Basic idea of how scanning electron microscope work

diagram of Scanning Electron Microscope (SEM)

Diagram of Kutztown University's scanning electron microscope (a Zeiss Gemini 300 FE-SEM with Oxford Ultim Max 100 EDS) (2.9 Mb image)

Basic concept of a scanning electron microscope

Imagine you're on a rescue boat searching for survivors in the water at night.  It's pitch black out, but you have a searchlight.  Rather than just pointing the searchlight in one direction and hoping the survivors happen to swim into view, you sweep the beam back and forth across the waters.  At any single moment, you only see the spot where the beam is pointing as the light rays fly from the searchlight to the water, and then back into your eye.  However, you create in your mind a complete picture of the scene by mentally stitching that mosaic of spots together into a whole.  An SEM works just like this, except:

This diagram illustrates the process in a step-by-step manner:

The letters in the diagram designate the different kinds of detectors.

(2.9 Mb original file)

diagram showing how electrons interact with a sample

Diagram showing how electrons penetrate to different depths and in different ways to create different signals that tell us different things about the sample (0.5 Mb image)

How does the electron beam interact with the sample?

This is a classic diagram that summarizes the different ways electrons interact with the atoms in a sample.  Much more complex that reflecting a searchlight beam, eh?  Different electron microscopes are configured to detect different kinds of interactions, depending on what the researchers are trying to study.

Our electron microscope is configured to detect:

Secondary electrons (SE) are electrons that are part of the sample that get knocked out of the sample by the electron beam.  This mainly happens to atoms at the topmost surface of the sample, so secondary electron images give us a view of the shapes on the surface of samples.  Detectors 🅐🅑🅒
Please click here to see examples of secondary electron imaging.

Backscatter electrons (BSD) are electrons from the original beam itself that "bounce" back in a special way called "backscattering" so that they don't lose any energy.  Because materials with more protons in their atomic nuclei are better at "bouncing" beam electrons back, backscatter electron imaging enables us to distinguish between things with different chemical compositions. Detectors 🅓🅔
Please click here to see examples of backscatter electron imaging.

Each element on the periodic table glows a characteristic x-ray energy.  The energy dispersive spectrometer (EDS) analyzes the precise chemical composition of any point on the sample by identifying what energies of x-rays glow from the sample (tells us what elements are present), and how brightly each energy x-ray glows (tells us concentrations of each element present). Detector 🅕
Please click here to see examples of x-ray spectral analysis and chemical mapping.

Transmitted electrons (STEM) pass all the way through the sample.  Some electrons run into atoms along the way and so do not make it all the way through the sample to the electron detector underneath.  The STEM detector is useful for studying organelles inside cells, viruses, and nanoparticles. Detector 🅖
Please click here to see examples of transmitted electron imaging.

(0.5 Mb original file)