High-density storage technology beyond hard disk drives and flash memory is required. Efforts are underway to develop new high-density storage technology based on scanning probe-based data storage. One of the candidates for scanning probe-type storage is thermomechanical data storage (also known as millipede, developed by IBM Zürich), and another is ferroelectric data storage. In this article, probe data-storage technologies are overviewed. Thermomechanical data storage and ferroelectric data storage are described in detail for next-generation high-density data-storage technology based on scanning probe microscopy. Ferroelectric data storage and scanning nonlinear dielectric microscopy-based and field-effect transistor-type probe-based probe data storage are also described.

In this article, scanning nonlinear dielectric microscopy (SNDM) with atomic resolution is reviewed. First, experimental results on the detection of ferroelectric domains are shown following a presentation about the theory and principle of SNDM. Next, a three-dimensional (3D) type of SNDM for measuring the 3D distribution of ferroelectric polarization and noncontact scanning nonlinear dielectric microscopy (NC-SNDM) are proposed. Using NC-SNDM under ultrahigh vacuum conditions, we clearly resolve the electric dipole moment distribution of Si atoms on a Si(111)7 × 7 surface. We also succeeded to resolve a fullerene (C60) molecule. Since the technique is applicable not only to semiconductors but also to both polar and non-polar dielectric materials, SrTiO3 and TiO2 surfaces were observed by NC-SNDM. Finally, we characterize an ultrahigh-density ferroelectric data storage system using SNDM as a pickup device and a congruent lithium tantalate single crystal as a ferroelectric recording medium.

We have demonstrated that scanning nonlinear dielectric microscopy (SNDM) exhibited high performance and high resolution in observing the dopant concentration profile of transistors. In this study, good quantitative agreement between the SNDM signals and dopant density values obtained by SIMS in standard Si samples, which dopant concentrations have been calibrated. We succeeded in visualizing high-resolution dopant profiles in n- and p-type MOSFET with 40 nm gate channels. It is considered that SNDM would be an effective method in measuring the quantitative two-dimensional dopant profiles of transistors. Finally, we have succeeded in detecting the dopant profiles of SRAM memory cell transistors.

In this study, we have developed ferroelectric data storage test systems based on scanning nonlinear dielectric microscopy (SNDM) to conduct various experiments concerning read/write capability. Nanodomain formation on ferroelectric recording media was studied using the data storage test system. A nanodomain dot array was successfully written on a single-crystal LiTaO3 recording medium. The diameter of the written dot was as small as 7 nm. Epitaxial-thin-film LiTaO3 recording media were also developed. Nanodomain dots with the diameter of 25 nm were written on the thin-film recording medium. In addition, a non-contact probe-height control technique was adopted to solve the problem of tip abrasion using higher-order nonlinear dielectric response detection method. Finally, a hard-disk-drive (HDD)-type ferroelectric data storage test system was developed for conducting read/write tests under conditions close to those of actual operation. Capabilities of reading at the bit rate of 2 Mbps and writing at the bit rate of 20 Mbps were confirmed using the HDD-type data storage test system.

Using a scanning nonlinear dielectric microscopy (SNDM), we observed two standard Si samples with epitaxial staircase structures, which have known dopant density values calibrated by using secondary ion mass spectroscopy (SIMS). As the result, good quantitative correlation between dopant density values and SNDM signals was obtained without the phenomenon of contrast reversal effect, which is associated with conventional scanning capacitance microscopy (SCM) measurements. Thus, it is expected that SNDM will be an effective method for observing the quantitative measurement of two-dimensional dopant profiling on semiconductor devices.

To investigate the long-term retention characteristics of a ferroelectric-data-storage system, 80-nm-thick congruent LiTaO3 plates with inverted-domain dot arrays composed of 100-nm-φpdots were baked at 220, 250, 280 and 300°C. After heat treatment over a range of different time intervals, the dots shrank. From the change in the dot radius data, the activation energy (Ea) and frequency factor (α), parameters of the Arrhenius equation, were determined to be = 0.76 eV, = 2.21×105. From these parameter we can predict competitive retention characteristics compared with general memory devices. The phenomenon of dot shrinking can be explained from the energy transition of the system based on wall energy.

Recently, we have developed Non-Contact Scanning Nonlinear Dielectric Microscopy (NC-SNDM) with a new height-control technique utilizing higher order nonlinear dielectric constant detection (ε(4) signal). In theoretically, NC-SNDM has quite high height sensitivity against the gap between tip and sample as well as STM technique and the simultaneous observation of the topography and ferroelectric polarization (local dipole moment) distribution with atomic resolution has been expected. To confirm such performance of NC-SNDM with atomic resolution, UHV-SNDM was developed and Si(111) cleaned surface was chosen as a specimen. As a result, we have succeeded in observing Si(111) 7×7 atomic structure. Moreover, distribution of tunneling current and lowest order nonlinear dielectric signal ε(3) could be observed simultaneously. To the best our knowledge, this is the first successful demonstration of the atomic resolution achievement in dielectric microscopy techniques.

Cross-sectional shapes of reversal nano-domain dots formed in a congruent LiTaO3 single-crystal recording medium were studied by scanning nonlinear dielectric microscopy (SNDM). Images obtained by SNDM measurements confirm that reversal nano-domain dots penetrate through the entire sample. The domain wall thickness was evaluated by the cross-sectional measurement results. The variation of the domain wall thickness as a function of sample thickness and of depth position was evaluated. It was found that thinner samples have a tendency to have thinner domain walls, and it was confirmed that the domain wall thickness at the bottom of the sample was thinner than at the front surface of the sample. A discussion of the measured result is also provided.

The surface potential distribution following polarization reversal by scanning probe microscopy is examined in order to determine the mechanism of anti-parallel polarization reversal and ring-shaped domain formation in single-crystal lithium tantalate. Lithium vacancy lattice defects are identified as charge-trapping sites, with the resultant surface potential distribution governing the formation of ring-shaped nano-domains. The behavior of this charge injection is shown to be dependent on atmospheric humidity.

Three-dimensional domain configuration of multi-domain LiTaO3 is revealed by Scanning Nonlinear Dielectric Microscopy (SNDM). SNDM can measure the polarization components both perpendicular and parallel to the surface of the specimen. These techniques are applied to the both congruent and stoichiometric LiTaO3 crystals. The images obtained by SNDM measurements allow us to confirm the various domain features of LiTaO3 and to understand both similarities and differences between the congruent and stoichiometric compositions.

In this study, a new method to achieve real information recording with a density of 1 Tbit/inch2 in ferroelectric data storage systems is proposed. In this system, data bits were written in the form of the polarization direction of nano domains. Local domain switching was carried out by applying a voltage pulse. A series of test writes revealed that the bit size varied according to the polarization direction of the bits in the immediate environment. Therefore, in order to keep the bit size constant (at 25.6 nm), the pulse amplitude required to create each individual bit was calculated as a function of the polarities of the surrounding data bits. As a result, real information storage at a density of 1 Tbit/inch2 was successfully achieved, with a low bit error rate of 1.8 × 10-2.

Ferroelectrics are expected to become one of the next generation ultra-high density data storage media. The requirements for pulse amplitude and the duration to switch the domain were both markedly decreased by using a new domain stabilizing method; offset voltage application method. Additionally, with this method it became possible to invert a smaller domain with a diameter of less than 10 nm. Finally, significant progress was made regarding the memory density for ferroelectric data storage, and an area density of 10.1 Tera-bit/inch2 was successfully achieved. This represents the highest memory density for rewritable data storage reported to date.

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 μC/cm2 and 36 kV/cm, respectively, and those of PZT on SrRuO3/SrTiO3 (111) were 32.7 μC/cm2 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.

A technique for measuring the absolute value of the ferroelectric polarization angle using scanning nonlinear dielectric microscopy (SNDM) is proposed and demonstrated. Using the technique, periodically poled lithium niobate (PPLN) with three-dimensional domain structure is observed. The measured polarization angles agreed well with the actual polarization orientations, and allowed precise visualization of the microdomain structure in PPLN. Through this experiment, we confirmed that SNDM is a useful tool for the absolute evaluation of the three-dimensional polarization direction.

This paper describes a newly developed ultra-high vacuum type scanning nonlinear dielectric microscope (UHV-SNDM) and the results from a ferroelectric LiTaO3 single crystal measured by the UHV-SNDM. In a cleaved (012) surface of LiTaO3 crystal, we can clearly observe a striped pattern with a period of about 0.3 nm and a granular pattern approximately corresponding to the sub-lattice period. From this image, we confirmed that the SNDM can be applied to lattice-level measurement in ferroelectric insulator materials.

A hydrothermal method is a unique method to deposit ferroelectric thin films utilizing chemical reaction in solutions. In addition to the low reaction temperature below 200°C, its simple process procedure, automatically aligned polarization and a three-dimensional deposition are advantages. In this study, a lead titanate epitaxial thin film was obtained on a strontium ruthenate bottom electrode sputterd on strontium titanate (100). The highresolution X-ray diffraction mapping showed the film was perfectly c-axis oriented. The transmission electron microscope observation revealed that the film had no lattice dislocation at the interface between lead titanate and strontium ruthenate. A remanent polarization of 96.5μC/cm2 was measured with the single crystal-like DE hysteresis curve. The observation of a scanning nonlinear dielectric microscopy indicated that this film did not contain any defect such as an a-domain and a grain boundary even on the nano scale. With various pulse parameters, nano-domain dots were patterned and the minimum dot radius of inverted domain was 12 nm corresponding to data storage density of 1Tbit/inch2.

Hydrothermal method has various advantages; low deposition temperature, high-purity, deposition on a three-dimensional structure and a large thickness. Although epitaxial PZT thin film deposition has reported, ferroelectric measurement has not been conducted due to the peel-off morphology of the film. The present 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, 200 nm SrRuO3 thin film was deposited on SrTiO3 single crystals, and the PZT thin was deposited on SrRuO3. The remanent polarization 2Pr for PZT on SrRuO3/SrTiO3 (001) was 19.5 μC/cm2 and that of PZT on SrRuO3/SrTiO3 (111) was 37.2 μC/cm2 respectively. The self alignment poling direction was confirmed via scanning nonlinear dielectric microscopy and is thought to have been related to the deposition mechanisms.

We used scanning nonlinear dielectric microscopy to observe the position of electrons and holes in the gate SiO2-Si3N4-SiO2 (ONO) film of metal-oxide-nitride-oxide semiconductor type Flash memory. The electrons were detected in the Si3N4 part of the ONO film. The holes, on the other hand, were found in the Si3N4 film as well as in the bottom SiO2 film.

The first prototype of high-density ferroelectric data storage system based on scanning nonlinear dielectric microscopy was developed in order to establish basic elemental technologies for actual read/write functions aiming for practical application. Using this system, a data transfer rate was evaluated. 9 kbps reading and 50 kbps writing were demonstrated with respect to 440 kbit/inch bit data array written on a lithium tantalate single crystal thin plate. Additionally, we considered future prospects for developing the data storage system with further fast data transfer rate.

It is well known that spontaneous polarization of ferroelectric material is an intrinsic property applied for nonvolatile memory devices. Poling direction can be reversed in a nanometer size area using the conductive cantilever of a scanning probe microscope. In order to detect nanodots patterns, scanning nonlinear dielectric microscope (SNDM) is superior to piezore-sponse microscope in terms of resolution. In this paper, a real-time measuring method of a poling direction is proposed. Using this method, the domain reversal process was observed and an unexpected phenomenon was found, namely, that the poling directions were aligned antiparallel to the poling electric field. This antiparallel poling reversal took place when the film thickness was more than 350 nm in the case of lithium tantalate. At present, the reason and mechanism of the antiparallel poling reversal are uncertain, although it might be related to the concentrated electric field near the cantilever tip.