PC Hardware in a Nutshell, 3rd Edition

CD-(M)RW Technical Details

If you want to know more about the technical details of how CD-(M)RW drives and discs work, read on. Otherwise, you can safely skip to the next section.

CD-RW discs use optical phase change technology that is similar to that used by magneto-optical drives, but does not use magnetism to aid the phase change. CD-RW discs are constructed similarly to CD-R discs, except for the recording layer, which is dramatically different. Like CD-R discs, CD-RW discs include a preformed pristine spiral groove that provides servo (tracking) information, absolute time in pregroove (ATIP) timing data, and so on. But rather than the simple dye layer used by CD-R discs, CD-RW discs use a recording layer sandwiched between two dielectric layers that absorb and dissipate excess heat generated during writes.

The recording layer comprises a crystalline compound of silver (chemical symbol Ag), indium (In), antimony (Sb), and tellurium (Te), and so is often referred to as the AgInSbTe layer. Together, these chemical elements form an exotic mix with a very special characteristic: when heated to a specific temperature and then cooled, the compound forms a crystalline matrix, but when heated to a higher temperature and then cooled, the compound assumes an amorphous (disordered) form. The reflectivity of the crystalline form lands, at 25% or so, is much lower than the equivalent on a pressed CD or CD-R disc (>70%). The reflectivity of the amorphous form pits, at 15% or so, ...