Material Characterization -Lecture 4

8/13/2019 Material Characterization -Lecture 4

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Material Characterization MethodsMaterial Characterization Methods

ELECTRON BEAM INSTRUMENTS

Lecture 4


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Energy-Dispersive X-Ray Spectroscopy, EDS

Electron Energy-Loss Spectroscopy in theTransmission Electron Microscope, EELS

Scanning Transmission Electron Microscopy,STEM

Electron Probe X-Ray Mircoanalysis, EPMA

Types of Electron beam microscopes


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Electron beam instruments are used to analyze differentcompositions using fine electron probes in the SEM and TEM

Electron beam columns specially constructed for analytical purpose(STEM)

The incoming electron beam interacts with the sample to produce anumber of signals that are subsequently detectable and useful foranalysis.

X-ray emission can be detected by Energy Dispersive Spectroscopy, EDS,

Wavelength Dispersive Spectroscopy, WDS;

visible or UV emission, which is known as Cathodoluminescence

(CL), Auger Electron Emission, which is the basis of Auger Electron

Spectroscopy


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AnalysisAnalysis of signals depends on:

the energy of the electron beam,

the nature of the material (high or low atomic number),

the type of information sought

elemental composition,

chemical composition (CL),

ultimate in spatial resolution,

information limited to the surface,

information throughout the bulk by transmission measurement


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Energy-Dispersive X-Ray Spectroscopy (EDS)

Its great advantage is its ability to routinely provide rapid multielement analysis for Z>11, with a detection limit of about 200

ppm for elements with resolved peaks.

If Z>11, the minimum detection limit is 0.02 wt.%, but in practicethe limit is 0.1wt% because of a high background count andbroad peaks.

If Z


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Sample requirements Any sample size within the size of the specimen chamber

(


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Mode of analysis Three modes of analysis commonly used: spectrum acquisition; spatial

distribution, or mapping of the elements; and elemental line scans.

Spectrum acquisition mode: The probe is either fixed in the spot mode or rasterscanned over a small area at high magnification and a complete spectrumacquired.

Spatial distribution mode: More than 15 elements can be collected

simultaneously and used to generate X-ray dot maps displaying the spatialdistribution of the elements in the sample.

These maps can be arranged in an overlay with the colors combined to showthe presence of various compounds.

Elemental line scans: It is much faster and often more used to simply scan one

line on a sample. it is usually possible to plot the data from many elements simultaneously. This

mode of display will usually show minor concentration differences much betterthan the brightness modulation technique used in mapping.


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Accuracy in EDS Better than 2% relative for major concentrations, if sample isprepared in very good conditions

It is expected to be 4-5% relative for general analysis using pureelement standards

Elements with concentrations less than 5 wt% will typically yieldrelative accuracies nearer 10% even with standards.

For samples with rough surfaces, such as fracture samples orsmall particles, the relative accuracy may be as bad as 50%.

To analyze a film only a few nm thick on a Si substrate, theaccelerating voltage should be set near 10 keV.

To analyze a bulk sample, more than a few micron, anaccelerating voltage of 20-25 keV should be used


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Applications It has been used for quality control and test analysis in many

industries: computers, semiconductors, metals, cement, paper,and polymers.

In medicine: Analysis of blood, tissues, bones, and organs

In pollution control: Asbestos identification

In field studies: ore prospecting, archeology, and oceanography;

For identification and forgery detection in the fine arts;

For forensic analysis in law enforcement.


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Principle process of spectrum analysis


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X-ray spectrum of a aluminum tungsten oxide on a carbon foil

supported on a copper grid


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Data Screen


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Comparison of SEM images


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Building material


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multi-layer ceramics


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Cross section of the Sn(Pb) solder bumps with a

under bump metallisation consisting of anAl/Ni(P) double layer on silicon


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Merits and limitations

With a radioactive source, an EDS system is easily portable and can beused in the field more easily than most other spectroscopy techniques.

Its speed of data collection; the detectors efficiency (both analytical andgeometrical); the ease of use; and the relative ease of interfacing toexisting equipment.

The disadvantages are: poor energy resolution of the peaks,

A relatively low peak-to-background ratio in electron-beam instrumentsdue to the high background radiation emitted by electrons suffering

Deceleration on scattering by atoms;

A limit on the input signal rate because of pulse processingrequirements.

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Material Characterization -Lecture 4