Download Scanning Probe Microscopy (SPM)

Introduction to
Atomic Force
Microscopy
Dr. Janelle Gunther
March 10, 1998
ACS Group and MENs, Beckman
Institute
adapted from the world wide web page at http://www.di.com
Digital Instruments, Santa Barbara, CA
Scanning Probe
Microscopy (SPM)
A family of microscopy forms where a
sharp probe is scanned across a surface
and some tip/sample interactions are
monitored
Scanning tunneling Microscopy (STM)
Atomic Force Microscopy (AFM)
•
contact mode
•
non-contact mode
•
TappingMode
Other forms of SPM
•
Lateral force
•
Force modulation
•
Magnetic or electric force
•
surface potential
•
scanning thermal
•
phase imaging
Multimode SPM
General AFM
“Beam Deflection” Detection
A
B
Solid State
Laser Diode
Cantilever and Tip
Used for Contact Mode, Non-contact and
TappingMode AFM
Laser light from a solid state diode is reflected off
the back of the cantilever and collected by a
position sensitive detector (PSD). This consists of
two closely spaced photodiodes. The output is
then collected by a differential amplifier
Angular displacement of the cantilever results in
one photodiode collecting more light than the
other. The resulting output signal is proportional
to the deflection of the cantilever.
Detects cantilever deflection <1A
Piezoelectric Scanners
SPM scanners are made from a piezoelectric material
that expands and contracts proportionally to an applied
voltage.
Whether they expand or contract depends upon the
polarity of the applied voltage. Digital Instruments
scanners have AC voltage ranges of +220 to -220V.
0V
No applied voltage


+V
-V
Extended
Contracted
In some versions, the piezo tube moves the sample
relative to the tip. In other models, the sample is
stationary while the scanner moves the tip.
AC signals applied to conductive areas of the tube
create piezo movement along the three major axes.
Contact Mode AFM
A tip is scanned across the sample
while a feedback loop maintains a
constant cantilever deflection (and
force)
The tip contacts the surface through the
adsorbed fluid layer.
Forces range from nano to micro N in
ambient conditions and even lower (0.1
nN or less) in liquids.
Tapping Mode™ AFM
A cantilever with attached tip is
oscillated at its resonant frequency and
scanned across the sample surface.
A constant oscillation amplitude (and
thus a constant tip-sample interaction)
are maintained during scanning. Typical
amplitudes are 20-100nm.
Forces can be 200 pN or less
The amplitude of the oscillations
changes when the tip scans over
bumps or depressions on a surface.
Non-contact Mode AFM
The cantilever is oscillated slightly above its
resonant frequency.
Oscillations <10nm
The tip does not touch the sample. Instead, it
oscillates above the adsorbed fluid layer.
A constant oscillation amplitude is maintained.
The resonant frequency of the cantilever is
decreased by the van der Waals forces which
extend from 1-10nm above the adsorbed fluid
layer. This in turn changes the amplitude of
oscillation.
Advantages and
Disadvantages of the 3 main
types of AFM
Contact Mode

Advantages:
High scan speeds
The only mode that can obtain “atomic
resolution” images
Rough samples with extreme changes in
topography can sometimes be scanned more
easily

Disadvantages:
Lateral (shear) forces can distort features in the
image
The forces normal to the tip-sample interaction
can be high in air due to capillary forces from the
adsorbed fluid layer on the sample surface.
The combination of lateral forces and high
normal forces can result in reduced spatial
resolution and may damage soft samples (i.e.
biological samples, polymers, silicon) due to
scraping
Lateral Force
Microscopy
The probe is scanned sideways. The
degree of torsion of the cantilever is
used as a relative measure of surface
friction caused by the lateral force
exerted on the probe.
Identify transitions between different
components in a polymer blend, in
Lateral Force
Microscopy
Images/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA
Natural rubber/
EDPM blend
20 micron scan
Polished polycrystalle silicon
carbide film.
Grain structures
30 micron scan
Magnetic recording
head
Al oxide grains
and contamination
800nm scan
Phase Imaging
Accessible via TappingMode
Oscillate the cantilever at its resonant
frequency. The amplitude is used as a
feedback signal. The phase lag is
dependent on several things, including
composition, adhesion, friction and
viscoelastic properties.



Identify two-phase structure of polymer blends
Identify surface contaminants that are not seen in
height images
Less damaging to soft samples than lateral force
microscopy
Phase Imaging
Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA
Compositepolymer
imbedded in a matrix
1 micron scan
Bond pad on an
integrated circuit
Contamination
1.5 micron scan
MoO3 crystallites
on a MoS2 substrate
6 micron scan
Magnetic Force
Microscopy
Special probes are used for MFM. These are
magnetically sensitized by sputter coating with a
ferromagnetic material.
The cantilever is oscillated near its resonant frequency
(around 100 kHz).
The tip is oscillated 10’s to 100’s of nm above the
surface
Gradients in the magnetic forces on the tip shift the
resonant frequency of the cantilever .
Monitoring this shift, or related changes in oscillation
amplitude or phase, produces a magnetic force image.
Many applications for data storage technology
Magnetic Force
Microscopy
Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA
Overwritten tracks on a textured hard disk, 25 micron scan
Domains in a 80 micron garnet film
LiftMode
•Two passes are made over the sample. The first measures
topography while the second measures a material property
(magnetic, electric, etc.)
•Eliminates “cross-contamination” of the images.
Force Modulation
Imaging
Oscillate the cantilever vertically at a rate that is
significantly faster than the scan rate.
The amplitude of the oscillations changes in response
to the sample stiffness.
Used in conjunction with LiftMode to separate
topography and elasticity data.
Force Modulation
Imaging
Other Techniques
Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA
Scanning capacitance microscopy

Apply a constant amplitude sine wave voltage to
the sample. The image is then reconstructed from
the changes in the amplitude of the capacitance
oscillations.

Location of defects in wafers (pinning of electrical
carriers)

Image carrier concentration
Scanning thermal microscopy

Uses a temperature sensitive probe with a special
holder.

Location of “hot-spot” defects in semiconductor
wafers
90 micron
scan size
Other Techniques
Nanoindenting and scratching

A diamond tip is mounted on a metal cantilever and
scanned either with contact or TappingMode.

Indenting mode presses the tip into the sample

Scratch mode drags the tip across the sample at a
specific rate and with a specified force.

The use of TappingMode makes it possible to
simultaneously image soft samples.
Imaging of biological samples

cells, DNA (TappingMode in solution)