Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2001-01-24
2004-03-23
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S945000, C428S064400, C369S275200, C369S275500, C369S275400
Reexamination Certificate
active
06709801
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an information recording medium for recording information by irradiation of a laser beam. More particularly, this invention relates to optical disks capable of recording information and typified by phase change type optical disks such as DVD-RAM and DVD-RW, magneto-optic disks such as MD and MO and write-once type optical disks such as DVD-R.
In this specification, the information recording medium is sometimes referred to as the “phase change optical disk”, the “magneto-optic disk” or merely the “optical disk”. However, the present invention can be applied to those types of information media that record information by generating heat by irradiating a laser beam and using this heat to cause a change in atomic configuration or a magnetic moment. For this reason, the present invention can be effectively applied to information recording media other than disk-like media, such as optical cards, without being particularly limited to the shapes of the information recording media.
The laser beam, too, will be called sometimes “laser light” or merely “light”. As described above however, the present invention uses a laser beam capable of generating heat on the information recording medium. Therefore, the effects of the present invention can be obtained so long as the laser beam can provide a multiple interference effect provided by a plurality of interference layers having mutually different refractive indices. Though the present invention is invented by using a red laser (having a wavelength of 645 to 660 nm) but is not particularly dependent on the wavelength of the laser. Therefore, the present invention exhibits its effects for high-density optical disks using laser having a relatively short wavelength such as blue laser and ultraviolet laser.
2. Description of the Related Art
Recently, phase change optical disks such as 2.6 GBDVD-RAM have been put on the market by utilizing their feature of high compatibility with reproduction-only optical disks such as DVD-ROM, DVD-Video, and so forth. However, the 2.6 GBDVD-RAM has not yet satisfied sufficiently the requirements of users in the aspect of its recording capacity, and hopes have been laid on 4.7 GBDVD-RAM and 4.7 GBDVD-RW. Since these phase change optical disks are equivalent to DVD-Video in the recording capacity, they are expected to accomplish optical disks for recording images that replace VTR.
However, many problems remain yet to be solved in order to accomplish the 4.7 GBDVD-RAM. Among others, suppression of cross-erase is a critical problem.
Generally, when a track pitch of an information recording medium is contracted to about 80% of a laser beam spot, leak of reproduction signals from adjacent information recording tracks occurs. Leak of the reproduction signals from the information recorded to the adjacent information recording tracks is referred to as “cross-talk”. A land-groove recording system that follows has been developed to solve this problem.
Dimples (groove shapes) are formed on a plastic substrate of a rewritable optical disk for tracking of a laser beam, and information is generally recorded to recesses or protuberances. To improve the recording density (narrower track pitch), however, a system that records the information to both recesses and protuberances has been developed in recent years. The recess and the protuberance of the dimple are called the “groove” and the “land”, respectively. Generally, when the track pitch of the information recording medium is narrowed to about 80% of the laser beam spot and the information is recorded to both land and groove, leak of a reproduction signal occurs from an adjacent information recording track (from the groove to the land and vice versa). When the information recorded to the land is reproduced, for example, the reproduction signal from the information recorded to the groove leaks and the information recorded to the land cannot be reproduced accurately.
To solve this problem, JP-A-6-338064 (hereinafter called the “reference 1”) teaches to set the groove depth to &lgr;/7 to &lgr;/5 (where &lgr; is a laser wavelength) in the land-groove recording system. This system has the feature in that even when the track pitch is narrowed to about 60% of the track pitch, cross-talk (leak from the adjacent information recording track) can be cancelled.
On the other hand, S. Maita et al “Erasable Phase Change Optical Disks for Recording at Low Linear Velocity (II)”, Proceedings of the 5
th
Phase Change Recording Research Symposium, pp9-14 (hereinafter called the “reference 2”) describes a method of improving reflectance. This method disposes a ZnS—SiO
2
layer and a SiO
2
layer having mutually different refractive indices on an energy beam incidence side of a recording layer and improves reflectance by a multiple interference effect.
SUMMARY OF THE INVENTION
The reference 1 does not sufficiently take into consideration a phenomenon (so-called “cross-erase”) in which a recording mark recorded to an adjacent information recording track (an adjacent groove in recording to a land or an adjacent land in recording to a groove) is erased. For instance, a distance between a recording layer and a heat sink layer (reflecting layer) is as small as 18 nm in the reference 1. Therefore, it has been found that heat diffuses into an adjacent information recording track through the heat sink layer during recording of information and cross-erase is likely to occur (problem 1).
To solve this problem, Japanese Patent Application No. 10-285008 as a prior application to the present application (but, Applicants do not intend to admit the prior application as the prior art therein.) describes that the distance between the recording layer and the heat sink layer must be greater than at least the groove depth. When the recording layer and the heat sink layer are spaced apart from each other to such an extent that cross-erase can be sufficiently suppressed (at least 65 nm when a laser wavelength is about 645 to about 660 nm), however, another problem develops in that reflectance drops due to the optical interference effect (problem 2).
To improve reflectance, the method described in the reference 2 may be utilized, in principle. However, since the recording layer and ZnS—SiO
2
keep mutual contact in this method, the sulfur (S) element in the ZnS—SiO
2
layer diffuses into the recording layer and invites the drop of reflectance when recording is repeatedly conducted thousands of times (problem 3).
To avoid the lowering of reflectance, JP-A-10-228676 describes a method that interposes an interface layer formed of a dielectric compound having a high melting point such as SiO
2
or Al
2
O
3
between a recording layer and a ZnS—SiO
2
dielectric protective layer. Nonetheless, it has been clarified that when a high temperature humidification test is conducted in this method, exfoliation develops between the recording layer and the interface layer (problem 4).
This exfoliation can be suppressed by using a dielectric material of a Cr—O type, a Ge—N type, etc, in place of the high melting point dielectric compound such as Al
2
O
3
, SiO
2
, etc, described in aforementioned JP-A-10-285008. However, the inventors of the present invention have found out that though these Cr—O and Ge—N type materials are resistant to exfoliation, they absorb a laser beam and eventually lower reflectance (similar problem to the problem 2). In this case, the problem can be suppressed to a certain extent when the film thickness of the interface layer is reduced. It has been further clarified, however, that when the film thickness of the interface layer is below 5 nm, diffusion of the S atoms from the ZnS—SiO
2
dielectric protective layer cannot be suppressed sufficiently (the occurrence of the problem 3).
When the technologies of the references 2 and 3 are combined, thin films of four layers, in all, exist on the laser beam incidence side of the recording layer, and the total number of thin films is undesirably great from the aspect of production. Furthermor
Andoo Keikichi
Anzai Yumiko
Fuchioka Tamotsu
Ikari Yoshihiro
Miyamoto Makoto
Angebranndt Martin
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
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