Optical disk substrates

Some of the key material requirements for optical disk substrates are mechanical integrity, dimensional stability over a range of temperatures and... 

Gore, M. P Bhatt, J. Kasperchik, V. P. Metal salt activators for use in leuco dye compositions and color coating optical disk substrates. U.S. Pat. Appl. Publ. US 2005053748, 2005 Chem. Abstr. 2005, 142, 281642. 

T. Tokuda and M. Hayashi, Optical disk substrate, optical disk and aromatic polycarbonate resin, U.S. Patent 5,633,060 (1997). 

Matsuda A., Matsuno Y., Mitsuhashi Y., Tohge N., Minami T. Optical disk substrate fabricated by the sol-gel metiiod. In Key Engineering Materials, Vol. 150, Schmidt H., ed. Switzerland Trans Tech Publications, 1998, pp. 111-120... 

With disk diameters above 5.25 in., all parameters, eg, water absorption and thermal expansion, become more critical which aggravates the expansion or warp of disks. If in the future disk rotation speeds have to be increased significantly to boost data transfer rates, higher demands will be placed on warp (tilt angle) and modulus to avoid creeping (ie, irreversible elongation in radial direction). A survey of the requirement profile for the substrate material of optical disks is given in Table 5 (182,186,187,189). 

The distance between the disk surface facing the optics and the memory layer naturally has to be much smaller than in common optical disks, where the memory layer is deposited behind a 1.2-mm thick transparent glass or polymer substrate disk. 

Fig. 26. Qualitative compatison of substrate materials for optical disks (187) An = birefringence IS = impact strength BM = bending modulus HDT = heat distortion temperature Met = metallizability WA = water absorption Proc = processibility. The materials are bisphenol A—polycarbonate (BPA-PC), copolymer (20 80) of BPA-PC and trimethylcyclohexane—polycarbonate (TMC-PC), poly(methyl methacrylate) (PMMA), uv-curable cross-linked polymer (uv-DM), cycHc polyolefins (CPO), and, for comparison, glass.

Two methods could be used to prepare the disk samples. One is spin coating. Compound 96 and poly (methyl methacrylate) (PMMA) were dissolved in cyclohexanone and the solution spun coated onto the disk substrate to prepare a photosensitive PMMA thin film on the disk doped with compound 96. The second method is direct evaporation of pure compound 96 to the disk under high vacuum conditions. The structure of the photochromic optical disk sample is shown in Figure 4.3. 

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. 

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... 

Matsumoto, M., Tsunematsu, N. and Ebisawa, K., Method and apparaters for manufacture of optical disks by bonding two disks substrates using cationic polymerizable UV-curable resin adhesives, Japanese Patent JP 219-608, CA 2003-97201, 2001. 

In the Air Sandwich design, the active layer is coated onto a transparent substrate and is located within an air gap sandwich structure. Thus, no overcoat is needed. The laser beam is focused through the substrate onto the active layer and consequently the mechanical and optical properties of the substrate play an important part in optical disk construction. The substrate should be highly transparent ("85X) at the laser wavelength and optically uniform. Optical uniformity not only includes constant, standard path length, but low, uniform birefringence as well. The requirement of low birefringence arises from the use of... 

High demands are placed on the substrate material of disk-shaped optical data storage devices regarding the optical, physical, chemical, mechanical, and thermal properties. In addition to these physical parameters, they have to meet special requirements regarding optical purity of the material, processing characteristics, and especially in mass production, economic characteristics (costs, processing). The question of recyclabiUty must also be tackled. 

The birefringence of substrate materials for optical data storage devices requires special attention, especially in the case of EOD(MOR) disks. Birefringence has no importance for glass substrates (glass does not exhibit any significant birefringence) and is only a subordinate factor for polymeric protective layers of aluminum substrates because of their reflective read/write technique. 

To efficiendy drive the development of improved substrate materials, the limiting values of birefringence have to be known this is especially tme for WORM and EOD(MOR) substrate disks. These limit values were laid down by the ANSI (American National Standard Institute) Technical Standard Committee (186—188). For 5.25 in. WORM disks, the ANSI document X 3 B 11/88-144 recommends a maximum LEP value of 9% this corresponds to an optical path difference perpendicular to the plane of the disk of not more than 80 nm/mm (double path). For 5.25 in. EOD(MOR) disks, more stringent conditions apply (ANSI-document X 3 B 11/88-049), which also allow calculation of the allowed range. 

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