Field ion microscope

The Field ion microscope (FIM) was invented by Müller in 1951 It is a type of microscope that can be used to image the arrangement of atoms at the surface of a sharp metal tip.

On October 11, 1955, Erwin Müller and his Ph.D. student, Kanwar Bahadur (Pennsylvania State University) observed individual tungsten atoms on the surface of a sharply pointed tungsten tip by cooling it to 21 K and employing helium as the imaging gas. Müller & Bahadur were the first persons to observe individual atoms directly.

In FIM, a sharp (<50 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 kilovolts is applied to the tip. Gas atoms 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 — 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.

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.

FIM like Field Emission Microscopy (FEM) consists of a sharp sample tip and a fluorescent screen (now replaced by a multichannel plate) as the key elements. However, there are some essential differences as follows:

Like FEM, the field strength at the tip apex is typically a few V/Å. The experimental set-up and image formation in FIM is illustrated in the accompanying figures.

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