1. Scanning Electron Microscope (SEM)
Major components:
Vacuum system
Electron beam generation system
Electron beam manipulation system
Beam specimen interaction system
Detection system
Signal processing system
Display and recording system
SEM operation principles:
SEM introduction

2. Vacuum system
Low vacuum pump (<10-3 Torr) (remove 99.99% of air), high vacuum pump(>10-3 Torr)
SEM operation need 10-4 to 10-6 Torr

3. E beam generation Three components:
1) cathode (filament or field emission)
2) grid cap that control the flow of electrons
3) anode that attracts and accelerates electrons
Voltage ranges from 0.1 to 40 Kv, (10kV the most common for biological specimen)
the higher the voltage the better the resoution (but
greater heat will be generated on the specimen)
On/off switch for high voltage (HV)
Beam current (the flow of electrons that hit a sample), controlled by bias voltage between filament and grip cap,
Increasing beam current results in deeper penetration of electron and a larger diameter spot)
Filament current control (adjustment of current filament, providing necessary heating current to filament)
Filed emission:
advantages: cool cathode, emitted beam is smaller in diameter (better resolution), longer life time
disadvantages: higher vacuum need (~10-7 Torr), cleaner microscope needed, not many x-rays generated (due to low beam currents and small beam diameters)

4. E beam manipulation
E-gun is controlled by electrostatic field, while the rest of SEM is controlled by magnetic lenses.
Electromagnetic lenses:
- condenser lenses: reduce spot size
spherical aberration limit resolution
- correct astigmatism (non-circular beam spot), due to
beam formed by filament is elliptical, dirt in column,
beam distorted on the aperture
using stigmator (control strength and azimuth)
- correct alignment
- two sets of magnetic coils (raster coils) that move the
seam scanning in the X and Y direction
- magnification: ratio of dimension of CRT to dimension
of the area being scanned
two ways of magnification adjustments:
1) change scanned area of the specimen
2) adjust focal point of the beam and working distance
(move Z-axis to bring sample to focal point)
Aperture: a round hole that control the passing through of scattered electrons
- small aperture for high resolution
- big aperture for low resolution with more electrons
needed

5. Beam interaction Backscattered electrons: original beam electrons,
high energy level,
useful when relative atomic density information with
topographical information is displayed
Secondary electrons:
generated by dislodging specimen e or other secondary e
a few eV,
detected near the surface, can obtain topographical and
high solution,
contrast and soft shadows of image resemble specimen
illuminated with light
X-rays:
can obtain elemental information (from wavelength and
energy characteristic of elements)
measurement of wavelength (wavelength dispersive
spectrometer (WDS)) or energy level (energy dispersive
spectrometer (EDS))
wavelength range: 10 ~ 0.01 nm (3x1016 ~ 3x1019 Hz),
soft x-ray:0.12ev~12 keV, hard x-ray:12kev~120 keV,

6. Cathode Luminescence:
specimen molecule’s florescence that produces light photon
Specimen current:
e energy decreases after scattering and e absorbed by sample
can build up negative charge and lead to charging
Transmitted electrons:
primary e pass through specimen
provide atomic density information displayed as a shadow
higher the atomic number the darker the shadow

9. Review for SEM Operation
Contamination : image become dark when irradiated
on a portion for a long time,
cause by residual gas being struck by e. probe (see the
guide on p.17)
Charge-up: negative charge collected that leads to
prevent normal emission of second e.

10. Signal Detection secondary e. attracted by 200V applied to ring around
Detector Structure
secondary e. attracted by 200V applied to ring around
the detector (Faraday Cup)
e. accelerated by 10 Kv applied to hit scintillator and
produce photons
photons travel down to photomultiplier (PM) where
signal is increased or amplified (PM control contrast)
Topological Features
density of emissive area determine signal strength
emissive area affected by topology such as flatness,
pointed structure, edge (see the guide on p. 9)
X-ray detection
x-ray loss energy by hitting another particles that produce
background
EDS detector gather spectrum from 0 to 30 ev
WDS is set to detect a small a range (more sensitive than
EDS)

11. Signal Manipulation brightness and contrast are main control of signal
brightness: actual value of each pixel
control by adding or subtracting value for each pixel
contrast: difference between two pixels
control by the amplifier of secondary e. (PM)

12. Display and Record System
Brightness
Contrast
Resolution: ability to distinguish between two points
determined factors: beam spot size, working distance, aperture size,
beam bias current/voltage, how cylindrical the beam is
Magnification: a function of area scanned and viewing size
adjusted by raster coils and location of focal point of the primary
beam to final lens
Depth of field: region of acceptable sharpness in front of and
behind focus points
Noise: can be controlled by
increasing signal (aperture size, bias voltage, etc)
decrease noise
increasing scanned time

13. Waveform Monitor
used to set appropriate brightness and contrast