Rewritable optical information medium

Details Rewritable optical information medium Rewritable optical information medium

1999-05-12

2001-02-20

Evans, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S064500, C428S457000, C428S913000, C430S270130, C430S495100, C430S945000, C369S283000, C369S288000

Reexamination Certificate

active

06190750

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the field of optical information medium for rewritable double layer recording using of a laser-light beam, the medium including a substrate having disposed on the same side thereof two recording stacks both having a phase change type recording layer.
BACKGROUND OF THE INVENTION
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 &lgr;, and/or increasing the numerical aperture (NA), because the laser spot size is proportional to (&lgr;/NA)
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)
80
(SiO
2
)
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.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a single-sided rewritable double layer optical information medium having recording stacks with an IPIM structure, in which the storage capacity is doubled with respect to a single-layered information medium.
This object is achieved in accordance with the invention by an optical information medium as described in the opening paragraph, which includes, in this order:
a first recording stack including a phase change type recording layer sandwiched between two dielectric layers, and a transparent metal layer and a further dielectric layer on the side of the first recording stack opposite the side of the first recording stack on which the laser-light beam is incident;
a transparent spacer layer having a thickness larger than the depth of focus of the laser-light beam; and
a second recording stack including a phase change type recording layer sandwiched between two dielectric layers, and a metal mirror layer on the side opposite the side of the second recording stack on which the laser-light beam is incident.
The optical information medium according to the invention has the following structure:
substrate |IPIMI
+
|S| IPIM
wherein the IPIMI
+
stack is the first recording stack with the further dielectric layer I
+
, S is a transparent spacer layer, IPIM is the second recording stack, and wherein I,P, and M have the above mentioned meaning. The laser-light beam is incident via the substrate.
The invention is based on the insight that the transmission of an IPIM stack is increased when the metal layer M is replaced by a thin metal film, which is transparent for the laser-light beam, in combination with a further dielectric layer. For example, the transmission of a recording stack with a 10 nm thick Ag layer can be increased by about 50% by the addition of a further dielectric layer, without adversely affecting the optical contrast between the amorphous and crystalline phase. For this reason the laser power for writing in the second recording stack can be reduced by about 50%.
The metal layer of the first recording stack is thin, i.e. it has a thickness between 10 and 30 nm. This thickness is sufficient for quenching the amorphous phase during writing, and allows sufficient transmission to ensure proper read/write characteristics of the second recording stack. The metal may be selected from Al, Cu, or Au, but is preferably made of Ag, because of its high transmissivity and good thermal conductivity.
The dielectric layers are preferably made of a mixture of ZnS and SiO
2
, e.g. (ZnS)
80
(SiO
2
)
20
. The layers may also be made of SiO
2
, Ta
2
O
5
, TiO2, ZnS, Si
3
N
4
, AlN, Al
2
O
3
, MgO, ZnO, SiC, including their non-stoichiometric compositions. Especially the last six members are preferred because of their good thermal conductivity.
For the metal mirror layer of the second recording stack, metals such as Al, Ti, Au, Ni, Cu, Ag, Rh, Pt, Pd, Ni, Co, Mn and Cr, and alloys of these metals, can be used. Examples of suitable alloys are AlTi, AlCr and AlTa. The thickness of this metal mirror layer is not critical, but preferably the transmission is zero for obtaining maximal reflection. For practical reasons the thickness is about 100 nm.
The recording layer includes a phase change material showing a crystalline-amorphous phase transition. Known materials are e.g. alloys of In—Se, In—Se—Sb, In—Sb—Te, Te—Ge, Te—Se—Sb, Te—Ge—Se, or Ag—In—Sb—Te. Preferably, the recording layer includes a GeSbTe compound. Especially useful are the compounds described in the above mentioned international patent application WO 97/50084. These compounds have a composition defined in atomic percentages by the formula:
Ge
50x
Sb
40−40x
Te
60−10x
,
wherein 0.166&lgr;x&lgr;0.444. These compositions a

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