Transcript Document

http://www.eaglabs.com/mc/sims-theory.html
http://www.youtube.com/watch?v=QTjZutbLRu0
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
 Basic Principles
Ip Primary
ion beam

Is
Depth Profiling
 Instrumentation
 Mass Resolution
 Modes of Analysis
 Applications
l
1 m
l
Solid sample and stable
in a vacuum
(Trace) element
analysis
Imaging and
mapping
http://www.youtube.com/watch?v=-7gSbaslRCU
to~0:50 SIMS sputtering
http://www.youtube.com/watch?v=UDFfEUZJ-Jo
Basic Principles: I. Ion Beam Sputtering
Primary ion beam:
Cs+, O2+, Ar+ and Ga+
at energies ~ a few keV.
Neutral & charged
(+/-) species
Ip
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.
Is
sample
Of ejected particles some are ionized (<10%), 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.
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.html#next
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.
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.
Instrumentation
http://www.eaglabs.com/mc/sims-instrumentation.html
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
Vacuum < 10−6 torr
• Magnetic sector analyzer
• Quadrupole mass analyzer
• Time of flight analyzer
Ion Detectors
• Faraday cup
• Dynode electron multiplier
Ion detectors
Is
Ip
Ion sources
Mass analyzers
SIMS CAMECA 6F
http://www.youtube.com/watch?v=IO-KCjxznLs
to~2:00
Basic Overview
http://www.youtube.com/watch?v=QTjZutbLRu0
at~0:45-1:45
Cameca SIMS
Duoplasmatron ion source
(O2)
http://en.wikipedia.org/wiki/Duoplasmatron
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.
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 (<50 nm) with moderate intensity, i.e., high spatial
resolution, which is important for mapping chemical
elements over the specimen surface.
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
Electrostatic
Sector
Mass Analyzer (MA)
Degree (r) of deflection of
ions by the magnetic filed
depends on m/q ratio.
Magnet Sector
Energy
Focal plane
mv2/r = qBv
High transmission efficiency
High mass resolution R  2000
Capable: R ~ 105
Imaging capability
r - radius of curvature of the path of the ion in the B field
http://www.youtube.com/watch?v=tOGM2gOHKPc&feature=relmfu
at~1:00-4:15
http://www.youtube.com/watch?v=lxAfw1rftIA
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?v=GSYueQzo2n8&feature=related
Time of Flight (TOF) SIMS - Reflectron
http://www.youtube.com/watch?v=TsxsVLcAGFY&feature=endscreen&NR=1
http://www.youtube.com/watch?v=ZoAUxsEBUnk
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?v=KAWu6SmvHjc
(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
Ion Detectors
http://www.eaglabs.com/mc/sims-secondary-ion-detectors.html#next
Faraday Cup
Secondary electron Multiplier
20 dynodes Current gain 107
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.
http://www.youtube.com/watch?v=k4mKDFPiBj8&list=PL212AE426663B340D
at~2:10-3:50
SIMS can do trace element analysis
Detection limit
is affected by
1 and 2 Static SIMS
3 Dynamic SIMS
Surface Analysis of Silicon Wafers
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.
Dynamic SIMS – Depth Profiling
Factors affecting
depth resolution
http://www.youtube.com/watch?v=-7gSbaslRCU&feature=related
Crater Effect
(a)
(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 analyzed area is usually required to
be at least a factor of 3  3 smaller than
the scanned area.
(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.
Sample Rotation Effect
Gate Oxide Breakdown
http://www.youtube.com/watch?v=IO-KCjxznLs&NR=1&feature=endscreen
2:10-2:45
Dynamic SIMS vs Static SIMS
http://www.youtube.com/watch?v=IO-KCjxznLs
at~2:45-3:18
http://www.youtube.com/watch?v=cspfWxnFwiM
3D TOF-SIMS
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.
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.
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 semiconductors 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.
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?
Next two lectures
• AES
• XPS
By Prof. Paul Chu