Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
1999-04-20
2001-07-03
Evans, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064500, C428S064600, C428S457000, C428S913000, C430S270130, C430S495100, C430S945000, C369S283000, C369S288000
Reexamination Certificate
active
06254957
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of rewritable phase-change type optical recording media.
BACKGROUND OF THE INVENTION
The invention relates to an optical information medium for erasable recording using a laser-light beam, the medium including a substrate carrying a stack of layers includes a first and a second carbide layer, a recording layer of a phase-change material arranged between the carbide layers, for recording amorphous bits when in its crystalline state, and a first dielectric layer arranged between the substrate and the first carbide 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.
The main problems in high speed phase-change optical recording related to the required capability for a large number of overwrite cycles (cyclability), i.e. the number of repeated writing (amorphization) and erasing (recrystallization) operations, and a proper crystallization 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, DVR (Digital Video Recorder) and optical tape, where the complete crystallization time (complete erase time: CET) has to be shorter than 50 ns. 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 cannot be completely removed (recrystallized) during DOW. This will cause a high noise level.
An optical information medium of the type mentioned in the opening paragraph is described in co-pending U.S. patent application 08/993,133 filed Dec. 18, 1997. The described 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 e.g. a GeSbTe compound, which recording layer is sandwiched between two relatively thin carbide layers of e.g. SiC, a second dielectric layer, and a reflective layer. Such a stack of layers can be referred to as an I
1
I
+
PI
+
I
2
M structure, wherein M represents a reflective or mirror layer, I
1
and I
2
represent the first and second dielectric layer respectively, I
+
represents a carbide layer, and P represents a phase-change recording layer. With such a stack, a CET value of 25 to 30 ns is reported. A CET value of 28 ns corresponds to a user data bit rate (UDBR) of 35 Mbit/s, or a data bit rate (DBR) of 41 Mbit/s. UDBR is DBR corrected for overhead, i.e. data used for addressing codes and error correction.
The above citations are hereby incorporated in whole by reference.
SUMMARY OF THE INVENTION
A higher UDBR value needs a shorter CET, e.g. a UDBR of 50 Mbit/s needs a CET of 20 ns. However, such a short CET is difficult to obtain.
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-RAM, DVD-ReWritable and DVR and optical tape, having a UDBR of more than 35 Mbit/s, e.g. 50 Mbit/s, without the need to lower the CET. 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 7.2 m/s, e.g. 18.3 m/s which is fifteen times the speed according to the Compact Disc standard. The jitter of the medium should be at a low, constant level. Moreover, the medium should have a good cyclability.
These objects are achieved in accordance with the invention by an optical information medium as described in the opening paragraph, in which a light-absorptive layer is arranged between the second carbide layer and a metal mirror layer and/or a second dielectric layer, causing the quantity of laser-light absorbed in the recording layer in the crystalline state to be higher than in the amorphous state. In optical recording media as described in the prior art, such as media having IPIM stacks, the quantity of laser-light absorbed in the recording layer in the crystalline state (A
c
) is lower than in the amorphous state (A
a
). When a light-absorptive layer is added to the stack, so that A
c
is higher than A
a
, it is found that a higher data rate can be obtained. When A
c
>A
a
, the crystalline portion of the recording layer is heated to a higher temperature than the amorphous marks when the recording layer is irradiated with laser light having a given pulse duration (dwell time). For erasing an amorphous mark, the temperature must be maintained above the crystallization temperature T
x
for a time t which is at least the CET value. Because the crystalline background will obtain a higher temperature than that of an amorphous mark to be erased, heat will diffuse to the mark, as a result of which the mark will cool down at a lower rate (slow cooling disc structure) and will remain above T
x
for a longer time. Because of the slow cooling disc structure, the amorphous marks can stay at a temperature above T
x
for a time t equal or longer than CET, using the same dwell time. With a disc structure having a light absorptive layer according to the invention, a higher data rate is achieved, without lowering CET. In contrast to this, a stack in which A
c
is lower than A
a
would result in a fast cooling disc structure, i.e. the crystalline background would have a lower temperature than an amorphous mark. Heat will then diffuse from the mark to the crystalline background. With the same dwell time and power, the time t during which the mark is above T
x
would be shorter than CET; the amorphous mark will not be erased completely.
The optical information medium according to the invention may have a stack of the following structure: I
1
I
+
PI
+
AM (
FIG. 2
) or I
1
I
+
PI
+
AI
2
(FIG.
3
), in which A is the light-absorptive layer, and I
1
and I
2
, I
+
, P and M have the above meaning. Preferred is the structure in which both a second dielectric layer I
2
and M are present: I
1
I
+
PI
+
I
2
AM (
FIG. 5
) or I
1
I
+
PI
+
AI
2
M (FIG.
1
). In the latter structure the second dielectric layer I
2
avoids alloying between A and M.
Preferred are the stacks in which a third dielectric layer
13
is arranged between the light-absorptive layer and the second carbide layer. Such a third dielectric layer can be used to optimise the thermal properties of the stack, and to avoid alloying between the second carbide layer and the light-absorptive layer. These stacks have the structure: I
1
I
+
PI
+
I
3
AI
2
M (
FIG. 4
) and I
1
I
+
PI
+
I
3
AI
2
(FIG.
6
).
In a preferred embodiment the material of the light-absorptive layer has a n/k ratio of 0.5 to 20, preferably 0.6 to 16, in which n is the refract
Evans Elizabeth
U.S. Philips Corporation
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