AFM

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AFM

• Atomic Force Microscopy

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Outline

• Motivation
• History
• How the AFM works
• 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

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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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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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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.

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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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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.

van der Waals force curve
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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
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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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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.
(www.nanodevices.com) Bottom Image Courtesy of
Stefanie Roes (www.fz-borstel.de/biophysik/
de/methods/afm.html)
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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

Image courtesy of (www.pacificnanotech.com)
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OUr AFM
We have a commercial Topometrix Explorer AFM.
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Some of Our Pictures
2D topographical image of Atomic Step
3D Image
Screw dislocations on InSb grown by MBE
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The Good Examples
Carbon Nanotube Used as a Conducting AFM Tip for
Local Oxidation of Si.
View of Silicon Surface Reconstruction
Right Image Courtesy of Dai, et al. from Stanford
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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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And the Ugly!
Teeny little dust mites, ultra tiny dust
mites about 2,000 in the average bed
Image courtesy of http//www.micropix.demon.co.uk/
sem/dustmite/article/page_2.htm
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Topography Scanning
Example of generated image upon scanning Pd
thermally evaporated on Si
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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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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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Thickness of a Thin Layerof Pd on Si Wafer
Si/Pd step
Step (where Pd coating ends)
Systematic error
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Surface Roughness
Roughness typically measured as root mean squared
(RMS)
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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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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
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Carbon Nanotube 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.
(www.nanodevices.com) and Wooley, et al., Nature
Biotech. 18, 760
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SPM Lithography

• STM can move atoms around on a surface.

Iron on Copper
Iron on Copper
Eigler, et al. from IBM
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SPM Lithography

• Dip Pen Lithography.

Mirkin, et al. from Northwestern University
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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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Million Cantilever Wafer
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Millipede Memory
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Millipede Memory
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Cantilever Gas Sensors (Noses)
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Cantilever Gas Sensors (Noses)