SIMS: Secondary Ion Mass Spectrometry

1
SIMS Secondary Ion Mass Spectrometry

• Paolo Ghigna, Dipartimento di Chimica Fisica M.
  Rolla, Università di Pavia

2
Introduction

• 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).
• The best SIMS reference is Secondary Ion Mass
  Spectrometry

3
Uses for SIMS

• Today, SIMS is widely used for analysis of trace
  elements in solid materials, especially
  semiconductors and thin films. The SIMS ion
  source is one of only a few to produce ions from
  solid samples without prior vaporization. The
  SIMS primary ion beam can be focused to less than
  1 um in diameter. Controlling where the primary
  ion beam strikes the sample surface provides for
  microanalysis, the measurement of the lateral
  distribution of elements on a microscopic scale.
  During SIMS analysis, the sample surface is
  slowly sputtered away. Continuous analysis while
  sputtering produces information as a function of
  depth, called a depth profile. When the
  sputtering rate is extremely slow, the entire
  analysis can be performed while consuming less
  than a tenth of an atomic monolayer. This slow
  sputtering mode is called static SIMS in contrast
  to dynamic SIMS used for depth profiles. Shallow
  sputtering minimizes the damage done to organic
  substances present on the sample surface. The
  resulting ion fragmentation patterns contain
  information useful for identifying molecular
  species. Only dynamic SIMS will be treated in
  this surface analysis computer aided instruction
  package because only dynamic SIMS yields
  quantitative information.

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 species useful in SIMS include Cs,
  O2, O , Ar, and Ga at energies between 1 and
  30 keV. Primary ions are implanted and mix with
  sample atoms to depths of 1 to 10 nm.
• 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.

5
Sputtering Effects

• The collision cascade model has the best success
  at quantitatively explaining how the primary beam
  interacts with the sample atoms. In this model, a
  fast primary ion passes energy to target atoms in
  a series of binary collisions.
• Energetic target atoms (called recoil atoms)
  collide with more target atoms. Target atoms that
  recoil back through the sample surface constitute
  sputtered material. Atoms from the sample's outer
  monolayer can be driven in about 10 nm, thus
  producing surface mixing.
• The term knock-on also applies to surface mixing.

6
Secondary Ion Energy Distributions

• The sputtering process produces secondary ions
  with a range of (translational) kinetic energies.
  The energy distributions are distinctly different
  for atomic and molecular ions.
• Molecular ions have relatively narrow
  translational energy distributions because they
  have kinetic energy in internal vibrational and
  rotational modes whereas atomic ions have all
  kinetic energy in translational modes. The figure
  shows typical energy distributions for mono, di,
  and triatomic ions.

7
Secondary Ion Yields

• The SIMS ionization efficiency is called ion
  yield, defined as the fraction of sputtered atoms
  that become ionized. Ion yields vary over many
  orders of magnitude for the various elements. The
  most obvious influences on ion yield are
  ionization potential for positive ions and
  electron affinity for negative ions. 

8
Secondary Ion Yields -- Primary Beam Effects

• Other factors affect the secondary ionization
  efficiencies in SIMS measurements. Oxygen
  bombardment increases the yield of positive ions
  and cesium bombardment increases the yield of
  negative ions.
• Oxygen enhancement occurs as a result of
  metal-oxygen bonds in an oxygen rich zone. When
  these bonds break in the ion emission process,
  the oxygen becomes negatively charged because its
  high electron affinity favors electron capture
  and its high ionization potential inhibits
  positive charging. The metal is left with the
  positive charge. Oxygen beam sputtering increases
  the concentration of oxygen in the surface layer.
• The enhanced negative ion yields produced with
  cesium bombardment can be explained by work
  functions that are reduced by implantation of
  cesium into the sample surface. More secondary
  electrons are excited over the surface potential
  barrier. Increased availability of electrons
  leads to increased negative ion formation.

9
Sensitivity and Detection Limits

• The SIMS detection limits for most trace elements
  are between 1012 and 1016 atoms/cc. In addition
  to ionization efficiencies (RSF's), two other
  factors can limit sensitivity.
• The output of an electron multiplier is called
  dark counts or dark current if no secondary ions
  are striking it. This dark current arises from
  stray ions and electrons in instrument 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. Mass interferences also
  cause background limited sensitivity.
•  

10
Depth Profiling

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

11
Bulk Analysis

• For samples with homogeneously dispersed analyte,
  bulk analysis provides better detection limits
  than depth profiling, usually by more than an
  order of magnitude.
• Faster sputter rates increase the secondary ion
  signal in bulk analysis. The fastest possible
  sputtering requires intense primary ion beams
  which sacrifice depth resolution because they
  cannot be focused as required for flat bottom
  (rastered) craters.
• Otherwise, bulk analyses are similar to depth
  profiles. Ion intensity data are displayed as a
  function of time. This provides a means for
  verifying that the sample is indeed homogenous.
  In a typical heterogeneous sample, the analyte is
  concentrated in small inclusions that produce
  spikes in the data stream.

12
Ion Imaging

• Ion images show secondary ion intensities as a
  function of location on sample surfaces. Image
  dimensions vary from 500 um to less than 10 um.
  Ion images can be acquired in two operating
  modes, called ion microscope or stigmatic
  imaging, and ion microbeam imaging or raster
  scanning. Ion microscopy requires a combination
  ion microscope/mass spectrometer capable of
  transmitting a mass selected ion beam from the
  sample to the detector without loss of lateral
  position information. Image detectors indicate
  the position of the arriving ions. Ion microscope
  images are usually round because the ion
  detectors are round. Lateral resolutions of 1 um
  are possible. A SIMS analyst selects images with
  higher lateral resolution at the expense of
  signal intensity and higher mass resolution at
  the expense of image field diameter.
• 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.
  Microbeam imaging uses standard electron
  multipliers and image shape follows raster
  pattern shape, usually square. Lateral resolution
  depends on microbeam diameter and extends down to
  20 nm for liquid metal ion guns. Some instruments
  simultaneously produce high mass resolution and
  high lateral resolution. However, the SIMS
  analyst must trade high sensitivity for high
  lateral resolution because focusing the primary
  beam to smaller diameters also reduces beam
  intensity.

13
Ion Imaging

• The example (microbeam) images show a pyrite
  (FeS2) grain from a sample of gold ore with gold
  located in the rims of the pyrite grains. The
  image on the right is 34S and the left is 197Au.
  The numerical scales and the associated colors
  represent different ranges of secondary ion
  intensities per pixel.