Method of producing a magneto-optical recording medium

Abstract: In a magneto-optical recording medium having a substrate, a recording layer having at least a rare earth - transition metal alloy film formed on the substrate, and a dielectric layer formed on the recording layer, a depth of an area in the recording layer containing an element derived from the dielectric layer is set to 70 Å or less from an interface between the recording layer and the dielectric layer in a depth-directional film structure analysis by Auger electron spectroscopy. Accordingly, mixing of impurities into the recording layer can be suppressed to thereby lower a recording magnetic field.

Description: The present invention relates to a method of producing magneto-optical recording medium having a rare earth - transition metal alloy film as a recording layer, and more particularly to an improvement for lowering of a recording magnetic field. In recent years, a magneto-optical recording system has been noticed as an overwritable high-density recording system. The magneto-optical recording system is such that a magnetic domain is written on a magnetic thin film to record information by using a thermal energy such as a semiconductor laser beam and this information is read by utilizing a magneto-optical effect. A typical recording material to be used in the magneto-optical recording system is known as a rare earth - transition metal alloy film (which will be hereinafter referred to as an RE-TM film) formed by combining a rare earth element such as Gd, Tb or Dy and a transition metal such as Fe or Co. As the magneto-optical recording medium having the RE-TM film as a recording layer, there has been proposed a recording medium having a four-layer structure consisting of an Si 3 N 4 dielectric film, TbFeCo magnetic film, Si 3 N 4 dielectric film and Al reflecting film, for example. Journal of Applied Physics vol. 67, no. 9, 1 May 1990, pages 5322-5324 discloses a magneto-optic recording medium, wherein a protective layer of SiN x or ALN is deposited on a TbCo magnetic layer. In manufacturing such a magneto-optical recording medium having the four-layer structure as mentioned above, it is necessary to sequentially form the Si 3 N 4 dielectric film, the TbFeCo magnetic film, the Si 3 N 4 dielectric film and the Al reflecting film in this order on a substrate by sputtering or the like. In general, the Si 3 N 4 dielectric film is formed by RF reactive sputtering in which an inert gas such as Ar and a reactive gas such as N 2 as a sputtering gas are introduced into a film forming chamber. However, in the magneto-optical recording medium to be manufactured by the above-mentioned technique, there is a problem such that unless recording is carried out in an external magnetic field of 150 Oe or more, a satisfactory CN ratio cannot be obtained. This problem is a large obstacle in case of applying the magneto-optical recording medium to an overwritable magnetic field modulation system. Accordingly, the elimination of this problem is a large subject for realizing the magnetic field modulation system. It is accordingly an object of the present invention to provide a method of producing magneto-optical recording medium which can suppress mixing of impurities into the TbFeCo magnetic film as a recording layer and lower a recording magnetic field. To achieve the above object, the present inventors have long studied to conclude that the characteristics are largely influenced by a degree of penetration of a constituent element of a dielectric layer into the TbFeCo magnetic film. The present invention has been accomplished on the basis of the above knowledge, and according to the present invention, there is provided a method of producing a magneto-optical recording medium the features of appended claim 1. In the magneto-optical recording medium produced according to the present invention, the RE-TM magnetic film is used as a recording layer, and at least a dielectric layer is formed on the RE-TM magnetic film. Specifically, the present invention may be applied to production of a magneto-optical recording medium having a four-layer structure consisting of an Si 3 N 4 dielectric film, TbFeCo magnetic film, Si 3 N 4 dielectric film and Al reflecting film. As the RE-TM magnetic film, all of known magnetic materials such as TbFeCo or GdFeCo for a recording film of a conventional magneto-optical recording medium may be used. A film thickness of the RE-TM magnetic film is set to about 100 - 1000 Å from the viewpoint of practical magneto-optical characteristics. The recording layer may be constituted of the RE-TM magnetic film only, or may be constituted of the RE-TM magnetic film and a rare earth element layer such as a Tb layer formed on the RE-TM magnetic film. The rare earth element layer has an influence upon recording and reproducing characteristics and enables recording and reproduction under a low magnetic field. However, if a film thickness of the rare earth element layer is too large, a noise tends to increase. Accordingly, it is preferable to set the film thickness to about 1 - 30 Å. The dielectric layer is provided for the purposes of improvement in corrosion resistance and enhancement of Kerr rotation angle and Faraday rotation angle due to multiple reflection. The dielectric layer is formed of oxides, nitrides or oxynitrides of Si or the like or zinc sulfide (ZnS), for example. A film thickness of the dielectric layer is set to about 100 - 2000 Å. In the case that the dielectric layer is laminated on the recording layer (RE-TM film) or two of the dielectric layers are formed on both surfaces of the RE-TM magnetic film, the present invention is effective. In the case that the dielectric film is laminated on the recording layer (RE-TM magnetic film), there occurs a phenomenon such that a constituent element of the dielectric film penetrates into the RE-TM magnetic film to some extent depending upon film forming conditions. However, such an RE-TM magnetic film containing a constituent element of the dielectric film is not desired in the point of characteristics, and a proportion of such a penetration area in the RE-TM magnetic film largely influences a magnitude of an external magnetic field required for recording. The interface between the recording layer and the dielectric layer is decided by a depth-directional film structure analysis by Auger electron spectroscopy, and it is defined in the present invention as follows: First, in a depth-directional Auger profile, an average peak level of Si in case of Si 3 N 4 dielectric film or Zn in case of ZnS dielectric film is obtained, and a position in the dielectric layer on the interface side with respect to the recording layer where a peak level of Si (or Zn) becomes 1/2 of this average peak level is obtained. On the other hand, an average peak level of a rare earth element (e.g., Tb) of the RE-TM magnetic film is obtained, and a position in the recording layer on the interface side with respect to the dielectric layer where a peak level of the rare earth element becomes 1/2 of this average peak level. Finally, a middle point between the above two positions is defined as the interface between the recording layer and the dielectric layer. As to the area in the recording layer containing an element derived from the dielectric layer, this area corresponds to an area containing N in case of Si 3 N 4 dielectric film, and it is determined by observing a degree of penetration of N from the interface into the recording layer. Similarly, in case of ZnS dielectric film, the area corresponds to an area containing S, and it is determined by observing a degree of penetration of S from the interface into the recording layer. The setting of such a degree of penetration of the element derived from the dielectric layer into the RE-TM magnetic film to 70 Å or less may be realized by providing a blocking layer at the interface between the dielectric layer and the recording layer. This blocking layer is a layer having a composition containing 3 % or more of a dissimilar element (e.g., oxygen) not existing in the RE-TM magnetic film. For example, an Auger signal of oxygen to be observed between the dielectric layer and the recording layer and an Auger signal of Tb to be observed by an increase in yield due to oxidation correspond to the blocking layer. A film thickness of the blocking layer may be obtained from a half width of a signal having a peak at the interface between the dielectric layer and the recording layer in the depth-directional Auger profile. The blocking layer is sufficiently effective even in the condition where an atom of the dissimilar element is arranged. From this viewpoint, the film thickness of the blocking layer may be several Å or more. However, if the film thickness of the blocking layer is too large, the characteristics of the recording layer (RE-TM magnetic film) is resultantly damaged, and it is therefore preferable to set the film thickness to 100 Å or less in terms of the above-mentioned half width. The formation of the blocking layer at the interface between the dielectric layer and the recording layer may be realized by the following manner. That is, after forming the RE-TM magnetic film or forming a rare earth element layer on the RE-TM magnetic film, the RE-TM magnetic film (or the rare earth element layer) is allowed to stand for 30 minutes or more. Alternatively, the RE-TM magnetic film (or the rare earth element layer) is exposed to an atmosphere having a predetermined oxygen partial pressure or an atmosphere having a predetermined oxygen partial pressure and a predetermined moisture partial pressure for a short time. The oxygen partial pressure may be set to about 1 x 10 -10 - 1 x 10 -1 Torr, and the steam partial pressure may be set to about 1 x 10 -7 - 1 x 10 -4 Torr. By setting the exposure time or other conditions within the above range, the blocking layer is formed. In manufacturing the magneto-optical recording medium having the four-layer structure as mentioned above, for example, the Si 3 N 4 dielectric film of the third layer is formed by sputtering after forming the TbFeCo magnetic film of the second layer. Accordingly, the TbFeCo magnetic film is allowed to stand however for a very short time in contact with a plasma of Ar or N 2 gas as a film forming atmosphere for the Si 3 N 4 dielectric film. Furthermore, the TbFeCo magnetic film is exposed to Si particles sputtered. At this time, there occurs a phenomenon such that an active element (especially, N or N + ) dissociated or ionized in the plasma penetrates into the TbFeCo magnetic film to cause a deterioration in characteristics. However, a depth of an area in the recording layer containing an element derived from the dielectric layer is set to 70 Å or less from the interface between the recording layer and the dielectric layer by a preferred technique of providing the blocking layer at the interface between the recording layer and the dielectric layer. Accordingly, the deterioration in characteristics can be suppressed to enable lowering of a recording magnetic field. That is, a degree of penetration of the element derived from the dielectric layer into the recording layer is set to 70 Å or less from the interface between the recording layer and the dielectric layer, thereby suppressing the deterioration in characteristics and improving a sensitivity to an external magnetic field. Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings. Fig. 1 is a graph illustrating an Auger profile in Example according to the present invention having a blocking layer; Fig. 2 is a graph illustrating an Auger profile in Comparison 1; and Fig. 3 is a graph illustrating an Auger profile in Comparison 2. The present invention will be more clearly understood with reference to the following examples. Example In Example, a magneto-optical recording medium having a four-layer structure consisting of an Si 3 N 4 dielectric film, TbFeCo magnetic film, Si 3 N 4 dielectric film and A1 reflecting film was prepared. In this four-layer structure, however, a Tb layer as a rear earth element layer was formed on the surface of the TbFeCo magnetic film. Further, a glass 2p substrate was used as a substrate of the recording medium. First, RF reactive sputtering was carried out by using an Si target in an Ar atmosphere containing 20 % of N 2 gas under the gas pressure of 3 x 10 -3 Torr to form the first Si 3 N 4 dielectric film. Then, direct current simultaneous two-element sputtering was carried out by using a Tb target and a FeCo alloy target to form the TbFeCo magnetic film having a film thickness of 225 Å. Then, direct current sputtering was carried out by using a Tb target to form the Tb layer having a film thickness of 5 Å. After forming the Tb layer, it was exposed to an atmosphere having a moisture partial pressure of 1 x 10 -6 Torr and an oxygen partial pressure of 2 x 10 -4 Torr for one minute to form a blocking layer. Then, RF reactive sputtering was carried out again by using an Si target to form the second Si 3 N 4 dielectric film. Subsequently, direct current sputtering was carried out by using an A1 target to form the A1 reflecting film. Finally, UV curing resin was applied to the A1 reflecting film to form a protective film covering the A1 reflecting film. Thus, a sample disk was prepared. Comparison 1 Similarly to Example, a sample disk was prepared with the exception that the film thickness of the TbFeCo magnetic film was set to 230 Å and the second Si 3 N 4 dielectric film was formed immediately after forming the TbFeCo magnetic film. Comparison 2 Similarly to Example, a sample disk was prepared with the exception that the second Si 3 N 4 dielectric film was formed immediately after forming the Tb layer. Using the sample disks prepared above, a depth-directional film structure analysis by Auger electron spectroscopy was carried out. There are shown Auger profiles of the sample disks in Figs. 1 to 3. Fig. 1 corresponds to Example; Fig. 2 corresponds to Comparison 1; and Fig. 3 corresponds to Comparison 2. The conditions of the Auger electron spectroscopy were as follows: Ion Energy 1 kV Emission Current 25 mA Measurement Area 0.3 x 0.2 mm 2 Sputter Rate 81 Å/min Acceleration Voltage 2 kV Beam Current 5 x 10 -7 A Measurement Pressure 6.8 x 10 -8 Pa Spot Radius 30 µm As apparent from comparison of these Auger profiles, it is observed in the Example that a peak of oxygen bonded to the Tb layer exists between the Si 3 N 4 dielectric film and the TbFeCo magnetic film. This peak of oxygen corresponds to the blocking layer, and it is understood that the blocking layer having a film thickness of about 50 Å is formed. It is also understood that a degree of nitrogen (N) into the RE-TM magnetic film is less in Example than in Comparison 1 and Comparison 2. More specifically, as shown in Figs. 1 to 3, the degree of penetration from the interface is 60 Å in Example. To the contrary, it is about 90 Å in Comparison 1. Yet in Comparison 2 providing the Tb layer, it is about 80 Å. Next, a recording and reproducing characteristic of these sample disks by a magnetic field modulation system was investigated. The recording and reproducing characteristic was measured under the conditions of 1.3 m/sec of medium linear velocity, 720 kHz of recording frequency, 4.5 mW of recording laser power and 0.6 mW of reproducing laser power. A spectral analysis for a whole frequency band of a reproduced waveform was carried out with a resolution band of 10 kHz to obtain a CN ratio in a recording external magnetic field of ± 70 Oe. The result is shown in Table 1. As apparent from Table 1, a magnetic field sensitivity in Example is improved. Magnetic Film Structure H 2 O Partial Pressure (Torr) O 2 Partial Pressure (Torr) Retention Time (min.) CN Ratio (dB) Example TbFeCo 225Å + Tb 5Å 1 x 10 -6 2 x 10 -4 1 48.6 Comparison 1 TbFeCo 230Å - - - 42.6 Comparison 2 bFeCo 225Å + Tb 5Å - - - 42.8 While the invention has been described with reference to specific embodiments, the description is illustrative and is not to be construed as limiting the scope of the invention. Various modifications and changes may occur to those skilled in the art within the scope of the invention as defined by the appended claims.

Claims: A method for producing a magneto-optical recording medium comprising the steps of: forming a recording layer having at least a rare earth - transition metal film; forming a blocking layer on said rare earth - transition metal film; forming a dielectric layer on said blocking layer; wherein said blocking layer is formed by exposing said rare earth - transition metal film to an atmosphere having a predetermined oxygen partial pressure for a short time after forming said rare earth - transition metal film. The method of claim 1, wherein said atmosphere has a predetermined oxygen partial pressure and a predetermined moisture or steam partial pressure. The method of claim 1, wherein said predetermined oxygen partial pressure is set about 1 x 10 -10 - 1 x 10 -1 Torr. The method of claim 2, wherein said predetermined oxygen partial pressure is set about 1 x 10 -10 - 1 x 10 -1 Torr, and said predetermined steam partial pressure is set about 1 x 10 -7 - 1 x 10 -4 Torr.

Inventors: IKEDA ETSURO, KIKKAWA MASAHIRO, KISHI TAKASHI, IKEDA, ETSURO, KIKKAWA, MASAHIRO, KISHI, TAKASHI, Ikeda, Etsuro, c/o SONY MAGNETIC PRODUCTS, INC., Kikkawa, Masahiro, c/o SONY MAGNETIC PRODUCTS, INC, Kishi, Takashi, c/o SONY MAGNETIC PRODUCTS, INC.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G11B11
IPC codes: G11B11