Download Energy Dispersive X

Amy Bryant
TU Nanomaterials Wokshop
Dr. Winton Cornell
Dr. Saibal Mitra
July 19, 2004
Energy Dispersive X-Ray Analysis (EDX)
Energy Dispersive X-ray Analysis (EDX) is one of the most useful
accessory features on a Scanning Electron Microscope (SEM). It is useful
because it allows simultaneous, non-destructive elemental analysis that is both
qualitative and quantitative. The EDX allows the SEM to perform elemental
analysis on the portion of the sample being imaged in selected areas as small as ½
a micron in size.
When the incident electron beam of the SEM hits atoms of the sample,
secondary and backscattered electrons are emitted from the sample’s surface.
When the incident beam penetrates the sample, electrons from atoms within the
sample are dislodged, creating secondary electrons. As the incident beam passes
through the sample, 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
the inner shells, the atoms stabilize by dropping electrons from the outer shells
Source: http://www.seallabs.com/howedx5.html
into the inner shells, releasing energy in the form of X-rays. The X-rays then hit
the lithium drifted silicon detector generating a photoelectron, which travels
through the silicon generating electron-hole pairs. An electric field within the
detector attracts the electrons and the holes to opposite ends. The energy of the
incoming X-ray determines the number of electron-holes created, which in turn
determines the size of the current pulse. An X-ray spectrum can therefore be
acquired qualitatively and quantitatively analyzing the elemental composition of
the sample. The elements in the sample are identifiable to the detector because
each X-ray spectrum is characteristically unique in energy and wavelength.
Moreover, the detector can identify the change within the atom because the X-rays
from the emitted atom are characteristic in energy and wavelength based on which
shells lost electrons and which shells replaced them.
The X-rays emitted from each element are characteristic in energy and
wavelength and produce a unique X-ray spectrum. Lower atomic number
elements have fewer X-ray peaks while higher atomic numbered elements have
more X-ray peaks. The area beneath the peak can be calculated to quantitatively
analyze each identified element. By analyzing multiple small sections of the
sample, it is possible to map the composition of the entire sample, creating an
image that mimics the SEM image.
Source: http://www.seallabs.com/howedx5.html