1. (Chapter 7) CHEM–E5225 Electron microscopy
How to see electrons
(Chapter 7)
CHEM–E5225 Electron microscopy

2. ELECTRON DETECTION AND DISPLAY
Different types of electrons
Types back-scattered
SE/BSE
Detector in chamber
Types forward-scattered
Detector in viewing chamber
Electron signal is digitized
Digital scanning image is presented on a fluorescent screen as an analog image.

3. VIEWING SCREENS Typically coated with ZnS
Emits light with ~450nm wavelength but doped to ~550nm for comfort
Main requirement for viewing screens is the ZnS particle grainsize be small enough so that the users eye cannot distinguish individual grains
Typical ZnS grain size is ~50µm
Other signals are also given off by the viewing screen, such as X-rays
Lead glass used for protection
NOTES
Wavelength is doped to 550nm due to it being the middle of the visible light spectrum
Minimum grain size used is 10µm for higher resolutions
In HVEMs this can amount to several tens of millimeters of glass

4. Semiconductor Detectors
Doped flat silicon wafer
Impacting electrons excite the Si electrons causing current to flow
Requires 3.6eV to form a electron-hole pair
Easy and cheap to manufacture
Sensitive to electron beam damage
Insensitivity to low energy electrons (SE)
Notes
With 100keV ~28000 electrons produced results in actual electron-hole pairs to form.
Electron beam damage can be minimized by minimizing exposure

5. B Scintillator-Photomultiplier Detectors/TV Cameras
Ascintillator emits visible light when struck by electrons due to cathodeluminesence
Scintillator detectors materials are Ce-doped yttrium-aluminum garnet (YAG) and various doped plastics and glasses.
Once the incoming electron signal has been converted to visible light, the light from the scintillator is amplified by a photomultiplier
High gain resulting in low DQE
More susceptible to radiation damage in comparison with semiconductor detectors
Notes
These materials have decay times on the order of nanoseconds rather than the microseconds needed for ZnS.
The scintillators that we use in STEMs or SEMs are often coated with a 100-nm-thick layer of Al to reflect any light generated in the microscope and stop it from entering the PM tube

6. Charge-Coupled Device (CCD) Detectors
CCD cameras are becoming the norm for real-time TV recording of images and DPs
CCDs are metal-insulator-silicon devices that store charge generated by light or electron beams.
Because such systems are so expensive astronomical telescopes for detecting very faint light sources
When cooled, they have very low noise and a good DQE
The drawbacks to CCDs are their cost, which is decreasing
Notes
The largest CCD arrays are gigapixels (10^9)
Ultrahigh-speed CCD cameras are available with >10^5 frames/second

7. Faraday Cup
Typically in TEM there isn’t much need to know the beam current, but for X-ray analysis it is essential
A Faraday cup is a detector that simply measures the total electron current in the beam
It is a way of characterizing the performance of the electron source
Notes
A Faraday Cup is a Black Hole for Electrons.
If you don’t have a Faraday cup, it is possible to get an approximate reading of the current by measuring the current through an insulated line from
a bulk region of your specimen and correcting for electron backscatter

8. Which detector do we use for which signal
Semiconductor detectors are only sensitive to electrons with sufficient energy (>5 keV) to penetrate the metal contact layer.
Scintillator may be coated with Al to prevent visible light
This coating would also prevent low-energy SEs from being detected
Penetration of the elctrons can be achieved by applying a high kV (>10 kV) positive bias to the scintillator.
Cathodoluminescencant specimens are sometimes studied under electron bombardment
Notes
TEMs have to be dedicated to CL detection alone and ignore other signals.
There are only a few such CL-TEMs in the world

9. Photographic Emulsions
Photographic film is the oldest recording method
Photographic emulsions are suspensions of silver halide grains in a gel
The emulsions are supported on a polymer film
Electrons strike the halide, ionize it, and transform it to silver.
Faster image film used to minimize electron beam damage
Notes
Before polymer films glass plates were used
They were heavy and occupied an enormous volume compared when to film
Difference in grain size between slow and fast film is 4µm and 5µm

10. Comparison of scanning images and static images
Either static images in conventional TEM mode or digital scanning images via electronic detection and display
Can form only BSE and SE images in a scanning mode
TEM image recorded directly onto photographic emulsion will have a high information density, with >10^7 pixels of information available in a 100 mm x 100 mm
Due to low dwell times STEM images are poorer in quality than static TEM images
Notes
Scanning images are always displayed on a computer screen, and this limits the amount of information in the image
Full HD tvs a total of only 2x10^6 pixels