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Energy Deposition Control during Cluster Bombardment: A Molecular Dynamics View Kathleen E. Ryan and Barbara J. Garrison* Department of Chemistry, 104 Chemistry Building, Pennsylvania State University, University Park, Pennsylvania 16802 Molecular dynamics simulations are performed to model C60 and Au3 bombardment of a molecular solid, benzene, in order to understand the energy deposition placement as a function of incident kinetic energy and incident angle. Full simulations are performed for 5 keV projectiles, and the yields are calculated. For higher energies, 20 and 40 keV, the mesoscale energy deposition footprint model is employed to predict trends in yield. The damage accumulation is discussed in relationship to the region where energy is deposited to the sample. The simulations show that the most favorable conditions for increasing the ejection yield and decreasing the damage accumulation are when most of the projectile energy is deposited in the near-surface region. For molecular organic solids, grazing angles are the best choice for achieving these conditions. Secondary ion mass spectrometry (SIMS) using cluster ion beams has been quite successful at molecular imaging and depth profiling experiments.1 Several factors influence the quality of the molecular depth profile and therefore the amount of information one can glean from these experiments. These factors are outlined by a model proposed by Cheng et al.2 Using quantities such as the total sputter yield, damage cross section, and altered layer thickness, the model allows one to estimate the level of damage accumulation with respect to the total sputter yield. For molecular depth profiling experiments, the total sputter yield should be large relative to the damage accumulation. Simulations of the cluster bombardment process have shown that to obtain the maximum yield that the energy must be deposited in a critical region near the surface.3–5 Gillen and Roberson also showed how the motion of a cluster projectile (SF5+) versus an atomic projectile (Ar+) can reduce the damage accumulation in an organic thin film because the cluster projectile is unable to penetrate the sample as deeply as the atomic projectile.6 Thus, the issue is to design means of controlling where the energy is deposited. Recently, there has been increased interest on the effect of the projectile angle of incidence on the sputtering yield and * Corresponding author. E-mail:
[email protected]. (1) Winograd, N. Anal. Chem. 2005, 77, 142A (2) Cheng, J.; Wucher, A.; Winograd, N. J. Phys. Chem. B 2006, 110, 8329. (3) Russo, M. F., Jr.; Garrison, B. J. Anal. Chem. 2006, 78, 7206. (4) Russo, M. F., Jr.; Szakal, C.; Kozole, J.; Winograd, N.; Garrison, B. J. Anal. Chem. 2007, 79, 4493. (5) Smiley, E. J.; Winograd, N.; Garrison, B. J. Anal. Chem. 2007, 79, 494. (6) Gillen, G.; Roberson, S. Rapid Commun. Mass Spectrom. 1998, 12, 1303.
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damage and, thus, on the energy deposition in the substrate.7–12 The influence of incident angle for atomic projectiles has been well-established.13 For atomic ion beams, the highest ejection yield is observed at incident angles of 40-80° after which the yield begins to decrease. For diatomic beams, such as O2+, the impact angle is considered to be a more important factor than primary ion energy due to changes in surface topology which alter the secondary ion yield.14 The greatest depth resolution for experiments of O2+ bombardment of silicon was observed at grazing angles (60°), but due to the reactivity of the sample, the highest ionization was seen at near-normal angles (0-20°).15,16 For cluster beams such as C60, on the other hand, only a few studies have begun to delve into this area. Experiments and molecular dynamics (MD) simulations of 5-20 keV C60 bombardment of silicon result in a higher yield at 45° incidence than at normal incidence.7,9 In a companion study to our simulations, experiments of 40 keV C60+ bombardment of cholesterol films are reported in which the highest yield is at 40°, but the lowest amount of molecular damage is at 73° incident angle.11,12 It is also found that grazing angles near 75° are the most effective for cleaning polymer surfaces with a 10 keV C60 ion beam.10 The anomaly lies with Postawa et al. who found with simulations of 15 keV C60 bombardment of Ag{111} that the highest yields were observed at normal incidence rather grazing angles. In this case, the sample atoms, Ag, are much more massive than the projectile atoms, C, and can easily reflect the projectile atoms back into the vacuum.8 These results highlight the importance of the projectile type with respect to the sample type. For heavy targets, off-normal projectiles can more easily backscatter into the vacuum, so the effective amount of projectile energy is decreased compared to projectiles aimed normal to the surface, and therefore, the total ejection yield should be decreased. Conversely, if a projectile is able to penetrate and deposit energy too deep into the sample, then some of the (7) Hill, R.; Blenkinsopp, P. Appl. Surf. Sci. 2004, 936. (8) Postawa, Z.; Czerwinski, B.; Szewczyk, M.; Smiley, E. J.; Winograd, N.; Garrison, B. J. J. Phys. Chem. B 2004, 108, 7831. (9) Krantzman, K. D.; Kingsbury, D. B.; Garrison, B. J. Nucl. Instrum. Methods Phys. Res., Sect. B 2007, 255, 238. (10) Sanada, N.; Miyayama, T.; Suzuki, M. Personal communication, 2007. (11) Kozole, J.; Willingham, D.; Winograd, N. Appl. Surf. Sci., in press. (12) Kozole, J.; Wucher, A.; Winograd, N. Anal. Chem., in press. (13) Carter, G.; Colligan, J. S. Ion Bombardment of Solids; American Elsevier Publishing Company, Inc.: New York, 1968. (14) Stevie, F. A.; Kahora, P. M.; Simons, D. S.; Chi, P. J. Vac. Sci. Technol., A 1988, 6, 76. (15) Wittmaack, K. J. Vac. Sci. Technol., A 1990, 8, 2246. (16) Jiang, Z. X.; Alkemade, P. F. A.; Algra, E.; Radelaar, S. Surf. Interface Anal. 1997, 25, 285. 10.1021/ac800287k CCC: $40.75 2008 American Chemical Society Published on Web 06/04/2008
projectile energy may be wasted in terms of influencing the ejection yield. We present here a series of MD simulations as a detailed analysis into the impact that angle of incidence has on the energy deposition in organic targets and the implications for the total sputtering yield and possible damage. MD simulations have been used extensively as a valuable tool for gaining insight into the physical phenomena that occur during cluster bombardment of a solid.17 Simulations are performed to model 5, 20, and 40 keV C60 and Au3 bombardment of a molecular solid, benzene, at varying angles from normal incidence, 0°, to grazing, 75°. COMPUTATIONAL DETAILS MD simulations are performed to investigate the angle of incidence effects on the ejection yield and damage accumulation in a molecular solid of benzene when bombarded with C60 or Au3 projectiles. The MD methods used have been described elsewhere.18 Briefly, Hamilton’s equations of motion are integrated to determine the positions and velocities of each atom as a function of time. A coarse-grained description for the benzene system as described previously has been used.19 Coarse-graining involves combining the carbon and hydrogen atoms in a benzene molecule into one united atom particle, CH, with a mass of 13 amu. LennardJones potentials are used to describe the CH-CH interactions between different molecules as well as the C-CH potential between projectile atoms and substrate atoms. A Morse potential is used to describe CH-CH interactions within one molecule so that the molecule may dissociate and still possess some internal energy. The REBO potential of Brenner is used to describe the C-C interactions of the C atoms in the C60 molecule.20 The Au-Au interactions for the Au3 projectile are described with a MD/MC-CEM potential,21 and the Au-CH interactions are described with a Molie`re potential. The system used is a molecular solid benzene crystal with an orthorhombic structure. The crystal measures 20 nm deep and 34 nm in each lateral direction while containing approximately 1.2 million CH beads (198 720 benzene molecules). The projectile angle of incidence is varied and is referenced with respect to the normal to the substrate; thus, an angle of 0° is normal incidence and an angle of 75° is near grazing. Full simulations (∼26 ps) were run for projectiles with 5 keV of incident kinetic energy, and yields are calculated. Short-time simulations, up to when 90% of the projectile energy is deposited to the surface, were run for 20 and 40 keV projectiles because the sample size needed to model the entire ejection process at these energies is computationally prohibitive. Therefore, the mesoscale energy deposition footprint (MEDF) model3,4,22 is used to predict trends in yield as well as extrapolate information about damage for the higher energy simulations. (17) Webb, R. P. Appl. Surf. Sci., in press. (18) Garrison, B. J. In ToF-SIMS: Surface Analysis by Mass Spectrometry; Vickerman, J. C., Briggs, D., Eds.; Surface Spectra: Manchester, U.K., 2001; pp 223-257. (19) Smiley, E. J.; Postawa, Z.; Wojciechowski, I. A.; Winograd, N.; Garrison, B. J. Appl. Surf. Sci. 2006, 252, 6436. (20) Brenner, D. W. Phys. Rev. B 1990, 42, 9458. (21) Kelchner, C. L.; Halstead, D. M.; Perkins, L. S.; Wallace, N. M.; Depristo, A. E. Surf. Sci. 1994, 310, 425. (22) Russo, M. F., Jr.; Ryan, K. E.; Czerwinski, B.; Smiley, E. J.; Postawa, Z.; Garrison, B. J. Appl. Surf. Sci., in press.
Figure 1. MEDF predicted yields for 5, 20, and 40 keV and the 5 keV calculated yields from simulations of C60 bombardment as a function of incident angle. Experimental yields from 40 keV C60+ bombardment of cholesterol from Kozole et al. (ref 12) is also included.
The MEDF model3,4,22 is derived from concepts of fluid dynamics calculations of Jakas et al.23 and relies on the observation that the position where the projectile energy is deposited has a direct influence on the amount of material ejected as well as the volume from where the material ejects. One approximates the volume where the energy is deposited to the substrate by a cylindrical track with a radius, Rcyl. The average excitation energy is measured with respect to the cohesive energy of the substrate ˜ ) Eexc/Uo, where Eexc is the excitation energy and Uo is so that E the cohesive energy of the substrate. Rcyl and can be used to outline an approximate ejection cone with a depth of Rcyl and a ˜ )1/2Rcyl (refer to Figure 3 of ref 3). The base radius of Rs ) (E yield can then be estimated using Y ) no(π/3)Rs2Rcyl, where Y is the yield and no is the number density of the substrate. RESULTS AND DISCUSSION C60 Bombardment. The yields calculated by MD simulations for 5 keV C60 bombardment are shown in black in Figure 1. For angles between normal and about 45° the yield is approximately constant. As the angle of incidence increases, the yield decreases because some of the projectile energy begins to be reflected back into the vacuum.24 Therefore, 5 keV C60 at grazing angles results in a shallower crater and a lower yield, similar to C60 bombardment of Ag{111}.8 The question remains, however, as to whether this trend can be extrapolated to higher energies such as 20 and 40 keV. For this analysis, we use the MEDF model. The MEDF model relies upon the notion that energy is deposited in a cylindrical track for a projectile aimed normal to the surface.3,4,22 However, it has not yet been applied to simulations where the projectile is aimed at an off-normal angle. The energy deposition as observed from the top of the sample for 0° and 60° is shown in Figure 2, parts a and b, while a side view for all angles is shown in Figure 3 at the time when 90% of the projectile energy is deposited to the sample. The dark blue regions represent molecules that are highly energized, whereas gray represents molecules in their initial state. Yellow to red represents molecules as they become increasingly energized. In both cases, the views are just a representative portion of the sample that focuses on the energized region. There is no apparent skewing for the off(23) Jakas, M. M.; Bringa, E. M.; Johnson, R. E. Phys. Rev. B 2002, 65, 165425. (24) Ryan, K. E.; Smiley, E. J.; Winograd, N.; Garrison, B. J. Appl. Surf. Sci., in press.
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Figure 2. Contour plots of the energy deposition as viewed from the top of the sample for (a) C60 aimed normal to the surface, (b) C60 aimed at 60°, (c) Au3 aimed normal to the surface, and (d) Au3 aimed at 60° at 5 keV incident energy. The dark blue regions represent highly energized molecules, yellow represents slightly energized molecules, and gray represents molecules in their initial state. The time specified is the time when 90% of the projectile energy is deposited to the surface.
Figure 3. Contour plots of the energy deposition in a 2 nm slice of the substrate below the point of impact following 40 keV C60 bombardment aimed from the left at varying angles of incidence. The color scheme is the same as provided for Figure 2, and the depth of energy deposition is also given.
normal angles in either view indicating that for C60, the projectile energy is deposited in an approximately cylindrical track for all angles at each energy: 5, 20, and 40 keV. Therefore, we feel it is appropriate to use the MEDF model to predict approximate yields. Figure 1 shows the predicted yields for the 5, 20, and 40 keV trajectories as well as the yield calculated from the simulation of 5 keV C60 bombardment. As the incident energy increases, the maximum yield shifts from occurring at near-normal angles of 0° and 15° to occurring at off-normal angles around 30-45°. It is optimal for ejection if the projectile energy is deposited in the near-surface region. The projectiles aimed at near-normal angles deposit their energy too deep into the substrate as the incident energy increases so that it is ineffectual for ejection. For grazing angles, such as 60° and 75°, the projectile energy is largely confined to the near-surface region, but approximately 15-35% of the energy is reflected into the vacuum for 60° and 55-65% for 75° for projectile energies between 5 and 40 keV with the lowest amount of reflection occurring at 40 keV. Thus, the effective incident projectile energy is less than the initial incident projectile energy, and the yield is decreased. At 0-45°, all of the projectile energy is deposited into the substrate. In particular, at 30° and 5304
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45°, all of the projectile energy is deposited near the surface, but due to the off-normal incident angles, the footprints, or Rcyl values, are slightly larger than those seen at 0° and 15°. The factor of Rcyl is cubed when calculating the yield, so even small increases in Rcyl will result in an enhancement of yield. Our interest is to compare to experimental results, so experimental data by Kozole et al.12 for 40 keV C60+ bombardment of cholesterol films are also given in Figure 1. For comparison, the cholesterol yields have been converted to units of benzene molecule equivalents by calculating the yield in molecular weight and converting to the corresponding number of benzene molecules. Fragmented and whole molecules were both included in the determination of yield and were not considered separately. The experimental yields were then scaled by a factor of 1.6 so that the peak yields were equivalent and the characteristics of each trend line could still be observed while plotted on the same scale. Previously, the MEDF model was shown to predict yields from C60 and Au3 bombardment of water ice within a factor of 2.4 Therefore, the MEDF predicted yields are remarkably similar to the experimental yields considering that the surfaces are composed of different molecules and also contain a different surface topology. The cholesterol molecule is much larger in both mass and size than a benzene molecule, and the film used in experiment is topologically rough, whereas the surfaces in simulation are always smooth. The experimental yields exhibit a similar trend to that predicted from simulation in that there is a drastic increase in yield until around 40° where the highest ejection yield is observed. The experimental yields show only a small decrease as the angle approaches grazing, 73°, where the simulation yields have a more significant decrease. It is difficult to identify the exact reasoning behind this; however, this is most likely attributed to the differences in systems (i.e., different molecules and surface topology) as well as that fact that the experimental yields are an average over several bombardments, whereas simulation yields are the result of one projectile bombardment. Both the experimental and simulation data indicate that off-normal angles between 30° and 45° will provide the largest yields. Estimating the damage to the substrate in the simulation is challenging because the coarse-grained model does not include many chemical effects and only the short-time dynamics were calculated for 20 and 40 keV bombardment. Thus, we estimate that the amount of damage to the sample is related to the projectile energy deposition.25 By analyzing the regions where energy is deposited, we can speculate about the amount of damage accumulation in the substrate. Figure 3 shows the energy deposition profiles for 40 keV bombardment at the time when 90% of the projectile energy has been deposited to the substrate. The profile is represented by a 2 nm slice of the sample centered around the point of impact. The region has been cut down to approximately half of the total crystal (10 nm deep and 15 nm wide) so that we can focus on the region of energy deposition. The depth of energy distribution is also labeled. As the angle of incidence becomes increasingly grazing, the depth of energy distribution decreases indicating that projectiles aimed at grazing angles are likely to leave less damage in the sample than (25) Ryan, K. E.; Wojciechowski, I. A.; Garrison, B. J. J. Phys. Chem. C 2007, 111, 12822.
Figure 6. Contour plots of the energy deposition in a 2 nm slice of the substrate below the point of impact following 40 keV Au3 bombardment aimed from the left at 0° and 60° angles of incidence. The color scheme is the same as used in Figure 2. The time specified is the time when 90% of the projectile energy is deposited to the surface.
Figure 4. Snapshots at 26 ps of a 2 nm slice of the sample centered around the point of impact for 5 keV Au3 projectiles aimed from the left at 0°,15°, 30°, 45°, 60°, and 75°.
Figure 5. Yield in benzene molecule equivalents as a function of incident angle for 5 keV Au3 bombardment.
projectiles aimed normal to the surface. Likewise, the damage cross section can be estimated by analyzing the energy deposition view from the top of the sample (Figure 2). For C60 the intensity of energy as well as the size of the deposition region is smaller at 60° than when aimed normal to the surface. This region is also the area where most material is likely to be removed so that there will be little damage accumulation over several bombardments. This conclusion is consistent with Kozole et al. who found that at 73-80° incidence, there is reduced molecule damage relative to 40° while the molecular signal is still retained.11,12 Au3 Bombardment. An initial visual analysis of the craters formed following 5 keV Au3 bombardment of coarse-grained benzene (Figure 4) reveal some immediate differences from the C60 simulations. Unlike the craters formed from C60 bombardment,24 these craters vary as a function of incident angle. As the angle becomes increasingly grazing, the craters become skewed in the direction of the projectile motion. The craters also become shallower so that, at an incident angle of 75°, it appears that only the top several layers of the surface are affected. The ejection yield as a function of incident angle is shown in Figure 5. Au3 bombardment has shown in the past to possess dynamics similar to atomic bombardment3,4,25,26 so that the gold atoms are each able to penetrate into the sample and deposit all of their energy at each angle rather than reflect energy like in the case of C60 (26) Russo, M. F., Jr.; Wojciechowski, I. A.; Garrison, B. J. Appl. Surf. Sci. 2006, 252, 6423.
bombardment. In these situations, the region where energy is deposited will have the greatest effect on the ejection yield. The simulation performed at a 75° angle of incidence has all of the projectile energy deposited near the surface and therefore is expected to result in the highest ejection yield. Indeed, as the angle becomes increasingly grazing, the ejection yield increases as shown in Figure 5. This trend is the same as observed for atomic ion bombardment,13 furthering that Au3 bombardment is more suitably compared to atomic bombardment than cluster bombardment. Once again, it is important to know whether this trend can be extrapolated to higher energies. In this case, it is difficult to apply directly the MEDF model because the track of energy deposition is no longer cylindrical as the angle moves away from normal as shown in Figure 2, parts c and d, and Figure 6. However, the principles of the MEDF model can still be used to interpret possible changes in yield. Figure 6 shows the energy deposition profiles in a 2 nm slice of the system for 40 keV Au3 bombardment of benzene at 0° and 60°. Here, approximately the whole crystal is shown (19 nm deep and 33.5 nm wide) and lines are drawn as guides to indicate the energized track cylinder and a depth of the cylinder radius, Rcyl. For the Au3 projectile that is aimed normal to the surface, the projectile penetrates the entire sample depositing its energy too deep to contribute effectively to ejection. The projectile aimed at 60°, however, deposits its energy closer to the surface, so it will contribute to a higher ejection yield. The energy contour plot at 60° is a good demonstration of how Au3 can begin to act more like three Au projectiles rather than a cluster. Two of the Au atoms have deposited energy in branches that appear independent of each other. In this case, the projectiles deposit their energy outside the optimum region for ejection, so damage may accumulate in the near-surface region. Subsequent bombardments are likely to uncover that damage. We estimate that the trend in yield for higher energies will be similar to the yield trend observed at 5 keV as shown in Figure 5 and may in fact be more pronounced. CONCLUSIONS We have performed simulations modeling C60 and Au3 bombardment of a coarse-grained benzene system using incident angles from 0° to 75°, as well as three incident energies: 5, 20, and 40 keV. The results from the higher energy simulations were interpreted using the MEDF model. The simulations show that angle of incidence is an effective means of controlling where the Analytical Chemistry, Vol. 80, No. 14, July 15, 2008
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energy is deposited in the substrate and that grazing angles of incidence should provide the best results in terms of decreasing damage accumulation and increasing molecular yields for both C60 and Au3 bombardment. However, the type of target material, projectile type, and incident energy all influence the ejection yield at varying angles by affecting how much projectile energy is deposited to the surface versus getting reflected into the vacuum as well as where the energy is deposited: in the near-surface region or deep in the sample. The optimal conditions for increasing ejection and decreasing damage accumulations revolves around the greatest amount of projectile energy being deposited near the surface, which is seen at higher projectile energies with grazing angles.
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ACKNOWLEDGMENT The authors thank the Chemistry Division of the National Science Foundation Grant No. CHE-0456514 for their financial support of this research. Computational support was provided by the Graduate Education and Research Services (GEaRS) group of Academic Services and Emerging Technologies (ASET) at Pennsylvania State University. We also acknowledge Nick Winograd and Joe Kozole for their insight and thoughtful scientific conversations as well as early access to their experimental data. Received for review February 11, 2008. Accepted May 5, 2008. AC800287K
