Specimen interaction is what makes Electron Microscopy possible. The energetic electrons in the microscope strike the sample and various reactions can occur as shown below. The reactions noted on the top side of the diagram are utilized when examining thick or bulk specimens (SEM) while the reactions on the bottom side are those examined in thin or foil specimens (TEM). A diagram showing the generation depths of the interactions is also available
The formation and uses of these reactions:
Bulk Specimen Interactions
Caused by an incident electron colliding with an atom in the specimen which is nearly normal to the incident's path. The incident electron is then scattered "backward" 180 degrees.Utilization
The production of backscattered electrons varies directly with the specimen's atomic number. This differing production rates causes higher atomic number elements to appear brighter than lower atomic number elements. This interaction is utilized to differentiate parts of the specimen that have different average atomic number. (See Interaction Volume) An example is shown in the SEM output section, specifically the mechanically alloyed specimen micrograph.
Caused by an incident electron passing "near" an atom in the specimen, near enough to impart some of its energy to a lower energy electron (usually in the K-shell). This causes a slight energy loss and path change in the incident electron and the ionization of the electron in the specimen atom. This ionized electron then leaves the atom with a very small kinetic energy (5eV) and is then termed a "secondary electron". Each incident electron can produce several secondary electrons.Utilization
Production of secondary electrons is very topography related. Due to their low energy, 5eV, only secondaries that are very near the surface (< 10nm, see Interaction Volume) 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 secondaries due to collection efficiencies. Collection of these electrons is aided by using a "collector" in conjunction with the secondary electron detector. The collector is a grid or mesh with a +100V potential applied to it which is placed in front of the detector, attracting the negatively charged secondary electrons to it which then pass through the grid-holes and into the detector to be counted.
Caused by the de-energization of the specimen atom after a secondary electron is produced. Since a lower (usually K-shell) electron was emitted from the atom during the secondary electron process an inner (lower energy) shell now has a vacancy. A higher energy electron from the same atom can "fall" to a lower energy, filling the vacancy. This creates and energy surplus in the atom which can be corrected by emitting an outer (lower energy) electron; an Auger Electron.Utilization
Auger Electrons have a characteristic energy, unique to each element from which it was emitted from. These electrons are collected and sorted according to energy to give compositional information about the specimen. Since Auger Electrons have relatively low energy they are only emitted from the bulk specimen from a depth of < 3 see Interaction Volume).
Caused by the de-energization of the specimen atom after a secondary electron is produced. Since a lower (usually K-shell) electron was emitted from the atom during the secondary electron process an inner (lower energy) shell now has a vacancy. A higher energy electron can "fall" into the lower energy shell, filling the vacancy. As the electron "falls" it emits energy, usually X-rays to balance the total energy of the atom so it.Utilization
X-rays or Light emitted from the atom will have a characteristic energy which is unique to the element from which it originated. These signals are collected and sorted according to energy to yield micrometer diameter) of bulk specimens limiting the point-to-point comparisons available (see Interaction Volume).
Thin Specimen Interactions
Incident electrons which are transmitted through the thin specimen without any interaction occurring inside the specimen.Utilization
The transmission of unscattered electrons is inversely proportional to the specimen thickness. Areas of the specimen that are thicker will have fewer transmitted unscattered electrons and so will appear darker, conversely the thinner areas will have more transmitted and thus will appear lighter.
Elasticity Scattered electrons:Source
Incident electrons that are scattered (deflected from their original path) by atoms in the specimen in an elastic fashion (no loss of energy). These scattered electrons are then transmitted through the remaining portions of the specimen.Utilization
All electrons follow Bragg's Law and thus are scattered according to Wavelength=2*Space between the atoms in the specimen*sin(angle of scattering). All incident electrons have the same energy (thus wavelength) and enter the specimen normal to its surface. All incidents that are scattered by the same atomic spacing will be scattered by the same angle. These "similar angle" scattered electrons can be collated using magnetic lenses to form a pattern of spots; each spot corresponding to a specific atomic spacing (a plane). This pattern can then yield information about the orientation, atomic arrangements and phases present in the area being examined.
Inelastically Scattered Electrons:Source
Incident electrons that interact with specimen atoms in a inelastic fashion, loosing energy during the interaction. These electrons are then transmitted trough the rest of the specimen.Utilization
Inelastically scattered electrons can be utilized two ways:
- Electron Energy Loss Spectroscopy: The inelastic loss of energy by the incident electrons is characteristic of the elements that were interacted with. These energies are unique to each bonding state of each element and thus can be used to extract both compositional and bonding (i.e. oxidation state) information on the specimen region being examined.
- Kakuchi Bands: Bands of alternating light and dark lines that are formed by inelastic scattering interactions that are related to atomic spacings in the specimen. These bands can be either measured (their width is inversely proportional to atomic spacing) or "followed" like a roadmap to the "real" elasticity scattered electron pattern.