Lead zirconate titanate(PZT) is a ferroelectric materials which has perovskite structure. The PZT film is one of the promising materials applicable to capacitors in memory devices, such as dynamic random access memory(DRAM) and ferroelectric non-volatile memory(FRAM), because of its high dielectric constant, high remanent polarization and P-E hysteresis properties. There has been much research on integrated PZT thin film memories for the past several year. In particular the fatigue properties of PZT thin films, which is a critical problem in applications for FRAM devices, has been extensively studied. To date, many approaches to overcome the switching fatigue problem have been proposed. However, the detailed doping effects have not been firmly established and Sb doping effects in PZT thin films have never been reported.
In this study, PZT thin films have been grown on platinized Si substrates at 550℃ by d.c. reactive sputtering using multi-target. The multi-target sputtering method leads to better control of the process in the case of multi-component films. Sb doped reactive sputtering-derived $Pb(Zr,Ti)O_3$ (Zr/Ti=48/52) thin films were investigated with the intention of improving ferroelectric properties. 5Ø Sb pellets were co-sputtered in order to add Sb in PZT film. Also, the atomic valence of Sb in PZT thin film was confirmed as trivalent cation $(Sb^{3+})$ by x-ray photoelectron spectroscopy(XPS). According to the electronegativity difference and tolerance factor t, $Sb^{3+}$ tends to occupy the B-site of $ABO_3$ perovskite structure and acts as an acceptor that generates oxygen vacancies and holes. Transmission Electron Microscope(TEM) was used to observe the structural changes of PZT thin films by Sb addition. The leakage current densities, Pr, Ec, Ei and polarization offset in lightly doped(＜ 1at%) PZT thin films increased as the Sb contents increased, but for heavily doped(＞ 1at%) PZT thin films, these parameters decreased. 0.7at% Sb doped PZT(PZST07) thin films exhibited improved fatigue properties (about 10% degradation of the remanent polarization after $10^9$ switching cycles).
In order to observe the grain size effect, PZT and PZST thin films were annealed in oxygen atmosphere at 650℃, 30min. Annealed PZT and PZST thin films have a good fatigue behavior with larged grain size. Annealed PZST thin film is fatigue free to $10^{11}$ switching cycles and have a larger remanent polarization(33μC/㎠) than $SrBi_2Ta_2O_9$(SBT) thin films.
Pt bottom electrodes with (111) and (200) preferred orientation were used in order to control the orientation of the PZT thin films. PZT thin film with (111) preferred orientation has a larger saturated polarization than that with (100) preferred orientation. Also this preferred orientation of PZT thin films did not affect the fatigue property.
Pb concentration was controlled by the variation of Pb target current. This relation was expressed as y=0.89x-11.09. The pyrochlore phase was transformed to perovskite phase as Pb concentration was increased. This phase transformation improved the electrical properties of PZT thin films. In perovskite phase, fatigue properties were not improved with Pb concentration. Pb concentration(Pb vacancy) did not affect the fatigue properties of PZT thin films.
Finally, main factor on fatigue properties was grain size. Microstructure(grain size) effect was predominant in fatigue behavior of PZT to ionic defects(oxygen vacancy and Pb vacancy).
