Sem & Afm Presentation
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Transcript of Sem & Afm Presentation
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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