Scanning Probe Microscopy
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
1
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
2
http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm
3
STM
It ~ Ve-cd
• Feedback loop keeps
current constant
• therefore d is constant
• Sample must conduct
electricity
• Capable of detecting
atomic scale defects
4
AFM Contact Mode
• Scan tip along surface
Hook’s law F=-kx
• Maintains constant cantilever
deflection (force) using a split
photo diode
k limits sensitivity
(want low k)
• Samples can be in liquid state
• Works in ambient conditions
5
AFM Tapping Mode
• Cantilever oscillates at or
below resonance frequency
• Maintains constant tip
movement
• amplitude
• Position of scanner stored to
create image
• Tip must breakthrough water
layer without getting stuck
• Also works in ambient or liquid
6
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. 7
Silicon Probe
8
AFM Imaging and Tip Shape
The radius of curvature of the tip limits the resolution of
the image that can be taken
9
AFM Imaging and Tip Shape
The probe cannot image a sidewall that is steeper than
the angle of the tip
Silicon nitride probe
10
AFM Imaging and Tip Shape
Silicon probe
11
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
1 N
R a   Z j  Z avg
N j 1
 Z
N
Image Rq : Root mean square
average of height deviations
taken from the mean plane.
Rq 
i 1
 Z avg 
2
i
N
12
NanoScope Software 6.13 User Guide, Section 6.4.3
AFM height image of a
semiconducting polymer
thin film on SiO2
AFM images of SiO2 substrates for FET’s
Uncleaned SiO2
5 nm
Mean Roughness: 0.44 nm
0 nm
-5 nm
5 nm
0 nm
-5 nm
No detergent, No Plasma SiO2
Mean Roughness: 0.19 nm
6nm tall particle on surface
8/4/08
AFM images of SiO2 substrates for FET’s
5 nm
No detergent, Airplasma 5min SiO2
0 nm
Mean Roughness: 0.27 nm
-5 nm
5 nm
0 nm
-5 nm
detergent, air plasma 5min SiO2
Mean Roughness: 0.17 nm
6nm tall particles on surface
8/4/08