AFM (ATOMIC FORCE MICROSCOPY)

8/4/2019 AFM (ATOMIC FORCE MICROSCOPY)

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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

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