Optical information medium and evaluation method

Details Optical information medium and evaluation method Optical information medium and evaluation method

2001-06-27

2003-06-10

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C430S270110

Reexamination Certificate

active

06576320

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to optical information media such as read-only optical disks and optical recording disks, and a method for evaluating the mar resistance of the same.
2. Background Art
To accommodate a vast quantity of information as typified by moving image data, advanced optical information media are required to increase their recording density. Active research and development works have been made to achieve a higher density for increasing the recording capacity. Among such research works, one proposal relating to digital versatile disks (DVD) is to shorten the wavelength of recording/reading light and increase the numerical aperture (NA) of an objective lens, thereby reducing the focused spot diameter of recording/reading light. As compared with compact disks (CD), DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength &lgr; from 780 nm to 650 nm and the numerical aperture from 0.45 to 0.60.
For long-term recording of moving images of quality, an attempt was recently made to achieve a recording capacity of at least 4 folds of that of DVD by reducing the recording/reading wavelength to about 400 nm and increasing the numerical aperture to about 0.85.
However, several problems arise in establishing such a recording/reading system having increased NA. One exemplary problem is a reduction of the tolerance for the tilt of the information recording layer of the medium. More particularly, as is well known in the art, the tilt margin, that is a permissible tilt of the information recording layer relative to incident light, is in proportion to &lgr;/[t×(NA)
3
] wherein &lgr; denotes the wavelength of recording/reading light and “t” denotes the thickness of a substrate. The tilt margin dramatically declines as the NA increases.
Also if the optical recording medium is inclined or tilted, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration W is represented by the formula I below.
W
=(½)
×t×{n
2
×sin &thgr;×cos &thgr;}×
NA
3
/(
n
2
−sin
2
&thgr;)
−5/2
  Formula I
In formula I, n denotes the refractive index of the transparent substrate (referred herein as light-transparent substrate) by which a recording/reading laser beam is transmitted before reaching the information recording layer and &thgr; is a tilt angle. It is appreciated from formula I that the thickness of the light-transparent substrate must be reduced in order to acquire a certain tilt margin.
For this reason, the DVD is given a tilt margin by reducing the thickness of the light-transparent substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the conventional CD substrate. For the system with a NA equal to 0.85, the thickness of the light-transparent substrate is reduced to about 0.1 mm.
Other problems associated with the increased NA are reduced focal depth and reduced working distance between the light-transparent substrate surface and the objective lens. The focal depth decreases in inverse proportion to (NA)
2
and so, the focusing servo system is likely to become unstable and consequently, very sensitive to mechanical precision, flaws and stains on the light-transparent substrate surface. Moreover, the working distance decreases as the NA increases, provided that the objective lens diameter is fixed, with the increased risk of collision of the pickup casing or objective lens against the light-transparent substrate surface. For example, the system with NA=0.85 will have a focal depth of ±0.3 &mgr;m and a working distance of about 100 to 300 &mgr;m so that the focusing servo system is likely to become very unstable.
Also, objective lenses are generally made of plastics and glass, and it is a common practice to provide a protective plate around the lens as a precaution to the possible contact of the lens with the light-transparent substrate surface. In the case of a plastic lens, a protective plate known as edge guard is integrally molded for the protection of the lens surface. In the case of a glass lens, a plate of plastic material such as acrylic resin or polypropylene is attached around the lens as a lens protector. The general design is such that even if the pickup contacts the disk, the protective plate portion comes in preferential contact to prevent the objective lens surface from being flawed.
Meanwhile, most optical disks now available in the market use rigid substrates of thermoplastic resins such as polycarbonate and polymethyl methacrylate as the light-transparent substrate. This means that data are recorded and reproduced with a laser beam which enters the information recording layer past the surface of these resinous substrates. These resins are optically homogeneous, highly transparent, easily moldable and excellent in mechanical strength, but have drawbacks including a low surface hardness and mar susceptibility. It is then a common practice to provide the resinous substrate on its surface with any hard coat layer as the mar-proof layer. In forming such hard coat layers, it is most customary to apply a polymerization curable compound having at least two polymerizable functional groups such as acryloyl groups in a molecule to the substrate surface, and irradiating thereto actinic radiation such as UV radiation for curing for thereby forming a hard coat layer.
However, these resins of the UV curing type have a certain limit of surface hardness achievable, despite superior abrasion resistance as compared with thermoplastic resins including polycarbonate, and do not always provide fully satisfactory abrasion resistance for use as optical information media. The system in which a high density is achieved by increasing the NA of the recording/reading optical system requires that disks be protected from not only damages during the user's handling of the disk, but also damages by the above-mentioned collision of the pickup. Accordingly, there is a need for a hard coat layer which has a dramatically improved hardness as compared with the aforementioned UV-curable resins.
To solve such a problem, methods for forming hard coat layers based on inorganic compounds have been proposed.
For example, JP-A 11-203726 discloses a method of forming a hard coat layer on an optical disk having a recording layer and a light-transparent layer successively stacked on a substrate, wherein recording and/or reading is performed with incident light from the light-transparent layer side, the method involving the steps of depositing inorganic materials such as SiN and SiO on the light-transparent layer surface as two or more layers by an ion beam sputtering or similar technique, the deposited layers serving as a protective film or hard coat layer. However, the inorganic film formed by such a technique as sputtering or evaporation has increased internal stresses. Then, when the film is grown to a thickness of more than several hundreds of nanometers in order to establish satisfactory abrasion resistance, the film becomes self-destructible. Then the inorganic film of this type is difficult to provide a hard coat layer having substantially satisfactory abrasion resistance.
JP-A 8-263878 proposes a method of forming a silica-based thin film on a substrate surface by a sol-gel process using a solution of alkoxysilane or the like, the thin film serving as a protective film or hard coat layer. When the inorganic compound film is formed by the sol-gel process, however, baking at a high temperature in extreme excess of 100° C. is necessary to promote the reaction to a full extent to form a consolidated film. It is then difficult to apply this method to the surface of light-transparent substrates made of less heat resistant resins.
JP-A 2000-132865 describes an information recording medium having a protective layer made of polysilazane. The subject matter of this patent publication is an information recording medium having a disk or tape-shaped support

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