Kazushi Amanuma

Preparation and ferroelectric properties of SrBi2Ta2O9 thin films

Ferroelectric SrBi2Ta2O9 thin films were synthesized on Pt/Ti/SiO2/Si substrates using a solution deposition process, and structural and electrical properties were investigated. The spin‐on films crystallized during firing above 700 °C. The films showed high diffraction peaks of (105) and (200), while little peaks from (00l) planes were observed. Good ferroelectric properties were obtained for a 280 nm thick film; Pr and Ec were 10.0 μC/cm2 and 38 kV/cm, respectively. Fatigue endurance was excellent; the hysteresis loop does not change up to 109 switching cycles. These properties are very attractive for nonvolatile memory application.

Ferroelectric Properties of Sol-Gel Derived Pb(Zr, Ti)O3 Thin Films

Pb(Zr, Ti)O3 thin films of thickness ranging from 55 nm to 625 nm were synthesized on Pt/Ti/SiO2/Si substrates using a sol-gel process. The film thickness dependence of both microstructure and electrical properties was investigated. The synthesized films showed columnar structure. The diameter of each column was around 100 nm regardless of the film thickness. The 328 nm-thick film exhibited a dielectric constant (e) of 1000, remanent polarization (Pr) of 20 µC/cm2, and coercive field (Ec) of 47 kV/cm. Both e and Pr decreased and Ec increased with decreasing film thickness. This behavior is attributed to the existence of a low-dielectric-constant interface layer. The results of leakage current measurement were in good agreement with the model of space-charge-limited current.

Fatigue Characteristics of Sol-Gel Derived Pb(Zr, Ti)O3 Thin Films

Fatigue mechanisms of sol-gel derived Pb(Zr-Ti)O 3 (PZT) thin films and the effects of La addition on the fatigue characteristics were investigated. For PZT(40/60) and PZT(53/47), the remanent polarization (P r ) and the coercive field (E c ) decreased with switching cycles. These properties were restored by heating the fatigued films above their Curie temperature. This result indicated that domain pinning is the dominant fatigue mechanism for PZT(40/60) and PZT(53/47). On the other hand, for PZT(70/30), P r was almost constant and E c increased with switching cycles; this implies that the fatigue mechanism for PZT(70/30) is different from that for PZT(40/60) and PZT(53/47)

Crystallization behavior of sol‐gel derived Pb(Zr,Ti)O3 thin films and the polarization switching effect on film microstructure

The microstructure of sol‐gel derived Pb(Zr,Ti)O3 thin films was investigated using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The AES depth profile revealed that the Ti content decreases and the Zr content increases from the bottom through the thickness as a result of the crystallization into perovskite phase. A cross‐sectional TEM micrograph showed that the PZT grains are columnar and grow epitaxially from the bottom to the surface. It was also shown that the 2.5‐nm‐thick interface layer appeared at the top Au electrode‐PZT interface after 108 polarization switching cycles, while there was no change at the bottom Pt‐PZT interface.

Capacitor-on-metal/via-stacked-plug (CMVP) memory cell for 0.25 /spl mu/m CMOS embedded FeRAM

A capacitor-on-metal/via-stacked-plug (CMVP) memory cell was developed for 0.25 /spl mu/m CMOS logic embedded FeRAM. Using 445/spl deg/C MOCVD Pb(Zr,Ti)O/sub 3/ process, a ferroelectric capacitor is formed after CMOS logic fabrication. Thus, FeRAM can be embedded without changing any logic devices and processes. Furthermore, this technology enables cell size reduction (3.2 /spl mu/m/sup 2/ for 1T1C), minimum process damage on ferroelectric, and low manufacturing cost.

Difference in microstructure between PZT thin films on Pt/Ti and those on Pt

Abstract PZT thin films were synthesized by sol-gel on a Pt/Ti double layer bottom electrode or on a Pt single layer bottom electrode to investigate the bottom electrode dependence of PZT film structure. On Pt/Ti, oriented perovskite grains with 50–100 nm lateral size were densely packed. On Pt, large perovskite grains (2−3 μm) were surrounded by fine pyrochlore grains (about 5 nm), and no certain orientation was observed. TEM and EDX analyses suggested that a fraction of Ti in the Pt/Ti layer diffused along Pt grain boundaries up to the Pt surface and was oxidized during the PZT annealing process. A model for the crystallization of sol-gel derived PZT was proposed, in which TiOx particles at the Pt surface act as nucleation sites for PZT crystallization. This model well explained the experimental results.

Low-Voltage Switching Characteristics of SrBi2Ta2O9 Capacitors

Ferroelectric capacitor arrays were fabricated using SrBi2Ta2O9 (SBT) thin films. Hysteresis and pulse responses were measured as functions of capacitor size and applied voltage. The remanent polarization (P r) at 5 V does not depend on capacitor size, though P r at low applied voltage decreases considerably as capacitor size decreases below 10 µ m. High-voltage pulse application enhances low-voltage polarization switching. Retention characteristics strongly depend on operating voltage. Switching charge at 2 V or above is stable up to 104 s retention, while that at 1 V operation decreases with increasing retention time.

Preparation of Pb(Zr, Ti)O3 Thin Films by Multitarget Sputtering

Thin lead zirconate titanate [PZT] (Zr/Ti=50/50) films (230–430 nm) were deposited by multitarget sputtering with three stoichiometric PZT targets and one PbO target. To control Pb content in film, the rf power for the PbO target was varied. The rf power for each PZT target was also varied to control the deposition rate. PZT films with single perovskite phase or dominant perovskite phase were obtained only when the film composition was self-controlled. Low deposition rate resulted in a wider range of PbO input power in which Pb/(Zr+Ti) ratio was almost constant. Therefore low deposition rate would enhance the self composition control (SCC) mechanism. A 240-nm-thick PZT film deposited under the SCC mechanism and low deposition rate showed remanent polarization of 21.5 µ C/cm2, dielectric constant of 600 and leakage current of 7.0×10-8 A/cm2.

Effect of interconnect layer on Pb(Zr,Ti)O 3 thin film capacitor degradation

Ferroelectric properties of a Pb(Zr,Ti)O/sub 3/ (PZT) thin film capacitor with a conventional Al/TiN/Ti interconnect layer are seriously degraded by annealing at around 400/spl deg/C. The degradation is observed even if a contact hole on the top electrode is not formed. This indicates that the cause of the degradation is not the diffusion of the interconnect material into the PZT film, and this is confirmed by secondary ion mass spectrometry. We suggest that it is the thermal strain of the interconnect layer which imposes tensile stress on the PZT film during the annealing that degrades the ferroelectric properties of the PZT capacitor.

Structural and Ferroelectric Properties of SrBi 2 Ta 2 O 9 Thin Films

Structural and electrical properties were investigated for chemically prepared SrBi 2 Ta 2 O 9 (SBT) thin films on Pt/Ti/SiO 2 /Si substrates. Good ferroelectric properties were obtained with a Pt top electrode: Pr=10.0μC/cm 2 and Ec-34kV/cm. Au top electrodes resulted in smaller Pr. However, no fatigue was observed up to 10 9 switching cycles regardless of the top electrode material. Grains were spherical, not columnar, and the average grain size was 200nm. A marked structural change took place in the bottom Pt/Ti electrode during film preparation. The SIMS analysis indicates the reaction between Bi and Pt