REU Wednesdays at One:
Scanning Probe Microscopy
Susan Enders
Department of Engineering Mechanics
June 30th, 2010
Microscopy
Optical
 uses visible light and
system of lenses to
magnify
 oldest and simplest
design
 new digital
microscopes use
CCD camera
 magnification up to
2000 times
Electron
Scanning Probe
 uses a particle beam
 forms images of surfaces
of electrons to
illuminate a
specimen
 create a highly-
magnified image
 uses electrostatic
and electromagnetic
lenses
 magnification up to
2 million times
using a physical probe that
scans the specimen
 surface image produced by
mechanically moving probe
in a raster scan of the
specimen and recording
probe-surface interaction
as a function of position
 atomic resolution
 was founded in 1981
SPM Types
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AFM, atomic force microscope
BEEM, ballistic electron emission microscope
EFM, electrostatic foce microscope
ESTM, electrochemical scanning tunneling microscope
FMM, force modulation microscope
KPFM, kelvin probe force microscope
MFM, magnetic force microscope
MRFM, magnetic resonance force microscope
NSOM, Near-Field scanning optical microscope (or SNOM, scanning near-field optical
microscopy)
PFM, Piezo Force Microscopy
PSTM, photon sanning tunneling microscope
PTMS, photothermal microspectroscopy/microscope
SAP, scanning atom probe
SECM, scanning electrochemical microscope
SCM, scanning capacitance microscope
SGM, scanning gate microscope
SICM, scanning ion-conductance microscope
SPSM, spin polarized tunneling microscope
SThM, scanning thermal microscope
STM, scanning tunneling microscope
SVM, scanning voltage microscope
SHPM, scanning Hall probe microscope
Scanning tunneling
microscope - STM

powerful technique for viewing surfaces at the atomic level
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invented in 1981 by Gerd Binnig and Heinrich Rohrer (at IBM
Zürich)
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Nobel Prize in Physics in 1986
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probes the density of states of a material using tunneling
current
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good resolution is considered to be 0.1 nm lateral resolution
and 0.01 nm depth resolution
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can be used not only in ultra high vacuum but also in air and
various other liquid or gas ambients, and at temperatures
ranging from near zero kelvin to a few hundred degrees
Celsius
based on the concept of quantum tunneling
Classic “Tunnel Effect”
Quantum Mechanics:
The Tunnel Effect
Using the Tunnel Effect
for Imaging
IT ~ exp(  d )
Binnig & Rohrer, 1982, Nobel Prize in Physics 1986
Using the Tunnel Effect
for Imaging
IT ~ exp(  d )
Binnig & Rohrer, 1982, Nobel Prize in Physics 1986
Using the Tunnel Effect
for Imaging
piezo-céramique
pm = 0.000000001 mm
pointe
échantillon
IT ~ exp(  d )
movie: by courtesy of Dr. Dirk Sander, Max Planck Institute Halle, Germany
sander@mpi-halle.de, www.mpi-halle.de
The Real Thing
2 inch
Surface of Au: atoms are visible!
Atoms and Molecules at Surfaces
4.2 Kelvin
Co atoms on a Copper surface
N. Knorr, A. Schneider
Dept. Prof. Kern, MPI Stuttgart, Germany
Moving Things Around
Fe atoms on Cu (111)
IBM Almaden , D. Eigler
Fe atoms on Cu (111)
IBM Almaden , D. Eigler
Xe / Ni(110)
D. Eigler & E. Schweizer,
Nature 344, 524 (1990)
Atomic Force Microscope
AFM
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very high-resolution type of SPM
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resolution of fractions of a nanometer - 1000 times
better than the optical diffraction limit
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STM precursor to the AFM
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Binnig, Ouate and Gerber invented the first AFM in
1986
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one of the foremost tools for imaging, measuring
and manipulating matter at the nanoscale
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information is gathered by "feeling" the surface
with a mechanical probe
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Piezoelectric elements facilitate tiny but accurate
and precise movements enable the very precise
scanning
Cantilevers and
their properties
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Typically made of
SixNy
Spring constants
in the range of 1 40 N/m
(Forces from 0,1nN
– 20 µN)
Tips range from a
pyramid to very
sharp, high aspect
ratio tips, to flat
punches.
J. E. Sader, Review of Scientific Instruments -- April 2003 -- Volume 74, Issue 4,
pp. 2438-2443
Basic principle
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cantilever with a sharp tip (probe) is used to scan the specimen surface
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tip is brought into proximity of a sample surface -> forces between tip and sample lead
to a deflection of the cantilever according to Hooke’s law
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forces measured in AFM include mechanical contact force, Van der Waals forces, capillar
forces, chemical bonding, electrostatic forces, magnetic forces, Casimir forces, solvation
forces etc…
deflection is measured using a laser spot reflected from the top surface of the cantilever
into an array of photodiodes
if tip was scanned at a constant height -> risk that the tip collides with the surface ->
damage -> feedback mechanism adjusts the tip-to-sample distance to maintain a
constant force between tip and sample
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sample is mounted on a piezoelectric tube which moves it z direction for maintaining a
constant force, and x and y for scanning
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AFM can be operated in a number of modes, depending on the application
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possible imaging modes are divided into static (also called Contact) modes and dynamic
(or non-contact) modes where the cantilever vibrates
AFM can be used to image and manipulate atoms and structures on surfaces
Atomic Force Microscope (AFM)
Laser Beam Deflection
Laser Beam Deflection
Laser Beam Deflection
Imaging modes
Static mode
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the static tip deflection is used as a feedback signal
measurement of a static signal is prone to noise
and drift -> low stiffness cantilevers used to boost
the deflection signal
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close to the surface of the sample, attractive forces
can be quite strong -> tip 'snaps-in' to the surface
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static mode AFM is almost always done in contact
where overall force is repulsive
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technique called 'contact mode‘
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force between the tip and the surface is kept
constant during scanning by maintaining a constant
deflection
Imaging modes -Dynamic mode
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tip of the cantilever does not contact the sample surface
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cantilever is oscillated at a frequency slightly above its resonance
frequency (amplitude ~ few nanometers (<10nm))
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van der Waals forces (strongest from 1nm to 10nm above surface)
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or other long range force which extends above the surface acts to
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decrease the resonance frequency of the cantilever
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decrease in resonance frequency combined with feedback loop
system maintains a constant oscillation amplitude or frequency by
adjusting the average tip-to-sample distance
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Measuring tip-to-sample distance at each (x,y) data point allows
software to construct topographic image of sample surface
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AFM does not suffer from tip or sample degradation effects
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non-contact AFM preferable to contact AFM for measuring soft
samples
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in case of rigid samples, contact and non-contact images may look
the same
Advantages and disadvantages
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AFM provides a true 3D surface profile
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samples viewed by AFM do not require special treatments
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Most AFM modes work perfectly well in ambient air or even a liquid
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Study of biological macromolecules and even living organisms
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gives true atomic resolution in ultra-high vacuum (UHV) and liquid environments
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high resolution AFM is comparable in resolution to STM and TEM
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disadvantage of AFM is image size (maximum height in mm and maximum scanning
area around 150 by 150 mm)
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incorrect choice of tip for required resolution can lead to image artifacts
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relatively slow rate of scanning during AFM imaging often leads to thermal drift in the
image
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AFM images can be affected by hysteresis of the piezoelectric material and cross-talk
between the (x,y,z) axes ->may require software enhancement and filtering
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filtering could "flatten" out real topographical features
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AFM probes cannot measure steep walls or overhangs
DualScope™ Microscope
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Sample size: Ø 50 mm
Sample height:
5 mm
Scan size:
Z range:
40 x 40 µm
2.7 µm
www.dme-spm.dk
Bearing
www.dme-spm.dk
Wooden Fibres
www.dme-spm.dk
Landing Zone of Hard Disk
www.dme-spm.dk
Motheye
www.dme-spm.dk
Near-field scanning optical
microscope NSOM/SNOM
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nanostructure investigation that breaks the far field resolution limit
by exploiting the properties of evanescent waves
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done by placing the detector very close (<< λ) to the specimen
surface
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allows for the surface inspection with high spatial, spectral and
temporal resolving power
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resolution of the image is limited by the size of the detector
aperture and not by the wavelength of the illuminating light
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lateral resolution of 20 nm and vertical resolution of 2-5 nm has
been demonstrated
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contrast mechanism can be adapted to study different properties
(refractive index, chemical structure, local stress)
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Dynamic properties can also be studied at a sub-wavelength scale
Operating Principle
optical microscopy
1870 Ernst Abbe :
d > λ / (2sinθ)
d = distance between the two
objects
λ = wavelength of the incident
light
2θ = angle through which the
light is collected.
best resolution with optical light
is about 200 nm
Operating Principle
SNOM
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if a subwavelength hole in a
metal sheet is scanned close
to an object, a super-resolved
image can be built up from
the detected light that passes
through the hole
light passes through a subwavelength diameter aperture
and illuminates a sample that
is placed within its near field
(distance much less than the
wavelength of the light)
achieved resolution is far
better than conventional
optical microscopy
Modes of Operation
1) Transmission mode imaging
- sample is illuminated through the probe
- light passing through the sample is collected and detected
2) Reflection mode imaging
- sample is illuminated through the probe
- light reflected from the sample surface is collected and detected
3) Collection mode imaging
- sample is illuminated with a macroscopic light source from the top or bottom
- probe is used to collect the light from the sample surface
4) Illumination/collection mode imaging
- probe is used for both the illumination of sample and for collection of reflected
signal
SNOM - Limit
Amount of light that can be transmitted by a small aperture poses a limit on
how small it can be made before nothing gets though
To a degree this can be lived with, as more optical power can be generated,
but the cutoff is so severe that it cannot be made smaller.
Next Step
Field Enhancement Microscopy
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instead of using a small
aperture, a metal tip is used to
provide local excitation
if a sharp metal tip is placed in
the focus of a laser beam, an
effect called local field
enhancement will cause the
electric field to become roughly
1000 times stronger
enhancement is localized to the
tip, which has a typical
diameter of 10 nm
as this tip is scanned over the
surface, an image can be
formed with a resolution as
fine as the tip
Optical scans made with
BioLyser SNOM
Shear Force Imaging
Bovine Kidney Cells,
non-contact mode: 10 µm scan
3D view of Bovine Kidney cell sample
Aluminum projection pattern on glas
Julien Toquant, University of Basel
SPM Advantages
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resolution not limited by diffraction, but only by the size of the
probe-sample interaction volume ( few picometers)
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ability to measure small local differences in object height (like
that of 135 picometre steps on <100> silicon)
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probe-sample interaction extends only across the tip atom or
atoms involved in the interaction
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interaction can be used to modify the sample to create small
structures (nanolithography)
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do not require a partial vacuum but can be observed in air at
standard temperature and pressure or while submerged in a
liquid reaction vessel.
SPM Disadvantages
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detailed shape of the scanning tip difficult to determine
(effect particularly noticeable if the specimen varies greatly
in height over lateral distances of 10 nm or less)
generally slower in acquiring images due to the scanning
process
embedding of spatial information into a time sequence
leads to uncertainties in metrology (lateral spacings and
angles) which arise due to time-domain effects like
specimen drift, feedback loop oscillation, and mechanical
vibration
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The maximum image size is generally smaller
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not useful for examining buried solid-solid or liquid-liquid
interfaces
The End