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

tel-00751814, version 1 - 14 Nov 2012
Chapter 2. Materials, experimental and simulation techniques
This temperature rise can initiate field evaporation by thermal activation during 1 to
10 ns [27,28].
(ii) Optical rectification model (OR). In this case, non linear response of the material is
considered. Interaction of the Laser with the material generated non symmetric
vibration of the electrons at the surface of the specimen. Thus, an electric field,
sufficient to promote field evaporation of surface atoms is generated. The pulsed field
induced by the OR may act exactly as a voltage pulse applied to the tip with sub-
picoseconds duration [29,30].
Recently, J. Houard and F. Vurpillot [31,32] have demonstrated that Laser assisted field
evaporation is clearly a thermal process in case of metals. Mechanisms occurring in the
insulators or semi-conductors are still under investigation.
II.3. Experimental devices and their performances
Experiments are carried out with two different Atom Probes: Laser-Assisted Tomographic
Atom Probe (LATAP) and Laser-Assisted Wide Angle Tomographic Atom Probe (LAWATAP).
Both of them are equipped with an advanced Delay Line position sensitive Detectors (aDLD).
Laser-Assisted Tomographic Atom Probe (LATAP) is known for its high mass resolution
that is achieved due to long distance between position sensitive detector and the specimen
(~40 cm). However, this technique allows a small volume of material to be analysed (up to
20×20×200 nm 3 ), limiting statistics. This instrument is equipped with a Ti:Sa femtosecond
Laser. Pulse length is 400 fs and the pulse repetition rate is equal to 2 kHz.
As for the Laser-Assisted Wide Angle Tomographic Atom Probe (LAWATAP), it is
equipped with a diode-pumped ytterbium femtosecond Laser with a pulse repetition rate of
100 kHz [23]. Undoubted benefit of this modification of TAP is that larger volume of
materials can be analysed (up to 60×60×200 nm 3 ) in comparison to LATAP. It is achieved by
reduction of the distance between position sensitive detector and the specimen (~20 cm).
However, the shorter flight-path lengths of ions lead to the degradation of the mass resolution.
The comparison of mass resolution of the LATAP and LAWATAP can be observed on
Figure 2.5.
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