The first electron microscope (EM) was developed in 1931 by German physicist Ernst Ruska (1986 Nobel prize in physics) and is an adaptation of traditional optical microscopes, with light being replaced by an electron beam and optical lenses replaced by electromagnetic “lenses”.
The Scanning Electron Microscope (SEM) was invented by German physicist Manfred von Ardenne around 1937, and may be considered a direct outgrowth of the then rapidly advancing television technology: it is very much an adaptation of a television camera electron tube.
Since then, both EM and SEM have evolved tremendously, and in particular the SEM is today a standard research workhorse. Modern SEMs are so easy to use that they are often preferred even if a conventional optical microscope would be sufficient to do the job!
In an SEM, a narrow beam of fast moving electrons is generated by an “electron gun”. The beam is then shaped and focused onto a tiny spot on the sample, using a system of apertures and electromagnetic lenses. The “response” of the sample is then measured. By steering the electron beam to another spot, a different response may be obtained. This process is repeated in a systematic way, and a “map” of the response of different sample regions is assembled pixel-by-pixel – the SEM image is formed.
There are many different types of “response” that can, in principle, be tested. The two most common ones are:
The electron beam is largely absorbed by the sample, but since negatively charged electrons are attracted to positively charged nuclei, some may “swing around” a nucleus of the sample and “bounce back”, similar to a comet being flung back into space after swinging by the sun. These backscattered electrons can be detected.
Because the backscatter process depends on the positive charge of nuclei, chemical elements with a higher atomic number (higher nuclear charge) give rise to a stronger response. In this way, an SEM backscattered electron image (BEI) gives not only information about the shape of the sample, but also hints at what elements microscopic features of the sample may be made from.
The sample, being made from atoms, already contains a large number of electrons. When the electron beam impinges on the sample, some of the sample's electrons may “splatter” out of the sample. This is called secondary electron emission and is quite similar to water droplets splashing out of a pail if one aims a water jet at it. The secondary electron image (SEI) generally provides better image quality than BEI.
CRADLΣ's electron microscope can provide both BEI and SEI images.