Overview of Electron Microscopy
by Tim Palucka
More detailed story here, summary below.
1. Early History of Electron Microscopy: 1931 to 1960
The invention of the electron microscope by Max Knoll and Ernst Ruska at the Berlin Technische Hochschule in 1931 finally overcame the barrier to higher resolution that had been imposed by the limitations of visible light. Since then resolution has defined the progress of the technology. The ultimate goal was atomic resolution - the ability to see atoms - but this would have to be approached incrementally over the course of decades. The earliest microscopes merely proved the concept: electron beams could, indeed, be tamed to provide visible images of matter. By the late 1930s electron microscopes with theoretical resolutions of 10 nm were being designed and produced, and by 1944 this was further reduced to 2 nm. (The theoretical resolution of a an optical light microscope is 200 nm.) Increases in the accelerating voltage of the electron beam accounted for much of the improvement in resolution. But voltage was not everything. Improvements in electron lens technology minimized aberrations and provided a clearer picture, which also contributed to improved resolution, as did better vacuum systems and brighter electron guns. So increasing the resolution of electron microscopes was a main driving force throughout the instrument's development. The progress achieved is discussed more fully in the decade-by-decade account of developments, and in the company-based spreadsheets accompanying this introductory article.
Of course, once the electron microscope became a commercial instrument, economic factors also figured into its development path, as can be seen by the variations offered by a manufacturer at one time. The highest-resolution offering (with its attendant high price tag) would be offered next to a lower-resolution instrument for researchers who did not need the ultimate resolution for their studies, or perhaps could not afford it. Advanced instruments requiring highly-trained technicians to operate them were offered next to simpler versions that could produce results after just a few hours of training. So a second driving force was the needs of the scientist and microscopist. While engineers might be driven to achieve the highest resolutions possible as a technological feat, they had to temper this drive by taking into account what could succeed in the marketplace.
With Knoll and Ruska leading the way, other researchers quickly joined in the development effort. In Brussels Ladislaus L. Marton made a primitive electron microscope to study the photoelectric effect, and went on to produce the first micrograph of a biological specimen. Manfred Von Ardenne in Berlin produced the earliest scanning-transmission electron microscope in 1937. At the University of Toronto in Canada, Cecil Hall, James Hillier, and Albert Prebus, working under the direction of Eli Burton, produced an advanced 1938 Toronto Model electron microscope that would later become the basis for Radio Corporation of America's Model B, the first commercial electron microscope in North America. Ruska at Siemens in Germany produced the first commercial electron microscope in the world in 1938.
The initial projections that a handful of electron microscopes would saturate the worldwide market proved to be grossly pessimistic, and many companies entered the fray. RCA was by far the leader in North America, with its electromagnetic lens technology, technological expertise, and corporate support for electron microscope development. General Electric tried to compete for a while with its line of electrostatic electron microscopes. Metropolitan Vickers in England (later Associated Electrical Industries) produced the E.M.1 invented by Professor L.C. Martin at Imperial University as early as 1936. Philips, Siemens, and Carl Zeiss each tried to grab a share of the European market. Starting in 1939, scientists in Japan gathered to decide on the best way to build an electron microscope. This group evolved into the Japan Electron Optics Laboratory (JEOL) that would eventually produce more models and varieties of electron microscopes than any other company. Hitachi and Toshiba in Japan also played a major role in the early development process.
Early progress in materials science in the 1940s was mostly limited to studies of small particles, such as the carbon black that mysteriously gave strength to automobile tires, and the pigments that were used to color paints and cosmetics. Their small size made it possible to analyze their outlines in the transmission electron microscope (TEM), and determine their size, shape, and number, but not reveal internal structure. In the 1950s, the microscopic analysis of thin foils (first described by Heidenreich in 1949) dominated the research. Studies of crystal defects such as stacking faults and dislocations in these thin films were popular, as well as studies of phase changes in samples subjected to a series of temperature regimes.
The 1940s and 1950s were decades of incremental improvements in instrumentation and technique, with resolution improving as power supplies and lenses were made more stable and brighter electron guns produced higher-energy electrons to probe the samples. Researchers learned how to prepare specimens of various kinds and to interpret the resulting micrographs. In biology, improved knife-edge microtomes produced thinner and thinner slices of sample that would not be damaged by the electron beam; in materials science, the invention of the replica technique by Mahl in 1941 provided thin impressions of the surface of a bulk sample that could then be analyzed in a TEM.
2. Recent History of Electron Microscopy:1960-2000
The 1960s through the 1990s produced many innovative instruments and trends. The introduction of the first commercial scanning electron microscopes (SEMs) in 1965 opened up a new world of analysis for materials scientists. Ultrahigh voltage TEM instruments (up to 3 MeV at CEMES-LOE/CNRS in Toulouse, France, and at Hitachi in Tokyo, Japan), in the 1960s and 1970s gave electrons higher energy to penetrate more deeply into thick samples. The evolution and incorporation of other detectors (electron microprobes, electron energy loss spectroscopy (EELS), etc.) made the SEM into a true analytical electron microscope (AEM) beginning in the 1970s. The development of brighter electron sources, such as the lanthanum hexaboride filament (LAB6) and the field emission gun in the 1960s, and their commercialization in the 1970s brought researchers a brighter source of electrons and with it better imaging and resolution. Tilting specimen stages permitting examination of the specimen from different angles aided significantly in the determination of crystal structure. In the late 1980s and throughout the 1990s, the environmental electron microscopes that allow scientists to examine samples under more natural conditions of temperature and pressure have dramatically expanded the types of samples that can be examined.
Other trends involved the simplification for the use of unskilled operators and the development of specialty instruments for the biological or the physical sciences. Electron microscopes specifically for use in the integrated circuit (IC) industry were developed in the 70s and 80s in response to that burgeoning industry. Computer technology for automated control of electron microscopes and for analysis of the resulting micrographs also added to the possibilities of the technology, especially with the miniaturization of computers in the 1980s.
This page was updated on 10 December 2002 by Arne Hessenbruch.