For high-density ferroelectric random access memory devices (FeRAMs) with capacitor over plug (COP) structure, oxygen diffusion barriers based on bi-layered Ir have been investigated. This paper describes the detailed characteristics of the barriers. A bi-layered Ir barrier was fabricated by repeating the deposition and RTO treatment of an Ir metal film, which was very effective to obtain excellent barrier properties against oxygen diffusion. Surface roughening was shown after RTO, but can be suppressed by lowering the RTO temperature. The roughening is caused by the formation of a gaseous phase of IrOx during RTO, and not by the formation of Ir hillocks. The stress of the bi-layered Ir barrier is tensile after RTO, which makes further stacking of more layers on the barrier with good adhesion possible. Performance of this barrier was checked using the post annealing in 18 O isotope ambient at 650 • C for 2 hours. SIMS profile showed the barrier prevented the diffusion of 18 O, effectively. The above results strongly suggest that the bi-layered Ir barrier can be applied to the COP structure for high-density FeRAMs.
The effect of thin Ti/PbZr0.4Ti0.6O3 seed layers on the properties of PbZr0.4Ti0.6O3 (PZT) capacitors has been investigated. The seed layer is based on a bi-layer of thin Ti and thin PZT with a total thickness ranging from 10 to 25 nm, which was deposited on Ir/Pt or Ir/IrO2/Pt by sputtering. After crystallization of the seed layers the main 130-nm-thick PZT film was deposited and crystallized. As a result, a highly preferred (111)-orientation of the PZT was obtained on a 10-nm-thick seed layer, where the peak intensity ratios of (111)/{100} and (111)/{110} are about 100 and 20, respectively. The 10-nm-thick seed forms a pyrochlore phase with a very smooth surface, where the formation of the pyrochlore phase is attributed to Pb diffusion, resulting in a Pb deficient stoichiometry. The seed layer transformed to the perovskite phase during the main PZT crystallization. It is shown that an IrO2 layer beneath the Pt can prevent Pt layer degradation related to the volume expansion due to the oxidation of Ir during the main PZT crystallization. Capacitors with the 10-nm-thick seed layer fabricated on the Ir/Pt and Ir/IrO2/Pt substrates showed typical 2 Pr values of 44.0 μC/cm2 and 41.2 μC/cm2, respectively. The voltage found for 90%-polarization saturation is about 3.0 V, and the capacitors are fatigue-free at least up to 1010 switching cycles.
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