Development of Holography Electron Microscope with Atomic Resolution

THE transmission electron microscope was invented in Germany in 1932. It works by directing a beam of electrons through a specimen and then using a lens to enlarge the image formed by their passage. In Japan, development and research into its applications began in 1939, and numerous companies around the world have competed to develop the instruments since Hitachi developed its own fi rst transmission electron microscope, the HU-1, in 1941. Hitachi supplied the first commercial model produced in Japan, the HU2, to Nagoya Imperial University (as it was then known) in 1942. A transmission electron microscope exhibited by Hitachi at the 1958 World’s Fair in Brussels won a grand prize, and since then Japanese instruments, including those made by Hitachi, have competed with the world’s best. Subsequently, the scanning electron microscope (SEM), which works by scanning a focused electron beam over the surface of the specimen, was successfully commercialized in 1965. The electron microscopes of that time used thermo electron guns, which are based on the thermionic emission of electrons by heating tungsten. Although the field emission effect (whereby electrons are emitted via the tunneling effect by applying a strong electric fi eld to a sharp metal tip) was a known source for obtaining the electrons used by these microscopes, it had not been adopted in practice because it required extreme high vacuum, making it difficult to use in practice. With assistance from Professor Albert V. Crewe of the University of Chicago, Hitachi successfully implemented a practical electron source, and an electron gun to use it, and incorporated it into an SEM that was released on the market in 1972. From 1984 onwards, this technology became part of a major business dealing with critical dimension-scanning electron microscopes (CD-SEMs) specifically designed for measuring semiconductor patterns, a product category in which Hitachi continues to have the leading share of the global market. Returning to the subject of transmission electron microscopes, since increasing the energy of the electron beam increases its ability to pass through specimens, the 100-keV energy of early instruments had increased ten-fold to 1 MeV (6) by 1966 to enable the observation of thicker specimens, with the subsequent development of larger models up to 3 MeV . As the history of the electron microscope is a long one that has been well documented elsewhere, this article will now move on to the subject of holography electron microscopes.