1. Field of the Invention
The present invention relates to an optical disc having an annular information recording area and a hub located substantially in the center thereof and which supports the optical disc, a method of manufacturing the optical disc, and a method of producing a stamper for a substrate for the optical disc.
2. Description of Related Art
Each of the optical discs comprises essentially a disc-shaped substrate made of plastic, and a recording layer a formed on the substrate. Some of the optical discs has a hub disposed substantially in the center thereof as a support member.
Concerning an optical disc of 130 mm (5.25 inches) in diameter, ISO standard prescribes an optical disc having a hub made of a magnetic-sensitive material. The optical disc is of an air-sandwich structure having a hole formed in the center thereof and two resin-made disc-shaped substrates, at least one of them having a recording layer formed thereon, and the two substrates being joined to each other with the recording layer placed inside. The hub is joined to both the substrates.
When an optical disc having such a hub is set on a disc table in an optical disc drive for recording and/or reproducing an information into and/or from the optical disc, the optical disc can be chucked to the disc table with the hub. That is to say, the hub is used to securely set and hold the optical disc on the disc table. More particularly, the hub is made of a metallic material and has formed in the center thereof a center hole through which a spindle shaft of the disc drive is introduced. When the optical disc is set on the disc table, the hub is attracted by a magnet in the disc table to chuck the optical disc.
Therefore, in manufacturing an optical disc having such a hub, the hub must be joined to the optical disc in such a manner that the center of rotation of the hub coincides highly precisely with that of an information recording area formed on a substrate of the optical disc.
Two typical conventional methods used to join a hub to an optical disc will be described herebelow by way of example:
The first one of the two methods is such that a center hole is formed in the center of an optical disc substrate to be concentrical with the center of rotation of an information recording area formed on the optical disc substrate and a hub is fitted and joined concentrically with the center hole. By this method, the hub can be joined rather easily. However, the center hole should be formed in thee optical disc substrate not to be eccentric to the center of rotation of the information recording area and to have a highly accurate diameter. Both these requirements are difficult to meet by this method.
The second method utilizes an X-Y table to position the hub and optical disc with reference to the boundary between the information recording area formed on the optical disc and the inner circumference of the information recording area and join the hub to the optical disc substrate so that the center of rotation of the information recording area coincides with the center of the hub.
Examples of the joining of hub to optical disc by the second method are disclosed in the Japanese Published Unexamined Patent Nos. 63-76131 and 4-49543. The principle of this second method will be described with reference to FIG. 1. As shown, an optical disc 100 has formed thereon an information recording area 101 in which convexities and concavities are formed, and a mirror area 102 located inside the inner circumference of the information recording area 101 and in which no convexities and concavities are formed. A difference in reflected quantity of light between the information recording area 101 and mirror area 102 is used to identify a boundary between the areas 101 and 102. In this case, the difference in reflected quantity of light is a difference in scattered quantity of light between the areas 101 and 102. Note that a common optical disc has a similar construction to that of the optical disc 100 shown in FIG. 1, and it has a center hole 103 formed substantially in the center thereof as will also be seen from FIG. 1.
In the field of optical data storage, however, the data storage density demanded for optical discs has been increasingly greater year after year, so that there is a tendency that the track pitch in the information recording area has become correspondingly narrower. In the case of an optical disc of 130 mm in diameter, for example, as the technology in consideration has been innovated, the track pitch of the information recording area has been reduced from 1.6 xcexcm, to 1.39 xcexcm, 1.15 xcexcm, and further to 0.85 xcexcm for a higher density of data storage.
In the second method, however, such a small track pitch will vary the angle of light scattering by the convexities and concavities in the information recording area changes, so the reflected quantity of light from the information recording area can be less detected. Thus, the difference in reflected quantity of light between the information recording area and mirror area on the optical disc is difficult to detect by the second method, thus it is not possible to accurately locate or identify the boundary between the information recording area and mirror area that is the reference for correct positioning of the hub.
The above will be described further with reference to FIGS. 2 and 3 being schematic diagrams, respectively, of a system for measuring the light reflection from an optical disc. The system in FIG. 2 measures the reflected light from one conventional optical disc 200 having a large track pitch in an information recording area 201 formed thereon. As illustrated, the optical disc 200 is irradiated with a light from a light source 203. The reflected light from the information recording area 201 on the optical disc 200 is detected by a CCD 204, and monitored on a display 205. FIG. 2 also shows how the light is reflected by the optical disc 200.
Similarly, the system in FIG. 3 measures the reflected light from another conventional optical disc 300 having a small track pitch in an information recording area 301 formed hereon. As illustrated, the optical disc 300 is irradiated with a light from a light source 203. The reflected light from the information recording area 301 on the optical disc 300 is measured by the CCD 204, and monitored on the display 305. FIG. 3 also shows how the light is reflected by the optical disc 300.
As seen from FIG. 2, the CCD 204 detects much reflected light, namely, much scattered light, from the information recording area 201 having the large track pitch. However, the CCD 204 detects less reflected light from the information recording area 301 having the small track pitch, as will be known from FIG. 3. This is because the small track pitch in the information recording area 301 causes a variation of the scattering angle of light or the like.
As mentioned above, the small track pitch in the information recording area will reduce the reflected quantity of light from the information recording area. Thus, a boundary between the information recording area and mirror area cannot disadvantageously be identified accurately by the second method unless a hubbing machine is adjusted concerning incident angle and incident quantity of the light correspondingly to each track pitch.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing an optical disc, method of manufacturing the optical disc and a method of producing a stamper for a substrate for the optical disc, by which the boundary between an information recording area and mirror area formed on an optical disc substrate can be identified more easily and accurately for an accurate joining of a hub to the optical disc substrate.
The above object can be accomplished by proving an optical disc having an annular information recording area and a hub fixed substantially in the center thereof.
In particular, the optical disc according to the present invention has formed inside the inner circumference of the information recording area concentric or spiral continuous or discontinuous recesses at a larger pitch than a track pitch in the information recording area.
Because of the above recesses formed inside the inner circumference of the information recording area and larger recess pitch than the track pitch in the information recording area, the recesses will reflect, in a constant and sufficient quantity for detection, a light irradiated from a hubbing machine during hubbing of (installation of a hub to) an optical disc substrate without the necessity of changing the incident angle and quantity of the light from the hubbing machine correspondingly to the magnitude of the track pitch in the information recording area.
Thus, it is possible to easily identify, based on the reflected light, the boundary inside the inner circumference of the information recording area with reference to the recesses, thereby permitting to join a hub to the optical disc in a correct position.
The above object can also be accomplished by providing a method of manufacturing the optical disc according to the present invention, in which to install a hub substantially in the center of an optical disc substrate having an annular information recording area formed thereon, concentric or spiral continuous or discontinuous recesses are formed inside the inner circumference of the information recording area at a larger pitch than a track pitch in the information recording area and the recesses are taken as a reference marker to position the hub with respect to the optical disc substrate.
In the above-mentioned optical disc manufacturing method, the recesses formed inside the inner circumference of the information recording area are taken as the reference marker to position the hub with respect to the optical disc substrate, so that the hub can be joined accurately to the optical disc substrate independently of the magnitude of the tack pitch in the information recording area.
Also, the above object can be accomplished by providing a method of producing a stamper for the optical disc substrate in which a substrate having a photoresist layer formed on the surface thereof is rotated and the photoresist layer is exposed while a laser beam is being scanned radially of the substrate, and the laser beam is selectively turned on and off to form, by exposure to the laser beam, inside the inner circumference of an information recording area concentric or spiral continuous or discontinuous recesses at a larger pitch than a track pitch Tp in the information recording area.
When exposing the photoresist layer while scanning the laser beam radially of the substrate, the laser beam is selectively turned on and off to form, by the laser exposure, the concentric or spiral continuous or discontinuous recesses at the larger pitch than the track pitch in the information recording area. Therefore, the recesses will reflect, in a constant and sufficient quantity for detection, a light irradiated from the hubbing machine during hubbing of the optical disc substrate without the necessity of changing the incident angle and quantity of the light from the hubbing machine correspondingly to the magnitude of the track pitch in the information recording area.
Thus, it is possible to accurately identify, based on the reflected light from the recesses, the boundary inside the inner circumference of the information recording area with reference to the recesses, thereby permitting to correctly position the hub with respect to the optical disc substrate.