Saeedeh Ghaffari
Nanofabrication
Fall 2011
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Outline
 Introduction
 X-Ray Generation
 Analysis
 Detection
 Reference
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Introduction
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Introduction
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Introduction
 An analytical technique used for the elemental
analysis or chemical characterization of a
sample
 Relies on the investigation of an interaction of
some source of X-ray excitation and a sample
 Determines characteristic of an atom due to
the a unique atomic structure
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Types of X-ray
 Continuous x-rays
background radiation
 must be subtracted
for quantitative analysis

 Characteristic x-rays


elemental identification
quantitative analysis
Example of Eds spectrum
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Continuous(Bremsstrahlung)
 The energy emitted as an
x-ray when the electron
incident on a specimen is
bent on its trajectory
 Decelerated by the
electrostatic field of a
nucleus
 This x-ray does not have
a value unique to an
element
 This background is
excluded for quantitative
analysis
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Characteristic
 An incoming high-energy
electron dislodges an inner-shell
electron in the target, leaving a
vacancy in the shell
 An outer shell electron then
“jumps” to fill the vacancy
 A characteristic x-ray
(equivalent to the energy
change in the “jump”) is
generated
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Characteristic
•The difference
in energy
between two
orbits has a
unique value
for each
element, the
energy of the
emitted x-ray is
also unique to
the element
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A typical spectrum obtained on mineral particles of up
to 2μm diameter. The peaks are labeled with the EDX
line of the corresponding element
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Pure Ge
Several examples
of EDS spectra
Characteristic
Al film on Si
Pure Al
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Silica glass
Graphite
Organic
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Electron Transition
•A variety of
characteristic energy Xrays is generated as
the various displaced
inner-shell electrons
are replaced by the
various outer-shell
electrons
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Electron Transition
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Characteristic
Typical characteristic x-ray and their names
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X-Ray Energies
 X ray Energies are a function of Z (atomic number)
 K lines: lighter elements
 L lines: heavier elements
 M, N .. lines: the heaviest elements
 Kα:
 Be (Z = 4) 110 eV
 Fe (Z = 26) 6.4 keV
 Au (Z = 79) 68.8 keV
 Lα:
 Fe 0.70 keV
 Au 9.71 keV
 A threshold energy to eject electron increases with atomic
number
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 Note: The EDX detectors work well only in the range 1-20 kev
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X-Ray Analysis
 Qualitative Analysis:
 Peak energy gives
qualitative information
about the constituent
elements
 Quantitative Analysis:
 Peak intensity gives
information about the
element composition to
find the changes in
concentration of elements
• Note: The minimum detection limits vary from approximately 0.1 to
a few atom percent, depending on the element and the sample
matrix.
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EDS Setup
• Four primary
components of the
EDS setup:
• Beam source
• X-ray detector
• Pulse processor
• Analyzer
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EDX Detector
• Crystal detects X-rays
• Liquid nitrogen cools
crystal to reduce noise
and also pumps dewar
• Window separates
detector from column
vacuum
• Collimator eliminates
stray x-rays
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EDX Detector
•X-rays pass through :
• collimator
• electron trap
• window
• gold layer
• dead layer into Li-drifted Si
crystal (SiLi)
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Solid State Detector in EDX
EDX
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Si(Li) Crystal
Anti-reflective
Al coating 30 nm
Ice
Gold electrode
Gold
electrode
20 nm
Silicon inactive layer (p-type) ~100 nm
X-ray
Holes
(+)
(–)
Window
Be, BN,
diamond,
polymer
0.1 mm — 7 mm
Electrons
Active silicon
(intrinsic)
3 mm
–1000 V bias
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20
X-Ray Detection
 Electron - hole pairs created.
 Each electron-hole pair requires a mean energy of 3.8 eV
 Bias voltage sweeps charge carriers to either side
 Charge proportional to Xray energy
 Note: Charge is small! Noise is a potential problem.
 Note: High energy X-rays may not be dissipated in the
active region of the crystal! Incomplete charge collection.
 (EDX spectrometers work best in the region 1-20 Kev)
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X-Ray Processing
1.
2.
3.
4.
5.
X-ray comes in, creates an e h pair
Charge pulse enters FET, converted to voltage pulse
Voltage pulse amplified several thousand times
Analog-to to-digital converter used to assign pulse to specific energy
Computer assigns x-ray as a ‘count’ in a multi-channel analyzer
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References
 Robert Edward Lee, Scanning




electron microscopy and X-ray
microanalysis, Prentice-Hall
(1993)
Goldstein book
Wikipedia: energy dispersive Xray spectroscopic
Let’s familiarize ourselves with
the SEM booklet
Microanalyst.net
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Thanks for your Concentration
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