SEM overview

1
SEM overview advantages and dis-

• Basic types of SEM information
• surface morphology and composition
• high depth of field gt1mm (light mic. lt15µm)
• high resolution lt 1 nm (light mic. gt500nm)
• Quantitative elemental analysis via X-ray
  spectroscopy
• SEM disadvantages
• specimen must be dry (variable pressure SEMs
  allow some moisture)
• lack of color
• surface information only (1 mm beam penetration)

2
SEM signal generation

• Water droplet analogy falling wave packet
  penetrates surface of water pool collisions
  ensue - signals emerge
• Electron/atom interactions analogous to
  collisions between incoming droplet molecules
  and molecules in the bucket
• Detectors measure the Splash (emerging signals)
• High energy splash (Backscattered Electrons or
  BSE)
• Low energy splash (Secondary electrons or SE)
• Quantum mechanical splash (X-ray photons)
• Ripples (radiative byproducts) dissipate excess
  energy
• Heat
• Waves (Electrical current)

3
Types of electron/atom collisions

• Elastic - incoming/outgoing energies are equal
• Backscattered electrons - high energy (not
  perfectly elastic usually have some loss from
  primary energy)
• Inelastic - some incoming energy is lost to
  radiative dissipation (heat, light, etc)
• Incoming electron collides with atom, causing
  emission of a conduction or bonding electron
• Emitted conduction electron is a secondary
  electron
• Emitted bonding or valence electron yields higher
  energy dissipation
• Xray photons
• Cathodoluminescent photons
• Auger electrons

4
Beam Interaction and detectors
The Electron Beam Penetrates the SampleThree-
Dimensionally at Each Scanned Pixel
5
Backscattered electron detection

• The BEI detector is a two-, three-, or
  four-piece annular detector located on the
  objective lens pole piece, directly above the
  sample
• No bias - electron trajectories must be towards
  detector
• Sensitive to shadowing and angle contrast

6
Solid State Back Scatter Detector

• 4 diodes of the Solid state BSD
• 2 adjacent quadrants 1 side (A or B) of
  detector
• Shadows can be top/bottom or left/right

BSD
A
B
Specimen
7
BSD Can be used for

• AB Compositional detail
• A-B Topographical detail
• Both methods can used in the Phenom

8
BSE detector - modes of operation

• Composition or full mode AB
• The higher the atomic Z, the more protons in the
  nucleus
• More protons -gt more coulombic force on beam
  electrons
• More force -gt larger change in electron
  trajectories
• Thus, higher Z turns more electrons back towards
  detector - Z contrast
• This mode dampens topographic contrast
• Topographic mode A-B
• Surfaces facing the A detector appear bright in
  the A signal and dark in the B signal
• Subtraction accentuates topographic contrast
• Subtraction dampens compositional contrast

9
Forming and scanning the electron probe
Source
Scan coils

• Aperture

Objective lens
Sample
Raster direction
10
Image formation not like your grandmothers
microscope!

• Rather than by a parallel array of detectors,
  e.g. CCD camera or film, we have one detector
  whose output signal is in sync with the scan
  coils a rastered image.
• Scan area determines magnification
• Resolution controlled by diameter of beam on
  sample
• Focus stays constant with magnification

11
Depth of field and Working Distance (WD)
final lens aperture
objective lens
OWD Optical WD
FWD Free WD
Specimen location 1
Increased WD at location 2 yields lower subtended
angle, thus greater DOF
Specimen location 2
12
Secondary electron imaging

• Main imaging mode for standard SEM, no detector
  present in table-top SEM
• Detector is positively biased sucks low-energy
  electrons from the sample region
• Images both sides of features
• Contrast mechanisms angle to detector, edge
  effect, shadowing
• Signal can be increased with appropriate tilting
• Edge effect gives SEM images their unique crisp
  look

13
Secondary Electron Detector for Topographical
Effects