Providing cutting edge microscopy services X-ray Analysis in the SEM and solutions for the life and physical sciences

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