered electron loses energy, which appears
as bremsstrahlung. Elastically scattered electrons (which
include backscattered electrons) are generally scattered
through larger angles than are inelastically scattered
electrons. (right) An incident electron ionizes the sample
atom by ejecting an electron from an inner-shell (the K
shell, in this case). De-excitation, in turn, produces
characteristic X-radiation (above) or an Auger electron
(below). Secondary electrons are ejected with low energy
from outer loosely bound electron shells, a process not
shown. (James H. Wittke)
X-Ray Detectors
The detection and measurement of x-rays by scanning electron
microscopy is preformed by a solid-state detector, also
called a SEMICONDUCTOR RADIATION DETECTOR, radiation
detector in which a semiconductor material such as a silicon
or germanium crystal constitutes the detecting medium. One
such device consists of a p-n junction across which a pulse
of current develops when a particle of ionizing radiation
traverses it. In a different device, the absorption of
ionizing radiation generates pairs of charge carriers
(electrons and electron-deficient sites called holes) in a
block of semiconducting material; the migration of these
carriers under the influence of a voltage maintained between
the opposite faces of the block constitutes a pulse of
current. The sensitivity of these detectors is increased by
operating them at low temperatures (commonly that of liquid
nitrogen, -164º C [-263º F]) to suppress the random
formation of charge carriers by thermal vibration. Such
pulses are amplified, recorded, and analyzed to determine
the energy, number, or identity of the incident charged
particles. (Encyclopedia Britannica)
WDS vs. EDS Detection
The energy dispersive (EDS) and wavelength dispersive (WDS)
systems both have benefits and disadvantages. The main
differences between the systems are in detector efficiency
and resolution. Schematic diagrams showing the components of
the WDS and EDS systems are shown in Figure A and B.

Figure A. Schematic representation of a
wavelength-dispersive spectrometer (Goldstein et al. 1981).

Figure B. Schematic
representation of an energy-dispersive spectrometer
(Goldstein et al. 1981). |
The overall efficiency of
collecting the X-rays produced is poor for both WDS (<0.2%)
and EDS (><2%) with the EDS better because the detector is
closer to the sample. Most X-rays produced from the sample
do not make it to the detection systems. In a WDS system
about 30% of the X-rays entering the detector are actually
counted; whereas, in an EDS system about 100% of the
incident X-rays are counted. As a consequence, the minimum
useful probe spot size is larger for WDS (about 2 µm). The
greater efficiency of EDS permits the use of beam diameters
as small as 50 Å (0.05 µm). ><0.2%) and EDS (<2%). The EDS
system is better because the detector is located closer to
the sample. However, the resolution of the WDS system is far
superior (Figure C). |

Figure C. Comparison
of the resolution of the proportional counter, SI(LI)
semiconductor detector, and several analyzing crystals. Note
that the axes are logarithmic (after Maurice et al. 1979).
Most X-rays produced from the sample do not make it to the
detection systems. In a WDS system, about 30% of the X-rays
entering the detector are actually counted; in an EDS
system, almost 100% are counted. As a consequence, the
minimum useful probe spot size for WDS is about 2 mm; the
greater efficiency of an EDS permits the use of beam
diameters as small as 50 Å (0.05 mm).
The instantaneous spectral acceptance range differs greatly
between the two systems. WDS spectrometers can only examine
the portion of the spectrum for which they are positioned,
whereas in EDS entire useful energy range can be examined
simultaneously. Finally, spectral artifacts produced by
stray X-rays and electrons are more rare with WDS. The EDS
detector, located close to the sample, receives many stray
X-rays and electrons and suffers peak distortion, peak
broadening, escape peaks, absorption, and internal Si
fluorescence. (James Wittke)
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|
Pronounced: i-am-uh |
IAMA is an acronym for the
International Association for Microanalysis and was formed
October of 1999. The organization is non-profit and was
created to provide forensic scientist with an informative
resource relating to the detection of primer gunshot residue
(P-GSR) by SEM/EDX. Currently, IAMA is internationally
recognized with over 150 members. Although, IAMA was
originally created to address a the specific topic of P-GSR
detection and analysis, eventually it is anticipated to grow
into a journal comprising all aspects of forensic scanning
electron microscopy.
The International Association for Microanalysis (IAMA) is a
not-for-profit organization. IAMA is funded by myself, the
generous donations of others and membership dues. If you or
your organization would like to contribute to IAMA, please
go to the Contact section of this web site and fill out the
information provided.
Other inquiries on retaining the services of forensic
experts in criminal or civil investigations can also be made
through the Contact section of this web site or visiting
www.iamaweb.com.
Interested individuals will be immediately contacted and
provided with the necessary information requested.
|
Primer Gunshot
Residue (P-GSR) Particle Containing Lead and Barium
Identified by Scanning Electron Microscopy (SEM)





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