1.  Field of the Invention
The present invention relates to a multi-information-layer recoding medium having a plurality of information layers and its manufacturing process. 
2.  Description of the Related Art
The following will explain the conventional method for manufacturing an optical disc substrate.  A plastic substrate material such as polycarbonate melt at high temperature is injected, with a high pressure, into a mold with a metal stamper placed therein, and thereafter the resultant is cooled and taken out.  Then a plastic substrate having physical patterns copied on its surface is completed.  This injection molding method (injection method) is one that is generally used in manufacturing plastic substrates for the currently-available media, such as DVD-ROM, DVD-R, DVD-RAM, DVD-RW, Blu-ray Disc, MO, in addition to CD-Audio, CD-R, and CD-ROM.  Then, a reflective layer or recording laminated film is deposited on the surface with the physical patterns by sputtering to form an information layer, and a protective layer is formed of ultraviolet (UV) curable resin for protection from damages.  As a result, the medium is completed. 
As a means for increasing the recording capacity of a medium, there are a method involving increasing a numerical aperture (NA) of a convergence lens and a method involving making an information layer of a medium multilayered.  Japanese Patent Application Publication No.  Hei 8-297861 discloses an optical recording medium with a four-layer structure in which two substrates, each having two recording layers, are adhered to, and face each other.  In a multi-information-layer recording medium, physical patterns including a recessed portion and a protruded portion formed of an optical spot groove and/or pits are required for each information layer.  An injection method and 2P method are mainly used to form patterns, but both methods are a pattern transfer method using a stamper as a mother die for pattern transfer.  A space, which is required to divide each information layer, is formed as a space layer, and therefore the space layer must be controlled to maintain a desired thickness as well as include physical patterns of the information layer at the same time. 
Regarding the space layer manufacturing method, Japanese Patent Application Publication No.  Hei 9-73671 discloses a method in which a space layer is formed by a spin coat method using an ultraviolet curable resin.  The spin coat method is a method in which liquid is dropped on a portion close to a central hole of a substrate through a nozzle, and the substrate is spun to spread the liquid by centrifugal force, and subjected to curing with ultraviolet light.  International Symposium on Optical Memory 2001 Technical Digest, p 312 describes following method.  An inner recording laminated film is formed on a thick substrate placed on an opposite side to a light incident side.  The formed recording laminated film is coated with ultraviolet curable resin having a good adhesive property.  In the meanwhile a polycarbonate stamper is coated with resin having good transferability and releasability from polycarbonate.  Both are adhered to each other as being spun and the resultant is cured with ultraviolet light.  When the polycarbonate stamper is separated after curing, a pattern is transferred on the surface, so that a recording laminated film close to the light incident side is formed thereon, and a cover layer having a thickness of 0.1 mm is further formed thereon.  Japanese Patent Application Publication No.  Hei 11-273147 discloses another space layer manufacturing method using a pressure-sensitive adhesive sheet or a dry photopolymer sheet.  Since the sheet having a predetermined thickness is used as a space layer, a space layer having no thickness unevenness can be formed.  Moreover, a four-layer optical disc having three space layers is reported in International Symposium on Optical Memory 2003 Technical Digest, p 10.  Dry photopolymer sheet is used as a space layer material. 
Apart from the aforementioned methods, there is another approach having been studied, of increasing the capacity with a high-density recording data obtained by increase of a numerical aperture (NA) of a lens.  Generally, the size of a condensed optical spot is proportional to λ/NA where a wavelength of an incident light is λ.  In other words, when the wavelength of the incident light is short and NA of the lens is large, the size of the optical spot becomes small, allowing a recoding mark to be reduced in size and high density to be achieved.  An attempt has been made to reduce the size of the optical spot by using a lens having NA higher than 1.  This type of lens is called as a solid immersion lens (SIL).  However, when NA is higher than 1, light having a component of NA>1 among light emitted from the lens is locally present in the vicinity of the light incident surface of the lens without being propagated.  The light locally present is called as an optical near field.  The optical near field is converted into propagation light when being in contact with a material that allows light to propagate therethrough.  For this reason, the SIL must be maintained to be always within about 20 nm from a medium surface in the use of the SIL. 
An attempt to combine the SIL with multilayer recording is reported in, for example, Proceedings of SPIE, vol.  6282, 62820Q. This shows a calculation result when four-layer recording is performed using a SIL having NA of 1.6.  According to this report, the thicknesses of the medium cover layer and first to third space layers may be 2.0 μm, 2.6 μm, 2.2 μm, and 2.4 μm, respectively.  Since NA is high, when an interlayer distance is changed, a signal from an adjacent layer is abruptly reduced.  This makes it possible to reduce the interlayer distance.  Conversely, when the interlayer distance is large and NA of the lens is high, light cannot be condensed on a point far from the lens.  Though the lens diameter must be enlarged to condense the lens on the point far therefrom, enlargement in lens increases weight of the lens, thus making it impossible to move the lens at high speed.  This causes, for example the lens and the medium to collide with each other, since a distance between the lens and the medium is not controlled at high speed.