Opt 307/407 Practical
Scanning Electron Microscopy
Considerations in any microscopy:Resolution
MagnificationDepth of field
Secondary information
Limits of Resolution (resolving power)
Unaided eye: 0.1mmLight microscope:0.2um
SEM: 1nmTEM: 0.2nm
Evolution of Resolution
Depth of Field
Light Microscope vs Electron Microscope
General Diagram of the SEM System
Light Microscopy vs Electron Microscopy
Advantages of EM:ResolutionMagnificationDepth of field
Disadvantages of EM:PriceyBetter if conductive (SEM)MaintenanceVacuum
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!
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
Opt 307/407Vacuum Systems
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
Opt 307/407Vacuum Systems
Measuring Vacuum in EM Systems
Thermocouple GaugePirani Gauge
Cold cathode GaugePenning Gauge
Ion pump current
Very Broad Range of Vacuum to Measure
Grouped Ranges for Vacuum Gauges
Vacuum Gauge Choices and Working Ranges
Thermocouple/Pirani Gauges
Ionization Gauges
Ion Gauge Collection
Hot Cathode Ion Gauge
Penning gauge
Penning gauge
Opt 307/407Vacuum Systems
Opt 307/407Vacuum Systems
Types of Vacuum Pumps
1- Rotary (Fore, Rough, Aux, Mechanical)
2- Turbomolecular (Turbo)
3- Diffusion (Diff)
4- Ion (Sputter-ion)
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
Opt 307/407Vacuum Systems
Opt 307/407Vacuum Systems
Opt 307/407Vacuum Systems
Rotary Pump Problems
Cannot pump <10-2 TorrNoisy
BackstreamsVibration
Maintenance
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
Opt 307/407Vacuum SystemsTurbo Pump Problems
Needs a Foreline pump
Costly
Can fail abruptly
Whine
Needs to be protected from solid material
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
Opt 307/407Vacuum Systems
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)
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)
Opt 307/407Vacuum Systems
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)
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
Opt 307/407Vacuum Systems
Opt 307/407Vacuum Systems
Typical TEMVacuum System
Opt307/407
Electron Sources and LensesElectron Sources and Lenses
Types of Electron SourcesTypes of Electron Sources
Thermionic SourcesTungsten filamentLanthanum Hexaboride (LaB
6) filament
CeB6
Field Emission sourcesColdSchottky
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
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)
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
Tungsten Hairpin Filament SaturationTungsten Hairpin Filament Saturation
Tungsten Hairpin FilamentTungsten Hairpin Filament
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
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
Thermionic Gun LayoutThermionic Gun Layout
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
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
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
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)
Typical Schottky Field Emission SourcesTypical Schottky Field Emission Sources
Schottky Field Emitter DiagramSchottky Field Emitter Diagram
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
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
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
Electron LensesElectron Lenses
ElectrostaticGun cap (Wehnelt cylinder)Totally inside vacuum
ElectromagneticAll other lenses and stigmatorsPartially outside of vacuum
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.
Electron OpticsElectron Optics
Refraction, or bending, of a beam of illumination is caused when the ray enters a medium of a different optical density.
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
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.
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.
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.
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
Electron OpticsElectrostatic lens
Must have very clean and high vacuum environment to avoid arcing across plates
Electromagnetic Lens
Passing a current through a single coil of wire will produce a strong magnetic field in the center of the coil
Three Electromagnetic LensesThree Electromagnetic Lenses
Electromagnetic Lens
Pole Pieces of ironconcentrate lines ofmagnetic force
Electromagnetic Lens
Electromagnetic Lens
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
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.
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.
Condenser Lens: Weak and Strong ConditionsCondenser Lens: Weak and Strong Conditions
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.
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
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
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.
Lens Defects
A few manufacturers have combined an electromagnetic (converging) lens with an electrostatic (diverging) lens to create an achromaticachromatic lens
LEO Gemini Lens
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
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
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.
Diffraction
Diffraction occurs when a wavefront encounters an edge of an object. This results in the establishment of new wavefronts
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.
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
If a lens is not completely symmetrical objects will be focussed to different focal planes resulting in an astigmaticastigmatic image
Astigmatism
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
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
Opt 307/407
The SEM Systemand
Electron Beam-Sample Interactions
The TEM system and components:
Vacuum Subsystem
Electron Gun Subsystem
Electron Lens Subsystem
Sample Stage
More Electron Lenses
Viewing Screen w/scintillator
Camera Chamber
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
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
Scan Generator Interface
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
Magnification Control in the SEM
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
Depth of Field and Defocus
DOF in the SEM
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
Note the large depth of field which is possible with small probe semi-angle ( .
DOF and Sample Tilt
DOF and Working Distance
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)
Spot Size Control in the SEM
Signal Detectors for the SEM
Electron Beam-Specimen Interactions
First thing: electrons are scattered in a near-forward direction
Electron Beam-Sample Interaction
Electron Flight Simulator Demo
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)
Electron Beam-Sample Interactions
Backscattered (Primary) Electrons
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
Backscattered Electron Detectors
Backscattered Electron Image
Backscattered Electron Detector Placement
For either solid-state Si detectors or Robinson type
Secondary Electrons and Detectors
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
Some Secondary Electron Characteristics
Types of Secondary Electrons/Origins
Secondary Electrons: Edge Effects
Everhart-Thornley (ET) Secondary Electron Detector
Photomultiplier Tube Electronics
Whole E-T Detector w/PMT Amplification
Secondary Electron Images
Auger Electron Generation
Auger Analytical Volume
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
X-ray Photon Production
Bremsstrahlung (Braking) radiation
Characteristic X-rays
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
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.
Characteristic X-ray Production
Energy Dispersive X-ray Spectrometer
X-ray Spectrum from EDS Spectrometer
Wavelength Dispersive (crystal) Spectrometer
X-ray Spectra Comparison EDS vs WDS
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
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
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
Relative Sizes of the Emission Zones (looking from above)
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
Image Collection
Proper subject identification
Proper subject orientation
Best selection of imaging conditions-HV-WD
-Spot size (aperture)-Scan rate
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
“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
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
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
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
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
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
<--- switch position 0
<--- switch position 1
Gamma Corrections
<----- switch position 3
<----- switch position 4
<----- switch position 5
<----- switch position 6
Gamma corrections
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
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
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)
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
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
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
Kernel Operations for Sharpening an Image
Different Kernels
Effect of Kernel Size on Operations
Contrast Enhancement
Original kernel Average kernel
Sharpen kernel Blur kernel
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
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
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
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
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
Sample Preparation for Electron Microscopy
Electrically Conductive SamplesElectrically Insulating SamplesBiological Samples“Odd” Samples
Why do samples need to be prepared???
Vacuum environment
Charged particle environment
Too big
Components migrate in response to the beam
Two General Samples Types
Bulk SamplesSEM only
Thin SamplesSEM and TEM
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
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
Air Drying
Can only be used on “rugged” samples
Biologicals like tough exoskeletons
Materials that won't change size/shape
Air Dried Sample
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
CPD Sample
Critical Point Dryer
More CPD Dried
HMDS drying
Water is replaced with a 2nd fluid
2nd fluid is replaced with HMDS
HMDS is allowed to dry leaving surfaces intact
HMDS Dried
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
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
Sample Coating
Things to watch out for:
Decoration artifactsX-ray emission lines
Sample deformation during deposition
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
Typical EM Lab Sputtering System
Cathode
Vacuum chamber
Samples
Vacuum gauge
HV control
Current monitor
Timer
Argon bleed
Sample Coating: Evaporation
Used when sputtering won't work wellCarbon
Making shadows
Line of sight deposition
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
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
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
Revealing Interior Portions of Samples
Toolsvarious types of knives and blades
Microtome
Polishing bench and wheels
Wet processing
Inside Structure
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
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
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
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
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
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
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
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
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
Sample Preparation Flowchart
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
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?
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
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
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
Cross Sections
Why?-to see interior or sub-surface details
How?-fracture-cleaving
-microtome-polishing
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
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)
Insulators
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
Electrically Conductive Samples
The best sample
The most unusual sample
Simply attach to sample stub and “go”
Beware of contaminated surfaces
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
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
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
Untouchable Samples
Historically significant samples
Forensic samples
Samples from litigations
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
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)
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).
Gunshot Residue
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
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
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
Manipulated Samples
Stressed in tension or compression
Samples irradiated to simulate high dose -exposure
Electron beam induced current (EBIC)
Voltage contrast
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
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
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
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
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
Hazardous Samples
Biohazards (DNA, Viruses, Bacteria, etc.)
Radioisotopes
Fine dust
Toxic materials (Be metal)
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
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!
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
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
TEM Sample Prep for Materials
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
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
Image Collection
Proper subject identification
Proper subject orientation
Best selection of imaging conditions-HV-WD
-Spot size (aperture)-Scan rate
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
“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
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
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
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
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
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
<--- switch position 0
<--- switch position 1
Gamma Corrections
<----- switch position 3
<----- switch position 4
<----- switch position 5
<----- switch position 6
Gamma corrections
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
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
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)
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
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
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
Kernel Operations for Sharpening an Image
Different Kernels
Effect of Kernel Size on Operations
Contrast Enhancement
Original Average kernel
Sharpen kernel Blur kernel
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
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
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
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
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
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
X-ray Generation
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
WDS System
Rowland Circle in WDS Spectrometer
Typical EMPA
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)
EDS Spectral Interpretation
Background Continuum
Characteristic x-rays
Excitation and absorption
Detector efficiency
Artifacts
Peak ID function (qualitative analysis)
Spectral matching
Structure of a Si(Li) Detector for X-rays
Nomogram ofE-beam Penetration
Beam Diameter vsBeam Current
Quantitative EDS Analysis
Clean spectrum
Standards vs. no-standards
K-ratio
Corrections-atomic # (Z)
-absorption (A)-fluorescence (F)
Advanced X-ray Techniques
X-ray image maps
Spectral Mapping
Particle and Phase Analysis
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
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
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
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
Micrograph Critique Session