1- Introduction to the Atom probe

Tomographic Atom Probe is a quantitative technique that provides atomic scale 3D elemental mapping of chemical heterogeneities in materials. As with STM (scanning tunneling microscopy), a single atom and its neighbours can be imaged. But the Atom Probe provides two big plusses:
    - Elemental analysis: each single atom is chemically identified.
    - Depth resolution, which makes the chemical map of the atoms truly 3D.
CAMECA is marketing, manufacturing, installing and servicing the LA-WATAP Atom Probe instrument, based on a collaboration and transfer technology agreement with the GPM (Groupe de Physique des Matériaux), at Rouen University, France. In this instrument, single atoms are field-evaporated from the sample surface and projected toward a position-sensitive detector. Atoms are chemically identified by Time-Of-Flight Mass Spectrometry. The in-depth investigation of the material reveals the 3D-distribution of atoms in the material on an atomic scale.
The sample is prepared in the form of a very sharp tip (radius = 10-70 nm). For metallic samples electro-erosion is used; for semiconductors Focused Ion Beam (FIB) milling is generally used: a rectangle of the wafer is protected with a metallization, a slice of sample of a few micron size is FIB-cut, extracted and attached (by FIB-metal deposition) in-situ onto a pilar support. It is then finely FIB-milled in a conical shape to obtain a sharp tip.
This tip is maintained under UHV condition at 15-70° K temperature during the analysis, to ensure reliable composition information.

 2- Atom probe application examples

Cottrel atmosphere: tube of boron located around a dislocation in a low Al content zone.	Nuclear science: 2nm copper enriched precipitates in Fe-1.4%Cu alloy irradiated with neutrons.	Nuclear science: nuclear reactor steel, precipitates.
Interraction of two Al and silver layers after thermal annealing: diffusion along grain boundaries.	Multilayer sample with depth profile reconstructed from an area of a few nanometers.	Thin wire (2nm in diameter) of copper in CuNb alloy (development for superconductive coil wires)

Courtesy for data above: University of Rouen, see the bottom of the page.
 
3- Working mode of the LASER- Assisted Atom Probe

The cooled tip is biased at a high DC voltage (5-20 kV) and positioned very near a counter electrode. The very small radius of the tip and the High Voltage induce a very high electrostatic field (tens MV/mm) at the tip surface, just below the point of atom evaporation.

A very short pulse of LASER on the tip gives 1) the extra energy required for evaporation of a few surface atoms 2) the Start signal for the flight  of the ions. Indeed under the high local electrostatic field, the evaporated atoms are immediately ionized by electron tunneling, and accelerated by the field into a Time Of Flight tube. They fly in Ultra High Vacuum and are projected straight on a Position-Sensitive Detector. The detector gives an accurate measurement of the ion impact positions and masses (= time of flight). The very high magnification of the instrument (about 10⁷) yields highly accurate (0.2 nm) impact coordinates, from which the atom original positions at the tip surface are derived. The time of flight gives the chemical nature of positioned ions. By repeating this sequence the atoms of tip are progressively removed and a 3D image of the material can be reconstructed on an atomic scale. The type of LASER technology used is crucial: CAMECA use a very fast (femto-second pulse) LASER in order to reduce the heating of the sample. Longer pulses (ex: pico-second LASER) degrade the mass resolution by thermal evaporation and can lead to redistribution/diffusion of the atoms during the pulse, specially if the sample thermal conductivity is low (semiconductors or insulators).  

  4- High Voltage mode
For conductive samples the LASER pulse can be replaced by a small HV pulse superimposed to the DC counter electrode HV. This technique is mature and has produced most of the results in metallurgy, nuclear and material science. The drawbacks are a relatively low rate of success as the HV pulsing induces fatigue on the sample often leading to a brakage of the tip during analysis. It is limited to conductive samples. This is why it is progressively being replaced by the more efficient and powerful LASER evaporation technique.

Courtesy for displayed data:
- Cottrel atmosphere: E. Cadel, university of Rouen. D. Blavette, E. Cadel, A. Fraczkiewicz, A.Menand, Science, 17 dec 1999, volume 286, p. 2317-2319.
-  Cu precipitate in Fe-Cul alloy : Ph. Pareige, univ. Rouen.
-  Irradiated nuclear reactor steel: Ph. Pareige, Univ. Rouen.
-  Ag/Al diffusion: Schleiwies & Schmitz, Göttingen univ.
-  Multi-layer deposition: Univ. Gottingen.
-  Cu nanowire: X. Sauvage, Univ. Rouen.