A first principle study of the atomic force microscope focussing on the forces being measured and their implication in experimentation.

1. Atomic Force Microscope
Fundamental Principles
-Joy Bhattacharjee
IIT Kanpur.
Co-Founder & Director,
Kanopy Techno Solutions

2. Microscopes
Optical
Simple
Compound
Stereoscopic
Confocal
X-Ray
STXM
TXM
XPEEM
Electron
TEM
SEM
STEM
REM
Probe
STM
EC-STM
MFM
CFM
EFM
AFM
C-AFM
Acoustic
SAM
C-SAM
Neutron
Invented by IBM Scientists in 1986
Gerd Binnig and
Heinrich Rohrer

3. AFM
The AFM (center) has inspired a variety of other scanning probe techniques. Originally the AFM was used to image
the topography of surfaces, but by modifying the tip it is possible to measure other quantities (for example, electric
and magnetic properties, chemical potentials, friction and so on), and also to perform various types of spectroscopy
and analysis. (Image: Christoph Gerber; copyright Nature Publishing Group)

4. Working of AFM

5. Working of AFM: Block Diagram

6. Significant Forces
Capillary
Van Der
Waals Chemical
Bond
Electro-
Static
Magnetic

7. Force in action

8. Operation
Signal for Post-Processing and Feedback
Non-
Contact
Tapping
Contact

9. Contact Mode
In contact mode the tip contacts the surface through the adsorbed fluid layer on the
sample surface.
The feedback circuit adjusts the probe height to try and maintain a constant force and
deflection on the cantilever. This is known as the deflection setpoint.
F = − k x (F = force, k = spring constant, x = cantilever deflection)
Advantage Disadvantage
Contact Mode • High scan speeds
• Rough samples with extreme changes
in vertical topography can sometimes
be scanned more easily
• Lateral (shear) forces may distort features in
the image
• In ambient conditions may get strong
capillary forces due to adsorbed fluid layer
• Combination of lateral and strong normal
forces reduce resolution and mean that the
tip may damage the sample, or vice versa

10. Feedback
Without Feedback With Feedback
Contact Mode

11. Tapping Mode
In tapping mode the cantilever oscillates at or slightly below its resonant frequency.
The resonant frequency of the cantilever is dependent on this separation.
The oscillation is also damped when the tip is closer to the surface. The feedback
circuit adjusts the probe height to try and maintain a constant amplitude of oscillation
i.e. the amplitude setpoint.
Advantage Disadvantage
Tapping Mode • Lateral forces almost eliminated
• Higher lateral resolution on most
samples
• Lower forces so less damage to soft
samples or tips
• Slower scan speed than in contact mode

12. Non-Contact Mode
• In non-contact mode the cantilever oscillates near the surface of the sample, but does
not contact it. The oscillation is at slightly above the resonant frequency.
• In ambient conditions the adsorbed fluid layer is often significantly thicker than the
region where van der Waals forces are significant. Therefore non-contact mode AFM
works best under ultra-high vacuum conditions.
Advantage Disadvantage
Non-Contact
Mode
• Both normal and lateral forces are
minimized, so good for
measurement of very soft samples
• Can get atomic resolution in a
UHV environment
• In ambient conditions the adsorbed
fluid layer may be too thick for
effective measurements
• Slower scan speed than tapping and
contact modes to avoid contacting the
adsorbed fluid layer

13. Effect of Shape of Tip

14. Details of Parts of AFM
Property Typical Value Desired Quality
Material Silicon, Silicon Nitride,
Silicon Oxide
Hard, Unreactive
Tip Radius < 10 nm Small
Tip Height 15-20 µm Mechanically stable
Cantilever
Length
100-250 µm Appropriate reach
Mean Width 20 – 70 µm Mechanically stable
Half Cone Angle 25° Sample dependent
Base Shape configurable Sample dependent
Apex Shape configurable Sample dependent
Resonant
Frequency
Several kHz, depends
on shape
Matching piezo’s
resonant frequency
Coating None, Gold, Platinum,
Diamond
Experiment
dependent
Cantilever and Tip

15. Details of Parts of AFM
Shapes of AFM Tip

16. Details of Parts of AFM
Shapes of AFM Tip
Protruding
from the Very
End
Positioned at
the Very End
Square-Based
Pyramid
Rectangular-
based
Pyramid
Circular
Symmetric
Spike

17. Details of Parts of AFM
High Aspect Ratio
Spike AFM Tips
Focused Ion
Beam
Electron Beam
Deposited
Carbon
Nanotube
Plateau Rounded Sphere
Critical
Dimension

18. Details of Parts of AFM
Scanner
In most AFMs piezoelectric materials are used to achieve this. These change dimensions
with an applied voltage. The diagram below shows a typical scanner arrangement.

19. Details of Parts of AFM
Scanner
The presence of electrical resonances and anti-resonances make the piezoelectric
impedance unique. The resonances result from the electrical input signal exciting a
mechanical resonance in the piezo element.
Equivalent Circuit Model

20. Details of Parts of AFM
Feedback
The feedback system is affected by three main parameters:
1. Setpoint
2. Feedback gains
3. Scan rate

21. Optical AFM
• Advanced Surface Topography technique avoids cantilever mechanism by use of optical
fiber based tips and using Fabry–Pérot Interferometry (or Etalon):
There is only one limitation of such an approach: surface of the sample should be
smooth enough and homogeneously reflecting.

22. Artefacts in AFM
Scanner Related
Hysteresis
The piezoelectric’s response to an applied voltage is not linear. This gives rise
to hysteresis.

23. Artefacts in AFM
Scanner Related
Scanner creep
If the applied voltage suddenly changes, then the piezo-scanner’s response is not all
at once. It moves the majority of the distance quickly, then the last part of the
movement is slower. This slow movement will cause distortion, known as creep.
Change in x-offset Change in y-offset Change in size

24. Artefacts in AFM
Scanner Related
Bow and Tilt
Because of the construction of the piezo-scanner, the tip does not move in a perfectly flat
plane. Instead its movement is in a parabolic arc (scanner bow). Also the scanner and sample
planes may not be perfectly parallel (tilt). Both of these artefacts can be removed by using
post-processing software.

25. Artefacts in AFM
Tip Related
Blunt tip: Use Feedback Mode
Tip picks up debris: Cleaning the sample with compressed air or N2 before use

26. Artefacts in AFM
Feedback Related
Poor tracking due to high scan rate
Gains are set too high, then the
feedback circuit can begin to
oscillate. This causes high
frequency noise

27. Artefacts in AFM
Vibration Related
AFMs are very sensitive to external mechanical vibrations, which generally show up as
horizontal bands in the image. These can be minimised by the use of a vibrational
isolation table, and locating the AFM on a ground floor or below.
Acoustic noise such as people talking can also cause image artefacts, as can drafts of
air. An acoustic hood can be used to minimise the effects of both of these.

28. Beyond just surface
Seeing the atomic orbital

29. Beyond just surface
Seeing the atomic orbital
Ref: Minghuang Huang, Martin Cuma, and Feng Liu. (27 June, 2003). Seeing the Atomic Orbital: First-
Principles Study of the Effect of Tip Termination on Atomic Force Microscopy. Physical Review Letters.
Volume 90, Number 25.

30. Beyond just surface
Seeing the reaction
Work done by Franz J. Giessibl at the Department of Physics,
University of Regensburg have been success to image
chemical reaction using AFM by having a carbon monoxide
molecule at the tip to obtain high spatial resolution.
Ref: Science Vol 340, 21 JUNE 2013