Compact Disk (CD) [5]

Making CDs

One reason for the success of the optical disk technology is the ease and cheapness of replicating them in large quantities. The making of a CD includes 2 main steps: Pre-mastering and Mastering.

Pre-mastering involves data preparation for recording. The data is indexed, organized, re-formatted (possibly with some ECC), and transferred to magnetic tape. Now, the data is ready to be imprinted onto the CD. Mastering involves physical transfer of the data into the pits and lands.

First, a layer of light-sensitive photoresist is spin-coated onto the clean glass master-disk from a solvent solution.

Then, the photoresist is exposed to a modulated beam of a short-wavelength light, which carries the encoded data. Next, the master is developed in a wet process by exposing it to the developer, which etches away exposed areas thus leaving the same pattern we will find later on the CD. Next, the master is coated (using electroplating technique) with a thick (about 300 um) metal layer to form a stamper - a negative replica of the disk. The photoresist layer is destroyed during this process, but the much more durable stamper is formed and can be used for CD replication. Usually, a stamper can be used to produce a few tens of thousands CDs before it wears out.

Finally, the process of injection molding is used to produce a surface of the compact disk. Hot plastic (PC) is injected into a mold, and then is pressed against the stamper and cooled, resulting in the CD. Other processes than injection molding could be used, but they all involve pressing the hot plastic against the stamper.

At the very end, the pits and lands on the surface of a CD are coated with a thin reflective metal layer (aluminum), then coated with lacquer and supplied with the label. Packaging usually finishes the process of making a CD.

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