Paul Ahern - Time of Flight Secondary Ion Mass Spectroscopy [ToF-SIMS] theory & practice

Paul Ahern - http://www.linkedin.com/in/paulahern1
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) – Theory & Practice
Abstract—Time of Flight (ToF) Secondary Ion Mass
Spectroscopy (SIMS) is an extremely sensitive surface analysis
technique where the mass to charge ratio of an ion or molecular
fragment is determined by its velocity in the time domain. The
commercially available ToF SIMS instruments available today
have their roots in close to a century’s worth of academic
research, and their ability to gather elemental and molecular
information with excellent depth resolution and high sensitivity is
incomparable.
Keywords — Nanomaterials, thin films, surface analysis, Time
of Flight, Secondary Ion Mass Spectroscopy, SIMS.
I. INTRODUCTION
The ever-shrinking geometries of new materials and novel
devices built at the nanoscale with ever thinner layers has
driven the requirement for ever more sensitive and insightful
analytical techniques capable of viewing and understanding
matter at closer to the atomic level. Surface science
techniques are now more than ever a pre-requisite to
characterize and understand the complex interaction between
chemical composition and surface morphology of materials.
One such surface analysis instrument which has found
increasing application in the mainstream in the past decade is
Secondary Ion Mass Spectroscopy (SIMS), an experimental
technique which allows the analysis of a material in terms of
its molecular, chemical and elemental structure.
Figure 1 – Photograph of the ToF-SIMS instrument located
at the Environmental Molecular Sciences Laboratory
(EMSL) in Washington, USA (Image courtesy of
www.flickr.com/EMSL under Creative Commons license).
A sub-set of this technique is the so-called “Time-of-Flight”
or “ToF” method, which uses a more sophisticated time-
sensitive detection system to separate ions based upon their
mass, and has the advantage of increased sensitivity compared
to the more traditional magnetic sector or quadrupole
detectors. Detection limits of better than 1 part per million
(ppm) are achievable with excellent isotropic sensitivity and
three dimensional imaging1
.
II. HISTORY OF TECHNIQUE DEVELOPMENT
The origins of the Time of Flight technique can be found in
the early experiments of J. J. Thomson in 1909. Thomson
observed that the discharge of a positive secondary ion could
be encouraged by bombardment of a metal surface with a
source of primary ions2
. Further refinement of his apparatus
allowed Thomson to separate differently charged isotopes of
Neon with divergent atomic masses of 20 and 223
.
K. S. Woodcock built upon the foundations laid by
Thomson, and in 1931 published his studies into the creation
of negative ion spectra from surfaces under positive ion
bombardment4
. Woodcock carried out his measurements by
reflecting lithium ions off of a metal surface, with the
innovative step of using an applied electric field to slow down
the positive ions so that the negative ion spectra could be
recorded, a principle that is still used to this day to decelerate
secondary ions for SIMS analysis5
.
In 1946 William Stephens of the University of Pennsylvania
had proposed the first concept of a linear “Time-of-Flight
Mass Spectrometer”6
at a meeting of the American Physical
Society. Stephens’s mass spectrometer separated ions based
on their relative speed as they travelled in a straight trajectory
towards a detector. Many would regard the work of Herzog
and Viehboeck of the University of Vienna published in 1949
as being similarly ground-breaking, as they were the first to
develop an instrument which used an electron impact ion
source7
to generate their primary beam, a progression that was
directly enabled by advances in vacuum pump technology in
the previous years.
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) -
Theory and Practice (April 2013)
Paul Ahern, School of Electronic Engineering, Dublin City University, Glasnevin, Dublin 9, Ireland.
paul.ahern3@mail.dcu.ie

Paul Ahern - ie.linkedin.com/in/paulahern1 1
Figure 2 – Schematic from William E. Stephens original
1952 patent for a “Time of Flight” spectrometer8
.
Meanwhile, Wiley and McLaren9
built upon Stephens’s
study and via their Bendix Corporation released the first Time-
of-Flight mass spectrometer with a similar “EI” or “electron
ionisation” source in 1955, capable of sensitivities of less than
100 amu for the first time. Another notable development was
the work of Richard Honig of RCA Laboratories in New
Jersey, who through the search for new hot cathode materials
for vacuum tubes led him to assemble first a two-stage mass
spectrometer, and then in 1958 a full secondary ion mass
spectrometry instrument10
.
Further advances came in the 1960’s under the stewardship
of NASA who funded Liebel and Herzog of Geophysics
Corporation of America (GCA) Corp to create an analytical
instrument11
to examine moon rock samples brought back to
earth from the Apollo space flights. In 1967 Liebel, now of
Applied Research Laboratories, created an instrument with an
improved design12
which now used a duoplasmatron as the ion
source and mass separation to improve the purity of the
primary ion beam; the mass spectrometer design used a new
arrangement with no entrance slit and an Einzel lens.
In the early 1970’s Wittmaack13
demonstrated that the
secondary ion yield could be increased to a high value using
O2
+
ions as the primary ion beam medium, and that there was
a self-induction effect due to the mechanism of recoil
implantation on the sample surface. The subsequent work by
Magee14
working again with Honig at RCA Labs showed
success with the first quadrupole mass analyser complemented
by a high current density 40
Ar+
primary beam, and presented
an instrument that was capable of speedy depth profiling
where an ultra-high vacuum environment helped to maximize
detection sensitivity. This was also the first time that the issue
of the large amount of output data from SIMS as an analytical
technique was addressed, as Magee’s system had computer
automated control and acquisition modules.
In the 1970’s Prof. Benninghoven15
and co-workers at the
University of Münster developed the first “static” SIMS
instrument, which was deemed to be much less harsh on the
surface under analysis than its dynamic SIMS counterpart.
The reason for the reduction in surface damage was the much
lower dose density of the primary ion beam on the sample
surface, with primary ion beam current densities of less than
1nA per cm2
being typical. This allowed slower rates of
material removal but maintained the high sensitivity which
ToF-SIMS would become noted for, as a sufficient amount of
spectral information could be captured quickly before the
sample surface was modified significantly16
, typically at levels
of less than 1%.
In common usage, though sometime the terms “static” and
“Time of Flight” are used interchangeably to describe this
subset of SIMS analysis techniques, they are actually very
different. The main difference being that in Time of Flight
SIMS, the atomic mass peaks are monitored and recorded
more or less simultaneously, whereas in static SIMS only one
peak can be measured at any one time. Benninghoven showed
the static technique was very useful to analyse organic
molecules which had been deposited onto conducting
substrates.
Subsequent improvements and refinements in the technique
have been directly due to the advances that have been made in
constructing high performance Time of Flight detectors, which
has seen it become a key technique in the study of surfaces,
especially those made up of organic materials.
III. OVERVIEW OF TECHNIQUE
To understand the operating principles of the ToF-SIMS
technique it is useful if we summarise the basic steps of the
process first, before we then break down the instrument into
it's constituent parts and describe the phenomenon happening
in each section in more detail.
Figure 3 – Atomic scale representation of the ion sputtering
process which takes place in SIMS when a primary ion
beam impinges up a sample surface17
.
Overview: In basic terms, the SIMS technique is one where a
vacuum is created and within this vacuum region a beam of
primary ions at an energy of hundreds to thousands of electron
volts (eV) is accelerated onto a focussed spot on the surface of
a sample to be analysed. Various elastic and inelastic
processes occur at the surface (and subsurface) atomic layers,

giving rise to the liberation of ionised atoms (secondary ions)
as part of a collision cascade, represented in figure 3.
As the cascade emanates outwards from the path of the
primary beam, its energy can affect atoms far away in the
surface layer with sufficient kinetic energy to break their
lattice bonds and be liberated from the surface as neutral ions,
ionised atoms or molecular clusters18
. The secondary ions are
extracted, accelerated, and fashioned into a beam and then
traverse a flight path of a known length which is long enough
that they have the opportunity to become time-focussed;
lighter ions arriving at a mess spectrometer detector and being
counted before their heaver contemporaries.
Figure 4 – Simplified diagram showing the instrument set-
up for secondary ion mass spectroscopy (SIMS)19
.
SIMS has the ability to detect all elements in the periodic
table, including Hydrogen. In SIMS, the analytical data is
contained in the current signal of an ion with a certain mass:
charge ratio, with little or no background signal to be removed
from the spectrum. Therefore, signal to noise is good and the
minimum detection limit in SIMS can be somewhere of the
parts per million (ppm) to parts per billion (ppb) scale.
A. Primary Ion Beam: The composition of the sample under
investigation is of prime importance, as bombardment from
the primary ion beam will give rise to the secondary ions
which must then be resolved in time. There is no set primary
ion beam medium which must be used for ToF-SIMS, and
many instruments employ the use of more than one beam to
give optimum material removal and sensitivity for the given
sample material
In terms of the atomic interaction of the primary ion beam
and the sample surface, the key factor is the difference in
electronegativity; that is, the ratio of the number of protons in
the nucleus and the number and separation distance of valence
electrons orbiting the atomic core. Consider the element
Boron, which has two shells, with 2 electrons in the innermost
{k} orbital and 3 in the outer {l} orbital. Compare Boron to
Aluminium, which is directly below it on the periodic table
and has 2 electrons in the {k} orbital, 8 in the{ l} orbital and 3
in the {m} orbital. The couloumbic attraction between the
nucleus and the electrons is stronger in Boron than in
Aluminium, as the atom is smaller and thus there is a smaller
distance between the electrons and the nucleus, leading to a
higher value of electronegativity (Boron has an
electronegativity value of 2 "Pauling units", while Aluminium
is 1.6).
In reality, it can be seen that electronegativity values have a
trend across the periodic table, increasing as you move to the
top right hand corner, as illustrated in figure 5 below.
Figure 5 – Pictorial representation of how electronegativity
values vary across the periodic table. Electronegativity is
the propensity for an atom to draw electrons from other
atoms, and rises from left to right and from the bottom to
the top of the periodic table. (Image © Wiley & Sons 2000,
all rights reserved).
The two most commonly used ion beam sources in ToF-
SIMS, Oxygen and Caesium, are widespread because they
generate a sufficiently high secondary ion yield for speedy
depth profiling are respectively positively and negatively
charged.; when using an Oxygen ion beam, the bombardment
of ions on the sample surface will increase the number of
positive ion fragments received at the detector, whilst for a
Caesium source you will increase the number of negative ions
collected.
In specialised instruments which are designed specifically
for image fine focussing techniques, a Gallium (Ga+
) source
may also be employed as it has a lower melting point and thus
a higher achievable brightness which makes it worthwhile
despite its relative lower sensitivity. Gallium ions are created
in a liquid metal ion source (LMIS), as shown in figure 6 - the
same method that is used to create a Caesium (Cs+
) beam.
Oxygen beams can be created by either using an electron
impact source or a duoplasmatron. In the case of a LMIS,
pulsing of the primary beam to achieve good mass resolution
can be achieved by the inclusion of a set of deflection plates to
quickly blank the beam on and off20
.

Figure 6 – Diagram of the parts of a liquid metal ion source
(LMIS)21
with the parts as labelled (a) emerging metal ions
(b) extractor plate (c) liquid metal film (d) capillary feed
tube (e) liquid metal reservoir (f) crucible needle.
In electron impact sources, oxygen or another noble gas
flows into an ionisation region where a filament resides.
Electrons from this filament are then accelerated by anodic
grid while an extraction cathode (such as a Weinhalt aperture)
accelerates the ions to the lens array where they are focussed
and rastered if in imaging mode. In the case of a
duoplasmatron, which is common in dynamic SIMS
instruments also, plasma is formed in the extraction region and
a pair of off-setted magnetic lenses is used to form the primary
beam and tune it to a usable outline before it is extracted by
the anode.
Figure 7 – Block diagram of the primary ion column in a
typical SIMS instrument22
.
In instruments which have only one ion source, it is
common to use an element in the middle of the
electronegativity table which can successfully liberate both
negative and positive ions at sufficient amounts. Recent
advance have shown that the use of primary cluster ion
sources such as those derived from Bismuth (Bi1+
, Bi3+
) and
polyatomic Carbon (C60+
) have benefits when used to analyse
biological samples, with Bi3+
showing the best surface
sensitivity for lower atomic-mass molecular fragments23
.
Other recent peptide analysis work with Argon (Arn+
) primary
ion beam sources24
has shown its usefulness in terms of
minimising surface damage and allowing the user to maintain
a constant sputter rate with an associated reduction in
secondary ionization mechanisms.
Once the ions have been formed, they must then be
extracted and focussed into the primary beam before they are
accelerated to the required energy25
.
B. Beam / Sample Interaction: Once the primary ion beam
has been formed, it is focussed and pulsed in short (typically
between a 2ns and 20ns interval) onto the sample surface,
giving rise to sputtering. One consequence of this pulsed ion
beam regime is that a sufficient interval must be allowed so
that the heaviest, and thus slowest-moving, secondary ions can
vacate the detection region before the next bundle of
secondary ion data can be accepted.
Figure 8 – Diagram of some of the myriad and complex
phenomena at play during the interaction between the
primary ion beam and the sample in SIMS26
. The
progression of the ion cascade started by a sole primary ion
can be seen.
Ideally the primary ion beam pulse is kept as short as
possible, by using a device known as an electrodynamic pulse
“buncher” in the primary ion column as shown in figure 9, to
time focus the sputter pulse to a Gaussian profile of typically
less than 1 nanosecond (1ns)27
. This primary beam pulse
width value is termed tbeam, and one other consequence of
this arrangement is that the AC current is only a smaller
fraction of the DC current which further preserves the static
conditions on the sample surface allowing for better chemical
mapping.

Figure 9 – Simplified schematic diagram of the construction
of a linear buncher for Time of Flight analysis.
Ions with a small first ionisation potential will be readily
ionised by the primary beam, while those with a greater
potential will form positive ions much less readily; ions with
high electron affinity will preferentially give rise to negative
ions more readily. The timespan for the creation of these
secondary ions is typically in the picosecond (ps) regime.
Figure 10 – representation of total ion yield during
sputtering, {S}, and a function of the incident ion energy
{E}28
.
Ion images are produced in a slightly different way,
whereby the primary ion beam is rastered over the area of
interest and the number of ions as a function of the {x,y} co-
ordinates is presented. Secondary electrons generated as a by-
product of his process can also be collected in a resident
Everhardt-Thornley photomultiplier and used to generate a
standard secondary electron image.
The total sputter yield of secondary ions for a particular
element depends on the incident ion energy as shown in figure
10 and also the angle of incidence when the primary beam
strikes the sample surface as shown in figure 12; the vast bulk
of the species evolved from the sputtered surface will actually
be neutrals but it is only the very small positively or
negatively charge portion of the secondary particle flux that is
detected.
A complete understanding of the formation of secondary
ion species in ToF-SIMS has not yet been accepted, although
many competing thesis have been proposed in the literature
and have had limited success in predicting experimental
results for a narrow subset of defined materials.
i. Local Thermal Equilibrium (LTE) model: This (now
defunct) historical model, sometimes also categorised as the
“surface excitation model” proposed that underneath the
bombardment area, surface plasma was created wherein the
sputtered atoms became ionised.
Under equilibrium conditions, the ionisation potential could
be calculated by the use of the Saha-Eggert ionisation
equation29
in the bombardment region. The only important
factor was deemed to be the plasma temperature30
and this
could be estimated by taking into account the amounts of each
element present.
The results from this model are strictly semi-quantitative as
the exponential term in the S-E equation is compatible with
quantum mechanical terms31
. As V. E. Krohn32
of the US
Dept. of Energy laboratory in Argonne, Illinois summarised
succinctly “Unfortunately, surface ionization is an
equilibrium process, whereas secondary-ion emission is not.”
ii. Electron Tunnelling model: This model of secondary ion
formation (sometimes called the Schroeer model33
) is based on
quantum mechanical principles, and describes how the
electron which sits in the conduction band (CB) has the ability
to tunnel into the valence band (VB) of the ejected atom34
.
The ionisation potential of the sputtered element governs
the probability statistics that govern this phenomenon, as well
as the adiabatic surface ionisation function and the velocity of
the sputtered atom.
However, in line with this model, the work function  is
autonomous of how the work function change, , is being
induced - as long as the external ions in the primary beam
used to induce  does not change the chemical state of the
target atoms35
, as shown in figure 11 below.
Figure 11 – Simplified energy diagram of a charged particle
separating from a metal surface36
. At the distance ZC, the
atomic level intersects the Fermi level of the metal and
charge exchange can occur by tunnelling.

iii. Broken Bond model: In this model, proposed initially by
Šroubek37
the electronic temperature, Te, within the region of
the collision cascade was calculated for different materials
from a starting point of electron transport theory, and these
values were compared to calculated values formulated in
conjunction with empirical SIMS data.
The resultant model tackles the process from the standpoint
of the creation of ionic compounds in an idealised electronic
lattice under bombardment from a primary ion beam of
oxygen, and a prerequisite is that there be present an oxide
layer on the surface of the sample to be analysed. In this
model, the binding electrons stay with the oxygen atom and
there is only emission of positively ionised species.
Figure 12 – Secondary ion sputter yield plotted versus the
angle of incidence, , of the primary ion beam (as measured
referenced to the normal plane)38
.
In the theoretical case whereby all the available secondary
ions could be ionised by the incoming beam, and detected by
the spectrometer, then the signal could be related to the
specimen composition by the equation39
below:
iS = iPS
where iS is the sputtered ion current,  is the atomic fraction of
the element of interest, iP is the primary ion beam current, S
the total effective sputter yield, and  the specific sensitivity
of the detector (which includes the previously explained effect
of the incident angle of detection). Each spectrometer
configuration will also have an inherent angular acceptance
range and a reduction in mass resolution can be observed
when angular divergence becomes too large.
C. Secondary Ion Acceleration: Typically less than 1% of the
sputtered ions from the primary beam are ionised, with the
resultant cloud of ejected atomic and molecular secondary
ions accelerated by a potential into the Time-of-Flight region.
Since the lighter ions travel faster, they thus arrive at the ion
detection module first and can be counted. The Time-of-
Flight relationship can be understood simply as the travel time
of a secondary ion being proportional to the square root of its
mass, and by this mechanism all secondary ions can be
isolated and detected discretely once they impinge upon the
detector.
The drawing out and collimating of the secondary ion signal
emitted from the sample surface is by means of a combination
of transfer and immersion lenses which control the image
amplification and, by way of apertures, serve to limit the
angular acceptance angle of the mass spectrometer detector.
Secondary particles are accelerated by an applied constant
voltage, Vacc such that they now have a fixed kinetic energy
{E} which overrides their initial kinetic energy at source. In
this case their accelerated velocity in the drift region dpends
solely upon their mass, by the equation40
qVacc = E = ½ mv2
The overall secondary ion transmission can thus be greater
than 40%. To further increase the detector effectiveness for
dense ions (m>1,000 Da) a post –acceleration area (10-20
keV) can be sited between the end of the drift region and the
entrance cone of the detector.
D. Charge Neutralisation: Since many of the common
samples analysed by ToF-SIMS are organic molecules or
polymers and are non-conductive, a separate source of
electrons is needed to provide a tuneable charge compensation
function; this is achieved by producing low energy electrons
and introducing them around the analysis area on the sample
surface.
A recurring issue in achieving good spectral quality with
SIMS is adequate monitoring and control of the sample’s
surface potential41
to ensure that as uniform an ion emission
profile as possible is generated, as even a very slight increase
in the electric field on the sample surface can significantly
shift the energy level accepted by the energy filter into an
undesirable region, whereupon the ion yield being generated is
negligible and a impractical spectra with large intensity losses
will be recorded.
Typically a LaB6 or Tungsten filament electron source is
used, and in some cases the removal of surface charge can be
further facilitated by placing an earthed metal grid (such as a
TEM grid) over or close to the area of interest on the sample
surface. This method is useful as the pulsed low energy
electrons automatically steady themselves further over time, as
a positive surface potential serves to minimise the losses of

secondary ions which would serve to decrease the overall
potential of the surface42
. Another complication with
instruments that operate in the time domain such as ToF-SIMS
is that charge neutralisation must typically done in a pulsed,
cyclical manner in between the repeated sequences of
secondary ion generation and acceleration.
E. Mass Analysis Detection: Early SIMS instruments relied
upon the quadrupole mass spectrometer for detection,
consisting of four elements at equal distances whereby
alternating pairs of DC and RF voltages were applied to act as
a filter allowing only ions with a specific charge to mass ratio
to traverse into the detector. However, the disadvantages of
poor transmission (typically <1%, which decreases further
with increasing mass number) and a” lossy” data collection
method as it can only operate in scanning mode, meant that
better detection methods were needed for ToF-SIMS to
advance further.
i. Reflectron Analyser
Today the most common in commercial ToF-SIMS
instrumentation, mass-reflectron analysers were first proposed
by Russian physicists Mamyrin & Karataev43
in 1972. A
modern reflectron detector offers a good balance of much
enhanced mass resolution in a smaller equipment footprint44
.
Figure 13 – Schematic representation of the nascent Time of
Flight instrument (complete with reflectron) used by
Benninghoven and co-workers at the University of
Münster45
. The parts are labelled as shown – (a) Electron
ion source (in this case, Ar+
). (b) Ga+
liquid metal ion
source (LMIS). (c) Sample holder (temperature
controllable). (d) Secondary ion acceleration lens. (e)
Reflectron of the gridless type design. (f) Mass
spectrometer detector.
The method of operation is that the secondary ions are
accelerated towards an ion “mirror” which functions as an
electrostatic reflector that then turns the ions and reflects them
back towards the direction of the detector in a “folding”
arrangement, preserving the time-focussed nature of the ions
and enhancing the mass resolution by an order or magnitude.
The travel time is governed by the equation -
√
√
This setup offers mass resolution of the order of 10,000 amu
when a deceleration & re-acceleration grid is used, but by
moving to a gridless design superior mass resolution of up to
50,000 amu is reliably achievable46
. As measuring the usable
secondary ion yield can be problematic, transmission is
instead estimated using a Thomson distribution47
based on the
incoming kinetic energy of the primary beam; however ,this
approximation neglects any influence from detector efficiency.
The detector itself is normally a combination of a Faraday
cup and microchannel plate48
which contains an array of
electron multipliers, similar in design to the historic
channeltron. The signal amplification is by means of an
electron cascade from the inner lining of leaded glass when it
is struck by an incoming secondary ion with secondary
excitation being provided by a phosphor screen.
Figure 14 - Diagram of secondary ion detection in a ToF-
SIMS instrument with a combination of a Faraday Cup and
an Electron Multiplier49
; the bias voltage is equally divided
across the cathodes & anodes.
To boost the detection of slow-moving, heavy ions a pre-
acceleration region is often incorporated immediately before
the detection plate. Between these two complimentary
detectors the full dynamic range of the mass window can be
sufficiently covered – the Faraday cup being useful at high
count rates of >5x104
c/s and the electron multiplier at lower
intensities of <5x106
c/s.

ii. TRIFT Analyser:
In a nutshell, a TRIFT (or TRIple Focussing Time-of-flight)
analyser uses three separate hemispherical electro static
analysers (ESA’s) to give an effective 270 drift path between
the secondary ion acceleration region and the detector. Each
of these ESA’s continually refocuses the secondary ions so
they can be resolved more accurately in the time domain at the
detector, allowing the maximum angular and energy
acceptance values.
TRIFT detectors excel at gathering high secondary ion
signal from all areas of the sample without appreciable
“shadowing”, and the resultant ion images have higher depth
of focus than reflectron captured images. High mass resolution
can be obtained with a low signal background, especially on
conductive samples with little topography such as the Silicon
wafers used for integrated circuit manufacturing (This mass
resolution can be of the order of ~ {m/m} >104
at 28
Daltons50
).
Figure 15 – Schematic of a typical set-up for static SIMS
with a TRIFT mass spectrometer51
. The 270 defection
path in the drift region can be clearly seen. (Image ©
Physical Electronic, MN, USA. All rights reserved)
F. Data Reduction & Analysis: All commercially available
SIMS instruments possess an in-built library of secondary ion
data for comparison to the sample being analysed. Sometimes
some ambiguity can be encountered when chemical
compounds of a similar mass overlap; one simple way of
sidestepping this limitation in a production environment is to
supply known “good” samples, so that the spectrum of known
material can be subtracted and only the difference signals of
interest remain to be further analysed. If this is not possible,
then another method of disambiguation is through the use of a
complimentary analytical technique such as Auger Electron
Spectroscopy (AES) or X-ray Photoelectron Spectroscopy
(XPS) may be needed.
There are numerous ways that the output data can be
presented from ToF-SIMS analysis. The most basic and easy
to understand is a standard {x-y} plot of positive or negative
ion intensity (counts at the detector) vs. a linear mass scale of
unified atomic mass units, (u), with the mass being recorded
either as atomic or molecular mass: charge ration (m/q) 52
also
called one “Dalton” – defined as “one twelfth of the mass of
an unbound neutral atom of carbon-12 in its nuclear and
electronic ground state” 53
, which is experimentally known to
be 1.660538921 (73 ) × 10−27
kg.
Sometimes it can be difficult to definitively allocate peaks
to one specific material due to the influence of isotopic effects
due to a large number of secondary isotopes for a particular
element; the sensitivity of the ToF-SIMS technique also
means that surface hydrocarbon contamination is readily seen
and the data must be refined in a sensible way before
meaningful analysis can proceed with the use of applicable
standards to help understand the complex spectra.
G. Vacuum System: Although not strictly a part of the
analytical technique, it is worth mentioning the importance of
the ultra high vacuum (UHV) environment and how it is
generated. Many of the early advances in sensitivity
improvement in mass spectroscopy can be related to
improvements in the vacuum environment, he most notable of
which came in the 1960's due to the invention of the turbo-
molecular pump by Becker54
. These pumps allowed the
realisation of an integrated vacuum system which allowed the
sample surface to be maintained free of contamination, and
maximised the inelastic mean free path that the primary and
secondary ion beams could traverse without encountering
losses due to gas phase scattering phenomena55
.
The vacuum in the analysis chamber of a modern ToF-
SIMS instrument can reach as low as 10-10 Torr and this is
achieved through a holistic vacuum system arranged in series
and using roughing pumps, turbo molecular pumps, ion pumps
and sublimation gettering pumps, all monitored by different
vacuum gauges and computer controlled. High temperature
“baking out” is also needed periodically to maintain adequate
system vacuum levels.

IV. SELECTED NOTABLE APPLICATIONS
As previously described, the complex nature of Time of Flight
spectral data means that often the technique is used in a
comparative rather than an absolute sense, with samples
analysed to show the differences between them as a method of
side-stepping the arduous task of trying to understand all the
uncertainties in the analytical results.
ToF-SIMS excels rather at being a very sensitive technique
with high surface specificity and resolution and it is in this
niche that it has found use in fields such as nanoelectronics,
composites, catalyst formulation, biomedical, and general
failure analysis. A select few of these applications are detailed
in the following sections.
Semiconductor Analysis: ToF-SIMS has found a natural home
in the labs of many semiconductor manufacturers where they
leverage the tools analytical capability very successfully,
especially in the arena of thin film analysis and defect
metrology. The ability of SIMS to perform analysis using
depth profiling has meant that three dimensional analyses of
defects and structures has become commonplace, if a little
time consuming. Several ToF-SIMS manufacturers provide
instruments with large, fast entry load locks with differential
pumping and five axes motorised stages which can hold and
navigate a 12 inch silicon wafers.
Figure 16 – Parallel ToF imaging used for defect identification
and analysis56
. Ion maps of C-
, Si3-
, InO-
and others used to
show the presence and composition of a defect on a thin film
transistor (TFT) array ( Imaged using a 25 keV Ga+
primary
ion source over 128µm2
area).
Indeed ToF-SIMS allows the analyst to understand
delicate properties, for instance how far an implant has
progressed into the silicon lattice or what exact layer of the
fabrication process that a particle has been introduced. When
used in conjunction with defect isolation information from in-
fab metrology tools, ToF-SIMS can be an excellent method to
understand subtle defects which arise in the manufacturing
process and accurately pinpoint where they come from as
depth profiling allows you to etch back to the layer where the
defect was originally detected in-line even if the wafer has
progressed all the way to end of line electrical test & sort.
Figure 17 – A typical high mass resolution positive ion
ToF-SIMS spectra of a contaminated silicon wafer57
. A
plethora of different contaminants are shown, especially
hydrocarbons which ToF-SIMS is especially sensitive to.
More general analysis of molecular contamination of
incoming virgin silicon and of thin films deposited during the
fabrication process can also be done, and increasingly the
results are used as part of routine process monitoring in certain
sensitive parts of the process; for instance, when selective
epitaxial growth is being carried out, to ensure the starting
substrate is free from contamination; or to monitor surface
oxidation levels on electroplated copper backend layers which
have a narrow time window in which they can be allowed to
progress to the next process step.
Molecular Analysis: ToF-SIMS makes it possible to amply
detect ppb levels of low-volatility molecules such as
hydrocarbons, which is one of the reasons why it is so useful
for molecular and polymer analysis at the nano scale.
Everything from the composition of self-assembled
monolayers, plant herbicides, interplanetary meteorites and
photocopier paper have been analysed by ToF-SIMS.
Polymer chemists have relied upon ToF-SIMS techniques
for many years to understand how they can adjust a polymer’s
composition through process alterations and variations in
formulation. ToF-SIMS ion imaging allows them to visualise
the molecular structure of the surface polymer while depth
profiling brings an understanding of chemical distribution
with submicrometer resolution, for instance when blend and
copolymer thin films undergo understated modification to
their molecular structure during annealing processes.

Figure 18 – Ion images obtained by ToF-SIMS from the
“Nakhla” Martian meteorite58
.
Pharmacological & Biomedical Analysis: ToF-SIMS is a
common analytical technique in the field of pharmaceuticals
as it is well suited to the parallel analysis of tablets and
formulations which have numerous organic and inorganic
ingredients.
In the biomedical field, advances in charge neutralisation
and data interpretation have seen ToF-SIMS become more
widely utilised, especially in the analysis of biocompatible
surfaces and coatings. Typically cluster ion sources such as
bismuth are used to generate the secondary ions while caesium
or oxygen are employed as a sputtering medium for rapid
depth profiling.
Figure 19 below shows a typical application, where ToF-
SIMS is used to understand the coating homogeneity and drug
loading of a stent with the aid of reducing inflammatory stent
thrombosis. By calibrating the {x} axis from sputter time to
depth, the film thicknesses can be readily measured.
Figure 19 – Negative ion ToF-SIMS depth profile of a stent
coated with Paclitaxel on a Parylene substrate59
.
Biological samples can also be tackled through the use of
appropriate pre-treatments of the sample. The major
roadblock with adoption of ToF-SIMS in this field is the
requirement for an ultra-high vacuum environment, and the
still improving sensitivity for high mass ions which are typical
in bio-molecular analysis specimens. Figure 20 shows high
resolution parallel ion imaging by ToF-SIMS of human cancer
cells.
Figure 20 – Positive ion images by ToF-SIMS60
of human
breast cancer cells pre-treated with 2-amino-1-methyl-6-
phenylimidazo[4,5-b]pyridine and a fluorescent
carbocyanine dye.
Aside from ion imaging for qualitative analysis, depth
profiling has also be used for the study of single or groups of
cells where the high lateral resolution of the technique has
been very beneficial. Breitenstein61
and co-workers used ToF-
SIMS to profile the levels and types of amino acids present in

cells from a rat kidney, while Fletcher62
showed in 3D a
similar result but constrained within a single freeze-dried cell
using a primary beam of clustered C60
.
Figure 21 – Fletcher’s 3D ToF-SIMS depth profile using
C60
clusters of a single freeze-dried lipid cell, showing the
amino acid fragments attributable to different proteins.
V. CONCLUSION & FUTURE DIRECTION
ToF-SIMS as an analytical technique can very successfully be
used for the characterisation of nano-materials and films.
Using it’s combination of depth profiling, ion imaging and
static mode analysis it can give huge amounts of information
relating to chemical, molecular and elemental composition of
the sample under investigation.
This has arisen due to decades of development of all the
constituent parts which together make up the instrument; from
the vacuum system to the ion source, an understanding of the
interaction between the primary beam and the generation of
secondary ions, charge neutralisation, signal detection and
data analysis.
The recent adoption of new techniques, such as the use of
exotic cluster sources with highly sensitive mass
fragmentation patterns for negative ion analysis of ultra thin
layers at low impact energies, should pay dividends and
ensure that ToF-SIMS remains a workhorse for nano-
characterisation for the foreseeable future.
In the longer term, the continuing efforts to refine new
techniques related to ToF SIMS, such as the EU FP7
supported 3D NanoChemiscope tool housed in Switzerland
which blends a Time of Flight detector with an AFM tip
scanning approach to manipulate and analyse matter atom by
atom for the creation of in-situ spatial 3D depth analysis,
should see the skillsets of ToF mass spectrometry scientists
remain useful and in demand for some time to come.
In the rapidly advancing biological space, the advent of
tandem mass spectroscopy has been applied to ToF
instruments where ions have multiple disassociative steps
taking place over time in a hybrid instrument with both a
quadrupole and a Time of Flight detector. This approach has
proven to be very useful in analysing molecular peptide chain
arrays rapidly63
, and may one day be able to easily sequence a
strand of DNA to check for genetic diseases. New
instrumental designs are on the horizon that will allow a
purely DC primary ion beam64
that has many advantages for
use with clustered polyatomic sources.
Across all disciplines the challenge remains of forging a
deeper understanding of the shortcomings presented by low
ionisation efficiency, and the data deconvolution processes
necessitated by sputtering matrix effects in the sample.
VI. ACKNOWLEDGMENT
The author would like to thank Dr. Rajani K. Vijayaraghavan
of the School of Electronic Engineering in Dublin City
University for her proposal of this review topic, as well as her
patience in explaining many of the important theoretical
principles and concepts that underlie the ToF-SIMS analytical
technique as a tool for nanomaterial characterisation.

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Briggs, D. and Seah, M.P. 1983; 1992. Ibid.
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Vickerman, J.C. and Gilmore, I.S.(.S. 2009. Ibid.

49
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50
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51
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