Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
1999-06-14
2004-05-11
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S275400, C369S283000, C428S064400
Reexamination Certificate
active
06735165
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical information medium for rewritable recording at constant angular velocity by means of a laser-light beam, said medium comprising a disc-shaped substrate carrying a stack of layers, which stack comprises in this order:
a first dielectric layer,
a recording layer of a phase-change material which is able to record amorphous marks when in the crystalline state, the recording layer forming an annular recording area with an inner and an outer radius,
a second dielectric layer, and
a metal mirror layer.
The invention also relates to the use of such an optical recording medium for recording at constant angular velocity.
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 in accordance with the coded, recorded digital information.
One of the problems in phase-change optical recording is to obtain a high erasing (recrystallization) speed. A high crystallization speed is particularly required in high-density recording and high data rate applications, such as disc-shaped DVD-RAM, DVD-Rewritable systems and DVR (Digital Video Recorder). If the crystallization speed is not high enough to match the linear velocity of the medium relative to the laser-light beam, the old data (amorphous marks) of the previous recording can not be completely removed (recrystallized) during DOW, causing a high noise level. For example, to successfully perform recording on a DVD-RAM disc at a constant recording speed of 7.2 m/s (i.e six times the speed according to the Compact Disc standard), the complete erasure time (CET) of the phase change recording layer should be 60 ns or shorter. The recording speed is the magnitude of the velocity between the recording layer of the recording medium and a spot formed by the laser-light beam on this recording layer. When recording data on a recording medium, the recording speed may change as a function of the position of the spot of the laser-light beam on the recording layer. Changes in recording speed are encountered when recording on a disc-shaped recording medium rotating at a constant angular velocity (CAV). Phase-change optical recording at CAV is especially important in computer and video applications. The access time from the outer radius to the inner radius is much shorter in CAV-recording than in constant linear velocity (CLV) recording, because in CLV-recording the rotational speed of the medium must be increased when the radial position of the spot of the laser-light beam on the recording layer is decreased. This adaptation of the rotational speed to the radial position is relatively slow due to the inertia of the medium.
When the recording is performed on a disc rotating at a constant angular velocity, the linear velocity decreases with decreasing radius. For example, in the case of a 120 mm disc rotating at a frequency of 20 Hz (DVD-RAM format B′ or DVD+RW 3.0), the linear velocity at the outer radius is about 7.3 m/s (at radius r=58 mm) and at the inner radius about 3 m/s (r=24 m/s). At such linear velocities, the CET of the recording layer should be 60 ns or shorter, so that the recording can be successfully performed at the outer radius where the linear velocity is highest. However, when the recording is performed at the inner radius of the disc (at the same rotating frequency), recrystallization occurs during DOW because of the high crystallization rate and the relatively low linear velocity. This leads to badly defined mark edges and thus a high jitter, a large increase of amorphous reflection and a decrease of modulation.
An optical information medium of the type mentioned in the opening paragraph is known from the international patent application WO 97/50084 (PHN 15881) (U.S. Application 08/795,819 now U.S. Pat. No. 5,876,822) filed by Applicants. The known medium of the phase-change type has a substrate carrying a stack of layers comprising a phase-change recording layer of a GeSbTe-compound sandwiched between two dielectric layers, and a metal mirror layer as a reflective layer. Such a stack of layers can be referred to as an IPIM-structure, wherein I represents a dielectric layer, P represents a phase-change recording layer, and M represents a metal mirror layer. With certain GeSbTe compounds and a well defined layer thickness of the recording layer and the second dielectric layer, CET-values of about 50-60 ns are obtained. The layer thicknesses are optimized for a constant linear velocity of 7.2 m/s. When such a recording is used at CAV, due to the difference in linear velocity over the radius of the disc, the above mentioned problems occur.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a rewritable optical information medium which is suitable for recording at constant angular velocity (CAV), such as DVD-RAM and DVR, and which is suitable for high data rate and high density optical recording.
This object is achieved in accordance with the invention by an optical information medium as described in the opening paragraph, characterized in that the recording layer has a gradually increasing thickness from the inner to the outer radius. It is known from the above mentioned WO 97/50084 that the CET-value of GeSbTe-compounds decreases with increasing layer thickness. Some of these materials, such as GeSb
2
Te
4
, show an almost linear decrease of the CET at an increase in layer thickness from 15 to 27 nm. In this thickness range the CET decreases by a factor of 3. The slope of the decrease depends on the composition of the GeSbTe-compound. Since the linear velocity difference between the outer and inner radius is of a similar order of magnitude as the CET difference, discs with a variable thickness of the phase change recording layer are suitable for CAV-recording, when the thickness of the recording layer at the inner radius is smaller than at the outer radius. When the thickness of the recording layer gradually increases from the inner to the outer radius, the increase in linear velocity between the spot of the laser-light beam and the recording layer can be met by an appropriate decrease in the CET, i.e. a faster crystallization rate of the recording layer.
The choice of the thickness range of the recording layer is determined by the CET vs thickness curve and the linear velocity difference between the inner and outer radius. If the CET vs thickness curve is (almost) linear, as for the compound GeSb
2
Te
4
in the thickness range between 15 to 27 nm, the thickness of the recording layer increases (almost) linearly from the inner radius to the outer radius.
The recording layer comprises 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, and Te—Ge—Se. Preferably, the recording layer comprises a GeSbTe-compound. Especially useful are the compounds described in the above mentioned patent applic
Van Woudenberg Roel
Zhou Guo-Fu
Bartlett Ernestine C.
Neyzari Ali
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