Questions/Problems on SEM microcharacterization

1
Questions/Problems on SEM microcharacterization
Explain why Field Emission Gun (FEG) SEM is
preferred in SEM? How is a contrast generated in
an SEM? What is the contrast for 1? change in the
surface feature if the beam angle is 60?? What
are the major surface imaging modes in SEM? What
are the SEM associated techniques that can give
elemental composition?
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Focused ion beam (FIB) technique

• In this technique, finely focused Ga ions are
  used to etch away selected regions in a circuit
  or a micro/nanostructure. The beam energy is 5
  50 keV, which is lower than that of electrons in
  SEM. The process is called ion-milling.
• Common applications are for etching materials so
  that they are suitable for imaging in optical
  microscope or even TEM
• The minimum spot size of ions is 10 nm, which is
  much larger than SEM.
• The region affected by the beam can be imaged by
  the secondary electrons just like in SEM. Note
  that the primary beam is no longer made of
  electrons.
• The technique can also be used to deposit metals
  like W, Pt, or Au. This is very important for
  rectifying small circuit errors or joining
  nanostructure to large metal pads for rapid
  device prototyping.
• The metals are deposited by delivering selected
  gases very close to the beam, which then gets
  adsorbed on the surface, get decomposed by the
  Ga ions and are deposited on the surface.

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Ion probe techniques

• Uses a variety of different materials to produce
  ions such as Cs, O2 or Ga
• Used in two common techniques SIMS and RBS

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SIMS 1

• Uses Ions to hit the material and produce
  secondary ions
• The secondary ions are selected by means of a
  tandem electric and magnetic filter so that a
  narrow range of ions with correct charge/mass
  ratio can emerge out
• This process is destructive but highly accurate
  provided a reference sample for comparison exists
• This is the only method that gives the actual
  dopant density and not just carrier concentration
  in a semiconductor
• All elements can be analyzed in this technique
• The mass resolution m/?m can be up to 40,000 so
  that elements differing in mass of 0.003 can be
  distinguished

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SIMS 2

• There are different types Ion Microprobe,
  TOF-SIMS, and Quadrupole SIMS. The first two are
  more important the first is also called dynamic
  SIMS where a complete depth profile can be done
  and uses q/m ratio to separate ions, and the 2nd
  used for static SIMS as only a few monolayers are
  removed, and is based on the different times of
  flight of the accelerated ions.
• All SIMS other than TOF does serial screening of
  the q/m ratio. TOF SIMS displays everything
  together based on the time taken to reach the
  detector (a fixed path length)

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Rutherford back scattering (RBS)

• This process uses light atoms typically He ions
  and measure the with energy 1 3 MeV to bombard
  the surface of the sample and measure the energy
  of the backscattered ions.
• Gives masses of elements in the sample upto a
  depth of 10 nm to a few microns
• This is non-destructive technique unlike SIMS
• Depth resolution is on the order of 10 nm or so
• The detection limit is in the range of 1017
  1020 /cm-3