2.2. Reactions exploited in SEM
2.2.1 Secondary electrons
When a sample is bombarded with electrons, the strongest region of the electron energy spectrum is as a result of secondary electrons. The secondary electron yield depends on many factors, and is by and large higher for high atomic number targets, and at higher angles of incidence. Secondary electrons are produced when an incident electron excites an electron in the sample and loses most of its energy in the process. The excited electron moves towards the surface of the sample experiencing elastic and inelastic collisions until it reaches the surface, where it can escape if it still has sufficient energy.
The production of secondary electrons is very topography related. Due to their low energy (5eV) only secondary electrons that are very near the surface (<10 nm) can exit the sample and be examined. Any changes in topography in the sample that are larger than this sampling depth will change the yield of secondary electrons due to collection efficiencies. Collection of these electrons is assisted by using a "collector" in conjunction with the secondary electron detector.
2.2.2. Backscattered electrons
Backscattered electrons constitute high-energy electrons originating in the electron beam that are reflected or backscattered out of the specimen interaction volume. The production of backscattered electrons differs directly with the specimen's atomic number. This inconsistent production rates causes elements of higher atomic number to appear brighter than lower atomic number elements. This interaction is exploited to differentiate parts of the specimen that have different average atomic number.
2.2.3. Relaxation of excited atoms
Inelastic scattering puts the atom in an excited (unstable) state. The atom “wants” to return to a ground or unexcited state. Therefore, at a later time the atoms will relax giving off the excess energy. Cathodoluminescence, X - Rays and Auger electrons are three types of relaxation. The relaxation energy is the fingerprint of each element.
When the sample is bombarded by the electron beam of the SEM, electrons are ejected from the atoms on the specimen's surface. A resulting electron vacancy is filled by an electron from a higher shell, and an X-ray is emitted to balance the energy difference between the two electrons. The X-ray detector measures the number of emitted x-rays versus their energy. The energy of the x-ray is characteristic of the element from which the x-ray was emitted.
Cathodoluminescence is the emission of photons of characteristic wavelengths from a material that is under high-energy electron bombardment. The electron beam is typically produced in an electron microprobe or scanning electron microscope. Auger electrons are electrons expelled by radiation - less excitation of a target atom by the incident electron beam. Auger electrons are a feature of the fine structure of the atom and have energies between 280 eV (carbon) and 2.1 keV (sulphur).
2.3. Advantages and disadvantages
The electrons in SEM penetrate into the sample within a small depth, and are, therefore, suitable for surface topology of every kind of samples (metals, ceramics, glass, dust, hair, teeth, bones, minerals, wood, paper, plastics, polymers, etc). It can also be employed for chemical composition of the sample's surface since the brightness of the image formed by backscattered electrons is increasing with the atomic number of the elements. This suggests that regions of the sample consisting of light elements (low atomic numbers) appear dark on the screen and heavy elements appear bright. Backscattered are used to form diffraction images that describe the crystallographic structure of the sample.
Consequently, SEM is only employed for surface images, and both resolution and crystallographic information are limited (because they are only referred to the surface). Other limitations are firstly the samples must be conductive, so non-conductive materials are carbon-coated and secondly, the materials with atomic number smaller than the carbon are not detected with SEM.