Lect2 Electron Interactions Ph451 551 2012

8/11/2019 Lect2 Electron Interactions Ph451 551 2012

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Chapter 3: Electron-Specimen Interactions

Electron Scattering Signals from Elastic Scattering

Signals from Inelastic Scattering

Interaction Volume

Influence of Beam Energy on Interaction Volume Influence of Atomic Number on Interaction Volume

Influence of Specimen Surface Tilt on Interaction Volume

X-Rays

Continuum X-Ray Production

Inner-Shell Ionization

X-Ray Absorption

X-Ray Fluorescence

Cathodoluminescence


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The versatility of scanning electron microscopy and of x-ray

microanalysis is derived in large measure from the rich variety of

interactions that the beam electrons undergo in a specimen. Theseinteractions can reveal information on the specimens composition,

topography, crystallography, electrical potential, local magnetic

field, and other properties.

1. Elastic scattering events

affect the trajectories of thebeam electrons inside the

specimen without changing

the kinetic energy of the

electron.

2. Elastic scattering is

responsible for thephenomenon of electron

backscattering that forms

an important imaging signal

in SEM.

3. Inelastic scattering events

result in a transfer of energyfrom the beam electrons to

the atoms of the specimen.

4. This event leads to the

generation of secondary

electrons; Auger electrons;

characteristic x-ray and

bremsstrahlung (continuum)

x-ray; electron-hole pairs in

semiconductor and

insulators.


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Elastic and Inelastic Scattering


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Five Major Inelastic Scattering Events

1. Phonon Excitation:Much of the energy deposited

into the specimen by the incident electron beam is

transferred to the solid by the excitation of lattice

oscillations (phonons), that is, heat. If the specimenis a good thermal conductor, the specimen and the

specimen holder will serve as an effective heat sink,

and significant heating will not occur. For

nonconductors, heating can be more significant. In

some cases, beam-induced heating is sufficient tocause phase transformations or recrystallization in

specimens.


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2. Plasmon Excitation:For metallic species such asaluminum or copper, the outermost atomic

electrons are so loosely bound that in a solidthese electrons are not localized to a specific atom

but rather form free-electron gas or sea, which

permeates the ion core. The incident electrons canexcite waves in this free-electron sea. Because of

the regularity of the atomic arrangement, theplasmon energy tends to have a specific value; in

aluminum, the excitation of a plasmon involves a

transfer of about 15 eVfrom the incident electronsto the free electron gas of the solid.


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3. Secondary Electron Excitation:The interaction of

the incident electron beam with the solid can leadto the ejection of loosely bound electrons in theconduction band. These ejected electrons are

referred to as secondary electrons, and most have

a kinetic energy of


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4. Bremsstrahlung or Continuum X-Ray Generation:

An incident electron beam can undergo deceleration in

the Coulombic field of the specimen atoms. Theenergy lost from the beam electron in this decelerationis converted into an x-ray photon known as abremsstrahlung x-ray. Deceleration is a continuousprocess, with most electrons losing their energythrough several interactions. The most energetic

continuum x-ray possible has the energy of theincident electron. In this case, the electron has lost allits energy in one scattering event. Consequently, thebremsstrahlung x-rays form a continuous spectrumfrom zero energy up to the incident-beam energy. Thecontinuum x-ray spectrum must be considered in

quantitative x-ray microanalysis. Because only thecharacteristic x-ray intensity above the continuum isimportant, the continuum must be removed to obtainonly the characteristic intensities.As such, thecontinuum x-ray serves no useful purpose inmicroanalysis.


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5. Inner-Shell Ionization:A sufficiently energeticelectron can interact with an atom and cause the

ejection of a tightly bound inner-shell electron,leaving the atom in an ionized and highly energeticstate (See next page). During subsequent de-excitation, an electron transition occurs in which anelectron from an outer shell drops inward to fill theinner shell vacancy. The electron transitioninvolves a change in energy. The energy releasedwill be in the form of a characteristic x-ray or anejected outer-shell electron termed an Augerelectron. Because the electron structure of eachatom is unique, the spectrum of possible

characteristic x-rays and Auger electrons is alsounique. The name assigned to a givencharacteristic x-ray, for example, K

, depends onthe electron shells involved in the transition.


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The principal quantum

number n is assigned

integral values 1, 2, 3, 4, 5,

that refer to the quantumshell to which the electron

belongs.

Quantum shells are

assigned a letter:

n=1 is designated K

n=2 is L

n=3 is M

n=4 is N


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Interaction Volume:

The combined effect of elasticand inelastic scattering is tolimit the penetration of thebeam into the solid. Theresulting region over whichthe incident electrons interactwith the solid, depositingenergy and producing thoseforms of secondary radiationis known as the interactionvolume.

An understanding of the sizeand shape of the interactionvolume as a function of

specimen and beamparameters is vital for properinterpretation of features ofSEM images and the spatialresolution of x-raymicroanalysis.


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Etching plastic can

directly reveal the

interaction volume for

the low atomic numbermaterials but can not for

intermediate- and high-

atomic number materials

such as metals.

Monte Carlo electron-

trajectory simulationprovides an indirect

method to visualize the

interaction volume for

metals.

A large number of

trajectories, typically

10,000 to 100,000, must

be calculated to achieve

statistical significance.


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Influence of Beam Energy on Interaction Volume

As the beamenergy is

increased, the

electrons can

penetrate to

greater depths.

The shape of the

interaction

volume does not

change

significantly withbeam energy.


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The influence of Atomic Number on Interaction Volume

In specimens of high atomic

number, the electrons

undergo more elasticscattering per unit distance

and the average scatteringangleis greater, as

compared to low-atomic-

number materials. Theelectron trajectories in high-

atomic-number materials

thus tend to deviate out of

the initial direction of travel

more quickly and reducethe penetration into the

solid. The shape of the interaction

volume also changes

significantly as a function

of atomic number.


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Influence of Specimen Surface Tilt on Interaction Volume

As the angle of tilt

of a specimen

surface increases(i.e., the angle of

the beam relativeto the surface

decreases), the

interaction volumebecomes smaller

and asymmetric.


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Interaction Volumes of Materials with Different Density


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SiO2Composition

The SiO2wires were found to be pure, the Au peakrepresents the nanoparticle in the center of the

nanoflower. This is a spectrum of the red spot.


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Lect2 Electron Interactions Ph451 551 2012

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