SEM/EDS : Scanning Electron Microscopy
with X-ray microanalysis
Technique Description
In scanning electron microscopy, (SEM) an electron beam is scanned across
a sample's surface. When the electrons strike the sample, a variety of signals
are generated, and it is the detection of specific signals which produces
an image or a sample's elemental composition. The three signals which provide
the greatest amount of information in SEM are the secondary electrons, backscattered
electrons, and X-rays.
Secondary electrons are emitted from the atoms occupying the top surface
and produce a readily interpretable image of the surface. The contrast in
the image is determined by the sample morphology. A high resolution image
can be obtained because of the small diameter of the primary electron beam.
Backscattered electrons are primary beam electrons which are 'reflected'
from atoms in the solid. The contrast in the image produced is determined
by the atomic number of the elements in the sample. The image will therefore
show the distribution of different chemical phases in the sample. Because
these electrons are emitted from a depth in the sample, the resolution in
the image is not as good as for secondary electrons.
Interaction of the primary beam with atoms in the sample causes shell transitions
which result in the emission of an X-ray. The emitted X-ray has an energy
characteristic of the parent element. Detection and measurement of the energy
permits elemental analysis (Energy Dispersive X-ray Spectroscopy or EDS).
EDS can provide rapid qualitative, or with adequate standards, quantitative
analysis of elemental composition with a sampling depth of 1-2 microns.
X-rays may also be used to form maps or line profiles, showing the elemental
distribution in a sample surface.
Secondary electron imaging of topographic features with
magnification up to 100,000X and spatial resolution better than 50 ÅBackscattered electron imaging of chemical phase differenceQualitative and quantitative elemental analysis with EDS
X-ray imaging: elemental line scans and maps
AES and ESCA are more sensitive to
light elementsAnalysis depth for AES and ESCA is
three orders of magnitude less than EDSEDS is only for elemental analysis;
ESCA gives chemical state information Suitable samples include most solids which are stable under vacuum (metals, ceramics, polymers, minerals). Sample must be less than 2 cm in diameter. Sample preparation: non-conducting samples are coated with a thin layer of carbon or gold. Metallographic embedding, polishing, and sectioning is available for samples requiring special preparation.
Samples are usually mounted and coated and introduced into the vacuum chamber. Digital images are acquired with an electron microscope image scanner. Images may be printed on a laser printer or transferred to disk or an e-mail account. Images and X-ray spectra may be accumulated within minutes.
Limited detection of elements below Na in the periodic
tableNo detection of elements below C in the periodic tableX-ray detection limit ~ 0.1% depending on the elementSamples must be compatible with vacuum (no fluids)Back to SCIC home page.