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Secondary Ion Mass Spectroscopy

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Bombardment of a sample surface with a primary ion beam followed by mass ... Ruthenium 104Ru and Palladium 104Pd. Sample Charging ... – PowerPoint PPT presentation

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Title: Secondary Ion Mass Spectroscopy


1
Secondary Ion Mass Spectroscopy
  • (SIMS)

Presented by Onur Karagöz
2
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.

3
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

4
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.

5
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.

6
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.

7
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)

8
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

9
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.

10
Secondary Ion YieldsPrimary Beam Effects
  • The variability in ionization efficiencies leads
    to different analysis conditions for different
    elements as indicated on the periodic table.

11
Relative Sensitivity Factors
  • Quantitative analysis by SIMS uses RSF functions
    as follows

12
Relative Sensitivity Factors
  • The matrix element is usually chosen as the
    reference. Substituting M (matrix) for R
    (reference) and rearranging gives the following
    equation.

13
Relative Sensitivity Factors
  • The matrix concentration can be combined with the
    elemental RSF to give a more convenient constant,
    RSF.

14
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.

15
Depth Profiling
  • Monitoring the secondary ion count rate of
    selected elements as a function of time leads to
    depth profiles.

16
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.

17
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.

18
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.

19
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.

20
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

21
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

22
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.

23
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

24
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

25
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.

26
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
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