1.  Field of the Invention
The present invention relates to a method for manufacturing an optical information recording medium having a process in which layers are formed on a substrate by a spin coat method using a center cap. 
2.  Description of the Related Art
In recent years and continuing to the present, there have been proposed high-density optical information recording media that reproduce and/or record information signals when irradiated with light from the side where the information layer of a substrate is formed.  FIG.  1 is a cross-sectional view showing the configuration of such an optical information recording medium having a double-layered structure.  In FIG.  1, reference numerals 10, 11, 12, 13, 14, 15, 16, and 17 denote an optical information recording medium, a substrate, a reflecting and/or recording layer, an intermediate layer, a second reflecting and/or recording layer, a cover layer, laser light, and an objective lens, respectively. 
In the optical information recording medium 10 shown in FIG.  1, the substrate 11 has a thickness of about 1.1 mm and is manufactured, for example, by injection-molding polycarbonate, etc.  In addition, the substrate 11 has a structure in which predetermined pits and/or guide grooves are formed on its one surface.  The reflecting and/or recording layer 12, the intermediate layer 13, the second reflecting and/or recording layer 14, and the cover layer 15 are successively laminated on the pits and/or guide grooves of the substrate 11. 
The reflecting and/or recording layer 12 and the second reflecting and/or recording layer 14 are information layers made of aluminum alloy, etc.  The second reflecting and/or recording layer 14 is configured to semi-transmit light so as to enable reproduction and/or recording of information signals with respect to the reflecting and/or recording layer 12 located far from the laser light 16.  The reflecting and/or recording layer 12 has a thickness of, for example, 20 nm, and the second reflecting and/or recording layer 14 has a thickness of, for example, 8 nm. 
The intermediate layer 13 separates the reflecting and/or recording layer 12 from the second reflecting and/or recording layer 14.  On the front surface of the intermediate layer 13, pits and/or guide grooves similar to those of the substrate 11 are formed by a 2P transfer method using a UV curing resin, etc.,  and its thickness is, for example, 25 μm. 
The cover layer 15 is made of a light-transmitting UV curing resin, etc.,  and has a thickness of, for example, 75 μm.  The laser light 16 emitted from a light source (not shown) is condensed by the objective lens 17.  The reflecting and/or recording layer 12 or the second reflecting and/or recording layer 14 is irradiated with the laser light 16 through the cover layer 15.  In the following description, the reflecting and/or recording layer is called an information layer as occasion demands. 
FIG.  2 is a view showing the appearance of the substrate 11 constituting the optical information recording medium 10.  In FIG.  2, the same constituents as those of FIG.  1 are denoted by the same reference numerals and their descriptions are omitted.  Reference numeral 11a denotes a center hole, and reference numeral 11b denotes a clamp part.  As shown in FIG.  2, the substrate 11 constituting the optical information recording medium 10 shown in FIG.  1 has generally a circular form having the center hole 11a as a through-hole at its center.  The clamp part 11b is provided at a predetermined area of the peripheral part of the center hole 11a and clamped when the substrate 11 completed as the optical information recording medium 10 is loaded into an information recording reproduction apparatus (not shown). 
In the high-density optical information recording medium 10 shown in FIG.  1, it is necessary to form the cover layer 15 having a uniform film thickness on the substrate 11.  Therefore, according to a proposed method, a UV curing resin is uniformly coated on the substrate 11 by a spin coat method and then cured, whereby the cover layer 11 having a uniform film thickness is formed on the substrate 11. 
Referring to FIGS.  3A-3C through 15, a description is now made of a method for forming the intermediate layer 13 and the cover layer 15 by the spin coat method in the optical information recording medium 10 having a double-layered structure.  FIGS.  3A through 3C show a conventional center cap 25 used for forming layers on the substrate 11 by the spin coat method.  FIG.  3A is a cross-sectional view of the center cap 25, FIG.  3B is a front view thereof, and FIG.  3C is a bottom view thereof.  The center cap 25 shown in FIGS.  3A through 3C has a fitting part 25c fitted in the center hole 11a of the substrate 11; a cone-shaped part 25b that is provided concentrically with the fitting part 25c, made parallel to the substrate 11 when the fitting part 25c is fitted in the center hole 11a of the substrate 11, and has an end surface 25e that is an annular-shaped plane having a diameter larger than that of the fitting part 25c; and a cylindrical-shaped projection part 25a provided concentrically with the cone-shaped part 25b at the upper part of the cone-shaped part 25b.  
The whole surface of the end surface 25e, which is brought into contact with the substrate 11, is a flat annular-shaped plane part.  Although a slight gap is caused when the end surface 25e of the center cap 25 is brought into contact with the substrate 11, such a case is taken into consideration for descriptive purposes.  Note that in FIGS.  3A through 3C, R1 denotes the radius of the center cap 25. 
FIGS.  4 through 15 are cross-sectional views for explaining a conventional method for forming the intermediate layer 13 and the cover layer 15 by the spin coat method in the optical information recording medium 10 having the double-layered structure.  In FIGS.  4 through 15, the same constituents as those of FIGS.  1 through 3A-3C are denoted by the same reference numerals and their descriptions are omitted.  First, as shown in FIG.  4, the center hole 11a of the substrate 11, on which aluminum alloy, etc.,  are sputtered as the reflecting and/or recording layer 12, is mounted on a rotating table 21 serving as a rotatable mounting board so as to be fitted in a substrate positioning part 22.  As a result, the fitting part 25c of the center cap 25 shown in FIGS.  3A through 3C is fitted in the center hole 11a of the substrate 11. 
Next, as shown in FIG.  5, a UV curing resin 33 is coated at the central part of the substrate 11, and then the rotating table 21 is rotated.  As a result, as shown in FIG.  6, the intermediate layer 13, which is made of the UV curing resin 33 having a uniform film thickness, is formed on the substrate 33.  Then, as shown in FIG.  7, the center cap 25 is removed upward.  After that, as shown in FIG.  8, a transparent stamper 34 having predetermined pits and/or guide grooves formed at its lower surface 34b, is mounted on the intermediate layer 13.  Moreover, as shown in FIG.  9, UV light 35 is radiated from above an upper surface 34a of the stamper 34, while the predetermined pits and/or guide grooves are transferred from the transparent stamper 34 to the intermediate layer 13.  Then, as shown in FIG.  10, the stamper 34 is removed upward.  The intermediate layer 13, on which the predetermined pits and/or guide grooves are formed, is thus formed. 
Moreover, aluminum alloy, etc.,  are sputtered on the intermediate layer 13 to form the second reflecting and/or recording layer 14.  After that, as shown in FIG.  11, the center hole 11a of the substrate 11 is mounted on the rotating table 21 so as to be fitted in the substrate positioning part 22, so that the fitting part 25c of the center cap 25 is fitted in the center hole 11a of the substrate 11.  Subsequently, as shown in FIG.  12, the UV curing resin 33 is coated at the central part of the substrate 11, and then the rotating table 21 is rotated.  As a result, as shown in FIG.  13, the cover layer 15, which is made of the UV curing resin 33 having a uniform film thickness, is formed on the substrate 11.  Then, as shown in FIG.  14, the center cap 25 is removed upward and detached.  After that, as shown in FIG.  15, the UV curing resin 33 is irradiated with the UV light 35 from above and cured.  As a result, the cover layer 15 made of the UV curing resin 33 having the uniform film thickness is formed.  The optical information recording medium 10 having the double-layered structure is thus completed (see, for example, Patent Documents 1 through 4). 
Patent Document 1: JP-A-11-213459
Patent Document 2: JP-A-2004-95108
Patent Document 3: JP-A-2006-59454
Patent Document 4: JP-A-2006-331566
However, according to the conventional method for manufacturing the optical information recording medium 10 having the double-layered structure shown in FIGS.  4 through 15, as shown in FIG.  16 that is an enlarged view of FIG.  13, when the liquid UV curing resin 33 is dropped onto the center cap 25 to form the intermediate layer 13 and the cover layer 15, it penetrates into a slight gap T1 between the end surface 25e of the center cap 25 and the substrate 11 due to capillary action.  Therefore, after the rotating table 21 is rotated, the liquid UV curing resin 33 is irradiated with the UV light 35 and cured.  As a result, as shown in FIG.  17 that is an enlarged view of FIG.  15, slight concave and convex portions 13a and 15a may be formed on the clamp part 11b of the substrate 11, or convex portions 13b and 15b may be formed at the peripheral part (end part of the center cap 25) of the clamp part 11b in the intermediate layer 13 and the cover layer 15. 
The convex portion 13b of the intermediate layer 13 formed at the peripheral part (end part of the center cap 25) of the clamp part 11b adversely affects the formation of the cover layer 15.  Aluminum alloy, etc.,  are sputtered on the intermediate layer 13, on which the convex portion 13b is formed, to form the second reflecting and/or recording layer 14.  After that, the UV curing resin 33 is dropped onto the center cap 25 as shown in FIG.  12 using the center cap 25 having the same radius R1 as that used for forming the intermediate layer 13.  Then, as shown in FIG.  13, when the rotating table 21 is rotated, the UV curing resin 33 is directly influenced by the convex portion 13b formed at the peripheral part (end part of the center cap 25) of the clamp part 11b and is not uniformly spread.  Consequently, as shown in FIG.  18, multiple radial linear defects 36 are caused in the cover layer 15, which in turn degrades optical characteristics such as jitter and error rates caused at the time of reproduction and/or recording. 
Such a problem cannot be solved, for example, by the center cap disclosed in Patent Document 3, of which a surface in contact with the substrate has a concave shape and which is structured to form a space between the center cap and the substrate.  In other words, although the center cap disclosed in Patent Document 3 can improve the poor appearance of the clamp part in the optical information recording medium having a single-layered structure, Patent Document 3 does not specifically disclose a method for applying the center cap to the optical information recording medium having the double-layered structure. 
In addition, the technique of changing the internal diameter of the intermediate layer and the cover layer, which is disclosed in Patent Document 4, is targeted for the optical information recording medium in which the intermediate layer is formed by bonding sheets together.  Therefore, the technique cannot solve the above problem occurring when the intermediate layer and the cover layer are formed by the spin coat method using the center cap.