Transcript AFM - University of Oklahoma Physics & Astronomy

AFM
Atomic Force Microscopy
OU NanoLab/NSF NUE/Bumm & Johnson
Outline
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Motivation
History
How the AFM works
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OUr AFM
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The Good
The Bad
And the Ugly
Uses
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Pictures
Examples
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Two modes
Contact Mode
Non-Contact Mode
Force Measurements
Raster the Tip: Generating an Image
Scanning Sample
Topographical Analysis
Thin Layer Depth
RMS Roughness Calculations
Other types of Microscopy
OU NanoLab/NSF NUE/Bumm & Johnson
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
OU NanoLab/NSF NUE/Bumm & Johnson
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
OU NanoLab/NSF NUE/Bumm & Johnson
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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van der Waals force curve
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.
OU NanoLab/NSF NUE/Bumm & Johnson
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.
OU NanoLab/NSF NUE/Bumm & Johnson
van der Waals force curve
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).
OU NanoLab/NSF NUE/Bumm & Johnson
van der Waals force curve
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.
OU NanoLab/NSF NUE/Bumm & Johnson
Raster Motion
Scanning Tip
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.
Top Image Courtesy of Nanodevices, Inc. (www.nanodevices.com)
OU NanoLab/NSF NUE/Bumm & Johnson
Bottom Image Courtesy of Stefanie Roes
(www.fz-borstel.de/biophysik/ de/methods/afm.html)
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
OU NanoLab/NSF NUE/Bumm & Johnson
Image courtesy of (www.pacificnanotech.com)
OUr AFM
We have a commercial Topometrix Explorer AFM.
OU NanoLab/NSF NUE/Bumm & Johnson
Some of Our Pictures
2D topographical image of
Atomic Step
3D Image
Screw dislocations on InSb grown by MBE
OU NanoLab/NSF NUE/Bumm & Johnson
The Good Examples
View of Silicon Surface Reconstruction
OU NanoLab/NSF NUE/Bumm & Johnson
Carbon Nanotube Used as a Conducting
AFM Tip for Local Oxidation of Si.
Right Image Courtesy of Dai, et al. from Stanford
The Bad Examples
Histogram shows level surface, but
scan is very streaky
The horizontal lines are due to tip hops –
where the tip picks up or loses a small
“nanodust”
OU NanoLab/NSF NUE/Bumm & Johnson
Typically the sample will have a slight tilt
with respect to the AFM. The AFM can
compensate for this tilt.
In this image the tilt have not yet
been removed.
And the Ugly!
Teeny little dust mites, ultra tiny dust mites
about 2,000 in the average bed
OU NanoLab/NSF NUE/Bumm & Johnson
Image courtesy of http://www.micropix.demon.co.uk/sem/dustmite/article/page_2.htm
Topography Scanning
Example of generated
image upon scanning
Pd thermally evaporated on Si
OU NanoLab/NSF NUE/Bumm & Johnson
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.
(Height)
extreme
OU NanoLab/NSF NUE/Bumm & Johnson
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 Layer
of 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)
OU NanoLab/NSF NUE/Bumm & Johnson
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
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
OU NanoLab/NSF NUE/Bumm & Johnson
Images taken from Nanodevices, Inc. (www.nanodevices.com)
and Wooley, et al., Nature Biotech. 18, 760
SPM Lithography
 STM can move atoms around on a surface.
Iron on Copper
OU NanoLab/NSF NUE/Bumm & Johnson
Iron on Copper
Eigler, et al. from IBM
 Dip Pen Lithography.
SPM Lithography
OU NanoLab/NSF NUE/Bumm & Johnson
Mirkin, et al. from Northwestern University
SPM Lithography
 Electrochemistry: carbon nanotube used as a conducting AFM tip for local
oxidation of Si.
OU NanoLab/NSF NUE/Bumm & Johnson
Dai, et al. from Stanford
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)
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