Optical recording medium and optical recording method therefor

Details Optical recording medium and optical recording method therefor Optical recording medium and optical recording method therefor

2001-10-02

2003-12-02

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064500, C428S064600, C430S270130

Reexamination Certificate

active

06656559

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to an optical recording medium having a phase change layer and a method for recording information in such medium.
Highlight is recently focused on optical recording media capable of recording information at a high density. Typical optical recording mediums include write once media which can be recorded only once and which can not be rewritten, and rewritable media wherein repeated rewriting has been enabled. Improvement in the recording density and increase in the data transmission rate are always required for an optical recording medium.
Write once media are unrewritable media which are adapted for use in documents wherein tampering of the information recorded therein may cause a serious problem as in the case of official documents. The most widely employed write once media are those using an organic dye for the recording material. Use of an organic dye, however, is associated with the difficulty of realizing a high transfer rate since recording sensitivity is likely to be insufficient when the recording is accomplished at a high speed by increasing the linear velocity of the medium. An organic dye also has relatively steep absorption spectrum and reflection spectrum, and a careful choice of the organic dye is required so that the organic dye chosen corresponds to the recording/reading wavelength. For example, when there is a high-end format which requires use of a recording/reading beam of shorter wavelength, a problem may arise that the medium of low-end format can not be recorded/read by the recording/reading beam adopted in the high-end format. There may also arise the problem that dyes corresponding to the recording/reading of shorter wavelength are difficult to design and purchase.
On the other hand, there are rewritable recording media of phase change type wherein the medium is recorded by changing the crystalline state of the recording layer by irradiating a laser beam, and read by detecting the change in the reflectivity induced in the recording layer by such change in the crystalline state.
In the phase change medium which can be rewritten by overwriting, amorphous record marks are formed by irradiating the medium with a laser beam of recording power level to melt the crystalline recording layer and quenching the molten recording layer to thereby form the amorphous record marks. In the erasure, the medium is irradiated with a laser beam of erasing power level to heat the recording layer to a temperature of not less than the crystallization temperature and less than the melting temperature followed by gradual cooling to thereby crystallize the amorphous record marks. Accordingly, the overwriting can be accomplished by irradiating a single laser beam with its intensity modulated. In the recording of such phase change medium at a high speed, the rate determining factor is crystallization speed of the recording layer, namely, the transformation speed from the amorphous to the crystalline phase. High speed recording can be realized by using a recording layer which crystallizes at a high speed while crystallization at an excessively high speed is likely to invite crystallization of the amorphous record marks, namely, destabilization of the record marks to adversely affect durability in the reading as well as storage stability. An excessively high crystallization speed may also invite the phenomenon of selferase wherein the record marks partly become recrystallized in the recording due to the heat conduction in in-plane direction as well as crosserase wherein record marks on the adjacent track are erased in the recording. As described above, it is difficult in a rewritable phase change medium to drastically increase the crystallization speed of the recording layer, and hence, remarkably increase the data transfer rate.
When a phase change medium is used as a write once medium, erasure (crystallization) is no longer required. When such medium is recorded at a high speed by increasing the linear velocity of the medium, increase in the crystallization speed of the recording layer in accordance with the increase in the linear velocity is unnecessary, and the problems as described above such as adverse effects on the storage reliability are alleviated. However, initialization of the recording layer will be difficult if the crystallization speed of the recording layer is excessively reduced for the purpose of increasing the storage reliability. The as-deposited phase change layer is generally amorphous, while the record marks formed by melting and quenching the crystalline recording layer are also amorphous. While both the as-deposited phase change layer and the record marks are amorphous, the stability of the amorphous phase is higher in the case of the as-deposited recording layer compared to the record marks. As a consequence, in the case of the overwriting of a phase change medium immediately after its production, crystallization of the area that has been irradiated with the laser beam of erasing power level is associated with difficulty. This is the reason why initialization (crystallization of the entire surface) is required before the overwriting. Difficulty in the initialization will invite increase of the production cost since a laser beam of high power is required and the medium can not be initialized at a high speed.
Also known in addition to those as described above are the write once media of the type wherein crystalline record marks are formed on the as-deposited recording layer, namely, on the as-deposited amorphous recording layer. In the medium of this type, the recorded medium has crystalline record marks formed in the amorphous recording layer, and the recorded state will be unstable if the stability of the amorphous phase is insufficient. However, when the stability of the amorphous phase is improved by reducing the crystallization speed, formation of the record marks will become difficult due to the difficulty of the crystallization. The medium of this type also suffers from the problem of difficulty in tracking servo in the recording since the amorphous recording layer of low reflectivity is irradiated with the recording laser beam.
Various proposals have been made to facilitate crystallization of the as-deposited amorphous recording layer or to speed up the erasure of the record marks. Proposals include provision of a layer in contact with the recording layer for promoting the crystallization of the recording layer, and constitution of the recording layer from a laminate of layers.
For example, Japanese Patent Application No. (JP-A) 92937/1989 discloses an optical recording medium comprising a recording layer containing Te or Se as its main component and a crystal nucleus-forming layer in contact with the recording layer, wherein apparent speed of nuclei formation near the melting point has been increased. There is also disclosed that the increase in the apparent nuclei formation speed of the recording layer enables erasure of the record marks at a higher speed. In claim 4 of JP-A 92937/1989, there is described that the crystal nucleus-forming layer is amorphous immediately after the production of the optical recording medium, and once crystallized by laser beam irradiation, the layer never becomes amorphous or immediately crystallized upon irradiation with the laser beam. In other words, the stable phase for this crystal nucleus-forming layer is the crystalline phase once the layer has been crystallized even if the layer went through repeated recording and erasing operations. JP-A 92937/1989 also describes that it is preferable that the crystalline phase of the crystal nucleus-forming layer after its crystallization is the same as the crystalline phase of the recording layer. Examples of JP-A 92937/1989 disclose combination of the recording layer comprising Te
57
In
18
Au
25
and the crystal nucleus-forming layer comprising Te
67
Au
33
.
WO98/47142 discloses an optical information recording medium wherein a crystallization-promoting layer is provided in contact with the recording layer comprising a Ge—Sb

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