Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process
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A two-step seeding process has been developed to lower the transformation temperature and modify the grain structure of ferroelectric lead zirconate titanate (PZT) thin films with high Zr/Ti ratio. Previous study has shown that nucleation is the rate-limiting step for the perovskite formation. Therefore, any process that enhances the kinetics of nucleation is likely to decrease the transformation temperature. In this process, a very thin (45 nm) seeding layer of PbTiO3, which has a low effective activation energy for perovskite formation, was used to provide nucleation sites needed for the low temperature perovskite formation. In this study, we have shown that the pyrochlore-to-perovskite phase transformation temperature of PbZr_cTi1_xO3 films of high Zr/Ti ratio (e.g., x = 53/47) can be lowered by as much as 100 °C. The grain size of these films can also be substantially modified by this two-step approach.
I. INTRODUCTION Recently, there has been a growing interest in developing ferroelectric thin films for nonvolatile memory applications. One of the promising materials for this application is the lead zirconate-lead titanate (PZT) solid solution. According to the phase diagram of PZT,1 the boundary between the ferroelectric and antiferroelectric phase is approximately 6 mole % of Ti. Any composition of PZT with Ti content higher than 6% is in the ferroelectric region. A commonly used PZT composition contains about 53 mole % Zr and 47 mole % Ti (53/47), which is located at the morphotropic phase boundary. Preliminary study has shown that ferroelectric films of composition close to MPB composition (from Zr/Ti ratio of 40/60 to 65/35) possess very good ferroelectric properties required for fast and reliable memory switching. In order to integrate PZT films into the existing semiconductor processes, many process problems have to be solved. One of the problems is the high temperature post-deposition annealing needed to form the desirable perovskite phase. This annealing is required because most of the as-deposited films are amorphous, and they form an intermediate nonferroelectric pyrochlore phase before the formation of the perovskite phase. The transformation temperatures for initial perovskite formation (7f er ) and complete perovskite formation (T^T) of the PZT films are functions of the compositions and of the type of substrate used for deposition. Typical annealing temperatures for the 53/47 films vary from 650 °C to 750 °C. At these annealing temperatures, interdiffusion among the PZT films, the contact electrodes, and the underlying metallization becomes a genuine concern; furthermore, thermal stress developed during the high J. Mater. Res., Vol. 8, No. 2, Feb 1993
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temperature annealing may affect the long-term reliability of the device. The objective of this study is to develop a novel process of thin film deposition which requires a lower perovskite transformation temperature. Chen and Mackenzie have commented qualitatively that the perovskite transfo
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