scanning electron microscopy · and system, and secondary electron detection (everhart - thornley...
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www.reading.ac.uk/EMLab
Scanning Electron Microscopy
Peter Harris
EM Workshops 2013
Scanning Electron Microscopy
What is scanning electron microscopy?
Looking at “wet” samples in the high vacuum SEM
Basic features of conventional SEM
Limitations of conventional SEM
Cryo-SEM and Environmental SEM
EM Workshops 2013
SEM & TEM
Scanning electron microscope
Electron gun
Specimen
Electron gun
Condenser lens
Objective lens
Specimen
Projector lens
Fluorescent screen
Electron gun
Digital camera
Condenser lens
Objective lens
Specimen
Projector lens
Fluorescent screen
Electron gun
Digital camera
Transmission electron microscope
Workshop: November 13
EM Workshops 2013
Electron sources for SEM
Tungsten thermionic source
Schottky Field Emission Source
Low brightness
Energy spread ~ 1-2eV
High brightness
Energy spread < 0.5 eV
EM Workshops 2013
Interaction of high-energy electrons with specimen
EM Workshops 2013
Interaction of electrons with specimen
• S = secondaries • B = backscattered • (A are auger)
EM Workshops 2013
Secondary and backscattered electrons
• What are backscattered electrons? • They are primary electrons that have entered the sample and then escaped back out:
• What are secondary electrons? • They are electrons dislodged from the specimen itself.
EM Workshops 2013
Detectors for SEM
Everhart Thornley detector: Scintillator – photomultiplier system.
Secondary electrons
Everhart Thornley or solid state detector (silicon diode).
Usually positioned around the final lens or inside.
Backscattered electrons
EM Workshops 2013
◊ Secondary electrons Topographic information
◊ Backscattered electrons
◊ X-rays
Compositional information
Spectra; Maps
Session on X-ray analysis: November 27
Information given by different signals
EM Workshops 2013
Origin of topographic contrast
Everhart Thornley detector
EM Workshops 2013
Early history of SEM
1942, Zworykin et al., RCA Laboratories, USA - first working SEM.
1950s, C.W. Oatley et al., Engineering Department, Cambridge - major improvements in electron optics and system, and secondary electron detection (Everhart - Thornley detector). First backscattered detector.
1965: first commercial SEM, Cambridge Instrument Company, Stereoscan Mark 1.
1935, M. Knoll, Germany – proposed concept of a scanning electron microscope.
EM Workshops 2013
EM Lab’s high vacuum SEM
EM Workshops 2013
Conventional SEM images
Compact disc
Polypropylene spherulites
Integrated circuit
EM Workshops 2013
Problems with conventional SEM
No wet samples
Surface must be electrically conducting
Non-conducting samples have to be coated
Can only operate at low pressure (10-5 Torr)
EM Workshops 2013
Looking at “wet” samples in the high vacuum SEM
Animal and plant tissues up to 98% water.
Options for examining these materials in the high vacuum SEM:
Chemical fixation, drying
Freeze drying
Critical point drying
EM Workshops 2013
Critical point drying
Specimen prepared by CPD: Hyphae and spores in Stilton cheese
• Evaporative drying of specimens can cause collapse of structures, mainly due to effects of surface tension.
• Effect can be reduced by substitution of water with a liquid with a lower surface tension
EM Workshops 2013
Cryo - SEM
Workshop: November 20
EM Workshops 2013
Cryo – SEM of food
Probiotic yoghurt with bacteria. Scale bar 5μm
Raw potato
EM Workshops 2013
Environmental SEM
Workshop: November 20
Cooled specimen
• Special pumping system allows water vapour to be introduced into chamber.
• Cooled specimen to retain moisture.
• New (backscattered) electron detector.
EM Workshops 2013
Introducing water vapour into the chamber enables uncoated samples to be examined
Positively charged water molecules are attracted to negatively charged sample.
Negative charge at sample surface is neutralized..
Secondary electrons from sample strike water molecules.
EM Workshops 2013
EM Lab’s FEI Quanta 600 ESEM
Pressures up to 20 Torr - samples typically cooled to 5oC.
Gaseous secondary electron detector – fits on objective lens.
EM Workshops 2013
Environmental SEM
• Raspberry buds : Hiroyuki Imanishi, Plant Sciences
EM Workshops 2013
Becoming an EMLab user
Sherrie Foo
EM Workshops 2013
Booking system
EM Workshops 2013
Remaining workshops
Date
Topic
13 November 2013 Transmission Electron Microscopy
20 November 2013 Environmental & Cryo SEM
27 November 2013 X-ray Analysis and Elemental Mapping
EM Workshops 2013
► Questions?
Electron Microscopy Laboratory
► Demo