this will be useful to those were instrested in imaging techniques and charecterising the samples at nano scale

1. -PRESENTED BY
A.Akila
1st yr M.TECH
Nano Science and Technology
Anna University –BIT Campus
Trichy

2.  Atom probe field ion microscopy is a combination
of field ion microscopy and time of flight mass
spectrometer.
 FIM is a projection type microscope of atomic
resolution.
 Time of flight : atom go through a small apecture
in the centre of screen makes it possible to
determine the mass to charge ratio.
 The atom probe microscope operates by removing
and analyzing individual atoms.

4.  The precursor to the atom probe microscope was
the field ion microscope (FIM)
 During FIM analysis, gas atoms adsorbed to the
surface of a sharply pointed specimen are ionized
by means of a strong electric field.
 They are then accelerated toward a phosphor
screen or other position-sensitive detector placed at
some distance from the specimen.

5. • Extremely high spatial resolution
• Equal detection efficiency for light elements
• The atom probe’s unique atom-by-atom analysis
provides a map of the elemental and isotopic
identity and position of individual atoms in
volumes of material of up to 100 nm in diameter
and 100 nm in depth.

6.  The atom probe analyzes three-dimensional
volumes of material, whereas the AFM analyzes
surface features only.
 The atom probe provides both imaging and
chemical analysis, whereas AFM provides imaging
only.

9.  Basic technique of preparing sample is electro
polishing technique.
 Advanced techniques - focused ion beam technique.

12.  Image is produced by applying high voltage to specimen
with respect to channel plate screen in presence of
imaging gas .
 The field strength required at the sample surface
reaches approximately 50v/nm.
 In order to achieve this order of magnitude needle
shaped specimens are prepared with average radius of
curvature R=10-100nm.
 The whole setup is kept in ultra high vacuum at a
pressure of 10ˆ-7 pa.
 During the operation tip is kept at temperature of 10-
150k
 Imaging system comprises of micro channel plate and
phosphor screen.

13.  The applied field at which field image results is called Best
Image Field (BIF) and the corresponding voltage is called
Best Image Voltage(BIV).
 Advantages of field evaporation
> Removal of oxides and impurities on the surface of
sample.
> smoothens the irregular surface of the sample until
the hemispherical surface is obtained.

16.  Chemical identification of the sample is achieved by
measuring the time of flight when used in combination
with the FIM.
 Difference between the BIF and evaporation field strength
is created by superimposing a short pulse Vp with respect
to width and amplitude of to that of the permanent
BIV.
 Vp must be high enough to remove the specimen atoms by
field evaporation.
 Pulse fraction is the ratio of pulse to permanent voltage.
 By the correction of selection of Vp and setting of the
analysis temperature the preferential evaporation of any
constituent with respect to the others is avoided and a
reliable analysis can be achieved.

17.  Synchron pulse in TDC (Time to digital converter) is used
to reset and start the clock
 When the field evaporation ion reaches the drift velocity ,
surface atoms are evaporated
 After the time of flight T , the ion hits the detector and the
pulse is used to stop the clock.
 Conversion of energy for drifting ion is given by
 From the above relation mass to charge ratio can be
measured for the field evaporation ion
 So the whole analysis is performed by adjusting the pulse
frequency
 The projection of probe hole onto the surface of the tip
defines the diameter of the sample and the length of the
sample is given by the hkl plane

18.  The resolution depends on the shape of applied pulse and
kind of TOF.
 The information obtained from the data is that only the
composition of the sample and it could not specify the
different distribution of the constituents.
 This can be overcome by using position sensitive detector.