Introduction to Atomic Force Microscopy

Dr. Janelle GuntherMarch 10, 1998

ACS Group and MENs, Beckman Institute

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

Scanning Probe Microscopy (SPM)

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

Scanning tunneling Microscopy (STM) Atomic Force Microscopy (AFM)

• contact mode• non-contact mode• TappingMode

Other forms of SPM• Lateral force• Force modulation • Magnetic or electric force• surface potential• scanning thermal• phase imaging

Multimode SPM

General AFM“Beam Deflection”

Detection

Used for Contact Mode, Non-contact and TappingMode AFM

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

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

Detects cantilever deflection <1A

Solid StateLaser Diode

Cantilever and Tip

A

B

Piezoelectric Scanners

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

Whether they expand or contract depends upon the polarity of the applied voltage. Digital Instruments 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 AFM

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

The tip contacts the surface through the adsorbed fluid layer.

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

Tapping Mode™ AFM

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

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

Forces can be 200 pN or less The amplitude of the oscillations changes when the

tip scans over bumps or depressions on a surface.

Non-contact Mode AFM

The cantilever is oscillated slightly above its resonant frequency.

Oscillations <10nm The tip does not touch the sample. Instead, it

oscillates above the adsorbed fluid layer. A constant oscillation amplitude is maintained. The resonant frequency of the cantilever is decreased

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

Advantages and Disadvantages of the 3

main types of AFM

Contact Mode» Advantages:

– High scan speeds– The only mode that can obtain “atomic resolution”

images– Rough samples with extreme changes in topography

can sometimes be scanned more easily

» Disadvantages:– Lateral (shear) forces can distort features in the image– The forces normal to the tip-sample interaction can be

high in air due to capillary forces from the adsorbed fluid layer on the sample surface.

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

TappingMode AFM» Advantages:

– Higher lateral resolution on most samples (1 to 5nm)– Lower forces and less damage to soft samples imaged

in air– Lateral forces are virtually eliminated so there is no

scraping

» Disadvantages:– Slightly lower scan speed than contact mode AFM

Lateral Force Microscopy

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

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

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

Lateral Force Microscopy

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 Imaging Accessible via TappingMode Oscillate the cantilever at its resonant frequency. The

amplitude is used as a feedback signal. The phase lag is dependent on several things, including composition, 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 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 Microscopy

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

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

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

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

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

Many applications for data storage technology

Magnetic Force 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

LiftMode

•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 Imaging

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

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

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

Force Modulation Imaging

Other Techniques

Scanning capacitance microscopy» Apply a constant amplitude sine wave

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

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

» Image carrier concentration

Scanning thermal microscopy» Uses a temperature sensitive probe with a

special holder. » Location of “hot-spot” defects in

semiconductor wafers

90 micronscan size

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

Other Techniques

Nanoindenting and scratching» A diamond tip is mounted on a metal

cantilever and scanned either with contact or TappingMode.

» Indenting mode presses the tip into the sample

» Scratch mode drags the tip across the sample at a specific rate and with a specified force.

» The use of TappingMode makes it possible to simultaneously image soft samples.

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