Yasuo Wagatsuma

Tbit/inch2 ferroelectric data storage based on scanning nonlinear dielectric microscopy

Nanosized inverted domain dots in ferroelectric materials have potential applications in ultrahigh-density rewritable data storage systems. Here, a data storage system based on scanning nonlinear dielectric microscopy and thin films of ferroelectric single-crystal lithium tantalite is presented. Through domain engineering, nanosized inverted domain dots have been successfully formed at a data density of 1.50 Tbit/in.2.

Ferroelectric properties of an epitaxial lead zirconate titanate thin film deposited by a hydrothermal method below the Curie temperature

Deposition of a hydrothermal method has a number of advantages; low deposition temperature, high-purity, deposition on a three-dimensional structure, and a large thickness. The present paper investigates the improvement of an epitaxial lead zirconate titanate (PZT) thin film deposited via hydrothermal synthesis. The critical parameter contributing to the film morphology was revealed to be the holding position of the reaction solution during deposition. In addition to this improvement, the SrRuO3 bottom electrode was confirmed to be able to withstand high alkali concentrations during the hydrothermal reaction. A hydrothermal synthesis time of 24 h yielded a PZT film with a thickness of 500 nm on SrRuO3/SrTiO3 (100) at 150 °C. The remanent polarization 2Pr and coercive electric field were 38.4 μC/cm2 and 21.8 kV/cm, respectively.

Ferroelectric property of an epitaxial lead zirconate titanate thin film deposited by a hydrothermal method

Deposition of thin films via hydrothermal method has various advantages: low deposition temperature, high purity, deposition on a three-dimensional structure, and a large thickness. Although an epitaxial lead zirconate titanate (PZT) thin-film deposition has been reported, the ferroelectric measurement has not been conducted due to the peel-off morphology of the film. The current paper investigates the improvement of an epitaxial PZT thin film deposited via a hydrothermal method. By adjusting the position at which the substrate was suspended in the solution, smooth morphology surface was successfully obtained. As a bottom electrode, a 200-nm SrRuO3 thin film was deposited on SrTiO3 single crystals, and the PZT thin film was deposited on SrRuO3. The remanent polarization 2Pr and coercive electric field for PZT on SrRuO3/SrTiO3 (001) were 17.1 muC/cm(2) and 36 kV/cm, respectively, and those of PZT on SrRuO3/SrTiO3 (111) were 32.7 muC/cm(2) and 59 kV/cm, respectively. The reason for large imprint electrical field, 91 kV/cm and 40 kV/cm for each film, was unclear at this stage, although it is associated with self-alignment poling direction. This self-alignment poling direction was confirmed via scanning nonlinear dielectric microscopy and is thought to have been related to the deposition mechanisms.

Ultrahigh-Density Ferroelectric Data Storage Using Scanning Nonlinear Dielectric Microscopy

Ferroelectrics have generated considerable interest as promising storage media. In this paper, an investigation of ultrahigh-density ferroelectric data storage based on scanning nonlinear dielectric microscopy (SNDM) was carried out. For the purpose of obtaining fundamental knowledge of high-density ferroelectric data storage, several experiments on nanodomain formation in lithium tantalate (LiTaO3) single crystal were conducted. As a result, a very small inverted domain with a radius of 6 nm was successfully formed in stoichiometric LiTaO3 (SLT), and in addition, a domain dot array with an areal density of 1.5 Tbit/inch2 was written on congruent LiTaO3 (CLT). Additionally, the first prototype high-density ferroelectric data storage system was developed. Using this system, reading and writing data transfer rates were evaluated.

Single crystal growth of KNbO3 and application to surface acoustic wave devices

Potassium niobate (KNbO 3 ) has large piezoelectric constants. The surface acoustic wave (SAW) substrates with high couplings are very important for wide-band SAW filters and SAW devices. Therefore, the large size KNbO 3 single crystals are required for SAW device applications. In this paper, the techniques of a large size of KNbO 3 using the top seeded solution growth techniques (TSSG) with large size of crucible of 60 mmo platinum are described. The results show that 50x50x 15 mm 3 single crystals are obtained. Also, the poling techniques of KNbO 3 crystal are investigated by using the nonlinear scanning dielectric microscope. The propagation characteristics of SAW and piezoelectric leaky surface waves in KNbO 3 single crystal were investigated theoretically. The results show that the electro-mechanical coupling coefficient of the surface wave propagating along the X-axis of the rotated Y-cut plane is extremely large: K 2 = 0.53, compared to K 2 of 0.055 for LiNbO 3 . The experimental results of the above KNbO 3 substrates agree with the theoretical ones. Also the KNbO 3 substrates showed experimentally the zero temperature coefficients of frequency (TCF) around 20 C.

Ferroelectric Single Crystal Recording Media Fabricated by Polarization Controlled Wet Etching Process

Ferroelectric single crystal thin plates are promising materials for ultrahigh-density storage devices. In order to fabricate recording media with large area and high homogeneity, a polarization controlled wet etching process was developed in this paper. Differential etch rates of ferroelectrics are useful in controlling the thickness during the wet etching process. By applying DC voltage to a substrate, the etching process can be stopped when the thickness reaches the desired value. A 455-nm-thick LiTaO3 thin plate with a diameter of 3 mm was prepared using this method. Nanodomains were artificially formed by applying voltage pulses to the substrate using a metal-coated cantilever top electrode.

Fundamental Study on Ferroelectric Data Storage with the Density Above 1 Tbit/inch2 Using Congruent Lithium Tantalate

Ferroelectrics have created considerable interest as promising storage media. In this paper, an investigation for ultra high-density ferroelectric data storage based on scanning nonlinear dielectric microscopy (SNDM) was carried out. For the purpose of obtaining the fundamental knowledge of high-density ferroelectric data storage, several experiments of nano-domain formation in lithium tantalate (LiTaO3 ) single crystal were conducted. As a result, domain dot array with areal density of 1.5 Tbit/inch2 was written in congruent LiTaO3 (CLT). We also found that quite fast nano second domain switching was possible by reducing a sample thickness even in CLT with a lot of Li vacancy defects which played a role of pinning sites against domain wall movement.

10 GHz Surface Acoustic Wave Filters with Narrow-Gap Interdigital Transducer Structure

Controllable well-insulated gaps between electrodes can be obtained by anodic oxidation of the edges of the regions of an Al film covered by photo resist. This technique is used to fabricate surface acoustic wave narrow gap interdigital transducers (NG-IDTs) and narrow gap floating electrode type unidirectional transducers (NG-FEUDTs). Experimental results indicate that the SAW NG-IDTs and NG-FEUDTs operate in the 10GHz range and have an electrode width of less than 0.2 µm.

Nanometer Electrode Fabrication Technology Using Anodic Oxidation Resist and Application to Unidirectional Surface Acoustic Wave Transducers

Nanometer lithography technology is very important for high density integrated circuits and higher-frequency-range surface acoustic wave (SAW) devices. The frequency ranges of SAW devices extend to around 10 GHz; therefore a sub-micron technology below 0.1 µm is required. In this paper, we propose new lithography techniques for nanometer fabrication using very thin anodic oxidation films. Since very thin resists (less than 50 nm) can be used in this process, a narrow resist pattern and electrodes can be obtained. The results show the 75 nm width of an interdigital transducer (IDT) made of aluminum with thickness of 30 nm on 128° Y-X LiNbO3. SAW experimental results show the 17 GHz-range characteristics. Also, new lithography technologies are applied to 2 GHz low-loss filters using a unidirectional transducer.

Tbit/Inch 2 Data Storage Using Scanning Nonlinear Dielectric Microscopy

Nano-sized inverted domain dots in ferroelectric materials is a technology with potential applications in ultrahigh-density rewritable data storage systems. Up to now, we have studied domain inversion characteristics of stoichiometric and congruent LiTaO 3 single crystals in nanoscopic area using scanning nonlinear dielectric microscopy (SNDM), which is the technique for observing ferroelectric polarization distribution with sub-nanometer resolution. In this study, we have revealed nano-sized inverted domain remained stably for a long time, and successfully formed inverted domain dots at a data density of 1.50 Tbit/inch 2, representing the highest memory density for rewritable electric data storage reported to date.