opt 307/407 practical scanning electron microscopy considerations in any microscopy: resolution...
TRANSCRIPT
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Opt 307/407 Practical
Scanning Electron Microscopy
Considerations in any microscopy:Resolution
MagnificationDepth of field
Secondary information
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Limits of Resolution (resolving power)
Unaided eye: 0.1mmLight microscope:0.2um
SEM: 1nmTEM: 0.2nm
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Evolution of Resolution
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Depth of Field
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Light Microscope vs Electron Microscope
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General Diagram of the SEM System
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Light Microscopy vs Electron Microscopy
Advantages of EM:ResolutionMagnificationDepth of field
Disadvantages of EM:PriceyBetter if conductive (SEM)MaintenanceVacuum
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Opt 307/407Vacuum Systems
Why do we need a vacuum anyway?
Electrons are scattered by gas (or any other) moleculesMFP at 1atm ~ 10cmMFP at 10-5T ~ 4m
Some samples react with gases (O2)
Helps keep things clean!
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Opt 307/407Vacuum Systems
Terminology
PressureUnits: atm, bar, mbar
Torr (mm of Hg)Pa (N/m2)
1atm=1Bar=1000mBar=760Torr=105Pa
Pumping speedl/min, l/sec
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Opt 307/407Vacuum Systems
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Opt 307/407Vacuum Systems
Quality of Vacuum
Low: 760-10-2 Torr
Medium: 10-2-10-5 Torr
High: 10-5-10-8 Torr
Ultrahigh: <~10-8 Torr
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Opt 307/407Vacuum Systems
Measuring Vacuum in EM Systems
Thermocouple GaugePirani Gauge
Cold cathode GaugePenning Gauge
Ion pump current
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Very Broad Range of Vacuum to Measure
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Grouped Ranges for Vacuum Gauges
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Vacuum Gauge Choices and Working Ranges
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Thermocouple/Pirani Gauges
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Ionization Gauges
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Ion Gauge Collection
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Hot Cathode Ion Gauge
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Penning gauge
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Penning gauge
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Opt 307/407Vacuum Systems
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Opt 307/407Vacuum Systems
Types of Vacuum Pumps
1- Rotary (Fore, Rough, Aux, Mechanical)
2- Turbomolecular (Turbo)
3- Diffusion (Diff)
4- Ion (Sputter-ion)
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Opt 307/407Vacuum SystemsRotary Pump Basics
Always in the Foreline of the system
Exhausts pumped gases to atmosphere
Pumping rate decreases as vacuum increases
Usually has a low VP oil as a sealant to facilitate pumping
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Opt 307/407Vacuum Systems
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Opt 307/407Vacuum Systems
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Opt 307/407Vacuum Systems
Rotary Pump Problems
Cannot pump <10-2 TorrNoisy
BackstreamsVibration
Maintenance
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Opt 307/407Vacuum Systems
Turbo Pump Basics
Direct drive electric motor-gas turbine
Rotor/stator assembly
Moves gas molecules through the assembly by sweeping them from one to another
High rotational speed (>10,000 RPM)
Very clean final vacuum
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Opt 307/407Vacuum SystemsTurbo Pump Problems
Needs a Foreline pump
Costly
Can fail abruptly
Whine
Needs to be protected from solid material
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Opt 307/407Vacuum SystemsDiffusion Pump Basics
No moving partsHeated oil bath and condensing chamberJet assembly to redirect condensing gas
Recycle of oil
Pressure gradient in condensing chamber/Foreline pump removes from high
pressure side
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Opt 307/407Vacuum Systems
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Opt 307/407Vacuum Systems
Diffusion pump problems
Heat up/cool down time
Needs foreline pump
Can make a mess in vacuum failures/overheating
Needs cooling water (usually)
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Opt 307/407Vacuum Systems
Ion Pump Basics
High voltage creates electron fluxIonizes gas molecules
Ions swept to titanium pole by magnetic fieldTitanium erodes (sputters) as ions become
embedded
Getters collect Ti atoms and more gas ionsCurrent flow indicates gas pressure (vacuum)
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Opt 307/407Vacuum Systems
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Opt 307/407Vacuum Systems
Ion Pump Problems
Cannot work until pressure is <10-5 Torr
Low capacity storage-type pump
Needs periodic bake-out
Hard to startup (sometimes)
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Opt 307/407Vacuum Systems
Summary
All electron microscopes require a vacuum system.Usually consists of rotary-(turbo, diff)-(ion) pumps.
System should provide clean oil-free vacuumat least 10-5 Torr or so.
Vacuum is usually measured with a combination of TC and ion gauges.
Vacuum problems are some of the most challenging to find and fix, and may even be caused by samples
outgassing
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Opt 307/407Vacuum Systems
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Opt 307/407Vacuum Systems
Typical TEMVacuum System
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Opt307/407
Electron Sources and LensesElectron Sources and Lenses
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Types of Electron SourcesTypes of Electron Sources
Thermionic SourcesTungsten filamentLanthanum Hexaboride (LaB
6) filament
CeB6
Field Emission sourcesColdSchottky
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Ideal Electron Source CharacteristicsIdeal Electron Source Characteristics
Low “work function” material so that it is easy toremove electrons from the material
High melting point
Chemically and physically stable at high temps
Low vapor pressure
Rugged
Cheap
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Thermionic Emission of ElectronsThermionic Emission of Electrons
Filament material is heated with an electrical currentso that the “work function” of the material is exceededand the electrons are allowed to leave the outermost orbital.
Generates a fairly broad source of electrons (cloud)
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Tungsten Hairpin FilamentsTungsten Hairpin Filaments
Most common of all filaments in electron guns
Low cost (~$20)
Lots of beam current
Not very intense illumination
Emission temperature ~2700K
Work function= 4.5ev
Can last about 100 hours
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Tungsten Hairpin Filament SaturationTungsten Hairpin Filament Saturation
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Tungsten Hairpin FilamentTungsten Hairpin Filament
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LaBLaB66 (and CeB (and CeB66) Filaments) Filaments
Lower work function thermionic source (2.4ev)
Lots brighter (~50x) than W-hairpin
Relatively costly (~$700)
Can be direct replacement for W-hairpin
Heated to about 1700K
Can last hundreds of hours
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LaBLaB66 Emitter Problems Emitter Problems
Need higher vacuum to reduce reactivity
More difficult to make
Heating/cooling must be slow (brittle material)
Heating is indirect through a graphite well
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Thermionic Gun LayoutThermionic Gun Layout
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Optimization of Thermionic Emitter LifetimeOptimization of Thermionic Emitter Lifetime
Keep vacuum system in good working order
Clean gun area
Do not oversaturate the filament
Minimize the number of heating/cooling cycles
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Field Emission Electron SourcesField Emission Electron Sources
Process proposed in 1954/Demonstrated in 1966
Usually a single crystal W-wire sharpened and shaped
Tip radius <1.0um
Usually includes a ZrO2 component to assist emission (if heated)
About 10,000 times brighter than W-hairpin
Small apparent source which helps obtain small probes with high temporal coherence
Decreased energy spread in the beam
Can last many thousands of hours
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Cold Field EmittersCold Field Emitters
Most intense (brightest) electron source
Tip radius very small (~0.1um)
Needs very high electric field intensity
Tips contaminate and need “flashing” to clean and/or anneal
Expensive (~$4000)
Requires ultrahigh vacuum in gun
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Schottky Field EmittersSchottky Field Emitters
More stable than cold field emitters
Self annealling as ions impact tip
Lower work function than cold field emitters
Extraction field intensity can be lower
Vacuum requirements lower
Still expensive (~$4000)
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Typical Schottky Field Emission SourcesTypical Schottky Field Emission Sources
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Schottky Field Emitter DiagramSchottky Field Emitter Diagram
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Suppressor Cap:limits the electron emission to the desired area of the tipactually blocks electrons from the heater and shaft
Heating Filament-tungsten hairpin:heats the tungsten tip to enhance emission (1800K)
Emitter:Single crystal W-needle w/ ZrO2 coating
Schottky Field Emitter PartsSchottky Field Emitter Parts
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Extractor Anode:applies voltage to the filament to extract electrons from the tip (1.8 - 7 keV)
Gun Lens:Electrostatic lens which forms a crossover of the electron source (acts similar to the C1 lens)
Schottky Field Emitter PartsSchottky Field Emitter Parts
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Optimizing Field Emission Emitter LifetimeOptimizing Field Emission Emitter Lifetime
Keep vacuum system in good working order
Leave the emitter heated
Don’t over-extract
Don’t overheat
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Electron LensesElectron Lenses
ElectrostaticGun cap (Wehnelt cylinder)Totally inside vacuum
ElectromagneticAll other lenses and stigmatorsPartially outside of vacuum
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Transmission Electron MicroscopeTransmission Electron MicroscopeOptical instrument in that it uses a lens to
form an image
Scanning Electron MicroscopeScanning Electron MicroscopeNot an optical instrument (no image forming
lens) but uses electron optics. Probe forming-Signal detecting device.
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Electron OpticsElectron Optics
Refraction, or bending, of a beam of illumination is caused when the ray enters a medium of a different optical density.
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Electron Optics
In light optics this is accomplished when awavelength of light moves from air into glassIn EM there is only a vacuum with an optical density of 1.0 whereas glass is much higher
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Electron Optics
In electron optics the beam cannot enter a conventional lens of a different refractive index. Instead a “force” must be applied that has the same effect of causing the beam of illumination to bend.
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Classical optics: The refractive index changes abruptly at a surface and is constant between the surfaces. The refraction of light at surfaces separating media of different refractive indices makes it possible to construct imaging lenses. Glass surfaces can be shaped.
Electron optics: Here, changes in the “refractive index” are gradual so rays are continuous curves rather than broken straight lines. Refraction of electrons must be accomplished by fields in space around charged electrodes or solenoids, and these fields can assume only certain distributions consistent with field theory.
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Converging (positive) lens: bends rays toward the axis. It has a positive focal length. Forms a real inverted image of an object placed to the left of the first focal point and an erect virtual image of an object placed between the first focal point and the lens.
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Diverging (negative) lens: bends the light rays away from the axis. It has a negative focal length. An object placed anywhere to the left of a diverging lens results in an erect virtual image. It is not possible to construct a negative magnetic lens although negative electrostatic lenses can be made
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Electron OpticsElectrostatic lens
Must have very clean and high vacuum environment to avoid arcing across plates
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Electromagnetic Lens
Passing a current through a single coil of wire will produce a strong magnetic field in the center of the coil
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Three Electromagnetic LensesThree Electromagnetic Lenses
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Electromagnetic Lens
Pole Pieces of ironconcentrate lines ofmagnetic force
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Electromagnetic Lens
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Electromagnetic Lens
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Forces Acting on an Electron Beam as Forces Acting on an Electron Beam as it goes through an Electromagnetic Lensit goes through an Electromagnetic Lens
...and the Result...and the Result
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The two force vectors, one in the direction of the electron trajectory and the other perpendicular to it, causes the electrons to move through the magnetic field in a helical manner.
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The strength of the magnetic field is determined by the number of wraps of the wire and the amount of current passing through the wire. A value of zero current (weak lens) would have an infinitely long focal length while a large amount of current (strong lens) would have a short focal length.
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Condenser Lens: Weak and Strong ConditionsCondenser Lens: Weak and Strong Conditions
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Lens DefectsSince the focal length f of a lens is dependent on the strength of the lens, if follows that different wavelengths will be focused to different positions. ChromaticChromatic aberration of a lens is seen as fringes around the image due to a “zone” of focus.
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Lens DefectsIn light optics wavelengths of higher energy (blue) are bent more strongly and have a shorter focal length
In the electron microscope the exact opposite is true in that higher energy wavelengths are less effected and have a longer focal length
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Lens Defects
In light optics chromatic aberration can be corrected by combining a converging lens with a diverging lens. This is known as a “doublet” lens
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The simplest way to correct for chromatic aberration is to use illumination of a single wavelength! This is accomplished in an EM by having a very stable acceleration voltage. If the e velocity is stable the illumination source is monochromatic. monochromatic.
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Lens Defects
A few manufacturers have combined an electromagnetic (converging) lens with an electrostatic (diverging) lens to create an achromaticachromatic lens
LEO Gemini Lens
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The effects of chromatic aberration are most profound at the edges of the lens, so by placing an aperture immediately after the specimen chromatic aberration is reduced along with increasing contrast
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Lens Defects
The fact that rays enter and leave the lens field at different angles results in a defect known as sphericalspherical aberration. The result is similar to that of chromatic aberration in that rays are brought to different focal points
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Spherical aberrations are worst at the periphery of a lens, so again a small opening aperture that cuts off the most offensive part of the lens is the best way to reduce the effect.
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Diffraction
Diffraction occurs when a wavefront encounters an edge of an object. This results in the establishment of new wavefronts
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Diffraction
When this occurs at the edges of an aperture the diffracted waves tend to spread out the focus rather
than concentrate them. This results in a decrease in resolution, the effect becoming more pronounced with ever smaller apertures.
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AperturesApertures
AdvantagesAdvantages
Increase contrast by blocking scattered electrons
Decrease effects of chromatic and spherical aberration by cutting off edges of a lens
DisadvantagesDisadvantages
Decrease resolution due to effects of diffraction
Decrease resolution by reducing half angle of illumination
Decrease illumination by blocking scattered electrons
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If a lens is not completely symmetrical objects will be focussed to different focal planes resulting in an astigmaticastigmatic image
Astigmatism
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The result is a distorted image. This can best be prevented by having a near perfect lens, but other defects such as dirt on an aperture etc. can cause astigmatism
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Astigmatism in light optics is corrected by making a lens with a offsetting defect to correct for the defect in another lens.
In EM it is corrected using a stigmator which is a ring of electromagnets positioned around the beam to “push” and “pull” the beam to make it more circular in cross-section
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Opt 307/407
The SEM Systemand
Electron Beam-Sample Interactions
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The TEM system and components:
Vacuum Subsystem
Electron Gun Subsystem
Electron Lens Subsystem
Sample Stage
More Electron Lenses
Viewing Screen w/scintillator
Camera Chamber
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The SEM System and Components:
Vacuum Subsystem
Electron Gun Subsystem
Electron Lens Subsystem
Scan Generator Subsystem
Scattered Signal Detectors
Observation CRT Display
Camera CRT/Digital Image Store
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SEM Scan Generation System
Sets up beam sweep voltage ramp in both X and Y directions (tells beam how far to move and the number of increments)
Synchronized between beam on sample and beam on CRT display
Can be analog or digital in format
Includes interface to magnification module for changing the beam sweep on the sample
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Scan Generator Interface
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Magnification control in the SEM
Beam sweep on sample is synchronized with beam sweepon display CRT
CRT size never changes
Sweep distance on sample can vary (using magnificationmodule)
Small distance on sample--> large magnification to CRTLarge distance on sample--> small magnification to CRT
Mag=CRT size/Raster Size
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Magnification Control in the SEM
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Depth of Field in the SEM
The single most important thing in making SEM imagespleasing to look at and interpret
Range of distances above and below the optimal focusof the final lens that produces acceptably focussed imagefeatures
DOF in the SEM is a few hundred times that of the LMat similar magnifications
DOF is inversely proportional to the aperture angle
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Depth of Field and Defocus
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DOF in the SEM
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DOF and Aperture Size
Table 1. Depth of Field at 10 mm working distance for SE images.
Magnification
30 1.9 mm
3000
30000
Table 2. Depth of Field at 25 mm working distance for SE images.
Magnification
30 4.9 mm 2.5 mm 1.6 mm
3000
100 m aperture
200 m aperture
300 m aperture
( = 0.005 rad) ( = 0.01 ( = 0.015
995 m 663 m
10 m 5 m 3 m
1 m 0.5 m 0.3 m
100 m aperture
200 m aperture
300 m aperture
( = 0.002 rad) ( = 0.004 rad)
( = 0.006 rad)
25 m 12.5 m 8.3 m
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Note the large depth of field which is possible with small probe semi-angle ( .
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DOF and Sample Tilt
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DOF and Working Distance
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Spot Size
Resolution is a direct function of (and limited by) the final spot size of the electron beam
This is a function of initial beam crossover size at the gunand the final spot formed by the beam shaping apertures and the condensing lenses
Shorter focal lengths produce smaller focussed spots
Short working distances have the smallest spots andthe best resolution
Smaller spots reduce the signals generated (S/N decreases)
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Spot Size Control in the SEM
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Signal Detectors for the SEM
Electron Beam-Specimen Interactions
First thing: electrons are scattered in a near-forward direction
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Electron Beam-Sample Interaction
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Electron Flight Simulator Demo
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Smorgasbord of Electron Beam Sample Interactions
Elastic ScatteringBackscattered Electrons
Inelastic ScatteringPlasmon Excitation (coherent oscillations in free electron “plasma”)
Secondary Electrons from conduction band
Electron Shell Excitation (photons, characteristic x-rays and Auger electrons)
X-ray Continuum (braking radiation)
Phonon Excitation (thermal)
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Electron Beam-Sample Interactions
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Backscattered (Primary) Electrons
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Backscatter Yield
n=-0.0254+0.016*A2-0.000186*A2*A2+0.00000083*A2*A2*A2
Backscatter Yield
0
0.1
0.2
0.3
0.4
0.5
0.6
0 20 40 60 80 100
Atomic #
Yie
ld
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Backscattered Electron Detectors
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Backscattered Electron Image
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Backscattered Electron Detector Placement
For either solid-state Si detectors or Robinson type
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Secondary Electrons and Detectors
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Secondary Electrons
Inelastic collision and ejection of weakly held conductionband electrons (need only few eV to exceed work functionof the sample atoms)
Always low in energy (<50eV)
Can also be formed from backscattered electrons. Ratio is Zdependent (SE
BS/SE
B increases with Z)
Usually a large fraction is produced within a region definedby the primary beam
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Some Secondary Electron Characteristics
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Types of Secondary Electrons/Origins
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Secondary Electrons: Edge Effects
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Everhart-Thornley (ET) Secondary Electron Detector
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Photomultiplier Tube Electronics
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Whole E-T Detector w/PMT Amplification
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Secondary Electron Images
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Auger Electron Generation
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Auger Analytical Volume
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Auger Electron Spectroscopy
Yielded inverse to BSE: lighter elements emit more
Electrons are VERY specific in energy...can indicatetype of bonding involved and oxidation state
MFP for typical Auger energies is about 0.1-2nm
Analytical volume is very small---> resolution is high
Signal is pretty weak
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X-ray Photon Production
Bremsstrahlung (Braking) radiation
Characteristic X-rays
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Bremsstrahlung Continuum X-rays
Formed by the release of energy from the primary electronbeam as it decelerates in the presence of the Coulombicfield of target (sample) atoms
Large energy spread (0-E0)
Not very useful
Forms a large portion of the x-ray spectral background
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Characteristic X-rays
Formed when inner shell electrons are ejected by the primary beam, followed by an outer shell electron
falling and filling the vacancy. Energy difference iscompensated by releasing a photon of “characteristic”energy, defined by the energy level differences of the orbitals, which is unique within a series of transitions.
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Characteristic X-ray Production
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Energy Dispersive X-ray Spectrometer
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X-ray Spectrum from EDS Spectrometer
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Wavelength Dispersive (crystal) Spectrometer
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X-ray Spectra Comparison EDS vs WDS
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Cathodoluminescence Signal Generation
Electron beam excitation of sample valence band electronsinto the conduction band (electron-hole pair production)
If allowed to recombine, the annihilation of the electron-hole paircreates a photon (sometimes in the visible range)
A high efficiency collector (usually a parabolic mirror) and aPMT are used to collect and amplify the signal
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Absorbed Current or Specimen Current
Sample is detector
IB~= I
SC+ I
BS + (I
SE + I
ph +I
etc)
SC image looks like an inverted BSE image
Very useful and easy to obtain
Resolution not so good
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Transmitted Electrons
In thin samples the beam may pass through the thickness
TED is located below the sample (like BSE detectors)
Sort of like TEM w/o the resolution
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Relative Sizes of the Emission Zones (looking from above)
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Image Collection, Recording and Presentation
Rule-of-thumb microscope conditions-best resolution-best depth of field-best sample preservation
Conventional Photographic Methods
Digital Methods
Presentation for:DisplayPublication
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Image Collection
Proper subject identification
Proper subject orientation
Best selection of imaging conditions-HV-WD
-Spot size (aperture)-Scan rate
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Subject Identification/Orientation
Representative of the whole
Image background
Not too busy
Important image information is centered and prominent
Many times a slight tilt conveys more information
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“Best” Imaging Conditions
High resolution-short working distance
-small spot size-high accel. Voltage-high magnifications
Depth of field-long working distance
-low magnifications-larger spot size
Low magnification-large spot
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Selection of Scan Rate for Imaging
Sensitive samples-may need to be fast
-low S/N-maybe TV integration mode
Insulating (charging) samples-decrease charging with small spot and
fast frame rate, maybe TV again-focus/stigmate in an area adjacent to the area recorded
-use image shift function to quickly move small amounts
Normally conductive samples-use slowest rate practical w/o degrading surface
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Old Technology
Analog scan SEMs
2nd CRT for viewing the image as it scans
Film based camera focused on this CRT (low persistence)
Almost always a 4x5 inch Polaroid sheet film camera
Very slow scan for about a 2000 line image (~3 minutes)
P/N film or just an instant positive image
About $3/shot now
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Generalized Photographic Processing
Needed for TEM image plates(Can be used for SEM film images too)
Exposure of silver halide grains (latent image)Development (reducing basic solution---> Ag0)Rinse (water) or Stop (acid)Fix (thiosulfates)Rinse (water)Dry
Scan or Print photographically
Good photographic processing results in the best imagesand are still the images that are used to compareother (newer) techniques
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Newer Technology
Digital raster SEMs
Frame buffer storage of image info
Image processing
Digital image storage-usually TIFF files so that header can contain
image and microscope specific data
Fully transportable formats
Easy incorporation of images into documents
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LEO 982 Specific Digital Imaging
Detectors-SEI (chamber)-SEI (column)-BSE
Signal mixer-brightness-ratio
Gamma correction-corrects for desired brightness and contrast I
out~=I
in
-power function deviation from 1:11.0 darkens and enhances lower greys1.0 lightens and enhances higher greys
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<--- switch position 0
<--- switch position 1
Gamma Corrections
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<----- switch position 3
<----- switch position 4
<----- switch position 5
<----- switch position 6
Gamma corrections
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LEO 982 Specific Digital Imaging
Slow scan rates 1-3 continuous scan
Slow scan rates 4-8 store one frame of data-dump to disk as image file (TIFF)
Choose image pixel matrix density from 512x512 to2048x2048 (lowest is usually OK)
Right mouse button will interrupt any scan and storeresults in the buffer (incl. TV)
TV rate integration of frames can reduce random noisein the final image at a fast scan rate
File path and naming convention
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LEO 982 Specific Digital Imaging
Variable small raster-used to increase scan rate for image adjustment
Can store multiple images in the same frame-variable frame
-split screen-kind of gimmicky.....don't use for important images
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Stereo Pair Images (Anaglyphs)
By collecting two images offset by about 4-100 in tilt
Display them side by side and cross eyes to converge
Build a blue-red image composite and use stereo glasses-In Photoimpact program:
convert images to RGBadjust color balance (red-right, blue-left)
perform image calculation (difference operator and merge)
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Special Scan Modes in the LEO 982
Line scan-disable Y-axis scan to see grey-level variations
on a line
Y-modulation-if very little Z-axis information this converts it
to Y-axis deflection (not very useful)
Spot scan-mostly for x-ray data acquisition
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Additional Scanning Features of the LEO 982
Dual magnification-useful for “looking around”-don't use for important images
Scan rotation-electronically rotates the raster on the sample-very useful for getting a good “presentation”
Dynamic focus-use to compensate for the portions of the sample that
fall outside the depth of field distance. Sets up aramp on the focus current +- the center of the field
Tilt correction-compensates for trapezoidal scan on highly tilted samples
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Image Processing
Generally use “kernels” which are arrays of arithmeticoperators on a pixel
Standard kernels are used to blur, average, and sharpenimages. 3X3, 5x5, array of operators.
Photoshop and PhotoImpact have custom and standardkernels
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Kernel Operations for Sharpening an Image
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Different Kernels
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Effect of Kernel Size on Operations
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Contrast Enhancement
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Original kernel Average kernel
Sharpen kernel Blur kernel
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Pitfalls of Image Processing
Images can be distorted and data lost
Pixelization of images
Ethical behavior dictates a minimum of processing
Always better off collecting the best image and eithernot processing or doing it only lightly
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Image Manipulation
Erosion of edge pixels-kernel operator to find edges
-erode or erase edge pixels one layer at a time-break apart and separate touching features
Dilation of edge pixels-kernel operator to find edges
-dilate or add edge pixels one layer at a time-fuse separate features
Most useful in particle and other small repeating features
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Presentation of Micrographs
Reports-probably least critical
-must convey information concisely
Journal-probably most critical
-size, grey-levels, resolution-must be specific and representative of the narrative
Posters-most variable in format-otherwise like journal
-conducive to point and discuss
Web-like journal
-can be interactive
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Presentation Media
Photographic paper
Photo quality printer output-dye sublimation
-ink jet....getting there!-laser...maybe...
-consider viewing distance in choice
Include TIFF or JPEG files in reports using word processor
Powerpoint for talks
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Micrographs as Art
Wonders of things small
Intricacies of natural samples
Subtle grey tones, like fine b/w photos
Can be psuedocolored to add interest
Comparisons to more familiar things
Explain phenomena in a “gee-whiz” way
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Sample Preparation for Electron Microscopy
Electrically Conductive SamplesElectrically Insulating SamplesBiological Samples“Odd” Samples
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Why do samples need to be prepared???
Vacuum environment
Charged particle environment
Too big
Components migrate in response to the beam
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Two General Samples Types
Bulk SamplesSEM only
Thin SamplesSEM and TEM
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Processes Common to Many Samples
Dehydration
Coating
Methods to reveal interior details
Stabilization of loose parts
Sample resizing
Methods to make similar measurements with other techniques
Special imaging circumstances
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Dehydration
Why? Samples are incompatible with the vacuum
Surfaces will be disrupted while forced-drying
How?Air dry
Critical Point DryHMDS Dry
What sample types?Biologicals
Hydrated geologicalsSynthetics like polymers or solgels/aerogels
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Air Drying
Can only be used on “rugged” samples
Biologicals like tough exoskeletons
Materials that won't change size/shape
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Air Dried Sample
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Critical Point Drying
Water is replaced with miscible 2nd fluid
Transitional fluid replaces 2nd fluid
Transitional fluid is driven past the “critical point”by increasing pressure and temperature
Pressure is relieved as gas escapes
Samples are left water, 2nd fluid, and transitional fluid dry
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CPD Sample
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Critical Point Dryer
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More CPD Dried
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HMDS drying
Water is replaced with a 2nd fluid
2nd fluid is replaced with HMDS
HMDS is allowed to dry leaving surfaces intact
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HMDS Dried
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Processes Common to Many Samples
Dehydration
Coating
Methods to reveal interior details
Stabilization of loose parts
Sample resizing
Methods to make similar measurements with other techniques
Special imaging circumstances
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Sample Coating
Why coat samples?Electrical insulators need to be made conductive
Increase rigidityIncrease SE emission
Usual coatingsMetals like Au, Ag, Pt, Pd, Cr, Os or alloys
Carbon
Typical coating methodsSputtering
Evaporation
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Sample Coating
Things to watch out for:
Decoration artifactsX-ray emission lines
Sample deformation during deposition
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Sputter Coating Samples
Usually a simple DC sputtering system
Low vacuum
Argon backfillinert and ionizable
relatively high massgood pumping character
Relatively simple time vs current rate of deposition
Slower coating--->smaller islands--->smoother film
Usually +-5nm is sufficient for conductivity
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Typical EM Lab Sputtering System
Cathode
Vacuum chamber
Samples
Vacuum gauge
HV control
Current monitor
Timer
Argon bleed
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Sample Coating: Evaporation
Used when sputtering won't work wellCarbon
Making shadows
Line of sight deposition
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Processes Common to Many Samples
Dehydration
Coating
Methods to reveal interior details
Stabilization of loose parts
Sample resizing
Methods to make similar measurements with other techniques
Special imaging circumstances
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Revealing Interior Portions of Samples
Why?Outside may be “weathered”
Inside may have different chemistry or morphology
Inside may have smaller pieces or details
Inside may be immature or undifferentiated
Inside may be source of problems or defects
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Revealing Interior Portions of Samples
How?Smash it! (don't make it any harder than necessary)
Cut it
Saw it
Grind it
Fracture it
Polish it (mechanical, electrochemical)
Etch it
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Revealing Interior Portions of Samples
Toolsvarious types of knives and blades
Microtome
Polishing bench and wheels
Wet processing
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Inside Structure
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Microtomes and Microtomy
Tool with very sharp blade and a sample translation stage
Ultramicrotome for EM
Usually a glass or diamond knifestationary cutting edge
moving samplecut pieces float off on water surface held adjacent
to the blade edge
Can use thin sections in TEM or cleaned bulk surfacein the SEM
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Processes Common to Many Samples
Dehydration
Coating
Methods to reveal interior details
Stabilization of loose parts
Sample resizing
Methods to make similar measurements with other techniques
Special imaging circumstances
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Stabilization of loose parts
Why?Loose stuff falls offLoose stuff changes other surface details
How?Use glues or tapesUse clipsMake sandwichesEmbed in other materialsSometimes a coating will do
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Processes Common to Many Samples
Dehydration
Coating
Methods to reveal interior details
Stabilization of loose parts
Sample resizing
Methods to make similar measurements with other techniques
Special imaging circumstances
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Sample Resizing
Why?Too darned big for the system
How?Similar to revealing interiors of samples
-smash, saw, cut, grind, polish, etc.
Concerns:Part left over is representative of the whole
You don't lose the interesting part
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Processes Common to Many Samples
Dehydration
Coating
Methods to reveal interior details
Stabilization of loose parts
Sample resizing
Methods to make similar measurements with other techniques
Special imaging circumstances
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Methods to make similar measurements with other techniques
Why?Complementary data
Comparisons
How?Use fiducial markings
Use sample holders with a grid of numbers/lettersFind a landmark
Use absolute or relative stage coordinatesCircle the area of interest
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Processes Common to Many Samples
Dehydration
Coating
Methods to reveal interior details
Stabilization of loose parts
Sample resizing
Methods to make similar measurements with other techniques
Special imaging circumstances
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Special imaging circumstances
Why?Want sample in particular positionNeed to see a certain area or side
Want proximity data to/from reference material
How?Be creative
Mount samples so they protrude from stageMake a multi-holder
Include a standard material on the stageSpring clips/tape/wire
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Sample Preparation Flowchart
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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How to Prepare Small Particles
Dispersion of single particles or groupings?
Mixture of sizes or monodisperse?
Potential to move around on stage?
Want compositional information? What about the substrate?
From a solid mass, dry powder, airborne, or liquidborne?
Reactive outside of their usual environment?
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Small Particle Dispersion
Agglomeration is a problem-camphor/napthalene method-sticky dot method-dust and remove method-filter onto membranes (Nuclepore filters)
Drying ring dispersions
Mortar and pestle size modification
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Small Particles
Most will stick electrostatically
Large ones may need some help to stay in place-carbon coating-metal coating
-sticky dots
Coatings often are not continuous-special stages for evaporators and sputter coaters
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Cross Sections
Why?-to see interior or sub-surface details
How?-fracture-cleaving
-microtome-polishing
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Electrically Insulating Materials
Four Choices:Try to view as-is w/low energy beam
-small aperture-vary accelerating voltage
Try a faster scan rate to limit electron dose
Make it conductive w/o destroying thesurface topography
Use a variable pressure instrument (we don't have one)
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Insulators
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Electrically Conductive Samples
The best sample
The most unusual sample
Simply attach to sample stub and “go”
Beware of contaminated surfaces
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Biological Materials
Generally require extensive preparation
Most important to remove water w/odestroying the surfaces
May need to ruggedize (fix) tissues
May be possible to freeze and view directly
Given rise to “environmental” or “low vacuum”systems to obviate need to dry samples
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Untouchable Samples
Historically significant samples
Forensic samples
Samples from litigations
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Preparing Samples for Automated SEM Scans
Usually a size/shape/compositional analysis
Usually requires a grey-level segmentation of the image
Usually needs some parameters to keep or discard data-edges
-too small-too big
Samples must be flat and relatively featureless except for your target
Examples:gunshot residue analysis
asbestos analysisbone implant analysis
small particle analysis (IPA, SPOT sampler)
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Gunshot Residue Analysis
When a gun is fired, small particles are generated during the explosion of the primer,
and leave the gun via the smoke.
The particles are deposited on parts of the body.
These small particles are called gunshot residue (GSR).
Particles are very characteristic, therefore presence of these particles forms evidence of firing a gun.
Particles normally consist of Pb (lead), Sb (antimony) and Ba (barium).
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Gunshot Residue
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Sample Preparation of Semiconductors
Usually Silicon
Increasingly III-V or II-VI compounds
Do not need conductive coatings unless a thick oxide,nitride or resist is present
p-type and n-type seem to image differently due tovariation in conductivity and dopant concentration
Some areas may be “floating” electrically and needseparate grounding
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Manipulated Samples
Stressed in tension or compression
Samples irradiated to simulate high dose -exposure
Electron beam induced current (EBIC)
Voltage contrast
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Temperature Controlled Viewing in the SEM
Some glasses have mobile components-Na+-Ag+
Cooling to <-140C seems to stabilize the electromigration
Some high VP or liquid samples can be frozen and viewedw/o a coating
Watch the crystallization of materials from solution
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Low Vacuum SEM
ESEM (environmental SEM)
Differentially pumped gun/column and chamber
High vacuum in former; adjustable vacuum in latter
Many types of backfill gasses and vapors
Up to about 1 Torr in chamber
Dissipates surface charging
Eliminates the need to fully dry samples
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Hazardous Samples
Biohazards (DNA, Viruses, Bacteria, etc.)
Radioisotopes
Fine dust
Toxic materials (Be metal)
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analyses
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Quick and Dirty Analyses
80% of what you'll ever know about something you learnin the first dirty experiment
Stabilize sample
Make it fit mechanically
Protect the instrument
Try it!
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Individual Processing of Samples for EM Observation
Small ParticlesCross-sectionsInsulatorsConductorsBiologicals“Untouchables”For automated analysesSemiconductor devicesManipulated samplesHigh or Low temperature processingLow vacuum observationHazardous materials“Quick-and-dirty” analysesMagnetic samples
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Magnetic Sample Materials
Deflect the electron beam
High mag work very difficult
Low mag work approachable
X-ray analysis OK
Make sure pieces are stable on stage
Small particles need to be FIRMLY adhered
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TEM Sample Prep for Materials
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Thin Sample Prep for TEM or SEM
Dispersion of small particlesSEM: sticky dots, conductive tabs or glueTEM: alcohol dispersion on thin film
Ultramicrotomy
Mechanical thinning
Chemical thinning
Ion thinning
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Image Collection, Recording and Presentation
Rule-of-thumb microscope conditions-best resolution-best depth of field-best sample preservation
Conventional Photographic Methods
Digital Methods
Presentation for:DisplayPublication
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Image Collection
Proper subject identification
Proper subject orientation
Best selection of imaging conditions-HV-WD
-Spot size (aperture)-Scan rate
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Subject Identification/Orientation
Representative of the whole
Image background
Not too busy
Important image information is centered and prominent
Many times a slight tilt conveys more information
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“Best” Imaging Conditions
High resolution-short working distance
-small spot size-high accel. Voltage-high magnifications
Depth of field-long working distance
-low magnifications-larger spot size
Low magnification-large spot
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Selection of Scan Rate for Imaging
Sensitive samples-may need to be fast
-low S/N-maybe TV integration mode
Insulating (charging) samples-decrease charging with small spot and
fast frame rate, maybe TV again-focus/stigmate in an area adjacent to the area recorded
-use image shift function to quickly move small amounts
Normally conductive samples-use slowest rate practical w/o degrading surface
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Old Technology
Analog scan SEMs
2nd CRT for viewing the image as it scans
Film based camera focused on this CRT (low persistence)
Almost always a 4x5 inch Polaroid sheet film camera
Very slow scan for about a 2000 line image (~3 minutes)
P/N film or just an instant positive image
About $3/shot now
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Generalized Photographic Processing
Needed for TEM image plates(Can be used for SEM film images too)
Exposure of silver halide grains (latent image)Development (reducing basic solution---> Ag0)Rinse (water) or Stop (acid)Fix (thiosulfates)Rinse (water)Dry
Scan or Print photographically
Good photographic processing results in the best imagesand are still the images that are used to compareother (newer) techniques
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Newer Technology
Digital raster SEMs
Frame buffer storage of image info
Image processing
Digital image storage-usually TIFF files so that header can contain
image and microscope specific data
Fully transportable formats
Easy incorporation of images into documents
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LEO 982 Specific Digital Imaging
Detectors-SEI (chamber)-SEI (column)-BSE
Signal mixer-brightness-ratio
Gamma correction-corrects for desired brightness and contrast I
out~=I
in
-power function deviation from 1:11.0 darkens and enhances lower greys1.0 lightens and enhances higher greys
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<--- switch position 0
<--- switch position 1
Gamma Corrections
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<----- switch position 3
<----- switch position 4
<----- switch position 5
<----- switch position 6
Gamma corrections
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LEO 982 Specific Digital Imaging
Slow scan rates 1-3 continuous scan
Slow scan rates 4-8 store one frame of data-dump to disk as image file (TIFF)
Choose image pixel matrix density from 512x512 to2048x2048 (lowest is usually OK)
Right mouse button will interrupt any scan and storeresults in the buffer (incl. TV)
TV rate integration of frames can reduce random noisein the final image at a fast scan rate
File path and naming convention
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LEO 982 Specific Digital Imaging
Variable small raster-used to increase scan rate for image adjustment
Can store multiple images in the same frame-variable frame
-split screen-kind of gimmicky.....don't use for important images
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Stereo Pair Images (Anaglyphs)
By collecting two images offset by about 4-100 in tilt
Display them side by side and cross eyes to converge
Build a blue-red image composite and use stereo glasses-In Photoimpact program:
convert images to RGBadjust color balance (red-right, blue-left)
perform image calculation (difference operator and merge)
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Special Scan Modes in the LEO 982
Line scan-disable Y-axis scan to see grey-level variations
on a line
Y-modulation-if very little Z-axis information this converts it
to Y-axis deflection (not very useful)
Spot scan-mostly for x-ray data acquisition
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Additional Scanning Features of the LEO 982
Dual magnification-useful for “looking around”-don't use for important images
Scan rotation-electronically rotates the raster on the sample-very useful for getting a good “presentation”
Dynamic focus-use to compensate for the portions of the sample that
fall outside the depth of field distance. Sets up aramp on the focus current +- the center of the field
Tilt correction-compensates for trapezoidal scan on highly tilted samples
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Image Processing
Generally use “kernels” which are arrays of arithmeticoperators on a pixel
Standard kernels are used to blur, average, and sharpenimages. 3X3, 5x5, array of operators.
Photoshop and PhotoImpact have custom and standardkernels
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Kernel Operations for Sharpening an Image
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Different Kernels
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Effect of Kernel Size on Operations
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Contrast Enhancement
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Original Average kernel
Sharpen kernel Blur kernel
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Pitfalls of Image Processing
Images can be distorted and data lost
Pixelation of images
Ethical behavior dictates a minimum of processing
Always better off collecting the best image and eithernot processing or doing it only lightly
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Image Manipulation
Erosion of edge pixels-kernel operator to find edges
-erode or erase edge pixels one layer at a time-break apart and separate touching features
Dilation of edge pixels-kernel operator to find edges
-dilate or add edge pixels one layer at a time-fuse separate features
Most useful in particle and other small repeating features
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Presentation of Micrographs
Reports-probably least critical
-must convey information concisely
Journal-probably most critical
-size, grey-levels, resolution-must be specific and representative of the narrative
Posters-most variable in format-otherwise like journal
-conducive to point and discuss
Web-like journal
-can be interactive
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Presentation Media
Photographic paper
Photo quality printer output-dye sublimation
-ink jet....getting there!-laser...maybe...
-consider viewing distance in choice
Include TIFF or JPEG files in reports using word processor
Powerpoint for talks
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Micrographs as Art
Wonders of things small
Intricacies of natural samples
Subtle grey tones, like fine b/w photos
Can be psuedocolored to add interest
Comparisons to more familiar things
Explain phenomena in a “gee-whiz” way
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Introduction to X-ray Microanalysis
Review of Physics of X-ray Generation
Hardware-EDS-WDS
-electron microprobe vs. SEM/EDS
Software-Spectral acquisition
-Spectral match-Qualitative analysis
-Quantitative analysis-X-ray images (maps)
-Spectral mapping-simulation of electron scattering/x-ray emission
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X-ray Generation
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Hardware for X-ray Microanalysis
WDS-Roland circle based Bragg-diffracting crystals and
detector arrangement-either horizontal or vertical design
EDS-cooled solid state detector-integrated FET and preamplifier
Computer accumulator/conditioner of signals
MCA output for energy vs intensity
Some hardware facility for control of the electron beamposition for mapping and DBC
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WDS System
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Rowland Circle in WDS Spectrometer
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Typical EMPA
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EDS Topics (from Notes)
Spatial Resolution
Directionality of Signals
Rough Surfaces
Hardware/Signal Processing-dead time and time constants
Microscope Parameters-overvoltage-TOA-WD (EA)
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EDS Spectral Interpretation
Background Continuum
Characteristic x-rays
Excitation and absorption
Detector efficiency
Artifacts
Peak ID function (qualitative analysis)
Spectral matching
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Structure of a Si(Li) Detector for X-rays
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Nomogram ofE-beam Penetration
Beam Diameter vsBeam Current
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Quantitative EDS Analysis
Clean spectrum
Standards vs. no-standards
K-ratio
Corrections-atomic # (Z)
-absorption (A)-fluorescence (F)
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Advanced X-ray Techniques
X-ray image maps
Spectral Mapping
Particle and Phase Analysis
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X-ray Image Maps
Edax Imaging and Mapping program
Process-take a look at your sample with eds
-look for elements of interest-setup ROI (region of interest) on the peaks
-start mapping function-DBC on
-dwell time-pixel density for map
-maps show up line by line in different colorsfor each ROI (element)
-color intensity is related to # of x-rays detected-can collect SE image simultaneously
Qualitative x-y spatial distribution of elements
Not very high resolution
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Spectral Mapping
Sort of like previous x-ray maps
Collect full spectra at each pixel
Store data in a raw form so that it can be massaged later
Take “phases” and additively process the spectra of all thepixels that determine that phase
-leads to pretty good quantitative analysis-averages small inhomogeneities in the phase
Huge file sizes (stores greylevel and data for each pixel)->30Mbytes
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Particle and Phase Analysis
Similar to mapping
Additional sizing information (area, feret diameters, calc. Volume...)
Mixes qualitative spectral matching info and morphological infoto come up with a particle or phase ID
Steers the beam on the sample to collect the data for binarized“white” areas (as determined by threshold setup)
Good for collecting statistically significant amount of data onfeature groups
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Imaging Artifacts
What is an “artifact”
Sources of Artifactssample preparation
vacuum compatibilityelectron beam “issues”
too low/too high KV (not really an artifact)vibrations
stray magnetic fieldsacoustic noise
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Micrograph Critique Session
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