Scanning Probe Microscope SPM

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Scanning Probe Microscope- SPM -
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Present Rony Levin Email levinbr_at_ee.bgu.ac.il
Course Nanotechnology Number 361-2-0826 Lecturer
Dr. Ilan Shalish
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Agenda

• Terms Definition
• Motivation
• Historical Overview
• SPM Overview
• STM Overview
• AFM Overview
• SPM Software
• Summary

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Surface Definition

• Surface is the shell of a macroscopic object
  (the inside) in contact with its environment
  (the outside world). An interface is the
  boundary between two phases. Object inside is
  called bulk
• In large objects with small surface area A to
  volume V ratio (A/V) the physical and chemical
  properties are primarily defined by the bulk
• In small objects with a large A/V-ratio the
  properties are strongly influenced by the surface

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Lotus Leaf Phenomenon
Hydrophobic effect
Water
Hydrophobic material Surface structure (20-100µm)
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Semiconductor Devices
Current flow near the surface
In modern semiconductor devices surface is
dominant
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Semiconductor Production
Layer by layer
Surface by surface
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Important Key Features of Surfaces

• surface topography
• valley, planes, hills

• physical materials behavior (conducting /
  insulating)
• polarity (hydrophilic / hydrophobic)
• tribological behavior (friction on "rough" or
  "smooth" surface)
• physical surface behavior (reflectivity)

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Application Examples of Surface Science

• understanding and inhibition of corrosion
• chip manufacturing / microelectronics
• hard disks (anti-friction, ultra-smooth,...)
• biological surfaces (patterned cell growth)
• sensors (chemical, biological, electrical)
• modification of anti-reflection (displays)
• modification of wetting (inkjet printing)

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Surface Research Motivation

• According to all the mentioned before our
  motivation as electrical engineers for surface
  research is clear. The modern microelectronics
  devices properties, in general, affected by
  surface.
• Moreover, we want not only passively receive
  information about the surface, but actively
  affect on it topology, moving the atoms

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Terms Definition

• Microscopy µ????s - small, s??p?s see
• Scanning Probe Microscopy (SPM) is a branch of
  microscopy that forms images of surfaces using a
  physical probe that scans the specimen. SPM able
  to receive 3D surface topography

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Terms Definition

• Local density of states (LDOS) is a physical
  space-resolved quantity that describes the number
  of states at each energy level that are available
  to be occupied. According to crystal's structure,
  this quantity can be predicted by computational
  methods, as for example with density functional
  theory
• Artifact is any perceived distortion or other
  data error caused by the instrument of observation

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Terms Definition

• Input transducer or sensor
• Convert nonelectrical signal to electrical one
• Output transducer or actuator
• Convert electrical signal to nonelectrical one
• Piezoceramic
• Ceramic that convert electrical field to
  mechanical deformation and vice versa
• Piezoelectric properties
• are time-depended

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Acronyms

• SPM Scanning Probe Microscope
• STM Scanning Tunneling Microscope
• AFM Atomic Force Microscope
• SFM Scanning Force Microscope
• FFM Force-Modulated AFM
• LFM Lateral Force Microscope
• MFM Magnetic Force Microscope
• SThM Scanning Thermal Microscope
• EFM Electrical Force Microscope

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Historical Overview

• 1981 STM was developed by Binnig and Rohrer,
  IBM, Zurich
• 1986 AFM was developed by Binnig, Quarter and
  Gerber
• 1988 Commercial SPM available
• 1990 Analog electronics replaced by digital
• 1990 Software for SPM based Microsoft
  Windows developed
• from 1990 SPM market wake up
• Agilent Technologies, nanoScience

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SPM Physical ModelThe Blind Mouse
Computer
Actuator
Probe
Sample
Sensor
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Blind Mouse Operational Principle

• The blind mouse cant see the object (sample),
  but using the stick (probe), he can scan it.
• Arm skin (sensor) send the received from the
  probe information to the brain (computer), the
  computer see the picture, if it need receive
  additional information about the sample (decision
  done using feedback), it send requirement to arm
  muscle (actuator), arm carefully moves the probe
  to required coordinate and vice versa

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SPM Operational Principle

• All of the SPM techniques are based upon scanning
  a probe (typically called the tip, since it
  literally is a sharp metallic tip) just above a
  surface whilst between scanned surface and probe
  exist interaction
• The nature of this chosen interaction defines a
  device accessory to this or that type within the
  family of Scanning Probe Microscope
• The information on a surface is taken by fixing
  (by means of feedback system) or monitoring of
  interaction of a probe and the sample

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SPM Operational Principle
Surface divided to matrix of N rows by M
columns Scanning performed row by row, as result
we receive per each couple of coordinates
xi,yj, vector with measurement results that
described surface at this point
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SPM Block Diagram
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SPM Operational Principle

• We will present scanning probe microscopes based
  on two kind of iteration
• Iteration is electrical current STM
• Iteration is atomic force AFM/SFM
• In general, as mentioned, SPM have two modes,
  defined by tip movement over the surface
• Fixed probe Z coordinate, iteration or parameter
  depended on iteration monitoring
• Fixed iteration, height change monitoring

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STM Operational Principle
Corral diameter 14 nm
Quantum Corral
Tunneling current
from site
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STM Schematics
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Probe Fabrication
The tips are made simply by pulling the wires
(typically 200 to 250 microns in diameter) using
a pliers. A more consistent way to fabricate tips
is using electrolysis. When the tip becomes very
thin, it breaks living a very sharp tip
appropriate to be used in the STM. The
electrolytic bath is usually a solution of NaOH
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Potential Barrier Schematics
V is bias voltage
EF is Fermi level
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Step Potential Barrier
Schrödinger time invariant equation
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Transfer Matrix
General solution
Using C1 connectivity
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Transfer Matrix
Transfer Matrix Definition
Now it can be written more simple
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Transfer Matrix
We received very powerful mathematical tool.
Using this algorithm and Matlab we can solve
complicated potential barriers
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Rectangular Potential Barrier
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Tunneling Probability
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Tunneling - Summary
Received result is not so suitable for classical
physics theory, were electron position defined as
to be or not to be In quantum mechanics theory
electron position defined as may be and
appropriate number from 0 to 1 that describe the
chance of electron to be in some
coordinate. Summarizing all the electrons over
all energy levels, that can pass the barrier,
will receive tunneling current expression
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Tunneling Current
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Distance Sensitivity
What happened if current will be changed, how
mach it will affect the distance? Assume that K4
eV, current precision is 2
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STM Operational Mode

• constant height mode the tip is scanned over the
  surface keeping the vertical tip position
  constant, topography/conductivity differences are
  mapped by recording variations in tunnel current
  with respect to x-y-position of tip
• constant current mode the vertical tip position
  is adjusted during scanning to keep tunnel
  current constant, topography/conductivity map is
  constructed from vertical tip position with
  respect to x-y-position

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LDOS Probing
Using the current, STM probes the local density
of states, and anything more Artifact, or another
atom can create LDOS similar to atom, but STM can
not distinguish it
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What are We Imaging in STM?

• Metals
• High density of states at atoms appear as
  bright protrusions
• Insulators
• No conduction possible, we crash
• Semiconductors and thin oxides
• Complex electronic structure at Fermi level ,
  be careful!

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Atom Manipulation
Pulling mode This mode uses the attractive forces
between the tip and the adatom. The tip is
positioned above the adatom and then brought
towards the surface. The tunnel current
increases. The tip is then moved horizontally.
The current subsequently falls off until the
adatom undertakes a hop towards the tip, while
remaining on the surface. The current increases
once more and the procedure continues
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Atom Manipulation
Pushing mode This is similar to the pulling mode
except that it makes use of repulsive forces
between the tip and the adatom. The tip is
brought towards the surface and moved
horizontally towards the atom. The latter jumps
to the neighboring surface site. The current
falls abruptly and the procedure continues
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Atom Manipulation
Sliding mode In this mode the forces between the
tip and the adatom are attractive, but the tip is
so close to the surface that the adatom is
attached onto it. As the tip approaches, the
current increases and the adatom jumps onto the
tip and remains there. When the tip moves
horizontally parallel to the surface the current
is related to the surface topography as seen by
the tip with the attached adatom. Finally the tip
is withdrawn and the adatom falls back onto the
surface
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Atom Manipulation
Written literally with atoms, the Japanese Kanji,
each just a few nanometers across - means atom
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STM Summary

• STM is powerful technique for 3D viewing
  surfaces at the atomic level
• STM require sample conductivity
• STM can be used not only in vacuum, but also in
  air and various other liquid or gas ambient, and
  at temperatures range
• STM allows manipulate atoms
• STM probes the local density of states

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STM Catalog example
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AFM Operational Principle
from site
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AFM Schematics
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Cantilever

• Cantilever is a beam supported on only one end

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Cantilever
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Cantilever Fabrication
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Cantilever Catalog example
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Cantilever Catalog example
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Cantilever Regimes
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Cantilever Modes

• Contact Mode
• Friction mode
• Tapping Mode
• Phase mode

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Contact Mode

• Contact (repulsive) mode tip makes soft
  "physical contact" with the sample, the contact
  force causes the cantilever to bend to
  accommodate changes in topography

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Contact Mode
Stoney's formula
s is applied stress E, v are known physical
parameters
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Friction Mode
Lateral force / friction mode AFM cantilever
in contact mode is laterally deflected in the
sample plane due to scanning motion
perpendicular to cantilever axes, lateral
deflection is measured and gives information on
surface material apart from topography
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Tapping Mode

• Intermittent contact AFM cantilever is vibrated
  near the surface of a sample with spacing on the
  order of tens to hundreds of angstroms
• In this case the oscillator is non linear,
  mathematical calculation is pretty complicated,
  it is out of scope of this presentation

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Linear Phase Mode

• Phase mode AFM cantilever is vibrated near the
  surface of a sample with known amplitude
  frequency equal to resonance cantilever one .
  Compare phase of driving signal and cantilever
  response

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AFM Image
Image size 3.3 nm x 3.3 nm.
Image of a Si surface imaged with a force sensor.
In the left half of the image, the cantilever
does not oscillate, in the right half it
oscillates with an amplitude of 0.09 nm, yielding
a double image of every atom
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Carbon Nanotube Manipulation
Lowering the tip and pushing
Van Der Waals forces hold the nanotube
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AFM Catalog example
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AFM Summary

• AFM is powerful technique for 3D viewing
  surfaces at the atomic level
• AFM doesnt require sample conductivity
• AFM can be used not only in vacuum, but also in
  air and various other liquid or gas ambient, and
  at temperatures range
• AFM allows manipulate atoms
• AFM interpretation is not straightforward

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Artifacts

• Convolution with other physics (sample
• charging, stiffness, contamination etc.)
• Feedback artifacts
• External noise and fields
• Capillary Forces
• .

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Artifact Example
AFM contact scanning
before scanning
after scanning
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Tests for Artifacts

• Repeat the scan to ensure that it looks the same.
• Change the scan direction and take a new image.
• Change the scan size and take an image to ensure
  that the features scale properly.
• Rotate the sample and take an image to identify
  tip imaging
• Change the scan speed and take another image

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Calibration

• Error as result of non orthogonal axes and
  reference point definition
• Error as result of probe geometric shape
• Error as result of sensor sensitivity

In order to avoid measurement errors depended on
setup, mandatory to perform pre measurement
system calibration
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SPM Software Requirements

• User-friendly interface, basic Windows tools
• Offline simulation tool
• Calibration tool, ability to see results
• Variety of supported formats
• Control unit monitoring
• Filtering control
• Mathematical tools
• Programming tools
• 3D studio capture and editing images and
  videos in real time

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Scanning Probe Image Processor
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SPIP Interface
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STM vs. AFM Summary

• Resolution of STM is better than AFM
• AFM wins in terms of versatility
• AFM allows to image materials that do not conduct
  a current

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SPM Standardization
SPM standardization has only recently begun as
part of an effort by the International
Organization for Standardization (ISO) !!! 25
years of scanning probe microscopy
www.nanowerk.com , 2007
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SPM Standardization
Today, Jan 2008, 25 years of STM, UNDER
DEVELOPMENT
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Any Question?
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Thank You for Attention