Optical information or data storage based on the phase change principle is attractive, because it combines the possibilities of direct overwrite (DOW) and high storage density with easy compatibility with read-only systems. Phase-change optical recording involves the formation of submicrometer-sized amorphous recording marks in a thin crystalline film, using a focused laser-light beam. During recording information, the medium is moved with respect to the focused laser-light beam which is modulated in accordance with the information to be recorded. Due to this, quenching takes place in the phase-change recording layer and causes the formation of amorphous information bits in the exposed areas of the recording layer, which remains crystalline in the unexposed areas. Erasure of written amorphous marks is realized by recrystallizing through heating with the same laser. The amorphous marks represent the data bits, which can be reproduced via the substrate by a low-power focused laser-light beam. Reflection differences of the amorphous marks, with respect to the crystalline recording layer, bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent (electronic signal) in accordance with the coded, recorded digital information.
One of the aims in optical recording is to increase the storage capacity of media like DVD-Rewritable and DVR (Digital Video Recorder) on a single-sided disc. This can be achieved by reducing the laser wavelength .lambda., and/or increasing the numerical aperture (NA), because the laser spot size is proportional to (.lambda./NA).sup.2. An alternative option is the application of multiple recording layers. When two recording layers on the same side of the optical disc are used, this is called double or dual layer recording.
An optical information medium of the phase change type having a single recording layer is known from the international patent application WO 97/50084 (PHN 15881) filed by Applicants. The known medium of the phase change type, includes a substrate carrying a stack of layers including a first dielectric layer of e.g. (ZnS).sub.80 (SiO.sub.2).sub.20, a phase change recording layer of a GeSbTe compound, a second dielectric layer, and a reflective metal mirror layer. Such a stack of layers can be referred to as an IPIM structure, wherein M represents a reflective or mirror layer, I represents a dielectric layer, and P represents a phase change recording layer. The metal layer serves not only as a reflective mirror, but also as a heat-sink to ensure rapid cooling for quenching the amorphous phase during writing. The known recording medium has a good cyclability, i.e. a large number of repeated writing and erasing operations are possible, is suitable for high speed recording, and shows a low jitter even after a large number of cycles. In that patent application, double layer recording is not disclosed.
For double layer recording, the first or upper recording stack must be sufficiently transmissive to ensure proper read/write characteristics of the second or lower recording stack. However, the known IPIM structure for rewritable phase change recording has a crystalline absorption of about 80% and a crystalline reflection of about 20%, thus ensuring a high temperature increase in the phase change recording layer, a high modulation, and proper tracking, with limited laser power. Since the IPIM structure is not transmissive, it is unsuitable as a first or upper recording stack, although it has a proper structure for the second or lower recording stack, where zero transmission is preferable. A possible solution could be the replacement of the mirror layer M by a transparent heat conductor such as aluminium nitride. However, the thermal conductivity of these materials in thin films still appears to be too low, and therefore their ability to rapidly reduce the temperature in the recording layer is insufficient.