Field ion microscopy (FIM) is an analysis technique in materials science. The field ion microscope is a kind of microscope which can be used to image the arrangement of atoms at the surface of a sharp metal tip. It was the first technique by which individual atoms could be spatially resolved. The technique was pioneered by Erwin Müller. Images of atomic structures of tungsten were first published in 1951 in the journal Zeitschrift für Physik.
In FIM, a sharp metal tip is produced and placed in a vacuum chamber which is backfilled with an imaging gas such as helium or neon. The tip is cooled to cryogenioc temperatures ( < 50K) . A positive voltage on the order of 5,000 volt to 10,000 volt is applied to the tip. Gas atoms adsorbed on the tip can be ionized by the strong electric field in the vicinity of the tip ( thus "field ionization"). These atoms are then positively charged and are repelled from the tip. The curvature of the surface near the tip causes a natural magnification -- ions are repelled in a direction roughly perpendicular to the surface ( a 'point projection' effect ). A detector is placed so as to collect these repelled ions: The image formed from all the collected ions can be of sufficient resolution to image individual atoms on the tip surface.
There is a common misunderstanding about the resolution of a microscope, that the best possible spatial resolution of a microscope is on the order of the wavelength of the particles which are used for imaging. While this is true for many types of microscopes, it is not true for the FIM or other microscopes where the magnification is due to point projection.