ou nanolab/nsf nue/bumm & johnson afm atomic force microscopy

OU NanoLab/NSF NUE/Bumm & Johnson

AFMAtomic Force Microscopy


Outline• Motivation• History• How the AFM works

– Two modes– Contact Mode– Non-Contact Mode– Force Measurements– Raster the Tip: Generating an Image– Scanning Sample

• OUr AFM– Pictures

• Examples– The Good– The Bad– And the Ugly

• Uses– Topographical Analysis– Thin Layer Depth– RMS Roughness Calculations

• Other types of Microscopy


Motivation

• Digitally image a topographical surface• Determine the roughness of a surface sample or

to measure the thickness of a crystal growth layer

• Image non-conducting surfaces such as proteins and DNA

• Study the dynamic behavior of living and fixed cells


History

• The Scanning Tunneling Microscope (STM) was invented by G. Binnig and H. Rohrer, for which they were awarded the Nobel Prize in 1984

• A few years later, the first Atomic Force Microscope (AFM) was developed by G. Binnig, Ch. Gerber, and C. Quate at Stanford University by gluing a tiny shard of diamond onto one end of a tiny strip of gold foil

• Currently AFM is the most common form of scanning probe microscopy


How the AFM Works

• The AFM brings a probe in close proximity to the surface

• The force is detected by the deflection of a spring, usually a cantilever (diving board)

• Forces between the probe tip and the sample are sensed to control the distance between the the tip and the sample.

van der Waals force curve


Two Modes

Repulsive (contact)• At short probe-sample

distances, the forces are repulsive

Attractive Force (non-contact)• At large probe-sample

distances, the forces are attractive

The AFM cantelever can be used to measure both attractive force mode and repulsive forces.


Non-Contact Mode

• Uses attractive forces to interact surface with tip

• Operates within the van der Waal radii of the atoms

• Oscillates cantilever near its resonant frequency (~ 200 kHz) to improve sensitivity

• Advantages over contact: no lateral forces, non-destructive/no contamination to sample, etc.

van der Waals force curve


Contact Mode

• Contact mode operates in the repulsive regime of the van der Waals curve

• Tip attached to cantilever with low spring constant (lower than effective spring constant binding the atoms of the sample together).

• In ambient conditions there is also a capillary force exerted by the thin water layer present (2-50 nm thick). van der Waals force curve


Force Measurement• The cantilever is designed with

a very low spring constant (easy to bend) so it is very sensitive to force.

• The laser is focused to reflect off the cantilever and onto the sensor

• The position of the beam in the sensor measures the deflection of the cantilever and in turn the force between the tip and the sample.


Raster the Tip: Generating an Image

• The tip passes back and forth in a straight line across the sample (think old typewriter or CRT)

• In the typical imaging mode, the tip-sample force is held constant by adjusting the vertical position of the tip (feedback).

• A topographic image is built up by the computer by recording the vertical position as the tip is rastered across the sample.

Sca

nn

ing

Tip

Ra

ste

r M

otio

n

Top Image Courtesy of Nanodevices, Inc. (www.nanodevices.com)

Bottom Image Courtesy of Stefanie Roes (www.fz-borstel.de/biophysik/ de/methods/afm.html)


Scanning the Sample

• Tip brought within nanometers of the sample (van der Waals)

Radius of tip limits the accuracy of analysis/ resolution

Stiffer cantilevers protect against sample damage because they deflect less in response to a small force This means a more sensitive

detection scheme is needed measure change in resonance

frequency and amplitude of oscillation

Image courtesy of (www.pacificnanotech.com)


OUr AFM

We have a commercial Topometrix Explorer AFM.


Some of Our Pictures2D topographical image of

Atomic Step 3D Image

Screw dislocations on InSb grown by MBE


The Good Examples

View of Silicon Surface Reconstruction

Carbon Nanotube Used as a Conducting AFM Tip for Local Oxidation of Si.

Right Image Courtesy of Dai, et al. from Stanford


The Bad ExamplesHistogram shows level surface, but scan is very streaky

Typically the sample will have a slight tilt with respect to the AFM. The AFM can compensate for this tilt.

The horizontal lines are due to tip hops – where the tip picks up or loses a small “nanodust”

In this image the tilt have not yet been removed.


And the Ugly!

Teeny little dust mites, ultra tiny dust mitesabout 2,000 in the average bed

Image courtesy of http://www.micropix.demon.co.uk/sem/dustmite/article/page_2.htm

http://www.micropix.demon.co.uk/sem/dustmite/article/12664_2.gif


Topography Scanning

Example of generated

image upon scanningPd thermally evaporated on Si


This targets the highest points of the sample and eliminates them

It then manipulates the image to create a smaller dynamic depth

extreme

(He

igh

t)

Centering on pt.

Elimination of Extreme Points


A Better View

Now:

• Removed extreme points

• Digitally decreased the height of analysis

• Less than 1/3 as high as initial scan

•Lose resolution and data by clipping off extreme points


Si/Pd step

Thickness of a Thin Layerof Pd on Si Wafer

Step (where Pd coating ends)

Systematic error


Surface Roughness

Roughness typically measured as root mean squared (RMS)


Other Types of SPM Techniques• Lateral Force Microscopy (LFM)

– Frictional forces measured by twisting or “sideways” forces on cantilever.

• Magnetic Force Microscopy (MFM)– Magnetic tip detects magnetic fields/measures magnetic properties

of the sample.• Electrostatic Force Microscopy (EFM)

– Electrically charged Pt tip detects electric fields/measures dielectric and electrostatic properties of the sample

• Chemical Force Microscopy (CFM)– Chemically functionalized tip can interact with molecules on the

surface – giving info on bond strengths, etc.• Near Field Scanning Optical Microscopy (NSOM)

– Optical technique in which a very small aperture is scanned very close to sample

– Probe is a quartz fiber pulled to a sharp point and coated with aluminum to give a sub-wavelength aperture (~100 nm)


STM modesconstant currentconstant height

AFM modescontactnon-contact

SPM lithographySTM lithographyAFM lithography – scratchingAFM lithography – Dynamic Plowing

SPM techniques (NT-MDT)visit these links for animations

http://www.ntmdt.ru/SPM-Techniques/STM_techniques/Constant_Current_mode_mode3.html

http://www.ntmdt.ru/SPM-Techniques/STM_techniques/Constant_Height_mode_mode4.html


Carbon Nanotube Tips Well defined shape and composition. High aspect ratio and small radius of curvature (“perfect” tip would be a delta

function tip). Mechanically robust. Chemical functionalization at tip.

DNA

CNT Tips

Images taken from Nanodevices, Inc. (www.nanodevices.com)and Wooley, et al., Nature Biotech. 18, 760


STM can move atoms around on a surface.

SPM Lithography

Iron on Copper Iron on Copper

Eigler, et al. from IBM


Dip Pen Lithography.SPM Lithography

Mirkin, et al. from Northwestern University


Electrochemistry: carbon nanotube used as a conducting AFM tip for local oxidation of Si.

SPM Lithography

Dai, et al. from Stanford


Million Cantilever Wafer


Millipede Memory


Cantilever Gas Sensors (Noses)

ou nanolab/nsf nue/bumm & johnson afm atomic force microscopy

Documents

tipsample force

attractive force mode

waals force curve slide

capillary force

surface sample

mode contact mode

probe tip

atomic force microscope