Sample preparation techniques for SEM to neutralize the effect of vacuum

By Luigi Raspolini - Aug 24, 2017

Scanning electron microscopes (SEM) scan the surface of the sample with an electron beam, collecting reflected electrons which carry information about the material the electrons interact with. If gas is in the sample chamber, its atoms interact with the beam, partly deflecting electrons and adding noise to the image. 

This is the reason why vacuum must be achieved in SEM before imaging. But while vacuum is crucial for proper analysis, it can also have a negative effect on certain types of materials. Read this blog to learn how you can neutralize vacuum and keep your samples intact.


Sample Preparation E-guide
Download this free Sample Preparation E-guide to obtain great results from the most common samples


Vacuum in SEM 

First, let's talk about vacuum in a SEM. The vacuum level (or the pressure value) can be controlled by the user. For desktop scanning electron microscopes, the vacuum level varies between 1-10 Pascals. For comparison purposes: atmospheric pressure is 100,000 Pascals.

 Vacuum gauge.jpg

When pressure drops below the atmospheric value, all liquids will undergo a phase change. Therefore, special materials — which are not affected by the vacuum — must be used for the internal mechanics of any SEM.

Samples can also be affected, and their behavior can vary: the most sensitive are samples containing water.

Samples with internal gas pockets, or those made of delicate materials, might also be affected, as they will outgas during the imaging process.

Any gas evaporating from the sample is a risk for the imaging tool, as it could contaminate the electron column or the detector, compromising functionality or affecting the image quality. For this reason, desktop SEM’s are usually equipped with a safety routine that will eject the sample when a sudden or unexpected increase in pressure level is detected.

How to prevent or limit the outgassing process

It is not always easy to predict the behavior of a sample: a plant leaf can usually be imaged without any particular precaution, while cheese will outgas. That’s why the key to good imaging is sample preparation. There are a couple of tricks that can prevent or limit the outgassing process. Many of them require pre-processing of the sample.

A desiccator, for example, can remove the liquids from the sample and therefore turn it ready for imaging.

But although this procedure will provide very nice images, the images will not be representative of the initial sample. Potentially water from the deeper layers of the sample, due to the low environmental pressure, will work its way through the structure of the sample to reach the atmosphere and evaporate.

As a consequence of this outgassing, the sample will show artifacts — or in a worst case scenario, the whole structure might collapse.

Other alternatives are available. Operating the SEM at a lower vacuum level (or increasing the pressure, which is the equivalent) will slow down the evaporation rate, while freezing the sample will almost completely stop the evaporation process and preserve the original structure of the sample. 

You will find more details on this and other sample preparation techniques in our Sample Preparation E-Guide that is available for download

Pick up your free copy here:

desktop-sem-sample-preparation-e-guide


About the author

Luigi Raspolini is an Application Engineer at Thermo Fisher Scientific, the world leader in serving science. Luigi is constantly looking for new approaches to materials characterization, surface roughness measurements and composition analysis. He is passionate about improving user experiences and demonstrating the best way to image every kind of sample.

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