Abstract:
A magneto-optical recording medium in a near-field optical storage system is provided. A flying optical head is suspended over the medium by a cushion of gas. The medium includes a magneto-optical recording layer including at least one recording track for magneto-optical recording of information. A tracking feature is associated with the recording track. An upper transparent dielectric layer is provided having an upper surface which is substantially planar over a recordable area of the medium above the recording track and the tracking feature. A reflector layer is positioned below the magneto-optical layer above a base substrate.

Description:
TECHNICAL FIELD 
     This invention relates to optical data storage, and more particularly to recordable magneto-optical storage media for use in a near field system. 
     BACKGROUND 
     A variety of optical storage media and technologies exist. These include media which may not be recorded on by the user (often referred as read-only memory (ROM) as in CD-ROM). Also included are user recordable media (frequently designated as write-once-read-many WORM) and user re-recordable media. One area of user recordable and/or re-recordable media involves magneto-optical technology. A typical magneto-optical disk drive features a magnetic recording head and at least one laser. Bits of information are recorded in discrete locations (“domains”) along the lengths of tracks spirally spanning the disk. In one form, the information is read via light from a read laser reflected by the disk. The nature of the reflected light is influenced by particles within the disk. To record a bit of information at a particular domain, a write laser may heat the domain to a condition wherein the magnetic head may apply a field to the domain to align the particles in that domain in a particular orientation corresponding to the state of the associated bit. Subsequently, with the particles frozen in the desired state, the read laser may be used to read the state of the bit. 
     FIG. 1 shows one conventional magneto-optical disk  20 . The bulk of the disk  20  may be formed by a substrate  22  such as a polycarbonate disk. The flat upper surface  24  of the substrate  22  forms the upper surface of the disk. The surface  24  is in close facing proximity to the underside  28  of the head  30  as is described in further detail below. For reference, unless specified to the contrary, the “upward” direction  500  shall refer to the local direction from the media to the head when the head is in position to read or write data from or to the media. It is understood that the media and head may be placed in a variety of absolute orientations. 
     The lower surface  32  of the substrate may be formed with a series of alternating spiral grooved and ungrooved areas  34  and  36 , respectively. A first dielectric layer  38  may be applied to the lower surface  32  of the substrate  22 . A magneto-optical layer  40  may be applied to the lower surface  42  of the first dielectric layer. A second dielectric layer  44  may be applied to the lower surface  46  of the magneto-optical layer. A reflective layer  48  may be applied to the lower surface  50  of the dielectric layer. A protective layer  52  may be applied to the lower surface  54  of the reflective layer. The portions of the magneto-optical layer  40  below the grooved areas  34  define the tracks  56  on which bits of information may be recorded. 
     In operation, the disk  20  is rotated at high speed about its central axis (not shown). The magneto-optical head  60 , which does not rotate with the disk, may be reciprocated approximately radially relative to the disk&#39;s central axis to access the various tracks on the disk. 
     A common head design for data storage systems is the “flying head”. With a flying head, the relative motion between the disk and head caused by the rotation of the disk produces a flow of air between the head and the upper surface of the disk. The flow of air prevents the head from colliding with the disk and allows the head to maintain its close facing relationship with the upper surface of the disk. This is achieved by providing the lower surface of the head with appropriate air bearing surfaces  62 . 
     The inconsistent spacing between the air bearing surfaces and the disk presents a number of difficulties. First, the stability of the head may be affected as the head moves across the disk. Second, the interaction of the head with the disk is harder to computationally model than with a uniformly flat upper disk surface. Such modeling is important if it is desired to economically alter the properties of the head, the geometry of the size and spacing of the tracks, or the rotational speed of the disk. 
     Accordingly, in one aspect, the invention is directed to a magneto-optical recording medium in a near-field optical storage system. A flying optical medium is suspended over the medium by a cushion of gas. The head includes a magneto-optical recording layer having at least one recording track for magneto-optical recording of information. At least one tracking feature is associated with the track. An upper transparent dielectric layer is provided with an upper surface which is substantially planar over a recordable area of the medium above the recording track and the tracking feature. A reflector layer is positioned below the magneto-optical layer above a base substrate. The upper dielectric layer has an upper surface which is substantially flat for presenting the flying optical head with a substantially uniform cushion of air between the upper surface of this dielectric layer and the air bearing surface of the flying optical head. 
     Implementations of the invention may include one or more of the following features. A lower transparent dielectric layer may be positioned below the magneto-optical recording layer and above the reflector layer. An upper surface of the recording track and the upper transparent dielectric layer may be separated by a distance between approximately 100 and 1 μm. The medium may be a disk. The reflector layer may be metallic. 
     The recording track and the tracking feature may each include annular features. The tracking feature may include a depression formed in an upper surface of at least one of: the substrate; the reflector layer; the lower transparent dielectric layer; and the magneto-optical recording layer. The depression may be formed by a groove in an upper surface of the substrate, the groove propagating the depression upward through the reflector layer, the lower transparent dielectric layer and the magneto-optical recording layer. 
     Either the upper or the lower dielectric layer may include a high index dielectric material, where the other of the layers includes a first sublayer of low index dielectric material and a second sublayer of high index dielectric material. 
     The high index dielectric material may be silicon nitride. The low index dielectric material may be silicon oxide. The magneto-optic layer may be made of a rare earth-transition metal compound including TbFeCo. 
     The groove has a groove width and a groove separation, a ratio of the groove width to groove separation may be less than about 7:15. The groove width may be less than about 0.175 μm. 
     In another aspect, the invention is directed to a near-field magneto-optical storage system. The system includes an optical head having an air-bearing surface, a laser for emitting a beam of light having a wavelength less than about 1 μm, a lens, a drive motor, and a magneto-optical storage disk. 
     The disk has an upper surface having a flat portion for interacting with the optical head via a substantially uniform cushion of air between the flat portion and the air bearing surface of the optical head. A magneto-optical recording layer is formed below the upper surface and includes at least one recording track for magneto-optical storage of information, which information is readable by the laser. The upper surface along the track is separated by a distance smaller than the wavelength from the lens. At least one tracking feature is associated with the recording track. A reflector layer is formed below the magneto-optical recording layer, below which is a substrate. 
     Implementations of the invention may include one or more of the following. An operational distance between the lens and the upper flat surface along the track is less than about 150nm. 
     In another aspect, the invention is directed to a medium in an optical storage system where a flying optical head is suspended over the medium by a cushion of gas. The medium includes a data layer having at least one track for storing information, at least one tracking feature associated with the at least one track, a dielectric layer having an upper surface substantially planar over a data area of the medium above the track and the tracking feature. The upper surface of the dielectric layer substantially forms an upper medium surface over the data area. A base substrate is provided below the data layer. The data layer may store read-only data or may store phase change media data. 
     Among the advantages made possible by the invention are improved dynamic coupling between the head and the exposed disk surface and improved tracking which may be balanced with a higher track density. 
     By presenting a substantially flat and smooth surface moving relative to the head, the aerodynamic properties of the layer of air trapped between the head become significantly uniform as the head moves radially. Furthermore, the aerodynamic properties become easier to model, both computationally and experimentally. This facilitates an easier process of designing heads and their associated actuation mechanisms. Additionally, changes may be made to the internal disk structure which would otherwise alter groove geometry without affecting the interaction between the disk and the head. 
     By reducing the portion of the disk between the groove and head occupied by air, the optical signal is increased. Additionally, because the dielectric layer filling the groove has a higher index of refraction than air, the optimal groove depth becomes smaller. 
    
    
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a partial schematic cross-sectional view of a prior art magneto-optical storage medium and head assembly. 
     FIG. 2 is a partial schematic cross-sectional view of a magneto-optical storage medium. 
     FIG. 3 is a partial schematic cross-sectional view of magneto-optical storage medium according to an embodiment of the invention. 
     FIG. 4 is a partial schematic cross-sectional view of an upper dielectric layer of a magneto-optical storage medium according to an embodiment of the invention. 
     FIG. 5 is a partial schematic cross-sectional view of a lower dielectric layer of a magneto-optical storage medium according to an embodiment of the invention. 
     FIG. 6 is a schematic view of a magneto-optical storage system according to an embodiment of the invention. 
     Like reference numbers and designations in the various drawings indicate like elements. 
    
    
     DETAILED DESCRIPTION 
     Commonly assigned, copending U.S. patent application Ser. No. 08/846,916, filed Apr. 29, 1997, the disclosure of which is incorporated herein by reference, discloses a pioneering form of magneto-optical medium for first surface recording utilizing the near field effect. One such disk  70  is shown in FIG.  2 . In the illustrated embodiment of the disk  70 , a substrate  72  is at the bottom of a stack of layers. Spiral grooves  74  are formed in the upper surface  76  of the substrate. In the illustrated embodiment, the grooves are flat-bottomed channels, evenly radially spaced so as to define a flat-topped land  78  between each pair of adjacent grooves  74 . A reflective layer  80  is formed atop the upper surface  76  of the substrate. The grooves  74  and lands  78  propagate through to the upper surface  82  of the reflective layer  80 . A lower dielectric layer  84  is formed atop the reflective layer  80 . The grooves and lands propagate through to the upper surface  86  of the lower dielectric layer. A magneto-optical layer  88  is formed atop the lower dielectric layer  84 . The grooves and lands propagate through to the upper surface  90  of the magneto-optical layer  88 . An upper dielectric layer  92  is formed atop the magneto-optical layer  88 . The grooves and lands propagate through to the upper surface  94  of the upper dielectric layer  92 . The upper surface  94  of the upper dielectric layer  92  forms the upper surface of the disk  70 . 
     As shown in FIG. 2, the upper surface of the disk has a portion  95 A above each land and a portion  95 B above each groove. The portion  95 A is at a height S 1  above the portion  95 B. In operation, a head  96  is at a nominally fixed height above the disk. As the head moves radially over the disk, the spacing between a given point on the head and the portion of the disk immediately therebelow will vary by the height S 1 . In the illustrated embodiment, the height S 1  is approximately the same as the land height D 1 . 
     The head  96  carries a lens  98  and has air bearing surfaces  99 . In the near-field regime, the spacing T 1  between the upper surface of the upper dielectric layer and the lens  98  is constrained to be less than λ. Exemplary values for these various parameters may be λ=685 nm, T 1 =100 nm, S 1 =70 nm, and R 1 =50 nm. 
     FIG. 3 shows a disk  170  which may be generally similar to disk  70  of FIG. 2, with the key exception that the upper surface  194  of the upper dielectric layer  192  is substantially planar. Thus, grooves  174  formed in the upper surface  176  of the substrate  172  propagate through to the respective upper surfaces  182 ,  186  and  190  of the reflective layer  180 , the lower dielectric layer  184  and the magneto-optical layer  188 . 
     The effective optical path length for a light ray traveling from the lens  98  to a layer of the disk and back will be twice the sum of the products of the distance traveled through each layer (including the layer of air between the head and the upper surface of the disk) and the index of refraction of such layer. The present invention influences the effective optical path length from the head to the magneto-optical layer along the disk grooves. Using vertical distance as an approximation, with reference to FIG. 3, the effective optical path length along a groove is equal to 2 (T 2 n air +S 2 n dielectric ). T 2  is the distance between the lens and the upper surface  194  of the dielectric layer and disk and S 2  is the height (thickness) of the upper dielectric layer  192  along a groove. For the disk of FIG. 2, the equivalent path length is 2 (T 1 n air +S 1 n air +R 1 n dielectric ). In this example T 1  is the height of the lens above the surface portion  95 A and S 1  is the distance between the portion  95 A and the upper surface  94  of the upper dielectric layer  92  along a groove. R 1  is the thickness of the upper dielectric layer (along both grooves and lands). In the exemplary embodiment, upper dielectric layers  92  and  192  may be formed of SiN, which has an index of refraction n dielectric ≈2. If it is desired that the effective path lengths be the same as each other, then T 1 n air +S 1 n air +R 1 n dielectric =T 2 n air +S 2 n dielectric . If T 2  is identical to T 1 , then S 1 n air +R 1 n dielectric =S 2 n dielectric . Substituting n dielectric +2 and n air =1, S 1 +2R 1 =2S 2 . Thus, S 2 =½S 1 +R 1 . The groove depth D 1  in the embodiment in FIG. 2 is approximately S 1 . In the embodiment of FIG. 3, if the height of the second dielectric layer  192  above the lands  178  is kept the same as the thickness R 1  of the second dielectric layer  92  of FIG. 2, the groove depth D 2  in the embodiment of FIG. 3 is approximately S 2 -R 1 . Substituting for S 2 , D 2 +R 1 =½S 1 +R 1 . Canceling and substituting for S 1 , D 2 =½D 1 . Thus, it can be seen that in the embodiment of FIG. 3, the groove depth D 2  may be reduced significantly relative to the groove depth D 1  of the embodiment of FIG.  2 . The reduction in groove depth facilitates a reduction in the groove width W 2  and in the pitch P 2  or distance between adjacent tracks. This in part arises as the shallower grooves may be formed more precisely than the deeper grooves causing less lateral distance to be lost in the transition between the flat bottom of the groove and the flat top of the adjacent lands. 
     In the exemplary embodiment, the substrate may be made of polycarbonate or a similarly lightweight and rigid material. The reflector layer may be made of aluminum. The magneto-optical layer may be made of a rare earth-transition metal compound including TbFeCo. The dielectric layers may be made of SiN. 
     In an alternate embodiment shown in FIG. 4, the second dielectric layer  192  may comprise a lower sublayer  192 A and an upper sublayer  192 B. In an exemplary embodiment, the lower sublayer  192 A may comprise a low index dielectric material formed on the magneto-optical layer and the upper sublayer  192 B may comprise a high index dielectric material. In an exemplary embodiment, the lower sublayer may comprise SiO 2  and the upper sublayer may be made of SiN. Various materials, compositions and dimensions for the various layers are described in the co-pending application identified above. 
     Similarly, as shown in FIG. 5, the first dielectric  184  may comprise lower sublayer  184 A and an upper sublayer  184 B. In an exemplary embodiment, the lower sublayer  184 A may comprise a high index dielectric material formed on the reflector layer and the upper sublayer  184 B may be made of a low index dielectric material. It should be noted that layer  184  may be omitted altogether if desired. 
     Preferred methods of applying a planarizing layer, e.g., the second dielectric layer in FIG. 4, include chemical vapor deposition (CVD), sputtering, and dipping or spin coating such as with a sol gel. In the illustrated embodiment, a preferred ratio of the groove width W 2  to the groove separation or pitch P 2  is less than about 7:15. In the illustrated embodiment, a preferred groove width W 2  is less than about 0.175 μm. 
     FIG. 6 shows a near field magneto-optical storage system  110  for use with the disk  170 . The system may incorporate the head  96  and includes a drive motor  112 . The disk  170  is carried within the system for rotation about its central axis  502  driven by the drive motor  112 . The head  96  and drive motor  112  are coupled to a control system  114  which may include a microprocessor, programmed with appropriate control software. 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, a variety of complex layerings and track structures are possible as are a variety of medium arrangements. The invention may be utilized in the context of read-only media and phase change media, besides the magneto-optic media described in the embodiments above. In these cases, a planarizing layer may be located at any point in the structure, not just at the topmost layer as described in the embodiments above. Accordingly, other embodiments are within the scope of the following claims.