Editing Field ion microscope (section)

==Introduction==

In FIM, a sharp (<50&nbsp;nm tip radius) metal tip is produced and placed in an [[ultra high vacuum]] chamber, which is backfilled with an imaging gas such as [[helium]] or [[neon]].  The tip is cooled to cryogenic temperatures (20–100 K). A positive [[voltage]] of 5 to 10 kilo[[volt]]s is applied to the tip.  Gas atoms [[adsorption|adsorbed]] on the tip are ionized by the strong [[electric field]] in the vicinity of the tip (thus, "field ionization"), becoming positively charged and being repelled from the tip.  The curvature of the surface near the tip causes a natural magnification — [[ion]]s 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.

Unlike conventional microscopes, 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.