How Does an SEM Microscope Work?
Preparing the Sample
Scanning electron microscopes (SEM) utilize electrons in place of light beams to achieve both higher magnification and greater clarity and depth of field over traditional microscopes. Preparing a sample of examination by an SEM is a complicated process. First, the sample must be dehydrated because it will be placed in a vacuum chamber. If it is a non-metal, it must then be covered with a thin conductive covering to prevent "charging" by the electrons, usually gold foil. This process is called "sputter coating."
Electron Beam
A beam of electrons is produced from the "electron gun" located at the top of the SEM. These electrons are usually produced by the heating of a metallic filament like tungsten. The electron beam from the "gun" follows a path down through the microscope past electromagnetic lenses which focus the beam towards the sample. When the beam hits the sample, other electrons are dispersed or ejected from the sample. These electrons are called back scattered or secondary electrons.
Producing the Image
Specialized detectors placed in the SEM collect the ejected electrons and convert them to a signal sent to a viewing screen. The screen assembles the signal into an image. SEM images can be controlled to a magnitude of x250,000 with a resolution around 10nm.
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