Process for producing substrate for optical disk by annealing substrate with gradient double refraction distribution

Abstract: In a process for producing a substrate for optical disks, first a disk-shaped substrate is formed with polycarbonate with such gradient distribution of double refraction that the value of double refraction is in a range from -10 to -30 nm in the vicinity of the external periphery of the substrate and is in a range from +10 to +30 nm in the vicinity of the center thereof, and then thus formed substrate is annealed.

Description: BACKGROUND OF THE INVENTION The present invention relates to a process for producing a substrate for an optical disk, comprising a forming step in which a disk-shaped polycarbonate substrate is formed by injection molding plastic material with a mold in such a manner that the absolute value of double refraction of the substrate is less than 30 nm, yet the sign of the double refraction of the substrate near its external periphery is opposite to the sign of the double refraction of the substrate near its center. Such a process is described in EP-A-0.155.452. As is realized by this publication, one important parameter in an optical disk is the value of double refraction: this value should be low, otherwise it becomes difficult to read the information written on the disk. It is for this reason that said publication aims to provide a substrate which is such prepared that the value of double refraction is low over the entire surface of the substrate when it is completed. However, the substrate is only an intermediate product in the fabrication of an optical disk. In order to prepare an optical disk from the substrate, further processing steps are made, such as the formation of a reflective film on a surface of the substrate, which involve exposing the substrate to high temperature. Due to such high temperature exposure, a conventional optical disk can show considerable double refraction in the vicinity of the external periphery and in an area somewhat distant said from external periphery, even if the substrate would be completely free from double refraction after its preparation. Factors such as transportation or extended storage can cause the same. OBJECT OF THE INVENTION The object of the present invention is to avoid the above-mentioned drawbacks in the prior technology. More specifically, the present invention aims to provide a substrate for an optical disk having a low value of double refraction, even after heating or prolonged storage. SUMMARY OF THE INVENTION The above-mentioned object can be achieved, according to the invention, by a process of the above-indicated kind, characterized in that after said forming step is completed the substrate thus formed is annealed; and that said forming step is effected in such a manner that, after said forming step is completed and before said annealing step is conducted, when considering the value of double refraction positive in a compression stress state in the radial direction of the substrate and negative in a tension stress state in the radial direction of the substrate, the value of double refraction of the substrate near its external periphery is in a range from -10 to -30 nm and the value of double refraction of the substrate near its center is in a range from +10 to +30 nm. The above-mentioned values of double refraction are usually defined in positions corresponding to the information recording area. Therefore, the above-mentioned molding of the substrate is made in such a manner that the value of double refraction is in a range from -10 to -30 nm at the external periphery of the information recording area and in a range from +10 to +30 nm at the internal periphery of said area. The magnitude of the double refraction is measured by a polarization microscope and is considered positive or negative, respectively, in a compression stress state or a tension stress state, respectively, in the radial direction of the substrate. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of an ordinary optical disk; Figs. 2A to 2C are schematic cross-sectional views showing steps of the process of the present invention for producing the optical disk substrate; Figs. 3A to 3C are charts showing the change in the distribution of double refraction in the optical disk substrate produced by the process of the present invention; and Figs. 4A and 4B are charts showing the change in the distribution of double refraction in the optical disk substrate produced by a conventional process. Related Background Art As schematically shown by a cross-sectional view in Fig. 1, an optical disk is prepared by forming, on a face of a substrate 1 of a disk form or doughnut form (in this text a disk form in which a central concentric portion is removed), a recording surface 2 on which information is recorded or which is capable of recording information. In case of a medium of read-only memory (ROM) type, such as the video disk, said information recording surface 2 can be formed by forming a train of pits representing information on the surface of the substrate and depositing thereon a reflective film composed for example of aluminum. In case of a DRAW (direct-read-after-write) type medium such as a magnetooptical disk, said information recording surface 2 can be formed by forming a recording layer, capable of optical information recording and reproduction, on the substrate. In this case spiral or concentric guide grooves are often formed on the surface of the substrate. Such recording surface 2 is usually formed in an annular area of a predetermined width on the substrate. The information recorded on an optical disk is read by irradiating the disk with a light beam from a side opposite to said recording surface, and converting the change in intensity of the light beam, which is transmitted in the substrate and reflected by the recording surface, into an electrical signal. In an optical disk it is an important target to reduce the loss in said intensity change, since a large double refraction in the light-transmitting member will reduce the intensity of light emerging from the disk, thus rendering the signal reading more difficult. Consequently in the conventional process, the substrate for optical disk has been so prepared that the double refraction is substantially zero over the entire surface when the substrate is completed. However, the substrate prepared in this manner has been associated with a drawback, even if it is free from double refraction immediately after the preparation of showing considerable double refraction in the vicinity of the external periphery and in an area somewhat distant from said external periphery, as the result of exposure to high temperature in the formation of the reflective film thereon or in the transportation of the optical disk, or as the result of extended storage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 2A to 2C illustrate the process of producing an optical disk substrate according to the present invention. At first, as shown in Fig. 2A, a substrate 1 is injection molded by means of molds 3, 4. In this operation the temperature of the molds is elevated to 110 - 130°C or selected higher by 10 - 20°C at the internal periphery than at the external periphery, in order that the substrate after molding has a double refraction in a range of -10 to -30 nm in the vicinity of the external periphery of the recording area and in a range of +10 to +30 nm in the vicinity of the internal periphery thereof. Otherwise the injection speed may be gradually increased from the internal periphery toward the external periphery. The double refraction should remain almost constant on any concentric circle, and, more specifically, the fluctuation on a concentric circle should not exceed 5 nm. Then the substrate is taken out from the molds 3, 4 as shown in Fig. 2B, and is annealed by heating for example in a drying oven 5 as shown in Fig. 2C. The heating is preferably conducted at a temperature lower by 10 - 60°C than the glass transition temperature of the plastic substrate, for a period of 1 to 6 hours, but these conditions are suitably regulated according to the plastic material employed and the thickness of the substrate. The substrate after annealing shows reduced double refraction at the external and internal peripheries, thus exhibiting limited double refraction over the entire surface. An optical disk is prepared by forming a recording layer, a reflective layer, a protective layer etc. on thus molded substrate. This substrate scarcely shows increase in the double refraction even by heating at the formation of the recording layer etc. or even after prolonged storage. In the following there will be given an example of the present invention and a reference example. Example Polycarbonate resin AD5503 (glass transition point 150°C) supplied by Teijin Kasei Co., Ltd. was injection molded with a 75-ton injection molding machine of Sumitomo Heavy Machinery Co., Ltd. to obtain a doughnut-shaped plastic substrate with an external diameter OD = 150 mm⊘, an internal diameter ID = 15 mm⊘ (having a central concentric hole) and a thickness t = 1.2 mm. This plastic substrate has an annular recording area from 25 mm to 64 mm in the radius R from the center, and showed a gradient distribution of double refraction from a position at R = 25 mm to a position at R = 64 mm. Said gradient distribution is illustrated in Fig. 3A, which shows the double refraction in a single pass (double refraction for the light transmitted once through the substrate) of said substrate immediately after molding, wherein the abscissa indicates the distance R from the center of disk, while the ordinate indicates the single-pass double refraction Δn. As shown in this chart, the double refraction shows a gradient distribution from +20 nm at a position on a circle at R = 25 mm to -20 nm on a circle at R = 64 mm. In the present invention, the double refraction preferably decreases gradually from the internal periphery of the recording area toward the external periphery and falls rapidly to the negative range in the vicinity of the external periphery, as shown in Fig. 3A. Said plastic substrate was then annealed (6 hours in an oven of 90°C) to obtain an optical disk substrate. Fig. 3B shows the double refraction of said substrate after annealing. In this manner the range of double refraction, which was as large as ±20 nm, could be reduced to ±10 nm by suitable heat treatment. Fig. 3C shows the double refraction of said substrate after it was again placed in an oven of 90°C for 6 hours. It is evident that the double refraction, after suitable heat treatment, no longer changes by repeated heat application. Reference Example A similar plastic substrate was prepared by injection molding in the same manner as in the foregoing example, except that the double refraction was maintained low over the entire surface according to the conventional process, as shown in Fig. 4A. Fig. 4B shows the double refraction obtained after said plastic substrate was subjected to heat treatment of 6 hours at 90°C. As will be apparent from Fig. 4B, the substrated prepared in the conventional process shows an area of undesirably high double refraction after heating. The present invention is not limited to the foregoing embodiment but can be used in various applications. For example it is applicable in the production of optical disks in which information is detected by the light transmitted by the recording surface. The present invention includes such modifications and applications, as long as it is within the scope and spirit of the appended claims.

Claims: A process for producing a substrate (1) for an optical disk, comprising a forming step in which a disk-shaped polycarbonate substrate is formed by injection molding plastic material with a mold (3, 4) in such a manner that the absolute value of double refraction of the substrate is less than 30 nm, yet the sign of the double refraction of the substrate near its external periphery is opposite to the sign of the double refraction of the substrate near its center; characterized in : that after said forming step is completed the substrate thus formed is annealed; and that said forming step is effected in such a manner that, after said forming step is completed and before said annealing step is conducted, when considering the value of double refraction positive in a compression stress state in the radial direction of the substrate and negative in a tension stress state in the radial direction of the substrate, the value of double refraction of the substrate near its external periphery is in a range from -10 to 30 nm and the value of double refraction of the substrate near its center is in a range from +10 to +30 nm. A process according to claim 1, characterized in that said annealing is conducted by heating said substrate at a temperature lower by 10 to 60 °C than the glass transition temperature of said substrate, for a period of 1 to 6 hours. A process according to claim 1, characterized in that said forming is conducted in such a manner that the fluctuation of double refraction on any concentric circle of the substrate does not exceed 5 nm. A process according to claim 1, characterized in that the temperature of molds used in said forming is in a range of 110 to 130 °C. A process according to claim 1, characterized in that the temperature of molds used in said forming is higher by 10 to 20 °C in an area thereof corresponding to the external periphery area of the substrate than in an area corresponding to the central area of the substrate. A process according to claim 1, characterized in that said forming is conducted by injection molding, of which injection speed is gradually increased from the central area of the substrate toward the external periphery area thereof.

Inventors: OKUDA TERUAKI, TAKADA KUNIO, OKUDA, TERUAKI, TAKADA, KUNIO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B7
IPC codes: B29C71, G11B7, B29C45, B29L11