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Alex Beck’s
Scanning Electron Microscope
Basic Manual
LN2
DEWAR
EDS
IR camera
Variable Aperture for fine
tuning electron column
Table of Contents
Page #
1.
Cover
2.
Table of Contents
3.
Sample Stage
4.
Determining Atomic Number
5.
Field of Focus
6.
Vacuum Systems
7.
Electron Gun
8.
Column Environment
9.
Beam Sample Interaction
10. Electron Emission
11. Backscattered and Auger Electrons
12. SEM images
13. Energy Dispersive Spectrometer
14. EDS signal processing
15. SEM mapping
16. Preparing Sample
17. Photography
18. Quantitative EDS analysis
MOTORIZED PROGRAMABLE SAMPLE STAGE
X-move
Y-move
Z-move
Left to right on Screen
Top to bottom on Screen
Vertical Motion up and down
Controls Working Distance.
41/2 mm minimum
Tilt
Rotate
Tilts toward ET Detector only
360 degrees on sample axis
Once Door is shut, activate ruffing pump to remove majority of air.
Activate Turbomolecular / oil diffusion pump to remove more air.
Once air pressure inside SEM falls below 10^-5 torr, turn on electron
stream, and begin analysis.
3 SEPARATE INSTRUMENTS for determining
atomic number:
Scanning Electron Microscope
SEM
Base Platform for determining atomic number:
:
Energy Dispersive X-ray Spectrometer
EDS
Wave Dispersive X-ray Spectrometer
WDS:
nλ = 2 d Sin Θ
Sin -1 λ Char = Θ [set Detector]
2d
The SEM is for Imaging
Surface Features and Topography
SE
Depth of Field Focus
Composition: At. #
and Shadow
BSE
Cathodlumniescence
Chemical Variation
The EDS is for Chemical Analysis
Qualitative and Quantitative Analyses
Point Analysis
Analysis
Line
Area Analysis
Elemental Mapping of Area
WHY SEM WORKS BETTER THAN OPTICAL
MICROSCOPE
Wavelength of Visible Light:
Violet 400 nm to Red 750 nm wavelength
DeBroglie Equation: λ = h/mv
h = Planck’s Constant
m = mass
v = velocity
For an electron wavelength this becomes:
λ = 1.23/Voltage
At 10 Kv acceleration voltage: 0.0123 nm
wavelength
At 30 Kv acceleration voltage: 0.0071 nm
wavelength
Optical field of focus:
SEM field of focus:
SEM - Sub Component Systems
Vacuum Systems
Electron Gun
Column Environment
Chamber Environment
Stage
Detectors
Beam - Sample Interaction
Software
SEM VACUUM SYSTEMS
Mechanical
Pump: [ 10 -3 torr]
760 mm Hg
Roughing Pump
Backing Pump
Vapor Pump: Oil Diffusion
Pump 10 –5 10 –6 torr
Turbo Molecular Pump
10 -7 torr
The Electron Gun:
Supplies electrons under variable acceleration voltage [
1000 to 30,000 volts]
Wehnelt Assembage
Wehnelt Cylinder
Filament
Wehnelt Grid
Electron Emission
Anode Plate
Filament Saturation
False Peak
THE COLUMN
ENVIRONMENT
Condensing Lenses
(Electromagnetic)
Controls “Spot
Size” [Probe Current]
Final Lens
(Electromagnetic)
Apertures: Fixed and
Variable
Scanning Coils:
Electromagnetic
Stigmator Coils:
Electromagnetic fixes
Astigmatism in beam
shape
Magnetic Lenses can become fixed on certain
settings and must be degaussed, every once in a
while to eliminate hysteresis memory, which can
cause beam misalignment and blurry images. This
effect is most pronounced when beam properties
are changed, and magnetic field is realigned.
BEAM - SAM PLE INTERACTION
Secondary Electrons (< 50 volts)
SE
Inelastic Collisions - 95% of electron population
Back Scatter Electrons (5 - 30 Kev)
BSE
Elastic Collisions - 5% of electron Population
Characteristic X-rays: Kalpha, K beta, etc.
Bremstrahlung [X-ray Continuum - Breaking Radiation]
Auger Electrons : Low Energy, top 1nm
Cathodluminescence: : visible light
Heat
Volume of Excitation
10Kv
20Kv
30 Kv
Monte Carlo
simulation of
interaction
volume with
changing beam
current
Electron Population
EVERHART - THORNLEY DETECTOR
Secondary Electrons Imaging 1960
BACK SCATTER DETECTOR - Centarus Detector
Annular Shape
Solid state scintillation detector
Directly above Sample and just below the Pole Piece
Responds to electrons with several Kev and higher.
Images appear without angular light perspective.
Quad Detectors can correct this artificially
Responds to Atomic Number
Auger Electrons
Low energy
Only come from around top 1nm
Require Auger Electron detector, somewhat new and expensive
BSE Image
BSE Image
BSE Image
SE Image
is organic and very fragile, requires
uniform pressure on both of its sides.
3.8 ev per hole-e
Energy Dispersive Spectrometer - EDS
Artifact Signals - Signal Processing
Summation Peaks: Produced by extremely high count rates. Small
peak appears at exactly twice the ev value of a major peak. Two counts
come in so fast that they are counted as one by the pulse Processor (50
nsec recovery)
Example: Al = 1.49 Kev.
2 Al = 2.98 Kev
Silicon Escape Peak : An incoming photoelectron causes the Si
detector to emit a characterisitic 1.74 Kev Kalpha X-ray. If this X-ray
escapes the system before being absorbed, it will take that energy
away, and a small peak will appear at exactly 1.74 Kev LESS than some
major peak
ZAF CORRECTIONS
Z = Atomic Number: Larger size atoms will
produce more X-rays than smaller
A = Absorbence of specific X- rays generated
by one element by another.
F = Fluorescence: X-ray emission induced by
emission of other atoms.
EDS MAPPING,
with user defined
color coding.
Takes a long time
to scan maps
even at lower
resolutions.
SE
CL
MAP
BSE
CL
MAP
Preparing Sample
Samples can be placed into SEM naked, but are normally mounted in
Epoxy.
Any Sample to be placed in SEM must either be conducting all the way
through or be sputtered with an extremely thin conductive coating,
usually gold or carbon. This is important as negation of conduction
through or around sample results in build up of charge which repels other
electrons and disrupts any analysis.
Samples are normally prepared in a cylindrical epoxy mold.
•Too make mold place sample at bottom of container and pour in epoxy
solution.
•Once epoxy has dried, remove mold from container.
•To grind mold down to sample use a very low grit sand paper, of for
example 240. Use Buchler 1,000 Minimet Polishing, or equivalent.
•After sample is exposed wash sample and hands thoroughly to make
sure no grains survive to a finer grit.
•Use an ultrasonic cleaner for approximately 10 minutes to shake out any
further grains.
•Grind mold at higher grit ex. 320, then repeat above two steps. Apply
even higher grit ex. 600, then repeat above two steps.
•Grind mold with 6 micrometer diamond solution apply same ultra sonic
cleaning.
•Polish mold with .05 micrometer diamond solution, and wash.
•Just look at that mirror shine as you attach the mold to a mound with
conducting carbon strips which both hold the mold in place and provide
conduction to ground.
•Use sputter coater to coat mold with conductor.
•Place sample in SEM and begin analysis.
Photography
Keep voltage around 20 kV
Low magnification images usually have magnification less than
5,000X, with less concern for beam sample interactions.
Long working distance normally around 20-30mm.
Moderate to large spot size 200-300 notwithstanding.
Higher magnification up to 100,000X, lessens depth of field and vice
versa.
Resolution is vital and should always be maximized. These are a few
ways to increase resolution:
Short working distance, 10-15mm NO LESS.
Small spot size 100 or less.
Small variable aperture.
Use of Stigmater very important.
Quantitative EDS data analysis
For optimal EDS analysis sample should be highly polished.
25V, 15 mm working distance, 400 spot size.
Standardize beam to Cu standard that you place on sample mount.
Processing time 5- 6 (slow), or 3-4 (fast)
Dead time is when EDS is overwhelmed with data. Dead time must
occupy less than 1/3 of time, or else recalibrate voltage and spot
size.
Aztec software user-friendly quick and easy, however lacks finer
intricacies and extra functionality.
Inca software is more professional and accurate and has a wide
assortment of advanced features including color coded elemental
mapping.