1° capitolo

Basic Principle of Scanning Probe Microscopy
•
•
•
Introduction to Scanning Probe Microscopy: the local probe
approach
Operational Principle: Atomic Force Microscope (AFM):
“contact and non-contact” mode
AFM cantilevers
AFM tips
Feedback loop
Operational Principle: Scanning Tunneling Microscope (STM):
topographic STM operation modes
STM instrumentation
STM designs
mechanical vibrations
electronics
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Zooming in atoms with Scanning Probe Microscope
10-3
10-6
10-9
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What is Scanning Probe Microscope (SPM)
SPM provides very high resolution images of various sample properties.
All of these microscopes work by measuring a local property - such as
height, optical absorption, or magnetism - with a probe or "tip“ placed
very close to the sample.
The small probe-sample separation (on the order of the instrument's
resolution) makes it possible to take measurements over a small area. To
acquire an image the microscope raster-scans the probe over the sample
while measuring the local property in question. The resulting image
resembles an image on a television screen in that both consist of many
rows or lines of information placed one above the other.
Unlike traditional microscopes, scanned-probe systems do not use
lenses, so the size of the probe rather than diffraction effects
generally limit their resolution.
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Varieties of SPM
Scanning Tunneling Microscope (STM)
First introduced by G. Binnig (1) and H. Rohrer in 1981
Stylus Profilometer (SP)
First introduced by J. B. P. Williamson
Scanning Probe
Microscope
(SPM)
(2)
in 1967
Atomic Force Microscope (AFM)
First introduced by G. Binnig (3) et al. in 1986
Magnetic Force Microscope (MFM)
First introduced by J. A. Sidles (4) et al. in 1992
Scanning Capacitance Microscope (SCM)
First introduced by J. R. Matey (5) et al. in 1985
Others
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Summary of progress in electron tunneling
Year
Investigator
Advancement
1928
1934
1937
1958
1960
Fowler, Nordheim
Zener
Muller
Eisaki
Gaiever
Explanation of field emission
Theory of interband tunneling
Field emission microscope
Tunneling in p-n junctions
Measurement of superc. energy gap
1961
1963
1966
Bardeen
Simmons
Jaklevic, Lambe
Many body effect in tunneling theory
Image forces in tunneling theory
Inelastic tunneling spectroscopy
1971
Young, Ward, Scire
Vacuum-tunneling in plane geometry
1982
Binnig, Rohrer, Gerber
Atomic resolution STM
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Comparison among different SPM
Type
Properties used for
scanning
Resolution
Used for
STM
Tunneling Current
between sample and probe
Vertical resolution < 0.1 Å
*Lateral resolution ~ 1 Å
=> Conductors
=> Solids
SP
Surface profile
Vertical resolution ~ 10 Å
*Lateral resolution ~1000 Å
⇒Conductors, insulators,
semiconductors
=> solids
AFM
Force between probe tip
and sample surface
(Interatomic or
electromagnetic force)
Vertical resolution < 1 Å
*Lateral resolution ~ 10 Å
=> Conductors, insulators,
semiconductor
=> liquid layers, liquid crystals
and solids surfaces
MFM
Magnetic force
Vertical resolution ~ 1 Å
*Lateral resolution ~ 100 Å
=> Magnetic materials
SCM
Capacitance developed in
the presence of tip near
sample surface
Vertical resolution ~ 2 Å
*Lateral resolution~ 5000 Å
=> Conductors
=> Solids
* Lateral resolution depends upon the resolution of mechanical XYZ stage
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Operational principle: Atomic Force Microscope (AFM)
• First generation AFM was the combination of Scanning tunneling Microscope
(STM) and Stylus Profilometer (SP). (G. Binnig et al. 1985)(1)
• The atomic force microscope measures topography with a force probe.
Two modes (1) contact mode (2) non-contact mode
• Laser beam deflection offers a convenient and sensitive method of measuring
cantilever deflection
• AFM cantilevers have high flexibility
• Tube piezoceramics position the tip or sample with high resolution
• Feedback is used to regulate the force on the sample
• AFM has alternate imaging modes.
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The atomic force microscope measures topography with a
force probe
AFM operates by measuring attractive or repulsive forces between a tip and the sample.
There are two ways to
scan.
1.
2.
Sample holder moves
and tip is fixed.
Tip moves and sample
holder is fixed.
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Modes of operation
• Contact Mode: direct physical contact with the sample
•Non-contact Mode:
Tip oscillating at constant distance above sample surface
• Tapping mode:
Intermittent contact, less damaging
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“contact” and “non-contact” mode
•In “contact mode” the AFM measures
hard-sphere repulsion forces between the
tip and sample.
Van der Waals force versus distance
•In “non-contact mode”, the AFM derives
topographic images from measurements of
attractive forces; the tip does not touch
the sample (Albrecht et al., 1991).
•In principle, AFM resembles the record
player as well as the stylus profilometer.
However, AFM incorporates a number of
refinements that enable it to achieve
atomic-scale resolution:
1.Sensitive detection
2.Flexible cantilevers
3.Sharp tips
4. High-resolution tip-sample positioning
5.Force feedback
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Tapping mode AFM
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Detection: laser beam
• AFM can generally measure the vertical
deflection of the cantilever with pico meter
resolution by using optical lever.
• The optical lever operates by reflecting a
laser beam off the cantilever. Angular
deflection of the cantilever causes a twofold
larger angular deflection of the laser beam.
• The reflected laser beam strikes a positionsensitive photodetector consisting of two sideby-side photodiodes.
• The difference between the two photodiode
signals indicates the position of the laser spot
on the detector and thus the angular deflection
of the cantilever.
• Because the cantilever-to-detector distance
generally measures thousands of times the
length of the cantilever, the optical lever
greatly magnifies motions of the tip.
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AFM cantilevers
•Flexible cantilever exerts lower force to sample thus less
distortion and less damage.
• AFM cantilevers have generally spring constant less than
0.1 N/m.
• Higher the resonance frequency of cantilever, faster and
better the imaging.
• To obtain both low spring constant and high frequency( >2
KHz), the mass of cantilever should be very small ~ 10-10
Kg.
• Microlithography process is used to make such cantilevers.
Si or Si3N4 are used for making tips. A thin gold layer is
deposited to upper side of tip for good reflectivity of laser
beam.
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Schematic illustration of the meaning
of "spring constant" as applied to
cantilevers. Visualizing the cantilever
as a coil spring, its spring constant k
directly affects the downward force
exerted on the sample.
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Commercially available AFM tips
(a)
(b)
(c)
SEM images of three common types of AFM tip. (a) normal tip (3 µm tall); (b) supertip; (c)
Ultralever (also 3 µm tall).
• Tips are generally evaluated by their “end radius” which limits the resolution of AFM.
• The "normal tip" ( Albrecht et al., 1990) is a 3 µm tall pyramid with ~30 nm end radius.
• The electron-beam-deposited (EBD) tip or "supertip" offers a higher aspect ratio (it is
long and thin, good for probing pits and crevices) and sometimes a better end radius
than the normal tip.
• The "Ultralever" is based on an improved microlithography process. Ultralever offers
a moderately high aspect ratio and on occasion a ~10 nm end radius.
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Resolution: apparent width
x 2 = (Rtip + Rsample ) − (Rtip − Rsample )
2
2
2
2
2
2
x 2 = Rtip
+ 2 Rtip Rsample + Rsample
− Rtip
+ 2 Rtip Rsample − Rsample
x = 2 Rtip Rsample
w = 2 x = 4 Rtip Rsample
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DNA: 2nm
tip∼ 20nm
tip ∼10nm
w=25nm
w=18nm
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Carbon nanotube at the end of a commercial AFM tip
Extremely tiny radius of curvature ~ 1nm
Extremely robust
Buckle reversibly
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Piezoceramics for tip-sample position
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Piezoceramics tubes for tip-sample position
Top view
•Small area scanning
•High resolution positioning
•Voltage applied to all quadrants: expansion
•Opposite voltage applied to two quadrants: bending
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Feedback loop to regulate the force on the sample
LASER
Detector
I
Sample
Tip
PZT
E
PID
Z
I
+I
-Z
E=I-Z
PID
Z
Z height signal
“Z” is used to
form image
PZT
• PID is compensation network, P=> Proportional gain, I=> Integral gain
D=>Differential gain.
• A feedback circuit integrates the signal coming from photodiode and applies a
feedback voltage to the z piezo (PZT) to exactly balance the cantilever bending.
• Since the probe force is proportional to the cantilever bending, this is constant.
• The image of the surface is built up as a series of scan lines, each displaced in
the y direction from the previous one. Each individual line is a plot of the
voltage applied to the z piezo as a function of the voltage applied to the x piezo.
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Feedback loop and contact mode
Force applied on the sample: F=-kd
k: cantilever force constant
d: cantilever deflection
Constant force applied on the sample:
•Force applied on the sample with deflection of cantilever
•Variation of deflection as tip scans
•Deflection measured by laser beam shift and fed back to scanner
•Scanner readjust height to keep deflection constant
•Consequently force applied on sample held constant
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Feedback loop and non-contact mode
Tip-sample distance held constant:
•Tip held above sample surface
•Cantilever oscillated at resonance frequency
•Oscillation amplitude and phase measured by laser detection system and fed to scanner
•Oscillation amplitude change due to Van der Waals interaction when tip reaches
10-100 Å above surface
•Scanner adjust height to keep amplitude constant.
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Advantages of AFM
Versatile technique: wide range of resolution, 100 µm -> Å
Atomic resolution on MICA
Scan area: 80 Å x 80 Å
Sb2O3 deposited on graphite
Scan area: 2 µm x 2 µm
Z. Wang, RHK Technology
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Advantages of AFM
Straightforward technique:
• No sample preparation
• Measurements in air, no vacuum
Versatile technique: wide variety of samples
•
•
•
•
•
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Conductor
Semiconductor
Insulator
Hard material: oxides, metals
Soft materials: wet cells
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AFM Resolution
Z resolution
•Fractional Å vertical sensitivity
• Limited only by electronic noise and mechanical vibrations
X,Y resolution (Atomic flat samples)
• Limited by diameter of atom at the probe tip
X,Y resolution (rough surface)
•Limited by tip geometry
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The Basics of Scanning Tunneling Microscopy
In 1981 G. Binnig and H. Rohrer at IBM Research on Zurich invented STM and
in 1986 they won the Nobel Prize
The basic components of the Microscope setup are:
☯ a sample
☯ a sharp tip to be placed in a very
close proximity to the sample
☯ a mechanism to control the location of the
tip in x-y plane parallel to the sample
surface (X-Y scan control)
Z
Feedback
Piezo
X
Y
I tunnel
Computer
tip
sample
Y-scan
V bias
X-scan
tip
Limited to conducting samples !!
sample
First instrument to generate real space images of surfaces with atomic resolution
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Tunneling current
vacuum
gap
tip
☯ Tunneling current is exponentially dependent upon the
distance between the tip and the sample:
sample
ψs
I ∝ exp(− Ad )
ψt
φ
EF
eV
ρs
ρt
d
Exponential dependence
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This means that for a small change
in the distance between tip and sample
there is a large change in current!
Extremely high z resolution
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Topographic STM operation modes
tip
height
tunnel
current
position
z-voltage
I
FB on
Constant current mode
Itunnel
I
FB off
Constant height mode
An individual atom can be "seen" as an increase in the tunneling current as the
tip is scanned across the surface of the sample.
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STM Instrumentation
Pohl, IBM J. Res. Dev. 30, 417 (1986)
Kuk and Silverman, Rev. Sci. Instrum. 60, 165 (1989)
1. Mechanical Construction
2. Electronics
3. Data acquisition
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Vibration isolation
The desired resolution in STM imaging of 0.1 Å vertically and 1.0 Å laterally,
imposes the requirement that noise from any source
be less than 0.01 Å in z and 0.1 Å in x and y.
Typical floor vibration ~ 0.1 – 1.0 µm in the range 0.1-50 Hz
Vibration isolation necessary
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Damping
• low frequency (< 20 Hz, building)
springs (resonance frequency ~ 1-5 Hz),
air table (resonance frequency ~ 1 Hz)
• medium frequency (20-200 Hz, motors, acoustic noise)
mounting on heavy plates
• external vibration isolation
system+ rigid STM design
can reduce external vibration
by a factor 10-6-10-7
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Sample-tip approach mechanisms :
Inchworm and Inertial slider
•Step size has to be small than the total range
of z piezodrive, step resolution of 50Å and
dynamical range of cm is required!
•An inchworm motor employs a piezotube and
piezoclamps to move an inner shaft with a
series of clamping/unclamping and
extension/retraction events
•Inertial motion of a free mass can be
achieved by asymmetric acceleration
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Scanner (inertial slider)
Inertial slider
approach mechanism
Sample holder
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Schematic and pictures of the scanner
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Scanner (inchworm)
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STM electronics
Schematic view
• the computer generates the scanning motion of the tip, overall program control
and image display
• the integrator, low pass filter optimizes the feedback performance
and prevents the system from resonating
• position control system that includes: digital-analog converters (DAC),
anolog-digital converters (ADC), high voltage amplifiers to control the scanner
• the z piezo is adjusted by the feedback system so that the current I stays
equal to the preset value Iref
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First stage amplifier
R
vout
Tunneling current is typically in the range
10pA-10nA. To keep it constant via feedback
is usually converted to a reference voltage ∼
1V. This requires a current-to-voltage
amplifier with a gain 108-1010 V/A and noise
below the equivalent of 1 pA.
punta
campione
bias
The second amplifier stage is a low pass
filter with a variable cut-off frequency
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Atomic images of graphite
a = 2.46 Å
Atomic resolution on graphite
Scan area: 53 Å x 53 Å
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