sem & afm presentation

8/3/2019 Sem & Afm Presentation

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SCANNING ELECTRON

MICROSCOPE (SEM)

&

Atomic Force

Microscope(AFM)

By: VIMAL KAURANI (090170111015)

DEEPAK TAURANI (090170111004)

Sub: Electronics Measurements &

Instrumentation laboratory


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MENA3100

SCANNING ELECTRON MICROSCOPE(SEM)

The Instrument in Brief


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WORKING In a typical SEM, an electron beam is thermionically emitted

from an electron gun fitted with a tungsten filament cathode

The electron beam, which typically has an energy ranging from0.5 keV to 40 keV, is focused by one or two condenser lenses to a

spot about 0.4 nm to 5 nm in diameter. The beam passes through

pairs of scanning coils or pairs of deflector plates in the electron

column, typically in the final lens, which deflect the beam in the x

and y axes so that it scans in a raster fashion (It's a systematicprocess of covering the area progressively, one square or line at a

time.) over a rectangular area of the sample surface.

The size of the interaction volume depends on the electron's

landing energy, the atomic number of the specimen and the

specimen's density The energy exchange between the electron beam and the sample

results in the reflection of high-energy electrons by elasticscattering, emission of secondary electrons byinelastic scatteringand the emission ofelectromagnetic radiation, each of which canbe detected by specialized detectors
http://en.wikipedia.org/wiki/Thermionicallyhttp://en.wikipedia.org/wiki/Electron_gunhttp://en.wikipedia.org/wiki/Tungstenhttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Electronvolthttp://en.wikipedia.org/wiki/Raster_scanhttp://en.wikipedia.org/wiki/Elastic_scatteringhttp://en.wikipedia.org/wiki/Elastic_scatteringhttp://en.wikipedia.org/wiki/Inelastic_scatteringhttp://en.wikipedia.org/wiki/Inelastic_scatteringhttp://en.wikipedia.org/wiki/Elastic_scatteringhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Inelastic_scatteringhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Inelastic_scatteringhttp://en.wikipedia.org/wiki/Elastic_scatteringhttp://en.wikipedia.org/wiki/Elastic_scatteringhttp://en.wikipedia.org/wiki/Raster_scanhttp://en.wikipedia.org/wiki/Electronvolthttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Tungstenhttp://en.wikipedia.org/wiki/Electron_gunhttp://en.wikipedia.org/wiki/Thermionically


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The beam current absorbed by the specimen can also be detected

and used to create images of the distribution of specimen current.

Electronic amplifiers of various types are used to amplify the

signals, which are displayed as variations in brightness on a

cathode ray tube.

Unlike optical and transmission electron microscopes, image

magnification in the SEM is not a function of the power of the

objective lens. SEMs may have condenser and objective lenses,

but their function is to focus the beam to a spot, and not to image

the specimen.

In a SEM, magnification results from the ratio of the dimensions

of the raster on the specimen and the raster on the display device.

Assuming that the display screen has a fixed size, higher

magnification results from reducing the size of the raster on the

specimen, and vice versa
http://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Cathode_ray_tubehttp://en.wikipedia.org/wiki/Objective_(optics)http://en.wikipedia.org/wiki/Condenser_(microscope)http://en.wikipedia.org/wiki/Condenser_(microscope)http://en.wikipedia.org/wiki/Objective_(optics)http://en.wikipedia.org/wiki/Cathode_ray_tubehttp://en.wikipedia.org/wiki/Electronics


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HOW DO WE GET AN IMAGE?

MENA3100

156 electrons!

Image

Detector

Electron gun


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SIGNALS FROM THE SAMPLE

MENA3100

Incoming electrons

Secondary electrons

Backscattered

electrons

Auger electrons

X-rays

Cathodo-

luminescence (light)

Sample


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COMPONENTS OF THE INSTRUMENT

MENA3100

electron gun (filament)

electromagnetic optics scan coils

sample stage

detectors

vacuum system

computer hardware

and software (not

trivial!!)


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The spatial resolution of the SEM depends on the size of the

electron spot, which in turn depends on both the wavelength ofthe electrons and the electron-optical system that produces the

scanning beam. The resolution is also limited by the size of the

interaction volume, or the extent to which the material interacts

with the electron beam. The spot size and the interaction volume

are both large compared to the distances between atoms, so theresolution of the SEM is not high enough to image individual

atoms, as is possible in the shorter wavelength (i.e. higher energy)

transmission electron microscope (TEM). The SEM has

compensating advantages, though, including the ability to image a

comparatively large area of the specimen; the ability to image

bulk materials (not just thin films or foils); and the variety of

analytical modes available for measuring the composition and

properties of the specimen. Depending on the instrument, the

resolution can fall somewhere between less than 1 nm and 20 nm.

Resolution of The SEM


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AN EXAMPLE OF SEM IMAGE


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SUMMARY OF SEM

The scanning electron microscope is aversatile instrument that can be used formany purposes and can be equipped withvarious accessories

An electron probe is scanned across thesurface of the sample and detectorsinterpret the signal as a function of time

A resolution of 1 2 nm can be obtained

when operated in a high resolution setupThe introduction of ESEM and the field

emission gun have simplified the imagingof challenging samples

MENA3100


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ATOMIC FORCE MICROSCOPE(AFM)

Block Diagram of AFM

The AFM is one of

the foremost tools for

imaging, measuring,

and manipulating

matter at the

nanoscale.
http://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Nanometre


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BASIC FUNCTIONING OFAFM

The AFM consists of a cantilever with a sharp tip (probe) at

its end that is used to scan the specimen surface. The

cantilever is typically silicon with a tip radius of curvature

on the order ofnanometers. When the tip is brought into

proximity of a sample surface, forces between the tip and

the sample lead to a deflection of the cantilever.

Typically, the deflection is measured using a laser spot

reflected from the top surface of the cantilever into an

array ofphotodiodes. Other methods that are used includeoptical interferometry, capacitive sensing or piezoresistive

AFM cantilevers. These cantilevers are fabricated with

piezoresistive elements that act as a strain gauge.
http://en.wikipedia.org/wiki/Cantileverhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Radius_of_curvature_(applications)http://en.wikipedia.org/wiki/Nanometershttp://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Photodiodeshttp://en.wikipedia.org/wiki/Capacitive_sensinghttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Capacitive_sensinghttp://en.wikipedia.org/wiki/Photodiodeshttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Nanometershttp://en.wikipedia.org/wiki/Radius_of_curvature_(applications)http://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Cantilever


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Imaging modes

The primary modes of operation for an AFM are static modeand dynamic mode.

In static mode, the cantilever is "dragged" across thesurface of the sample and the contours of the surface aremeasured directly using the deflection of the cantilever.

In the dynamic mode, the cantilever is externally oscillatedat or close to its fundamental resonance frequency or aharmonic. The oscillation amplitude, phase and resonancefrequency are modified by tip-sample interaction forces.These changes in oscillation with respect to the externalreference oscillation provide information about the sample'scharacteristics.
http://en.wikipedia.org/wiki/Oscillationhttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Interactionhttp://en.wikipedia.org/wiki/Interactionhttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Oscillation


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AFM CANTILEVER DEFLECTION

MEASUREMENT

Laser light from a solid state diode is reflected off the back of the

cantilever and collected by a position sensitive detector (PSD)consisting of two closely spaced photodiodes whose output signal

is collected by a differential amplifier. Angular displacement of

the cantilever results in one photodiode collecting more light than

the other photodiode, producing an output signal (the difference

between the photodiode signals normalized by their sum) which is

proportional to the deflection of the cantilever
http://en.wikipedia.org/wiki/Photodiodeshttp://en.wikipedia.org/wiki/Differential_amplifierhttp://en.wikipedia.org/wiki/Differential_amplifierhttp://en.wikipedia.org/wiki/Photodiodes


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AN EXAMPLE OFAFM IMAGE

Figure shows the

Atomic force

microscope

topographical scan ofa glass surface. The

micro and nano-scale

features of the glass

can be observed,

portraying the

roughness of the

material


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ADVANTAGES & DRAWBACKS

Advantages AFM has several advantages over the scanning electron

microscope (SEM). Unlike the electron microscope which

provides a two-dimensional projection or a two-dimensional

image of a sample, the AFM provides a three-dimensionalsurface profile

While an electron microscope needs an expensive

vacuum environment for proper operation, most AFM

modes can work perfectly well in ambient air or even

a liquid environment. This makes it possible to studybiological macromolecules and even living organisms.

In principle, AFM can provide higher resolution than

SEM.
http://en.wikipedia.org/wiki/Scanning_electron_microscopehttp://en.wikipedia.org/wiki/Scanning_electron_microscopehttp://en.wikipedia.org/wiki/Vacuumhttp://en.wikipedia.org/wiki/Vacuumhttp://en.wikipedia.org/wiki/Scanning_electron_microscopehttp://en.wikipedia.org/wiki/Scanning_electron_microscope


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Disadvantages

A disadvantage of AFM compared with the scanningelectron microscope (SEM) is the single scan image size. In

one pass, the SEM can image an area on the order of squaremillimetres with a depth of field on the order of millimetres,whereas the AFM can only image a maximum height on theorder of 10-20 micrometres and a maximum scanning areaof about 150150 micrometres.

The scanning speed of an AFM is also a limitation.Traditionally, an AFM cannot scan images as fast as aSEM, requiring several minutes for a typical scan, while aSEM is capable of scanning at near real-time, although atrelatively low quality.
http://en.wikipedia.org/wiki/Scanning_electron_microscopehttp://en.wikipedia.org/wiki/Scanning_electron_microscopehttp://en.wikipedia.org/wiki/Millimeterhttp://en.wikipedia.org/wiki/Depth_of_fieldhttp://en.wikipedia.org/wiki/Depth_of_fieldhttp://en.wikipedia.org/wiki/Millimeterhttp://en.wikipedia.org/wiki/Scanning_electron_microscopehttp://en.wikipedia.org/wiki/Scanning_electron_microscope

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