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

2004-05-11

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064500, C428S064600, C430S270130

Reexamination Certificate

active

06733858

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 are 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.
Of the rewritable recording media, those of phase change type are recorded by changing the crystalline state of the recording layer by irradiating a laser beam, and read by detecting the change induced in the recording layer by such change in reflectivity 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 state. The change from the crystalline to the amorphous states can be accomplished within the period of several nano seconds while crystallization from the amorphous to the crystalline state requires maintenance at a temperature at or above the crystalline temperature for at least a predetermined period.
An as-deposited phase change layer is generally amorphous. In the meanwhile, record marks formed by melting and quenching the crystalline recording layer are also amorphous. The as-deposited phase change layer and the amorphous record marks share the common feature that they are amorphous. The amorphous state of the as-deposited amorphous recording layer, however, is more stable than the amorphous record marks, and even when the as-deposited phase change medium were overwritten as described above, crystallization of the region which had been irradiated with the laser beam of erasing power level is difficult. Accordingly, there is a need to complete the initialization of the recording layer (initialization of the entire surface) before the overwriting operation. If the initialization is difficult, production cost will be increased since the initialization should be conducted by using a laser beam of higher power and at a lower speed. Also known is a write once medium wherein crystalline record marks are formed in the as-deposited recording layer, namely, in the amorphous recording layer. The crystallization of the as-deposited recording layer is quite difficult as described above, and there is a high demand for a means capable of readily crystallizing the as-deposited amorphous recording layer. In addition, if amorphous record marks can be crystallized (erased) at a higher speed, data transfer rate in the overwriting operation can be increased.
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, 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-Te-based alloy. This crystallization-promoting layer has a crystal structure of face centered cubic lattice which is the same as that of the recording layer, or a rhombohedral lattice which does not include Te. Initialization of the recording layer is not required in this medium since the recording layer is crystallized at the time of its formation owing to the provision of the crystallization-promoting layer and the recording layer in contact with each other. There is disclosed that the adjacent crystallization-promoting layer and recording layer turns out to be in mixed state. In Examples of WO98/47142, the recording layer comprises a composition based on Ge
2
Sb
2
Te
5
, and the crystallization-promoting layer contains PbTe, Bi
2
Te
3
, Sb, or Bi. In Comparative Examples, the crystallization-promoting layer contains W (body centered cubic lattice), Te (hexagonal system), Sb
2
TeSe
2
(rhombohedral lattice), Sb
2
Te
3
(rhombohedral lattice), or Ag
2
Te (monoclinic system), CrTe (hexagonal system).
JP-A 185289/1999 discloses a write once optical information-recording medium which has a phase change recording layer on at least one surface of the substrate, and a layer comprising a semiconductor material immediately on and/or under the recording layer. In this medium, when the recording layer is crystallized, the shape of the unit cell constituting the crystal face parallel to the substrate in the recording layer matches with the shape of the unit cell constituting the most dense face of the semiconductor material layer. The invention described in JP-A 185289/1999 attempts to reduce the jitter by providing such semiconductor material layer, and adequately selecting the material used for each layer so that absolute value of the lattice mismatch between the recording layer and the semiconductor material layer does not exceed 10%. JP-A 185289/1999 does not explicitly indicate the crystallization-promoting effect realized by providing the semiconductor material layer in contact with the recording layer. JP-A 185289/1999, however, describes that it has been estimated that, when the recording layer had been crystallized, deformation of the lattice that takes place at the boundary with the adjacent layer prevents crystallization, and hence, invites increase in the jitter. The compounds indicated in JP-A 185289/1999 as exemplary compounds for use in the semiconductor material layer include BaO, AgCl, BeTe, GaAs, AlAs, YSb, YP, ZnSe, ThS, SnAs, YSe, AgBr, ThP, LaS, ScSb, ThSe, CaSe, PbS, ScBi, ThAs, BiSe, InAs, YTe, GaSb, PbSe, SnSb, AlSb, CuI, SrSe, SnTe, ThSb, CaTe, BaS, LaTe, PbTe, BiTe, SrTe, AgI, InSb, CdTe, Sb
2
Te
3
, Bi
2
Se
3
, and Bi
2
Te
3
. Th

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