Download Secondary Ion Mass Spectrometry

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AP 5301/8301
Instrumental Methods of Analysis
and Laboratory
Lecture 8
Secondary ion mass spectrometry
(SIMS)
Prof YU Kin Man
E-mail: [email protected]
Tel:
3442-7813
Office: P6422
Lecture 9: outline
 Introduction: general features of secondary ion mass spectrometry
 SIMS theory:
─ Ion-solid interactions
─ Sputtering process
─ Ion yield
─ Quantification: relative sensitivity factor
 Instrumentation:
─ Ion sources
─ Mass spectrometer
─ Ion detector
─ Time-of-flight SIMS
 Common modes of SIMS:
─ Static SIMS
─ Dynamic SIMS
 Depth profiling:
─ Crater effects
─ Depth resolution
 Strengths and weaknesses
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Secondary ion mass spectrometry (SIMS)
A well established analytical technique that was first pioneered in 1949
SIMS is generally used for
surface, bulk, microanalysis,
depth profiling, and
impurity analysis.
Primary ion beam
(O-, O2+, Ar+, Cs+, Ga+ are often
used with energies between 1
and 30 keV)
http://atomika.com/
Primary ions are implanted and
mix with sample atoms to depths
of 1 to 10 nm.
The technique
bombarding
primary
ion beam produces
monatomic
and polyatomic
particles
involves
bombarding
the surface
of a sample
with a beam
of
of
sample
material secondary
and re-sputtered
primary
along
with electrons
ions,
thus emitting
ions. These
ions ions,
are later
measured
with a and
photons.
The secondary
particleseither
carrythe
negative,
positive,
and composition
neutral charges
mass spectrometer
to determine
elemental
or isotopic
of
and
they have
kinetic
energies that range from zero to several hundred eV.
the surface
of the
sample.
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SIMS analysis
Secondary ion mass spectrometry (SIMS) is a technique used to analyze the
composition of solid surfaces and thin films by sputtering the surface of the
specimen with a focused primary ion beam and collecting and analyzing
ejected secondary ions with a mass spectrometer to determine the elemental,
isotopic, or molecular composition of the surface to a depth of 1 to 2 nm.
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SIMS analysis
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Cameca IMS 6f secondary ion mass spectrometer
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SIMS: comparison with other techniques
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Ion-solid interaction
Cs+, O2+, Ar+ and Ga+
at energies ~ 1-30 keV
negative, positive, and neutral
charges with kinetic energies ranging
from zero to a few hundred eV.
Sputtered species:
 Monatomic and
polyatomic particles of
sample material (+ve,
-ve or neutral)
 Re-sputtered primary
species (+ve, -ve or
neutral)
 Electrons
 photons
Energy is transferred from the energetic primary ions to atoms in the sample.
Some of these atoms receive enough energy to escape the sample
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Sputtering
Sputtering is a process whereby particles are
ejected from a solid target material due to
bombardment of the target by energetic particles.
The kinetic energy of the incoming particles is
typically hundreds eV to keV, leading erosion of the
target materials.
Sputtering is commonly used as a
tool for thin film deposition
 Eroding material from a target
source onto a substrate using a
gaseous plasma (Ar)
targets
substrate
For thin film analysis:
 Mass analyze the sputtered ejected
ions─SIMS
 To expose atoms underneath the
surface for analysis─depth profiling
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The Sputtering Process
Sputter rates in typical SIMS experiments vary between 0.5 and 5 nm/s.
Sputter rates depend on sputter yield, which in turn depends on the primary
beam species, energy, intensity, sample material, and crystal orientation.
 Sputter yield: ratio of number of atoms sputtered
to number of impinging ions, typically 5-15
─ Commonly in SIMS, oxygen or cesium is used as a
primary ion source, which chemically changes the
surface and the sputter rate.
Sputter yields of silicon as a function of ion energy for
noble gas ions at normal incidence.
The variation of the sputter yield with angle
for the three metals. Below approximately 60
degrees, the sputter rate increases with
angle before passing through a maximum
Secondary ion yield
The number of secondary particles (atoms/ions) emitted by the surface for each
impinging primary ion is defined as sputtering yield and can range between 5
and 15. The fraction of ionized emitted particles is called secondary ion yield
and ranges typically between 10-4 to 10-6.
In SIMS, it is the secondary ions that are eventually detected by the mass
spectrometer
Secondary ion current of species 𝑚
𝐼𝑠𝑚 = 𝐼𝑝 𝑦𝑚 𝛼 + 𝜃𝑚 𝜂
𝐼𝑝 = primary particle flux
𝑦𝑚 = sputter yield
𝛼 + = ionization probability to positive ions
𝜃𝑚 = fractional concentration of m in the layer
𝜂 = transmission of the analysis system
Ion yield is influenced by
─ Matrix effects
─ Surface coverage of reactive elements
─ Background pressure
─ Orientation of crystallographic axes with respect to the sample surface
─ Angle of emission of detected secondary ions
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Secondary ion yields: primary ion beams
Secondary ion yield depends critically on the primary ion beam species. Typically
𝐶𝑠 + and 𝑂2+ ion beams are used in SIMS measurements.
 𝑶+
𝟐 ions beam:
─ Oxygen tends to bind with metal (Me)
atoms, if present in the sample.
─ During secondary emission the Me-O
bonds break thus generating 𝑀𝑒 𝑛+
 𝑪𝒔+ ions beam:
Selection of primary ions:
 Inert gas (Ar, Xe, etc.)
─ Minimize chemical modification
 Oxygen
─ Enhance positive ions
 Cesium
─ The implanted Cs ions lower the
─ Enhance negative ions
sample work function
 Liquid metal (Ga)
─ More secondary electrons are excited
─ Small spot for enhanced
over the surface potential barrier
lateral resolution
─ Increased availability of electrons
leads to increased negative ion
formation especially for elements with
high electron affinity.
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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Secondary ion yields: primary ion beam
 Oxygen bombardment increases the yield of positive ions
 Cesium bombardment increases the yield of negative ions.
The increases can range up to four orders of magnitude.
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Relative secondary ion yield
16.5 keV Cs+
13.5 keV O-
108
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Relative secondary positive ion yield
Relative secondary negative ion yield
107
106
105
104
105
104
103
103
102
102
0
10
20
30
40
50
60
Atomic number
70
80
90
10
20
30
40 50 60 70
Atomic number
80
90
100
Quantification in SIMS
One of the main obstacles preventing the derivation of a universal theory of the
secondary ion emission is a fact that the secondary ion yield of any chemical
element strongly depends on its chemical environment─matrix effect.
 This may cause variations in the ion yield over several orders of the
magnitudes, from one matrix to another.
 For example, yields of Al+ ions from Al2O3 and Al metal differ by a factor of 100;
Si+ ion emission from SiO2 is 2500x higher than that from Si.
The SIMS signal intensity for a particular element M (𝐼𝑀 ) is related to its
concentration in the analyzable layer (𝐶𝑀 ) by several parameters:
𝐼𝑀 = 𝐽𝑝 𝐴𝑆𝛽𝑀 𝑇𝐶𝑀
𝐽𝑝 = primary ion current
𝐴 = analyzed surface area
𝑆 = sputtering yield
𝛽𝑀 = secondary ion yield for element M
𝑇 = transmission of SIMS spectrometer
Since many of these parameters are not known, an approach based on
relative sensitivity factors is adopted in SIMS to evaluate atomic
concentrations of minor constituents when that of the major constituent is
known.
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Relative sensitivity factor (RSF)
For absolute quantification using SIMS, standards as similar as possible to
the real sample are needed. It is typical to use implanted samples as
standards.
 For example for the concentration profile of an impurity 𝑖 (𝐶𝑖 ) in a matrix
(𝑚𝑎𝑡), a standard with a known dose (𝐷 𝑖𝑛 𝑎𝑡𝑜𝑚𝑠/𝑐𝑚2 ) of 𝑖 in the same
matrix is created. So that the relative sensitivity factor
𝐷𝐶𝐼𝑚𝑎𝑡 𝑡
𝑅𝑆𝐹 =
,
𝑧𝐼𝑖
𝑠𝑡𝑑
where 𝐶 is the number of data cycles, 𝐼𝑚 is the matrix element secondary ion
intensity (counts/sec), 𝑡 is the count time/cycle, 𝑧 is the depth of the crater, 𝐼𝑖 is the
summation of secondary ion intensity of 𝑖 in counts.
𝐶𝑖 =
𝐼𝑖
𝐼𝑚𝑎𝑡
∙ 𝑅𝑆𝐹
𝑠𝑎𝑚𝑝𝑙𝑒
 Implanted standards have the advantages of:
─ Any element (isotope) can be implanted into any matrix
─ Depth and peak concentration can be tuned by the energy and the dose
─ Multiple element can be implanted
─ A detection limit can be established
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SIMS: ion implanted standards
 The procedure is based on the exposure, for a controlled time, of the
matrix to a beam of primary ions of the element of interest.
 The primary ion energy usually ranges between 50 and 300 keV, whereas
the dose is about 1013-1016 ions/cm2.
 After implantation the sample is analyzed under Dynamic SIMS conditions
and the element signal is monitored as a function of time (e.g. of depth
reached due to erosion).
 After the SIMS measurement
the crater is measured to reveal
the real depth
 the implantation dose/crater
depth ratio provides an estimate
for the average atomic
concentration (atoms/cm3) of
the element in the matrix
 A RSF can be established
𝐷𝐶𝐼𝑚𝑎𝑡 𝑡
𝑅𝑆𝐹 =
𝑧𝐼𝑖
𝑠𝑡𝑑
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SIMS: 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
Vacuum < 10−6 torr
SIMS CAMECA 6F
Schematic Diagram of a SIMS instrument
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Ion source: Duoplasmatron
A duoplasmatron is an ion source with electron impact ionization
 A cathode filament emits electrons into a vacuum chamber
 Small quantity of gas (Ar, O2, Ne, etc.) leaks into the chamber and interacts
with the electrons forming a plasma
 The plasma is accelerated through a series of highly charged grids to the
desired energy and extracted through the aperture.
 It can operate with almost
any gas
 When O2 is used, O-, O2- or
O2+ can be extracted
depending on the electrical
polarity selected
 Probe diameter typically
between 5 mm to 1 mm
 Ion current densities >10
mA/cm2
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Ion source: Cs+ source
 Cs metal (or compound) is heated in the reservoir (~400oC) forming a vapor
 The Cs vapor flows through a feed tube to a porous tungsten plug
 The Cs vapor diffuses through the pores in the plug to the front of surface
which is maintained at >1100oC by the ionizer heater
 The Cs atoms are ionized
during evaporation because
the work function of W (4.52
eV) is substantially greater
then the ionization potential of
Cs (3.88 eV)
 The Cs+ ions are extracted and
accelerated to an energy up to
10 keV.
 Depending on the gun design,
fine focus or high current can
be obtained.
 Cs gun is typically more
expensive to operate
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Ion source: Liquid Metal Ion Source (LMIS)
Capillary
W
500 mm
 Operates with low melting point metals or metallic alloys, which are liquid at
room temperature or slightly above (Ga, Cs).
 The liquid metal covers a W tip and emits ions under influence of an intense
electric field.
 Ion current densities > 1A/cm2 with sub mm probe diameter.
 Beam can be focused to <50 nm with moderate intensity and rastered to provide
secondary electron image or elemental mapping over the specimen surface.
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Dual source SIMS
 Many SIMS spectrometers are
equipped with two sources,
usually a Cesium gun and an
Oxygen Duoplasmatron
source.
 A mass filter (typically a
quadrupole), enables the
selection of the ion of interest.
 Selected ions are then focused
and accelerated towards the
sample by electrostatic
lenses.
 In the final stage of the dual
source electrostatic deflectors
drive primary ions towards
specific regions of the sample
surface.
Electrostatic Analyzer and Mass Spectrometer
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The sputtering process produces ions with 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.
 ESA is to minimize fluctuation
of kinetic energy of ions so as
to reduce the interference of
ions, providing a higher
mass resolution of mass
spectrometers
 All paraxial ions of particular
energy will follow the central
lines to be focused in a plane
of the ESA slit
 Fluctuation in kinetic energy
of ions is substantially
suppressed
Magnet Sector
 The mass analyzer select the particular
species according to the mass-to-charge ratio
2
𝑚 = 𝐵 𝑟2
𝑞 2𝑉
where B is the magnetic field, V is the ion
accelerating voltage, r is the radius of curvature
of the ion
Mass spectrometer
For an analogy, think of how a prism
refracts and scatters white light separating it
into a spectrum of rainbow colors.
In a mass spectrometer, ions travel different paths
through the magnet to the detector due to their
mass/charge ratios. A mass analyzer sorts the
ions according to mass/charge ratios and the
detector records the abundance of each ratio.
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Ion Detectors
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.
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Mass resolution
 Mass resolution is usually specified in terms of 𝑚/∆𝑚 where 𝑚 is the mass of
the ion and ∆𝑚 is the FWHM of the detected signal.
─ For example, 56Fe+ and 28Si2+ (𝑚/𝑞=55.9349 and 55.9539) require 𝑚/∆𝑚 of
2,950 for separation while Au and 133Cs32S2 (𝑚/𝑞=196.9666 and 196.8496)
require 𝑚/∆𝑚 of 1700.
∆𝒎
FWHM
𝑚/∆𝑚 for the two
species is 21160
 Typically a higher mass resolution will accompany a loss of ion intensity
Time of flight SIMS
 Time-of-Flight SIMS (ToF-SIMS) uses a pulsed ion beam to remove
molecules from the very outermost surface of the sample. These particles are
then accelerated into a "flight tube" and their mass is determined by
measuring the exact time at which they reach the detector (i.e. time-of-flight).
 ToF-SIMS is based on the fact that ions with the same energy but different
masses travel with different velocities.
 mass resolutions >18,000 can be achieved
 It also has extremely high transmission with the parallel detection of all
masses and the unlimited mass range.
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Time-of-flight mass analyzer
𝑙
During a short pulse of ion beam,
sputtered ions are accelerated
and acquire a constant kinetic
energy:
𝐾𝐸 = 𝑚𝑣 2 /2
with different 𝑚/𝑞 and velocity 𝑣.
The ions arrive to the detector in
time sequence (𝑡) after traveling
a distance 𝑙.
𝑙
𝑙
𝑡= =
𝑣
2𝑞𝑉 2
𝑚
𝑚 2𝑉𝑡 2
= 2
𝑞
𝑙
In order to provide higher resolution the pulse should be as narrow as 1-10 ns.
The pulse repetition frequency is usually in a kHz range. Typical flight times
10 ns to 800 µs
Reflectron ToF spectrometer
The kinetic energy distribution in the direction of ion flight can be corrected by
using a reflectron. The reflectron uses a constant electrostatic field to reflect
the ion beam toward the detector.
 The more energetic ions penetrate deeper into the reflectron, and take a
slightly longer path to the detector.
 Less energetic ions of the same mass-to-charge ratio penetrate a shorter
distance into the reflectron and, correspondingly, take a shorter path to the
detector.
 Twice the flight path is achieved in a given length of instrument.
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SIMS: modes of operation
According to the primary ion energy and current, the SIMS technique can be
divided into two variants:
 Static SIMS: 0.1-10 keV ions are
employed, with current surface
densities in the nA/cm2 range,
Under these conditions the total
erosion of the sample first
monolayer (1 nm) may take even
an hour.
 Dynamic-SIMS: 10-30 keV ions,
with current surface densities in
the mA-mA/cm2 range, are used.
Under these conditions the sample
is eroded continuously and the
acquired mass spectra enable the
monitoring of constituting elements
along the sample depth (depth
profiling).
 Ultra surface
analysis
 Elemental or
molecular analysis
 Analysis completed
before significant
fraction of molecules
destroyed
 Profiling
 Material removal
 Elemental analysis
Static SIMS
 Under the ion bombardment, fragment ions or
even intact molecular ions are emitted from
the top monolayer.
 If the primary ion dose is limited to a level at
which every primary ion should (statistically)
always hit a fresh area, the (static) SIMS
spectrum reveals molecular information.
 Progressively, as the ion dose increases, the molecular signal decreases then
vanishes when the whole area has been damaged.
 To stay in static SIMS mode, the primary ion dose must be < 𝟏𝟎𝟏𝟐 𝒊𝒐𝒏𝒔/𝒄𝒎𝟐
 Static SIMS gives rise to a fingerprint mass spectrum that contains "low mass"
(< 500 amu) ion fragments, identifying organic surface composition.
 Due to the complexity of the static SIMS mass spectrum, it is mostly used as a
qualitative characterization of the molecular composition of the top surface.
 By focusing and scanning the primary ion beam, molecular information can be
obtained with sub-micron lateral resolution, and molecular surface distribution
can be imaged.
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Static SIMS
 Range of elements
H to U: all isotopes
 Destructive
Yes, if sputtered long enough
 Chemical bonding
Yes
 Depth probed
Outer 1 to 2 monolayers
 Lateral resolution
Down to below 100 nm
 Imaging/mapping
Yes
 Quantification
Possible with suitable standard
 Mass range
Typically up to 1000 amu, 10000 amu (ToF)
 Main application
Surface chemical analysis, organics, polymers
Positive ion TOF mass spectrum of polydimethylsiloxane
contaminated polyethylene terephthalate
Silicon wafer contaminated with copper, iron and chromium
Dynamic SIMS
 Ion dosage and sputter rates are high resulting more
fragmentation.
 Must be equipped with Oxygen and Cesium primary
ion beams in order to enhance, respectively, positive
and negative secondary ion intensity by 2 to 3 orders
of magnitude compared to the use of noble gas ions.
 As the primary ion dose implanted in the target increases, the primary
species concentration (oxygen or cesium) will reach an equilibrium and this
corresponds to a sputtering steady state when reliable quantification is
possible with reference standard samples, using RSF.
 One of the main application of dynamic SIMS is the in-depth distribution
analysis of trace elements (for example, dopant in semiconductors).
 Impact ion energy is adjusted depending on the applications.
─ Low energy (down to 150eV) is used to reduce atomic mixing and
improve depth resolution down to the sub-nanometer level.
─ High energy (up to 20 keV) is chosen to investigate deeper (10-20
microns), faster (sputter rate of µm per min range), and improve
detection limits and image resolution.
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Dynamic SIMS
 Range of elements
H to U: all isotopes
 Destructive
Yes, material removed during sputtering
 Chemical bonding
In rare cases only
 Depth probed
Depth resolution 2-30 nm, probe into mm below surface
 Quantification
Standard needed
 Accuracy
2%
 Detection limits
1012-1016 atoms/cm3 (ppb-ppm)
 Imaging/mapping
Yes
 Sample requirements Solid; vacuum compatible
Near surface B depth profiles
from a 2.2 keV BF implant in
Si using different energies O2+
primary beam
SIMS depth profiling: example
Phosphorus doped Silicon
Sputter time: 700 sec
Depth: 9310 Å
Erosion rate:13.3 Å/sec
Using an ion implanted
sample: P dose 1015 P/cm2
RSF: Relate the intensity to
atomic concentration:
𝑑𝑜𝑠𝑒
𝑅𝑆𝐹 =
𝐼 𝑥 𝑑𝑥
RSF: 1counts/s=3.4x1015 P/cm3
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Dynamic SIMS – Depth Profiling
Factors affecting depth resolution:
 Crater edge rejection:
─ Raster beam for flat bottomed crater
─ Accept ions only from the center of crater
 Ion beam mixing
─ Primary ion mass
─ Impact energy
─ Impact angle
 Surface roughness
─ Metal worse than single crystal materials
The depth profile can be affected by:
 Redeposition by sputtering from
the crater wall onto the analysis
area
 Direct sputtering from the crater
wall
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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 much 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.
Depth resolution
Simulation of the effect of 1% and 10%
unevenness on crater bottom for a sinusoidal
dopant distribution, according to the uneven
etching model
D. S. McPhail, et al., Scanning Microscopy 2, 639 (1989)
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1.5 keV O2+ beam incident at 60° from
normal on a delta doped Be in GaAs sample
FWHM depth
resolution <3 nm
Reducing ion penetration depth can reduce
effects of ion mixing, this can be achieved by
─ Larger angle of incidence from normal
─ Lower bombarding energy
─ Increased mass of primary beam
Luftman et al., J. Vac. Sci. Technol. B10, 323 (1992)
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Sample Rotation Effect
SEM micrographs of a)
aluminum surface, b) bottom of
crater sputtered through 1 μm
aluminum layer into underlying
silicon without rotation and c)
with rotation
F. A. Stevie and J. L. Moore, Surf. Interf.
Anal. 18, 147 (1992)
No rotation
Al
Al
B
Si
With rotation
Si
B
SIMS profiles of 11B ion implantation into 1 μm Al/Si. With sample rotation, B at interface is clearly
defined and silicon from Al-Si-Cu layer shows movement to Al/Si interface
Reduction of preferential sputtering of different grains of polycrystalline materials
Sensitivity and resolution
The SIMS detection limits for most trace elements are between 1012 and
1016 atoms/cm3.
 The primary limiting factor is ionization efficiencies.
 The dark current (or dark counts) arises from stray ions, electrons in
vacuum systems, and from cosmic rays
 Count rate limited sensitivity:
─ When sputtering produces less secondary ion signal than the detector dark current.
─ If the SIMS instrument introduces the sample element, then the introduced level
constitutes background limited sensitivity, e.g. Oxygen, present as residual gas in
vacuum systems
 Atoms sputtered from mass spectrometer parts by secondary ions constitute
another source of background.
Typically, sensitivity and depth resolution cannot be optimized simultaneously
 Best sensitivity is achieved with high sputtering rate and large detected
area
 Best depth resolution is achieved with low impact energy, reduced ion
penetration into sample, low sputtering rate and small detected area
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Detection Limits: in InP, GaAs, GaN
For electropositive elements:
Element
Li
Be
B
Na
Mg
Al
K
Ca
Ti
V
Cr
Mn
Fe
Ni
Cu
Zn
Sr
Y
Zr
Nb
Mo
Cd
In
M+ (O2+)
3E13
3E14
1E15
3E14
1E14
2E15
2E14
3E14
2E14
1E14
1E15
3E14
1E15
1E16
3E16
1E16
5E15
1E17
1E15
1E16
1E16
5E16
3E15
M- (Cs+)
1E16
1E20
3E15
2E17
1E20
1E17
2E18
1E20
1E18
1E17
2E17
1E18
3E17
5E17
1E16
1E20
1E20
1E20
4E17
1E18
1E18
1E21
3E17
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(atoms/cm3)
For electronegative elements:
Element
H
C
N
O
F
P
Si
S
Cl
Ge
Se
Br
Te
Ag
Au
M- (Cs+)
2E17
1E16
5E15 (NGa-)
1E16
2E14
2E15
2E15
1E15
3E15
5E15
5E14
5E13
1E15
2E16
1E15
M+ (O2+)
2E18
2E18
5E18
1E20
5E16
1E16
1E16
1E19
2E17
2E16
2E17
1E17
2E17
2E16
1E17
Comparison: static and dynamic SIMS
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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.
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45
Imaging SIMS
Scanning ion image of granite from the Isle of Skye.
-University of Arizona SIMS 75 x 100 micrometers.
Imaging SIMS
Detection of micrometric spots due to an organic contaminant (pentaerythritoltetraoctanoate, C37 H68 O8 , a lubricant) on an hard disk surface.
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Charging of insulating samples in SIMS
 A positive charge is accumulated on the sample surface during a
SIMS analysis, due to ionic bombardment.
 In insulating samples this charge cannot be neutralized by electrons
drawn from the ground through the sample stage.
 Sample charging diffuses the primary beam and diverts it from the
analytical area, changes the energy distribution and direction of
secondary ions.
 Several techniques are available to manage sample charging:
─ Flooding the sample surface with a low energy (a few eV) electron
beam, like in the case of XPS
─ Placing a conducting grids over the sample, similarly samples are
often coated with conducting materials such as gold or carbon.
─ Bombarding the sample with negative ions (for example O-)
─ Applying a continuously variable voltage offset to the accelerating
voltage for samples that are only slightly charging.
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Example: Gate oxide breakdown
48
Example: GaAs quantum well structure
Negative secondary ions with 5keV
Cs primary ion bombardment
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Example: ion beam mixing in isotope superlattice
310 keV Ga+ 1.0 × 1015 /cm2
SIMS concentration profiles of the stable isotopes 74Ge (upper
solid line) and 70Ge (lower solid line) in crystalline (natGe/70Ge)10
and amorphous (natGe/73Ge)10 as-grown multilayers
 The structures were implanted with 310 keV
Ga ions with a dose of 1.0 × 1015 and 3.3 ×
1014 /cm2.
 Self-atom mixing in crystalline Ge is mainly
controlled by radiation enhanced diffusion
during the early stage of mixing before the
crystalline structure turns amorphous
310 keV Ga+ 3.3 × 1014 /cm2
Bracht et al., J. Appl. Phys. 110, 093502 (2011).
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Example: self-diffusion un GaSb
Ga and Sb profiles of the
as-grown 69Ga121Sb/
71Ga123Sb heterostructure
After annealing the
isotope structure under
Sb-rich conditions at
700oC for 105 min
After annealing at 700oC
for 18 days
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Near the melting
temperature, Ga diffuses
more rapidly than Sb by
over three orders of
magnitude. This surprisingly
large difference in atomic
mobility is a consequence of
reactions between defects
on the Ga and Sb
sublattices, which suppress
the defects that are required
for Sb diffusion.
Bracht et al., Nature 408, 69 (2000).
Advantages and weaknesses of SIMS
Advantages
Weaknesses
 Excellent sensitivity, especially for
light elements
 Destructive method
 High surface sensitivity
 Element specific selectivity
 Depth profiling with excellent depth
resolution (nm) (dynamic)
 Standards needed for quantification
 Good spatial resolution (<1-25 mm)
 Sample must be vacuum compatible
 Small analyzed volume (down to
0.3mm3) so little sample is needed
 Sample mist have a flat surface
 Information about the chemical
surface composition due to ion
molecules (static)
 High equipment cost (>1M-3M USD)
 Elements from H to U can be
detected with excellent mass
resolution
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