Scanning Electron Microscope (SEM)

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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 http//www.youtube.com/w
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SEM introduction http//www.youtube.com/watch?vc7
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Vacuum system

• Low vacuum pump (lt10-3 Torr) (remove 99.99 of
  air), high vacuum pump(gt10-3 Torr)
• SEM operation need 10-4 to 10-6 Torr

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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)

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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

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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

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• Cathode Luminescence
• specimen molecules 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

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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

• 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)

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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)

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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

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Waveform Monitor
used to set appropriate brightness and contrast