A New Version of the Millbrook MiniSIMS

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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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A New Version of the Millbrook MiniSIMS



The Millbrook MiniSIMS is the only instrument to provide a fast, comprehensive surface analysis
capability in a convenient desktop size. Millbrook have now introduced a second, high
-
performance, version of the Min
iSIMS incorporating a time of flight mass spectrometer
(ToFSIMS). By using the latest spectrometer and detector technology, the improved performance is
actually achieved with an increase in analysis speed and a reduction in operating complexity. The
nove
l design also means the ToF MiniSIMS has some significant advantages over much more
expensive SIMS systems.



The MiniSIMS


The MiniSIMS is shown in Figure 1. It uses the well
-
established analysis technique of secondary
ion mass spectrometry or SIMS (see
Appendix A and Figure 2). Despite its apparent simplicity,
this one instrument can generate mass spectra, images and depth profiles to build up a complete
picture of the surface of a material. With a sample loading time of just a few minutes, and data
ac
quisition times measured in seconds rather than minutes, the MiniSIMS has allowed fast, cost
-
effective analysis of samples. Its small size means it can be located wherever is most convenient


next to a production line, close to an experimental process, o
r out in remote locations. In
universities and companies throughout the world, it has become an indispensable tool, operated
either in isolation or as a partner to an existing SIMS, XPS, Auger or SEM/EDS instrument.





Figure 1 The Millbrook MiniSIMS (
ToF version), a cost
-
effective desktop instrument
for the routine identification of surface contaminants. Further details can be found at

www.minisims.com


The original MiniSIMS was based on a quadrupole mass anal
yser. The higher performance version
of the MiniSIMS uses a time of flight (ToF) mass spectrometer (ToFSIMS). The major advantage
is that a ToF spectrometer collects and measures ions of all masses simultaneously, whereas the
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quadrupole only measures ion
s of one mass at any one time (see Appendix B and Figure 3 for
further details). In the extreme case of acquiring a quadrupole mass spectrum over its full 300 amu
range, the quadrupole scans each mass in turn and so is actually only using 1/300th of the a
vailable
material. The ToF spectrometer would collect all 300 potential ions in parallel and therefore makes
much more efficient use of the sputtered material. This allows data to be collected with a much
lower cumulative primary ion dose (see below).



A novel design of ToF analyser


Most ToFSIMS instruments are designed for maximum efficiency, and use a pulsed incoming
primary beam. In fact, the primary beam is typically only active for <1% of the elapsed analysis
time. Therefore, although cumulative

primary ion dose is minimised, long analysis times are
required. The ToF MiniSIMS uses a novel design of detector and analyser, in which pulsing is
instead applied to the secondary ions as they enter the analyser. Although some secondary ions are
lost i
n the pulsing process, the parallel detection feature means that overall there is still much more
efficient use of the sputtered material than in a quadrupole. The great benefit of this unconventional
approach is that secondary pulsing allows a continuous

primary beam to be used. This maximises
data acquisition rates and minimises the time taken to complete the analysis. The increased
efficiency of the ToF analyser therefore translates directly into shorter analysis times, which can be
more than 100 time
s faster than the quadrupole version of the instrument.


As well as the capability for faster analysis, the efficiency of the ToF spectrometer also means
improved analysis in situations where the bombarding primary ion flux must be kept to a minimum.
Thi
s requirement becomes increasingly important when working at higher magnifications,
especially when organic materials are involved. One such case in a typical failure analysis
laboratory would be the identification of a small defect or surface contaminan
t. Sometimes
SEM/EDS can give the answer, but often the contaminant is of a nanoscale thickness, and is
therefore too thin to provide a meaningful EDS analysis. It is even more frustrating when the
contaminant is organic in nature
-

even when an EDS spec
trum can be obtained it does little more
than reveal the presence of carbon without any information on molecular structure. What is needed
in such cases is a localised mass spectrometry technique with a small probe size and high surface
specificity, which

is exactly what SIMS has to offer.



Improved organic analysis


The original quadrupole MiniSIMS provided good identification of organic material from large (e.g.
>1 mm
2
) contaminated areas. However, in smaller areas (<0.1 mm
2
) the amount of material
ava
ilable in the surface layer is reduced by several orders of magnitude. The limited efficiency of
the quadrupole mass spectrometer meant that only part of the mass spectrum could be acquired
before the molecules remaining on the surface had undergone signi
ficant fragmentation. This
partial spectrum was often sufficient to confirm or exclude a suspected source of contamination, but
completely unknown contaminants were not always identifiable.


In such cases the additional efficiency of the ToF spectrometer
comes into its own. By analysing
secondary ions of all masses simultaneously, a full spectrum can be acquired from these smaller
areas before the molecular information starts to degrade. In the example shown in Figure 4,
complete positive and negative sp
ectra were acquired in less than 30 seconds from an area of just
200
µ
m x 200
µ
m. The characteristic peaks can be compared with library spectra over the full mass
range, improving the quality of the identification. In this case the contaminant was identi
fied as a
proprietary cutting fluid from the Ecocut™ range.

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Figure 4 Positive and negative SIMS spectra of a cutting fluid acquired in < 30 seconds
from a contaminated area 200
µ
m in size. The characteristic fragmentation pattern can
be used to i
dentify the contaminant by comparison with library data.

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Other advantages of ToFSIMS


The spectra in Figure 4 also demonstrate other advantages of the ToF spectrometer, notably the
extended mass range (over 1000 amu). The secondary ions seen at lower mas
ses (100


300 amu)
represent characteristic fragments of the whole molecule, and therefore provide significant
information on molecular structure. However, the peaks observed at higher masses give even more
detailed information on organic structure. For

many biological molecules and polymer additives,
this extended mass range means that a peak corresponding to the complete molecule can be
observed, allowing an unambiguous identification.


A further advantage is that the ToF spectrometer can be operated i
n a high mass resolution mode to
separate peaks at the same nominal integer mass. This is particularly useful to differentiate between
elemental and organic species, for example to identify aluminium (Al
+
) in the presence of surface
hydrocarbons (C
2
H
3
+
).

Although both these ions have a nominal mass of 27 Da, at higher mass
resolution the peak can be split into the two component peaks at 26.982 and 27.024 respectively.
This high mass resolution capability again reduces ambiguity in data interpretation, an
d effectively
improves the minimum detectable concentration level for many elements.



Retrospective experiments


Higher performance is normally gained at the expense of user friendliness. Unexpectedly, in the
MiniSIMS ToF all this additional capability
is actually gained with a reduction in complexity for
the user! This is because the simultaneous detection of all secondary ions means that the operator
does not have to make on
-
the
-
spot choices during the analysis about which secondary ions to
monitor or

in what order. Instead, one complete data set can be acquired, consisting of a set of pixel
co
-
ordinates and a full mass spectrum associated with each pixel. The information can then be
derived from the data afterwards, with no need to return to the ins
trument to run further
experiments. The speed of the instrument is again impressive, with acquisition of the full data set
shown in Figure 5 taking less than 30 seconds to acquire.


The example in Figure 5 shows how localised spectra and individual seco
ndary ion maps can be
extracted from the data set to identify a fluorocarbon
-
based lubricant contaminant and reveal its
distribution. This “retrospective experiment” is extremely powerful. Many individual pixel data
points can be added together to give a

composite spectrum associated with a contaminated area.
Mapping the characteristic peaks may then reveal other features, such as the second smaller 50
µ
m
contaminant spot seen here. Images of several characteristic secondary ions can be added together
t
o increase overall intensity. Often a commonly occurring element such as sodium can be used to
mark the contaminant boundaries, but the composite spectrum derived from the bounded areas will
reveal additional components. This all adds up to a detailed an
alysis result, allowing the specific
origin of the contaminant to be traced.


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Figure 5 A full data set (acquired in under 30 seconds) used to derive extensive
information about the distribution and identity of a fluorocarbon contaminant present on
t
he surface. There is no need to return to the instrument to run further experiments.

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Superior depth profiling


This retrospective data analysis can be extended to the third physical dimension of depth, showing
how the chemical composition changes beneat
h the original surface. The data set then becomes a
3D pixel array, with a full mass spectrum associated with every point. The use of a continuous
primary beam in the ToF MiniSIMS is especially beneficial when this type of depth analysis is
required. In

a conventional ToFSIMS, the pulsing primary beam is inactive for over 99% of the
analysis time. Etch rates are therefore too slow to be useful, so most ToFSIMS instruments
interleave etching (with a continuous primary beam) with analysis (with a pulsing
primary beam).
This has the major disadvantage that the majority of material is sputtered during the etching part of
the cycle, but all this potential information is simply wasted. From this it can be deduced that even
the most expensive ToFSIMS system
may not be the ideal choice if depth profiling is a frequent
requirement.


In the ToF MiniSIMS, the continuous primary beam means significant erosion rates can be
achieved without interrupting secondary ion collection and analysis. All sputtered material
can
therefore contribute to the analysis. As well as maximising sensitivity, the continuous analysis
ensures that unexpected features are not missed. Although the data can simply be displayed as a
conventional depth profile showing the changing intensity

of specific secondary ions, the lateral
distribution of each species can also be examined at different depths. Retrospective spectra can be
extracted at points of interest such as the interface between two layers, which is ideal for detecting
contaminant
s that were present on a substrate before a coating was deposited. Previously it was
necessary to make a judgement when the interface had been exposed, and interrupt the depth profile
to acquire spectra.



Summary


It can be seen that the ToF MiniSIMS re
tains the benefits of size and cost
-
effectiveness of the
original quadrupole MiniSIMS instrument, but with a greatly improved performance when
analysing organic materials. The advantages are especially significant when working at higher
magnifications. A
mazingly, this improvement has been achieved with an increase in analysis speed
and a reduction in operating complexity. In contrast to much more expensive SIMS instruments,
either version of the MiniSIMS provides a rounded capability for spectral acquisi
tion, imaging and
depth profiling rather than being optimised for one mode of operation. It is therefore a great
addition to any laboratory, either as a stand
-
alone instrument or to complement other surface
analysis equipment.


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Appendix A What is Second
ary Ion Mass Spectrometry?





Figure 2 The SIMS sputtering process. The image is one frame taken from a computer
-
generated simulation (courtesy of Dr Zbigniew Postawa, Jagiellonian University,
Poland).



Secondary Ion Mass Spectrometry (SIMS) is based
on a process known as
sputtering

(Figure 2).
The sample is placed in a vacuum. A beam of high energy keV ions (
primary ions
) is directed on
to the surface of interest. The primary ions are implanted beneath the surface and initiate chains of
collisions
between the atoms in the near surface layers. Some of the atoms are lifted out the
surface; many are liberated as neutral species, but some are positively or negatively charged. These
are known as the
secondary ions
, which can then be analysed by
mass sp
ectrometry
. The
secondary ions range in size from individual atoms to complete organic molecules that were present
at the surface.


Only ions from the outermost surface atomic layers can escape, so a SIMS spectrum represents the
composition of the extreme

surface. In this mode, the
cumulative primary ion dose

is kept to a
minimum so that on an atomic scale only a small fraction (<1%) of the surface layer is affected.
The surface under investigation is then effectively unchanging, and so the analysis is c
alled
static
SIMS
. This is especially relevant for organic materials, where prolonged ion bombardment causes
fragmentation of the molecules remaining on the surface long before the material is actually
sputtered away.


The primary beam can also be focused

to a small spot to localise the analysis on a specific feature.
If the ion beam bombardment time is then deliberately increased, significant etching occurs. This
allows the sub
-
surface composition to be progressively analysed, which is called
dynamic SI
MS
or
depth profiling
.


The primary ion beam can be raster scanned over the surface, or the spatial distribution of
secondary ions can be preserved during mass analysis. Both techniques yield a map of chemical
composition based on the origin of secondar
y ions from different parts of the surface, and the
technique is then called
imaging SIMS
.

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Appendix B How do SIMS Mass Analysers Work?














Figure 3 Different types of mass analyser used for SIMS a) quadrup
ole, b) magnetic
sector, c) time of flight (reflectron geometry). Each has specific advantages for certain
types of SIMS analysis.




A
quadrupole

mass analyser (Figure 3a) consists of four longitudinal rods. A voltage with rf and
dc components is applie
d to the rods, generating a rapidly varying electrostatic field at the centre.
Secondary ions entering the analyser are sent into oscillating trajectories. Ions of a particular
mass/charge (m/z) ratio come into resonance and adopt stable trajectories. O
nly these ions are
transmitted through the quadrupole to reach the detector. Scanning the applied voltages changes the
field to transmit ions of each m/z value in turn. A complete mass spectrum can therefore be built up
sequentially.


Since a quadrupol
e is acting more as a mass filter than as a true mass analyser, it is inherently
inefficient. In practical terms, it has a limited mass range. Transmission is poor and normally
decreases at higher m/z values. Achievable mass resolution is also limited,
and a quadrupole will
typically only be used to separate ions at adjacent integer m/z values. Despite these performance
limitations, the quadrupole has significant advantages in terms of its low cost and high stability.




A
magnetic sector

analyser (Figu
re 3b) uses a magnetic field to bend the trajectory of incoming
secondary ions into a circular path. The radius of the path depends on the m/z ratio for the ion.
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9

Although the ions are separated, many magnetic sector spectrometers use a single detector lo
cated
behind an exit slit, and scan an electromagnetic field to transmit ions of each m/z value in turn.
More complex spectrometers have multiple detectors so that ions of up to 10 different m/z values
can be detected simultaneously.


In addition to the c
apability for some degree of parallel detection, overall transmission of the
analyser is much better than for a quadrupole. Magnetic sector spectrometers can also have a high
upper limit for the mass range. This type of analyser is well suited to separat
ing two ions of
different species at the same nominal m/z value, such as two isotopes of different elements. A
magnetic sector spectrometer can also be designed with ion focusing to produce a direct image of
the surface.



A
time of flight (ToF)

analyser,

in its simplest form, is a field
-
free drift region of known length. If
a secondary ion enters the analyser with a given kinetic energy (
½
mv
2
), its velocity will be
inversely proportional to m
½
. The time taken for the ion to reach the detector at the end

of the drift
region can therefore be used to determine its mass. If ions of different masses enter the analyser at
the same time, the heavier ions will arrive progressively later, but it is important to note that all ions
are transmitted and measured. M
any ToF analysers use a reflectron design (as shown in Figure 3c),
where ions of the same mass but higher energy follow a longer flight path, which relaxes the
restriction on the ions to have exactly the same starting energy.


In a conventional ToF SIMS sy
stem, the primary ion beam is pulsed to set a specific time at which
the secondary ions enter the analyser and the timing process is started. Only when all ions have
reached the detector does the next primary ion pulse occur. The pulse duration (nanoseco
nds) is
short compared to the flight time (microsconds), so the duty cycle of the system is often <<1%.
This leads to artificially long analysis times. In the ToF analyser of the MiniSIMS the primary
beam is continuous and the secondary ions are pulsed i
nto the analyser. During the flight time of a
packet of ions, more secondary ions are continuing to arrive at the entrance of the analyser; these
ions are stored awaiting the next pulse into the flight region. There are therefore many more
secondary ions

in each pulse, which means analysis times are kept short. The very high data
acquisition rate means that a sophisticated detector and timing system is required.


ToF analysers therefore feature parallel rather than sequential detection of ions of differe
nt masses.
This improved utilisation of secondary ions is a major advantage in static SIMS where the
cumulative primary ion dose must be kept to low levels. Transmission is uniformly good. There is
theoretically no upper limit to the mass range, althoug
h slower ions are more difficult to detect.
The timing process is sufficiently precise to distinguish between some ions at the same nominal m/z
value. One design of spectrometer produces a direct image of the surface.