When using an electron microscope, image formation requires the interaction of the electron beam with the sample. The mechanisms behind this interaction will determine the contrast that is seen, and also determine how it should be interpreted. There are three main signals created in this electron beam interaction which are commonly used to form an image.
The Secondary Electron (SE) image has a strong topographic component. The reason for this is that, the secondary electrons have low energy and consequently come from the near surface of the sample. The SE originate are generated by the inelastic interaction of the primary electron beam with the outer shell electrons and have energies less than 50eV. Due to the low energy of the secondary electrons, they only have a small escape depth of approximately 10 to 100 nm’s, depending on whether the sample is an insulator or a conductor. This small escape depth causes the SE signal strength to have a strong dependence on the angle of incidence of the primary beam to the sample; and as such, the SE signal primarily carries information about the local topography. The SE yield is also approximately proportional to the number of outer shell electrons in the atom.
The Backscattered Electron (BE) signal, exhibits more information relating to composition and phase and has less sample topography dependence than the SE signal. The BE yield is approximately proportional to the product of the sample density and the square root of the average atomic number of the sample. This signal is commonly called "atomic number contrast or Z contrast". The reason for the decreased topographic dependence is that the BE have higher energy and come from deeper within the sample therefore they are less affected by surface features. The most commonly used aspect of the BE signal, is the atomic number dependence, this arises because the backscattered electrons originate from the elastic interaction of the primary electron beam and the nucleus of the atom, that is in effect they bounce of the more massive nucleus of the atom; and the bigger the nucleus the more chance it has of being reflected. Similarly, as the density increases there will also be a direct effect on BE yield. The escape depth of a BE is approximately 0.5 to 10 micrometres depending on the energy of the beam and the stopping power of the samples. Using atomic number contrast it is possible under certain circumstances to resolve a difference of 0.1 in average atomic numbers.
When the primary electron beam is inelastic scattered by the nucleus, it also looses some energy to the electrons within the atoms that they are hitting. This energy loss of teh beam and energy absorption of the atoms, subsequently causes the atom to emit an x-ray whose energy carries information about the atom. These elemental x-ray signal’s are created when the incident electrons causes the ejection of a inner shell electron from the atoms in the sample, which is followed by the subsequent relaxation of the outer shell electrons, which in turn generates characteristic x-ray’s. EDX analysis is possible within the volume over which the electron beam interacts (approximately four cubic micrometers). With a typical EDX system it is possible to detect an x-ray signal for all elements of atomic number greater than 6, with a detection limit in the order of 0.1 to 5 wt% depending on the energy of the characteristic x-ray line. Where the elemental; composition of the sample is required characteristic x-ray signals are collected at selected positions for qualitative or quantitative Energy Dispersive X-ray (EDX) analysis.
Other signals that are used are; Absorbed current, which is similar to a negative image of the BE signal, Cathodoluminescence which has a similar production mechanism to x-rays but involving transitions between conduction and valence bands which result in light photons being emitted.
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