Akira Kamisawa

Preparation of Pb(Zr,Ti)O3 thin films on electrodes including IrO2

The development of ferroelectric memory devices requires an improvement of the fatigue properties of ferroelectric thin films. Pb(Zr,Ti)O3(PZT) thin films obtained by the sol‐gel method on Pt/Ti electrodes have reduced residual polarization by continuous polarization reverses about of 108 cycles. The electric characteristics such as fatigue properties have mostly depended on electrode materials. We propose Ir, IrO2, and these layer films as electrode materials and evaluate electric characteristics of PZT thin film capacitors. PZT thin films using Ir/IrO2 and Pt/IrO2 electrodes show no fatigue up to 1012 cycles of ±5 V switching pulses. Moreover, good properties of PZT capacitors, not only on SiO2 but also on polycrystalline silicon, were obtained by using IrO2.

Preparation of Pb(Zr,Ti)O3 Thin Films on Ir and IrO2 Electrodes

Pb(Zrx Ti1-x )O3 (PZT) thin films were prepared on Ir and IrO2 electrodes. Ir has very similar properties to Pt, and IrO2 is a conductive oxide. Perovskite single-phase PZT thin films were obtained on their electrodes. PZT thin films were grown by the conventional sol-gel method with rapid thermal annealing (RTA) at 700° C. When Pt thin films were deposited directly on poly-Si, PtSi layers were formed, and PZT thin films on the Pt had very poor crystallinity. When an IrO2 layer was formed between PZT and poly-Si, a high-quality PZT thin film was obtained. Moreover, when electrodes including the IrO2 layer were used, fatigue properties of PZT thin films were drastically improved.

Surface morphology of lead-based thin films and their properties

Surface morphology of lead-zirconate-titanate [PZT(52/48)] thin films prepared by sol-gel processing on Pt/Ti/ SiO2/Si substrates was studied. When the atomic ratio [Pb/(Zr+Ti)] of PZT gel films was 1, rosette structure was observed in the films after annealing at 600° C for one hour. The crystal structure of the rosette was identified as perovskite and that of the other area was nonperovskite, by electron diffraction analysis. Lead deficiency in the non-perovskite phase caused by lead diffusion into the bottom electrode was detected by energy dispersive X-ray spectroscopy and Auger electron spectroscopy. In order to prepare PZT films with a smooth surface, the following four means were efficient: addition of lead excess, rapid thermal annealing (RTA), adoption of buffer layer, and preparation of crystal nucleus on the substrate.

Electrical Properties of Pb(Zr,Ti)O3 Thin Film Capacitors on Pt and Ir Electrodes.

Pb(Zr x Ti 1-x )O 3 (PZT) thin films were prepared on Ir/IrO 2 and Pt/IrO 2 electrodes. We have already reported that fatigue properties of PZT thin films were improved by using these electrodes due to barrier effects of IrO 2 . In this present paper we describe a study of the imprint characteristics of PZT thin films on these electrodes. There is little difference in fatigue properties between the PZT films on Pt/IrO 2 and Ir/IrO 2 . However, we find some difference in imprint characteristics between the PZT films on these electrodes. In the case of Ir/IrO 2 electrode, improvements in the electrical properties are observed in the measurements of the imprint characteristics. Moreover, improvements in imprint characteristics were also obtained with good (111)-axis orientation of PZT films when Pt x Ir 1-x /IrO 2 electrodes were used.

Study on ferroelectric thin films for application to NDRO non-volatile memories

Abstract In this paper we describe an experimental study on ferroelectric non-volatile memory. The test devices using a MFMIS (metal-ferroelectric-metal-insulator-semiconductor) structure were developed for 1-Tr-type NDRO (non-destructive readout) memory. PZT (Lead-zirconate-titanate) and PT (lead-titanate) thin films were deposited on various bottom electrodes by solgel method. As electrodes, Pt, Pt/Ti, Pt/Ti/Ta and Pt/IrO2 were deposited by RF magnetron sputtering. We found out that a variation in the electrodes caused drastic changes in the C-V characteristics and fatigue characteristics of MFMIS capacitors. The I-V characteristics of Pt/PZT/ Pt/SiO2/p-Si MFMIS FET showed a hysteresis loop and the direction of the loop corresponded to ferroelectric polarization in the PZT film. A 2.7 V memory window was achieved using a 6 V programming voltage. PZT thin films on Pt/IrO2 electrodes showed no fatigue after 1012 switching cycles.

Development of Low Dielectric Constant Ferroelectric Materials for the Ferroelectric Memory Feild Effect Transistor

In this paper we discuss ferroelectric materials suitable for a metal-ferroelectric-metal- insulator-semiconductor feild effect transistor (MFMIS FET). It is important for a ferroelectric material to have a low dielectric constant to enable the application of sufficient electric field to a ferroelectric layer. Films of Sr 2 Nb 2 O 7 and Sr 2 (Ta 1-x Nb x ) 2 O 7 were prepared by the sol-gel method on Pt/IrO 2 electrodes for an MFMIS FET. The ferroelectricities of Sr 2 (Ta 1-x Nb x ) 2 O 7 films were confirmed to be in the range of x = 0.1-0.3. In the case of x = 0.3, the largest remanent polarization was obtained in the hysteresis loop. The values of the remanent polarization and the coercive field are 0.5 μC/cm 2 and 44kV/cm, respectively. The film has a low dielectric constant (e r = 53). The characteristics of Sr 2 (Ta 1-x Nb x ) 2 O 7 thin films are suitable for MFMIS FET.

Micro-Patterning of PbZrxTi1-xO3 Thin Films Prepared by Photo Sensitive Sol-Gel Solution

The photo sensitivity of sol-gel solution of PbZr x Ti 1-x O 3 (PZT) was studied. A coated film of the sol-gel solution on Pt/Ti/SiO 2 /Si substrates was exposed to an excimer laser and developed with water. The film was finally annealed at 700 o C for 60 s by rapid thermal annealing (RTA). Ferroelectric perovskite phase was observed in the PZT thin films. The 130-nm-thick film showed P r of 11.2 μzC/cm 2 and E c of 93.8 kV/cm. From this process, half-micron patterns of PZT films were obtained

FABRICATION TECHNOLOGY OF FERROELECTRIC MEMORIES

Two types of ferroelectric memories have been proposed. One is a 1 transistor and 1 capacitor (1T1C) type and another is an FET (field effect transistor) type. The fabrication technique for fabrication ferroelectric capacitors on polycrystalline Si (poly-Si) was very effective for high density 1T1C ferroelectric memory and metal-ferroelectric-metal-insulator-semiconductor (MFMIS) FET with stable properties. MFMIS FET is one type of FET ferroelectric memory which we proposed. To obtain good ferroelectric capacitors on poly-Si, we used Ir based materials as electrodes of the capacitors. By using Ir based electrodes, the characteristics of Pb(Zi,Ti)O3 (PZT) capacitors on poly-Si were equivalent to those on SiO2. Moreover, the PZT capacitors with Ir based electrodes showed dramatic improvement in fatigue and imprint properties.

Application of Sr2Nb2O7 Family Ferroelectric Films for Ferroelectric Memory Field Effect Transistor

The compounds of the Sr2Nb2O7 (SNO) family are suitable for use as ferroelectric materials for ferroelectric memory field effect transistors (FETs), because these substances have a low dielectric constant, low coercive field and high heat-resistance. In this study, we succeeded in preparing Sr2(Ta,Nb)2O7 (STN) capacitors on polycrystalline silicon (poly-Si). From SIMS profiles, no interdiffusion in the STN metal ferroelectric metal insulator semiconductor (MFMIS) structure was confirmed. C–V and ID–VG hysteresis curves which were dependent on ferroelectric polarization were obtained. These capacitors were applied to floating gate type ferroelectric random access memory (FFRAM) cells. The degradation in ferroelectricity of STN capacitors was not observed during FFRAM cell fabrication process. We succeeded in operating FFRAM cells with a lower voltage than that required for PZT and confirmed the drain current difference of 1 or 2 orders at the 30 s after applying write pulses of ±5 V or ±10 V.

MgxZn1-xO-Based Schottky Photodiode for Highly Color-Selective Ultraviolet Light Detection

Spectral response of Schottky photodiodes consisting of a MgxZn1-xO (x≤0.43) thin film and a transparent conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), was characterized under a zero-bias condition at room temperature. The cut-off wavelength in each photodiode was systematically tuned by Mg content (x), while its high quantum efficiency was kept near unity. The steepness in the photo-response around the cut-off wavelength was maintained for higher x samples, implying a small alloy broadening effect in this system.