Modelling of Atomic Force Microscope(AFM)
Download
Report
Transcript Modelling of Atomic Force Microscope(AFM)
Deng Wenqi
Supervisors: Guang-Ming Zhang,Dave
Harvey, Francis Lilley
Outline
Background of AFM
Brief introduction of Comsol Multiphysics
Simulation and results
Project plan
How AFM works?
•The x and y-piezos provide lateral scanning(~30nm) ,
while the z-piezo extends and retracts to follow the
surface topography(up to 0.1nm) .
Image mode
Non-contact mode
•The cantilever is oscillated near its
resonant frequency at a position just
above the sample surface
•Measures the topography by detecting
the vibration amplitude affected by the
attractive forces
•Difficult to use in normal ambient
conditions
•Better to use in ultra high vacuum
conditions
Image mode
Contact mode
•Constant force during scanning
•Damage soft sample
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.
First model
•Model consists of a piezo actuator,
a cantilever and the tip.
• A sinusoidal voltage signal is
applied to piezo( purple block)
Free vibration results
•Vibration curve is not smooth
Contact model
•Define contact pair between
the tip and sample
•Omit the inertial effect
•Start with linear
discretization model
Displacement and contact force curves
•Bouncing effect
Quasi-static study
Bouncing effect disappear
Smooth curves
•Free vibration
Quasi-static study
Different materials such as
cooper,GaAs,Ge,Si,andpolymer are tested
Contact force curve of cooper
Phase curve
Phase
shift
Light contact
Young’s modulus
Phase
shift
Deeper contact
Young’s modulus
Phase curve
Phase
shift
Phase
shift
Tip sample separation(Polymer)
Tip sample separation(Copper)
Problems of static study
It omits the inertial effect of the whole model
Does not reflect the resonant frequency of the
cantilever
No matter what frequency you apply, the amplitude is
almost the same.
Problem of resonant frequency
When using linear discretization model, the resonant
frequency is not right.
Different mesh lead to different results
1.15e5 Hz
2.69e5 Hz
This problem will not occur when changing to
quadratic discretization.
82424.8 Hz
82425 Hz
New model
AFM Probe Type: PNP-DB
Manufacturer: NanoWorld
Geometry built in COMSOL
lLength
Width
Thickness
Resonant
Frequency
100 µm
13.5um
0.6um
67kHz
Eigenfrequency Study
Resonant frequency is 82424Hz
Time dependent study 1
Fixed boundary
Edge load=sin(2*pi*f*t)*100[nN]
Mesh
Free vibration
Time dependent study 2
•Apply the contact force on the tip
1.Treat the contact force as
spring
2.Relate the contact force with
Young’s modulus of tip and
sample
Displacement curves of the cantilever
•Phase shift between green and blue is 3.8 degree
•Phase shift between red and blue is is 4 degree
Time dependent study 2
Load a force on the sample for a short period
Unload the force, sample almost recovered
Problems of simulation
Difficult to define the contact force equation
It’s better to model tip and sample together
However, it is still difficult to solve contact
modelling in time dependent study.
Contact model
Project Plan
Aim:
1) Phase interpretation
2) Nano-NDE(non-destructive evaluation) for defect
detection, imaging of internal micro-structures,
measurement of mechanical properties.
Test samples:
1) Single layer samples with surface defects.
2) Multiple layers samples with defects
Project Plan
Methodologies
Elastic deformation force only
Elastic deformation force + Van Der Waals
Curve of phase shift vs height
Curve of phase shift vs material
Image restoration using these curves: material
phase image
Project Plan
Future work
Solve the problem of contact modelling
Define the relationship between phase shift and
material parameter, such as phase shift vs young’s
modulus, phase shift vs thickness
Observe how phase change in non-destructive
evaluation, eg. The sample has a defect inside
Test soft material like cells and compare the results
with real experiment
Thank you!!!
Any Questions?