Introduction to Introduction to Atomic Force Atomic Force MicroscopyMicroscopy

Dr. Janelle GuntherDr. Janelle Gunther

March 10, 1998March 10, 1998ACS Group and MENs, Beckman ACS Group and MENs, Beckman

InstituteInstitute

adapted from the world wide web page at http://www.di.com adapted from the world wide web page at http://www.di.com Digital Instruments, Santa Barbara, CADigital Instruments, Santa Barbara, CA

Scanning Probe Scanning Probe Microscopy (SPM)Microscopy (SPM)

A family of microscopy forms where a sharp probe is scanned across A family of microscopy forms where a sharp probe is scanned across a surface and some tip/sample interactions are monitoreda surface and some tip/sample interactions are monitored

Scanning tunneling Microscopy (STM)Scanning tunneling Microscopy (STM)

Atomic Force Microscopy (AFM)Atomic Force Microscopy (AFM)• contact modecontact mode• non-contact modenon-contact mode• TappingModeTappingMode

Other forms of SPMOther forms of SPM• Lateral forceLateral force• Force modulation Force modulation • Magnetic or electric forceMagnetic or electric force• surface potentialsurface potential• scanning thermalscanning thermal• phase imagingphase imaging

Multimode SPMMultimode SPM

General AFMGeneral AFM“Beam Deflection” Detection“Beam Deflection” Detection

Used for Contact Mode, Non-contact and Used for Contact Mode, Non-contact and TappingMode AFMTappingMode AFM

Laser light from a solid state diode is reflected off Laser light from a solid state diode is reflected off the back of the cantilever and collected by a the back of the cantilever and collected by a position sensitive detector (PSD). This consists of position sensitive detector (PSD). This consists of two closely spaced photodiodes. The output is two closely spaced photodiodes. The output is then collected by a differential amplifierthen collected by a differential amplifier

Angular displacement of the cantilever results in Angular displacement of the cantilever results in one photodiode collecting more light than the one photodiode collecting more light than the other. The resulting output signal is proportional other. The resulting output signal is proportional to the deflection of the cantilever.to the deflection of the cantilever.

Detects cantilever deflection <1A Detects cantilever deflection <1A

Solid StateLaser Diode

Cantilever and Tip

A

B

Piezoelectric ScannersPiezoelectric ScannersSPM scanners are made from a piezoelectric material SPM scanners are made from a piezoelectric material that expands and contracts proportionally to an applied that expands and contracts proportionally to an applied voltage.voltage.

Whether they expand or contract depends upon the Whether they expand or contract depends upon the polarity of the applied voltage. Digital Instruments polarity of the applied voltage. Digital Instruments scanners have AC voltage ranges of +220 to -220V.scanners have AC voltage ranges of +220 to -220V.

0 V - V+ V

No applied voltage Extended Contracted

In some versions, the piezo tube moves the sample relative to the tip. In other models, the sample is stationary while the scanner moves the tip.

AC signals applied to conductive areas of the tube create piezo movement along the three major axes.

Contact Mode AFMContact Mode AFM

A tip is scanned across the sample while a A tip is scanned across the sample while a feedback loop maintains a constant cantilever feedback loop maintains a constant cantilever deflection (and force)deflection (and force)

The tip contacts the surface through the The tip contacts the surface through the adsorbed fluid layer.adsorbed fluid layer.

Forces range from nano to micro N in ambient Forces range from nano to micro N in ambient conditions and even lower (0.1 nN or less) in conditions and even lower (0.1 nN or less) in liquids.liquids.

Tapping Mode™ AFMTapping Mode™ AFM

A cantilever with attached tip is oscillated at its A cantilever with attached tip is oscillated at its resonant frequency and scanned across the sample resonant frequency and scanned across the sample surface.surface.

A constant oscillation amplitude (and thus a constant A constant oscillation amplitude (and thus a constant tip-sample interaction) are maintained during tip-sample interaction) are maintained during scanning. Typical amplitudes are 20-100nm.scanning. Typical amplitudes are 20-100nm.

Forces can be 200 pN or lessForces can be 200 pN or less

The amplitude of the oscillations changes when the tip The amplitude of the oscillations changes when the tip scans over bumps or depressions on a surface.scans over bumps or depressions on a surface.

Non-contact Mode AFMNon-contact Mode AFM

The cantilever is oscillated slightly above its resonant The cantilever is oscillated slightly above its resonant frequency.frequency.

Oscillations <10nmOscillations <10nm

The tip does not touch the sample. Instead, it oscillates above The tip does not touch the sample. Instead, it oscillates above the adsorbed fluid layer.the adsorbed fluid layer.

A constant oscillation amplitude is maintained.A constant oscillation amplitude is maintained.

The resonant frequency of the cantilever is decreased by the The resonant frequency of the cantilever is decreased by the van der Waals forces which extend from 1-10nm above the van der Waals forces which extend from 1-10nm above the adsorbed fluid layer. This in turn changes the amplitude of adsorbed fluid layer. This in turn changes the amplitude of oscillation.oscillation.

Advantages and Advantages and Disadvantages of the 3 main Disadvantages of the 3 main

types of AFMtypes of AFM

Contact ModeContact Mode Advantages:Advantages:

High scan speedsHigh scan speeds

The only mode that can obtain “atomic resolution” imagesThe only mode that can obtain “atomic resolution” images

Rough samples with extreme changes in topography can Rough samples with extreme changes in topography can sometimes be scanned more easilysometimes be scanned more easily

DisadvantagesDisadvantages::Lateral (shear) forces can distort features in the imageLateral (shear) forces can distort features in the image

The forces normal to the tip-sample interaction can be high in The forces normal to the tip-sample interaction can be high in air due to capillary forces from the adsorbed fluid layer on the air due to capillary forces from the adsorbed fluid layer on the sample surface.sample surface.

The combination of lateral forces and high normal forces can The combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft result in reduced spatial resolution and may damage soft samples (i.e. biological samples, polymers, silicon) due to samples (i.e. biological samples, polymers, silicon) due to scrapingscraping

TappingMode AFMTappingMode AFM Advantages:Advantages:

Higher lateral resolution on most samples (1 to 5nm)Higher lateral resolution on most samples (1 to 5nm)

Lower forces and less damage to soft samples imaged in airLower forces and less damage to soft samples imaged in air

Lateral forces are virtually eliminated so there is no scrapingLateral forces are virtually eliminated so there is no scraping

DisadvantagesDisadvantages::Slightly lower scan speed than contact mode AFMSlightly lower scan speed than contact mode AFM

Lateral Force Lateral Force MicroscopyMicroscopy

The probe is scanned sideways. The degree of torsion The probe is scanned sideways. The degree of torsion of the cantilever is used as a relative measure of of the cantilever is used as a relative measure of surface friction caused by the lateral force exerted on surface friction caused by the lateral force exerted on the probe.the probe.

Identify transitions between different components in a Identify transitions between different components in a polymer blend, in composites or other mixturespolymer blend, in composites or other mixtures

This mode can also be used to reveal fine structural This mode can also be used to reveal fine structural details in the sample. details in the sample.

Lateral Force Lateral Force MicroscopyMicroscopy

Magnetic recordinghead

Al oxide grainsand contamination

800nm scan

Natural rubber/EDPM blend

20 micron scan

Polished poly-crystalle silicon carbide film.

Grain structures

30 micron scan

Images/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

Phase ImagingPhase Imaging

Accessible via TappingModeAccessible via TappingMode

Oscillate the cantilever at its resonant frequency. The Oscillate the cantilever at its resonant frequency. The amplitude is used as a feedback signal. The phase lag amplitude is used as a feedback signal. The phase lag is dependent on several things, including composition, is dependent on several things, including composition, adhesion, friction and viscoelastic properties.adhesion, friction and viscoelastic properties.

Identify two-phase structure of polymer blends Identify surface contaminants that are not seen in

height images Less damaging to soft samples than lateral force

microscopy

Phase ImagingPhase Imaging

Compositepolymerimbedded in a matrix

1 micron scan

Bond pad on anintegrated circuit

Contamination

1.5 micron scan

MoO3 crystalliteson a MoS2 substrate

6 micron scan

Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

Magnetic Force Magnetic Force MicroscopyMicroscopy

Special probes are used for MFM. These are Special probes are used for MFM. These are magnetically sensitized by sputter coating with a magnetically sensitized by sputter coating with a ferromagnetic material.ferromagnetic material.

The cantilever is oscillated near its resonant frequency The cantilever is oscillated near its resonant frequency (around 100 kHz). (around 100 kHz).

The tip is oscillated 10’s to 100’s of nm above the The tip is oscillated 10’s to 100’s of nm above the surfacesurface

Gradients in the magnetic forces on the tip shift the Gradients in the magnetic forces on the tip shift the resonant frequency of the cantilever .resonant frequency of the cantilever .

Monitoring this shift, or related changes in oscillation Monitoring this shift, or related changes in oscillation amplitude or phase, produces a magnetic force image.amplitude or phase, produces a magnetic force image.

Many applications for data storage technologyMany applications for data storage technology

Magnetic Force Magnetic Force Microscopy Microscopy

Overwritten tracks on a textured hard disk, 25 micron scan

Domains in a 80 micron garnet film

Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

LiftModeLiftMode

•Two passes are made over the sample. The first measures topography while the second measures a material property(magnetic, electric, etc.)

•Eliminates “cross-contamination” of the images.

Force Modulation Force Modulation ImagingImaging

Oscillate the cantilever vertically at a rate that is Oscillate the cantilever vertically at a rate that is significantly faster than the scan rate.significantly faster than the scan rate.

The amplitude of the oscillations changes in response The amplitude of the oscillations changes in response to the sample stiffness.to the sample stiffness.

Used in conjunction with LiftMode to separate Used in conjunction with LiftMode to separate topography and elasticity data.topography and elasticity data.

Force Modulation Force Modulation ImagingImaging

Other TechniquesOther Techniques

Scanning capacitance microscopyScanning capacitance microscopy Apply a constant amplitude sine wave voltage to Apply a constant amplitude sine wave voltage to

the sample. The image is then reconstructed from the sample. The image is then reconstructed from the changes in the amplitude of the capacitance the changes in the amplitude of the capacitance oscillations.oscillations.

Location of defects in wafers (pinning of electrical Location of defects in wafers (pinning of electrical carriers)carriers)

Image carrier concentrationImage carrier concentration

Scanning thermal microscopyScanning thermal microscopy Uses a temperature sensitive probe with a special Uses a temperature sensitive probe with a special

holder. holder. Location of “hot-spot” defects in semiconductor Location of “hot-spot” defects in semiconductor

waferswafers

90 micronscan size

Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

Other TechniquesOther Techniques

Nanoindenting and scratchingNanoindenting and scratching A diamond tip is mounted on a metal cantilever and A diamond tip is mounted on a metal cantilever and

scanned either with contact or TappingMode.scanned either with contact or TappingMode. Indenting mode presses the tip into the sampleIndenting mode presses the tip into the sample Scratch mode drags the tip across the sample at a Scratch mode drags the tip across the sample at a

specific rate and with a specified force.specific rate and with a specified force. The use of TappingMode makes it possible to The use of TappingMode makes it possible to

simultaneously image soft samples.simultaneously image soft samples.

Imaging of biological samplesImaging of biological samples cells, DNA (TappingMode in solution)cells, DNA (TappingMode in solution)