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Scanning probe microscopes
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Atomic Force Microscope (AFM)
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Atomic Force Microscopy
Measures the atomic forces between theprobe and sample
Image formed by feedback loop which
maintains constant tip-sample force duringscanning
Scans both insulators and conductors
Most widely used form of SPM
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Interatomic Force versus Distance Curve
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Atomic Force Microscopy
Three primary modes of operation:
Contact Mode AFM
Non-contact Mode AFM
TappingMode AFM
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Contact Mode AFM
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Contact Mode AFM
Tip is in contact with the surface.
The bending of the cantilever is proportional to the
contact force. The bending of the cantilever is monitored
through
the vertical deflection of the reflected laser on
thephotodetector.
Feedback loop looks at the vertical deflection tomaintain
constant contact force during scanning.
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Contact Mode AFM
Scans probe on the end of a soft cantilever (springconstant =
0.01 - 1.0N/m) across the samplesurface
Feedback loop maintains constant cantilever
deflection during scanning
Tip contacts surface through adsorbed fluid layer
Tip-sample force of 10-7 to 10-11N
Tip radius of curvature = 5 to 40nm
Operates in air and fluid
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Contact Mode AFM
Applications:
Atomic and molecular resolution imaging
Force-Distance Measurements
Characterization of hard, rigid surfaces (metals,thin films)
Some Biological Surfaces
Electrochemical AFM
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Contact Mode AFM Image of Al/Cu alloy film deposited on Si
10m scan
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Contact Mode AFM
Disadvantages
Lateral (shear) forces can damage soft orfragile samples
Imaging force can be increased by:
Surface tension forces due to adsorbedfluid layer
Electrostatic forces
Operation under fluid alleviates some of theseproblems
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Contact Mode in Fluid
Advantages
Reduces surface tension and electrostatic
forces Allows samples to be imaged in native
fluid environment
Disadvantages
Only partially alleviates lateral forces Not practical to
submerge many surfaces
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Contact Mode in Air Contact Mode in Fluid
Contact Mode in Fluid
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Non-contact Mode AFM
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Non-Contact / Intermittent Contact AFM
The cantilever oscillates at its resonant frequency.Typically
frequency ranges from 70 KHz to 350KHzdepending on the dimension of
the cantilever.
The contact force is related to the amplitude ofoscillation.
The amplitude of oscillation is monitored using the
Photodetector. The feedback loop maintains a constant contact
force
via keeping the Amplitude of oscillation constant.
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Non-contact Mode AFM
Cantilever oscillated at or near its resonancefrequency to
obtain AC signal
Scans probe on the end of oscillating cantilever
above the adsorbed f luid layer on the samplesurface
Probe responds to Van der Waals forces whichextend from 1nm to
10nm above the adsorbed
layer Feedback loop maintains constant oscillation
amplitude or frequency during scanning
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Contact Mode vs Non-Contact Mode (Pros and Cons)
Contact Mode
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Contact Mode vs Non-Contact Mode (Pros and Cons)
Non-Contact Mode
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Non-contact Mode AFM
Advantage Nondestructive measurement
Disadvantages
Resolution limited by tip-sample
separation
Must scan slowly to avoid contacting andgetting stuck in
adsorbed layer
Due to these disadvantages, applications for non-
contact mode imaging have been limited
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TappingMode AFM
Incorporates benefits of contact mode and non-contact mode while
overcoming their drawbacks
Scans probe on end of oscillating cantilever acrossthe sample
surface
Oscillation amplitude typically >20nm Large oscillation
amplitude allows tip to "tap" on
the sample surface without getting stuck inadsorbed layer
Feedback loop maintains constant oscillationamplitude
Operates in air and fluid
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TappingMode AFM
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Etched Silicon Cantilever/Tip
100nm
Nominal radius ofcurvature = 5-10nm
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TappingMode AFM
Amplitude reduced
"Free" Amplitude
"Tapping"Fluid layer
10-100 nm
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TappingMode Advantages
Virtually eliminates shear (lateral) forces
Reduces normal (vertical) forces
Provides the high resolution of a contacting
technique Allows imaging of soft, fragile, and adhesive
surfaces without risk of sample damage
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, -Mode, and TappingMode
Contact Mode Noncontact Mode TappingMode
Resulting image profiles
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TappingMode Applications
Semiconductor surfaces
Data Storage Devices
Thin Films Polymers
Biological Surfaces (airand fluid environments)
Photoresist
Optics
Metals and insulators
Composites Numerous other
applications
