Yumiko Anzai

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.

Scalable 3-D vertical chain-cell-type phase-change memory with 4F2 poly-Si diodes

A three-dimensional (3-D) vertical chain-cell-type phase-change memory (VCCPCM) for next-generation large-capacity storage was developed. The VCCPCM features formation of memory holes in multi-layered stacked gates by using a single mask and a memory array without a selection transistor. As a result of this configuration, the number of process steps for fabricating the VCCPCM is reduced. The excellent scalability of the VCCPCMs new phase-change material makes it possible to reduce the cell size beyond the scaling limit of flash memory. In addition, a poly-silicon selection diode makes it possible to reduce the cell factor to 4F2. Consequently, relative cost of the VCCPCM compared to 3-D flash memory is reduced to 0.2.

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.

An efficient probe with a planar metallic pattern for high-density near-field optical memory

A probe with a single or double triangle-shaped planar metallic pattern is proposed to increase efficiency. The probes optical property was investigated using a three-dimensional finite-difference time-domain calculation method.

GeSbTe Phase Change Material for Blue-Violet Laser at High Linear Speed

High-speed recording and read-out were carried out under the condition of numerical aperture (NA) 0.65 and substrate thickness 0.6 mm using a blue-violet laser of 405 nm. Using a GeSbTe phase change material of the crystal nucleus generation dominant type, we obtained a carrier-to-noise ratio (CNR) of 50 dB with 0.24 µm mark and a DC erase ratio of 30 dB at a velocity of 18 m/s. It was proven that the realization of the 100 Mbps data transfer rate was possible by optimizing record compensation.

Interlayer crosstalk reduction of a multilayer Blu-ray Disc using a grating in a three beam optical system.

In multilayer optical discs, light reflected by out-of-focus layers, which we call interlayer crosstalk, causes the tracking error signal to fluctuate, making the readout signal unstable. We previously proposed a novel method to use a grating along the optical axis in the return path of a pickup to suppress the fluctuation of a differential push-pull (DPP) signal. We develop a pickup and evaluate its performance to stabilize the DPP signal experimentally. DPP signal fluctuation is suppressed to one-third (6% to 2%), and also satisfactory readout jitters (about 8%) are obtained for a triple-layer Blu-ray Disc (BD), which demonstrate the validity of this method to reduce interlayer crosstalk of multilayer optical discs.

Analyses of signals from dual-layer phase change optical disks

The signals obtained from dual-layer optical disks were analyzed experimentally and theoretically with emphasis on reducing layer cross talk (LCT) and spherical aberration (SA). Using the dual-layer phase change optical disk, the DC element of the LCT was measured and phase change marks were written on both layers. Diffraction calculations were used to explain the experimental results and to determine the amount of LCT and signal degradation caused by SA as functions of spacer layer thickness and optical detector size. Analytical methods to estimate LCT and SA were proposed, and their predictions turned out to agree well with the experimental and diffraction calculation results. These findings indicate that for a blue laser, the conventional techniques cannot simultaneously keep the LCT and SA to within practical values. Accordingly, designs for disks and drives for dual-layer optical disks are discussed.

2.8-GB/s-write and 670-MB/s-erase operations of a 3D vertical chain-cell-type phase-change-memory array

A high-programming-throughput three-dimensional (3D) vertical chain-cell-type phase-change memory (VCCPCM) array for a next-generation storage device was fabricated. To increase the number of write cells at one time by reducing resistance of bit and source lines, the VCCPCM array includes plate electrodes and double-gate vertical-chain-selection MOSs with 5-nm-thick poly-Si channels. In addition, CO2 laser annealing enhances the drivability of a poly-Si cell MOS to 680 μA/μm to suppress energy loss in the cell MOS. In addition to write throughput, erase throughput is increased by erasing memory cells in a “bundle” by channel heating (called “bundle erase”). GeSbTe CVD with high uniformity is also developed.

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.

Use of Grating in Reading Multilayer Discs to Reduce Amount of Interlayer Crosstalk

Crosstalk from adjacent layers affects the tracking error signal of conventional optical pickups when a multilayered disc is read. The tracking error signal fluctuates and makes precise tracking difficult. We propose the use of a grating in the return path to reduce the amount of crosstalk. Using a grating with an uneven plane on the optical axis reduces the amount of stray light causing the crosstalk and allows the target light to pass through. We show that using a grating with an appropriate pitch prevents diffracted light higher than an order of ±1 from going back to detectors. As the shape and depth of the grating are important factors for the zeroth-order light, we chose a deep triangular grating. We measured the distribution of the transmitted intensity through the grating as a focused beam position was scanned and found that the grating can reduce the amount of crosstalk.