Optical recording medium and optical recording apparatus

Details Optical recording medium and optical recording apparatus Optical recording medium and optical recording apparatus

2000-10-20

2002-06-18

Mulvaney, Elizabeth Evans (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064500, C430S270130

Reexamination Certificate

active

06406771

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to an optical recording medium and an optical recording apparatus that allow information to be recorded, erased and reproduced by irradiation with a laser beam. Particularly, this invention relates to a rewritable phase change type optical recording medium that allows information signals to be recorded at high speeds and high densities.
A rewritable phase change type optical recording medium has a recording layer mainly composed of tellurium, etc., and at the time of recording, the recording layer in the crystalline state is irradiated with focused laser beam pulses for a short time, to be partially molten. The molten portion is quickly cooled by thermal diffusion and solidified, to form recorded marks of the amorphous state. The light reflectance of the recorded marks is lower than that of the crystalline state and can be optically reproduced as recorded signals. For erasure, the mark portions are irradiated with a laser beam, to be heated to a temperature lower than the melting point of the recording layer and higher than the crystallization temperature, for crystallizing the recorded marks of the amorphous state for return into the original non-recorded state. As the materials of recording layers of such rewritable phase change type optical recording media, alloys such as Ge
2
Sb
2
Te
5
(N. Yamada et al., Proc. Int. Symp. on Optical Memory, 1987, p. 61-66) are known.
In an optical recording medium using a Te alloy as the recording layer, the crystallization rate is high, and simply by modulating the irradiation power, high speed overwriting by one circular beam can be executed. In an optical recording medium using such a recording layer, usually, one each heat resistant and translucent dielectric layer is formed on both the sides of the recording layer, to prevent that the recording layer is deformed or opened at the time of recording. Furthermore, a technique in which a light-reflecting metallic reflection layer of light-reflecting Al, etc. is laminated on the dielectric layer on the side opposite to the incident optical light falling side, to improve the signal contrast at the time of reproduction by an optical interference effect is known.
The rewritable phase change type optical recording medium has such a problem in the repetition durability of the disc, that repeated overwriting causes the reproduced signal amplitude (contrast) to decline for aggravating the jitter characteristics or causes the burst defect due to the delamination or breaking of the protective layer. As a means for improving the repetition durability, it is known to form a diffusion prevention layer in contact with the recording layer, for example, as described in JP, 11-115315, A.
However, as the optical recording medium becomes higher in linear velocity and higher in density, there arises a problem that the erasure characteristics are aggravated in the conventional optical recording medium. That is, if overwrite recording is carried out on a track having already recorded signals, the forms and positions of recorded marks are modulated by the signals existing before overwriting, to lower the erasure characteristics. As a result, there arises a problem that compared to the recording of the first time, the jitter characteristics are aggravated.
As a means for improving the erasure characteristics, a technique for forming an absorbable layer for absorbing the light. transmitting the recording layer is proposed, for example, in JP, 5-159360, A. However, the absorbable layer proposed here is an absorbable layer composed of a metal such as Ti, Ni, W, Mo, V, Nb, Cr or Fe is insufficient as a means for improving the erasure characteristics.
Furthermore, as the optical recording medium becomes higher in linear velocity and higher in density, there arises a problem that since the recorded marks become smaller in size, the signal contrast becomes lower to aggravate the jitter.
Moreover, in the conventional optical recording media, it can happen that if a disc having signals recorded is allowed to stand for a long time, the recorded marks vanish. Furthermore, if an optical recording medium having signals recorded is allowed to stand for a long time and has other signals overwritten, it can happen that the jitter characteristics are aggravated compared to a case of immediate overwriting. So, the optical recording medium has a problem of storage durability.
Furthermore, the phenomenon called the cross erasure that if the track width is narrowed to achieve a higher density, a laser beam also acts on an adjacent track, to affect the recorded marks in the adjacent track, for aggravating the jitter characteristics is also a serious problem. This problem becomes more serious especially when the track width is 0.7×d (d is the laser beam diameter on the recording surface) or less.
It can also happen as a problem that if the laser beam is repetitively applied for reproduction, the recorded marks are partially crystallized to deteriorate the signal quality (so-called reproducing light deterioration). In an optical recording medium with a higher crystallization rate for allowing higher linear velocity recording, the problems of cross erasure and reproducing light deterioration are more likely to occur.
SUMMARY OF THE INVENTION
The object of this invention is to provide a rewritable phase change type optical recording medium and an optical recording apparatus that are good in the erasure characteristics and small in jitter, and unlikely to cause the cross erasure and the reproducing light deterioration and good also in the storage durability even if recording is executed at a high linear velocity at a high density. An object of this invention can be achieved by an optical recording medium, that allows information to be recorded, erased and reproduced by laser beam irradiation, and in which the recording and erasure of information are achieved by reversible phase change between the amorphous phase and the crystalline phase, characterized in that at least a first dielectric layer, a first boundary layer, a recording layer, a second boundary layer, an absorption correction layer and a reflection layer are provided in this order on a substrate, that the composition of said recording layer is represented by general formula
[{(Ge
1−k
Sn
k
)
0.5
Te
0.5
}
x
(Sb
0.4
Te
0.6
)
1−x
]
1−y−z
Sb
y
A
z
(where A denotes an element selected from the elements belonging to the group 3 through group 14 of the 3
rd
period through 6
th
period of the periodic table excluding Ge, Sb and Te), that x, y, z and k are in the ranges respectively represented by the following formulae (1) or (2)
0.5≦x≦0.95, 0≦y≦0.08, 0<z≦0.2, k=0  (1)
0.5≦x≦0.95, 0.01≦y≦0.08, z=0, 0≦k≦0.5  (2)
that the first boundary layer and the second boundary layer are respectively mainly composed of at least one selected from carbon, carbides, oxides and nitrides, and that the absorption correction layer is 1.0 to 4.0 in refractive index and 0.5 to 3.0 in attenuation coefficient.
Another object of this invention can be achieved by an optical recording apparatus, having an optical head and an optical recording medium, in which the laser beam from said optical head is applied to allow information to be recorded, erased and reproduced by reversible phase change between the amorphous phase and the crystalline phase in said optical recording medium, characterized in that the linear velocity of laser beam irradiation is 7.5×10
6
x d (d is the laser beam diameter on the recording surface) or more per second, that the length of the shortest mark of the recorded marks recorded according to the mark edge method by the laser beam is 0.55×d or less in the laser beam propagation direction, that the track width of the optical recording medium is 0.7×d or less, and that said optical recording medium is the above-mentioned optical recording medium.
In this invention, “mainly composed of” means that the ingredie

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