afm (atomic force microscopy)
TRANSCRIPT
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ATOMIC FORCE
MICROSCOPY
WORKING PRINCIPLE ANDAPPLICATIONS
Presented By Engr. Asif Shah
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Introduction
AFM is a mechanical imaging
instrument that measures the threedimensional topography as well as
physical properties of a surface with a
sharpened probe.
Sharpened probe is positioned closeenough to the surface such that can
interact with the force fields associated
with the surface
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Probe is very sharp, typically less than
50 nanometers in diameter and theareas scanned by the probe are less
than 100 um.
Magnifications of the AFM may be
between 100 X and 100,000,000 X.
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Comparison to Other Microscopes
Because of the sharper probe, greater
resolution occur in AFM (App. 0.05NM)
Minimum sample preparation is required
with an AFM
AFM is good for measuring mechanical
properties of surfaces.
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SEM / TEM AFM
SamplesMust beconductive
Insulating /Conductive
Magnification 2 Dimensional 3 Dimensional
Environment Vacuum Vacuum / Air / Liquid
Time for image 0.1 - 1 minute 1 - 5 minuteHorizontalResolution
0.2 nm (TEM)5 nm (FE-SEM)
0.2 nm
Vertical Resolution n/a .05 nm
Field of View100 nm (TEM)1 mm (SEM)
100 um
Depth of Field Good Poor
Contrast on FlatSamples Poor Good
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Operation Of AFM
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Sample Preparation
Sample must be clean, If the surface isdirty with a thick contamination layer, itcauses severe distortion in the image.
Sample must be rigidly mounted in stage
If the sample is not mounted rigidly, it
can vibrate. Vibrations reduced theresolution of the microscope and make it
impossible to see small surface features.
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Cantilever
Tip radius: 20nm
legs: 50um X 10um X, 0.5um
Platform. 10um X 5um, X 0.5um
Stiffness, 1 N/m
Resonant frequency, 200KHz
Thermal time constant, 1us
Tip height: 1.7um
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AFM Modes ofOperation
Topographic Mode
Field Mode
Material Sensing ModeElectrical Mode
Lithographic Mode
Mechanical Measurements ModeThermal Measurements Mode
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Topographic Mode
Three basic regions of interactionbetween the probe and surface.
a) Free space
b) Attractive region
c) Repulsive region
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Repulsive forces increase as the probebegins to "contact" the surface.
The repulsive forces in the AFM tendto cause the cantilever to bend up.
Two primary methods for establishingthe forces between a probe and asample when an AFM is operated.
a) Contact Modeb) Vibrating Mode
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In contact mode the deflection of the
cantilever is measured.
A constant force is applied to the
surface while scanning.
Contact mode is typically used forscanning hard samples and when a
resolution of greater than 50
nanometers is required
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Resonant frequencies of contact mode
cantilevers are typically around 50 KHz
and the force constants are below 1 N/m
In vibrating mode the changes in
frequency and amplitude are used tomeasure the force interaction.
Make more sensitive measurementsrequiring better signal/ noise ratios in
scientific instruments
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Vibrating methods are used when the
highest resolution is required or if very
soft samples are being scanned.
The probes used for vibrating mode
are often less than 10 nm in diameter.
The cantilevers have natural resonant
frequency o given by
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Contact Mode ImagesLeft: Bits on a compact disk. Center: Image of a metal surface.
Right: Nano-particles on a surface.
Vibrating Mode ImagesLeft: Silicon wafer. Center: Cancer cells. Right: Proteins
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Field Modes
Field modes are used to measure theelectrostatic or magnetic fields above a
surface.
In field modes require a tip that is
coated with either an electrically
conductive or magnetic material.
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Resolution of the field modes are
typically much less than the resolutionof the topography modes.
The success of making fieldmeasurements ultimately depends on
getting a high quality coating on aprobe.
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Material Sensing Modes
In this mode we check the physicalproperties of materials.
The amount of motion is in some wayrelated to the differences in the
chemical/physical properties of a surface
Cantilever tends to twist while scanning
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Twisting of the cantilever is measured
in the light lever AFM by monitoring the
left to right motion of the deflected
laser light with a photo detector
El i l M d
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Electrical Mode
Conductive AFM probe may be used for
measuring the electrical properties of a
surface or structures located on the
surface
Probe itself can be conductive or it can
be made to be conductive by coating it
with a metal layer
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Electrical properties are measured
with a parametric tester
Parametric tester is attached
directly between the conductive AFM
probe and the sample
Mechanical M i M d
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Mechanical Measuring Mode
Probe in an AFM may interact with the
hard forces at a surface Such interaction
allow making nano-mechanical
measurements on a surface Primary method for making mechanical
measurement is the force/distance curve
When the probe is pressed firmly into a
surface, it may cause a nano-indentation
of the surface
i
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Thermal Measuring Mode
By placing a small temperature sensing
device at the end of an AFM probe it is
possible to make a number of thermal
measurements of a surface
Two primary types of probes used in an
AFM for making thermal measurementsare the thermocouple and the resistive
probe
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From thermocouple and resistive probe
its easy to measure surface temperature
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AFM Applications
Physical Sciences
Life Sciences
High Technology
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Physical Sciences
a) Polymer Composites
AFM can readily measure images of
composite polymers with little or nosample preparation
Differences in the composition incomposite polymers can be measured
b) Ph T iti
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b) Phase Transitions
As materials undergo phase transitions,
they under go changes in their surface
structure that can be readily imaged
with an AFM
With a small heater located at the end
of the AFM probe
) S f T t d D f t
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c) Surface Texture and Defects
AFM have good contrast which is notpossible with any other type of
microscope
AFM measures three dimensional surface
structure, it is possible to measure not
only the area of the defect but also the
volume of a defect at a surface
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e) Crack Propagation
AFM is ideal for studying crackpropagation in surfaces because the
AFM gives great contrast on flat samples
To place a device for creating stresses
and strains in materials directly in the
AFM stage. In such a case, direct images
of crack formation are measurable
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g) Nano Particles and Carbon NanoTubes
It is possible to measure the size of
individual nano particles as well as
measure the parameter distribution ofnano particles.
Parameters such as particle size,volume, circumference and surface area
are readily measured
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High resolution images of both single wall
and multi-wall carbon nano tubes are
measurable with the AFM
The nano tubes must be dispersed on a flat
surface for imaging
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h) Crystal Structure
It is possible to measure atomic
structure with an AFM in an ultra high
vacuum environment
Atomic terraces on crystal surfaces are
readily measured with an AFM in
ambient air
2) Lif S i
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2) Life Sciences
The AFM has great ability to measure
surface structure of biological material
AFM is the only microscopy techniquethat allows making nanometer scale
images with the sample submerged in
liquid
) C ll
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a) Cells
AFM can readily measure images of cells
both in ambient air and submerged in a
liquid
It is possible to measure the mechanical
activity of a cell by simply placing the
probe on the surface of the cell and
monitoring the motion of
the AFM cantilever
b) Bi M l l
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b) Bio-Molecules
Bio-molecules images such as DNA can be
easily measured as the bio-molecules are
directly attached to a surface
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3) High Technology
The high technology industries, including
semiconductors, data storage and
advanced optical devices, use AFM for
product/process development
these industries already rely on
manufacturing procedures that create
structures with nanometer sized
dimensions.
) S i d
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a) Semiconductor
AFM has played a critical role in thedevelopment of many semiconductor
processes
Advanced applications include: electrical
testing, verification and secondary
defect review
b) Data Storage
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b) Data Storage
AFM is utilized for product development
of both magnetic and optical mass storagedevices
In the magnetic storage area include thevisualization of magnetic bits, pole tip
recession, and platter surface texture
Bits and tracks in DVD and CD-R/W
storage media are visualized with an AFM
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Software is available for automatically
calculating the critical dimensions of
DVD bits
DVD bits 11.7 x 11.7 u, B: CD R/W 23 x
23 u.
) Ad d O ti l
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c) Advanced Optical
AFM can be very helpful for metrologicalmeasurements when optical profilers
cannot be used because the specimen
under study is transparent
Optics play a critical role in many high
technology products such as cameras,
video recorders, and flat panel displays
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AFM give image of micro-optical devices
are often created from insulating material
and cannot be viewed in an SEM/TEM,
because they are transparent
AFM image of a micro lens
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THANKS