As a compact disc
spins, it is the transitions between pits and lands
(and thus the lengths of each pit and land) that code the information (interleaved binary
). A silver factory disc is pressed from a mould of pits and lands, but ReWritable discs do not have these physical pits and lands like a pressed disc, because they need to be erasable and written to many times over. Instead they use the interesting phase-change properties of particular chemicals. Where the simple factory CD
is made of just three or four layers, erasable media uses a complex six-layer design, which is composed of (top down): a protective UV-resistant lacquer coat, an aluminium
alloy reflector; an upper dielectric layer; a phase-change alloy; a lower dielectric layer; and the clear polycarbonate substrate. The phase-change alloy is the recording layer, consisting of an alloy of silver
, indium, antimony and tellurium.
The recording layer has polycrystalline properties – it works because it can be interchanged between a crystalline state (high reflectivity) and an amorphous state (low reflectivity). The two states can thus be optically distinguished. A pressed CD reflects the laser light with variable intensities due to destructive interference of the laser, while a CD-RW is made of materials that absorb or transmit light depending on their phase.
To record onto the disc, a short pulse of 11 mW laser heats areas of the recording layer to about 600°C. This is a hot enough to melt the crystals. Rather than recrystallising, they cool back into an amorphous state – these marks are analogous with pits. To erase data, a more gentle 6 mW laser massages the amorphous material to its ‘glass transition temperature’ (200°C), which transforms it back into crystalline state (a process called annealing) – the crystalline blocks resemble lands.
The dielectric layers, among a few other functions, act as a thermal insulator for the recording layer. This keeps the heat efficiently concentrated on the recording layer, and also protects the other layers of the disc.
Lifetime? Many manufacturers say their disc can provide 1000 writes. Some manufacturers even claim as much as 100,000 writes, so a CD-RW disc might live longer than you!