Highly Stable Ir-Ta-O Electrode for Ferroelectric Material Deposition
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Highly Stable Ir-Ta-O Electrode for Ferroelectric Material Deposition Fengyan Zhang, Sheng Teng Hsu, Jer-shen Maa, Yoshi Ono, Ying Hong, Weiwei Zhuang, Shigeo Ohnishi*, Wendong Zhen* and Norito Fujiwara* Sharp Laboratories of America, Inc., 5700 NW Pacific Rim Blvd. Camas, WA 98607, USA * Sharp Corporation, Tenri-city, Nara632-8567, Japan ABSTRACT Ir-Ta-O composite bottom electrode has extraordinary high temperature stability. It can maintain good conductivity and integrity even after 5min annealing at 1000 °C in oxygen ambient. The thermal stability of Ir-Ta-O on different substrates has been studied. It shows that Ir-Ta-O is also very stable on Si and SiO2 substrates. No hillock formation and peelings of the bottom electrode were observed after high temperature and long time annealing in O2 ambient. SEM, TEM, XRD, and AES have been used to characterize the Ir-Ta-O film and the interfaces between Ir-Ta-O bottom electrode and Si or SiO2 substrate. The composition and conductivity changes of the electrode during oxygen ambient annealing and the interdiffusion issue will be discussed. Furthermore, Ir-TaO/SiO2/Si capacitor with 30Å gate oxide was fabricated and the C-V and I-V characteristics were measured to confirm the stability of Ir-Ta-O on thin gate oxide. INTRODUCTION Thin films of Ir, Pt, Ru, IrO2 and RuO2 have been extensively studied for the application as electrode materials in FeRAM and DRAM devices[ 1-10]. In addition to the advantages of chemical stability and low resistivity, noble metal oxides can also improve the fatigue property of ferroelectric material such as PZT by preventing space charge formation at the electrode and ferroelectric material interface. However, some problems, such as hillock formation and film peeling caused by stresses, poor adhesion, and oxidation of the barrier layer, have limited the application of these electrodes in very high temperature in oxygen ambient, which is the required deposition and annealing condition for some ferroelectric materials such as SrBi2Ta2O9. It has been reported previously that grain boundary precipitation can inhibit hillock formation, improve the structural stability, and enhance the barrier property of thin films[11]. Yoon et al [12, 13] found RuO2 stuffed Ta barrier was more resistant to oxidation and oxygen diffusion than nitrogen stuffed polycrystalline nitride. There was no increase in sheet resistance even after 800°C annealing for 30 min in air. They also found RuO2 stuffed Pt can prevent the oxygen diffusion up to 650°C for 30 min. A Ru-Ti alloy with high Ru composition was found to have better thermal stability and better barrier properties against interdiffusion of Si and oxygen than Ru metal alone[14]. Comparing with Pt and Ru, Ir has been reported to have better barrier property against oxygen diffusion. Ta and TaN are more resistant to oxidation than Ti and TiN are. The refractory nature of both Ta and Ir, and the high formation temperature of their compounds have assumed their increased thermal stability of the Ta-Ir system [15] . It is
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