Literature review: AC Impedance measurements using atomic force microscopy

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Literature reviewAC Impedance measurements
using atomic force microscopy

• Anne Clemencon
• January 21st, 2005

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Reviewed articles

•  Resistance measurements at the nanoscale
  scanning probe ac impedance spectroscopy 
• A. Layson, S. Gadad, D. Teeters (The University
  of Tulsa)
• Electrochimica Acta 48 (2003) 2207-2213
•  Local impedance imaging and spectroscopy of
  polycrystalline ZnO using contact atomic force
  microscopy 
• R. Shao, S. Kalinin, and D. Bonnell (University
  of Pennsylvania, ONL)
• Applied Physics Letters 82 (2003) 1869-1871
•  Ionic and electronic impedance imaging using
  atomic force microscopy 
• R. OHayre, M. Lee, F. Prinz (Stanford
  University)
• Journal of Applied Physics 95 (2004) 8382-8392

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AC impedance

• Apply an alternating voltage perturbation to a
  sample, and measure the current response
• E(t)Eo ei?t I(t)Ioei(?t-F)
• Z(?)E(?)/I(?) Z(?) i Z(?)
• Impedance spectroscopy measure impedance for a
  range of ?.

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Motivation
A number of AFM techniques exist for studying
local electronic properties of materials Conducti
ve AFM, Tunneling AFM, Scanning Surface Potential
Microscopy, Scanning Spreading Resistance
Microscopy, Scanning Capacitance Microscopy. All
of these techniques use dc potentials or ac
potential with a single frequency. AFM probes
can have tip diameters of 20nm or smaller. They
can measure local electronic properties on the
surface with nanoscale resolution. Microprobe AC
impedance measurements cannot achieve as good a
resolution as AFM.
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1  Resistance measurements at the nanoscale
scanning probe ac impedance spectroscopy  A.
Layson, S. Gadad, D. Teeters
Experimental setup The surface topography is
first measured by AFM. The tip is then brought in
contact with the surface of the sample, and ac
impedance measurements are taken.
The AFM tip is used as an electrode. The
substrate is used as the other electrode. Tips
silicon AFM tips sputter-coated with chromium
and gold nominal force constant 0.2 N
m-1 typical tip diameter 20 nm AC impedance
measurements 100 MHz - 400 Hz
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1  Resistance measurements at the nanoscale
scanning probe ac impedance spectroscopy  A.
Layson, S. Gadad, D. Teeters
Experimental results Samples PEO-lithium
triflate films on Ni foil AC impedance
spectroscopy measurements are taken at four
different locations on the sample
The ionic conductivity varies greatly from region
to region.
3.8µm
3.8µm
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1  Resistance measurements at the nanoscale
scanning probe ac impedance spectroscopy  A.
Layson, S. Gadad, D. Teeters

• Comparison with macroscale ac impedance
  measurements
• Samples
• PEO film on Ni foil
• PEO filled porous polycarbonate membrane, on Ni
  foil

AFM nanoscale ac impedance data
AFM macroscale ac impedance data
AFM surface scan
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1  Resistance measurements at the nanoscale
scanning probe ac impedance spectroscopy  A.
Layson, S. Gadad, D. Teeters
Conclusions AC impedance data shows variations
in electronic conductivity. PEO films are known
to be heterogeneous, with more conductive
amorphous regions and less conductive crystalline
regions. Noisy data in the low-frequency.
Shielding wires and protecting the AFM from
electro-magnetic interference would decrease
noise. Critique This article serves mostly as a
proof that AFM can be used for AC impedance. The
experimental data could have been collected using
other AFM techniques.
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2  Local impedance imaging and spectroscopy of
polycrystalline ZnO using contact atomic force
microscopy  R. Shao, S. Kalinin, and D. Bonnell

• This article describes the same basic concept as
  the first article nanoimpedance
  microscopy/spectroscopy (NIM).
• However, it proposes several new ideas
• Two different configurations
• Two different modes
• Spectroscopic mode local ac impedance
  spectroscopy measurements are taken at fixed
  locations .
• Imaging mode a single frequency impedance map
  is acquired across the surface.

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2  Local impedance imaging and spectroscopy of
polycrystalline ZnO using contact atomic force
microscopy  R. Shao, S. Kalinin, and D. Bonnell

• Experimental results
• Sample sectioned polycrystalline ZnO varistor

The three grains have different impedances. The
bias voltage has an effect on the impedances of
regions 2 and 3, indicative of varistor behavior.
Vdc35 V
Vdc40 V
F10 kHz, Vac0.1V
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Advantages and limitations of nanoimpedance
spectroscopy

• Advantages
• This technique is a unique way of measuring local
  electronic properties at the nanometer scale. The
  AFM-ac impedance coupling can achieve a
  resolution below 100 nm (the resolution is
  governed by the size of the tip).
• Limitations
• Quantitative measures are difficult, because they
  involve measuring the contact area between the
  tip and the sample. This is possible using
  nanoindentation to relate the applied force and
  the contact area between the tip and the sample.
• The tip coating is degraded due to wear and/or
  high current density.
• The applied force between the tip and the sample
  can influence the measures it can change the
  contact area.

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3  Ionic and electronic impedance imaging using
atomic force microscopy R. OHayre, M. Lee, F.
Prinz
This article develops in more detail the
limitations and capabilities of AFM impedance
imaging.

• Limitations
• Measurement speed and resolution
• It takes a long time (minutes to hours) to
  measure single-frequency impedance at all the
  points of a grid, especially at low-frequency.
  Drift can become an issue.

• Small contact points lead to high impedances .
  This is not a problem for metals, but can be an
  issue for ionic conductors.

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3  Ionic and electronic impedance imaging using
atomic force microscopy R. OHayre, M. Lee, F.
Prinz
Repeatability Impedance measurements were taken
for two applied forces, while the tip is in a
fixed position. The variability of the
measured impedance decreases significantly at the
higher force setpoint.
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3  Ionic and electronic impedance imaging using
atomic force microscopy R. OHayre, M. Lee, F.
Prinz

• Applications
• Gold-nitride test structures
• ZnO varistors
• Nafion solid polymer electrolyte

A platinum-coated tip is used as a probe of
proton density on the surface.
A dc bias is applied between the tip and the
anode. The protons go through the membrane and
combine with oxygen from the air to produce
water.
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3  Ionic and electronic impedance imaging using
atomic force microscopy R. OHayre, M. Lee, F.
Prinz
Nafion is not homogeneous it consists of
hydrophilic and hydrophobic regions. The contrast
shown in the impedance and phase plots could
correspond to these two phases
Hydration has a significant effect on the
measured impedances.
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Conclusions

• Scanning probe ac impedance spectroscopy is a
  powerful tool to measure impedance data with
  resolution better than 100nm.
• Some of the current limitations of this technique
  can be overcome by better experimental setup tip
  coating, shielding from electro-magnetic noise,
  characterization of tip-sample contact area
  (nanoindentation).
• Applications solid electrolytes,
  semi-conductors, electroceramics, coatings and
  corrosion research, electrochemical systems.

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Conductive AFM Option (C-AFM)

• Conductive Atomic Force Microscopy is a is an
  imaging mode derived from contact AFM that
  characterizes conductivity variations across
  medium- to low-conducting and semiconducting
  materials.
• CAFM performs general-purpose measurements, and
  has a current range of 2 pA to 1 µA. CAFM employs
  a conductive probe tip. Typically, a DC bias is
  applied to the tip, and the sample is held at
  ground potential. While the z feedback signal is
  used to generate a normal contact AFM topography
  image, the current passing between the tip and
  sample is measured to generate the conductive AFM
  image.

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Tunneling AFM (TUNA)

• Tunneling AFM (TUNA) works similarly to
  Conductive AFM, but with higher sensitivities.
  TUNA characterizes ultra-low currents (lt1 pA)
  through the thickness of thin films. The TUNA
  application module can be operated in either
  imaging or spectroscopy mode.
• Applications include gate dielectric development
  in the semiconductor industry.

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Scanning Capacitance Microscopy (SCM)

• Scanning Capacitance Microscopy is an imaging
  mode derived from contact AFM that maps
  variations in majority electrical carrier
  concentration (electrons or holes) across the
  sample surface (typically a doped semiconductor).
  
• As in Electric Force Microscopy, an ac bias
  voltage is applied between the tip and sample.
  The tip scans across the sample surface, and
  changes in capacitance between the tip and the
  sample surface are monitored by an extremely
  sensitive high-frequency resonant circuit.SCM
  is commonly used for two-dimensional profiling of
  dopants in semiconductor process evaluation and
  failure analysis.

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Scanning Spreading Resistance Microscopy (SSRM)

• Scanning Spreading Resistance Microscopy (SSRM)
  is an imaging mode derived from contact AFM that
  maps two-dimensional carrier concentration
  profiles (resistance) in semiconductor materials.
  
• A conductive probe is scanned in contact mode
  across the sample, while a DC bias is applied
  between the tip and sample. The resulting current
  between the tip and sample is measured using a
  logarithmic current amplifier providing a range
  of 10 pA to 0.1 mA.

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Surface Potential

• Surface Potential (SP) imaging is an imaging
  mode derived from Tapping Mode that maps the
  electrostatic potential on the sample surface.
• SP is a nulling technique. As the tip travels at
  a distance above the surface, the tip and the
  cantilever experience a force wherever the
  potential on the surface is different than the
  potential of the tip. The force is nullified by
  varying the voltage of the tip so that the tip is
  at the same potential as the region of the sample
  surface underneath it.