Takanobu Higuchi

Multilayer Optical Read-Only-Memory Disk Applicable to Blu-ray Disc Standard Using a Photopolymer Sheet with a Recording Capacity of 100 GB

We have been investigating a multilayer optical disk applicable to Blu-ray Disc (BD) standard using a photopolymer (2P) sheet in order to realize a recording capacity of 100 GB on a 12-cm-diameter disk. We had already developed a 2P sheet utilizing the multilayer disk. In this study, we developed a BD-type quad-layer disk using the 2P sheet, and investigated readout signals from each layer. The jitter values of layer 0 (L0), layer 1 (L1), layer 2 (L2) and layer 3 (L3) in the quad-layer disk were 6.5%, 7.1%, 7.3% and 6.6%, respectively. According to the analysis of the components of jitter values, the main factor behind the deterioration of jitter values is the crosstalk from adjacent layers. However, the jitter value of each layer was sufficient to reproduce the recorded data. From these results, we proved the feasibility of realizing the BD-type multilayer read-only-memory (ROM) disk with the recording capacity of 100 GB.

Multilayer 500 Gbyte Optical Disk

To confirm the feasibility of realizing subterabyte or terabyte optical memories, we evaluated a multilayer read-only disk having 20 information layers. We propose a new disk structure, which is an alternative structure to reduce interlayer crosstalk, optimized by computer simulations. We fabricated multilayer read-only optical disks using a conventional manufacturing process for Blu-ray disk (BD) dual-layer media. Moreover, we improved the reproduction system to compensate for the large spherical aberration. From the experimental results, we obtained sufficiently low jitter values reproduced from a 20-layer disk. We confirmed the feasibility of the multilayer read-only disk with a capacity of 500 Gbytes.

27.4 Gbyte Read-Only Dual-Layer Disc for Blue Lasers

We prototyped a next-generation digital-versatile-disc (DVD) dual-layer disc system using a blue laser diode. Numerous technologies were investigated for our proposal, such as new signal-processing techniques, a disc mastering process using photobleachable dye and silver alloy as a semireflective material. The dual-layer disc for the blue laser system has a 27.4 Gbyte capacity on a 12 cm disc. Where the threshold level of the jitter value was provided at 15%, the radial tilt margin was within ±0.66 degrees on layer 0 and ±0.64 degrees on layer 1, while the tangential tilt margin was within ±0.73 degrees on layer 0 and ±0.70 degrees on layer 1. These values are sufficiently large for the playback system when compared with the DVD system.

25 Gbyte Read-Only Memory Disk by Injection-Compression Molding Process

We studied the feasibility of fabricating a disk replica at a recording capacity of 25 Gbytes by a conventional injection molding process. This work is the first attempt at fabricating a disk replica using a stamper recorded by an electron beam recorder. The bottom jitter value of the injection-molded disk was 9.3% when using a conventional equalizer and 6.5% when using an additional limit equalizer. These values were highly similar to those of the photo-polymer (2P) disk. The residual error on the axial tracking of the injection-molded disks was smaller than that of the 2P disk, and the residual error on the radial tracking was almost the same as that of the 2P disk.

High Density Optical Disc Mastering Using Photobleachable Dye

The authors earlier developed a UV laser beam recorder (UV-LBR) and the mastering process for a high-density optical disc. Although it has sufficient resolution for the current digital versatile disc (DVD), the available resolution is not sufficient for the next generation DVD. Therefore, the authors have developed a new mastering process that can extend the resolution limit of the UV-LBR to realize the next generation DVD. It uses photobleachable dye (PBD) coated over a photoresist layer. In this paper, the effectiveness of the new multilayer mastering process is discussed, especially the writing ability of random data using 8/16-modulation. From the results of the experiments, the authors confirmed that the new process, which consists of a multilayer using PBD, has an advantage over the conventional process in the high-density recording, for a recording capacity of 13 Gbytes or greater.

High Recording Density Optical Disc Mastering

Aiming at obtaining a high CNR (carrier-to-noise ratio) and four times higher recording density than that of the conventional Laser Disc, we have developed a mastering process for a Laser Beam Recorder utilizing an ultraviolet laser. A photoresist claiming high resolution and high CNR has been screened out from the photoresists available on the market. By the improvement of the photoactive component in the photoresist, a CNR of 61.5 dB was obtained in the case of the 4-times higher density.

Superlarge-capacity optical disk with multilayer structure fabricated using a photopolymer sheet

We have been studying a multilayer disk in order to realize a 12-cm-diameter optical disk with the recording capacity of 100 GB or more. We tried to fabricate a multilayer disk using a photopolymer sheet. We developed a new sheet, and confirmed the performance of this sheet. A single layer disk made using this sheet had a value of 5.3%, and a 25 GB capacity. Then we fabricated a dual layer disk. The thickness variations from the disk surface to each recording layer were ±0.6 µm and ±0.7 µm. These results indicated the possibility of realizing a multilayer disk for a high-numerical-aperture (high-NA) system. We will evaluate the signals reproduced from the multilayer disk in the next step.

Bd-type write-once disk with pollutant-free material and starch substrate

We realized an inorganic write-once disk for an optical recording system of the Blu-ray disk format. We developed a new Al alloy for the reflective layer and a Nb-compound oxide nitride material for the dielectric layer. By adopting these materials for the reflective layer and the dielectric layer of our write-once disk, we achieved complete exclusion of toxic substances specified in the pollutant release and transfer register (PRTR) law. That is, this disk did not contain any substances specified in the PRTR law. We confirmed this disk to be compatible with 1× to 2× recording at the user capacity of 25.0 GB. The bottom jitter values of both 1× and 2× were less than 6.0%. In addition, we developed another kind of substrate, which was made of a natural polymer derived from corn starch. The bottom jitter value was 6.0% at the user capacity of 25.0 GB with the limit equalizer.

Study of Chemically Amplified Resist Using an Electron Beam Recorder

We have been developing an electron beam recorder for next-generation optical disk mastering. Using a ZEP-520 resist that had high resolution, we fabricated read-only memory disks and obtained sufficient reproduction performance. But the recording velocity was 0.7 m/s to obtain sufficient jitter of the disk. We could not expect such low recording velocity to be used in mass production. Therefore we decided to use a chemically amplified resist, which had high sensitivity. To reduce the recording time, we adopted the resist to the optical disk mastering and investigated the process conditions. We examined the effect of development power and post-exposure banking (PEB) temperature on the jitter of the reproduced signals and obtained 6.4% jitter at a 2.5 m/s recording velocity.

Low-Noise and Low-Environmental-Impact Reflective Material for High-Density Optical Data Storage

We designed a new reflective material for high-density optical data storage to manage both recording and reproducing properties and environmental impact. We achieved suitable characteristics using an Al based alloy including two additional elements. In the case that AlPdSnO2 was applied in a Blu-ray disc (BD) type write-once disk, recording and reproducing properties equivalent to those for the conventional Ag alloy of AgPdCu were achieved. Moreover, in the case that it was applied for a BD-type read-only disk, jitter was improved compared with the conventional Al alloy of AlTi.