Early developments

The field ion microscope (FIM) is the first microscope to have achieved atomic resolution. It is a surprisingly simple instrument (though the simplicity is somewhat deceptive), consisting of a sample in the form of a tip and a phosphorus screen some 10 cm away. To obtain an atomic image, one has only to introduce about 10−4 Torr of an image gas such as He into the system, cool down the sample tip below liquid nitrogen temperature and apply a positive voltage to the tip and raise the tip voltage gradually. When the field at the tip surface reaches about 4 V/Å, an atomic image will start to appear. This simple instrument is an outgrowth of field emission microscope, invented by Miiller in 1936. In a field emission microscope, the sample is also a sharp tip. In ultra-high vacuum, a negative voltage is applied to the tip. When the field at the nearly hemispherical tip surface reaches a value above ˜0.3 V/Å, electrons are emitted out of the surface by the quantum mechanical tunneling effect. These electrons are projected onto a phosphorus screen some 10 cm away to form an image of the surface. As the electron current density depends very sensitively on the work function of the surface, the greatly magnified, radial projection image of the field emitted electrons represents a map of the work function variation of the surface. These field emitted electrons originate mostly from the vicinity of the Fermi level. They have a relatively large kinetic energy and therefore a relatively large velocity component in the lateral direction of the surface.