Seigen Otani

Interface between electrode and PZT memory device

Abstract PbZrTiO 3 (PZT) is one of the most promising ferroelectric materials for ferroelectric random access memory (FeRAM). But, there is a problem that the PZT ferroelectric properties are degraded with platinum (Pt) electrodes after annealing in a hydrogenous atmosphere during LSI fabrication. This degradation has been attributed to the catalytic nature of Pt, which dissociates H 2 into protons which then migrate into the PZT and reduce it at elevated temperature. In this theory, it is unclear how Pt and PZT physically interact at the interface. Also, the mechanism concerning the generation of an interfacial layer of oxygen vacancies has not been addressed, which creates some ambiguity in the model. We researched the interface using secondary ion mass spectroscopy (SIMS) and field-emission type transmission electron microscopy (FE-TEM) in order to more clearly understand its impact on the degradation mechanism. We have verified atomically that annealing a Pt/PZT/Pt capacitor results in Pb diffusion from the PZT into the Pt electrode and Pt diffusion from the electrode into the PZT. We next verified that the interdiffusion is not a usual thermal interdiffusion process, but appears rather to be generated by the reaction with hydrogen on both top and bottom Pt electrodes. Finally, we present a model of how the effects of hydrogen reduction combine with the Pt catalysis to form oxygen vacancies. Lead and Pt interdiffuse easily through these vacancies at both interfaces. The model presented here can predict the distribution of vacancies and demonstrates the limitations of the recovery anneal. It also supplements our understanding of the degradation process and provides additional credibility to the above theory.

Hysteresis variations of (Pb, La)(Zr, Ti)O3 capacitors baked in a hydrogen atmosphere

Baking (Pb, La)(Zr, Ti)O3 capacitors in a hydrogen atmosphere causes a significant loss of remanent polarization even at 150 °C. The hysteresis variations depend on the polarization states during baking. The hysteresis loop showed voltage shifts when the capacitor was polarized before baking, whereas it became a cramped shape when the baking was carried out on a virgin capacitor. Although remanent polarization diminished in all cases, saturation polarization was not suppressed. The clamped hysteresis loop can be described as an average of two loops shifted to positive and negative voltages. The results indicate that the loss of remanent polarization is not due to the suppression of switching, but due to the shift of the hysteresis of each domain larger than the coercive voltage.

Evaluation of PZT capacitors with Pt/SrRuO3 electrodes for feram

Abstract (Pb, La)(Zr, Ti)O3[PLZT] films were prepared by CSD on sputtered electrodes of Pt/IrO2 on SiO2/Si wafers. Top electrodes consisting of Pt/SrRuO3(SRO) were sputter deposited and the Pt/SRO/PLZT/Pt capacitors were annealed at 600°C. Evaluation of fatigue endurance revealed that more than 6% excess Pb was necessary to produce a fatigue free capacitor. However, the FeCap leakage current increased in proportion to the film excess Pb content. SIMS analysis of the FeCap containing 10% excess Pb revealed that Sr from the 70 nm thick SRO electrode diffused into the PLZT film to the bottom electrode during the anneal resulting in high leakage. FeCap leakage current was greatly reduced by decreasing the PLZT film excess Pb content and SRO film thickness. SRO electrodes with thicknesses of 5 and 15 nm were found to be sufficient to produce a capacitor with high fatigue endurance and little static imprint. These results indicate that the PLZT leakage current was greatly influenced by the Sr interdiffusion and...

Evaluation of (Pb, La)(Zr, Ti)O3 (PLZT) Capacitors of Different Film Thicknesses with Pt/SrRuO3 Top Electrodes

(Pb, La)(Zr, Ti)O3 [PLZT] films with thicknesses of 150, 225 and 300 nm were prepared by chemical solution deposition (CSD) on Pt/IrO2 coated SiO2/Si wafers. Top electrodes of Pt/SRO were sputter deposited and annealed at 600°C to form a capacitor. All three PLZT films were highly (111) oriented and showed high switchable polarization of >40 µCcm-2 at 200 kVcm-1. A coercive field of 40 kVcm-1 was observed for all three films regardless of thickness. Little fatigue degradation was observed up to 1010 cycles. These results indicate that it is possible to combine both oxide and metallic contacts in a high endurance ferroelectric capacitor for low voltage applications.

Laser annealing of SrTiO3 thin films deposited directly on Si substrates at low temperature

SrTiO3 films sputter deposited directly on a Si substrate at 200 °C were annealed at a scanning cw Ar laser beam at a substrate temperature of 200 °C. The laser annealing improved the crystallinity of the SrTiO3 films and minimized the formation of a SiO2 layer between SrTiO3 and Si. These results are confirmed by x‐ray diffraction analysis and secondary ion mass spectrometry. The dielectric constant of the SrTiO3 films increased monotonically with laser power up to 1.2 W. The dielectric constant was improved from the as‐grown value of 26–118 for a 160‐nm‐thick SrTiO3 film annealed at a 1.2 W laser power. At 1.0 W laser power, the annealed dielectric constant varied from 55 to 101 with increasing film thickness from 110 to 380 nm. From the film thickness dependence constant, it is shown that the intrinsic dielectric constant for the SrTiO3 films is about 150.

RuO2 Thin Films as Bottom Electrodes for High Dielectric Constant Materials

Ruthenium dioxide ( RuO2) thin films are evaluated as a bottom electrode for SrTiO3. It was found that a thin RuO2(50 nm)/Ru(20 nm) layer on Si is quite effective as a barrier layer for both orygen atoms and metals when depositing SrTiO3 at a relatively low temperature of 450° C. To test its suitability for high-temperature processes such as CVD of SrTiO3, the RuO2/Ru electrode on Si was annealed in air at 600° C for 1 hour. Even under this severe condition, the electrode using 100-nm-thick RuO2 was sufficient for preventing oxygen diffusion into Si.

Influence of Electrode Contacts on Leakage Current of SrTiO3 Capacitors

The current-voltage (I-V) characteristics of capacitors using SrTiO3 film were investigated. Rectification characteristics were observed, either when oxygen gas was introduced during sputter deposition of top electrodes, or when the SrTiO3 film was annealed in oxygen. These I-V characteristics are attributed to blocking contacts between SrTiO3 and the electrodes. It is considered that such contacts are formed because of the reduction of the crystal defects in SrTiO3 films, and they have decisive influence on the leakage current of the capacitor.

Microstructure evolution and leakage phenomena of CSD PLZT thin films

Abstract (Pb,La)(Zr,Ti)O3 [PLZT] thin films were deposited by chemical solution deposition (CSD) on sputtered Pt/IrO2 electrodes on SiO2/Si wafers. A relationship between PLZT grain size and leakage was observed with films of 150 nm thick. Large-grained films showed high leakage, whereas fine-grained films showed low leakage. Limited nucleation sites led to pyrochlore at grain boundaries, which may act as an electrical pathway. Thin CSD PLZT film with lower leakage was prepared by shortening the total pyrolysis time. From these results, pyrolysis time was an important parameter used to control film microstructure and leakage.

Process Degradation of a Ferroelectric Capacitor

Ferroelectrics used in a memory device such as (Pb,La)(Zr,Ti)O 3 (PLZT) are vulnerable to reducing atmosphere and lose remanent polarization easily. In the semiconductor processes, hydrogen gas is generated both from deposition gas of interlayer dielectric and from reaction between metals and moisture in the dielectric. Improving the ferroelectrics resistance to reducing environments is required for the planarization and multi-layer interconnections in future devices. Loss of remanent polarization is related to the imprint properties of the capacitor and can be improved by controlling the deposition condition of sol-gel PLZT and annealing IrO x electrode in oxygen.