An 8-Mbit 0.18-µm CMOS 1T1C ferroelectric RAM (FeRAM) in a planar ferroelectric technology was developed. Even though the cell area of 2.48 µm2 is almost equal to that of a 4-Mbit stacked-capacitor FeRAM (STACK FeRAM) 2.32 µm2[1], the chip size of the developed 8-Mbit FeRAM, including extra 2-Mbit parities for the error correction code (ECC), is just 52.37 mm2, which is about 30% smaller than twice of the 4-Mbit STACK FeRAM device, 37.68mm2×2[1]. This excellent characteristic can be attributed to the large cell matrix architectures of the sectional cyclic word line (WL) that was used to increase the column numbers, and to the 1T1C bit-line GND level sensing (BGS)[2][3] circuit design intended to sense bit lines (BL) that have bit cells 1K long and a large capacitance. An access time of 52 ns and a cycle time of 77 ns in RT at a VDD of 1.8 V were achieved.

It is well known that PZT material properties are strongly dependent on the Zr/Ti ratio. The reliability characteristics, such as the retention and imprint properties, of PZT thin film correlate with the reliability of FRAM(R). PZT films with various Zr/Ti ratios, 60/40, 52/48, 45/55, 40/60 and 30/70 were prepared on Pt/IrO2 and Pt/Ti electrodes by a sol-gel process. With lower Zr/Ti ratio, the grain size becomes smaller and the film was highly oriented to (111) crystallographic plane. But, pyrochlore phase in the PZT films on Pt/IrO2 electrode was detected by SEM and XRD. Hysteresis and pulse responses were measured on the capacitors. With lower Zr/Ti ratio, Pr and Vc become larger. It was found that the preferential (111) orientation played an important role in determining Pr. Voltage shifts which are related to imprint are dependent on the Zr/Ti compositional ratio. Increasing the Ti concentration causes the voltage shift to increase due to more oxygen vacancies. But, this tendency was not in accordance with the results of Qos which were measured by capacitor test simulation of imprint properties for FRAM operation, because the results of Qos did not change monotonically with Ti concentration. However, the capacitor for 45/55 film grown on Pt/IrO2 had good imprint property, similar to the capacitor for sputtered PLZT. In addition, H2 degradation of PZT capacitor with Pt/IrO2 was studied. It was found that H2 annealing degraded the PZT capacitors even at temperatures as low as 150. Thus, imprint and H2 degradation are serious problems for PZT capacitors and these will be a key issue in the reliability of FRAM.