Recordable optical element using low absorption materials

Details Recordable optical element using low absorption materials Recordable optical element using low absorption materials

1995-03-07

2001-01-02

Angebranndt, Martin (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Radiation sensitive composition or product or process of making

C430S945000, C430S273100, C430S275100, C428S064500, C428S064400, C369S288000, C369S275200

Utility Patent

active

06168901

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to optical recording elements, particularly to those that are useful as recordable compact disks.
BACKGROUND OF THE INVENTION
There are many types of optical information carrying elements that are known. One of the popular forms of optical information containing element is the compact disk or CD. Digital information is stored in the form of low reflectivity marks or pits on an otherwise reflective background. Stringent specifications on CD formats have been published by Sony and Philips, and these formats are used as standards throughout the world. One of the most important format requirements is the background reflectivity which is specified to be greater than 70% at about 780 nm. This high reflectivity value is unusual when compared with other optical recording disks. In the latter case, the reflectivity normally is designed to be low in order to improve the absorption of the writing laser power to facilitate the information recording process.
In CD's, the optical information is most often in the form of read only memory or ROM. Optical information is usually not recorded in real time but rather is produced by press molding. In a typical process, the recording substrates are first mass produced by press molding using a stamper containing the digital information to be reproduced. The press molded substrate is then overcoated with a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the reflectivity is lower than in those areas not have the deformations.
It is desirable to produce optical recording elements which are capable of being recorded in real time and producing a record that mimics the conventional CD on read out. In this manner, information can be added to the CD and the CD can be used on a conventional CD player.
It has been difficult to produce such optical recording elements because the recorded elements have to meet the strict specifications for CD. In particular, it has been difficult to produce recordable elements that will meet the >70% reflectivity requirement.
One method for forming a recordable element that mimics conventional mold pressed CD elements is to provide a transparent, heat deformable support having thereon, in order, a layer that absorbs recording radiation and a reflective layer. When radiated through the transparent support, the reflectivity varies with the thickness of the absorbing layer as a result of the light interference effect (FIG.
1
). When an absorbing layer of very small thickness (much less than that corresponding to R
min
) is used, the reflectivity is high, but such structure is not useful for recording purposes because of low thermal efficiency. The reflective layer is a very effective heat sink. Most of the writing energy absorbed next to the reflector in the recording layer is conducted away by the reflector. It is generally observed that the smallest useful thickness is that which produces reflectivity in the neighborhood of the first minimum in reflectance. To produce useful recording elements, therefore, requires materials which will produce >70% reflectance with thickness larger than this minimum useful thickness. Such materials are characterized by low optical absorption coefficients, contrary to the materials used in conventional recording structures where high optical absorption is needed. These low absorption materials when used in conventional media structure without a reflector generally do not perform well. Adequate sensitivity and contrast can only be achieved when incorporated in a complete optical interference structure using the reflector. Thus, generally speaking, materials that are appropriate for conventional recording structure are not appropriate for recordable CD structure, and vice versa.
Materials of this type based on organic dyes are described in U.S. Pat. No. 4,940,618, European Patent Application 0,353,393, and Canadian Patent 2,005,520.
One of the undesirable features of elements based on such organic dyes is their wavelength sensitivity. The desirable optical properties can only be obtained at wavelengths near the absorption edges of such dyes. As a result, the reflectivity and other properties of such elements depend strongly on wavelength. It is very difficult to meet all the stringent CD specifications throughout the entire range of wavelengths that the CD's are designed to function. It is nearly impossible to operate such elements using shorter wavelengths which are to be used in future generation CD's to increase the recording density.
SUMMARY OF THE INVENTION
Thus, it is an object of this invention to provide a recording element that has high reflectivity, recording sensitivity, and contrast to meet the CD specifications over a wider wavelength range.
This object is achieved by a recordable element having a substrate and on the surface of the substrate, a recording layer and a light reflecting layer, the improvement comprising:
a) a recording layer including material given by the formula
(Te
a
Ge
b
C
c
H
d
O
e
),
a, b, c, d, and e are atomic percents and (c+d)>40, d>10, a>5, b>5, and e≧0 such that a+b+c+d+e=100; and
b) the reflecting layer and the recording layer being selected such that the element R
min
or R
max
(element reflectivity) is about or greater than 70% at about 780 nm.


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patent: 4985349 (1991-01-01), Ohkawa et al.
patent: 5242784 (1993-09-01), Chew et al.
patent: 5273861 (1993-12-01), Yamada et al.
patent: 5294523 (1994-03-01), Nagata et al.
patent: 2005520 (1990-06-01), None
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patent: 1134437 (1989-05-01), None
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Translation of JP 1-137437 (Kuroiwa et al.)(Publ. May 1989).
Hamada et al., “CD-Compatible write once disc with high reflectivity” SPIE 1078 (1989) pp. 80-87.
Optical Recording, Allan B. Marchant (1990) pp 343-371.

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