UIC

Atomic Force Microscopy (AFM)

Stephen Fahey

Ph.D. Advisor: Professor Sivananthan

October 16, 2009

UICOutline

• Scanning Tunneling Microscope (STM)

• Atomic Force Microscope (AFM)

• Today’s Sample

UICTunneling

Charge carriers can apparently violate E conservation!

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)

UICSTM ObservationsSi (111) 7x7 reconstruction (1983)

Nobel Prize (1986)

(1982)

UICMore STM

• Scanning Tunneling Spectroscopy (STS)

• Single atom manipulation• Multiple probes

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

UICPrevalence of the Harmonic Oscillator

Taylor Series and Potential Energy

Inter-atomic potential

Separation

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

UICTip-Sample Forces

• Pauli and Coulomb repulsive

• Van der Waals attractive

Tip-Sample

Cantilever Deflection

Piezo Height

UICTypical AFM data

GaNNorthwestern U.

2 μm

Carbon Nanotubes Unidym

BaF2 / CaF2 / Si

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)

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

UIC

Interferrometrically Patterned

Today’s Sample

UIC

Pattern Transfer and Oxide Removal

UIC

5 μm

Final Sample

UICConclusion

• STM ~ 1982 led to AFM ~ 1986

• Both are powerful and beautiful characterization techniques