Optical Storage - Upgrading and Repairing PCs - page 675

Instead of burning an organic dye as with CD-R, the recording layer in a CD-RW disc
is made up of a phase-change alloy consisting of silver, indium, antimony, and tellurium
(Ag-In-Sb-Te). The reflective part of the recording layer is an aluminum alloy, the same
as used in normal stamped discs. As a result, the recording side of CD-RW media looks
like a mirror with a slight blue tint. The read/write laser works from the underside of the
disk, where the groove again appears like a ridge, and the recording is made in the phase-
change layer on top of this ridge. The recording layer of Ag-In-Sb-Te alloy normally has
a polycrystalline structure that is about 20% reflective. When data is written to a CD-RW
disc, the laser in the drive alternates between two power settings, called P-write and P-
erase.Thehigherpowersetting(P-write)isusedtoheatthematerialintherecordinglayer
to a temperature between 500°C and 700°C (932°-1,292°F), causing it to melt. In a liquid
state the molecules of the material flow freely, losing their polycrystalline structure and
taking what is called an amorphous (random) state. When the material then solidifies in
thisamorphousstate,itisonlyabout5%reflective. Whenbeingread,theseareaslowerin
reflectivity simulate the pits on a stamped CD-ROM disc.
To return the material back to a polycrystalline state, the laser is set to the lower-power P-
erase mode. This heats the active material to approximately 200°C (392°F), which is well
below the liquid melting point but high enough to soften the material. When the materi-
al is softened and allowed to cool more slowly, the molecules realign from a 5% reflect-
ive amorphous state back to a 20%reflective polycrystalline state. These higher reflective
areas simulate the lands on a stamped CD-ROM disc.
NotethatdespitethenameoftheP-eraselaserpowersetting,thediscisnoteverexplicitly
“erased.” Instead, CD-RW uses a recording technique called direct overwrite , in which a
spot doesn't have to be erased to be rewritten; it is simply rewritten. In other words, when
data is recorded, the laser remains on and pulses between the P-write and P-erase power