Etude par Sonde Atomique Tomographique de la formation de nano ...

tel-00751814, version 1 - 14 Nov 2012
Chapter 2. Materials, experimental and simulation techniques
Another important characteristic of an Atom Probe is its mass resolution. It characterizes
the capability of an instrument to resolve two neighbouring peaks on the mass spectrum. It is
convenient to define the mass resolution as the ratio between the peak position (in amu) and
the peak width at 10% of its height. Mass resolution depends on the type of AP and on
experimental conditions. This will be discussed in a next section.
II.2. Laser assisted field evaporation
As it was shown in previous section, conventional Atom Probes use electrical pulses in
order to evaporate surface atoms. In this case, analysis of materials that are brittle under the
applied electric field (which is the case of ODS and NDS samples obtained from powder
grains), could be very difficult. Indeed, these electrical pulses submit the material to cyclic
stress that frequently leads to the rupture of the specimen [19].
In the 1980`s Tsong and co-wokers designed a pulsed Laser Atom Probe (PLAP) [20,21].
This work was done on the basis of one-dimentional Atom Probe that differs from 3DAP by
much smaller field of view and the use of the detector non sensitive to the position. In the
PLAP, because of duration of Laser pulses (nanosecond or subnanosecond) the evaporation
field of atoms was strongly thermally assisted. It was shown that the peak temperature of the
specimen under nanosecond Laser pulse can reach 500 K [22]. In that case, the spatial
resolution has been degraded due to surface diffusion of atoms prior their field evaporation.
Since 2004, a new generation of Atom Probes, using femtosecond Laser pulses have been
developed in the GPM laboratory [19,23]. In this case poor conductive materials (ceramics,
oxide layers, etc…) as well as brittle samples can be analysed [24,25]. Another significant
advantage is the higher mass resolution of the Laser assisted TAP in comparison to electrical
[26].
As it was already mentioned, the evaporation of an atom is assisted either by i) increasing
the electric field at the tip surface (case of applied electrical pulses) or ii) by increasing the
temperature. Since few years, many efforts have been devoted to the understanding of
physical processes involved in field evaporation under action of femtosecond Laser. Up to
now, two models are generally accepted:
(i) Thermal pulse model (TMP). According to this model, a part of the Laser pulse energy
is absorbed in a fine area close to the tip apex, generating a localised thermal spike.
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