After the master disc has been cut, it must be transformed into a configuration from which replicas can be made. This is done by transforming the essentially "two-dimensional" master record, which consists of holes in a thin metal film, into a "three-dimensional" configuration which can be used to stamp or form inexpensive, plastic replica discs.

2.1 Master Transformation The master is coated with a photoresist material and is exposed through the rear (under-surface) of the disc. The ultraviolet light source exposes (polymerizes) the photoresist through the information holes. The uncut metal film shields the photoresist where there are no holes. This results in an array of hardened areas which coincide with the initial array of information holes. The unpolymerized photoresist material is then washed away with an appropriate solvent leaving bumps over the holes. Depending upon the photoresist used, the hardening program and other parameters, the height and profile of these bumps may be tailored to optimize the optical contrast between these bumps and the surrounding flat area when they are illuminated by the high numerical-aperture, diffraction limited optical scanning system of the player.

2.2 Replica Forming The prime method of producing good quality, inexpensive replica discs uses a polyethylene terephthalate ("Mylar") material and is a proprietary process at this time. It will be treated elsewhere when the patent circumstances and the proprietary elements permit. It has among its advantages a better quality, tougher record, a shorter production cycle-time, and web or automated belt handling of the entire disc replication process. An alternate process which has also been employed involves treating the transformed master described above by electroless and electrodeposition to form a metal tool ("stamper") from which replicas are thermoformed, typically from polyvinylchloride (PVC), by a method close to that used to make audio records.

2.3 Post Forming Operations The plastic discs are finally metallized with a reflective coating and coated with a transparent plastic for protection against degradation by handling. An alternate process to the final protective coating is to produce the discs using a transparent plastic permitting optical reading of the record through the transparent back side. Due to the limited depth of focus of the read-out optical system, typically ±1 micron, scratches or dirt on the surface of the protective coating are out of focus and have no degrading effect on the record playback. Optically read records of this type actually require less care in handling than ordinary audio LPs. The replicated discs are typically 5 to 10 mils in thickness. They may be configured either with information bumps as indicated in the master transformation section, or information holes made by forming these bumps into a mating surface - depending upon how many generations or reversals are involved between the transformed master and the final plastic replica. These two configurations are both satisfactory from an optical read-out standpoint.