Site: Hitachi Central Research Laboratory (HCRL)
(optical storage presentations)
Kokubunji, Tokyo 185, Japan
Date Visited: 12 March 1998
The main business of Hitachi, Ltd. and its subsidiaries is a combination of electrical, electronics and information systems. Hitachi is active in information and electronics (mainframes to microcomputers, magnetic and optical disks, telecommunication, medical electronics and chip making), power systems (nuclear and thermal power plant, transmission systems, pollution control equipment), industrial systems, transportation systems (elevators escalators, automotive), and consumer products (air conditioners, refrigerators, TV, VCR, video camera).
The Hitachi Central Research Laboratory (HCRL) was established in 1942 by Mr. N. Odaira, founder of Hitachi, Ltd. for "creating new basic technologies for the coming 10 to 20 years, as well as pursuing development work for today's business." HCRL is active in information and media, electronic devices, and medical electronics. Hitachi pursues joint development partnerships and alliances with other companies and universities in Japan and overseas to make use of R&D resources.
Seven corporate research laboratories are directly attached to the president's office. These labs are the following:
In addition, R&D for product development is also carried out under the business groups. Hitachi also funds research facilities throughout North America, Europe and Asia.
Research in HCRL is carried out in 13 departments. These include the following:
Dr. Yamaguchi described HCRL's effort in developing positioning technologies for 10 Gb/in2 areal densities with tpi=31.6 K and Tp= 0.8 Ám. HCRL's present work aims at TW/TWR=0.63/0.50, with a settling of 0.08 Ám, non-repeatable runout (NRRO) = 0.096 (m and max TMP = 15%Tp. He noted that presently the dominant disturbances are of mechanical origin and the key is to increase the servo bandwidth. Present crossover frequency of 500 Hz produces an NRRO of 0.183 Ám. Future systems are aimed at 1 kHz with NRRO of 0.132 (m. At higher bandwidths the detection noise becomes dominant.
Several directions are being pursued to increase servo bandwidth including dedicated servos, multi-sensing and multi-stage approaches. Presently, for multi-sensing, sensors and accelerometers are placed at different locations on the actuator arm. In the future two stage actuators are being contemplated. However, this increases the number of servo systems. Piezoelectric actuators are considered to be promising, while the reliability of MEMS actuators remains a concern.
Mr. Sukeda discussed the future directions envisioned for optical data storage at HCRL. He indicated that the next generation of DVD would be aimed at satisfying the needs of HDTV, requiring 15 GB capacity for two hours video stream and approximately three times faster data rate than current DVD performance. HCRL does carry on research on both PC and MO media and on probe storage for the future. During the visit, the discussion centered on PC media and probe storage.
The technologies that HCRL is developing for PCs include adaptive recording control to improve signal-to-noise ratios (for product insertion in 1999) of 4.7 GB DVDs and super resolution and blue laser recording (for product insertion in 2001) for 15 GB DVDs.
Adaptive recording control relies on contrast enhancement and thermal engineering of the media for more homogeneous absorption of heat and better reflectivity. To this end a thermal buffer layer and a contrast enhancement layer are introduced into the media as shown in Figure C.3. This optimized media design should allow for 4.7 GB capacity per layer.
Fig. C.3. Improvement of SNR (~1999).
Fig. C.4. Super Resolution (~2001).
To keep backward compatibility, for the near future, increasing the NA of the optical system is not envisioned. Instead, media PSR is being investigated for 15 GB products. The principle of super-resolution (PSR) for PC media is based on affecting the intensity profile of the spot to be recorded by introducing a non-linear phase mask as described in Figure C.4. A more conventional technique that was first studied was to introduce a photoreactive organic film to produce the mask. However, because of the limited cycle ability of this film, HCRL researchers have developed a new oxide glass film, 200 nm thick, that contains Si, Na, Ca, Co, and O (proprietary composition). This film produces a very large index change ((n~0.5) as a function of temperature (dn/dT). The persistence of the change in the index is less than the time it takes for one disk rotation, and no damage is observed during reading and recording. However, 30% more laser power is required. Future efforts will be concentrated on improving recording sensitivity.
The structure of the new disk media is shown in Figure C.5. It was also noted that blue lasers may be more important for PC media. But for next generation products, because of back compatibility considerations, the time is not considered ripe for introducing blue lasers in a product.
Fig. C.5. Cross-section of a PC disk with phase mask layer (L.) and HCRL roadmap for DVD products (R.).
Dr. Hosaka described HCRL's effort in probe storage to reach areal densities in the 0.1-1 Tb/in2 range with 1 Mb/s data rate for ROM application. Both phase change and plastic media that can be deformed under heat are being studied using AFM pit recording. Using 15 mW from an SNOM probe and a 30 nm GeSbTe layer 200 Gb/in2 densities have been demonstrated. Using a plastic deformation approach with a heated AFM tip, pits separated by 10 nm have demonstrated the feasibility of 1 Tb/in2 densities. Read out is carried out with a probe oscillating at 2.3 MHz with a rotation speed approaching mm/s. For faster readout the oscillation frequency needs to be increased using a shorter and stiffer probe. Using this method with 25 nm x 12 nm pits separated by 20 nm on a disk rotating at 100 rpm, HCRL researchers have achieved a 1.25 Mb/s data rate. However, at this point reading from the same location with good repetition appears difficult. The WTEC team learned that Canon, Epson, Sharp and Matsushita are also involved in probe storage with their own approaches.
HCRL researchers believe that PC can address many high performance applications with areal densities on the order of 20 Gb/in2. The main issue is to keep media compatibility for different applications. The researchers feel that the future of PC beyond 20 Gb/in2 is not clear but point out that MO should be able to reach 100 Gb/in2 densities because of an extra degree of freedom introduced by the magnetic field.