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SiO2–Si under swift heavy ion irradiation: A comparison between normal and grazing incidence features
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2981–2985 www.elsevier.com/locate/nimb
SiO2–Si under swift heavy ion irradiation: A comparison between normal and grazing incidence features Aminata M.J.F. Carvalho a, Antoine D. Touboul a,*, Mathias Marinoni a, Jean-Francßois Carlotti a, Cathy Guasch a, Michel Ramonda b, Henning Lebius c, Fre´de´ric Saigne a, Jacques Bonnet a a
IES, UMR-CNRS 5214, cc082 Universite´ Montpellier 2, 34095 Montpellier Cedex 5, France b LMCP, Universite´ Montpellier 2, 34095 Montpellier Cedex 5, France c CIRIL-GANIL, Rue Claude Bloch, BP 5133, 14070 Caen Cedex 5, France Available online 31 March 2008
Abstract A comparison between experimental results on SiO2–Si under swift heavy ion irradiation at normal and grazing incidence is dealt with. After irradiation, the samples were gradually etched in an aqueous hydrofluoric acid solution. Atomic force microscopy has been used to study the evolution of the tracks regarding their extension and shape. At normal incidence, the latent tracks are revealed by chemical etching. Increasing the etching time, nano-hillocks appeared in the depths of each of the conical holes formed at the beginning of the chemical process. When the SiO2 layer is completely removed, nano-bumps are clearly visible at the interface between the two materials. Under grazing incidence (1° of beam inclination), elongated discontinuous tracks are directly observable after irradiation on the SiO2 surface. While with a sequential chemical etching, the intermittent tracks leave room to lengthy craters of decreasing depth, which disappear when the oxide layer is completely etched-away. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.82.Ms; 61.82. d; 61.80.Jh; 68.37.Ps; 68.47.Gh Keywords: Swift heavy ion irradiation; Silicon dioxide; Normal incidence; Grazing incidence; Nano-hillocks; Elongated tracks
1. Introduction Material damage induced by swift heavy ion irradiation has been intensively studied for many decades. To date, energetic heavy ions are of prime interest in the synthesis and modification of materials [1–5]. From the nanotechnological point of view to ultrathin MOS gate oxide reliability issues, studying SiO2 behavior under swift heavy ion irradiation is crucial for a better understanding of the basic mechanisms of the defect production at the nanometric scale. In this respect, many studies have been devoted, for some decades, to radiation effects on SiO2 because of *
Corresponding author. Tel.: +33 4 67 14 47 41; fax: +33 4 67 52 15 84. E-mail address: [email protected] (A.D. Touboul). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.206
its manifold applications and its paramount role in microelectronics and optoelectronics. Track formation induced by energetic heavy ion bombardment of SiO2 structures at normal incidence have been reported by many authors. Experimental results have shown that a single swift heavy ion can induce a morphological modification in SiO2 [6– 14]. The latent tracks are revealed by a selective chemical etching in hydrofluoric acid (HF). Since the pioneer results of Young concerning the first fission fragment individual track chemically revealed on LiF substrates [15], this technique has been of widespread use in solid state detectors [16–18] and continues to do so when studying radiation defect production in solids [10–12,18–20]. Even if the selective chemical etching is a smart technique to achieve nanostructures on the surface or in the bulk of the irradiated material, it is a destructive method, which does not act only
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A.M.J.F. Carvalho et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2981–2985
through the ion wake, but enlarges towards the surrounding material of the ion path. The purpose of this paper is to compare the evolution of the ion-induced tracks in SiO2–Si structures irradiated under normal and grazing incidence in the electronic excitation regime. The irradiated samples have been etched in an aqueous HF solution and the evolution of the features in both of the cases is reported. 2. Experimental procedure All surface analysis (before and after irradiation) have been done using a Digital 3100 AFM Veeco Instruments working in ambient air conditions and at room temperature in tapping mode. On the one hand, 31.3 nm SiO2 layers thermally grown on (1 1 1) p-type Si substrates have been irradiated at normal incidence at room temperature with a VIVITRON accelerator (Strasbourg, France) delivering 210 MeV Au+ ions under normal incidence, corresponding to electronic linear energy transfer (LETelec) of 16.6 keV/nm in Si and of 18.3 keV/nm in SiO2 according to the SRIM2006 code [21]. To avoid overlap of the conical holes that were revealed after the chemical etching, the fluence of the beam was fixed to 2 109 cm 2, corresponding to an average
areal ion impact density of 20 per lm2. Before irradiation, the root mean square (RMS) oxide surface roughness was typically in the range of 0.2 nm. After irradiation, the measured oxide surface roughness did not exhibit significant change after irradiation. The irradiated samples have been thereafter etched in an aqueous HF solution (5% vol.) for different times (15 s, 25 s, 35 s, 41 s, 51 s, 61 s and 71 s). After etching, the samples were immediately rinsed in deionised water and dried under dry argon. Regarding the grazing incidence experiment, 11.8 nm SiO2 films thermally grown on (1 0 0) p-type Si substrates have been irradiated at room temperature with a 131 MeV 208 Pb32+ ion beam supplied by the IRRSUD facility at GANIL (Caen, France). The samples were titled to an 1° ± 0.4° angle between the sample surface and the beam. The LETelec was about 13.6 keV/nm in Si and 15.5 keV/ nm in SiO2 [21]. The beam fluence was fixed to 5 1010 cm 2, matching with an average areal ion impact density of 8.5 per lm2. This fluence was chosen to give a reasonable ion impacts number and to avoid overlap. As for the normal incidence, samples have been characterized before and after irradiation using a Digital 3100 AFM Veeco Instruments working in ambient air conditions and at room temperature in tapping mode. Subsequently, they were
Fig. 1. Two-dimensional (2D) AFM topographical view of 31.3 nm SiO2 layer on Si substrate irradiated under normal incidence with 210 MeV Au+ ions at a fluence of 2 109 Au/cm2; (a) before etching, (b) after 15 s etching with aqueous HF vol. 5%, (c) after 25 s etching time and (d) after 35 s etching time.
A.M.J.F. Carvalho et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2981–2985
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gradually etched in an aqueous HF solution (5% vol.). The etching times were 10 s, 20 s, 30 s, 40 s, 50 s and 60 s. The samples were immediately washed in deionised water and dried under dry argon before being observed by AFM. 3. Results and discussion After normal incidence irradiation, samples did not exhibit any significant change after irradiation. The RMS roughness (about 0.2 nm) remained constant and no surface tracks have been directly observed at the SiO2 surface (Fig. 1(a)). As reported in the literature [10–13,18–20], the tracks induced in the material were only revealed after proper chemical etching with HF. During the gradual chemical etching, nanometric conical holes of growing diameter have been revealed at each ion impact point (Fig. 1(b)–(d)). A nanodot appeared in the depths of each of the holes as the chemical etching exceeded about 30 s of etching time. Fig. 2 depicts such observation. The conical hillocks became clearly distinguishable at the interface when the SiO2 layer was almost or completely etched-away (Fig. 3). At the end of the chemical process, the average nanodot size was 8 nm high and 20 nm large [22]. In contrast with irradiation performed at normal incidence, when the ion beam was different at grazing incident
Fig. 4. AFM Topographical top view of the 11.8 nm SiO2 coated sample after 208Pb32+ ion irradiation. The beam inclination was 1° ± 0.4°with reference to the plan surface of the samples and the fluence was 5 1010 cm 2. Ions have traversed from bottom to top (white arrow). Elongated trail of nano-hillocks have been formed at the oxide surface as shown in the 3D-picture that refers to the track outlined in white.
angle of 1° relative to the surface plane, elongated surface tracks (about 500 nm long and 2 nm wide) were directly observable at the SiO2 surface (Fig. 4). Their number was
Fig. 2. Three-dimensional (3D) AFM topography (a) of the irradiated surface after 41 s etching time and height profile and (b) that shows the emergence of the nanodots in the depths of the conical holes.
Fig. 3. Three-dimensional (3D) AFM topography (a) of the irradiated surface after 71 s etching time showing the nanodots revealed at the interface when the SiO2 layer is completely etched away. The AFM topographical height profile (b) illustrates the formation of the bumps in loco of the conical holes chemically revealed.
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in good agreement with the expected areal density. Each of these tracks consists of a trail of nano-hillocks formed in parallel to the direction of the ion beam at the impact point on the oxide surface [23]. In addition, the general topographical modification of the surface has been quantified by a roughness analysis showing a RMS roughness increase
from 0.18 nm before irradiation to 0.40 nm after grazing irradiation. When gradually etched afterwards in aqueous HF solution, extended craters have been formed in loco of the elongated surface tracks at the first steps of the chemical process (Fig. 5(a) and (b)). The average crater was 3 nm
Fig. 5. AFM top view of the SiO2–Si sample irradiated under grazing incidence (a) after HF etching during 10 s, (b) after 30 s etching time, (c) after 50 s etching time and (d) after 60 s etching time. In (a) and (b) the elongated tracks visible in Fig. 4 have developed into extended craters of increasing width and decreasing depth. Increasing the etching time, the craters vanished progressively (c) until disappearing completely for about 60 s of etching time (d).
Fig. 6. 3D AFM topographical view of the SiO2–Si sample irradiated under grazing incidence (a) after HF etching during 20 s. The height profile (b) shows a crater formed at this step of the chemical process referenced by the white line in (a). The average crater size is 800 nm long and 3 nm high.
A.M.J.F. Carvalho et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2981–2985
deep and 800 nm long after 20 s etching (Fig. 6). Increasing the etching time, the grooves become less and less visible (Fig. 5(c)) until disappearing completely (Fig. 5(d)) when the SiO2 layer is completely etched away. No residual features have been formed at the interface at the end of the chemical etching. 4. Conclusion SiO2–Si structures have been irradiated under normal and grazing incidence with swift heavy ion. In both the cases, the irradiated samples were gradually etched in an aqueous hydrofluoric acid (HF) solution at room temperature and studied by AFM. Normal incidence impingement has led to the formation of individual nanodots at the interface between the SiO2 layer and the Si substrate when the oxide layer was completed etched away. However, concerning the grazing incidence irradiation, elongated discontinuous tracks were directly observable at the SiO2 surface after irradiation. When subsequently etched in aqueous HF solution, the long surface tracks developed into extended grooves that decreased visibly until disappearing completely when the SiO2 film was completely removed. Hence, through such experiments with swift heavy ion irradiation, we have addressed under normal incidence, the formation of single nanodots at the interface between the two materials. Besides, we have put forward the formation of elongated discontinuous tracks at the oxide surface when the sample is tilted to very grazing incidence. Swift heavy ion irradiation at grazing incidence is therefore a suitable technique to create nanostructures that can be observed directly – by means of near-field microscopy – without any chemical amplification. Owing to a proper chemical etching of these nanostructures, one can form long nanometric trenches in the oxide which size can be controlled by the etching time. Acknowledgements The authors are very grateful to the Grand Acce´le´rateur National d’Ions Lourds (GANIL, Caen-France) for the Pb ion beam supply and to its technical staff who have contributed to the success of this experiment. We are indebted to J.-P. Stoquert from the INESS laboratory for the Au ion
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beam irradiation at the VIVITRON tandem accelerator (Strasbourg-France). References [1] R.G. Musket, J.M. Yoshiyama, R.J. Contolini, J.D. Porter, J. Appl. Phys. 91 (9) (2002) 5760. [2] P.S. Chaudhari, T.M. Bhave, D. Kanjilal, S.V. Bhoraskar, J. Appl. Phys. 93 (2003) 3486. [3] D. Rodichev, P. Lavallard, E. Dooryhe´e, A. Slaoui, J. Perriere, M. Gandais, Y. Wang, Nucl. Instr. and Meth. B 107 (1996) 259. [4] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Nucl. Instr. and Meth. Phys. Res. B 216 (2004) 1. [5] B. Canut, M.G. Blanchin, S. Ramos-Canut, V. Teodorescu, M. Toulemonde, Nucl. Instr. and Meth. B 245 (2006) 327. [6] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [7] A. Meftah, F. Brisard, J.M. Constantini, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 12457. [8] M. Toulemonde, S. Bouffard, F. Studer, Nucl. Instr. and Meth. Phys. Res. B 91 (1994) 108. [9] S. Bouffard, J. Cousty, Y. Pennec, F. Thibaudau, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Radiat. Eff. Def. Solids 126 (1993) 225. [10] N. Khalfaoui, C.C. Rotaru, S. Bouffard, E. Jacquet, H. Lebius, M. Toulemonde, Nucl. Instr. and Meth. Phys. Res. B 209 165 (2003). [11] C.C. Rotaru, Ph.D. Dissertation, University of Caen, 2004 (in French). [12] K. Awazu, S. Shii, K. Shima, S. Rooda, J.L. Brebner, Phys. Rev. B 62 (2000) 3689. [13] J. Jensen, A. Razpet, M. Skupinski, G. Possnert, Nucl. Instr. and Meth. Phys. Res. B 243 (2006) 119. [14] S. Klaumu¨nzer, Nucl. Instr. and Meth. Phys. Res. B 225 (2004) 136. [15] D.A. Young, Nature 182 (1958) 375. [16] R.L. Fleischer, P.B. Price, J. Appl. Phys. 34 (9) (1963) 2903. [17] P.B. Price, R.M. Walker, J. Appl. Phys. 33 (1962) 3407. [18] K. Awazu, S. Shii, K. Shima, S. Rooda, J.L. Brebner, Phys. Rev. B 62 (2000) 3689. [19] J. Jensen, A. Razpet, M. Skupinski, G. Possnert, Nucl. Instr. and Meth. Phys. Res. B 243 (2006) 269. [20] G. Saint Martin, O.A. Bernaola, G. Garcı`a Bermu`dez, Nucl. Instr. and Meth. Phys. Res. B 245 (2006) 274. [21] J.F. Ziegler, J.P. Biersack, U. Littmark (Eds.), The Stopping and Range of Ions in Solids, Pergamon, New York 1985. (SRIM is freely available at http://www.srim.org. The current version is 2006.01). [22] A.D. Touboul, J.-F. Carlotti, M. Marinoni, M. Caussanel, M. Ramonda, C. Guasch, G. Bruguier, J. Bonnet, F. Saigne´, J. Gasiot, J. Non-Cryst. Solids 351 (2005) 3834. [23] A.M.J.F. Carvalho, M. Marinoni, A.D. Touboul, C. Guasch, H. Lebius, M. Ramonda, J. Bonnet, F. Saigne´, Appl. Phys. Lett. 90 (2007) 073116.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2981–2985 www.elsevier.com/locate/nimb
SiO2–Si under swift heavy ion irradiation: A comparison between normal and grazing incidence features Aminata M.J.F. Carvalho a, Antoine D. Touboul a,*, Mathias Marinoni a, Jean-Francßois Carlotti a, Cathy Guasch a, Michel Ramonda b, Henning Lebius c, Fre´de´ric Saigne a, Jacques Bonnet a a
IES, UMR-CNRS 5214, cc082 Universite´ Montpellier 2, 34095 Montpellier Cedex 5, France b LMCP, Universite´ Montpellier 2, 34095 Montpellier Cedex 5, France c CIRIL-GANIL, Rue Claude Bloch, BP 5133, 14070 Caen Cedex 5, France Available online 31 March 2008
Abstract A comparison between experimental results on SiO2–Si under swift heavy ion irradiation at normal and grazing incidence is dealt with. After irradiation, the samples were gradually etched in an aqueous hydrofluoric acid solution. Atomic force microscopy has been used to study the evolution of the tracks regarding their extension and shape. At normal incidence, the latent tracks are revealed by chemical etching. Increasing the etching time, nano-hillocks appeared in the depths of each of the conical holes formed at the beginning of the chemical process. When the SiO2 layer is completely removed, nano-bumps are clearly visible at the interface between the two materials. Under grazing incidence (1° of beam inclination), elongated discontinuous tracks are directly observable after irradiation on the SiO2 surface. While with a sequential chemical etching, the intermittent tracks leave room to lengthy craters of decreasing depth, which disappear when the oxide layer is completely etched-away. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.82.Ms; 61.82. d; 61.80.Jh; 68.37.Ps; 68.47.Gh Keywords: Swift heavy ion irradiation; Silicon dioxide; Normal incidence; Grazing incidence; Nano-hillocks; Elongated tracks
1. Introduction Material damage induced by swift heavy ion irradiation has been intensively studied for many decades. To date, energetic heavy ions are of prime interest in the synthesis and modification of materials [1–5]. From the nanotechnological point of view to ultrathin MOS gate oxide reliability issues, studying SiO2 behavior under swift heavy ion irradiation is crucial for a better understanding of the basic mechanisms of the defect production at the nanometric scale. In this respect, many studies have been devoted, for some decades, to radiation effects on SiO2 because of *
Corresponding author. Tel.: +33 4 67 14 47 41; fax: +33 4 67 52 15 84. E-mail address: [email protected] (A.D. Touboul). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.206
its manifold applications and its paramount role in microelectronics and optoelectronics. Track formation induced by energetic heavy ion bombardment of SiO2 structures at normal incidence have been reported by many authors. Experimental results have shown that a single swift heavy ion can induce a morphological modification in SiO2 [6– 14]. The latent tracks are revealed by a selective chemical etching in hydrofluoric acid (HF). Since the pioneer results of Young concerning the first fission fragment individual track chemically revealed on LiF substrates [15], this technique has been of widespread use in solid state detectors [16–18] and continues to do so when studying radiation defect production in solids [10–12,18–20]. Even if the selective chemical etching is a smart technique to achieve nanostructures on the surface or in the bulk of the irradiated material, it is a destructive method, which does not act only
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A.M.J.F. Carvalho et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2981–2985
through the ion wake, but enlarges towards the surrounding material of the ion path. The purpose of this paper is to compare the evolution of the ion-induced tracks in SiO2–Si structures irradiated under normal and grazing incidence in the electronic excitation regime. The irradiated samples have been etched in an aqueous HF solution and the evolution of the features in both of the cases is reported. 2. Experimental procedure All surface analysis (before and after irradiation) have been done using a Digital 3100 AFM Veeco Instruments working in ambient air conditions and at room temperature in tapping mode. On the one hand, 31.3 nm SiO2 layers thermally grown on (1 1 1) p-type Si substrates have been irradiated at normal incidence at room temperature with a VIVITRON accelerator (Strasbourg, France) delivering 210 MeV Au+ ions under normal incidence, corresponding to electronic linear energy transfer (LETelec) of 16.6 keV/nm in Si and of 18.3 keV/nm in SiO2 according to the SRIM2006 code [21]. To avoid overlap of the conical holes that were revealed after the chemical etching, the fluence of the beam was fixed to 2 109 cm 2, corresponding to an average
areal ion impact density of 20 per lm2. Before irradiation, the root mean square (RMS) oxide surface roughness was typically in the range of 0.2 nm. After irradiation, the measured oxide surface roughness did not exhibit significant change after irradiation. The irradiated samples have been thereafter etched in an aqueous HF solution (5% vol.) for different times (15 s, 25 s, 35 s, 41 s, 51 s, 61 s and 71 s). After etching, the samples were immediately rinsed in deionised water and dried under dry argon. Regarding the grazing incidence experiment, 11.8 nm SiO2 films thermally grown on (1 0 0) p-type Si substrates have been irradiated at room temperature with a 131 MeV 208 Pb32+ ion beam supplied by the IRRSUD facility at GANIL (Caen, France). The samples were titled to an 1° ± 0.4° angle between the sample surface and the beam. The LETelec was about 13.6 keV/nm in Si and 15.5 keV/ nm in SiO2 [21]. The beam fluence was fixed to 5 1010 cm 2, matching with an average areal ion impact density of 8.5 per lm2. This fluence was chosen to give a reasonable ion impacts number and to avoid overlap. As for the normal incidence, samples have been characterized before and after irradiation using a Digital 3100 AFM Veeco Instruments working in ambient air conditions and at room temperature in tapping mode. Subsequently, they were
Fig. 1. Two-dimensional (2D) AFM topographical view of 31.3 nm SiO2 layer on Si substrate irradiated under normal incidence with 210 MeV Au+ ions at a fluence of 2 109 Au/cm2; (a) before etching, (b) after 15 s etching with aqueous HF vol. 5%, (c) after 25 s etching time and (d) after 35 s etching time.
A.M.J.F. Carvalho et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2981–2985
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gradually etched in an aqueous HF solution (5% vol.). The etching times were 10 s, 20 s, 30 s, 40 s, 50 s and 60 s. The samples were immediately washed in deionised water and dried under dry argon before being observed by AFM. 3. Results and discussion After normal incidence irradiation, samples did not exhibit any significant change after irradiation. The RMS roughness (about 0.2 nm) remained constant and no surface tracks have been directly observed at the SiO2 surface (Fig. 1(a)). As reported in the literature [10–13,18–20], the tracks induced in the material were only revealed after proper chemical etching with HF. During the gradual chemical etching, nanometric conical holes of growing diameter have been revealed at each ion impact point (Fig. 1(b)–(d)). A nanodot appeared in the depths of each of the holes as the chemical etching exceeded about 30 s of etching time. Fig. 2 depicts such observation. The conical hillocks became clearly distinguishable at the interface when the SiO2 layer was almost or completely etched-away (Fig. 3). At the end of the chemical process, the average nanodot size was 8 nm high and 20 nm large [22]. In contrast with irradiation performed at normal incidence, when the ion beam was different at grazing incident
Fig. 4. AFM Topographical top view of the 11.8 nm SiO2 coated sample after 208Pb32+ ion irradiation. The beam inclination was 1° ± 0.4°with reference to the plan surface of the samples and the fluence was 5 1010 cm 2. Ions have traversed from bottom to top (white arrow). Elongated trail of nano-hillocks have been formed at the oxide surface as shown in the 3D-picture that refers to the track outlined in white.
angle of 1° relative to the surface plane, elongated surface tracks (about 500 nm long and 2 nm wide) were directly observable at the SiO2 surface (Fig. 4). Their number was
Fig. 2. Three-dimensional (3D) AFM topography (a) of the irradiated surface after 41 s etching time and height profile and (b) that shows the emergence of the nanodots in the depths of the conical holes.
Fig. 3. Three-dimensional (3D) AFM topography (a) of the irradiated surface after 71 s etching time showing the nanodots revealed at the interface when the SiO2 layer is completely etched away. The AFM topographical height profile (b) illustrates the formation of the bumps in loco of the conical holes chemically revealed.
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in good agreement with the expected areal density. Each of these tracks consists of a trail of nano-hillocks formed in parallel to the direction of the ion beam at the impact point on the oxide surface [23]. In addition, the general topographical modification of the surface has been quantified by a roughness analysis showing a RMS roughness increase
from 0.18 nm before irradiation to 0.40 nm after grazing irradiation. When gradually etched afterwards in aqueous HF solution, extended craters have been formed in loco of the elongated surface tracks at the first steps of the chemical process (Fig. 5(a) and (b)). The average crater was 3 nm
Fig. 5. AFM top view of the SiO2–Si sample irradiated under grazing incidence (a) after HF etching during 10 s, (b) after 30 s etching time, (c) after 50 s etching time and (d) after 60 s etching time. In (a) and (b) the elongated tracks visible in Fig. 4 have developed into extended craters of increasing width and decreasing depth. Increasing the etching time, the craters vanished progressively (c) until disappearing completely for about 60 s of etching time (d).
Fig. 6. 3D AFM topographical view of the SiO2–Si sample irradiated under grazing incidence (a) after HF etching during 20 s. The height profile (b) shows a crater formed at this step of the chemical process referenced by the white line in (a). The average crater size is 800 nm long and 3 nm high.
A.M.J.F. Carvalho et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2981–2985
deep and 800 nm long after 20 s etching (Fig. 6). Increasing the etching time, the grooves become less and less visible (Fig. 5(c)) until disappearing completely (Fig. 5(d)) when the SiO2 layer is completely etched away. No residual features have been formed at the interface at the end of the chemical etching. 4. Conclusion SiO2–Si structures have been irradiated under normal and grazing incidence with swift heavy ion. In both the cases, the irradiated samples were gradually etched in an aqueous hydrofluoric acid (HF) solution at room temperature and studied by AFM. Normal incidence impingement has led to the formation of individual nanodots at the interface between the SiO2 layer and the Si substrate when the oxide layer was completed etched away. However, concerning the grazing incidence irradiation, elongated discontinuous tracks were directly observable at the SiO2 surface after irradiation. When subsequently etched in aqueous HF solution, the long surface tracks developed into extended grooves that decreased visibly until disappearing completely when the SiO2 film was completely removed. Hence, through such experiments with swift heavy ion irradiation, we have addressed under normal incidence, the formation of single nanodots at the interface between the two materials. Besides, we have put forward the formation of elongated discontinuous tracks at the oxide surface when the sample is tilted to very grazing incidence. Swift heavy ion irradiation at grazing incidence is therefore a suitable technique to create nanostructures that can be observed directly – by means of near-field microscopy – without any chemical amplification. Owing to a proper chemical etching of these nanostructures, one can form long nanometric trenches in the oxide which size can be controlled by the etching time. Acknowledgements The authors are very grateful to the Grand Acce´le´rateur National d’Ions Lourds (GANIL, Caen-France) for the Pb ion beam supply and to its technical staff who have contributed to the success of this experiment. We are indebted to J.-P. Stoquert from the INESS laboratory for the Au ion
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