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

tel-00751814, version 1 - 14 Nov 2012
Chapter 2. Materials, experimental and simulation techniques
where Ni is the number of atoms of the element i and Nt is the total number of atoms in the
volume. The uncertainty of the concentration estimate due to counting statistics given by:
C ( 1�
C)
2�
� 2
(2.5.)
N
The detector on which evaporated ions arrive is a position sensitive Advanced Delay Line
Detector (aDLD) [17]. It consists in two microchannel plates (located in front of aDLD
detector), which amplify the signals by creating electron bundle for each ion (see Figure 2.2.).
This electron bundle creates electric signal in vertical and horizontal wire windings. The
electrical signal propagates up to the end of each wire. The recording of signals in each wire
end allows to measure difference of propagation delay between the top, the bottom, the left
and the right of the detector and to deduce accurately the position of impact. The aDLD also
allows the deconvolution of coordinates if group of ions arrive at the same time.
Once the position of ion impacts (x, y) on the detector is known, the coordinate of the
atoms at the specimen surface may be computed by inverse projection. As shown in Figure
2.4, the image magnification is given by:
G
L
( m �1)
R
t
� (2.6.)
where m is the image compression factor related to the position of the projection point P. R is
the tip radius and L is the free flight length of an ion (the estimated specimen to detector
distance).
Specimen
P O
(m+1)R
R
(x', y')
Detector
Figure 2.4. Shows that the derivation of atom positions from impact coordinates
simply involves a point projection P.
L
(x,y)
d
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