Master plate for optical memory device and manfacturing method thereof

Abstract: A master plate of an optical memory device has a substrate having a plurality of guiding grooves on a surface thereof with a predetermined interval. The respective edge portions of the guiding grooves are inclined with respect to the surface of the substrate at an angle not more than 30°. According to a manufacturing method of the master plate, the post-baking process is carried out at a baking temperature of not less than 150°C and an etching gas containing O₂ gas of 4 (SCCM) to 10 (SCCM) is used. Thus, even when the copying is carried out with respect to the substrate of the optical memory device according to the injection molding process wherein the stamper, which is manufactured based on the above-mentioned master plate, is used, the mold release of the stamper can be carried out without being caught by the substrate. Thus, it is possible to keep the respective shapes of the guiding grooves of the stamper, thereby enabling of manufacturing the substrate of the optical memory device with high repeatability.

Description: FIELD OF THE INVENTION The present invention relates to a master plate for an optical memory device for manufacturing the optical memory device to or from which recording and reproducing of information are optically carried out and also relates to a manufacturing method of the master plate for the optical memory device. BACKGROUND OF THE INVENTION An optical memory device has been focussed on since it is a memory device having high density and large capacity. The reason why the optical memory device has the high density and large capacity is that a bit, which is a recording unit for information, is restricted only to a light beam diameter and thus the light beam can be set to have a size of about 1 micron. However, this gives many restrictions to the optical memory device. More specifically, the light beam should be positioned on the optical memory device with a high accuracy in order to record information on a predetermined recording area and in order to reproduce recorded information from a predetermined recording area. In general, an optical memory device, which is exclusively used for reproducing of information, can be arranged such that address information is in advance contained in a recorded bit. Therefore, it is possible to position the light beam with the reproducing of recorded information. But, it is impossible to carry out recording of information on a write once-type memory or a rewritable memory so as to contain the address information therein during the recording of information. Accordingly, in the write once-type memory or rewritable memory, there is employed some guiding signal and guiding address which are in advance contained in a memory substrate of the write once-type memory or rewritable memory. For example, the following is a general procedure: the memory substrate of the write once-type memory or rewritable memory is provided with grooves of physical protrusions and recessions; and recording and reproducing of information are carried out along the grooves. The grooves of the physical protrusions and recessions are discrete in a circumference direction of the respective memories, thereby presenting pit information indicative of each groove's address. When the substrate of the optical memory device is made of polycarbonate, the injection molding process, wherein a stamper having a pattern such as guiding grooves is used, is widely employed. The following deals with a conventional method for manufacturing the stamper with reference to Fig. 3. First, a master plate 11 having a pattern such as guiding grooves is prepared (see Fig. 3(a)). The master plate 11 is coated with an electrode member 14 such as Ni, the electrode member 14 being a thin film having a thickness of the range between 20 nm and 100 nm (see Fig. 3(b)). A Ni electrocasted thin film 15, having a thickness of the range between 200 µm and 400 µm is formed on the electrode member 14 (see Fig. 3(c)) by the electrocasting. Then, the Ni electrocasted thin film 15 is removed from the master plate 11. Thereafter, the other side of the Ni electrocasted thin film 15 is polished (see Fig. 3(d)), thereby finishing the manufacturing of the stamper. As mentioned above, the pattern such as the guiding grooves of the master plate 11 is exactly copied to the stamper so as to reversely form a pattern such as the guiding grooves on the stamper. A good master plate 11 should be manufactured in order to obtain a good stamper. The following two methods for manufacturing the master plate 11 have been widely known: (1) a method for forming a protrusion and recession pattern 52 on a glass substrate 51 by the use of the photo resist (see Fig. 4(a) ); and (2) a method for directly forming grooves of a protrusion and recession pattern on a glass substrate 51 according to the following steps, i.e., (a) making a mask by forming a pattern by the photo resist on the glass substrate 51, (b) carrying out the dry etching to the glass substrate 51 and thereafter (c) removing the photo resist (see Fig. 4(b)). When the master plate manufactured according to the method of Fig. 4(a) is observed by the STM (Scanning Tunnel Microscope), it is confirmed that there exist a plurality of minute physical protrusions and recessions on the surface of the protrusion and recession pattern 52 of the photo resist. In contrast, when the master plate manufactured according to the method of Fig. 4(b) is observed by the STM, it is confirmed that smooth pattern such as the guiding grooves is formed by removing the photo resist on which there exist the residual minute physical protrusions and recessions after the dry etching. Accordingly, the master plate made by the etching is more preferable than that by the photo resist in order to decrease the noise. The following conventional methods (1) to (3) are well known as the method for directly forming the guiding grooves and the like on the glass substrate. The following deals with the conventional methods (1) to (3) with reference to Figs. 5 through 7. According to the conventional method (1), guiding groove patterns 22 are formed on a glass substrate 21 by the photo resist (see Fig. 5(a)). After the photo resist, the dry etching is carried out by the use of a gas such as CF 4 , CHF 3 (see Fig. 5(b)). After the dry etching, the residual photo resist is removed (see Fig. 5(c)), thereby directly forming physical protrusions and recessions on the glass substrate 21. According to the conventional method (2), guiding groove patterns 32 are formed on a glass substrate 31 by the photo resist (see Fig. 6(a)). The guiding groove patterns 32 are baked at the temperature of more than the softening point of the photo resist (see Fig. 6(b)). Thereafter, the dry etching is carried out by the use of a gas such as CF 4 , CHF 3 (see Fig. 6(c)). After the dry etching, the residual photo resist is removed (see Fig. 6(d)), thereby directly forming physical protrusions and recessions on the glass substrate 31. According to the conventional method (3), guiding groove patterns 42 are formed on a glass substrate 41 by the photo resist (see Fig. 7(a)). The dry etching is carried out with respect to the guiding groove patterns 42 by the use of a gas such as CF 4 , CHF 3 with O 2 gas (see Fig. 7(b)). After the dry etching, the residual photo resist is removed (see Fig. 7(c)), thereby directly forming physical protrusions and recessions on the glass substrate 41. In order to copy the shape of the guiding grooves of the stamper to the polycarbonate substrate according to the injection molding process, it is required that the mold release of the polycarbonate substrate from the stamper is carried out while each shape of the guiding grooves of the stamper is kept unchanged, the polycarbonate substrate being formed by carrying out the injection molding with respect to the stamper. However, according to the conventional method (1), each edge portion of the guiding grooves rises upward substantially perpendicularly. Therefore, the polycarbonate substrate is likely to be caught by the edge portion of the guiding groove when the mold release of the polycarbonate substrate from the stamper is carried out, thereby making it impossible for the polycarbonate substrate to keep the shape of the guiding groove unchanged. The object of the conventional methods (2) and (3) is to improve the reliability, endurability and other functions of the optical disk memory substrate by keeping the respective continuity of a recording medium, a protecting layer, and other materials which are formed on the guiding grooves. Therefore, it can be achieved by the conventional methods (2) and (3) that each edge portion of the stamper inclines so as to keep the continuity of the formed thin film. However, the conventional methods (2) and (3) do not ensure that each edge portion of the stamper greatly inclines so as to avoid being caught by the polycarbonate substrate during the mold release of the polycarbonate substrate from the stamper. More specifically, as shown in Fig. 8, it is assumed that the injection molding is carried out by the use of the stamper which is obtained by the etched master plate according to the conventional methods (1) to (3), the stamper being composed of the electrode member 14 and the Ni electrocasted thin film 15. When the mold release of a polycarbonate substrate 16 from the stamper is carried out in a direction of the arrow of Fig. 8, each edge portion of the guiding grooves is caught by the polycarbonate substrate 16. Accordingly, the guiding groove pattern on the polycarbonate substrate 16 is deformed (see the reference numerals 16a and 16b of Fig.8), thereby arising a problem, i.e., the deformed grooves are not appropriate for leading a light beam which is converged onto the substrate. EP-A-0 168 763 discloses the formation of a master recording disc having tapered-wall data pits whose walls have an optimum angle of inclination of 45°. This document is used for the delimitation of claims 1 and 7. SUMMARY OF THE INVENTION It is an object of the present invention to provide a master plate for an optical memory device that can form a stamper by which a substrate of the optical memory device can be obtained with repeatability. In one aspect, the present invention provides the master plate defined by claim 1. Claims 2 to 6 are directed to embodiments of the master plate of claim 1. According to the master plate of the invention, even when copying is carried out with respect to the substrate of the optical memory device using the injection molding process wherein the stamper, which is manufactured based on the above-mentioned master plate, is used, the mold release of the stamper can be carried out without being caught by the substrate. Thus, it is possible to keep the respective shapes of the guiding grooves of the stamper thereby enabling manufacture of the substrate of the optical memory device with high repeatability. It is another object of the present invention to provide a method suitable for manufacturing such a master plate for the optical memory device. Therefore, in another aspect, the present invention provides the manufacturing method defined by claim 7. Claims 8 to 14 are directed to embodiments of the manufacturing method of claim 7. According to the manufacturing method of the invention, by baking the resist thin film at a temperature of more than the deforming temperature of the photo resist during the post-baking process, side portions of the resist thin film are inclined with respect to the surface of the substrate. Further, by simultaneously carrying out etching of the resist thin film having the above-mentioned shape in the forming process of the guiding grooves on the substrate during the etching process, the master plate of the optical memory device can be manufactured wherein the guiding grooves are formed such that each edge portion is inclined with respect to the substrate at an angle of not more than 30°. Accordingly, even when copying is carried out with respect to the substrate of the optical memory device using the injection molding process wherein the stamper, which is manufactured based on the above-mentioned master plate, is used, the mold release of the stamper can be carried out without being caught by the substrate. Thus, it is possible to keep the respective shapes of the guiding grooves of the stamper, thereby enabling manufacture of the substrate of the optical memory device with high repeatability. In further aspects, the present invention provides the stamper defined by claim 15 and the optical memory device substrate defined by claim 17. Claim 16 is directed to an embodiment of the stamper of claim 15. Claim 18 is directed to an embodiment of the substrate of claim 17. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an explanatory diagram showing the main steps in a manufacturing method of a master plate for an optical memory device in accordance with the present invention; Fig. 2 is an explanatory diagram showing the condition where the copying is carried out with respect to a polycarbonate substrate according to the injection molding process wherein the stamper, which is manufactured based on the above-mentioned master plate, is used, and thereafter the mold release of the polycarbonate substrate is carried out; Fig. 3 is an explanatory diagram showing the difference between a conventional etched master plate and a conventional photo resisted master plate; Fig. 4 is an explanatory diagram showing the step wherein the stamper is produced from the etched master plate according to the conventional manufacturing method; Fig. 5 is an explanatory diagram showing one example of the conventional manufacturing methods of the optical memory device; Fig. 6 is an explanatory diagram showing another example of the conventional manufacturing methods of the optical memory device; Fig. 7 is an explanatory diagram showing a still another example of the conventional manufacturing methods of the optical memory device; and Fig. 8 is an explanatory diagram showing the condition where the copying is carried out with respect to a polycarbonate substrate according to the injection molding process wherein the stamper, which is manufactured based on the conventional master plate, is used, and thereafter the mold release of the polycarbonate substrate is carried out. DESCRIPTION OF THE EMBODIMENTS The following description deals with one preferred embodiment of the present invention with reference to Figures 1 and 2. The following deals with a manufacturing method of a master plate for an optical memory device in accordance with the present embodiment with reference to Fig. 1. More specifically, a substrate 1 is first coated with a photo resist having a thickness of the range between 200 nm and 400 nm. The photo resist is of a positive type such as AZ-1400 (trade name) which is produced by "SHIPLEY CO. INC.". The coating is made according to a method such as the spin coating method. The substrate 1 is made of glass or quartz. After the formation of the photo resist thin film, the photo resist thin film is exposed, by the laser cutting method or the contact printing method, so as to have patterns such as guiding grooves, and thereafter is developed. Fig. 1(a) shows photo resist patterns 2 to which the development has been made. The developed photo resist pattern 2, as shown in Fig. 1(a), are defined such that respective edge portions 3 of the photo resist patterns 2 rise upward substantially perpendicularly with respect to the surface of the substrate 1. In that case, the photo resist patterns 2 are disposed spirally or concentrically such that the pitch between the photo resist patterns 2 is 1.6 µm. In case where information is recorded on respective bottom portions of the guiding grooves, the respective photo resist patterns 2 are formed such that each width is substantially 0.6 µm. On the other hand, in case where information is recorded on respective land portions of the guiding grooves, the respective photo resist patterns 2 are formed such that each width is substantially 1.0 µm. Next, post-baking is carried out with respect to the developed photo resist patterns 2 at about 150 C for 30 minutes. In that case, the post-baked photo resist patterns 2 become softened to be fluidized. Thus, the post-baked photo resist patterns 2 change their shapes like Fig. 1(b) due to the surface tension exerted on the substrate 1. Note that the temperature of the post-baking is preferably a temperature of not less than that at which the photo resist patterns soften. Then, the reactive ion etching (hereinbelow referred to as dry etching) is carried out for five minutes under the conditions where the etching gas pressure of 10 m Torr is introduced and the high frequency output power of 400 Watt is applied. Note that CF4 gas is used during the dry etching process as the etching gas to be introduced and this CF 4 gas s contains a predetermined amount of O 2 gas. According to the present embodiment, the flow rate of CF 4 gas is about 10 (SCCM) and the flow rate of O 2 gas falls in the range between 4 (SCCM) and 10 (SCCM). Fig. 1(c) shows the condition after the dry etching process. Respective edge portions of the guiding grooves are greatly inclined with respect to the surface of the substrate 1 according to Fig. 1(c). Note that the depths of the guiding grooves fall in the range between 50 nm and 80 nm. Note also that CHF3 gas is preferably used as the etching gas. More specifically, each edge portion 3 is greatly inclined with respect to the surface of the substrate 1 based on the fact that (1) before the dry etching process is carried out (see Fig. 1(b)), the photo resist patterns 2 have already been inclined with respect to the surface of the substrate 1 due to the softening of the photo resist patterns 2, and (2) isotropic etching is carried out with respect to the photo resist patterns 2 because a predetermined amount of O 2 gas is mixed during the dry etching process. At the final step, the residual photo resist is removed. Thus, as shown in Fig. 1(d), an etching master plate is obtained wherein inclined angle  of the edge portions 3 with respect to the surface of the substrate 1 falls in the range between 23° and 27°. The inclined angle  is hereinbelow referred to as an edge angle . A stamper of Fig. 2 is provided according to the etched master plate manufactured by the above-mentioned manner. The stamper is manufactured according to the following steps. More specifically, the etched master plate is first prepared and is coated with an electrode member 4 such as Ni, the electrode member 4 being a thin film having a thickness of the range between 20 nm to 100 nm. A Ni electrocasted thin film 5, having a thickness of the range between 200 µm and 400 µm, is formed on the electrode member 4. The Ni electrocasted thin film 5 is removed from the master plate and the other surface of the Ni electrocasted thin film 5 is polished, thereby manufacturing the stamper. The following deals with the comparison between the edge angle of the stamper in accordance with the present embodiment and that in accordance with the earlier mentioned conventional methods (1) to (3) with reference to Table 1. As is clear from Table 1, the respective stampers which are manufactured by the master plates based on the conventional methods (1) through (3) have their great edge angles  , i.e., 49° to 83°. Accordingly, the edge portions of the polycarbonate substrate are caught by the injection molded polycarbonate substrate when the mold release of the stamper from the injection molded polycarbonate substrate is carried out, thereby deforming the corresponding portions of the polycarbonate substrate. Note that according to the conventional methods of Table 1, edge angle  is greater in the case where O 2 gas is not mixed than in the case where O 2 gas is mixed during the dry etching process. Further, as is clear from Table 1, even when the master plates are made based on the same conventional method (3), the more amount of O 2 gas is mixed, the smaller are the respective edge angles  of the provided stampers. According to the comparative example of Table 1, the flow rate of O 2 gas is set to be 2 (SCCM), but the temperature of the post-baking and the flow rate of CF 4 gas are the same as those of the present embodiment. Even the comparative example, edge angle  is 42°. Therefore, the edge portions of the polycarbonate substrate are caught by the stamper when the mold release of the injection molded polycarbonate substrate from the stamper is carried out, thereby deforming the corresponding portions of the polycarbonate substrate (see reference numerals 16a and 16b of Fig. 8). In contrast, the stampers manufactured by the master plate based on the method of the present embodiment have edge angle  which is less than 30°, i.e., 23° to 27°, the stampers being manufactured under the conditions where the temperature of the post-baking process is set to be about 150 °C and the flow rate of O 2 gas is set to be at least 4 (SCCM). Accordingly, no edge portion of the polycarbonate substrate 6 is caught (see Fig. 2) by the stamper when the mold release of the injection molded polycarbonate substrate from the stamper, which is composed of the electrode member 4 and the Ni electrocasted thin film 5, is carried out in the direction of the arrow of Fig. 2. Thus, the respective shapes of the guiding grooves of the stamper can be kept unchanged, thereby enabling to obtain the substrates for use in the optical memory device with repeatability. As mentioned above, according to Table 1, the required flow rate of O 2 gas to be mixed during the dry etching process is at least 4 (SCCM) and the required temperature of the post-baking process is about 150 °C. The resisted thin film softens so as to be fluidized since the temperature of the post-baking process is set to be 150 °C, thereby making the side portions of the post-baked resist thin film be inclined with respect to the substrate surface. In the process of the formation of the guiding grooves on the substrate surface during the dry etching process, the etching of the resist thin film having a shape like above is simultaneously carried out, thereby making the respective edge portions of the guiding grooves greatly be inclined with respect to the substrate surface. Thus, the master plate of the optical memory device having the inclined angle of not more than 30° can be manufactured. The master plate of the optical memory device in accordance with the present invention, as mentioned above, is arranged such that the respective edge portions of the guiding grooves are greatly inclined with respect to the substrate surface and the inclined angles with respect to the substrate surface can fall within 30°. Therefore, even when the copying is carried out with respect to the substrate of the optical memory device according to the injection molding process wherein the stamper, which is manufactured based on the master plate of the optical memory device in accordace with the present invention, is used, the mold release of the stamper can be carried out without being caught by the substrate. Thus, it is possible to keep the respective shapes of the guiding grooves of the stamper, thereby enabling manufacture of the substrate of the optical memory device with high repeatability. The manufacturing method of the master plate of the optical memory comprises the following steps: (1) coating a substrate with photo resist thin film; (2) exposing guiding groove patterns of the master plate of the optical memory device by projecting light onto the photo resist thin film; (3) developing the resist thin film having the exposed guiding groove patterns; (4) post-baking the developed resist thin film at the temperature which causes the resist thin film to be deformed; (5) carrying out dry etching after the post-baking by the use of mixed gas including an etching gas for the substrate and an etching gas for the resist thin film; and (6) removing the resist thin film after the dry etching process. Therefore, the side portions of the resist thin film are inclined due to the deformation thereof during the post-baking process. The respective edge portions of the guiding grooves are greatly inclined with respect to the substrate surface during the dry etching process since the isotropic etching is carried out with respect to the photo resist thin film having the above-mentioned shape. Accordingly, the master plate of the optical memory device having the inclined angle which is not more than 30° can be manufactured. Moreover, even when the copying is carried out with respect to the substrate of the optical memory device according to the injection molding process wherein the stamper, which is manufactured based on the master plate of the optical memory device in accordace with the present invention, is used, the mold release of the stamper can be carried out without being caught by the substrate. Thus, it is possible to keep the respective shapes of the guiding grooves of the stamper, thereby enabling manufacture of the substrate of the optical memory device with high repeatability. POST-BAKE TEMP.(°C) CF 4 FLOW (SCCM) O 2 FLOW (SCCM) EMPLOYED METHOD  (°) EDGE IS CAUGHT ? 100 10 0 METHOD (1) 83 YES 10 2 METHOD (3) 74 YES 10 4 METHOD (3) 70 YES 10 6 METHOD (3) 67 YES 10 8 METHOD (3) 64 YES 10 10 METHOD (3) 62 YES 150 10 0 METHOD (2) 49 YES 10 2 COMPARATIVE EMBODIMENT 42 A LITTLE BIT 10 4 PRESENT EMBODIMENT 27 NO 10 6 PRESENT EMBODIMENT 26 NO 10 8 PRESENT EMBODIMENT 24 NO 10 10 PRESENT EMBODIMENT 23 NO Note that methods (1) to (3) of Table 1 correspond to conventional methods respectively and  indicates an edge angle.

Claims: A master plate for an optical memory device, the master plate comprising a substrate having a plurality of guiding grooves on a surface thereof, the guiding grooves being spaced at a predetermined interval, each guiding groove having edge portions which are inclined with respect to the surface of said substrate, characterized in that the inclination angles of said edge portions with respect to the surface of said substrate are not more than 30°. A master plate according to claim 1, wherein the inclination angles of said edge portions with respect to the surface of said substrate fall in the range of 23° to 27°. A master plate according to claim 1 or claim 2, wherein said substrate is of glass. A master plate according to claim 1 or claim 2, wherein said substrate is of quartz. A master plate according to any preceding claim, wherein said guiding grooves are disposed spirally or concentrically. A master plate according to any preceding claim, wherein said guiding grooves have depths in the range of 50 nm to 80 nm. A method of manufacturing a master plate for an optical memory device, the method comprising: (a) coating a substrate with a photo resist thin film; (b) exposing guiding groove patterns to be formed on the master plate, by projecting light onto the photo resist thin film; (c) developing the photo resist thin film having the exposed guiding groove patterns; (d) post-baking the developed photo resist thin film at a temperature which causes the photo resist thin film to be deformed; (e) carrying out dry etching after the post-baking; and (f) removing the photo resist thin film after the dry etching process; characterized in that: (g) in step (e), the dry etching is carried out using a mixed gas including a first etching gas for the substrate and a second etching gas for the photo resist thin film; and (h) the guiding grooves are formed in the substrate such that the edge portions thereof are inclined with respect to the substrate at an angle of not more than 30°. A manufacturing method according to claim 7, wherein the photo resist thin film is formed such that the substrate is coated with a positive-type photo resist having a thickness of 200 nm to 400 nm using the spin coating method. A manufacturing method according to claim 7 or claim 8, wherein step (d) comprises the step of inclining side portions of the photo resist thin film with respect to the surface of the substrate, and step (e) comprises simultaneously forming the guiding grooves on the surface of the substrate and etching the photo resist thin film. A manufacturing method according to claim 9, wherein the post-baking of step (d) is carried out at a temperature of about 150°C for 30 minutes. A manufacturing method according to claim 9 or claim 10, wherein the dry etching of step (e) is carried out using reactive ion etching. A manufacturing method according to claim 11, wherein the first etching gas of step (e) is selected from the group consisting of CF 4 gas and CHF 3 gas, and the second etching gas of step (e) is O 2 gas. A manufacturing method according to claim 12, wherein the mixed gas of step (e) includes CF 4 and O 2 gases, and the mixing ratio of the CF 4 and O 2 gases falls in the range of 1:0.4 to 1:1. A manufacturing method according to claim 11, wherein the reactive ion etching is carried out under the conditions where a flow rate of CF 4 gas is about 10 SCCM, a flow rate of O 2 gas falls in the range of 4 SCCM to 10 SCCM, an etching gas pressure is about 10 m Torr, and the applied high frequency output power is about 400 Watt. A stamper for the manufacture by injection molding of a substrate of an optical memory device, the substrate to be manufactured having one or more guide grooves, characterized in that the stamper has a surface configuration such that said one or more guide grooves of the substrate are produced with edges which are inclined relative to the substrate surface by an angle of no more than 30°. A stamper according to claim 15, comprising an electrocasted Ni thin film. An optical memory device substrate having one or more guide grooves formed in at least one surface thereof, characterized in that said one or more guide grooves have edges which are inclined relative to the substrate surface by an angle of no more than 30°. An optical memory device substrate according to claim 17, made of injection-molded polycarbonate.

Inventors: HIROKANE JUNJI, KATAYAMA HIROYUKI, MIEDA MICHINOBU, NAKAYAMA JUNICHIRO, OHTA KENJI, HIROKANE, JUNJI, KATAYAMA, HIROYUKI, MIEDA, MICHINOBU, NAKAYAMA, JUNICHIRO, OHTA, KENJI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G11B7
IPC codes: G11B7, G01Q80