A DIY Scanning Electron Microscope

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By Ben Krasnow

For fun and profit, I have built many different kinds of electronic and electromechanical device over the years in my shop. Throughout all of this time, I have always wanted to build a device that makes direct use of particle physics, and very recently completed this goal. I built a primitive scanning electron microscope (SEM) by combining power supplies that I purchased on eBay and machining parts from materials bought from McMaster-Carr and the local hardware store.

Scanning electron microscopes work by accelerating electrons in a vacuum and focusing them into a tight beam which impacts the sample under study. The impacting electrons cause additional secondary electrons to be liberated from the surface of the sample, and many SEMs detect these secondary electrons in order to produce an image of the sample. By scanning the electron beam across the sample in a raster pattern, and measuring the number of secondary electrons emitted from the sample for each beam position in the raster, the image of the sample is constructed and displayed in real time. The surface topology of the sample has a large effect on the number of secondary electrons that are detected so that the resulting image is similar to a photograph taken with the only light being supplied by a single directional light source. For example, if examining a highly textured item, all of the item surfaces that face away from the secondary electron detector will appear to be more shadowed than the surfaces that face the detector.

Nearly all commercial SEMs use electromagnets to shape and direct the electron beam across the surface of the sample. The reason is that magnetic fields are powerful tools for controlling electron trajectory. However, building very uniform electromagnets in a small workshop is difficult, as any asymmetries would degrade the microscope performance. Alternatively, electron beams can be controlled by electrostatic fields, which is the method that I used in my SEM. The electrostatic fields are not as effective as magnetic fields for the purpose of beam focusing and deflection, but are easier to implement since it only involves machining thin metal plates and cylinders (as opposed to machining more complex iron pole pieces, and winding them with wire).

My SEM makes use of a standard analog oscilloscope to display the image formed by beam scanning and secondary electron detection in the microscope. The oscilloscope is operated in X-Y mode, with the X and Y signals generated in synchronization with the SEM beam deflection. The signal from the secondary electron detector is used to modulate the oscilloscope trace brightness, thus translating secondary electron signal strength to brightness on the oscilloscope display. This arrangement will generate the high-contrast images that are typical of SEMs.

My ultimate goal in undertaking this project was to learn more about particle physics, especially electron beam handling, material science, and high-precision machining. Since the project has gone better than expected so far, I will soon be taking the next steps of refining my design and generating some images of insects, small mechanical parts and other every-day items.

Editor’s Note: I want to thank Ben for sharing this remarkable project with the CSL blog. We look forward to hearing more from Ben as he continues to work on it. -SG

This entry was posted in General Interest, Microscopy, Physics. Bookmark the permalink.

4 Responses to A DIY Scanning Electron Microscope

  1. This is a first class project. I very much hope that Ben will post updates about any new YouTubes of this DIY SEM. I also hope to learn more about his other projects.

  2. Dave says:

    Excellent article! I hope to see more details in the future.

    Regarding the use of electrostatic deflection for the electron beam rather than electromagnetic deflection, some of the same concerns apply for CRTs. One of the reasons that oscilloscopes typically use electrostatic deflection of the electron beam rather than the electromagnetic deflection that televisions typically use, is that it’s easier to get proper response/deflection at the varying frequencies that oscilloscopes typically use. Plus, I don’t know that I would want to try an incorporate a television style deflection yoke into a SEM; in that case, the electrostatic deflection plates may be quite superior for their simplicity.

    For that matter, one wonders if a CRT electron gun could have been used, although the learning experiences may not have been quite as good.

    I’d like to know a bit more about the secondary electron capture circuitry. Are the secondary electrons collected directly, or are they amplified using a dynode structure?

    For an interesting comparison, consider the operation of the Image Orithicon tube that was used for television cameras (One such tube was the RCA 5820.):

    http://en.wikipedia.org/wiki/Image_orthicon_tube#Image_orthicon

    Dave

  3. Pingback: The Electron Microscope

  4. RICHARD IDES says:

    Hello Ben,
    thank you so much for this very interesting article
    can you tell me what the different is between the SEM and TEM electron Microscope is
    let me know when you have more videos
    i been for many years in electronics and for the last 10 years in micro and cell biology and looking for a used electron microscope on EBAY
    may be you can give me some ideas
    thanks again

    RICHARD IDES

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