Patent Publication Number: US-7586833-B2

Title: Magneto-optical recording medium

Description:
The subject matter of application Ser. No. 10/488,386 is incorporated herein by reference. The present application is a continuation of U.S. application Ser. No. 10/488,386, filed Sep. 13, 2004 now U.S. Pat. No. 7,257,076, which is a 371 U.S. National Stage filing of PCT application No. PCT/JP03/08536, filed Jul. 4, 2003, which claims priority to Japanese Patent Application No. JP 2002-195729, filed Jul. 4, 2002. The present application claims priority to these previously filed applications. 

   TECHNICAL FIELD 
   This invention relates to a magneto-optical recording medium, particularly to a magneto-optical recording medium in which, for example, grooves are formed to obtain tracking servo signals, employing a groove recording method in which a signal recording area is in the grooves and reproduction is performed by means of a so-called DWDD (Domain Wall Displacement Detection) method. 
   BACKGROUND ART 
   In magneto-optical recording media, the magneto-optical recording medium employing the DWDD method has particularly been regarded as significant, because of the capacity of high density recording. Reproduction by means of the DWDD method is performed such that thermal distribution in the magneto-optical recoding medium with irradiation of reproducing laser light causes domain wall displacement of recording marks to expand and the expanded recording marks are read out; and by doing so, recording marks are minimized to be smaller than the limit of the diameter of the spot size of the reproduction light and the high density recording is achieved. 
   When the above DWDD method is employed, in order to realize the magneto-optical recoding medium having a large capacity, it is necessary to realize the smooth displacement of domain walls for the improvement of the jitter and the bit error rate. 
   Further, in order to appropriately perform the high density recording, it is necessary to obtain the enough power margin for recording. 
   DISCLOSURE OF THE INVENTION 
   The object of the present invention is to provide a magneto-optical recording medium employing the DWDD method to perform recording in grooves, in which displacement of domain walls can be performed reliably and smoothly to reduce the jitter and the bit error rate, and at the same time, to enlarge the power margin for appropriately performing high density recording. 
   Namely, the present invention is the magneto-optical recording medium, in which recording is performed in grooves and reproduction is performed by means of DWDD method; and it is found that each shape of curved portions of the side edges of grooves and lands that separate the grooves affects characteristics of this medium, particularly, affects the above-described displacement of domain walls and power margin for recording, so that by specifying the shapes, the above-described object can be achieved. 
   Specifically, a curvature of the side edge of the land, that is, a curvature radius R 1  in the cross section of the shoulder portion of the land that is adjacent to the grooves, which is perpendicular to the recording track, is set 30 nm or less, and a curvature radius R 2  in the cross section, which is perpendicular to the recording track, of the side edge of the groove on which recording is performed, that is, the inside edge of the groove adjacent to the land, is set 20 nm or more. 
   Here, both the lower limit of the R 1  and the upper limit of the R 2  are respective values approximately with which concave and convex shapes and stepped differences are practically determined to obtain required tracking servo signals. 
   As mentioned above, according to the magneto-optical recording medium of the present invention, since the shoulder portion of the land that separates the grooves is made steep compared to that of media employing the conventional DWDD, thermal transmission to the land at the recording in the grooves, specifically, at the thermal recording with the irradiation of the laser light, furthermore thermal conduction to the other adjacent grooves over the land therebetween can be reduced, so that the cross talk at the time of the recording and the over-write are improved and the expansion of the power margin for the recording can be made; and at the same time, since the both side edges of the bottom portion of the groove are made not steep compared to that of media employing the conventional DWDD, displacement of domain walls can be performed smoothly at the time of reproduction, so that the jitter and the bit-error rate can be reduced. 

   
     THE BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view showing an example of a magneto-optical recording medium according to the present invention; 
       FIG. 2  is a constitutional view showing an example of formed layers including an information recording layer of the magneto-optical recording medium according to the present invention; 
       FIG. 3  is a schematic cross-sectional view showing the relevant part of the structure provided to explain the shape of a substrate of the magneto-optical recording medium according to the present invention; 
       FIGS. 4A and 4B  are schematic perspective views showing the lands and grooves of type-A and type-B in enlarged scale illustrated based upon the ATM (Atomic Force Microscope) photographs; 
       FIGS. 5A and 5B  are diagrams showing the measurement results of a margin of the magneto-optical recording medium using the substrates shown in  FIGS. 4A and 4B , respectively; 
       FIGS. 6A through 6E  are process diagrams showing an example of steps of a method for manufacturing the magneto-optical recording medium according to the present invention; 
       FIGS. 7A and 7B  are process (diagrams showing the subsequent steps to those shown in  FIG. 6 ; and 
       FIGS. 8A and 8B  are enlarged cross-sectional views showing the relevant parts of  FIGS. 6C and 6D . 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   An embodiment of the magneto-optical recording medium by means of the DWDD method according to the present invention will be explained. 
   The magneto-optical recording medium according to the present invention, as relevant parts of the medium are shown, for example, in a schematically cross-sectional view of  FIG. 1 , comprises: a recording medium substrate  1 , for example, a disc substrate, on at least one main surface of which a concave and convex surface  4  provided with grooves  2  in, for example, a spiral shape or a concentric circle shape to obtain tracking servo signals and lands  3  separating the grooves  2  are formed; and on the concave and convex surface  4  a laminated film  5  having at least a recording layer and a reproducing layer respectively composed of a magnetic layer is formed. 
   As shown in the cross sectional view of  FIG. 2 , on the recording medium substrate  1 , the laminated film  5  is formed by sequentially laminating, for example, a dielectric layer  6  which is a non-magnetic layer, and a displacement layer  7  having the characteristic of low domain wall coercive force, a separating layer  8  having low Curie temperature Tc and a recording layer  9  having the characteristic of high domain wall coercive force, each of which is a magnetic layer. 
   Further, a protective layer  11  is formed on the laminated film  5 , such that the concave and convex surface of the film is filled. 
   In the normal condition of this magneto-optical recording medium, the recording layer  9  and the displacement layer  7  are in the condition of magnetic exchange coupling through the separating layer  8 , wherein the recording marks recorded on the recording layer  9  are transferred to the displacement layer  7 . 
   Then, on reproduction, laser light L is irradiated for example from the rear side of the magneto-optical recoding medium substrate  1 , the separating layer  8  where recording marks to be reproduced by means of DWDD, namely, by irradiation of the laser light are formed is heated up to temperature equal to its Curie temperature or higher, and the exchange coupling between the displacement layer  7  and recording layer  9  is released to displace domain walls in the displacement layer  7  along the extended direction of the grooves  2  by means of the distribution of temperature within the spot of the laser light, so that the recording marks are expanded and the necessary and sufficient reproduction signal output can be obtained. In this manner, since the recording marks are expanded on reproduction, the recording marks themselves can be miniaturized, thereby enabling the high density recording to be performed. 
   Further, in the magneto-optical recording medium according to the present invention, as shown in the cross sectional view of the groove  2  and adjacent land  3  in  FIG. 3 , curvature radius R 1 , in the direction perpendicular to recording tracks that is perpendicular to the extended direction of grooves, of a curved portion, that is, a shoulder portion  3 S of the land  3  which is adjacent to the groove  2 , and curvature radius R 2 , in the direction perpendicular to recording tracks that is perpendicular to the extended direction of grooves, of a curved portion, that is, the inside edge portion  2 C of the groove  2  which is adjacent to the land  3  are respectively specified. 
   Specifically, the curvature radius R 1  is set to 30 nm or less, and the curvature radius R 2  is set to 20 nm or more. 
   Further, as mentioned above, both the lower limit of R 1  and the upper limit of R 2  are practically determined as values approximately with which the shape and stepped differences to obtain required tracking servo signals are obtained. 
   In order to clarify the characteristics of the magneto-optical recording medium according to the present invention, magneto-optical recording media of type-A and type-B are prepared. 
   In type-A, the curvature radiuses R 2  of both the side edges  2 C of the groove  2  are set to 20 nm, and the curvature radius R 1  of the shoulder portion  3 S of the land  3  is set to 69 nm. 
   Also, in type-B, the curvature radius R 1  of the shoulder portion  3 S of the land  3  is set to 30 nm, and the curvature radiuses R 2  of both the side edges  2 C of the groove  2  are set to 7 nm. 
     FIGS. 4A and 4B  are the schematic perspective views of lands and grooves of the recording medium substrates  1  of the type-A and type-B that are drawn based upon AFM (Atomic Force Microscope) photographs. 
   Each of the magneto-optical recording media of those type-A and type-B, comprises the substrate  1  composed of polycarbonate of 0.6 mm in thickness, on which a dielectric film made of an SiN film of 40 nm in thickness, a displacement layer made of a GdFeCo film of 40 nm in thickness, a control layer made of a TbFeCo film of 3 nm in thickness, a separating layer made of a TbFeCoAl film of 10 nm in thickness, a recording layer made of a TbFeCo film of 50 nm in thickness, a recording auxiliary layer made of a GdFeCo film of 10 nm in thickness, a dielectric layer made of an SiN film of 30 nm in thickness, and a reflective film or temperature control film made of an Al alloy film of 300 nm in thickness are formed sequentially. 
   Here, the control layer functions as a layer that controls displacement of the domain walls to reduce noise at the time of reproduction, and the recording auxiliary layer is a layer to enhance the sensitivity to the magnetic field at the time of recording. 
   The measurement results of recording power margin of the magneto-optical recording media of the type-A and type-B are shown in  FIGS. 5A and 5B , respectively. 
   In this case, the track pitch of 0.44 μm, the pit length of 0.09 μm, and the duty ratio of the recording pulse of 33% are each set. 
   Characteristic curves  51 A and  51 B in  FIGS. 5A and 5B , respectively are the curves in which measurement results of jitter of the reproduction signals of a main groove are plotted, in the case where data recording is respectively performed on the main groove and on adjacent grooves next to the main groove and over writing (O. W.) of another recording data is performed onto the data written in the main groove. 
   Further, characteristic curves  52 A and  52 B are the curves in which, first, data recording is performed on the main groove, and subsequently the data recording is performed onto both the adjacent grooves, and then returned to the main groove and jitter is measured to plot the results thereof. Accordingly, in this case, influence caused by the recording onto the next groove, namely, cross write (C.W.) is measured. 
   Here, if the jitter of 15% or lower is allowable, in the type-A, the range in which both the jitters of  51 A and  52 A become 15% or lower is approximately 6.7 mW±13%, compared with that, the range thereof with respect to  51 B and  52 B is 7.6 mW±26%, so that the recording power margin is expanded in the type-B. 
   Specifically, in the type-B in which the shoulder portion  3 S of the land  3  has a steeper slope of 30 nm than conventional ones, the recording power margin is expanded compared with the type-A in which the shoulder portion  3 S of the land  3  has a gentle slope. 
   It is understood that when the curve of the shoulder portion  3 S of the land  3  is steep, the heat caused by irradiating the groove  2  with laser light at the time of recording is not easily diffused over the land. Therefore, as the improvement of the recording density may be attempted and the track pitch is getting narrower than now, the curvature radius of the shoulder portion  3 S of the land  3  is desired to be smaller than 30 nm of type-B. In other words, the inside angle of this shoulder portion  3 S may be smaller than 90°. 
   However, with respect to jitter bottom, the type-A is lower than the type-B. This is considered that in the type-A, because the curvature radiuses R 2  of both the edge portions  2 C of the groove  2  are set to 20 nm with gentle curve compared with the type-B in which the curvature radius R 2  is 7 nm, displacement of the domain walls of the recording marks can occur easily at the time of reproduction. Thus, in order to lower the jitter bottom, it is desirable for the curvature radiuses of both the edge portions  2 C of the groove  2  to be set to at least 20 nm or more. 
   As a result, in the construction according to this invention, in the magneto-optical recording medium substrate  1  by means of the DWDD the curvature radius R 1  of the shoulder portion  3 S of the land  3  is selected to be 30 nm or less, and the curvature radiuses R 2  of both the edge portions  2 C of the groove  2  is at least 20 nm or more. 
   Next, an example of a method for manufacturing a magneto-optical recording medium having the curvature radius R 1  of the shoulder portion  3 S of the land  3  and the curvature radiuses R 2  of both the edge portions  2 C of the groove  2  according to the present invention is explained referring to  FIGS. 6 through 8 . 
   In this case, the concave and convex surface  4  having the groove  2  where the tracking servo signals are obtained at the time of recording and reproducing and the land  3  that separates those grooves is formed in the magneto-optical recording medium substrate  1 , and the concave and convex surface  4  is formed such that, similarly to a conventional method, when the substrate  1  is injection-molded out of plastics such as polycarbonate, with placing the stamper that has the inverted concave and convex surface  4  in a mold cavity to integrally form the concave and convex surface  4  with the formation of the substrate  1 . 
   Alternatively, the concave and convex surface  4  is formed by the so-called 2P (Photo Polymerization) method, in which the substrate  1  is coated with ultra-violet curing resin to form a resin layer, onto which a stamper having the inverted concave and convex surface  4  is pressed to transfer and form the surface  4 , and then ultra violet ray is irradiated to cure the resin. 
   In this method, processes for fabricating the master of this stamper are employed. A method for manufacturing the stamper is explained referring to process views in  FIGS. 6 and 7 . 
   First, as shown in  FIG. 6A , a substrate  21 , for example, a glass substrate having a plane surface to form the master is provided, and the plane surface of the substrate is coated with a laminated resist layer  33  comprising a lower resist layer  31  formed of, for example, Si-based photo-resist and an upper resist layer  32  formed of, for example, novolak-based photo-resist. 
   As shown in  FIG. 6B , electron beam lithography or pattern exposure by optical exposing is performed onto the laminated resist layer consisting of the lower resist layer  31  and upper resist layer  32 , and development processing is performed to form openings  33 W at the portions where grooves of the above aimed concave and convex surface  4  are formed. RIE (Reactive Ion Etching) is performed from the upper side of the laminated resist layer  33 , and as shown in  FIG. 6C , when the upper resist layer  32  is etched, the surface of the substrate  21  that exposes to the outside through the opening  33  is etched at the same time. 
   Then, oxygen plasma processing is performed. With this, as shown in enlarged scale in  FIG. 8A , a portion marked with a circle a in  FIG. 6C  remains with the resist layer  31  of a small etching rate, and in the portion where the opening  33 W is formed the oxygen plasma processing is performed under the condition of the concave portion being formed, so that in a convex portion  213 , a shoulder portion thereof becomes steep and in a concave portion  212 , the side edge thereof having a gentle curve is formed. 
   Subsequently, as shown in  FIGS. 6D and 8B , the resist layer  33  is dissolved and removed. As described above, the concave portion  212  having the shape and depth corresponding to the aforementioned groove  2 , and the land  213  corresponding to the land  3  that separates the concave portions  212  are formed. Consequently, a master  35  for manufacturing the stamper is formed. 
   As shown in  FIG. 6E , a nickel plating layer  36 , for example, is formed on the master  35  by electroless plating and electrolytic plating, for example. 
   As shown in  FIG. 7A , the plating layer  36  is separated from the master  35 , and the stamper  36  with the concave and convex surface having an inverted pattern of the master is obtained. Alternatively, using the above as a master stamper, the stamper  36  is formed by repeating similar plating and separating processes. 
   Then, as shown in  FIG. 7B , using the stamper  36  in the 2P method or the injection molding, the aimed magneto-optical recording medium  1  having the concave and convex surface  4  provided with the grooves  2  and lands  3  is obtained. 
   Accordingly, the substrate  1  having the above requiring concave and convex surface  4  in which the land  3  that has the shoulder portion  3 S with the small curvature radius R 1  and the groove  2  that has the side portion  2 C with the large curvature radius R 2  are formed is provided. Further, curvature radiuses R 1  and R 2  can be determined as R 1 ≦30 nm and R 2 ≧30 nm respectively by selecting conditions of the above mentioned RIE and oxygen plasma processing, for example, by selecting processing time and others. 
   Then, with forming the coating layer  5  having the film structure shown in, for example,  FIG. 2  on this substrate  1 , the magneto-optical recording medium is obtained. 
   As mentioned above, according to the magneto-optical recording medium by means of the DWDD of the present invention, the curvature radius R 1  of the shoulder portion  3 S of the land  3  and the curvature radius R 2  of the both edge portions  2 C of the groove  2  are specified, so that a wide recording power margin can be obtained, the bottom jitter can be low, and the error rate can be improved. 
   Furthermore, according to the manufacturing method of the present invention, by performing RIE for the laminated resist layer  33  that has the resist layer  31  with the small etching rate in its lower layer, the magneto-optical recording medium of the present invention can be manufactured without increase in production processes. 
   Although in the above described embodiment the photoresist layer is formed by laminating two layers of the lower and upper thereof, for practical purposes, since the groove has the shallow depth of approximately 35 nm, the following method can be used in which the single resist layer is employed, RIE is performed and the single resist layer remains with the required thickness, and then the oxygen plasma processing is performed. 
   DESCRIPTION OF REFERENCE NUMERALS 
   
       
         1  . . . RECORDING MEDIUM SUBSTRATE 
         2  . . . GROOVE 
         2   c  . . . SIDE EDGE PORTION 
         3  . . . LAND 
         3   s  . . . SHOULDER PORTION 
         4  . . . CONCAVE AND CONVEX SURFACE 
         5  . . . LAMINATED FILM 
         6  . . . DIELECTRIC LAYER 
         7  . . . DISPLACEMENT LAYER 
         8  . . . SEPARATING LAYER 
         9  . . . RECORDING LAYER 
         11  . . . PROTECTIVE LAYER 
         21  . . . MASTER SUBSTRATE 
         31  . . . LOWER RESIST LAYER 
         32  . . . UPPER RESIST LAYER 
         33  . . . LAMINATED RESIST 
         34  . . . CONCAVE PORTION 
         35  . . . MASTER 
         36  . . . PLATING LAYER 
         37  . . . STAMPER