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.


Summary of Instrument Capabilities

*Secondary electron imaging of topographic features with magnification up to 100,000X and spatial resolution better than 50 Å

*Backscattered electron imaging of chemical phase difference

*Qualitative and quantitative elemental analysis with EDS

*X-ray imaging: elemental line scans and maps

Comparison With Related Techniques

*AES and ESCA are more sensitive to light elements

*Analysis depth for AES and ESCA is three orders of magnitude less than EDS

*EDS is only for elemental analysis; ESCA gives chemical state information


Samples/Sample Preparation

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.

Limitations

*Limited detection of elements below Na in the periodic table

*No detection of elements below C in the periodic table

*X-ray detection limit ~ 0.1% depending on the element

*Samples must be compatible with vacuum (no fluids)