1. Magnification
Unlike ordinary optical microscopes, in SEM, the magnification is controlled by controlling the size of the 3-scan area. If higher magnification is required, just scan a smaller area. The magnification is obtained by dividing the screen/photo area by the scan area. Therefore, in SEM, the lens has nothing to do with magnification.
2. Field depth
In SEM, the sample points located in a small layer area above and below the focal plane can be well focused and imaged. The thickness of this small layer is called the depth of field and is usually a few nanometers thick, so SEM can be used for 3D imaging of nanoscale samples.
3. Action volume
The electron beam not only interacts with the atoms on the surface of the sample, it actually interacts with the atoms in the sample within a certain thickness range, so there is an interaction "volume". The thickness of the action volume varies depending on the signal:
Ou Ge Electronics: 0.5~ 2nm.
Secondary electrons: 5A, for conductors, λ=1 nm; for insulators, λ=10 nm.
Backscattered electrons: 10 times that of secondary electrons.
Characteristic X-rays: micron scale.
X-ray continuum: slightly larger than characteristic X-rays, also on the micrometer scale.
4. Working distance
Working distance refers to the vertical distance from the objective to the highest point of the sample.
If the working distance is increased, a larger depth of field can be obtained under the condition that other conditions remain unchanged.
If the working distance is reduced, higher resolution can be obtained ceteris paribus.
The commonly used working distance is between 5mm and 10mm.
5. Imaging
Secondary electrons and backscattered electrons can be used for imaging, the latter is not as good as the former, so secondary electrons are usually used.
6. Surface analysis
The generation process of Og electrons, characteristic X-rays, and backscattered electrons are all related to the atomic properties of the samples, so they can be used for composition analysis. However, since the electron beam can only penetrate a very shallow layer of the sample surface (see action volume), it can only be used for surface analysis.
Characteristic X-ray analysis is the most commonly used surface analysis, and two types of detectors are used: energy spectrum analyzer and spectrum analyzer. The former is fast but not accurate, the latter is very accurate and can detect the presence of trace elements but takes too long.
