A ferroelectric material, utilized for FRAM devices, is a kind of functional dielectric, which has a perovskite crystal structure, with bi-stable states, each of which corresponds to the logic states, 1 or 0. Among the various ferroelectric materials, $Pb(Zr,Ti)O_3$ (PZT) is the best candidate for the application to the FRAM devices. FRAMs are not only nonvolatile but also have stronger read/write endurance, and lower programming voltage and faster writing speed than other nonvolatile memories. FRAMs are in great demand for IC card, mobile computer and robots that have learning algorithms.
However, in order to use FRAMs for electronic equipments, we have to overcome a few issues of reliability. They are as follows: 1) relaxation: the decrease in polarization charge immediately after applying voltage. 2) external depolarization: the degradation of remnant polarization by the sense capacitor in a Sawyer-Tower circuit 3) imprint: the shift in a specific polarized direction in the hysteresis curve. 4) size effect: the degradation of polarization characteristics with decreasing the thickness of the ferroelectric films. In this study, the mechanisms and the solutions of the reliability problems in PZT thin film capacitors were investigated.
The retained charge (memory window) of a ferroelectric capacitor measured by pulse method is smaller than that measured by continuous wave method. This is attributed to the relaxation phenomenon which represents the fast loss of the remanent polarization after removal of the applied voltage. The relaxation phenomenon is induced by the depolarization field which is affected by various parameters such as the ferroelectric properties and thickness of the ferroelectric film, measurement temperature, electrode material and the formation of interfacial layer between the ferroelectric film and the electrodes. In this study, the effects of each parameter and the causes of the relaxation phenomenon were investigated. The magnitude of polarization loss increases with that of the initial remanent polarization. The increased polarization loss with increasing measurement temperature is attributed to the reduced coercive field with temperature. The fatigue stressing and the use of $RuO_2$ top electrode cause the formation of nonswitching interfacial layer between the ferroelectric film and the electrodes, resulting in the large relaxation and thus large polarization loss. The reduction in the thickness of the ferroelectric film also results in the increasing polarization loss by increasing the magnitude of depolarization field.
The external depolarization phenomenon, which is the degradation of remnant polarization by the sense capacitor in a Sawyer-Tower circuit, was investigated with a physical model. In the Sawyer-Tower circuit, different P-V curves are obtained with \alpha value, which is the capacitance ratio between the sense capacitor and the ferroelectric capacitor (i.e $α=C_{sense}/C_f)$. As α decreases, the apparent polarization values decrease due to the effect of the external depolarization field. In this study, the degradation mechanism of remnant polarization in the ferroelectric capacitor by the external depolarization field is investigated with varying modeling parameters. As α decreases, the external depolarization field lowers the polarization values by changing the voltage across the ferroelectric film. The external depolarization property is closely related to the ferroelectric property of the film.
The dynamic and static imprint characteristics of the PZT film capacitors were studied. The cause of the imprint is the internal field generated by the trapping of the electronic charges injected from the electrodes by the electric field in the interfacial layers. The dynamic imprint is induced by the repetitive application of the unipolar pulses to the ferroelectric capacitors, which means repetitive storage of the same data in the cells. The cumulative time when the unipolar pulse is applied is more responsible for the dynamic imprint characteristics than the unipolar pulse cycles. When the unipolar pulses with high amplitudes are applied or the unipolar pulses are applied at high temperatures, the voltage shift is reduced. This is because that a large number of charges are injected into the ferroelectric film and thus result in the trapping of the charges not onlay at the interfacial layers but also in the film. Holding the remanent polarization without external bias induces static imprint, which represents storing one digital information for a long time. When the absolute value of the poling voltage is larger than 3 V, there is no remarkable difference in the static imprint characteristics with the magnitude and the polarity of the poling voltage. The voltage shift is increased as the temperature of the static imprint stressing is increased. Voltage shift can be easily removed by applying bipolar pulses at high temperatures. The estimation method of lifetime limited by the read failure in 1T/1C FRAMs is proposed and the lifetime of the PZT film capacitor fabricated in this study is about 82 years. Ti rich PZT film has a relatively longer lifetime than Zr rich film, which is because the Ti rich film has a comparatively square hysteresis loop.
The mechanism of the size effect in the PZT films with thickness ranging from 60 nm to 145 nm was investigated. The PZT films with thicknesses over 115 nm show excellent electrical properties. Whereas the leakage current characteristics of the films with thicknesses below 85 nm are severely degraded. The coercive voltage of the PZT films is decreased as the thickness of the films is decreased, which implies that it is necessary that the thickness of the ferroelectric film must be decreased for the stable operation at low voltages. However, the saturation property and the squareness of the P-V curve are decreased with decreasing the film thickness. The size effect, which is the decrease in the remanent polarization and the increase in the coercive field, is induced by the charge injection from the electrodes during the measurement of the P-V curves. The injected charges interrupt the switching process of the ferroelectric film. The variation of the imprint characteristics with the thickness of the PZT film demonstrates that the imprint characteristics are closely related to the polarization values of the films.
