OPTICAL RECORDING MEDIUM

Abstract: An optical recording medium (1) comprising a substrate (2) which is transparent to a high energy beam capable of recording optical information on and reproducing it from the medium (1) and having tracking guides (4) for servo control on one side of the substrate. A manganese phthalocyanine layer (3) is formed on the one side of the substrate (2), so that a good signal-to-noise ratio is ensured at reproduction.

Description: TITLE OF THE INVENTION OPTICAL RECORDING MEDIUM BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an optical recording medium in which optical information is recorded and reproduced by means of high energy optical beams such as a semiconductor laser beam. Description of the Prior Art Direct-read-after-write (hereinafter referred to simply as DRAW) systems have been developed and are now in use, in which high energy beams such as laser beams, are used to record and reproduce optical information. Intensive studies have now been made to develop disc mediums for use in the DRAW systems. Broadly, the disc medium can be classified into two groups. One group makes use of inorganic metals as a recording layer, whereas the other group uses organic pigments or dyes. The former medium has the great advantage that because information is recorded in a high contrast, the signal-to-noise ratio becomes high when the medium is reproduced. However, the metal layer is apt to undergo oxidation with the attendant disadvantage that the medium is not durable. In addition, the metals used are, in most cases, harmful to the human body, thus presenting the problem of public nuisance. On the other hand, the latter medium undesirably requires an additional reflective film since organic dyes are not generally so high in reflectivity. This disadvantageously makes the manufacture process complicated and invites a high manufacture cost. SUMMARY OF THE INVENTION It is an object of the invention to provide an optical recording medium which overcomes the drawbacks of both types of optical recording mediums and which comprises a recording layer of manganese phthalocyanine having a high reflectance whereby no reflective film is necessary. It is another object of the invention to provide an optical recording medium which is more inexpensive than known counterparts using organic dyes. It is a further object of the invention to provide an optical recording medium which has a high signal-to-noise ratio at the time of reproduction and can be reproduced at a low output power of a semiconductive laser. The optical recording medium of the invention is suitable for recording and reproducing optical information by application of a high energy beam. The medium is of the type in which an optical information signal is encoded as physical variations or pits. The medium comprises a substrate which is tranparent to a high energy beam and has tracking guides for servo control on one side thereof, and a layer of manganese phthalocyanine formed on the one side of the transparent substrate. In order to prevent deposition of dust on the phthalocyanine layer, a protective layer may be provided over and arranged parallel at a distance from phthalocyanine layer to establish a gap between the phthalocyanine layer and the protective layer. A double-sided recording medium can be readily fabricated using two medium units of the type described above. More particularly, a pair of the above medium units are assembled such that the manganese phthalocyanine films are facing each other at a distance to allow a gap therebetween which is equal to or larger than the thickness of the manganese phthalocyanine layer. The resultant medium can be recorded on opposite sides, i.e. recording on double sides becomes possible. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of part of an optical recording medium according to one embodiment of the invention; Fig. 2 is a schematic view of part of an optical recording medium according to another embodiment of the invention; Fig. 3 is a schematic view of part of a double-sided optical recording medium according to a further embodiment of the invention; Fig. 4 is a graphical representation of absorbance of optical recording mediums of the invention and for comparison in relation to variation in wavelength; and Fig. 5 is a graphical representation of regular reflectance of optical recording mediums of the invention and for comparison in relation to variation in wavelength. DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION As described above, the present invention is characterized by using, as an optical recording layer, manganese phthalocyanine. Manganese phthalocyanine has the following formula EMI4.1 We have found that manganese phthalocyanine has a fairly high reflectance when used as an optical recording layer. This means that any additional reflective film is not necessary as will be used in prior optical recording mediums using organic dyes. This is very advantageous in that the manufacture process becomes simple and the medium can be manufactured at lost cost. To a surprise, it has also been found that the signal-to-noise ratio becomes much better than the ratio attained by the case where a reflective film is provided as in prior art mediums. Manganese phthalocyanine of the above formula has a high reflectance and an appropriate absorbance in a wavelength range of from 700 to 900 nm, which is a wavelength usually generated from semiconductor lasers ordinarily used for these purposes. Especially, in the case of an optical recording medium of the DRAW type. it is desirable that the absorbance of the recording medium in the wavelength of a semiconductor laser beam be rather high. This is because the output power for the laser at the time of recording can be decreased, so that a substrate do not undergo an undesirable thermal history. In this sense, manganese phthalocyanine is very useful. Since manganese phthalocyanine exhibits a high reflectance upon application of semiconductor laser beams, no additional reflective film is necessary as mentioned above. In addition, it has been unexpectedly found that the sisnal-to-noise ratio of the recording medium of the invention is very high. For instance, an optical recording medium comprising an aluminum reflective layer provided between a substrate and a manganese phthalocyanine film has a signal-to-noise ratio of 30dB, whereas an optical recording medium of the invention which is free of the aluminum reflective film, has a signal-to-noise ratio as high as 48 dB. The manganese phthalocyanine of the foregoing formula is a phthalocyanine complex in which Mn is present as a central metal. A number of metalo-phthalocyanines are known but are completely different in nature from manganese phthalocyanine when applied as an optical recording layer. For example, metalo-phthalocyanines containing, as the central metal, Cu, Co, Ni, Fe, Zn, Sn and the like, are known. However, when these metalo-phthalocyanines are used as the recording layer, good results cannot be obtained as will be seen from comparative examples appearing hereinafter. Reference is now made to the accompanying drawings and particularly, to Figs. 1 to 3, in which like reference numerals indicate like parts. In Fig. 1, there is shown an optical recording medium 1 which comprises a substrate 2 and a manganese phthalocyanine layer 3 formed on the substrate 2. The substrate 2 has, on one side thereof, tracking guides or grooves 4 which serve for servo control as is well known in the art. These tracking guides 4 are formed by molding of the substrate 2 and have usually a depth of 0.1 micron and pitches of 1.6 microns. On the side of the substrate 2 where the guides are formed1 is formed the layer 3. The substrate 2 is generally in the form of a disk but may have any other forms, if desired. The substrate 2 should be made of materials which are transparent to high energy beams such as semiconductor laser beams and include, for example, glasses and synthetic resins such as methacrylic resin, polycarbonate resins and the like. The manganese phthalocyanine layer 2 is readily formed by vacuum deposition under conditions of a vacuum of about 10 2 to 10 3 Pa and a temperature of about 300 to 500 C. The layer 2 is formed to have a thickness of about 50(2K -1) to 90(2k - 1) nm ,preferably 60(2k - 1) to 80(2k - 1) nm in which k is a positive integer and preferably, k is 1. The recording medium using manganese phthalocyanine is advantageous in that the recording speed is high and it is free of any problem of toxicity as in the case of lead phthalocyanine. It will be noted that in Fig. 1, a high energy beam is applied from the side indicated by arrow A. The recording medium 1 as shown in Fig. 1 may have a protective layer 5 formed through a gap 6 at the side of the manganese phthalocyanine layer 2 as particularly shown in Fig. 2. By the formation of the protective layer 5, dust is prevented from deposition on the layer 2, permitting reproduction characteristics not to deteriorate. The protective layer 5 is usually made of a transparent material such as vinyl chloride resins, methacrylate resins, polycarbonate resins and the like. The gap 6 is arranged to be equal to or larger than the thickness of the manganese phthalocyanine layer 2. The protective layer 5 and the manganese phthalocyanine layer-bearing substrate 1 are fixedly secured, for example, at the peripheral margin thereof. In the above embodiments, the optical recording medium is a single-sided medium. However, a double-sided recording medium may be readily fabricated, in which a pair of medium units of the type described above, i.e. each unit has the same construction as shown in Fig. 1, are assembled so that the manganese phthalocyanine layers of the respective units are facing each other at a distance equal to or larger than the thickness of the manganese phthalocyanine layer. This is particularly shown in Fig. 3, in which there is shown a unit la which comprises a substrate 2a and a manganese phthalocyanine layer 3a formed on the substrate Ea. Similarly, another unit lb has a substrate 2b and a manganese phthalocyanine layer 3b. The two units are assembled in such a way that the layers 3a, 3b are in faceto-face relation through a gap 7. The gap 7 is determined to be equal to or larger than the thickness of the layer 3a or 3b. In this case, the paired units are bonded at inner and/or outer marginal portions thereof by a suitable means. In this double-sided medium, the units la and lb are recorded and reproduced by means of high energy beams indicated by arrows a and b, respectively. The high energy beams used in the practice of the invention may be any laser beams having a wavelength ranging from 700 to 900 nm. For this purpose, semiconductor lasers such as semiconductive GaAs-AlGaAs lasers are generally used. The present invention is more particularly described by way of example. Example A polycarbonate resin disk having a diameter of 12 cm was placed in a vacuum evaporator having a boat for evaporation of a sample target. Fifty milligrams of manganese phthalocyanine of the afore-indicated formula (product of Dainichi Seika K.K.) was placed on the boat. The vacuum evaporator was kept at a vacuum of 10## 3 Pa and 0 the boat was heated to 500 C, so that manganese phthalocyanine was evaporated on one surface of the disk, thereby obtaining an optical disk as shown in Fig. 1. The thickness of the phthalocyanine layer was measured by an optical interference measuring instrument with the result that it was 80 nm. The manganese phthalocyanine layer of the optical disc was subjected to measurement of an absorbance characteristic. The results are shown by solid line in Fig. 4. From the figure, it will be seen that the curve has an absorption peak at 760 nm at which the absorbance is about 0.9. At 780 nm, the absorbance is about 0.8. From this, it will be seen that the manganese phthalocyanine layer has an alpha-type crystal form. Upon measurement of a regular reflectance characteristic, the results are just as shown by solid line in Fig. 5. The regular reflectance, determined by an integrator, was found to be about 40% at 780 nm. Next, an information signal was recorded on and reproduced from the optical disk using a semiconductor laser beam having a wavelength of 780 nm. When the disk was reproduced, the sisnal-to-noise ratio at a frequency of 1 MHz was 48 dB. It will be noted that the recording was conducted under conditions of a frequency of the optical disk of 900 r.p.m., a laser beam pulse frequency of 1 MHZ at a linear velocity of 4.8 mfsecond, and an average output power of 3.5 mW. After the recording, the disk was reproduced at a laser output power of 0.5 mW. Comparative Example 1 The general procedure of Example was repeated except that the polycarbonate resin disk was first provided with a 50 nm thick aluminum reflective film and then placed in the vacuum evaporator, thereby obtaining a reflective filmbearing optical disk. This optical disk was subjected to recording by the use of a semiconductor laser capable of outputting a beam with a wavelength of 780 nm, at an average output power of 4 mW. Thereafter, the disk was reproduced at a laser output power of 0.5 mW. As a result, it was found that the signal-tonoise ratio at the reproduction operation was as low as 30 dB. From the above, it will be appreciated that the signalto-noise ratio of the reflective film-bearing medium at 1MHz is lower by 18 dB than the medium of the invention. Thus, the optical disk of the invention is much superior to the optical disk of this comparative example. Comparative Example 2 The general procedure of Example was repeated except that phthalocyanine was formed, instead of manganese phthalocyanine, in a thickness of 90 nm, thereby obtaining a reflective film-free optical disk. The absorbance characteristic of the phthalocyanine film was measured with the results as shown by broken line in Fig. 4. More particularly, the regular reflectance at 780 nm is 23. In the same manner as in Example, a semiconductor laser was used for recording on the disk at a wavelength of 780 nm and an output power of 8 mW, followed by reproduction at a laser output power of 1 mW. As a result, it was found that the signal-to-noise ratio was 25 dB. Comparative Example 3 The general procedure of Example was repeated except that iron phthalocyanine was formed, instead of manganese phthalocyanine, in a thickness of 70 nm, thereby obtaining a reflective film-free optical disk. The absorbance and regular reflectance of the optical disk at 780 nm were measured and found to be 0.2 and 22%, respectively. The disk was subjected to recording by the use of a semiconductor laser capable of emitting a beam with a wavelength of 780 nm, at an output power of 8 mW and then reproduced at an output power of 1 mW. As a result, it was found that the signal-to-noise ratio was as low as 20 dB. Comparative Example 4 The general procedure of Example was repeated except that zinc phthalocyanine was used instead of manganese phthalocyanine, thereby obtaining an optical disk. The absorbance and regular reflectance of the zinc phthalocyanine layer at 780 nm were, respectively, 0.2 and 16%. The signal-to-noise ratio was found to be as low as 15 dB when determined in the same manner as in Comparative Example 3, Comparative Example 5 The general procedure of Example was repeated except that neodymium phthalocyanine was used instead of manganese phthalocyanine, thereby obtaining an optical disk. The absorbance and regular reflectance of the disk were, respectively. 0.2 and 26. The signal-to-noise ratio was found to be as low as 25 dB when determined in the same manner as in Comparative Example 3. Comparative Example 6 The general procedure of Example was repeated except that cobalt phthalocyanine was used instead of manganese phthalocyanine, thereby obtaining an optical disk. The absorbance and regular reflectance of the disk were, respectively, 0.2 and 25%. The signal-to-noise ratio was found to be as low as 25 dB when determined in the same manner as in Comparative Example 3. Comparative Example 7 The general procedure of Example was repeated except that copper phthalocyanine was used instead of manganese phthalocyanine, thereby obtaining an optical disk. The absorbance and regular reflectance of the disk were, respectively, 0.2 and 21. The signal-to-noise ratio was found to be as low as 25 dB when determined in the same manner as in Comparative Example 3.

Claims: CLAIMS 1. An optical recording medium for use in recording and reproducing optical information by application of a high energy beam and in which the information is recorded as physical variations, the medium (1) comprising a substrate (2) which is transparent to a high energy beam and has tracking guides (4) for servo control on one side thereof and a layer (3) of manganese phthalocyanine formed on said one side. 2. An optical recording medium according to Claim 1, wherein said manganese phthalocyanine has the following formula EMI14.1 3. An optical recording medium according to Claim 1 or 2 wherein the manganese phthalocyanine layer (3) has a thickness of 50(2k - 1) to 90(2k - 1) nm in which k is a positive integer. 4. An optical recording medium according to Claim 3, wherein the thickness is from 60(2k - 1) to 80(2k - 1). 5. An optical recording medium according to claim 3 or 4 wherein k is 1. 6. An optical recording medium according to any one of the preceding claims which also comprises a protective layer (5) provided over and arranged parallel to the manganese phthalocyanine layer (3) at a distance providing a gap (6) between said protective layer (5) and said manganese phthalocyanine layer (3). 7. An optical recording medium according to Claim 6, wherein the distance is equal to or larger than the thickness of said manganese phthalocyanine layer. 8. A double-sided optical recording medium which comprises a pair of units (la and lob), each unit being an optical recording medium as defined in any one of claims 1 to 4, the paired units being assembled such that the manganese phthalocyanine layers (3a and 3b) are facing each other at a distance providing a gap (7) therebetween which is equal to or larger than the thickness of the manganese phthalocyanine layer (3a or 3b).

Inventors: MARUTA, HIDEKI, NISHIZAWA, AKIRA, SUEYOSHI, NAOZI
Applicants: VICTOR COMPANY OF JAPAN, LIMITED
Application year: 1985
Main classification: G11B7
IPC codes: G11B7, B41M5