Although identical in appearance to CD-DAs, CD-ROMs store data instead of (or in addition to) audio. The CD-ROM drives in PCs that read the data discs are almost identical to audio CD players, with the main changes in the circuitry to provide additional error detection and correction.

This is to ensure data is read without errors because what would be a minor—if not unnoticeable—glitch in a song would be unacceptable as missing data in a file. A CD is made of a polycarbonate wafer, 120mm in diameter and 1.2mm thick, with a 15mm hole in the center.

This wafer base is stamped or molded with a single physical track in a spiral configuration starting from the inside of the disc and spiraling outward. The track has a pitch, or spiral separation, of 1.6 microns (millionths of a meter, or thousandths of a millimeter). By comparison, an LP record 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.

Mass-Producing CD-ROMs

Commercial mass-produced CDs are stamped or pressed and not burned by a laser as many people believe. 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 discs would be impractical for the reproduction of hundreds or thousands of copies.

The steps in manufacturing CDs are as follows:

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 may 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 2–3 seconds in a modern stamping machine.

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. This protects the aluminum from oxidation.

9. Finished Product. Finally, a label is affixed or printing is screen-printed on the disc and cured with UV light.

This manufacturing process is identical for both data CD-ROMs and audio CDs.