An interactive journey into the history and technology of electron microscopy
Traditional light microscopes cannot resolve two points closer than about 200 nm apart. In the life sciences, superresolution microscopes and certain software manipulations can edge us past this limit, but subcellular structures still defy good definition—as do those fine structural features in the materials sciences, semiconductor, and other key industries. Electrons, by contrast, have wavelengths about four to five orders of magnitude shorter than visible light and using them, electron microscopes can capture images with extraordinary detail.
SEMs, developed largely thanks to the work of von Ardenne in the 1930s, and Oatley from the 1940s into the 1960s, create surface images of bulk samples – as opposed to the thin samples used in transmission electron microscopy – by scanning an electron beam over a sample, recording the resulting echoes and electrical interactions point by point.
Anton van Leeuwenhoek produced the first light microscope in the mid-1600’s using a new technique he developed to create high quality, if small, lenses. About 200 years later, Ernst Karl Abbe, working with German engineer and entrepreneur, Carl Zeiss, published (in 1873) the seminal observation that the resolution of a microscope could be defined as the wavelength of the light used, divided by twice the numerical aperture. Although there is some debate as to whether Abbe was the first to make this discovery, he has historically been credited. The resulting equation implies that there is a point, known as the Abbe diffraction limit, at which two objects viewed under a light microscope cannot be separately distinguished, and is roughly half the wavelength of the light used. As a consequence, the fine details of any objects smaller than that limit remained tantalizingly out of reach.
For many years, this limit was seen as immutable, but this did not stop researchers from dreaming of one day pushing past it. It took until 1931 for this dream to become a reality, when Ernst August Friedrich Ruska, while working at Siemens-Reiniger-Werke AG (precursor to present-day Siemens AG) built the first transmission electron microscope. Using electrons, which have a far shorter wavelength than light, it was possible to resolve individual objects at a far greater magnification, up to 12,000x. It was not an easy path to this milestone and Ruska undoubtedly stood on the shoulders of giants when developing his microscope, but it set the foundation for the development of electron microscopy technology. Four years later, Max Knoll discovered a means to sweep an electron beam over the surface of a sample, creating the first scanning electron microscope (SEM) images.
Although both of these developments were a huge step forward, it was another 30 years, in 1965, before the first commercial SEM became available to scientists, revolutionizing high-magnification microscopy. In the ensuing 50 years the field has experienced both gradual progress as well as quantum leaps. Many of these milestones are laid out in the illustrated historical timeline of SEM development, to be seen on the front of this poster. They accompany a primer on SEM, explaining the basics of the technology, the types of signals that can be detected, and how these are applied today in a research setting.
Sean Sanders, Ph.D.