The Development of a Very High Mass Polyatomic Ion Beam System for Large Molecule Desorption in
Static SIMS
N.P Lockyer, SCC Wong, A. Henderson and J.C. Vickerman
Surface Analysis Research Centre,
Department of Chemistry, UMIST, Manchester M60 1QD
1. The Need
Among the surface analysis techniques only static SIMS has the precise chemical specificity to have any chance of tackling the
difficult problems of understanding surface behaviour in the enormously complex surface systems thrown up in both human
technology and in the biosciences, be they coatings on polymers; metals or textiles; biomolecules on cells or body tissue; toxic
herbicides on foliage; synthesised molecules on combinatorial library beads etc. The incentive therefore to achieve the best
sensitivity is enormous. The requirement is clear: Improve secondary ion yield from surface species in the sub-2000 Da
regime which is relatively easily accessible already, and extend the molecular detection regime into the region of 10 000 Da
and beyond. Thus the basic need was to increase the secondary ion yield of high mass components from the surface of complex
organic and inorganic materials while at the same time exercising more control over molecular fragmentation of species
sputtered from molecular solids.
A good deal of research has been devoted to finding methods of improving the efficiency of ion formation and mass
3
analysis in static SIMS. The move from quadrupole to time-of-flight mass analysers improved analysis efficiencies by 10 to
4
10 . This was a tremendous advance which we pioneered in the UK 1. It not only improved the overall efficiency of detection,
it dramatically increased the size (molecular mass) of species which could be detected and thus enabled materials of much
greater chemical complexity to be analysed. However the yield of ions in the SIMS process is very low. Less than 1% of
species sputtered from the surface are ionised. Photo-ionisation of the sputtered neutrals is proving to be an effective approach
to increasing yields and the area is the subject of active research in this laboratory and others2, 3. The proposed project was
based on emerging evidence that polyatomic primary beams could deliver very marked increases in secondary ion yield per
unit of surface damage generated.
2. Status of Primary Ion Beam Development
Laser desorption and MALDI have been rather successful in generating high yields of high mass molecular ions from large
organic species. However, neither approach is applicable for true surface analysis or in surface imaging mode at sub-µm
spatial resolution. What is required is to try to attain the benefits of the soft desorption evident in the laser techniques
combined with the surface sensitivity and micro-area capability of static SIMS.
2.1 Atomic primary ions To date the vast majority of static SIMS systems have used atomic primary projectiles, either
+
+
the inert gas ions Ar , Xe for non-spatially resolved analysis or liquid metal ion sources, which generate beams which are
+
+
easily focused to sub-micron levels, for small area analysis. Ga has been most widely used, but more recently In has become
+
available. Even in the early days of static SIMS it was recognised that the higher mass Xe (av m/z 131) beam generated a
+
+
+
higher yield of ions to higher m/z than Ar (m/z 40) of a similar energy 4. The shift to In (m/z 115, cf Ga m/z 69) in spatially
resolved analysis is stimulated by the desire to increase ion yields, however, whilst yields do increase by three to five times,
the benefits are not dramatic.
2.2 Poly-atomic primary ions Sputtering theory has been dominated by the linear cascade theory due to Sigmund 5.
This was based on atomic bombardment of homogeneous solids. Cascades of binary elastic collisions between the atoms
composing the solid transferred the energy deposited by the primary projectile at the surface into the sub-surface layers of the
solid. Some cascades terminated back at the surface causing emission of the surface atoms. Recent experiments and
molecular dynamics theory suggest that the efficient emission of large molecular species requires the correlated action of a
number of cascades such that a number of soft impacts contribute to the ‘lift-off’ of the molecule6. Atomic primary particles
focus the energy deposition in a very small area and there is evidence that a high proportion of the energy is deposited quite
deep in the solid. The idea behind the application of a polyatomic primary particle is that whilst it would dissociate on
collision with the surface, because of the spatial spread there would be the correlated impact of a number of atoms over a wider
area - a splash effect7. The energy deposited per atom would be lower and hence most of the energy would be deposited in the
1
JA Eccles and JC Vickerman, J. Vac. Sci. Technol. A7, 234 (1989)
2
3
C. Brummel, KF. Wi lley, JC Vickerman and N. Winograd Int. J. Mass Spectrom. Ion Processes 143, 257 (1995)
N.P. Lockyer and J.C. Vickerman, Laser Chemistry 17, 139-159, 1997; N.P. Lockyer and J.C. Vickerman, Int.J. Mass Spectrom., 197,
197-209, 2000.
4
5
D. Briggs and MJ Hearn, Int. J. Mass Spectrom. Ion Processes 67, 47 (1985)
P. Sigmund in 'Sputtering by Particle Bombardment', ed R. Behrisch, Springer Series in Applied Physics, Springer, Berlin 1981, vol 47,
p9.
A. Delcorte, P.Bertrand, J.C. Vickerman and B.J. Garrison, in Secondary Ion Mass Spectrometry, SIMS XII, p27, (2000); A. Delcorte
and B. J. Garrison, J. Phys. Chem. B. 104, 6785 (2000).
T. C. Nguyen, D. W. Ward, J. A. Townes, A. K. White, K. D. Krantzman, and B. J. Garrison, J. Phys. Chem. B, 105, 8221, (2000).
6
7
1
surface region. Thus the use of such beams could be anticipated to give rise to higher yields of large molecules from the
surface region.
Recent research from the Benninghoven group has demonstrated that a simple polyatomic such as SF5+ can result in
rather significant rises in real secondary ion yield as compared to atomic projectiles such as Ar+, Xe+ and Ga+.8 However, it
was the research carried out by groups in Texas and at Orsay in France into the enhancement of the secondary ion yield using
gold cluster ions Aun+, compared to ions formed from a number of large organic molecules including C60, C24H12 (coronene) 9,
10 which has formed the principal basis of the present project. Au + ions where n =1 to 5 from a heated gold source were shown
n
to deliver strongly non-linear increases in secondary ion yields from organic surfaces such as phenylalanine or fatty acid LB
films such that the yield of the (M-H) per gold atom at the same energy per gold atom varied as 1:4: 8: 12 for Au:Au2:Au3:Au4.
This yield enhancement is even more dramatic for dimers of the molecular ion. To compare the behaviour with organic cluster
+
+
ions such as C60 and C24H12 the primary beams had to be generated by bombardment of the rear of a foil coated with a film of
252
the organic materials using fission fragments from the decay of Cf. The required primary ions were formed and accelerated
to the sample in a ToF analyser. The yield of primary ions using this method was very low so it did not provide a very
practical ion source, however the ion yield results were clear. The yield per unit mass of the projectile was 4 to 5 times higher
for the organic projectiles than for the gold projectiles of similar mass 11.. Whilst there is a considerable yield enhancement, a
study of a series of large organic projectiles shows that the non-linear effect seen for gold is not observed for these large
multiatomic particles, rather the yield rises linearly with molecular mass. However, at a given total kinetic energy, the amount
of energy deposited per volume unit in the solid appears to depend on the number of atoms in the projectiles. A TRIM
calculation suggests that 20 keV C60 deposits most of its energy within 30 Å of the surface, whereas Au4 which has a very
similar mass deposits its energy very much deeper in the solid - 115 Å. This is a consequence of the energy per atom for C60
being only 333 eV, whereas for Au4 it is 5 keV. Ion yields do increase with projectile energy, as the does the ability to focus
the ion beam for high spatial resolution analysis. The shallow penetration depth and high SI yield make these two ion systems
very attractive as extremely surface sensitive high yield primary projectiles for static SIMS analysis and molecular imaging.
C60 provides the opportunity to investigate and optimise the effect of projectile energy (from a few eV/C atom to hundreds of
eV/C atom) on the yield of molecular ions and their fragmentation. Gold polyatomic beams from a liquid metal source offer
the possibility of optimising yield with good spatial resolution. It was decided that initial work in this project would focus on
the development of a C60 based ion beam system. In late 1999 we also initiated a separate CASE project with Ionoptika’s
support to develop a gold liquid metal ion source to incorporate in the liquid metal ion gun already on our BioTofSIMS
system. As will be seen the C60 development has been entirely successful and an impressive liquid gold source has been
demonstrated in June 2001 using a gold germanium alloy.
3. The Development Programme
The clear benefits which will accrue to surface analysis by static SIMS from the development of a high mass polyatomic
primary beam source were the stimulation for the proposed programme. The arrangements used to produce the high mass
organic and CsI based beams used in the above studies could not form the basis of practical ion beams systems for use in
commercial instruments or in the R&D laboratories in which ToF-SIMS is increasingly to be found. The aim of the project
was to produce a practical and robust primary ion beam system for incorporation on current and future ToF-SIMS systems
based on C60. The design specification called for a primary ion energy range of 300 eV to 20 keV delivering about 1 nA
continuous beam flux into a beam focus of less than 100 µm. The primary ion energy range would allow the effect of the
energy/C atom to be explored from 5 eV to 666 eV (using C602+).
3.1 The Design and Build of the C60 Ion Beam System The ion beam design is shown in the Figure 1. See Figure
1, Appendix for photograph of built up gun.
The Source. Three potential source designs were considered - (a) A plasma source in which the solid C60 coating the source
walls or a special electrode would be subject to plasma bombardment generating ions which would be extracted to form the ion
beam. (b) An effusive source in which the materials would be heated to a suitable temperature in a small oven. 500 C is
required for C6012. A molecular beam would emerge which can then be ionised by simple electron bombardment or in a
plasma. (c) Direct sputtering of the source materials by a small external ion beam, followed by extraction of the sputtered ions
could be an alternative route.
Our early research indicated that the most promising method of ionisation was low energy electron bombardment. At
the outset, there was evidence that such a source would work, but that it would suffer the drawback of low brightness. To
overcome this problem, we experimented with a source using direct injection of the vapour into the centre of the ionisation
cell. The final source design is thus based on the effusive source (b). It has proved to be simple to implement and as will be
seen it has enabled us to exceed our design specifications. 0.5 g C60 powder is loaded into a reservoir that is held in a copper
8
F. Kötter and A. Benninghoven, Appl. Surf. Sci., 113, 47 (1998);
K. Boussofiane-Baudin, G. Bolbach, A. Brunelle, S. Della-Negra, P. Håkansson and Y. Le Beyec, Nucl. Ints. and Meth. B88, 160
(1994)
10 RG Kaercher, EF da Silveira, CV Barros Leite and EA Schweikert, Nucl. Ints. and Meth. B94, 207 (1994)
11 R. D. Harris, M. J. Van Stipdonk and E. A. Schweikert, Int. J. Mass Spectrom. Ion Proc. 174, 167 (1998); R. D. Harris, W. S. Baker, M.
J. Van Stipdonk, R. M. Crooks and E. A. Schweikert, Rapid Commun. Mass Spectrom. 13, 1374 (1999)
12 J de Vries, H. Steger, B. Kamke, C. Menzel, B. Weisser, W. Kamke and IV Hertel, Chem. Phys. Lett., 188, 159 (1992)
9
2
block at the rear of the source. This assembly is heated by a 150 W halogen bulb which is normally operated at about 13.5 V,
or 62 W. Under these conditions the block temperature is about 475 C. C60 evaporates (~ 8*10-8 mbar) and passes into the
centre of an ionisation cell through a small
nozzle. A circular filament surrounds this cell,
Figure 1
with a concentric cylindrical grid that
accelerates electrons from the filament into the
centre of the source.
In the collisions between low energy
electrons and C60 molecules the predominant
ionisation process is the formation of C60+, with
lesser cross sections for multiply charged C60
ions and for positively charged fragments13.
Ion Optics. There were a number of different
possible configurations for extraction optics.
The use of a small diameter extraction and
lensing system close to the exit aperture of the
source was initially appealing, as this could limit
the spatial spread of the beam through the
column. However, ion-optical modelling
showed that such a lens introduced excessive
aberration into the beam. The final choice was
Figure 2. 2 D SIMION Ion-Optical Model
an extraction electrode followed by a large
diameter acceleration electrode, in turn followed by a large diameter einzel lens. The extraction electrode is held at negative
voltage relative to the source. Figure 2 shows the 2D SIMION ion-optical model including this configuration of extraction
optics. The beam of C60+ is extracted from a fairly diffuse region within the source and brought to a strongly demagnified field
image just inside the extractor. The first lens then focuses the beam to an intermediate field image near the mid-point of the
column. The beam is apertured for angular acceptance in this part of the column. The left hand side of the figure shows the
second lens bringing the beam to a final focus at the sample. This lens is strongly demagnifying to optimise spot size. The
demagnifying nature of the optics means that a reasonably small spot size can be attained despite the relatively diffuse nature
of the source. For operation at high spatial resolution, a lot of beam current is necessarily rejected within the column.
However, the output from the source has proved sufficient to give a more than adequate beam for ToF SIMS.
In order to use the ion beam for ToF-SIMS analysis, it is necessary to chop the beam into short pulses, and the probe
should not move across the sample at the beginning or end of the pulses. This is achieved by using lens 1 (Figure 1) to focus
the ion beam to an intermediate field image between a pair of pulsing plates (FIRST PULSER). Lens 2 then transmits this
intermediate image into the final focus on the sample. Hence, when the beam is deflected by these plates, there is no apparent
change in position lens 2’s object, and so no movement of the final image. In reality, aberrations in the beam originating from
the source geometry cause some movement or change in beam shape, but these effects are not severe in relation to the target
spatial resolution of the system. It is desirable to eliminate any neutrals from the beam, as they will not be focussed. This is
achieved by a 1° bend in the optical column which is positioned at the first pulsing plates, in order to minimise chromatic beam
distortion at the bend.
Mass Filtering In order to obtain clean, short pulses of uniform intensity, a means of mass-filtering the beam is required.
In ion guns operating with lower mass ions (i.e. elemental atoms or gas molecules), it is common practice to mass filter using a
Wien filter, in which orthogonal magnetic and electric fields are adjusted to cancel out for the selected mass, other masses
being bent out of the beam line. Unfortunately, the mass resolution of such a filter becomes very poor for slow-moving, more
massive particles. The C60 ion gun therefore incorporates a double chopping system for mass filtering.
When there are ions of different masses or charges in the beam, they travel down the column at different velocities.
Hence, the pulse spreads in both space and time, with the light (or multiply charged) ions leading. By placing a SECOND
PULSER further down the column, operated at a controlled time delay after the FIRST PULSER, sections of the pulse
corresponding to a particular mass range can be selected. The limitation of this technique is that the second pulser must operate
in an interval between two discrete masses if 'motionless' pulsing is to be preserved. Based on earlier work with ionisation of
C60, we anticipated the presence of a number of charged species with multiple charging or formation of discrete species such as
C50 being predominant. The double chopping system installed in the column can separate C50 from C60 with pulses up to 150 ns,
and doubly charged from singly charged C60 with pulses up to 600 ns. These figures apply when running the gun at 20 keV;
longer pulses can be used with lower energies.
Double Deflection Beam Scanning A critical feature of the ion optics is that the second lens should be strongly
demagnifying, and this meant placing it as close to the sample as possible. Therefore, it was desirable to dispense with the use
of raster scanning plates after the lens. The alternative was to use double deflection before the lens. This is a conventional
method of scanning the beam in which there are two sets of plates for each scan direction before the lens. To make the beam
pass through the centre of the lens the first pair of plates are double the distance from the centre of the lens as the second
plates, but only half the length of the second plates. The second plates have opposite polarity to the first.
13
S. Matt et al, J. Chem. Phys. 105, 1. (1996)
3
This technique has been used historically for scanning one of the two axes, or for scanning both with the plates in a
quadrupole arrangement. For the C60 gun , it was desirable to use double deflection on both axes, but not to accept the beam
distortion resulting from quadrupole plates. Therefore, we conceived a novel design using four sets of successive deflectors in
the sequence X-short, Y-short, X-long, Y-long. Plate lengths and separations were calculated such that the net deflections
would pass through the lens centre and provide the same amplitude in each axis. The final design of the beam scanning is
highly compact and free of quadrupole field distortion.
Other elements of the ion beam system shown in the schematic are: (a) Two alignment units. These are quadrupole
deflection plates used to bring the beam back on to the optical axis, compensating any drift off-axis in the extraction/lens 1
region. (b) Beam shaping. This is an octupole arrangement for optimising the probe shape. (c) Pulse buncher. This is a
chamber with front and rear electrodes. If a pulse is applied to the rear electrode as the beam pulse passes through the
chamber, the beam pulse is compressed into a shorter time. This is used when high mass resolution analysis is required and
where the primary ion beam pulse is the time reference for the ToF analyser. (d) Selectable apertures. A selection of 5
apertures can be moved onto the optical axis to change the angular acceptance of the column. Thus, the column can run with
high current and low spatial resolution, or lower current and high spatial resolution.
Electronics. A high voltage power supply unit was designed to meet the special requirements of the ion gun. It consists of a
6U high 19” rack containing a number of plug in cards. These include a low voltage power supply, high voltage power
supplies, data acquisition system and embedded PC104 controller. Control is from a PC via an RS422 serial connection.
The main high voltage outputs are the anode and the two lenses. Supplies
for the extractor and the various elements of the source float on the anode supply.
The deflector card provides a series of ground referenced outputs to drive the
alignment units, the stigmator, and beam blanking. In addition, the unit provides
3 current monitors for checking beam currents through the column, it is equipped
with a TTL input for switching the blanking voltage, and also has a high voltage
safety interlock loop. Replacement cards of opposite polarity for the anode,
extractor and lenses allows performance with negative ions.
The pulser units in the gun (the beam pulser and the mass filtering pulser)
are driven by two high-speed pulsers which are triggered by TTL pulses from the
instrumental control system.
Ion beam control software At present the electronics module is over 12
months late (see section 5). All testing and proving studies have used modified
standard controllers supplied by Ionoptika. However, when the dedicated
electronics are supplied the instrument will be controlled from a PC. As part of
the project, software has been produced and tested to fulfil this task.
Figure 3 Schematic of C60 high voltage
The RS422 serial port communicates with the power supply via a
power supplies,
predetermined command set that uses hexadecimal values to represent zero to
full-scale deflection of any given electrode. Each of the electrodes has a different operating range and stability.
The operating system targeted was Microsoft’s 32-bit Windows family (95, 98, NT4 and 2000). The language chosen
was C++ due to its object-oriented design allowing a real world system to be readily modelled in code. Error handling is
accomplished through the use of exceptions. Borland C++Builder was chosen as the preferred compiler. The software is
multithreaded to allow asynchronous communication with the controller. This allows real-time feedback of current values.
Two modes of operation are allowed – visual and manual. In visual mode, each electrode is represented on-screen by a
scrollbar slider with digital readouts of its voltage and current. Typically, a voltage parameter is adjusted and the current value
is read-only. The user adjusts the voltage by means of either the slider, using the mouse to click on the scrollbar arrows or
selecting fine control buttons for maximum sensitivity. This system allows four levels of adjustment from coarse to fine. The
electrodes in the ion gun have different response times depending on their capacitance rise-time and relative stability, and the
sensitivity levels are compensated accordingly. Manual mode allows the user the highest level of control where individual
commands may be issued to the control unit and its responses analysed numerically rather than by updating a slider. An option
is available to query every available electrode for both its voltage and current readings. Previous commands are stored for
quick recall.
Electrodes are periodically polled to maintain an accurate view of the stability of the ion gun. This refresh period is
adjustable. The status of a number of parameters is displayed on a bar at the foot of the display. Values may be stored and
recalled later at the push of a button by means of a series of seven presets. In order to observe stability of each electrode over
an extended period, a data logging system was incorporated. The time-base is selectable. Output is to a tab-delimited file to
allow for data analysis using spreadsheet-type software. Options are available to store the headers for columns, insert
comments into the file and to append data to a file stored previously. Both the last set of parameters used and a default set may
be stored. At any time, a snapshot of the data may be copied to the clipboard.
The command set and various values relating to the operation of the electrodes are stored in an initialisation file. This
may be modified in a text editor, allowing the manner in which the software operates to be modified without the need to
recompile the source code. This allows the same software to be used on other ion guns of similar design in the future.
4
4. Performance of C60 Ion Beam system
The operation of the ion beam system has been explored under three headings:
Operational parameters With the temporary electronics the maximum beam energy attainable was 12 keV. The C60
powder (0.5 g) is normally heated to about 475 C, delivering a partial pressure of 8*10-8 mbar C60 vapour into the electron
bombardment source region. Two parameters are of primary
C60+
interest, the beam composition and the current attainable at the
sample target. When the alignment of the source and lens voltages
were optimised an electron beam filament current of 1.8 A gives a
maximum total target current of between 0.6 and 1 nA using the
C602+
1000 µm aperture. Raising the source heater temperature to 500 C
increased the target current to 3 nA. The source grid voltage, which
controls electron energy, influences both target current and beam
composition. A grid voltage of 75 V delivers maximum target
current. Beam composition can be estimated from the various
contributions to a SIMS spectrum of aluminium surface generated
Figure 4 Variation of beam composition with grid voltage
by an unfiltered primary ion beam, see Figure 2, Appendix. By
varying the grid voltage the beam composition can be controlled, Figure 4 (assumes SI yield
from singly charged and doubly charged C60 is similar). Thus a grid voltage of 25 V gives a
relatively clean continuous C60+ beam. Using the mass filtering by beam chopping higher C60+
target currents can be achieved at 50 V grid voltage. Using the 1.95 µs delay pure C602+ beams
can also be selected which effectively doubles the beam energy. The C60 charge lifetime is well
in excess of 500 hours.
The beam diameter at the target was originally specified as less than 100 µm. Our ion
optical design has enabled less than 10 µm spatial resolution to be attained with the largest
beam forming aperture, Figure 5 and see Figure 3, Appendix. Thus operationally the ion beam
system has more than met all the design specifications.
Figure 5 In + image of 150
µm mesh Cu grid set in
indium (square 169 x 169
µm); 1000mm beam
aperture; FOV 300 µm
Yield enhancement. The development of the new ion beam system was primarily directed
towards increasing secondary ion yields particularly in the high mass ‘molecular’ region. A set
of materials representative of the types we will be interested in future research have been
studied (Irganox 1010; Polymers: PET, PTFE, Cellulose: Peptide - Gramicidin D; Lipid dipalmitoyl phosphatyidylcholine, DPPC) with the aim of comparing yield and fragmentation
data under 18 keV Ga+ and 10 keV C60+ ion bombardment. Two types of sample were studied – thin monolayer films
supported on a silicon wafer, or thick self-supporting films. The spectrum of gramicidin D in figure 6 gives an impression of
the enhancement obtained at high mass. The dose for gallium was 10 times that for C60. Other specimen spectra from cellulose
and the lipid, DPPC are provided in Figures 4 and 5, Appendix. The general conclusion is that under C60+ bombardment
yields overall increase by in excess of 50 times, with some ion yields increasing by more than 5000, see Table 1, Appendix. It
can be seen that yield enhancements vary across the mass scale, in the majority of cases increasing with mass, which was a
major aim of the project.. Table 1 demonstrates that these are real yield increases. It is interesting note that the yield
enhancements are greater from thick films than from thin
monolayer films. The impact energy will be partitioned
17 keV Ga+
between the silicon and the film. Because the component C
atoms in C60 only have 166 eV, and organic films are
monomolecular a fair proportion will be lost in the silicon
without giving rise to sputtering. In the thick films Ga
primary energy will penetrate deep into the film, some will
be lost to a sputtering effect, whereas for C60 all the energy
will be dissipated in the top two or three layers into the
vibrations of the organic film, enhancing yield.
PTFE shows almost no yield enhancement.
Benninghoven et al8 observed the same lack of
10 keV C60 +
enhancewment under SF5+ bombardment. At the time it was
thought that this might be due to the fact that fluorine
formed part of the primary ion. This is clearly not the case.
The mechanism of sputtering of PTFE must be different
from other polymers.
We have sought to compare the yield enhancements
obtained in this work with C60 with those obtained by
Benninghoven et al with SF5+. It is exceedingly difficult to
make very accurate comparisons when the absolute
detection efficiency of the two instruments used is not
Figure 6 Spectra of Gramicidin D under Ga+ and C 60+ primary
known. We can cross compare yields obtained with
ion bombardment. Ga dose 10x C60
5
standards and then extrapolate. Using this approach we estimate that 10 keV C60 delivers at least a 10 times greater yield than
10 keV SF5. When the full 20 keV supplies are available even higher yield enhancements can be expected with C60.
Damage cross-sections and fragmentation. It is important to
assess whether the very considerable yield enhancements have been
obtained at the expense of a concomitant increase in the rate of
generation of surface damage. Earlier work using SF5 showed that real
increases in ion yield per unit of surface damage could be obtained8.
1
-14
-2
σ= (13±2)*10
cm
A detailed study of the variation of fragment yield under
σ
(13 ± 2) x 10 cm
continuous 10 keV C60+ bombardment of PS and PET demonstrates that
damage cross sections, σ, from most ions are about 15*10-14 cm2, see
figure 7. This compares with about 6*10-14 cm2 for 10 keV SF58 and
2.4*10-14 cm2 for 15 keV Ga14, Table 2, Appendix. Hence real ion
yield enhancements of at least 10 to 100 are obtainable.
While high mass yield enhancements with C60 were observed by
the Texas group, there were suggestions of greater molecular
dissociation to low mass fragments. To date we only have limited
preliminary data, but this suggests that there is no more fragmentation
with C60 than with other primary ions. Table 3, Appendix reports the
yields from PET in five m/z ranges corresponding to small fragments
(m/z 10 to 50), larger fragments (50 to 80), monomer and large
fragments (100 to 200), dimers and fragments (200 to 400) and larger
0
1x10
2x10
3x10
4x10
multimers (400 to 1000). The overall yield using 10 keV C60 compared
2
Ion fluence (ions/cm
)
to 17 keV Ga increases by 1500 (which considering the relative
damage cross-sections converts to real increase of 300), however the
distribution of ions across the mass range is very similar, with just a small increase in the relative yield of high multimers.
However, there is much to learn about C60 sputtering which our future research will clarify.
Figure 7 Decay of the integrated peak intensity of a
selected secondary ion mass range (50-200 Da) from
poly(ethylene terephthalate) (PET) under C 60+ (12.5
keV) bombardment.
-14
2
Relative intensity
50-200
12
12
12
12
5. Project Execution
The successful execution of the project has had to overcome two major challenges: (a) recruiting experienced and
knowledgeable RAs and (b) delivery of hardware from sub-contractors.
The start of the project was delayed for 9 months due to difficulties in recruiting a suitable RA. Dr Lozhkin from Russia
was appointed 29/6/98. He was able to define the source design, but he clearly saw this job as a stepping stone to a more
permanent appointment in the UK and he resigned as from 31/01/99.
As an equipment-build project periods of design were followed by periods of construction by external contractors
before testing and development in house. In view of the scarcity of suitable RAs, it was decided to pro-actively make shortterm appointments of experts from the UMIST group to fulfil the specific tasks required to enable Ionoptika to get the
construction work underway. It was also decided to use some of the consumables support to fund more of the design work at
Ionoptika (hence the apparent underspend on consumables). Having defined the ion source, an initial pass at the ion beam optic
design was carried out by Ionoptika staff in early 1999. Dr Lockyer, a SIMS expert from the UMIST group was appointed
from 1/6/99 for a 6 months to fully refine the beam and electronics system design with Ionoptika and place the hardware
component orders with sub-contractors. An appointment break at this point was appropriate while components were
manufactured and delivered. However, the specialist sub-contractors able to carry out this work are scarce and greatly in
demand. Their delivery times proved to be wildly optimistic and despite constant pressure the ion beam hardware was not
complete until November 2000. The electronics hardware delivery is projected now to be September 2001. In the interim Dr
Henderson, a computational EO in UMIST, had been appointed part-time to complete the computer control software. EPSRC
extended the project end date to 28/03/01. In December 2000 Mr Steve Wong from the UMIST group was appointed to
complete the ion beam construction and testing. With his skilled and dedicated work together with first class support from
Ionoptika and the assembly of a set of temporary ‘scratch’ electronics, the ion beam system has been built, installed on the
BioTofSIMS at UMIST and tested successfully to demonstrate that even in its ‘prototype’ state it more than meets all its
design specifications. It is clear that the post-project delivery of the electronics enabling full 20 keV operation, and
implementation of the software control will enhance performance further, and make available a very powerful addition to the
capability of the BioTofSIMS system that will contribute greatly to our success in extending the capability of ToF-SIMS into
the area of bio-sciences research. Two publications are now being prepared on the functioning and capability of the C60 ion
beam system. A detailed programme of research on the mechanism of sputtering using C60 is planned.
The testing period has shown that some modifications to the source design are advisable to reduce carbon
contamination of electrode insulators that can lead to source break-down. However the performance of the ion beam system
has so exceeded our expectations that early commercial exploitation under a licence with the researchers is certain (see
attached letter from Ionoptika).
14 A. Delcorte, BG Segda, BJ Garrison and P Bertrand, Nucl. Inst. Meth. Phys. Res. B, 171, 277 (2000)
6
Appendix
Figure 1 Picture of the fully assembled ion gun
Figure 2
Positive SSIMS spectrum of copper sputtered using 10 keV C60+, (a.) without the chopper, and with 100 ns
chopper pulses at (b.) 0.6 s – (due to an unknown fragment species), (c.) 1.95 s, (due to C602+) and (d.)
2.75 s delay with respect to the blanking pulse, (due to C60+).
a.
Unfiltered
beam
b.
Low mass
fragment
C602
c.
d.
Na+
Cu+
+
C60+
1
Figure 3 A 250x500 um image of the edge of a DPPC droplet on Si. The m/z 184+ headgroup ion is shown.
The edge resolution is measured to indicate the spatial resolution. The largest (1 mm) gun aperture was used
(ie this gives the worst beam character) without the operation of the chopper.
QuickTime™ and a
BMP decompressor
are needed to see this picture.
Figure 4a. Positive SSIMS spectrum of cellophane film, sputtered using 18 keV Ga +.
Note Ga+ dose > 10 x C60+ dose.
Figure 4b. Positive SSIMS spectrum of cellophane film, sputtered using 10 keV C 60+.
2
Figure 5 Positive ion Spectra of dipalmitoyl phosphatyidylcholine, DPPC, obtained under (a) 18 keV Ga +
bombardment and (b) 10 keV C60+ bombardment. Note Ga+ dose > 10 x C60+ dose.
Ga
C60+
3
Table 1
C60/Ga Yield Comparisons
+
+
Yields and relative yields from standards, using the 18 keV Ga LMIG and 10 keV C60 ion gun on the BioToF
Material
Cellulose
+
+
Rel. Yield (C60/Ga)
29.5
267.9
62.7
Ion (m/z)
45
158
869
Yield (Ga )
1.47E-04
1.12E-06
3.83E-07
Yield (C60 )
4.33E-03
3.00E-04
2.40E-05
PET
27
76
104
149
193
237
385
429
577
621
769
2.04E-05
1.19E-05
2.30E-05
9.04E-06
3.30E-06
7.60E-07
4.34E-07
1.47E-07
1.30E-07
3.42E-08
6.67E-08
1.27E-03
1.27E-03
1.43E-03
8.42E-04
2.80E-04
8.22E-05
4.47E-05
2.03E-05
2.00E-05
8.39E-06
3.60E-06
62.3
107.2
62.2
93.2
84.8
108.2
103.0
138.1
153.5
245.3
54.0
PTFE
31
69
131
231
281
1.12E-03
6.99E-04
7.36E-04
4.19E-05
1.36E-05
5.13E-03
2.16E-03
1.27E-03
3.03E-05
1.83E-05
4.6
3.1
1.7
0.7
1.3
Irganox 1010
(thick film)
57
219
527
728
1.72E-04
1.86E-04
6.41E-07
6.86E-07
1.00E-02
1.39E-02
1.29E-04
2.80E-04
58.1
74.7
201.2
408.2
-232
-279
-1174
4.26E-06
2.91E-06
1.04E-07
6.64E-04
2.48E-04
1.89E-04
155.9
85.2
1817.3
57
219
527
728
3.56E-04
3.74E-04
7.24E-07
8.23E-07
5.40E-03
7.10E-03
3.40E-05
6.99E-05
15.2
19.0
47.0
84.9
72
86
130
159
170
1267.5
2.45E-05
1.79E-05
1.26E-05
4.23E-06
3.90E-06
5.47E-09
2.33E-03
2.35E-03
2.99E-03
1.10E-03
8.41E-04
3.09E-05
95.1
131.3
237.3
260.0
215.6
5649.0
-77
-179
-970
4.10E-06
1.95E-07
2.65E-09
1.68E-04
7.70E-05
7.63E-06
41.0
394.9
2879.2
41
184
224
551
734
6.28E-4
4.3E-5
3.41E-6
1.26E-7
2.1E-7
3.4E-2
1.58E-2
8.68E-4
1.75E-6
6.7E-5
54.1
367.4
254.5
14.0
324
(thin film)
Gramicidin D
(thick film)
DPPC
4
Table 2. Damage cross sections,  (Xi+), for several primary ions bombardment of polystyrene (PS)
spin-coated films.

(10
-14
Ar+(*)
10 keV
2
cm )
Ga+(#)
15 keV
SF5+(*)
10 keV
Xe+(*)
10 keV
C60+
12.5 keV
51+
0.7
0.65
2.8
1.1
14  2
103+
2.7
2.4
5.6
3.3
16  2
178+
2.8
5.8
3.6
15  2
51-215
1.8
5.8
3.0
12  1
*: F Kötter and A. Benninghoven, Appl. Surf. Sci. 113 (1998) 47-57
#: A. Delcorte, B.G. Segda, B.J. Garrison and P. Bertrand, Nucl. Inst. Meth Phys. Res B 171 (2000) 277-290.
Figure 6 Decay of the integrated peak
intensity of a selected secondary ion
mass range (51-215 amu) from
polystyrene (PS) under C60+ (12.5 keV)
bombardment.
Relative intensity
1
-
51-215 = (12 ± 1) x 10 14 cm
0
12
1x10
12
2x10
12
3x10
2
12
4x10
12
5x10
12
6x10
2
Ion fluence (ions/cm )
Table 3 Comparison of ion yields from PET under 17 keV Ga+ and 10 keV C60+ bombardment
Relative secondary ion yields for 2.25  109 17 keV Ga+
m/z range
counts
yield
rel. yield
total
909221
0.000404
10-50
310251
0.000138
0.341
50-80
247484
0.00011
0.272
100-200
240035
0.000107
0.264
200-400
50591
2.25E-05
0.055
400-1000
11988
5.33E-06
0.013
Relative secondary ion yields for 1.5  108 10 keV C60+
m/z range
total
10-50
50-80
100-200
200-400
400-1000
counts
6477178
1721540
2210306
1671109
344017
109722
yield
0.043181
0.011477
0.014735
0.011141
0.002293
0.000731
rel. yield
0.266
0.341
0.258
0.053
0.017
5