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Applied Surface Science 256 (2010) 6480–6487

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

ormation of Sim+ and SimCn+ clusters by C60

+ sputtering of Si

an Lyon ∗, Torsten Henkel, Detlef Rostchool of Earth, Atmospheric and Environmental Sciences, Williamson Building, The University of Manchester, Oxford Road, Manchester M13 9PL, UK

r t i c l e i n f o

rticle history:eceived 27 February 2010eceived in revised form 29 March 2010ccepted 13 April 2010vailable online 18 April 2010

eywords:OFSIMS60

putteringilicon

a b s t r a c t

The secondary ion mass spectrum of silicon sputtered by high energy C60+ ions in sputter equilibrium is

found to be dominated by Si clusters and we report the relative yields of Sim+ (1 ≤ m ≤ 15) and variousSimCn

+ clusters (1 ≤ m ≤ 11 for n = 1; 1 ≤ m ≤ 6 for n = 2; 1 ≤ m ≤ 4 for n = 3). The yields of Sim+ clusters upto Si7+ are significant (between 0.1 and 0.6 of the Si+ yield) with even numbered clusters Si4+ and Si6+

having the highest probability of formation. The abundances of cluster ions between Si8+ and Si11+ are

still significant (>1% relative to Si+) but drop by a factor of ∼100 between Si11+ and Si13

+. The probabilityof formation of clusters Si13

+–Si15+ is approximately constant at ∼5 × 10−4 relative to Si+ and rising a little

for Si15+, but clusters beyond Si15 are not detected (Sim≥16

+/Si+ < 1 × 10−4). The probability of formationof Sim+ and SimCn

+ clusters depends only very weakly on the C60+ primary ion energy between 13.5 keV

and 37.5 keV. The behaviour of Sim+ and SimCn+ cluster ions was also investigated for impacts onto a fresh

+

ilicon clusters Si surface to study the effects that saturation of the surface with C60 in reaching sputter equilibriummay have had on the measured abundances. By comparison, there are very minor amounts of pure Sim+

clusters produced during C60+ sputtering of silica (SiO2) and various silicate minerals. The abundances

for clusters heavier than Si2+ are very small compared to the case where Si is the target.The data reported here suggest that Sim+ and SimCn

+ cluster abundances may be consistent in a qual-cal mexpe

itative way with theoretiatoms in the sample. This

. Introduction

Secondary ion mass spectrometry (SIMS) uses energetic pri-ary ions (such as Cs+, Ar+, O+,−, Ga+) to sputter secondary ions

rom materials for analysis. The sputtering process is relatively wellnderstood as a transfer of energy in a collision cascade from therimary ion to various secondary species: atoms, molecules, clus-ers and ionized versions of these species. The primary ion is finallymplanted up to 50 nm deep in the surface [1]. Over recent yearshere has been a growing interest in using cluster ions (e.g. Aun

+,in+, and C60

+) as the primary ion species because of the differenthysical mechanisms that occur during cluster impact comparedo single atomic ion sputtering. This gives rise to new sputteringroperties [1–7].

The use of C60+ as a primary ion for sputtering has generated

great deal of interest since upon impact its energy is sharedetween its 60 constituent carbon atoms, none implanting them-elves deeply into the sample and causing alteration within theurface only to depths of a few nm. Large organic and biogenic

∗ Corresponding author. Tel.: +44 1612753942; fax: +44 1613069360.E-mail addresses: Ian.Lyon@manchester.ac.uk (I. Lyon),

orsten.Henkel@manchester.ac.uk (T. Henkel), Detlef.Rost@manchester.ac.ukD. Rost).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.04.038

odelling by others which predicts each carbon atom to bind with 3–4 Sirimental data may now be used to improve theoretical modelling.

© 2010 Elsevier B.V. All rights reserved.

molecules on the target surface are also sputtered efficiently andwith little fragmentation compared to sputtering by single atomprimary ions leading to greatly enhanced detection sensitivity forthese species [8].

The sputtering of Si with energetic C60+ ions is of particular

theoretical interest because modelling [9] indicates that the C60+

cluster disintegrates into its constituent carbon atoms within 5 ps ofimpact and each atom bonds with 3–4 Si atoms of the target, form-ing strong covalent bonds. Indeed the Si–C bond formed is strongerthan the Si–Si bond [9,10]. The modelling indicates that almost all ofthe carbon atoms are implanted into the target although these maybe sputtered by subsequent impacts. Krantzman et al. [10] mod-elled these reactions at primary ion energies between 5 keV and20 keV and Gillen et al. [11] studied the process experimentallyin this energy range. We here study the primary ion energy rangebetween 13.5 keV and 37.5 keV to further the study of Si cluster for-mation by C60

+ sputtering and acquired measurements of Sim+ andSimCn

+ cluster abundances in order to benchmark theoretical mod-elling of the understanding of the processes that take place duringthe C60

+ impact.

2. Apparatus and techniques

We used Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) to analyze the sputtered secondary ions and clusters. The

I. Lyon et al. / Applied Surface Science 256 (2010) 6480–6487 6481

Fig. 1. The relative abundances of Sim+ and SimCn+ clusters normalized to the Si+

abundance as a function of the number of Si atoms and C atoms at an energy of3Sfi

i[imw(

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Fig. 2. The relative abundances of the Sim+ clusters normalized to the Si+ abundanceas a function of ion energy (given in keV in the legend).

Fig. 3. The yields of Sim+/Si+ clusters up to Si11+ normalized to the yield of the same

clusters at 37.5 keV primary ion impact plotted in such a way that the behaviour of+

7.5 keV. The secondary ion spectrum is dominated by Si clusters up to Si15+ and

imCn clusters (1 ≤ n ≤ 4). Data acquired at other energies are not shown on thisgure as they appear very similar in this type of plot.

nstrument used was ‘IDLE 2′ which has been described elsewhere12,13]. Briefly, this instrument consists of a 40 keV C60

+ primaryon gun (Ionoptika UK Ltd) [4], a sample stage in which the sample

ay be pulsed to high voltage, and extraction and focusing opticsith a reflectron time-of-flight secondary ion mass spectrometer

R500, Kore Technology).There are two principle modes of operation: ‘normal extraction’

nd ‘delayed extraction’. Both were used during this study. Dur-ng ‘normal extraction’ a short pulse of primary ions (typically aew ns in length) impacts the surface whilst it is at high poten-ial (+2500 V in this case) relative to ground. The secondary ionsormed are accelerated through this potential difference into thextraction optics. The advantage of this mode of operation is thatll species are accelerated into the extraction optics almost equallyo that there is little fractionation between species. The disadvan-age of this mode of operation is that the mass resolution is definedy the pulse length of the primary ion pulse and so signal ratesan be very low when high mass resolution is required, for exam-le to resolve many of the isobars encountered during this study.

Normal extraction’ was used in this study where absolute mea-urements of species’ abundances across a wide mass range wereequired (the data shown in Tables 1–6 and Figs. 1–4), and where itas important that there was no fractionation during extraction or

n detection efficiency between different species. During ‘normalxtraction’, the impact energies of C60

+ ions onto the surface areherefore 2.5 keV less than the accelerating potential used in theon gun.

During ‘delayed extraction’ the primary ion pulse impacts theample whilst it is at ground potential and the secondary ions leavehe surface for the duration of the primary ion pulse which can beeveral tens of nanoseconds long. After a suitable delay of a few tensf nanoseconds after the end of the primary ion pulse, the sampleoltage is pulsed on with a rise time of only a few nanosecondso that the ions, which have dispersed to different heights abovehe surface according to their sputtered energy distribution and theime at which they were sputtered during the primary ion pulse,ain different energies in being accelerated towards the extractionptics. The ions may then be time focused at the detector by tuninghe sample pulse delay and the time/energy focusing properties ofhe reflectron. The advantage of this method is that high mass res-

lution can be gained whilst also using long primary ion pulses sohat data acquisition rates can be 10–100 times higher than duringnormal extraction’. The disadvantage of this method is that therean be variable transmission into the extraction optics across the

the different Sim clusters at each impact energy can be more easily seen. The datafor clusters Si12

+–Si15+ are not shown since poor counting statistics for these low

abundance species yield very large error bars on this scale which add little to aidinterpretation.

mass spectrum, particularly between lighter species (such as Si+)and heavier species such as the Si clusters and this leads to fraction-ation across the mass spectrum. The delay time of the sample pulsein this study was chosen to select for high mass resolution whilstminimising mass fractionation (the ratio of heavier Si clusters atmasses >200 u relative to Si+ was within 20% variation of the sameratio determined by ‘normal extraction’). Some data reported hereused ‘delayed extraction’, for example where ratios of SimCn

+/Si+

were calculated relative to the same SimCn+/Si+ ratio for different

conditions, for example such as during the examination of howdifferent species’ abundances evolve with cumulative C60

+ dosage(Figs. 5 and 6). Any fractionation therefore cancels out during theseparticular calculations.

The secondary ions were detected by a channel plate detector

biased at up to 25 kV relative to the ions’ energy so that even heavysecondary ions could be detected with an efficiency of ∼100% [1].The C60

+ ions impact the surface at an angle of 50◦ and ion pulses oftypically <10 ns duration were used during ‘normal extraction’ and

6482 I. Lyon et al. / Applied Surface Science 256 (2010) 6480–6487

Fig. 4. The yields of Sim+/Si+ clusters up to Si11+ normalized to the yield of the same

clusters at 37.5 keV primary ion impact energy so that relative changes in yield withprimary ion energy can be more easily seen. The data for clusters Si12

+–Si15+ are

not shown since poor counting statistics for these low abundance species yield verylarge error bars on this scale which add little to aid interpretation.

Fdt

∼bfnn

<trotttie1u∼

over a 200 �m square for a few seconds. The C ion beam DC cur-

ig. 5. The yields of silicon (Sim+) cluster ions relative to Si+ as a function of C60+

ose on the surface normalized to the yields of the same silicon cluster ions relativeo Si+ at equilibrium values.

40 ns for ‘delayed extraction’. Mass filtering of the primary ioneam was used to exclude species such as C70

+ or C60+ fragments

rom the primary ions impacting the sample. Sample charging wasot observed and so no electron beam charge compensation wasecessary.

The Si sample was high purity Si (impurities ∼few ppm O2,100 ppb of all other elements) used to sample solar wind onhe NASA Genesis mission [14]. For some of the measurementseported here, the surface had been thoroughly sputter-cleanedver a square area of 200 �m on each side using 40 keV C60

+ ionso remove surface contaminants. The pulsed beam of C60

+ ions washen rastered over an area of ∼100 �m on each side at the cen-er of the sputter-cleaned area and the mass-resolved secondaryon spectra recorded in repeated measurements using primary ion

nergies (impact energy onto the surface) of 37.5, 32.5, 27.5, 22.5,7.5 and 13.5 keV. Primary beam currents were very dependentpon the accelerating voltage of the C60 ion gun, ranging from100 pA (direct current) for 37.5 keV C60

+ to a few pA (direct cur-

Fig. 6. The yields of SimCn+ cluster ions relative to Si+ as a function of C60

+ doseon the surface normalized to the yields of the same SimCn

+ ions relative to Si+ atequilibrium values.

rent) for 13.5 keV C60+ ions. Secondary ion counts were corrected

for deadtime effects of the detector. The high post-acceleration ofthe detector yields ∼100% detection efficiencies across the massspectrum in the mass range used here (up to ∼500 u) [1] so rela-tive yields of the species are given as measured, corrected only fordeadtime and for the isotopic composition of Si and C. Each clusterobserved in the mass spectrum consisted of a series of peaks due tothe presence of the 3 Si isotopes 28Si, 29Si and 30Si and the carbonisotopes 12C and 13C as well as variants with H added. The abun-dances of clusters reported in the tables were therefore obtainedfrom the measured abundances of the 28Sim12Cn

+ mass peak aloneof each cluster and calculating the equivalent abundance of thewhole cluster by correcting the abundance of that peak for all thevariations with all the isotopes (28Si/Si ∼ 0.91, 12C/C ∼ 0.989).

Further analyses were acquired from a fresh area of the surfacewithout presputtering to investigate whether the measured rela-tive abundances of silicon clusters may have been influenced by thepresputtering of the surface by C60

+ ions in the previous measure-ments. Abundances of the SimCn

+ and Sim+ ions were recorded asa function of the C60

+ dose on the surface to see whether the mea-sured abundances of the different clusters changed in going froma situation where individual C60

+ impacts were essentially isolatedand each impact was into unaltered Si surface to where the sur-face had reached a sputter equilibrium after saturation with C60

+

bombardment. These data are reported in Figs. 5 and 6.The data acquisition here was necessarily for much shorter anal-

ysis times than in the earlier reported analyses on varying primaryion energy in order that these spectra could be acquired for C60

+

doses equivalent to much less than a monolayer on the Si surface.Total counts were therefore much smaller and errors higher than inthe earlier reported measurements. For this reason, data on someclusters (Si12

+–Si15+ and some of the less abundant SimCn

+ clusters)had to be omitted because they were not adequately resolved oraccurately measured in the mass spectrum whereas they could bemeasured in the far more extensive data sets shown in Tables 1–6.

These data were acquired by firstly acquiring mass spectra froma completely fresh Si surface with no previous exposure to C60

+ ionbombardment. A direct current (DC) C60

+ beam was then rastered+

60rent was measured by firing it into a Faraday cup. A mass spectrumwas then acquired using the pulsed beam over a sufficient lengthof time to acquire good statistical accuracy on the cluster peaks upto Si11

+ but not for long enough to contribute more than a small

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I. Lyon et al. / Applied Surfac

raction of the C60+ dose introduced by the previous DC sputtering.

he DC beam was then used once again to sputter the surface forfew more seconds and a further mass spectrum obtained. This

equence was repeated until no further changes were observed inhe relative abundances of cluster ions to Si+ and the surface hadome into sputter equilibrium. In the data plots in Figs. 5 and 6,he last few measurements showed little change and so these last 5

easurements were averaged and taken as the equilibrium valueor the ratio of the various Sim+ and SimCn

+ clusters relative to Si+.he measured value for each individual cluster at each C60

+ doseas then ratioed to the equilibrium value for this particular cluster.

. Results

The experimental data are gathered in Figs. 1–6 and in appendixs Tables 1–6. The error bars quoted in the tables are 1� and derivedrom counting statistics.

Fig. 1 shows the relative shapes of the abundance curves fororming Sim+ and SimCn

+ clusters for C60+ ions impacting the sur-

ace with 37.5 keV of energy. The shape of the abundance curveor SimCn

+ ions for increasing numbers of Si atoms in the cluster isot the same relationship as for Sim+ abundances. The abundancesf the SimC+ clusters show a distinct peak for 3 Si atoms and, forarger numbers of Si atoms within the cluster, the probability of anyarbon atoms being attached becomes very low. The relative abun-ances of the SimCn

+ (n > 1) clusters relative to Si+ show a similarehaviour, peaking at 2–3 Si atoms in the cluster and then drop-ing off rapidly for higher numbers of Si atoms so that they becomendetectable at the level of sensitivity available here. The yield ofim+ clusters relative to Si+ for clusters up to Si11

+ is significant>1%) and remains detectable up to Si15

+ as shown in Fig. 2. Theven clusters Si4+ and Si6+ are higher in abundance relative to Si+

han the clusters with a similar but odd number of Si atoms: Si3+,i5+ and Si7+. There is a sharp drop of a factor of ∼100 in the proba-ility of forming clusters higher than Si11

+ and then the probabilityf forming clusters Si13

+ to Si15+ plateaus at ∼5 × 10−4 relative to

i+ but with a slightly increased probability for Si15+. The yield of

lusters with more than 15 Si atoms was too small to be detectedSim≥16

+/Si+ < 1 × 10−4). These observations are shown in Fig. 2.It is clear from Fig. 2 that to first order the yield of Sim+ clusters

elative to Si+ is very similar across the range of primary ion impactnergies 13.5–37.5 keV. However, to assess whether there is anyystematic variation of the relative yield of the clusters with pri-ary C60

+ energy, we normalized the relative yield of each clusterim+/Si+ at each different energy to the relative yield of Sim+/Si+ at7.5 keV. (This choice of ion beam energy to normalize against isrbitrary and done here for 37.5 keV as the ion gun gave the largeston current at this energy and therefore the largest secondary ionignal with the smallest uncertainties due to counting statistics.)hese data are plotted in Figs. 3 and 4. Although these figures con-ain the same data, the two different ways of looking at the data arehown to aid interpretation. Fig. 3 shows that there is some varia-ion in relative yields of clusters up to Si11

+ with primary ion energy.he abundances of clusters for Sim+ (m < 11) are lowest for 13.5 keVnd rise steadily up to 32.5 keV. The cluster abundances relative toi+ then fall again at 37.5 keV of energy. Fig. 4 shows the same dataut with the abundance data for different clusters plotted againstrimary ion energy so that data acquired at a particular energy cane easily compared across the range of clusters. The clusters rang-

ng from Si2+ to Si11+ do not behave equally however. As is perhaps

ore clear from Fig. 3, Si2+, Si3+ and Si9+–Si11

+ show the smallestariation between different primary ion impact energies whereashe clusters Si4+–Si8+ show a much larger variation and this differ-nce is quite characteristic across the mass range of all the differentlusters. We discuss this observation further below.

nce 256 (2010) 6480–6487 6483

Much of the behaviour described here in which abundant Sim+

and SimCn+ clusters are observed may arise from the fact that the

surface has been pre-saturated with C60+ ions in order to achieve

a clean surface which is in sputter equilibrium. We therefore alsomeasured the abundances of Sim+ and SimCn

+ clusters relative to Si+

as a function of the cumulative C60+ dose on a fresh surface. This has

a necessary caveat however that no surface is truly composed solelyof the substrate atoms and it will have some elemental contami-nation (often Na and K) and organic molecules upon it, usually inthe form of hydrocarbons. The measured abundances of each Sim+

and SimCn+ cluster relative to Si+ and normalized to the equilibrium

value of that same cluster as a function of the C60+ cumulative dose

are shown in Figs. 5 and 6.Fig. 5 shows data on the Sim+ clusters. For those Si-only clusters

that have high and low numbers of Si atoms (m = 2, 3 and 11 in Sim+),their abundances rise up from ∼0.5 of the equilibrium value to justover the equilibrium ratio and fall back to it with increasing C60

+

dose. (The equilibrium value is defined here as 1 since all valuesare normalized to the average equilibrium values.) Intermediatemass clusters, particularly around Si6+ and Si7+ increase to valuesconsiderably above the equilibrium value before decreasing to theequilibrium value for increasing C60

+ dose. The behaviour seemsmore or less symmetrical about Si6+, Si7+ for heavier and lighterclusters.

SimCn+ clusters (Fig. 6) behave differently with increasing C60

+

dose (these measurements are from the same raw mass spectraas the Sim+ abundances reported in Fig. 5). Here, all the mea-sured species with carbon as a constituent rise from low valuesup towards the equilibrium value and do not exceed it, unlike theSim+ clusters. Si3C+ and Si4C+ rise up to the equilibrium value after aC60

+ dose of ∼3 × 1014 cm−2 whereas the other clusters containingC achieve this only after ∼5 × 1014 cm−2.

4. Discussion

The detailed behaviour of cluster formation described here willonly be fully understood by comprehensive theoretical modellingwhich would need to include the ionization process which is notwell understood.

However, for the time being we may take the model put forwardby Krantzman et al. [9,10]. For 15 keV C60

+ ions, they predict a dis-rupted hemispherical volume of radius ∼3 nm containing ∼2830Si atoms and the 60 C atoms spread through this volume bond-ing with 3 or 4 Si atoms each because the bond strength betweenSi and C is stronger than between Si and Si. Krantzman et al. pre-dict that when impacting into a Si surface, all of the C atoms of theC60

+ impactor are implanted and only Si is sputtered. Of course,a subsequent impact by another C60

+ on the same area is likelyto excavate some of the implanted C atoms as well as Si atomsfrom the sputtered surface. We may therefore expect significantdifferences in the production of SimCn

+ clusters between the ‘fresh’and pre-sputtered surfaces. Some Si atoms will be sputtered froma fresh surface and we may estimate this number from Fig. 1 of[9] and extrapolate the energy to ∼37.5 keV. As this extrapolationis considerably out of the range of the graph, then this estimate isuncertain. However we may estimate that between ∼500 ± 200 Siatoms are sputtered per impact at this energy.

We can try and quantitatively view our data in the light of Kratz-mann et al.’s [9,10] model, both for the pre-sputtered surface whichwill be saturated with C60

+ and for the measurements on fresh Si

with no previous exposure to C60

+. According to the model, eachC atom rapidly bonds to 3 or 4 Si atoms producing a C-rich cen-tral volume surrounded by disrupted Si atoms for each C60 impact.The peak for the SimC+ distribution at 3 silicon atoms agrees verywell with the model prediction suggesting mainly the formation

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484 I. Lyon et al. / Applied Surfa

f Si3C from a C-rich central zone though there are more pure Silusters sputtered from the surrounding area, which contains no.

We assume that the probability of ionization of Sim+ and SimCn+

lusters is the same and that these species are the main ways inhich Si and C atoms may be sputtered as ionized species. These

ssumptions are clearly not completely correct as there are SimOn+

ons observed (for low n, m) and Cm+ and CmHn

+ ions. The measuredbundance of these other positively ionized species containing Sind C are however small compared to the abundances of the Sim+

nd SimCn+ cluster ions so this first assumption seems reasonable.

eutral species were not detected but the assumptions would onlye invalidated if the ionization yields of species that contained Siere very different to the yield of species containing C.

If these assumptions are valid, we may look at the relative yieldsf Si and C atoms sputtered either as part of a silicon only clusterSim+) or as part of a SimCn

+ cluster. We do this by looking at theelative yields of these species in each case to the Si+ yield. Sum-ing up all relative numbers of Si atoms in Sim+ and SimCn

+ clustersompared to the relative numbers of C atoms in SimCn

+ clusters, wend a factor of ∼12 times more Si sputtered than C summing overhese sputtered clusters.

If we multiply the 60 carbon atoms per C60+ primary ion with

he Si/C ratio of ∼12, this gives the number of Si atoms sputtered inach impact event of ∼725 and a total number of sputtered atomss ∼785. This is at the high end of our previous estimate of theumber of sputtered atoms per impact from the Krantzman modelut within the wide uncertainty limits of the estimates that haveecessarily been made.

We may compare this figure with an estimate derived from ourarlier work [15] on sputtering silicate glasses with 40 keV C60

+

ons. Sputtering of silicate glasses by C60+ ions produced complex

pectra involving all the elemental species present in the miner-ls but not extensive Si clusters in the same way as with purei. These data are not presented here as they were obtained inrder to quantify relative sensitivity factors for elements sputteredrom silicate glasses by C60

+ and are presented elsewhere [15].n that work we measured a figure of 44 ± 8 nm of sputtering for× 1015 cm−2 C60

+ ions. The silicate glasses analyzed in that workave a mean atomic mass of approximately 26 g mol−1 comparedo 28 g mol−1 for pure Si analyzed here. We therefore estimate a fig-re of ∼260 atoms sputtered per C60

+ impact from that work whichs a little lower than the estimate made from the extrapolation ofig. 1 of [9].

We speculate that the behaviour of the Si4–9+/Si+ abundance

atios as a function of C60+ dose as shown in Fig. 5 and the abun-

ances of SimCn+ clusters starting off quite low relative to the

quilibrium value (Fig. 6) arises as a result of the transition fromsurface composed solely of Si atoms, to one where most Si atomiconds have been disrupted and many of the Si atoms are bondedo carbon atoms.

An added complication to evaluating this picture however is theact that the ‘fresh’ Si surface will not be truly pure Si, there areydrocarbons on the surface. These supply carbon atoms whichould interact with sputtered Si+ ions and cluster ions as they areeaving the surface to form SimCn

+ clusters during the very low C60+

oses in the very first measurements. These hydrocarbons will beleaned away as the surface is sputtered but then SimCn

+ clustersould be formed by the different mechanism of excavating C atomsrom the previous C60

+ impacts. The interpretation is therefore aittle difficult at low C60

+ doses, as the extent of this effect and

hether it occurs at all is uncertain but the fact that SimCn clus-

ers are detected in the very first measurements corroborates thisdea.

A C60 impact on the surface therefore leaves the 60 C atomspread through a volume with ∼2300 Si atoms giving a Si/C ratio

nce 256 (2010) 6480–6487

in these disrupted volumes of ∼40. If we accept a disruption radiusof ∼3 nm then it will take >5 × 1012 C60

+ impacts cm−2 to disruptthe whole of the surface if the C60

+ impacts were spread uniformly.Given that some will overlap previous impact sites, ∼2 × 1013 cm−2

should therefore disrupt almost all of the surface. The density ofC in the surface will still be fairly low however and subsequentimpacts over previous impact sites will raise the C/Si ratio. Ourdata shown in Figs. 5 and 6 show that a dose of ∼1 × 1014 cm−2 isrequired to remove the ‘dirt’ layer on top of the Si and that a dose of∼5 × 1014 cm−2 is required before sputter equilibrium is achieved.Although there may be some small systematic errors in our calcu-lation of C60

+ dose (probably no more than a factor of ∼2 in total),then these figures imply that of the order a factor of 25 more C60

+

impacts cm−2 are required to achieve sputter equilibrium than todisrupt the surface with single impacts.

If we take our estimate of the number of atoms that each impactwill sputter ∼785 atoms, then at first these will all be Si but thenumber of C atoms sputtered will rise proportional to the C/Si ratioat that instant. After ∼25 C60

+ impacts at any particular point onthe surface then of course the surface will have eroded consider-ably. When the C/Si ratio in the impact region reaches ∼0.1, thenthe number of sputtered C atoms in the sputtered atoms will equal60 so that the number of C atoms in the impact volume will reachequilibrium and the C/Si ratio in the impact region will not changefurther. By implication this will be the point at which sputter equi-librium will be reached. This simple conceptual model assumes thatall the 60 C atoms from each impact are implanted and only exca-vated by subsequent impacts. If however some of the C atoms ofthe impactor are sputtered within the same impact then of coursethe C/Si ratio of the impact zone will rise more slowly. The low C/Siratio at low primary ion densities shows that most primary ionsare implanted as otherwise the C/Si ratio would be high from thebeginning.

The estimate above of ∼785 sputtered atoms per C60+ impact

into Si is divergent from the estimate we make above of ∼260atoms sputtered per C60

+ impact into silicate glasses. We cannotrigorously explain this difference but note that C60

+ sputtering ofsilicate glasses does not produce high abundances of Si cluster ions.Since a high proportion of the sputtered Si atoms from pure Si isreleased in clusters, the propensity for forming clusters from Simay increase the total sputtered yield and mass removed per C60

+

impact.We have previously reported on the effects of sputtering sili-

cate glasses with C60+ ions [15] and so do not repeat these data

here. Clusters composed of the main constituent elements of theglasses (e.g. Si, O, Ca, Na, Mg, and Al) were observed but pure Sim+

clusters were rare and clusters heavier than Si2+ were of very lowabundance compared to the abundances observed here with Si asthe target. The explanation would seem obvious that sputteredspecies are composed of a mixture of the different species thatexist in the glass and so it is rare that a number of Si atoms canbe sputtered without other species being attached. Such systemsare therefore far more complex to understand theoretically and wefocus upon the simpler system of Si and C which can be treatedtheoretically.

5. Conclusions

Our experimental data are in good agreement with theoreticalmodels in that relative abundances of SimCn

+ clusters peak at Si3C+.

This is consistent with modelling that shows that each C atom fromthe impactor C60

+ ion binds strongly with, on average, 3 Si atoms.Furthermore, the estimated sputter rate of ∼785 Si atoms per C60

+

impact is not very divergent from the extrapolation of numbers ofsputtered atoms from impacts by C60

+ into Si from previous work

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I. Lyon et al. / Applied Surfac

lthough it is significantly higher than the number we derive fromputtering of silicate glasses. This difference may arise from the lackf Si clusters formed by C60

+ impacts into silicate glasses comparedith pure Si.

We find that the relative abundances of the Sim+ and SimCn+ ions

re, to first order, independent of primary ion energy between theanges of 13.5 keV and 37.5 keV. There are subtle differences how-ver that need to be explained by more sophisticated modelling.urther points that are not explained by the current models are:he rise in the C/Si ratio is slower than would be expected from aimple model calculating the change in sample composition andight be connected to amorphization of the sample but further

etailed modelling would be necessary to test this; the overabun-

ance of Si clusters produced with 4–9 Si atoms by primary ionoses of ∼2 × 1014 C60

+ cm−2 before reaching sputter equilibriums not understood. It might be simply due to the stability of Si clus-ers of these sizes but cannot be explained with current, published

odelling data.

able 1ives the measured secondary ion abundances of the various clusters relative to Si+ for 37

Silicon atoms Carbon atoms

0 1 2

1 1.0000 ± 0.0000 0.0114 ± 0.0100 0.0100 ± 0.0002 0.4974 ± 0.0010 0.1656 ± 0.0006 0.1102 ± 0.0003 0.4714 ± 0.0010 0.5580 ± 0.0011 0.1020 ± 0.0004 0.6229 ± 0.0012 0.2430 ± 0.0008 0.0202 ± 0.0015 0.3342 ± 0.0010 0.1201 ± 0.0006 0.0022 ± 0.0006 0.6491 ± 0.0014 0.0014 ± 0.0003 0.0007 ± 0.0007 0.2718 ± 0.0020 0.0063 ± 0.0003 0.0003 ± 0.0008 0.0922 ± 0.0006 0.0006 ± 0.00019 0.0609 ± 0.0011 0.0005 ± 0.0001

10 0.0727 ± 0.0016 0.0007 ± 0.000111 0.0438 ± 0.002212 0.0023 ± 0.000113 0.0003 ± 0.000114 0.0004 ± 0.000115 0.0006 ± 0.0001

able 2ives the measured secondary ion abundances of the various clusters relative to Si+ for 32

Silicon atoms Carbon atoms

0 1 2

1 1.0000 ± 0.0000 0.0119 ± 0.0001 0.0085 ± 0.02 0.5916 ± 0.0005 0.1772 ± 0.0003 0.1134 ± 0.03 0.5912 ± 0.0005 0.6276 ± 0.0008 0.0984 ± 0.04 0.7950 ± 0.0007 0.2641 ± 0.0007 0.0165 ± 0.05 0.4265 ± 0.0012 0.1137 ± 0.0002 0.0015 ± 0.06 0.8192 ± 0.0012 0.0217 ± 0.0001 0.0004 ± 0.07 0.3348 ± 0.0038 0.0041 ± 0.00018 0.1076 ± 0.0003 0.0004 ± 0.00019 0.0693 ± 0.0009 0.0003 ± 0.0001

10 0.0769 ± 0.0020 0.0005 ± 0.000111 0.0466 ± 0.002012 0.0020 ± 0.000113 0.0002 ± 0.000114 0.0003 ± 0.000115 0.0005 ± 0.0001

nce 256 (2010) 6480–6487 6485

Acknowledgements

This work was supported through the United Kingdom Cos-mochemical Analytical Network (UKCAN) with the Science andTechnology Facilities Council funding a research assistantship forDR and the University of Manchester funding the C60 primary iongun. STFC also funded the research assistantship of TH. We thankProfessor D. Burnett and the NASA Genesis mission team for theserendipitous provision of pure Si samples and D. Blagburn andB. Clementson for continuing assistance with construction andmaintenance of our TOFSIMS instruments. We are grateful to P.Blenkinsopp and R. Hill for assistance with the operation of the C60primary ion gun and to two anonymous reviewers for constructive

reviews.

Appendix A. Appendix

See Tables 1–6.

.5 kV primary ion impact energy.

3 4 5

1 0.0010 ± 0.0001 0.0011 ± 0.00005 0.006 ± 0.00015 0.0117 ± 0.0002 0.0018 ± 0.00015 0.0061 ± 0.00010 0.0010 ± 0.0001111

.5 kV primary ion impact energy.

3 4 5

001 0.0006 ± 0.0001 0.0004 ± 0.0001 0.003 ± 0.0001002 0.0083 ± 0.0001 0.0009 ± 0.0001002 0.0045 ± 0.0001001 0.0005 ± 0.0001001001

6486 I. Lyon et al. / Applied Surface Science 256 (2010) 6480–6487

Table 3gives the measured secondary ion abundances of the various clusters relative to Si+ for 27.5 kV primary ion impact energy.

Silicon atoms Carbon atoms

0 1 2 3 4 5

1 1.0000 ± 0.0000 0.0141 ± 0.0002 0.0104 ± 0.0002 0.0006 ± 0.0001 0.0006 ± 0.0001 0.0043 ± 0.00012 0.5776 ± 0.0014 0.1865 ± 0.0007 0.1160 ± 0.0006 0.0096 ± 0.0002 0.0012 ± 0.00013 0.5667 ± 0.0013 0.6231 ± 0.0015 0.1020 ± 0.0006 0.0051 ± 0.00024 0.7412 ± 0.0016 0.2616 ± 0.0011 0.0182 ± 0.0003 0.0008 ± 0.00015 0.3925 ± 0.0015 0.1184 ± 0.0007 0.0018 ± 0.00016 0.7593 ± 0.0020 0.0246 ± 0.0004 0.0004 ± 0.00017 0.3152 ± 0.0035 0.0054 ± 0.00028 0.1037 ± 0.0008 0.0004 ± 0.00019 0.0675 ± 0.0013 0.0003 ± 0.0001

10 0.0761 ± 0.0012 0.0007 ± 0.000111 0.0467 ± 0.0019 0.0002 ± 0.000112 0.0017 ± 0.000213 0.0003 ± 0.000114 0.0003 ± 0.000115 0.0004 ± 0.0001

Table 4gives the measured secondary ion abundances of the various clusters relative to Si+ for 22.5 kV primary ion impact energy.

Silicon atoms Carbon atoms

0 1 2 3 4 5

1 1.0000 ± 0.0000 0.0147 ± 0.0003 0.0102 ± 0.0002 0.0010 ± 0.0001 0.0008 ± 0.0001 0.0078 ± 0.00022 0.5116 ± 0.0018 0.1637 ± 0.0010 0.1006 ± 0.0008 0.0090 ± 0.0002 0.0014 ± 0.00013 0.4783 ± 0.0018 0.5240 ± 0.0020 0.0893 ± 0.0008 0.0040 ± 0.00024 0.6269 ± 0.0022 0.2257 ± 0.0015 0.0160 ± 0.0004 0.0010 ± 0.00015 0.3384 ± 0.0018 0.1063 ± 0.0010 0.0018 ± 0.00016 0.6705 ± 0.0026 0.0226 ± 0.0005 0.0007 ± 0.00017 0.2845 ± 0.0039 0.0052 ± 0.0004 0.0005 ± 0.00018 0.0978 ± 0.0011 0.0007 ± 0.00029 0.0644 ± 0.0015 0.0003 ± 0.0001

10 0.0731 ± 0.0025 0.0008 ± 0.000211 0.0454 ± 0.0021 0.0002 ± 0.000112 0.0016 ± 0.000213 0.0003 ± 0.000114 0.0003 ± 0.000115 0.0007 ± 0.0002

Table 5gives the measured secondary ion abundances of the various clusters relative to Si+ for 17.5 kV primary ion impact energy.

Silicon atoms Carbon atoms

0 1 2 3 4 5

1 1.0000 ± 0.0000 0.0165 ± 0.0002 0.0137 ± 0.0002 0.0013 ± 0.0001 0.0014 ± 0.0001 0.0074 ± 0.00022 0.4798 ± 0.0015 0.1855 ± 0.0009 0.1145 ± 0.0007 0.0130 ± 0.0002 0.0020 ± 0.00013 0.4212 ± 0.0014 0.5394 ± 0.0017 0.1036 ± 0.0007 0.0066 ± 0.00024 0.5211 ± 0.0017 0.2255 ± 0.0012 0.0197 ± 0.0003 0.0011 ± 0.00015 0.2645 ± 0.0013 0.1188 ± 0.0009 0.0028 ± 0.00016 0.5570 ± 0.0020 0.0288 ± 0.0005 0.0010 ± 0.00017 0.2517 ± 0.0032 0.0074 ± 0.0003 0.0003 ± 0.00028 0.0884 ± 0.0009 0.0007 ± 0.00029 0.0608 ± 0.0011 0.0007 ± 0.0001

10 0.0719 ± 0.0031 0.0009 ± 0.000311 0.0434 ± 0.002212 0.0022 ± 0.000213 0.0001 ± 0.000114 0.0003 ± 0.000115 0.0004 ± 0.0001

I. Lyon et al. / Applied Surface Science 256 (2010) 6480–6487 6487

Table 6gives the measured secondary ion abundances of the various clusters relative to Si+ for 13.5 kV primary ion impact energy.

Silicon atoms Carbon atoms

0 1 2 3 4 5

1 1.0000 ± 0.0000 0.0191 ± 0.0004 0.0175 ± 0.00042 0.4536 ± 0.0023 0.1860 ± 0.0015 0.1146 ± 0.00123 0.3751 ± 0.0022 0.5069 ± 0.0025 0.1022 ± 0.00124 0.4561 ± 0.0026 0.2124 ± 0.00175 0.2339 ± 0.0021 0.1165 ± 0.00146 0.4905 ± 0.0030 0.0310 ± 0.00097 0.2280 ± 0.0056 0.0080 ± 0.00068 0.0799 ± 0.0013 0.0008 ± 0.00039 0.0530 ± 0.0016 0.0006 ± 0.0002

10 0.0559 ± 0.005111 0.0341 ± 0.0009

R

[

[

[

[

12 0.0010 ± 0.000213 0.0001 ± 0.000114 0.0001 ± 0.000115 0.0001 ± 0.0001

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