Post on 21-Mar-2020
Electron microscopes
MEF3100 Spring 2007
Bombard and analyze
MEF3100 Spring 2007
Sample
Electron gunDetector
SEM – Scanning electron microscope
MEF3100 Spring 2007
Quanta 200F from FEIField emissionESEMEDS
General outline for next 2 weeks - SEM
MEF3100 Spring 2007
Warming Up:• SEM History• Why use electron microscopes?• Some SEM examples
Beam specimen interaction
Images•Detectors •Contrast - Brightness•Image disturbances and their causes
Introduction:•How does the SEM work?•Resolution•Signal to noise• Wavelength, energy, speed ANALYTICAL SEM/Micro probe:
• Spectroscopy• EDS• WDSInstrument:
• Electron guns• Electron lenses• Probe size versus current
History
MEF3100 Spring 2007
1953-McMullan's Ph.D. Thesis 1962 -Cambridge InstrumentsThe first modern scanning electron microscope, constructed by D. McMullan in the Cambridge University Engineering Laboratory in1951.
Source: Electron Optics and Electron Microscopy, P.W. Hawkes.
Why use SEM and TEM?
MEF3100 Spring 2007
We will concentrate on:
MEF3100 Spring 2007
Secondary electrons =SE= picture, topographyBack scattered electrons =BSE= picture, chemistryCharacteristic X-rays =EDS= chemistry
Limits to Resolution
MEF3100 Spring 2007
• Unaided eye ~ 0.1 mm• Light microscope ~ 0.2 µm• Scanning EM ~ 1.0 nm• Transmission EM ~ 0.1 nm
Why use SEM?
MEF3100 Spring 2007
Optical microscope SEM
Example 1
MEF3100 Spring 2007
Secondary electron image of PbZr-oxide
Example 2 – BSE – Chemical contrast
MEF3100 Spring 2007
Back scatter image of gold on a polymer
Example 3 – EDS – chemistry:
MEF3100 Spring 2007
SEM is a strong tool for material characterization
MEF3100 Spring 2007
How does the SEM work?
MEF3100 Spring 2007
Probe gun
Secondary electron detector
Raster system
SEM cross section
MEF3100 Spring 2007
Magnification in SEM:
MEF3100 Spring 2007
MEF3100 Spring 2007
Increasing the magnification by reducing the size of the area being scanned
Abbe and resolution
MEF3100 Spring 2007
Signal to noise
MEF3100 Spring 2007
Signal to noise – scan time
MEF3100 Spring 2007
Wave length for electrons
MEF3100 Spring 2007
Wavelength of e-beam
MEF3100 Spring 2007
ρλ
emh
2= Substituting actual values:Accelerating
Voltage (kV)Nonrelativistic
wavelength (nm)
5 0.01830 0.007
Increasing the accelerating voltage decreases the wavelength of the electrons.
)2(mev ρ
= For 30 kV v~ 0.3 c
Instrument
MEF3100 Spring 2007
• Vacuum• Electron gun• Optics • Detectors
• Sample• Operator
Elektronkanon og Brightness
MEF3100 Spring 2007
For å kunne detektere maksimum signal velgervi materialer som gir så kraftig elektronstrålesom mulig.
brightness= maksimum signaler avhengig av flere parametere:ie - emisjonsstrømmenDo - elektronstrålens diameter – første cross-overαo - spredningen av elektronene
Vi vil ha høy emisjonsstrøm og liten diameter på kilden.
Lens system
MEF3100 Spring 2007
Condensor lens -determines beam current and "possible" spot sizeObjective lens, or final or probe forming lens,
determines the final spot size.Fixed Condensor aperture(s)Selectable final (objective) apertureElectrostatic lens in gun forms the first
crossover.Electromagnetic lens as condensor and
objective.EM are converging lenses –(i.e. parallel electron beam will converge to a
focal point at focal length f).–Diverging beam will be made to converge.
Condensor lens – spot size
MEF3100 Spring 2007
What happens if we make C1 weaker?
Condensor lens – spot size
MEF3100 Spring 2007
What happens if we make C2 stronger?
Condenser lens aperture
MEF3100 Spring 2007
Aperturen bestemmer hvor stor andel av strålen som treffer prøven. Den kontrollerer intensiteten. I SEM kontrollerer den også dybde i fokus.
Electron optics
MEF3100 Spring 2007
Summing up general concepts
MEF3100 Spring 2007
Interaction
MEF3100 Spring 2007
Sample – beam – Signals
MEF3100 Spring 2007
SEM – electron detection – charging
MEF3100 Spring 2007
Back scatter I
MEF3100 Spring 2007
Back scatter II
MEF3100 Spring 2007
Secondary electron
MEF3100 Spring 2007
SE II
MEF3100 Spring 2007
SE III
MEF3100 Spring 2007
SE I, II; III og BSE I ,II– influence on resolution
MEF3100 Spring 2007
Excitation of SE and BSE from within a specimen by the primary beam. SE and BSE trajectories are shown.(a) High accelerating voltage applied to the primary beam. BSEI emerge from close proximity to the beam impact area.BSEII come from a greater depth from within the specimen after undergoing multiple accumulative elastic interactions, ultimately emerging spatially disconnected from the point of impact of the beam. (b) Low accelerating voltage appliedto the primary beam. The primary electrons penetrate less into the specimen and therefore the BSEII emerge closer to the beam impact area and are more sensitive to the surface topology than at higher accelerating voltages.
Beam scattering – broadening
MEF3100 Spring 2007
Detektorer
MEF3100 Spring 2007
Everhart-Thornley Detector
MEF3100 Spring 2007
Everhart-Thornley Detector
MEF3100 Spring 2007
Everhart-Thornley Detector
MEF3100 Spring 2007
Back scatter electron detector
MEF3100 Spring 2007
A solid-state (semi-conductor) backscattered electron detector is energized by incident high energy electrons (~90% E0), wherein electron-hole pairs are generated and swept to opposite poles by an applied bias voltage. This charge is collected and input into an amplifier (gain of ~1000). The detector is positioned directly above the specimen, surrounding the opening through the polepiece