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Surface patterning by heavy ion lithography using self-assembled colloidal masks
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 257 (2007) 777–781 www.elsevier.com/locate/nimb
Surface patterning by heavy ion lithography using self-assembled colloidal masks M. Skupin´ski a, R. Sanz b, J. Jensen b
a,*
a Department of Engineering Sciences, The A˚ngstro¨m Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas, Cantoblanco, 28049 Madrid, Spain
Available online 19 January 2007
Abstract Heavy ion lithography using self-assembled colloidal particles as a mask enables micro- and nano-patterning of surfaces. The resulting patterns can be tuned by varying the mask configuration, i.e. packing geometry of the colloidal particles and number of particle layers. In this work we present several patterns, which can be transferred to rutile TiO2 single crystals by irradiating through self-assembled layers of silica micro-spheres with 25 MeV Br ions. As the induced ion tracks in TiO2 have a very high etching selectivity the patterns can be developed in HF with very high contrast. This makes it possible to prepare large patterned areas which can be of interest for e.g. optical applications. 2007 Elsevier B.V. All rights reserved. PACS: 61.72.Ff; 61.80.Jh; 81.16. c; 81.16.Nd; 81.65.Cf; 82.70.Dd Keywords: Ion tracks; TiO2; Ion lithography; Ion irradiation; Colloidal mask; Nano-patterning
1. Introduction Ion tracks induced in matter by MeV ions may result in very localized material transformation and thus yield one of the highest contrasts of all irradiation technologies [1,2]. A lithography technique that can harness ion tracks may thus transfer exact micro- or nano-patterns, providing high aspect ratios and making patterned material modifications of technological relevance. Fabricating periodic micro- and nano-structures is of great interest because of their potential applications in photonic crystals, data storage and biological sensors [3,4]. As a lithographic mask, self-assembled materials with nano- or micro-scale features can be used. The major challenges are to make a mask of high enough absorption and still of high resolution, to make it stable during irradiation and when it is of high aspect ratio to align it with respect to the ion beam. One example of a self-assembled template is
*
Corresponding author. Fax: +46 18 555736. E-mail address: [email protected] (J. Jensen).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.099
the porous anodic alumina membrane [3,4]. Only a few studies on pattern transfer using ion track lithography have until now been undertaken with this type of mask [5–7]. We have recently shown ion track pattern transfer to amorphous SiO2 through a mask of porous alumina resulting in closely packed features down to 40 nm in size at 100 nm nearest centre-to-centre distance [5]. In single crystalline rutile TiO2 we have shown the possibility of high aspect ratio structures by this method [6]. Another kind of mask that has received much attention due to its potential technological application consists of micro to nanometre size colloidal particles [3,4]. With this kind of mask a hexagonally closed packed monolayer or multilayers of colloidal particles is formed on a surface and the open interstices between the particles can be used as mask openings in lithography [8,9]. There have only been a few investigations on ion beam-induced modification of the underlying substrate after high-energy irradiation through a single colloidal layer [10]. Swift heavy ion irradiation through colloidal layers has, to our knowledge, not been used previously to ‘sculpture’ the underlying substrate into analogue patterns. However, several studies
M. Skupin´ski et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 777–781
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have been performed on the response of colloidal particles to MeV ions. Under exposure to MeV ions colloidal particles can undergo extreme deformations induced by the deposited energy, leading to the modification of their shape [9,11,12]. Here we present results on ion track formation and pattern transfer into rutile TiO2 after MeV ion irradiation through self-assembled colloidal masks of silica microspheres. TiO2 is an oxide semiconductor with a wide band gap and one of the highest dielectric constants, both suitable characteristics in electronic and optical applications [13]. In addition, the surface of crystalline TiO2 has interesting properties as a photo catalyst in some chemical reactions [14]. A periodic array of dielectric media such as TiO2 with micro to submicron repeated features have been investigated for photonic components such as waveguides [15,16]. A variation in refractive index between neighbouring dielectric regions is necessary to govern the propagation of light in these photonic crystal structures. Many techniques have been used to fabricate high-contrast periodic dielectric arrays, i.e. photolithography [15]. In photolithography the precision of the structures is, however, limited by the wavelength and the coherence of the light source. As ion tracks in TiO2 has a very high etching selectivity [16,17], the induced damage can be developed in HF with very high contrast. This makes it possible to prepare well-ordered arrays or patterns of TiO2 with variation in refractive index. Furthermore large surface areas and highly active sites useful in photo catalytic work can be fabricated.
self-assembling process the sample was tilted by 20, making the evaporation process start from the upper parts of the samples [18]. This resulted in the build-up of a monolayer of hexagonally ordered micro-spheres. However, the layer is not perfect as dislocation and unordered spots are present in addition to quadrangular ordered microspheres, see Fig. 1(a). Also some parts of the sample are covered with two and more monolayers. Irradiations of the samples were carried out at the Tandem Laboratory, Uppsala University, with Br ions delivered from the 5 MV tandem accelerator. The irradiations were performed at room temperature under normal incidence with respect to the sample surface. The ion fluences were in the range 1.0 · 1013–1.0 · 1014 ions/cm2 and homogeneous irradiation over an area of 30 · 30 mm2 was achieved by means of an electrostatic raster scanner. The energy used in this study and main irradiation parameters are listed in Table 1. SRIM-2003 simulations [19] indicate that the ions completely pass through the silica spheres. To remove the micro-sphere layers from the surface after irradiation the samples were immersed in ethanol and kept for 5 min in an ultrasound bath. Thereafter the samples were cleaned in a 1:1 H2SO4:H2O2 solution for one minute and rinsed with deionised water. Subsequently the samples were etched in a 20% HF solution for 35 min. After etching the samples were cleaned in deionised water. The surfaces of the samples were investigated before and
2. Experimental procedure Rutile single crystals TiO2 (Crystal Gmbh) with (1 0 0) direction were used as substrates. Samples were exposed to UV irradiation for about 1 hour to make the surface hydrophilic, which is important for subsequent mask placement. After UV exposure a droplet of a colloidal suspension was deposited on the TiO2 surface by means of a micropipette. The colloidal suspension, containing water and silica spheres with a mean diameter of 1.57 ± 0.06 lm, was purchased from Duke Scientific Corporation. To slow down the evaporation process – important for homogeneity of the colloid layer – samples left for drying were covered with a glass beaker and to enhance the
Fig. 1. Scanning electron micrographs of TiO2 samples: (a) with one monolayer of unirradiated silica spheres on the surface; (b) showing a HF etched surface after irradiation with 25 MeV Br ion at a fluence of 1.0 · 1014 ions/cm2 through one monolayer of silica spheres; (c) as (b), but sample tilted 30.
Table 1 Characteristics of the Br ion irradiation used in this study. For the chosen initial ion energy, the range in electronic (dE/dx)e and nuclear (dE/dx)n stopping powers within the 1.5 lm silica spheres and in TiO2 after passing a single silica sphere are shown. Also shown are the maximum thickness of silica which an ion can penetrate and still have an energy above the track etching threshold in TiO2 (ds) and the range of etchable depth in TiO2 after penetrating a single silica sphere (de). The values were calculated using the SRIM-2003 code [19], with a density of the silica spheres and TiO2 set to q = 2.0 g/cm3 and 4.25 g/ cm3, respectively Silica spheres Ion energy (MeV) 25
(dE/dx)e (keV/nm) 5.2–4.4
TiO2 (dE/dx)n (keV/nm) 0.07–0.096
ds (lm) 3.1
(dE/dx)e (keV/nm) 10.3–8.5
(dE/dx)n (keV/nm) 0.14–0.18
de (lm) 1.6–0.9
M. Skupin´ski et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 777–781
after etching by a LEO 1550 FEG high resolution scanning electron microscope (SEM).
3. Results and discussion Rutile TiO2 single crystals are not etchable by hydrofluoric acid (HF) prior to swift heavy-ion irradiation. Ion track studies on TiO2 suggest that the threshold in electronic stopping power for highly selective chemical etching of irradiated areas on rutile single crystals is 6.2 keV/nm [16,17]. No etching was observed for electronic stopping powers below this value. These studies furthermore indicated that the irradiated area consist of amorphous and stressed rutile phases. It is thus the amorphous region and the stressed lattice region created by swift heavy ions with energies above the threshold, which are highly soluble in HF. Fig. 1(b) and (c) show one kind of observed pattern on a TiO2 surface after etching a sample irradiated through a colloidal mask with 25 MeV Br ions at a fluence of 1.0 · 1014 cm 2. The pattern is the result of irradiating through one monolayer of hexagonally ordered silica
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spheres. Large areas (several hundreds of lm2) of ordered structures were obtained. When seen from above, the diameter of the observed round structures is similar to that of the silica spheres. Tilting the sample in the microscope the structures look like TiO2 spheres lying on the surface. However, the SEM images are difficult to interpret due to e.g. shadow effects. From a simple simulation the shape should be a semisphere, with a height of 0.7–0.8 lm (see Fig. 2(e)). This can be understood by the fact that the projected mass distribution of the silica sphere resembles a semi-sphere. All ions going through a monolayer of silica spheres deposit enough energy in TiO2 to induce etchable damage. Nevertheless, the deposited energy gradually decreases from the centre of the silica spheres (see Table 1). Besides the hexagonal ordered structures seen in Fig. 1(b) and (c), we also observed more complex structures. In Fig. 2(a) a gradual change in the etched structures is seen from left to right. The pattern changes from relatively well ordered ‘round’ structures (left), over features looking like flowers with four petals (centre), to features resembling flowers with six petals (right). Tilting the sample in the microscope it becomes easier to see that the struc-
Fig. 2. (a–d) Scanning electron micrographs of a HF etched TiO2 sample following irradiation with 25 MeV Br ion at a fluence of 1.0 · 1014 ions/cm2 through different configurations (from left to right) of the colloidal mask; (b)–(d) corresponding to the left, middle and right regions shown in (a), viewed with the sample tilted 30; (e) Simulation showing the resulting pattern after irradiating through one monolayer of hexagonal ordered silica spheres (cf. Fig. 1(c)); (f) Simulation showing the resulting pattern after irradiating through two monolayers of quadrangular ordered silica spheres (see text); (g) Simulation showing the resulting pattern after irradiating through two monolayers of hexagonal ordered silica spheres. All length scales in the simulations are given in lm. Note that the simulations have been slightly tilted.
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M. Skupin´ski et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 777–781
tures have a three dimensional appearance. Especially the flowerlike features have a protruding character, Fig. 2(c) and (d). In Fig. 3(a) we show an area which looks like a hexagonally ordered array of holes with a diameter of 300 nm. The depth of these holes is less than 1 lm. What is not visible here is that the surface is not flat, but has a bumpy appearance. This pattern is very similar to the well-ordered pattern of an anodic alumina mask we have transferred to TiO2 using nanolithography [6]. Using high aspect ratio alumina masks, mapping by Rutherford Backscattering Spectrometry is used to align the mask with the ion beam [5,6]. However, for irradiation through a selfassembled pattern of silica micro-spheres no special beam alignment is needed. The reason for the patterns shown in Figs. 2(a) and 3(a) is that there are areas on the samples where the mask consists of more than one monolayer of silica spheres. The structures that are obtained after irradiating through one monolayer with 25 MeV ions are relative straightforward to understand. However, to help understanding the com-
Fig. 3. (a) Scanning electron micrograph of a HF etched TiO2 sample following irradiation with 25 MeV Br ion at a fluence of 1.0 · 1014 ions/ cm2 through colloidal layers of silica spheres. Note the ’defect’ visible in the lower part (due to a missing silica sphere in the mask); (b) simulation showing contour of the resulting pattern after irradiating through three monolayers of hexagonal ordered silica spheres (see text). All length scales are in lm.
plex structures, which can be obtained when ions pass through several monolayers of micro-spheres, we did simple simulations of the possible etchable patterns using the SRIM-2003 [19] and MATLAB software [20]. First, the lateral distribution of silica material for a given geometric configuration of spheres, which the ions penetrate, was obtained with MATLAB. We assume contact between all the spheres. Then, using the SRIM-2003 code, the energy loss, DE, of an ion with a certain initial energy penetrating a given amount of silica material was calculated. The evaluated DE-values were subtracted from the initial ion energy. In this way the lateral distribution of ion energies hitting the TiO2 surface after penetrating the chosen geometric configuration of silica spheres was obtained. For this ion energy distribution we then calculated with SRIM-2003 the distribution in ion ranges in TiO2 for which the energy stays above the proposed threshold for chemical etching of ion tracks [16,17]. There is no etching in pristine TiO2 and in addition no etching is observed when the ion energy is below the threshold value [6,16,17]. The result of this calculation are contour plots showing the lateral distributions of etchable depth. Figs. 2(e)–(g) and 3(b) show simulations explaining the pattern seen in Figs. 2(b)–(d) and 3(a), respectively. The pattern seen to the left of Fig. 2(a) (i.e. Fig. 2(b) and also in Fig. 1(c)) results from ions going through one monolayer of hexagonally ordered silica spheres. This is shown by the simulation in Fig. 2(e), which also indicated that the resulting structures look like semi-spheres that resemble the projection mass distribution of the mask. The middle part of Fig. 2(a) (i.e. Fig. 2(c)) is understood by the simulation shown in Fig. 2(f). The pattern of protrusions looking like a flower with four petals can be reproduced by assuming a mask consisting of two layers of quadrangular ordered silica spheres. That is, in the first layer the spheres are packed in a square arrangement, whereas the spheres of the second layer are placed in the interstices of the first layer. The right part of Fig. 2(a) (i.e. Fig. 2(d)) is explained by the simulation shown in Fig. 2(g), which reproduces the protruding pattern of a flower with six petals. Here the mask consists of two hexagonally ordered monolayers. Spheres in the first layer are placed as in Fig. 2(e) and spheres in the second layer are placed above alternate interstices in the first layer. Fig. 3(b) shows a simulation that reproduces the pattern seen in Fig. 3(a). The calculation was done by assuming that the mask consisted of three layers of hexagonally ordered silica spheres. The two first layers are arranged in the same way as in Fig. 2(g). The spheres in the third layer are placed directly above those interstices in the first layer that were not covered with spheres in the second layer. The bumpy appearance between the holes, which is seen in the SEM, is also seen in the simulations. The colloidal particles can be arranged in other configurations leading to other patterns not shown here. In addition, the colloidal mask is in some places not perfect leading to patterns which are more difficult to simulate.
M. Skupin´ski et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 777–781
That is, gap between spheres, the boundary between two mask configurations and also missing spheres act as ‘defect’ zones (see the Figs. 2(a) and 3(a)). It should be noted that the maximum etchable depth in TiO2 not only depends on the ion energy. The applied fluence also plays a role [16,17], as the etching properties depend on how large a fraction of the irradiated area is covered by induced amorphous and stressed lattice regions. This will influence the resulting etched patterns. For this reason, we applied a fluence (1.0 · 1014 ions/cm2) where a saturation value in etchable depth seems to have been reached [16,17]. We checked this by irradiating TiO2 samples covered with copper grids (used for transmission electron microscopy) with Br ions of the used energy, applying fluencies in the interval 1 · 1013–1 · 1014 ions/cm2. For 25 MeV Br ions a maximum depth of 2 lm was obtained, which is more than the 1.6 lm given by a simple threshold estimate (see Table 1). This indicates a lower etching threshold. Indications of a lower etching threshold were also observed when irradiating TiO2 through an anodic alumina mask [6]. On the other hand, as the deformation of the silica spheres has been observed to be energy, temperature and fluence dependent [9] and as changing the shape of silica spheres influences the transferred pattern, we wanted to keep the applied fluence as low as possible. Before removing the silica spheres from the irradiated TiO2 surfaces, the samples were thus investigated by SEM to check if the silica spheres were subjected to any deformation. For the used fluence of 1.0 · 1014 cm 2 we only saw a small change in shape of the silica spheres. Experiments are in progress in order to study the effect of the initial ion energy and also the applied ion fluence on the transferred pattern.
4. Conclusion and outlook By using heavy ion lithography in combination with monolayers of self-ordered silica spheres we have transferred well-ordered patterns to surfaces of rutile TiO2 single crystals. The shape of the patterns which can be obtained depends on the geometry and number of layers of the colloidal mask, as the observed structures are replicas of the mass distribution of the mask. The present combination of ion irradiation through a colloidal mask and etching is a precise method with a very high resolution, capable of being applied as a micro- and nano-fabrication method. Especially the use of nano-spheres will allow us to create even smaller features. Positioning colloidal particles in desired patterns [21,22] is a feasible way of creating desired micro- and nano-patterns over large surface areas. Together with the possibility of ion beam shaping of the colloidal mask constituents, either by ion-hammering [9,22] or ion etching [23], interesting new patterns may be possible. The patterned surface is believed to have interesting optical properties, due to a var-
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iation in refractive index, which will be studied in a forthcoming paper. Acknowledgments We are grateful to the staff at the Tandem Laboratory, Uppsala University, for valuable technical assistance. Dis˚ ngstro¨m Laboratory, Uppcussions with K. Hjort, The A sala University, are gratefully acknowledged. We thank N. Darwish for help with the MATLAB code. References [1] R. Spohr, in: Ion Tracks and Microtechnology, Principles and Applications, Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig, 1990. [2] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Nucl. Instr. and Meth. B 216 (2004) 1. [3] H.S. Nalwa, Handbook of Nanostructures Materials and Nanotechnology, Elsevier, Amsterdam, 1999. [4] B. Bhushan (Ed.), Handbook of Nanotechnology, Springer Verlag, 2004. [5] A. Razpet, A. Johansson, G. Possnert, M. Skupin´ski, K. Hjort, A. Halle´n, J. Appl. Phys. 97 (2005) 44310. [6] R. Sanz, A. Johansson, M. Skupin´ski, J. Jensen, G. Possnert, M. Boman, M. Va´zquez, K. Hjort, NanoLetter 6 (2006) 1065. [7] M. Skupin´ski, J. Jensen, A. Johansson, A. Razpet, G. Possnert, M. Boman, K. Hjort, Journal of Vacuum Science and Technology, submitted for publication. [8] F. Burmeister, W. Badowsky, T. Braun, S. Wieprich, J. Boneberg, P. Leiderer, Appl. Surf. Sci. 144–145 (1999) 461. [9] T. van Dillen, A. van Blaaderen, A. Polman, Mater. Today 7–8 (2004) 40, and references therein. [10] C. Strohho¨fer, J.P. Hoogenboom, A. van Blaaderen, A. Polman, Adv. Mater. 14 (2002) 1815. [11] T. van Dillen, E. Snoeks, W. Fukarek, C.M. van Kats, K.P. Velikov, A. van Blaaderen, A. Polman, Nucl. Instr. and Meth. B 175–177 (2001) 350. [12] J.C. Cheang-Wong, U. Morales, A. Oliver, L. Rodrı´guez-Ferna´ndez, J. Rickards, Nucl. Instr. and Meth. B 242 (2006) 452. [13] K. Rajeshwar, N.R. de Tacconi, C.R. Chenthamarakshan, Chem. Mater. 13 (2001) 2765. [14] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, NanoLetter 5 (2005) 191. [15] A. Shishido, I.B. Diviliansky, I.C. Khoo, T.S. Mayer, S. Nishimura, G.L. Egan, T.E. Mallouk, App. Phys. Lett. 79 (2001) 3332, and references therein. [16] K. Awazu, M. Fujimaki, Y. Ohki, T. Komatsubara, Rad. Measure. 40 (2005) 722. [17] K. Nomura, T. Nakanishi, Y. Nagasawa, Y. Ohki, K. Awazu, M. Fujimaki, N. Kobayashi, S. Ishii, K. Shima, Phys. Rev. B 68 (2003) 064106. [18] R. Micheletto, H. Fukuda, M. Ohtsu, Langmuir 11 (1995) 3333. [19] J.F. Ziegler, J.R. Biersack, U. Littmark, The Stopping and Ranges of Ions in Matter, vol. 1, Plenum Press, New York, 1985; J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027. [20] MATLAB 6.5.1 (Release 13) The language of technical computing. The Mathworks, Inc., 3 Apple Hill Derive, Natick, MA 017 60-2098, USA, 2003. [21] J.P. Hoogenboom, D.L.J. Vossen, C. Faivre-Moskalenko, M. Dogterom, A. van Blaaderen, Appl. Phys. Lett. 80 (2002) 4828. [22] D.L.J. Vossen, D. Fific, J. Penninkhof, T. van Dillen, A. Polman, A. van Blaaderen, NanoLetters 5 (2005) 1175. [23] Y.B. Zheng, S.J. Wang, A.C.H. Huan, Y.H. Wang, J. Appl. Phys. 99 (2006) 034308.
Nuclear Instruments and Methods in Physics Research B 257 (2007) 777–781 www.elsevier.com/locate/nimb
Surface patterning by heavy ion lithography using self-assembled colloidal masks M. Skupin´ski a, R. Sanz b, J. Jensen b
a,*
a Department of Engineering Sciences, The A˚ngstro¨m Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas, Cantoblanco, 28049 Madrid, Spain
Available online 19 January 2007
Abstract Heavy ion lithography using self-assembled colloidal particles as a mask enables micro- and nano-patterning of surfaces. The resulting patterns can be tuned by varying the mask configuration, i.e. packing geometry of the colloidal particles and number of particle layers. In this work we present several patterns, which can be transferred to rutile TiO2 single crystals by irradiating through self-assembled layers of silica micro-spheres with 25 MeV Br ions. As the induced ion tracks in TiO2 have a very high etching selectivity the patterns can be developed in HF with very high contrast. This makes it possible to prepare large patterned areas which can be of interest for e.g. optical applications. 2007 Elsevier B.V. All rights reserved. PACS: 61.72.Ff; 61.80.Jh; 81.16. c; 81.16.Nd; 81.65.Cf; 82.70.Dd Keywords: Ion tracks; TiO2; Ion lithography; Ion irradiation; Colloidal mask; Nano-patterning
1. Introduction Ion tracks induced in matter by MeV ions may result in very localized material transformation and thus yield one of the highest contrasts of all irradiation technologies [1,2]. A lithography technique that can harness ion tracks may thus transfer exact micro- or nano-patterns, providing high aspect ratios and making patterned material modifications of technological relevance. Fabricating periodic micro- and nano-structures is of great interest because of their potential applications in photonic crystals, data storage and biological sensors [3,4]. As a lithographic mask, self-assembled materials with nano- or micro-scale features can be used. The major challenges are to make a mask of high enough absorption and still of high resolution, to make it stable during irradiation and when it is of high aspect ratio to align it with respect to the ion beam. One example of a self-assembled template is
*
Corresponding author. Fax: +46 18 555736. E-mail address: [email protected] (J. Jensen).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.099
the porous anodic alumina membrane [3,4]. Only a few studies on pattern transfer using ion track lithography have until now been undertaken with this type of mask [5–7]. We have recently shown ion track pattern transfer to amorphous SiO2 through a mask of porous alumina resulting in closely packed features down to 40 nm in size at 100 nm nearest centre-to-centre distance [5]. In single crystalline rutile TiO2 we have shown the possibility of high aspect ratio structures by this method [6]. Another kind of mask that has received much attention due to its potential technological application consists of micro to nanometre size colloidal particles [3,4]. With this kind of mask a hexagonally closed packed monolayer or multilayers of colloidal particles is formed on a surface and the open interstices between the particles can be used as mask openings in lithography [8,9]. There have only been a few investigations on ion beam-induced modification of the underlying substrate after high-energy irradiation through a single colloidal layer [10]. Swift heavy ion irradiation through colloidal layers has, to our knowledge, not been used previously to ‘sculpture’ the underlying substrate into analogue patterns. However, several studies
M. Skupin´ski et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 777–781
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have been performed on the response of colloidal particles to MeV ions. Under exposure to MeV ions colloidal particles can undergo extreme deformations induced by the deposited energy, leading to the modification of their shape [9,11,12]. Here we present results on ion track formation and pattern transfer into rutile TiO2 after MeV ion irradiation through self-assembled colloidal masks of silica microspheres. TiO2 is an oxide semiconductor with a wide band gap and one of the highest dielectric constants, both suitable characteristics in electronic and optical applications [13]. In addition, the surface of crystalline TiO2 has interesting properties as a photo catalyst in some chemical reactions [14]. A periodic array of dielectric media such as TiO2 with micro to submicron repeated features have been investigated for photonic components such as waveguides [15,16]. A variation in refractive index between neighbouring dielectric regions is necessary to govern the propagation of light in these photonic crystal structures. Many techniques have been used to fabricate high-contrast periodic dielectric arrays, i.e. photolithography [15]. In photolithography the precision of the structures is, however, limited by the wavelength and the coherence of the light source. As ion tracks in TiO2 has a very high etching selectivity [16,17], the induced damage can be developed in HF with very high contrast. This makes it possible to prepare well-ordered arrays or patterns of TiO2 with variation in refractive index. Furthermore large surface areas and highly active sites useful in photo catalytic work can be fabricated.
self-assembling process the sample was tilted by 20, making the evaporation process start from the upper parts of the samples [18]. This resulted in the build-up of a monolayer of hexagonally ordered micro-spheres. However, the layer is not perfect as dislocation and unordered spots are present in addition to quadrangular ordered microspheres, see Fig. 1(a). Also some parts of the sample are covered with two and more monolayers. Irradiations of the samples were carried out at the Tandem Laboratory, Uppsala University, with Br ions delivered from the 5 MV tandem accelerator. The irradiations were performed at room temperature under normal incidence with respect to the sample surface. The ion fluences were in the range 1.0 · 1013–1.0 · 1014 ions/cm2 and homogeneous irradiation over an area of 30 · 30 mm2 was achieved by means of an electrostatic raster scanner. The energy used in this study and main irradiation parameters are listed in Table 1. SRIM-2003 simulations [19] indicate that the ions completely pass through the silica spheres. To remove the micro-sphere layers from the surface after irradiation the samples were immersed in ethanol and kept for 5 min in an ultrasound bath. Thereafter the samples were cleaned in a 1:1 H2SO4:H2O2 solution for one minute and rinsed with deionised water. Subsequently the samples were etched in a 20% HF solution for 35 min. After etching the samples were cleaned in deionised water. The surfaces of the samples were investigated before and
2. Experimental procedure Rutile single crystals TiO2 (Crystal Gmbh) with (1 0 0) direction were used as substrates. Samples were exposed to UV irradiation for about 1 hour to make the surface hydrophilic, which is important for subsequent mask placement. After UV exposure a droplet of a colloidal suspension was deposited on the TiO2 surface by means of a micropipette. The colloidal suspension, containing water and silica spheres with a mean diameter of 1.57 ± 0.06 lm, was purchased from Duke Scientific Corporation. To slow down the evaporation process – important for homogeneity of the colloid layer – samples left for drying were covered with a glass beaker and to enhance the
Fig. 1. Scanning electron micrographs of TiO2 samples: (a) with one monolayer of unirradiated silica spheres on the surface; (b) showing a HF etched surface after irradiation with 25 MeV Br ion at a fluence of 1.0 · 1014 ions/cm2 through one monolayer of silica spheres; (c) as (b), but sample tilted 30.
Table 1 Characteristics of the Br ion irradiation used in this study. For the chosen initial ion energy, the range in electronic (dE/dx)e and nuclear (dE/dx)n stopping powers within the 1.5 lm silica spheres and in TiO2 after passing a single silica sphere are shown. Also shown are the maximum thickness of silica which an ion can penetrate and still have an energy above the track etching threshold in TiO2 (ds) and the range of etchable depth in TiO2 after penetrating a single silica sphere (de). The values were calculated using the SRIM-2003 code [19], with a density of the silica spheres and TiO2 set to q = 2.0 g/cm3 and 4.25 g/ cm3, respectively Silica spheres Ion energy (MeV) 25
(dE/dx)e (keV/nm) 5.2–4.4
TiO2 (dE/dx)n (keV/nm) 0.07–0.096
ds (lm) 3.1
(dE/dx)e (keV/nm) 10.3–8.5
(dE/dx)n (keV/nm) 0.14–0.18
de (lm) 1.6–0.9
M. Skupin´ski et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 777–781
after etching by a LEO 1550 FEG high resolution scanning electron microscope (SEM).
3. Results and discussion Rutile TiO2 single crystals are not etchable by hydrofluoric acid (HF) prior to swift heavy-ion irradiation. Ion track studies on TiO2 suggest that the threshold in electronic stopping power for highly selective chemical etching of irradiated areas on rutile single crystals is 6.2 keV/nm [16,17]. No etching was observed for electronic stopping powers below this value. These studies furthermore indicated that the irradiated area consist of amorphous and stressed rutile phases. It is thus the amorphous region and the stressed lattice region created by swift heavy ions with energies above the threshold, which are highly soluble in HF. Fig. 1(b) and (c) show one kind of observed pattern on a TiO2 surface after etching a sample irradiated through a colloidal mask with 25 MeV Br ions at a fluence of 1.0 · 1014 cm 2. The pattern is the result of irradiating through one monolayer of hexagonally ordered silica
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spheres. Large areas (several hundreds of lm2) of ordered structures were obtained. When seen from above, the diameter of the observed round structures is similar to that of the silica spheres. Tilting the sample in the microscope the structures look like TiO2 spheres lying on the surface. However, the SEM images are difficult to interpret due to e.g. shadow effects. From a simple simulation the shape should be a semisphere, with a height of 0.7–0.8 lm (see Fig. 2(e)). This can be understood by the fact that the projected mass distribution of the silica sphere resembles a semi-sphere. All ions going through a monolayer of silica spheres deposit enough energy in TiO2 to induce etchable damage. Nevertheless, the deposited energy gradually decreases from the centre of the silica spheres (see Table 1). Besides the hexagonal ordered structures seen in Fig. 1(b) and (c), we also observed more complex structures. In Fig. 2(a) a gradual change in the etched structures is seen from left to right. The pattern changes from relatively well ordered ‘round’ structures (left), over features looking like flowers with four petals (centre), to features resembling flowers with six petals (right). Tilting the sample in the microscope it becomes easier to see that the struc-
Fig. 2. (a–d) Scanning electron micrographs of a HF etched TiO2 sample following irradiation with 25 MeV Br ion at a fluence of 1.0 · 1014 ions/cm2 through different configurations (from left to right) of the colloidal mask; (b)–(d) corresponding to the left, middle and right regions shown in (a), viewed with the sample tilted 30; (e) Simulation showing the resulting pattern after irradiating through one monolayer of hexagonal ordered silica spheres (cf. Fig. 1(c)); (f) Simulation showing the resulting pattern after irradiating through two monolayers of quadrangular ordered silica spheres (see text); (g) Simulation showing the resulting pattern after irradiating through two monolayers of hexagonal ordered silica spheres. All length scales in the simulations are given in lm. Note that the simulations have been slightly tilted.
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tures have a three dimensional appearance. Especially the flowerlike features have a protruding character, Fig. 2(c) and (d). In Fig. 3(a) we show an area which looks like a hexagonally ordered array of holes with a diameter of 300 nm. The depth of these holes is less than 1 lm. What is not visible here is that the surface is not flat, but has a bumpy appearance. This pattern is very similar to the well-ordered pattern of an anodic alumina mask we have transferred to TiO2 using nanolithography [6]. Using high aspect ratio alumina masks, mapping by Rutherford Backscattering Spectrometry is used to align the mask with the ion beam [5,6]. However, for irradiation through a selfassembled pattern of silica micro-spheres no special beam alignment is needed. The reason for the patterns shown in Figs. 2(a) and 3(a) is that there are areas on the samples where the mask consists of more than one monolayer of silica spheres. The structures that are obtained after irradiating through one monolayer with 25 MeV ions are relative straightforward to understand. However, to help understanding the com-
Fig. 3. (a) Scanning electron micrograph of a HF etched TiO2 sample following irradiation with 25 MeV Br ion at a fluence of 1.0 · 1014 ions/ cm2 through colloidal layers of silica spheres. Note the ’defect’ visible in the lower part (due to a missing silica sphere in the mask); (b) simulation showing contour of the resulting pattern after irradiating through three monolayers of hexagonal ordered silica spheres (see text). All length scales are in lm.
plex structures, which can be obtained when ions pass through several monolayers of micro-spheres, we did simple simulations of the possible etchable patterns using the SRIM-2003 [19] and MATLAB software [20]. First, the lateral distribution of silica material for a given geometric configuration of spheres, which the ions penetrate, was obtained with MATLAB. We assume contact between all the spheres. Then, using the SRIM-2003 code, the energy loss, DE, of an ion with a certain initial energy penetrating a given amount of silica material was calculated. The evaluated DE-values were subtracted from the initial ion energy. In this way the lateral distribution of ion energies hitting the TiO2 surface after penetrating the chosen geometric configuration of silica spheres was obtained. For this ion energy distribution we then calculated with SRIM-2003 the distribution in ion ranges in TiO2 for which the energy stays above the proposed threshold for chemical etching of ion tracks [16,17]. There is no etching in pristine TiO2 and in addition no etching is observed when the ion energy is below the threshold value [6,16,17]. The result of this calculation are contour plots showing the lateral distributions of etchable depth. Figs. 2(e)–(g) and 3(b) show simulations explaining the pattern seen in Figs. 2(b)–(d) and 3(a), respectively. The pattern seen to the left of Fig. 2(a) (i.e. Fig. 2(b) and also in Fig. 1(c)) results from ions going through one monolayer of hexagonally ordered silica spheres. This is shown by the simulation in Fig. 2(e), which also indicated that the resulting structures look like semi-spheres that resemble the projection mass distribution of the mask. The middle part of Fig. 2(a) (i.e. Fig. 2(c)) is understood by the simulation shown in Fig. 2(f). The pattern of protrusions looking like a flower with four petals can be reproduced by assuming a mask consisting of two layers of quadrangular ordered silica spheres. That is, in the first layer the spheres are packed in a square arrangement, whereas the spheres of the second layer are placed in the interstices of the first layer. The right part of Fig. 2(a) (i.e. Fig. 2(d)) is explained by the simulation shown in Fig. 2(g), which reproduces the protruding pattern of a flower with six petals. Here the mask consists of two hexagonally ordered monolayers. Spheres in the first layer are placed as in Fig. 2(e) and spheres in the second layer are placed above alternate interstices in the first layer. Fig. 3(b) shows a simulation that reproduces the pattern seen in Fig. 3(a). The calculation was done by assuming that the mask consisted of three layers of hexagonally ordered silica spheres. The two first layers are arranged in the same way as in Fig. 2(g). The spheres in the third layer are placed directly above those interstices in the first layer that were not covered with spheres in the second layer. The bumpy appearance between the holes, which is seen in the SEM, is also seen in the simulations. The colloidal particles can be arranged in other configurations leading to other patterns not shown here. In addition, the colloidal mask is in some places not perfect leading to patterns which are more difficult to simulate.
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That is, gap between spheres, the boundary between two mask configurations and also missing spheres act as ‘defect’ zones (see the Figs. 2(a) and 3(a)). It should be noted that the maximum etchable depth in TiO2 not only depends on the ion energy. The applied fluence also plays a role [16,17], as the etching properties depend on how large a fraction of the irradiated area is covered by induced amorphous and stressed lattice regions. This will influence the resulting etched patterns. For this reason, we applied a fluence (1.0 · 1014 ions/cm2) where a saturation value in etchable depth seems to have been reached [16,17]. We checked this by irradiating TiO2 samples covered with copper grids (used for transmission electron microscopy) with Br ions of the used energy, applying fluencies in the interval 1 · 1013–1 · 1014 ions/cm2. For 25 MeV Br ions a maximum depth of 2 lm was obtained, which is more than the 1.6 lm given by a simple threshold estimate (see Table 1). This indicates a lower etching threshold. Indications of a lower etching threshold were also observed when irradiating TiO2 through an anodic alumina mask [6]. On the other hand, as the deformation of the silica spheres has been observed to be energy, temperature and fluence dependent [9] and as changing the shape of silica spheres influences the transferred pattern, we wanted to keep the applied fluence as low as possible. Before removing the silica spheres from the irradiated TiO2 surfaces, the samples were thus investigated by SEM to check if the silica spheres were subjected to any deformation. For the used fluence of 1.0 · 1014 cm 2 we only saw a small change in shape of the silica spheres. Experiments are in progress in order to study the effect of the initial ion energy and also the applied ion fluence on the transferred pattern.
4. Conclusion and outlook By using heavy ion lithography in combination with monolayers of self-ordered silica spheres we have transferred well-ordered patterns to surfaces of rutile TiO2 single crystals. The shape of the patterns which can be obtained depends on the geometry and number of layers of the colloidal mask, as the observed structures are replicas of the mass distribution of the mask. The present combination of ion irradiation through a colloidal mask and etching is a precise method with a very high resolution, capable of being applied as a micro- and nano-fabrication method. Especially the use of nano-spheres will allow us to create even smaller features. Positioning colloidal particles in desired patterns [21,22] is a feasible way of creating desired micro- and nano-patterns over large surface areas. Together with the possibility of ion beam shaping of the colloidal mask constituents, either by ion-hammering [9,22] or ion etching [23], interesting new patterns may be possible. The patterned surface is believed to have interesting optical properties, due to a var-
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iation in refractive index, which will be studied in a forthcoming paper. Acknowledgments We are grateful to the staff at the Tandem Laboratory, Uppsala University, for valuable technical assistance. Dis˚ ngstro¨m Laboratory, Uppcussions with K. Hjort, The A sala University, are gratefully acknowledged. We thank N. Darwish for help with the MATLAB code. References [1] R. Spohr, in: Ion Tracks and Microtechnology, Principles and Applications, Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig, 1990. [2] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Nucl. Instr. and Meth. B 216 (2004) 1. [3] H.S. Nalwa, Handbook of Nanostructures Materials and Nanotechnology, Elsevier, Amsterdam, 1999. [4] B. Bhushan (Ed.), Handbook of Nanotechnology, Springer Verlag, 2004. [5] A. Razpet, A. Johansson, G. Possnert, M. Skupin´ski, K. Hjort, A. Halle´n, J. Appl. Phys. 97 (2005) 44310. [6] R. Sanz, A. Johansson, M. Skupin´ski, J. Jensen, G. Possnert, M. Boman, M. Va´zquez, K. Hjort, NanoLetter 6 (2006) 1065. [7] M. Skupin´ski, J. Jensen, A. Johansson, A. Razpet, G. Possnert, M. Boman, K. Hjort, Journal of Vacuum Science and Technology, submitted for publication. [8] F. Burmeister, W. Badowsky, T. Braun, S. Wieprich, J. Boneberg, P. Leiderer, Appl. Surf. Sci. 144–145 (1999) 461. [9] T. van Dillen, A. van Blaaderen, A. Polman, Mater. Today 7–8 (2004) 40, and references therein. [10] C. Strohho¨fer, J.P. Hoogenboom, A. van Blaaderen, A. Polman, Adv. Mater. 14 (2002) 1815. [11] T. van Dillen, E. Snoeks, W. Fukarek, C.M. van Kats, K.P. Velikov, A. van Blaaderen, A. Polman, Nucl. Instr. and Meth. B 175–177 (2001) 350. [12] J.C. Cheang-Wong, U. Morales, A. Oliver, L. Rodrı´guez-Ferna´ndez, J. Rickards, Nucl. Instr. and Meth. B 242 (2006) 452. [13] K. Rajeshwar, N.R. de Tacconi, C.R. Chenthamarakshan, Chem. Mater. 13 (2001) 2765. [14] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, NanoLetter 5 (2005) 191. [15] A. Shishido, I.B. Diviliansky, I.C. Khoo, T.S. Mayer, S. Nishimura, G.L. Egan, T.E. Mallouk, App. Phys. Lett. 79 (2001) 3332, and references therein. [16] K. Awazu, M. Fujimaki, Y. Ohki, T. Komatsubara, Rad. Measure. 40 (2005) 722. [17] K. Nomura, T. Nakanishi, Y. Nagasawa, Y. Ohki, K. Awazu, M. Fujimaki, N. Kobayashi, S. Ishii, K. Shima, Phys. Rev. B 68 (2003) 064106. [18] R. Micheletto, H. Fukuda, M. Ohtsu, Langmuir 11 (1995) 3333. [19] J.F. Ziegler, J.R. Biersack, U. Littmark, The Stopping and Ranges of Ions in Matter, vol. 1, Plenum Press, New York, 1985; J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027. [20] MATLAB 6.5.1 (Release 13) The language of technical computing. The Mathworks, Inc., 3 Apple Hill Derive, Natick, MA 017 60-2098, USA, 2003. [21] J.P. Hoogenboom, D.L.J. Vossen, C. Faivre-Moskalenko, M. Dogterom, A. van Blaaderen, Appl. Phys. Lett. 80 (2002) 4828. [22] D.L.J. Vossen, D. Fific, J. Penninkhof, T. van Dillen, A. Polman, A. van Blaaderen, NanoLetters 5 (2005) 1175. [23] Y.B. Zheng, S.J. Wang, A.C.H. Huan, Y.H. Wang, J. Appl. Phys. 99 (2006) 034308.