AN OPTICAL DISC

Abstract: In an optical disc according to the present invention, first (5) and second (8) discs comprise a hub half (14) inserted into the center portion of a disc substrate respectively, the hub half (14) is arranged at the center of the disc substrate, and a recording layer (9) is formed on the disc substrate by using the center portion of the hub half as a reference point. Therefore, the center of the recording layer can not be shifted from the center of the hub half and the optical disc with high reliability can be provided when recording and playing back an information.

Description: TITLE OF THE INVENTION AN OPTICAL DISC FIELD OF THE INVENTION The present invention relates to an optical disc and a process of making the optical disc. BACKGROUND OF THE INVENTION Fig. 1 is a sectional view of a dual sided recordable type of an optical disc made by a prior art process. As shown in Fig. 1, two discs 53 are arranged through a hub 51 and an outer spacer 52 so that the disc 53 faces to another disc 53. The disc 53 comprises a transparent disc substrate 54 and a recording layer 55 arranged on the one side of the disc substrate 54, wherein a plurality of pregrooves and/or prepits 56 are formed on the recording layer 55, and the two disc 53 are arranged so that the recording layer 55 faces another recording layer 55. On the other hand, the hub 51 is made of a material with an abrasion resistance characteristic such as a metal, and a center hole 57 is formed at the center of the hub 51 in order to position the optical disc at a predetermined position by inserting a spindle of a disc drive device into the center hole 57 of the hub 51. In Fig. 1, there is a spacing 58 between the two discs 53. Figs. 2 through 4 show a process of making the optical disc shown in Fig. 1. First of all, as shown in Fig. 1, a resin layer 61 using ultraviolet rays hardening resin with fluidity before hardened is coated on a stamper 60, on the surface of which a number of concentric or spiral grooves 59 corresponding to the pregrooves and/or prepits 56 are formed. The disc substrate 54 is pressed on the resin 61 on the condition that the disc substrate 54 is curved so that the center portion of the substrate 54 is lower than the other portion of the substrate 54. That is, firstly, the center portion of the disc substrate 54 is pressed on the center of the stamper 60, and subsequently the outer portion of the disc substrate 54 is pressed on the resin 61 arranged on the stamper 60. By pressing the disc substrate 54 on the resin 61 as described above, the resin 61 arranged between the disc substrate 54 and the stamper 60 is spread toward the outer side of the disc substrate 54 without bubble, and the resin 61 fills the grooves 59. Then, an ultraviolet rays radiate onward the disc substrate 54 in the direction of an arrow 62 as shown in Fig. 3, thereby the ultraviolet rays hardening resin hardens. Next, when the disc substrate 54 is exfoliated from the stamper 60, the hardened resin 61 is separated from the stamper 60 with the disc substrate 54. Then, the recording layer 55 is formed on the hardened resin 61 arranged on the disc substrate 54 by a vapor evaporation process, thereby the recording layer 55 with the pregrooves and/or prepits 56 is formed on the disc substrate 54. Then the replica process is completed. The two disc substrates 54 are respectively constructed with a hub 51 and an outer spacer 52 by positioning the two disc substrates 54 in accordance with the formed pregrooves and/or prepits 56 so that the first disc substrate 54 faces the second disc substrate 54, thereby an optical disc is made as shown in Fig. 1. However, the aforementioned making process in the prior art has the following disadvantage. That is, it is difficult to arrange the center lines of the two disc substrates 54 on the center line of the hub 51 in a straight line, therefore the center lines of the both disc substrates 54 are actually shifted from the center line of the hub 51. The shift between the center lines of the both disc substrates 54 and the center line of the hub 51 decreases the reliability of the optical disc, and the constructing process, including the arranging process of the center line of the both disc substrates 54 on the center line of the hub 51 in a straight Ine decreases the productivity of the whole making process of the optical disc. That is, in general, the disc substrate 54 is made of a transparent substrate made of polycarbonate resin, epoxy resin or glass, becauseJthe disc substrate 54 has to be transparent in order to write and read an information to and from the recording layer 55 arranged on the disc substrate 54. On the other hand, in general, the hub 51 is made of metal, because the hub 51 has to be made of a material with abrasion resistance in order to set and take off the optical disc on and from a turn table of recording and playing back device repeatedly. Thus the coefficient of thermal expansion of the hub 51 made of the metal is larger nearly ten times than the coefficient of thermal expansion of the disc substrate 54, therefore a clearance is undesirably formed between the disc substrate 54 and the hub 51 in order not to change the shape of the disc substrate 54 by the thermal expansion of the disc substrate 54, and also the clearance is required in order to make the engagement between the disc substrate 54 and the hub 51 easy. When there is the clearance as described above, the relative shift between the center lines of the disc substrates 54 and the center line of the hub 51 easily occurs by the clearance. In case the hub 51 is bonded on the disc substrate 54 on the condition that the center line of the disc substrate 54 is slightly shifted from the center line of the hub 51, the optical disc is positioned by engaging the hub 51 in the spindle of the turn table of the recording and playing back device as described above, and the optical disc is arranged on the turn table and an information is written and read in and from the optical disc, the pregrooves and/or prepits 56 arranged on the recording layer 55 rotate on the condition that the center of the pregrooves and/or prepits 56 is slightly shifted from the rotation center of the optical disc. Therefore, the pregrooves and/or prepits 56 cannot be tracked properly and the information included in the recording layer 55 of the optical disc cannot be written and read properly. Particularly recently, the information is recorded in the recording layer 55 of the optical disc at higher density, wherein the width of the pregrooves and/or prepits 56 of the optical disc is very narrow such as 0.4 pm - 0.8 pm and the pitch between the pregrooves and/or prepits 56 of the optical disc is very short such as about 1.6 pm, therefore the very small shift between the rotation center and the center of the pregrooves and/or prepits 59 directly influences the writing and reading characteristic of the optical disc. In the prior art process, it is necessary to bond the hub 51 on the disc substrate 54 by positioning the hub 51 very carefully using a microscope, therefore the bonding process decreases the productivity of the whole making process. SUMMARY OF THE INVENTION An essential object of the present invention is to provide an optical disc with high reliability when recording and playing back an information and with high productivity in a process of making the optical disc. According to the present invention, an optical disc comprises a first disc comprising a first hub half having a cylindrical portion and a first disc substrate with a center hole, wherein the first hub half and the first disc substrate constructed as one body by inserting the cylindrical portion of the first hub half into the center hole of the first disc substrate, and a second disc comprising a second hub half having a cylindrical portion and a second disc substrate with a center hole, wherein the second hub half and the second disc substrate are constructed as one body by inserting the cylindrical portion of the second hub half into the center hole of the second disc substrate, wherein a surface of the first hub half and a surface of the second hub half are connected so that the first hub half and second hub half are connected as one body, and the first disc faces the second disc. Furthermore, according to the present invention, a process of making an optical disc comprising steps of arranging a hub half at the center of a disc substrate, and forming a recording layer having at least a tracking information on the disc substrate by using the center portion of the hub half as a reference point. In the optical disc according to the present invention, the first and second discs comprise the hub half inserted into the center portion of the disc substrate respectively, the hub half is arranged at the center of the disc substrate, and the recording layer is formed on the disc substrate by using the center portion of the hub half as a reference point. Therefore, the center of the recording layer can not be shifted from the center of the hub half and an optical disc with high reliability can be provided when recording and playing back an information. Moreover, in the process of making the optical disc according to the present invention, the recording layer is formed on the disc substrate by using the center portion of the hub half arranged at the center of the disc substrate as a reference point, therefore it is not necessary to arrange the center line of the disc substrate-on the center line of the hub half in a straight line by using a microscope. Furthermore, it is easy to position the both disc substrates because the both disc substrates are positioned in accordance with the center hole of the hub half, therefore, the process of making the optical disc increases the productivity. BRIEF EXPLANATION OF THE DRAWINGS Fig. 1 is a sectional view of an optical disc according to a prior art, Figs. 2 through 4 are sectional views showing a making process of the optical disc shown in Fig. 1, Fig. 5 is a sectional view of an optical disc of a first preferred embodiment according to the present invention, Fig. 6 is a sectional view of a cylindrical hub half of the optical disc shown in Fig. 5, Fig. 7 is a plane view of an outer spacer of the optical disc shown in Fig. 5, Figs. 8 (A) and 8 (B), and 9 through 12 are sectional views showing a process of making the optical disc shown in Fig. 5, Fig. 13 is a sectional view of an optical disc of a second preferred embodiment according to the present invention, Fig. 14 is a plane view of an inner spacer of the optical disc shown in Fig. 13, Figs. 15 and 16 are sectional views showing a process of making the optical disc shown in Fig. 13, Fig. 17 is a sectional view of an optical disc of a third preferred embodiment according to the present invention, Figs. 18 and 19 are sectional views of optical disc of a fourth preferred embodiment according to the present invention, Fig. 20 is a perspective view of a hub half body of the optical disc shown in Fig. 18, Fig. 21 is a perspective sectional view of the hub half body on the line of A - A' shown in Fig. 20, Fig. 22 is a perspective sectional view of a hub half body of the optical disc shown in Fig. 19, Fig. 23 is a perspective sectional view of a hub half body constructed by screwing and bonding a flange portion on a cylindrical portion. Fig. 24 is a sectional view of an optical disc of the fifth preferred embodiment according to the present, and Fig. 25 is a sectional view of an optical disc of the sixth preferred embodiment according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 5 iS a sectional view of a dual sided recordable type of an optical disc of a first preferred embodiment according to the present invention. In Fig. 5, a first disc 3 faes a second disc 4 through an inner spacer 1 and an outer spacer 2, each spacer having an annular shape. The first disc 3 comprises a first transparent disc substrate 5 and c first recording layer 6 arranged on one side of the first transparent disc substrate 5. A first grooves 7 and pre:its in the spiral or concentric shape are formed on the fir ; recording layer 6. The second disc II comprising a scold disc substrate 8, a second recording layer 9 arranged cn the second disc substrate 8, and second grooves 10 on the second recordihg layer 9 is provided similar to the first disc 3. The two discs 3 and 4 are arranged so that the first recording layer 6 faces the second recording layer 9. A first hub half 1t is bonded at the center of the first disc substrate 5, and a second hub half 12 is bonded at the center of the second disc substrate 8. The hub half halves 11 and 12 are made of a metal whose coefficient of thermal expansion is nearly equal to the disc substrate 5 and 8, for example, the hub half halves 11 and 12 are made of an alloy of iron, nickel and chromium, containing nickel at 42 percent by weight and chromium at 6 percent by weight. As shown in Fig. 6, each of the hub half halves 11 and 12 has the same shape and comprises a cylindrical portion 14 having a center hole 13 and an inner flange portion 15 in the shape of ring and arranged at the end of the cylindrical portion 14. The hub half halves 11 and 12 are formed by processing an original member by a pressing process or a cutting process. A bonding recess 16 in an annular shape is formed in the end surface of the inner flange portion 15, and the inner and outer rim portion of the cylindrical portion 14 of the hub half halves 11 and 12 are chamferred at 13a and 13b. Fig. 7 is a plane view of the outer spacer. The outer spacer 2 and the inner spacer 1 are respectively made of a metal such as stainless and aluminum etc., ceramics, or composed resin such as epoxy resin and polycarbonate resin. One or more grooves 26 for letting air out of the inside of the optical disc are formed on the upper surface of the outer spacer 2 so that the grooves 26 pass through from the inside to the outside of the optical disc, as shown in Fig, 7. A process of making the optical disc will be described below, referring to Figs. 8 through 12. First of all, a center hole 13 is formed precisely at the center of the hub half 11, as shown in Fig. 8 (B), and a through hole 17 is formed at the center of the surface of the disc substrate 5, on which the recording layer 5 is not arranged, as shown in Fig. 8 (A). On the other hand, the upper surface of the inner flange portion 15 and the outer rim surface of the cylindrical portion 14, of the hub half 11, are coated with a bonding material 18, then the cylindrical portion 14 is inserted into a through hole 17 of the disc substrates 5, thereby the hub half 11 is bonded on the disc substrate 5 as shown in Fig. 9. For example, a transparent bonding material such as ultraviolet rays hardening resin etc. is used as the bonding material 18, because the transparent bonding material 18 can not influence the appearance of the optical disc. As shown in Fig. 10, there is provided a stamper 19 having concentric or spiral grooves 20 corresponding to the pregrooves and/or prepits 7, wherein the grooves 20 are formed on the upper surface of the stamper 19 with a positioning pin 21 projected from the surface of the stamper 19 at the center of the stamper 19 to position the disc substrates 5 on the stamper 19, and a predetermined quantity of the fluidity ultraviolet rays hardening resin not hardened is coated on the surface of the stamper 19. Then, the disc substrate 5 with the hub half 11 is pressed on the stamper 19. As shown in Fig. 10, the disc substrate 5 is placed on the surface of the stamper 19 on the condition that the disc substrate 5 is curved so that the center portion of the disc substrate 5 is lower than the outer portion of the disc substrate 5, and the positioning pin 21 is inserted into the center hole 13 of the hub half 11, thereby it is possible to position the disc substrate 5 on the surface of the stamper 19. On that condition, when the disc substrate 5 is pressed on the surface of the stamper 19, the ultraviolet rays hardening resin 22 is spread toward the outer side without bubbles, the resin 22 not hardened is flowed into the grooves 20, and the grooves 20 is filled with the resin 22 not hardened. After that, as shown in Fig. 11, the disc substrate 5 is radiated uniformly by the ultraviolet rays on the condition that the disc substrate 5 is pressed on the surface of the stamper 19, thereby the ultraviolet rays hardening resin 22, which is arranged between the stamper 19 and the disc substrate 5, hardens. Next, the disc substrate 5 is exfoliated from the stamper 19, then the hardened resin 22 is separated from the stamper 19 with the disc substrate 5, thereby the hardened resin 22, which become a transparent resin layer with the pregrooves and/or prepits 7 and the prepits not shown, is formed on the surface of the disc substrates 5. The disc substrate 5 with the transparent resin layer 22 is arranged in the vacuum vapor evaporation device, which is the vacuum evaporator, the recording layer 6 is formed on the surface of the transparent resin layer 22 by the known vapor evaporation method. A metal with high optical reflectivity such as aluminum or an alloy of tellurium, selenium and lead etc. is used as the recording layer 6. On the other hand, another disc 4 comprising the hub half 12, the disc substrate 8 and the recording layer 9 with the pregrooves and/or prepits 10 is made by the same process as the making process of the disc 3. After the disc 3 comprising the recording layer 6 arranged on the transparent resin layer 22 and the disc LI comprising the recording layer 9 arranged on the transparent resin layer 22 are made by the aforementioned replica process, the second disc 4 is arranged on the base stand 24, when the second disc 4 is positioned on the base stand 24 by inserting the center pole 25 arranged on the upper surface of the base stand 24 into the center hole 13 of the second hub half 12. Next, the bonding material 18 is coated in the bonding recess 16 of the second hub half 12 and near the recess 16, then the inner spacer 1 and the outer spacer 2, whose top and bottom surfaces are coated with the bonding material 18, are arranged at the predetermined position of the disc 4. When the inner spacer 1 is arranged at the said position, the inner spacer 1 is positioned by engaging the inner spacer 1 in the flange portion 15 of the second hub half 12. The bonding material 18 is not coated at the grooves 26 and near the grooves 26 on the surface of the outer spacer 2 on which the grooves 26 are formed, in order not to close the grooves 26 with the bonding material 18. Then, the first disc 3 comprising the first hub half 11 whose bottom surface is coated with the bonding material 18 is arranged on the inner spacer 1 and the outer spacer 2, the center pole 25 is inserted into the center hole 13 of the first hub half 1. Thereby, the first disc 3 and the second disc 4 are positioned at the proper position, and the hub halves 11 and 12 of two discs 3 and LI, the inner spacer 1 and the outer spacer 2 are bonded as one body, as a result, the optical disc is made as shown in Fig. 5. The optical disc made as described above has spacings, which are the grooves 26 for letting air out of the inside of the optical disc, arranged between the outer spacer 2 and the disc substrate 5. The grooves 26 are useful for letting the gas, evolved when the bonding material 18 is firmed, out of the inside of the optical disc to the outside of the optical disc. The gas is preferably exhausted as described above, because the gas causes the recording layers 6 and 9 to flange in quality when the gas evolves when the bonding material 18 is firmed, i.e. the mainly organic dissolving material remains in the inside of the optical disc. As the grooves 26 for letting the gas out of the inside of the optical disc are closed after the bonding material 18 is firmed, the inside of the optical disc is made airtight. Fig. 13 is a sectional view of an optical disc of a second preferred embodiment, Fig. 14 is a plane view of an inner spacer used for the optical disc shown in Fig. 13, and Figs. 15 through 16 are a sectional views showing a making prowess of the optical disc shown in Fig. 13. In Fig. 13, the first hub half 11 and the second hu half 12 comprise a cylindrical portion 14 having a center hole 13 processed precisely and an outer flange portion 27, respectively, and the first hub half 11 and the sec:nd hub half 12 of the optical disc of the second pre'erred embodiment are made of an alloy of iron, nickel and hromium,-containing nickel at 1'2 percent by weight and chrolium at 6 percent by weight. As shown in Figs. 13 throngh 15, the bottom surface of the outer flange portion 27 ali the outer surface of the cylindrical portion 14 are coated with the bonding material.18, then the first hub half 11 is bonded on the first disc substrate 5 as one body by inserting the cylindrical portion 14 of the hub half 17 into the center hole of the first substrate 5, on the other hand, the second hub half 12 is bonded on the second disc substrate 8 as one body by inserting the cylindrical portion 14 of the hub half 12 into the center hole of the second disc substrate 8. In the optical disc of the second preferred embodiment, bonding recesses 16 in the shape of ring are formed on the top and bottom surface of the inner spacer 1. One and more grooves 26 used for letting the gas out of the inside of the optical disc are formed at the middle position in the axial direction so that the grooves 26 pass through from the inside of to the outside of the optical disc. Arranging the grooves 26 at the middle position in the axial direction prevents the bonding material 18 described below from closing the grooves 26. The making process of the optical disc of the second preferred embodiment will be described below. First of all, the hub half 17 is bonded at the center portion of the disc substrate 5 on which the recording layer 5 is not formed. In Fig. 15, the stamper 19 comprises concentric or spiral grooves 20, corresponding to the pregrooves and/or prepits 7, and a positioning pin 21 arranged at the center of the grooves 20 and projected from the surface of the stamper 19. As shown in Fig. 15; the aforementioned disc substrate 5 comprising the hub half 11 is arranged on the stamper 19 by inserting the positioning pin 21 into the center hole 13 of the hub half 11 of the disc 3, on the condition that thedisc substrate 5 with the hub half 11 is curved so that the center portion of the disc substrate 5 become lower than the other portion of the disc substrate 5. Next, the disc substrate 5 is pressed completely on the stamper 19, then the ultraviolet rays radiate toward the upper surface of the disc substrate 5 pressing as described above, thereby the ultraviolet rays hardening resin 22, filled between the disc substrate 5 and stamper 19, hardens by the ultraviolet rays. Then, the disc substrate 5 comprising the hub half 5 and the resin 22 are exfoliated from the stamper i19. The disc substrate 5 comprising the hub half 5 and the resin 22 are arranged in the vacuum vapor evaporation device, then the recording layer 6 made of the metal with high optical reflectivity is formed by the same vapor deposition method as the method of the optical disc of the first preferred embodiment. On the other hand, another disc 4 comprising a disc substrate 8, a recording layer 9 with pregrooves and/or prepits 10 and a hub half 12 is made by the same process as the leaking process of the disc 3 as described above. As shown in Fig. 16, the second disc substrate 4 is arranged on the base stand 24 so that the center pole 25 is inserted into the center hole 13.of the second hub half 12. Then, the inner spacer 1 and the outer spacer 2, whose top and bottom surface are coated with the bonding material 18, are arranged at the predetermined position of the disc 4, thereby the-inner spacer 1 is positioned at the predetermined position by engaging the inner spacer 1 in the outer surface of the cylindrical portion of the second hub half 12. Then, the first disc 3 is arranged on the inner spacer 1 and the outer spacer 2 so that the center pole 25 is inserted into the center hole 13 of the first hub half 11, thereby the first disc 3 and the second disc LI are positioned at the predetermined proper position so that the center of the first disc 3 corresponds to the center of thesecond disc 4, and the two discs 3 and 4, and the inner spacer 1 and outer spacer 2 are bonded as one body, as a result, the optical disc of the second preferred embodiment shown in Fig. 13 is made. Fig. 17 is a sectional view of an optical disc of a third preferred embodiment according to the present invention, in which the inner spacer 1 used in the optical discs of the first and second preferred embodiments is omitted. In the optical disc shown in Fig. 17, each of the disc halves 3 and 4 is provided with a hub half 11 which is similar to the hub half 11 ( or 12 ) of the optical disc of the first and second preferred embodiments except that the diameter of the flange portion 15 is larger than that of the hub half 11 ( or 12 ). By using the hub half 11, the large diameter flange 15 can serve as the inner spacer to support the substrates 5 and 7, thus there is no need to use the separate inner spacer 1. In Fig. 17, the outer surface of the cylindrical portion 14 and the inside surface of the flange portion 15 of the hub half 11, which is not the end surface, and the inner surface and the outer surface of the flange portion 15, are coated with the bonding material 18, then the cylindrical portion 14 of the two cylindrical hub halves 11 and 12 is inserted into the center hole of the disc substrates 5 and 8, respectively, thereby the cylindrical hub half 11 is bonded on the disc substrates 5 and 8, respectively. After that, the recording layers 6 and 9 are formed on the one side surface of the disc substrates 5 and 8 comprising the hub halves 11 and 12 made as described above by the same process as the process of the optical discs of the first and second preferred embodiments, respectively. After the bottom surface of the flange portion 15 of the two hub halves 11 and 12, facing each other, is coated with the bonding material 18, and the outer spacer 2 is arranged on the outer rim portion between the one side surface of the the disc substrates 3 and 8, the optical disc of the third preferred embodiment is cons.ucted so that the recording layer 6 faces the recorting layer 9, as well as the optical discs of the first and second preferred embodiments: Fig. 17 shows the sectional view or the constructed optical disc of the third preferred embodiment. The optical disc of the third preferred embodiment has the two advantage described below. That is, one advantage is to simplify the making process because it is not necessary to bond the inner spacer 1 on the disc substrates 5 and 8 with bonding material 18, and another advantage is that the first disc 3 is bonded more strongly to the second disc 4 than the optical discs of the first and second preferred embodiments comprising the inner spacer 1, because the bonding area of the optical disc of the third preferred embodiment is wider than the bonding area of the optical discs of the first and second preferred embodiments. Figs. 18 and 19 are sectional views of the optical disc of the fourth preferred embodiment. In the fourth preferred embodiment, each of the hub halves 11 is formed by a flange body 15a and a cylindrical body i1'a, each body is prepared by separate members. The two flange bodies 15a and the two cylindrical bodies 14a are made respectively, so that the outer diameter of the flange body 15a is larger than the outer diameter of the flange portion 15 of the cylindrical hub half 11 of the optical disc of the third preferred embodiment shown in Fig. 17. Then the flange body 15a is bonded on the end surface of the cylindrical body 14a with the bonding material 18. The flange body 15a may be coupled with the cylindrical body 14a by being welded or screwed. Thus, the cylindrical hub half 11 comprising the cylindrical body 14a and the flange body 15a can be made. In the making process of the optical disc of the fourth preferred embodiment, after the two cylindrical hub - halves 11 and 12 are made and the surfaces of the hub halves 11 and 12 are coated with bonding material 18 similar to the hub halves 11 and 12 of the optical disc of the third preferred embodiment, the cylindrical body lila of the hub half 11 is inserted into the center hole of the disc ubstrate -5- of'he-first disc disc 3, and the cylindrical body 14a of another hub half 11 is inserted into the center hole of the disc substrate 8 of the second disc 4. After the end surfaces of the flange bodies 15 of the two hub halves 11 and 12 are coated with the bonding material, the first disc 3 and the second disc 4 are connected together as one body with the outer spacer 2, so that the end surface of the cylindrical body 15a of one hub half 11 is bonded onto the end surface of the cylindrical body 14a of another hub half 12 and the recording layer 6 formed on the disc substrate 5 faces the recording layer 9 formed on the the disc substrate 8, thereby the optical disc of the fourth preferred embodiment shown is made without the inner spacer 1. In the optical disc of the fourth preferred embodiment, the bonding area between the flange body 15a and the disc substrates 3 or 4 and between the cylindrical body 14a and the disc substrates 3 or 4 are wider than the bonding area of the optical disc of the third preferred embodiment shown in Fig. 17, because the outer diameter of the flange body 15a is greater than the outer diameter of the flange portion 15 of the hub half 11 shown in Fig. 17. The flange portion 15 of the hub half 11, with such a large outer diameter as the flange body 15a formed by punching a plate and shown in Figs. 18 and 19, cannot be made, since the flange portion 15 of the hub half 11, e.g. whose thickness is 0.3 mm, is formed by a cutting process. Therefore, the first disc 3 can be bonded more strongly through the two flange bodies 15a to the second disc 4 than the optical disc shown in Fig. 17, the optical disc of the fourth preferred embodiment can be rotated at the high speed such as 1200 - 3600 rpm and it is possible to access to the optical disc at high speed rate. Fig. 18 is the sectional view of one of the optical discs of the fourth preferred embodiment. In Fig. 18, each of hub halves 11 and 12 is composed of a first cylindrical body 14a of a small diameter and a second cylindrical body 15a of a large diameter acting as the flange body 15a, The cylindrical bodies 14a and 15a are molded individually and they are bonded together to provide the hub halves 11 and 12 as shown in Fig. 20. There are defined, in the hub halves 11 and 12, a first hole 14x having a relatively large diameter enough to receive a standard pin of the recording and playing back device and a second hole 14y of a small diameter to provide a main cylindrical portion and a sub cylindrical portion. The end surface of the main cylindrical portion of the first cylindrical body 14a is bonded on the second cylindrical body 15a whose inner diameter is equal to the diameter of the hole 14y of the main cylindrical portion of the first cylindrical body The The outer diameter of the flange body 15a is larger than the outer diameter of the second cylindrical portion 15 of the hub half 11 shown in Fig. 17, thereby, the bonding area between the disc substrates 3 or 4 and the end surface of the second cylindrical body 15a is wider than the bonding area between the disc substrate 3 or 4 and the flange portion 15 of the cylindrical hub half 11 shown in Fig. 17. Also the bonding area of both second cylindrical bodies 15a shown in Fig. 18 is wider than the bonding area of the second cylindrical portions 15 of the hub half 11 shown in Fig. 17. Therefore, two disc substrates 5 and 8 can be bonded together strong through the flange bodies 15a and the outer spacer 2 as shown in Fig. 18. Figs. 19 and 22 show a modification of the hub half body of the optical disc of the fourth preferred embodiment. In Figs. 19 and 22, the cylindrical hub half body 14a comprises a main cylindrical portion and a sub cylindrical portion, wherein the outer diameter of the sub cylindrical portion is shorter than the outer diameter of the main cylindrical portion of the cylindrical body 14a and the inner diameter of the sub cylindrical body is equal to the inner diameter of the main cylindrical portion, and the thickness of the flange body 15a is equal to the axial -length of the-sub-cylindrical portion. The sub cylindrical portion is inserted into the central hole of the flange body 15a and the flange body 15a is bonded onto the cylindrical body lila, so that the inner surface of the flange body 15a faces the outer surface of the sub cylindrical portion of the cylindrical body lila, thereby the cylindrical hub half body 11 is constructed. Fig. 23 is a perspective sectional view of a screwed hub half body of the cylindrical body 14a and the flange body 15a of the optical disc shown in Fig. 19. The flange body 15a may be screwed and bonded on the cylindrical body 14a with one or more screws 90 as shown in Fig. 23, and also the flange body 15a may be only screwed on the cylindrical body lila. According to the fourth preferred embodiment shown in Figs. 18 and 19, the optical disc has the advantage that it is possible to adjust the length of the flange of the flange body 15a corresponding to the outer diameter minus the inner diameter of the flange body 15a, so that the optical disc has a required mechanical strength, whereby it can be accessed at high speed. The bonding recess 16 may be formed on the end surface of the flange portion 15 Or the cylindrical hub half 11 shown in Fig. 17, or the end surface of the flange body 15a shown in Figs. 18 and 19, which are facing each other. Fig. 24 shows an optical disc of a fifth preferred embodiment according to the present invention, comprising two cylindrical hub halves 11 and 12 having a cylindrical portion 14 and a flange portion 15 arranged on the end surface of the cylindrical portion 14. In Fig. 24, the end surface of the cylindrical portion 14 of the hub half 11, where the flange portion 15 is not formed, is bonded on the the end surface of the cylindrical portion of another hub half 12 and another inside surface of the flange portion 15 of the bylindrical hub halves 11 and 12 is bonded on the outer surface of the disc substrates 5 and 8, respectively. The outer surfaces of the cylindrical portions 14 of the hub halves 11 and 12 are bonded on the inner surfaces of the center holes of the disc substrates 5 and 8, respectively. As shown in Fig. 24, as the flange portion 15 of the cylindrical hub half 11 is arranged at the outside of the optical disc, an adapter arranged on the turn table is required when the optical disc is set on the turn table. However, the coefficient of friction between the turn table and the optical disc become bigger than the aforementioned optical disc such as the optical discs of the first, third and fourth preferred embodiments shown in Figs. 5, 17, 18 and 19, because the outside area of the hub half 11 shown in Fig. 24 is wider than the outside area of the hub half 11 shown Figs. 5, 17, 18 and 19. Therefore, the optical disc shown in Fig. 24 can be accessed at high speed. Fig. 25 shows an optical disc of a sixth preferred embodiment according to the present invention, comprising an inner spacer 1 and two cylindrical hub halves 11 and 12 without a flange portion. In Fig. 25, the outer surface of the two cylindrical hub halves 11 and 12 are bonded on the inner surface of the inner spacer 1, and the top and bottom surfaces of the inner spacer 1 are bonded on the disc substrates 5 and 8 respectively, thereby the inner spacer 1 acts as well as the flange portion 15 of the cylindrical hub half 11 of the optical discs of the third and fourth preferred embodiments. That is, bonding among the inner spacer 1, the disc substrates 5 and 8, and the cylindrical hub halves 11 and 12 can improve the bonding intensity between the disc substrates 5 and 8. The pregrooves and/or prepits in the optical disc for recording may be formed in a spiral or concentric shape.

Claims: WHAT IS CLAIMED IS: 1. An optical disc comprising a first disc comprising a first hub half having a cylindrical portion and a first disc substrate with a center hole, wherein the first hub half and the first disc substrate are constructed as one body by inserting the cylindrical portion of the first hub half into the center hole of the first disc substrate, and a second disc comprising a second hub half having a cylindrical portion and a second disc substrate with a center hole, wherein the second hub half and the second disc substrate are constructed as one body by inserting the cylindrical portion of the second hub half into the center hole of the second disc substrate, wherein a surface of the first hub half and a surface of the second hub half are connected, so that the first hub half and second hub half are connected as one body and the first disc faces the second disc. 2. The optical disc as fined in claim 1, wherein the first hub half of the first disc further comprises a flange portion arranged at the end of the cylindrical portion of the first hub half, the second hub half of the second disc further comprises a flange portion arranged at the end of the cylindrical portion of the second hub half, and the first hub half is connected to the second hub half as one body. 3. The optical disc as defined in claim 2, wherein the first and second hub halves comprise a cylindrical body and a flange body in an annular shape respectively, and the flange body is connected on the cylindrical body as one body. 4. The optical disc as defined in claim 3, wherein the flange body is connected on the cylindrical body with a bonding material. 5. The optical disc as defined in claim 2, wherein a recording layer is formed on at least one side surface of the first and second disc substrates. 6. The optical disc as defined in claims 2 or 5, wherein the flange potions of the first and second hub halves are constructed as one body between the first and second substrates so that the flange portion of the first hub half faces the flange portion of the second hub half. 7. The optical disc as defined in any of claims 2 through 6, wherein the flange portion of the first hub half is connected to the flange portion of the second hub half with a bonding material on the condition that the flange portion of the first hub half faces the flange portion of the second hub half. 8. The optical disc as defined in claim 6, wherein a bonding recess is formed on at least one end surface of the flange portions of the first and second hub halves, facing each other. 9. The optical disc as defined in claim 1, wherein the first disc substrate is connected to the first hub half with a bonding material, the second disc substrate is connected to the second hub half with a bonding material, an inner spacer is arranged between the first and second discs engaging in the outer surfaces of the cylindrical portions of the first and second hub half on the condition that the first disc substrate faces the second disc substrate, the inner spacer is connected to the first and second disc substrates with a bonding material, the inner spacer is connected to the outer surfaces of the first and second hub halves, thereby the first and second discs are connected as one body. 10. The optical disc as defined in claim 2, wherein the first disc substrate is connected to the first hub half with a bonding material, the second disc substrate is connected to the second hub half with a bonding material, an inner spacer is arranged at the outside of the flange portions of the first and second hub halves, the inner spacer is connected to the first and second disc substrates as one body with a bonding material, and the inner spacer is connected to the flange portions of the first and second hub halves as one body with a bonding material. 11. The optical disc as defined in claim 2, wherein the first disc substrate is connected to the first hub half with a bonding material, the second disc substrate is connected to the second hub half with a bonding material, an inner spacer is arranged at the outside of the wcXyTiXndeOcal-port f'the first and second hub halves, the inner spacer is connected to the first and second disc substrates as one body with a bonding material, and the inner spacer is connected to the cylindrical portions of the first and second hub halves as one body with a bonding material. 12. The optical disc as defined in claims 1, 2, or 9 through 11, wherein the first and second disc substrates are made of glass, and the first and second hub halves are made of an alloy of iron, nickel and chromium, containing nickel at 42 percent by weight and chromium at 6 percent by weight. 13. A process of making an optical disc comprising steps of arranging a hub half at the center of a disc substrate, and forming a recording layer having at least a tracking information on the disc substrate by using the center portion of the hub half as a reference point. 14. The process as defined in claim 13 further comprising steps of arranging a spindle projected from a surface of a stamper on the stamper having at least one groove arranged on the surface of the stamper, and positioning the disc substrate relative to the stamper by inserting tepindle into a center hole of the hub half. 15. A process of making an optical disc comprising steps of arranging a first hub half at the center of a first disc substrate, ,,,,, ,arra.nging a second hub half at the center of a second disc substrate, forming a first recording layer having at least a tracking information on the first disc substrate by using the center portion of the first hub half as a reference point, forming a second recording layer having at least a tracking information on the second disc substrate by using the center portion of the second hub half as a reference point, positioning the first and second substrates by using a center hole of the first hub half and center hole of the second hub half after making the first substrate face the second substrate so that the first recording layer faces the second recording layer, and constructing the first substrate with the first hub half and the second substrate with the second hub half as one body.

Inventors: GOTOH, AKIRA, HIGASIHARA, TOSIO, SUZUKI, MASAHIRO, TUBURAYA, YOSHITANE, YOSHIZAWA, KEN
Applicants: HITACHI MAXELL LTD.
Application year: 1986
Main classification: G11B7
IPC codes: G11B7