Trends in Optical Disk Mastering
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essary, or at least désirable, in order to fulfill thèse requirements with a high enough quality. It is expected that in the future higher information densities on optical disks will become necessary, e.g., for high-definition télévision or for large data storage applications. It is generally believed that the photoresist technique is the most promising for a higher resolution mastering process and many companies are currently exploring its limits.
The Mastering Process A schematic overview of the disk manufacturing process is shown in Figure 2. The séquence starts with very flat, ground-and-polished glass disks. They are carefully cleaned and coated with an adhésion promoting layer.4 Then a layer is added, into which the relief structures are created according to the information or the format that has to be recorded. This article will refer to this layer as the "active" layer. The relief formation process is called "mastering." At this point the active layer is covered with a thin métal film e.g., by evaporation, sputtering, or a wet çhemical process. This layer is needed to make the surface conducting so that a thick nickel shell can be grown on top of it in a galvanic bath. When the shell is separated from the master disk and the residues of the active material are removed, the surface shows a highquality négative of the relief structure on the master. This nickel shell, usually called the "father," can be used as a stamper in the replication processes. It is also possible to make nickel copies of the father by two additional galvanic steps. The intermediate product carries a positive copy of the master and is called a "mother." The final stampers, négatives again, are called "sons." Instead of the galvanic stamper making process, it is also possible to use a photopolymerization technique that results in plastic stampers.5 In the early
days of optical recording many différent techniques were suggested for the replication step.1'6 Today, injection or compression molding bf polymethylmethacrylate (PMMA) or polycarbonate (PC) is commonly used for the largescale production of read-only disks. 7 For write-once or erasable data disks a photopolymerization technique 89 is also used. In the mastering step, a Laser Beam Recorder (LBR) is used in almost ail cases; i.e., the information is written onto the master disk by exposing the active material to a focused beam of laser light (Figure 3a). In this process either the relief structures may be created immediately (in a direct-effect mastering technique) or a latent image may be formed via a photochemical reaction in the material. A second step is needed then to transform the latent image into a relief structure. The photoresist technique is a well-known example of this second possibility. Other mastering processes using électron beams10 and electromechanical techniques3 hâve also been proposed but hâve found only limited application in the production of optical disks. The information on read-only disks is contained in the length of the pits. (the width should be const
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