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ELECTRON

DETECTORS FOR SEMPRESENTED BY

Engr. Naeem Basra(MM-06, 2006-07)

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What is a SEM?

Is a microscope that uses electrons rather than light

to form an image and to examine objects on a very finescale.

This examination can yield the following information:

Topography surface features

Morphology shape and size

Composition elements and compounds/ relativeamount of them

Crystallographic information how the atoms are arranged in the sample

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How the SEM Works

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Microstructure evaluationSmall feature measurements

Fracture mode preliminary identification

Grain size

Corrosion failure inspection

Surface contamination evaluationChemical Composition

Crystallographic information

Applications

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Interaction between electron beam and sampleThe interactions may be elastic or inelastic.

The elastic interactions:incident electrons and nucleus a large-angle deflection

of incident electrons. little energy loss

The inelastic interactions: incident electrons andorbital shell electrons a small-angle deflectionof incident electrons. heavy energy loss

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Secondary electrons (SE)

Generated when a primary electron dislodges a

specimen electron from the specimen surface. Produced by inelastic interactions. SE energy level is only 3 ~ 5 eV. Can be easily collected

The maximum escape depth is about 5 nm inmetal and 50 nm in insulators.

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Backscattered electrons (BSE)

Produced by elastic interactions. BSE energy level is about 60 ~80 % of incident

one. Special detector is required to collect BSE. The maximum escape depth varies inversely with

the average atomic number,

the range from a fraction of micrometer to severalmicrometers.(Larger atomic number, larger escape depth).

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X-rays

Generated when electron are dislodge fromspecific orbits of an atom in the specimen.

Large escape depth due to difficulty of x-rayabsorption

Energy and wavelength of x-ray can be used

for chemical analysis.1.energy-dispersive spectroscopy, EDS,2.wavelength-dispersive spectroscopy, WDS

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Electron Detectors

Hi hi SEM

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Hitachi SEMDetectors

View from inside, looking up obliquelyima e taken b handheld di ital camera

Annular

BSEdetector

IRChamber

scope

E-T SEdetector

EDS

detector

ESED

detector

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Detectors used to amplify the signalrather than detected secondary and

backscattered electrons directly

i.e. Electrons Photons Electrons

Backscattered electrons are high energy

(i.e. 1 - 40 keV) Secondary electrons are low energy

(~0 - 50 eV)

Detecting electrons

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Must use different detectors to collect thebackscattered and secondary electronsignals.

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Detecting backscattered

electrons

Two specialized detectors are used todetect BSE

Detector sits directly beneath theobjective lens.

Large surface area to maximize collection

efficiency.

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Backscatter electron detector

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Scintillator detectors

These comprise a scintillator (phosphor)with a light pipe and a photomultiplier

Fast response time and high gain, which

makes them suitable for use at TV rates.

Bulky and may restrict the workingdistance of the microscope.

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Solid state detectors

Si p-n junction which forms electron-holepairs on the impingement of a BSE.

A reverse bias separates the electron holepairs, giving rise to a detectable current.

Much smaller than a scintillator

Cheap to make Slow response time makes them unsuitable

for operation at TV scan rates

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Limitations

Slow response time

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Detecting secondary electrons

used an Everhardt-Thornley detector

based on a scintillator-photomultipliersystem

Energy of the secondary electrons is toolow to be detected directly

accelerated by applying a bias of ~ +10kV to a thin Al film on the scintillator

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A collector grid which is biased to ~200 Vis also used to attract the secondaryelectrons

This collector screens the scintillator fromthe incident beam and improves thecollection efficiency of the detector

Light is converted into pulses of electronsin the photomultiplier which are used tomodulate the intensity on a CRT

Detecting secondary electrons

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Secondary ElectronDetector

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THANKS