Optical disc with corrosion-resistant metallic reflective layer

Abstract: An optical disc (1) for use as a compact disc, an optical video disc, or the like has a disc substrate (2) of a light-transmissive material such as polycarbonate, PMMA, or the like. The disc substrate (2) has a pattern (3) of pits and lands formed as representing an information signal on at least one surface thereof. A thin reflective layer (4) of a corrosion-resistant metallic material such as an alloy of Fe, Ni, and Co is deposited on the disc substrate over the pattern (3) of pits and lands. The optical disc (1) is relatively inexpensive to manufacture as no protective layer is deposited on the reflective layer (4) and is also highly resistant to aging because the reflective layer (4) is made of a corrosion-resistant metallic material.

Description: The invention relates to an optical disc. Such an optical disc can, for example, be used for storing various information signals such as musical tone signals, video signals, data signals, etc., which can optically be read out for reproduction. Figure 1 of the accompanying drawings shows one conventional read-only optical disc which stores various information signals such as musical tone signals, video signals, data signals, etc., which can optically be read out for reproduction. The optical disc, designated by the reference numeral 11, comprises a disc substrate 12 having a signal pattern 13 which has been formed as a pattern of pits and lands on one surface thereof according to an information signal, a thin reflective layer 14 deposited on the signal pattern 13, and a protective layer 15 deposited on the reflective layer 14 for protecting the same. The disc substrate 12 is injection-moulded of a light-transmissive synthetic resin such as polycarbonate, PMMA, or the like by an injection moulding machine with a stamper which has a pattern of pits and lands based on the information signal to be recorded on the optical disc 11. Specifically, the light-transmissive synthetic resin is injected into a cavity in the injection moulding machine, and the pattern of pits and lands on the stamper is transferred onto one surface of the cake of light-transmissive synthetic resin in the cavity, thereby forming the signal pattern 13 on the surface of the disc substrate 12. In another known optical disc fabrication process, a layer of ultraviolet-curing resin is coated on one surface of a flat glass substrate. Then, a stamper having a pattern of pits and lands according to an information signal to be recorded is pressed against the coated layer of ultraviolet-curing resin. While the stamper is being pressed against the coated layer of ultraviolet-curing resin, an ultraviolet radiation is applied to the layer of ultraviolet-curing resin through the glass substrate, thus curing the layer of ultraviolet-curing resin on the glass substrate. Thereafter, the stamper is detached from the cured layer of ultraviolet-curing resin. In this manner, the pattern of pits and lands on the stamper is replicated on the layer of ultraviolet-curing resin on the glass substrate, thus producing a disc substrate. Generally, the former disc substrate injection-moulded of light-transmissive synthetic resin is widely used because it is less costly to manufacture than the latter disc substrate with the layer of ultraviolet-curing resin. The thin reflective layer 14 is deposited on the signal pattern 13 of the disc substrate 12 which may be fabricated according to either the former or the latter fabrication process. The reflective layer 14 may be made of A1, Au, or the like, but is most commonly made of A1 due to its lower cost. Using an evaporation source of Al, the thin reflective layer 14 is formed by evaporation or sputtering over the signal pattern 13 on one surface of the disc substrate 12. Subsequently, the protective layer 15 is deposited on the reflective layer 14 to protect the same. The protective layer 15 is made of ultraviolet-curing resin. Specifically, the ultraviolet-curing resin is dropped in an annular pattern on a radially inner region of the reflective layer 14 on the protective layer 15, and then the disc substrate 15 is rotated at high speed by a rotary actuator to cause the dropped ultraviolet-curing resin to spread radially outwardly as a thin layer under centrifugal forces. The radially outwardly spread thin layer serves as the protective layer 15. The read-only optical disc, typically a compact disc, an optical video disc, or the like, is manufactured in the manner described above. One problem with the conventional optical disc 11 is as follows: Due to aging of the optical disc 11, air tends to find its way into the optical disc between the disc substrate 12 and the protective layer 15, and to oxidize the aluminum of the reflective layer 14 when held in contact therewith. When the aluminum of the reflective layer 14 is oxidized, it turns to aluminum oxide which is transparent, and the reflective layer 14 can no longer perform its function. In addition, since the reflective layer 14 of metallic material and the protective layer 15 of ultraviolet-curing resin do not adhere to each other with high bonding strength, the protective layer 15 is liable to peel easily from the reflective layer 14. WO82/01098 discloses the provision of a reflective or conductive layer of metals or alloys which are not readily oxidised or corroded, such layer being provided before the introduction of the information by embossing of pits is stored into the discs. The reflective layer is preferrably highly reflective. US-A-4 956 216 discloses use of a single layer of highly reflective corrosion resistant metal in combination with other rare substances. JP 58196641 discloses stainless steel as a reflective layer for a magneto-optical disk. Between the substrate and the stainless steel a rare earth element and a transition metal layer acting as a magneto-optical Kerr effect recording layer is provided together with a further transparent film. The stainless steel layer also increase the recording speed thanks to its low thermal conductivity and protects the recording layer against corrosion. According to the invention, there is provided a read-only optical disc comprising: a disc substrate of a light-transmissive material and having a pattern of pits and lands formed as representing an information signal on at least one surface thereof; and a thin reflective layer of a corrosion-resistant metallic material deposited on said pattern of pits and lands of said disc substrate; characterised in that the thin reflective layer comprises a stainless steel material deposited directly over the pattern of pits and lands after the pattern of pits and lands has been formed in said substrate, the stainless steel layer having a reflectance that allows information recorded on said read-only disc to be read by a signal reproducing circuit for magneto optical read/write discs. Such an optical disc can be relatively inexpensive to manufacture and highly resistant to aging. Two of the above optical discs may be bonded to each other by an adhesive layer coated on the reflective layers of the optical discs. The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 is a fragmentary cross-sectional view of a conventional optical disc; Figure 2 is a fragmentary cross-sectional view of an optical disc according to a first embodiment of the invention; Figure 3 is a block diagram of an apparatus for manufacturing an optical disc according to the invention; and Figure 4 is a fragmentary cross-sectional view of an optical disc according to a second embodiment of the invention. Figure 2 fragmentarily shows a read-only optical disc 1 according to a first embodiment of the invention. The optical disc 1 has a diameter of 64mm and a thickness of 1.2mm, for example, and serves to store, on one surface thereof, various information signals such as musical tone signals, video signals, data signals, etc., which can optically be read out for reproduction. The optical disc 1 comprises a disc substrate 2 having a signal pattern 3 which has been formed as a pattern of pits and lands on one surface thereof according to an information signal. The disc substrate 2 is injection-moulded of a light-transmissive synthetic resin such as polycarbonate, PMMA, or the like by an injection moulding machine with a stamper which has a pattern of pits and lands complementary to the signal pattern 3 of pits and lands. The optical disc 1 also has a thin reflective layer 4 deposited on the signal pattern 3 up to a thickness of 1000 A or less by way of sputtering, evaporation, or the like. The reflective layer 4 is made of a corrosion-resistant stainless steel material which is an alloy of Fe, Ni, and Co. The reflective layer 4 is formed so as to cover at least an entire signal recording area of the disc substrate 2 where the signal pattern 3 is formed. After the reflective layer 4 has been deposited on the disc substrate 2, the optical disc 1 may be printed with identification and description data, known as a label, regarding the recorded information signal by the silk screen printing, as with usual compact discs and optical video discs. As shown in Figure 2, the reflective layer 4 on the signal pattern 3 of the disc substrate 2 is so thin that it also has a pattern of pits and lands on its outer surface. The reflective layer 4 with such pattern of pits and lands has a relatively low reflectance. Figure 3 shows in block form an apparatus for manufacturing the optical disc according to the invention. The apparatus has a moulding machine 101 which moulds a disc substrate of a light-transmissive synthetic resin, with a pattern of pits and lands transferred from a stamper to one surface of the disc substrate. The disc substrate moulded by the moulding machine 101 is then fed by a first feeder 102 to a layer depositing machine 103 which comprises a sputtering machine. In the sputtering machine, the disc substrate is placed in a layer depositing chamber in the form of a vacuum container, and a thin reflective layer is deposited on the disc substrate by a sputtering source in an argon gas atmosphere. The sputtering source comprises a target of a stainless steel material positioned in confronting relationship to the disc substrate that is placed in the layer depositing chamber. While DC sputtering electric power is being supplied between the target and the disc substrate, the stainless steel material from the target is deposited on the disc substrate by way of sputtering in the argon gas. The DC sputtering electric power supplied between the target and the disc substrate is preferably controlled to grow the reflective layer up to a predetermined thickness on the disc substrate, i.e., to cause the reflective layer to have a reflectance ranging from 20 to 50% with respect to the wavelength ranging from 380 to 820nm of a light beam which will be applied from a light source to the optical disc for reading the recorded information signal from the optical disc. The optical disc with the reflective layer thus deposited on the disc substrate is thereafter fed by a second feeder 104 to an inspecting machine 105 which inspects the layer growth of the reflective layer deposited on the disc substrate. The inspecting machine 105 determines the inspected optical disc as either an accepted optical disc or a rejected optical disc. An optical disc according to an inventive example and a conventional optical disc according to a comparative example, which was manufactured by the conventional process, will be described below. According to the inventive example, a disc substrate was injection-moulded of polycarbonate by an injection moulding machine, with a pattern of pits and lands transferred from a stamper to one surface of the disc substrate. Thereafter, a reflective layer of stainless steel material (SUS304) was deposited on the pattern of pits and lands under a pressure of 5 x 10 -3 Torr by a sputtering device. No protective layer was deposited on the reflective layer. According to the comparative example, a reflective layer of A1 was deposited on a disc substrate of polycarbonate, and thereafter coated with a photo-setting acrylic protective coating layer by the spin coating process. The coated photo-setting acrylic protective coating layer was then set by exposure to an ultraviolet radiation, thereby producing a protective layer on the reflective layer. The following table shows the corrosion resistance of the inventive and comparative examples. Immersed for 12 hours in tap water boiled to 100°C Immersed for 12 hours in 10% saline solution boiled to 100°C Placed for 40 days in thermostatic chamber at temperature of 60°C and humidity of 90% Inventive example No change No change No change Comparative example A1 was oxidized and became transparent A1 was oxidized and became transparent A1 was partly oxidized and become turbid The optical disc according to the inventive example was tested for reflective layer adhesion. An adhesive Cellophane Tape (trademark) or a gummed tape was applied to the reflective layer of the optical disc, and then peeled off. The reflective layer was not peeled. The same test was conducted on the optical disc according to the comparative example, and the reflective layer was peeled. The reflectance of the reflective layer of the optical disc according to the inventive example remained unchanged after it was immersed in tap water or saline solution boiled to 100°C. For compatibility with magnetooptical discs which comprise a thin layer of magnetooptical recording material coated on a light-transmissive disc substrate, since the reflective layers of presently available magnetooptical discs have a reflectance ranging from 15 to 30%, the reflective layer of the optical disc according to the invention can be deposited by the above manufacturing device such that its reflectance ranges from 20 to 35% in order to compensate for a reflectance loss of about 5% which is caused by the disc substrate. If the reflective layer of the read-only optical disc according to the invention has substantially the same reflectance as that of the magnetooptical discs, they can be played back by a common reproducing circuit with a constant servo gain without suffering various problems such as the need for switching between reproducing or recording circuits due to different reflectances and the need for switching between gains for a servo control circuit associated with an optical pickup, which problems would otherwise arise out of the difference between the reflectance of 80% or more of optical discs with reflective layers of A1 and the reflectance of about 40% of magnetooptical discs. Figure 4 fragmentarily shows an optical disc according to a second embodiment of the invention. The optical disc according to the second embodiment comprises two read-only optical discs 1 according to the first embodiment which are bonded to each other by an adhesive layer 5 coated on the reflective layers 4 of the optical discs 1. If two conventional optical discs with protective layers were bonded to each other by an adhesive layer coated on the protective layers, then the peelability of the resultant optical disc would be governed by the bonding strength between the protective and reflective layers and also between the protective layers and the adhesive layer. According to the second embodiment shown in Figure 4, however, since only the bonding strength between the reflective layers 4 and the adhesive layer 5 determines the peelability of the optical disc, the optical disc is less liable to get peeled off than the conventional optical disc composed of two optical discs bonded to each other would be. As described above, a read-only optical disc comprises a disc substrate of light-transmissive synthetic resin with a pattern of pits and lands formed on one surface thereof based on a recorded information signal, and a reflective layer of a stainless steel material deposited on the pattern of pits and lands of the disc substrate. However, the read-only optical disc may be composed of a glass substrate having a flat surface which is fabricated as follows: The flat surface of the glass substrate is coated with a layer of ultraviolet-curing resin. Then, a stamper having a pattern of pits and lands according to an information signal to be recorded is pressed against the coated layer of ultraviolet-curing resin. While the stamper is being pressed against the coated layer of ultraviolet-curing resin, an ultraviolet radiation is applied to the layer of ultraviolet-curing resin through the glass substrate, thus curing the layer of ultraviolet-curing resin on the glass substrate. Thereafter, the stamper is detached from the cured layer of ultraviolet-curing resin. The reflective layer of the optical disc according to the invention does not need to be protected as it is made of a corrosion-resistant metallic material. Particularly, in the case where the optical disc is stored in a disc cartridge, since the optical disc is protected as a whole by the disc cartridge, any protective layer for protecting the reflective layer is not required at all. If the optical disc is not stored in a disc cartridge, however, the reflective layer may be protected by a protective layer because the thin metallic reflective layer deposited on the disc substrate may possibly be peeled off when it comes into contact with a sharp pointed object. In the above embodiments, the reflective layer may be formed so as to cover at least the entire signal recording area of the disc substrate. However, the reflective layer may be formed so as to cover either the entire surface of the disc substrate where the signal pattern is formed or the surface of the disc substrate where the signal pattern is formed except for a disc region that will be gripped by a disc rotating unit of a recording or reproducing device for the optical disc. The reflective layer of stainless steel material, which is deposited on the pattern of pits and lands of the transparent disc substrate, is effective to reduce the cost of the optical disc, and also to reduce aging of the optical disc.

Claims: A read-only optical disc (1) comprising: a disc substrate (2) of a light-transmissive material and having a pattern (3) of pits and lands formed as representing an information signal on at least one surface thereof; and a thin reflective layer (4) of a corrosion-resistant metallic material deposited on said pattern (3) of pits and lands of said disc substrate (2); characterised in that the thin reflective layer (4) comprises a stainless steel material deposited directly over the pattern (3) of pits and lands after the pattern (3) of pits and lands has been formed in said substrate (2), the stainless steel layer having a reflectance that allows information recorded on said read-only disc to be read by a signal reproducing circuit for magneto optical read/write discs. A read-only optical disc (1) comprising: a first optical disc member comprising a first disc substrate (2) of a light-transmissive material and having a pattern (3) of pits and lands formed as representing an information signal on at least one surface thereof, and a first thin reflective layer (4) of a corrosion-resistant metallic material deposited on said pattern (3) of pits and lands of said first disc substrate (2); a second optical disc member comprising a second disc substrate (2) of a light-transmissive material and having a pattern (3) of pits and lands formed as representing an information signal on at least one surface thereof, and a second thin reflective layer (4) of a corrosion-resistant metallic material deposited on said pattern (3) of pits and lands of said second disc substrate (2); and an intermediate layer (5) of an adhesive bonding said first and second reflective layers (4,4) to each other; characterised in that the first and second thin reflective layers (4) each comprise a stainless steel material deposited directly over the respective patterns (3) of pits and lands after the respective patterns have been formed in said first and second substrates (2), the stainless steel layers each having a reflectance that allows information recorded on said read-only disc to be read by a signal reproducing circuit for magneto optical read/write discs.

Inventors: IKEDA JIRO, YANAGISAWA YOSHITAKE, IKEDA, JIRO, YANAGISAWA, YOSHITAKE, Ikeda, Jiro, c/o Patents Division, Yanagisawa, Yoshitake, c/o Patents Division
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G11B7
IPC codes: G11B7