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