-PRESENTED BY

A.Akila

1st yr M.TECH

Nano Science and Technology

Anna University –BIT Campus

Trichy

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.

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.

• 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.

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.

Basic technique of preparing sample is electro

polishing technique.

Advanced techniques - focused ion beam technique.

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.

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.

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.

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

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.