Atomic Force Microscope

Fundamental Principles

-Joy BhattacharjeeIIT Kanpur.

Co-Founder & Director,Kanopy Techno Solutions

MicroscopesOptical

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

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)

Working of AFM

Working of AFM: Block Diagram

Significant ForcesCapilla

ry

Van Der

Waals Chemical Bond

Electro-Static

Magnetic

Casimir

Solvation

Force in action

Operation

Signal for Post-Processing and Feedback

Non-Contact

TappingContact

Contact ModeIn 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

Feedback

Without Feedback With Feedback

Contact Mode

Tapping ModeIn 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

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

Effect of Shape of Tip

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

Details of Parts of AFMShapes of AFM Tip

Details of Parts of AFMShapes of AFM Tip

Protruding from the Very

End

Positioned at the Very End

Square-Based Pyramid

Rectangular-based

Pyramid

Circular Symmetric Spike

Details of Parts of AFMHigh Aspect Ratio Spike

AFM Tips

Focused Ion Beam

Electron Beam Deposited

Carbon Nanotube

Plateau Rounded Sphere

Critical Dimension

Details of Parts of AFMScanner

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.

Details of Parts of AFMScanner

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

Details of Parts of AFMFeedback

The feedback system is affected by three main parameters:

1. Setpoint2. Feedback gains3. Scan rate

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.

Artefacts in AFMScanner Related

Hysteresis

The piezoelectric’s response to an applied voltage is not linear. This gives rise to hysteresis.

Artefacts in AFMScanner 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

Artefacts in AFMScanner 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.

Artefacts in AFMTip Related

Blunt tip: Use Feedback Mode

Tip picks up debris: Cleaning the sample with compressed air or N2 before use

Artefacts in AFMFeedback 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

Artefacts in AFMVibration 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.

Beyond just surfaceSeeing the atomic orbital

Beyond just surfaceSeeing 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.

Beyond just surfaceSeeing the reactionWork 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