Compact Disc

A compact disc is a thin wafer of clear polycarbonate plastic and metal measuring 4.75 in (120 mm) in diameter with a small hole in its center. The metal layer is usually pure aluminum that is spread on the polycarbonate surface in a layer that is only a few molecules thick.

The metal reflects light from a tiny infrared laser as the disc spins in the CD player. The reflections are transformed into electrical signals and then further converted to meaningful data for use in digital equipment.

Information (either audio on a music CD or data of many kinds on a CD-ROM) is stored in pits on the CD that are 1-3 microns long, about 0.5-micron wide, and 0.1-micron deep. There may be more than 3 mi (4.8 km) of these pits wound around the center hole on the disc. The CD is coated with a layer of lacquer that protects the surface. By convention, a label is usually silkscreened on the backside.

Compact discs are made in a multistep process. First a glass master is made using photolithographic techniques. An optically ground glass disc is coated with a layer of photoresist material 0.1-micron thick. A pattern is produced on the disc using a laser; then the exposed areas on the disc are washed away, and the disc is silvered to produce the actual pits. The master disc is next coated with single-molecule-thick layers of nickel, one layer at a time, until the desired thickness has been achieved. The nickel layer is next separated from the glass disc and used as a metal negative.

For low production runs, the metal negative is used to make the actual discs. Most projects require that several positives be produced by plating the surface of the metal negative. Molds or stampers are then made from the positives and used in injection molding machines.

Plastic pellets are heated and injected into the molds, where they form the disc with pits in it. The plastic disc is coated with a thin aluminum layer for reflectance and with a protective lacquer layer. The disc is then given its silkscreened label and packaged for delivery. Most of these operations take place in a cleanroom because a single particle of dust larger than a pit can destroy data. Mastering alone takes about 12 hours of work.