Junko Ushiyama

Nanosize fabrication using etching of phase-change recording films

An etching technique called phase-change etching was developed. In this technique, only crystalline regions in a phase-change recording film are selectively etched by an alkaline solution, and amorphous regions remain on the sample surface, which means that a phase-change recording film can be used as a resist for pattern formation. By combination of this technique and phase-change recording, fabrication of the dot pattern with a size of about 1∕10 of the fabricating spot was demonstrated. This result indicates the possibility of nanosize fabrication using the phase-change etching technique.

Sub-Terabyte-Data-Capacity Optical Discs Realized by Three-Dimensional Pit Selection

To realize optical discs with the sub-terabyte data capacity, we propose the three-dimensional pit selection (3DPS) method where a single data pit to be read out in a multi-layer disc is selected three-dimensionally to obtain super-resolution in the disc plane and to reduce layer cross-talk. To examine the feasibility of this method, the phase-change pit capsule method was tested where the data pits consist of a phase-change material which melts during readout. The super-resolution effect was observed for both layers of a dual-layer disc. It was shown that a quadric-layer disc can be designed because of the high transmittance of each layer. Thus, 3DPS is considered to have the potential for a data capacity of hundreds of gigabytes with a conventional optical system.

Fabrication of Discs for Three-Dimensional Pit Selection Using Damascene Process

The damascene process using chemical mechanical polishing was introduced to embed a super-resolution material in the pits of a read-only memory (ROM) substrate to fabricate discs using a next-generation large-capacity optical-disc technique called three-dimensional pit selection. GeSbTe was used as the super-resolution material. Experimental results using scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that GeSbTe remained only in the pits. The readout signal from a disc tester demonstrated that GeSbTe in the damascene disc changed its phases between amorphous and crystal. The polishing rate could be controlled by adjusting the conditions for the concentration of colloidal silica and the pH of the slurry. The test equipment we fabricated for mass producing the discs demonstrated that the degree of polishing could automatically be determined by detecting the distortion in the polishing arm caused by friction between the polishing head and the sample surface.

Read/Write Technology for 50 GB/layer at 100 Mbps

We developed read/write technology for high-density optical data storage of 50 GB/layer at a data transfer rate of 108 Mbps. We demonstrated the writing and reading capabilities of a 50 GB/layer optical disk recording system at 36 Mbps with a sufficient power margin, using a three-beam crosstalk canceller with optical response equalizers, using a phase-change write-once disk. Bit error rates were below 1×10-4 at 108 Mbps. Experimental results showed that the technology realizes writing and reading densities two times higher and data transfer rates three times higher than those of Blu-ray with a simple optical component such as Blu-ray.

8-

Three-dimensional optical/thermal spot profiles obtained by thermally assisted magnetic recording (TAMR) on bit-patterned media (BPM) with dot densities of 6 to 15 Tb/in2 were numerically analyzed. Introduction of a spacing layer with higher thermal conductivity than that of the recording dots leads to narrow temperature distribution (i.e., steep temperature profile) in the dots. A temperature profile with FWHM of less than 5 nm was obtained on a patterned dot array under areal densities of 6 to 15 Tb/in2. In addition, introduction of a thermal-control layer beneath the recording layer decreased vertical temperature difference within a recording bit while keeping a narrow temperature distribution. Feasibility of 8-Tb/in2-class TAMR on a BPM was verified by LLG simulation with a triangular antenna-type near-field optical element.

{\rm Tb/in}^{2}

To reduce interlayer crosstalk caused by the ghost spot which appears in a multilayer optical disk with more than three information layers, a multilayer disk structure which reduces interlayer crosstalk with a wide disk-fabrication margin was proposed in which the backward reflectivity of the information layers is sufficiently low. It was confirmed that the interlayer crosstalk caused by the ghost spot was reduced to less than the crosstalk from the adjacent layer by controlling backward reflectivity. The wide disk-fabrication margin of the proposed disk structure was indicated by experimentally confirming that the tolerance of the maximum deviation of the spacer-layer thickness is four times larger than that in the previous multilayer disk.

-Class Bit-Patterned Medium for Thermally Assisted Magnetic Recording

Showed the feasibility of 4.7-GB DVD-RAM media for 5/spl times/ recording. In order to maintain compatibility with current 2/spl times/ drives, we reduced jitter in both 2/spl times/ and 5/spl times/ recording. The most important design concept is to control the crystal growth velocity and crystal nucleation frequency: the former should be reduced and the latter increased. To control these values, we prepared a medium having a Ge-Sb-Te recording film and a nucleation promoting film. It showed jitter lower than 10% in both 2/spl times/ and 5/spl times/ recording.

Multilayer Disk Reduced Interlayer Crosstalk with Wide Disk-Fabrication Margin

A method is proposed to reduce interlayer crosstalk in multilayer optical discs by controlling backward reflectivity of information layers, which can lead to wider tolerances of disc fabrication accuracy. Reduction of the backward reflectivity reduces the signal from the ghost spot even if thicknesses of spacer layers are equal. Experimental results showed that the ratio of the signals obtained by the readout spot and the ghost spot is less by about one order for a disc with controlled backward reflectivity than for a conventional disc. A rough estimate of the crosstalk caused by the ghost spot agrees qualitatively with the experimental results.

High-transfer-rate 4.7-GB DVD-RAM

We experimentally studied an optical disk with a large capacity and high data-transfer rate that can record high-definition television signals. A high rotation speed was used to achieve the high data-transfer rate. Using Ge–Sb–Te phase change material, a carrier-to-noise ratio of about 50 dB was achieved at a linear velocity of 15–30 m/s with a 0.4-µm-long mark. High-speed crystallization was necessary to erase recording marks. Therefore, by appropriately designing layers that assist nucleation, we obtained a DC erase ratio of 29 dB with a 0.4-µm mark and 1-mW power margin for over-25-dB erase ratio at a linear velocity of 18 m/s. Furthermore, the 0.4-µm mark-space repetition jitter was less than 7% even after 1000 cycles. A data transfer rate of 60 Mbps was obtained by optimizing the write strategy using a multi-pulse with an optical system equivalent to that of current digital versatile disk (DVD).

Interlayer Crosstalk Reduction by Controlling Backward Reflectivity in Multilayer Optical Discs

Using spin‐polarized scanning electron microscopy, the laser power dependence of recorded mark shapes and sizes is studied for the TbFeCo, magneto‐optical recording medium, prepared on a land/groove‐shaped substrate. The bit width which increases with laser power is suppressed to the land or groove width during higher power, showing that the land/groove border acts as a barrier to bit expansion. These results confirm the advantages of a land/groove shaped medium for high density recording.