Substrates for optical discs

The substrate for an optical disc is typically 1.2 mm thick and 120 or 130 mm diameter, a shape and size that can readily be made via injection moulding or casting. But the requirements on the polymer are very complex, as shown in Table 9.3. 

Residual monomer, catalyst residues and plasticizers may tend to migrate into the recording layer and cause pinholes or other defects. So every effort must be made to reduce or avoid such low-molecular-weight material. 

Finally, it must be remembered that discs have to be manufactured at the highest possible speed, so slow moulding cycles are unacceptable. Acrylic discs can be moulded with a 5 s cycle. Polycarbonate discs require a cycle time of 20 s. Some epoxy thermoset substrates require a 1 h cure time It can become difficult to balance the needs for quality against the needs for throughput. 

Many polymers have been evaluated as optical disc substrates. Acrylics have outstanding optical properties, but are ruled out by their water absorption characteristics. Of the aromatic polymers, only polycarbonate 

At present, the dominant substrate material is polycarbonate, and large amounts of high-purity, high-quality material are daily consumed in the manufacture of compact discs. There is no doubt, however, that the industry would greatly welcome an alternative to polycarbonate. 

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J. Henning, Polymer substrates for optical discs, Japanese Journal of Applied Physics, Vol. 26, suppl. 26-4,... 

Taiyo Yuden, Taiyo Yuden Recycling Business of Polycarbonate Substrates for Optical Disc Launched, press release, Oct. 27, 2004. 

The excellent insulating and dielectric properties of BN combined with the high thermal conductivity make this material suitable for a huge variety of applications in the electronic industry [142]. BN is used as substrate for semiconductor parts, as windows in microwave apparatus, as insulator layers for MISFET semiconductors, for optical and magneto-optical recording media, and for optical disc memories. BN is often used as a boron dopant source for semiconductors. Electrochemical applications include the use as a carrier material for catalysts in fuel cells, electrodes in molten salt fuel cells, seals in batteries, and BN coated membranes in electrolysis cells for manufacture of rare earth metals [143-145]. 

Qualitative comparison of a few substrate materials for optical discs. 

We will therefore concentrate the discussion on birefringence brought about by orientation stresses. One measure to reduce this is to bring down these stresses as far as possible. From the material suppliers side this has been done by developing a polycarbonate grade especially for optical disc substrate production. This has an improved flowability due to a considerable reduction of the molecular weight, however to such a value that the thermal and mechanical properties still remain acceptable (9). Other necessary modifications have been an improvement of heat stability and optical purity. 

Substrates for optical tape and floppy optical discs... 

The structures that have evolved for ablative-mode optical discs make use of interference effects to minimize the reflectance (R) of the disc in the absence of a hole. A typical ablative-mode optical disc has the structure shown in Figure 5.51. The substrate is an optically transparent material such as polycarbonate, poly(methyl methacrylate), poly(ethylene terephthalate), or poly(vinyl chloride), topped by a subbing layer to provide an optically smooth (to within a fi-action of a nanometer) surface for the recording layer. A metal reflector (typically aluminum) is then incorporated next to a transparent dielectric medium such as spin-coated poly(a-methyl styrene) or plasma-polymerized fluoropolymers. This dielectric spacing layer serves both to satisfy the quarter-wave (2/4) antireflection conditions and to insulate thermally the A1 reflector from the top absorbing layer where the information pits are created. 

Since the major role that polymers play in optical recording media is in substrates, the remaining part of this review will focus on this aspect. The most imporant requirements on polymers for optica disc substrates are, in a qualitative sense, the following ... 

Polymers play a vital part in optical recording media, mainly as the substrate material. Polycarbonate is the material of choice for Compact Disc substrates. However, in particular for magneto-optic rewritable disc substrates it could be attractive to reduce the birefringence not only by optimizing the moulding process, but also by modifying the material towards a lower orientational stress optical coefficient. Work at our laboratories has shown that this can be achieved... 

An interesting alternative to the use of 1 mm of substrate for providing immunity is a low-birefringence polycarbonate film used as a protective overcoat stretched like a drum skin above a high-performance 14 in optical disc (Kodak Co., Rochester, New York). However, this has not proved to be generally applicable in the marketplace. 

Observation of absorption bands due to LO phonons in RAIR spectra of thin, silica-like films deposited onto reflecting substrates demonstrates an important difference between RAIR and transmission spectra. Berreman has shown that absorption bands related to transverse optical (TO) phonons are observed in transmission infrared spectra of thin films obtained at normal incidence [17]. However, bands related to LO phonons are observed in transmission spectra of the same films obtained at non-normal incidence and in RAIR spectra. Thus, it is possible for RAIR and transmission spectra of thin films of some materials to appear very different for reasons that are purely optical in nature. For example, when the transmission infrared spectrum of a thin, silica-like film on a KBr disc was obtained at normal incidence, bands due to TO phonons were observed near 1060,790,and450cm [18]. 

Spin coating utilizes centrifugal forces created by a spinning substrate to spread a liquid evenly over its surface. Current applications are in photoresist technology for the microelectronic industry and in the manufacture of protective overcoats and adhesives for the optical storage industry (compact discs and DVDs).5... 

Electrical conductor wire, printed circuit board coatings, optical fibers, magnetic media, coatings, computer disc clearcoats, coatings for metallized substrates... 

Figure 23B presents IR transmission spectra for (5) an optically pure and mechanically polished white diamond disc, (6) an undoped and polished (both sides) Si substrate, and (7 and 8) moderately and heavily boron-doped microcrystalline diamond thin films deposited on the undoped Si. The white diamond is relatively free of structural defects and chemical impurities. There is reduced transparency between 2500 and 1500 cm due to the two-phonon absorption. Diamond films with more... 

FIG. 23. Transmission spectra for different materials in the (A) UV/Vis and (B) IR regions of the electromagnetic spectrum. The electrodes in (A) are (1) a thin film of ITO on quartz, (2) a thin film of boron-doped nanocrystalline diamond on quartz, (3) a thin film of mechanically polished and boron-doped diamond on an optically pure, white diamond substrate, and (4) a free-standing, boron-doped, and mechanically polished diamond disc. The electrodes in (B) are (5) an optically pure and mechanically polished white diamond disc, (6) an undoped and polished (both sides) Si substrate, and (7 and 8) moderately and heavily boron-doped microcrystalline diamond thin films deposited on the undoped Si. (Reprinted with permission from Interface 2003, 12, 33. Copyright (2003) The Electrochemical Society, Inc.) (From Ref. 158.)... 

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