The writing process: CD-RW
In the CD-RW disc, the recording layer is made of an alloy of silver, indium, antimony and tellurium. In its original state, this layer has a polycrystalline structure. During the recor-ding process, the laser selectively heats tiny areas of the recording track to a temperature above the layer’s melting point (500 - 700°C). For CD-RW writing, the laser power used is in the range 8 to 14 mW.
The pulsed energy delivered by the laser beam melts the crystals in the heated areas into a non-crystalline amorphous phase (‘pits’), which has a much lower reflectance than the remaining crystalline areas (‘lands’).This difference in reflectance allows the recorded data to be read-out, producing a signal similar to that obtained from a standard CD. The physical characteristics of the amorphous phase are ‘frozen-in’ during cooling, making the recording just as permanent as any standard CD.
Erasing of a CD-RW disc is performed by returning the material in the recording layer which has been changed to the amorphous state back to the crystalline state. This is
done by an annealing process, consisting of heating the layer to a temperature of about 200°C (i.e. less than the melting point) and maintaining that temperature for an extended period (in practice, this takes some 37 minutes for a complete disc). The disc is then returned to its original, completely unrecorded state. A much faster ‘on the fly’ erasing facility is also available, allowing the last recorded track to be erased simply by
erasing the subcode reference to that track while leaving the recorded data in place in the recording layer.
A direct overwrite strategy is obtained by combining the write and erase techniques. In this case, new pits are written in the recording layer using the same pulsed laser
beam energy as in the standard writing strategy. However, in the areas between the newly recorded pits, a lower-energy, non-pulsed laser beam is used to write new crystalline lands. The laser beam is repeatedly switched to the lower-energy
erase level between the new pits, resulting in complete erasure of the audio data that was formerly contained in these areas.
As in the writing of a CD-R disc, a higher energy level is used initially to create the temperature increase required to melt the recording layer. Between the pits, the temperature is reduced to the annealing (erase) level. This provides a higher starting temperature, so less energy is subsequently needed each time the melting temperature has to be reached.