Field Emission Scanning Electron Microscope (FE-SEM)

Optical microscopes have a maximum magnification of 2000 times the magnification that is often not enough. To see particles that are smaller than the wavelength of light, we need to have very small particles so that they can be traced back to the target and visualized very small particles. The best particle for this purpose is electron. Electron is a charged and fundamental particle that can improve the magnification up to a million times.

Scanning electron microscopy is an efficient and non-destructive technique that provides detailed information on the morphology, composition and structure of the studied materials. The first scanning electron microscope was invented in 1942. It was shown at that time that Secondary Electrons (SE) produced topographic contrast using the collector positive bias rather than the sample. After that, many changes were made until in 1965 the first commercial sample of scanning electron microscope was introduced.

In scanning electron microscopy (SEM), two classes of electrons are detected: secondary electrons (SE) and backscattered electrons (BSE). Backscattered electrons are electrons that resonate after the elastic collision of the electron beam with the sample, leading to the electron redirection. Secondary electrons originate from the sample atoms and result from the inelastic collision of the electron beam with the sample and have less energy than backscattered electrons. As a result of the inelastic collision of the electron beam with the sample surface, the energy of the electron beam electrons is transferred to the conduction band electrons and sometimes the valence band of the sample which separates these electrons from the sample atom, which is called secondary electrons. BSEs return from deeper points of the sample and depend on the atomic number of the material, the larger the atomic number, the material appears in the brighter image. The SEs are sampled from the more superficial areas and provide a lot of surface detail information. As a result, these two groups of electrons carry different information.

With the advancement of technology and the advent of Field Emission Scanning Electron Microscopes (FE-SEM), higher resolution images became available. The method of operation of these microscopes is similar to that of conventional scanning electron microscopes (SEM), in that the surface is scanned by an electron beam. The biggest difference between SEM and FE-SEM is the electron generation system. FE-SEMs use field effect guns. These guns concentrate low-energy and high-energy electrons at a low electrical potential (about 0.02 to 5 kV) and increase spatial resolution.

In conventional electron microscopy by heating the tungsten filament the electrons are separated from the filament, which contaminates the sample surface, but the field emission electron gun does not require thermal energy to overcome the surface potential and by applying a very high electric field to the sample surface. , Separates electrons from the surface. In the SEM method, a resolution of 3-7 nm is achievable, and in the FE-SEM the resolution is 1.5 nm or better.

In the case of samples that are not electrically conductive, such as dielectrics and semiconductor materials, a sample preparation process is required prior to imaging. Due to the collision of the electron beam in the imaging process using electron microscopy, the non-conductive structure of these specimens traps electrons on the surface of the material and thus causes the surface to temporarily become charged. This phenomenon causes white areas to be seen in the image captured by the electron microscope. The conductive layer deposited on the sample prior to the microscopic process acts as a channel and eliminates the charges created on the sample surface.

The conductive layer shown on a sample to be subsequently imaged by field emission scanning electron microscopes must be very thin and uniform and have fine grain size so in addition to removing excess electrical charges on the sample, it also improves contrast in low-density materials. The presence of this conductive thin film is especially important when it is a biological sample. In order to achieve this thin layer which is only achievable by vacuum deposition methods, selecting the appropriate material and controlling the deposition conditions such as pressure and also the substrate type is very important. Usually the thickness appropriate for the preparation of field emission scanning electron microscopes samples is 0.5 to 3 nm. In addition to the above, the presence of a conductive layer on the sample stimulates secondary electrons, especially at the surface. A conductive coating is appropriate when the topographic features of the specimens are not significantly enlarged or hidden. The selected material must have a suitable secondary electron emission coefficient to help improve image contrast.

The optimum thin film conditions for coating scanning electron microscopy specimens are obtained when the thin film is of minimum thickness and contains grains with no surface mobility such that they do not affect the sample appearance and be smaller than the probe diameter. Researchers have found interesting results by comparing field emission scanning electron microscopy images of a same sample coated with various materials: chromium, gold / palladium and platinum. At the preparation stage, the gold / palladium thin film is coated with different conditions on the specimen. The images obtained from SEM, FE-SEM and TEM are as follows:


The images on the right belong to TEM and the scale bar is 50 nm. In the first row the thickness of the thin film is 3 nm and for the other rows it is 1.5 nm. The first row on the left is the SEM image and the other row is the FE-SEM on the left. The deposition of gold / palladium thin film is provided with sputtering machines from different companies. According to the figures, it can be said that even under the same pressure and power conditions in different coating systems, different results are obtained. As can be seen, the layer shown in the second row is more uniform than the first row and has a finer grain size. In the third row, the space between the particles is clearer than the second row. Next, platinum was applied to the sample at 1, 1.5 and 2 nm thickness and the images were investigated.

The images on the right belong to TEM and the scale bar is 50 nm. In the first row the thickness of the thin film is 3 nm and for the other rows it is 1.5 nm. The first row on the left is the SEM image and the other row is the FE-SEM on the left. The deposition of gold / palladium thin film is provided with sputtering machines from different companies. According to the figures, it can be said that even under the same pressure and power conditions in different coating systems, different results are obtained. As can be seen, the layer shown in the second row is more uniform than the first row and has a finer grain size. In the third row, the space between the particles is clearer than the second row. Next, platinum was applied to the sample at 1, 1.5 and 2 nm thickness and the images were investigated.

In the first row, the sample is coated with a 2 nm platinum film. As can be seen, it has a much larger grain size than gold / palladium. The second row has more space between the grains but the particles are larger than gold / palladium.

Next, chromium of different thicknesses was coated as a conductive thin film on the sample to be imaged by field emission scanning electron microscope and the following images were obtained.

By comparing images of a sample prepared with chromium, gold / palladium and platinum, chromium is more suitable for high resolution imaging applications. The thin layer of chromium created is relatively smooth and have no sub-structured relative to the other thin films. But the use of chrome thin film also has its problems. For example, it is not suitable for biological samples; the high vacuum environment is required for its deposition process and contains less emission secondary electrons coefficient than platinum and gold/palladium thin films. Chromium is rapidly oxidized so the deposited specimens must be stored in a vacuum medium.

For more information, see the following links:

Summary
Field Emission Scanning Electron Microscope (FE-SEM)
Article Name
Field Emission Scanning Electron Microscope (FE-SEM)
Description
this paper is about Field Emission Scanning Electron Microscope (FE-SEM).
1
Hello
How can we help you? for talking with human and asking your question, please push the below green circle and go to the WhatsApp.
Powered by