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Large-area silica nanotubes with controllable geometry on silicon substrates
Applied Surface Science 255 (2009) 3563–3566
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Large-area silica nanotubes with controllable geometry on silicon substrates Mingzhe Hu a,b,*, Rong Yu a, Judith L. MacManus-Driscoll a, Adam P. Robinson a a b
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Faculty of Electronic Science and Technology, Hubei University, Xueyuan Road, Wuhan 430062, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 July 2008 Received in revised form 31 August 2008 Accepted 1 September 2008 Available online 14 October 2008
The synthesis of a highly uniform, large-scale nanoarrays consisting of silica nanotubes above embedded nanohole arrays in silicon substrates is demonstrated. In situ anodized aluminium oxide (AAO) thin film masks on Si substrates were employed, and the nanotubes were fabricated by Ar ion milling through the masks. The geometries of the nanoarrays, including pore diameter, interpore distance and the length of both nanopores and nanotubes could be controlled by the process parameters, which included that the outer pore diameter of silica tube was tuned from 80 nm to 135 nm while the inner tube diameter from 40 nm to 65 nm, the interpore distance of the nanotube arrays was from 100 nm to 180 nm and the length of silica tube changed from 90 nm to 250 nm. The presented nanostructure fabrication method has strong potential for application in intensity and frequency adjustable high luminescence efficiency optoelectronic devices. ß 2008 Elsevier B.V. All rights reserved.
PACS: 81.16.Rf 81.16. c Keywords: Silica nanotubes AAO thin film Ion beam technology
1. Introduction Large-scale nanopatterns embedded in indirect bandgap silicon substrates have been greeted with great research interest recently [1,2]. Since the quantum confinement effect caused by these nanofeatures (<100 nm) increases the bandgap of silicon and results in an increased probability of carrier recombination and, consequently, an enhanced luminescence efficiency [3]. Additionally, a nanopore or nanowire structure has a significantly larger specific surface area than the surface of a polished silicon wafer. These advantages have propelled an explosion of work focused on the potential applications of nanopatterned Si and SiO2 in optoelectronic devices, in catalyst production and in chemical sensors [4–8]. A great deal of effort has been devoted to the synthesis of nanoarray structures in silicon, such as pulsed laser deposition [9], thermal evaporation [10], HF-based solutions chemical etching [11] or electrochemical anodization [12,13]. However the main challenges related to the fabrication of such arrays are poor size distribution, non-uniform pore or wire diameter, and low array density. Although recently ordered nanoarray structure have been produced in silicon by HF electrochemical anodization, the necessary electron beam litho-
* Corresponding author at: Faculty of Electronic Science and Technology, Hubei University, Xueyuan Road, Wuhan 430062, PR China. E-mail address: [email protected] (M. Hu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.09.084
graphy (EBL) patterning process makes the device processing expensive and time consuming [14]. By contrast, here we propose that the use of nanoporous AAO (a well-known hard template) combined with a dry etching process could be an inexpensive and simple solution to these problems. The pores of AAO templates are uniform over large areas, are able to self-organize into ordered honeycomb arrays perpendicular to the substrate, and the pore geometry (pore diameter, density and length) is highly controllable [15]. Thus, it is an ideal technology for the fabrication of size and spacing controllable nanopatterns on Si substrates, which consequently, allows possible precise control of the luminescence efficiency and emission strength and frequency of optoelectronic devices [16]. In order to carry out such patterning, it is necessary to either place thin AAO templates on the surface of the substrate to be patterned or to produce the template in situ. However, owing to the brittle nature of thin AAO templates, the former option would prove difficult. In the present paper, we focus on the production of large-scale novel nanostructures, consisting of silica nanotubes on silicon, by combining Ar ion milling through AAO template masks anodized in situ, and demonstrate controllable geometries of SiOx nanotubes by using different synthesis parameters. The process has the advantage over wet chemical etching methods in that it has a more uniform distribution and better control of nanopatterning [17]. We demonstrate the fabrication of large-area arrays of silica nanotubes with geometries controlled by the anodizing conditions.
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2. Experimental procedure Fig. 1 shows a schematic flow chart of the fabrication process. Aluminium thin films were deposited onto n-type (1 0 0) Si wafers using dc sputtering followed by a two-step anodization process [15] which was carried out using a Keithley2400 source-meter controlled by a Labview based program, using 0.3 M oxalic acid as the electrolyte. Unless indicated otherwise, the anodization voltage used was 40 V, which is the most commonly employed for oxalic acid solution anodizations. The conditions produce regular pores with ordering by self assembly. The interpore distance is 100 nm and the pore diameter is approximately 50 nm. Anodizing voltages greater than 40 V were employed to alter the geometry of the arrays, and will be indicated where used. The oxide formed in the first step was etched in 2%CrO3 + 5%H3PO4 at 60 8C for 10 min, leaving indentations in the underlying aluminium at the base of the pores. The second anodization was continued down to the underlying silicon substrate, forming silica nanodots at the base of the AAO pores. In order to remove the AAO barrier layer and to control the diameter of the pores, a 5%H3PO4 aqueous solution was used to etch the samples. After anodization, pattern transfer was carried out by Ar ion milling through the template and into the underlying silicon. An Ar+ + 2%O2 plasma with 20 mA beam current (4.85 mA/cm2 beam current density) and 500 V beam voltage was employed. After pattern transfer, the template was removed using aqueous NaOH solution. The microstructures of the surfaces and cross sections of the samples were observed using JEOL-6, LEO SEM and a Tecnai F20 TEM. EDX measurements were performed using an Oxford Instrument 7426 EDS system. The nanopore geometry and distribution was analyzed using the UTHSCSA Image Tool 3.0 software. The step height of silicon film after milling was measured using a Dektak profilometer.
Fig. 1. Schematic flow chart of the fabrication process, (a–c) anodization and (d–f) pattern transfer and template removal.
3. Results and discussion In order to carry out pattern transfer with precise control over the milled pattern, the ion milling rates of (a) a bare silicon wafer,
Fig. 2. Micostructure of silica nanotubes on Si produced at 40 V template anodizing voltage. SEM images of the surface (a) and oblique angle view (b) of the nanotubes. (c) TEM cross section image of silica nanotubes and (d) a thinner section of the same sample.
M. Hu et al. / Applied Surface Science 255 (2009) 3563–3566
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Fig. 3. (a) Diameters of pores in AAO templates and of transfer patterned silica pores and (b) full width half maximum (FWHM) of the transferred pore diameter distribution as a function of AAO pore widening time.
(b) an AAO template and (c) a silicon wafer through an AAO template were determined by both profilometery and SEM studies. The values found were 35 nm/min, 40 nm/min and 31 nm/ min 2 nm, respectively. Knowing these values it was possible to mill the silica structures without removal of the template. It should be noted that the milling rate through the AAO templates is dependent on the template pore aspect ratio. However, the dependence was found to be relatively weak below an aspect ratio of 10, and all samples studied here were prepared under these conditions. Fig. 2 shows SEM images of arrays of silica nanotube arrays produced by pattern transfer after Ar ion milling for 10 min through a 500 nm thick AAO template anodized at 40 V, and pore widened for 60 min, giving an AAO pore size of 65 nm. Fig. 2(a) shows an SEM image of the surface of the silica nanotube array, with a pore distribution similar to that observed in the AAO template indicative of a successful pattern transfer. Continuous and uniform coverage was observed over the entire anodized area of 0.5 cm2. Fig. 2(b) shows an oblique angle SEM image of the same sample. The nanotube structure of the silica features can be seen on the surface of the silicon substrate. Fig. 2(c) shows a cross-sectional TEM image of the silica nanotube structure on top of the underlying silicon substrate. The tubes have a length of 80– 90 nm and an outer diameter of approximately 80–90 nm, 10– 20 nm less than the interpore distance. Quantification by electron energy loss spectroscopy (EELS) indicated an Si:O ratio of approximately 1:2 for the walls of the nanotubes. Fig. 2(d) shows a thinner section of the same sample. The silica tubes are milled away but the
pores beneath the silica tubes are emphasized more clearly. The pore channel in the silicon substrate is tapered with a trapezoidal shape, and is believed to be caused by a shadowing effect during pattern transfer [18] which results in a build-up of milled material in the AAO pores during milling causing the diameter of the pores to decrease with the ion milling time. And, with this trapezoidal shape, the aspect-ratio of the silica nanotube was calculated using the mean value of the top and bottom diameter of the silica tubes, and was found to be 1.5 in the present case, which, however, could be adjusted in the range of 1.5–4 by controlling the processing parameters, such as ion milling time, AAO thin film thickness and its pore diameter. To determine to what extent the silica tube diameter can be controlled by the template microstructure, the diameter of the transferred pattern as a function of pore diameter of the AAO template was studied. A template with a thickness of 300 nm was fabricated by anodizing at 40 V. It was then fractured into several pieces and each pore-widened in 5 wt% H3PO4 at 20 8C for a different length of time before carrying out pattern transfer. Fig. 3(a) shows a plot of AAO pore diameter as a function of pore widening time, along with a plot of the pore size of the transferred pattern as a function of the same variable. For AAO templates anodized at 40 V the pore widening rate vs. etching time follows the relationship PD (nm) = 36 + 0.45TH, where TH is the etching time in 5 wt% H3PO4 in minutes [20]. The average diameter of the transferred pattern follows a similar linear gradient to that observed in the AAO template. However an offset of approximately
Fig. 4. (a and b) SEM images of the surface of silica nanotube arrays produced by AAO anodization at 57 V and 76 V, respectively (both pore widened for 60 min). (c) Plot of the silica nanotube dimensions as a function of anodizing voltage.
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20 nm is observed. This decrease in the transferred pore diameter can be ascribed to a shadowing effect of the template during pattern transfer. Fig. 3(b) shows a plot of the full width half maximum (FWHM) of the transferred pattern pore diameter as a function of template pore widening time. As expected, the distribution width of the transferred pore size is observed to narrow as the AAO pore diameter increases. The geometry of the silica array can also be controlled by altering the voltage used to anodize the template. It is well known that both the interpore distance (ID) and pore diameter (PD) of AAO templates vary linearly with anodizing voltage [18]. In bulk, the parameters are ID (nm) = 2.5 V and PD (nm) = 1.29 V [19]. In a previous investigation, we showed that for anodization of thin (<1 mm) aluminium films on silicon substrates, ID and PD vary with anodizing voltage as ID (nm) = 15.8 + 2.17 V and PD (nm) = 0.905 V respectively [20]. Fig. 4 shows silica nanotube arrays can be produced with different geometries produced by milling through templates of different geometries. Fig. 4(a and b) shows SEM images of the surface of arrays produced using (a) an AAO template anodized at 57 V pore widened for 60 min, yielding an interpore distance of 140 nm and pore diameter of 80 nm, and (b) using an AAO template produced at 76 V and pore widened for 60 min, yielding an interpore distance of 180 nm and a pore diameter of 95 nm. For both array spacings highly uniform coverage was observed over the entire area of 0.5 cm2. Comparing these with a sample made under the same conditions but using a 40 V anodizing voltage (Fig. 2(a)) shows that the increasing anodizing voltage not only increases the interpore distance but also increases both the diameter and length of the nanotubes. Fig. 4(c) shows a plot of the dimensions for the silica nanostructures produced using AAO templates anodized at 40 V, 57 V and 76 V, determined by analysis of both TEM and SEM images. Each template was pore widened for 60 min. The inner and outer diameter and the tube length are shown to increase with increasing anodizing voltage. 4. Conclusion A new pattern transfer technique using AAO template masks combined with ion milling has been demonstrated for the synthesis of geometry controllable nanoarray patterns on silicon
substrates. The geometry of the nanostructures may be precisely controlled by altering the processing parameters such as applied anodization potential, H3PO4 etching time (TH) and ion milling time. The interpore distance of the nanotube arrays was tuned from 100 nm to 180 nm, the tube length from 90 nm to 250 nm, the outer tube diameter from 80 nm to 135 nm and the inner tube diameter from 40 nm to 65 nm. The average pore diameter of the nanotube arrays after pattern transfer is approximately 20 nm less then the diameter of the AAO templates. Acknowledgements The authors would like to thank the European Commission for the financial support of the Marie Curie Excellence Grant NanoFen contract number EXT- 014156. The same gratification is also indebted by the author to the financial support from China Scholarship Council (CSC) under granted number [G]2005842133. This work reflects only the views of the author and not the views of the European Commission or the University of Cambridge. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
X.W. Du, Y. Jin, N.Q. Zhao, Y.S. Fu, S.A. Kulinich, Appl. Surf. Sci. 254 (2008) 2479. S.G. Cloutier, P.A. Kossyrev, X. Jimmy, Nat. Mater. 4 (2005) 887. A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909. D. Almawlawi, K.A. Bosnick, A. Osika, M. Moskovits, Adv. Mater. 12 (2002) 1252. L. Tian, R.K. Bhargava, I. Ahmad, L. Menon, M. Holtz, J. Appl. Phys. 97 (2005) 026101. Q.P. Wu, D.Z. Li, Z.X. Chen, X.Z. Fu, Photochem. Photobiol. Sci. 5 (2006) 653. D. Riabinina, C. Durand, F. Rosei, M. Chaker, Phys. Stat. Sol. 204 (2007) 1623. N.S. Subramanian, R.V. Sabaapathy, R. Sriram, B. Santhi, Proc. ISSS, 2005, Bangalore, India, SC92-8. D.Q. Yang, V. Ethier, E. Sacher, M. Meunier, J. Appl. Phys. 98 (2005) 024310. N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev. B 58 (1998) R16024. H. Asoh, A. Oide, S. Ono, Appl. Surf. Sci. 252 (2005) 1668. D.A. Kima, S.I. Imb, C.M. Whanga, W.S. Choa, Y.C. Yooa, N.H. Choa, Appl. Surf. Sci. 230 (2004) 125. H.W. Lau, G.J. Parker, R. Greef, M. Holling, Appl. Phys. Lett. 67 (1995) 1877. S. Borini, A.M. Rossi, L. Boarino, G. Amato, Mater. Res. Soc. Symp. Proc. 872 (2002) J13.9.1. H. Masuda, K. Fukuda, Science 268 (1995) 1466. H. Asoh, M. Matsuo, M. Yoshihama, Appl. Phys. Lett. 83 (2003) 4408. J. Zhou, Q.X. Yuan, T.L. Wen, J.H.S. Bethanie, Appl. Phys. Lett. 89 (2006) 093106. M. Nakao, S. Oku, T. Tamamura, K. Yasui, H. Masuda, Jpn. J. Appl. Phys. 38 (1999) 1052. A.P. Li, F. Mu¨ller, A. Birner, K. Nielsch, U. Gosele, J. Appl. Phys. 84 (1998) 6023. P.R. Adam, B. Gavin, H. Mingzhe, L.M.D. Judith, Appl. Phys. Lett. 91 (2007) 143123.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Large-area silica nanotubes with controllable geometry on silicon substrates Mingzhe Hu a,b,*, Rong Yu a, Judith L. MacManus-Driscoll a, Adam P. Robinson a a b
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Faculty of Electronic Science and Technology, Hubei University, Xueyuan Road, Wuhan 430062, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 July 2008 Received in revised form 31 August 2008 Accepted 1 September 2008 Available online 14 October 2008
The synthesis of a highly uniform, large-scale nanoarrays consisting of silica nanotubes above embedded nanohole arrays in silicon substrates is demonstrated. In situ anodized aluminium oxide (AAO) thin film masks on Si substrates were employed, and the nanotubes were fabricated by Ar ion milling through the masks. The geometries of the nanoarrays, including pore diameter, interpore distance and the length of both nanopores and nanotubes could be controlled by the process parameters, which included that the outer pore diameter of silica tube was tuned from 80 nm to 135 nm while the inner tube diameter from 40 nm to 65 nm, the interpore distance of the nanotube arrays was from 100 nm to 180 nm and the length of silica tube changed from 90 nm to 250 nm. The presented nanostructure fabrication method has strong potential for application in intensity and frequency adjustable high luminescence efficiency optoelectronic devices. ß 2008 Elsevier B.V. All rights reserved.
PACS: 81.16.Rf 81.16. c Keywords: Silica nanotubes AAO thin film Ion beam technology
1. Introduction Large-scale nanopatterns embedded in indirect bandgap silicon substrates have been greeted with great research interest recently [1,2]. Since the quantum confinement effect caused by these nanofeatures (<100 nm) increases the bandgap of silicon and results in an increased probability of carrier recombination and, consequently, an enhanced luminescence efficiency [3]. Additionally, a nanopore or nanowire structure has a significantly larger specific surface area than the surface of a polished silicon wafer. These advantages have propelled an explosion of work focused on the potential applications of nanopatterned Si and SiO2 in optoelectronic devices, in catalyst production and in chemical sensors [4–8]. A great deal of effort has been devoted to the synthesis of nanoarray structures in silicon, such as pulsed laser deposition [9], thermal evaporation [10], HF-based solutions chemical etching [11] or electrochemical anodization [12,13]. However the main challenges related to the fabrication of such arrays are poor size distribution, non-uniform pore or wire diameter, and low array density. Although recently ordered nanoarray structure have been produced in silicon by HF electrochemical anodization, the necessary electron beam litho-
* Corresponding author at: Faculty of Electronic Science and Technology, Hubei University, Xueyuan Road, Wuhan 430062, PR China. E-mail address: [email protected] (M. Hu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.09.084
graphy (EBL) patterning process makes the device processing expensive and time consuming [14]. By contrast, here we propose that the use of nanoporous AAO (a well-known hard template) combined with a dry etching process could be an inexpensive and simple solution to these problems. The pores of AAO templates are uniform over large areas, are able to self-organize into ordered honeycomb arrays perpendicular to the substrate, and the pore geometry (pore diameter, density and length) is highly controllable [15]. Thus, it is an ideal technology for the fabrication of size and spacing controllable nanopatterns on Si substrates, which consequently, allows possible precise control of the luminescence efficiency and emission strength and frequency of optoelectronic devices [16]. In order to carry out such patterning, it is necessary to either place thin AAO templates on the surface of the substrate to be patterned or to produce the template in situ. However, owing to the brittle nature of thin AAO templates, the former option would prove difficult. In the present paper, we focus on the production of large-scale novel nanostructures, consisting of silica nanotubes on silicon, by combining Ar ion milling through AAO template masks anodized in situ, and demonstrate controllable geometries of SiOx nanotubes by using different synthesis parameters. The process has the advantage over wet chemical etching methods in that it has a more uniform distribution and better control of nanopatterning [17]. We demonstrate the fabrication of large-area arrays of silica nanotubes with geometries controlled by the anodizing conditions.
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2. Experimental procedure Fig. 1 shows a schematic flow chart of the fabrication process. Aluminium thin films were deposited onto n-type (1 0 0) Si wafers using dc sputtering followed by a two-step anodization process [15] which was carried out using a Keithley2400 source-meter controlled by a Labview based program, using 0.3 M oxalic acid as the electrolyte. Unless indicated otherwise, the anodization voltage used was 40 V, which is the most commonly employed for oxalic acid solution anodizations. The conditions produce regular pores with ordering by self assembly. The interpore distance is 100 nm and the pore diameter is approximately 50 nm. Anodizing voltages greater than 40 V were employed to alter the geometry of the arrays, and will be indicated where used. The oxide formed in the first step was etched in 2%CrO3 + 5%H3PO4 at 60 8C for 10 min, leaving indentations in the underlying aluminium at the base of the pores. The second anodization was continued down to the underlying silicon substrate, forming silica nanodots at the base of the AAO pores. In order to remove the AAO barrier layer and to control the diameter of the pores, a 5%H3PO4 aqueous solution was used to etch the samples. After anodization, pattern transfer was carried out by Ar ion milling through the template and into the underlying silicon. An Ar+ + 2%O2 plasma with 20 mA beam current (4.85 mA/cm2 beam current density) and 500 V beam voltage was employed. After pattern transfer, the template was removed using aqueous NaOH solution. The microstructures of the surfaces and cross sections of the samples were observed using JEOL-6, LEO SEM and a Tecnai F20 TEM. EDX measurements were performed using an Oxford Instrument 7426 EDS system. The nanopore geometry and distribution was analyzed using the UTHSCSA Image Tool 3.0 software. The step height of silicon film after milling was measured using a Dektak profilometer.
Fig. 1. Schematic flow chart of the fabrication process, (a–c) anodization and (d–f) pattern transfer and template removal.
3. Results and discussion In order to carry out pattern transfer with precise control over the milled pattern, the ion milling rates of (a) a bare silicon wafer,
Fig. 2. Micostructure of silica nanotubes on Si produced at 40 V template anodizing voltage. SEM images of the surface (a) and oblique angle view (b) of the nanotubes. (c) TEM cross section image of silica nanotubes and (d) a thinner section of the same sample.
M. Hu et al. / Applied Surface Science 255 (2009) 3563–3566
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Fig. 3. (a) Diameters of pores in AAO templates and of transfer patterned silica pores and (b) full width half maximum (FWHM) of the transferred pore diameter distribution as a function of AAO pore widening time.
(b) an AAO template and (c) a silicon wafer through an AAO template were determined by both profilometery and SEM studies. The values found were 35 nm/min, 40 nm/min and 31 nm/ min 2 nm, respectively. Knowing these values it was possible to mill the silica structures without removal of the template. It should be noted that the milling rate through the AAO templates is dependent on the template pore aspect ratio. However, the dependence was found to be relatively weak below an aspect ratio of 10, and all samples studied here were prepared under these conditions. Fig. 2 shows SEM images of arrays of silica nanotube arrays produced by pattern transfer after Ar ion milling for 10 min through a 500 nm thick AAO template anodized at 40 V, and pore widened for 60 min, giving an AAO pore size of 65 nm. Fig. 2(a) shows an SEM image of the surface of the silica nanotube array, with a pore distribution similar to that observed in the AAO template indicative of a successful pattern transfer. Continuous and uniform coverage was observed over the entire anodized area of 0.5 cm2. Fig. 2(b) shows an oblique angle SEM image of the same sample. The nanotube structure of the silica features can be seen on the surface of the silicon substrate. Fig. 2(c) shows a cross-sectional TEM image of the silica nanotube structure on top of the underlying silicon substrate. The tubes have a length of 80– 90 nm and an outer diameter of approximately 80–90 nm, 10– 20 nm less than the interpore distance. Quantification by electron energy loss spectroscopy (EELS) indicated an Si:O ratio of approximately 1:2 for the walls of the nanotubes. Fig. 2(d) shows a thinner section of the same sample. The silica tubes are milled away but the
pores beneath the silica tubes are emphasized more clearly. The pore channel in the silicon substrate is tapered with a trapezoidal shape, and is believed to be caused by a shadowing effect during pattern transfer [18] which results in a build-up of milled material in the AAO pores during milling causing the diameter of the pores to decrease with the ion milling time. And, with this trapezoidal shape, the aspect-ratio of the silica nanotube was calculated using the mean value of the top and bottom diameter of the silica tubes, and was found to be 1.5 in the present case, which, however, could be adjusted in the range of 1.5–4 by controlling the processing parameters, such as ion milling time, AAO thin film thickness and its pore diameter. To determine to what extent the silica tube diameter can be controlled by the template microstructure, the diameter of the transferred pattern as a function of pore diameter of the AAO template was studied. A template with a thickness of 300 nm was fabricated by anodizing at 40 V. It was then fractured into several pieces and each pore-widened in 5 wt% H3PO4 at 20 8C for a different length of time before carrying out pattern transfer. Fig. 3(a) shows a plot of AAO pore diameter as a function of pore widening time, along with a plot of the pore size of the transferred pattern as a function of the same variable. For AAO templates anodized at 40 V the pore widening rate vs. etching time follows the relationship PD (nm) = 36 + 0.45TH, where TH is the etching time in 5 wt% H3PO4 in minutes [20]. The average diameter of the transferred pattern follows a similar linear gradient to that observed in the AAO template. However an offset of approximately
Fig. 4. (a and b) SEM images of the surface of silica nanotube arrays produced by AAO anodization at 57 V and 76 V, respectively (both pore widened for 60 min). (c) Plot of the silica nanotube dimensions as a function of anodizing voltage.
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20 nm is observed. This decrease in the transferred pore diameter can be ascribed to a shadowing effect of the template during pattern transfer. Fig. 3(b) shows a plot of the full width half maximum (FWHM) of the transferred pattern pore diameter as a function of template pore widening time. As expected, the distribution width of the transferred pore size is observed to narrow as the AAO pore diameter increases. The geometry of the silica array can also be controlled by altering the voltage used to anodize the template. It is well known that both the interpore distance (ID) and pore diameter (PD) of AAO templates vary linearly with anodizing voltage [18]. In bulk, the parameters are ID (nm) = 2.5 V and PD (nm) = 1.29 V [19]. In a previous investigation, we showed that for anodization of thin (<1 mm) aluminium films on silicon substrates, ID and PD vary with anodizing voltage as ID (nm) = 15.8 + 2.17 V and PD (nm) = 0.905 V respectively [20]. Fig. 4 shows silica nanotube arrays can be produced with different geometries produced by milling through templates of different geometries. Fig. 4(a and b) shows SEM images of the surface of arrays produced using (a) an AAO template anodized at 57 V pore widened for 60 min, yielding an interpore distance of 140 nm and pore diameter of 80 nm, and (b) using an AAO template produced at 76 V and pore widened for 60 min, yielding an interpore distance of 180 nm and a pore diameter of 95 nm. For both array spacings highly uniform coverage was observed over the entire area of 0.5 cm2. Comparing these with a sample made under the same conditions but using a 40 V anodizing voltage (Fig. 2(a)) shows that the increasing anodizing voltage not only increases the interpore distance but also increases both the diameter and length of the nanotubes. Fig. 4(c) shows a plot of the dimensions for the silica nanostructures produced using AAO templates anodized at 40 V, 57 V and 76 V, determined by analysis of both TEM and SEM images. Each template was pore widened for 60 min. The inner and outer diameter and the tube length are shown to increase with increasing anodizing voltage. 4. Conclusion A new pattern transfer technique using AAO template masks combined with ion milling has been demonstrated for the synthesis of geometry controllable nanoarray patterns on silicon
substrates. The geometry of the nanostructures may be precisely controlled by altering the processing parameters such as applied anodization potential, H3PO4 etching time (TH) and ion milling time. The interpore distance of the nanotube arrays was tuned from 100 nm to 180 nm, the tube length from 90 nm to 250 nm, the outer tube diameter from 80 nm to 135 nm and the inner tube diameter from 40 nm to 65 nm. The average pore diameter of the nanotube arrays after pattern transfer is approximately 20 nm less then the diameter of the AAO templates. Acknowledgements The authors would like to thank the European Commission for the financial support of the Marie Curie Excellence Grant NanoFen contract number EXT- 014156. The same gratification is also indebted by the author to the financial support from China Scholarship Council (CSC) under granted number [G]2005842133. This work reflects only the views of the author and not the views of the European Commission or the University of Cambridge. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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