An FIM specimen in the form of a sharp needle is cooled to cryogenic temperatures (20 to 100K) in an ultrahigh vacuum (UHV) chamber. Controlled amounts of imaging gas (typically neon, helium, hydrogen, argon, or combinations of these) are admitted into the vacuum system. High voltage (~10 kV) applied to the tip results in a high electric field near the apex, where the imaging gas atoms become polarized and are attracted onto the specimen surface. At protruding sites on the surface (i.e., on atomic ledges) the localized field is sufficient to field ionize (by quantum-mechanical tunneling) these imaging gas atoms. The resulting ions are then radially repelled from the tip toward the phosphor screen where they produce brightly imaged spots. Thus individual atoms may be imaged by this technique.
Unlike conventonal micrsopes, where the spatial resolution is limited by the wavelength of the particles which are used for imaging, the FIM is a projection type microscope with atomic resolution and an approximate magnification of a few million times.