Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of coating supply or source outside of primary...
Reexamination Certificate
2000-01-03
2002-03-12
Padgett, Marianne (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of coating supply or source outside of primary...
C427S568000, C427S255140, C427S162000, C427S168000, C427S422000, C427S901000
Reexamination Certificate
active
06355311
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to a method for making an optical recording medium for high-density recording, and more particularly, to a method for making an optical disk of the read-only, write-once or rewriteable type. The invention also relates to such an optical recording medium as mentioned above.
2. Description of the Prior Art
In recent years, read-only optical disks, in which information is recorded in high density, write-once optical disks capable of recording information only once, and rewriteable magnetooptical (MO) disks or phase change (PC) disks, have started to rapidly come into wide use, not to mention compact disks. Among them, those optical disks, which comprise a recording layer containing organic materials, have been extensively developed and now put into practical use.
Organic compounds are more versatile in kind than metals or inorganic compounds, and enable a diversity of functions to be developed by structurally designing same at a molecular level, making it easy to realize high-density recording and high sensitivity. For instance, as a write-once CD (CD-R), there is provided an inexpensive optical disk of high productivity, whose recording layer is formed by spin-coating an organic dye, dissolved in solvent, on a transparent resin substrate having a spiral, continuous groove cut therein.
Further, studies have been made on so-called “superresolution” optical disks, wherein an optically transparent substrate, in which information is encoded as pits beforehand, is formed thereon with a mask layer containing a reversible dye such as a thermochromic or photochromic compound.
In the optical disk formed with the mask layer, the nonlinear change of light transmittance against light intensity inherent to reversible dyes, such as thermochromic or photochromic compounds, are utilized in a manner as follows. The spot size of a laser beam, which is restricted depending the wavelength of an irradiated laser beam and the numerical aperture (NA) of a pickup lens, can be made smaller than that of the irradiated beam by causing the laser beam to be transmitted only at an intense region of a beam intensity distribution thereof. This enables smaller pits to be reproduced, and as a result, higher densification can be achieved.
As is known in the art, a layer containing such an organic compound as mentioned above has been formed according to the aforesaid spin-coating technique. In a write-once optical disk making use of an organic dye in the recording layer, such as CD-R, usual practice is to form a recording layer by spin-coating an organic dye, dissolved in a solvent, onto a transparent resin substrate having a spiral, continuous groove therein. In this connection, however, an optical disk of next generation, which is so designed as to record in a density higher than conventional optical disks, such as a compact disk, the size and depth of pits and the dimensions such as of a width and depth of the continuous groove tend to become smaller. When a solution of a dye dissolved in solvent is spin-coated onto a substrate formed with such small and shallow pits and such a fine and shallow groove therein, the pits and the groove are filled up with the coated dye, thereby presenting the problem of causing signal characteristics to be degraded owing to the difficulty in tracking the groove and pits at the time of recording and reproduction.
From the standpoint of costs and mass-productivity, most of the recent optical disks make use, as a substrate, of non-crystalline, light-transmitting thermoplastic polymer materials such as polycarbonates. However, the non-crystalline thermoplastic polymer materials, in most cases, exhibit a relatively low resistance to organic solvents. This causes the following problems.
(1) The types of organic solvents, which are usable for the formation of an organic matter-containing layer on the substrate by spin coating and which do not attack the substrate, are limited.
(2) The types of organic matters, such as dyes, to be dissolved in such organic solvents incapable of attacking the substrate are more restricted.
On the other hand, aside from the spin-coating method of forming an organic compound-containing layer, a vacuum deposition method has been studied in order to provide optical disks. For instance, according to Japanese Laid-open Patent Application No. 7-18693,there is disclosed a so-called “superresolution” optical disk wherein its mask layer having a reversible dye, such as a thermochromic dye, is formed by vacuum deposition under heat. The formation, by vacuum deposition under heat, of such a mask layer as mentioned above on a substrate having small pits therein as formed in a “superresolution optical disk” disenables the pits to be filled up as experienced in the spin-coating method, realizing a high-quality superresolution optical disk.
However, the vacuum deposition under heat has limitations described below and are not always satisfactory in this regard.
(1) Organic compounds, which can be stably deposited, are limited in type. For instance, since organic ionic dyes such as cyanine dyes and polymer compounds do not exhibit any sublimation, they cannot be formed as a film according to the vacuum deposition under heat.
(2) Even if organic compounds used have sublimating properties, they are ordinarily more decomposable than inorganic materials on heating, so that the film-forming speed cannot increase so much, and it undesirably takes a relatively long time for the film formation.
(3) It is difficult to carry out deposition from a single target obtained by mixing a plurality of types of organic compounds while controlling the sublimation speeds of the respective compounds. For the simultaneous vacuum deposition of a plurality of types of organic compounds, a vacuum deposition source for every organic compound has to be provided.
In order to overcome the drawbacks of the vacuum decomposition methods, spraying methods have been proposed, for example, in Japanese Laid-open patent Application Nos. 6-306181 and 7-252671 and also by T. Hiraga et al (J. Vac. Sci. Technol., A12(3), pp. 876-878 (1994) ) and by T. Hiraga et al (Jpn. J. Appl. Phys., 33(9A), 5051-5059 (1994)).
In the methods of the Japanese references, it is stated that a substrate is heated to a temperature not exceeding a thermal decomposition temperature of an organic deposit to remove volatile matter therefrom. The substrate is heated by means of a heater in contact therewith. Under conditions where the substrate placed in a high-vacuum chamber is heated to a temperature lower than the thermal deformation temperature of the substrate, e.g. 150° C., if a sprayed mist arrived at the substrate has a large amount of a solvent left therein (e.g. 50wt %), the substrate is rapidly cooled owing to the heat of vaporization of the solvent. When the substrate is made, for example, of a metal or glass having high thermal conductivity, the heat energy corresponding to the heat of vaporization can be immediately supplied from the heater to the substrate. However, where an organic material, e.g. polycarbonate, having low thermal conductivity is used as the substrate, the surface temperature of the substrate considerably lowers, requiring a long time before the vaporization of the solvent left in the deposit on the surface.
We have experimentally confirmed that the above methods are applied to a substrate, which has been encoded with information in the form of pits beforehand or which has a groove or grooves to form a mask layer on the substrate, the pits and/or groove is significantly deformed. More particularly, a thin film of an organic material cannot be formed while keeping the pits and/or groove substantially in original form. Presumably , this is considered due to the fact that when an organic polymer material is used as the substrate, an organic solvent left in contact with the substrate deforms the surface profile of the substrate, and that the deposit containing a large amount of a solvent is caused, more or less, to flow owing to the pre
Tanaka Norio
Tsujita Kouji
Ueno Ichiro
McDermott & Will & Emery
Padgett Marianne
Victor Company of Japan Ltd.
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