"Total" Ion Beam Analysis – 3D imaging
of complex samples using MeV ion beams
© C.Jeynes, 3rd April 2012
University of Surrey Ion Beam Centre, Guildford GU2 7XH, England
An article in the ION BEAM METHODS Chapter of the Wiley
Characterisation of Materials (2nd edition) on-line book
Introduction
In this Chapter the synergy between a number of closely related techniques for thin
film depth profiling are described; they all use ion beams from MV accelerators as
probes. These include the nuclear methods: RBS, EBS, ERD, NRA (and see
PARTICLE SCATTERING in the COMMON METHODS Chapter). But they can also include
PIXE (see ATOMIC EXCITATIONS in the COMMON METHODS Chapter). See Table 1
for the expansion of the acronyms and references to the list of the detailed articles on
individual techniques: this article will not describe the techniques themselves but will
concentrate specifically on the synergisms available. I will use acronyms for
complementary techniques freely: a Glossary for these can be found in the
INTRODUCTION to this Chapter (ION BEAM METHODS).
"TOTAL-IBA" is operating when multiple IBA techniques are being handled selfconsistently to obtain more information than the sum of that available from each
technique handled separately [1]. We will show that the sum of the whole is far more
than the sum of the parts, to the extent that large new classes of samples become
tractable and powerful new types of characterisation become feasible: the various IBA
techniques are in fact strongly complementary. Indeed, we believe that even
chemical tomography is feasible with these new techniques.
The alert reader will object that we are here only stating the obvious: it is easy to find
examples showing that this complementarity has always been recognised. For
example, Feldman et al presented a paper combining He-RBS and He-PIXE to the
first Ion Beam Analysis Conference nearly forty years ago in 1973 [2]. The Abstract
(not available electronically) is informative for us :Anodic oxide films on GaAs have been studied by the combined use of He back-scattering
[sic] and He-induced X-rays. Back-scattering is hampered by the lack of mass resolution
between Ga and As. X-ray analysis has excellent mass resolution but poor depth resolution.
This poor depth resolution is overcome by increasing the effective thickness of the films by
entering at grazing angles and making use of the property that the He-induced X-ray crosssections fall steeply with decreasing energy. This technique and the methods of data analysis
are discussed in detail. The anodic oxide films are found to be deficient in As within 200Å of
the surface and to have a Ga:As ratio of approximately 1:1 for the rest of the oxide. On
heating to 650°C most of the As diffuses out of the films.
This early use of RBS/PIXE is exemplary, and includes an explicit awareness of the
strengths and weaknesses of each technique. We shall underline these below, and
show why it is only recently that the idea of using the IBA techniques selfconsistently has been picked up and made usable by the analytical community.
We will first briefly survey the individual techniques, particularly with 3D chemical
imaging in mind. This survey will overlap surprisingly little with the separate articles
treating each techniques. We will then show why the nuclear and atomic
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communities have pursued largely separate courses over the last 40 years. We will
describe the recent advances that have made TOTAL-IBA possible. Finally we will
show the extraordinary power of the new technique. We expect that IBA computed
tomography (IBA-CT) over sample sizes ~20 m with deep sub-micron voxel sizes
will become available in the next five years or so. (Provided civilisation survives the
current crises.)
IBA Techniques not considered as " TOTAL-IBA "
TOTAL-IBA envisages an MeV ion beam striking a sample surrounded by detectors
for all the various reaction products: backscattered (RBS and EBS), forward
scattered ("RBS" or off-axis STIM) and forward recoiled (ERD) particles; particles
from nuclear reactions (NRA), and photons from both nuclear (PIGE) and atomic
(PIXE) excitation.
LEIS and MEIS are both RBS techniques, but they are low energy and usually
applied to surface science problems involving a few nm at most. TOTAL-IBA is
applicable to thin film applications involving surface layers (or samples) up to
~20 m thick. LEIS and MEIS are complex and are usually used alone (or with other
surface science instrumentation – XPS, LEED etc).
Dynamic (depth profiling) SIMS is a destructive (sputtering) technique using entirely
different methods which we will not consider here. However depth information from
SIMS is commensurate with TOTAL-IBA information, and in principle (and practice
too [3]) could be incorporated. Note here that ELLIPSOMETRY in the OPTICAL
IMAGING AND SPECTROSCOPY chapter is also potentially (and practically [4]) a
commensurate technique, as are polymer diffusion studies ([5] [6]; see SMALL ANGLE
NEUTRON SCATTERING in the NEUTRON TECHNIQUES chapter) and protein
crystallography ([7]; see SINGLE-CRYSTAL X-RAY STRUCTURE DETERMINATION in
the X-RAY TECHNIQUES chapter).
AMS, IBIC, He-Ion- and Field-Ion-Microscopy, and the Radiation Damage studies
are quite different types of characterisation or materials modification applications.
Strengths and Weaknesses of Single IBA Techniques
All the analysis methods with particle resultants (RBS, EBS, ERD, NRA) are
intrinsically sensitive to depth directly through the energy loss of the ion beam in the
sample, since the detected particle energy is always analysed. (There are exceptions
for NRA in many cases either where there is little depth information intrinsically in
the signal or where it is degraded by range foils or the kinematical broadening of large
detectors.).
However, none of the analysis methods with photon resultants (PIXE, PIGE) are
usually very sensitive to depth. The exceptions in PIXE are where closely adjacent
matrix elements bring the absorption edges into play, and in PIGE where resonant
nuclear reaction allow depth profiling by stepping the beam energy.
Elastic (Rutherford and non-Rutherford) backscattering
RBS is the simplest technique. The yield is proportional to the square of the atomic
number and inversely proportional to the square of the scattering particle energy, and
the kinematical factor for a head-on scattering event of a nucleus mass M1 on a target
nucleus mass M2 is {(M2-M1)/(M2+M1)}2 (see Eqs.1,3 in ELASTIC BACKSCATTERING
2
OF IONS FOR
COMPOSITIONAL ANALYSIS). Thus light element signals are at a lower
energy than (and therefore superimposed on) heavy element signals. This means that
the sensitivity for light elements is low both because the absolute cross-section is low
(1 barn/sr at 170° scattering angle for 1 MeV 4He on Si) and also because the light
element signal frequently has a heavy element background.
The great strength of RBS is that because the cross-sections are accurately calculated
with a Coulomb potential, the spectra have absolutely traceable quantitation [8] [9].
Because the charge solid-angle product (see Eq.8 in ELASTIC BACKSCATTERING OF
IONS FOR COMPOSITIONAL ANALYSIS) can be readily determined from the spectral data
and the material stopping power, and because in one important case (He RBS of Si)
the stopping power can be accurately determined from a certified standard material
[10] we expect that 1% traceable accuracy (that is, 2% at 95% confidence)
For EBS there may be greatly enhanced cross-sections many times Rutherford which
frequently are highly useful to improve the sensitivity to light elements [11] [12]. On
the other hand, protons on Al or alphas on Si (for example) have complicated crosssection functions with very many sharp resonances but average cross-sections close to
Rutherford. In these cases EBS is distinctly unhelpful, being very complicated but
just as insensitive as RBS.
Elastic Recoil Detection and Nuclear Reactions
ERD and NRA are good TOTAL-IBA techniques when used with light particle beams
for which other signals (RBS/EBS and PIXE) are also available. He-ERD is a very
useful beam for determining H depth profiles in a variety of materials, and NRA is
indispensible for sensitivity to certain light atoms (Z<14) in certain contexts. Both of
these are standard techniques in use for decades. The limitation of both light ion ERD
and NRA is that they are usually sensitive to only one isotope present in the sample,
so that they both need a separate calibration and the analysis of the sample depends on
complementing with other techniques.
Particle-Induced X-ray Emission
PIXE is not usually considered to be sensitive to depth since the detected X-rays at any
particular energy have been integrated along the whole beam path. Most published
PIXE work has been either on samples effectively homogeneous in depth, or on
samples where the depth profile is not important, or on very simple layered samples
whose structure is already known. Indeed, one major PIXE code (GUPIX [13]) does
not currently support the analysis of diffusion (or any other sort of complex) profiles.
This is not due to a basic limitation of the code: it is just the way it has been
implemented – the authors took the view that such complexity was of no interest!
However, because PIXE does not have direct depth information does not mean that
there is no depth information folded into the signal. SEM-EDS and XRF are
analytical methods comparable to PIXE (using respectively electrons and X-rays as
the probe beam: see the ATOMIC EXCITATIONS article) and commercial software
packages for both SEM-EDS and XRF are commonly used routinely to give "layer
thicknesses". And in surface science angle-resolved XPS is also used routinely to
give ultra-high depth resolution. "Differential" PIXE using beam energy ([14] [15]
[16]) or beam geometry (that is, an "angle-resolved" method [ref.1] [17]) variation has
been recognised and used for a very long time. Ahlberg [18] showed that a single
measurement using these equivalent effects had sensitivity to the depth distribution of
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an element through the yield ratio within characteristic line groups.
sensitivity is limited, as he showed rather elegantly.
But the
On the other hand, the sensitivity of PIXE is enormous. For 3 MeV protons on Si the
K-shell production cross-section is about 87 barns/sr. The RBS cross-section for this
beam on Si (170° scattering) is 7 mb/sr, four orders of magnitude smaller!
Barriers to Synergy
If TOTAL-IBA is so wonderfully powerful, and has been recognised as such for nearly
40 years, why make so much fuss about it now? Why isn't everyone doing it? There
are essentially three main reasons for this (two good and one bad) and a few minor
reasons (we are here talking mostly about the synergy between the particle and the
photon techniques). The position is that, from a practical point of view, TOTAL-IBA
has only been routinely feasible in the last five years or so.
Barrier 0: Separation of Nuclear and Atomic Communities
The most obvious point is that atomic (PIXE) and nuclear (RBS/EBS/ERD/NRA)
processes are entirely different, and have completely different formulations. Their
descriptions (see the appropriate individual articles) use different physics and involve
a completely different literature. And technical problems in both the atomic and
nuclear physics communities persist until today. Neither field was mature enough
until quite recently to admit a usable synthesis between them
Barrier I: Code Limitations
TOTAL-IBA is only needed for relatively complex samples. For such samples the
particle scattering codes must take many second order effects into account. These are
described in detail in the recent IAEA-sponsored intercomparison and review of
particle-scattering codes [19], with only two codes recognised as "new generation",
that is, able to model all these effects. These are the DataFurnace [20] and SIMNRA
[21] codes, which are both only just over a decade old. Recent reviews are available
(respectively [22] and [23]). The fitting accuracy available with these new codes is
extraordinary, especially compared to what was considered acceptable forty years
ago (see Fig.14 in ELASTIC BACKSCATTERING OF IONS FOR COMPOSITIONAL
ANALYSIS: this is Fig.1 in [ref.19]; the other Figures in the EBS article are also
impressive).
We should note parenthetically that the knowledge of stopping powers of the probing
beam in the materials being analysed is critical to interpreting particle scattering
spectra, and the semi-empirical database of these data regularises a massive
experimental effort, much of which is fairly recent. Modern knowledge is far more
accurate than was available a generation ago (again, see the EBS article for more
details, and [24]).
Lastly, and crucially, integrated codes allowing analysts to use TOTAL-IBA routinely
have only recently become available. OMDAQ [25] (also see [ref.6]) has been
available for over 15 years, but it is mainly a microbeam PIXE code with only rather
simple facilities for fitting particle spectra. IBAlab [26] does allow simulation of
TOTAL-IBA analyses, which is an important step. But DataFurnace, one of the "new
generation" particle-scattering codes, only acquired a PIXE module in 2006 [27]. It is
clear, from all the published TOTAL-IBA work so far, that good fitting of the particle
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spectra is essential to be able to make full use of TOTAL-IBA synergies. Moreover,
self-consistent fitting of multiple spectra is crucial to TOTAL-IBA applications.
Barrier II: EBS cross-sections
Table 1 in the ELASTIC BACKSCATTERING OF IONS FOR COMPOSITIONAL ANALYSIS
article shows that the boundary between RBS and EBS is exceeded for 2 MeV proton
beams onto all atoms lighter than Fe (see also [ref.10]). This is where the beam
energy is so high that the Rutherford approximation of the Coulomb interaction of
point charges breaks down, and a proper quantum mechanical treatment is needed for
the interaction. The EBS article explains that in the last century, ion beam analysts
were generally forced to use empirical scattering cross-section functions to make use
of EBS. These are very frequently a strong function of scattering angle, so that
typically everyone measured their own data.
However, 2 MeV (and higher energy) proton beams are typical for PIXE analysis
(see the PARTICLE-INDUCED X-RAY EMISSION article), so that to interpret particle
spectra collected simultaneously with X-ray data was often difficult. Much data did
exist of course, but its quality was always suspect even where the scattering angle
was appropriate. Therefore, there was a strong disincentive for using TOTAL-IBA
techniques on the grounds that the particle data were too often intractable.
Barrier III: Low statistics particle spectra
The other perceived barrier to doing PIXE and particle scattering simultaneously was
that, as mentioned above, the X-ray production cross-sections for proton beams
hugely exceed the particle scattering cross-sections. Thus, for microbeam (imaging)
PIXE applications, where the beam current is usually far smaller than for normal
RBS, the particle spectra typically had very few counts.
The temptation is to consider that the amount of information in a spectrum is
proportional to the number of counts in it. Of course, this is entirely false! Figure 1
demonstrates this quantitatively in the case of a mixed silicide. If this were a
TOTAL-IBA analysis, the particle spectra would be needed to determine the layer
structure of the sample so that the PIXE spectrum could be properly quantified.
Different layer structures can give vastly different absorption behaviour, which is what
is needed to interpret the relative line intensities. Even extremely noisy spectra can give
qualitatively correct layer structures, with quite accurate thicknesses. For such mixed
silicides with adjacent elements the absorption corrections can be very large.
Other Barriers
Because of the other barriers to TOTAL-IBA, and in particular because of the
unavailability of codes able to handle multiple spectra self-consistently, ion beam
analysts frequently did not put multiple detectors into their measurement systems.
There was another justification for this too: that the problems amenable to PIXE and
those amenable to particle scattering tended to be in distinct classes.
So, for whatever reason, IBA has historically been split into the "PIXE" and the
"RBS" camps, roughly speaking. Of course, many labs did both, but not usually
together. It is telling that only very recently a "TOTAL-IBA" paper from a major lab
in a high impact journal was published, but where the techniques reported did not
include self-consistent treatment of the data ([28]: this is despite the fact that the same
lab did previously report such a self-consistent treatment on some of the same
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samples [29]!)! Forty years after Feldman's paper there is again recognition that our
historic methods are heavily limited.
TOTAL-IBA: Synergies between IBA methods
Much work has been published recently using powerful methods involving synergies
between various of these IBA techniques used together self-consistently. We shall
survey these applications fairly systematically, leading towards the goal of 3D
elemental and chemical imaging.
RBS is good for heavy elements in a light matrix and typically the mass resolution is
not very good, so that only fairly simple things can be said about fairly simple
samples. On the other hand, PIXE on its own cannot compete on price against the
almost equivalent XRF (there is even an explicit comparison of PIXE with XRF
showing their near-equivalence [30]). But putting these techniques together allows
the strengths of the one to compensate for the weaknesses of the other so that the
combination is extraordinarily powerful.
In the previous section we surveyed the reasons for these synergies to have been
avoided until now: here we survey existing TOTAL-IBA examples – that is, examples
of the synergistic use of multiple IBA methods.
Ambiguity
Of course, the underlying reason for using multiple spectra self-consistently is that
individual spectra are always more or less ambiguous. Trivially, in Fig.1 the spectra
do not identify Fe and Co since the mass resolution is too poor and in any case in RBS
there is always a mass-depth ambiguity: we know these metals are present since they
were used to make the samples! But the combined use of PIXE would unequivocally
identify the metals, and a self-consistent data treatment of the particle and photon
spectra would confirm that the PIXE line intensities were consistent with the numbers
of atoms counted by RBS. A similar but much more complex example (requiring
quantitative as well as qualitative analysis) is the measurement of the
functionalisation of carbon nanotubes, where the catalyst signal must be accounted
for to extract the desired light element signal [31].
Multiple detectors or, equivalently, multiple beam incidence angles are regularly
used in RBS to identify surface signals and relieve the worst mass-depth ambiguities.
So Fig.17 in the EBS article shows a zirconia/silica multilayer sample whose
spectrum is unequivocal only because the analysis had data from two beam incidence
angles [32]. Fig.16 in the EBS article shows a complex case where two detectors and
two incidence angles are used, and all the information is used to obtain unequivocal
information about the samples. In both of these examples it was essential to impose
chemical constraints on the data to interpret them unequivocally, as discussed long
ago by Butler (1990) [33].
There are many ways of overcoming the ambiguity of any particular measurement,
and these are discussed in depth in the review by Jeynes et al (2003) [34]. We show
examples below of "TOTAL-IBA", that is, the use of multiple techniques analysed
self-consistently.
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Synergy Examples I: RBS/ERD
Fig.1 in the EBS article shows a case where it was essential to take the hydrogen
content of a sample into account, even though it was not the required measurand,
because the uncertainty of the final result needed to be as small as possible. With
He-RBS one needs only to tilt the sample such that the H-recoils can escape and be
detected to obtain simultaneous H-ERD data. One "invisible" element can of course
be inferred from spectral data, but this depends on accurate knowledge of the energy
loss (which in any case is very low for H), and usually only rough information can be
extracted in the absence of a direct signal.
Synergy Examples II: RBS/EBS/ERD/NRA
Figure 2 shows a complex analysis applied to an important case. the Joint European
Torus (JET) is a long-running tokamak experiment where many detailed analyses are
needed as a function of position of the fusion vessel linings. The distribution of the
light elements (and the heavy contaminants) is of great interest, and this analysis
using data from multiple beams can be done entirely automatically using simulated
annealing [35] on the whole (very large) dataset.
In this example of TOTAL-IBA the heavy element profiles are given by both He-RBS
and H-RBS, where the H-RBS has greater information depth but lower sensitivity. O
and Be profiles are given by EBS (with a significant cross-section enhancement over
Rutherford), and the H and D profiles are obtained by He-ERD (using a range foil)
with the D profile being obtained independently by 3He-NRA.
Synergy Examples III: RBS/EBS/PIXE
A fully self-consistent and convenient PIXE/BS analysis code based on the
DataFurnace [ref#35] and DATTPIXE [36] codes was introduced in 2006 [37]. This
was used to analyse Niépce's heliograph of 1827 [38], a 19th century reproduction of
Frans Hals' La Bohémienne [ref#29], oxidation of carbon nanotubes [ref#30] and
photovoltaic and ferroelectric materials [39] [40] [41], and so-called "Darwin glass"
samples (see Figs. 4 & 5, discussed below). In all these cases the PIXE signal was
crucial in quantifying signals that were either trace elements with no significant
particle scattering signal, or elements inextricable from others in the particle
scattering signal. In all cases the samples had more or less complex layering, so that
the PIXE signals could not be quantified without the depth information in the particle
scattering spectra. For example, the important heliograph of 1827, as the "first
photograph", is a priceless record in the history of photography now in the collection
of the Louvre museum. It suffers from corrosion and the conservators wanted to
know what exactly was the nature of the damage. The surface is a Pb/Sn alloy, and
the questions are : Is the corrosion oxidation? If so which species is oxidising? What
is the thickness of the modified layer? RBS is able to give the Pb/Sn ratio at the
surface, PIXE can give the total Pb and Sn content, and EBS gives the O (and C!)
profile. Using TOTAL-IBA with the external beam that is standard at the Louvre (see
the recent review [42]) it is clear that the tin is oxidising, and the depth of the
corroded layer is also determined. This information could not easily be obtained by
IBA without self-consistent data analysis, although this particular case is simple
enough that iterative methods would have worked [ref#28]. Note that these samples
are relatively rough, and in principle the particle scattering data can even quantify the
roughness [43].
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A further example is shown in Figure 3 of a significantly harder case which required
both differential PIXE and high energy resolution PIXE (HR-PIXE) as well as the
particle scattering spectra. This work aimed to evaluate the use of SrTiO3 as a
temperature compensator for MgTiO3 films used for filters or oscillators in telecomms
devices. A thin layer of SrTiO3 is deposited on the Pt electrode and under the final
MgTiO3 film, and the analytical question is: what happens to it? There is too little
Sr for the particle spectra to have any sensitivity for it, but it is clearly visible in
PIXE. To find out where the Sr is, differential PIXE used 250, 325, 700, 1000 and
1960 keV H beams at glancing exit geometries. For these low energy beams there is
no usable cross-section for the Sr K X-rays, and the L lines must be used instead (not
shown in Fig.3). But Sr L lies between the strong Si K and K lines, and with the
normal EDX detectors the Sr L signal must be inferred with a subtraction of the
interfering Si signal that assumes knowledge of the Si K/K ratio. In this work the
Si K/K ratio was measured directly using a high energy resolution EDS (energy
dispersive) X-ray detector of a microcalorimeter design based on superconducting
transition-edge sensors. Then, using the Si K/K ratio measured directly by HRPIXE, the Sr L signal could be extracted from the differential PIXE spectra, and the
depth profile obtained.
There is a further problem that complicates this analysis and is also quantified by
HR-PIXE: there is a Pt radiative Auger emission (RAE) satellite peak at about the
same energy as the Sr L signal. Auger electrons originate in a radiation-less process:
these RAE satellites are "forbidden" transitions requiring both photons and "Auger"
electrons to be emitted [44]. Detailed use of PIXE data requires a detailed knowledge
of PIXE physics, which is not always available despite the "maturity" of X-ray physics!
The recent availability of HR-EDS X-ray detectors has underlined this problem.
Synergy Examples IV: RBS/PIXE/MeV-SIMS
Figure 4 shows a remarkable image from an unexpected TOTAL-IBA application
using SIMS with an MeV ion beam (MeV-SIMS). Where regular SIMS using a keV
ion beam generates sputtered ions by a nuclear displacement process, MeV-SIMS
sputtering is due to the electronic energy deposition, and therefore occurs appreciably
only for insulating samples. It has already been demonstrated that MeV-SIMS
produces a significantly higher proportion of high molecular weight sputtered ions
than does keV-SIMS, even using molecular primary ion beams such as C60 [45]. And
of course, MeV beams can be used in an external beam analysis, that is, in
atmosphere. Since ion beams can be readily focussed it seems that not only is high
spacial resolution chemical imaging in atmosphere feasible, but it can also be
combined with simultaneous complementary methods than can quantify or otherwise
complete the data. MALDI (matrix-assisted laser desorption ionisation) is a powerful
and popular technique with similar capabilities, also used at ambient pressure, but it
is without either spacial resolution or the complementary information like the PIXE
that naturally accompanies MeV-SIMS.
Chemical information, that is, information about the chemical state of the elements
present in the sample, is in principle readily available with X-ray techniques. But the
measurement of chemical shifts requires an energy resolution at or below 1 eV. This
has historically been available only with electron spectrometers (XPS, AES, EELS) or
with wavelength-dispersive (WDS) X-ray spectrometry. But third-generation HREDS X-ray detectors are expected to also achieve energy resolution comparable to
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WDS in the near future [46]. We have already cited exciting HR-EDS-PIXE using
such (first-generation) detectors; there is no reason not to expect a much more
powerful capability to emerge.
TOTAL-IBA: Tomography
Tomography is a 3D imaging technique based on taking a series of slices of a sample.
With computed tomography (CT), physical slices are not taken, but a series of
images are obtained (by any technique) and subsequently reconstructed by
calculation. X-ray tomography (XR-CT) images by radiography, that is, the contrast
mechanism is from differential absorption due to density variation. XR-CT is long
established, with Cormack & Hounsfield taking the 1979 Nobel prize in medicine.
STIM-CT is an almost equivalent (and solved) problem [47].
There has been very significant recent interest in tomographic methods sensitive to
the chemistry (stoichiometry) of the sample under investigation using both
microfocussed confocal XRF and XRF-CT [48] [49]. We should comment that
synchrotron XRF is not essential to this: there have also been serious reports of XRFCT on desktop tools [50]. There has long been interest in XRF-CT: for example,
Brunetti & Golosio in 2001 [51] published an open code [52] capable of this using
Hogan’s 1991 algorithm [53]. Great strides have also been made towards a PIXE-CT:
see the summary in a recent review [54].
In principle, where radiation damage is an issue, XRF-CT is preferable to confocal
XRF for obtaining 3D information about samples, since confocal techniques throw
away all information not from the confocal plane and therefore a full analysis of the
sample must take much longer by confocal methods. Similarly, we believe that
IBA-CT will be found preferable to XRF-CT because the particle scattering that must
accompany the PIXE signal carries much information not available in XRF.
The best sy-XRF-CT spacial resolution so far reported (200 nm, determined by the spot
size of the X-ray nanobeam) is by Silversmit et al [55]. This used optimal stepping
(pixel size) of 100 nm and 180° scan in 4.5° steps per sinogram. The largest dimension
in the sample (a cometary fragment from the Stardust space mission) was 2 m and the
measurement took 26 hours beam time. Synchrotron XRF has many very powerful
variants. For example, for some samples XANES is very powerful for chemical
speciation tomography (see for example Blute et al [56]), but de Jonge & Vogt
comment that “this approach to chemically resolved tomography is not generally
applicable due to the need for a strong, unambiguous XANES feature for contrast” [57].
There are (at least) two big problems in XRF-CT, one algorithmic and one practical.
Fully quantitative XRF-CT will require self-absorption to be properly taken into
account. In the XRF community this has been approximated, but the discretised realspace reconstruction algorithm (DISRA [58]), permitting a more exact treatment, is
currently used only in the ion microprobe community.
Also, as hinted above, the classical tomography algorithms require much data and
many slices, and, even with a relatively non-destructive X-ray primary beam, the
radiation sensitivity of the sample dominates the applicability of the method.
Moreover, the sy-XRF facilities all depend on moving the sample through the beam,
which is a) slow and b) gives extra mechanical problems where very high spacial
resolution is the aim. Therefore the IBA methods still appear very interesting for the
following reasons: a) a scanning microbeam (or nanobeam) is easy, both being much
9
faster and also avoiding the mechanical problems of moving the sample, b) the
spacial resolution is dominated by the beam size, and a 100 nm proton beam for
analytical purposes appears to be entirely feasible [59], c) there are more IBA
microbeam facilities in the world than there are synchrotrons, and they could become
as common as SEMs if desktop IBA tools based on small cryogenic synchrotrons
become commercially successful, d) many of the advances happening at the
synchrotron facilites are equally applicable to IBA facilities (fast detectors for
example [60]).
However, new algorithms incorporating particle scattering data may well be orders of
magnitude faster than the classical tomography algorithms, and in addition it may be
that an order of magnitude higher spacial resolution is possible, given by the energy
resolution of the particle spectra (for samples where 180° rotation is feasible). An
interesting example is an analysis of a 20 m geological sample [61]. Figure 4 shows
PIXE maps for one orientation of the sample, together with various BS spectra from
various parts of the sample. Figure 5 shows the spacial distribution of one principal
component of the PIXE data cube in Fig.4, together with the PIXE and BS spectra for
that component. But the depth profile can be obtained explicitly for the PIXE/BS
data, using TOTAL-IBA, so that from the projection at just one angle the entire 3D
elemental map of the sample can be reconstructed, combining the principal
components appropriately for each pixel. Many problems remain before the
algorithm for this reconstruction can be specified in detail, an algorithm which in any
case is quite different from the classical CT algorithms. If it exists, this algorithm
will be hugely more efficient. And it seems very likely that it exists.
We have shown that IBA-CT (that is, using the BS signals as well as the PIXE
signals) may already be achievable, and should be orders of magnitude more efficient
(and therefore much faster!) than pure XRF-CT (or PIXE-CT) since a single slice
already has very substantial depth information from the particle spectrum. This is
important since tomography is rather slow, and its importance is increased since it
seems that beam damage severely limits the use of a pure PIXE-CT for important
classes of samples [62]. This is also true for XRF-CT [ref#41]. In principle, using
the depth information available explicitly in IBA-CT (from the particle signals) must
be quicker than unfolding the depth information available only implicitly (and at
much lower depth resolution) in the PIXE signals.
Conclusions
In the last five or six years there has been a step-function change in the capability of
ion beam analysis, as we have found out how to put the various IBA techniques
together in a powerful synergy. The most critical step has been the incorporation of
PIXE into TOTAL-IBA, since PIXE and the particle methods complement each other
perfectly, and have historically been the most completely separated. Where one has
excellent depth resolution the other has excellent elemental discrimination, and a
TOTAL-IBA analysis inherits the strengths of both. And since ion beams are readily
focussed, TOTAL-IBA mapping (in 3D because of the depth sensitivity) is
straightforward and should also have a powerful tomographic capability.
Analysts today should always include both particle and photon detectors in their target
chambers, and users should expect them to do so!
10
Figure 1: Noisy spectra can contain crucial information!
1.5MeV He RBS spectra of a mixed Fe:Co silicide on a Si substrate are simulated from an
initial structure (top right), given various charge solid-angle products (left hand column).
These simulated spectra are treated as data, fitted (lines) and hence inverted back to depth
profiles, with ±1 uncertainties given by a Bayesian analysis (right hand column). Modified
from Figs.1&2 of Barradas et al, 2000 [63]. (Reproduced from Fig.27 of Jeynes et al, 2011 [64].)
11
Figure 2: "TOTAL-IBA" of JET samples using sequentially collected RBS, EBS,
ERD, NRA data.
Top left: 2 MeV 4He RBS; Top right: 2.45 MeV 1H EBS; Bottom left: 2 MeV 4He ERD with
inset of 2.3 MeV 3He NRA. Bottom right shows the depth profile obtained through a selfconsistent treatment of all the data. (Reproduced from Fig.2 of Alves et al, 2010 [65])
12
Figure 3: RBS/EBS/PIXE analysis of MgTiO3/Sr TiO3/ bilayer on Pt/Si
Data from Fig.2 of Reis et al, NIMB 268, 2010, 1980-5 [66], with extracted depth profile
(bottom, right) reproduced from Fig.4 of Reis et al, XRS 40, 2011, 153-156 [67]
1 MeV H
2 MeV He
Line area (counts)
2 MeV He
106
103
1
Ti K Sr K Pt L
13
Figure 4: Images of a grid obtained by TOTAL-IBA using 10 MeV O4+
Ta La PIXE and RBS show the metal grid with shadowing contrast depending of the detector
position, and MeV-SIMS signals (from the insulating carbon tape) for two different
molecular weight fragments. Reproduced from Fig.3 of Jones et al, NIMB 268, 2010, 17141717 [68]
14
Figure 5: "TOTAL-IBA" of an inclusion in a Darwin Glass.
Above: selected PIXE maps showing distribution of Si, Fe, Cu; Centre: BS spectra at varying energies of the
resin region showing the 12C(p,p0)12C resonance at 1734 keV; Below: BS spectra at 1.9 MeV for three areas
marked on the Si PIXE map (above, left). (See Bailey et al, 2009 [ref#]). After Fig.1 of [ref#1]. Reproduced from
Fig.28 of Jeynes, Webb & Lohstroh, 2011 [ref#47]
15
Figure 6: Principal component decomposition of the data cube of Fig.3.
One component from the principal component decomposition of the data cube of Fig.3 using AXSIA
(see Doyle et al, 2006 [69]). This component is one of the several Si-rich components. Reproduced from Fig.2 of
[ref#1].
sdfsdfs
16
Table 1: Glossary, with names of other articles in the ION BEAM METHODS Chapter
Term
Expansion
Reference
IBA
Ion Beam Analysis
INTRODUCTION
Total-IBA
(RBS, EBS, ERD, NRA, PIXE,
MeV-SIMS; using MeV ion (micro)beams for elemental (and chemical) depth
profile (including 3D) information.
This article
RBS
Rutherford backscattering spectrometry
ELASTIC BACKSCATTERING OF IONS FOR
COMPOSITIONAL ANALYSIS
EBS
Elastic (non-Rutherford) backscattering
spectrometry
ELASTIC BACKSCATTERING OF IONS FOR
COMPOSITIONAL ANALYSIS
ERD
Elastic recoil detection
ELASTIC RECOIL DETECTION ANALYSIS
NRA
Nuclear reaction analysis
NUCLEAR REACTION ANALYSIS AND
PARTICLE-INDUCED GAMMA EMISSION
PIXE
Particle-induced X-ray emission
PARTICLE-INDUCED X-RAY EMISSION
PIGE
Particle-induced gamma-ray emission
NUCLEAR REACTION ANALYSIS AND
PARTICLE-INDUCED GAMMA EMISSION
Channelling Well-collimated beam aligned with MEDIUM ENERGY ION SCATTERING
crystallographic axes in single crystals for
crystalline defects and lattice location of
impurities.
SIMS
Secondary Ion Mass Spectrometry (with a
keV ion beam) and also MeV-SIMS
SECONDARY ION MASS SPECTROMETRY
LEIS
Low Energy Ion Scattering
LOW ENERGY ION SCATTERING
MEIS
Medium Energy Ion Scattering
MEDIUM ENERGY ION SCATTERING
IBIC
Ion-Beam-Induced Charge
ION-BEAM-INDUCED CHARGE
AMS
Accelerator Mass Spectrometry
ACCELERATOR MASS SPECTROMETRY
17
References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
C.Jeynes, M.J.Bailey, N.J.Bright, M.E.Christopher, G.W.Grime, B.N.Jones, V.V.Palitsin, R.P.Webb,
"Total IBA" – where are we? Nucl. Instr. Methods B, 271 (2012) 107-118
L.C. Feldman, J. M. Poate, F. Ermanis and B. Schwartz, Combined use of He backscattering
and He-induced X-rays in study of anodically grown oxide films on GaAs, Thin Solid Films 19
(1973) 81-89
Barradas, N.P., Likonen, J., Alves, E., Alves, L.C., Coad, P., Hakola, A., Widdowson, A,
Integration of SIMS into a general purpose IBA data analysis code, AIP Conference
Proceedings 1336 (2011) 281-285
Barradas N P, Keddie J L and Sackin R, Bayesian inference analysis of ellipsometry data Phys.
Rev. E 59 (1999) 6138–51
Payne, R. S.; Clough, A. S.; Murphy, P.; Mills, P. J., Use of the d(3He,p)4He reaction to study
polymer diffusion in polymer melts Nucl. Instrum. Methods B, 42 (1989) 130–134
Thompson RL, Hardman SJ, Hutchings LR, Narrainen AP, Dalgliesh RM, pH-Controlled
Polymer Surface Segregation, Langmuir 25(5) (2009) 3184-3188
Elspeth F. Garman, Geoffrey W. Grime, Elemental analysis of proteins by microPIXE,
Progress in Biophysics and Molecular Biology 89 (2005) 173–205
G.Boudreault, C.Jeynes, E.Wendler, A.Nejim, R.P.Webb, U.Wätjen, Accurate RBS measurement of
ion implant doses in a silicon, Surf.Interface Anal., 33 (2002) 478-486
C.Jeynes, N.Peng, N.P.Barradas, R.M.Gwilliam, Quality assurance in an implantation
laboratory by high accuracy RBS, Nucl. Instr. Methods B, 249 (2006) 482–485
N.P. Barradas, K. Arstila, G. Battistig, M. Bianconi, N. Dytlewski, C. Jeynes, E. Kótai, G. Lulli,
M. Mayer, E. Rauhala, E. Szilágyi and M. Thompson, "Summary of 'IAEA intercomparison of
IBA software' ", Nucl. Instr. Methods B, 266 (2008) 1338-1342
A.F.Gurbich, Evaluated differential cross-sections for IBA, Nucl. Instrum. Methods B, 268
(2010) 1703-1710
D.Abriola, N.P.Barradas, I.Bogdanović-Radović, M.Chiari, A.F.Gurbich, C.Jeynes,
M.Kokkoris, M.Mayer, A.R.Ramos, L.Shi, I.Vickridge, Development of a reference database
for Ion Beam Analysis and future perspectives, Nucl. Instrum. Methods B, 269 (2011) 20112016
J.L.Campbell, N.I.Boyd, N.Grassi, P.Bonnick, J.A.Maxwell, The Guelph PIXE software
package IV, Nucl. Instrum. Methods B, 268 (2010) 3356–3363
M.E Fedi, M Chiari, L Giuntini, F Lucarelli, P.A Mandò, The differential PIXE set-up at the
Van de Graaff laboratory in Florence, Nucl. Instrum. Methods B, 189 (2002) 56-61
G. Demortier, J.L. Ruvalcaba-Sil, Differential PIXE analysis of Mesoamerican jewelry items,
Nucl. Instrum. Methods B, 118 (1996) 352-358
F. William Reuter and Harold P. Smith, Full range solution for the measurement of thin film
surface densities with proton excited x rays, J. Appl. Phys. 43 (1972) 4228-4232
W. Pabst, Ion-induced X-ray spectroscopy as a method to determine depth distribution of trace
elements, Nucl. Instr. and Meth. 124 (1975) 143-147
M.Ahlberg, Simple depth profile determination by proton-induced X-ray emission, Nucl. Instr.
and Meth. 131 (1975) 381-384
N.P.Barradas, K.Arstila, G.Battistig, M.Bianconi, N.Dytlewski, C.Jeynes, E.Kótai, G.Lulli,
M.Mayer, E.Rauhala, E.Szilágyi, M.Thompson, IAEA intercomparison of IBA software, Nucl.
Instr. Methods B, 262 (2007) 281–303
N.P.Barradas, C.Jeynes, R.P.Webb, Simulated annealing analysis of RBS data, Appl. Phys. Lett. 71
(1997) 291–293
M.Mayer, SIMNRA, a simulation program for the analysis of NRA, RBS and ERDA, AIP
Conference Proceedings 475 (1999) 541-544
N.P.Barradas, C.Jeynes, Advanced physics and algorithms in the IBA DataFurnace, Nucl. Instr.
Methods B, 266 (2008) 1875-1879
M. Mayer, W. Eckstein, H. Langhuth, F. Schiettekatte, U. von Toussaint, Computer simulation
of ion beam analysis: Possibilities and limitations, Nucl. Instr. Methods B, 269 (2011) 30063013
James F. Ziegler, M.D.Ziegler, J.P.Biersack, SRIM – The stopping and range of ions in matter
(2010), Nucl. Instrum. Methods B, 268 (2010) 1818-1823
18
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
G.W. Grime, The “ Q factor” method: quantitative microPIXE analysis using RBS
normalisation, Nucl. Instr. Methods B, 109-110 (1996) 170-174
I. Orlić, Shijun Zhou, J.L. Sanchez, F. Watt, S.M Tang, Virtual PIXE and RBS laboratory, Nucl.
Instr. Methods B, 150 (1999) 83-89
C.Pascual-Izarra, M.A.Reis, N.P.Barradas, Simultaneous PIXE and RBS data analysis using
Bayesian inference with the DataFurnace code, Nucl. Instr. Methods B, 249 (2006) 780-783
L.de Viguerie, L.Beck, J.Salomon, L.Pichon, Ph.Walter, Composition of Renaissance paint
layers: simultaneous PIXE and BS, Anal. Chem., 81 (2009) 7960-7966
L.Beck, C.Jeynes, N.P.Barradas, Characterization of paint layers by simultaneous selfconsistent fitting of RBS/PIXE spectra using simulated annealing, Nucl. Instr. Methods B, 266
(2008) 1871-1874
Giulia Calzolai, Massimo Chiari, Franco Lucarelli, Federico Mazzei, Silvia Nava, Paolo Prati,
Gianluigi Valli, Roberta Vecchi, PIXE and XRF analysis of particulate matter samples: an interlaboratory comparison, Nucl. Instr. Methods B, 266 (2008) 2401–2404
J.C.G. Jeynes, C. Jeynes, K.J. Kirkby, M. Rümmeli, S.R.P. Silva, RBS/EBS/PIXE measurement
of single-walled carbon nanotube modification by nitric acid purification treatment, Nucl. Instr.
Methods B, 266 (2008) 1569–1573
C. Jeynes, N. P. Barradas, H. Rafla-Yuan, B. P. Hichwa, R. Close, Accurate depth profiling of
complex optical coatings, Surf. Interface Anal. 30 (2000) 237–242
J.W.Butler, Criteria for validity of Rutherford scatter analysis, Nucl. Instr. Methods B, 45
(1990) 160–5
C Jeynes, N P Barradas, P K Marriott, G Boudreault, M Jenkin, E Wendler and R P Webb,
Elemental thin film depth profiles by ion beam analysis using simulated annealing—a new tool,
J. Phys. D: Appl. Phys. 36 (2003) R97–R126
N.P.Barradas, C.Jeynes and R.P.Webb, Simulated annealing analysis of Rutherford
backscattering data, Appl. Phys. Lett. 71 (1997) 291–3
M.A. Reis, L.C. Alves, A.P. Jesus, Matrix effects correction for quantitative TTPIXE analysis,
Nucl. Instr. Methods B, 109-110 (1996) 134-138
C. Pascual-Izarra, M.A. Reis, N.P. Barradas, Simultaneous PIXE and RBS data analysis using
Bayesian inference with the DataFurnace code, Nucl. Instr. Methods B, 249(1-2) (2006) 780783
C. Pascual-Izarra, N. P. Barradas, M. A. Reis, C. Jeynes, M. Menu, B. Lavedrine, J. J. Ezrati, S.
Röhrs, Towards truly simultaneous PIXE and RBS analysis of layered objects in cultural
heritage, Nucl. Instr. Methods B, 261(2007)426-429
V. Corregidor, P. C. Chaves, M. A. Reis, C. Pascual-Izarra, E Alves and N. P Barradas,
Combination of IBA techniques for composition analysis of GaIn AsSb films, Mat. Sci. Forum,
514-516 (2006) 1603-1607
Chris Jeynes, G.Zoppi, I.Forbes, M.J.Bailey, N.Peng, Characterisation of thin film
chalcogenide PV materials using MeV ion beam analysis, IEEE Proceedings of Supergen
Conference, Nanjing, April 2009, DOI:10.1109/SUPERGEN.2009.5348162
Reis MA, Barradas NP, Pascual-Lzarra C, Chaves PC, Ramos AR, Alves E, Gonzalez-Aguilar
G, Costa MEV, Salvado IMM, Holistic RBS-PIXE data reanalysis of SBT thin film samples,
Nucl. Instr. Methods B, 261 (2007) 439-442
L.Giuntini, A review of external microbeams for ion beam analyses, Anal.Bioanal.Chem. 401
(2011) 785-793
S L Molodtsov, A F Gurbich, C Jeynes, Accurate ion beam analysis in the presence of surface
roughness, J. Phys. D: Appl. Phys. 41 (2008) 205303 (7pp)
F.Bloch, Double Electron Transitions in X-Ray Spectra, Phys. Rev. 48 (1935) 187-192
Y.Wakamatsu, H.Yamada, S.Ninomiya, B.N.Jones, T.Seki, T.Aoki, R.P.Webb, J.Matsuo,
Biomolecular Emission by Swift Heavy Ion Bombardment, Ion Implantation Technology (AIP:
CP1321, 2010, edited by J.Matsuo, M. Kase, T. Aoki, and T. Seki) 233-236
T. Jach, J.N.Ullom, W.T.Elam, The microcalorimeter X-ray detector: A true paradigm shift in
X-ray spectroscopy, Eur. Phys. J. Special Topics 169, 237–242 (2009)
T.Satoh, M.Oikawa, T.Kamiya, Three-dimensional measurement of elemental distribution in
minute samples by combination of in-air micro-PIXE and STIM, Nucl. Instr. Methods B,
267 (2009) 2125-2127
19
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
Janssens K, De Nolf W, Van Der Snickt G, Vincze L, Vekemans B, Terzano R, Brenker FE,
Recent trends in quantitative aspects of microscopic X-ray fluorescence analysis. Trends Anal.
Chem. 29(6) (2010) 464–478
Ursula Elisabeth, Adriane Fittschen & Gerald Falkenberg, Confocal MXRF in environmental
applications, Anal. Bioanal. Chem. 400 (2011) 1743–1750
Brian M. Patterson, John Campbell and George J. Havrilla, Integrating 3D images using
laboratory-based micro X-ray computed tomography and confocal X-ray fluorescence
techniques, X-Ray Spectrom. 39 (2010) 184–190
A. Brunetti, B. Golosio, Software for X-ray fluorescence and scattering tomographic
reconstruction, Computer Physics Communications 141 (2001) 412–425
http://cpc.cs.qub.ac.uk/summaries/ADPH
J.P. Hogan, R.A. Gonsalves, A.S. Krieger, Fluorescent computer tomography: a model for
correction of X-ray absorption, IEEE Trans. Nucl. Sci. 38 (1991) 1721–1727
C.G.Ryan, PIXE and the nuclear microprobe: Tools for quantitative imaging of complex natural materials,
Nucl. Instr. Methods B, 269 (2011) 2151-2162
Geert Silversmit, Bart Vekemans, Frank E. Brenker, Sylvia Schmitz, Manfred Burghammer,
Christian Riekel, and Laszlo Vincze, X-ray Fluorescence Nanotomography on Cometary Matter
from Comet 81P/Wild2 Returned by Stardust, Anal. Chem. 81 (2009) 6107–6112
N.K. Blute, D.J. Brabander, H.F. Hemond, S.R. Sutton, M.G. Newville, M.L. Rivers, Arsenic
sequestration by ferric iron plaque on cattail roots, Environ Sci Technol, 38 (2004) 6074–6077
Martin D de Jonge, Stefan Vogt, Hard X-ray fluorescence tomography — an emerging tool for
structural visualization, Current Opinion in Structural Biology, 20 (2010), 606–614
Sakellariou A, Cholewa M, Saint A, Legge GJF, An accurate reconstruction algorithm for
tomography experiments that involve complex probe–sample interactions, Meas Sci Technol. 8
(1997) 746-758
D.N.Jamieson, New generation nuclear microprobe systems, Nucl Instrum Methods B, 181
(2001) 1-11
Ryan CG, Siddons DP, Moorhead G, Kirkham R, Dunn PA, Dragone A, De Geronimo G, Large
detector array and real-time processing and elemental image projection of X-ray and proton
microprobe fluorescence data, Nucl Instrum Methods B, 260 (2007) 1-7
M.J.Bailey, K.T.Howard, K.J.Kirkby, C.Jeynes, Characterisation of inhomogeneous inclusions in
Darwin glass using IBA, Nucl. Instr. Methods B, 267 (2009) 2219–2224
T.Andrea, M.Rothermel, R.Werner, T.Butz, T.Reinert, Limited angle STIM and PIXE
tomography of single cells, Nucl. Instr. Methods B, 268 (2010) 1884–1888
N.P.Barradas, C.Jeynes, M.Jenkin, P.K.Marriott, Bayesian error analysis of Rutherford
backscattering spectra, Thin Solid Films, 343–344 (1999) 31–4
Chris Jeynes, Roger P. Webb, Annika Lohstroh, Ion Beam Analysis: A Century of Exploiting
the Electronic and Nuclear Structure of the Atom for Materials Characterisation, Reviews of
Accelerator Science and Technology, 4 (2011) 41–82
L.C. Alves, E. Alves, N.P. Barradas, R. Mateus, P. Carvalho, J.P. Coad, A.M. Widdowson, J.
Likonen, S. Koivuranta, Erosion and re-deposition processes in JET tiles studied with ion
beams, Nucl. Instrum. Methods B 268 (2010) 1991–1996
M.A. Reis, L.C. Alves, N.P. Barradas, P.C. Chaves, B. Nunes, A. Taborda, K.P. Surendran, A.
Wu, P.M. Vilarinho, E. Alves, High Resolution and Differential PIXE combined with RBS,
EBS and AFM analysis of magnesium titanate (MgTiO3) multilayer structures, Nucl. Instr.
Methods B, 268 (2010) 1980–1985
M. A. Reis, N. P. Barradas, P. C. Chaves and A. Taborda, PIXE analysis of multilayer targets,
X-Ray Spectrom., 40 (2011) 153–156
B.N.Jones, V.Palitsin, R.P.Webb, Surface analysis with high energy time-of-flight secondary
ion mass spectrometry measured in parallel with PIXE and RBS, Nucl. Instr. Methods B, 268
(2010) 1714-1717
B.L.Doyle, P.P.Provencio, P.G.Kotula, A.J.Antolak, C.G.Ryan, J.L.Campbell, K.Barrett, PIXEquantified AXSIA: Elemental mapping by multivariate spectral analysis, Nucl. Instr. Methods
B, 249 (2006) 828-832
20