Secondary Ion Mass Spectroscopy

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Secondary Ion Mass Spectroscopy

• (SIMS)

Presented by Onur Karagöz
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What is SIMS?

• Bombardment of a sample surface with a primary
  ion beam followed by mass spectrometry of the
  emitted secondary ions constitutes secondary ion
  mass spectrometry (SIMS).
• A failure analysis technique used in the
  compositional analysis of a sample.

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Uses for SIMS

• Trace elements in solid materials in
  semiconductors and thin films.
• The SIMS primary ion beam can be focused to less
  than 1 um in diameter.
• Microanalysis provides the measurement of the
  lateral distribution of elements on a microscopic
  scale.
• Continuous analysis while sputtering produces
  information as a function of depth, called depth
  profile.
• Static SIMS
• Dynamic SIMS

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Ion Beam Sputtering

• 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.
• Primary beam Cs, O2, O , Ar, and Ga with
  depths of depths 1 to 10 nm between 1 and 30 keV.

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Ion Beam Sputtering

• Sputter rates in typical SIMS experiments vary
  between 0.5 and 5 nm/s. Sputter rates depend on
  primary beam intensity, sample material, and
  crystal orientation.
• The sputter yield is the ratio of the number of
  atoms sputtered to the number of impinging
  primary ions. Typical SIMS sputter yields fall in
  a range from 5 and 15.

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Sputtering Effects

• The collision cascade model has the best success
  at quantitatively explaining how the primary beam
  interacts with the sample atoms.
• Sputtering leads to surface roughness in the
  sputter craters.

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Secondary Ion Energy Distributions

• The sputtering process produces secondary ions
  with a range of (translational) kinetic energies.
  
• Molecular Ions (internal vibrational and
  rotational modes)
• Atomic Ions (only in translational modes)

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Secondary Ion YieldsElemental Effects

• The SIMS ionization efficiency is called ion
  yield, defined as the fraction of sputtered atoms
  that become ionized.
• Positive Ions Ionization potential
• Negative Ions Electron affinity

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Secondary Ion YieldsPrimary Beam Effects

• Oxygen bombardment increases the yield of
  positive ions and cesium bombardment increases
  the yield of negative ions. The increases can
  range up to four orders of magnitude.
• Oxygen enhancement occurs as a result of
  metal-oxygen bonds in an oxygen rich zone.

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Secondary Ion YieldsPrimary Beam Effects

• The variability in ionization efficiencies leads
  to different analysis conditions for different
  elements as indicated on the periodic table.

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Relative Sensitivity Factors

• Quantitative analysis by SIMS uses RSF functions
  as follows

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Relative Sensitivity Factors

• The matrix element is usually chosen as the
  reference. Substituting M (matrix) for R
  (reference) and rearranging gives the following
  equation.

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Relative Sensitivity Factors

• The matrix concentration can be combined with the
  elemental RSF to give a more convenient constant,
  RSF.

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Sensitivity and Detection Limits

• The SIMS detection limits for most trace elements
  are between 1x1012 and 1x1016 atoms/cc.
• Dark Current The output of an electron
  multiplier if no secondary ions are striking it.
• Oxygen present in vacuum systems or analyte
  atoms sputtered from mass spectrometer parts.

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Depth Profiling

• Monitoring the secondary ion count rate of
  selected elements as a function of time leads to
  depth profiles.

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Standarts for RSF Measurement

• Quantitative SIMS analysis requires standard
  materials from which to measure RSF values.
• Because ion yields depend on the analyte element,
  the sputtering species, and the sample matrix,
  separate RSF's must be measured for each. Ion
  implants are good standards.
• Typical implant ion energies range from 50 to 300
  keV.
• The shaded area is the total signal from the
  phosphorous implant.

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Standarts for RSF Measurement

• The implant dose (1e15 ions per square centimeter
  in this example) and the crater depth (0.74 um)
  are required to calculate the average implant
  concentration, Cl.

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Bulk Analysis

• Samples with homogeneously dispersed analytes are
  analyzed by bulk analysis technique.
• Faster sputtering rates increase the secondary
  ion signal.
• In a typical heteregenous sample, the analyte is
  concentrated in small inclusions that produce
  spikes in the data stream.

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Mass Spectra

• Mass spectra sample the secondary ions in a
  preselected mass range by continuously monitoring
  the ion signal while scanning a range of
  mass-to-charge (m/z) ratios.
• The mass spectrum detects both atomic and
  molecular ions.

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Mass Interferences

• Mass interferences occur whenever another ion has
  the same nominal mass as the analyte ion. Such
  interferences are called isobaric.
• 28Si2 and 56Fe

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Elemental Interferences

• In a few cases, an isotope of one element has the
  same nominal mass as an isotope of another.
• Ruthenium 104Ru and Palladium 104Pd

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Sample Charging

• The SIMS primary ion beam, secondary ions, and
  secondary electrons produce a net electric
  current at the sample surface.
• Conductors no problem.
• Insulators charge buildup occurs.
• Sample charging diffuses the primary beam and
  diverts it from the analytical area, often
  eliminating the secondary ion signal entirely.

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Solution?

• Electron Bombardment
• To compensate the positive charge buildup
• Adjacent Conductors
• Samples are coated with conducting materials such
  as gold or carbon to compensate charge buildup

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Ion Imaging

• Ion images show secondary ion intensities as a
  function of location on sample surfaces.
• Ion images can be acquired in two operating
  modes,
• Ion Microscope
• Ion Microbeam Imaging

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Microbeam Imaging

• For ion microbeam imaging, a finely focused
  primary ion beam sweeps the sample in a raster
  pattern and software saves secondary ion
  intensities as a function of beam position.

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References

• http//www.lpdlabservices.co.uk/analytical_techniq
  ues/surface_analysis/sims.php
• http//en.wikipedia.org/wiki/Secondary_ion_mass_sp
  ectrometry
• http//www.eaglabs.com/en-US/references/tutorial/s
  imstheo/caistheo.html
• http//www.siliconfareast.com/SIMS.htm