Field ion microscope

The field ion microscope (FIM ) is an analyzer of materials science. It is a special microscope, which makes the arrangement of atoms on the visible surface of a sharp needle tip. The method by which individual atoms could be resolved for the first time, was developed by Erwin Müller, he developed his field electron microscope so on. Images of the atomic structure of tungsten were first published in 1951 in the Journal of Physics.

Operation

For the analysis of a sample in a field ion microscope, a sharp metal tip is produced and placed in a vacuum chamber, which is filled with an inert gas (e.g., helium or neon ). Are used noble gases, because one needs a higher electric field to ionize ( closed shells ). Higher electric field then also means higher resolution. The tip is cooled to a temperature between 20 and 100 K in order to reduce the anxiety of the atoms due to the thermal motion. A positive high voltage of 5 to 10 kV is applied between the tip and a detector, such as a combination of the microchannel plate and the phosphor screen is applied. A negative voltage at the tip would cause an unwanted field emission of electrons.

Gas atoms are ionized by the strong electric field near the tip (hence field ionization ), positively charged and repelled from the tip. The electric field strength must indeed be sufficiently large to allow Feldionisiation of the gas atoms, but its so small that a detachment of atoms of the tip surface ( field evaporation or field desorption ) is avoided.

In contrast to other types of microscope ( optical microscope, electron microscopy) in which the magnification can be adjusted by means of optical elements ( lenses), it depends on the field ion microscope is substantially the curvature of the tip surface, and the applied voltage. The resolution is in this case not adversely affected by aberrations of optical elements. A wave-optical resolution limitation is due to the small de Broglie wavelength of the ions thus practically. The ionized gas atoms are accelerated radial ( perpendicular to the surface ) from the top surface to the detector and provide a central projection of the top surface at a magnification which is in the range of 106 ... 105. This makes it easily possible to image individual atoms of the tip surface and lattice defects such as dislocations observed.