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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?
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 ionmilling.
• 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.
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
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
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)
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