A Study on the Selective Nucleations for Formation of Large Single Grains in PZT Thin Films
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problems of ferroelectric films affecting lifetime. [4,5] There are many reports about the degradation phenomena in ferroelectric materials and the main source is thought to be the pinning of domain walls, which inhibits switching of the domains. A variety of mechanisms for domain wall pinning have been proposed, including pinning due to electronic charge trapping, pinning by oxygen vacancies, as well as pinning by extended defects. The interface between the electrode and ferroelectric film is thought to be the main site of these charges and defects. So metal electrodes, especially Pt, are not acceptable for high density devices that require a large number of bipolar cycles. It is well known that the grain boundaries, which are randomly generated during phase transformation, are very weak points with regard to their electrical characteristics because grain boundaries have many defects and are not fully crystallized structure. [6] However grain boundaries are not thought to be the main source of the degradation because it is impossible to remove grain boundaries or control the location of grain boundaries in ferroelectric thin films. In this study, we have investigated the electrical characteristics of the grain boundaries in PZT thin films to determine their effects on degradation, and by development of a novel process, we made artificial nucleation sites on the Pt electrode so the locations of grain boundaries could be pre-determined by controlling nucleation sites. Thereby, we could measure the polarization and electrical properties of the PZT thin films without grain boundary in the area studied. The growth behavior and size of grains are investigated according to annealing temperature and time, and the growth mechanisms are discussed in detail. The ferroelectric and electrical performance 217 Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
of thus obtained PZT thin films were characterized. EXPERIMENT PZT(65/35) thin films were formed on Pt/SiO 2/Si substrates by RF magnetron sputtering at 350 IC using multi-metal targets of Pb, Zr, and Ti. Base pressure before sputtering was below I x 10-5 Torr and the working pressure was 20 mTorr during sputtering. PZT thin films (100 rum) for seeding layer were deposited on Pt/SiO 2/Si substrates and transformed into perovskite phase at 700 IC by rapid thermal annealing (RTA). Seeding islands (area: 13 Inx 13 Im, space : 40 un) were patterned by a conventional lithographic and etching method. PZT thin film (200 rnm) was deposited on the substrates having PZT seeding islands. Figure 1 shows a schematic diagram of the fabrication process of the single-grained PZT array using PZT island seeds. Post annealing of the films was carried out in a furnace in air. The growth behavior and the crystal structures of the films were investigated with an optical microscope and X-ray diffractometer, respectively. The conduction properties were measured using an HP4140B pA meter and the polarization properties were examined by RT66A. Fatigue tests were performed at roo
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