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Physica E 37 (2007) 274–278 www.elsevier.com/locate/physe
Bi3.25La0.75Ti3O12 (BLT) nanotube capacitors for semiconductor memories B.I. Seoa, U.A. Shaislamova, S.-W. Kima, H.-K. Kima, B. Yanga,, S.K. Hongb a
Kumoh National Institute of Technology, Department of Advanced Nano Materials for Information Technology, 1 Yangho-dong, Gumi-si, Gyeongbuk 730-701, Korea b Hynix Semiconductor Inc., Memory R&D Division, New Device Team, Icheon-si, Kyoungki-Do 467-701, Korea Available online 30 October 2006
Abstract We report results of fabrication and examination of Bi3.25La0.75Ti3O12 (BLT) ferroelectric nanotubes. BLT nanotubes are suggested for developing 3D ferroelectric nanotube capacitors which could be used in high-density memory applications. BLT nanotubes were prepared by template-wetting process using polymeric sources where anodic aluminum oxide had been used as a template. After annealing, tubular BLT structures were crystallized inside the pores of the template. By selective etching of the template, released BLT nanotubes have been obtained. Crystallization and nucleation of the nanotubes were analyzed by XRD and FE-SEM techniques. r 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 78.67.Ch; 07.85.Jy Keywords: Anodization; Ferroelectric; Nanotube; Porous alumina; BLT
1. Introduction Ferroelectric random access memory devices (FeRAMs) have attracted a great deal of interest as a potential application for nonvolatile stand-alone memory devices or as a memory element in system-on-chip devices where a high density (416 mega bit) is preferred for the device performance and cost effectiveness. The limited markets and the frail cost competition of FeRAMs mostly resulted from the low density of current commercial FeRAMs. The main reasons for the low density have been due to the limited design rule of FeRAMs. Furthermore, a study of 3D capacitors with superior charge in limited area becomes more important, because the size of capacitors will be significantly reduced down to 0.1 mm2. Thus, for commercialization of FeRAMs with ultra-high density level, stable thin film process, based on CVD and etching technologies for capacitor formations are required. However, these technological difficulties worldwide have rarely been overcome. Therefore, in this study, it is suggested that Corresponding author. Tel.: +82 54 478 7741; fax: +82 54 478 7769.
E-mail address:
[email protected] (B. Yang). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.09.003
ferroelectric nanotubes are applied for the development of 3D nanotube capacitors, as an alternative to conventional technologies showing the technological limitation. The ferroelectric nanotube technology has several advantages such as cost and technological effectiveness due to the simple process as described in Table 1, where key parameters of conventional and nanotube capacitors are compared. Here we report fabrication of Bi3.25La0.75Ti3O12 (BLT) [1–3] nanotubes by template wetting process [4,5]. This convenient method consists of wetting of pore walls of porous template with polymeric precursors. Synthesis of anodic aluminum oxide (AAO) has been performed by two-step anodization process [6]. 2. Experiments and discussion For the preparation of the self-organized pore arrays, high purity (99.99%) aluminum sheets were used for anodization. After careful cleaning, annealing and electropolishing, the aluminum substrates were anodized under constant cell potential in aqueous phosphoric acid using experimental setup shown in Fig. 1. During the anodization process, the pores nucleate at the surface at almost
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Table 1 Comparison of memory cells using conventional and ferroelectric nanotube capacitors Capacitor type
Conventional 3D
Ferroelectric nanotube
Conventional process
Tera bit cell size No
Cell structure
Advantage
photolithography
No capaitor ething Low cost Disadvantage
Difficulty in capacitor etch process High cost
New process
Process issues
Capacitor etch angle (4801) Elimination of capacitor
Capacitor crystallization Capacitor planarisation Nano-pore process on wafer
side-wall polymer
level
OU T L E T IN L E T
DR AI N CI RC UL A T OR
A N O D IZ A T IO N BA T H
DC P O W E R SU PPL Y 220 V A C
Fig. 1. Schematic of experimental setup for anodization process.
random positions, and as a result pores on the surface occur randomly and have a broad size distribution. However, under some specific anodization conditions, hexagonally ordered pore domains can be obtained at the bottom of the layers. The porous alumina was prepared in aqueous 10 wt% phosphoric acid at 160 V and 3 1C. To fabricate ordered pore arrays in which the holes are straight and regularly arranged throughout the film, a twostep anodization process was used, which was described previously. Subsequently, the oxide layer is removed by wet chemical etching in a mixture of phosphoric acid (6 wt%) and chromic acid (1.8 wt%) at 60 1C. The remaining periodic concave patterns on the aluminum substrate act as self-assembled masks for the second anodization process. An ordered pore array is obtained after anodizing
for the second time by using the same parameters as in the first step. We have prepared BLT nanotubes by using simple and convenient template wetting method [10]. This approach consists of wetting of the pore walls of porous alumina, which was used as a template, by precursor solutions. The driving force of the process leads to reduction of the surface energy of the system. BLT nanotubes have been prepared using mixture containing BLT and polymer in proper quantities [11,12]. Fabrication of BLT nanotubes has been accomplished using two kind of porous alumina template, commercial nanopore by ‘‘Whatman’’, and experimentally prepared one. After the solutions had been brought into the contact with template, the wetting time needed for pore walls to be covered by BLT source using commercial nanopore was 4 min. In the case of experimentally prepared nanopore, wetting time for the complete covering of pore walls was considerably longer than that for the commercial one. This is because both surfaces (top and bottom side) of commercial nanopore membrane are open (through hole pores), whereas the experimentally prepared membrane had bottom side closed structure. The first baking was performed at 100 1C for 1 h and the second baking at 280 1C for 1 h. In the baking process precursor solution will evaporate. Then dried samples were annealed in rapid thermal annealing (RTA) in oxygen ambient at 540 1C for 6 min followed by furnace annealing (FA) at 700 1C for 30 min. To observe and characterize the fabricated nanotubes, after the heat treatments have been finished, templates were selectively etched. Resulting FE-SEM images of commercial and experimentally prepared nanoporous alumina membranes are shown in Fig. 2. In Fig. 2(a) is shown the surface view of
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(a)
(b)
(c)
Fig. 2. FE-SEM images of porous anodic alumina: (a) surface image of commercial nanoporous alumina, (b) and (c) surface and cross-section images of nanoporous alumina prepared using bulk Al in 10 wt% phosphoric acid at 160 V DC.
commercial nanopores with diameters of about 200 nm purchased from Whatman. Figs. 2(b) and (c) show surface and tilting views of nanoporous alumina fabricated using two-step anodization in phosphoric acid at 160 V, respectively. Right after second step, pore diameter was about 150 nm, subsequent wet chemical etching in 5 wt% phosphoric acid lead to the enlargement of pore diameter up to 200 nm [10]. It should be pointed out that even after two-step anodization the pore arrangement is not highly regular. This is due to the fact that first anodization time was not long enough, only 30 min. Usually to achieve high regular pore arrangement, either first step anodization should continue for long time period (usually 15–20 h) or using nano-imprint method [7–9,11]. Organic materials and most polymers are considered as low-energy materials with respect to their surface energies, whereas inorganic materials are referred to as high-energy materials. Low-energy liquids spread rapidly on highenergy surfaces. Therefore, the pore walls will be covered with a mesoscopic film if they exhibit a high surface energy. The underlying driving forces are due to short-range as well as long-range Van der Waals interactions between the
wetting liquid and the pore walls. In our case polymer added to BLT source to improve wetting property of the overall system. After the solvent had been evaporated, the solidified BLT source formed a thin film covering both the pore walls and the surface of the template. To investigate morphology of the BLT nanotubes, we removed the alumina template by selectively etching with 30 wt% KOH at room temperature. Figs. 3(a) and (b) show surface image of BLT tubes in low and large magnification prepared using commercial nanopore membrane, after partially etching the template. Figs. 3(c) and (d) represent openings and closed capped ends of nanotubes obtained by wetting AAO prepared at 160 V DC in phosphoric acid solution, respectively. From the open ends of BLT tubes it is easy to estimate diameter of tubes. They correspond to the diameter of applied template about 200 nm. The crystal structures of BLT nanotubes were investigated by XRD. In Fig. 4 the XRD patterns of BLT nanotubes embedded to alumina template are represented. XRD investigations were performed after the annealing at 540 and 700 1C and the results are shown in Figs. 4(a) and (b) showing corresponding (1 1 7), (2 0 0), (1 1 1) peaks. From Fig. 4(b)
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(b)
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Fig. 3. FE-SEM images of BLT nanotubes, (a) top view of the BLT nanotube arrays using commercial nanopore, after partially removing of template, (b) cluster of BLT nanotubes after complete etching of template, (c) closed-end and (d) open end views of released BLT nanotubes using AAO prepared bulk Al.
(a) 400 350
(b) 400
BLT(540˚C)
350
Intensity (a.u)
Intensity (a.u)
(117)
300
300 250 200 150 100
250 200
(200)
(111)
150
(220)
100
50 0 10
BLT(700˚C)
50 BLT-05-02-11
BLT-05-02-11
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20
25
30
35
40
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50
Angle (2 )
0 10
15
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25
30
35
40
45
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Angle (2 h)
Fig. 4. X-ray diffraction pattern of BLT nanotubes inside the alumina template after annealing at (a) 540 and (b) 700 1C.
it is apparent that crystallization of BLT nanotubes occurred near 700 1C. 3. Conclusion In this work fabrication of ferroelectric BLT (Bi3.25La0.75Ti3O12) nanotubes were accomplished by tem-
plate-wetting process. We applied both commercial and experimentally fabricated nanoporous templates. During experiments we found out that complete wetting of pore walls in close ended template requires much more time than open ended one. FE-SEM and XRD analysis have been performed to examine the tube structures. Diameter of tubes is adjustable by using templates with appropiate pore
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diameter. Released tubes have been obtained by chemical etching of the template. The possibility of the BLT nanotube process for ferroelectric nanotube capacitor in memory devices is shown in this work. Acknowledgments This work was supported by the Ministry of Commerce, Industry and Energy in Korea under the System IC 2010 Project. References [1] B. Yang, S.S. Lee, Y.M. Kang, K.H. Noh, S.W. Lee, N.K. Kim, S.Y. Kweon, S.J. Yeom, Y.J. Park, J. Appl. Phys. 42 (2003) 1327. [2] W.S. Yang, N.K. Kim, S.J. Yeom, S.Y. Kweon, E.S. Choi, J.S. Roh, J. Appl. Phys. 40 (2001) 5569.
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