The document discusses the scanning electron microscope (SEM). It was invented in 1935 by Max Knoll and uses a focused beam of electrons to generate high-resolution 3D images of samples, with magnifications from 20-30,000x. The SEM works by scanning a focused electron beam across a sample, detecting signals from electrons interacting with the sample surface, and using those signals to construct an image. Common signals detected are secondary electrons for topography and backscattered electrons for information about atomic number/chemistry. Samples must be dry, conductive, and fit in the vacuum chamber. Modern SEMs generate digital data that is highly portable.

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ANGEL ANNA LAL
1st YEAR
MSc Biotechnology

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3. Scanning Electron Microscope
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4.  Invented by Max Knoll in 1935.
 Uses a focused beam of high-energy electrons to
generate images of a sample.
 3-Dimentional images are obtained.
 Magnification ranging from 20X to approximately
30,000X.
 Provides 250 times larger image than light microscope.
 Used in high or low vacuum in wet conditions and
even at wide range elevated temperatures.
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5.  The basic principle is that a beam of electron
is generated by a suitable source, typically a
tungsten filament or a field emission gun.
 The electron beam is accelerated through a
high voltage[20 kV] and pass through a
system of aperture and electromagnetic
lenses to produce thin beam of electrons.
 Then beam scans the surface of specimen.
 Electrons are emitted from specimen by
action of scanning beam and collected by
suitably positioned detector.
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8.  Appropriate size & should be dry.
 Specimens should be electrically
conductive.
 Coated with ultrathin layer of electrically
conducting method.
 Eg: Gold, GoldPalladium
alloy,Platinum,Osmium,Tungsten,Graphite
etc.
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10. Electron gun fitted with tungsten
filament
Electron beam focused by one or two
condenser, Passes through scanning
coilsdeflector plates
Primary electron beam interacts with
sample-repeated random scattering.
Beam current absorbed by specimen
is detected & electronic amplifier
amplifies signal & image is displayed.
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11.  Signals:
 Secondary electrons (SE): mainly topography
 Low energy electrons, high resolution
 Surface signal dependent on curvature
 Backscattered electrons (BSE): mainly
chemistry
 High energy electrons
 “Bulk” signal dependent on atomic number.
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Sample
Secondary electrons
Backscattered
electrons
Incoming electrons
X-rays

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Backscattered electron
detector:
Secondary electron
detector

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14.  Most SEM's are comparatively easy to operate,
with user-friendly interfaces.
 Many applications require minimal sample
preparation.
 For many applications, data acquisition is rapid
[less than 5 minutes/image for SEI, BSE.]
 Modern SEMs generate data in digital formats,
which are highly portable.
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15.  Samples must be solid and they must fit into
the microscope chamber.
 Maximum size in horizontal dimensions is
usually on the order of 10 cm; vertical
dimensions are generally much more limited
and rarely exceed 40 mm.
 For most instruments samples must be stable
in a vacuum on the order of 10-5 - 10-6 torr.
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