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