Rewritable Triple-Layer Phase-Change Optical Disk Providing 100 Gbyte Capacity

Abstract

A rewritable phase-change optical disk providing a large capacity of 100 Gbyte on a 120 mm disk was first demonstrated using the multilayer Blu-ray DiscTM (BD-XL) format. The doubled capacity of this optical disk compared with that of a conventional dual-layer disk was achieved firstly by stacking triple recording layers and secondly by increasing the recording capacity per layer from 25 to 33.4 Gbyte at 33.6%. The high transmittances of 50% (middle layer) and 60% (front layer) were achieved by thinning a Ge–Sb–Te phase-change film to 7.5 and 6 nm and also by thinning a Ag-alloy film to 9 and 7 nm, respectively. An additional TiO2-based film formed on the Ag-alloy film was effective in improving the transmittance at 3%, compared with the structure using a conventional TiO2 film. Furthermore, a transmittance-balanced structure was adopted for these layers in order to stabilize the recording-reading properties. To improve cyclability, ZrO2–Cr2O3-based interface films were provided on both sides of the phase-change film for the middle and front layers. The increase in recording capacity per layer was achieved by reducing the minimum mark length from 0.149 to 0.112 µm. Since the optical changes degrade with the reductions in the mark lengths and thicknesses of the Ge–Sb–Te and Ag-alloy films, a phase-change material with a GeTe-rich composition on a GeTe–Sb2Te3 pseudo-binary line was adopted for every layer to compensate it. It was confirmed that the sample disk successfully satisfies all the requirements of the BD-XL format.