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Nanofabrication of self-organized periodic ripples by ion beam sputtering
Microelectronic Engineering 155 (2016) 50–54
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanofabrication of self-organized periodic ripples by ion beam sputtering Erica Iacob a, Rossana Dell'Anna a,⁎, Damiano Giubertoni a, Evgeny Demenev a, Maria Secchi a, Roman Böttger b, Giancarlo Pepponi a a b
Fondazione Bruno Kessler, Center for Materials and Microsystems, Micro Nano Facility, Via Sommarive 18, I-38123 Trento, Italy Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Modification, Bautzner Landstraße 400, 01328 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 6 November 2015 Received in revised form 11 February 2016 Accepted 16 February 2016 Available online 17 February 2016 Keywords: Surface nanopatterning Ion beam sputtering Surface morphology Silicon AFM SEM
a b s t r a c t Ion beam sputtering can induce the formation of regular self-assembled surface nanostructures on different materials. Several experimental variables, such as the ion energy, the ion incidence angle, the ion fluence, together with the sample surface properties and temperature, have been proven to control in a complex and not completely codified manner the formation of periodically arranged dot, hole or ripple nanopatterns. In this work, we have studied how to apply ion beam sputtering to induce periodic ripples of specific aspect ratio on silicon surfaces, namely ripple height a of about 10 nm and period λ ≤ 50 nm, since these topographies can be appealing for technological applications. Silicon surfaces were irradiated with increasing O+ and Xe+ ion fluences at fixed ion energy and incidence angle. We have verified that some combinations of the chosen parameter values produced the desired structures. The obtained results also indicate that with a further refinement of those parameter values a better control of the aspect ratio of the obtained ripples is possible. Therefore, this work is a contribution to the final aim of exploiting ion beam sputtering as a fast, cost-effective and single-step method to fabricate well-controlled patterns over large surface areas at length scales beyond those of both standard and e-beam lithography. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In 1962 Navez et al. [1] first reported the development of periodic structures by Ion Beam Sputtering (IBS) of glass surfaces and discovered their dependence on the incidence angle of the ions. In the following years, many studies have been performed in order to understand the physical mechanisms that regulate the development of periodically arranged dot or ripple nanopatterns on the surface of different materials [2–7]. This spontaneous topography formation represents an unwanted surface flatness degradation in IBS applications for surface analysis/ depth profiling (e.g. secondary ion mass spectrometry [8,9], ion milling [10]). On the other side, being essentially a self-assembling phenomenon, it can be exploited to build sub-micrometer and nanometer structures on semiconductor, metal and insulator surfaces. In fact, the nanostructuring of surfaces by IBS has some attractive advantages over the top-down lithographic techniques, as it is a fast, cost-effective and single-step method to fabricate self-organized nanopatterns over large surface areas. This may lead to consider IBS as a bottom-up technique alternative or complementary to the conventional lithography for the patterning of materials at the nanoscale. The formation and the characteristics of self-organized topographies depend on several experimental parameters related both to the incident ion beam (such as ion species, kinetic energy, incidence angle and ion ⁎ Corresponding author. E-mail address: [email protected] (R. Dell'Anna).
http://dx.doi.org/10.1016/j.mee.2016.02.025 0167-9317/© 2016 Elsevier B.V. All rights reserved.
fluence), and to the sample conditions (such as pristine surface and sample temperature). Considerable advances have been made to reach a comprehensive theoretical understanding of the dot and ripple formation process. In 1988, Bradley and Harper proposed a theoretical model to explain the formation of sputtering-induced ripple topographies [11]. These result from the competition of two phenomena: a curvatureinduced roughening instability, i.e. an increasing amplitude instability due to the sputter yield variation with surface curvature, and a smoothening process by surface diffusion. On a short time/low fluence scale, the process can be approximated by a linear continuum equation, which predicts a constant wavelength and an exponential growth of the amplitude. By increasing the bombarding time/fluence or, alternatively, the incidence angle [12,5], the non-linear effects are no longer negligible and important surface profile changes appear. First, the coarsening of the ripple wavelength and the ripple amplitude saturation can be observed [12]. Secondly, the patterns become more and more faceted at the downstream side with respect to the ion beam direction and typical sawtooth profiles emerge [7,13]. Eventually, for grazing incidence angles or longer bombarding times, the surface smoothing is observed [14]. Despite the valuable theoretical studies, a common general framework for all the different experimental observations is still missing [6], whereas for some technological applications it is instead mandatory to understand how to effectively tune the experimental parameters to produce on large areas periodic nanostructures with the required dimensions. For this reason, the aim of this work was to see if it was
E. Iacob et al. / Microelectronic Engineering 155 (2016) 50–54
possible to systematically order the knowledge gained with experiments described in literature and derive an empirical model allowing the prediction of the outcome when pushing parameters out of the already explored ranges. In particular, the choice of the experimental parameters controlling IBS on silicon surfaces was considered, in order to obtain periodic nano-ripples with a pre-defined aspect ratio AR = a/ λ ≥ 1/5, where a is the height and λ is the wavelength (period). Ripples of a period of about 50 nm and height a close as much as possible to 10 nm would be potentially useful for nanoelectronics and energy storage applications [15]. For ripples of lower height, different applications have been already envisaged, namely electrochemical devices [16], plasmonics [17], and nanostructured materials for bio-devices [18]. 2. Materials and methods Six different irradiation conditions (see Table 1) were chosen after comparing and integrating many published experimental results, allowing the definition of a “reference empirical schema”, describing how to change the experimental parameters to control the ripple sizes. Among others, references [13,19,20] provided the starting point of the six experiments, as the ripple heights and wavelengths therein discussed were already in ranges close to those expected. The same incident ion species were chosen, i.e. Xe+ and O+, and the empirical model was applied, with the aim to obtain ripples of period λ ≤ 50 nm and height a close as much as possible to 10 nm, therefore modifying the ripple sizes reported in [13,19,20]. Ion irradiations were performed using a low energy ion implanter (Danfysik A/S, Denmark, Model1050) equipped with a gas-fed Chordis ion source. 5 keV Xe+ and 1 keV O+ ion beams, with fluxes of 0.6 × 1013 and 1.1 × 1013 cm−2 s−1 respectively, were electrostatically raster scanned (fx, fy ~ 1 kHz) full area over 1 × 1 cm2 Si(100) samples, which were tilted 50° or 55° with respect to the ion beam. Ion fluences between 8.0 × 1016 and 2.0 × 1018 cm−2 were irradiated on six different samples. A vacuum b 2 × 10−6 mbar was maintained in the implantation chamber. Table 1 gives an overview of all irradiation conditions of investigated samples. All the samples were irradiated at the Institute of Ion Beam Physics and Materials Research at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. Atomic Force Microscopy (AFM) images were acquired with a scanning probe microscope (Unisolver P47H, NT MDT Co.). The analyses were performed in semi-contact mode with a silicon tip having a nominal radius of less than 10 nm. Scan areas of 2 × 2 μm2, 5 × 5 μm2 and 10 × 10 μm2 were acquired with a resolution up to 1024 × 1024 pixels. The 10 × 10 μm2 images (not shown) were used to confirm on a larger spatial scale the hereinafter discussed results. The root mean square (RMS) surface roughness, representing the standard deviation of surface heights, was calculated for each scanned area and used to compare the roughness of the irradiated surfaces. The periodicity of the patterns was evaluated by the discrete two-dimensional Fast Fourier transformation (2D-FFT), as well as by inspecting the cross sections of the AFM topographies, which were taken along the projected irradiation beam direction. The same irradiated samples were also analyzed by field-emission Scanning Electron Microscopy (SEM, Jeol JMS 7401F). Analyses were carried out in plan-view, setting an acceleration voltage of 5 kV. Table 1 The irradiating sample conditions. Sample name
Implanted ion species
energy (keV)
Fluence 1018 (ions/cm−2)
Incidence angle (°)
Xe_1 Xe_2 Xe_3 O_1 O_2 O_3
Xe+ Xe+ Xe+ O+ O+ O+
5 5 5 1 1 1
0.080 1.5 2.0 1.0 2.0 2.0
55 55 55 50 50 55
51
3. Results Fig. 1 shows the AFM topography images acquired on the three Xe+ irradiated samples. For each image, the corresponding 2D-FFT image and a vertical linear profile are shown. Fig. 1A shows the presence of a pattern with an irregular short range correlation and structure heights of about 1–2 nm. On the contrary, the surfaces of Fig. 1B and C show a regime of surface smoothing, since the resulting RMS roughness is comparable with the initial RMS roughness The SEM images are not shown, since the modified surfaces have morphological features below the instrument detection limit. The presence of a regular topography is instead recognizable in all the samples irradiated with O+ ions. In general, following the ion fluence increase from Fig. 2A to C, the analysis shows the development of periodic ripples (with a distribution of wavelengths as described by the 2D-FFT images shown in the insets of Fig. 2), with a nearly constant wavelength of ~40 nm, 50 nm and 60 nm respectively, and corresponding heights of ~ 2.5 nm, 3 nm, and 15 nm. These patterns are superimposed on a less regular topography of longer wavelength and higher amplitude, which can be associated to the long time-scale and non-linear regime proper of the sputtering process [5]. In fact, in Fig. 2C, which corresponds to the highest fluence, the larger wavelength structure definitely dominates on the high-frequency rippled structure, and the cross section clearly shows a sawtooth profile, which is typical of long bombarding times [5,7]. In Fig. 3, the SEM images of the same samples of Fig. 2(A–C) show, on a larger field of view, the regularity of the obtained nano-ripples. In Fig. 3C, which corresponds to the highest fluence, shingle-like faceted surfaces are clearly visible in place of the regular ripples of Fig. 3(A,B), which are indeed typical of lower ion fluences. 4. Discussion Starting from the experimental settings given in [13,19,20], in order to modify the ripple features to values of λ ≤ 50 nm and of a of about 10 nm, the empirical model derived from the literature prescribes values of fluence and incidence angle that move towards a quasilinear experimental regime, so that a weak dependence of λ on irradiation time is expected while the height a increases [5,12]. In addition, the ion energy has to be chosen to have λ values in the required nanometer range, as it is known that at room temperature the ripple wavelength increases with the ion energy [5]. It is generally acknowledged that the surface remains flat up to a critical incidence angle. Some authors also gave evidence that this critical angle shifts to lower values with decreasing ion energy [5], and that the linear instability develops on a faster time scale for increasing incidence angles, leading to an earlier onset of nonlinear effects [5,12]. Therefore, by choosing low values of the ion energy and incidence angle, we aimed at increasing the aspect ratio and at delaying as much as possible the onset of nonlinear features, such as the coarsening of the wavelength, the appearance of large triangular and faceted hillocks or of shingle-like faceted surfaces, and, on longer times, the surface smoothing regime. A different outcome of the Xe+ experiments with respect to the O+ experiments is clearly outlined in Figs. 1 and 2. Because of a large mass difference between Xe and O, the absolute value of the (physical) sputtering rate and its incident angle dependence are different. The oxygen reactivity might also give additional differences in the fabrication process of ripple structures [21]. In Fig. 1 (B–C), the silicon surfaces after Xe+ irradiation show the effects of a long-term non-linear regime. In fact, only for the first ion fluence (Fig. 1A) a pattern was produced having an irregular short range correlation and structure heights of about 1–2 nm. Therefore, in this case the quasi-linear regime probably starts and ends at definitely lower fluence values. In Fig. 2(A–C), showing the results of O+ irradiation, a sufficiently stabilized non-linear regime is present as well, but, for samples of
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E. Iacob et al. / Microelectronic Engineering 155 (2016) 50–54
Fig. 1. Silicon surfaces after Xe+ irradiation as reported in Table 1. Left: AFM and 2D-FFT images. Right: one corresponding cross section. Sputtering conditions: (A) E = 5 keV, α = 55°, Φ = 8.0 × 1016 ions/cm2; (B) E = 5 keV, α = 55°, Φ = 1.5 × 1018 ions/cm2; (C) E = 5 keV, α = 55°, Φ = 2.0 × 1018 ions/cm2. The arrows indicate the projection of the ion beam direction.
Fig. 2(A–B), which in fact correspond to the first two ion fluences and to the lowest incidence angle, the structures meet the expected AR. Therefore, these samples are in principle suitable to be used in some of the envisaged technological applications. On the other hand, the patterns illustrated in Figs. 2C and 3C show how the increase of the incidence angle accelerated the onset of non-linear effects. In fact, for the same values of energy and fluence of Figs. 2B and 3B, Figs. 2C and 3C describe the appearance of shingle-like faceted surfaces, typical of the long-term ion bombardment. However, also in this case, lower ion fluence values should be tested (given the same values of energy and incidence angle), to possibly reduce the non-linear effects, responsible of the periodic structure of longer wavelength and larger amplitude shown in the cross sections of Fig. 2(A-B). 5. Conclusions The self-organized ripple formation on silicon surfaces induced by low-energy ion sputtering at the nanometer length scale has been
investigated. An empirical recipe has been proposed to produce ripples with desired geometries, trying to gain a stricter control over the sizes of the nano-structures, as sometimes required by technological applications. For the specific purposes here discussed, Xe+ and O+ ions were chosen and different ion fluences at fixed incidence angle and energy were considered. Overall, this work has shown the still-existing difficulty of properly tuning ripple sizes. In the case of O+ irradiation, AFM and SEM analyses showed the development of regular ripples with wavelengths of about 40–50 nm and heights of about 2–3 nm, superimposed on a less regular topography of longer wavelength and higher amplitude. These structures have sizes compatible with those previously discussed in works dealing with the use of nanoripples in some cutting edge technological applications: electrochemical devices [16], plasmonics [17], and nanostructured materials for bio-devices [18]. In both experiments here discussed, obtained results have shown an early achievement of the non-linear regime which indicates the possibility of lowering the ion fluence to obtain better-shaped periodic ripples. These new recipes will be investigated in forthcoming experiments.
E. Iacob et al. / Microelectronic Engineering 155 (2016) 50–54
53
Fig. 2. Silicon surfaces after O+ irradiation as reported in Table 1. Left: AFM and 2D-FFT images. Right: one corresponding cross section. Sputtering conditions (A) E = 1 keV, α = 50°, Φ = 1.0 × 1018 ions/cm2; (B) E = 1 keV, α = 50°, Φ = 2.0 × 1018 ions/cm2; (C) E = 1 keV, α = 55°, Φ = 2.0 × 1018 ions/cm2. The arrows indicate the projection of the ion beam direction.
Fig. 3. SEM images on silicon substrate after O+ irradiation. Sputtering conditions (A) E = 1 keV, α = 50°, Φ = 1.0 × 1018 ions/cm2; (B) E = 1 keV, α = 50°, Φ = 2.0 × 1018 ions/cm2; (C) E = 1 keV, α = 55°, Φ = 2.0 × 1018 ions/cm2.
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Acknowledgments We would like to thank the Ion Beam Center at Helmholtz-Zentrum Dresden-Rossendorf for the ion irradiation. References [1] M. Navez, C. Sella, D. Chaperot, Etude de l'attaque du verre par bombardement ionique, C. R. Acad. Sci. Paris 254 (1962) 240–248. [2] S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, H.L. Hartnagel, Formation of ordered nanoscale semiconductor dots by ion sputtering, Science 285 (1999) 1551–1553. [3] W.L. Chan, E. Chason, Ion beam well established way to create nanopatterning, J. Appl. Phys. 101 (2007) 121301. [4] D.J. Ghose, Ion beam sputtering induced nanostructuring of polycrystalline metal films, J. Phys. Condens. Matter 21 (2009) 224001. [5] M. Teichmann, J. Lorbeer, B. Ziberi, F. Frost, B. Rauschenbach, Pattern formation on Ge by low energy ion beam erosion, New J. Phys. 15 (2013) 103029. [6] A. Keller, S. Facsko, Ion-induced nanoscale ripple patterns on Si surfaces: theory and experiment, Materials 3 (2010) 4811–4841 (and references therein). [7] T. Basu, D.P. Datta, T. Som, Transition from ripples to faceted structures under lowenergy argon ion bombardment of silicon: understanding the role of shadowing and sputtering, Nanoscale Res. Lett. 8 (2013) 289 (and references therein). [8] E. Iacob, E. Demenev, D. Giubertoni, M. Barozzi, S. Gennaro, M. Bersani, Development of nano‐roughness under SIMS ion sputtering of germanium surfaces, Surf. Interface Anal. 45 (2013) 409–412. [9] E. Iacob, M. Bersani, A. Lui, D. Giubertoni, M. Barozzi, M. Anderle, Topography induced by sputtering in a magnetic sector instrument: an AFM and SEM study, Appl. Surf. Sci. 238 (2004) 24–28. [10] D.P. Adams, M.J. Vasile, T.M. Mayer, V.C. Hodges Adams, Focused ion beam milling of diamond, J. Vac. Sci. Technol. B 21 (2003) 2334–2343.
[11] R. Mark Bradley, James M.E. Harper, Theory of ripple topography induced by ion bombardment, J. Vac. Sci. Technol. A 6 (1988) 2390–2395. [12] M. Castro, R. Gago, L. Vázquez, J. Muñoz-García, R. Cuerno, Stress-induced solid flow drives surface nanopatterning of silicon by ion-beam irradiation, Phys. Rev. B 86 (2012) 214107. [13] M. Teichmann, J. Lorbeer, F. Frost, B. Rauschenbach, Ripple coarsening on ion beameroded surfaces, Nanoscale Res. Lett. 9 (2014) 439. [14] S. Macko, F. Frost, B. Ziberi, D.F. Förster, T. Michely, Is keV ion-induced pattern formation on Si(001) caused by metal impurities? Nanotechnology 21 (2010) 085301. [15] Shih-Yang Lin, Shen-Lin Chang, Feng-Lin Shyu, Jian-Ming Lu, Ming-Fa Lin, Featurerich electronic properties in graphene ripples, Carbon 86 (2015) 207–216. [16] T. Kumar, U.B. Singha, M. Kumarb, S. Ojhaa, D. Kanjilala, Tuning of ripple patterns and wetting dynamics of Si (100) surface using ion beam irradiation, Curr. Appl. Phys. 14 (2014) 312–317. [17] Application of ion beam produced patterned substrates in plasmonics, M. Ranjan, T. W., H. Oates and Stefan Facsko, Chapter 10, in: Dinakar Kanjilal (Ed.), Nanofabrication by Ion-Beam Sputtering, Fundamentals and Applications, Pan Stanford Publishing 2012, pp. 297–327, http://dx.doi.org/10.1201/b13726-11 (Print ISBN: 978-981-4303-75-0, eBook ISBN: 978-981-4303-76-7). [18] B. Teshome, S. Facsko, A. Keller, Topography-controlled alignment of DNA origami nanotubes on nanopatterned surfaces, Nanoscale 6 (2014) 1790–1796. [19] S. Sarkar, B. Van Daele, W. Vandervorst, Impact of repetitive and random surface morphologies on the ripple formation on ion bombarded SiGe-surfaces, New J. Phys. 10 (2008) 083012. [20] Z.X. Jiang, P.F.A. Alkemade, The complex formation of ripples during depth profiling of Si with low energy, grazing oxygen beams, Appl. Phys. Lett. 73 (1998) 315–317 (Here molecular oxygen ions at 1 keV were used to produce ripples of wavelength of 40–45 nm. This is comparable to 500 eV atomic ions and doubled fluence. Therefore, with respect to the work of Z.X. Jiang et al., the experiments reported in this paper were carried out in the low flux regime). [21] Z.X. Liu, P.F.A. Alkemade, Flux dependence of oxygen-beam-induced ripple growth on silicon, Appl. Phys. Lett. 79 (2001) 4334–4336.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanofabrication of self-organized periodic ripples by ion beam sputtering Erica Iacob a, Rossana Dell'Anna a,⁎, Damiano Giubertoni a, Evgeny Demenev a, Maria Secchi a, Roman Böttger b, Giancarlo Pepponi a a b
Fondazione Bruno Kessler, Center for Materials and Microsystems, Micro Nano Facility, Via Sommarive 18, I-38123 Trento, Italy Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Modification, Bautzner Landstraße 400, 01328 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 6 November 2015 Received in revised form 11 February 2016 Accepted 16 February 2016 Available online 17 February 2016 Keywords: Surface nanopatterning Ion beam sputtering Surface morphology Silicon AFM SEM
a b s t r a c t Ion beam sputtering can induce the formation of regular self-assembled surface nanostructures on different materials. Several experimental variables, such as the ion energy, the ion incidence angle, the ion fluence, together with the sample surface properties and temperature, have been proven to control in a complex and not completely codified manner the formation of periodically arranged dot, hole or ripple nanopatterns. In this work, we have studied how to apply ion beam sputtering to induce periodic ripples of specific aspect ratio on silicon surfaces, namely ripple height a of about 10 nm and period λ ≤ 50 nm, since these topographies can be appealing for technological applications. Silicon surfaces were irradiated with increasing O+ and Xe+ ion fluences at fixed ion energy and incidence angle. We have verified that some combinations of the chosen parameter values produced the desired structures. The obtained results also indicate that with a further refinement of those parameter values a better control of the aspect ratio of the obtained ripples is possible. Therefore, this work is a contribution to the final aim of exploiting ion beam sputtering as a fast, cost-effective and single-step method to fabricate well-controlled patterns over large surface areas at length scales beyond those of both standard and e-beam lithography. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In 1962 Navez et al. [1] first reported the development of periodic structures by Ion Beam Sputtering (IBS) of glass surfaces and discovered their dependence on the incidence angle of the ions. In the following years, many studies have been performed in order to understand the physical mechanisms that regulate the development of periodically arranged dot or ripple nanopatterns on the surface of different materials [2–7]. This spontaneous topography formation represents an unwanted surface flatness degradation in IBS applications for surface analysis/ depth profiling (e.g. secondary ion mass spectrometry [8,9], ion milling [10]). On the other side, being essentially a self-assembling phenomenon, it can be exploited to build sub-micrometer and nanometer structures on semiconductor, metal and insulator surfaces. In fact, the nanostructuring of surfaces by IBS has some attractive advantages over the top-down lithographic techniques, as it is a fast, cost-effective and single-step method to fabricate self-organized nanopatterns over large surface areas. This may lead to consider IBS as a bottom-up technique alternative or complementary to the conventional lithography for the patterning of materials at the nanoscale. The formation and the characteristics of self-organized topographies depend on several experimental parameters related both to the incident ion beam (such as ion species, kinetic energy, incidence angle and ion ⁎ Corresponding author. E-mail address: [email protected] (R. Dell'Anna).
http://dx.doi.org/10.1016/j.mee.2016.02.025 0167-9317/© 2016 Elsevier B.V. All rights reserved.
fluence), and to the sample conditions (such as pristine surface and sample temperature). Considerable advances have been made to reach a comprehensive theoretical understanding of the dot and ripple formation process. In 1988, Bradley and Harper proposed a theoretical model to explain the formation of sputtering-induced ripple topographies [11]. These result from the competition of two phenomena: a curvatureinduced roughening instability, i.e. an increasing amplitude instability due to the sputter yield variation with surface curvature, and a smoothening process by surface diffusion. On a short time/low fluence scale, the process can be approximated by a linear continuum equation, which predicts a constant wavelength and an exponential growth of the amplitude. By increasing the bombarding time/fluence or, alternatively, the incidence angle [12,5], the non-linear effects are no longer negligible and important surface profile changes appear. First, the coarsening of the ripple wavelength and the ripple amplitude saturation can be observed [12]. Secondly, the patterns become more and more faceted at the downstream side with respect to the ion beam direction and typical sawtooth profiles emerge [7,13]. Eventually, for grazing incidence angles or longer bombarding times, the surface smoothing is observed [14]. Despite the valuable theoretical studies, a common general framework for all the different experimental observations is still missing [6], whereas for some technological applications it is instead mandatory to understand how to effectively tune the experimental parameters to produce on large areas periodic nanostructures with the required dimensions. For this reason, the aim of this work was to see if it was
E. Iacob et al. / Microelectronic Engineering 155 (2016) 50–54
possible to systematically order the knowledge gained with experiments described in literature and derive an empirical model allowing the prediction of the outcome when pushing parameters out of the already explored ranges. In particular, the choice of the experimental parameters controlling IBS on silicon surfaces was considered, in order to obtain periodic nano-ripples with a pre-defined aspect ratio AR = a/ λ ≥ 1/5, where a is the height and λ is the wavelength (period). Ripples of a period of about 50 nm and height a close as much as possible to 10 nm would be potentially useful for nanoelectronics and energy storage applications [15]. For ripples of lower height, different applications have been already envisaged, namely electrochemical devices [16], plasmonics [17], and nanostructured materials for bio-devices [18]. 2. Materials and methods Six different irradiation conditions (see Table 1) were chosen after comparing and integrating many published experimental results, allowing the definition of a “reference empirical schema”, describing how to change the experimental parameters to control the ripple sizes. Among others, references [13,19,20] provided the starting point of the six experiments, as the ripple heights and wavelengths therein discussed were already in ranges close to those expected. The same incident ion species were chosen, i.e. Xe+ and O+, and the empirical model was applied, with the aim to obtain ripples of period λ ≤ 50 nm and height a close as much as possible to 10 nm, therefore modifying the ripple sizes reported in [13,19,20]. Ion irradiations were performed using a low energy ion implanter (Danfysik A/S, Denmark, Model1050) equipped with a gas-fed Chordis ion source. 5 keV Xe+ and 1 keV O+ ion beams, with fluxes of 0.6 × 1013 and 1.1 × 1013 cm−2 s−1 respectively, were electrostatically raster scanned (fx, fy ~ 1 kHz) full area over 1 × 1 cm2 Si(100) samples, which were tilted 50° or 55° with respect to the ion beam. Ion fluences between 8.0 × 1016 and 2.0 × 1018 cm−2 were irradiated on six different samples. A vacuum b 2 × 10−6 mbar was maintained in the implantation chamber. Table 1 gives an overview of all irradiation conditions of investigated samples. All the samples were irradiated at the Institute of Ion Beam Physics and Materials Research at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. Atomic Force Microscopy (AFM) images were acquired with a scanning probe microscope (Unisolver P47H, NT MDT Co.). The analyses were performed in semi-contact mode with a silicon tip having a nominal radius of less than 10 nm. Scan areas of 2 × 2 μm2, 5 × 5 μm2 and 10 × 10 μm2 were acquired with a resolution up to 1024 × 1024 pixels. The 10 × 10 μm2 images (not shown) were used to confirm on a larger spatial scale the hereinafter discussed results. The root mean square (RMS) surface roughness, representing the standard deviation of surface heights, was calculated for each scanned area and used to compare the roughness of the irradiated surfaces. The periodicity of the patterns was evaluated by the discrete two-dimensional Fast Fourier transformation (2D-FFT), as well as by inspecting the cross sections of the AFM topographies, which were taken along the projected irradiation beam direction. The same irradiated samples were also analyzed by field-emission Scanning Electron Microscopy (SEM, Jeol JMS 7401F). Analyses were carried out in plan-view, setting an acceleration voltage of 5 kV. Table 1 The irradiating sample conditions. Sample name
Implanted ion species
energy (keV)
Fluence 1018 (ions/cm−2)
Incidence angle (°)
Xe_1 Xe_2 Xe_3 O_1 O_2 O_3
Xe+ Xe+ Xe+ O+ O+ O+
5 5 5 1 1 1
0.080 1.5 2.0 1.0 2.0 2.0
55 55 55 50 50 55
51
3. Results Fig. 1 shows the AFM topography images acquired on the three Xe+ irradiated samples. For each image, the corresponding 2D-FFT image and a vertical linear profile are shown. Fig. 1A shows the presence of a pattern with an irregular short range correlation and structure heights of about 1–2 nm. On the contrary, the surfaces of Fig. 1B and C show a regime of surface smoothing, since the resulting RMS roughness is comparable with the initial RMS roughness The SEM images are not shown, since the modified surfaces have morphological features below the instrument detection limit. The presence of a regular topography is instead recognizable in all the samples irradiated with O+ ions. In general, following the ion fluence increase from Fig. 2A to C, the analysis shows the development of periodic ripples (with a distribution of wavelengths as described by the 2D-FFT images shown in the insets of Fig. 2), with a nearly constant wavelength of ~40 nm, 50 nm and 60 nm respectively, and corresponding heights of ~ 2.5 nm, 3 nm, and 15 nm. These patterns are superimposed on a less regular topography of longer wavelength and higher amplitude, which can be associated to the long time-scale and non-linear regime proper of the sputtering process [5]. In fact, in Fig. 2C, which corresponds to the highest fluence, the larger wavelength structure definitely dominates on the high-frequency rippled structure, and the cross section clearly shows a sawtooth profile, which is typical of long bombarding times [5,7]. In Fig. 3, the SEM images of the same samples of Fig. 2(A–C) show, on a larger field of view, the regularity of the obtained nano-ripples. In Fig. 3C, which corresponds to the highest fluence, shingle-like faceted surfaces are clearly visible in place of the regular ripples of Fig. 3(A,B), which are indeed typical of lower ion fluences. 4. Discussion Starting from the experimental settings given in [13,19,20], in order to modify the ripple features to values of λ ≤ 50 nm and of a of about 10 nm, the empirical model derived from the literature prescribes values of fluence and incidence angle that move towards a quasilinear experimental regime, so that a weak dependence of λ on irradiation time is expected while the height a increases [5,12]. In addition, the ion energy has to be chosen to have λ values in the required nanometer range, as it is known that at room temperature the ripple wavelength increases with the ion energy [5]. It is generally acknowledged that the surface remains flat up to a critical incidence angle. Some authors also gave evidence that this critical angle shifts to lower values with decreasing ion energy [5], and that the linear instability develops on a faster time scale for increasing incidence angles, leading to an earlier onset of nonlinear effects [5,12]. Therefore, by choosing low values of the ion energy and incidence angle, we aimed at increasing the aspect ratio and at delaying as much as possible the onset of nonlinear features, such as the coarsening of the wavelength, the appearance of large triangular and faceted hillocks or of shingle-like faceted surfaces, and, on longer times, the surface smoothing regime. A different outcome of the Xe+ experiments with respect to the O+ experiments is clearly outlined in Figs. 1 and 2. Because of a large mass difference between Xe and O, the absolute value of the (physical) sputtering rate and its incident angle dependence are different. The oxygen reactivity might also give additional differences in the fabrication process of ripple structures [21]. In Fig. 1 (B–C), the silicon surfaces after Xe+ irradiation show the effects of a long-term non-linear regime. In fact, only for the first ion fluence (Fig. 1A) a pattern was produced having an irregular short range correlation and structure heights of about 1–2 nm. Therefore, in this case the quasi-linear regime probably starts and ends at definitely lower fluence values. In Fig. 2(A–C), showing the results of O+ irradiation, a sufficiently stabilized non-linear regime is present as well, but, for samples of
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Fig. 1. Silicon surfaces after Xe+ irradiation as reported in Table 1. Left: AFM and 2D-FFT images. Right: one corresponding cross section. Sputtering conditions: (A) E = 5 keV, α = 55°, Φ = 8.0 × 1016 ions/cm2; (B) E = 5 keV, α = 55°, Φ = 1.5 × 1018 ions/cm2; (C) E = 5 keV, α = 55°, Φ = 2.0 × 1018 ions/cm2. The arrows indicate the projection of the ion beam direction.
Fig. 2(A–B), which in fact correspond to the first two ion fluences and to the lowest incidence angle, the structures meet the expected AR. Therefore, these samples are in principle suitable to be used in some of the envisaged technological applications. On the other hand, the patterns illustrated in Figs. 2C and 3C show how the increase of the incidence angle accelerated the onset of non-linear effects. In fact, for the same values of energy and fluence of Figs. 2B and 3B, Figs. 2C and 3C describe the appearance of shingle-like faceted surfaces, typical of the long-term ion bombardment. However, also in this case, lower ion fluence values should be tested (given the same values of energy and incidence angle), to possibly reduce the non-linear effects, responsible of the periodic structure of longer wavelength and larger amplitude shown in the cross sections of Fig. 2(A-B). 5. Conclusions The self-organized ripple formation on silicon surfaces induced by low-energy ion sputtering at the nanometer length scale has been
investigated. An empirical recipe has been proposed to produce ripples with desired geometries, trying to gain a stricter control over the sizes of the nano-structures, as sometimes required by technological applications. For the specific purposes here discussed, Xe+ and O+ ions were chosen and different ion fluences at fixed incidence angle and energy were considered. Overall, this work has shown the still-existing difficulty of properly tuning ripple sizes. In the case of O+ irradiation, AFM and SEM analyses showed the development of regular ripples with wavelengths of about 40–50 nm and heights of about 2–3 nm, superimposed on a less regular topography of longer wavelength and higher amplitude. These structures have sizes compatible with those previously discussed in works dealing with the use of nanoripples in some cutting edge technological applications: electrochemical devices [16], plasmonics [17], and nanostructured materials for bio-devices [18]. In both experiments here discussed, obtained results have shown an early achievement of the non-linear regime which indicates the possibility of lowering the ion fluence to obtain better-shaped periodic ripples. These new recipes will be investigated in forthcoming experiments.
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Fig. 2. Silicon surfaces after O+ irradiation as reported in Table 1. Left: AFM and 2D-FFT images. Right: one corresponding cross section. Sputtering conditions (A) E = 1 keV, α = 50°, Φ = 1.0 × 1018 ions/cm2; (B) E = 1 keV, α = 50°, Φ = 2.0 × 1018 ions/cm2; (C) E = 1 keV, α = 55°, Φ = 2.0 × 1018 ions/cm2. The arrows indicate the projection of the ion beam direction.
Fig. 3. SEM images on silicon substrate after O+ irradiation. Sputtering conditions (A) E = 1 keV, α = 50°, Φ = 1.0 × 1018 ions/cm2; (B) E = 1 keV, α = 50°, Φ = 2.0 × 1018 ions/cm2; (C) E = 1 keV, α = 55°, Φ = 2.0 × 1018 ions/cm2.
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