HIGH-DENSITY INFORMATION RECORDING CARRIER

Abstract: A high-density information recording carrier such as an optical disk which is used for recording and/or playing back information carried on a plastic substrate by means of laser beams has an improved protective layer between a recording layer and the plastic substrate. The protective layer is made of inorganic glass made of alkali-free glass containing less than 1 wt % of alkali metal oxides. The resulting optical disk shows improved durability, higher C/N, lower BER and higher film forming rate.

Description: The present invention relates to a high-density information recording carrier which is used to record and play back information on a recording layer by a laser beam, more particularly, relates to an optical disk, and more particularly a magneto-optical recording disk, having a plastic substrate. PRIOR ART What is technically most important in the optical disk is to improve the durability or reliability when a plastic substrate is used. Although a plastic substrate has many advantages over a glass substrate, it has a serious drawback. The recording layer formed on a plastic substrate is degraded by residual water and monomer in a plastic substrate and also by water infiltrating through a plastic substrate. To eliminate this drawback, the present inventors proposed a recording carrier made up of a plastic substrate and a recording layer, with a thin film of alkali-free glass interposed between them, see Japenese Patent Application JP-A-59 32230, "Optical Information Recording Carrier", filed on February 22, 1984 〈(EP-A-163810)〉. The thin film of alkali-free glass greatly improves the durability of an optical disk and provides a satisfactory optical disk. To improve the previous invention further, the present inventors carried out further experiments and found preferred conditions which can reduce BER (bit error rate) without deteriorating the durability. As the result, they completed the present invention. OBJECTS OF THE INVENTION It is an object of the present invention to provide an optical information recording carrier which has a lower BER and which is stable over a long period of time because the plastic substrate has improved smoothness, hardness and the moisture resistance like a glass substrate so that the functional film or recording layer is protected from degradation by the substrate. It is another object of the present invention to provide a high-density information recording carrier in which the functional film or recording layer of rare earth-transition metal amorphous alloy is greatly improved in magneto-optical recording properties. DESCRIPTION OF THE INVENTION In accordance with the invention, the above mentioned objects are achieved by a high-density information recording carrier including a transparent plastic substrate, a high-density information recording layer formed on the substrate, and a thin film layer of inorganic glass interposed between the substrate and the recording layer, the high-density information recording carrier being characterized in that said thin film layer of inorganic glass is made of a glass (a) which is an alkali-free glass containing less than 1 wt% of alkali metals in terms of alkali metals oxides and (b) further contains from 20 wt% to 35 wt% of alkaline earth metals and terms of alkaline metal oxides, and (c) has a coefficient of linear expansion of 40 to 70 x 10⁻⁷/°C. Fig. 1 is an illustrative cross-sectional view of an optical disk to which the present invention is applicable. Fig. 2 is a graph showing relationship between the value of BER of magneto-optical recording disks having a variety of thickness of the inorganic glass protective layer described in Example 2 and the value of the thickness after 30 days under the conditions of 45°C and 90% relative humidity. The alkali-free glass contains less than 1 wt% of alkali metals, namely Li, Na, K, Rb, Cs, preferably less than 0.8 wt% of alkali metals in terms of alkali metal oxides. The transparent plastic substrate and the high-density information recording layer used in the present invention are described in details in our co-pending Japanese patent application. JP-A-59-32230 which is referred to in this specification and hence are not explained now. The common structure of an optical disk is schematically shown in Fig. 1. An optical disk is made up of a transparent plastic substrate 1, a protective layer 2, a recording medium layer 3, a surface protective layer 4, and an external protective layer 5 (which is provided as required). The substrate 1 is made of any transparent plastic such as acrylic resin, polycarbonate resin, epoxy resin and styrene resin. The first three resins are preferred because of the birefringence, moldability, and stability. The high-density information recording medium 3 may be of any known type such as DRAW (direct read after write) and E-DRAW (erasable direct read after write). In the conventional optical disk, the protective layer 2 is made of an oxide such as SiO₂ and TiO₂ or an organic polymer such as organopolysiloxane. The feature of this invention resides in the protective layer 2 which is made of inorganic glass having special characteristics. The protective layer 2 of inorganic glass has a thickness of 10 to 200 nm. With a thickness less than 10 nm, protection is not satisfactory; and a protective layer thicker than 200 nm is uneconomic and poses a problem such as the absorption of laser light. The inorganic glass layer 2 may be formed by vapor deposition, sputtering or ion plating. The inorganic glass is preferably so-called alkali-free glass containing no or little amount of alkali metals. Alkali-free glass is commercially available from, for example, Corning Glass Works under a product number 7059. It is composed of SiO₂ (49.9%), BaO (25.1%), B₂O₃ (10.5%), Al₂O₃ (10.3%) and CaO (4.3%). This is not a limitative example of the inorganic glass in this invention. The inorganic glass is not specifically limited so long as it permits the formation of thin film thereon by physical vapor deposition. According to this invention, the above-mentioned inorganic glass is used to make the protective layer 2 interposed between the plastic substrate 1 and the recording layer 3. It may also be used to make the protective layer 4 external to the recording layer 3 and to make the protective layer 5 external to the plastic substrate 1. The above-mentioned inorganic glass layer is superior, particularly in resistance to moisture absorption, to the conventional protective layer made of simple substance such as SiO₂ and TiO₂. The present invention produces the best result when applied to a magneto-optical disk in which the high-density information recording layer is made of rare earth-transition metal amorphous alloy. The recent rapid increase in the amount of information necessitated high-density recording and high-speed access. To meet this requirement, there has been developed a thin film of amorphous alloy composed of rare earth elements and transition metals, e.g. TbFe, TbFeCo and GdTbFe. It has the easy magnetization axis in the direction perpendicular to the film surface. Therefore, it is drawing public attention as a high-density information recording medium capable of recording, retrieving and erasing. The information recording disk will find use as document files, still picture files, moving picture files and computer memories. For their improvement, it is important to reduce the error rate and to increase the signal-to-noise ratio. These factors are associated with the recording medium, and they are adversely affected by (1) the uneven film thickness and alloy composition, (2), the uneven magnetic properties and (3) the degradation by oxidation. The performance of the recording medium depends on the cleanliness, smoothness and flawlessness of the substrate. The above-mentioned three troubles can be eliminated all at once when the inorganic glass layer 2 is interposed between the plastic substrate 1 and the recording medium layer 3 of rare earth-transition metal amorphous alloy. As the result, it is possible to obtain a magneto-optical recording carrier (referred to as magneto-optical disk hereinafter) of practical use. The magneto-optical recording medium to which the present invention can be applied is a rare earth-transition metal amorphous alloy of any known type. Examples of the alloy include Tb-Fe alloy (Japanese Patent Publication No. 20691/1982), Dy-Fe alloy (Japanese Patent Publication No. 20692/1982), Gd-Tb-Fe alloy (Japanese Patent Laid-open No. 126907/1981), Gd-Tb-Dy-Fe alloy (Japanese Patent Laid-open No. 94948/1982), Gd-Co (Japanese Patent Laid-open No. 121719/1979) and Tb-Fe-Co alloy. The layer of rare earth-transition metal alloy can be formed on the inorganic glass layer by deposition, sputtering or ion plating. The thickness of the layer is usually 50 to 150 nm. According to this invention, the inorganic glass layer 2 has the same composition and thickness as mentioned above. Heretofore, much has been investigated on the formation of an amorphous layer on a glass substrate, and it is now possible to form an amorphous layer capable of perpendicular magnetization on a glass substrate. However, there is no established technology for forming a stable amorphous layer capable of perpendicular magnetization on a plastic substrate. In the case of an amorphous layer formed on a glass substrate, the Kerr rotary angle is greater than 0.3 degrees, whereas in the case of an amorphous layer formed on a plastic substrate, the Kerr rotary angle is 0.1 degrees at the best. It may be sometimes possible to obtain an amorphous layer having a Kerr rotary angle greater than 0.2 degrees but it is not stable for a long period of time and it is poor in reproducibility. Thus, there is a strong demand for the technology of forming a stable amorphous layer capable of perpendicular magnetization on an easy-to-handle plastic substrate in place of the conventional glass or aluminum substrate. According to the present invention, there is provided a magneto-optical disk of practical use, which is based on a plastic substrate. The completion of the new magneto-optical disk is attributable to the above-mentioned inorganic glass layer 2, which permits a stable amorphous alloy layer to be formed on a plastic substrate. The magneto-optical disk in accordance with this invention has almost the same C/N ratio as the conventional magneto-optical disk based on a glass substrate. In what follows, the invention will be described with reference to a magneto-optical disk, although the invention will be applicable to any known recording medium of DRAW type. The magneto-optical recording medium was chosen as an example for explanation because it requires higher performance, particularly in moisture resistance, than a recording medium of DRAW type. Any one that satisfies the stringent requirements of a magneto-optical recording medium would be satisfactorily applicable to a recording medium of DRAW type. Apparently, it is also applicable to any E-DRAW medium of phase-change type and form-change type which needs durability. The first feature of the present invention resides in that the amount or proportion of alkali metals in the alkali-free glass is less than 1 wt%, preferably less than 0.8 wt%, in terms of alkali metal oxides. In other words, the alkali-free glass contains less than 1 wt%, preferably less than 0.8 wt%, of alkali metals (Li, Na, K, Rb and Cs) in terms of their oxides. If the amount of alkali metal is greater than 1 wt% in terms of alkali metal oxide, cracks occur in the recording layer, destroying the recorded data which result in lower BER. The reason for this is not known. It appears, however, that the alkali metal reacts with the moisture that has infiltrated through the plastic substrate, the monomer that has migrated from the plastic substrate, or the moisture in the air, to degrade the recording layer. The amount of the alkali metal should preferably be less than 0.8 wt% in terms of alkali metal oxide, and the less the amount of alkali metal, the more desirable, unless the properties of the glass are adversely affected. It is industrially impossible to eliminate alkali metal completely; and alkali metal up to 1 wt% will not pose any practical problems. The alkali-free glass used in this invention does not include silica glass composed almost entirely of SiO₂. The protective layer formed by the deposition of SiO₂ alone has been known; but the thin film of silica glass has almost no moisture resistance. The thickness of the thin film of the inorganic glass should be 10 to 200 nm. With a thickness less than 100 Å, the thin film does not perform the protective function satisfactorily and with a thickness greater than 200 nm, the thin film absorbs laser light and is uneconomical. The present invention is advantageously applied to an optical disk in which the high-density information recording medium is a magneto-optical recording material of rare earth-transition metal amorphous alloy, e.g. TbFeCo. Apparently, it is applicable also to other high-density information recording carrier on which is formed a recording material susceptible to the monomer and moisture in the plastic substrate and/or the moisture in the air. It is a matter of course that the thin protective layer of inorganic glass can be formed on the outside of the recording layer or on the opposite side of the substrate to isolate the recording layer from the atmosphere. The second feature of the present invention resides in that the inorganic glass contain further from 20 wt% to 35 wt% of alkaline earth metals in terms of alkaline earth metal oxides. The alkaline earth metal oxide is at least one of oxides selected from a group consisting of BaO, CaO, MgO, SrO and BeO. Preferably, the oxides consist mainly of CaO and BaO and more preferably is BaO. Remaining components which constitute the inorganic glass can be such materials that are used to form glass along with the alkaline earth metals and may be oxides of silicon aluminum, boron, lead or the like such as SiO, Al₂O₃, B₂O, PbO. If the amount of alkaline earth metal oxides is not greater than 20 wt%, it is difficult to produce the inorganic glass because of rise of melting point as well as decrease of sputtering rate. If the amount of the oxide exceed 35 wt%, the resistance to water and acids decrease, so that a thin film layer made of the resulting inorganic glass shows a lower resistance to humidity and hence the final optical recording disk has a lower performance. In fact, durability of optical recording disk can be improved as well as the speed to form the thin film layer of the inorganic glass can be increased when the amount of alkaline earth metals is limited within the above-mentioned range according to the present invention. The reason why the durability is improved by such limitation is not clear, but the amount of the alkaline earth metals is concerned with formation of the inorganic glass layer judging from the increase of sputtering rate. The third feature of the present invention resides in that the inorganic glass has a coefficient of linear expansion in the range of 40 to 70 x 10⁻⁷/°C so as to improve further BER of the resulting disk. If the coefficient is outside this range, cracks occur in the recording layer resulting in lowering the BER. The coefficient of linear expansion of inorganic glass can be varied to any desired value by changing the composition of the glass. The thin film layer of the inorganic glass can be produced by any conventional physical vapor deposition technique such as sputtering, vacuum deposition, or ion-plating. Theoretically, the thin film layer of inorganic glass can be produced on a plastic substrate by using any suitable method such that a plurality of components are vacuum-deposited from more than two sources which will form the inorganic glass having the proportions of the alkali metals and/or alkaline earth metals defined in the claim. But, in case of industrial scale, a source of inorganic glass including above-mentioned alkaline earth metals and/or alkali metals is prepared and is vapor-deposited physically. The invention is now described with reference to the following examples. EXAMPLE 1 Optical disks were prepared as follows: The plastic substrate is a 1.2 mm thick polymethyl methacrylate (PMMA) substrate or polycarbonate (PC) substrate. A thin film of inorganic glass was formed on the substrate by sputtering using an RF magnetron sputtering apparatus made by Nippon Shinku Gijutsu Co., Ltd. The thickness of the thin film is 50 nm. The composition of the inorganic glass is shown in Table 1. A recording layer of amorphous alloy was formed on the inorganic glass film using the same sputtering apparatus as mentioned above. The amorphous alloy has a composition of Gd 0.12 Tb 0.12 Fe 0.76 . The thickness of the recording layer is 100 nm. A protective thin layer of inorganic glass was formed on the recording layer by sputtering in the same manner as mentioned above. The inorganic glass has the same composition as mentioned above. The thickness of the thin layer is 100 nm. The optical disks thus obtained were evaluated by measuring the change of BER and C/N ratio that took place after the disks had been kept at 45°C and 90% RH for 30 days. (The change of BER is expressed by comparison with the initial value (=1), and the change of C/N ratio is expressed in terms of decrease (in dB) from the initial value.) Table 1 shows such a fact that BER can be reduced when the amount of alkali metal (Na) is reduced to less than 1 wt% in terms of alkali metal oxide (Na₂O). EXAMPLE 2 The same procedure as Example 1 is repeated, but in this case, the thickness of the thin film layer of inorganic glass having the same glass components as the Sample No. A in the Example 2 is changed to 25, 50, 75, 100 and 200 nm respectively on a substrate of polycarbonate resin. Fig. 2 shows the value of BER of the resulting magneto-optical disk when the BER is measured after 30 days during which the disk is maintained under conditions of 45°C and 90% relative humidity. It is apparent from Fig. 2 that the thickness of the thin film layer of inorganic glass is advantageously within the range of 50 to 100 nm. EXAMPLE 3 The same procedure as Example 1 is repeated, but in this case, the composition of the inorganic glass is changed as is shown in Table 2. The optical disks thus obtained were evaluated by measuring the change of BER and C/N ratio that took place after the disks had been kept at 45°C and 90% RH for 30 days and the deposition rate. (The change of BER is expressed by comparison with the initial value (=1), the change of C/N ratio is expressed in terms of decrease (in dB) from the initial value, and the deposition rate is expressed by sputtering rate respectively.) The result is shown in TABLE 2. As is shown in TABLE 2, alkali-free glass having the limited proportions of alkali metals and/or alkaline earth metals defined by the present invention can produce optical disks having improved durability and productivity. EXAMPLE 4 The same procedure as Example 1 is repeated, but in this Example 4, the composition and/or the coefficient of linear expansion of the inorganic glass is varied as is shown in Table 3 which also show the result of the readout properties of the disk. The optical disks thus obtained were evaluated by measuring the change of BER and C/N ratio that took place after the disks had been kept at 45°C and 90% RH for 30 days. (The change of BER is expressed by comparison with the initial value (=1), and the change of C/N ratio is expressed in terms of decrease (in dB) from the initial value.) It is noted from Table 3 that where inorganic glass having a coefficient of linear expansion in the range of 40 to 70 x 10⁻⁷ is used the resulting optical disks have a small change in C/N ratio. It is also noted that the optical disks ore improved in BER where the alkali-free glass is one which contains less than 1 wt% of alkali metal in terms of oxides thereof.

Claims: A high-density information recording carrier including a transparent plastic substrate, a high-density information recording layer formed on the substrate, and a thin film layer of inorganic glass interposed between the substrate and the recording layer, characterized in that said thin film layer of inorganic glass is made of a glass a) which is an alkali-free glass containing less than 1 wt% of alkali metals in terms of alkali metal oxides and b) further contains from 20 wt% to 35 wt% of alkaline earth metals in terms of alkaline earth metal oxides and c) has a coefficient of linear expansion of 40 to 70 x 10⁻⁷/°C. A high-density information recording carrier as set forth in claim 1, wherein the amount of said alkali metals is less than 0.8 wt% in terms of alkali metal oxides. A high-density information recording carrier as set forth in claim 2 wherein said alkaline earth metal oxide is at least one of compounds selected from a group comprising BaO, CaO, MgO, SrO and BeO. A high-density information recording carrier as set forth in any of claims 1 to 3, wherein said high-density information recording layer is made of magneto-optical recording material. A high-density information recording carrier as set forth in claim 4, wherein said magneto-optical recording material is amorphous alloy composed of rare earth elements and transition metals.

Inventors: SATO, MASARU, TSUKANE, NAGAYOSHI
Applicants: DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1986
Main classification: G11B7
IPC codes: G11B7, G11B11