Optical-recording disk substrates

The largest volume use of polymers in optical recording media is in the disk substrate. A schematic of a typical double-sided disk substrate is shown in Figure 1.38, which illustrates the properties required of the substrate material ... 

This chapter will review the present position and opportunities for polymeric materials in write-once and reversible optical recording. At present, no consensus has been reached on the optimal recording or disk substrate materials. Several alternatives are still being actively investigated. 

Optical information storage, which has been a dream since the discovery of the laser, is now becoming a commercial reality. Read-only consumer products (video and digital audio disks) have provided a solid technological base for the development and introduction of the more sophisticated write-once and erasable recording systems. This chapter will review the current status of polymeric materials as substrates, protective layers, and active recording media in laser recording. 

In the air-sandwich structure, which consists of two disks bonded face to face and separated by a spacer-defined air gap, the substrate becomes the protective layer. Very high quality optical substrates are necessary because the write and read functions take place through the disk. The air gap is advantageous for ablative media. Other related structures have been disclosed in which a thin polymer film is supported physically or aerodynam-ically some distance above the recording surface (12). 

In addition, the surface should be free of contaminating particles and occlusions that would interfere with the information retrieval process. A typical ablative-mode optical disk has the structure shown in Figme 16.14. The substrate is an optically transparent material such as polycarbonate, PMMA, polyfethylene terephthalate), or polyfvinyl chloride) topped by a subbing layer to provide an optically smooth surface for the recording layer. A metal reflector (typically aluminum) is then incorporated next to a transparent dielectric medium such as poly(a-methyl styrene) and, finally, the absorbing layer where the information pits are created is added. The latter can be a metal-polymer composite (silver particles in a gel) or a dye molecule dispersed in a polymer matrix such as squaryllium dyes, which act as infrared absorbers for GaAs lasers, typically... 

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