Download ION BEAM TECHNIQUES • Ion beam characterization techniques

ION BEAM TECHNIQUES
• Ion beam characterization techniques are illustrated in Fig. 11.21.
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ION BEAM TECHNIQUES
• Incident ions are absorbed, emitted, scattered, or reflected leading
to light, electron or X-ray emission.
• Aside from characterization, ion beams are also used for ion
implantation.
• We discuss two main ion beam material characterization methods
1. Rutherford backscattering spectrometry (RBS)
2. Secondary ion mass spectrometry (SIMS)
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2. Secondary Ion Mass Spectrometry (SIMS)
• Secondary ion mass spectrometry, also known as ion microprobe
and ion microscope, is one of the most powerful and versatile
analytical techniques for semiconductor characterization.
• It was developed independently by Castaing and Slodzian at the
University of Paris and by Herzog and collaborators at the GCA Corp.
in USA in the early 1960s.
⇒ One of the major purposes: moon rock analysis.
• The technique is element specific and is capable of detecting all
ions
elements as well as isotopes and molecular species.
• Lateral resolution is typically 100 μm, but can be as small
as 0.5 μm with depth resolution of 5 to 10 nm.
RBS
• The depth resolution is on the order of 10 ~ 20 nm for film thicknesses ≤ 200 nm.
• Beam diameters are commonly around 1 to 2 mm but microbeam backscattering
with beam diameters as small as 1 μm is possible.
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Hydrogen Isotopes
# of neutrons
zero
one
two
Atomic mass
1.0 u
2.0 u
3.0 u
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Working mechanism
• A primary ion beam impinges on the sample and atoms from the
sample are sputtered or ejected from the sample.
• Most of the ejected atoms are neutral and cannot be detected by
conventional SIMS, but some are positively or negatively charged.
⇒ This fraction was estimated as about 1 % of the total.
• RBS: around 10−6 of the number of incident ions backscattered (elastically).
• The mass/charge ratio of the ions is analyzed, detected as a mass
spectrum, as a count, or displayed on a fluorescent screen.
• The detection of the mass/charge ratio can be problematic, since
various complex molecules form during the sputtering process
between the sputtered ions and light elements like H, C, O, and N
typically found in SIMS vacuum systems.
⇒ The mass spectrometer only recognizes the total mass/charge
ratio and can mistake one ion for another.
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• The basis of SIMS, is the destructive removal of material from the
sample by sputtering and the analysis of the ejected material by
a mass analyzer.
Static mode
Image mode
Dynamic mode
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A%2F%2Fwww.geos.ed.ac.uk%2Ffacilities%2Fionprobe%2FSIMS4.pdf&ei=tn21UIuwKI7KmAX9tIC4Aw&u
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Primary ion sources
Three basic types of ion guns are employed.
1. Ions of gaseous elements: for instance noble gases (Ar+, Xe+), oxygen
(O-, O2+), or even ionized molecules such as SF5+ (generated from
SF6) or C60+. (by duoplasmatrons or electron ionization)
⇒ This type of ion gun is easy to operate and generates roughly
focused but high current ion beams.
=> An electron ionization ion source is typically used to ionize noble
gas atoms such as He, Ne or Ar.
⇒ Oxygen primary ions are often used to investigate electropositive
elements due to an increase of the generation probability of
positive secondary ions.
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SIMS Apparatus: ion gun (1)
(Ions of gaseous elements)
Electron ionization
(Ar)
(Ar+)
• a cathode: e- into a vacuum chamber.
• Ar is introduced (very small quantities).
Ar + eAr+ + 2eforming a plasma.
• The plasma is then accelerated through a series of highly charged
grids, and becomes a high speed ion beam.
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Primary ion sources
2. The surface ionization source, generates Cs+ primary ions. Cesium
atoms vaporize through a porous tungsten plug and are ionized
during evaporation. Depending on the gun design, fine focus or
high current can be obtained.
⇒ while cesium primary ions are often used when electronegative
elements are being investigated.
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Cesium surface ionization source (2)
• Cs is heated to approximately 900°C => Cs vapor is formed.
• Cs vapor moves to an enclosed region between the cathode, which is
cooled, and the ionizer, which is heated.
• Some of Cs condenses onto the cool surface of the cathode, some of
Cs comes in contact with the ionizer surface (immediately ionized).
• Cs+ leaves the ionizer is accelerated toward and focused onto the
cathode, sputtering material from the cathode at impact.
• Some of the sputtered material
gains an electron in passing
through Cs coating on the
surface of the cathode, and
forms the negatively charged
beam. This negative beam is
accelerated out of the source.
heated
cool
electropositive
the University of Notre Dame
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3. The 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 tungsten
tip and emits ions under influence of an intense electric field.
⇒ LMIS: a field ion emission source
⇒ Generate highly bright positive ion beams from neutral
atoms or molecules.
• Taylor cone's tip sharper
⇒ the electric field becomes
stronger.
• LMIS: particularly used in ion
implantation or in focused
ion beam (FIB).
W
http://www.hindawi.com/journals/ijae/2011/361215/fig2/
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⇒ A gallium source (able to operate with elemental gallium), recently
developed sources for gold, indium and bismuth use alloys which
lower their melting points.
• Ga: Tm: ~ 30 °C, liquid at rt.
• Low vapor pressure: good for high vacuum
⇒ The LMIG provides a tightly focused ion beam (< 50 nm) with
moderate intensity and is additionally able to generate short
pulsed ion beams. It is therefore commonly used in static SIMS.
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• The secondary ion yield is significantly lower than the total yield, but
can be influenced by the type of primary ion.
⇒ Electronegative oxygen (O2+) enhances species for electropositive
elements (e.g., B and Al in Si) which produce predominantly
positive secondary ions.
⇒ O2+ easily strip off electrons from the sputtered atoms.
⇒ Electronegative elements (e.g., P, As and Sb in Si) have higher
yields when sputtered with electropositive ions like cesium (Cs+).
⇒ Cs easily to donate an electron to the sputtered atoms.
• The secondary ion yield
for the elements varies
over five to six orders of
magnitude.
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SIMS: sputtering process
Primary ions
Sputtered particles:
Positive/negative or neutral
Vacuum
tens of nm
Cascade mixing (later)
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• Sputtering is a process in which incident ions lose their energy
mainly by momentum transfer as they come to rest within the solid.
(displace atoms within the sample.)
• The primary ion loses its energy in the process and comes to rest
tens of nm below the sample surface.
• Sputtering takes place when atoms near the surface receive
sufficient energy from the incident ion to be ejected from the
sample.
• The escape depth of the sputtered atoms is generally a few
monolayers for primary energies of 10 to 20 keV typically used in
SIMS.
• Ion bombardment leads not only to sputtering, but also to ion
implantation and lattice damage.
• The sputtering yield is the average number of atoms sputtered per
incident primary ion; it depends on the sample or target material,
its crystallographic orientation, and the nature, energy, and
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incidence angle of the primary ions.
• The yield for SIMS measurements with Cs+, O2+, O−, and Ar+ ions of
1 to 20 keV energy ranges from 1 to 20 (sputtered atoms per ion).
• Sputtering yield (Y) = mean number of emitted atoms/incident particle
1. mass, energy of incident ion beam
2. properties (structure, composition, etc.) of target
3. incident angle
Sputtering yield of Si as a function of energy of Ar+
Why?
Energy
Prof. F. Ernst
Case Western Reserve University
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The yield usually increases with the ion energies.
• However, sometimes, at very high energies
=> the yield decreases because of the increasing penetration
depth; thus, increasing energy loss below the surface
=> not all the bombarded atoms are able to reach the surface
to escape.
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Sputtering yield of Si as a function of different ion mass
• What is important, however, is not the total yield, but the yield of
ionized ejected atoms or the secondary ion yield, because only ions
can be detected.
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RBS vs. SIMS
Different primarily in the energy range of ion beam
RBS: high energy ions (MeV)
A small fraction (10−6 ) : elastic
collisions, backscattered
SIMS: low energy ions (keV)
1 % charged ions
Prof. F. Ernst
Case Western Reserve University
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Energy loss of energetic ions
• Several semi-empirical stopping power formulas have been developed.
• The most popular: the Ziegler, Biersack and Littmark model.
Prof. Kai Nordlund
• Both charged and uncharged particles lose energy while passing
through matter. The stopping power depends on the type and
energy of the radiation and on the properties of the material it
passes.
J. Lindhard, M. Scharff, and H. E. Shi|tt. Range concepts and heavy ion ranges.
Mat. Fys. Medd. Dan. Vid. Selsk., 33(14):1, 1963.
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• Electronic stopping refers to the slowing down of a projectile ion
due to the inelastic collisions between bound electrons in the
medium and the ion moving through it. The collisions may result
both in excitations of bound electrons of the medium, and in
excitations of the electron cloud of the ion as well.
• Number of collisions an ion experiences with electrons is large.
• In the beginning at high energies, the ion slows down mainly by
electronic stopping. When the ion has slowed down sufficiently, the
collisions with nuclei become more and more probable, finally
dominating the slowing down.
• Nuclear stopping power refers to the interaction of the ion with the
nuclei in the target. Nuclear stopping increases when the mass of
the ion increases. Nuclear stopping is larger than electronic stopping
at low energy. However, for very light ions slowing down in heavy
materials, the nuclear stopping is weaker than the electronic at all
energies.
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• At extremely high ion energies, one also has to consider radiative
stopping power which is due to the emission of bremsstrahlung
in the electric fields of the particles in the material traversed.
Important only for electrons.
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Preferential sputtering issue (1)
• Selective or preferential sputtering can occur in multi-component or
polycrystalline targets when the components have different
sputtering yields.
• The component with lowest yield becomes enriched at the surface
while that with the highest yield becomes depleted.
⇒ However, once equilibrium is reached, the sputtered material
leaving the surface has the same composition as the bulk material
and preferential sputtering is not a problem in SIMS analysis.
⇒ as time progresses, a steady state is reached and eventually
produces sputtering yields representing the bulk concentration.
• Rich sputtered species to allow direct measurement of composition.
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Matrix effect (2)
• SIMS has not only a wide variation in secondary ion yield among
different elements, it also shows strong variations in the secondary
ion yield from the same element in different samples or matrices
- the matrix effect (2).
• For example, the secondary ion yield for oxidized surfaces is higher
than for bare surfaces by as much as 1000.
• A striking example is a SIMS profile of B or P implanted into oxidized
Si obtained by sputtering through an oxidized Si wafer.
⇒ The yield of Si in SiO2 is about 100 times higher than the yield of
Si from the Si substrate.
⇒ A plot of yield versus sputtering time shows a sharp drop when
the sample is sputtered through the SiO2-Si interface.
Si
⇒ Chemical bonding environment.
SiO2
(B or P) Si
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Cascade mixing (3)
• For SIMS, the most important type of atomic mixing is “cascade
mixing,” resulting from primary ions striking sample atoms and
displacing them from their lattice positions, leading to
homogenization of all atoms within the depth affected by the
collision cascade.
⇒ Dopant atoms originally present at a given depth in the sample
will distribute throughout this “mixing depth” as sputtering
proceeds and the dopant profile will give a deeper distribution
than the true distribution.
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Cascade mixing
Incident primary ion
θ
Secondary ions
Transient
depth
Cascade
mixing
Steady state
depth
staff.science.nus.edu.sg/~pc4250/2006/lectures/1.%20SIMS.ppt
• It is important that the primary ion penetration depth be kept to
a minimum for shallow dopant profiling.
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Side-wall effect (4)
• Another instrumentation effect that complicates SIMS analysis is
the edge or wall effect.
• To obtain good depth resolution, it is important that only the signal
from the flat, bottom portion of the sputtered crater be analyzed.
⇒ Atoms are also ejected from the crater bottom as well as from the
sidewalls during sputtering. But the sidewalls of an ion-implanted
sample, especially near the top surface, contain a much higher
doping density than the crater bottom.
• If secondary ions from the edge of the crater are included in the
analytical signal, the depth profile will have poor depth resolution.
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How to avoid side-wall effect
1. the detectors are only open, when the beam reaches a central
region of the sputter-crater. With this method it is possible to
avoid edge effects and to get only a signal of a defined
sputter-depth.
2. Using electronic gating of the secondary ion yield signal or a lens
system, it is possible to detect only those ions from the central part
of the crater.
SIMS crater
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• A high vacuum is very important for SIMS (10−6 torr).
⇒ This is needed to ensure that secondary ions do not collide with
background gases on their way to the detector.
⇒ It also prevents surface contamination by adsorption of
background gas particles during measurement.
⇒ This is particularly important for low mass species like hydrogen.
Standards
• The usual approach is one of using standards with composition and
matrices identical or similar to the unknown.
• Ion implanted standards are very convenient and also very accurate.
⇒ The implant dose of an ion-implanted standard can be controlled
to an accuracy of 5% or better.
• When such a standard is measured, one calibrates the SIMS system
by integrating the secondary ion yield signal over the entire profile.
• Calibrated standards are, therefore, very important for accurate
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SIMS measurements.
Static mode
Image mode
Dynamic mode
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SIMS can give three types of results.
1. Static SIMS (surface analysis)
• The aim is to obtain sufficient
signal to provide a compositional
analysis of the surface, without
actually removing a significant
fraction of a monolayer.
• This requires the use of very low
ion fluxes (around 1012 cm-2) to
ensure that each ion is statistically-likely to impact upon fresh,
undamaged surface and that the sputtered secondary ions are
representative of the original surface, rather than surface that has
already been "damaged" by earlier ion impacts.
• In this form, the technique is capable of providing information about
adsorbed molecular layers or the topmost atomic layer of the solid
surface.
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http://www.chem.qmul.ac.uk/surfaces/scc/scat5_5.htm
2. Dynamic SIMS (Depth Profiling)
• The aim of depth profiling is to obtain information on the variation
of composition with depth below the initial surface. Such information
is obviously particularly useful for the analysis of layered structures
such as those produced in the semiconductor industry.
• Since the SIMS technique itself relies upon the removal of atoms from
the surface, it is by its very nature a destructive technique, but also,
ideally suited for depth profiling applications.
• A depth profile of a sample may be obtained simply by recording
sequential SIMS spectra as the surface is gradually eroded away by
the incident ion beam probe.
• A plot of the intensity of a given mass signal as a function of time, is
a direct reflection of the variation of its abundance/concentration
with depth below the surface.
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B in Si
11B+
• The time-to-depth conversion is usually made by measuring the
sputter crater depth after the analysis is completed.
⇒ An example of the conversion of yield or intensity versus time to
density versus depth profile is given in Fig. 11.23, showing both
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the raw SIMS plot and the dopant density profile.
• One of the main advantages that SIMS offers over other depth
profiling techniques (e.g. Auger depth profiling) is its sensitivity to
very low (sub-ppm, or ppb) concentrations of elements.
• This is particularly important in the semiconductor industry where
dopants are often present at very low concentrations.
• The depth resolution achievable (e.g. the ability to discriminate
between atoms in adjacent thin layers) is dependent upon a ions
number of factors, including:
http://www.sensoft.ca/FAQ.aspx
1. the uniformity of etching by the incident ion beam
2. the absolute depth below the original surface to which etching has
already been carried out
3. the nature of the ion beam utilized (i.e. the mass & energy of the
ions) as well as effects related to the physics of the sputtering
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process itself (e.g. ion-impact induced burial).
• With TOF-SIMS instruments the best depth resolution is obtained
using two separate beams; one beam is used to progressively etch
a crater in the surface of the sample under study, whilst short-pulses
of a second beam are used to analyze the floor of the crater.
• This has the advantage that one can be confident that the analysis is
exclusively from the floor of the etch crater and not affected by
sputtering from the crater-walls.
wall
floor
SIMS crater
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3. 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 while 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.
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Chemical imaging with ToF-SIMS. The primary ion source is
rastered across the sample surface and a mass spectrum is collected
at each pixel. For every mass range of interest, an intensity plot, which
maps the distribution of that signal across the sample, can be
generated. The intensity plots can be color coded and overlaid. In this
example, an electronic device composed of titanium (m/z 48, green)
and barium (m/z 138, blue) is chemically imaged.
http://what-when-how.com/nanoscience-and-nanotechnology/single-cell-level-mass37
spectrometric-imaging-nanotechnology/
Instrumentation Approaches
• There are two instrumentation approaches to SIMS:
(1) the ion microprobe; (2) the ion microscope.
(1) The ion microprobe
• An ion analog of the electron microprobe.
• The primary ion beam is focused to a fine spot and rastered over
the sample surface.
• The secondary ions are mass analyzed and the mass spectrometer
output signal is displayed on a CRT in synchronism with the primary
beam to produce a map of secondary ion intensity across the
surface.
• The spatial resolution is determined by the spot size of the primary
ion beam and resolutions lower than 1 μm are possible.
• The mass spectrometer consists of electrostatic and magnetic sector
analyzers in tandem.
usually referred to as MS/MS
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Magnetic Analyzer
Electrostatic Analyzer
(focus mass angular dispersions)
(an ion beam is focused for energy)
rB
rv
Force Equations:
Electrical force: Fe = -q E where E =
mv2
Centrifugal force: Fc =
rv
2
V
mv
2KE
Fe = -q E = -q
=
d
rv = rv
qVrv
KE =
2d
V
d
Force Equations:
Magnetic force: Fm = qvB
mv2
Centrifugal force: Fc =
rB
2
mv
qvB =
mv = qBrB
rB
m2v2 = q2rB2B2
qrB2B2
m
=
q
mv2
qrB2B2
=
2KE 39
• In the electrostatic analyzer, the ions travel between two parallel
plates separated a distance d with a radius of curvature rV .
• A potential V between the two plates permits only those ions with
the proper energy E to be transmitted without striking either plate,
where KE is
K
d: a distance between two plates
rV: a radius of curvature
V: a potential between the two plates
(1)
• In the magnetic sector spectrometer, a magnetic field B curves the
ion of mass m, charge q, and energy E into a path of radius rB
according to
qB2rB2
B2rB2d
=
(2)
qVrv =
Vr
v
2
2d
• Substituting Eq. (1) into (2) gives
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(2) The ion microscope
• A direct imaging system, analogous to an optical microscope
or a TEM.
• The primary ion flood beam illuminates the sample and secondary
ions are simultaneously collected over the entire imaged area with
a resolution on the order of 1 μm.
• The spatial distribution of the secondary image is preserved through
the system using an electrostatic and magnetic sector analyzer in
tandem, amplified by a microchannel plate, and displayed on a
fluorescent screen.
• A small aperture may be inserted to select an area for analysis.
• The lateral resolution of this imaging method is dependent on the
beam size, which can be as small as 50 nm.
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• Proper mass resolution is essential for unambiguous SIMS analysis.
⇒ For example, a SIMS mass/charge (m/e) spectrum for high-purity
Si obtained with an O2+ primary ion beam contains 28Si+, 29Si+, and
30Si+ isotopes, polyatomic Si + and Si + as well as many molecular
2
3
species involving oxygen.
⇒ Oxygen is not from the sample itself, but are due to the oxygen
primary beam causing oxygen implantation and subsequent
sputtering.
• The mass resolution can be as high as 40,000, equivalent to resolving
two masses differing by only 0.003%.
• Such high mass resolution is required for detecting ions for which
there are interferences.
⇒ For example, 31P (31.9738 amu) has a very similar mass/charge
ratio as 30Si1H (31.9816 amu) and 29Si1H2 (31.9921 amu).
⇒ 54Fe is similar to the 28Si2 dimer.
• This plethora of signals requires a high resolution spectrometer.
e
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Detectors: Quadrupole SIMS (1)
• Quadrupole SIMS: a quadrupole mass analyzer consists of four
parallel rods with an oscillating electric field through which the
ions pass.
⇒ Robust and less expensive than the electrostatic-magnetic
sector analyzers, but has lower resolution.
⇒ Due to lower extraction potentials, it is suitable for analyzing
insulating samples, but it cannot distinguish between ions with
close mass/charge ratios.
• DC and RF (180 ° out of phase) voltages.
• Only ions with a limited range of
m/z reach the transducer.
⇒ All the others strike the rods
and converted to neutral.
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• The trajectory in the oscillating electric fields applied to the rods.
• Each opposing rod pair is connected electrically.
• A RF + a DC voltages are applied between one pair of rods and
the other.
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• Electrostatic or magnetic spectrometers depend on serial scanning
of an electrostatic or magnetic field, requiring narrow slits for only
those ions with the correct mass/charge ratio to be transmitted.
⇒ This reduces the transmittance of the spectrometer substantially
to values as low as 0.001%.
• A permanent magnet or an
electromagnet to cause the
beam to travel in a circular
path (60°, 90°, or 180°)
• Ions of different m/z can be
scanned across the exit slit
by varying the field strength
of the magnet or the
accelerating potential
between slits A and B.
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MS/MS in tandem (2)
Magnetic sector
http://www.cif.iastate.edu/mass-spec/ms-tutorial#Tandem MS
• Higher-mass ions are deflected less than lower-mass ions.
• Scanning the magnet enables ions of different masses to be
focused on the monitor slit.
=> the ions have been separated only by their masses.
Electrostatic sector
• To obtain a spectrum of good resolution (the same m/z)
• Ions have to be filtered by their kinetic energies.
=> Their energy distributions corrected for and are focused at
the double focusing point on the detector slit.
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Time-of-flight SIMS (TOF-SIMS) (3)
slit
• A SIMS approach without this limitation is time-of-flight SIMS
(TOF-SIMS): more ion collection.
• Separation of ions by mass occurs during the transit of
the ions to the detector located at the end of the tube.
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• Instead of continuous sputtering by an ion beam, in TOF-SIMS,
the incident beam consists of pulsed ions from a liquid Ga+ gun
with beam diameters as small as 0.3 μm.
⇒ These pulses typically have a frequency of 10 to 50 kHz and a
lifetime of 0.25 ms.
⇒ Ions are sputtered in brief bursts and the time for these ions to
travel to the detector is measured.
Equating the kinetic and potential energy gives
v: the ion velocity
V: the potential
• The transit time tt is simply L/v, where L is the path length from
sample to detector, leading to the expression
2V
m
2Vtt2
= 2 =
v
q
L2
48
L
v
=
tt
=
mL2
2qV
• Because all ions entering the tube have the same kinetic energy,
their velocities in the tube vary inversely with their masses,
with the lighter particles arriving at the detector earlier than
the heavier ones.
=> Assuming the species with the same charges
• Typical flight time is in the microsecond range for a 1 m tube.
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• A major advantage of TOF-SIMS is the absence of narrow slits in
the spectrometer increasing the ion collection by 10 – 50 %.
⇒ This allows the incident beam current to be reduced
significantly compared to conventional SIMS, which reduces
the sputtering rate greatly.
⇒ In fact, the sputtering rate is so low that it may take an hour
to remove a fraction of a monolayer. Such low sputtering
rates allow characterization of organic surface layers.
• Furthermore, since m/q is determined by time of flight, very
large and small ion fragments can be detected, much larger
than in other SIMS approaches.
⇒ a TOF-SIMS spectrum of an organic layer contains hundreds
of peaks.
• TOF-SIMS has also proven very sensitive to surface metals.
Surface densities as low as 108 cm−2 have been detected for
Fe, Cr, and Ni on Si.
⇒ In contrast to conventional RBS with a sensitivity of around
1013 cm−2, HIBS can reduce that to the 109–1010 cm−2 range.
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• 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.
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Applications I
• A major source of the limited sensitivity of SIMS is the fact that
most of the sputtered material is neutral and cannot be detected.
• Secondary neutral mass spectrometry (SNMS) or resonance
ionization spectroscopy (RIS)
⇒ Analyze the sputtered neutral atoms.
⇒ Ions are still required for mass analysis system.
⇒ Ionize sputtered neutral atoms after they have left the specimen
⇒ The neutral atoms are ionized by a laser or by an electron gas
and then detected.
⇒ The pulsed laser volatizes and ionizes a small volume of the
sample and the ions are analyzed
in a time-of-flight mass
spectrometer.
ACS Nano, 2011, 5 (4), pp 3059–3068
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⇒ Significant sensitivity enhancements over conventional SIMS
are achieved. High speed of operation, is applicable to
inorganic as well as organic samples and has microbeam
capability with a spatial resolution of ∼1 μm.
⇒ It is primarily used in failure analysis where chemical differences
between contaminated and control samples must be rapidly
assessed.
Applications II
• SIMS has found its greatest utility in semiconductor characterization,
especially for dopant profiling.
• Because matrix effects are minor and ion yields can be assumed
to be linearly proportional to densities.
⇒ Furthermore, the substrate sputters very uniformly, at least for Si.
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n-type As
p-type B
Poly-Si
Poly-Si
Si
Si
• An example profile in Fig. 11.24 shows that arsenic, boron, and
oxygen can be determined in a single measurement.
⇒ This sample was formed by diffusing As and B from a poly-Si layer
deposited on the Si substrate.
⇒ The plot shows the location of the junction (NAs = NB) and the
location of the poly-Si/substrate interface (oxygen peak).
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ToF SIMS depth profiling
HfO2
Ge
Ge
HfO2
Er2O3
HfO2
(13%)
Ge
No Er
• as grown and annealed
With Er
Microelectronic Engineering 88 (2011) 415–418
• Ge diffusion: decrease when Er is introduced in HfO2
• In HfO2: more Ge migrates to the high-k surface than in
Er-HfO2.
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Pros
Conclusions
• High sensitivity: especially for light elements (hydrogen is possible.)
• High surface sensitivity: important for depth profiling.
• Information about the chemical surface composition due to ion
molecules.
• Detection of the various isotopes of an element.
Cons
• Destructive method.
• High selectivity, depending on the element.
• Secondary ion yield for an element varies with the surrounding
elemental composition (matrix dependence).
• Interference of molecules and isotopes in the mass spectrum
• Quantitative analysis is quite complicated
http://www.condensed-matter.uni-tuebingen.de/cms/parser/parser.php?file=/en/
facilities/sims.htm
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• Factors that need to be considered in data analysis are crater
wall effects, ion knock-on, atomic mixing, diffusion, preferential
sputtering, and surface roughening.
• Some of these are instrumental and can be alleviated to some
extent, but others are intrinsic to the sputtering process.
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Sensitivity Versus Detection Limit
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• Sensitivity alone cannot be related to system performance, since it
is only an indication of signal strength.
• Detection limits are direct indicators of system performance, since
both detection limits and performance are functions of the
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signal-to-noise (S/N) ratio.