SIMS

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SIMS
Viraj Jayaweera Department of Physics
Astronomy Georgia state University
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Outline

• Theory
• Strength
• Accuracy
• Limitations
• Application using GaN as example

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SIMS Theory
A well established analytical technique that was
first pioneered in 1949
Primary ion beam (O-, O2, Ar, Cs, Ga are
often used with energies between 1 and 30 keV)
Primary ions are implanted and mix with sample
atoms to depths of 1 to 10 nm.
SIMS is generally used for surface, bulk,
microanalysis, depth profiling, and impurity
analysis.
http//atomika.com/
The bombarding primary ion beam produces
monatomic and polyatomic particles of sample
material and resputtered primary ions, along with
electrons and photons. The secondary particles
carry negative, positive, and neutral charges and
they have kinetic energies that range from zero
to several hundred eV.
The technique involves bombarding the surface of
a sample with a beam of ions, thus emitting
secondary ions. These ions are later measured
with a mass spectrometer to determine either the
elemental or isotopic composition of the surface
of the sample.
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Schematic Diagram of a SIMS instrument
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Cameca IMS 6f secondary ion mass spectrometer
Faraday Cup
Florescence Screen
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SIMS Theory
The detected secondary ion current for ith element
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Mass Spectrometer
For an analogy, think of how a prism refracts and
scatters white light separating it into a
spectrum of rainbow colors. In a mass
spectrometer, ions travel different paths through
the magnet to the detector due to their
mass/charge ratios. A mass analyzer sorts the
ions according to mass/charge ratios and the
detector records the abundance of each ratio.
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Mass Spectrometer
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Time-of-Flight Mass Analyzer
Typical flight times 10 ns to 800 µs
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The Quadrupole Mass Analyzer
www.chm.bris.ac.uk/ms/theory/quad-massspec.html
The two opposite rods in the quadrupole have a
potential of (UVcos(wt)) and the other two
-(UVcos(wt)) where 'U' is the fixed potential
and Vcos(wt) is the applied RF of amplitude 'V'
and frequency 'w'. This results in ions being
able to traverse the field free region along the
central axis of the rods but with oscillations
amongst the poles themselves. These oscillations
result in complex ion trajectories dependent on
the m/z of the ions. Specific combinations of the
potentials 'U' and 'V' and frequency 'w' will
result in specific ions being in resonance
creating a stable trajectory through the
quadrupole to the detector. All other m/z values
will be non-resonant and will hit the quadrupoles
and not be detected. The mass range and
resolution of the instrument is determined by the
length and diameter of the rods.
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SIMS Imaging
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Depth Profiling

• The measurement of dopant and impurity
  concentrations with depth in compound
  semiconductor is often accomplished by SIMS.
• Monitoring the secondary ion count rate of
  selected elements as a function of time leads to
  depth profiles.

The raw data for a measurement of phosphorous in
a silicon matrix. The sample was prepared by ion
implantation of phosphorous into a silicon wafer.
The analysis uses Cs primary ions and negative
secondary ions.
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Depth Profiling
To convert the time axis into depth, the SIMS
analyst uses a profilometer to measure the
sputter crater depth. Total crater depth divided
by total sputter time provides the average
sputter rate. Relative sensitivity factors
convert the vertical axis from ion counts into
concentration.
Previous phosphorous depth profile plotted on
depth and concentration axes.
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Depth Profiling

• Depth Resolution
• Depth resolution depends on flat bottom craters.
• Modern instruments provide uniform sputter
  currents by sweeping a finely focused primary
  beam in a raster pattern over a square area.
• In some instruments, apertures select secondary
  ions from the crater bottoms, but not the edges.
• Alternatively, the data processing system ignores
  all secondary ions produced when the primary
  sputter beam is at the ends of its raster pattern.

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Sensitivity and Detection Limits
The SIMS detection limits for most trace elements
are between 1012 and 1016 atoms/cm3. In
addition to ionization efficiencies (RSF's), two
other factors can limit sensitivity. The dark
current (or dark counts) arises from stray ions,
electrons in vacuum systems, and from cosmic
rays Count rate limited sensitivity occurs when
sputtering produces less secondary ion signal
than the detector dark current. If the SIMS
instrument introduces the analyte element, then
the introduced level constitutes background
limited sensitivity. Oxygen, present as residual
gas in vacuum systems, is an example of an
element with background limited sensitivity.
Analyte atoms sputtered from mass spectrometer
parts back onto the sample by secondary ions
constitute another source of background.
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Advantages and Weaknesses
Advantages Weaknesses
High sensitivity, especially for light elements Destructive method
High surface sensitivity, important for depth profiling High selectivity, depending on the element
Information about the chemical surface composition due to ion molecules Secondary ion yield for an element varies with the surrounding elemental composition (matrix dependence)
Can detect all elements and isotopes, including H Interference of molecules and isotopes in the mass spectrum
Can detect all elements and isotopes, including H Quantitative analysis quiet complicated
SIMS is best fitted to SIMS is best fitted to
Surface analysis Micro analysis Doping profiles (i.e. semiconductors) Analysis of the isotopes (i.e. meteorite, isotopic labeling) Surface analysis Micro analysis Doping profiles (i.e. semiconductors) Analysis of the isotopes (i.e. meteorite, isotopic labeling)
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SIMS Application using GaN as an Example
Detection Limits of Selected Elements in GaN
Source
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SIMS Application using GaN as an Example
Detection Limits of Selected Elements in GaN
Chu, Gao, and Erickson Characterization of III
nitride materials J. Vac. Sci. Technol. B, Vol.
16, No. 1, Jan/Feb 1998
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Al Depth Profiles at the Interface of an
AlGaN/GaN
Al depth profiles at the interface of an
AlGaN/GaN sample. One profile was acquired on a
sample with a high density of visible surface
pits, whereas the other curve was obtained on a
sample with few if any visible
Chu, Gao, and Erickson, J. Vac. Sci. Technol. B,
Vol. 16, No. 1, Jan/Feb 1998
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Si and Mg Doping Profile of GaN
SIMS depth profile of a p-n homojunction in GaN
using Mg and Si as p- and n-type dopants. Common
dopants in GaN such as O and C and some
transition metals such as Fe, Mo, Cr, and Ni are
also measured.
Chu, Gao, and Erickson, J. Vac. Sci. Technol. B,
Vol. 16, No. 1, Jan/Feb 1998
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GaN/InGaN/GaN LED Device
Optical micrograph depicting the post-SIMS
measurement crater on the lower left-hand side
and the remnant of the setup crater on the upper
right-hand side of the device.
SIMS analysis of a finished LED chip after
de-encapsulation
Chu, Gao, and Erickson Characterization of III
nitride materials J. Vac. Sci. Technol. B, Vol.
16, No. 1, Jan/Feb 1998
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Depth Profiles of GaN/InGaN/GaN LED Device
SIMS depth profiles of dopants compositional
profile for the GaN/InGaN/GaN LED device.
Chu, Gao, and Erickson, J. Vac. Sci. Technol. B,
Vol. 16, No. 1, Jan/Feb 1998
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END