1. Secondary Ion Mass Spectrometry
TOF-SIMS
Zaahir Salam

2. Basic Overview
What is Spectroscopy?
 It is the study of the interaction between matter and
radiation.
What is an Ion?
 It is a charged atom or molecule. It is charged because the
number of electrons are not equal the number of protons in
the atom or molecule.
 An atom can acquire a +ve or a -ve charge depending on
whether the number of electrons in an atom is greater or less
then the number of protons in the atom.

3.  When an atom is attracted to another atom because it has an
unequal number of electrons and protons, the atom is called
an ION.
 If the atom has more electrons than protons, it is a
negative ion, or ANION.
 If it has more protons than electrons,it is a positive ion.

4. Secondary Ion
A small percentage of these ejected atoms leave as either
positively or negatively charged ions, which are referred to as
'secondary ions’

5. Mass Spectrometry (MS)
 It is an analytical technique used for the determination of
qualitative and quantitative information about both atomic
and molecular composition of inorganic and organic
compounds.
 Mass spectrometers use the difference in mass-to-charge
ratio (m/e) of ionized atoms or molecules to separate them
from each other.

6. Mass Spectra
• When the electron beam ionizes the molecule, the species
that is formed is called a radical cation, and symbolized as
M+•.
• The radical cation M+• is called the molecular ion or parent
ion.
• The mass of M+• represents the molecular weight of M.
• Because M is unstable, it decomposes to form fragments of
radicals and cations that have a lower molecular weight than
M+•.

7. • The mass spectrometer analyzes the masses of cations.
• A mass spectrum is a plot of the amount of each cation (its
relative abundance) versus its mass to charge ratio (m/z,
where m is mass, and z is charge).
• Since z is almost always +1, m/z actually measures the mass
(m) of the individual ions.
Mass Spectrometry
• Though most C atoms have an atomic mass of 12, 1.1% have a mass of 13. Thus,
13CH is responsible for the peak at m/z = 17. This is called the M + 1 peak.
4

8. Why SIMS Is Used
The technique offers the following advantages:
 The elements from H to U may be detected.
 Most elements may be detected down to concentrations of 1ppm or 1ppb.
 Isotopic ratios may be measured, normally to a precision of 0.5 to 0.05%.
 2-D ion images may be acquired.
 Using a high-energy and high primary beam densities (dynamic SIMS) a
volume of a 100 to 1000 μm is analysed. In contrast, using low energy and
low primary beam densities (static SIMS) the material sputtered is
exceedingly small, with surface mono-layers lasting hours or days.

9. Advantages Continued…
 3-D ion images may be acquired by scanning (rastering) the primary beam
and detecting the ion signal as the sample is gradually eroded.
 Little or no sample preparation may be needed.
 For all semiconductor products, from IC’s to solar cells to LED’s, it is crucial
to control the concentration depth profiles of dopants and contaminants.
 Secondary Ion Mass Spectrometry (SIMS) is the most important technique
for measuring depth profiles of all elements with extreme sensitivity, huge
dynamic range and very good depth resolution.
 The technique is also used to measure dilute element profiles in specialty
glasses, ceramics, and metal alloys, and –by measuring isotope ratios– in
art history, geology and extra-terrestrial research.

11. SIMS is an analytical technique based on the measurement of
the mass of ions ejected from a solid surface after the surface
has been bombarded with high energy (1-25 keV) primary ions.

12. Collision Cascade
The primary ions deposit energy into the surface layers. Around the impact site and
to the depth of ~3 nm, many bonds are broken and there is much random
displacement and movement of atoms. This region is called the collision cascade
• The physical effects of primary ion bombardment: implantation and sputtering

13. SPUTTERING
• The sputtering, or ejection, of target atoms and molecules occurs
because much of the momentum transfer is redirected toward the
surface by the recoil of the target atoms within the collision cascade
• The bombardment of a solid surface with a flux of energetic
particles can cause the ejection of atomic species, it causes erosion
or etching of the solid.
• The incident projectiles are often ions, because this facilitates
production of an intense flux of energetic particles that can be
focused into a directed beam.
• However, in principle, sputtering (and secondary ion emission) will
also occur under neutral beam bombardment.

15. 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.

16. • To convert the time axis into depth, the SIMS analyst uses a profilometer to
measure the sputter crater depth. A profilometer is a separate instrument that
determines depth by dragging a stylus across the crater and noting vertical
deflections. At the end of the above phosphorous depth profile, profilometry gives
0.74 um for the crater depth. Total crater depth divided by total sputter time
provides the average sputter rate.
• Relative sensitivity factors (RSFs) convert the vertical axis from ion counts into
concentration. The appropriate RSF value for the above phosphorous implant is
1.07E23 atoms per cubic centimeter and the matrix current (IM) is 2.2E8 silicon ion
counts per second. The following figure shows the above phosphorous depth
profile plotted on depth and concentration axes.

17. Quantitative Analysis using
Relative Sensitivity Factors

18. 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)

19. Species Monitored
• The choice for impurity species for analysis is
usually concerned with detection limits, but
also involves sensitivity and dynamic range.
The best detection limit can be found with
knowledge of the mass interferences and the
relative sensitivity factors for atomic and
molecular ions. The use of rarer isotopes for
analysis can be an aid.

20. Ion Implantation
• The primary ion undergoes a continuous energy loss due to
momentum transfer, and to the electronic excitation of target
atoms.
• Thus, the primary ion is eventually implanted tens to hundreds of
angstroms below the surface.
• In general, then, the ion bombardment of a solid surface leads not
only to sputtering, but also to electronic excitation, ion
implantation, and lattice damage.

21. • Under typical SIMS conditions most of the sputtered material
(>95%) emanates from the uppermost two atomic layers, so
the sampling depth can be taken to be ~1 nm.
• Large organic fragments are more likely to come
predominantly from the surface monolayer, whilst the most
energetic atomic fragments may come from significantly
deeper layers.
• Primary ion bombardment also causes the emission of low
energy electrons and these can be detected to provide sample
visualization (topographic information) in a manner analogous
to a Scanning Electron Microscope (SEM).

22. Sputtering Yield
Sputtering yield is the average number of
sputtered particles per incident ion.
• Depends on the ion incident angle, the
energy of the ion, the masses of the ion
and target atoms, and the surface binding
energy of atoms in the target
Ion sputtering yield is the average number of Secondary ion cluster
ions emitted per incident primary ion. spectrum from Ar ion
Many factors affect the ion yield. The bombardment of Al. Note
most obvious are that the ordinate is in a log
scale. Predominant species
• Intrinsic tendency to be ionized
are Al+ ions; Al2+ and Al3+ are
Positive ion : Ionization potential (IP) also abundant
Negative ion: Electron affinity (EA)
• Matrix effects
Al+ from Al2O3 versus Al+ from Al metal

23. The 3 SIMS Analysis Modes
• Static SIMS are used to determine surface concentrations of
elements and molecules without significantly altering the
analyte.
• Imaging SIMS like static SIMS does not alter the analyte
appreciably. This mode is used to generate images or maps of
analytes based upon concentrations of one secondary ion
representing either an element or molecule.
• Dynamic SIMS involves the use of a much higher energy
primary beam (larger amp beam current). It is used to
generate sample depth profiles.

24. Static SIMS
• Low ion flux is used. This means a small amount of primary ions is used to
bombard the sample per area per unit time. Sputters away approximately
only a tenth of an atomic monolayer.
• Ar+, Xe+, Ar, and Xe are the commonly used particles present in the primary
particle beam, which has a diameter of 2-3 mm.
• The analysis typically requires more than 15 minutes.
• This technique generates mass spectra data well suited for the detection of
organic molecules.

25. Static SIMS: Organic Analytes
• Fragmentation and subsequent ion formation of the
sample can reveal the overall structure of the molecule
through mass spectrometry.
Polymethylmethacrylate SIMS (a) positve and (b) negative
SIMS data. -http://www.siu.edu/~cafs/surface/file6.html

26. Imaging SIMS
• The mass spectrometer is set
to only detect one mass.
• The particle beam traces a
raster pattern over the sample
with a low ion flux beam, much
like Static SIMS.
• Typical beam particles consists of Ga+ or In+ and the beam
diameter is approximately 100 nm.
• The analysis takes usually less than 15 min.
• The intensity of the signal detected for the particular mass
is plotted against the location that generated this signal.
• Absolute quantity is difficult to measure, but for a
relatively homogeneous sample, the relative
concentration differences are measurable and evident on
an image.
• Images or maps of both elements and organics can be
generated.

27. Images created using the Imaging SIMS mode.
Scanning ion image of granite from the Isle of Skye.
-University of Arizona SIMS 75 x 100 micrometers.

28. Dynamic SIMS
• The higher ion flux used in dynamic SIMS eats away at the
surface of the analyte, burying the beam steadily deeper into
the sample and generating secondary ions that characterize the
composition at varying depths.
• The beam typically consists of O2+ or Cs+ ions and has a
diameter of less than 10 μm.
• The experiment time is typically less than a second.
• Ion yield changes with time as primary particles build up on the
analyte effecting the ejection and path of secondary ions.

29. Dynamic SIMS generated depth Profile
Depth profile of hydrogen
embedded in Silicon.
The intensity of a secondary ion signal as a function of sputtering time is measured. By
suitable calibration measurements, this can later be translated into a concentration
depth profile.

30. The active part of a fast-switching bipolar transistor contains a single crystalline stack of Si / SixGel-x
/Si (see figure 1). The p-type dopant Boron is included during layer growth. The n-type dopant
Arsenic is introduced by deposition of an As-doped poly –crystalline Si layer. Upon a thermal
anneal, the As and B dopants diffuse, yielding the profiles shown in figure 1. The shape of the As
and B dopant profiles and their position with respect to the SixGe1-x layer are very critical for the
performance of the transistor. To define the position of the pn-junction, it is essential to measure
the n- and p-type dopant depth profiles simultaneously with the Ge concentration depth profile.

31. Instrumentation
Ion Sources
• Ion sources with electron impact
ionization - Duoplasmatron: Ar+,
O2+, O-
• Ion sources with surface
ionization - Cs+ ion sources
• Ion sources with field emission -
Ga+ liquid metal ion sources
Mass Analyzers
• Magnetic sector analyzer
• Quadrupole mass analyzer
• Time of flight analyzer
Ion Detectors
• Faraday cup
• Dynode electron multiplier

32. Selection of Primary Ions

33. Mass Analyzers
• The secondary mass spectrum is obtained by
collecting the secondary ions and subjecting
them to mass filtration prior to detection.
• Three types of mass filter (or mass analyser)
are used in SIMS – 1) Magnetic sector.
2) Quadrupole.
3) Time-of-flight.

34. Magnetic Sector Analyzer
R 2000
Capable: R ~ 105
High transmission efficiency
High mass resolution
Imaging Capability

35. MASS RESOLUTION
Charge
Mass

36. • The mass filter must be capable of separating secondary ions
that differ in mass by one atomic mass unit (amu or dalton)
over the whole mass range. The mass resolution or resolving
power (R) of the mass spectrometer is given by
R= m/∆m
• where m is the mass of the detected species and ∆ m is the
peak width. ∆ m is often quoted as the full width at half
maximum peak height (fwhm) but in the case of the
quadrupole it is more useful to quote ∆ mb, the full width at
10% maximum peak height i.e. the width at the base of the
peak.

37. Quantitative Analysis
• The secondary ion current Is for a selected ion of mass m (or
more correctly of mass/charge ratio m/z, where z is almost
always unity in SIMS) is given by
Is(m) = Ip y αT C(m)
where Ip is the primary ion current, y is the sputter yield, α is
the ionization probability, T is the overall transmission of the
energy and mass filters (i.e. the fraction of the sputtered ions
of a given mass which are actually detected), and C(m) is the
concentration of the detected species in the sputtered
volume.

38. • For a quadrupole ,T is of the order 0.1% and is approximately
inversely proportional to m. Therefore the sensitivity of the
instrument decreases as higher mass ions are selected by the
mass filter.
• y is the total yield of sputtered particles, neutral and charged,
of mass m per incident primary ion; y is typically between 1
and 20 for atomic species and it is a function of the mass and
energy of the primary ion and its angle of incidence (peaking
strongly at ~ 60º to the surface normal).
• Under the constant primary beam conditions in the
MiniSIMS, y only varies by a factor of 5 for different elements.

39. • α+ and α - are the respective probabilities that the sputtered
particle will be a positive or negative ion. Ionization
probabilities vary dramatically across the elements and are
additionally very sensitive to the electronic state of the
surface.
• The secondary ion yield (y) can therefore vary by over four
orders of magnitude for different elements and may be very
matrix sensitive.
– For example, the yield of Mg+ from clean Mg versus MgO is 0.01/0.9
(= 0.01) whereas the yield of W+ from clean W versus WO3 is
0.00009/0.035 (= 0.003).

40. Summary

41. Time-of-Flight Mass Spectrometer (TOF-MS)
• In a more sophisticated design,
the TOF analyser corrects for
small differences in initial energy
and angle in order to achieve high
mass resolution. Combinations of
linear drift paths and electrostatic
sectors or ion mirrors are used
and results with mass resolutions,
M/∆M, above 10,000 can be
achieved.
• Major advantages of this
approach over quadrupole and
magnetic sector type analysers
are the extremely high
transmission, the parallel
detection of all masses and the
unlimited mass range.

42. Time-of-Flight (TOF) SIMS

43. SECONDARY ION DETECTORS
• Most modern mass spectrometers have more than one
detector.
• The Cameca ims-4f has four secondary ion detectors; an
electron multiplier, a Faraday cup, an image plate and a
resistive anode Encoder.
• In the Cameca ims-1270 the instrument is equipped with
eleven detectors some of which can be used simultaneously
to give higher precision and rapid acquisition.

44. Electron Multiplier
• An electron multiplier consists of a
series of electrodes called dynodes.
Each dynode is connected to a
resistor chain.
• The first dynode is at ground
potential, so that both positive or
negative ions may be detected. The
last dynode can be between +1500 to
+3500 V depending on the age and
type of multiplier.
• When a particle (electron, neutral,
ion etc.) strikes the first dynode it
may produce a few (1, 2 or 3)
secondary electrons.
• These secondary electrons are
accelerated to the second dynode
that is held at a slightly higher
positive potential. On impact more
secondary electrons are generated
and a cascade of secondary electrons
ensues.

45. • For optimum performance, the electron multiplier should
operate at sufficiently high voltage so that every ion arrival
produces a pulse. This pulse is then amplified and as long as it
is above a set threshold, it will be passed to the counting
circuit.
• The time taken for the multiplier, amplifier and discriminator,
to process a pulse is known as the dead time (t). With fast
pulse-processing circuitry, this is in the order of 15 to 20 ns
and limits the electron multipliers maximum count rate to
about 5x106 c/s if the dead time correction is to be kept low.
• At a count rate of 5x105 c/s and a dead time of 25 ns the
percentage correction is about 1.3%.
• The true count rate (n) may be calculated from the observed
count rate (no) by the equation:

46. • Pulse counting detectors follow Poisson
statistics which require that each ion arrives
independently of all other ions. If, in a fixed
interval of time, n counts are detected, the
standard deviation of the measurement is
given by:
• and the relative standard deviation is given by:

47. Faraday Cups
• A Faraday cup detector can detect count rates from 5x104 c/s
upwards. Unlike the electron multiplier it does not
discriminate between the type of ion or its energy. It is simple
and cheap, but its response time is slow

48. • The Faraday cup detector consists of a hollow conducting
electrode connected to ground via a high resistance. The ions
hitting the collector cause a flow of electrons from ground
through the resistor.
• The resulting potential drop across the resistor is amplified. A
plate held at about -80 V in front of the collector, prevents any
ejected secondary electrons from escaping and causing an
anomalous reading.
• A single charge on a single ion is 1.6x10-19 C. Therefore a count
rate of 1x106 c/s would produce a current of 1.6x10-13 Amps.
With a resistor of 10 MW connected to ground, the amplifier
must be able to detect a potential drop of 1.6x10-6 (0.0016
mV).
• The detection limit of the Faraday cup is limited by the
thermal noise in the resistor and the quality of the amplifier.
Often these components will be enclosed within an
evacuated, thermally controlled chamber.

49. Technique Comparison
AES XPS D-SIMS TOF-SIMS
Probe Beam Electrons Photons Ions Ions
Analysis Beam Electrons Photons Ions Ions
Spatial 8nm 9µm 2µm 0.1µm
Resolution
Detection 0.1-5 atom% 0.01-.1 atom% 1ppm 1ppm
Limits
Quantification Good Excellent Challenging Challenging
Information Elemental Elemental Elemental Elemental
Content Chemical Molecular
bonding
Dept Profiling Excellent for Excellent for Excellent for Excellent for
small areas insulating Speed and sensitivity
materials Sensitivity

50. Surface Imaging using SIMS
• If the aim of the measurement is to obtain compositional
images of the surface formed from the secondary ion
spectrum with minimum possible damage to the surface, then
the main problem is to ensure that sufficient signal is
obtained at the desired spatial resolution whilst minimizing
the ion flux incident on any part of the surface.
• This is most easily achieved by switching from the traditional
instrumental approach of using continuous-flux ion guns and
quadrupole mass spectrometer detectors, to using pulsed ion
sources and time-of-flight (TOF) mass spectrometers. The TOF
mass spectrometers are a much more efficient way of
acquiring spectral data, and also provide good resolution and
sensitivity up to very high masses. Using such instruments,
SIMS images with a spatial resolution of better than 50 nm are
obtainable.

51. Imaging
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. High lateral resolution is required for mapping chemical elements.
197 AU 34 S
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 numerical scales and
associated colors represent different ranges of secondary ion intensities per pixel.

52. Cross-Sectional Imaging
Cross-sectional 27Al- Image depth profile of SiO2 capped GaAs/AlGaAs
superlattice with a 4 micrometer laser melt strip

53. Thank You for
Kind Attention
Any Questions?