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Physica E 37 (2007) 279–282 www.elsevier.com/locate/physe
Fabrication of electrode Pt nanotubes for semiconductor capacitors B.I. Seoa, U.A. Shaislamova, S.-W. Kima, H.-K. Kima, S.K. Hongb, B. Yang a
Kumoh National Institute of Technology, Department of Advanced Nano Materials for Information Technology, 1 Yangho-dong, Gumi-si, Gyeongbuk 730-701, Republic of Korea b Hynix Semiconductor Inc., Memory R & D Division, New Device Team, Icheon-si, Kyoungki-Do 467-701, Republic of Korea Received 6 May 2006; received in revised form 21 July 2006; accepted 26 July 2006 Available online 11 September 2006
Abstract Template-wetting process was used to fabricate Pt electrode nanotubes for the further application in 3D nanotube capacitors. Anodic alumina oxide (AAO) was used as a template, which can be fabricated by means of two-step anodization process. After the wetting process, released Pt nanotubes have been obtained by selective etching of the template using KOH solution. Pt nanotubes formation and tube walls morphology were investigated by FE-SEM technique. Furnace annealing (FA) effects at temperatures ranging from 200 to 600 1C with 100oC step on nucleation and crystal growth condition of Pt nanotubes have been examined. Crystallization of the Pt nanotube inside the template has been examined by step-by-step annealing at different temperatures, and confirmed by XRD. r 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 78.67.Ch; 07.85.Jy Keywords: Pt; Anodization; Nanotube; Polymer; Template wetting
1. Introduction Pt materials are used as an electrode in ferroelectric semiconductor memories applications due to its chemical stability and low leakage. A study of 3D capacitors for ferroelectric random access memory (FeRAM) with superior charge in limited area become more important, because the size of capacitors will be significantly reduced such as less than 0.1 mm2, which is essential for the commercial FeRAM with higher density than 64 Mb using 0.13 mm technology node within 3–4 years. Thus, for commercialization of FeRAMs with ultra density level, it is required for a stable thin film process based on CVD and etching technologies for capacitor formations [1]. However, there has been rarely overcome yet these technological difficulties in worldwide. The ferroelectric nanotube technology using template-wetting process [2–8] has several advantages such as cost and technological effectiveness due to the simple process [9]. Consequently, in this paper, basic studies of grain growths of electrode Pt nanotube inside alumina 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.2006.07.033
nanopores for development of high-density semiconductor memories are reported. Anodic aluminum oxide (AAO) of 200 nm pore size was fabricated for templates. Polymer metallic sources were used for improvement of wetting properties in templates. Grain growth process of Pt nanotubes during baking, rapid thermal annealing (RTA), and furnace annealing was examined by FESEM, XRD. 2. Experiments and discussion In order to fabricate nanoporous alumina template we anodically oxidized a high-purity aluminum sheet (99.99%, 0.5 mm thick) [10]. First, Al substrate was cleaned in acetone then to reduce the surface roughness it was electropolished in a solution of H3PO4:H2SO4:H2O to a mirror finish. Clean aluminum sheets were anodized in a 10 wt% of phosphoric acid solution [11] at 3 1C using specially designed experimental set-up shown in Fig. 1. Through anodization process constant anodizing voltage was used and the value was 160 V DC. The resultant aluminum oxide film was subsequently removed by dipping the anodized sheet into an aqueous mixture of phosphoric acid (6 wt%)
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and chromic acid (1.8 wt%) at 60 1C. The surface of the remaining aluminum had a highly ordered indented holes array upon it. Anodization of the remaining aluminum layer, under the same conditions, resulted in AAO nanoporous arrays of better uniformity and straighter pore channels. After second step anodization, nano-porous alumina with closely paced hexagonally ordered pores has been obtained. To enlarge the pore diameter, pore widening process was used. Fabrication of Pt nanotubes was accomplished by using template-wetting process [12]. Generally, template-wetting process consists of wetting the pore walls of the used template by polymeric source. Due to the high surface energy of the template, polymeric source having low surface energy rapidly spreads on the walls of the template leaving thin film [13]. After selectively etching of the template tubular structures can be obtained. Pt solution was mixed with polymeric source in proper quantities to prepare Pt metallic-polymeric source for wetting process. Then solution dropped onto the porous alumina template which had pores of diameter 200 nm. After few minutes of wetting, during which the solution covers the pore walls,
Fig. 1. Experimental setup for anodization.
wetted template subjected to the heat treatment. During baking process, Pt polymeric source inside the alumina template solidified and produces thin film on the pore walls. Subsequent annealing at high temperatures led to the completely removing of the polymer, which was added to improve wetting properties. To study crystallization stages of the Pt nanotubes, templates with embedded nanotubes were annealed step-by-step in tube furnace at different temperatures and checked via XRD. For investigation of tube wall morphology, templates were etched with 30 wt% KOH solution. Resulting FE-SEM image of AAO templates fabricated by using two-step anodization process presented in Fig. 2(a). From the face image of AAO (Fig. 2(a)) it is clear to observe hexagonally ordered pores. Pore diameter has been enlarged by pore widening process in 5 wt% H3PO4 solution up to 230 nm, pore length was measured to be 700 mm, which is time controlled. In Fig. 2(b) is shown actually used commercial nanoporous membrane. To investigate wall morphology of the individual nanotube, template was completely etched with 30 wt% KOH solution at room temperature. Then solution with nanotubes was thoroughly rinsed with DI water and dropped onto the Si wafer for FE-SEM investigation. Fig. 3(a) shows single Pt nanotube lying on Si substrate, baked at 200 1C for 24 h. It can be seen from the open ends, that tube has 200 nm diameters which corresponds to the diameter of used template. Fig. 3(b, c) shows low and high magnification image of several fee-laying nanotubes on the Si wafer. After the baking process, templates with solidified Pt source inside the walls were annealed at 350 1C for 1 h. Crystallization of Pt nanotubes has been confirmed by XRD. To check the grain growth temperature, Pt nanotubes embedded in AAO template were annealed sequentially at different temperatures from 200 to 600 1C. During investigation of XRD patterns of Pt nanotubes we observed that reflections coinciding to the (111), (200) planes were increased by increasing the temperature. XRD patterns of Pt nanoubes annealed at temperature range from 200 to 600 1C, respectively, are shown in Fig. 5(a–c). After annealing at 600 1C, Pt nanotubes showed strong intensity of XRD peaks (Fig. 4).
Fig. 2. Scanning electron micrographs of the nanoporous alumina template: (a) surface view after two-step anodization followed by pore widening process and (b) commercial nanoporous alumina membrane.
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Fig. 3. FE-SEM images of Pt nanotubes prepared by template-wetting process: (a) individual Pt nanotube and (b, c) cluster of Pt nanotubes after baking at 200 1C for 24 h, respectively.
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Fig. 4. XRD patterns of Pt nanotubes embedded in AAO template after annealing successively at different, (a) 200 1C, (b) 350 1C, (c) 400 1C for 24 h, (d) 500 1C, (e) 600 1C, temperatures for 1 h, respectively.
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3. Summary We report fabrication results of Pt nanotubes by wetting the pore walls of porous alumina by Pt polymer solution. It is shown in this work that Pt nanotubes by template-wetting process are applicable for semiconductor memory fabrication. It is also shown that Pt sources mixed with polymer improve wetting properties of the template wall. FE-SEM images conformed that after annealing the wetted templates at 200 1C for 24 h tubular structures can be obtained, which are stable even after etching the template. Crystallization of Pt nanotubes occurred at higher annealing temperatures than 350 1C, which was evaluated by successive annealing.
Acknowledgment This work was supported by the Ministry of Commerce, Industry and Energy in Korea under the System IC 2010 Project.
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