Mold for cast molding of substrate for information recording medium, method for preparing substrate for information recording medium and method for preparing information recording medium by use thereof

Abstract: A mold (3) for cast molding of a substrate for information recording medium (5) has an uneven preformat pattern, wherein said uneven preformat pattern formed on the mold surface has at least a pattern corresponding to tracking grooves (1) and information pits (2), each information pit portion has a size with a length b in the direction parallel to the pattern of the tracking groove (1) of 2 to 15 µm and a width a in the direction perpendicular thereto of 1 to 6 µm and a height c of 300 to 3500 Å, and said pit pattern has a pitch which is 1.8-fold or more of said pit length b . A method for preparing a substrate for information recording medium (5) having an uneven preformat uses a mold (3) for cast molding which is characterized in that an uneven preformat pattern corresponding to said uneven preformat formed on the mold surface has at least a pattern corresponding to tracking grooves (1) and information pits (2), each information pit portion (2) has a size with a length b in the direction parallel to the pattern of the tracking groove of 2 to 15 µm and a width a in the direction perpendicular thereto of 1 to 6 µm and a height c of 300 to 3500 Å, and said pit pattern has a pitch which is 1.8-fold or more of said pit length b . A method for preparing an information recording medium (5) comprises the following steps: the step of preparing a substrate for information recording medium (5) on which uneven preformat with at least tracking grooves (1) and information pits (2) is formed by use of a mold for cast molding (3), said mold for cast molding (3) having an uneven preformat pattern corresponding to said uneven preformat formed on the mold surface, the information pit portion (2) of said uneven preformat pattern having a size of a length b of 2 to 15 µm and a width a of 1 to 6 µm and a height c of 300 to 3500 Å, and said pit pattern having a pitch which is 1.8-fold or more of said pit length b ; and the step of forming a recording layer on said substrate for information recording meidum (2).

Description: This invention relates to a mold and to a method for cast molding of a subtrate for information recording medium, particularly to a mold for casting having a preformat pattern of information pit. In the prior art, a substrate for information recording medium has an uneven preformat such as tracking grooves or pits for information exhibiting address, etc. for recording and reproduction by an optical information recording and reproducing device formed on its information recording surface. As the method for forming such uneven preformat on the substrate for information recording medium, there have been known (1) the method in which an uneven preformat pattern of the stamper mold is transferred by the injection method or the hot press method, when the substrate comprises a thermoplastic resin or (2) the method according to the so called 2P method in which a photocurable resin composition is coated on a transparent resin plate comprising a thermosetting resin or a thermoplastic resin, then a stamper mold is adhered thereon, an energy such as UV-ray is imparted from the transparent resin side to cure the above photocurable resin composition, thereby transferring the replica of the stamper mold onto the transparent resin or (3) the so called cast molding method in which a monomer of a resin or a prepolymer containing a solvent is injected into a mold for cast molding in which an uneven preformat pattern corresponding to the uneven preformat is previously formed and cured to effect transfer of the pattern. However, the thermoplastic resin substrate obtained by the injection method or the hot press method of the above (1) suffers from warping of the substrate after molding or occurrence of optical anisotropy due to residual stress or orientation of molecules by the thermal hysteresis during molding, and therefore the problems still remain as the substrate for information recording medium. On the other hand, the substrate obtained by the 2P method of the above (2) causes frequently the problem that the polymerization initiator and the monomer used during photopolymerization are liable to remain in the resin after photopolymerization, and they affect the optical recording layer of the recording medium, thereby causing deterioration of the recording characteristis to occur. As the method for preparing a substrate for information recording medium free from these problems, there may be sometimes used the cast molding method of the above (3) as disclosed in Japanese Laid-open Patent Publication No. 61-92450. The substrate obtained by this method is substantially free from pressure applied during molding as compared with the methods of the above (1) and (2), and also as the resin for cast molding, all of thermoplastic resins, thermosetting resins, photocurable resins are available, and therefore it is possible to select a resin which does not have deleterious influence on the recording layer. However, the substrate obtained by the injection molding method had the problem that the tracking groove and the edge portion of the information pit are lacked during mold release. Such lacking of the substrate occurring during mold release may be considered to be due to the stress applied primarily on the edge portion through the adhesive force between the substrate and the mold for cast molding and thermal shrinkage of the substrate. Since this stress is liable to be applied on the edge portion of the information pit which is in the direction perpendicular to the tracking groove, because the tracking groove exists lengthy in the lateral direction of the substrate, lacking is liable to occur in the information pit rather than in the tracking groove. Fig. 3 shows a sectional plan view of a preformat on the substrate having the defect generated during cast molding by use of a mold for cast molding of the prior art as described above. Particularly, between the tracking grooves 1′ of the preformat, defects 8 such as formless lacking or crack which damage the pit shape at the edge portion of the information pit 2′ formed in parallel to said tracking groove 1′ are formed. Such defects may cause reading incapability or reading error during reproduction of the preformat signal, resulting in damage of reliability of the information recording medium. In other words, generation of defects of such lacking, etc. is a problem which is a great obstacle in preparing an information recording medium of high reliability. Particularly, when the yield of the substrate becomes 50% or less on account of such lacking, the production cost of the information recording medium will be also lowered. On the other hand, in such an information recording medium that the recording layer is formed by coating of a dye, etc. on a substrate having an uneven preformat, the recording medium is penetrated into the pit for preformat during coating, and particularly at the portion where the pits are provided approximate to each other, the thickness of the recording layer at the convexity between the information pits of the substrate (hereinafter called the land portion) becomes thinner. Also, since the reflected light quantity when a laser beam is irradiated is determined depending on the thickness of the recording layer, there has been involved the problem that no sufficient reflected light quantity can be obtained. Also, U.S. Patent 4,209,804 discloses a recording carrier which performs recording of recording information by varying the length of the information region. However, when such recording is performed, the pit length becomes longer and the land portion between the pits becomes shorter, whereby the land portion is very susceptible to be lacked during mold release in cast molding. The present invention has been accomplished in order to improve such problems of the prior art, and its object is to provide a mold for cast molding of a substrate for information recording medium which can reduce generation of defect during mold release of the substrate for information recording medium during preparation of a substrate for information recording medium having an uneven preformat pattern by cast molding method, and also can obtain good signal characteristics, and a method for preparing a substrate for information recording medium. Another object of the present invention is to provide a method for preparing an optical recording medium which is free from defect at the information pit portion and can reproduce the information pit at high contrast. More specifically, the mold for cast molding of the present invention is a mold according to claim 1. The method for preparing a substrate for information recording medium of the present invention is a method according to claim 3. Thus, according to the present invention, it may be considered that the stress caused by the heat shrinkage during curing of the cast molded resin and the adhesive force between the resin and the mold for casting applied on the edge portion of the information pit can be dispersed, and further by making the pit interval at least equal to 0,8 times the length of the pit length, concentration of the above-mentioned stress on the edge of the information pit portion can be inhibited, whereby defects such as a lack on the substrate, etc. can be inhibited. Also, by making the pit interval longer than 0,8 times the length of the pit length, reduction of the recording layer thickness at the land portion by inflow of the recording layer material into the pit portion when a recording layer is provided by coating can be made smaller, and therefore lowering in contrast around the information pit portion can be prevented. Fig. 1A is a partial plan view showing an embodiment of the mold for cast molding of a substrate for information recording medium of the present invention; Fig. 1B is a sectional view taken along the line Y-Y′ of the mold for cast molding in Fig. 1A; Fig. 2A is a partial plan view of the information recording medium according to the present invention; Fig. 2B is a schematic illustration showing the change in reflected light quantity when the information pit portion of the information recording medium shown in Fig. 2A is reproduced; Fig. 3 is a partial plan view showing the defect generated at the information pit portion of the substrate for information recording medium of the prior art. Fig. 4 shows the relationship between reflectance (%) and dye layer thickness (Å). DETAILED DESCRIPTION OF THE INVENTION The present invention is now described by referring to the drawings. Fig. 1A is a partial plan view showing an embodiment of the mold for cast molding of a substrate for information recording medium of the present invention, and Fig. 1B is a sectional view taken along the line Y-Y′. In the same Figure, the mold 3 for casting of the substrate for information recording medium of the present invention has an uneven preformat pattern with a convexity 2 corresponding to the information pit provided along the tracking groove between the convexities 1 which are the pattern corresponding to the tracking grooves provided in parallel. In the present invention, the size of individual information pit portions 2 forming the pattern of the information pit formed on the mold surface of the mold 3 for casting, namely the length b , the width a , the height c and the pitch of the information pattern, is defined within a specific range, whereby the above objects could be accomplished. However, in the present invention, the defect generation ratio should be preferably 50% or lower, particularly 20% or lower, and the contrast 0.25 or more, particularly 0.35 or more. In Fig. 1, when the size of the information pit portion is such that the length b in the direction parallel to the pattern 1 of the tracking groove is 2 to 15 µm, the width a in the direction perpendicular thereto is 1 to 6 µm, the height c is 300 to 3500 Å and the pitch d is 1.8-fold or more of the pit length b , particularly when b is, 3 to 12 µm, a is 2 to 4 µm, c is 1000 to 3000 Å, and the pitch is 2-fold or more of the pit length b , the defect generation ratio becomes 20% or less, and also when a recording layer is coated, the relationship between the film thickness of the pit portion and the film thickness of the land portion can be made optimum, whereby the contrast becomes also 0.35 or higher to give an optical recording medium further improved in reliability. The mold for cast molding of the present invention can be prepared by conventional photolithographic technique, for example, by forming a Cr film on a glass substrate to the height of a pattern, coating a photoresist thereon, and then performing exposure, development and etching. Next, the method for forming the substrate for information recording medium may be practiced by use of the mold for cast molding as described above according to conventional procedures of the cast molding method known in the art. As the material of the substrate for information recording medium may be used, for example, acrylic resins, epoxy resins, unsaturated polyesters, etc. By providing a recording layer on the uneven preformat surface of the substrate for information recording medium can be obtained which has an uneven preformat thus obtained, and adhering a protective substrate through an adhesive layer, an information recording medium can be obtained which has an uneven preformat with the information pit portion having a size of a length b of 1 to 15 µm, a width a of 1 to 6 µm, a height of 300 to 3500 Å and a pitch d of 2-fold or more of the information pit length b . As the recording layer for the present invention may be used, one which is varied in optical characteristics (e.g. refractive index, reflectance, transmission) by irradiation of recording light, and examples of such recording layer material may include organic dyes such as polymethine dyes, cyanine dyes, anthraquinone dyes, melocyanine dyes, transition metal complexes (e.g. diamine type metal complexes, dithiol type metal complexes), etc. or those incorporated into polymers, and further Bi, Te or chalcogenide compounds such as alloys of these, etc. Otherwise, magnetic films such as TbFeCo, GdFeCo, which perform recording by use of the magneto-optical effect, by reversing magnetization on the Kerr effect or the Faraday effect, can be also used. Such recording layer may be formed by coating (e.g. dip coating, spray coating, spinner coating, bar coating, blade coating, roll coating, curtain coating, etc.), or the gas phase deposition method such as vapor deposition, sputtering, etc. Particularly, in the present invention, even by inexpensive formation of recording layer by coating which can be prepared easily, reduction in film thickness at the land portion by inflow of the recording material into the information pit portion can be inhibited to give good contrast. More specifically, when 1,1,5,5-tetrakis-(p-diethylaminophenyl)-2, 4-pentadienyl-perchlorate is used as the recording layer material, the relationship between its film thickness and the reflectance is known to be as shown in Fig. 4. In this case, the film thickness of the land portion is generally coated to 900 to 1000 Å which exhibits the maximum reflectance. At this time, the coating is effected with inflow into the tracking groove, etc. being previously considered, and also it has been practiced to improve the contrast of tracking signals by increasing the film thickness of the concavity by utilizing the inflow by controlling the depth, width of the tracking groove, thereby lowering reflectance. However, when the information pit portion is provided continuously, much inflow of the recording material occurs into the information pit, tracking groove in the recording layer at the land portions therebetween, and the film thickness becomes thinner than the defined value to lower the contrast, whereby troubles may be sometimes given to reproduction of information pit. In the substrate for information recording medium according to the present invention, it can be inhibited and therefore the substrate for information recording medium according to the present invention can be used more preferably for the information recording medium of which recording layer is formed by coating. In the present invention, the uneven preformat pattern to be formed on the mold surface for cast molding corresponds to the information pit or tracking groove which becomes the positional information such as start pit or end pit as described above, and the unevenness relationship is inversed to the uneven preformat formed on the substrate surface for information recording medium. The pattern of the tracking groove may be specifically, for example, a spiral or concentric tracking groove for optical disc with a width of 0.5 µm to 2 µm and a pitch of 1.0 µm to 5 µm, or a pattern of tracking groove for parallel optical card with a width of 2 µm to 5 µm and a pitch of 8 µm to 15 µm. The optical card here refers to an information recording medium of portable size, having generally an area of 63.0 cm² or less, particularly 46.5 cm² or less, a thickness of 1.0 mm or less, particularly 0.85 mm or less, and a weight of 7 g or less, particularly 2 g to 6 g. As described above, according to the present invention, an information recording medium of high reliability can be obtained which can inhibit generation of defect or lack, particularly around information pit, of the uneven preformat of the substrate for information recording medium during mold release in preparing a substrate for information recording medium having an uneven preformat by cast molding method. Also, according to the present invention, an information recording medium which can reproduce information pit with high contrast can be obtained. [Examples] The present invention is described in more detail below by referring to Examples. However, the present invention is not limited to these Examples at all. Example 1 First, by use of a glass plate having a mirror surface with a thickness of 2.3 mm as the substrate, a chromium film was formed on said glass plate by the vacuum vapor deposition method. This thickness is a value taking into consideration the shrinkage of the resin for cast molding cured to the height of the preformat pattern. Next, on said chromium film was coated a posi-type photoresist (trade name: AZ-1350, manufactured by Hoechst Japan) by spin coating. Subsequently, the resist was subjected to exposure by photoirradiation through a mask formed correspondingly to the preformat pattern, and then developed with a developer (trade name: AZ-312, manufactured by Hoechst Japan) to have the chromium film beneath the photoresist exposed. The preformat pattern was made a preformat pattern corresponding to an optical card having a tracking groove with a width of 3 µm and a pitch of 12 µm and a rectangular information pit with a length b , a height c , a constant pitch and a width a which is variously varied as shown in Table 1. Next, the chromium film portion having the preformat pattern portion exposed thereon is etched with 6 N hydrochloric acid until the glass plate of the substrate was exposed, and then throughly washed, followed by drying. The remaining photoresist was then removed by ashing according to the oxygen plasma ashing method to form a preformat pattern. Similarly, 100 sheets each of the mold having the same pattern were prepared. By use of the resulting molds for cast molding, a glass plate of the same size of the mold having a mirror surface with a thicknes of 3 mm was placed thereon through a spacer with a thickness of 0.4 mm interposed therebetween to prepare a cast molding device. Into the cast molding device was injected as the liquid prepolymer a liquid acrylic resin having a formulated composition shown below. (Formulated composition) Methyl methacrylate 70 parts by weight t-Butyl methacrylate 25 parts by weight Polyethylene glycol dimethacrylate (M.W. 620) 5 parts by weight After the liquid resin was injected into the cast molding device, curing was effected by heating at 135 °C for 6 hours to obtain a substrate for optical card in a size of 85 mm x 54 mm. For each mold, a substrate for optical card was prepared in the same manner, and the ratio of defect generation on the substrate for information recording medium was examined to obtain the results shown in Table 1. The ratio of defect generation is a value of the number of the substrates on which one or more defects are generated divided by the total number of substrates and multiplied by 100. Examination of the defect on the substrate was conducted by observation with eyes and an optical microscope (x 200). Example 2 Molds for cast molding were prepared in the same manner as described in Example 1 except for changing the dimensions of the information pit portion to the constant values of width a and height c , and variously varied length b as shown in Table 2, and substrates for information recording medium were prepared by use thereof. The defect generation ratios at this time are shown in Table 2. Example 3 Molds for cast molding were prepared in the same manner as described in Example 1 except for changing the dimensions of the information pit portion to the constant values of width a , length b and pitch d , and variously varied height c as shown in Table 3, and substrates for information recording medium were prepared by use thereof. The defect generation ratios at this time are shown in Table 3. Example 4 Molds for cast molding were prepared in the same manner as described in Example 1 except for changing the dimensions of the information pit portion to the constant values of width a , length b and height c , and variously varied pitch d as shown in Table 4, and substrates for information recording medium were prepared by use thereof. The defect generation ratios at this time are shown in Table 4. Example 5 Next, on the substrates for optical card obtained in Examples 1 to 4, as the recording layer, a 2 wt.% diacetone alcohol solution 1,1,5,5-tetrakis(p-diethylaminophenyl)-2,4-pentadienyl perchlorate was coated by a gravure coater so that a film thickness of the land portion without information pit around the substrate is 1000 Å, and reproduction of the information pits without defect at this time was effected and the contrast of the signal was examined. However, here, since the value of the pit length b is deeply correlated with the reproduction device, the contrast when a, c and d were varied was observed. The respective results at this time are shown in Tables 1 to 4. Te contrast was obtained as described below. That is, the reflected light intensity when the information pit of the optical card prepared in Example 5 was irradiated with a laser beam for reproduction of a spot diameter of 3 µm and a power of 0.3 mW and scanned as shown in Fig. 2A is represented on an oscilloscope as shown in Fig. 2B. Vb/Va at this time is made the contrast. The values of contrast shown in Tables 1 to 4 are average values of contrast for 100 sheets of optical cards having the information pit of the same pit size. a b c d Defect generation ratio (%) Contrast 0.5 µm 3µm 1000Å 6 µm 75 0.20 1 3 1000 6 25 0.36 2 3 1000 6 15 0.39 3 3 1000 6 10 0.44 6 3 1000 6 5 0.31 a b c d Defect generation ratio (%) Contrast 2 µm 0.5 µm 1000Å 1 µm 20 0.16 2 1 1000 2 18 0.23 2 2 1000 4 17 0.30 2 3 1000 6 15 0.39 2 6 1000 12 14 0.42 2 12 1000 24 14 0.43 2 15 1000 30 12 0.43 a b c d Defect generation ratio (%) Contrast 2 µm 3 µm 200Å 6 µm 10 0.18 2 3 300 6 8 0.31 2 3 1000 6 15 0.39 2 3 3000 6 20 0.50 2 3 3500 6 40 0.48 2 3 4000 6 80 0.47 a b c d Defect generation ratio (%) Contrast 2 µm 3 µm 1000Å 4 µm 52 0.20 2 3 1000 5.0 30 0.24 2 3 1000 5.4 27 0.29 2 3 1000 6 15 0.39 2 3 1000 9 14 0.44 2 3 1000 12 15 0.43 From the above results of Examples 1 to 4, concerning defect generation ratio, the size of the pit for preformat should be preferably greater for a , is substantially irrelevant for b and preferably smaller for c . Also, when d is 1.8-fold or lower of the pit length b , defect generation ratio is higher. As to the contrast of the reproduced signal of the information pit when used for an information recording medium, if c is small, the difference in thickness between the pit portion and the portion therearound is small, whereby the contrast is not sufficient. On the other hand, with a value of c of 1000 A or more, the contrast becomes substantially unchanged. Concerning a , it is correlated with the beam diameter of light, and the contrast is high in the vicinity thereof. When d is 1.8-fold or less of the pit length, the contrast is low.

Claims: A mold for cast molding of a substrate for information recording medium having an uneven preformat pattern on the mold surface, said uneven preformat pattern having at least a pattern corresponding to tracking grooves (1) and a pattern corresponding to information pits (2), characterized in that each information pit portion (2) forming said pattern of information pits has a length b of 2 to 15 µm in the direction parallel to the pattern of the tracking grooves (1), a width a of 1 to 6 µm in the direction perpendicular thereto, and a height c of 300 to 3500 Å, and that said pit pattern (2) has a pitch d in the direction parallel to the pattern of the tracking grooves (1), said pitch d being at least 1.8 times as large as said pit length b . A mold for cast molding of a substrate for information recording medium according to claim 1, characterized in that said uneven preformat pattern formed on the mold surface has at least a pattern corresponding to tracking grooves (1) and information pits (2), each information pit portion (2) has a size with a length b in the direction parallel to the pattern of the tracking groove (1) of 3 to 12 µm and a width a in the direction perpendicular thereto of 2 to 4 µm and a height c of 1000 to 3000 Å, and said pit pattern (2) has a pitch width which is 2-fold or more of said pit length b . A method for preparing an information recording medium characterized by the following steps: the first step of preparing a substrate for said information recording medium on which uneven preformat with at least tracking grooves (1) and information pits (2) is formed by use of a mold for cast molding (3), said mold for cast molding (3) having an uneven preformat pattern corresponding to said uneven preformat formed on the mold surface, the information pit portion (2) of said uneven preformat pattern having a size of a length b of 2 to 15 µm in the direction parallel to the pattern of the tracking grooves (1), a width a of 1 to 6 µm in the direction perpendicular thereto, and a height c of 300 to 3500 Å, and said pit pattern (2) having a pitch d being at least 1.8 times as large as said pit length b ; and the second step of forming a recording layer by applying a material for said recording layer on said uneven preformat surface of the substrate for information recording medium which has been obtained by the first step. A method for preparing an information recording medium according to claim 3, said mold (3) being characterized in that said information pit portion (2) of said uneven preformat pattern has a size, a length b of 3 to 12 µm, a width a of 2 to 4 µm and a height c of 1000 to 3000 Å, and the pitch d is 2-fold or more of the pit length b . A method for preparing an information recording medium according to claim 3, characterized in that said recording layer is formed by coating. A method for preparing an information recording medium according to claim 5, characterized in that the material for said recording layer is an organic dye. A method for preparing an information recording medium according to claim 3, characterized in that said information recording medium is an optical card.

Inventors: YASHIMA MASATAKA, YASHIMA, MASATAKA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: B29D17
IPC codes: G11B7, B29D17, B29C39, B29C33