Ferroelectric materials have two stable polarization states even when the electric field is removed. A ferroelectric random access memory (FRAM) is a memory that makes use of this bistable states as two logics, “0” and “1”. FRAM has attracted considerable attention because it has many advantages such as nonvolatility, fast read and write speed, high endurance, and high potential for integration due to its dynamic random access memory (DRAM)-like cell structure. Lead zirconate titanate (PZT) is the most promising material for use as a dielectric for the capacitor of FRAM devices because it has a high phase transition temperature($T_c$), a high remnant polarization ($P_r$), and a relatively low fabrication temperature.
One of the most important issues in integrating a high density nonvolatile memory using PZT ferroelectric capacitors is that the polarization hysteresis characteristics of the capacitors are degraded by exposure to a hydrogen-containing environment such as the deposition of interlayer dielectric (ILD), and the passivation process. This hydrogen-induced degradation depends strongly on the top electrode material and, in particular, the degradation is quite severe when Pt is used as top electrode.
The main purpose of this study is to clarify the mechanism for the degradation of ferroelectric properties of Pt/PZT/Pt capacitors during hydrogen annealing and to improve the resistance of PZT capacitors to hydrogen-induced degradation. For this purpose, we have investigated the degradation behavior of Pt/PZT/Pt capacitors under several conditions of hydrogen annealing and suggested a new electrode system showing a good resistance to hydrogen-induced degradation.
The degradation of ferroelectric properties for PZT capacitors with Pt top electrode is caused by the catalytic effect of Pt, which dissociates $H_2$ molecule into atomic hydrogen diffusing into the PZT film and creating oxygen vacancies that are responsible for the loss of polarization due to domain pinning. At the initial stage of hydrogen annealing, a large shift of polarization hysteresis curve toward the negative voltage direction occurs, while the polarization value of PZT capacitors decreases slightly. This is because the degradation occurs mainly at the vicinity of top-interface. However, as the hydrogen annealing time increases, the polarization value of capacitors decreases further and the degree of voltage shift decreases because the degradation also occurs in the PZT film as well as at the both side of interfaces.
The polarization value of PZT capacitors pre-poled to a positive or negative [Pr] state shows a smaller decrease than that of the virgin capacitors without pre-poling does, but the value of voltage shift in the polarization hysteresis curve was very large. The direction and magnitude of remnant polarization of PZT film determined by pre-poling prior to hydrogen annealing affects the motion of hydrogen ions and the degradation behavior of PZT capacitors depends on the motion of hydrogen ions for compensating the polarization charge during hydrogen annealing. For the PZT capacitors pre-poled to a positive and negative [Pr] state, hydrogen ions which have diffused into the PZT film create the oxygen vacancies at the vicinity of the top-interface and bottom-interface, respectively and form the positive space charge layer at that place, resulting in a voltage shift in polarization hysteresis curves. On the contrary, for the PZT capacitors of virgin state, hydrogen ions create the oxygen vacancies in the bulk of PZT film as well as at the both side of interfaces, resulting in a severe degradation of polarization characteristics.
The resistance of PZT capacitors to hydrogen-induced degradation was largely improved by adopting $LaNiO_3$ top electrode so that no degradation of polarization characteristics occurs after hydrogen annealing at 200℃ for 30 min. However, the capacitors with $LaNiO_3$ top electrode shows a severe degradation after hydrogen annealing at 400℃ for more than 20 min. This is believed to be due to the catalytic effect of Pb, which has out-diffused onto the surface of $LaNiO_3$ top electrode during post-annealing in O$_2$ atmosphere. Therefore, by inserting a thin Pt layer between $LaNiO_3$ electrode and PZT film to prevent the out-diffusion of Pb component from PZT film, we can fabricate the PZT capacitors which are not nearly degraded even after hydrogen annealing at 400℃ for 30 min. The PZT capacitors with $LaNiO_3$ top electrode shows a very good fatigue resistance. This is because $LaNiO_3$ film acts as a sink for oxygen vacancies accumulated at the interface by repeated cycling. The $LaNiO_3$/Pt/PZT/Pt capacitors with the Pt layer interposed between $LaNiO_3$ and PZT shows a better resistance to fatigue cycling than the Pt/PZT/Pt capacitors does. The fatigue resistance of the capacitors with $LaNiO_3$/Pt top electrode was further improved by reducing the thickness of the inserted Pt layer. Therefore, by adopting $LaNiO_3$/Pt top electrode that has been prepared by optimizing the thickness of the inserted Pt layer and the processing conditions, the PZT capacitors having a good resistance not only to hydrogen-induced degradation but also to fatigue cycling can be fabricated.
