Cluster Primary Ion Sputtering: Secondary Ion ... - ACS Publications

Oct 14, 2009 - 4 of the 4 arms of the molecule largely intact and to unity for those ... Much historical data and data in spectral libraries18-20 are ...
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J. Phys. Chem. C 2010, 114, 5351–5359

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Cluster Primary Ion Sputtering: Secondary Ion Intensities in Static SIMS of Organic Materials† M. P. Seah, F. M. Green,* and I. S. Gilmore Quality of Life DiVision, National Physical Laboratory, Teddington TW11 0LW, U.K. ReceiVed: May 29, 2009; ReVised Manuscript ReceiVed: July 15, 2009

+ Positive secondary ion spectra from Irganox 1010, sputtered by Ar+, Bi+, Bi+ 3 , and Bi5 primary ions at 25 keV impact energy, are analyzed in detail. Irganox 1010 consists of a central carbon atom with 4 identical side chains or “arms”, each of C18H27O3. First, it is shown that the previously established relation, in which the secondary ion yield of the molecular species is proportional to the square of the sputtering yield, is accurately validated but, this time, for the positive, protonated molecular secondary ion instead of the previous negative deprotonated molecular secondary ion. Next, it is shown, for the first time, that the spectral ratios for Bi+ + + + primary ions to that for Ar+ primary ions, for Bi+ 3 to Ar and Bi5 to Ar , for 389 mass channels, are comprised of the product of two basic spectra called H*Irg and L*Irg raised to powers as defined in the text. These descriptions are valid to a remarkably low relative standard deviation of 2.3% over the 389 mass channels. The above squared dependence is inherent in both H*Irg and L*Irg so that it is likely that all similar primary ion sources will exhibit the same squared dependence on the sputtering yield as found earlier for the (M-H)- yields. The power of 2 is reduced for lower-mass secondary ion fragments, falling to ∼1.5 for fragments that have 3 or 4 of the 4 arms of the molecule largely intact and to unity for those fragments that are parts of an arm. This work also shows that, for the G-SIMS of peaks with m/z e 300 u it is recommended to use Bi+ and Mn+ (or + + Ar+) whereas, for higher mass peaks, cluster primary ions, such as Bi+ 3 or Bi5 , should be used with either Bi + + or Mn (or Ar ) to obtain the best signal quality.

I. Introduction In the static SIMS analysis of organic materials, many researchers now find it useful to obtain detailed quantitative information from the secondary ion yields of specific fragment ions, as well as the molecular ion intensity that may be either protonated or deprotonated. Thus, end groups in polymers may be used to measure the degree of cross-linking,1,2 polar and nonpolar groups may be related to wettability,3,4 etc. As part of the armory of techniques to handle these many intensities are the multivariate methods.5,6 In all such work, intensities are combined linearly. That is often correct in inhomogeneous materials where domains of different materials will contribute signals in proportion to the relevant areas although this linearity is not always the case in mixtures of materials.7,8 The above linearity does not appear to translate to the intensities in secondary ion spectra from different primary ion sources. Seah showed that the yield of the deprotonated molecular negative secondary ions, for monolayers of Irganox 1010 on polystyrene, increased as the square of the sputtering or total secondary ion yields as one progressed from one primary ion cluster to another.9,10 This, of course, means that at least part of the static SIMS spectrum (the deprotonated molecular secondary ion) will vary in intensity relative to the dominant intensities at low mass, from source to source, so that there is no unique spectrum applicable to all primary ion sources for a given organic solid. The squared dependence of the molecular secondary ion yield on the sputtering or total secondary ion yields has not been shown, yet, for systems other than Irganox 1010 and some

analysts have claimed it is, in some way, unique or untypical. That may or may not be true and does need investigation. These analysts, like us, use modern commercial SIMS systems operating at energies typically less than 20 keV and occasionally up to 60 keV. However, those operating in the swift ion regime with primary ion energies greater than 0.5 MeV/nucleon often see such effects and power dependencies on the rate of primary ion energy loss in the range up to an index of 4 have been reported.11-17 Much historical data and data in spectral libraries18-20 are for inert gas primary ions. Over the years, the primary ion sources used have changed and changed again. Different spectrometer manufacturers fit different sources. It is important, therefore, if analysts are to make the best use of their instruments and of the published literature and data sources that the relationships for the secondary ion spectra from different primary ion sources are characterized and understood. Very detailed studies of spectral intensities have been made in the studies for, and development of, the G-SIMS method. There, it is shown that, on moving from, say, argon primary ions at 4 keV to argon at 10 keV, the spectra for m/z < 200 u (u is the unified atomic mass unit) are very similar but the relative intensities at successive C or H atom losses in the series CjH2j+2-i would be progressively a few percent weaker in the lower energy spectrum. Thus, if the intensity ratios, F12,i of two spectra, I(1,m/z) and I(2,m/z) were plotted, it was seen that the data exhibit linear regions when F12,i is plotted on a log10 scale. This was interpreted to show that21,22

F12,i ) F12,0 exp(-β*i)

†

Part of the Barbara J. Garrison Festschrift. * To whom correspondence should be [email protected].

addressed.

10.1021/jp905037k

E-mail:

where β* is given by Published 2010 by the American Chemical Society Published on Web 10/14/2009

(1)

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(

β* ) ∆u

1 1 kTp2 kTp1

)

Seah et al.

(2)

and ∆u is the energy required to remove successive hydrogen atoms from the fragment CjH2j+2-i. In this formulation, it is assumed that spectrum 1 has an effective plasma temperature Tp1, that characterizes the hydrogen stripping of CjH2j+2-i, and similarly for spectrum 2 and Tp2. Of course, in the process of sputtering there are different plasma temperatures in different spatial regions around the original primary ion track and these regions, in turn, change with time. Nevertheless, the intensity of each peak appears to be characterized by a single effective plasma temperature. Studies using many different primary ions have been made using polystyrene, (CH2-CHPh)q, where Ph is a pendant phenyl ring and q shows the number of repeat groups or the degree of polymerization.21,23 The spectra are referenced to that for argon at 7 keV beam energy and values of β () 0.431β*)22 can then be measured. In polystyrene, in the mass range 12 e m/z e 160 u, for which 1 e j e 12 and for 0 e i e 10, it is found that β is effectively constant.22 Thus, the characteristic plasma temperature appeared to be constant for a given primary ion over this fragment range. Polystyrene is a very effective material for characterizing primary ion sources and, recently, data for many monatomic ions have been correlated with the energy deposition density, in a semiempirical theory, showing that the lower the energy deposition density, the higher the value of β, as found experimentally.22 From the above studies of monatomic primary ions, it was found that in the primary ion energy range 4 e E e 25 keV used in much SIMS work, Bi+ primary ions exhibited the greatest value of β and Mn+ the lowest. The values for primary ions with atomic masses from that of Ne+ to that of Bi+ at 25 keV are described by

β ) 0.405x2 - 1.427x + 1.234

(3)

where x is log10(M) and M is the atomic mass of the monatomic primary ion in u. In G-SIMS, β* from eq 1 is used to calculate colder and colder spectra that are less and less degraded until, eventually, only a few peaks remain, indicating the mass of the parent molecule or of major fragment peaks.21 This leads to the method of G-SIMS FPM24-26 where the procedure allows the identification of the intermediate fragments, so permitting one to work toward identifying which fragments generated which daughter fragments in order to reconstruct the original molecular structure. Thus, in the above, we have a detailed analysis of spectra using monatomic primary ions with many different ions impacting polystyrene and with selected ions impacting a range of organic molecules. One of the most studied of these molecules is Irganox 1010, and this has become an effective reference point for many researchers. In the present work, therefore, we study, in more detail, the full secondary ion spectrum for the primary ions Ar+, Bi+, Bi3+, and Bi5+ impacting pure bulk layers of Irganox 1010. The study will largely be in terms of ratios of one spectrum to another to generate FXY spectra as above since, in this process, the major changes in intensity, including those arising from ionization probabilities, are removed. 2. Experimental Section SIMS spectra were recorded for pure samples of Irganox 1010 deposited onto a cleaned Si wafer using an Edwards AUTO

Figure 1. Schematic of the molecular structure of Irganox 1010.

Figure 2. Example of Bi+ 5 positive secondary ion spectrum for Irganox 1010 (a) with the mass ranges for groups involving R, 2R, 3R, and 4R indicated where R represents one of the four arms of the Irganox 1010 molecule, (b) expansion for 900 e m/z e 1200 u.

306 vacuum coater. Positive secondary ion spectra were measured using a TOF-SIMS IV time-of-flight secondary ion mass spectrometer (ION-TOF, Mu¨nster, Germany) with 25 keV primary ions. Four spectra were recorded for Ar+, three for Bi+, and two each Bi3+ and Bi5+ using raster areas of 200 µm × 200 µm. The time-averaged beam currents were 1.06, 0.475, 0.12, and 0.02 pA, respectively, with total acquisition times of 80, 139, 133, and 914 s, respectively, to provide similar doses. The doses relative to Ar+ were 1.032, 0.249, and 0.286, for Bi+, Bi3+, and Bi5+, respectively. For calculating spectral ratios, the average spectra for each primary ion source were then scaled to the same effective dose of one incident ion. This gives the yield in each case. For the peaks with intensities greater than 1000 counts in the original spectra, the average standard deviations of the repeat spectra were 3% for Ar+ and Bi+, and 1.5% for Bi3+ and Bi5+ primary ions. 3. Results and Discussion All of the measurements reported here are for bulk samples of pure Irganox 1010 the structure of which is shown in Figure 1. An example spectrum for Bi+ 5 is shown in Figure 2. In Figure 2, for convenience, are shown the ranges of mass for which parts of the molecule can occur. The most intense fragments

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are at masses (denoted by m/z) below 250 u and relate to parts of one of the 4 identical side chains or “arms”, R, of C18H27O3, of the molecule. Of much lower, but significant, intensity are parts that may be associated with the whole molecule with small gains or losses or with one arm lost. In this section it is helpful to consider, first, the constituent intensities of the measured secondary ion spectra before then interpreting the G-SIMS for cluster sources. 3.1. Intensities in the Spectra. To consider the structure of the SIMS spectra, it may be thought that they are generated in two essential parts: a rapid part IC, for example, arising from the initial effects of the primary ion transit, and a later part IT arising from thermal and other secondary events causing emission slightly later in the process. From this, one may consider that the spectra could be separated into two parts:

I ) IC + IT

(4)

Figure 3. Positive secondary ion yield for the protonated molecule, Y(M + H)+, as a function of the total ion yield, Y(TIY), for the four primary ion sources. These yields are the emitted numbers of ions per incident ion. The dashed line represents eq 5.

where the ratio of IT/IC would rise through the primary ion series Ar+, Bi+, Bi3+, and Bi5+. However, there is ample evidence that the shape of the spectrum alters for different primary ions. For instance, in many publications, the dependence of the secondary ion intensities of a fragment with t repeats of a component changes as the primary ion cluster size, n, increases, with the higher t value secondary ion fragments becoming progressively stronger.27-36 In the study by Seah,9 it was shown that, for a monolayer of Irganox 1010 on low-density polyethylene, the secondary ion yield of the deprotonated molecular ion Y(M - H)- was found to be proportional to the square of the sputtering yield or of the total ion yield, Y, i.e., -

Y(M-H) ∝ Y

2

(5)

Clearly such spectra could only be a linear combination of spectra if the deprotonated molecular ion and any associated peaks formed a spectrum, say IT, independent of the intensities for those at m/z < 250 u comprising the majority of the total ion yield, say IC. In a later work,10 it was postulated that the above power of 2 could change with the secondary ion cluster size for organic materials. One could expect it to be unity for molecules with m/z e 250 u and, perhaps rising monotonically in the range 250 to 1176 u. In this work for positive secondary ions, the squared law described by eq 5 is upheld for the yield of the protonated molecular ions, Y(M + H)+, as shown in Figure 3, with great precision. The dashed line is the function of eq 5. This differs slightly from our previous analyses for Irganox 10109,10 in which it was deposited as a monolayer, instead of the bulk, and in which the intensities were for the deprotonated molecular negative ion, instead of the protonated positive molecular ion recorded here. These changes are expected to have, and appear to have, little effect. To study the spectra in more detail we work with spectral ratios since the dramatic changes from channel to channel arising from ionization and other effects are then removed. This is the process also used to generate G-SIMS spectra. Thus, in Figure 4 are shown the ratio spectra, Y(X, m/z)/Y(Ar, m/z) for the primary ion X ) Bi+, Bi3+, and Bi5+ to that for Ar+ where the spectra are the respective averages of the several measurements described for each primary ion. To reduce the contribution of noise, all the data in mass channels with 50 u. The resulting average of HIrg5 and HIrg3, HIrg, is shown in Figure 7a and in more detail at low masses in Figure 7b. Observing the behavior here is not too clear and so we shall return to this later. In the detail in Figure 7b, note the runs of data in HIrg with negative slopes for ∼10 or so data

Cluster Primary Ion Sputtering

Figure 6. Correlation of [log10(I5) - c5 log10(I1)] vs [log10(I3) - c3 log10(I1)] for the HIrg spectrum, (O) m/z ) 0-199 u, (0) m/z ) 200-599 u, and (∆) m/z ) 600-1200 u. The gradient of the solid straight line is Q ) 1.272.

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Figure 8. Correlation of [log10(I1) - d1 HIrg] vs [log10(I3) - d3 HIrg] for the LIrg spectrum, (O) m/z ) 0-199 u, (0) m/z ) 200-599 u, and (∆) m/z ) 600-1200 u. The gradient of the solid straight line is S ) 1.431.

points in a row. These are observable for 40 < m/z < 200 u. Figure 7c shows the high-mass regime. The LIrg spectrum may be generated in the same way that we generated the HIrg spectrum. Here we know that the Bi+ and Bi3+ data will exhibit this most strongly and so we consider log10 I1 - d1HIrg and log10 I3 - d3HIrg. Figure 8 shows the correlation for these data with d1 ) 0.150 and d3 ) 0.994. The standard deviation of the scatter for S(log10 I1 - d1HIrg) - (log10 I3 - d3HIrg) is 0.012, a rather better result than that for Figure 6. In this relation S ) 1.431. The correlation of I1 and I3 may be repeated similarly with I1 and I5. This gives three results for the LIrg spectrum that are averaged. The original guess that there is only one significant low-mass spectrum is upheld. The average LIrg is plotted earlier in Figure 7 with the average HIrg spectrum. The two spectra appear almost mirror images of each other, particularly at the low masses. Finally, we compare the HIrg and LIrg spectra with the original Y(Bin+, m/z)/Y(Ar+, m/z) data. Here we find

log10[Y(Bi+, m/z)/Y(Ar+, m/z)] ) 0.1501HIrg + 0.9999LIrg SD ) 0.001 (12) + log10[Y(Bi+ 3 , m/z)/Y(Ar , m/z)] ) 0.9948HIrg + 1.4294LIrg SD ) 0.014 (13) + log10[Y(Bi+ 5 , m/z)/Y(Ar , m/z)] ) 1.2118HIrg + 1.3589LIrg SD ) 0.015 (14)

Figure 7. Average HIrg and LIrg spectra (a) m/z ) 0-1200 u and (b) m/z ) 0-250 u, (∆) HIrg and (O) LIrg, and (c) m/z ) 900-1200 u using the traditional stick plot style.

where the SDs, the scatter standard deviations, are for m/z g 50 u. The average standard deviation of these descriptions as a relative percent for the log10 intensity ratios is 0.6%, which is excellent. In terms of absolute intensities, this converts to an average scatter of 2.3%. This is exceptionally good since the original data involved signals as low as 144 in the Ar+ spectrum. Calculating the standard deviation of the intensities of the mass peaks used in the repeat Ar+ spectra and averaging these gives a scatter of 2.0% to which the similar scatter of the Bin+ spectra should be added. The description of eqs 12-14 thus appear to be within the very low experimental error of the present spectra. The above analysis could have been conducted efficiently, but with less transparency, using principal components or factor analysis for the logarithms of the spectral intensities. The results would be different in that the two main components would be

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log10 FX1 )w log10 FX2

(16)

where w is some constant. From this,

FX1 ) (FX2)w

Figure 9. Plot as in Figure 2 for various combinations of a and b; with a as indicated and b rising from 0.5 to 1.55 in increments of 0.15 starting from the lowest point in each series. The power for the solid straight line is 2.

linear combinations of the log spectra HIrg and LIrg. Of course, an infinite set of pairs of spectra is possible, formed of linear combinations of HIrg and LIrg.The method used here leads to positive terms and emphasizes the high mass range in HIrg. We can plot the synthetic spectra in the form of Figures 2 or 4 for comparison but there is little point since these data involving the spectral HIrg and LIrg factors are too close to the original data to see any discrepancies. Could we similarly establish two spectra suitable for linear addition as in eq 4? A similar approach to that above for linear addition gives two results for the couterpart to HIrg that already differ by 16%, an order of magnitude worse than for product spectra. It is clear that linear combinations cannot generate eqs 5 or 6 or Figures 3 or 5. From eqs 12 to 14, the measured spectra for Bi+, Bi3+, and Bi5+ primary ions can be generated using only the set of six coefficients, the Ar+ spectrum and the HIrg and LIrg spectra, the latter being spectral factors covering 389 data channels. For the range of possible spectra generated using the spectral factors HIrg and LIrg, Figure 9 shows the result in a similar format to Figure 3 for various values of b for four values of a. Figure 9 shows that, for a range of values of a and b from eq 9, the results would conform very closely to the experimental observation of Figure 3. Why is this so? If we inspect Figure 2, we see that the total ion yield is dominated by 0 e m/z e 260 u. If we consider the spectra HIrg and LIrg shown in Figure 7a, we may evaluate the intensity of the (M + H)+ peak and the average intensity from say 0 to 260 u. In both cases, this ratio is close to 2. Thus, whatever the total ion yield is scaled by, the (M + H)+ term is scaled by that factor squared since the HIrg and LIrg spectra are constituents for log10[Y(Bin+, m/z)/ Y(Ar+, m/z)]. Thus, eq 5 is a direct result of the forms of the HIrg and LIrg spectra. 3.2. G-SIMS and Fragmentation. It is now appropriate to return to the G-SIMS aspect of this work. In G-SIMS, we have

Y(X1, m/z) ) FX1 Y(Ar, m/z)

(15)

where, from the log10(FX1) result, the gradients of the data for polystyrene for CjH2j+2-i gave a characterization of the primary ion source, X1. For a second primary ion source, X2, FX2 is similar to FX1 but is just scaled on the plot of log10 [Y(X1, m/z)/ Y(Ar, m/z)]. Thus

(17)

Thus, the conclusion that all of the spectra derive from product terms is already implicit in the G-SIMS approach. The value of FX for X ) Bi+ was about twice that for FX with X ) Xe+. The gradients were double and the log10 FX values were double. Similar gradients to those for polystyrene are seen in Figure 4 for 0 e m/z e 300 u for the ratio of the Bi+ and Ar+ spectra. From the molecular structure of Irganox 1010 shown in Figure 1, we may expect similar series for C1 to C18 with successive hydrogen removals which occupy the mass range 12 e m/z e 291 u for one arm R of the Irganox 1010 molecule. This is just what is seen for Y(Bi+, m/z)/Y(Ar+, m/z) in Figure 4 and shown in more detail in Figure 10a. For CjH2j+2-i the gradients are similar for 1 e j e 10 but above 10 they seem to weaken. For Y(Bi3+, m/z)/Y(Ar+, m/z) the gradients are much steeper for low values of j and seem to weaken and invert around m/z ) 140 u. + For Y(Bi+ 5 , m/z)/Y(Ar , m/z) the situation again changes and the inversion occurs around m/z ) 70 u. A similar effect may be seen in the work of Straif and Hutter.37 This would indicate that Bi3+ and Bi5+ primary ion sources, while being excellent for high (M + H)+ intensities and associated fragment yields, may be less useful for G-SIMS at low m/z. What is happening, of course, is that increasing amounts of the HIrg spectrum shown in Figure 7b are being added to the LIrg spectrum which was the main contributor to the Y(Bi+, m/z)/Y(Ar+, m/z) spectrum. The HIrg inversion occurs around m/z ) 40 u and, as more and more HIrg is added, the slopes around m/z ) 30 u increase while those at higher mass decrease and eventually invert. Thus, for masses < ∼250 u, sources that give strong LIrg spectra and weak HIrg are most effective for G-SIMS. In broad terms, in the low-mass regime emitted from the core of the sputtered region, the increase in n for cluster primary ions causes an increase in the energy density such that β falls.22 Thus, in general, cluster SIMS is less helpful for low-mass G-SIMS. However, in the high-mass regime, shown in Figure 10b, the situation is changed. Here, the cluster sources give very much higher emission of the molecular fragments than does Ar+. For m/z > 900 u, the intensity of the secondary ion peaks in the Ar+ spectrum in Figure 10b average around 240 counts with an associated signal-to-noise ratio of only 15. This is too low for effective G-SIMS unless the ratio of the two spectra gives a sufficiently large effect. It appears from Figure 10b that whereas there are no lines of data to give β slopes between Bi+ and Ar+, there are between Bi3+ and Ar+ and also between Bi5+ and Ar+. This is shown more clearly in Figure 10c with just the Y(Bi5+, m/z)/Y(Ar+, m/z) data and with the gradients shown by the set of parallel lines. The equivalent set for Y(Bi3+, m/z)/ Y(Ar+, m/z) are, of course, weaker by the scaling factor 0.9948/ 1.2118. These slopes are about double those for Y(Bi+, m/z)/ Y(Ar+, m/z) for the low-mass fragments in Figure 10a. It is easy to generate G-SIMS spectra to see what all this causes in practice. From the original data,

[G-SIMS spectrum] ) [Y(X1, m/z)](FX1)g

(18)

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Figure 11. Fractions of the total G-SIMS spectral intensities, calculated from equation 18 as a function of the index, g: (a) for Bi+ and Ar+ and (b) for Bi5+ and Ar+. The dashed data in (a) represent data with inadequate signal-to-noise ratios. The boxes to the right show the approximate masses of the fragments. The smaller abscissa scale in (b) results from the greater β gradients for the cluster ions in Figure 10.

Figure 10. Details of Figure 4 for (O) Y(Bi+, m/z)/Y(Ar+, m/z), (0) Y(Bi3+, m/z)/Y(Ar+, m/z), and (∆) Y(Bi5+, m/z)/Y(Ar+, m/z), (a) lowmass range with j values marked under the lower spectrum and (b) the + high-mass range, (c) the high-mass range for Y(Bi+ 5 , m/z)/Y(Ar , m/z) with parallel β gradient lines added.

where X1 is the primary ion source for one of the SIMS spectra, as previously.21 The G-SIMS spectra have only a few major peaks, but as before,25 the most intense peak depends on the value of g. The peak that maximizes at the highest value of g is generally for the fragment that is most representative of the parent molecule and those that maximize at lower values of g generally represent daughter fragments. However, in rare cases, daughter fragments can have a parent with a lower maximum g. Figure 11a shows the plot for the fractional intensity of selected peaks contributing to the G-SIMS spectrum as a function of g for X1 ) Bi+. It is clear that there are different groups of peaks with those 150 < m/z < 250 u having maxima around g values, gmax, of 1.5 and those with 700 < m/z < 850 u having gmax around 7. All the peaks with 50 < m/z < 250 u could be plotted and this would show gmax for CjH2j+2-i rising from -1 at j ) 4 to ∼3.2 at j ) 14 for i ) 0 but the gmax values are all around -1 for the higher i values near the ends of the runs of data. Note that for g ) 0 the G-SIMS spectrum is just

that for Bi+ and for g ) -1 it is that for Ar+. As noted above, the ratio intensities for the positive secondary ions from Irganox 1010 with m/z > 1000 u are too noisy for effective G-SIMS, particularly as higher g values are being used and this noise is then further amplified. Care must then be exercised when interpreting the higher masses, as shown by the dashed lines in Figure 11a. Figure 11b shows the results for X1 ) Bi5+. Now we see that, for 50 < m/z < 250 u, we are already beyond gmax at g ) 1 (the Bi5+ spectrum) and that the peaks with 700 < m/z < 900 u are better defined. Now the (M + H)+ peak is identified as that most characteristic of the parent molecule. Note that the peak with mass 1189.79 u, although higher in mass than (M + H)+ at 1177.79 u, is identified as a special daughter that is M with a captured CH; it has a lower value of gmax than (M + H)+. Table 1 identifies the fragments, shown in Figure 12, which clearly comprise simple parts of the molecule. In our earlier study of Irganox 1010 using G-SIMS,26 primary ion sources of Cs+ and Ar+ were used. This is similar to, but half the effect of, Bi+ with Ar+. More details of fragmentation pathway mapping can be found there. Also, importantly, that work with the negative secondary ions shows that if the important fragments have sufficient intensity, the use of cluster sources is not always necessary. Unfortunately, for the positive secondary ions from Irganox 1010 and for both positive and negative secondary ions for many other molecules, these peaks are too weak for the monatomic ions to be effective. Comparison of panels a and b of Figure 11 shows that the effect of Bi+ 5 generates g values about 3 times as strong as those for Bi+. The effect for Bi3+, not shown, is about 2.5 times as

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TABLE 1: G-SIMS Fragment Masses and Their Identificationa

The spectra from the different primary ion sources may be split very precisely into a spectrum from a single source and two further spectral terms H*Irg and L*Irg which generate the final spectrum according to

mass

identity

mass

identity

1189.79 1187.78 1177.79 1176.78 1175.78 1173.76 1122.74 1120.72 899.60 843.54b 787.48 731.41

M + CH (M - H) + C M+H M M-H M - H3 M - C 4H 6 M - C 3H 8 δ δ - (But - H) δ - (But - H)2 δ - (But - H)3

259.17 233.12 220.18 163.11 57.07 41.04

R - CH4O R + CΗ2 R+H R - But + H But C 3H 5

a R, β, and γ are defined in Figure 12. represents the butyl group.

b

The symbol But

an bn I(Bi+ n , m/z) ) I(Ar,m/z)[H*Irg(m/z)] [L*Irg(m/z)]

(19) Here, this has been shown for Irganox 1010 for 389 mass channels, m/z, to a standard deviation of 2.3% for m/z > 50 u. In eq 19, log10 H*Irg ) HIrg and log10 L*Irg ) LIrg. A similar formalism, using a sum of spectra, cannot be found with a similar level of agreement. Since the analogue of eq 5, that Y(M ( H)( ∝ Y2, is upheld by the constituent parts HIrg(m/z) and LIrg(m/z), eq 5 will be valid for all spectra generated in this way and this agrees with the observation for many different primary ion species, as noted earlier.9 To the extent that eq 19 is valid for other materials, it may be used as a basis to aid the analyst extrapolate or interpolate literature data for relevant spectra. For the use of G-SIMS for m/z e ∼250 u it is recommended that Bi+ with Mn+ or, say Ar+, are used as the two sources, whereas for m/z > 750 u, if the relevant intensities are weak, it will be found that Bi3+ or Bi5+ with Bi+ or Mn+ are more effective. This arises from the higher yields available at high mass with the cluster primary ions. More work is needed to evaluate the behavior described here for the negative secondary ions, for other large and small organic molecules, and for inorganic solids.

Figure 12. Fragments of the Irganox 1010 molecule for Table 1.

strong as those for Bi+. These effects are consistent with the relative positions of the data plotted in Figure 3. Thus, for large organic molecules, it appears that cluster sources have an important role for G-SIMS in the high-mass regime, although interpreting the low-mass zone may be more effectively conducted with the monatomic ions. Clusters from the combined Bi/Mn source23 could be used in this way with traditional G-SIMS at lower masses using the Bi+ and Mn+ primary ions. 4. Conclusions Part of the importance of this work is for the many users of SIMS, which today sees a growing number of primary ion sources being used. Libraries of data do exist;18-20 they are extensive but are mainly for the inert gas primary ion sources such as argon. At first inspection, the spectra from modern cluster sources look very different from those and the user may feel that the experience gained from libraries of data or from the published literature is not applicable and reference data for molecular identification must be conducted with the correct primary ion source and energy. Different manufacturers use different cluster primary ions, and comparable data may be difficult to find. This work shows, using spectra for for Ar+, + Bi+, Bi+ 3 , and Bi5 primary ions and Irganox 1010 as the sample, how these spectra are all closely related but that the intensity of the (protonated or deprotonated) molecular ion rises more rapidly than that of other fragments as the total yield rises; rising very precisely with a squared power. This also occurs, but more weakly for molecular fragments, falling to a power of around 1.4 to 1.6 for fragments in the range 700 < m/z < 1100 u and to unity for m/z e 400 u.

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