A brief introduction to SIMS

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					A BRIEF INTRODUCTION TO
SECONDARY ION MASS
SPECTROMETRY (SIMS)
Jiangjiang Gu
OUTLINE
 What is SIMS?
 Principle

 Application

 Some Examples
WHAT IS SIMS?
 SIMS = Secondary Ion Mass Spectrometry, also
  known as ion microprobe and ion microscope
 Used to analyze the composition of solid surfaces
  and thin films
 Material surface is first sputtered with a focused
  primary ion beam and the ejected secondary ions
  are then collected and analyzed
WHAT IS SIMS?
HISTORY OF SIMS
 1910: British physicist
  J.J. Thomson observed
  release of positive ions
  and neutral atoms from a
  solid surface induced by
  ion bombardment
 1940s: Improved vacuum
  technology appear
 1949: First prototype
  experiments on SIMS by
  Herzog and Viehböck at      1949: Herzog and Viehböck

  the University of Vienna,
  Austria.
HISTORY OF SIMS
   Early 1960s: two SIMS
    instruments are
    developed independently
    by
     Castaing and Slodzian at
      the University of Paris for
      the PhD thesis of Slodzian
     Herzog and Liebel at GCA
      Corp., funded by NASA
      for analyzing moon rocks

                                    1963: Liebel and Herzog
HISTORY OF SIMS
   1970s:
     K.Wittmack and C. Magee developed SIMS
      instruments with quadruple mass analyzers
     A. Benninghoven introduced method of static SIMS

 1980s: ‘Time of Flight’ mass spectrometers were
  developed by Benninghoven and cooperators.
 Recent developments: focus is on novel primary
  ion species like C60 or cluster ions of gold and
  bismuth.
    PRINCIPLE                Typically 10-20keV

   Classical SIMS
    device consists of:
       Primary ion gun
       Primary ion column
       High vacuum (<10-6
        torr) sample
        chamber
       Mass analyzer
       Ion detection unit

   The basis of SIMS is the destructive removal of
    material from the sample by sputtering and the
    analysis of the ejected material by a mass analyzer.
PRINCIPLE



 A primary ion beam impinges on the sample and
  atoms from the sample are sputtered or ejected
  from the sample.
 Most of the ejected atoms are neutral and cannot
  be detected by conventional SIMS, but some are
  positively or negatively charged. This fraction
  was estimated as about 1% of the total.
 The mass/charge ratio of the ions is analyzed,
  detected as a mass spectrum, as a count, or
  displayed on a fluorescent screen.
SPUTTERING PROCESS
   Sputtering is a process in which incident ions lose their energy
    mainly by momentum transfer.
   In the process they displace atoms within the sample. Sputtering
    takes place when atoms near the surface receive sufficient energy
    from the incident ion to be ejected form the sample. The escape
    depth of the sputtered atoms is generally a few monolayer for the
    primary energies of 10 to 20keV.
   The primary ion loses its energy in the process and rest tens of nm
    below the sample surface.
SPUTTERING YIELD
   Sputtering yield is the average number of atoms
    sputtered per incident primary ion. It depends on the
    sample, its crystallographic orientation, and the
    nature, energy and incidence angle of the primary
    ions. The yield for SIMS measurements with Cs+, O2+,
    O- and Ar+ ions of 1 to 20keV energy ranges from 1 to
    20.
   What is important is not the total yield, but the yield
    of ionized ejected atoms or the secondary ion yield.
    The secondary ion yield is significantly lower than
    total yield, but can be influenced by primary ion.
   For example: Electronegative oxygen O2+ enhances
    species for electropositive elements (e.g., B and Al in
    Si) which produce predominantly positive secondary
    ions. Electropositive ions like cesium (Cs+) enhances
    species for electronegative elements )e.g., P, As and
    Sb in Si)
    STATIC AND DYNAMIC SIMS
   SIMS can give three types of results: (1) mass
    spectrum (2) depth profile (3) imaging
   Static SIMS offers mass spectrum
    measurement
   Dynamic SIMS offers depth profile and 2-D
    imaging option.
   Main difference between static and dynamic
    SIMS is the incident ion beam current or
    sputtering rate. Static SIMS has much lower
    incident current and much slower sputtering
    rate compared to dynamic SIMS.                   Various SIMS output signal

   Static SIMS measure a spectrum of peaks,
    while dynamic SIMS focus on one peak.
STATIC SIMS
   For low incident ion beam current or low
    sputtering rate (~0.1nm per hour), a complete
    mass spectrum can be recorded for surface
    analysis of the outer 0.5nm or so. This mode of
    operation is known as static SIMS.




Example of a SIMS spectrum   SIMS spectrum of Mineral Arsenopyrite
DYNAMIC SIMS
 In dynamic SIMS, the intensity of one peak for on
  particular mass is recorded as a function of time
  as the sample is sputtered at a higher sputter
  rate (~10µm per hour), yielding a depth profile.
 It is also possible to display the intensity of one
  peak as a 2-D or 3-D image.




Depth profile example:
Negative Ion SIMS Depth Profile of Ni Layer   Imaging example:
on Cu Substrate                               a Tinplated Steel Sample Used for Food Canning
DEPTH PROFILE APPLICATION
 Show impurities present in a particular layer,
  their concentrations, and their depth
  distributions.
 Measure the thickness of a film.

 Monitor a diffusion profile across an interface.

 Show whether segregation occurs at interface.

 Detect concentration levels <1e17 atoms/cm3
  while providing depth resolution of <5 nm.
MASS ANALYZERS
   Depending on the SIMS type, there are three basic
    analyzers available: sector, quadrupole, and time-of-
    flight.
   A sector field mass spectrometer uses a combination
    of an electrostatic analyzer and a magnetic analyzer
    to separate the secondary ions by their mass to charge
    ratio.
   A quadrupole mass analyzer separates the masses by
    resonant electric fields, which allow only the selected
    masses to pass through.
   The time of flight mass analyzer separates the ions in
    a field-free drift path according to their kinetic
    energy. It requires pulsed secondary ion generation
    using either a pulsed primary ion gun or a pulsed
    secondary ion extraction.
QUADRUPLE SIMS
   Quadruple SIMS uses a quadruple mass analyzer
    instead of electrostatic-magnetic sector analyzers.
   A quadruple mass analyzer consists of four parallel
    rods with an oscillating electric field through which
    the ions pass.
   Pros: more robust, less expensive
   Cons: lower resolution, cannot distinguish between
    ions with close mass/charge ratios
   Quadruple SIMS is suitable for analyzing insulating
    samples
TIME-OF-FLIGHT SIMS (TOF-SIMS)
 Time-of-flight SIMS is a method of using Time-
  of-flight mass analyzer.
 A major advantage of TOF-SIMS is the absence
  of narrow slits in the spectrometer increasing the
  ion collection by 10-50%. This reduces the
  sputtering rate greatly, allowing characterization
  of organic surface layers.
 TOF-SIMS is the standard method for static
  SIMS instruments.
 Pros: highly sensitive

 Cons: qualitative
BEYOND SIMS
 A major source of the limited sensitivity of SIMS
  is the fact that most of the sputtered material is
  neutral and cannot be detected.
 Secondary neutral mass spectrometry (SNMS)
  provides significant sensitivity enhancements,
  where neutral atoms are ionized by a laser or by
  an electron gas and then detected.
 Laser microprobe mass spectrometry (LAMMA)
  replaces the primary ion beam in SIMS with a
  pulsed laser, providing high sensitivity, high
  speed of operation, and is applicable to inorganic
  as well as organic samples.
SIMS APPLICATION
    Semiconductor (primary)                     Displays
    Aerospace                                   Electronics
    Automotive                                  Lighting
    Compound Semiconductor                      Photonics
    Data Storage                                Solar Photovoltaic
    Defense                                     Telecommunications

For semiconductor characterization:
        Dopant and impurity depth profiling
        Composition and impurity measurements of thin films (metal,
         dielectric, SiGe, III-V, and II-V)
        Ultra-high depth resolution profiling of shallow implants and ultra
         thin films (ULE implants and gate oxides)
        Bulk analysis, including B, C, O, and N in Si
        High-precision matching of process tools (ion implanters)
SOME NOTES TO CONSIDER
   Reference standards are required beforehand
    because secondary ion yields are substrate
    dependent.
   SIMS is a destructive technique.
   A flat surface is required to obtain the best
    lateral and depth resolution.
   Samples must be <25 mm in diameter and
    preferably <5 mm thick.
COMPANIES DOING SIMS
EXAMPLE: ION-IMPLANTATION PROFILE
EXAMPLE (CONT’D)
   The primary ion beam was Cs+ with an energy range
    between 0.5 and 3keV, depending on the depth, with an
    angle of 60o. The secondary ions were P-, As- or Si-. The
    base vacuum was less than 10-8 Pa, and the raster scan
    area was 120µm×120µm.
OTHER DEPTH PROFILE EXAMPLES


                                          Ultra Shallow Junctions




 Ultra thin Oxynitride gate dielectrics



                                                         25
EXAMPLE: TOF-SIMS IMAGING OF VIAS




  Ti ion image   Si ion image   Overlay
EXAMPLE: SIMS MASS SPECTRUM




   Microscopic image
                       Residue ion image




                        Corrosion image
    Mass Spectrum
SUMMARY
 The technique of SIMS, its history, basic
  principles and its state-of-the art variants are
  overviewed.
 The various applications of SIMS are introduced.

 SIMS as a surface analysis, depth profiling and
  imaging technique is inevitable in the
  semiconductor industry, especially in the micro
  and nano regime.
SOURCE
 Wikipedia
 Google

 Semiconductor material and device
  characterization – 3rd Edition (K. Schroder)
THANK YOU FOR YOUR ATTENTION




      PHI TRIFT III Time of Flight SIMS