Measurement of latent tracks in amorphous SiO2 using small angle X-ray scattering

Available online at www.sciencedirect.com

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 266 (2008) 2994–2997 www.elsevier.com/locate/nimb

Measurement of latent tracks in amorphous SiO2 using small angle X-ray scattering P. Kluth a,*, C.S. Schnohr a, D.J. Sprouster a, A.P. Byrne b, D.J. Cookson c, M.C. Ridgway a b

a Department of Electronic Materials Engineering, Australian National University, Canberra ACT 0200, Australia Department of Nuclear Physics, Faculty of Physics, Australian National University, Canberra ACT 0200, Australia c Australian Synchrotron Research Program, Building 434, 9700 South Cass Avenue, Argonne, IL 60439, USA

Available online 23 March 2008

Abstract In this paper we present preliminary yet promising results on the measurement of latent ion tracks in amorphous, 2 lm thick SiO2 layers using small angle X-ray scattering (SAXS). The tracks were generated by ion irradiation with 89 MeV Au ions to fluences between 3  1010 and 3  1012 ions/cm2. Transmission SAXS measurements show distinct scattering from the irradiated SiO2 as compared to the unirradiated material. Analysis of the SAXS spectra using a cylindrical model suggests a core–shell like density distribution in the ion ˚ is in tracks with a lower density core and a higher density shell as compared to unirradiated material. The total track radius of 48 A very good agreement with previous experiments and calculations based on an inelastic thermal spike model. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.05.cf; 61.80.Jh; 61.43.Fs Keywords: SiO2; Swift heavy ion irradiation; SAXS; Latent tracks

1. Introduction Swift heavy ion irradiation (SHII) of a solid can leave a trail of permanent damage along the ion path, a so-called latent track. The formation of these tracks is governed by the interaction of the projectile ion with the target electrons and involves a material dependent threshold value of electronic energy loss [1,2]. Tracks have been observed in various crystalline and amorphous materials including semiconductors [3,4], insulators [5–8] and various metals [9,10]. Particular attention has been focused on tracks in amorphous SiO2 (a-SiO2) [11–14] due to its technological relevance and a number of interesting applications, for example in nanofabrication, have been demonstrated. In crystalline substrates, the formation and morphology of tracks (which are generally amorphous with radii 2– 10 nm) can be studied using transmission electron micros*

Corresponding author. Tel.: +61 2 6125 0358; fax: +61 2 6125 0511. E-mail address: [email protected] (P. Kluth).

0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.182

copy [15]. In a-SiO2 no direct measurements of the track dimensions exist up to date. Only indirect measurements using infrared spectroscopy [13] and etching [16,17] have been employed. The latter method takes advantage of a significantly increased etching rate of the SiO2 in the tracks, however, it is critically dependent on the etching conditions and requires the tracks to be continuous. Small angle X-ray scattering (SAXS) has previously been employed for studying tracks in polymers and LiF [18–20]. In this paper we will present preliminary yet promising results on the measurement of latent tracks in amorphous SiO2 by means of SAXS. A detailed study will be presented elsewhere [21]. 2. Experimental For our experiments, we have irradiated 2 lm thin films of thermally grown SiO2 on Si(1 0 0) substrates with 89 MeV 197Au ions to fluences between 3  1010 and 3  1012 ions/cm2. At this energy, the energy loss in the thin

P. Kluth et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2994–2997

Scattering intensity [arbitrary units]

3

2

unirradiated 89 MeV 3×10

1000

11

ions/cm

2

9 8 7 6 5 4 3 2

100 2

3

4

0.01

5

6

7

8 9

2

0.1

-1

q [Å ]

Fig. 2. Recorded SAXS spectra of SiO2 prior to and after irradiation with 89 MeV 197Au to a fluence of 3  1011 ions/cm2.

the unirradiated SiO2 standard was subtracted from all spectra. 3. Results and discussion Fig. 3 shows scattering spectra from samples after background removal for a variety of irradiation fluences. At low fluences (3  1010 and 6  1010 ions/cm2) the scattering scales with the ion fluence or equivalently the number of tracks generated in the SiO2. This indicates that track overlap effects are negligible at these fluences. This is confirmed by an overlap model: d = 1  exp(pR2t) [24], where d is the area of modified material, R is the track radius and t is the ion fluence. The area of track overlap is then estimated as ˚ the overlap pR2t  d. For an estimated track radius of 50 A 10 is less than 0.1% at a fluence of 6  10 ions/cm2. At higher fluences, a qualitative shape change (weakening/disappearance of the oscillations) in the scattering spectra with

Scattering intensity [arbitrary units]

SiO2 layer is approximately constant at 12 keV/nm as estimated by TRIM [22] calculations and virtually entirely due to electronic interactions. For transmission SAXS measurements, the Si substrate was locally removed by mechanical polishing, dimple grinding and subsequent selective wet chemical etching using a KOH solution. Using this method, we can isolate an area of the thin SiO2 film of approximately 1 mm Ø supported by the surrounding Si substrate. This enables transmission SAXS measurements on the thin SiO2 layer only. Furthermore, precision alignment of the X-ray beam with respect to the SiO2 layer is possible. A detailed description of the sample preparation techniques is presented in [23]. The SAXS measurements were performed at the ChemMatCARS beamline 15ID-D at the Advanced Photon Source, Argonne National Laboratories, USA using an X-ray ˚ (10.27 keV) and a camera length of wavelength of 1.1 A 1894 mm. Images were taken with an exposure time of 10 s. An unirradiated SiO2 film was measured as a standard for background removal. Fig. 1 shows SAXS images of an SiO2 layer irradiated to a fluence of 6  1011 ions/cm2 with (a) the X-ray beam incident normal to the sample surface (parallel to the direction of SHI irradiation) and (b) the sample surface tilted by 45° with respect to the X-ray beam. The radially symmetrical image in (a) is consistent with tracks aligned normal to the surface of the SiO2 and as such the scattering contains information on the radial track dimensions only. When the sample is tilted (Fig. 1(b)) narrow streaks are apparent in the image reflecting the comparably ‘‘large” track lengths of 2 lm leading to vanishing of scattering in the vertical at the given camera length. For quantitative analysis samples were aligned such that the X-rays incident normal to the sample surface and images were radially integrated around the beam center. Fig. 2 shows SAXS spectra of SiO2 prior to and after irradiation to a fluence of 3  1011 ions/cm2. The distinct significant scattering originated by the ion irradiation is clearly apparent. AFM measurements (not shown) confirm flat surfaces after irradiation and as such surface scattering can be ruled out. To isolate scattering from the tracks, scattering from

2995

1000

100

10

1 12

3×10

11

6×10

10

0.1

6×10

10

3×10

ions/cm ions/cm ions/cm ions/cm

2 2 2 2

0.01 2

197

Fig. 1. SAXS images of an SiO2 layer irradiated with 89 MeV Au to a fluence of 6  1011 ions/cm2 with (a) the X-ray beam incident normal to the sample surface (parallel to the direction of SHI irradiation) and (b) the sample surface tilted to 45° with respect to the X-ray beam.

3

4

5

6

7

8

-1

9

0.1

2

q [Å ]

Fig. 3. SAXS spectra of irradiated SiO2 after background removal as a function of irradiation fluence.

2996

P. Kluth et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2994–2997

increasing fluence indicates that overlap effects become important. For quantitative analysis we assume a cylindrically symmetrical density distribution in the ion track as consistent with continuous tracks and nearly constant energy loss throughout the SiO2 layer. Furthermore radial symmetry can be assumed in amorphous material. The most appropriate density distribution was found to be that of a core–shell cylinder. The scattering amplitude f(qr) can be derived to [21,25]  Rc f ðqr Þ ¼ 2pL ðq0  qs Þ J 1 ðRc qr Þ qr  ðRc þ Rs Þ þ qs J 1 ððRc þ Rs Þqr Þ ; ð1Þ qr where L is the track length, q0 is the density contrast in the core, qs the density contrast in the shell, Rc the core diameter, Rs the shell thickness, and J1 the Bessel function of first order. The scattering intensity can then be written as   Z R 2 Iðqr Þ / exp  ð2Þ jf ðqr Þj dR 2r with R = Rc + Rs, assuming a narrow distribution of track radii R. Fig. 4 shows the scattering intensity of the sample irradiated with 6  1010 ions/cm2 and the corresponding fit to Eq. (2). Clearly both are in very good agreement. The refined track dimensions yield a core radius Rc = 18 ± ˚ and a shell thickness of Rs = 30 ± 0.5 A ˚ and thus a 0.3 A ˚ . We note that the reported total track radius of R = 48 A uncertainties denote the errors from the non-linear least squares fitting. The total track radius is in very good agree˚ reported by Toulemonde ment with the value of 50 A et al. [14], calculated from an inelastic thermal spike model, extrapolated from experimental values using infrared mea˚ is less than 10% of surements [13]. The fitted r = 3.5±0.3 A the track radius and accounts not only for a narrow distri-

4. Conclusions In conclusion we have demonstrated that SAXS is capable of measuring a previously unresolved fine structure in latent tracks in amorphous SiO2. A core–shell cylinder structure of the tracks is apparent with a lower density core and a higher density shell when compared to unirradiated SiO2. The total extracted track radius is in good agreement with previous experimental and theoretical results and generally supports the validity of an inelastic thermal spike model. However, refinements of existing models are required to explain the observed track fine structure. Our measurements thus show a high potential to reveal new insights into latent track formation and aid in identifying the mechanisms responsible therefore. Furthermore they are not limited to continuous tracks and as such likely to also be suited for determination of threshold energies for track formation. Acknowledgements

100

Scattering intensity [arbitrary units]

bution of radii but also for other deviations to the model of perfectly aligned, monodisperse cylinders including the possibility that the density changes between core and shell and shell and substrate are not perfect step functions as assumed. Probably the most striking feature of the fitted track structure is the opposite sign of the density contrast of the shell compared to the core qs = 0.56 ± 0.05qc. This means that assuming a net compaction of the SiO2 as previously observed by others [11], the track consists of a lower density core and a higher density shell when compared to unirradiated SiO2. This observed track fine structure might indeed fit within the broad framework of previous theoretical descriptions of latent tracks in SiO2 as elastic inclusions formed in a thermal spike [26], however, at this early stage of the investigation no definitive conclusion as to the mechanism responsible for the track fine structure is attempted.

P.K. and M.C.R. thank the Australian Research Council for support. The authors’ were supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. ChemMatCARS Sector 15 at the Advanced Photon Source is principally supported by the National Science Foundation/Department of Energy under Grant No. CHE0087817 and by the Illinois Board of Higher Education. The Advanced Photon Source is supported by the US Department of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-Eng-38.

10

1

0.1 10

6×10 Fit

ions/cm

2

References

0.01 2

3

4

5

6

7

-1

8

9

0.1

q [Å ]

Fig. 4. SAXS spectrum of SiO2 irradiated with 6  1010 ions/cm2 (open triangles) and the corresponding fit to the core–shell model (solid line).

[1] A. Dunlop, D. Lesueur, Radiat. Effects Defects Solids 126 (1993) 123. [2] A. Meftah, J.M. Costantini, N. Khalfaoui, S. Boudjadar, J.P. Stoquert, F. Studer, M. Toulemonde, Nucl. Instr. and Meth. B 237 (2005) 563.

P. Kluth et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2994–2997 [3] M. Levalois, P. Bogdanski, M. Toulemonde, Nucl. Instr. and Meth. B 63 (1992) 14. [4] W. Wesch, A. Kamarou, E. Wendler, Nucl. Instr. and Meth. B 225 (2004) 111. [5] C. Trautmann, M. Toulemonde, K. Schwartz, J.M. Costantini, A. Mueller, Nucl. Instr. and Meth. B 164–165 (2000) 365. [6] M. Toulemonde, E. Balanzat, S. Bouffard, J.C. Jousset, Nucl. Instr. and Meth. B 39 (1989) 1. [7] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Nucl. Instr. and Meth. B 216 (2004) 1. [8] A. Meftah, F. Brisard, J.M. Costantini, E. Dooryhee, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 12457. [9] A. Barbu, A. Dunlop, D. Lesueur, R.S. Averback, Europhys. Lett. 15 (1991) 37. [10] C. Dufour, A. Audouard, F. Beuneu, J. Dural, J.P. Girard, A. Hairie, M. Levalois, E. Paumier, M. Toulemonde, J. Phys.: Condens. Matter 5 (1993) 4573. [11] S. Klaumuenzer, Nucl. Instr. and Meth. B 225 (2004) 136. [12] K. Awazu, S. Ishii, K. Shima, S. Roorda, J.L. Brebner, Phys. Rev. B 62 (2000) 3689. [13] M.C. Busch, A. Slaoui, P. Siffert, E. Dooryhee, M. Toulemonde, J. Appl. Phys. 71 (1992) 2596.

2997

[14] M. Toulemonde, C. Dufour, E. Paumier, F. Pawlak, Mat. Res. Soc. Symp. Proc. 504 (1998) 99. [15] F. Studer, M. Hervieu, J.-M. Costantini, M. Toulemonde, Nucl. Instr. and Meth. B 122 (1997) 449. [16] J. Jensen, A. Razpet, M. Skupinski, G. Possnert, Nucl. Instr. and Meth. B 243 (2006) 119. [17] C. Milanez Silva, P. Varisco, A. Moehlecke, P.P. Fichtner, R.M. Papaleo, J. Eriksson, Nucl. Instr. and Meth. B 206 (2003) 486. [18] D. Albrecht, P. Armbruster, R. Spohr, M. Roth, K. Schaupert, H. Stuhrmann, Appl. Phys. A 37 (1985) 37. [19] C. Trautmann, K. Schwartz, J.M. Costantini, T. Steckenreiter, M. Toulemonde, Nucl. Instr. and Meth. B 146 (1998) 367. [20] Y. Eyal, S.A. Saleh, J. Appl. Cryst. 40 (2007) 71. [21] P. Kluth et al., in preparation. [22] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, Pergamon Press, New York, 1985. [23] R. Giulian, P. Kluth, B. Johannessen, D.J. Cookson, M.C. Ridgway, in: Proceedings of the Fifth International Conference on Synchrotron Radiation in Materials Science, SRMS 5, No. 155, 2006. [24] C. Riedel, R. Spohr, Radiat. Effects 42 (1979) 69–75. [25] A. Guinier, & G. Fournet, Small-Angle Scattering of X-rays, John Wiley, New York, 1955. [26] H. Trinkaus, A.I. Ryazanov, Phys. Rev. Lett. 74 (1995) 5072.