Electron Microscope

The prototype of the first electron microscope was devised by Max Knoll and Ernst Ruska, who were German engineers, in 1931. It was the discoveries and ideas of Louis de Broglie, a French physicist, that it was based on.

The electron microscope is a kind of device wherein electrons are used to produce an image of the specimen. Its power of resolving or magnification is much higher compared to an ordinary light microscope. Even though the electronic microscopes that are in use today have the ability of magnifying objects up to two million times, yet they are still based on the prototype that was first made by Ruska and the correlation he made between resolution and wavelength.

The electron microscope is one of the most essential components of many modern laboratories. It is used by researchers to examine cells and microorganisms, samples of medical biopsy, various large molecules, crystalline and metal structures, and the inherent features of a variety of surfaces.

The electron microscope is extensively used for various applications in industry such as the analysis of failure, quality assurance, and inspection, especially in the fabrication of semiconductor devices. There are various types of electron microscopes available these days, given below are a few of them.

Types of Electron Microscopes

Transmission Electron Microscope, or TEM: In this instrument a beam of electrons is used to form the image of the specimen, with no light being involved. The specimen’s image can be studied on a screen that is phosphor coated, and the electron micrographs, or the pictures, are made on film or photographic plates. An extremely sharp metal tip, a single crystal of Lanthanum Hexaboride, or a heated tungsten filament is the source of the beam of electrons. A high voltage is used to accelerate the beam of electron down a vertical column, which is done under vacuum, and electro-magnetic lenses are used to focus it.

It is not possible to focus electrons using glass lenses. As a matter of fact, an electron beam can be stopped by even an extremely thin piece of glass. The voltages that are used to accelerate the electron beam can range from a few thousand to even up to a million volts. The specimens that are examined by a Transmission Electron Microscope have to be extremely thin, such s 50 nanometers, or even less.

Scanning Electron Microscope, or SEM: This electron microscope is quite different from the Transmission Electronic Microscope. In this, the image of the specimen is not created by using electrons directly. Instead, electrons are used in a such a manner as to excite it, so that it gives off secondary electrons, which detectors collect and use to create an image. Most electron microscope images that are seen in books and articles are secondary electron images. However, apart from the generating secondary electrons, the beam also results in scattering them as well. An image can also be formed from these backscattered electrons. As a matter of fact, there are several beam interactions of the specimen which create useful information. For instance, the electron beam can also cause X-Rays to be emitted by the specimen. A separate detector can collect these, which can be used to study the elemental components that the specimen is made up of, or can also be used for creating the specimen’s elemental map on a video screen.

Scanning Tunneling Microscope, or STM: This was an invention of Heinrich Rohrer and Gerd Binnig at IBM’s lab in Zurich, Switzerland. This electron microscope is for obtaining images of surfaces that are conductive at the atomic scale of 0.2 nanometer or 2 x 10-10 m. It is also used for altering the material that is observed by the manipulation of individual atoms, which helps to trigger off chemical reactions. And, also to create ions by individual electrons being removed from atoms and then reverting them back to atoms by the replacement of the electrons.

The Scanning Transmission Electron Microscope, or STEM: This is just a regular Transmission Electron Microscope with a scanning system added to it. Rather than using a single electron beam, a small spot is used for scanning the specimen with the image being collected on a detector under the specimen. This instrument is particularly efficacious for the X-Ray microanalysis of tiny parts of thin slices of specimens.