Digital video disk mastering

Mastering refers to the process of forming the original pattern for replicating structures to provide the specified opto-electronic signals. Several technologies have been explored since the start of the optical storage industry. Mechanical cutting, as used in the conventional audio industry, turned out to be unpractical. Mastering dynamics could be best reached by laser beam recording. Literature reveals two types of possible processes for laser beam recording; photo-resist based and non-photoresist (ablative) based technology. The photo-resist technology is based upon a process using well established lithographic technologies. The master disc is composed of a glass substrate on which a layer of photo-resist material has been applied. The submicron sized portions of this photo-resist layer, exposed by the focused laser beam, give the requested opto-electronic structure after development and rinsing of the master substrate. The photo-resist based process is a microscopically localized process. The pit shape depends on the exposure of that area only and no contributions from adjacent spot positions are present. The photo-resist process has proven flexibility towards a large variety of groove and pit shapes. The final pit structure is controlled during the development process by pit-formation monitoring. Non photo-resist mastering deals with forming structures by local heating of specific areas of a substrate covered with an ablative material. The pit formation and the shape of the pit depend strongly on the temperature profile. This pit-formation process depends not only on the (heat) exposure of the laser beam for that given minute portion but also on the heat flow from neighbouring areas. If a nearby area has just been heated, heat will diffuse to that given area and cause a different pit shape. This highly unwanted phenomenon is known in optical recordable systems as "inter symbol interference". In order to prevent uncontrolled pit formation due to this interference from adjacent pits, complex write strategies have to be used. These write strategies are strongly dependent on recording speeds, layer thickness, spot quality and layer quality. In order to certify the performance of this process for its pit characteristics, the pit formation has to be verified by a second read beam. This read laser adds to the systems complexity while its reliability has to be certified systematically. This explains why more complex pit structures, e.q. MO, are preferably realized by the photoresist mastering process.