TOF-SIMS Analysis Using C60. Effect of Impact Energy on Yield and

C60 has been shown to give increased sputter yields and, hence, secondary ions when used as a primary particle in SIMS analysis. ... TOF-SIMS with Arg...
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Anal. Chem. 2006, 78, 1827-1831

TOF-SIMS Analysis Using C60. Effect of Impact Energy on Yield and Damage John S. Fletcher,* Xavier A. Conlan, Emrys A. Jones, Greg Biddulph, Nicholas P. Lockyer, and John C. Vickerman

Surface Analysis Research Centre, School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M60 1QD, UK

C60 has been shown to give increased sputter yields and, hence, secondary ions when used as a primary particle in SIMS analysis. In addition, for many samples, there is also a reduction in damage accumulation following continued bombardment with the ion beam. In this paper, we report a study of the impact energy (up to 120 keV) of C60 on the secondary ion yield from a number of samples with consideration of any variation in yield response over mass ranges up to m/z 2000. Although increased impact energy is expected to produce a corresponding increase in sputter yield/rate, it is important to investigate any increase in sample damage with increasing energy and, hence, efficiency of the ion beams. On our test samples including a metal, along with organic samples, there is a general increase in secondary ion yield of high-mass species with increasing impact energy. A corresponding reduction in the formation of low-mass fragments is also observed. Depth profiling of organic samples demonstrates that when using C60, there does not appear to be any increase in damage evident in the mass spectra as the impact energy is increased. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a powerful tool in surface science and analysis,1 offering high spatial resolution imaging with the generation of information-rich mass spectra. Application of the technique to the analysis of organic and biologically relevant samples has provided an impetus for the development of methods for the generation of higher mass ions from the sample. The generation of such ions is dependent on the sputtering conditions, and molecular dynamics simulations have identified a number of pathways that increase the sputter yield of varying mass species. The mechanisms are described by Delcorte et al.2 as low action/low yield, low action/high yield, high action/low yield, and high action/high yield. The latter of these mechanisms produces the greatest yields of higher-mass fragments. Novel ion sources have been employed that result in an increase in the number of high-mass ions generated during the sputtering process and, therefore, yielding more chemically unique * Corresponding author. E-mail: [email protected]. (1) Vickerman, J. C., Briggs, D., Eds. ToF-SIMS Surface Analysis By Mass Spectrometry; Surface Spectra, I M Publications: Manchester and Chichester, 2001. (2) Delcorte, A.; Garrison, B. J. J. Phys. Chem. B 2004, 108, 15652. 10.1021/ac051624w CCC: $33.50 Published on Web 02/14/2006

© 2006 American Chemical Society

data with a significant increase in the number of molecular ions generated. The recent developments of Au3+ and C60+ cluster ion sources have demonstrated secondary ion yield (measured secondary ions per primary ion) enhancements in the range of 30-100 times, as compared with gallium monatomic analysis. In some cases, an infinite secondary ion yield increase is possible. For example, in the analysis of the peptide gramicidin, the molecular ion is present in the Au3+ and C60+ spectrum but absent from spectra acquired using gallium.3,4 Molecular modeling simulations5,6 have been used to visualize the impact of the primary ion on a surface. Comparison between gallium and C60+ impacting a silver surface has illustrated a number of distinct differences in the sputtering process. The monatomic gallium ion penetrates into the sample to a much greater depth than the C60+ ion, causing the disruption of many subsurface layers, with only a small proportion of the impact energy being transferred to the surface region of the sample and producing only a small yield of secondary particles. C60+, however, is expected to break up on impact and, thus, is equivalent to the simultaneous impact of 60 carbon atoms, each with 1/60th of the energy of the primary ion. The result is a massive ejection of material from the impact crater and, because each individual atom has relatively low energy, a reduction in the penetration depth of the primary particle with a corresponding decrease in disruption/ damage of the subsurface region. SIMS is used in the semiconductor industry to map changes in concentration of atomic species, such as silicon and gallium, both laterally and as a function of depth. This has not been possible for larger molecular species because the sputtering process, in addition to damaging the surface and producing a carbonized graphitic layer, also causes subsurface damage. Recent reports of experiments using C60+ ion beams have demonstrated not only an increase in yield of secondary ions but also a substantial reduction in chemical damage of the sample, possibly due to the removal of any damage generated with each subsequent impact. Low-energy C60+ bombardment has been used to remove surface contamination from polymers prior to analysis (3) Davies, N.; Weibel, D. E.; Blenkinsopp, P.; Lockyer, N. P.; Hill, R.; Vickerman, J. C. Appl. Surf. Sci. 2003, 223, 203. (4) Weibel, D. E.; Wong, S.; Lockyer, N. P.; Blenkinsopp, P.; Hill, R.; Vickerman, J. C. Anal. Chem. 2003, 75, 1754. (5) Nguyen, T. C.; Ward, D. W.; Townes, J. A.; White, A. K.; Krantzman, K. D.; Garrison, B. J. J. Phys. Chem. 2000, 104, 8221. (6) Postawa, Z.; B. Czerwinski, B.; Szewczyk, M.; Smiley, E. J.; Winograd, N.; Garrison, B. J. J. Phys. Chem. B 2004, 108, 7831.

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using X-ray photoelectron spectroscopy with no observable chemical change to the sample.7 C60+ ion beams have recently been used to analyze a number of biological test systems, such as multilayer samples, for example, Langmuir-Blodget films8 and biomolecules suspended in high sputter yield matrixes, such as histamine in water-ice.9 SF5+ ion beams have also shown reduced damage characteristics,10,11 particularly when analyzing molecules contained within a suitable matrix. Reduced damage effects have been observed in polymer samples, particularly PMMA,12 and the successful depth profiling of a number of biopolymers having application as drug delivery systems using SF5+ has been reported.13 The considerable increase in sputter rate and corresponding secondary ion yield increase available using C60 offer obvious benefits over SF5. Even larger macromolecular beams, such as glycerol clusters,14 have been shown to remove surface contamination from analyte while leaving underlying molecular species intact, although practical ion beam systems have not resulted from these studies. In this paper, we investigate the variation in secondary ion yield and the depth profiling characteristics of a C60 ion beam when increasing the impact energy of the particle. It is assumed that increasing the impact energy will lead to an increase in the sputter yield from the sample surface, although from the point of view of mass spectrometric analysis, particularly of biological samples, we are particularly interested in any increase in the secondary ion yield of the higher mass species that are often the most chemically/biologically characteristic. The ability of TOF-SIMS to analyze intact proteins, for example, would allow the technique to be applied to samples for which MALDI is the only practical alternative, albeit with the advantage of the greater spatial resolution. If the plume of ejected material in the SIMS experiment becomes sufficiently dense, then the probability of gas-phase interaction/reaction (e.g., proton transfer) is more likely, thus increasing the experimental possibilities available through matrixassisted SIMS. A further consideration when increasing the impact energy of the C60 particle is whether the low damage characteristics of the projectile are maintained. High-energy projectiles are expected to penetrate more deeply into the surface and may increase subsurface damage. In this study, the highest impact energy used was 120 keV, and this still results in a relatively low (2 keV) energy per carbon atom. EXPERIMENTAL SECTION The analysis was performed on a Bio-TOF SIMS instrument that has been described in detail elsewhere.15 In brief, the instrument comprises a fast entry load lock, sample preparation chamber, and a main analysis chamber. A reflectron TOF mass (7) Sanada, N.; Yamamoto, A.; Oiwa, R.; Ohashi, Y. Surf. Interface Anal. 2004, 36 (3), 280. (8) Sostarecz, A. G.; Sun, S.; Szakal, C.; Wucher, A.; Winograd, N. Appl. Surf. Sci. 2004, 231-232, 179. (9) Wucher, A.; Sun, S.; Szakal, C.; Winograd, N. Anal. Chem. 2004, 76, 7234. (10) Kotter, F.; Benninghoven, A. Appl. Surf. Sci. 1998, 133, 47. (11) Gillen, G.; Roberson, S. Rapid Commun. Mass Spectrom. 1998, 12, 1303. (12) Wagner, M. S.; Gillen, G. Appl. Surf. Sci. 2004, 231-232, 169. (13) Mahoney, C. M.; Roberson, S.; Gillen, G. Appl. Surf. Sci. 2004, 231-232, 174. (14) Dookeren, N. M.; McNahon, J. M.; Short, R. T.; Todd, P. J. Rapid Commun. Mass Spectrom. 1995, 9, 1321. (15) Braun, R. M.; Blenkinsopp, P.; Mullock, S. J.; Corlett, C.; Willey, K. F.; Vickerman, J. C.; Winograd, N. Rapid Commun. Mass Spectrom. 1998, 12, 1246.

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Figure 1. Plot of the secondary ion yield of the silver clusters, Agn, for n ) 1, 3, 5, 7 for C60 impact energies of 20, 40, 80, and 120 keV.

analyzer is used, and when required to prevent sample charging, low energy (25 eV) electrons can be flooded onto the sample between ion gun pulses. The instrument has recently been equipped with a new C60 ion gun capable of providing ions with energies up to 120 keV (Ionoptika Ltd. UK). The ion gun runs at anode voltages up to 40 kV, and the gas-phase ionization process produces singly, doubly, and triply charged C60 ions. By selecting these ions independently using a wien filter, analyses can be performed using primary particles with impact energies up to 120 keV. Under standard operating conditions, DC currents of ∼1 nA of C60+ are routinely available at 40 keV. Samples were depth-profiled by performing a sequence of etch/ analysis cycles, with the analysis area set to half of the etch area to avoid any influence from the edge of the crater. The C60 ion beam was used for both analysis and etching of the samples during depth profiling, and the depth profile sequence was computercontrolled to maximize accuracy and stability. Samples. A number of materials with distinctly different physical properties were chosen for the study. We present data obtained from a metal, silver; an antioxidant, Irganox 1010; a biopolymer, polycaprolactone (PCL); a phospholipid, dipalmitoyl phosphatidylcholine (DPPC); and a peptide, gramicidin. RESULTS AND DISCUSSION Silver. The SIMS analysis of silver results in a spectrum that comprises a number of sliver clusters of the type Agn+. The secondary ion yield of the clusters has been measured for n ) 1, 3, 5, and 7, providing information relating to the change in secondary ion yield with increasing impact energy over the mass range 100-800. Impact energies of 40, 80, and 120 keV were used. Both the Ag1+ and the Ag3+ silver species reach a maximum secondary ion yield when analyzed with the 80-keV C60, although this maximum is most pronounced for the Ag1+. However, for the higher mass clusters Ag5+ and Ag7+, there is a continuing increase in secondary ion yield with increasing energy, with the Ag7+ species demonstrating a linear increase of yield with impact energy (Figure 1). It is clear that simply hitting the sample harder with C60 does not increase the secondary ion yield of all species evenly, although this may be the case at lower energies. Molecular dynamics simulations of 5, 10, 15, and 20-keV C60 impacting upon a Ag{111} surface show a linear yield increase with energy for the Ag1, Ag2, Ag3, and Ag4 species, although the latter has a much

Figure 2. Plot of the secondary ion yield of characteristic peaks in the Irganox 1010 spectrum, including the molecular ion for C60 impact energies of 20, 40, and 80 keV.

greater yield increase than the first three, which show an approximately equal gradient of increase with increasing impact energy.6 At high energies (120 keV in this example), the overall secondary ion yield is reduced due to a loss of the intense lowmass Ag clusters, despite an increase in the yield of higher mass species. In the SIMS analysis of organic and biological samples, it is often these higher mass fragments that are the most chemically distinct and, therefore, the most significant for data interpretation. The variation in cluster yields from metals is not expected to be influenced by the surface structure of the sample,16 and so the preference for the formation of higher mass clusters at increased impact energy is expected to result from an increase in sputtering events in the sample, leading to the ejection of these higher mass clusters. The corresponding reduction in lower mass species indicates a stabilization of the emitted clusters, resulting in reduced fragmentation. Irganox 1010. Irganox 1010 is a large tetrahedral molecule that is often used as an antioxidant. A central carbon atom is bound to four arms, with each arm having a mass of 219 Da. The TOFSIMS spectra show a series of peaks of increasing mass up to the molecular ion. For the secondary ion yield calculations, peaks were selected that have been previously assigned and are listed in the commercially available Static SIMS Library.17 The secondary ion yield data have been determined for C60 impact energies of 20, 40, and 80 keV, and the yields of the selected peaks show an increase with increasing impact energy, as illustrated in Figure 2. This is in agreement with the results obtained from the silver sample because the peaks used for this calculation fall in the mass range 500-1200 and is the Ag5 and Ag7 clusters that also show increased secondary ion yield with increasing impact energy. The increase in secondary ion yield with increasing impact energy becomes more apparent for the higher mass fragments, again corroborating the results from the silver. Depth profiles were performed at each of the energies, and a number of common observations have been made. The higher mass peaks, for example, the molecular ion, initially show a rapid reduction in intensity at all energies until a more stable, gradual (16) Wucher, A.; Ma, Z.; Calaway, W. F.; Pellin, M. J. Surf. Sci. Lett. 1994, 304, L439. (17) Vickerman, J. C., Briggs, D. Henderson, A. Eds. The Static SIMS Library, Version 3; Surface Spectra Ltd.: Manchester, 2003.

Figure 3. TOF-SIMS spectra generated using a 40-keV C60 ion beam: the initial spectrum, top, and that spectrum acquired after etching to a dose of 1 × 1014 ions/cm2 (bottom).

loss region is reached. At 20 keV, the signal from the molecular ion diminishes to ∼55% of the original intensity. The loss of intensity at 40 keV is to ∼16% of the initial intensity, and at 80 keV, the signal is reduced to 10% of the starting value. Following this initial loss of signal, both the 40 and 80 keV experiments showed a leveling of the loss of the molecular ion, whereas the 20 keV experiment resulted in a continued loss of signal, although this was at a reduced rate from the initial drop, with a total reduction in intensity to 15% of the initial value being reached following a dose of 1.8 × 1014 ions/cm2. Despite the loss of intensity, characteristic mass fragments remain in the spectrum following ion doses well beyond traditional static conditions with peaks evident up to the molecular ion following ion doses greater than 1 × 1014 ions/cm2, as illustrated in Figure 3. Although the molecular ion signal from the lower energy spectra show less reduction in intensity, yield effects must also be considered. Normalizing the depth profile to the secondary ion yields from the initial spectra illustrates that, in reality, the yield of the molecular ion from the etched sample is the same for all energies. This indicates that the removal of any subsurface damage occurs at the same rate, independent of impact energy. A plot of the secondary ion yield of the molecular ion normalized to the yield for 80 keV impact energy is presented in Figure 4. Polycaprolactone. Polycaprolactone (PCL) is a biopolymer of the general formula (R-CO-O-R) the bombardment of which is known to produce high sputter yields due to the ease with which the linking oxygen bonds are cleaved. The TOF-SIMS spectrum is very simple with only five major fragments being present at m/z 41, 55, 69, 97, and the monomer ion at m/z 115. Depthprofiling experiments using polyatomic SF5 primary ions have Analytical Chemistry, Vol. 78, No. 6, March 15, 2006

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Figure 4. Variation of the intensity of the molecular ion peak, normalized to initial secondary ion yield at 80-keV impact energy, with increasing ion dose during depth profiling using 20, 40, and 80 keV.

Figure 5. Variation of the intensity of the monomer peak, relative to total ion counts, with increasing ion dose during depth profiling using 20, 40, 80, and 120 keV.

been reported to be successful for samples of this type, and we therefore expect the PCL film to be stable under C60 bombardment. It is, however, of interest to investigate whether very high impact energies result in damage effects being observed in the spectra. Presented in Figure 5 is the variation in intensity of the m/z 115 monomer fragment with increasing ion/dose during depth profiling with 20, 40, 80, and 120-keV C60 primary ions. At all impact energies, the signal rapidly reaches a steady state and then remains stable until the substrate is reached. Although, with our current experimental setup, it was not possible to accurately assess the amount of material removed (by either mass or film thickness), assuming the spun cast film to be approximately homogeneous in thickness where the analyses were performed, a clear increase in sputter rate with impact energy was observed based on the appearance of the m/z 28 signal from the silicon substrate. Hence, increasing the impact energy of the C60 results in an increase in sputter rate/yield with no apparent increased generation of subsurface damage. PS 2000. Secondary ion yields have been calculated for four regions of the spectra generated from a sample of PS 2000. Yields have been obtained for the mass ranges m/z 10-50, 50-100, 100200, and 200-400. In agreement with the results discussed for the silver sample, there are different optimum impact energies for the removal of ions in different mass ranges. For the mass regions below m/z 200, the maximum yield is observed when using an 80 keV primary particle, whereas the higher mass 1830 Analytical Chemistry, Vol. 78, No. 6, March 15, 2006

Figure 6. Relative secondary ion yield from four regions of the spectra generated from PS 2000 (m/z 10-50, 50-100, 100-200, and 200-400) using 20-, 40-, 80-, and 120-keV C60. Spectra were integrated over each area and then normalized relative to the 40 keV data to compensate for variation in target current.

secondary ions show an increase in secondary ion yield with impact energy (Figure 6). DPPC. DPPC is a common phospholipid that is present in many of the biological samples to which SIMS analysis has been employed. The spectrum contains an intense peak at m/z 184 corresponding to the PC headgroup of the lipid. Such is the intensity, due to a high proton affinity and, therefore, high ionization probability, that the peak is one of considerable diagnostic significance in SIMS analysis of tissue and cellular samples. Depth profiles of the DPPC were performed using C60 impact energies of 20, 40, and 80 keV, and in all cases, a similar trend was seen in the variation of the intensity of the m/z 184 signal with increasing dose. The signal shows an initial reduction in intensity, ∼1 order of magnitude, before a steady state is reached, with little or no change until the silicon substrate was reached. The rate at which the steady-state region was reached did vary with increasing beam energy, however, with the m/z 184 peak in the 20-keV depth profile reaching a plateau after an ion dose of 1 × 1014 ions/cm2, the 40-keV experiment after a dose of 7.5 × 1013 ions/cm2, and the 80 keV profile after a dose of ∼5 × 1013 ions/ cm2. Specific peaks from depth profile results using 40-keV C60 are plotted in Figure 7, where the [M + H]+ peak is that at m/z 735. Secondary ion yield from the DDPC sample following analysis using impact energies of 20, 40, 80, and 120 keV show an increasingly familiar trend. The lower mass species show an increase in secondary ion yield with increasing impact energy, with a maximum yield at 80 keV, while in the higher mass region of the spectrum, the 120-keV beam begins to have an increased effect and provides the greatest secondary ion yield for the species present above m/z 300 (Figure 8). Gramicidin. The secondary ion yield variations with impact energy for a range of peaks present in the SIMS spectrum of gramicidin have been calculated and are presented relative to 20keV C60 in Figure 9. The variation in the optimum impact energy over the mass range is again present. The lower mass peaks (below m/z 400) show a trend of increasing optimum impact energy for increasing mass. For the peaks between m/z 50 and 200, the 20- and 40-keV C60 beams give the greatest secondary ion yield, while there is

Figure 7. Variation in normalized intensity of peaks in the TOFSIMS spectrum of DPPC during depth profiling with 40-keV C60.

Figure 8. Secondary ion yield relative to 20-keV C60 for peaks present in the SIMS spectrum of DPPC for 40-, 80-, and 120-keV C60.

an increase in the yield from the 80-keV beam between m/z 200 and 400. Above m/z 400, continuing up to the molecular ion, the yield for all species increases with increasing impact energy. CONCLUSIONS We have demonstrated an enhancement of secondary ion yield from a variety of samples with increasing impact energy of C60. Although the nature of the samples differs considerably, some clear trends are evident. One of these is that the yield enhancement is not linear across the mass range, and with an impact energy of 120 keV, there appears to be an increase in the highmass fragments, with a corresponding loss of intensity of the lowmass fragments. Damage effects following prolonged ion beam exposure during depth profiling have been assessed, and both the DPPC and the PCL show no significant increase in damage. Irganox 1010 produces a similar secondary ion yield under (18) Sun, S.; Szakal, C.; Smiley, E. J.; Postawa, Z.; Wucher, A.; Garrison, B. J.; Winograd, N. Appl. Surf. Sci. 2004, 231-232, 64.

Figure 9. Secondary ion yield relative to 20-keV C60 for peaks present in the SIMS spectrum of Gramicidin for 40-, 80-, and 120keV C60.

interrogation with 20-, 40-, and 80-keV beams once the surface layer has been removed, an intriguing phenomenon that obviously requires further investigation on a wider range of samples. The increased sputter yield associated with the higher impact energy is expected to arise from the ejection of material from deeper within the sample, because molecular modeling studies suggest that as the impact energy of C60 on silver is increased, the increase in the diameter of the impact crater is diminished while the increase in the crater depth is augmented. As a practical consideration, shallow depth profiling may be less accurate at high impact energies, whereas the profiling of thick biological samples for example tissue sections may be facilitated by the increased rate of erosion. We postulate that the possibility for gas-phase reaction in the plume is enhanced when using high-energy C60. Investigation into the kinetic energy of sputtered silver (Ag1 and Ag2) with 15-keV C60 has shown an energy distribution confined to lower energies (compared with gallium), suggesting an adiabatic expansion from a superheated volume with all the species leaving the surface at the same velocity.18 This, combined with the increased yield without increased lateral distribution, would be expected to increase the collision probability of species within the plume. This may also result in gas-phase cooling, leading to a reduction in fragmentation, an effect observed on a number of the samples using 120-keV impact energy. ACKNOWLEDGMENT This work was funded by The Engineering and Physical Sciences Research Council (EPSRC) and The Biotechnology and Biological Sciences Research Council (BBSRC), UK. Received for review September 11, 2005. Accepted January 23, 2006. AC051624W

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