1. Atomic Force Microscopy
AFM
Atomic Force Microscopy
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2. Outline Motivation History How the AFM works OUr AFM Examples Uses
Two modes
Contact Mode
Non-Contact Mode
Force Measurements
Raster the Tip: Generating an Image
Scanning Sample
OUr AFM
Pictures
Examples
The Good
The Bad
And the Ugly
Uses
Topographical Analysis
Thin Layer Depth
RMS Roughness Calculations
Other types of Microscopy
Find SEM of cockroach
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3. Motivation Digitally image a topographical surface
Determine the roughness of a surface sample or to measure the thickness of a crystal growth layer
Image non-conducting surfaces such as proteins and DNA
Study the dynamic behavior of living and fixed cells
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4. History
The Scanning Tunneling Microscope (STM) was invented by G. Binnig and H. Rohrer, for which they were awarded the Nobel Prize in 1984
A few years later, the first Atomic Force Microscope (AFM) was developed by G. Binnig, Ch. Gerber, and C. Quate at Stanford University by gluing a tiny shard of diamond onto one end of a tiny strip of gold foil
Currently AFM is the most common form of scanning probe microscopy
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5. How the AFM Works
The AFM brings a probe in close proximity to the surface
The force is detected by the deflection of a spring, usually a cantilever (diving board)
Forces between the probe tip and the sample are sensed to control the distance between the the tip and the sample.
van der Waals force curve
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6. Two Modes Repulsive (contact)
At short probe-sample distances, the forces are repulsive
Attractive Force (non-contact)
At large probe-sample distances, the forces are attractive
The AFM cantelever can be used to measure both attractive force mode and repulsive forces.
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7. Non-Contact Mode Uses attractive forces to interact surface with tip
Operates within the van der Waal radii of the atoms
Oscillates cantilever near its resonant frequency (~ 200 kHz) to improve sensitivity
Advantages over contact: no lateral forces, non-destructive/no contamination to sample, etc.
Find out where the image is actually from
van der Waals force curve
OU NanoLab/NSF NUE/Bumm & Johnson

8. Contact Mode
Contact mode operates in the repulsive regime of the van der Waals curve
Tip attached to cantilever with low spring constant (lower than effective spring constant binding the atoms of the sample together).
In ambient conditions there is also a capillary force exerted by the thin water layer present (2-50 nm thick).
van der Waals force curve
OU NanoLab/NSF NUE/Bumm & Johnson

9. Force Measurement
The cantilever is designed with a very low spring constant (easy to bend) so it is very sensitive to force.
The laser is focused to reflect off the cantilever and onto the sensor
The position of the beam in the sensor measures the deflection of the cantilever and in turn the force between the tip and the sample.
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10. Raster the Tip: Generating an Image
The tip passes back and forth in a straight line across the sample (think old typewriter or CRT)
In the typical imaging mode, the tip-sample force is held constant by adjusting the vertical position of the tip (feedback).
A topographic image is built up by the computer by recording the vertical position as the tip is rastered across the sample.
Scanning Tip
Raster Motion
Top Image Courtesy of Nanodevices, Inc. (
Bottom Image Courtesy of Stefanie Roes
( de/methods/afm.html)
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11. Scanning the Sample
Tip brought within nanometers of the sample (van der Waals)
Radius of tip limits the accuracy of analysis/ resolution
Stiffer cantilevers protect against sample damage because they deflect less in response to a small force
This means a more sensitive detection scheme is needed
measure change in resonance frequency and amplitude of oscillation
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Image courtesy of (

12. OUr AFM We have a commercial Topometrix Explorer AFM.
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13. Some of Our Pictures 2D topographical image of Atomic Step 3D Image
Screw dislocations on InSb grown by MBE
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14. The Good Examples Carbon Nanotube Used as a Conducting
AFM Tip for Local Oxidation of Si.
View of Silicon Surface Reconstruction
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Right Image Courtesy of Dai, et al. from Stanford

15. The Bad Examples
Histogram shows level surface, but scan is very streaky
Typically the sample will have a slight tilt with respect to the AFM. The AFM can compensate for this tilt.
The horizontal lines are due to tip hops – where the tip picks up or loses a small “nanodust”
In this image the tilt have not yet been removed.
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16. And the Ugly! Teeny little dust mites, ultra tiny dust mites
about 2,000 in the average bed
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Image courtesy of

17. Topography Scanning Example of generated image upon scanning
Pd thermally evaporated on Si
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18. Elimination of Extreme Points
This targets the highest points of the sample and eliminates them
It then manipulates the image to create a smaller dynamic depth
Centering on pt.
extreme
(Height)
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19. A Better View Now: Removed extreme points
Digitally decreased the height of analysis
Less than 1/3 as high as initial scan
Lose resolution and data by clipping off extreme points
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20. Thickness of a Thin Layer of Pd on Si Wafer
Si/Pd step
Step (where Pd coating ends)
Systematic error
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21. Surface Roughness
Roughness typically measured as root mean squared (RMS)
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22. Other Types of SPM Techniques
Lateral Force Microscopy (LFM)
Frictional forces measured by twisting or “sideways” forces on cantilever.
Magnetic Force Microscopy (MFM)
Magnetic tip detects magnetic fields/measures magnetic properties of the sample.
Electrostatic Force Microscopy (EFM)
Electrically charged Pt tip detects electric fields/measures dielectric and electrostatic properties of the sample
Chemical Force Microscopy (CFM)
Chemically functionalized tip can interact with molecules on the surface – giving info on bond strengths, etc.
Near Field Scanning Optical Microscopy (NSOM)
Optical technique in which a very small aperture is scanned very close to sample
Probe is a quartz fiber pulled to a sharp point and coated with aluminum to give a sub-wavelength aperture (~100 nm)
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23. SPM techniques (NT-MDT) visit these links for animations
STM modes
constant current
constant height
AFM modes
contact
non-contact
SPM lithography
STM lithography
AFM lithography – scratching
AFM lithography – Dynamic Plowing
OU NanoLab/NSF NUE/Bumm & Johnson

24. Carbon Nanotube Tips DNA CNT Tips Well defined shape and composition.
High aspect ratio and small radius of curvature (“perfect” tip would be a delta function tip).
Mechanically robust.
Chemical functionalization at tip.
DNA
CNT Tips
Images taken from Nanodevices, Inc. (
and Wooley, et al., Nature Biotech. 18, 760
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25. SPM Lithography Iron on Copper Iron on Copper
STM can move atoms around on a surface.
Iron on Copper
Iron on Copper
Eigler, et al. from IBM
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26. SPM Lithography Dip Pen Lithography. OU NanoLab/NSF NUE/Bumm & Johnson
Mirkin, et al. from Northwestern University

27. SPM Lithography
Electrochemistry: carbon nanotube used as a conducting AFM tip for local oxidation of Si.
Dai, et al. from Stanford
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28. Million Cantilever Wafer
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29. Millipede Memory
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30. Millipede Memory
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31. Cantilever Gas Sensors (Noses)
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32. Cantilever Gas Sensors (Noses)
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