Universal Equation for Argon Cluster Size-Dependence of Secondary

May 26, 2014 - Martin P. Seah,* Rasmus Havelund, and Ian S. Gilmore. National Physical Laboratory, Teddington, Middlesex TW11 0LW, U.K.. ABSTRACT: A ...
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Universal Equation for Argon Cluster Size-Dependence of Secondary Ion Spectra in SIMS of Organic Materials Martin P. Seah,* Rasmus Havelund, and Ian S. Gilmore National Physical Laboratory, Teddington, Middlesex TW11 0LW, U.K.

ABSTRACT: A study has been made of the fragmentation of three organic molecules, fluorenylmethyloxycarbonyl-Lpentafluorophenylalanine, tris(8-hydroxyquinolinato)aluminum, and Irganox 1010 under bombardment by argon cluster ions with sizes, n, between 500 and 10000 and for 10 and 20 keV energies, E. It is shown that the intensities of the fragments are governed by the same relation as that for the total volume sputtering yield but that, importantly, the parameter AM that governs the transition in behavior between low and high values of E/n is thought to be related to the energy required to remove that particular fragment from the ensemble of molecules. This causes a great reduction in intensity for the more strongly bound fragments as n is increased, such that, at n = 10000, the intensities of the molecular group peaks, which are weakly bound to the substrate, then dominate the spectrum. For studies involving weakly bound molecules, the low E/n values give the most intense molecular group peaks, but high E/n values give a greater number of small, characteristic fragments. For molecules more strongly bound to the bulk or substrate, it is not as likely that the low E/n data will be so helpful.

1. INTRODUCTION The secondary ion mass spectrometry (SIMS) of organic materials has been a major activity since soon after the technique was established in 1969.1 Spectra were sufficiently reliable that databases of the spectra of organic materials were established.2−4 However, it has not been possible to predict the SIMS spectra from first-principles or to predict accurately how spectra change from one source primary ion to another. Therefore, the databases, which used the primary ion sources popular at that time and which entailed significant labor, have become less useful. In 2000, Gilmore and Seah established the G-SIMS technique5 which showed that the spectra for different primary ion sources were related and that the way that spectra changed from source-to-source depended on the source and its ion impact energy.6 Later, it was shown7 that secondary ion spectra from Irganox 1010 for Ar+, Bi+, Bi+3 , and Bi+5 primary ion sources could all be related, over many orders of magnitude, to an accuracy of 2.3% for 389 fragment peaks. These spectra involved the product of just two spectra, one representing a low degree of fragmentation and one a high degree. A major advance in the analysis of organic materials came with the application of C+60 as a primary ion source since this permitted effective depth profiling of organic layers.8 A range of other carbon-based sources were used,9 including coronene,10 but C+60 remained the most important source for some 6 to 8 Published 2014 by the American Chemical Society

years despite the extent of damage that ensued to the organic material being analyzed.11,12 The spectra that were related to each other in the G-SIMS and similar studies were all in the static mode for undamaged material. Spectra for damaged surfaces, as in these depth profiles, were, of course, significantly different and highly dependent on the precise level of damage. They could not, therefore, be readily related. The situation changed dramatically with the work of Matsuo’s group,13,14 who showed that the argon gas cluster source used for polishing silicon wafers was a good source for SIMS primary ions and that excellent depth profiles could be established with high depth resolution15,16 and very little remaining damage.14 Depth profiles could now be made efficiently and routinely for many organic materials. The argon gas cluster ion beams (GCIBs) had an additional advantage that the cluster size, n, as well as its energy, E, could be set or changed by the user. These two parameters allow the analyst to optimize the quality of the depth resolution15−17 or the speed of profiling. It was also clear that the spectra, themselves, changed significantly as n and E were altered. Received: March 17, 2014 Revised: May 20, 2014 Published: May 26, 2014 12862

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Matsuo et al.18 in 2008 showed that, for 8 keV Ar+n , studies of positive secondary ions from arginine exhibited a similar fragmentation for both n = 300 and n = 1 but that at n = 1500, the intensity of the fragment ions reduces 1 to 2 orders of magnitude when compared with the molecular ion (Arg + H)+. Similar results using typically 3 different n or E values show a greater relative molecular signal for high n or low E for glycyl− glycyl−glycine,19 Alq3,20,21 leucine,21 PMMA,21 phenylalanine,21 and polystyrene.22 In a more extensive study, Rabbani et al.23 show for angiotensin III that 20 keV C+60 and Ar+60 give similar results. Furthermore, that the relative intensity of the protonated molecular signal, (M + H)+, to that of the smaller fragments grows as n increases to 2000 but that a reduction of E to 5 keV gives little further change. On the other hand, Gnaser et al.24 show a significant enhancement for (M + H)+ relative to an intense fragment of about half that mass for n increasing from 300 to 1500 with the effect being significantly stronger at 5.5 keV than at 11 keV. Above n = 1500 there is little further benefit. Kayser et al.25 show similar results for Irganox 1010 using 10 and 20 keV primary ions where the benefits increase beyond n = 1500 up to n = 10000 and possibly further. In the molecular dynamics calculations of Delcorte and coworkers,26−28 it is shown that below 1 eV/nucleon (i.e., 40 eV/ Ar atom) the sputtering process changes from being more atomistic with bond breaking to a situation in which the primary ion cluster remains essentially intact, and the fragmentation induced in the molecule becomes minimal (i.e., lower energies and higher n values below 1 eV/nucleon favor lower fragmentation). However, at the present time, no systematic study has been concluded to show precisely how the spectra are inter-related from one n,E combination to another. In the present work, we seek to do this for three archetypal organic materials in order that users can readily appreciate what changes may occur and how to optimize their instrumental settings to obtain the desired information most efficiently.

Figure 1. Structures of the molecules studied in this work: (a) fluorenylmethyloxycarbonyl- L -pentafluorophenylalanine, FMOC (C24H16F5NO4, Mr = 477.4), (b) tris(8-hydroxyquinolinato)aluminum, ALQ3 (C30H18AlN3O3, Mr = 459.4), and (c) Irganox 1010 (C73H108O12, Mr = 1177.6).

reference, spectra were also acquired using 25 keV Bi+3 . All the spectra were normalized to spectra for 107 primary ions and it is these, rather than the recorded spectra, that are used in the analysis that follows.

2. EXPERIMENTAL SECTION Smooth molecular films of 40 to 60 nm thicknesses were prepared by vapor depositing the three compounds fluorenylmetryloxycarbonyl-L-pentafluorophenylalanine (relative molar mass, Mr = 477), tris(8-hydroxyquinolinato)aluminum (Mr = 459), and Irganox 1010 (Mr = 1177) onto pieces of clean Si wafer. We will use the abbreviated names FMOC, ALQ3, and Irganox 1010, respectively, for these compounds. The structures of these compounds are given in Figure 1. Static SIMS analysis of the surfaces of these films was conducted using a TOF-SIMS IV (ION-TOF GmbH, Muenster, Germany) equipped with an argon cluster ion source and a Bi liquid metal ion source. The argon cluster ion source25 uses a 90° pulsing system to provide a mass-selected ion pulse (n/Δn between 60 and 120). The ion pulse can be further bunched to obtain a short pulse useful for TOF-SIMS analysis. The mass resolution obtained deteriorates at high n and low E values being ∼1 Da for n = 7500 at 10 keV. In this study, argon cluster sizes ranging from n = 500 to n = 10000 at 20 keV and, for Irganox 1010 also at 10 keV energy, were used, incident at 45° to the surface normal. The dc ion beam current was measured in a Faraday cup on the sample holder for each beam condition. All beam currents were below 0.1 pA and, typically, below 0.05 pA. Positive and negative ion spectra were acquired from fresh areas (100 × 100 μm) on the thin film surfaces to avoid any effect of previous measurements. For

3. RESULTS AND DISCUSSION 3.1. The Yields for FMOC. A set of spectra have been recorded for FMOC for a range of cluster sizes for 20 keV argon clusters for negative secondary ions. These data introduce some of the interesting effects. There is no published generic analysis for secondary ion yields, but if we start by assuming that the ionization coefficients, α+M and α−M, for a fragment of mass M are independent of both the beam energy, E, and also of the number of atoms in the primary ion cluster, n, then at first we only have to deal with the changes in the intensity of the secondary ion fragments with E and n. In the negative ion spectra for FMOC, the intensities up to mass 900 Da are measured. If α−M is independent of E and n, these total secondary ion intensities should reflect, approximately, the sputtering yield. The sputtering yield per primary ion has been shown to follow the universal equation:29 B(E /An)q Y = n 1 + (E /An)q − 1

(1) 3

where the sputtering yield, Y, is a volume measured in nm , B is a coefficient of the order 0.006 to 0.017 nm3 for organic materials, A is a parameter ∼1 to 4 eV, and q is ∼3. Rather than simply using the sum of the secondary ion intensities or the 12863

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CH2 right up to the molecular mass of the organic material. Of course, as discussed later, we also need to allow for the higher probability of selecting the (CH2) fragment compared with (CH2)x. If we have a sample that is composed of molecules deposited by condensation, the binding energy of the whole molecule to the rest of the sample, U, will be very much less than 3 to 4 eV, indeed, it will be less than 1 eV or it could not be evaporated without significant decomposition. That A, as found above, is 5.72 eV, instead of the 50 to 60 eV found for elements, arises because the SIMS spectrum is composed of large fragments such that, for instance, the average size in Irganox 1010 is equivalent to (CH2)1329 (i.e., x̅ = 13). This generates an average A value that is related to U/x̅ (i.e., is ∼1/13 of that for elements). To analyze the emitted spectra, therefore, we should first consider that individual ions may follow eq 1 but that the value of A for that particular fragment, AM, may depend on the fragment size as well as the energy to create that fragment. The value of AM is shown most clearly in the plot of YM/n for each secondary ion versus E/n. For very low AM, this would be a straight line that, on a plot with log axes, has unit gradient. As AM increases, the low E/n data fall below this line on a line of gradient q. As AM increases further, the apparent junction of the lines with gradient 1 and gradient q moves to higher and higher E/n values, the effective junction occurring at an E/n value of AM. Figure 3a shows this plot for 92 of the more intense mass fragment ions from FMOC normalized, for display purposes, at

total ion yield in a spectrum, it is clear that the volume contribution of the different ions should be included since, here, they range from 0.015 to 7 nm3. We ignore signals for fragments of mass less than 12 Da. The above volumes are roughly proportional to the mass, and so eq 1 is fitted to the sum of the products of mass, the ion count per primary ion for that mass, and the volume per unit mass. For simplicity, for this part, the spectra are binned to 1 Da to do this. If we assume a volume of (0.25 nm)3 for each 14 Da of mass in a typical organic material,30,31 the application of eq 1 will be correct dimensionally except that B will be reduced by an average of the α−M values for all fragments and also by the mass analyzer detection efficiency for negative secondary ions. We shall assume the detection efficiency to be around unity but that may be less true for masses above 10000 Da32 that are not studied here. Unfortunately, α−M will vary significantly with the fragment species detected, but we assume, for now, that this varies randomly through the spectrum. We shall return to this later. Using the above approach, we find the data for FMOC, calculated as above, may be fitted with eq 1 to 7%, as shown in Figure 2, with B = 34 × 10−6 nm3, A = 5.72 eV, and q = 3. If we

Figure 2. Data for the total volume of negative secondary ions per primary ion from FMOC for 20 keV argon cluster ions as a function of the cluster number, n. The fitted curve is eq 1 with B = 34 × 10−6 nm3, A = 5.72 eV, and q = 3.

had fitted just to the total counts or the summed fragment intensities, q would not really change much but A increases to ∼6.6. Those fits, while interesting, are not really appropriate. That B is about 0.1% of the values found for the sputtering yields of organic materials29 indicates an average value for α−M of the order of 0.1%, a value similar to or slightly higher than that often quoted or assumed.33 Unfortunately, the exact value of B for the sputtering yield has not yet been measured for FMOC. The sample, if elemental, would be considered as a set of atoms with a given binding energy, U, to the bulk material. In that case, U is ∼3 to 5 eV and A is ∼50 to 60 eV.29 The present organic sample is very different from this and could be considered as, say, an ensemble of CH2 entities of mass 14 Da and binding energy 3 to 4 eV to the next CH2 entity. But, unlike the elemental systems, the fragment (CH2)x is emitted with a similar binding energy to that for CH2 instead of the much higher binding energy that would arise for an equivalent metallic cluster. The approximately linear nature of organic molecules makes a significant difference. If the probability of emission were solely related to the binding energy to the bulk, the Ar GCIB SIMS spectra for our (CH2)x molecule would have a population of similarly high intensity fragments from

Figure 3. Fitting the individual FMOC negative secondary ion intensities to eq 1. (a) Negative secondary ion yields per primary ion, divided by n and normalized to the value at E/n = 40, as a function of E/n and (b) deduced values of AM for each negative secondary ion and the curve for eq 2. In (a), the lines are to guide the eye and simply join points for the same mass. 12864

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the highest E/n value used. Here we have not binned the data to 1 Da bins since that would average several fragments with differing absolute intensities and differing AM values in each bin. The lowest curves plotted have low mass and high AM values, whereas the highest curves, approaching the unity gradient line, have high mass and low AM values. Earlier,29 it was suggested that A would fall approximately inversely with the number of carbon atoms in the cluster, all else being equal (i.e., fall with the total fragment release energy per carbon atom in the fragment). Figure 3b shows the deduced values of AM from Figure 3a for those mass fragments above 25 Da contributing more than 250 counts to the spectra normalized to 107 primary ions and for which the relative standard deviation for eq 1 is then better than 25% (to remove some of the variable contaminants inevitably present, but this does not significantly change the plot). In the fitting, it is found that q is in the range from 4 to 6, rather than the 3 found for the total negative secondary ion volume in Figure 1. To simplify matters, q is assumed to be 5 for all the fits. It is clear that the fragments shown in Figure 3a do follow eq 1 in a general way such that AM generally falls as the fragment mass in Da, M, increases. The smooth red curve in Figure 3b is a fit based on −0.5 ⎡ 1 ⎛ M ⎞2 ⎤ ⎟ ⎥ AM = ⎢ 2 + ⎜ ⎝ 1000 ⎠ ⎦ ⎣ 20

Figure 4. Negative secondary ion spectra per 107 primary ions from FMOC for 20 keV argon cluster ions as a function of mass. The six spectra are for n = 500, 1000, 2500, 5000, 7500, and 10000 using the colors purple, blue, dark green, light green, orange, and red, respectively. Successive spectra for the 92 masses are shifted up by 1 Da for clarity.

keV Bi+3 , not shown, are similar to those for argon clusters with n = 500. In Figure 4, it is clear that the high mass peaks have reduced a little in intensity per primary ion, but from Figure 2, it is also clear that the overall consumption of molecules may have reduced rather more. The absolute sputtering yield is required here, and we shall return to this later. In single-beam SIMS, the secondary ion yield of the high mass intensities, per molecule consumed, may have improved as n increases; at low E/n values, proportionately fewer strong bonds are broken. This is important and demonstrates that, for observing the highest masses for weakly bound molecules with minimal sample consumption, the high n values are best, whereas for observing a characteristic low mass for, say, imaging, a low value of n may be best (there are simply many more small ions available per nanometer cubed than larger ones). In dual-beam SIMS, if Bi3+ is used for imaging, the molecular signals are relatively weak and low mass fragments are generally used. Let us now look more closely at the mass spectra by removing much of the fragment-to-fragment change in intensity that involves α−. This is done by dividing all of the spectra by a reference spectrum which, here, is formed by the cube root of the product of the intensities per 107 primary ions for the n = 500, 1000, and 2500 spectra. This is simply the geometric average of those three most intense spectra and is done to reduce the noise of the reference spectrum. These ratios are shown in Figure 5. Note that the reference spectrum is close to n = 1000 and so, in Figure 5b, the data for n = 1000 are close to unity. In Figure 5a, at n = 500, the data are above unity, up to a mass of ∼170 Da and then fall close to unity. In Figure 5c, the data for n = 2500 largely mirror this, being reduced by a factor similar to the amount that the n = 500 data are raised. For n = 5000, 7500, and 10000, the data form a monotonic series where, for example in Figure 5b relative to n = 1000, the data for n = 7500 are a factor of 300 reduced at low mass but only 1.2 times reduced at high mass. If these figures could be normalized by the total negative ion yield volume shown in Figure 2, the low mass fragments are reduced by ∼60 times, but those at high mass would be increased by a factor of 4. In comparing the n = 10000 and n = 500 data, the gain of the high mass intensities exceeds a factor of 6, while the low mass intensities are even further reduced. These show, clearly, the

(2)

(i.e., at high masses, AM ∼1000/M as expected if AM is related to U/x̅ and x̅ is proportional to M). Values of AM below 3 are less certain since they depend solely on the n = 10000 spectrum, which is weak for masses below 200 Da and therefore involves more variability. From the discussion above, this general dependence of AM from a highish value for small fragments to values closer to unity at high mass is expected if AM falls with the fragment release energy per carbon atom in the fragment. The scatter of the data around this curve simply reflects the fact that the energies to remove the fragments are not all the same but will vary widely in any given fragment size range. A strongly bound fragment, even with high mass, will have a high value of AM and vice versa. These strongly bound fragments rapidly gain in intensity as n falls or E/n rises and quickly dominate the intensity of the weakly bound high mass fragments. In the study of the fragment intensities from polystyrene using argon gas cluster ions by Moritani et al.,22 detailed plots are provided for the relative intensities of masses up to 131 Da as a function of some 30 E/n values. If their data are reanalyzed as described above, the fragment data fit eq 1 very well, allowing AM values to be determined. These show a scatter with mass with the AM values found to be in the range from 0 to 20 for the masses given. The specific fragments that they characterize to have a greater energy for creation all, and only those, can be shown to have the higher AM values, supporting the view that the AM values are related to the fragment release energy per carbon atom in the fragment. As AM falls, the intensities in the mass spectrum can be observed to alter with the greater strength shifting to higher and higher mass. This is shown in the negative secondary ion spectra for FMOC in Figure 4. It is not easy to see in that plot, but there are many mass intensities up to 300 Da and those at high mass are fairly similar in intensity for all cluster sizes whereas, at low mass, the intensities for n = 7500 and 1000 are 2 to 3 orders of magnitude weaker than for n = 500. Data for 25 12865

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those of n = 500 being typically 60% of the intensity for the same dose. This shows that the fragmentation is minimized for E/n ≤ 2, or an energy per nucleon ≤0.05, a significantly lower figure than that derived from the molecular dynamics calculations.26−28 The distributions, as a function of mass, of the high n data in Figure 5 basically reflect the reciprocal of AM in Figure 3b and arise from the different energies to create each fragment. We shall revisit this in more detail for Irganox later in Figure 14. Figure 5d compiles all of the data but, so that the trends are clear for presentational purposes, includes a running 5-point geometrical smooth. We have not yet completed our study of FMOC, but it is useful to illustrate what is being seen by the use of a simple model. We shall return to the FMOC analysis later. 3.2. Illustrative Simple Model. To help readers understand the overall behavior, below we consider a very simple model of a molecule with repeats −(CH2)− with all fragments having identical ionization probabilities. Assuming that AM for the components is given by eq 2, we may use eq 1 with q = 5 to deduce the fragment dependence on n. We use q = 5 here since that was found for FMOC and is also seen by fitting eq 1 to the molecular dynamics calculations for the fragments sputtered by argon clusters from benzene crystals.34 However, we also need to consider the value of BM. This should depend on the fragment in two ways. The probability for generating the CH2 fragment, compared with (CH2)p, must be, from the number of ways that we can extract (CH2) from (CH2)p, p times larger. However, in removing a fragment (CH2)x in a CH2 line of length p units, the respective number is p − x, which tends to p as x becomes much smaller than p. The maximum value of p would of course be defined by the size of the energetic impact zone and may increase with both E and n. From this, we expect BM to be inversely dependent, in some way, on the mass of the fragment. However, the emitted fragment has a volume dependent linearly on the mass and so we try the function BM = B0 M (1 − r) nm 3

(3)

where M is the fragment mass and r is a parameter greater than zero and likely to be greater than unity. BM will also be affected by the necessity to sum all the contributions within the total energy envelope so that as one fragment intensity rises others must fall. Additionally, to simulate a typical spectrum, we permit (CH2) additions up to mass 140 Da, then, to spread the masses out, (C3H6) additions up to 476 Da. We then sum the total intensities and compare that with the total intensity for eq 1 with B = 34 × 10−6 nm3, A = 5.72 eV, and q = 3 as found for FMOC. Of course, the exact numbers are unimportant but do help to illustrate the behavior. Here we find, to match the sums of the fragments to the total emission, that BM = 0.000063M −0.7 nm 3

Figure 5. Spectral intensities per primary ion, divided by the geometric average for the 20 keV argon spectra with n = 500, 1000, and 2500 also per primary ion, (a) n = 500 and 5000, (b) n = 1000 and 7500, (c) n = 2500 and 10000, and (d) all six n values as in (a) to (c) but with a running geometrical smooth of 5 adjacent masses in each spectrum.

(4)

In practice, such a simple relation will not be found since the ionization probabilities will not all be the same but will vary widely. However, for showing general trends we may ignore that. Figure 6a shows how the fragment intensities change with n and Figure 6b how they separate on the normalized Y/n versus E/n plot for 20000 eV data. Figure 6c shows the dependence of the spectra on the n values. Note the similarity of Figures 6c and 5d.

advantage in using high n values for observing the molecular mass entities in the case, as here, when they are weakly bonded to the substrate. The results for Bi+3 at 25 keV are similar to 12866

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and arise from the lack of coupling of the equations (e.g., an increase in smaller fragments must come at the expense of the larger fragments that are destroyed and for explaining some data, this issue may need future inclusion). This simple approach is sufficient to illustrate the important issues. The power q is much larger for the fragments but the way their contributions add up in Figure 6c leads to the lower value of q in the overall sum. The general behavior is quite clear here but may be missed in observing the measured FMOC data. In Figure 6b, the two solid lines are similarly presented. The curve for eq 1, when plotted as Y/n versus E/n on log axes shows two regions, each of a straight line, one of gradient q and one of unity gradient, that blend at E/n = AM. Thus, in Figure 6b, the joining region is at E/n = 19 eV for M = 14 and reduces here by ∼2 eV for each successive increase of 14 Da in M (i.e., the steeper parts shift to the left by this amount). The value of AM may be read directly from such plots. The data recorded by Moritani et al.,22 if replotted, also confirm this analysis, showing good, very linear parts on the log/log plots both above and below the AM value with very many data points covering up to 4 orders of magnitude in the low E/n range. We can now see why the product spectra for eq 3 are not as precise as for the series Bi+, Bi+3 , and Bi+5 . For product spectra to work in that way, the spectra in Figure 6c, instead of peeling away from the near horizontal line for n = 100 with the departure point rising in mass, should all have fractions of the same shape on this log plot, (i.e., a fixed departure point). Figure 5d does approximate to this and so the product spectra work reasonably well, but not as well as for the small Bi clusters. 3.3. The Secondary Ion Spectra for FMOC. We return, now, to an extension of the consideration of the FMOC data. In a detailed analysis of spectra for Irganox 1010, we showed that the spectra for the series Bi+, Bi+3 , and Bi+5 could be expressed7 log10[Y (n , m /z)] = an H + bn L + cn

(5)

where Y(n, m/z) is the secondary ion spectrum for an argon cluster of size n. The parameters an and bn are coefficients that are independent of mass but are dependent on n. The spectra H and L are the logarithms to base 10 (simply for convenience) of the basis spectra, here, of the 92 mass fragments above 11 Da involving predominantly high and low masses, respectively. The parameter cn simply scales the whole spectrum and includes any variability in the beam current measurement. Equation 3 describes the spectra as intrinsically the product of two parts where the intensities are described by these parts raised to the relevant powers, an and bn (i.e., they comprise linear sums on log plots). The spectra of normalized intensities in Figure 4 indicate that L would be strong in the n = 500 spectrum and H may be taken directly as the n = 1000 spectrum. Previously, using the spectra normalized to 107 primary ions and putting H = log10[Y(n = 10000)], the factor an was deduced for the remaining spectra and then using log10[Y/n] − anH, the factor bn was deduced. Here, instead, for reasons that will become clear later, we simply use H as above but put L = log10[Y(n = 500)] and deduce an and bn. The average standard deviation in this fitting is 0.18, equivalent to a scatter factor of 1.5. Figure 7a shows the results for an and bn and Figure 7b the basis spectra used for H and L (as 10H and 10L) which cover over 4 orders of magnitude. The average standard deviation in the fitting is approximately the lettering size in Figure 7b and, although small, it is over an order of magnitude higher than observed for

Figure 6. Results for the model system of a series of fragments all with identical binding energies and ionization coefficients. (a) The secondary ion intensities for various mass fragments from 14 to 476 Da as a function of n together with their sum and the best fit of eq 1, (b) the plot of ion yield/n versus E/n, normalized at E/n = 40 for various mass fragments to show the effect of the falling AM value as the ion mass increases together with their sum and the best fit of eq 1, and (c) the secondary ion spectra for n = 100 and for n = 750 to 10000 in 9 equal geometric steps there being little change between n = 100 and n = 750.

In Figure 6a, it is clear that the major change in the spectrum occurs between n = 1000 and n = 5000 where, as shown in Figure 6c, the relative intensity of the low and high masses changes by 2 orders of magnitude. At the top of Figure 6a are two solid lines. The black line is the sum of the spectra shown, and the red line (which differs by ∼7%) is the fit for FMOC for eq 1 with B = 34 × 10−6 nm3, A = 5.72 eV, and q = 3. The small differences in the two lines are unimportant in the illustration 12867

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Figure 8. Data for the total volume of negative secondary ions per primary ion from ALQ3 for 20 keV argon cluster ions as a function of the cluster number, n. The fitted curve is eq 1 with B = 70 × 10−6 nm3, A = 4 eV, and q = 1.8.

The AM values here are typically half the values found for FMOC.

Figure 7. Decomposition of the FMOC spectra according to eq 3. (a) The factors an and bn as a function of n and (b) the basis spectra H and L. In (a), the green points close to the ordinate axis are for 25 keV Bi+3 primary ions at n = 3.

the series Bi+, Bi+3 , and Bi+5 .7 Figure 7b shows H to be dominated by high mass intensities and L by both high and low mass intensities. Clearly L−H would be dominated by just low mass intensities and would be a better basis spectrum than L; we shall return to this later. Figure 7a shows that as n increases, an rises and bn falls, confirming the smooth ascendancy of the high mass intensities at high n values for FMOC. Note that, within 6%, an + bn = 1. If an + bn = 1 had been used, the average standard deviation remains at 0.18. The values for Bi+3 at 25 keV are shown in Figure 7a, plotted at n = 3 to the left as green points and are close to the argon values around n = 1000. If the difference between argon and bismuth can be ignored, it may indicate that there may be little spectral change for argon for n < 500. Likewise, the tailing off for the change in an and bn between n = 7500 to 10000, may indicate little spectral change for higher n values. The range selected here, of 500 ≤ n ≤ 1000, thus illustrates the major changes. 3.4. ALQ3. The analysis of the ALQ3 data follows that for FMOC but here the volume for each 14 Da is closer to (0.26 nm)3. For the overall secondary ion yield, eq 1 is valid with B = 70 × 10−6 nm3, A = 4 eV, and q = 1.8 for 20 keV argon cluster ions with 500 ≤ n ≤ 10000 to a relative standard deviation of 8%. The curve for the total volume of negative secondary ions, binned to 1 Da bins, per primary ion, is similar to Figure 1, but the volume yield is over twice that for FMOC, as shown in Figure 8. The results are generally similar to FMOC in all respects except that the slightly lower A value leads to the lower AM values in the equivalent of Figure 3b, as shown in Figure 9.

Figure 9. Deduced values of AM for each negative secondary ion from the ALQ3 samples.

As for FMOC, the high-resolution spectra may be decomposed into a part represented by the spectrum at n = 10000 and a part at n = 500. Using the procedure discussed for FMOC, the parameters an and bn of eq 3 may be deduced and are shown in Figure 10. This is very similar to Figure 7a and also confirms the relative position for Bi+3 shown by the green points at n = 3. In the fitting for Figure 10, the average standard deviation of the fits was 0.10, significantly better than for FMOC and, to within 6%, an + bn = 1. The results in Figure 10 are more linear than those for FMOC. 3.5. Irganox 1010. Irganox 1010 has been the subject of many studies and has much published data, particularly for the sputtering yields that are not available for FMOC and ALQ3. It therefore merits a closer set of experiments. Here, both positive and negative ion spectra have been recorded for both 10 and 20 keV beam energies. At 20 keV, the data are for 500 ≤ n ≤ 10000 but, for 10 keV, data above n = 5000 gave spectra that were very poorly mass resolved and so were not used except for the results binned to 1 Da for the total volume yields. Again, the 25 keV Bi+3 data were recorded. The total volume of, separately, the positive and negative secondary ions per primary 12868

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determined earlier from the sputtering yields since the damage created by Bi+3 makes any total sputtering yield measurements uncertain. Below E/n = 10, the intensities at high mass become more dominant and they, with their lower AM values, give curves closer to but flatter than the blue dashed curve. The reason for the divergence, at low E/n values, of the total volume ion yields from the measured volume ion yield scaled by 0.00034 may arise from a combination of the change in spectral shape indicated in Figure 7c together with a general reduction of α+ and α− with increasing mass. However, a more likely contribution to the divergence could be a reduction in α+ and α− as E/n reduces below the relevant A value, here, 4.5. At such low E/n values, the emitted fragments may be ejected with a lower velocity and be less ionized. This seems reasonable and would cause the q values for ions to be greater than for neutrals or for the total volume sputtering yield. The results do indicate that the higher q value is consistent for all fragments and operates, for each fragment if E/n < AM, thus the reduction in α+ and α− may occur as E/n reduces below the relevant AM value rather than the global A value for the material. Plots for Irganox 1010 for Y/n versus E/n for the individual positive and negative secondary ions appear much as Figure 3a except that the power, q, is 6. These plots allow us to evaluate the AM values from the data for both energies for the different ions as shown in Figure 12 (panels a and b). In both cases, AM values below a value of 2 are uncertain since the minimum value of E/n in the data is 2. The results do differ a little from those of Kayser et al.25 in that the intensities of the higher

Figure 10. Factors an and bn as a function of n in the decomposition of the ALQ3 spectra according to eq 3. The green points close to the ordinate axis are for 25 keV Bi+3 primary ions at n = 3.

ion from Irganox 1010 are shown in Figure 11, but not as Y but Y/n versus E/n where Y is in nanometers cubed. Here, the

Figure 11. Data for the ratio, to n, of the total volume of, separately, the negative and positive secondary ion yields per primary ion from Irganox 1010 for 10 and for 20 keV argon cluster ions as well as 25 keV Bi+3 as a function of E/n. The fitted curve is eq 1 with B = 17 × 10−6 nm3, A = 4.5 eV, and q = 3.9. The dashed blue curve is eq 1 with B = 6 × 10−6 nm3, A = 1.6 eV, and q = 2.76 as published29 for the sputtering yield with B reduced by the average ionization coefficient set here at 0.00034.

volume per 14 Da of fragment has been taken as (0.26 nm)3.29 On this plot are both the positive and negative secondary ions for both the 20 and 10 keV data as well as, in the top right corner, the 25 keV Bi+3 data. This shows that the 10 and 20 keV data are consistent, the positive and negative secondary ion data are consistent, and that the Bi+3 data are also consistent with these and is situated well into the unity gradient regime. Note that, since the Bi+3 points are on the unity gradient region, if this were a plot per nucleon instead of per atom,26 the Bi+3 points would shift down relative to the argon data by a factor of 5.23 on both axes and stay in contact with the line. The red curve is eq 1 with B = 17 × 10−6 nm3, A = 4.5 eV, and q = 3.9. The dotted blue curve is the previously published curve for the sputtering yield29 reduced by a factor of 0.00034. This shows ionization coefficients α+ and α− are each around 0.034%, slightly smaller than the estimated value for FMOC. The underlying positive and negative contributions are approximately equal. The excellent consistency of the Bi+3 data with that for the argon clusters is unexpected and could not be

Figure 12. Deduced values of AM from the combined 10 and 20 keV data for Irganox 1010 as a function of mass (a) negative and (b) positive secondary ions. 12869

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masses do not fall here when n is reduced from 2500. Such a fall would lead to a very noticeable reduction below the red line in the equivalent of Figure 3a by a factor of 3 between E/n = 8 and E/n = 40. This difference may arise from the use, by Kayser et al.,25 of a sputtering preclean to remove contaminants, using a dose of argon gas cluster ions with E/n = 2 sufficient to remove ∼10 nm. This may also remove any weakly bound Irganox 1010 molecules. In the present work, precleans were avoided. Note that, at high mass, the values of AM for the positive secondary ions are lower than those for the negative and that the latter are similar to the value (1.6 eV) for bulk sputtering. This means that, at values of E/n < 10, the yield of molecular ions per molecule consumed decreases slightly for negative secondary ions but increases for positive secondary ions. This apparent analytical advantage is, unfortunately, overshadowed by the significantly lower ionization coefficient such that the absolute yield for the positive secondary ions only just matches that for the negatives at E/n = 2.67 (the lowest value measured). As before, we may split the spectra into high and low mass components, H and L, according to eq 3, but here we treat the positive and negative secondary ion spectra for any E and n combination as one spectrum so that the same an and bn values describe both. The parameter cn is a constant that scales the whole spectrum. Figure 13a shows how the factors an and bn scale not with n but with n/E. Figure 13b could show the basis spectra H and L for the negative secondary ions and Figure 13c for the positive secondary ions, but a more basic set of spectra would be H and L−H as discussed for FMOC. This is what is shown, and the predominantly high and low mass characters are much clearer. In Figure 13a for the argon clusters, to within 5%, an = 1 − bn. Assuming this equivalence, eq 3 becomes log10[Y (n , m /z)] = (1 − bn)H + bn L + cn = H + bn(L − H) + cn

(6)

This result shows that the log10 spectral intensities are based on H (from the 20 keV n = 10000 spectrum) with the addition of b times a spectrum, as seen before, of (L−H) where b = 1 − 2n/E. This is precisely the basis of the G-SIMS procedure. From Figure 7b, if an individual fragment has a high (L−H) value then it should have a high value of AM. Figure 14 shows a plot of these parameters for both negative and positive secondary ions together with the theoretical prediction from a fitting of the different curves over the 1 < E/n < 100 range. The correlation between the application of the universal equation and the G-SIMS approach is clear. The importance of splitting the spectra into the factors H and L−H is that this procedure allows the use of the G-SIMS procedure and concepts,5,35,36 where the n = 500 spectrum could be taken as a condition that is more degrading and a n = 10000 one that is less degrading. The ratio of these spectra [which here is 10−(L−H)] is used to progressively multiply the n = 10000 spectrum (or whatever good quality low E/n spectrum is available) to show the larger and larger fragments and eventually the intact molecule. The strength of (L−H) for a given fragment is thought, above, to reflect the bonding to the sample, and this is also true for the G-SIMS studies.5,35,36 AM also reflects the bonding of the fragment to the bulk of the sample, and so Figure 14 confirms the consistency of these two

Figure 13. Decomposition of the Irganox 1010 high-resolution spectra according to eq 3. (a) The factors an and bn as a function of n/E, (b) the basis spectra H and L−H for the negative secondary ions, and (c) the basis spectra H and L−H for the positive secondary ions. In (a), the green points close to the ordinate axis are for 25 keV Bi+3 primary ions at n = 3, the more solid color points are for Ar GCIB at 20 keV, and the lighter colors are for 10 keV.

separate approaches for both the small and large cluster primary ion regimes. From these studies, it appears that the very gentle sputtering conditions at high n values provides a spectrum given here as 10H. This spectrum incorporates all of the fragmentation and ionization contributions. As n/E falls, the impact energy density rises and the more strongly bound species are liberated with a gain in intensity mainly governed by the value of n/E and the binding energy of that fragment to the solid. The latter is characterized either by (L−H) or AM. These alternative 12870

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ACKNOWLEDGMENTS

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REFERENCES

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The authors would like to thank Helena Stec for assistance in recording spectral data, Alex Shard for helpful comments, and Steve Spencer and Steve Smith for provision of the samples. This work forms part of the Chemical and Biological programme of the National Measurement System of the UK Department of Business, Innovation and Skills and with additional funding from the European Union through the European Metrology Research Programmes (EMRP) projects NEW01 TreND and IND15 SurfChem. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

Figure 14. Relation between (L−H) for both negative and positive secondary ions and the value of AM reflecting the average binding energy from the sputtering yield model. The red line is the theoretical prediction based on the universal equation, eq 1 with q = 6, and eq 5.

(1) Benninghoven, A.; Jaspers, D.; Sichtermann, W. Secondary-Ion Emission from Amino Acids. Appl. Phys. 1976, 11, 35−39. (2) The Static SIMS Library, version 4; Vickerman, J. C., Briggs, D., Henderson, A., Eds.; SurfaceSpectra: Manchester, 2006 (http:// surfacespectra.com/simslibrary/). (3) Schwede, B. C.; Heller, T.; Rading, D.; Niehius, E.; Wiedmann, L.; Benninghoven, A. The Munster High Mass Resolution Static SIMS Library; ION-TOF: Munster, 2003. (4) Briggs, D.; Brown, A.; Vickerman, J. C.; Handbook of Static Secondary Ion Mass Spectrometry (SIMS); Wiley: Chichester, 1989. (5) Gilmore, I. S.; Seah, M. P. Static SIMS: Towards Unfragmented Mass Spectra - The G-SIMS Procedure. Appl. Surf. Sci. 2000, 161, 465−480. (6) Seah, M. P.; Gilmore, I. S.; Green, F. M. G-SIMS: Relative Effectiveness of Different Monatomic Primary Ion Source Combinations. Rapid Commun. Mass Spectrom. 2009, 23, 599−602. (7) Seah, M. P.; Green, F. M.; Gilmore, I. S. Cluster Primary Ion Sputtering: Secondary Ion Intensities in Static SIMS of Organic Materials. J. Phys. Chem. C 2010, 114, 5351−5359. (8) Wong, S. C. C.; Hill, R.; Blenkinsopp, P.; Lockyer, N. P.; Weibel, D. E.; Vickerman, J. C. Development of a C60+ Ion Gun for Static SIMS and Chemical Imaging. Appl. Surf. Sci. 2003, 203−204, 219− 222. (9) Mahoney, C. Cluster Secondary Ion Mass Spectrometry. Mass Spectrom. Rev. 2010, 29, 247−293. (10) Seah, M. P.; Spencer, S. J.; Shard, A. Angle-Dependence and the Sputtering Yield of Irganox 1010 by Coronene. J. Phys. Chem. B 2013, 117, 11885−11892. (11) Cheng, J.; Wucher, A.; Winograd, N. Molecular Depth Profiling with Cluster Ion Beams. J. Phys. Chem. B 2006, 110, 8329−8336. (12) Shard, A. G.; Brewer, P. J.; Green, F. M.; Gilmore, I. S. Measurement of Sputtering Yields and Damage in C60 SIMS Depth Profiling of Model Organic Materials. Surf. Interface Anal. 2007, 39, 294−298. (13) Ninomiya, S.; Nakata, Y.; Ichiki, K.; Seki, T.; Aoki, T.; Matsuo, J. Measurements of Secondary Ions Emitted from Organic Compounds Bombarded with Large Gas Cluster Ions. Nucl. Instrum. Methods Phys. Res., Sect B 2007, 256, 493−496. (14) Ninomiya, S.; Ichiki, K.; Nakata, Y.; Seki, T.; Aoki, T.; Matsuo, J. The Effect of Incident Cluster Ion Energy and Size on Secondary Ion Yields Emitted from Si. Nucl. Instrum. Methods Phys. Res., Sect B 2007, 256, 528−531. (15) Lee, J. L. S.; Ninomiya, S.; Matsuo, J.; Gilmore, I. S.; Seah, M. P.; Shard, A. G. Organic Depth Profiling of a Nanostructured Delta Layer Reference Material using Large Argon Cluster Ions. Anal. Chem. 2010, 82, 98−105. (16) Shard, A. G.; Havelund, R.; Seah, M. P.; Spencer, S. J.; Gilmore, I. S.; Winograd, N.; Mayo, D.; Miyayama, T.; Niehuis, E.; Rading, D.; et al. Argon Cluster Ion Beams for Organic Depth Profiling: Results from a VAMAS Interlaboratory Study. Anal. Chem. 2012, 84, 7865− 7873.

approaches govern how the spectra change for different E and n settings.

4. CONCLUSIONS Fragmentation results are reported for FMOC, ALQ3, and Irganox 1010 when sputtered in SIMS using argon gas cluster ions with cluster sizes between 500 and 10000 atoms and beam energies of 10 and 20 keV. It is shown that the secondary ion species behave as well-defined components of the total emitted particles, and the intensities may be treated using the same eq 1 as the total sputtering yield. Each component contributes to the total yield but in different proportions as E/n reduces. The important parameter AM in the relevant equation generally reduces as the fragment mass in organic materials increases as a result of the reduction in the average binding energy to the sample of the emitted fragments per unit mass. This leads to the relative reduction of intensity for the low mass fragments as E/n reduces. Three orders of magnitude reduction occurs for the low mass ions as n increases from 500 to 10000 for 20 keV argon gas cluster ions but less than 1 order of magnitude for the high mass ions. The intensity of high mass fragments per molecule consumed, for molecules weakly bound to the substrate, then increases up to an order of magnitude over this range. This behavior is generic to all such organic molecules and indicates the benefits of high n values for observing the largest molecular fragments or protonated/ ionized molecular entities and the low n values for characteristic fragments for imaging. These effects are illustrated by a simple model. In this work, we have not studied molecules that are strongly bonded to a substrate such as those with a thiol bond to gold substrates. There, the larger entities will probably have a higher AM than discussed above since AM rises as the energy to remove the fragment increases. For such entities, it is unlikely that equivalent benefits of high n values will be observed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +442089436634. Notes

The authors declare no competing financial interest. 12871

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(17) Niehuis, E.; Moellers, R.; Rading, D.; Cramer, H.-G.; Kersting, R. Analysis of Organic Multilayers and 3D Structures Using Ar Cluster Ions. Surf. Interface Anal. 2013, 45, 158−162. (18) Matsuo, J.; Ninomiya, S.; Nakata, Y.; Honda, Y.; Ichiki, K.; Seki, T.; Aoki, T. What Size Cluster is Most Appropriate for SIMS. Appl. Surf. Sci. 2008, 255, 1235−1238. (19) Ninomiya, S.; Nakata, Y.; Honda, Y.; Ichiki, K.; Seki, T.; Aoki, T.; Matsuo, J. A Fragment-free Ionization Technique for Organic Mass Spectrometry with Large Ar Cluster Ions. Appl. Surf. Sci. 2008, 255, 1588−1590. (20) Ninomiya, S.; Ichiki, K.; Yamada, H.; Nakata, Y.; Seki, T.; Aoki, T.; Matsuo, J. Analysis of Organic Semiconductor Multilayers with Ar Cluster Secondary Ion Mass Spectrometry. Surf. Interface Anal. 2011, 43, 95−98. (21) Ichiki, K. Study on Size Effect of Cluster Ion Beam Irradiation, Ph.D. Thesis, Kyoto University, 2012. (22) Moritani, K.; Mukai, G.; Hashinokuchi, M.; Mochiji. EnergyDependent Fragmentation of Polystyrene Molecule Using SizeSelected Ar Gas Cluster Ion Beam Projectile. Surf. Interface Anal. 2011, 43, 241−244. (23) Rabbani, S.; Barber, A. M.; Fletcher, J. S.; Lockyer, N. P.; Vickerman, J. C. TOF-SIMS with Argon Cluster Ion Beams: A Comparison with C60+. Anal. Chem. 2011, 83, 3793−3800. (24) Gnaser, H.; Ichiki, K.; Matsuo, J. Strongly Reduced Fragmentation and Soft Emission Processes in Sputtered Ion Formation from Amino Acid Films Under Large Arn+ (n ≤ 2200) Cluster Ion Bombardment. Rapid Commun. Mass Spectrom. 2012, 26, 1−8. (25) Kayser, S.; Rading, D.; Moellers, R.; Kollmer, F.; Niehuis, E. Surface Spectroscopy Using Large Argon Clusters. Surf. Interface Anal. 2013, 45, 131−133. (26) Delcorte, A.; Garrison, B. J.; Hamraoui, K. Dynamics of Molecular Impacts on Soft Materials: From Fullerenes to Organic Nanodrops. Anal. Chem. 2009, 81, 6676−6686. (27) Delcorte, A.; Restrepo, O. A.; Czerwinski, B.; Garrison, B. J. Surface Sputtering with Nanoclusters: The Relevant Parameters. Surf. Interface Anal. 2013, 45, 9−13. (28) Czerwinski, B.; Postawa, Z.; Garrison, B. J.; Delcorte, A. Molecular Dynamics Study of Polystyrene Bond-Breaking and Crosslinking under C60 and Arn Cluster Bombardment. Nucl. Instrum. Methods Phys. Res., Sect B 2013, 303, 22−26. (29) Seah, M. P. Universal Equation for Argon Gas Cluster Sputtering Yields. J. Phys. Chem. C 2013, 117, 12622−12632. (30) Seah, M. P. An Accurate and Simple Universal Curve for the Energy-Dependent Electron Inelastic Mean Free Path. Surf. Interface Anal. 2012, 44, 497−503. (31) Seah, M. P. Simple Universal Curve for the Energy-Dependent Electron Attenuation Length for All Materials. Surf. Interface Anal. 2012, 44, 1353−1359. (32) Gilmore, I. S.; Seah, M. P. Ion Detection Efficiency in SIMS: Energy, Mass and Composition Dependencies for Microchannel Plates used in Mass Spectrometers Int. J. Mass Spectrom. 2000, 202, 217− 229. (33) Vickerman, J. C. Prologue: ToF-SIMS: An Evolving Mass Spectrometry of Materials in TOF-SIMS: Materials Analysis by Mass Spectrometry, 2nd ed.; Vickerman, J. C., Briggs, D.; IMPublications LLP, Chichester, and Surface Spectra Ltd: Manchester, 2013; Chapter 1, pp 1−38. (34) Postawa, Z.; Paruch, R.; Rzeznik, L.; Garrison, B. J. Dynamics of Large Ar Cluster Bombardment of Organic Solids. Surf. Interface Anal. 2013, 45, 35−38. (35) Gilmore, I. S.; Seah, M. P. Organic Molecule Characterisation: G-SIMS. Appl. Surf. Sci. 2004, 231−232, 224−229. (36) Gilmore, I. S.; Green, F. M.; Seah, M. P. G-SIMS-FPM: Molecular Structure at Surfaces: A Combined Positive and Negative Secondary Ion Study. Appl. Surf. Sci. 2006, 252, 6601−6604.

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