2 The Principle of the Electron Microscope

How to Make a Microscope, Chapter 2

Unlike the optical microscope, the scanning electron microscope uses accelerated electrons in a vacuum to act as light to view the sample. An electron is the negatively charged particle of an atom orbiting around a nucleus. It can be released by heat or an electric field. Electrons are 2,000 times lighter than the smallest atom, thus they can be easily stopped or diverted when striking materials. Because electrons move freely only in a vacuum, there must be a vacuum in the entire column. To achieve this state, vacuum pumps are used along with various levels of vacuum. Dozens of millions to billions of electrons hit the sample per second. Electromagnetic lenses then focus the beam on the examined sample in an optimal way. During its journey, the electron beam passes through a number of apertures with various diameters, which stop electrons undesirable for creating an image. The electrons can either scan the sample step by step or go through the sample to show its inner structure, giving us the distinction between a scanning and a transmission electron microscope. Modern electron microscopes not only display, but also analyze, measure, and modify in 2D, 3D, and 4D.

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