Three and Four Data Layer Disk Structures and Corresponding Drive Systems for High Capacity DVD Applications
Original Publication Date: 2002-May-02
Included in the Prior Art Database: 2003-Jun-21
Digital Versatile Disk (DVD) products are rapidly becoming ubiquitous in PC and consumer video players. Current standard DVD-ROM (DVD Read Only Memory) disk capacities are at 4.7 GB (single-layer, single-sided), 8.5 GB (dual-layer, single-sided), 9.4 GB (single-layer, dual-sided) and 17 GB (dual-layer, dual-sided). While a number of consumer electronics companies have been proposing a next generation standard based on blue/violet lasers and high numerical aperture (NA) optics, these lasers devices are possibly still far from mass production at prices required for consumer DVD products. Further, significant technical obstacles in their implementation in reliable and cost effective players may need to be overcome. An alternative to blue/violet lasers is an extension of the current DVD specifications to multiple data layers as a means of increasing effective disk capacity. Such capacity improvements are required for upcoming High Definition Television (HD-TV) applications and to keep pace with other forms of storage such as hard disk drives in set-top boxes. This article describes a three-layer and four-layer disk structure, as shown in the following figures. The disk structure is composed of two substrate layers referred to as top and bottom. These two substrate layers each have a thickness in the range of 0.5-0.6 mm and have a birefringence essentially of 1.5 +/-0.1. In between these two substrate layers are a single 2P layer for a three-layer disk and two 2P layers for a four-layer disk, and an adhesive layer. The photopolymer process commonly referred to as 2P is defined as follows. A stamper is placed on a liquid, commonly acrylate-based, coating which was previously applied to a substrate. Exposure of the liquid to ultraviolet light causes the acrylate to cross-link and solidify, forming a layer which has very low birefringence and which replicates with extremely high fidelity, the features such as pits, track, etc contained in the stamper. Sometimes a bake step is added after the UV exposure. A key element is the recording through the top substrate to access by laser light, in the range of 630-660 nm, all of the data layers from the same side in a multilayer disk structure employing the 2P process to impart the intermediate data layers shown in the figure. Such a structure maintains the needed 1.2 mm overall disk thickness and the desired 0.5-0.6 mm cover-plate thickness of the substrate layers. Further the substrate layers are bonded together in a spaced apart relationship with an adhesive material. The 2P and adhesive layers are in the range of 20-120 microns and used materials with essentially the same refractive index as the substrate material; and, with data pits depths optimized for maximum signal consistent with the lambda/4 rule for signal cancellation in a pit.