Atom probe

The atom probe is an analyzer of materials science. It enables the identification of the mass of individual atoms, which are followed by a very sharp tip of an electrically conductive material by field evaporation. Is it possible by using a suitable detector to obtain three-dimensional data of many atoms, so they are referred to as tomographic atom probe (s: tomographic atom probe TAP) or three-dimensional atom probe ( 3DAP ). Although there are atomic probes, which allow only a one-dimensional analysis, the probe is usually meant tomographic atom.

Operation

One can understand the TAP as a development or amendment of the field ion microscope, the experimental setup is similar. A very sharp tip of an electrically conductive material having a tip radius of the order of 10 to 100 nm is produced electrochemically or by focused ion beam system ( FIB). Under ultra high vacuum conditions and at temperatures at the head of 20 to 100 K, an electrical field is applied with a voltage of 2 to 15 kV, the field strength is not sufficient to cause detachment of the atoms ( field evaporation ) from the top. In addition to this base voltage, a very short voltage pulse of the order of 10 to 25% of the base voltage is applied, so that the field strength is sufficient short in order to cause field evaporation of atoms. This pulse is so short that on average only once every 100 pulses is replaced an atom. Is the number of the detached atoms is too low or too high, the base voltage is varied during the measurement. The separated positively charged ion atom is directed through the electric field to a detector. Since the time at which it was replaced ( the time of the last voltage pulse ) is known, from the flight time of the mass of the atom be determined ( as with other flight mass spectrometers ). The x and y position of the atoms can be determined from the arrival at the detector. Determining the z- position, the order of arrival of atoms is used. Later arrived atoms were located further down inside the tip as previously arrived atoms. In addition to this simple principle still corrections to be made, which are due to the (usually assumed to be semi- spherical) tip geometry.

The position determination in the z- direction is accurate enough to distinguish individual lattice planes in the reconstructed data ( a few hundredths of nanometers). The resolution in the x and y direction is a few tenths of nanometers.

A total of several million atoms can thereby be measured at the latest atomic probes, which are characterized in particular by large detector areas, 50 million atoms per measurement are possible. This corresponds to a volume of several hundred cubic nanometers.

Areas of application

Some applications of the TAP ( not a complete list )

• Multilayer systems (for example, TMR, GMR systems)
• Cluster precipitates, especially in early stages, not examinable by transmission electron microscopy
• Order determining

Examine can in principle only materials with a certain electrical conductivity. With additional laser pulses is also less conductive materials can be investigated.