nano 225 micro/nanofabrication characterization: scanning probe microscopy 1
TRANSCRIPT
History
• Scanning Tunneling Microscope invented in 1982 by, Binning, Rohrer, Gerber and Weibel
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http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm
• Binning and Rohrer won Nobel prize in 1986
• AFM developed in 1986 by Binning, Quate and Gerber
Hierarchy of Techniques
Scanning Probe Microscopy
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http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm
• Scanning Tunneling Microscopy
• Atomic Force Microscopy • Contact Mode
• Tapping Mode• Non-contact Mode
STM
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• Feedback loop keeps current constant• therefore d is
constant
• Sample must conduct electricity
• Capable of detecting atomic scale defects
It ~ Ve-
cd
AFM Contact Mode
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• Scan tip along surface
• Tip contacts sample through adsorbed fluid layer
• Maintains constant cantilever deflection (force) using a split photo diode
• Samples can be in liquid state
• Works in ambient conditions
Hook’s law F=-kx
k limits sensitivity(want low k)
AFM Tapping Mode
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• 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
AFM Non-Contact Mode
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• 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
Silicon Nitride ProbeSpring 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
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(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.
AFM Imaging and Tip Shape
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The radius of curvature of the tip limits the resolution of the image that can be taken
AFM Imaging and Tip Shape
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The probe cannot image a sidewall that is steeper than the angle of the tip
Silicon nitride probe
Surface Roughness Measurements
N
1a N
1R
javgj ZZ
N
ZZN
iavgi
1
2
qR
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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.
NanoScope Software 6.13 User Guide, Section 6.4.3
Four Point Probe
15
University of CaliforniaEECS 143 Manual
• Constant current in two outer probes
• Voltage measurement on two inner probes eliminates contact resistance
• Four point technique required for precision resistance measurements
Four Point Probe
I
Vt
2ln
I
V
I
V
tRs 53.4
2ln
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University of CaliforniaEECS 143 Manual
• Thin film case• Current flow is
restricted to a thin film• t<<s t