Scannedproximity Probe Microscopy SPM Background - PowerPoint PPT Presentation

Title: Scannedproximity Probe Microscopy SPM Background


1
Scanned-proximity Probe Microscopy (SPM)
Background

• Emphasis on Atomic Force Microscopy (AFM)

• Reading
• SPM Features
• AFM Specifics
• AFM Operation (Conceptual)
• AFM and Nanotechnology

2
Reading

• Window on a Small World, McGuire, Todays
  Chemist, June 2002Vol. 11, No. 5 pp 2124
  http//pubs.acs.org/subscribe/journals/tcaw/11/i06
  /html/06inst.html
• AFM Technology Overview From Veeco (Digital
  Instruments) the manufacturer of the AFM that
  will be used.
• http//nano.nd.edu/SC190/scanningprobe.pdf
• AFM Nanomanipulation From Veeco on the use of
  the AFM in direct nanotechnology applications.
  http//www.veeco.com/pdfs.php/70
• Scanning Probe Microscopy (SPM) Instructions
  http//frontpage.okstate.edu/nanotech/Lab/lab3/SPM
  _Instructions_082003.pdf

3
Scanned Proximity Probe Microscopy Common Features

• Piezoelectric (PZ) positioning
• PZ crystals expand/contract under applied voltage
  (d11 4x10-10 m/V)
• Example A commercial linear actuator produces 15
  mm displacement for 100 V applied with sub nm
  resolution
• PZ crystals generate voltages under applied force
  (g33 1x10-2 V/Nm)
• Deflection Feedback Loop
• Height (z) dependent signal
• Example Tunneling current for Scanning Tunneling
  Microscope (STM)
• Example Van der Waals forces for Atomic Force
  Microscope (AFM)
• Probe/Tip
• Photolithographic Fabrication Techniques
• Resolution determined by probe size (not
  diffraction)

4
AFM Capabilities (Advantages)

• Able to achieve a resolution of 10 pm with
  special tips (typical resolution 10 nm)
• Able to image samples in air and under liquids
• Able to measure in 3 D (within limits)
• Able to measure non-conductive surface (unlike
  SEM or STM)

5
AFM Capabilities (Disadvantages)

• Incorrect tip choice can lead to measurement
  artifacts or sample damage
• Depth of field limited by cantilever / z
  positioning PZ
• Scan area limited by PZ scanners
• Slow scan rate compared to SEM

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AFM Components
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AFM Probe

• Tip
• Modifies measurements
• Typically Si or SiN for ease of fabrication
• Many variations depending on application
• Cantilever (Tip at the end)
• Low spring constant (Hookes Law ? F -kz)
• Low weight for high resonant frequency (
  )
• Coated for reflectivity

Tip on apex of cantilever
Images from http//stm2.nrl.navy.mil/how-afm/how-a
fm.htmlGeneral20concept
8
AFM Modes

• Contact mode (Repulsive-Static)
• AFM tip rides on the sample in close contact with
  the sample surface (low k)
• The force in the feedback loop is friction
• May interact with the sample surface
• Non-contact mode (Attractive-Dynamic)
• AFM tip hovers 5-15 nm away from the sample
  surface
• The force in the feedback loop is typically van
  der Waals (VDW) forces
• Applied force (dependent on height z) changes
  cantilever oscillation frequency.
• Tapping mode (Repulsive-Dynamic)
• AFM tip taps surface as it maps z.
• Eliminates lateral forces or hysteresis due to
  the tip sticking on the sample.
• Less likely to damage the sample

9
AFM Measurement
Changes of the surface properties along the scan
line
Changes of interaction forces between the probe
tip and sample surface
Contact Mode
Non-Contact Mode
Deflection of the cantilever
Change of oscillation amplitude and phase of the
cantilever
Laser beam position oscillates on the PSPD
Change in laser position on PSPD
Electronic signal to control and recording
electronics
Signal processing to form image
10
AFM Visualization (Contact Mode)
PSPD measures change optical beam position ?
change in cantilever height
Sample is raster scanned
11
Forces at Work

• Atomic Forces (approx.)

U
Repulsive
Attractive
B and A coefficients depend on the surfaces
involved.
Detectable forces for an AFM 1 nN in the contact
regime and 1 pN in the noncontact regime
(Theoretical Limit 10-18 N with heroic measures.)
R. Wiesendanger, "Chapter 11. Future
Nanosensors." In H. Meixner, R. Jones, eds.,
Volume 8 Micro- and Nanosensor Technology /
Trends in Sensor Markets. In W. Gopel, J. Hesse,
J.N. Zemel, eds., Sensors A Comprehensive
Survey, VCH Verlagsgesellschaft mbH, Weinheim
Germany, 1995 pp. 337-356.
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Approach-Retract Phenomenon
Approach
Snap On
Impress
Force (proportional to cantilever deflection)
Retract
Snap Off
(NOT the tip Z position!)

• ApproachVDW forces pull tip toward surface
• Snap OnWhen close enough, VDW grow stronger than
  restoring spring force in the cantilever.
• ImpressTip pressed into sample with positive
  force after reaching the set point deflection
• RetractTip adheres to surface giving rise to
  hysteresis on measured force/position curve
• Snap OffSpring force dominates the tip-sample
  adhesion and tip leaves surface

13
Alternate Scanning Probe Systems

• Magnetic ForceMaps surface magnetic field (think
  hard drives)
• Electrostatic ForceMaps surface potential (100
  nm resolution)
• Lateral ForceMaps friction force experienced by
  scanning probe on surface orthogonal to scanning
  direction
• Magnetic ResonancePossible 3D imaging of
  individual molecules via resonant electron spin
  flipping with an antenna on the cantilever
• Near Field Optical Microscopy
• Probe tip has a small aperture (radius ltlt l) for
  optical wavelength measurements
• And many, many more

14
Nanotech Applications

• Measurement of nanostructures
• Nano indention of surfaces
• Dip Pen Nanolithography
• Manipulating nanoparticles (building
  nanostructures)
• Precision electro chemistrysupply electrons by
  applying voltage across AFM tip and substrate