Gas field ion source and liquid metal ion source charged particle material interaction study for semiconductor nanomachining applications

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1 Gas field ion source and liquid metal ion source charged particle material interaction study for semiconductor nanomachining applications Shida Tan, a Richard Livengood, and Darryl Shima Intel Corporation, MS: SC9-68, Santa Clara, California John Notte and Shawn McVey Carl Zeiss SMT, 1 Corporation Way, Peabody, Massachusetts Received 13 July 2010; accepted 11 October 2010; published 1 December 2010 Semiconductor manufacturing technology nodes will evolve to the 22, 15, and 11 nm generations in the next few years. For semiconductor nanomachining applications, further beam spot size scaling is required beyond what is capable by present generation Ga + focused ion beam technology. As a result, continued Ga + beam scaling and/or alternative beam technology innovations will be required. In this work, several alternative ion beam technologies are explored and compared to Ga + beam for key nanomachining and substrate interaction attributes. First, thorough Monte Carlo simulations were conducted with various ion species incident on silicon and copper. Additionally, silicon and copper substrates were experimentally exposed to ion beams of helium, neon, and gallium. These substrates were subsequently analyzed to determine the sputter yields and subsurface damage American Vacuum Society. DOI: / I. INTRODUCTION For over 20 years, gallium liquid metal ion source LMIS technology has been the workhorse charged particle beam for the focused ion beam FIB industry. 1 Within the semiconductor industry, the gallium FIB is used for failure analysis, transmission electron microscope TEM sample preparation, and circuit edit CE applications. Steady progress in reducing the ion beam probe size has been an ongoing requirement in order for the FIB to keep pace with Moore s law two times transistor density increase every 2 years. 2 Indeed, gallium FIB probe sizes have scaled from 50 nm in the mid- 1980s, when they were first introduced commercially, to less than 10 nm full width at half-maximum in today s most advanced FIB tools. 3 For circuit edit applications, FIB probe size scaling is particularly critical, since high aspect ratio via milling is required with minimum geometries on the order of 100 nm wide and several hundred nanometers deep. Figure 1 shows a cross section of a FIB via, which was nanomachined down through the silicon backside to a metal-2 node in an integrated circuit IC. The via was first milled and then backfilled with FIB deposited tungsten to enable rewiring of the signal to an alternative location signal node in the IC. FIB via applications like this are typical and require very high acuity nanomachining capabilities. According to Moore s law, the process technology node will evolve to the 11 nm generation in the next 4 5 years. In order to enable the continued use of FIB for circuit edit applications, the spot size for the existing gallium FIB must also continue to reduce or alternative beam technologies must be developed. Several promising alternative novel ion source technologies have emerged over the past few years, such as gas field ion source GFIS, 4 alloy LMIS, 5 magneto-optical trap ion source MOTIS, 6 and multicusp plasma ion source. 7 For the purpose of silicon nanomachining, the critical FIB attributes include small probe size, high brightness, high secondary electron yield, reasonable sputter yield 1 5 atoms per incident ion, shallow implant depth 200 nm, low vacancies per sputtered atom, limited electrical circuit invasiveness, and the ability to operate over a wide range of beam currents e.g., 500 fa 10 na and beam energies e.g., 5 30 kv. The focus of the work presented in this article is to perform a detailed analysis of a wide range of ion source technologies and analyze their performance attributes for the pura Electronic mail: shida.tan@intel.com FIG. 1. SEM micrograph of a cross section of a FIB via nanomachined on a 32 nm IC. C6F15 J. Vac. Sci. Technol. B 28 6, Nov/Dec /2010/28 6 /C6F15/7/$ American Vacuum Society C6F15

2 C6F16 Tan et al.: GFIS and LMIS charged particle material interaction study C6F16 FIG. 2. Color online Modeled sputter yield, interaction volume, and implant depth of He +,Be +,Ne +,Ar +,Cr +,Ga +,Xe +, and Au + at 30 kev beam energies in a silicon substrate. Throughout this figure, the incident ions are shown in red, and green is used to represent the dislocated silicon atoms. pose of performing CE on future process technologies. This includes performing detailed modeling as well as empirical analysis of beam probe size, implant depth, sputter yields, and electrical invasiveness. Although we intend to empirically study the broad range of sources discussed above, for the purposes of this article, we limited our study to gallium LMIS and noble GFIS. II. HELIUM AND NEON GFIS The gas field ion source is a technology that has been commercialized over the past 4 years as the ion source for the helium ion microscope HIM. 8 The HIM and the GFIS technologies have some very impressive properties, such as a sub-0.5-nm probe size, high brightness, low energy spread, and an image resolution of 0.35 nm. Recently, the same helium ion beam has been used for lithography 9,10 and beam induced chemical deposition and etching. 11,12 Other noble gases e.g., neon, argon, and krypton can perhaps also be adapted to the GFIS, but are not yet commercially available. As a precursor to our study, we developed a novel neon source that was sufficiently stable in the laboratory environment to enable empirical experimentation. Like the helium GFIS, the neon ions are generated through creating a high electric field above a single atom in the presence of a noble gas. 13 The subsequent ion beam is then collimated, focused, and scanned on the sample substrate using traditional electrostatic optics. However, unlike helium, the neon ion beam has sufficiently higher mass amu, resulting in preferred material interaction attributes relative to lighter element ions, such as helium. Two such attributes that are particularly compelling are the shallower implant depth and the increased sputter yield. These attributes were studied thoroughly and will be discussed in the following sections. III. TRIM MODELING In this section a theory-based understanding of ion beam implantation and sputtering are introduced with simulations using the TRIM Monte Carlo simulation software. 14 The forms of damage that will be considered here are 1 the dislocation of sample atoms from their original lattice positons, 2 the implantation of atoms from the incident ion beam, and 3 surface sputtering of atoms from the sample. The ion mass and beam energy determine the material interaction volume and the material sputter yields. Ideally, the damage induced by the ion is limited to the surface of the substrate, with a sufficient momentum transfer at or near the surface producing the sputtering of the desired material. This is critical to achieving high nanomachining acuity with limited subsurface dislocations and implantations. The ion species that were modeled include several established source technologies and with an atomic weight ranging from 4 to amu: He +,Be +,Ne +,Ar +,Cr +,Ga +, Xe +, and Au +. For modeling purposes, all bombarding ions strike the substrate at normal incidence, with energies spanning from 1 to 50 kev. Figure 2 shows the penetration of these ion species striking silicon with a fixed incident energy of 30 kev. Graphically, it is quite clear that low mass ions e.g., He + and Be + penetrate deep into the sample but produce relatively low sputter yield SY sputtered silicon atoms per incident ion, whereas the high mass ions e.g., Xe + and Au + penetrate much less deeply but produce higher sputter yields. In Fig. 3, the simulated sputter yield for He +,Be +,Ne +, Ar +,Cr +,Ga +,Xe +, and Au + at 1, 2, 5, 10, 20, and 50 kev on silicon and copper was plotted against atomic weight. For lighter ions e.g., He + and Be +, the sputter yield increases with decreasing beam energies; whereas for the heavier ions e.g., Xe + and Au +, the sputter yield increases with increasing beam energies. The reversal in trend occurs for ion species with atomic weight between 20 and 40 amu. It is relevant to note that many common nanomachining applications use heavier ions with an energy between 30 and 50 kev. J. Vac. Sci. Technol. B, Vol. 28, No. 6, Nov/Dec 2010

3 C6F17 Tan et al.: GFIS and LMIS charged particle material interaction study C6F17 Considering the vacancies produced per sputtered atom, Ne + may well be less invasive than Ga + at beam energies less than 10 kev. For higher beam energies, as neon s sputter yield is reduced, Ne + creates overall more damage for each sputtered atom compared to Ga +. At 10 kev, neon s sputter yield on silicon is 1.16 atoms per incident ion and on copper is 4.40 atoms per incident ion, both of which are roughly 60% that of the sputter yield for Ga +. Therefore, while helium is not suitable for nanomachining applications, neon appears to be well suited and may even perform better than gallium in some energy regimes. FIG. 3. Color online Simulated sputter yield of silicon and copper using 1, 2, 5, 10, 20, and 50 kev He +,Be +,Ne +,Ar +,Cr +,Ga +,Xe +, and Au + at 0 incident angle. The sputter yield is plotted against atomic weight of the incident ion species. As previously discussed, for CE nanomachining, it is desirable to limit the ion-atom interaction to the target material surface, thus maintaining an optimal sputter yield while limiting the subsurface dislocation damage. Plotted in Fig. 4 are the vacancies created per sputtered atom for incident He +, Ne +, and Ga + ions with energies of 1, 2, 5, 10, 20, 50, and 100 kev. Due to its low atomic weight and sputter yield 0.1 atoms per incident ion, a much higher dose of He + is required to sputter one silicon atom compared with the heavier ion species. This results in massive subsurface damage for each sputtered silicon atom. Our previous research has also shown that nanoscale to microscale helium bubbles are created a few hundred nanometers below the surface of both silicon and copper for helium dosages exceeding ions cm Although recent progress has been reported on the precision sputtering of thin Au films with He + ions, 16 the creation of subsurface damage we observed will likely limit the use of helium for CE nanomachining applications. IV. NEON AND GALLIUM ION SUBSURFACE DAMAGE One of the key sources of invasiveness for circuit edit nanomachining applications is subsurface damage caused by the ion beam. As previously discussed, this is an area that we have modeled quite thoroughly. In addition to the modeling, this has also been a major focus for empirical analysis. In this section, we present our experimental results of damage induced by incident neon ions on silicon and copper substrates at various beam energies and dosages. A comparison data set was also taken for gallium ion beam to compare and contrast neon with the better known properties of the gallium FIB. Characterization results clearly showed the progression of defect formation with increasing dose and the dependency of implant depth and sputter yield for Ne + ion on beam energy. Experiments were performed on silicon and copper substrates. The silicon sample used was a 100 Czochralski grown, nondoped silicon wafer with a resistivity of cm. The copper sample used in this work is a 1 m thick e-beam evaporated polycrystalline copper film on a silicon wafer. The silicon and copper samples were implanted at room temperature by an engineering test stand, based on the Zeiss ORION instrument, 8 which had been specially modified for neon operation. Ga + implant experiments were carried out using an FEI Vectra FIB. Implant dosages ranged from to ions cm 2, and the incident energy varied from 9 to 35 kev. The Ne + beam current used was approximately 7.5 pa for neon and 5 pa for gallium. An FEI Strata 400 Dual Beam was used to prepare TEM samples using a lift-off technique. High resolution scanning electron microscope SEM cross-section images at the implant sites were collected using an FEI Altura 855 Dual Beam. TEM samples were characterized using a JEOL JEM TEM. Before cross-section and TEM sample preparation, 500 nm of platinum were deposited onto the implant region surface by e-beam followed by a 1000 nm platinum deposition by Ga + ion beam to protect the implant sites from being damaged by the 30 kev Ga + during the cross-section and TEM sample preparation process. Figure 5 shows a progression of the TEM cross-section images of a silicon substrate implanted with neon at 12 kv and dosages between and ions cm 2. The progression shows that as the dose increases, the total depth JVST B-Microelectronics and Nanometer Structures

4 C6F18 Tan et al.: GFIS and LMIS charged particle material interaction study C6F18 FIG. 4. Color online Simulations of vacancies created per sputtered atom per nm of implant depth for 1, 2, 5, 10, 20, and 50 kev He +,Ne +, and Ga + at normal incidence to silicon. The distribution of vacancies created by He + is shown in ; the distribution of vacancies created by Ne + is shown in ; the distribution of vacancies by Ga + is shown in. and damage density also increase. For example, with neon ion exposure of ions cm 2, a 36.8 nm thick crystalline silicon layer close to the substrate surface becomes amorphous Fig. 5 a, but as we progress to higher dosages ions cm 2, the amorphous layer depth increases to 57.1 nm Fig. 5 c. At this dose level, an additional defect phenomenon begins to appear in the form of a subsurface nanobubble layer with the bubble diameter mostly in the sub-2-nm regions, extending down about 48 nm within the amorphous silicon region. This slight porosity is visible at 50k magnifications and causes a slight swelling of the substrate. When the dose increased to ions cm 2, the nanobubbles that formed under the substrate surface became larger in diameter, with the largest bubbles around 20 nm in diameter Fig. 5 d. At this dose, surface sputtering also becomes noticeable. At ions cm 2, about 160 nm of silicon was removed from the substrate Fig. 5 e. The micrograph shown in Fig. 5 e is collected at the bottom of the silicon trench created by neon ions. A porous neon bubble layer is clearly present, the thickness of which is similar to that shown in Fig. 5 d. Overall, at 12 kev neon beam energy, a slight surface swelling is present for doses higher than ions cm 2, but the amount of neon implantation and sputtering seem to reach equilibrium beyond a dose of ions cm 2. In Fig. 6, a similar set of data shows the neon implant damage in silicon substrate with a neon beam with an energy of 34 kev. The amorphous layer created in silicon substrate at ions cm 2 is 62.4 nm deep Fig. 6 a, nearly twice as deep as in Fig. 5 a. At a dose of ions cm 2, the amorphous layer is 120 nm with no apparent porosity Fig. 6 b. At a dose of ions cm 2, a porous region with bubbles extends FIG. 5. Color online TEM images of implant range and subsurface damages created by 12 kev Ne + with dosages of a , b , c , d , and e ions cm 2. FIG. 6. Color online TEM images of implant range and subsurface damages created by 34 kev Ne + with dosages of a , b , c , d , and e ions cm 2. J. Vac. Sci. Technol. B, Vol. 28, No. 6, Nov/Dec 2010

5 C6F19 Tan et al.: GFIS and LMIS charged particle material interaction study C6F19 Ebeam Ga+ Ebeam Ne+ TEOS He+ Ga+ staining AmorphousSi Ne nano-bubbles AmorphousSi AmorphousSi 0nm 51 nm 108 nm CrystallineSi CrystallineSi Beam Energy: 20kV Dose = 1.6E+17 ions/cm 2 He nano-bubbles CrystallineSi 283 nm FIG. 7. Color online TEM images of implant range and damage created by 20 kev He +,Ne +, and Ga + at normal incidence to silicon. The dose used is ions cm 2. FIG. 8. Color online STEM micrograph and EDS spectra collected from a neon implanted sample a nanobubble region, b amorphous silicon region, and c crystalline silicon region. about nm below the surface and an additional 32.5 nm of amorphous silicon is seen below the porous region Fig. 6 c. The nanobubbles are 1 2 nm in diameter. Figure 6 d shows the silicon surface slightly elevated with a neon exposure of ions cm 2. A porous region with bubbles extends about nm below the surface with an additional 28.1 nm of amorphous silicon below the porous region. Finally, with a dose of ions cm 2 Fig. 6 e, the silicon surface appears to have been sputtered by the neon beam, resulting in a depression of 70 nm. Here, the neon subsurface implant creates bubbles up to 100 nm in diameter. This significantly distorted the implant region with amorphous silicon, and the bubbles extend about 250 nm below the surface. At 34 kev, Ne + creates massive damage and significant surface swelling at high doses. Comparing Figs. 5 e and 6 e, more neon implantation and substrate distortion at higher beam energies are very evident. Thus, our conclusion is that for sputter applications, the lower beam energy regime is preferred for neon. When comparing neon to gallium and helium ion beams, as shown in Fig. 7, one can clearly see the depth and nanobubble distribution differences for each ion species. The TEM micrographs collected on silicon samples were exposed to gallium, neon, and helium ions at 20 kev. The exposure dose was ions cm 2 in all three experiments. At 20 kev, the defects created by helium ions have an asymmetric distribution, with the majority of the defects occurring near the stopping range of the ions. This is apparent from the micrograph in Fig. 7, where the nanobubbles formed in the substrate mostly distribute in the last 100 nm of the implant depth. For neon and gallium ions, the defects mostly form close to the substrate surface at 20 kev. Gallium staining appears to be close to the top 30 nm of the overall 50 nm of the amorphized silicon region. Unlike helium, implanted neon bubbles in silicon formed close to the substrate surface instead of close to the stopping range. Some energy dispersive spectroscopy EDS point scans were collected to understand the neon implant characteristics. A 20 kev neon implanted silicon sample was tested using the FEI Tecnai F20 TEM in scanning transmission electron microscopy STEM mode with an EDS detector. The micrograph shown on the left side of Fig. 8 is a STEM image of the regions under test. Three EDS spectra were collected on the nanobubble, the amorphous silicon, and the crystalline silicon regions of the sample. The neon EDS peak is present only in the nanobubble region, and no neon is detected in underlying amorphous or crystalline silicon region. This suggests that implanted neon is trapped within these bubbles and has a very low diffusion rate. A similar analysis completed on gallium implanted samples show an overall amorphized region that is 51 nm in depth at 20 kv, with a distinct subregion consisting of silicon and implanted gallium corresponding to the gallium staining band shown in Fig. 7. Both these data agree well with the SRIM simulation data showing an implant depth region and an additional cascade event induced damage region extending several tens of nanometers beyond the implant depth. V. NEON AND GALLIUM ION SPUTTER YIELD The next area of focus of our empirical study was the sputter yield properties for neon as compared with gallium. For CE nanomachining applications, the ion beam must have a sputter component because many of the materials we encounter in nanomachining in semiconductors are not easily removed with ion beam induced chemical etch alone; thus, it becomes necessary to sputter the material away. Figures 9 a and 9 b show cross-section micrographs of silicon and copper samples that were exposed to 9, 20, and 34 kev neon beams and 10, 20, and 35 kev gallium beams. In all of the silicon experiments, the ion beam dose was at ions cm 2. In the copper experiments, the neon beam dosage was ions cm 2 and the gallium dose was ions cm 2 due to gallium s high sputter yield and the limited thickness of the copper sample. Parameters were adjusted to normalize the dose delivery rates for all data sets. Just by looking at these cross-section micrographs, it is apparent that gallium s sputter yield increases with beam energy, whereas neon s sputter yield decreases. Interestingly, as the neon sputter yield decreases, the subsurface defect density is observed to increase at a rate that is inversely proportional to the sputter rate decrease. This is particularly evident in the silicon sample, where the increased subsurface neon trapping and surface swelling with increasing neon beam energy are quite clear. A similar trend for the sputter yield is also observed for the copper samples. However, for copper, we do see an additional irregularity at the bottom of the JVST B-Microelectronics and Nanometer Structures

6 C6F20 Tan et al.: GFIS and LMIS charged particle material interaction study C6F20 Dose = 1.3E+18 ions/cm cm 2-2 Ne+ 9kV 20kV 34kV Dose = 1.3E+18 ions/cm cm 2-2 Si Ga+ 500 nm 10 kv 20 kv 35 kv Si )*+ Dose = 1.3E+18 ions/cm cm 2-2 Ne+ 9kV 20kV 34kV Dose = 1.6E+17 ions/cm cm 2-2 Ga+ 500 nm 10 kv 20 kv 35 kv Cu Cu ),+ FIG. 9. Color online SEM cross section of a silicon and b copper samples sputtered by Ne + at a beam energy of 9, 20, and 34 kev and by Ga + at a beam energy of 10, 20, and 35 kev. milled region. This is due to ion channeling effect, which causes differential sputtering and asymmetry in subsurface defect formation. The resulting silicon substrate sputter yields for neon and gallium ion beams are shown in Figs. 9 a and 9 b, respectively. These values are based on the volumetric sputtered area removed from our samples and were calculated using the following formula: total no. of sputtered atoms sputter yield = total incident ions density volume A molar weight =. dose area In this equation, A represents Avogadro s constant. The material density used for silicon is g cm 3 and for copper is 8.96 g cm 3. When estimating the material removal volume, the sidewall profile was taken into account. In the cases for neon, part of the neon implanted region is hollow and filled with neon; half of this volume is considered to be the material removed. Both the empirically derived sputter yield and the TRIM modeled sputter yield for neon and gallium are shown in Fig. 10 a for silicon and Fig. 10 b for copper. The empirically derived data and the TRIM modeling results are in agreement to within 30%. VI. DISCUSSIONS AND FUTURE WORK From previous research, the helium beam did not show any appreciable sputtering of silicon or copper substrates for beam energies ranging from 9 to 34 kev. An increase in the helium dose only introduces further helium implantation and more subsurface damage. Of course, the depth of the damage could be modulated by He + beam energy. Ion beam chemical FIG. 10. Color online Comparison of empirical and simulated sputter yields for silicon and copper using Ne + and Ga + ions at beam energy from 9 to 35 kev. assisted material removal has been demonstrated using helium ions and even electrons for oxide films and silicon. 11 Thus, the use of light element ions such as helium or beryllium is a viable technology for many materials. However, lighter element ions are still a concern for comprehensive nanomachining applications due to their high implant depth, potential for excessive subsurface damage, and low sputter yields. More comprehensive studies of chemical assisted ion beam milling will need to be done in order to better understand the potential of light element ion nanomachining for CE. From the empirical data, neon ions show appreciable sputtering on silicon and on copper at a dose of ions cm 2 at low beam energies and surface swelling with the same dose at high beam energies. With a dose 8 times higher ions cm 2, the sputtering is very significant at all energy levels. In order to minimize neon implantation and substrate subsurface distortion, a lower beam energy e.g., 10 kev is desirable. The substrate surface distortion in silicon is more pronounced compared to other target materials, such as copper or silicon dioxide, due its single crystalline nature. At 10 kev, the neon sputter yield for silicon is 1.18 atoms per incident ion and for copper is 4.40 atoms per incident ion. These values are about 30% higher compared to neon s sputter yield at 35 kev and are about 60% of the gallium sputter yield at 10 kev. Based on the simulation results shown in Fig. 4, the vacancies created J. Vac. Sci. Technol. B, Vol. 28, No. 6, Nov/Dec 2010

7 C6F21 Tan et al.: GFIS and LMIS charged particle material interaction study C6F21 for each sputtered atom at energies below 10 kev are lower for neon compared with gallium. These data encourage the continued evaluation of neon as a CE nanomachining candidate. As with the lighter element ions, a significant area for future research for neon is to analyze the material removal rates of neon when assisted by chemical etch chemistry and understand the dose requirements and subsurface damage events that occur in a chemically enhanced environment. Based on the modeling data, other elements that broadly fall in the medium mass ion category e.g., amu are also good candidates for CE nanomachining based on implant depth and sputter yield properties and should be investigated for other key attributes, such as brightness and probe size. Beam technologies such as MOTIS-chromium, GFISargon, and alloy-silicon may be very good candidates for future nanomachining capabilities if they can be demonstrated to be usable outside of a development laboratory environment. Finally, larger mass ions, such as Xe and Au, may also be suited for bulk material removal applications. Broad beam material removal using inductively coupled plasma xenon beams, for example, has been demonstrated for FA cross-section applications; however, the probe size is far too large for present or future generation nanomachining needs. Based on all of these considerations, the immediate focus for future research will be to analyze intermediate mass ion beams. In particular, future research is to continue to develop and evolve gallium LMIS and neon GFIS for possible 22, 15, and 11 nm technology node applications. For these ion species, machining acuity and gas chemistry enhancement factors will be analyzed. Last, but not least, in order for an ion species to be well suited for circuit edit applications, electrical circuit invasiveness such as timing degradation on metal-oxide-semiconductor field-effect transistor devices needs to be carefully understood. ACKNOWLEDGMENTS The authors would like to thank Steve H. Smith and Mary Martinez for their excellent FIB cross-section support. The authors would also like to thank Mario Baca, Kechang Yu, and Steve Hield for his outstanding TEM preparation support. 1 M. Utlaut, Handbook of Charged Particle Optics, edited by J. Orloff, 2nd ed. CRC, New York, 2009, Vol. Chap. 11, p G. Moore, Electronics 38, See 4 V. N. Tondare, J. Vac. Sci. Technol. A 23, L. Bischoff, Ultramicroscopy 103, J. L. Hanssen, S. B. Hill, J. Orloff, and J. J. McClelland, Nano Lett. 8, Q. Ji, X. Jiang, T.-J. King, K.-N. Leung, K. Standiford, and S. B. Wilde, J. Vac. Sci. Technol. B 20, The ORION helium ion microscope is offered by Carl Zeiss. For product specifications, see 9 V. Sidorkin, E. v. Veldhoven, E. v. d. Drift, P. Alkemade, H. Salemink, and D. Maas, J. Vac. Sci. Technol. B 27, L D. Winston et al., J. Vac. Sci. Technol. B 27, P. F. A. Alkemade, P. Chen, E. V. Veldhoven, H. W. M. Salemink, D. Maas, P. D. Rack, and D. A. Smith, J. Vac. Sci. Technol. B C. A. Sanford, L. Stern, L. Barriss, L. Farkas, M. DiManna, R. Mello, D. J. Mass, and P. F. A. Alkemade, J. Vac. Sci. Technol. B 27, F. F. Rahman, C. Sanford, J. Notte, S. Tan, and R. Livengood, Proceedings of the EIPBN, 2010 unpublished. 14 J. F. Zeigler, J. P. Biersack, and U. Littmark, The Stopping and Range of Ions in Solids Pergamon, New York, 1984, Vol. 1 see 15 R. Livengood, S. Tan, Y. Greenzweig, J. Notte, and S. McVey, J. Vac. Sci. Technol. B 27, L. Scipioni, D. Ferranti, V. Smentkowski, and R. Potyrailo, J. Vac. Sci. Technol. B JVST B-Microelectronics and Nanometer Structures