e-, hv, ione-, hv, ion tip
Characterization Techniques
Primary Secondary
Investigate:• Structure/morphology
• Surface analysis
• Compositional/elemental and molecular
•Physical properties
•?
•?
Microscopy
Scanning ProbeOptical Electron
▪ uses visible light and
system of lenses to
magnify
▪ oldest and simplest
design
▪ new digital
microscopes use
CCD camera
▪ magnification up to
2000 times
▪ uses a particle beam
of electrons to
illuminate a
specimen
▪ create a highly-
magnified image
▪ uses electrostatic
and electromagnetic
lenses
▪ magnification up to
2 million times
▪ forms images of surfaces
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
Scanning probe microscopy has been essential in the development
of nanotechnology:
Characterisation/visualisation tool at nanoscale.
Nanomanipulator and modificator.
… a family of tools for the nanotechnology world..
Scanning Probe Microscopy (SPM)
Operates by measuring the interaction force between the tip
and sample
STM Modes of Operation• Constant Current
• Constant Height
AFM Operation Modes• Contact Mode
• Tapping Mode
• Non-Contact Mode
Wide range of applications
Topography/Atomic Structure
Magnetic/Electric fields
Surface temperatures
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
• Change in current is a result of
a change in the tip-sample
separation
E
Classical
mechanics
Impenetrable
barrier
Quantum
mechanicsTunneling
Quantum Tunnelling
➢ In classical physics e flows are not possible without a direct connection by a wire between two surfaces
➢ On an atomic scale a quantum mechanical particle behaves in its wave function.
➢ There is a finite probability that an electron will “jump” from one surface to the other of lower potential.
➢ A sharp conductive tip is brought to within a few Angstroms of the surface of a conductor (sample).
➢ Bias voltage is applied, Fermi levels shift
➢ The wave functions of the electrons in the tip overlap those of the sample surface
➢ Electrons tunnel from one surface to the other of lower potential.
Tunneling Current
The direction of current flow is determined by the polarity of the bias. If the sample is biased -Ve with respect to the tip, then electrons will flow from the surface to the tip, whilst if the sample is biased +Ve with respect to the tip, then electrons will flow from the tip to the surface.
)exp( cdVI −=
The probability of tunnelling is exponentially-dependent upon the distance
of separation between the tip and surface : the tunnelling current is
therefore a very sensitive probe of this separation
Ideally a STM probe tip is very pointed (1-2
atoms at the end) and has a relatively low work
function. Etched tungsten crystals are ideal and
are nearly identical to field emitters
➢Tunneling current exhibits an
exponentially decay with an
increase of the separation
distance!
➢Exponential dependence leads to fantastic resolutions. Order of 10-12 m in the perpendicular direction and ~10-10 m in the parallel directions
Resolution
Constant Height
Constant Current
If the tunneling current is kept
constant the Z position of the tip
must be moved up and down. If
this movement is recorded then
the topography of the specimen
can be inferred.
Alternatively if the Z position of
the tip is kept constant the
tunneling current will change as
it moves across the surface. If
the changes in current are
recorded the then the
topography of the specimen can
be inferred
STM Modes of Operation
STM tip:
atomically sharp needle and terminates in a single atom
➢Pure metals (W, Au)
➢Alloys (Pt-Rh, Pt-Ir)
➢Chemically modified conductor (W/S, Pt-Rh/S, W/C…)
➢Preparation of tips: cut by a wire cutter and used as is
cut followed by electrochemical etching
➢Electrochemical etching of tungsten tips. A tungsten wire, typically 0.25 mm in diameter, is vertically inserted in a solution of 2M NaOH. A counter electrode, usually a piece of platinum or stainless steel, is kept at a negative potential relative to the tungsten wire.
➢The etching takes a few minutes. When the neck of the wire near the interface becomes thin enough, the weight of the wire in electrolyte fractures the neck. The lower half of the wire drops off.
The two most commonly used tips are made from either a Pt-Ir (80/20) alloy or tungsten wire.
➢ Control of environment vibration:
➢ building the instrument with sufficient mechanical
rigidity;
➢ hung on a double bungee cord sling to manage
vibration;
➢ vibration isolation systems have also been made with
springs and frames;
➢ operate at night with everything silent.
➢ Ultrahigh vacuum (UHV): to avoid contamination of the
samples from the surrounding medium. (The STM itself
does not need vacuum to operate; it works in air as well as
under liquids.)
➢ Using an atomically sharp tip.
Forces applied to a segment of
material lead to the appearance of
electrical charge on the surfaces of
the segment. The specific distribution
of electric charges in the unit cell of a
crystal is the source of this
phenomenon.
The Piezoelectric Effect:
Piezoelectric Effect
• Piezoelectricity is the ability of some materials (notably
crystals and certain ceramics) to generate an electric field in
response to applied mechanical stress.
• If the material is not short-circuited, the induced charge
generates a voltage across the material.
• The piezoelectric effect is reversible, that is, the piezo
materials exhibit:
• the direct piezoelectric effect – the production of electricity
when stress is applied,
• the inverse piezoelectric effect – the production of stress
and/or strain when an electric field is applied.
•PZT (Lead zirconium
titanate) ceramics must
be poled at an elevated
temperature.
•The ceramic now
exhibits piezoelectric
properties and will
change dimensions
when an electric
potential is applied.
Tube
Piezoelectric scanners
+y
-x +x
-y
Piezo
tube
A(111) S(√3x√3)R30o S8
Ru(0001)
Grafit
Ge (100)
Concluding remarks
➢Disadvantage of STM:
1.Making atomically sharp tips remains something of a dark art!
2.External and internal vibrations from fans, pumps, machinery, building movements, etc. are big problems.
3.UHV-STM is not easy to built and handle.
4.The STM can only scan conductive surfaces or thin nonconductive films and small objects deposited on conductive substrates. It does not work with nonconductive materials, such as glass, rock, etc.
5.The spatial resolution of STM is fantastic, but the temporal resolution is typically on the order of seconds, which prevents STM from imaging fast kinetics of electrochemical process.
➢STM is one the most powerful imaging tools with an unprecedented precision.
ATOMIC FORCE MICROSCOPY
Tips are typically made from Si3N4 or Si, and extended down from the end of a
cantilever. A diode laser is focused onto the back of the reflective cantilever. As
the tip scans the surface of the sample, moving up and down with the contour
of the surface, the laser beam is deflected into a dual element photodiode which
measures the difference in light intensities between the upper and lower
photodetectors, and then converts to voltage.
1. Laser
2. Mirror
3. Photodetector
4. Amplifier
6. Sample
7. Probe
8. Cantilever
Force Sensors - Cantilevers
Si , Si3N4,
Diomand
There are some simple criteria to be considered, when cantilevers are fabricated:
• resonance frequency fR > 100Hz (building vibrations), > 10kHz (sound waves)
• high force sensitivity requires low spring constants (MFM:, 0.1 N/m, mRFM: 0.001
N/m)
• atomic resolution requires spring constant to be in range of atomic spring constants
> 10N/m
• thermal vibrations of the cantilever < 0.1nm, i.e. k>0.4N/m @ 300K
It can be shown that only cantilevers of dimensions in the micrometer range fulfil
these design criteria.
Generally, higher resonance frequencies require smaller cantilevers.
P. Grutter, McGill University
Spring constants k and resonant frequency f of cantilevers
Spring constant k :
typical values: 0.01 - 100 N/m
Young’s modulus EY ~ 1012
N/m2
Resonant frequency fo:
typical values: 7 - 500 kHz
W
L
t
3
3
4 L
wtEk Y=
YE
L
tf
20 162.0=
Basics
• most widely used sensor.
• adjustment of Laser or Super-LED
beam to the cantilever (2 directions)
• adjustment of 4Q-diode to reflected
beam
Advantages:
• vertical & lateral deflection
• easy to use
• Robust and cheap
Disadvantages
• not intrisically quantitative, requires
calibration
• requires large volume
Deflection Sensors
A laser beam is reflected off the rear side of the cantilever. Angular deflections of the
laser beam are measured with a position sensitive detector (4-quadrant photo diode).
The A-B-signal is proportional to the normal force or topography and the C-D-
signal is proportional to the torsional force.
Electron Tunneling: original concept, potentially sensitive, practically problematic
Beam Deflection: most widely used, robust, high sensitivity, not directly quantitative,
requires calibration
Interferometry: best sensitivity, quantitative, uses limited space, complicated
Capacitance: sensor can be microfabricated, strong force from sensor, limited
sensitivity
Piezoresistance: ideal for microfabrication & integration, limited sensitivity, heating of
cantilever
Piezoelectric: mostly quartz tuning forks, good for true atomic resolution, limited
sensitivity
STM
AFM
Relevant Forces
typical long-range forces (> 1nm):
• van der Waals
• electrostatic, magnetic, …
forces in liquids:
• hydrophobic / hydrophilic forces
• steric forces
• solvation forces
typical short-range forces
(contact / near contact):
• short-range repulsive forces
(Pauli exclusion) or ionic repulsion forces
• short-range chemical binding forces
Refs.:
J. Israelachvili
Intermolecular and Surface Forces with Applications to Colloidal and
Biological Systems, Academic Press (1985)
D. Tabor
Gases, liquids and solids, Cambridge University Press (1979)
P. Grutter, McGill University
FvdW = AR/6z2
A…Hamaker const.
R…Tip radius
z…Tip - sample separation
A depends on type of materials (polarizability). For most materials and vacuum A~1eV
Krupp, Advances Colloidal Interface Sci. 1, 113 (1967)
R~100nm typical effective radius
-> FvdW ~ 10 nN at z~0.5 nm
Dipole-dipole forces between non-permanent dipoles (dispersion forces). These act
between dipoles that arise from fluctuations and dipoles induced in their electric field.
They are always present and attract even chemically inert noble gas atoms. The
range of dispersion forces is limited. When the distance between molecules is larger
than the way light can travel during the characteristic lifetime of the fluctuations, the
dispersion forces are weakened. This effect is called retardation. The van der Waals
force at short distances decays like F = 1/r6, beyond r > 5 nm this power law reduces
to F = 1/r7.
Because the range of van der Waals forces is limited, the tip-sample geometry of the
force microscope can be well approximated as a sphere approaching a semi-infinite
body.
van der Waal's Forces
Short-Range Forces
Due to overlap of electron wave functions and from the repulsion of the ion cores.
can be both attractive:
• attractive when the overlap of electron waves reduces the total energy. These
situations are comparable to molecular binding.
• around 0.5 - 1 nN per interacting atom at tip-sample distances typical for STM
operation.
• decay length of the order of atomic units, i.e. 0.05 nm for metallic adhesion, but
around 0.2 nm for covalent bonding.
true atomic resolution AFM operates with these forces
or repulsive:
• repulsive when strong electron wave overlap (Pauli exclusion principle). These
forces are directly connected to the total electron density. The ionic repulsion acts
for small distances, where the screening of the ion cores by the electrons falls
away.
• usual contact AFM operates with these forces.
• Model potentials like the Lennard-Jones or the Morse potential are used to describe
shortrange forces.
Electrostatic forces
Felectrostatic = p e0 RU2/ z
U…Potential difference
R…Tip radius
z…Tip - sample separation
R~100nm typical effective radius
U=1V
-> Felectrostatic ~ 5 nN at z~0.5 nm
•The strength and distance dependence of
electrostatic forces obey Coulombs law.
Chemical forces
FMorse = Ebond / z • (2e-k(z-s) - e-2k(z s))
Ebond …Bond energy
k …decay length radius
s…equilibrium distance
Electrostatic interaction: Caused by both
the localized charges and the polarization of
the substrate due to the potential difference
between the tip and the sample. It has been
used to study the electrostatic properties of
samples such as microelectronic structures,
charges on insulator surfaces, or
ferroelectric domains.
Solvation Forces
Magnetic Forces
Fmagntic = mtip • Hsample
Magnetic interaction: Caused by magnetic dipoles both on the tip and the sample.
This interaction is used for Magnetic Force Microscopy to study magnetic domains
on the sample surface.
P. Grutter, McGill University
Capillary forces (water layer)
There is always a water layer on a surface in air!
Fcapillary = 4p R g cos
g …surface tension, ~10-50 mJ/m2
…contact angle Surface
Water
Tip
Can be LARGE (several 1-10 nN)
Total force on cantilever = sum of ALL forces
At very small tip-sample distances (a few angstroms) a very strong repulsive force
appears between the tip and sample atoms. Its origin is the so-called exchange
interactions due to the overlap of the electronic orbitals at atomic distances. When this
repulsive force is predominant, the tip and sample are considered to be in “contact”.
Noncontact (NC) AFM
Both are based on a Feedback Mechanism
of constant oscillation amplitude.
•Non-contact mode: amplitude set as ~
100% of “Free” amplitude;
•Tapping mode: amplitude set as ~ 50 -
60% of “Free” amplitude.
•Tapping mode provides higher resolution
with minimum sample damage.
•Most of times, non-contact mode is
operated as tapping mode.
• Frictional Force
Lateral Force Microscopy
Low
Friction
Low
Friction
High
Friction
Topography Profile
Friction Profile
Non-ContactTappingContact
•Polymer
latex
•particle on
•mica
smallsmalllarge•Damage
>10 nm~1 nm<0.2 nmDistance
smallsmalllargeFriction
0.1-0.01nN0.1-0.01nN1-10 nNForce
•hard•hard•softCantilever
•High resolution topography (top)
and Lateral Force mode (bottom)
images of a commercially avail-
able PET film. The silicate fillers
show increased friction in the
lateral force image
Laterally twisted due to friction
www.tmmicro.com/tech/modes/lfm.htm
Lateral Force Microscope
Force Modulation Microscopy
•Contact AFM mode
•Periodic signal is applied to either to
tip or sample
•Simultaneous imaging of
topography and material properties
•Amplitude change due to elastic
properties of the sample
Magnetic Force Microscopy
•Ferromagnetic tip: Co, Cr
•Noncontact mode
•vdW force: short range force
•Magnetic force: long range force; small
force gradient
•Close imaging: topography
•Distant imaging: magnetic properties
http://www.tmmicro.com/tech/index.htm
Glass Hard Disk Sample
• The tip is magnetized (a soft-magnet
coating tip is used)
• Cantilever deflects when it scans over
magnetized domains of sample
• MFM images locally magnetized domains.
• Magnitude of deflection (up/down) is
proportional to sample magnetization
• MFM is used to determine the local
magnetization variation.
•• Ferroelectric
materials
•• Charge
distribution on
surfaces
•• Failure analysis
on the device
Electrostatic Force Microscopy(EFM)
• Voltage is applied between tip and sample while
cantilevers hovers above sample.
• Cantilever deflects when it scans over static charges
• EFM plots locally charged domains similar to MFM plots
of magnetic domains.
• Magnitude of deflection is proportional to the charge
density.
• EFM is used to determine the local charge density
variation.
Metallic tip
Bias Voltage
Chemical Force Microscopy
A. Noy et al, Ann. Rev. Mater. Sci. 27, 381 (1997)
• (A) topography
• (B) friction force using a tip modified with a COOH-terminated SAM,
• (C) friction force using a tip modified with a methyl-terminated SAM.Light
regions in (B) and (C) indicate high friction; dark regions indicate low
friction.
-COOH-CH3
Force / Distance Curves in AFM using adhesion forces to obtain chemical specificity
COOH CH3 scanned in air CH3 tip air H2O COOH CH3 hydrophobic tip [CH3 covered]
Contrast Reversal
Electrochemistry
Surface structure
Electrochemical deposition
Corrosion
Scanning Electrochemical Microscopy
Nearfield Scanning Optical Microscopy
hv
Al
Cam
f=60-200nm
Detektör
Topography and Optical properties
d
Fluorescence image of
a single DiIC molecule.
Scanning Thermal Microscopy
• Subsurface defect review
•Semiconductor failure analysis
•Measure conductivity differences in copolymers,
•surface coatings etc
P. Grutter, McGill University
Blunt tip :
Imaging Artifacts
‘High’ resolution and double tip:
Image Artifacts
•Tip imaging
•Feedback artifacts
•Convolution with other
physics
•Imaging processing
How to check ?
•Repeat the scan
•Change the direction
•Change the scan size
•Rotate the sample
•Change the scan speed
www.psia.co.kr/appnotes/apps.htm
AFM image of random noise. Left
image is raw data. Right image is a
false image, produced by applying a
narrow bandpass filter to the raw data
SPM Advantages
• resolution not limited by diffraction, but only by the size of the probe-sample interaction volume ( few picometers)
• ability to measure small local differences in object height (like that of 135 picometre steps on <100> silicon)
• probe-sample interaction extends only across the tip atom or atoms involved in the interaction
• interaction can be used to modify the sample to create small structures (nanolithography)
• 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
• 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
• The maximum image size is generally smaller
• not useful for examining buried solid-solid or liquid-liquid interfaces
SPM has eyes to see the geometry and properties of
nanostructures and fingers to manipulate and build
nanostructures
There is a great tendency to “see what you want” to in
AFM images, although multimode operation helps to
reduce interpretations. This technique has not-so-
obvious limitations:
• Tip contamination
• Piezo non-linearities and drift
• Tips are rarely characterized for spring constant, geometry
• Artifacts (double tip)
• topography / phase convolution in lateral force
• “near sightedness”