Secondary Ion Mass Spectrometry (SIMS)

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http//www.eaglabs.com/mc/sims-theory.html http//
www.youtube.com/watch?vQTjZutbLRu0
Secondary Ion Mass Spectrometry (SIMS)
Bombardment of a sample surface with a primary
ion beam (Ip) followed by mass spectrometry of
the emitted secondary ions (Is) constitutes
secondary ion mass spectrometry. SIMS is a
surface analysis technique used to characterize
the surface and sub-surface region of materials
and based on m/e ratio measurement of ejected
particles under ion bombardment.
To mass spectrometer
?
Ip
Primary ion beam
?

• Basic Principles
• Instrumentation
• Mass Resolution
• Modes of Analysis
• Applications

Is
Depth Profiling (Trace) element
analysis Imaging and mapping
1 ?m
l l
Solid sample and stable in a vacuum
http//www.youtube.com/watch?v-7gSbaslRCU
to050 SIMS sputtering
http//www.youtube.com/watch?vUDFfEUZJ-Jo
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Basic Principles I. Ion Beam Sputtering
Neutral charged (/-) species
Primary ion beam Cs, O2, Ar and Ga at
energies a few keV.
Ip
Is
Bombarding Ip (sputtering) produces monoatomic
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 ranging from zero to
a few hundred eV.
sample
Of ejected particles some are ionized (lt10),
these are the secondary ions. Only ions ejected
from the surface are employed for analysis.
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.
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Al from Al2O3 versus Al from Al metal
The ion sputtering yield of any considered
element varies with the alternation of other
components within the specimen surface.
http//www.eaglabs.com/mc/sims-seconday-ion-yields
-elemental.htmlnext
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Matrix Effects
DI I - ICLEAN
Absolute secondary ion yields as a function of
atomic
number, under high vacuum conditions (a) and
under


oxygen saturation (b) 3keV Ar
, incident angle 60o,
beam density 10-3 ?A/cm-2 pressure 10-10 Torr.
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Selection of Primary Ions
Oxygen works as a medium which strips off
electrons from the speeding sputtered atoms when
they leave surface, while Cesium prefers to load
an electron on the sputtered atoms.
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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

http//www.eaglabs.com/mc/sims-instrumentation.htm
l
Vacuum lt 10-6 torr
Ion detectors
Ip
Is
Ion sources
SIMS CAMECA 6F
Mass analyzers
http//www.youtube.com/watch?vIO-KCjxznLs
to200
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Basic Overview
http//www.youtube.com/watch?vQTjZutbLRu0
at045-145
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Cameca SIMS
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Duoplasmatron ion source
(O2)
http//en.wikipedia.org/wiki/Duoplasmatron
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Cs Ion Source
Cs ion sources are used to enhance negative ion
yield, such as C, O, and S etc. which is based on
the surface ionization of vapors on hot surfaces
and extraction of ions by an electric field.
In general Cs beams are smaller than those
generated by the duoplasmatron and sputter
material more effectively due to their greater
mass. However, the Cs gun is expensive to operate
and is only routinely used for O, S or C isotopic
analysis.
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Liquid Metal Ion Source (Ga or metal alloys)
W
Liquid metal ion source (LMIS), operates with
metals or metallic alloys, which are liquid at
room temperature or slightly above. The liquid
metal covers a W tip and emits ions under
influence of an intense electric field. The LMIS
provides a tightly focused ion beam (lt50 nm) with
moderate intensity, i.e., high spatial
resolution, which is important for mapping
chemical elements over the specimen surface.
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Magnetic Sector Analyzer
ESA bends lower energy ions more strongly than
higher energy ions. The sputtering process
produces a range of ion energies. An energy slit
can be set to intercept the high energy ions.
Sweeping the magnetic field in MA provides the
separation of ions according to mass-to-charge
ratios in time sequence.
E
Mass Analyzer (MA)
Magnet Sector
Degree (r) of deflection of ions by the magnetic
filed depends on m/q ratio.
Electrostatic Sector
Energy Focal plane
High transmission efficiency High mass resolution
R ? 2000 Imaging capability
mv2/r qBv
Capable R 105
r - radius of curvature of the path of the ion in
the B field
at100-415
http//www.youtube.com/watch?vtOGM2gOHKPcfeature
relmfu
http//www.youtube.com/watch?vlxAfw1rftIA
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Quadrupole Mass Filter
In a QMS the quadrupole is the component of the
instrument responsible for filtering sample ions.
It consists of 4 circular rods with a direct
current voltage and a superimposed
radio-frequency (RF) potential. The A rods are
connected and are at the same DC and superimposed
RF voltages. The same is true of the B rods but
in the opposite DC voltage with respect to the A
rods, and RF field is phase shifted by 180o. Ions
travel down the quadrupole between the rods. Only
ions of a certain mass-to-charge ratio m/z will
reach the detector for a given ratio of voltages
other ions have unstable trajectories and will
collide with the rods. This permits selection of
an ion with a particular m/z or allows the
operator to scan for a range of m/z-values by
continuously varying the applied voltage.
http//www.youtube.com/watch?vGSYueQzo2n8feature
related
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Time of Flight (TOF) SIMS - Reflectron
http//www.youtube.com/watch?vTsxsVLcAGFYfeature
endscreenNR1 http//www.youtube.com/watch?vZoA
UxsEBUnk
TOF SIMS is based on the fact that ions with the
same energy but different masses travel with
different velocities. Basically, ions formed by a
short ionization event are accelerated by an
electrostatic field to a common energy and travel
over a drift path to the detector. The lighter
ones arrive before the heavier ones and a mass
spectrum is recorded. Measuring the flight time
for each ion allows the determination of its mass.
http//www.youtube.com/watch?vKAWu6SmvHjc

• (TOF) SIMS enables the analysis of an unlimited
  mass range with high sensitivity and
  quasi-simultaneous detection of all secondary
  ions collected by the mass spectrometer.

Schematic of time of flight (TOF) spectrometer -
reflectron
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Ion Detectors
http//www.eaglabs.com/mc/sims-secondary-ion-detec
tors.htmlnext
A Faraday cup measures the ion current hitting a
metal cup, and is sometimes used for high current
secondary ion signals. With an electron
multiplier an impact of a single ion starts off
an electron cascade, resulting in a pulse of 108
electrons which is recorded directly. Usually
it is combined with a fluorescent screen, and
signals are recorded either with a CCD-camera or
with a fluorescence detector.
Faraday Cup
Secondary electron Multiplier 20 dynodes Current
gain 107
http//www.youtube.com/watch?vk4mKDFPiBj8listPL
212AE426663B340D at210-350
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SIMS can do trace element analysis
Detection limit is affected by
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1 and 2 Static SIMS
3 Dynamic SIMS
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Surface Analysis of Silicon Wafers
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Dynamic Secondary Ion Mass Spectrometry
Dynamic SIMS involves the use of a much higher
energy primary beam (larger amp beam current). It
is used to generate sample depth profiles. The
higher ion flux eats away at the surface of the
sample, 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 material effecting the ejection
and path of secondary ions.
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Dynamic SIMS Depth Profiling
Factors affecting depth resolution
http//www.youtube.com/watch?v-7gSbaslRCUfeature
related
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Crater Effect
(a) (b)
(a) Ions sputtered from a selected central area
(using a physical aperture or electronic gating)
of the crater are passed into the mass
spectrometer. (b) The beam is usually swept over
a large area of the sample and signal detected
from the central portion of the sweep. This
avoids crater edge effects.
The analyzed area is usually required to be at
least a factor of 3 ? 3 smaller than the scanned
area.
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Sample Rotation Effect
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Gate Oxide Breakdown
http//www.youtube.com/watch?vIO-KCjxznLsNR1fe
atureendscreen 210-245
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Dynamic SIMS vs Static SIMS
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http//www.youtube.com/watch?vIO-KCjxznLs at245
-318
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http//www.youtube.com/watch?vcspfWxnFwiM
3D TOF-SIMS
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Mapping Chemical Elements

• 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.

34 S
197 AU

• 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.

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Summary

• SIMS can be used to determine the composition of
  organic and inorganic solids at the outer 5 nm of
  a sample.
• To determine the composition of the sample at
  varying spatial and depth resolutions depending
  on the method used. This can generate spatial or
  depth profiles of elemental or molecular
  concentrations.
• These profiles can be used to generate element
  specific images of the sample that display the
  varying concentrations over the area of the
  sample.
• To detect impurities or trace elements,
  especially in semi-conductors and thin filaments.
• Secondary ion images have resolution on the order
  of 0.5 to 5 µm.
• Detection limits for trace elements range between
  1012 to 1016 atoms/cc.
• Spatial resolution is determined by primary ion
  beam widths, which can be as small as 100 nm.

SIMS is the most sensitive elemental and isotopic
surface microanalysis technique (bulk
concentrations of impurities of around 1
part-per-billion). However, very expensive.
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Review Questions for SIMS

• What are matrix effects?
• What is the difference between ion yield and
  sputtering yield?
• When are oxygen and cesium ions used as primary
  ions?
• What is mass resolution?
• How can depth resolution be improved?

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Next two lectures

• AES
• XPS

By Prof. Paul Chu