Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2001-06-25
2004-05-11
Maki, Steven D. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S074000, C156S275500, C156S275700, C156S295000, C156S356000, C156S379800, C156S538000, C156S578000
Reexamination Certificate
active
06733604
ABSTRACT:
FIELD OF TECHNOLOGY
The present invention relates to a manufacturing method and manufacturing apparatus for an optical disc recording medium comprised of a plurality of laminated substrates, and relates more particularly to a method and apparatus.
BACKGROUND TECHNOLOGY
To achieve a higher recording density in an optical disc recording medium, it is necessary to shorten wavelength of laser used for recording and reproduction while concomitantly increasing a numeric aperture (NA) of an objective lens. However, when tilt occurs between the disc and the laser beam axis as a result of disc rotation or deformation, the focal point of the laser becomes off set from the correct position on the information signal surface of the disc. Primary types of tilting include so-called radial tilt, in which the optical disc recording medium sags of its own weight in a conical manner when the disc is loaded in an optical disc drive apparatus, and so-called tangential tilt, in which the optical disc recording medium tilts in the circumferential direction as a result of the attitude of the optical disc drive apparatus itself or the dimensional precision of loading the optical disc recording medium in the optical disc drive apparatus.
It is necessary to make the recording pits larger to compensate for an offset in the laser beam focal point caused by tilting, and this makes it extremely difficult to increase the recording density. The offset in the laser beam focal point can be reduced if the thickness of the optical disc recording medium substrata is made thinner. However, because disc rigidity drops if the substrata of the optical disc recording medium is made thinner, even greater tilt easily occurs, and not only is the effect of making the substrata thinner impaired, the offset in the laser beam focal point becomes even greater.
Increasing the mechanical strength of an optical disc recording medium by laminating two substrates together is an extremely effective means for preventing tilting of a disc as a result of making the substrates thinner. The recording capacity of a single optical disc recording medium can also be doubled by disposing an information signal surface on two sides of the laminated substrates. For example, when an optical disc recording medium is made by laminating two substrates, it is possible to manufacture a single-sided, single-layer disc OD
1
in which a method signal surface is disposed on only one surface of the disc, a single-sided, double-layer disc OD
2
in which an information signal surface is disposed in two layers on a single side of the disc, or a double-sided, single-layer disc OD
3
in which a single information signal surface is disposed on opposite sides of the disc. An example of a single-sided, single-layer disc OD
1
is shown in
FIG. 13
, a single-sided, double-layer disc OD
2
is shown in
FIG. 14
, and a double-sided, single-layer disc OD
3
is shown in FIG.
15
. Note that, in each figure, Ls indicates the recording/reproducing laser beam, RS indicates the information recording surface, AS indicates an adhesive layer, and PL indicates a protective layer.
In the case of a single-sided, double-layer disc OD
2
as shown in
FIG. 14
, the laser beam LS
2
for recording and reproducing information recording surface RS
1
in
FIG. 1
must pass through the adhesive layer AS. In addition, a label layer for displaying a title and other disc contents cannot be provided in the case of a double-sided, single-layer disc OD
3
shown in FIG.
15
.
Methods for manufacturing an optical disc recording medium by laminating a plurality of substrates in this manner can be broadly classified in two based on the coating method of the adhesive for laminating the substrata, i.e., one is a printing method and the other is a spin-coating method. First describing the printing method below with reference to FIG.
11
and
FIG. 12
, thereafter the spin-coating method is described with reference to
FIG. 16
,
FIG. 17
,
FIG. 18
, and FIG.
19
.
An optical disc recording medium manufacturing method using the printing method is simply shown in FIG.
11
and FIG.
12
. Using a thermoplastic resin having a high viscosity at room temperature, an adhesive PP is uniformly coated through a screen SP onto the entire surface of a substrate
6
while moving a spatula
60
in a particular direction Ds; and two such substrates are then positioned with the thermoplastic resin PP coated surfaces thereof in mutual opposition, and heated until the thermoplastic resin becomes soft, and then pressure is applied to press the two substrates together, thereby bonding the two substrates to produce an optical disc recording medium.
However, because the thermoplastic resin is non-transparent, it cannot be used in the manufacture of a single-sided, double-layer disc OD
2
. In addition, when another substrate is pressed to the thermoplastic resin PP coated onto the entire surface of a substrate, a large number of bubbles necessarily occur between the resin PP and the substrate because of the surface contact therebetween, and the substrates are bonded with these bubbles trapped in the adhesive layer. Because bubbles trapped in this adhesive layer disperse the laser beam and interfere with data recording and reproducing, the printing method cannot be applied to the manufacture of an optical disc recording medium comprising an information recording layer written and read by a laser beam passing through an adhesive layer as in a single-sided, double-layer disc OD
2
, even if a transparent thermoplastic resin becomes possible in the future.
Moreover, because thermoplasticity is a reversible reaction, exposure to a temperature exceeding the thermoplastic temperature even after substrates are laminated and a finished optical disc recording medium is produced may loosen the adhesive layer and the laminated substrates may warp of their own weight, even resulting in the worse case the top and bottom substrates are displaced or separated. Because the viscosity of the thermoplastic material is high, adhesive that protrudes when the substrates are laminated must be mechanically removed. In addition, the thickness distribution of print layer PP varies between discs in the diametric direction or in the direction Ds of the spatula
60
movement as a result of print coating as shown in FIG.
12
. That is, if the distance between substrate
6
and screen SP at diametrically opposite edges of substrate
6
is S
1
and S
2
, the thickness of the adhesive layer on substrate
6
will vary in the diametric direction by |D
1
−D
2
|. As a result, because the thickness varies in the radial direction at the same circumference, quality problems arise in discs that are rotated for use.
In one printing method a light-permeable light-setting resin is used in place of a thermoplastic resin as the adhesive. In this case, a light-setting resin with a high viscosity similar to the thermoplastic resin is used and printed to the substrates using a method such as shown in FIG.
11
and FIG.
12
. The adhesive surface side of the substrate is then exposed to light to slightly set the adhesive, and pressure is applied in a manner pressing the two substrates together, thereby bonding the two substrates and producing an optical disc recording medium. While a light-permeable resin can be used as the adhesive in this method, printing the entire surface again results in bubbles being sealed in the adhesive layer when the substrates are put together. That is, this method cannot be used for the manufacture of a single-sided, double-layer disc OD
2
. In addition, qualitative problems in the finished optical disc recording medium are involved as a result of variations in adhesive thickness in the radial direction and at the same circumference of the substrate.
A manufacturing method for a single-sided, double-layer disc OD
2
using a conventional spin coating method is described briefly below with reference to
FIG. 16
,
FIG. 17
,
FIG. 18
,
FIG. 19
, and FIG.
20
. It should be noted that the optical disc recording medium OD
2
is
Inoue Kiyoshi
Itou Hirokazu
Kozono Toshikazu
Miyamoto Hisaki
Greenblum & Bernstein P.L.C.
Maki Steven D.
Matsushita Electric - Industrial Co., Ltd.
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