Optical storage medium
FIELD OF THE INVENTION
The present invention generally relates to storage media and to a method of storing data. In particular, the invention is directed to optical storage media for storing data in a ROM format and a recordable format, to a stamper for structuring an optical storage medium, and to a method of producing an optical storage medium.
BACKGROUND OF THE INVENTION
Next generation optical storage devices are for example based on blue laser diodes as a light source for reading and writing processes. There are two competing technological concepts: HD-DVD (High Definition DVD) and blu-ray disc (BD) differing in technical aspects like information storage capacity, numerical aperture, etc. For each concept several data storage formats for different applications and disc structures have been developed. For example, besides the pre-recorded BD-ROM there are BD-R (recordable) and BD-RE (re-writable) formats in single layer and dual layer technology available. Recently, a hybrid format combining BD-ROM and BD-R/RE functionality is under development. Such a format is highly appreciated in many high-density applications, since it allows for reading pre-recorded data and writing/reading new, personalized data. For example, eBooks (electronic/digital books) or computer and video games are ROM based applications. Usually, there are several possibilities of storing personal data in an internal memory like flash memories, hard discs, etc. However, integrating recording and ROM functionality in a single so-called hybrid disc is advantageous for several reasons. Regarding the eBook application mentioned above, the user, e.g. a student, is able to take the hybrid disc anywhere and is able to store notes or essays directly on the disc. Thus, the digital content becomes device independent and the user is able work at home and at school without the need of taking a reading/writing device with him. The same holds for computer game applications. Computer and video games are highly sophisticated software, which typically comprises different game levels and corresponding scenes. When a user has to interrupt the game, he may want to continue the game at the point where he stopped playing. The settings and levels

of the game can then be stored on the game disc and the user can take the disc to a different location to continue playing.
Such a hybrid medium as described above may be created either by providing at least two storage layers on a disc (multi-layer disc), wherein one layer is structured in a ROM format and the other layer in a recordable format, or by designing discs with a single layer containing both ROM and recordable sections. Combining a ROM and a recordable data storage functionality in a single layer seems to be promising. However, integrating the two different formats including the shape of the pre-grooves for writing and the pits of the ROM format as well as realizing the corresponding readout and recording stack in a single layer is a difficult task. The challenge lies in finding an information stack that can simultaneously be used for the readout of ROM data and recording of new data.
It is an object of the invention to provide an optical storage medium and a method of storing optical data supporting a ROM and a recordable format for a single layer disc.
SUMMARY OF THE INVENTION
This object is solved by the features of the independent claims. Further developments and preferred embodiments of the invention are outlined in the dependent claims.
In accordance with a first aspect of the invention, there is provided an optical storage medium comprising
a substrate layer having
a pit structure storing optical data, and
a groove structure for storing optical data by projecting a laser light beam thereon; and
a recording layer stack deposited above the substrate layer, comprising a first dielectric layer, a first recording layer, a second recording layer, and a second dielectric layer, characterized by
the absence of a metal layer between the recording layer stack and the substrate layer, or
the presence of an aluminum layer having a thickness of between 5nm and 30nm between the recording layer stack and the substrate layer.
The recording stack may be deposited on the substrate layer comprising areas with pre-recorded ROM data - the pit structure - and providing areas for user recordable data - the groove structure. The first and the second recording layer are responsive to energy applied by a laser light beam. The recording layers change their reflectivity by applying energy exceeding a certain level. This change in reflectivity is detected during the readout operation of the laser light beam with reduced light power and can thus be utilized for storing data in the groove structure. Compared with recording stacks currently applied in optical recording media there is, according to a first embodiment of the invention, no metal layer provided between the substrate layer and the first dielectric layer. This allows for combining the ROM and the recording functionality in a single layer medium in a efficient way. The variation in reflectivity takes place in or on the pre-grooves, whereas the information in the pre-recorded pit structure can be read out independently of the information stored in the groove structure. The absence of a metal layer in between the substrate layer and the recording stack reduces the influence of the recording stack on the readout properties of the pit structure, thus providing an excellent readout functionality. According to a second embodiment of the invention, an aluminum layer with a significantly reduced thickness compared with prior art layers like heat sinks in between the recording layer stack and the substrate layer is provided. The aluminum layer with a thickness of between 5nm and 30nm leads to a higher stack absorption and improves the recording properties of the recording stack.
In this regard, it is advantageous that the substrate layer comprises at least one first region comprising adjacent tracks having a pit structure, and at least one second region comprising adjacent grooves. Separating areas having a pit structure from areas having a groove structure enables an adaptation of the data storage capacity of the pit structure related to the groove structure, according to the intended application. In some cases, only a small portion of the total data storage capacity has to be allocated for user recordable data, whereas for other applications it may be the other way round.
Preferably, one of the recording layers comprises Cu and the other recording layer comprises Si. The Cu/Si technology provides a write-once characteristic by changing the layer reflectivity after applying laser energy.
According to a particular embodiment of the present invention, the pit structure conforms with the BD-ROM format (BD = Blu-ray Disc) and the groove structure

conforms with the BD-R/RE format (BD-R = Blu-ray Disc Recordable/REwriteable). Thus, the advantages of the invention can be utilized in the context of the Blu-ray system and in particular for Blu-ray discs.
In accordance with an embodiment of the invention, the dielectric layers comprise ZnS-Si02. The ZnS-SiCO2 layers act as protective layers for the recordable layer stack.
According to a preferred embodiment of the present invention, the layer thickness of the dielectric layers is between lOnm and 50nm, and the layer thickness of the recording layers is between 4hm and 15nm. In a highly preferred embodiment the thickness of the dielectric layers is between lOnm and 20nm and the thickness of the recording layers is 7nm. With these parameters good recording and readout performance can be realized.
According to a preferred embodiment of the invention, the mean depth of the pits of the pit structure is greater than the mean depth of the grooves of the groove structure. The usage of two different depths for the groove structure and the pit structure in a hybrid optical storage medium ensures a better compatibility with optical storage media either comprising only pre-recorded data or only pre-grooves for recording. In this context, preferably one or more sections of the substrate are dedicated to the pit structure and one or more other sections are dedicated to the groove structure.
Accordingly, the pits of the pit structure have a mean depth of between 40nm and l00nm, preferably of between 60 and 80nm. This depth values correspond to the standard target depth of pit structures for optical storage media with pre-recorded data.
Preferably, the grooves of the groove structure have a mean depth of between l0nm and 40 nm, more preferably of between 25nm and 27nm. Thus, the hybrid disc is compatible with non-hybrid optical storage media as far as its groove structure is concerned.
According to an alternative embodiment of the invention, the mean depth of the pits of the pit structure substantially equals the mean depth of the grooves of the groove structure. The same depth for the pit structure and the groove structure facilitates the manufacturing of the hybrid optical storage media.
Particularly, the pits of the pit structure and the grooves of the groove structure have a mean depth of between 40nm and l00nm, preferably of between 60 and 80nm. As mentioned above, this depth corresponds to the standard target depth of pit structures for optical storage media with pre-recorded data.
In accordance with a second aspect of the invention, there is provided a stamper for structuring an optical storage medium according to the present invention.

In accordance with a third aspect of the invention, there is provided a method of producing an optical storage medium, comprising the steps of: providing a substrate layer structuring the substrate layer with
a pit structure storing optical data, and
a groove structure for storing optical data by projecting a laser light beam thereon;
- ' depositing above the substrate layer a recording layer stack comprising
a first dielectric layer, a first recording layer, a second recording layer, and a second dielectric layer, characterized by
depositing the recording layer stack directly on the substrate layer, or depositing an aluminum layer having a thickness of between 5nm and 30nm between the recording layer stack and the substrate layer.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows in a schematic diagram a view of a central cross-section of a first embodiment of a hybrid BD-ROM/R disc according to the present invention.
Figure 2 shows in a schematic diagram a view of a central cross-section of a second embodiment of a hybrid BD-ROM/R disc according to the present invention.
Figure 3 shows in a schematic diagram a view of a central cross-section of a third embodiment of a hybrid BD-ROM/R disc according to the present invention.
Figure 4 shows in a schematic diagram a view of a central cross-section of a fourth embodiment of a hybrid BD-ROM/R disc according to the present invention.
Figure 5 shows a perspective view of a surface section of a DVD-RW stamper with structures according to the invention.
Figure 6 shows a top view of a surface section of a DVD-RW stamper with structures according to the invention.
Figure 7 shows a flow diagram for producing an optical storage medium according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS
The invention is described in terms of several preferred embodiments with reference to the attached figures wherein like reference numerals refer to like elements throughout. It should be noted that the dimensions of the schematic diagrams of Figures 1 to 4 are not necessarily drawn to scale.
Figure 1 shows in a schematic diagram a view of a central cross-section of a first embodiment of a hybrid BD-ROM/R disc according to the present invention. A substrate 12 is structured with two different structures having different depths. The first structure is a pit structure 14 storing pre-recorded ROM data in the BD-ROM format. The pit structure 14 is formed by a pit land structure. The target depth of the pit structure 14 is 80nm. The second structure is a pre-groove structure 18 having pits with a target depth of 25nm. On the substrate 12 a recording stack 24 is deposited, comprising a Cu-Si stack 28, 30 sandwiched by two dielectric layers 26, 32. Accordingly, the resulting recording stack 24 has the following composition: A first dielectric layer 26 of ZnS-SiO2 having a thickness of l0nm, followed by a first recording layer 28 of Cu having a thickness of 7nm and a second recording layer 30 of Si having a thickness of 7nm, completed by a second dielectric layer 32 of ZnS-Si02 having a thickness of 20nm. According to the invention, no metal layer like a heat sink is arranged in between the substrate layer 12 and the recording stack 24. As already mentioned above, the thicknesses of the different layers may vary. For example, the dielectric layers 26, 32 may have a thickness of between l0nm and 50nm, and the recording layer 28, 30 may have a thickness of between 4nm and 15nm. Additionally, onto the recording stack 24 a cover layer 50 having a thickness of l00um is provided for protecting the recording stack 24. Perpendicular to the drawing plane the pit structure 14 and the groove structure 18 extend in the plane of the optical storage disc (not shown) and form a track structure, as will be explained in relation to Figures 5 and 6. The distance between adjacent tracks, the track pitch, is 320nm in this example.
The pit structure 14 contains the pre-recorded data information and can be read out by applying a laser beam with a certain reading power. In the groove structure 18 the data to be recorded by the user are generated by applying a laser light beam 22 with a higher power as compared with the reading power onto the recording stack 24 of the groove structure 18. Thereby, the reflectivity of the recording layers 26, 28 changes irreversibly. By projecting a laser beam having a reading power onto the groove structure 18 this change in reflectivity is detected and interpreted accordingly. Although with this embodiment the
ecording stack 24 also extends over the pit structure 14, an excellent readout quality can be achieved. At the same time, despite the absence of a metal layer good recording result can be expected. For verifying the quality of the pre-recorded ROM data, a Cu-Si recording stack was sputter-deposited on a 25GB BD-ROM substrate resulting in a layer stack as described above. For comparison, a reference mirror as used in standard BD-ROM discs (15nm AITi) was utilized. The stack was provided with a l00µm cover layer to analyze the data. By an eye-pattern diagram analysis of the readout signal and an inter-symbol interference analysis of the data distribution the following results were obtained: The corresponding jitter value was 6.2% (limit equalizer jitter, 25GB BD-ROM, track pitch 320 nm, channel bit length CBL=74.5 nm, asymmetry 0.11%). The asymmetry of the eye-pattem was considered to be still on the low side indicating the possibility of further improvements. In order to qualify the recording quality of the groove structure 18, marks were also written in the Cu-Si stack. The contrast was quite high and the modulation was still on the low side (0.24).
Figure 2 shows in a schematic diagram a view of a central cross-section of a second embodiment of a hybrid BD-ROM/R disc according to the present invention. This second embodiment comprises in addition to the first embodiment a metal (Al) layer 34. This additional Al layer 34 is arranged in between the substrate layer 12 and the first dielectric layer 26, i.e. the recording stack 24. The thickness of this metal layer 34 is 8nm and may vary between 5nm and 30nm and results in a higher stack absorption of 77%, which improves the recording characteristics of the hybrid recording layer stack 24.
Figures 3 and 4 show in schematic diagrams views of central cross-sections of third and fourth embodiments of a hybrid BD-ROM/R disc according to the present invention. The layer structure of the third and fourth embodiments corresponds to that of the first and second embodiments, respectively. The substrate 12 is structured with a pit structure 16 and with a groove structure 20. Compared with the structures 14, 18 of the first and second embodiments, the pit structure 16 and the groove structure 20 have essentially the same target depth of 80nm. Thus, the manufacturing of the hybrid BD-ROM/R may be alleviated, as already discussed above.
Figures 5 and 6 show a perspective and a top view of a surface section and a corresponding section analysis of a stamper 44 with structures according to the invention. The stamper 44 shows a first area 36 having adjacent first tracks 52 with bumps 40 for forming a pit structure 14 (see figures 1 to 4), and a second area 38 having adjacent second tracks 54 with bumps 42 for forming a groove structure 18 (see figures 1 to 4). The first and second tracks 52, 54 are arranged in a concentric manner in relation to the optical storage
8 disc symmetry. The height of the bumps 40 for forming the pit structure 14 is 83nm, the height of the bumps 42 for forming the groove structure is 30nm.
Figure 7 shows a flow diagram for producing an optical storage medium according to the invention. In step SO1, a substrate layer is provided. The substrate layer is structured by stamper embossing in step S02. In this step, the pit structure for ROM data and the groove structure for recordable data are generated. Both structures are arranged in tracks. Usually, a spiral arrangement is used for arranging the tracks on the disc. After step S02, an Al layer with a thickness of between 5nm and 30nm is optionally deposited on the structured substrate (S03). Alternatively, a recording stack comprising of several subsequently deposited layers is directly deposited on the substrate layer (S04). As discussed above, the recording stack comprises a Cu/Si layer stack sandwiched by two dielectric layers. Afterwards, in step S05 a cover layer is provided.
Equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

CLAIMS:
1 • An optical storage medium (10) comprising
a substrate layer (12) having
a pit structure (14, 16) storing optical data, and a groove structure (18, 20) for storing optical data by projecting a laser light beam (22) thereon; and
a recording layer stack (24) deposited above the substrate layer (12), comprising
a first dielectric layer (26), a first recording layer (28), a second recording layer (30), and a second dielectric layer (32), characterized by
the absence of a metal layer between the recording layer stack (24) and the substrate layer (12), or
the presence of an aluminum layer (34) having a thickness of between 5nm and 30nm between the recording layer stack and the substrate layer (12).
2. The optical storage medium according to claim 1, characterized in that the substrate layer (12) comprises at least one first region (36) comprising adjacent tracks having a pit structure (14, 16), and at least one second region (38) comprising adjacent grooves (18, 20).
3. The optical storage medium according to claim 1, characterized in that one of the recording layers (28) comprises Cu and the other recording layer (30) comprises Si.
4. The optical storage medium according to claim 1, characterized in that the pit structure (14, 16) conforms with the BD-ROM format (BD = Blu-ray Disc) and the groove structure (18, 20) conforms with the BD-R/RE format (BD-R = Blu-ray Recordable, BD-RE =Blu-ray REwriteable).

5. The optical storage medium according to claim 1, characterized in that the dielectric layers (26, 32) comprise ZnS-SiO2.
6. The optical storage medium according to claim 1, characterized in that the layer thickness of the dielectric layers (26, 32) is between l0nm and 50nm, and the layer thickness of the recording layers (28, 30) is between 4nm and 15nm.
7. The optical storage medium according to claim 1, characterized in that the mean depth of the pits (40) of the pit structure (14) is greater than the mean depth of the grooves (42) of the groove structure (18).
8. The optical storage medium according to claim 1, characterized in that the pits (40) of the pit structure (14) have a mean depth of between 40nm and l00nm, preferably of between 60 and 80nm.
9. The optical storage medium according to claim 1, characterized in that the grooves (42) of the groove structure (18) have a mean depth of between lOnm and 40 nm, preferably of between 25nm and 27nm.
10. The optical storage medium according to claim 1, characterized in that the mean depth of the pits of the pit structure (16) substantially equals the mean depth of the grooves of the groove structure (20).
11. The optical storage medium according to claim 1, characterized in that the pits of the pit structure (16) and the grooves of the groove structure (20) have a mean depth of between 40nm and 1 OOnm, preferably of between 60 and 80nm.
12. A stamper (44) for structuring an optical storage medium (10) according to one of the claims 1 to 11.
13. A method of producing an optical storage medium, comprising the steps of:
providing a substrate layer
structuring the substrate layer with
a pit structure storing optical data, and
a groove structure for storing optical data by projecting a laser light beam thereon;
depositing above the substrate layer a recording layer stack comprising a first dielectric layer, a first recording layer, a second recording layer, and a second dielectric layer, characterized by
depositing the recording layer stack directly on the substrate layer, or depositing an aluminum layer having a thickness of between 5nm and 30nm between the recording layer stack and the substrate layer.