Early development of electron microscopy

The word microscopy contains two parts with Greek origins; micro- (small) and -skopion, from skopein "to look, see". So we have "an instrument for viewing what is small" Light microscopes which best suited for the human eye were developed first and are still very useful as they reveal much information on a specimen. However, the resolving power of a microscope is limited by physics. The wave characteristics of light leads to diffraction, as a consequence the image of any infinitely small spot will be transformed in a dot or disc of finite dimension. The diameter of such a disc is proportional to the wavelength used. Using green light of 550 nm wavelength you can’t do much better then 200 nm, usually the resolving power is less. In order to create images of significantly higher resolution, we need to use a wave of shorter wavelength. Among other options, this wave can be an electron beam. Here the wavelength decreases with increasing acceleration voltage. Using an acceleration voltage of 10 KV the wavelength of the electron beam with 12 pm significantly smaller than the wavelength of visible light. At least diffraction should not limit the resolving power of an “electron microscope” as it does for an optical microscope.

McMullan, in an account of the early history of SEM, traces the SEM development back to 1843 with the invention of the first fax machine, based on a clock pendulum. [1][2] The first images using electron beams were produced in the earlier 1930s by Ernst Ruska and Max Knoll, who produced a photo with a 50 mm object-field-width showing channelling contrast by the use of an electron beam scanner,These first images were taken in the “transmission” mode. Ernst Ruska received the Nobel Prize in 1986 for his work on electron [3]. In 1937, Manfred von Ardenne proposed [4] a microscope with high magnification. Images were produced by scanning a very small raster with a demagnified and finely focused electron beam. Von Ardenne applied the scanning principle to both achieve magnification and eliminate the chromatic aberration otherwise inherent in the electron microscope. Further work was reported by Zworykin's group in the 1940s ,from the RCA labs in the US. However, the real breakthrough in terms of electron microscopy was achieved by the group of Charles Oatley at the university of Cambridge in the 1950s and early 1960s . This work finally led in 1965 to the marketing of the first commercial instrument, the “Stereoscan”, by Cambridge Scientific Instrument Company.

  1. McMullan, D. "Scanning electron microscopy 1928–1965". ; Scanning 17 (3); 1995; 175–185. DOI 
  2. Link retrieved 07.2016 www-g.eng.cam.ac.uk/125/achievements/mcmullan/mcm.htm
  3. Link retrieved 07.2016 nobelprize.org/.../1986/ruska-facts.html
  4. von Ardenne, Manfred . "Das Elektronen-Rastermikroskop. Theoretische Grundlagen". Zeitschrift für Physik, 109 (9–10); 1938; 553–572. DOI:10.1007/BF0134158

Basic operation

The basic operation of an electron – microscope can be summarized as follows; The electron beam produced by the electron gun is focussed and manipulated by a set of electromagnetic lenses before interacting with the specimen.The main components required for an electron microscope are:

  • an electron source
  • an electron optical column,
  • detector systems,
  • a vacuum system,
  • the necessary control electronics,
  • and obviously control software.

The microscope may be fully enclosed to reduce electromagnetic interference and vibration from environmental sources, and operated remotely.

 The light source of the light microscope is replaced by an electron gun. Electromagnetic lenses replace the glass lenses. The focal length of electro-magnetic magnetic lenses can be varied by changing the applied voltages and currents. A set of different detectors replaces the eyepiece and/or ocular. Depending on the detector type and location, a large variety of information on the specimen can be obtained.

The earliest electron microscopes were Transition Electron Microscopes (TEM). In this variant, a “large” beam of highly energetic electron is directed towards the sample. A portion of the electrons passes through the specimen and can be passed through electromagnetic lenses and directed towards a screen, to produce an “image”, which contains information on the sample. However, as many electrons interact with the matter, while passing through the sample, they loose some of there energy. Electromagnetic lenses show relatively strong chromatic aberration, which limited the achievable resolution and/or requires extremely this samples.

Using a very fine electron beam and scanning this across the sample, one can go around this limitation induced by chromatic aberration, obviously at the expense of analysis time. Using the scanning technique, not only the transmitted electrons can be used for “microscopy” but also the “reflected” electrons. The term Scanning Electron Microscopy (SEM) generally relates the electron microscopes operating in the “reflection” mode, whereas the others are referred to as Scanning Transmission Electron microscopes (STEM). It is the SEM variant that has been turned, after several decades of development, into the “workhorse” of electron microscopy, making its way from highly specialized laboratories to routine operation in industrial environment.