Download X-ray Analysis in the SEM

Providing cutting edge microscopy services
and solutions for the life and physical sciences
X-ray Analysis in the SEM
X-ray Analysis in the SEM
Operation
X-ray analysis in a scanning electron microscope
(SEM) provides quantitative information about
the elemental composition of the sample. The xrays are produced when the electrons hit the
sample surface. As a consequence the
technique can be used to image the spatial
variations in composition within the sample.
Electron column
Liquid nitrogen
reservoir to
keep detector
cooled
Each of the SEMs in CfAM is equipped with Xray analysis systems. The details of each system
vary from instrument to instrument but each is an
Oxford Instruments INCA system. Each SEM is
equipped with an energy dispersive detector.
The environmental SEM also has a wavelength
dispersive system which runs in parallel with the
energy dispersive detector. The key difference is
the resolution, the wavelength system is
considerably higher (peak widths as low as 2eV)
than the energy dispersive detector (~ 150eV).
However, this higher resolution comes at the
price of much longer data accumulation times.
X-ray detector
The INCA systems have a powerful software
system which is integrated with the hardware
of the SEM. Since the X-rays are formed by
the electron beam interaction with the
sample surface, what ever area of the
sample being imaged is analyzed. This
allows the SEM to perform elemental
analysis in very selected areas. The X-rays
are emitted from a depth equivalent to how
deep the secondary electrons are formed.
Depending on the sample density and
accelerating voltage of the incident beam,
this is usually from 1/2 to 2 microns in depth.
Detectibility limits can be as low as 0.2% for
the higher atomic number elements.
It is also possible to map the elements found
in an SEM image by X-ray analysis. The
INCA system can provide overlay maps
mimics the SEM image, except the contrast
is formed by the strength of the elemental
composition. As well as maps, variations in
compositions along lines or in selected areas
can be displayed. The central image (right)
compares the spectrum obtained with an
energy dispersive detector with that from a
wavelength dispersive system..
The INCA energy dispersive system can detect
x-rays from all elements from atomic number 5
(boron) up to 92 (uranium) and analyze them
simultaneously.
How does it work?
The incident electron beam is composed of
highly energized electrons. If one of these
electrons collides with a sample atom electron,
it will knock the electron out of its shell. This
electron is called a secondary electron and is
weak in energy (nearly 100 volts). If these
secondary electrons are close enough to the
sample surface, they can be collected to form
an SEM image.
How does it work continued?
If the innermost shell (the K shell) electron
of an iron atom is replaced by an L shell
electron, a 6.4 keV K alpha X-ray is emitted
from the sample. Whereas if the electron is
replaced by an M shell electron or an L shell
electron, a 7.057 keV K beta or a 0.704
keV L alpha X-ray is emitted. Since lower
atomic number elements have fewer filled
shells, they have fewer X-ray peaks. Carbon
for example, has only one peak, a K alpha
X-ray at 0.282 keV.
When the incident beam passes through the
sample creating secondary electrons, it leaves
thousands of the sample atoms with holes in
the electron shells where the secondary
electrons used to be. If these "holes" are in
inner shells, the atoms are not in a stable
state. To stabilize the atoms, electrons from
outer shells will drop into the inner shells,
however, because the outer shells are at a
higher energy state, to do this the atom must
lose some energy. It does does this in the
form of X-rays.
Schematics from www.seallabs.com
Essentially, each element has characteristic
X-ray line(s) that allow a sample's elemental
composition to be identified by a nondestructive technique.
For more information contact 0118 378 6118 [email protected] or www.reading.ac.uk/cfam