Optical information medium

Details Optical information medium Optical information medium

2001-07-05

2003-03-04

Mulvaney, Elizabeth (Department: 1773)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064500, C428S064600, C430S270130

Reexamination Certificate

active

06528138

ABSTRACT:

The invention relates to an optical information medium for rewritable recording by means of a laser-light beam, said medium comprising a substrate having disposed on a side thereof:
a first recording stack comprising a phase change type recording layer, sandwiched between two dielectric layers,
a second recording stack comprising a phase change type recording layer, sandwiched between two dielectric layers,
a transparent spacer layer, interposed between the first and the second recording stack, having a thickness larger than the depth of focus of the laser-light beam, and
a metal mirror layer proximate the second recording stack and at a side of the second recording stack remote from the transparent spacer layer.
Such an optical information medium is known from U.S Pat. No. 6,190,750. Said patent describes an optical information medium for single-sided double layer recording.
Optical 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 optical data storage systems. Phase-change optical recording involves the formation of submicrometer-sized amorphous recording marks in a crystalline film using a focused relatively high power laser-light beam. During recording information, the medium is moved with respect to the focused laser-light beam that 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 marks in the exposed areas of the recording layer that remains crystalline in the unexposed areas. Erasure of the amorphous marks is realized by recrystallizing through heating with the same laser at an intermediate power level, without melting the recording layer. The amorphous marks represent the data bits, which can be read, e.g. via the substrate, by a low-power focused laser-light beam. Reflection differences of the amorphous marks with respect to the crystalline unexposed areas of the recording layer bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the recorded digital information.
It is an aim to increase the storage capacity of optical recording media like DVD-Rewritable and DVR (Digital Video Recorder) on a single-sided disc. This can be achieved by reducing the laser-light wavelength &lgr;, and/or increasing the numerical aperture (NA) of a recording lens, because the laser-light spot size is proportional to (&lgr;/NA)
2
. As a consequence of a smaller laser-light spot size the marks which are recorded are smaller. Therefore the storage capacity of a disc increases because more marks fit per unit area of the disc. An alternative option is the application of multiple recording layers. This is called double or dual layer recording, when two recording layers on the same side of the optical disc are used. When more than two recording layers on the same side of the optical disc are used, it is called multi layer recording
For these new discs the complete erasure time (CET) has to be at most 60 ns. CET is defined as the minimum duration of an erasing pulse for complete crystallization of an amorphous mark in a crystalline environment, which is measured statically. For DVD-Rewritable, which may have a 4.7 GB recording density per 120 mm diameter disc, a user data bit rate of 33 Mbits/s is needed, and for DVR-red and -blue, wherein red and blue refer to the used laser-light wavelength, said data bit rate is 35 Mbits/s and 50 Mbits/s respectively. Each of these data bit rates rates can be translated to a maximum CET which is influenced by several parameters, e.g. thermal design of the recording stacks and the recording layer materials used.
For double layer recording, the first recording stack must be sufficiently transmissive to ensure proper read/write characteristics of the second recording stack. The known medium from U.S. Pat. No. 6,190,750 has a |IP
1
IM
1
I
+
|S|IP
2
IM
2
structure for rewritable phase change recording which has two metal layers of which the first, M
1
, is relatively thin and has a high optical transmission and of which the second, M
2
, is a mirror with high optical reflection. I represents a dielectric layer, I
+
represents a further dielectric layer, P
1
, on which the laser light is incident first, and P
2
represent phase change recording layers, and S represents a transparent spacer layer. The metal layers not only serve as a reflective mirror, but also as a heat sink to ensure rapid cooling for quenching the amorphous phase during writing. The recording and erasing behavior of the two recording layers P
1
and P
2
, which are made of the same or of very similar material, is different due to their position in the stack. The P
1
layer is present proximate a relatively thin metal layer M
1
with limited heat sink capacity while the P
2
layer is present proximate a relatively thick metal mirror layer M
2
which causes substantial cooling of the P
2
layer during recording. The cooling behavior of a recording layer determines to a large extend the required laser-light write pulse strategy and the required recording-velocity of the laser-light beam during recording. Furthermore, the relatively thin metal layer M
1
inevitably blocks a substantial part of the laser-light causing a reduced recording power at the P
2
layer.
It is a disadvantage of the known medium that the recording and erasing behavior of the first and second recording layer is substantially different. This requires a different laser light write pulse strategy and recording velocity for each of the recording layers, which causes the recording apparatus to be more complex.
It is an object of the invention to provide an optical information medium of the kind described in the opening paragraph, in which an optimal recording and erasing behavior is achieved with a laser-light write pulse strategy which is substantially equal for the recording layers and a recording velocity which is substantially equal for the recording layers.
This object is achieved in that
the first, recording stack comprises a phase change type recording layer of a kind selected from a kind with substantially growth dominated crystallization and a kind with substantially nucleation dominated crystallization and
the second recording stack comprises a phase change type recording layer of a kind different from the kind selected for the first recording stack.
The principle of the optical information medium of the invention can be schematically explained by e.g. the following layer structure:
|IP
1
I|S|IP
2
I|M|
wherein IP
1
I is the first recording stack, IP
2
I is the second recording stack, wherein I and S have the above-mentioned meaning, M is a metal mirror layer and P
1
and P
2
are phase change type recording layers of a different kind. During recording and reading the laser-light beam of an optical recorder is incident via the first recording stack. The substrate on which the layer structure is disposed can either be present adjacent the metal layer M, in which event the laser-light beam enters via the first recording stack without passing the substrate, or adjacent the first recording stack, in which event the laser-light beam enters via the first recording stack after passing the substrate layer. At the side of the layer structure, which is remote from the substrate, a cover layer may be present, which protects the layer structure from the environment.
The invention is based on the insight that the crystallization kinetics of the recording layers has to be matched with the thermal and/or optical properties of the layers adjacent the recording layers by the choice of material of the recording layer. Two mechanisms of crystallization are known: growth dominated and nucleation dominated crystallization. The presence of a metal mirror M causes the second recording stack to be a relatively fast cooling struc

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