Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
2000-08-15
2003-10-28
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S064100, C428S064900
Reexamination Certificate
active
06638594
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical information recording medium for rewritable high-speed recording by means of a laser-light beam, said medium comprising a substrate carrying a stack of layers, which stack comprises, in this order, a first dielectric layer, a recording layer comprising a phase-change recording material comprising Sb and Te, a second dielectric layer and a metal mirror layer.
The invention also relates to the use of such an optical recording medium in high storage density and high data rate applications.
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 most important demands in phase-change optical recording is a high data rate. A high data rate requires the recording layer to have a high crystallization rate, i.e. a short crystallization time. To ensure that the previously recorded amorphous marks can be recrystallized during direct overwrite, the recording layer should have a proper crystallization time to match the linear velocity of the medium relative to the laser-light beam. 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) from the previous recording cannot be completely erased (recrystallized) during DOW. This will cause a high noise level. High crystallization speed is particularly required in high-density recording and high data rate applications, such as disc-shaped DVD+RW, DVR-red and blue and CD-RW, where the complete erasure time (CET) has to be shorter than approximately 60 ns. For DVD+RW, which has a 4.7 GB recording density per 120 mm disk, a user data bit rate of 33 Mbits/s is needed, and for DVR-red a user data bit rate of 35 Mbits/s is needed. For rewritable phase change optical recording systems such as DVR-blue (digital video recording operated with a blue laser-light beam), a user data bit rate higher than 50 Mbits/s is required.
An optical information medium of the type mentioned in the opening paragraph is known from United States patent U.S. Pat. No. 5,876,822, filed by Applicants. The known medium of the phase-change type comprises a disc-shaped substrate carrying a stack of layers consisting of, in succession, a first dielectric layer, a recording layer of a phase-change Ge—Sb—Te recording material, a second dielectric layer and a metal reflective 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. Said patent discloses that the CET-value can be shortened by optimizing the composition of the recording material. The CET-value of the recording layer comprising such a recording material decreases as the thickness of the recording layer increases. The shortest CET-value is obtained at a thickness between 25 and 35 nm. Experiments have shown that a user data bit rate of 40 Mbits/s can be achieved with such a recording layer when oxygen is added to the recording material and a SiC cap layer is applied to the stack.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a rewritable optical information medium which is suitable for high speed optical recording, such as DVD+RW, DVD-red and DVD-blue, and for DOW. The CET-value of the recording layer should be 60 ns or lower. In particular, the medium should have a user data bit rate of at least 50 Mbits/s. High speed recording is to be understood to mean in this context a linear velocity of the medium relative to the laser-light beam of at least 4.8 m/s, which is four times the speed according to the compact disc standard.
These objects are achieved in accordance with the invention by an optical information medium as described in the opening paragraph, which is characterized in that the recording material consists of a composition defined by the formula
Q
a
In
b
Sb
c
Te
d
(in atomic percentages), wherein
Q is selected from the group consisting of Ag and Ge;
2<a<8
0<b<6
55<c<80
15<d<30; a+b+c+d=100, and
the recording layer having a thickness of 5 to 15 nm.
In contrast with a recording layer based on Ge—Sb—Te, a recording layer based on Ag—In—Sb—Te or Ge—In—Sb—Te shows a decreasing CET-value as its thickness decreases. The shortest CET-value is found at a thickness smaller than 15 nm, said value being below 60 ns, or even below 40 ns. The CET-value of compositions having a-, b-, c- or d-values outside the claimed range does not become shorter than 60 ns, even at a thickness of 8 nm. For practical reasons, the thickness of the recording layer is at least 5 nm. Optimum results are achieved with a recording layer thickness between 7 and 14 nm.
It is noted that in European patent EP-B-569664 an optical information recording medium is disclosed having a recording layer with a recording material (AgSbTe
2
)
0.43
(InSb
0.77
)
0.57
which can be rewritten as Ag
15.7
In
20.9
Sb
31.9
Te
31.5
. The preferred layer thickness is between 20 and 300 nm. There is no indication of a CET-value or a user data bit rate in that patent specification.
Preferably, the recording material is a Ge—In—Sb—Te alloy (Q=Ge in the above formula), because this material shows a thermally more stable behaviour than Ag—In—Sb—Te material.
The cyclability of the medium can be represented by the value M
50000
/M
0
when 50000 cycles are demanded, which is the relative change of optical contrast M after 50000 cycles and 0 cycles. M is defined as (R
H
−R
L
)/R
H
, wherein R
H
and R
L
are the reflections of the crystalline and amorphous recording material. In every cycle the written amorphous marks are erased by recrystallizing through heating with a laser-light beam while the new amorphous marks are written. The ideal value of M
50000
/M
0
is 1.0, i.e. the optical contrast remains unchanged after cycling.
There is no observable influence of the thickness of the first dielectric layer, i.e. the layer which is adjacent to the servotrack, and therefore in general between the substrate and the phase-change recording layer, on CET and the cyclability value M
50000
/M
0
. Thus, it is possible to vary the thickness of this layer for other, for example optical, reasons without affecting the thermal properties of the stack. This layer protects the recording layer against humidity and the substrate against thermal damage, and optimizes the optical contrast M.
From the viewpoint of jitter, the thickness of the first dielectric layer is preferably at least 70 nm. In view of e.g. optical contrast, the thickness of this layer is preferably limited to 500 nm.
An optimum thickness range for the second dielectric layer, i.e. the layer between the recording layer and the metal mirror layer, is found between 15 and 50 nm, preferably between 20 and 40 nm. Wh
Bartlett Ernestine C.
Ferguson L.
Kelly Cynthia H.
Koninklijke Philips Electronics , N.V.
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