New method of ferroelectric thin film characterization using a polarization instability invoked by a voltage cycling
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0902-T06-05.1
New method of ferroelectric thin film characterization using a polarization instability invoked by a voltage cycling Yuki Yamada, Susumu Shuto and Iwao Kunishima SoC R&D Center, TOSHIBA Corporation, 8, Shinsugita-cho, Isogo-Ku, Yokohama, Japan ABSTRACT The polarization change invoked by bipolar pulse cycling stress (voltage cycling) was investigated in detail for a Pt/PZT/Pt ferroelectric capacitor. The remnant polarization increased after voltage cycling, then, it decreased during storage and approached to the initial value. The estimated activation energy of the decay of the gained-polarization during the storage was 0.52eV. This value suggests that the valence change between Ti3+ and Ti4+ is the key of this phenomenon. The model where Ti-related defects pin the polarization was introduced to explain the result. Based on the findings and the model, a combination of voltage cycling and storage is useful to characterize internal defects. We propose this characterization method, named voltage cycling method, as an effective way to analyze internal defects in a ferroelectric capacitor. INTRODUCTION Since reliability is one of the most important issues for ferroelectric memory devices, a lot of studies have been carried out on the failure mechanisms, such as fatigue, imprint and retention. It is widely believed that these failure mechanisms are closely related to internal defects. One group previously reported that the polarization of ferroelectric capacitor increased at the early stage of a fatigue cycling [1]. It is also reported that the capacitor which was degraded by process-induced damage rejuvenated its polarization by applying a high voltage stress, and lost the rejuvenated polarization during high temperature storage [2]. It is anticipated that those results are related to the defects in the ferroelectric film and give us a hint on a new way of reliability characterization. However, no detailed investigation has been reported on these phenomena. In this work, we focused on the polarization change of the Pt/PZT/Pt ferroelectric capacitor due to voltage cycling and high temperature storage. Based on our experimental results, we discussed an inherent mechanism of defect kinetics and proposed a new method as a good way to analyze the internal defect. EXPERIMENTAL PROCEDURE The capacitor stack of Pt/PZT/Pt structure was used for this experiment. A 220nm-thick PZT film was deposited on a Pt bottom electrode by sputtering, followed by 700°C RTA in oxygen atmosphere for crystallization. Then, a Pt top electrode was deposited by sputtering. The capacitor was defined by photolithography and reactive ion etching (RIE). After the capacitor fabrication, the sample was integrated with 2-layer Al metallization. Capacitor size used for electrical measurements was 35um x 35um. The ferroelectric properties were measured by HFE-HS1 (toyo-technica). A hysteresis loop of the capacitor was measured with 1kHz triangular pulse. The amplitude of the pulse was ranging from 1V to 5V. In the following part of this paper, t
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