Hiroshi Ashida

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.

Millimeter-wave beam multiplexing method using hybrid beamforming

A millimeter-wave beam multiplexing method using a hybrid beamforming is proposed. We theoretically show that inter-subarray coding at the subarray type interleaved configuration can reduce interference and multiple beams of the same high gain as the full connection type. The condition that a hybrid beamforming of the subarray type can create the multiple high gain beams is clarified. For the case of beam multiplexing, the interleaved configuration with inter-subarray coding is confirmed to be robust for interference increasing by errors of beam direction estimation. Therefore, we think that the interleaved configuration is suitable for millimeter-wave beam multiplexing.

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.

System validation of millimeter-wave beam multiplexing with interleaved hybrid beam-forming antennas

This paper describes beam division multiplexing at millimeter wave band using interleaved hybrid beam-forming (BF) antennas. The antenna elements are located alternately to make narrow beam-width of about 5 degrees, and the user multiplexing calculation on the digital signal processing part can suppress the undesired grating-lobe. The authors manufacture the interleaved hybrid beamforming antennas at 60GHz, and conduct antenna beam pattern measurements and transmission experiments. The measurement results show the desired-undesired ratio of 18dB after the suppression of the grating lobe by digital weight multiplication, and show good communication performance from the results of received constellation patterns of OFDM 16QAM modulation scheme.

Robust beamforming method for SDMA with interleaved subarray hybrid beamforming

This paper considers millimeter-wave access network systems with space division multiple access using an interleaved subarray hybrid beamforming structure, where antenna elements in each subarray scatter uniformly over the whole antenna array. The structure can provide high system capacity by multiplexing users in the space domain with narrow beams. However, imperfect user direction estimation might cause system capacity degradation due to interference between the beams. In particular, severe interference is experienced in the interleaved subarray configuration because many grating lobes are observed due to the wide element spacing in each subarray. Therefore, we propose a robust beamforming method in order to provide high system capacity even if user direction estimation is inaccurate. In the proposed method, multiple subarrays form beams to a common direction when the peaks of beams directed to the respective user directions are overlapped. We conduct computer simulations to evaluate the proposed method with a non-ideal beam search. The simulation results demonstrate that the proposed method can improve system capacity in the SDMA systems with the interleaved subarray hybrid beamforming structure.