CHAPTER 13

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690 Chapter 13 Optical Storage has a physical track pitch of about 125 microns. When viewed from the reading side (the bottom), the disc rotates counterclockwise. If you examined the spiral track under a microscope, you would see that along the track are raised bumps, called pits, and flat areas between the pits, called lands. It seems strange to call a raised bump a pit, but that is because when the discs are pressed, the stamper works from the top side. So, from that perspective, the pits are actually depressions made in the plastic. The laser used to read the disc would pass right through the clear plastic, so the stamped surface is coated with a reflective layer of metal (usually aluminum) to make it reflective. Then, the aluminum is coated with a thin protective layer of acrylic lacquer, and finally a label or printing is added. Note CD-ROM media should be handled with the same care as a photographic negative. The CD-ROM is an optical device and degrades as its optical surface becomes dirty or scratched. Also it is important to note that, although discs are read from the bottom, the layer containing the track is actually much closer to the top of the disc. Writing on the top surface of a disc with a ballpoint pen, for example, easily damages the recording underneath. You need to be careful even when using a marker to write on the disc. The inks and solvents used in some markers can damage the print and lacquer overcoat on the top of the disc, and subsequently the information layer right below. Use only markers designed for writing on CDs. The important thing is to treat both sides of the disc carefully, especially the top (label) side. Mass-Producing CD-ROMs Commercial mass-produced CDs are stamped or pressed and not burned by a laser as many people believe (see Figure 13.1). Although a laser is used to etch data onto a glass master disc that has been coated with a photosensitive material, using a laser to directly burn copies would be impractical for the reproduction of hundreds or thousands of copies. The steps in manufacturing CDs are as follows (use Figure 13.1 as a visual): 1. Photoresist Coating. A circular 240mm diameter piece of polished glass 6mm thick is spin-coated with a photoresist layer about 150 microns thick and then hardened by baking at 80°C (176°F) for 30 minutes. 2. Laser Recording. A Laser beam recorder (LBR) fires pulses of blue/violet laser light to expose and soften portions of the photoresist layer on the glass master. 3. Master Development. A sodium hydroxide solution is spun over the exposed glass master, which then dissolves the areas exposed to the laser, thus etching pits in the photoresist. 4. Electroforming. The developed master is then coated with a layer of nickel alloy through a process called electroforming. This creates a metal master called a father. 5. Master Separation. The metal master father is then separated from the glass master. The father is a metal master that can be used to stamp discs, and for short runs, it can in fact be used that way. However, because the glass master is damaged when the father is separated, and because a stamper can produce only a limited number of discs before it wears out, the father often is electroformed to create several reverse image mothers. These mothers are then subsequently electroformed to create the actual stampers. This enables many more discs to be stamped without ever having to go through the glass mastering process again. 6. Disc Stamping Operation. A metal stamper is used in an injection molding machine to press the data image (pits and lands) into approximately 18 grams of molten (350°C or 662°F) polycarbonate plastic with a force of about 20,000psi. Normally, one disc can be pressed every 3 seconds in a modern stamping machine.
What Is a CD-ROM? Chapter 13 691 7. Metalization. The clear stamped disc base is then sputter-coated with a thin (0.05–0.1 micron) layer of aluminum to make the surface reflective. 8. Protective Coating. The metalized disc is then spin-coated with a thin (6–7 micron) layer of acrylic lacquer, which is then cured with UV (ultraviolet) light to protect the aluminum from oxidation. 9. Finished Product. Finally, a label or printing is screen-printed on the disc and also cured with UV light. This manufacturing process is identical for both data CD-ROMs and audio CDs. Figure 13.1 CD manufacturing process. Pits and Lands Reading the information back is a matter of bouncing a low-powered laser beam off the reflective layer in the disc. The laser shines a focused beam on the underside of the disc, and a photosensitive receptor detects when the light is reflected back. When the light hits a land (flat spot) on the track, the light is reflected back; however, when the light hits a pit (raised bump), no light is reflected back.
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Recordable DVD Standards Chapter 13
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Recordable DVD Standards Chapter 13
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CD/DVD Software and Drivers Chapter
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Tip Making a Bootable CD for Emerge
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Caring for Optical Media Chapter 13
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