CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

the surface of the sample while the tip movements normal to the surface are measured. The
deflections occurred in the tip, due to the interaction forces between the tip and sample
surface, can be measured by focussing a laser beam onto cantilever and detecting the
reflected light from the cantilever using a position sensitive detector. Schematic diagram of
atomic force imaging is shown in Fig 15 (a). As the tip is rastered over the surface, a
feedback mechanism is employed to ensure that the piezo-electric motors maintain a constant
tip force or height above the sample surface. The tip movements normal to the surface are
digitally recorded and can be processed and displayed in three-dimensions by a computer.
This technique has a lateral resolution of 1 to 5 nm. AFM is typically used to obtain a threedimensional
surface image or to determine the surface roughness of thin films and crystal
grains. There are mainly two types of AFM modes, namely the contact mode and the semi
contact/taping mode, which are used for imaging the samples. The schematic representation
of the different types forces acting between tip and sample in the two methods of imaging is
shown in Fig.15 (b) and are described below.
Tip
cantilever
(a)
(b)
Fig.15. (a) Principle of AFM imaging, (b) variation of interaction force versus distance
between the AFM tip and substrate.
(1) Contact mode: Here, the force between the probe tip and substrate is repulsive, and it is
within the range of 10 -8 to 10 -7 N. The force is set by pushing a cantilever against the sample
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