Ferroelectric random access memory (FRAM) has been commercialized for about 20 years and its reliability has been well proven all over the world. In the recent Internet of Things (IoT) era, it also plays important roles to particularly in edge computing because of its high writing speed, high rewriting endurance, and low writing energy consumption. We review the history of semiconductor memories using ferroelectrics and overview the progresses of the new ferroelectrics and promising ferroelectric applications in the future.

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We investigated the crystallization mechanisms of sputter-deposited La-doped Pb(Zr,Ti)O3 (PLZT) on a Pt/Ti metal stack in the postdeposition annealing (PDA) at 600 °C in O2-mixed Ar ambient. As-deposited amorphous PLZT generally transforms to a perovskite phase over 550 °C through a metastable pyrochlore phase during the PDA. We found that the O2 content of the PDA ambient crucially affects the pyrochlore-perovskite transformation (PPT) speed. While an O2 content much higher than 2% of the PDA ambient suppresses PPT, an O2 content much lower than 2% enhances PPT. An O2 content around of 2% of the PDA suppresses PPT near the surface of PLZT and simultaneously keeps PPT fast in the inner regions of PLZT in the pyrochlore phase because of the O2 diffusion limit from the PLZT surface, eventually resulting in almost only the growth of highly {111} oriented columnar PLZT on Pt, which reveals better electric properties than those obtained by the PDA with the ambient of O2 contents much higher or lower than 2%.

we have developed ferroelectric capacitor fabrication technique and a new sensing amplifier circuit to realize lowvoltage and high-density FRAM. Improvement of IrO x top electrode near the ferroelectric interface successively lowers operation voltage. And our developed "Dual Reference Sensing Amplifier" enables to commercialize highly-reliable FRAM with memory density of 4Mb or larger. FRAM, Feroelectric, PZT, IrO, Dual SensingI.

In our previous works on La-doped Pb(Zr,Ti)O 3 (PLZT) growth on a Pt/Ti bottom electrode, the O 2 content in postdeposition annealing (PDA) was found to play an important role in obtaining good electrical characteristics and high manufacturing yield of ferroelectric random access memory (FeRAM). The optimal O 2 content of around 2% inhibits the growth of randomly oriented La-doped Pb(Zr,Ti)O 3 (PLZT) grains near the PLZT surface, resulting in the growth of highly {111}-oriented PLZT. We found that the Pt bottom electrode grown on an AlO x layer can further suppress the formation of randomly oriented PLZT grains near the PLZT surface and increases the optimal O 2 content range from 2 to 50%, which can enlarge the manufacturing process margin of PDA. It is proven that the AlO x layer blocks the diffusion of lead oxides (PbO x ) from PLZT to SiO 2 interlayers through Pt and promotes pyrochlore-perovskite transformation near the bottom electrode during PDA.

We developed a lanthanum-doped Pb(Zr0.4,Ti0.6)O3 (PLZT)-based ferroelectric capacitor for low-voltage operation of ferroelectric memory for use in edge devices in the internet of things. The structure, consisting of PLZT stacked layers 30 and 90 nm thick, showed drastically improved performance in three main measures: low leakage current, high saturation polarization, and low saturation voltage. Secondary ion mass spectroscopy indicated that atomic interdiffusion between PLZT and IrOx in the top electrode (TE) was suppressed by the 30 nm thick PLZT, which plays the role of a diffusion barrier, producing a low leakage current and high saturation polarization. The higher oxygen atomic density of the PLZT near the TE interface lowers the oxygen vacancy, which should also suppress the leakage current and the P–V hysteresis shift (imprint). The large grain size and high crystalline quality of PLZT near the TE interface in the PLZT structure provide a high saturation polarization with low leakage current.

we have developed ferroelectric capacitor fabrication technique and a new sensing amplifier circuit to realize lowvoltage and high-density FRAM. Improvement of IrO x top electrode near the ferroelectric interface successively lowers operation voltage. And our developed "Dual Reference Sensing Amplifier" enables to commercialize highly-reliable FRAM with memory density of 4Mb or larger. FRAM, Feroelectric, PZT, IrO, Dual SensingI.