Effect of conductive and non-conductive substrates on ...

Microelectronic Engineering 157 (2016) 1–6

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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee

Effect of conductive and non-conductive substrates on the formation of anodic aluminum oxide (AAO) template for mask-less nanofabrication Debabrot Borgohain a, Raj Kishora Dash a,⁎, Ghanashyam Krishna Mamidipudi b a b

School of Engineering Sciences & Technology, University of Hyderabad, Hyderabad, TS, India Centre for Advanced Studies in Electronics Science and Technology, School of Physics, University of Hyderabad, India

a r t i c l e

i n f o

Article history: Received 23 March 2015 Received in revised form 8 July 2015 Accepted 21 January 2016 Available online 23 January 2016 Keywords: Nanopores Anodic aluminum oxide (AAO) Nanostructure Mask-less nanofabrication

a b s t r a c t Anodic aluminum oxide (AAO) templates are generally the most cost-effective mask less nanofabrication processes for synthesis of nanowires, nano-meshes and nanoperiodic structures for several applications. In this present work, the effect of conductive and non-conductive substrates on the nanopores formation by anodic aluminum oxide (AAO) template method was investigated by using the same electrolyte, anodization voltages and time. The experimental results revealed that under same anodization conditions, the pore size, uniformity and aspect-ratio of the nanopores can be controlled better over the non-conductive substrates as compared to the conductive substrates. Furthermore, it was confirmed that under different electrolytic bath conditions (potentiostatic and galvanostatic),the potentiostatic condition causes formation of higher density of nanopores and leads to better uniformity resulting in higher aspect ratio AAO templates in comparison to the galvanostatic condition. Nanopore widening studies were also carried out and results are presented. The results of the present study can open a pathway to develop nanophotonics, optical-MEMS and sensor applications. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, nanostructure fabrications have attracted much interest due to the development of different nanodevices, nanosensors, quantum dots etc. [1–5]. Several conventional lithographic processes are well established for this purpose [6–9]. However, all of these processes are very expensive and cannot be scaled up easily. In contrast, the anodic aluminum oxide (AAO) template based nanofabrication process which is more cost effective and has the potential for device fabrication with possibility of large scale integration had been used by several researchers for different applications such as nanosensors and quantum dots [10–14]. However, due to the lack of periodicity, uniformity and control over the formation of barrier layer during the nanopores formation, the AAO template method is still evolving for several applications starting from sensors [15], photonics, nanodevices [16] and Nanoelectromechanical systems (NEMS) [17]. It was also observed that the formation of nanopores based on AAO template method depends upon the type of substrates used, i.e. conductive or nonconductive. It was reported that periodicity, uniformity and control over the formation of barrier layer during the nanopores formation can be improved by choosing proper substrate. Therefore, the main objective of the present work is to investigate the effects of conductive and non-conductive substrates, used in the AAO template method, on ⁎ Corresponding author. E-mail addresses: [email protected] (D. Borgohain), [email protected] (R.K. Dash), [email protected] (G.K. Mamidipudi).

http://dx.doi.org/10.1016/j.mee.2016.01.023 0167-9317/© 2016 Elsevier B.V. All rights reserved.

the nanopores formation, nanopores size, density, uniformity and aspect ratio. For this purpose Aluminum (Al) thin films were deposited on both conductive (SiO2/Si) and non-conductive substrates (glass) and anodized in an electrolytic bath in an oxalic acid (0.3 M) as electrolytes. Different potentiostatic and galvanostatic conditions were considered to investigate the effect of current and voltages in the formation of nanopores. The nanoporous AAO templates were characterized by field emission scanning electron microscope (FESEM) for uniformity, pore size determination, density and aspect ratio. Further, the postanodization process was carried out by treating the AAO templates in

Fig. 1. Schematic diagram of an electrolytic bath for AAO template.

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D. Borgohain et al. / Microelectronic Engineering 157 (2016) 1–6

Fig. 2. The FESEM images of Al thin films deposited for 30 s on (a) Cr (30 nm)/Glass and (b) SiO2/Si substrates.

Fig. 3. Surface profilometer reading of Al thin film on (a) non-conductive (glass) and (b) conductive (SiO2/Si) substrates.


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Fig. 4. The EDS spectrum of the Al thin film coated on (a) non-conductive and (b) conductive substrates.

concentrated H2SO4 for the pore widening and removal of any barrier layer.

Aldrich. Oxalic acid which was used as an electrolyte, purchased from Sisco Research Laboratory.

2. Materials and methods

2.2. Preparation of AAO template

2.1. Materials and chemicals

In the process of making AAO templates, an electrolytic bath was used as shown in Fig. 1. The thermally evaporated Al thin films on both conductive [18–20] and non-conductive substrates [21,22] were kept for anodization in the acidic electrolytic bath. An electrolytic cell composed of 0.3 M electrolytic solution (oxalic acid 0.3 M), two electrodes (2 × 2 cm2 Al/Cr/Glass or Al/SiO2/Si substrates as anode and 2 × 2 cm2 platinum mesh as cathode), a DC power supply, a voltmeter to measure the voltage and an ammeter to measure the current throughout the anodization process were used. Two different electrolytic bath conditions, i.e.potentiostatic (where anodization voltage was kept constant throughout the process) and galvanostatic (where anodization current was kept constant throughout the anodization process) conditions were considered to study the effect of bath conditions on the pore growth rate and the depth profile of the pores. An optimization study of the effect of different DC voltages (e.g. 10 V, 15 V, 20 V and 30 V) carried out to obtain the optimum condition for the nanopores formation on both the conductive and non-conductive substrates. It was observed that an anodization voltage of 30 V was the best. Simultaneously four different anodization times, 20, 25, 30 and 90 s, were considered for 30 DCV in the electrolytic bath. For 20 and 25 s the anodization was not observed. As the anodization time was increased, the formation of nanopores on aluminum thin film occurred as evident from the good and uniform nanopores observed after 30 s anodization process. After the anodization process was completed, sample was further treated in post-anodization process by dipping in a highly acidic sulphuric acid bath having pH = 1 for 10 s, for pore widening and removal of any barrier layer left over during anodization process. FESEM (FESEM Model ULTRA55 of CARL-ZEISS) was used to study the morphology as well as to obtain the cross-sectional images of the AAO templates for both conductive and non-conductive substrates. Composition of the thin film was characterized by energy dispersive X-ray spectroscopy (EDS) spectrum. Thin film thicknesses were measured by using a surface profilometer (XP 100 of Ambios Technology, USA). A high resolution x-ray diffractometer (Discover D8 of Bruker, Germany) equipped with Cu Kα radiation of wavelength 1.5408 Å was used to record the X-ray diffraction patterns of the Al and Cr thin films.

Chromium (Cr) granules were purchased from Atomergic Chemetals Corp. whereas Aluminum (Al) wires were purchased from Sigma-

3. Results and discussion Fig. 5. An X-ray diffraction pattern of the composite film of (a) Al/Cr/glass (b) Al/SiO2/Si substrate.

Fig. 2(a) and (b) show the FESEM images of the Al thin films on nonconductive (glass) and conductive (SiO2/Si) substrates respectively.

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Fig. 6. The FESEM images of AAO templates at optimum anodization voltage 30 V for 1 h anodization time on (a) non-conductive and (b) conductive substrates.

It was seen from the Fig. 2 that Al thin films were uniform and smoother in the case of non-conductive substrate. Fig. 3(a) and (b) show the surface profilometer data for both non-conductive (Al/Cr/ glass) and conductive (SiO2/Si) substrates respectively. For thickness measurement the steps were created during the deposition of the thin films by thermal evaporation. In this regard the substrates were hold by clips and which covered some portion of the substrate restricting deposition of Al thin film. In case of SiO2/Si substrate, it was heated at 400– 4500 C as pure Al metal reacts readily with oxygen to form Al2O3 and the energy required for this is −378.1 kcal/mol. In contrast, the energy of

formation of SiO2 is −205 kcal/mol [23]. Therefore, Al can be reduced by the reaction as shown here 3SiO2 þ 4Al→3Si þ 2Al2 O3 : From the above reaction the good adhesion between Al and SiO2/Si can be attributed to the fact that the deposited Al atoms react with the native SiO2 layer of the substrate reducing it to Si and leading to a better contact between Al and Si. For this reason, the Al thin film deposition was carried out in two steps wherein after the first deposition

Fig. 7. The cross-sectional FESEM image of AAO template anodized at 30 V for 1 h on (a, b) non-conductive and (c, d) conductive substrates.


Fig. 8. Pore size distribution of nanopores formed in non-conductive and conductive substrate during anodization at 30 V for 1 h.

(~ 20 nm) the thin film was heated to 400–4500 C for 4 h. In the next step, another layer of Al (~190 nm) was deposited to develop the AAO template. The profilometer reading shown in Fig. 3b verifies the thicknesses of the Al thin film. Fig. 4(a) and (b) show the EDS spectrum of Al thin film deposited on non-conductive and conductive substrates respectively. Structural analysis of Al thin films was done by X-ray diffraction analysis for both non-conductive and conductive substrates which is shown in Fig. 5(a) and (b). In Fig. 5(a), the three peaks (2θ = 38.28, 65.08 and 78.22°) are indexed to the face centered cubic (FCC) structure of Al with a lattice constant 4.05 Å, having (hkl) values of (111), (220) and (311) respec-

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tively (verified with JCPDS card no. 89–4037).Similarly, a peak at 2θ = 44.68° is indexed to the body centered cubic (BCC) structure of Chromium (Cr) with lattice constant 2.87 Å, having (hkl) value of (110) (verified with JCPDS card no. 01–1261). The four peaks (where 2θ = 38.56, 44.8, 65.16 and 78.28) for the thin film of Al/SiO2/Si, as shown in Fig. 5(b), are indexed to the FCC structure of Al with lattice constant 4.04 Å, corresponding to the (111), (200), (220), (311) planes, respectively. The peak at 2θ = 51.48° is indexed with the (217) plane of the rhombohedral structure of Al2O3 having lattice constants a = b = 4.844 Å and c = 13.270 Å (verified with JCPDS card no. 71-1684). The lower indices planes for Al2O3 are not appeared in Fig. 5b. as the surface roughness and thermally induced stress increase during the heating process to attain good adhesion between the Al and SiO2/Si substrate [24]. After anodization of Al thin films in the electrochemical cell, the AAO templates were characterized under FESEM and it was found that the nanopore formation occurred at 30 V as shown in Fig. 6(a) and (b). Fig. 7(a, b) and (c, d) show the cross-sectional FESEM images of AAO template on non-conductive (glass) and conductive (SiO2/Si) substrates respectively. The nanopore sizes were in the range of 23–45 nm for nonconductive and 38–69 nm for conductive substrates and can be attributed from Fig. 8. Similarly, pore depths were approximately 90–100 nm for non-conductive and 60–100 nm for conductive substrates. From Figs. 6 and 7 it is clearly seen that the AAO templates made from two different substrates i.e. Al/SiO2/Si and Al/Cr/Glass are completely different. The nanopores formed on the non-conductive exhibited better uniformity and control over the size. In case of nonconductive substrate, most of the nanopores were having uniform inner diameter. The cross-sectional view of the same AAO template made on both non-conductive and conductive substrates as shown in Fig. 7(a, b) and (c, d). It is easily visible that the numbers of nanopores formed along the cross-section throughout the film thickness are more in case of the non-conductive substrate than the conductive substrate.

Fig. 9. The FESEM images of AAO templates anodized at potentiostatic condition @ 30 V with 1 hour anodization time for non-conductive substrate (a) top view and (b) cross-sectional view.

Fig. 10. The FESEM images of the AAO templates anodized at galvanostatic condition @ 60 mA with 1 hour anodization time for non-conductive substrate (a) top view and (b) crosssectional view.

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Fig. 11. The FESEM images of the AAO template on Al/Cr/glass (a) after anodization at 30 V and (b) After post-anodization.

Since, the non-conductive substrate was showing better result as compared to conductive substrate, potentiostatic and galvanostatic electrolytic conditions were applied, on these substrates. The FESEM images shown in Figs. 9 and 10, revealed that the nanopore formation was greater for longer period of anodization in potentiostatic condition than the galvanostatic condition and the nanopores were forming along the cross-section throughout the film thickness. However, the pore diameters (Dp) were smaller in size in case of the potentiostatic condition than that for the galvanostatic condition. Post-anodization treatment was performed by dipping the AAO template on the non-conductive substrate (as prepared in potentiostatic condition) in sulfuric acid (pH = 1) for 10 s and then was cleaned 5 times with deionized water. The FESEM images of the AAO templates before and after post-anodization with acid treatment are shown in Fig. 11. From the figure it is seen that the pore diameter (Dp) increases from 30 to 50 nm to 50–65 nm, after the post anodization acid treatment.

Acknowledgment This work is supported by UPE-II (Sec. No R-57) project, University of Hyderabad, India. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

4. Conclusions Nanopores based on AAO template process are successfully formed on both conductive and non-conductive substrates. The nanopores on non-conductive substrates are more or less uniform in size with better control over size and higher depth and the pore growth was found in the direction of the film thickness. The potentiostatic condition is better than the galvanostatic condition in the aspects of higher density of pore formation and also for the better control over the uniformity and aspect ratio of the pores. It is also confirmed that the pore diameters can be varied by following a post anodization acid treatment, which is also helpful to remove the barrier layers in the AAO template. Hence, by controlling the different process parameters desired nanopores can be formed on different substrates and can be utilized for patterning nanodevices starting from nanosensors, quantum dots to synthesis of nanomaterial.

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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