SCANNING ELECTRON MICROSCOPE (SEM) - Working Principle and Applications

 SCANNING ELECTRON MICROSCOPE

·       The first Scanning Electron Microscope (SEM) debuted in 1942 with the first commercial instruments around 1965. Its late development was due to the electronics involved in "scanning" the beam of electrons across the sample.

Working Principle

·       In Scanning Electron Microscopy (SEM), the Specimen (Sample) is coated with a thin film of a Heavy metal, typically Gold.

·       The Specimen (Sample) to be examined is made extremely thin, at least 200 times thinner than those used in the Optical light microscope.

·       In Scanning Electron Microscopy (SEM), an Electron gun produces a finely focused beam of Electrons called the Primary electron beam.

·       The Primary electron beam passes through the two Electromagnetic Condenser lenses (First Condenser Lens and Second Condenser Lens).

·       The First Electromagnetic Condenser lens largely determines the "spot size"; the general size range of the final spot that strikes the sample. The Second Electromagnetic Condenser lens changes the size of the spot on the sample, changing it from a wide dispersed spot to a pinpoint beam.

·       The beam from two Electromagnetic Condenser lens is directed over the surface of the Sample through Aperture, Scanning coil and Objective lens.

·       The Specimen (Sample) is usually placed on Aluminium stubs.

·       When the Primary electron beam knocks electrons out of the surface of the Sample, interaction occurs with the Sample and produce the Secondary electron beam.

·       The produced Secondary electron beam are transmitted to Detector, Amplified and used to produce an image on a Viewing screen or Photographic plate. The image is called as Scanning Electron Micrograph.

Uses

·       SEM is especially useful in studying the surface morphology of the specimens, macromolecular aggregates and tissues (usually magnified 1000 – 1,00,000 X).

SEM has a variety of applications in a number of scientific research and industry - related fields, especially where characterizations of solid materials is beneficial.