Retention characteristics of Pb(Zr, Ti)O 3 films deposited by various methods for high-density non-volatile memory
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Retention characteristics of Pb(Zr, Ti)O3 films deposited by various methods for highdensity non-volatile memory Sangmin Shin*, Mirko Hofmann, Yong Kyun Lee, Choong Rae Cho, June Key Lee, Youngsoo Park Materials & Devices Laboratory, Samsung Advanced Institute of Technology, Suwon 440-600, Korea Kyu Mann Lee and Yoon Jong Song Process development team, Samsung Electronics, Yongin 449-900, Korea * e-mail: [email protected] ABSTRACT Retention loss is a significant issue for an application of ferroelectric thin films to highdensity non-volatile memory devices. We investigated the polarization retention characteristics of ferroelectric Pb(Zr,Ti)O3 (PZT) thin films which were fabricated on Pt/IrO2/Ir substrates by different deposition methods. In thermally-accelerated retention failure tests, Pb(Zr,Ti)O3 (PZT) films which were prepared by a chmeical solution deposition (CSD) method showed rapid decay of retained polarization charges as the films became thinner down to 1000
Å , while the films
which were grown by metal organic chemical vapor deposition (MOCVD) showed relatively large nonvolatile charges at the same thickness. We concluded that in the CSD-grown films, the relatively large interfacial passive layer compared with the MOCVD-grown films had an unfavorable effect on retention behavior. We observed the existence of such interfacial layers by extrapolation of the total capacitance with thickness of the films and the capacitance of this layer was larger in MOCVD-grown films. It means that the possibility of the accumulation of space charges at the interface was reduced, so that less imprint and less retention loss could be observed in the MOCVD-grown films. KEYWORDS: PZT, MOCVD, CSD(sol-gel), FRAM, retention loss, ferroelectric, thin film.
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INTRODUCTION Ferroelectric random access memory (FRAM) is a promising candidate for future memory device because of its non-volatility, fast access time and hardness from the radiation [1]. However, there are some crucial issues on the reliability of FRAM such as thermal stability, fatigue, and retention. In a real application field of using PbZrxTi1-xO3 (PZT) as a material inside the ferroelectric capacitor of FRAM, fatigue, the reduction of the remnant polarization with the numerous cycles of polarity switching in the capacitor, could be much alleviated by using oxide electrodes like IrO2, RuO2 and La1-xSrxCoO3 [2,3]. But, when a ferroelectric capacitor is kept at a higher temperature or for a long time in a specific polarization state, the decrease of the nonvolatile charge in the capacitor becomes quite a hard problem [4,5,6], and even much severer in thin ferroelectric films whose thickness is less than 100 nm. The decay of non-volatile charge can be termed as retention loss. In order to see long-time behavior of retention loss, temperature-accelerated retention test is used. One first writes the data bit of “0” or “1” in a capacitor with electrical pulses of opposite polarities, bakes the capacitor at a higher temperature, and reads the data a
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