1. EDS Energy Dispersive Spectroscopy

2. Background Theory Introduction to the EDS System
Hardware & Software
X-Ray Signal Generation
Signal Origin, Spatial Resolution, Direction of Signal, Sample Surface
EDS Instrumentation & Signal Generation
Detector and geometry efficiency, Signal processing, Energy Resolution, Collimation

3. Introduction to the EDS System
Hardware
Software

4. Hardware Schematic SEM Column Monitor (MCA Display) Dewar FET Preamp
Chamber
Pole
Piece HP
Computer
Detector
EDAM
III
Window
PCI
Collimator
Sample
Stage

5. Processing Schematic Spectrum Interpretation Electron Beam Signal
Specimen
Interaction
Signal
Detection
X-Ray
Signal

6. X-Ray Signal Generation
Signal Origin
Spatial Resolution
Directionality of Signals
Analysis of Rough Surfaces or Particles

7. Bohr Model of the Atom (a simplified view) ---where X rays come fromLa Lb Ka Kb Ma
Real life spectra are more complex because there are multiple orbitals (esp. for the L, M and N orbitals). L-series spectra in EDS can have 6 or 7 peaks.
Nucleus

8. Atomic Number Order for the K Series Peaks

9. Chart of Lines visible 0-10 kV
K Lines - Be (Z=4) to Ga (Z=31)
L Lines - S (Z=16) to Au (Z=79)
M Lines - Zr (Z=40) to the highest occurring atomic numbers.
Every element (Z>3) will have at least one line viewable between
0.1 and 10 keV. In some overlap conditions it might be necessary to
examine the area between 10 and 20 keV.

10. Interaction Volume Regions
primary
beam
sample
surface se
bse
This diagram is somewhat misleading. High-energy and low-energy x rays behave very differently (just like e-).
x-rays
High energy x rays can not be excited at great depths. Low energy x rays can be excited at great depths, but will most likely be absorbed and will not escape.

11. SE vs BSE Images SE -- Edge effect, charge sensitive,
very little Z contrast.
BSE --Z contrast dominates, no
edge effect, no charging seen.

12. X-Ray Spatial Resolution
Low Z
Spot size does not determine the reso-lution but kV and Z are more significant.
High Z
Low kV
High kV

13. Signal Resolution x-ray se bse sample surface se bse x-rays
Signal resolution (se) is determined by the width of the electron beam (spot size) and is proportional to the signal depth.
x-ray se
bse
sample
surface se
bse
x-rays

14. Directionality of Signals
SE Signal - attracted to positive voltage on wire mesh network in front of detector.
BSE Signal - Detector is arranged to collect signals from a large, symmetrical area.
X-ray Signal - most directional of all signals, only one detector with no way to influence the trajectory of x-rays

15. Spectrum Anomalies Specimen Matrix Detector Electron Beam Backscatter
electrons
Fluorescence
X-rays
Specimen Matrix
Interaction
volume
Absorption of x-rays

16. Directionality of X-ray Signal
Detector
Direction
A B C
stage/mount
sample
Topography has a significant effect on spectrum count rate
and on composition (take-off angle and absorption effects)

17. A= Lower low end peaks B= Normal C= Higher low end peaks B C A
3 different spectra at 3 locations on the same particle with a uniform composition.
Take-off angle is highest at C and lowest at A.

18. Effects of Tilt (FeCO3)
Peaks are autoscaled to the O K peak. Q: What if they were scaled to the background area? A: FeK same height, C K, O K and FeL would be higher at +30 degrees.

19. EDS Instrumentation & Signal Detection
X-Ray Detectors
The Detector Efficiency
Geometrical Efficiency
Signal Processing
The Signal Processor
Energy Resolution
Collimation

20. X-section of window & crystal (sapphire)
-500 to 1000 volts
+,-
charges
x-ray
(photon)
microscope
vacuum to
preamplifier
(FET)
Detector
Vacuum
Detector
SiLi
Detector
Window
8u Be or 0.3u Polymer
Metallization Layer,(85 angstroms) plus
the Si dead layer

21. Detector Efficiency Window Transmission Capabilities
I / Io = e -(mr t)
Where :
I = Final Intensity
Io = Initial Intensity
m = mass absorption coefficient
r = density
t = thickness

22. Transmission of K x-rays through various windows

23. Mass Absorption Coefficient
N Ka Energy
Absorption edge
or critical excita-
tion energy
Absorption
(Kab) C
C Ka Energy
0.284
X-ray Energy (keV)

24. Absorption evidence in Spectra
The background is lower on the high-energy side due to
absorption in the sample.

25. W = A/d 2 Solid Angle Where: A= detector area, mm 2
d = the sample to detector distance
The solid angle (omega) is in steradians. Count rate at 70 mm scale setting = 1/4 that at 50 mm.

26. The Preamplifier C Output FET Reset Detector 50 ns/x-ray event
Ultimate peak measurement time will be about 50 us (1000x 50 ns)
Reset
Detector

27. Output signal of an X-Ray Event (or 3 events)
Higher dead time (all rejected)
Voltage
(mv)
Lower dead time v
Time
Multiple x-ray events too close to each other will be rejected.

28. Throughput Curves
Lesson: High count rates and high dead times actually give fewer counts and poorer spectra. You might consider a faster time constant.

29. Multichannel Analyzer

30. FWHM= SQRT[(FWHM)noise2 + (2.35 FEe)2]
Resolution Equation
FWHM= SQRT[(FWHM)noise2 + ( FEe)2]
Where:
F = fano factor= 0.11
E = energy of the x-ray, ev
e = 3.8 ev/charge pair (Si), ev/charge pair (Ge)

31. Resolution vs Energy for 70ev noiseMn

32. Collimators Be Window with no magnets (BSE do not penetrate)
SUTW or UTW Window
with magnets (shown in yellow) to deflect BSE
If BSE reach the detector they will produce
background anomalies --a hump in the
background at high energies.