Optical recording medium

Details Optical recording medium Optical recording medium

2002-10-07

2004-12-07

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S064500, C428S064600, C430S270130

Reexamination Certificate

active

06828000

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium utilizing a phase change by laser light irradiation, and more particularly to an optical recording medium adapted to higher recording densities.
2. Description of Related Art
Many devices are currently used as recording media for recording pieces of information. Among them, phase change type recording media are known, such as main recording media, which support the practical use of DVD's (Digital Versatile Discs) which are recording media of voice, image and information.
Such a phase change type recording medium is formed with a recording layer including chalcogens as main components, in which the recording layer is locally irradiated by laser light so as to cause a phase change between a crystal phase and a noncrystal phase, to thereby conduct recording by utilizing the difference between optical characteristics in the respective phase states.
FIG. 4
shows a film constitution of a conventional phase change type-recording medium. In
FIG. 4
, the phase change type recording medium comprises a light reflecting side substrate
11
sequentially laminated thereon with: a reflecting film layer
12
, a second dielectric layer
13
, a phase-change recording layer
14
and a first dielectric layer
15
, all of which layers are formed by a film-forming method such as a resistive heating vacuum deposition method, an electron beam vacuum deposition method, or a sputtering method; and a light incident side substrate
16
provided thereon by adhesion or by coating and curing.
The light reflecting side substrate
11
and light incident side substrate
16
are generally transparent in the visible light range, and it is possible to adopt therefor those made of: glass; a plastic resin such as polycarbonate; or an ultraviolet curable resin. Typically, the light reflecting side substrate
11
is provided thereon with a tracking-aimed guide groove serving as a rail for precisely guiding the travel of the light beam.
The reflecting film layer
12
reflects the laser light transmitted through the phase-change recording layer
14
so as to cause the thus reflected laser light to interfere with the laser light reflected by an upper surface of the phase-change recording layer
14
, and adopted for the reflecting film layer
12
is a single metal material having a higher reflectivity such as Al, Au, Ag, Cu, Cr, an alloy including a plurality of kinds of such metals, and a mixture thereof.
The phase-change recording layer
14
is a material having a reflectivity to be changed by a phase change reversibly caused by laser light irradiation, and there is concretely adopted therefor an alloy mainly including Te such as Sb—Te, Ge—Sb—Te, Ag—In—Sb—Te, and Ge—In—Sb—Te. Optical recording media including phase-change recording layers made of such Te alloys have a higher crystallization speed and thus a shorter erasure time, thereby enabling a high-speed overwrite based on a single circular beam by simply modulating an irradiation power of laser light. The state of the phase-change recording layer just after film formation is an amorphous or noncrystal phase state. Thus, there is conducted an initialization treatment for bringing the whole of phase-change recording layer into a crystal phase, so as to form a recorded portion upon recording a piece of information into the phase-change recording layer. The recording is achieved by forming an amorphous phase portion within a crystallized state.
The first dielectric layer
15
and second dielectric layer
13
are arranged on both sides of the phase-change recording layer
14
, respectively, so as to have: a protective function for avoiding a change of optical characteristics of the phase-change recording layer
14
due to a chemical change thereof such as oxidation; and an optical adjusting function for adjusting the reflectivities of recorded portion and erased portions in the phase-change recording layer
14
by film thicknesses, refractive indexes and optical absorptivities of these dielectric layers, respectively. Adopted as these dielectric layers are those materials having an excellent adhesive property to the phase-change recording layer
14
and reflecting film layer
12
, and durability causing no cracks even due to long-term storage. Particularly, there have been conventionally adopted mixtures of ZnS and SiO
2
, since they have smaller film stresses and excellent adhesive properties to adjoining layers.
In the aforementioned conventional phase change type recording medium having the film constitution shown in
FIG. 4
, rewriting for a great number of times result in diffusion of constituent atoms of the first dielectric layer
15
and second dielectric layer
13
into the phase-change recording layer
14
to thereby change the composition of the phase-change recording layer
14
, thereby causing a possibility of an occurrence of fluctuation such as of recording characteristic and erasure characteristic due to repetitively conducted rewriting.
To avoid such a deterioration of characteristics due to repetitive rewriting, there has been proposed a phase change type-recording medium having anti-diffusion layers (see JP-A-10-275360 [275360/1998], and JP-A-11-115315 [115315/1999], for example).
The phase change type recording medium having such anti-diffusion layers includes the anti-diffusion layers made of silicon oxide, aluminum nitride, germanium nitride, between the phase-change recording layer
14
and first dielectric layer
15
and between the phase-change recording layer
14
and second dielectric layer
13
, respectively, in a manner to interpose the phase-change recording layer
14
therebetween, to thereby avoid: mutual diffusion of constituent atoms of the first dielectric layer
15
and second dielectric layer
13
and the phase-change recording layer
14
; and a timewise change of the recording layer composition.
Meanwhile, JP-A-10-326434 (326434/1998) discloses a thick-film based phase change type recording medium provided with a high hardness layer having a sufficiently large film thickness and a hardness higher than that of the first dielectric layer
15
, in a manner to be contacted with the laser light incident side of the first dielectric layer
15
, so as to increase the mechanical strength of the medium and to reduce the deterioration at a write starting portion and a write ending portion of each of sectors in repetitive rewriting.
Recently, requirements for higher recording densities in optical recording media have become increasingly severe, and beam diameters of laser light are also promoted to be diminished for higher recording densities. One way to achieve the above includes shortened wavelengths of laser light.
Namely, in focusing a laser beam by an optical lens, the minimum beam diameter depends on the wavelength of the laser, so that the shorter wavelength allows further diminishing of the beam diameter. This means that recording densities of optical recording media can be increased inversely proportionally to the laser wavelength, so that there is a trend to replace light sources from currently used laser beams in the red color range to those in the violet range having wavelengths near 400 nm.
Meantime, in the aforementioned conventional phase change type recording medium, the laser light having a high energy density is locally irradiated onto the recording layer upon recording, thereby causing considerable mechanical strains in the recording layer. As such, repetitive rewriting for a great number of times result in repeated melting and solidification of the recording layer thereby easily causing a flow of the melted recording layer, to thereby cause problems such as deterioration of jitter characteristics, decreasing of amplitudes of reproduced signals, and a due number of rewriting operations is not sufficiently ensured.
Particularly, in case of adopting mark edge recording, the recording layer tends to flow due to recording and erasing more easily than pit position recording, thereby leadi

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