Electron Beam Optics: (Image Formation) Electron Beam MicroAnalysis- Theory and Application
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UofO- Geology 619 Electron Beam MicroAnalysis Theory and Application Electron Probe MicroAnalysis (EPMA)
Electron Beam Optics: (Image Formation)
(modified from The Smithsonian Institution, 2006) and National Institute of Technology (NIST, 1997)
Why Electrons?
• They are easily created from various materials (work function). This makes it possible to create an electron beam in the first place. • They have a negative electrical charge and are attracted to a positive one. This allows us to give them a very high speed by attracting them with a very highly charged anode.
• They are affected by magnetic fields. This characteristic allows us to move the beam around and use magnetic fields as electron lenses in place of ground glass lenses for visible light optics.
wavelength
Light Vs SEM
Based on Abbes theory you cannot resolve structure below about ½ the wavelength of the probe.
Electron beam
@
50pm Visible light
@
400-700nm
The Story So Far
1. Electron Optics 2. Electron Beam -Specimen Interactions/Detection 3. Image Formation
1. Electron Optics 2. Electron Beam-Specimen Interactions/Detection 3. Image Formation
Critical Concept Backscattering Coefficient,
Cu,
~ 30%
Electron beam - specimen interactions define what information is available
SE 3 Sampling depth BSEs ~ 0.25 R K-O SE 1 ~ 10 nm SE 2 , SE 3 follow BSEs BSEs: composition & topography Specimen current = Beam - BSE - SE BSE SE 1 SE 2 Secondary Coefficient,
Cu,
~ 10% Lateral Resolution BSEs ~ 0.5 R K-O SE 1 ~ beam SE 2 follow BSEs SEs: topographic contrast R K-O (
m) = 0.0276 AE 1.67
/(
R K-O (nm) = 27.6 AE 1.67
/(
Z Z 0.89
0.89
) ) 2 nd critical equation!
The SEM
• Gun – Filament – Wehnelt – Anode • Electromagnetic Lenses – Usually three • Spray apertures – At least one. More with higher resolution • Scanning coils – Two sets X and Y deflection • Stigmators – 1-2 sets • Detectors – Could be several
Creating Those Electrons
• First the filament is heated until no further increase in brightness/signal is seen. This is saturation.
• The Wehnelt and filament are held at negative potential. • Wehnelt cap maintained at slightly more negative potential than the filament forcing the cloud to condense and pass through the aperture.
• The cloud of electrons is accelerated towards the positively charges anode and pass through a hole down the column
The Filament & Thermionic Emission •High current •Produces heat ( 1800-2500K ) •Broad emission points around150-100nm •lower resolution instruments
LaB 6 Tungsten wire
Filament & Thermionic Emission cont’d
Chromatic Aberration
http://www.mellesgriot.com/glossary/
Determining “W” Filament Saturation
Filament Current 1 st saturation point (false peak) 2 nd saturation point the sweet spot Beam Current
Electrons Need a Vacuum
Vacuum Air No scattering Complete scattering
The SEM
(a little lower) • • • • • Gun – Filament – Wehnelt – Anode Electromagnetic Lenses – Usually three Spray apertures – At least one. More with higher resolution Scanning coils – Two sets Stigmators – 1-2 sets • • Detectors Sample
Electromagnetic Lenses
Spherical Abberation
http://www.mellesgriot.com/glossary/
The Objective Lens
• Final lens in the system is a highly modified condenser lens called the objective. The objective lens is the workhorse and houses: – Scan coils – Stigmator coils – A limiting aperture
A One-Slide Summary of the Electron Optics Lecture
Brightness Equation
i
/
( 2 d 2 2 )
i = probe current d = probe diameter
= probe divergence Critical trade-off: To get a small beam for high resolution, we must sacrifice current.
The Rest of the Story….
• The filament is saturated and producing a fine cloud of electrons and an image of the filament tip. This first crossover image must be defocused, condensed into a tight, coherent, beam and scanned across the sample surface. • The final spot that rasters across the specimen will be about 1000 times smaller than it started at the first focus near the gun. (de-magnification)
The Scan Generator
Ratio of the area viewed to the area being scanned is magnification
The Basic Resolution Limitation of SEM Imaging:
We form the image a pixel at a time. We can only see details down to the smallest volume from which the signal is isolated.
SEM CRT Is the smallest volume limited by the probe, by the specimen scattering, or both?
How it Works
low mid high
Magnification and Pixel Size Area sampled as a function of magnification (assume 10 cm 2 display)
Magnification
10x 100x 1,000x 10,000x 100,000x 1,000,000x
Area on sample
1 cm 2 1 mm 2 100 um 2 10 um 2 1 um 2 100 nm 2 (0.1 um 2 ) Pixel element size as a function of magnification (assume 1000 pixels 2 and 10 cm 2 display)
Magnification
10x
Pixel Size
1 cm/1000 = .01mm or 10um 100x 1,000x 10,000x 100,000x 1,000,000x 1 mm/1000 = .001mm or 1um 100 um/1000 = 0.1um or 100 nm 10 um/1000 = 0.001um or 10 nm 1 um/1000 = 1nm 100 nm/1000 = 0.1nm
The SEM
(in the chamber) • • • • • Gun – Filament – Wehnelt – Anode Electromagnetic Lenses – Usually three Spray apertures – At least one. More with higher resolution Scanning coils – Two sets Stigmators – 1-2 sets • • Detectors Sample
Coating
• Sputtering (non uniform) – Gold – Palladium – Gold/Palladium alloy – Platinum – Chromium – Osmium Tetroxide • Evaporation (uniform) – Carbon – Platinum
Affect of kV on Interaction Volume
Incident beam sample 1kV 10kV 25kV
Secondary Electrons
Movies courtesy of Oxford Instruments
Beam Interaction & Resolution
Primary electron beam Sample surface Secondary electrons ~100A-10nm Backscatter electrons 1 2µm Characteristic X-rays 2-5um Interaction volume
Atomic Number Contrast
Raney Ni-Al Al-Cu eutectic Obsidian 50
m 2
m 10
m
SE 20kV SE 5kV
Reality
BSE BSE
Alternate Reality
So is it a bump or is it a hole?
keV & Penetration
5 kV
keV & Penetration
25 kV
keV & Fine Structure
5 kV 25 kV
Objective Aperture and Depth of Field
aperture objective sample Small aperture E.g. 20µm Large aperture E.g. 300µm
Working Distance
5mm 34mm
Depth of Focus
By simply shortening the working distance the background is blurred drawing the viewers eye to the bugs proboscis.
Imaging Unprepared Samples
VPSEM Capabilities
• Conventional HIGH VACUUM – Coated/conductive specimens – Critical point dried specimens – Cryo-microscopy • Low Vacuum – Charge reduction for non-conductors – Surface imaging in a gas – Wet or dirty specimens (ESEM)
The Holy Trinity
I.
Working Distance II.
Gas Pressure III. Accelerating Voltage
Considerations for Electron Imaging in Low Vacuum
High Vacuum ESEM Vacuum No Vacuum Minimal Scattering Scatter <5%
Images compliments of FEI
Partial Scattering Scatter 5% to 95% Complete Scattering Scatter >95%
SE Detection in Low Vacuum
Primary beam GSED Collection ring at high voltage + +
Detected electron signal
Signal amplification by gas ionisation + + + ground non-conductive specimen ground
Pressure/Temperature Phase Diagram for H
2
O
15 10 Solid phase Liquid phase Gaseous phase 5 0 -10 0 10 20 Temperature - Celsius 30