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UIC
Atomic Force Microscopy (AFM)
Stephen Fahey
Ph.D. Advisor: Professor Sivananthan
October 16, 2009
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UICOutline
• Scanning Tunneling Microscope (STM)
• Atomic Force Microscope (AFM)
• Today’s Sample
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UICTunneling
Charge carriers can apparently violate E conservation!
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UICScanning Tunneling Microscope
• Exponential dependence of J
• Piezoelectric materials
• Amplifier circuitry
• Low oxide tip
1981 Binnig and Rohrer observe tunneling between a metal tip and a flat surface
This observation led to the first STM shortly thereafter (~ 1982)
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UICSTM ObservationsSi (111) 7x7 reconstruction (1983)
Nobel Prize (1986)
(1982)
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UICMore STM
• Scanning Tunneling Spectroscopy (STS)
• Single atom manipulation• Multiple probes
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UICAtomic Force Microscope (AFM)
• STM not good for insulators
• ~ 1985 Binnig, Quate, and Gerber used STM as feedback of first AFM
• AFMs can typically be operated in either tapping-mode or contact-mode
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UICPrevalence of the Harmonic Oscillator
Taylor Series and Potential Energy
Inter-atomic potential
Separation
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UICDamped and Driven Harmonic Oscillator
• Complementary Function ( F = 0 ), plus Particular Solution
• Motion has resonance frequencies• Motion is phase shifted• Less damping increases “Q” and
sharpens phase vs. frequency curve
• More damping decreases resonance frequency
• Resonance frequency shifts under an external applied field gradient
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UICTip-Sample Forces
• Pauli and Coulomb repulsive
• Van der Waals attractive
Tip-Sample
Cantilever Deflection
Piezo Height
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UICTypical AFM data
GaNNorthwestern U.
2 μm
Carbon Nanotubes Unidym
BaF2 / CaF2 / Si
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UICArtifacts
• Large scale tip shape– Tip radius– Tip side-wall angles
• Double tip (or more)• Contamination of tip• Bow from Piezoelectric
elements• Background noise• Feedback laser interference• Response time limited errors
(t ~ 10-4 seconds)
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UICAFM System from Veeco
• Dimension 3100
• Etched Si probe tips with Al back coating
• Movable sample stage with optical microscope for course positioning
• Stepper motors for micrometer postioning of tip
• Friendly software and feedback control circuitry
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UIC
Interferrometrically Patterned
Today’s Sample
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UIC
Pattern Transfer and Oxide Removal
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UIC
5 μm
Final Sample
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UICConclusion
• STM ~ 1982 led to AFM ~ 1986
• Both are powerful and beautiful characterization techniques