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Focused ion beam machined nanostructures depth profiled by macrochannelling ion beam analysis
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 249 (2006) 747–751 www.elsevier.com/locate/nimb
Focused ion beam machined nanostructures depth profiled by macrochannelling ion beam analysis S.M. Orbons a
a,*
, L. van Dijk b,c, M. Bozkurt b,c, P.N. Johnston b, P. Reichart a, D.N. Jamieson a
Microanalytical Research Centre, School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia b Department of Applied Physics, Eindhoven University of Technology, The Netherlands c Applied Physics, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia Available online 19 May 2006
Abstract High aspect ratio sub-lm periodic structures fabricated by focused ion beam (FIB) lithography have been characterised by Rutherford backscattering spectrometry (RBS) using the macrochannelling technique. The technique overcomes the limitations of complementary techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), which can provide images with sub-lm resolution of just the surface features and not of the deep sub-surface structures, without destructive cross sectioning of the sample. Here RBS macrochannelling with a 2 MeV He+ ion beam is used to analyse a diffraction grating fabricated by FIB milling an array of 100 nm wide trenches in a 300 nm thick Ag film on a Si substrate. Using the surface structure imaged by SEM and AFM as a starting point, a numerical model for the RBS spectrum from the grating is fitted to the experimental spectrum as a function of the sub-surface structure. This process allows the width of the trenches to be determined as a function of depth even though the lateral structure is not resolved by the ion beam. 2006 Elsevier B.V. All rights reserved. PACS: 82.80.Yc; 61.85.+p; 07.10.Cm; 42.55.Tv Keywords: RBS; Macroscopic channelling; High aspect ratio structure; Sub-surface structure; Nanomachining
1. Introduction Ion beam channelling, with Rutherford backscattering spectrometry (RBS) is well established as a technique for providing information about the sub-microscopic structure of materials. Inspired by this method, and the need to characterise high aspect ratio periodic structures on a scale too small for nuclear microprobe analysis, we have developed a macroscopic ion beam channelling technique for the characterisation of micro and nanoscale high aspect ratio periodic structures. The technique exploits the fact
*
Corresponding author. Tel.: +61 3 8344 8123; fax: +61 3 9347 4783. E-mail address: [email protected] (S.M. Orbons).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.179
that samples exhibiting porosity or surface roughness have RBS spectra of characteristic shape [1–4] and this shape may be calculated for simple periodic sample geometries. Complementary techniques such as scanning electron microscopy (SEM) and scanning probe microscopy (SPM) can be used to image sub-lm structures, but they both suffer the disadvantage that important information about sub-surface structure can be difficult to obtain. Especially difficult to analyse are high aspect ratio structures because of the limited reach of the scanning probe into deep wells, or the shallow penetration of the electron beam. We have adapted the macrochannelling technique of [5], which we previously applied to simple gratings, to the experimental study of more complex samples, which include a number of different materials.
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S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
2. Sample fabrication and SEM/AFM characterization The sample under investigation was fabricated by thermally evaporating a 300 nm thick Ag film onto a commercial grade n-type Si wafer substrate. An Orsay Physics Canion 31MPlus focused ion beam (FIB) system, driven by a Raith Elphy Quantum pattern generator was used to mill 250 trenches, each 100 lm long, in the Ag film, with a period of 400 nm and a dose of 0.25 C/cm2, using 30 keV Ga+ ions at normal incidence. The FIB probe size was nominally 50 nm for this work. Fig. 1 shows an SEM image of the surface of the resulting structure which reveals the period and width of the trenches to be nominally 400 and 100 nm, respectively. Regions of the sample patterned in this way showed dramatic diffraction colours to the unaided eye which confirmed the high quality of the grating. To confirm the SEM results, atomic force microscopy (AFM) was used to measure the sample topography. AFM images were obtained using a NT-MDT stand alone SMENA SPM in non-contact mode, using a MikroMasch NSG01/Pt tip. Fig. 2 illustrates the resulting images. Upon first inspection of Fig. 2(a), the image looks remarkably
similar to the SEM image shown in Fig. 1. Furthermore, the line profile shown in Fig. 2(b) seemingly reveals that the trenches possess a depth and width of approximately 80 nm and 100 nm, respectively. However, the tip diameter is 70 nm, which is of the order of the trench width. This tip diameter, combined with a cone half angle of 22, means that the trenches are too narrow for the tip to reach the trench bottom. As a result, the observed line profile is simply revealing the shape of the tip. In order to accurately measure the topography of such a structure using AFM, a tip with a higher aspect ratio than the trenches themselves is required. Such tips are commercially available, but are expensive and extremely fragile, making them far too cumbersome for everyday use. Conversely, RBS is a common and non-destructive tool for probing the elemental depthdependent composition of a sample. Used in conjunction with the numerical macrochannelling technique, it has enormous potential to characterize 3D periodic structures. 3. Macrochannelling analysis The macrochannelling experiment was performed on the Melbourne nuclear microprobe system using 2 MeV He+
Fig. 1. SEM images of the structure under investigation.
Fig. 2. AFM image and line profile of the structure under investigation.
S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
ions focused to a spot size of approximately 3 lm, with detector angles hs of 135 and 145. Ions of this energy loose a convenient amount of energy in the high aspect ratio structures to allow for convenient analysis with conventional Si surface barrier detectors (energy resolution better than 15 keV). The RBS spectrum was measured with the beam oriented at hi = 0, 2, 3 and 5 to the sample. The beam was scanned over an area approximately 180 · 180 lm2 in size, with a step size of approximately 700 nm and 1 pC of charge was collected at each point. A total of 0.5 lC was collected for each spectrum. Fig. 3 provides a 2D map of (in order), the RBS signal from the front Si edge, the rear Ag edge, the X-ray signal for the Ag-L lines and the Ga-K lines. Inspection of Fig. 3 clearly reveals the area patterned with the grating over the central 100 · 100 lm2 region where the RBS signal from the Si front edge is much higher within the patterned area, indicating an increase in the number of Si atoms near the sample surface. The Ga RBS signal and Ga-K X-ray signals only appear within the 100 · 100 lm2 area due to inherent implantation of Ga during the milling process. Finally, the X-ray signal corresponding to the Ag-L lines are diminished within the patterned area, owing to the material removed by FIB milling. Using the multiparameter data acquisition system on the Melbourne microprobe, the RBS spectrum from both the unmilled homogeneous Ag film and the patterned region could be extracted and these are shown in Fig. 4. Discrepancies between the two spectra are immediately obvious. As expected, an apparent decrease in Ag film thickness is observed in the spectrum from the patterned region owing to the trenches where the Ag was milled away with the FIB. In addition, the presence of a tail at the back edge of the Ag peak appears for the patterned film. This tail is caused by the Ga residue, implanted during the FIB milling process. More noticeable in the spectrum for the patterned region is the shift of the Si front edge to higher energies compared to the corresponding position for the unmilled film. This shift indicates the Si substrate is not covered by the full Ag layer or even exposed on the surface as expected following the milling process. We note that even if exposed, the energy loss along the outgoing paths would
749
Fig. 4. RBS spectra for the bare (thick line) and patterned (thin line) Ag film (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
result in a shift due to the backscattered angle of hs = 145. In future experiments, an additional annular detector with a detection angle close to 180 will be used. The Si signal from the patterned region also displays oscillations in the yield. These oscillations are the signature for macrochannelling effects and are due to structural variations in the ion beam incident and exit path length. The numerical technique used to simulate RBS spectra for periodic, high aspect ratio structures is described in [5]. Although we have used the same general method in this work, here we have extended the model so that much more complicated structures can be characterized. Fig. 5 shows the first attempt to simulate the experimental RBS spectrum for the patterned film by assuming an ideal rectangular profile of the ion milled trenches. As mentioned earlier, it is a commonly observed fact, that FIB milling leaves an often undesired layer of implanted Ga ions in the surface of the sample [6]. For this reason, the model included a thin Gallium layer over the sample surface. As shown in
Fig. 3. 2D map of the RBS signal from the Si front edge, Ag back edge and the X-ray signal corresponding to the Ag-L lines and the Ga-K lines (left to right; the grey scale on the right indicates the yield in arbitrary units). Each figure clearly illustrates the 100 · 100 lm2 area where the FIB milling occurred. Image scale approximately 180 · 180 lm2.
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S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
Fig. 5. (a) Schematic illustration of the structure parameters used to simulate the RBS spectrum. (b) Comparison of the simulated (thin line) and the experimental (thick line) RBS yield (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
Fig. 5(b), the first attempt with the structure dimensions of Fig. 5(a) appears to approach the Si and Ga peaks with the macrochannelling features quite well, however, significant discrepancies between the experimental and simulated Ag signal cannot be eliminated with the rectangular model for the trenches. Trenches created by FIB milling often possess sloped sidewalls, particularly close to the surface [7]. This observation, combined with the observed discrepancy evident in the Ag peak shown in Fig. 5(b) suggests that incorporating sloped sidewalls would improve the match between the experimental and simulated spectra. To test this hypothesis, the numerical model was altered to allow for sloped sidewalls. Fig. 6(a) shows the altered structure parameters, whilst Fig. 6(b) illustrates the results from the simulation compared to the experimental values. It is now possible to obtain an excellent fit for energies above 1.4 MeV (superposition of Ag and high energy Ga signal). However there exists a significant discrepancy for the Si signal and the tail of the Ag/Ga peak.
Comparing the results of these two models, it appears as though the true structure shape is actually a combination of a rectangular profile with sloped side walls. The numerical model was altered accordingly so as to calculate the RBS spectrum for the structure shown in Fig. 7(a). The results are shown in Fig. 7(b), along with the experimental spectrum. Comparison of the two spectra reveals a good fit throughout the entire spectrum, suggesting that the dimensions shown in Fig. 7(b) provide an accurate description of the sample structure. The remaining discrepancies in the height of the Si macrochannelling ‘‘waves’’ and the Ga tail might be due to straggling and non-uniform trenches or different Ga thickness on the side walls compared to surface and edges. These issues will be addressed in future work as suggested below. 4. Conclusion It has been shown that the numerical macrochannelling technique first presented in [5] can be altered to simulate
Fig. 6. (a) Schematic illustration of the structure parameters used to simulate the RBS spectrum. (b) Comparison of the simulated (thin line) and the experimental (thick line) RBS yield (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
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Fig. 7. (a) Schematic illustration of the structure parameters used to simulate the RBS spectrum. (b) Comparison of the simulated (thin line) and the experimental (thick line) RBS yield (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
the expected RBS spectra from a number of different sample structures. In particular, we have shown that the technique is capable of characterizing the sub-surface structure of a high aspect ratio diffraction grating fabricated by FIB milling lines in an Ag film deposited on a Si substrate. As the determination of Ga contamination is of large interest for FIB nanomachining, our future work will focus on the accurate determination of the implanted Ga profile caused by the FIB milling process. Further extensions of the numerical model work towards defining the structure parameters on a layer by layer basis, allowing for the accurate simulation of any 3D periodic structure, with arbitrary unit cell dimensions. Acknowledgements The authors acknowledge useful discussions with Ann Roberts of the University of Melbourne and Darren Free-
man of the Australian National University. This work is supported by a grant from the Australian Research Council and Shannon Orbons is supported by an Australian Postgraduate Research Award. References [1] Z. Hajnal, E. Szilagyi, F. Paszi, G. Battistig, Nucl. Instr. and Meth. B 118 (1996) 617. [2] F. Paszti, E. Szilagyi, Vacuum 50 (1998) 451. [3] M. Mayer, Nucl. Instr. and Meth. B 194 (2002) 177. [4] I.M. Yesil, W. Assmann, H. Huber, K.E.G. Lobner, Nucl. Instr. and Meth. B 136 (1998) 623. [5] L. van Dijk, M. Bozkurt, A. Alves, P.N. Johnston, T.J. Davis, P. Reichart, D.N. Jamieson, Nucl. Instr. and Meth. B 231 (2005) 130. [6] H. Ryssel, L. Frey, C. Lehrer, Appl. Phys. A 76 (2003) 1017. [7] A. Tseng, J. Micromech. Microeng. 14 (2004) R15.
Nuclear Instruments and Methods in Physics Research B 249 (2006) 747–751 www.elsevier.com/locate/nimb
Focused ion beam machined nanostructures depth profiled by macrochannelling ion beam analysis S.M. Orbons a
a,*
, L. van Dijk b,c, M. Bozkurt b,c, P.N. Johnston b, P. Reichart a, D.N. Jamieson a
Microanalytical Research Centre, School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia b Department of Applied Physics, Eindhoven University of Technology, The Netherlands c Applied Physics, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia Available online 19 May 2006
Abstract High aspect ratio sub-lm periodic structures fabricated by focused ion beam (FIB) lithography have been characterised by Rutherford backscattering spectrometry (RBS) using the macrochannelling technique. The technique overcomes the limitations of complementary techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), which can provide images with sub-lm resolution of just the surface features and not of the deep sub-surface structures, without destructive cross sectioning of the sample. Here RBS macrochannelling with a 2 MeV He+ ion beam is used to analyse a diffraction grating fabricated by FIB milling an array of 100 nm wide trenches in a 300 nm thick Ag film on a Si substrate. Using the surface structure imaged by SEM and AFM as a starting point, a numerical model for the RBS spectrum from the grating is fitted to the experimental spectrum as a function of the sub-surface structure. This process allows the width of the trenches to be determined as a function of depth even though the lateral structure is not resolved by the ion beam. 2006 Elsevier B.V. All rights reserved. PACS: 82.80.Yc; 61.85.+p; 07.10.Cm; 42.55.Tv Keywords: RBS; Macroscopic channelling; High aspect ratio structure; Sub-surface structure; Nanomachining
1. Introduction Ion beam channelling, with Rutherford backscattering spectrometry (RBS) is well established as a technique for providing information about the sub-microscopic structure of materials. Inspired by this method, and the need to characterise high aspect ratio periodic structures on a scale too small for nuclear microprobe analysis, we have developed a macroscopic ion beam channelling technique for the characterisation of micro and nanoscale high aspect ratio periodic structures. The technique exploits the fact
*
Corresponding author. Tel.: +61 3 8344 8123; fax: +61 3 9347 4783. E-mail address: [email protected] (S.M. Orbons).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.179
that samples exhibiting porosity or surface roughness have RBS spectra of characteristic shape [1–4] and this shape may be calculated for simple periodic sample geometries. Complementary techniques such as scanning electron microscopy (SEM) and scanning probe microscopy (SPM) can be used to image sub-lm structures, but they both suffer the disadvantage that important information about sub-surface structure can be difficult to obtain. Especially difficult to analyse are high aspect ratio structures because of the limited reach of the scanning probe into deep wells, or the shallow penetration of the electron beam. We have adapted the macrochannelling technique of [5], which we previously applied to simple gratings, to the experimental study of more complex samples, which include a number of different materials.
748
S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
2. Sample fabrication and SEM/AFM characterization The sample under investigation was fabricated by thermally evaporating a 300 nm thick Ag film onto a commercial grade n-type Si wafer substrate. An Orsay Physics Canion 31MPlus focused ion beam (FIB) system, driven by a Raith Elphy Quantum pattern generator was used to mill 250 trenches, each 100 lm long, in the Ag film, with a period of 400 nm and a dose of 0.25 C/cm2, using 30 keV Ga+ ions at normal incidence. The FIB probe size was nominally 50 nm for this work. Fig. 1 shows an SEM image of the surface of the resulting structure which reveals the period and width of the trenches to be nominally 400 and 100 nm, respectively. Regions of the sample patterned in this way showed dramatic diffraction colours to the unaided eye which confirmed the high quality of the grating. To confirm the SEM results, atomic force microscopy (AFM) was used to measure the sample topography. AFM images were obtained using a NT-MDT stand alone SMENA SPM in non-contact mode, using a MikroMasch NSG01/Pt tip. Fig. 2 illustrates the resulting images. Upon first inspection of Fig. 2(a), the image looks remarkably
similar to the SEM image shown in Fig. 1. Furthermore, the line profile shown in Fig. 2(b) seemingly reveals that the trenches possess a depth and width of approximately 80 nm and 100 nm, respectively. However, the tip diameter is 70 nm, which is of the order of the trench width. This tip diameter, combined with a cone half angle of 22, means that the trenches are too narrow for the tip to reach the trench bottom. As a result, the observed line profile is simply revealing the shape of the tip. In order to accurately measure the topography of such a structure using AFM, a tip with a higher aspect ratio than the trenches themselves is required. Such tips are commercially available, but are expensive and extremely fragile, making them far too cumbersome for everyday use. Conversely, RBS is a common and non-destructive tool for probing the elemental depthdependent composition of a sample. Used in conjunction with the numerical macrochannelling technique, it has enormous potential to characterize 3D periodic structures. 3. Macrochannelling analysis The macrochannelling experiment was performed on the Melbourne nuclear microprobe system using 2 MeV He+
Fig. 1. SEM images of the structure under investigation.
Fig. 2. AFM image and line profile of the structure under investigation.
S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
ions focused to a spot size of approximately 3 lm, with detector angles hs of 135 and 145. Ions of this energy loose a convenient amount of energy in the high aspect ratio structures to allow for convenient analysis with conventional Si surface barrier detectors (energy resolution better than 15 keV). The RBS spectrum was measured with the beam oriented at hi = 0, 2, 3 and 5 to the sample. The beam was scanned over an area approximately 180 · 180 lm2 in size, with a step size of approximately 700 nm and 1 pC of charge was collected at each point. A total of 0.5 lC was collected for each spectrum. Fig. 3 provides a 2D map of (in order), the RBS signal from the front Si edge, the rear Ag edge, the X-ray signal for the Ag-L lines and the Ga-K lines. Inspection of Fig. 3 clearly reveals the area patterned with the grating over the central 100 · 100 lm2 region where the RBS signal from the Si front edge is much higher within the patterned area, indicating an increase in the number of Si atoms near the sample surface. The Ga RBS signal and Ga-K X-ray signals only appear within the 100 · 100 lm2 area due to inherent implantation of Ga during the milling process. Finally, the X-ray signal corresponding to the Ag-L lines are diminished within the patterned area, owing to the material removed by FIB milling. Using the multiparameter data acquisition system on the Melbourne microprobe, the RBS spectrum from both the unmilled homogeneous Ag film and the patterned region could be extracted and these are shown in Fig. 4. Discrepancies between the two spectra are immediately obvious. As expected, an apparent decrease in Ag film thickness is observed in the spectrum from the patterned region owing to the trenches where the Ag was milled away with the FIB. In addition, the presence of a tail at the back edge of the Ag peak appears for the patterned film. This tail is caused by the Ga residue, implanted during the FIB milling process. More noticeable in the spectrum for the patterned region is the shift of the Si front edge to higher energies compared to the corresponding position for the unmilled film. This shift indicates the Si substrate is not covered by the full Ag layer or even exposed on the surface as expected following the milling process. We note that even if exposed, the energy loss along the outgoing paths would
749
Fig. 4. RBS spectra for the bare (thick line) and patterned (thin line) Ag film (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
result in a shift due to the backscattered angle of hs = 145. In future experiments, an additional annular detector with a detection angle close to 180 will be used. The Si signal from the patterned region also displays oscillations in the yield. These oscillations are the signature for macrochannelling effects and are due to structural variations in the ion beam incident and exit path length. The numerical technique used to simulate RBS spectra for periodic, high aspect ratio structures is described in [5]. Although we have used the same general method in this work, here we have extended the model so that much more complicated structures can be characterized. Fig. 5 shows the first attempt to simulate the experimental RBS spectrum for the patterned film by assuming an ideal rectangular profile of the ion milled trenches. As mentioned earlier, it is a commonly observed fact, that FIB milling leaves an often undesired layer of implanted Ga ions in the surface of the sample [6]. For this reason, the model included a thin Gallium layer over the sample surface. As shown in
Fig. 3. 2D map of the RBS signal from the Si front edge, Ag back edge and the X-ray signal corresponding to the Ag-L lines and the Ga-K lines (left to right; the grey scale on the right indicates the yield in arbitrary units). Each figure clearly illustrates the 100 · 100 lm2 area where the FIB milling occurred. Image scale approximately 180 · 180 lm2.
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S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
Fig. 5. (a) Schematic illustration of the structure parameters used to simulate the RBS spectrum. (b) Comparison of the simulated (thin line) and the experimental (thick line) RBS yield (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
Fig. 5(b), the first attempt with the structure dimensions of Fig. 5(a) appears to approach the Si and Ga peaks with the macrochannelling features quite well, however, significant discrepancies between the experimental and simulated Ag signal cannot be eliminated with the rectangular model for the trenches. Trenches created by FIB milling often possess sloped sidewalls, particularly close to the surface [7]. This observation, combined with the observed discrepancy evident in the Ag peak shown in Fig. 5(b) suggests that incorporating sloped sidewalls would improve the match between the experimental and simulated spectra. To test this hypothesis, the numerical model was altered to allow for sloped sidewalls. Fig. 6(a) shows the altered structure parameters, whilst Fig. 6(b) illustrates the results from the simulation compared to the experimental values. It is now possible to obtain an excellent fit for energies above 1.4 MeV (superposition of Ag and high energy Ga signal). However there exists a significant discrepancy for the Si signal and the tail of the Ag/Ga peak.
Comparing the results of these two models, it appears as though the true structure shape is actually a combination of a rectangular profile with sloped side walls. The numerical model was altered accordingly so as to calculate the RBS spectrum for the structure shown in Fig. 7(a). The results are shown in Fig. 7(b), along with the experimental spectrum. Comparison of the two spectra reveals a good fit throughout the entire spectrum, suggesting that the dimensions shown in Fig. 7(b) provide an accurate description of the sample structure. The remaining discrepancies in the height of the Si macrochannelling ‘‘waves’’ and the Ga tail might be due to straggling and non-uniform trenches or different Ga thickness on the side walls compared to surface and edges. These issues will be addressed in future work as suggested below. 4. Conclusion It has been shown that the numerical macrochannelling technique first presented in [5] can be altered to simulate
Fig. 6. (a) Schematic illustration of the structure parameters used to simulate the RBS spectrum. (b) Comparison of the simulated (thin line) and the experimental (thick line) RBS yield (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
S.M. Orbons et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 747–751
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Fig. 7. (a) Schematic illustration of the structure parameters used to simulate the RBS spectrum. (b) Comparison of the simulated (thin line) and the experimental (thick line) RBS yield (2 MeV He+, hi = 0, hs = 145, solid angle of detection X = 27.5 msr).
the expected RBS spectra from a number of different sample structures. In particular, we have shown that the technique is capable of characterizing the sub-surface structure of a high aspect ratio diffraction grating fabricated by FIB milling lines in an Ag film deposited on a Si substrate. As the determination of Ga contamination is of large interest for FIB nanomachining, our future work will focus on the accurate determination of the implanted Ga profile caused by the FIB milling process. Further extensions of the numerical model work towards defining the structure parameters on a layer by layer basis, allowing for the accurate simulation of any 3D periodic structure, with arbitrary unit cell dimensions. Acknowledgements The authors acknowledge useful discussions with Ann Roberts of the University of Melbourne and Darren Free-
man of the Australian National University. This work is supported by a grant from the Australian Research Council and Shannon Orbons is supported by an Australian Postgraduate Research Award. References [1] Z. Hajnal, E. Szilagyi, F. Paszi, G. Battistig, Nucl. Instr. and Meth. B 118 (1996) 617. [2] F. Paszti, E. Szilagyi, Vacuum 50 (1998) 451. [3] M. Mayer, Nucl. Instr. and Meth. B 194 (2002) 177. [4] I.M. Yesil, W. Assmann, H. Huber, K.E.G. Lobner, Nucl. Instr. and Meth. B 136 (1998) 623. [5] L. van Dijk, M. Bozkurt, A. Alves, P.N. Johnston, T.J. Davis, P. Reichart, D.N. Jamieson, Nucl. Instr. and Meth. B 231 (2005) 130. [6] H. Ryssel, L. Frey, C. Lehrer, Appl. Phys. A 76 (2003) 1017. [7] A. Tseng, J. Micromech. Microeng. 14 (2004) R15.