Transcript X-ray detection

Scanning Electron Microscope (SEM)
Major components:
1.Vacuum system
2.Electron beam generation system
3.Electron beam manipulation system
4.Beam specimen interaction system
5.Detection system
6.Signal processing system
7.Display and recording system
SEM operation principles:
http://www.youtube.com/watch?v=PNHn4YM7yfc&fea
ure=related
SEM introduction
http://www.youtube.com/watch?v=c7EVTnVHNs&feature=related
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
•
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)
•
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
•
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 (3x10 16 ~ 3x1019 Hz),
soft x-ray:0.12ev~12 keV, hard x-ray:12kev~120 keV,
http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.
html
http://www.physics.isu.edu/radinf/xray.htm
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
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.
Signal Detection
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)
Signal Manipulation
brightness and contrast are main control of signal
manipulation
 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)

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


Waveform Monitor
used to set appropriate brightness and contrast