Characteristic X-Rays are generated when excited sample atoms undergo a relaxation process. For that the atoms need to be excited first and this can be done with high energy electromagnetic radiation (in XRF) or accelerated particles such as electrons (in SEM). The primary beam kicks out an inner shell electron and a vacant spot is left behind. As this state is unstable, a higher shell electron will soon move into this vacant spot and during this process energy is emitted in the form of X-Rays. This emitted radiation has a specific energy which depends on the binding energies of the two electrons that participated in this process. If this emitted ( characteristic ) x-ray radiation is detected then the composition of the material can be measured.
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Scanning Electron Microscopy (SEM)
The scanning electron microscope is a powerful tool used by many scientists for studying and developing materials. This device uses electrons instead of photons and that allows the operator to visualize even nano-scaled surface details. Therefore it is very useful for also studying corrosion processes and creating efficient thin coatings which have a thickness of less than a micrometer.
In the video above you will see the working principle of this microscope and how it is operated.
The electrons are generated in the electron gun on top of the microscope. There are different source types but one of the oldest used is a tungsten filament that is heated over 2400 degrees celsius by passing a current through it. At this temperature electrons emit from the filament and are pulled down the column with the help of an anode. The operator selects the voltage that accelerates the electrons. It can be up to 30 kV in conventional scanning electron microscopes. The accelerated electrons are scattered and need to be focused – this is done with the help of multiple focusing lenses. These lenses also tune the amount of electrons that reach the sample. For example by broadening the electron beam, the electrons are absorbed by the column and dont reach the sample. In order to scan across the sample row by row, the electron beam needs to be moved in such a manner. This is achieved with the help of scanning coils that are placed below the focusing lenses. The final focusing of the beam is done with objective lenses located below the scanning coils. In order to get an image of the substrate the surface is scanned row by row with the electron beam. In each spot signals like secondary electrons, backscattered electrons or characteristic X-Rays are generated. By detecting the signal emitted from each scanned spot, an image of the surface is generated. So for example by detecting secondary electrons a secondary electron image is generated.
Secondary electrons are generated when the primary beam electrons kick out electrons from the substrate atoms. In that process primary beam electrons lose energy as it is transfered to the secondary electrons. These secondary electrons generaly have low energy and escape only from near the surface, giving a good image of the substrates topography. The electrons are collected with a special detector that has a positively charged collector for pulling the negatively charged electrons towards it.
Backscattered electrons are primary beam electrons that are scattered back in a similar direction as they interact with the substrate atoms nucleous. In that process they dont lose much energy and can be emitted even from deep layers of the substrate. Therefore they carry the bulk information of the sample and generated images are not completely topographic. The electron yield strongly depends on the atomic number of the substrate. For example regions on substrate with higher atomic number appear brighter on the image as more electrons are backscattered in these regions.
Characteristic X-Rays are generated when sample atoms exited by primary beam electrons are undergoing a relaxation process where the inner shell vacant spot is filled with an outer shell electron. The emitted radiations energy depends strictly on the atoms number and on the electrons binding energies that are involved in this process. Therefore this signal can be used to detect different elements and do quantitative element analysis.
For a real SEM study the sample first needs to be prepared – it has to be dry, conductive and with a suitable size. Non-conductive samples can be coated with a thin (couple nanometers) layer of metal (Au, Pt..), which allows SEM studies as they conduct away the heat and negative charge caused by electrons. The sample is moved into the microscope through an airlock or the main chamber door and placed on a stage with a special holder. The stage can move in any desired direction (x,y,z, rotate, tilt). The sample is then moved under the electron column to a suitable working distance. After focusing and other adjustments the first electron image can be taken.
Element microanalysis can be done in several ways but first an electron image of the sample is obtained. The most common way to do element analysis is selecting a characteristic spot on the sample and then bombard it with the narrow (couple nm wide) electron beam. As a result X-Rays are emitted from the couple micrometer wide interaction volume. This gives information from a very localized area but in some cases an average composition of the material is needed. For that a larger area (lets say 50 x 50 microns) is bombarded by electrons and the X-Rays emitted from that area are detected. It is also possible to map the distribution of elements on a larger area by scanning across the surface with the electrons row by row and collect X-Rays from each scanned point. Based on the detected X-Rays an image is created that shows the distribution of certain elements.
In order to see what is inside a material a focused ion beam (FIB) is used to make a cross-section or a thin lamella. The cross-sections or lamella are then studied with electrons at a suitable angle by tilting it with the help of the stage.
Scanning electron microscopes generally work in a high vacuum in order to prevent surface contamination and electron or x-ray interactions with the gas in the chamber which would affect the quality of the image. However there are also specially designed environmental scanning electron microscopes (ESEM-s), that work in low vacuum. The ESEM can therefore even be used to study living cells or bacteria.