Download Scanning Probe Microscopy

Scanning Probe Microscopy
Colin Folta
Matt Hense
ME381R 11/30/04
Outline
Background and History
 AFM

– MFM
– EFM
– SThM
STM
 SHFM
 SNOM

Background
First scanning probe microscope
invented in 1981 by Binning and Roher
 Wide range of applications

– Topography/Atomic Structure
– Magnetic/Electric fields
– Surface temperatures
Branches of Scanning Probe Microscopy
http://spm.phy.bris.ac.uk/
Operation

Scanning probe
microscopes
operate by detecting
the deflection in the
cantilever
 Modern scanning
probe microscopes
use a split photo
diode to detect the
deflection
http://spm.phy.bris.ac.uk/
Atomic Force Microscopy (AFM)
Most widely used branch of scanning
probe microscopy
 Operates by measuring the interaction
force between the tip and sample

AFM Operation Modes

Contact Mode
– Tip remains in the repulsive regime of the intermolecular force curve

Tapping Mode
– Tip is oscillated at a high frequency
– Deflections in the oscillations are observed

Non-Contact Mode
– Tip is oscillated outside of the repulsive regime
Image Defects

Broadening
– Occurs when feature is roughly the same
size as the radius of curvature
– Side wall of tip comes into contact before
the tip itself

Compression
– The forces involved actually change the
shape of the specimen (ex. DNA)
Image Defects Cont.

Aspect Ratio
– Steep walled
features become
distorted
– The tip can not follow
a perfectly vertical
wall
http://spm.phy.bris.ac.uk/
Magnetic Force Microscopy (MFM)
Coated with a magnetic covering
 Two modes of operation

– Non-vibrating for larger magnetic fields
– Vibrating for weaker fields that require a
greater sensitivity
MFM Cont.

Uses a two pass
technique
– First pass finds
topography of sample
– Second pass finds the
magnetic field

On the second pass tip
is kept at a constant
height
http://www.ntmdt.ru/SPM-Techniques/SPM-Methodology/
Magnetic_Force_Microscopy_MFM/text45.html
Electrostatic Force Microscopy (EFM)
A bias is used to create an electrostatic
field between the tip of the probe and
the sample
 Two uses

– Determine which regions are conducting
and which are insulating
– Determine the electric potential at different
points
Scanning Tunneling Microscopy (STM)

Electrons are
transferred between the
tip and the sample due
to overlapping orbitals
– A net transfer can be
sustained by applying a
voltage across the gap

http://stm1.phys.cmu.edu/stm/si5x5s.gif
Change in current is a
result of a change in the
tip-sample separation
http://www.d.umn.edu/~jmaps/stm1.html
STM Modes of Operation

Constant Current
– Maintain a constant tunneling current by
adjusting the separation

Constant Height
– Maintain a constant height and measure
the current change
Scanning Thermal Microscopy (SThM)

Thermocouple is placed on the tip of the
probe
 Combined with AFM, SThM can associate
thermal properties with surface features
 By heating the tip ~30K higher than the
sample, local thermal conductivity can be
determined
 Thermocouple can be used conventionally to
measure temperature distribution along the
sample
Scanning Near Field Optical Microscopy
(SNOM)

Typical optical
microscopes
– Limited by the Abbe
diffraction barrier
– Resolution equal to
one half of the
wavelength of the
light
http://molebio.iastate.edu/~p_haydon/nsom.html
SNOM Cont.

SNOM
– Uses a very small
aperture
– Keeps the specimen
in the near field
regime
– Resolution is
determined by the
aperture diameter
http://spm.phy.bris.ac.uk/
Shear Force Microscopy (ShFM)

Probe oscillates parallel to the specimen
 Oscillation changes because of Van der
Waals interactions. Topography can be
determined from these changes.
 Advantages
– More rigid set up
– “Jump to contact” problem is almost eliminated

Disadvantages
– Can be very difficult to set up
– Probe tip is very hard to reproduce reliably
Questions?