When using a scanning electron microscope (SEM), the electron beam can, over time, permanently alter or degrade the sample that is being observed. Sample degradation is an unwanted effect as it can alter — or even destroy — the details you want to see, and consequently change your results and conclusions. In this blog, I will explain what can cause sample degradation, and how you can slow down the process.
In a SEM, a focused electron beam is used to scan the surface of your sample to create an image. Electrons are generated by an electron source and accelerated through the column by an electric field. This field varies from 1 kV to 30 kV with typical beam currents in the nano-Ampere range.
Accelerated electrons interact within the sample and, when analyzing beam sensitive materials, this interaction can damage and degrade the sample. The degradation can be seen in the form of cracks on the surface, or it can appear that the material is melting or boiling. The speed at which the degradation becomes visible varies with accelerating voltage, beam current and magnification level.
In samples where the material appears to be melting or boiling, you might assume that the material is being heated by the electron beam. However, simulations of samples show that the melting point of materials can only be reached in extreme cases. These extreme cases are samples with a very low heat transfer coefficient, high beam current or high zoom level. Degradation can also set in at low electron beam currents and low magnification levels, but it will just occur over a longer time frame.
What is causing the sample degradation?
Depending on the accelerating voltage, electrons from the electron beam can interact with electrons in the atoms of the sample. If a valence electron — an electron that can participate in the formation of a chemical bond — happens to be knocked out of the atom, it will leave an electron hole. This hole must be filled by another electron within 100 femtoseconds (i.e. the typical time period of an atomic vibration), or the bond will be broken.
In conductive materials, this is not a problem as the electron hole is filled within 1 femtosecond (fs). But for non-conductive materials it can take up to several microseconds to fill up the electron hole, potentially breaking the bond and chemically altering the material and its morphology.
How to slow sample degradation down
- The speed at which the degradation becomes visible varies according to the material. With some samples, you might not see it at all. If samples do degrade and this interferes with your results, then here are a few tips to slow degradation down:
- Coat the samples with a conductive (gold) layer to slow down the degradation. The thicker the layer, the better the effect. But be careful not to cover up details with the (conductive) gold layer.
- Lower the beam current and acceleration voltage.
- Limit viewing time by adjusting your image settings such as focus and contrast on a non-important part of your sample. When these are correct, move to the area of interest, immediately take a picture and move away again.
For more suggestions on how to improve image quality with difficult samples, you can download this sample preparation e-Guide.
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