Tapping mode AFM

Report
Wenqi Deng
Supervisors: Guang-Ming Zhang,Dave
Harvey, Francis Lilley
Outline
 Background of AFM
 Simulation and results
 Experiment and results
 Discussion
 Project plan
What is AFM?
•Image topography
•Mechanical, electrical, and
magnetic properties of surfaces
•First atomic force microscope
was invented in 1986 by Binnig.
How AFM works?
•The x and y-piezos provide lateral scanning(up to
19um), while the z-piezo extends and retracts to follow
the surface topography(up to 12um) .
Image mode
 Tapping mode
•The cantilever is vibrated at its
resonant frequency
•Intermittent contact with sample
• Reduce sample destruction
Phase imaging
•Measure the phase lag of the
cantilever oscillation (solid
wave) relative to the phase of
the piezo drive (dashed wave).
•The amplitude signal is used
simultaneously by the
controller for Tapping Mode
feedback.
• Spatial variations in sample
properties cause shifts in the
cantilever phase (bottom)
Why phase image provides more
information of structure?
Topography (left) & phase (right) images of a composite polymer
•Higher spots in the AFM topography images should correspond to the hard
phase polymer?
•Brighter areas correspond to harder materials?
•We don’t have an agreement yet.
Simulation model
 The cantilever is treated to be a massless spring of
stiffness k having an effective mass at the end and
equivalent damping c.
1D Lumped model of tapping mode AFM
 The tip–sample forces are modeled by three springs
with spring constants kN and kS for vertical and
lateral contact stiffness, respectively.
3D model tapping mode AFM
Geometry
Type
AC mode air
Spring k (N/m)
2.8 (0.5 - 9.5)
Freq (kHz)
75 (45 - 115)
Length (µm)
225 (215 - 235)
Width (µm)
28 (20 - 35)
Thickness (µm)
Shape
3.0 (2.0 - 4.0)
rectangular
Material
Silicon
Reflex Coating (nm)
none
A piezo (red) is attached behind the end of the cantilever. A voltage of
sin(2*pi*f*t)*4 is applied to the piezo to cause the vibration of the cantilever. The
free vibration amplitude is about 28nm.
Eigenfrequency study
Resonant frequency : 65208.9 Hz
Contact model
•Equilibrium position, the
initial tip sample separation
•Cantilever, tip and sample
are defined as elastic
deformation model
Time dependent study
 Initial tip sample separation: 19 nm(green line)
Displacement curve
Experiment results
Discussion
Limitation of simulation and experiment
 The simulation just provides the first cycle vibration,
which means it has not reached steady state yet.
 It may take hundreds of cycles to reach steady state,
hence, based on the simulation time, it is not realistic
to carry out a simulation until the steady state.
 However, due to the limitation of experiment, the
AFM data is captured under the steady state, and it is
difficult to determine the contact time.
 It seems a challenge to identify the bouncing effect.
Project Plan
Future work
 Set up a different experiment to determine the
contact time from the deflection data.
 Modify the simulation model.
 Simulate tapping mode AFM separately. Calculate
the contact stiffness between tip and sample using
the model below.
Tip-sample model
 Investigate what parameters affect contact stiffness,
such as the radius of the tip, deformation of the
sample, indentation force, young’s modulus of tip
and sample.
 In real experiment, the sample surface is not always
flat. The tip may not indent normally to the surface.
 Especially, the tip scans across the sidewall.
 It is worthwhile to study how forces from different
direction affect contact stiffness.
 The calculated contact stiffness can be applied to
the tip.
 Phase vs contact stiffness curve
 Might help to select what kind of cantilevers(soft or
stiff?) for phase imaging
Cantilever-tip model
 While using cantilever-tip model, computation time is
reduced. It is possible to simulate tapping mode until
it becomes steady.
 Calculate the steady time, then we might be able to see
how scan rate affects phase imaging.
Thank you!!!
Any Questions?

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