Electron Microscopes Handout Changes in atomic theory have led to numerous new medical technologies including: Electron Microscopes Biochips (biological microchips) Nuclear Magnetic Resonance Magnetic Resonance Imaging-(MRI) How does an electron microscope differ from a regular light microscope? An electron microscope uses electrons instead of light to magnify specimens. Instead of glass lenses directing the light, electromagnets are used to make the electrons bend and focus, just like glass lenses make light bend for optical microscopes. There are different types of electron microscopes including: Scanning Electron Microscope (SEM) Transmission Electron Microscope (TEM) How Does a SEM Work? This imaging technology uses electrons instead of light to form an image and can be used with almost any type of specimen. Magnifications up to 300,000x can be achieved. The image is formed by a very fine beam of electrons that is focused on the surface of a specimen through a series of magnetic lens. The beam of electrons is scanned over a small area of the specimen in a series of lines and frames. Several things may happen to the electrons bombarding the specimen all of which can be used to produce an image. The most common means of image formation is by the beam of electrons being absorbed by the specimen which gives rise to secondary electrons of very low energy (together with X-rays). The secondary electrons pass through a grid and strike a disc that causes emission of light from the chemical coating the disc. The grid and disc form the detection system. The light emitted from the disc is converted to a voltage. The strength of this voltage is dependent on the number of secondary electrons that are striking the disc. The voltage is converted to a point of brightness on a television (cathode ray tube) screen. An image is built up by simply scanning the electron beam across the specimen in exact synchrony with the scan of the electron beam in the television screen. Page 1 of 5
SCHEMATIC DRAWING OF A SEM Medical Applications of SEM Comparison of healthy and unhealthy blood and tissue samples to determine the cause of illness in a patient. Study the effect of medicine on a patient by observing the differences between unhealthy patients given the medicine and those patients not given the medicine. Crime scene analysis of metal fragments, paints, inks, hair, and fibers. How does a TEM (transmission EM) differ from a SEM (scanning electron microscope)? A TEM transmits electrons through a specimen instead of imaging the surface of the specimen. Magnifications up to 500,000Xx can be achieved with TEM. The TEM provides high resolution, two-dimensional views of a specimen that has sufficient transparency. Internal cell structures can be imaged and in some cases individual atoms. Page 2 of 5
Hitachi S800 SEM at USF College of Engineering Metrology Laboratory SEM of a nanolithograph of USF logo on a polymer substrate Page 3 of 5
SEM of a filter paper with 0.45 micron (10-6 meters) pores at 1000x magnification. White particles are aluminum oxides. SEM of the same filter paper with 0.45 micron (10-6 meters) pores at10,000x magnification. White particles are aluminum oxides. Page 4 of 5
SEM of a nickel film test pattern on silicon carbide. (The white squares are the nickel film.) Note the micron scale on the image. Page 5 of 5