NANO 225
Micro/Nanofabrication
Characterization: Scanning Probe Microscopy
1
History
• Scanning Tunneling Microscope
invented in 1982 by, Binning,
Rohrer, Gerber and Weibel
• Binning and Rohrer won Nobel
prize in 1986
• AFM developed in 1986 by Binning,
Quate and Gerber
2
http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm
Hierarchy of Techniques
Scanning Probe Microscopy
• Scanning Tunneling
Microscopy
• Atomic Force Microscopy
• Contact Mode
• Tapping Mode
• Non-contact Mode
3
http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm
4
STM
It ~ Ve-cd
• Feedback loop keeps
current constant
• therefore d is constant
• Sample must conduct
electricity
• Capable of detecting
atomic scale defects
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AFM Contact Mode
• Scan tip along surface
• Tip contacts sample through
adsorbed fluid layer
• Maintains constant cantilever
deflection (force) using a split
photo diode
Hook’s law F=-kx
k limits sensitivity
(want low k)
• Samples can be in liquid state
• Works in ambient conditions
6
AFM Tapping Mode
• Cantilever oscillates at or
below resonance frequency
• Maintains constant RMS of tip
movement
• amplitude
• Position of scanner stored to
create image
• Tip must breakthrough water
layer without getting stuck
• Also works in ambient or liquid
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AFM Non-Contact Mode
• Tip oscillates but does not
touch sample
• above resonance frequency
• The resonant frequency is
decreased by van der
Waals forces
• Position of scanner used to
map 3-D plot of surface
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Silicon Nitride Probe
Spring Constant
(k)
0.58, 0.32, 0.12,
0.06 N/m (1)
Nominal Tip
Radius of
Curvature
20 - 60nm
Cantilever
Lengths
100 & 200μm
Cantilever
Configuration
V-shaped
Reflective
Coating
Gold
Sidewall angles
35° on all 4 sides
(1)Calculated spring constant values are based on the 0.6μm silicon nitride
thickness; however, this value can actually vary from 0.4μm to 0.7μm. Thickness is
cubed in the spring constant calculation, thus, actual values can vary substantially. 9
Silicon Probe
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AFM Imaging and Tip Shape
The radius of curvature of the tip limits the resolution of
the image that can be taken
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AFM Imaging and Tip Shape
The probe cannot image a sidewall that is steeper
than the angle of the tip
Silicon nitride probe
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AFM Imaging and Tip Shape
Silicon probe
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Surface Roughness Measurements
Rz : average difference in height between the five highest
peaks and five lowest valleys relative to the mean plane
Image Ra : average of the
absolute values of the surface
height deviations measured from
the mean plane
Image Rq : Root mean square
average of height deviations
taken from the mean plane.
1 N
R a   Z j  Z avg
N j 1
 Z
N
Rq 
i 1
 Z avg 
2
i
N
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NanoScope Software 6.13 User Guide, Section 6.4.3
Four Point Probe
• Constant current in two
outer probes
• Voltage measurement on
two inner probes
eliminates contact
resistance
• Four point technique
required for precision
resistance measurements
University of California
EECS 143 Manual
15
Four Point Probe
• Thin film case
• Current flow is
restricted to a thin film
• t<<s

 t V 

t
 
ln 2  I 
 V 
V 
Rs  
   4.53 
t ln 2  I 
I
University of California
EECS 143 Manual
16
Four Point Probe
• Bulk resistivity case
• Current flows through
bulk material
• s<<t
t
V 
  2  s 
I
University of California
EECS 143 Manual
17