WO2004055798A1

WO2004055798A1 - Storage medium for the optical storage and retrieval of information

Info

Publication number: WO2004055798A1
Application number: PCT/IB2003/005286
Authority: WO
Inventors: Christopher Busch; Alexander M. Van Der Lee; Johannes T. A. Wilderbeek
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-12-18
Filing date: 2003-11-19
Publication date: 2004-07-01
Also published as: TW200416696A; AU2003276618A1

Abstract

The invention relates to a storage medium for the storage and retrieval of information, comprising: a substrate (1), an active layer (2) for retention of data, the active layer employing bit-position encoding for the storage and retrieval of information, and a plurality of micro-optical elements (24, 24', ...) for receiving illumination from an external source of illumination. According to the invention, the micro-optical elements (24, 24', ...) are provided according to a pre-determined pattern of bit positions, preferably at each of the possible bit positions. Preferably, the micro-optical elements comprise lenses, preferably hemispherical or stigmatic lenses. The invention also relates to a method of manufacturing the storage medium. According to the invention, the storage medium has a relatively high data density.

Description

Storage medium for the optical storage and retrieval of information

The invention relates to a storage medium for the optical storage and retrieval of information.

In addition, the invention relates to a method of manufacturing a storage medium for the optical storage and retrieval of information and to a record carrier having information written thereon.

The information age has led to an explosion of information available to users. (Personal) computers are omnipresent and connected via a worldwide network of computer networks. The decreasing costs of storing data, and the increasing storage capacities of the same small device footprint, have been key enablers of this revolution. While current storage needs are being met, storage technologies continue to improve in order to keep pace with the rapidly increasing demand.

Media for optical storage of the kind mentioned in the opening paragraph are well known in the art. However, both magnetic and conventional optical data storage technologies, where individual bits are stored as distinct magnetic or optical changes on the surface of a recording medium, are approaching physical limits beyond which individual bits may be too small and/or too difficult to store and/or to distinguish. Inter-pixel or inter-symbol interference is a phenomenon in which intensity at one particular pixel contaminates data at nearby pixels. Physically, this interference arises from the band-limit of the (optical) channel, originating from optical diffraction or from time-varying aberrations in the lens system.

The invention has for its object to provide a storage medium with a higher data density. According to the invention, a medium for optical storage of the kind mentioned in the opening paragraph for this purpose comprises: a substrate, an active layer for retention of data, the active layer employing bit-position encoding for the storage and retrieval of information, and a plurality of micro-optical elements for receiving illumination from an external source of illumination, the micro-optical elements being provided according to a pre- determined pattern of bit positions.

An active layer in the present description and claims is understood to be a layer in which information can be stored (coded) and changed.

In a conventional one-dimensional (optical) storage medium a single bit row is written along a spiral. In general, the track pitch is chosen large enough to reduce thermal cross talk between neighboring tracks to acceptable levels. In addition, a recording dye layer is or, alternatively, inorganic phase change layers are distributed homogeneously across the medium.

By providing a plurality of micro-optical elements, wherein the micro-optical elements are provided according to the pre-determined pattern of bit positions, the track density on the storage medium can be significantly improved. The inventors have had the insight to provide above every possible bit position a micro-optical element, for instance a so-called nano-lens, resulting in a significant gain in optical resolution of the storage medium. By embedding micro-optical elements (nano-optics) in the storage medium close to or on top of the active layer, cross-talk between bit positions is largely reduced. When information is stored (recorded or coded) in the storage medium, the spot size of the storage means, preferably, is such that only the active layer at the desired bit position is activated or de-activated and that the adjacent bit positions are (practically) not affected by the storing means.

US patent 5,910,940 describes a storage medium employing an optical layer provided with cylindrical micro-lenses embedded in the storage medium in close proximity of the active layer. In the known (one-dimensional) optical storage medium, a single bit row is written along a spiral employing the bit-length encoding as encoding concept. In the so- called near-field optical recording as described in the known storage medium, the cylindrical micro-lenses stretch along the track direction. When a pre-determined pattern comprising a plurality of micro-optical elements are provided according to a pre-determined pattern of bit positions, the preferred encoding concept is bit-position encoding.

Preferably, the micro-optical elements are provided at each of the possible bit positions. This has the additional advantage that the optical constraints are the same for every possible bit position. Patterning the micro-optical elements largely facilitates the manufacturing of the storage medium according to the invention.

A preferred embodiment of the storage medium according to the invention is characterized in that the micro-optical elements are made from a material with a relatively high refractive index and that the micro-optical elements are embedded in a cover layer, the cover layer being made of a material with a relatively low refractive index. The higher the difference in refractive index between the cover layer and the micro-optical elements, the higher the gain in optical resolution. Preferably, the refractive index n_cι of the cover layer is ri_ci ≤ 1.5 and the refractive index n_me of the micro-optical elements is n_me > 1.5. Preferably, the refractive index of the micro-optical elements is n_me > 1.75. High gains in optical resolution are possible by choosing high refractive index materials for the micro-optical elements and/or low index material for the cover layer.

Preferably, the micro-optical elements comprise lenses (e.g. nano-lenses). Preferably, each of the possible bit positions in the active layer is provided with an individual lens. Preferably, the curvature of the lenses is hemispherical or stigmatic. For micro-optical elements based on hemispherical lenses, the achieved gain in optical resolution is proportional to the ratio of the refractive index of the lens material as compared to the refractive index of the cover layer material. For micro-optical elements based on stigmatic lenses, an even higher gain is possible than with a hemispherical lens set-up, in this case, the optical resolution being proportional to the square of the ratio between the respective refractive indexes.

A preferred embodiment of the storage medium according to the invention is characterized in that the pre-determined pattern comprises a two-dimensional strip of micro- optical elements. In the known (one-dimensional) optical storage medium, a single bit row is written along a spiral employing bit-length encoding as encoding concept. When a predetermined pattern comprising a two-dimensional strip of bit positions is used, bit-position encoding is employed. Preferably, a strip is aligned horizontally and consists of a number of rows and columns. Preferably, code words do not cross boundaries of a strip.

A preferred embodiment of the storage medium according to the invention is characterized in that the pre-determined pattern comprises an at least partial quasi-hexagonal or quasi-square pattern. With a quasi-hexagonal or quasi-square pattern is meant a pattern of micro-optical elements that may be ideally arranged hexagonally or square, respectively. However, small position distortions from the ideal pattern may be present. The number of nearest neighbors is six for the hexagonal pattern whereas it is four for a square pattern. The bit error rate is smaller for the quasi-hexagonal and quasi-square pattern as compared to the known storage medium. The higher packing density of the quasi-hexagonal pattern provides a higher storing efficiency than the quasi-square pattern. The quasi-hexagonal or quasi-square patterns are very suitably employed in a storage medium comprising a two-dimensional strip of bit positions.

The invention has for its further object to provide a method of manufacturing a storage medium for the optical storage and retrieval of information providing alignment of the micro-optical elements with the pre-determined pattern of bit positions. According to an embodiment of the invention, a method of manufacturing a storage medium for the optical storage and retrieval of information comprises the following steps. A hydrophilic wetting material is deposited on a substrate provided with an active layer for retention of data. The hydrophilic wetting material is provided on the substrate according to a pre-determined pattern. As a next step, a hydrophilic optical layer material is deposited on the patterned hydrophilic wetting material. Subsequently, the hydrophilic optical layer material is fixated, forming a micro-optical element according to the pre-determined pattern. By applying a hydrophilic optical layer material on a patterned hydrophilic wetting material, the surface tension properties are influenced in a favorable manner, such that droplets of optical layer material are formed on the hydrophilic wetting material. These droplets of hydrophilic optical layer material can be fixated to form the micro-optical elements. The method according to the invention is efficient, low-cost and high-yield and is very well suited for manufacturing. A preferred embodiment of the method of manufacturing a storage medium according to the invention is characterized in that the hydrophilic wetting material is deposited on a pressing tool provided with protrusions according to the pre-determined pattern. Preferably, the optical layer is deposited by means of spin coating or dip coating. Due to the characteristic properties of the wetting material, the droplets are be formed only on top of the patches of hydrophilic wetting material

According to an alternative embodiment of the invention, a method of manufacturing a storage medium for the optical storage and retrieval of information comprises the following steps. A hydrophobic (wetting) material is deposited on a substrate provided with an active layer for retention of data. The hydrophobic (wetting) material is provided according to a pre-determined pattern. As a next step, a hydrophobic optical layer material is deposited on the patterned hydrophobic (wetting) material. Subsequently, the hydrophobic optical layer material is fixated, forming a micro-optical element according to the pre-determined pattern. By applying a hydrophobic optical layer material on a patterned hydrophobic (wetting) material, respectively, the surface tension properties are influenced in a favorable manner, such that droplets of optical layer material are formed on the hydrophobic (wetting) material, respectively. These droplets of hydrophobic optical layer material can be fixated to form the micro-optical elements. The method according to the invention is efficient, low-cost and high-yield and is very well suited for manufacturing. A preferred embodiment of the method of manufacturing a storage medium according to the invention is characterized in that the hydrophobic (wetting) material is deposited on a pressing tool provided with protrusions according to the pre-determined pattern, and in that the hydrophobic optical layer material is deposited on the hydrophobic wetting material by positioning the pressing tool on the substrate. By employing a pressing tool, the micro-optical elements are provided on the storage medium according to a pattern which can be determined beforehand such that recording or storing (coding) information in the active layer is possible only at pre-determined positions.

Preferably, the micro-optical elements comprise lenses. Preferably, the curvature of the lenses is hemispherical or stigmatic. The surface tension properties during and after the deposition of the hydrophilic optical layer material on the patterned hydrophilic wetting material and the hydrophilic properties of the active layer largely determine the shape of the lenses. The method of manufacturing according to the invention is particularly suitable for providing stigmatic lens layout on the storage medium. Such lens shapes are difficult to produce by conventional methods.

A favorable embodiment of the method of manufacturing a storage medium according to the invention is characterized in that the storage medium is covered by a transparent cover layer. Such a cover layer largely has a protective purpose.

The storage medium according to the invention can be a record carrier having information written thereon, e.g. an optical disc, a CD, a CD-Rom, a CD-R, a CD-RW, and a DVD, BD, optical memory cards, and similar products.

Preferably, a record carrier having information written thereon, is coded in an active layer and that micro-optical elements are embedded in the record carrier, said micro- optical elements being provided by a method of manufacturing. Preferably, the record carrier is an optical disc.

Other advantageous embodiments and further developments are defined in the dependent claims. The invention will now be explained in more detail with reference to a number of embodiments and accompanying drawing figures in which:

Fig. 1 A shows a storage medium for optical storage and retrieval of information according to the invention; * Fig. IB shows a detail of the storage medium of Figure 1A;

Fig. 2 A shows a side view of an embodiment of the storage medium according to the invention,

Fig. 2B shows a side view of an alternative embodiment of the storage medium according to the invention, Figs. 3A-3F show steps of the method of manufacturing a storage medium according to a preferred embodiment of the invention, and

Fig. 4 shows the geometry and direction of the surface tensions for a droplet of hydrophilic optical layer material forming the micro-optical elements.

The Figures are purely diagrammatic and not drawn true to scale. Some dimensions are particularly strongly exaggerated for reasons of clarity. Equivalent components have been given the same reference numerals as much as possible in the Figures.

Figure 1 A shows very schematically a storing medium for optical storage and retrieval of information according to the invention. In Figure 1A a substrate 1 is provided by a strip or track in the form of a spiral of possible bit positions. Upon storing and retrieving of information the spiral is followed by the storage or retrieval means, respectively. Figure IB shows very schematically a detail of the storing medium of Figure 1 A. A pre-determined pattern 4 of micro-optical elements 24, 24', ... is shown. So-called guard bands 3 are shown between the strips or tracks of micro-optical elements 24, 24', ...; the direction in which information is stored and retrieved from a strip of micro-optical elements 24, 24', ... is indicated by a bold arrow. In the example of Figure IB, the pattern 4 of micro-optical elements 24, 24', .-.. is a quasi-hexagonal pattern for which the number of nearest neighbors is six. In an alternative embodiment, the pattern of micro-optical elements is a quasi-square pattern for which the number of nearest neighbors is four. It is well known that hexagonal patterns provide the highest packing fraction. In particular, the packing fraction for the hexagonal pattern is approximately 15% higher than that of a square pattern with the same distance between nearest-neighbor micro-optical elements. In addition, other patterns can be employed. Periodic two-dimensional patterns can be built up using triangles with arbitrary angles as basic building blocks. In addition, patterns with parallelograms and hexagons can be used.

When a pre-determined pattern of micro-optical elements comprising a two- dimensional strip of bit positions as shown in Figure 1 A and IB is employed, information is stored and retrieved in the storage medium employing bit-position encoding. Reliable readout at such a high packing density of the information bits is only possible by the synchronized detection and processing of signals from several rows of micro-optical elements. This can e.g. be done by using an array of light spots that simultaneously detects (or writes) the two- dimensional (2D) encoded information, thereby dramatically increasing the data rate. Using the obtained 2D signal information, the large signal energy present in inter-symbol interference (which in standard optical recording largely is considered as part of the noise) can be coherently used in the reconstruction of the original 2D bit patterns. So-called two- dimensional coding enhances the speed of data coding and decoding. The location of the micro-optical elements is known to a high accuracy beforehand. Figure 2 A shows a side view of an embodiment of the storage medium according to the invention. In the example of Figure 2A, the micro-optical elements 24, 24', ... comprise hemispherical lenses. The hemispherical lenses are deposited on the substrate 1 provided with an active layer 2.

Preferably, the[0] active layer 2 is a recording dye layer (typical for a WORM medium). Preferably, these layers are deposited by conventional techniques such as spin coating, embossing, molding, (photo)lithography, micro-contact printing or vapor deposition. Organic dye layers can be easily patterned. Alternatively, inorganic phase change layers may also be used as re-writable medium. Preferably, the latter layers are deposited by sputtering. Patterning organic dyes is preferred as compared to patterning re-writable rare earth recording layers.

Preferably, the cover layer 18 is made of a material with a relatively low refractive index, and the micro-optical elements 24, 24', ... have a relatively high refractive index. Preferably, the refractive index n_s of the cover layer 18 is n_cv < 1.5 and the refractive index n_mo of the micro-optical elements 24, 24', ... is n_mo ≥ 1.5. Preferably, the refractive index n_mo of the micro-optical elements 24, 24', ... is n_mo > 1.75. For micro-optical elements based on hemispherical lenses, the achieved gain in optical resolution is proportional to the ratio of the refractive index of the lens material as compared to the refractive index of the substrate material. By way of example, if the refractive index of the cover layer n-ρl.5 (e.g. polycarbonate) and the refractive index of the hemispherical lens n_me=2, an optical resolution increase of 1.33, or equivalently an effective numerical aperture of NA=1.13, is obtained.

Figure 2B shows a side view of an embodiment of the storage medium according to the invention. In the example of Figure 2B, the micro-optical elements 24, 24', ... comprise stigmatic lenses, giving even higher optical resolutions than with the hemispherical lenses, the optical resolution being proportional to the square of the ratio between the respective refractive indexes. By way of example, if the refractive index of the cover layer material ri_cpl.5 (e.g. polycarbonate) and the refractive index of the hemispherical lens n_me=2, an optical resolution increase of 1.78, or equivalently an effective numerical aperture NA=1.51, is obtained. Higher gains in optical resolution are possible by choosing higher refractive index materials for the micro-optical elements and/or lower index materials for the cover layer (e.g. fluorinated polymers n « 1.3).

Figure 3A-3F show steps of the method of manufacturing a storage medium according to a preferred embodiment of the invention. Figure 3 A shows diagrammatically a cover layer 18 provided with an active layer 2 for retention of data and a pressing tool 10 provided with protrusions 11, 11', ... according to a pre-determined pattern 4. A hydrophilic wetting material 13 is deposited on the pressing tool 10.

Suitable materials for the hydrophilic wetting material can be recruited from the class of materials that are conventionally used for wetting and non- wetting purposes as are known to persons skilled in the art, such as thiols, protein derivatives, long-chain fatty acid derivatives, silane derivatives, or other surfactant-like molecules, clusters, complexes, or aggregates, all optionally provided with functional head, and/or tail and/or side groups. The mentioned material classes are solely included as non-limitative examples.

The pressing tool 10 is lowered (according to arrow in Figure 3 A) and brought into contact with the substrate (see Figure 3B). The hydrophilic wetting material 13 is transferred on the surface of (the active layer 2 on) the substrate 1. In Figure 3C the pressing tool 10 is removed from the substrate (according to arrow in Figure 3C) leaving on the surface of (the active layer 2 on) the substrate 1 the hydrophilic wetting material 13 at a plurality of locations according to the pre-determined pattern 4. In practice, the wetting layer is a molecular monolayer with a thickness substantially smaller than that of the other layers. As a next step of the method of manufacturing a storage medium, a hydrophilic optical layer material 23 is deposited on the hydrophilic wetting material 13 (see Figure 3D). In this manner, the droplets of hydrophilic wetting material forming the micro- optical elements 24, 24', ... (also see Figure 4) are provided according to the pre-determined pattern 4. Preferred deposition methods are spin coating or dip coating. Other suitable deposition techniques are embossing, molding, (photo)lithography, micro-contact printing or vapor deposition.

In Figure 3E the hydrophilic optical layer material is fixated, forming a plurality of micro-optical elements 24, 24', ... according to the pre-determined pattern 4. Preferred fixating methods are radiation (e.g. UN-light, visible light) or thermal energy (e.g. thermally initiated polymerization). Photo-initiators, photo-sensitizers and/or thermal initiators, along with other additives (e.g. inhibitors, stabilizers, nucleating agents (so-called clarifying agents) can be added to the optical layer material before enclosure in the pressing tool.

In an alternative embodiment of the invention, a hydrophobic (wetting) material is used in stead of a hydrophilic wetting material and a hydrophobic optical layer material in stead of a hydrophilic optical layer is deposited on the patterned hydrophobic (wetting) material. The micro-optical elements manufactured according to the method according to the invention are, optionally, covered with a transparent cover layer consisting of a low refractive index material (n«1.3) (see Figure 3F). Preferably, (amorphous) fiuorinated polymers are employed.

The optical elements are restricted in size by the lattice parameter d_c. If a system with a blue laser (λ=405nm) and a ΝA=0.85 is used, a typical parameter is d_c « 140 nm. The result is a significantly higher bit density for the storage medium according to the invention as compared to the known storage media. Compared to the so-called Blu-ray Disc standard, the physical bit density is increased roughly by a factor or two. By employing a recording medium with the array of micro-optical elements, writing cross-talk between bit positions is significantly reduced.

The pressing tools used for the manufacturing process as described here are obtained by standard methods commonly used for the preparation of stamps for micro- contact printing. By way of example, a soft polydimethylsiloxane (PDMS) stamp is cast from a preformed mold. This allows the exact replication of the designated recording areas both in size, depth and position. Preferably, the widths of the protrusions 11, 11 ', ... are approximately 100-300 nm which can be readily made using this replication process.

The concept of the surface tension determining the shape of a droplet is given by Young's equation: Ysv = YL_S + YLV cos(θ),

where γ_Sv is the solid- vapor interfacial tension, γ_Ls and γ v those of liquid-solid and liquid vapor, respectively. Figure 4 shows the geometry and direction of the surface tensions for a droplet of hydrophilic optical layer material forming the micro-optical elements 24 on the active layer 2 on the substrate (not shown in Figure 4). In order to form a stigmatic (hemispheric) lens, the solid- vapor tension of the interfaces has to be smaller (equal) to the liquid-solid interfaces term. In that case, the effective force tries to minimize the surface of the optical layer material, resulting in the correct shape. It has to be kept in mind though, that the actual situation is more difficult than indicated by the simple formula because the extra interface between the wetting layer 13 and the active layer 2.

Preferably, a transparent high refractive index material is used for the micro- optical elements (nano-lenses). By a proper choice of materials, the shape of the droplet can be determined. Also, additional means such as electro-wetting concepts might be used to further control the effect. Suitable materials for the hydrophilic optical layer material are predominantly polar high refractive index materials. Optionally, mixtures of one or more different reactive (e.g. monomers) and one of more inert materials may be employed as to adjust the wetting and refractive index properties of the mixture and of the fixated optical layer material. Non-limitative examples of monomers resulting in high refractive index polymers are high index polymers are pentabromophenyl acrylate

pentabromophenyl methacrylate (n_om=1.71), 2-vinylnaphtalene (n_om=1.68), 2-naphtyl methacrylate (n_om=1.64), N-vinylphtalimide (n_0m ⁼l-62), and pentachlorophenyl acrylate (n_om«1.6).

Preferably, a two-dimensional strip of micro-optical elements 24, 24', ... in the form of a spiral is provided on the substrate by the method of manufacturing a storage medium (see Figure 1 A and IB).

The scope of the invention is not limited to the embodiments. The invention is embodied in each new characteristic and each combination of characteristics. Any reference sign do not limit the scope of the claims. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. Use of the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A storage medium for the storage and retrieval of information, the storage medium comprising: a substrate, an active layer for retention of data, the active layer employing bit-position encoding for the storage and retrieval of information, and a plurality of micro-optical elements for receiving illumination from an external source of illumination, the micro-optical elements being provided according to a predetermined pattern of bit positions.

2. A storage medium as claimed in claim 1, characterized in that the micro-optical elements are provided at each of the possible bit positions.

3. A storage medium as claimed in claim 1 or 2, characterized in that the micro-optical elements comprise lenses.

4. A storage medium as claimed in claim 3, characterized in that the curvature of the lenses is hemispherical or stigmatic.

5. A storage medium as claimed in claim 1 or 2, characterized in that the micro-optical elements are made from a material with a relatively high refractive index and that the micro-optical elements are embedded in a cover layer, the cover layer being made of a material with a relatively low refractive index.

6. A storage medium as claimed in claim 5, characterized in that the refractive index n_cι of the cover layer is n_cι < 1.5 and the refractive index n_me of the micro-optical elements is n_me > 1.5.

7. A storage medium as claimed in claim 1 or 2, characterized in that the predetermined pattern comprises a two-dimensional strip of micro-optical elements.

8. A storage medium as claimed in claim 1 or 2, characterized in that the pre- determined pattern comprises an at least partial quasi-hexagonal or quasi-square pattern.

9. A method of manufacturing a storage medium for the optical storage and retrieval of information, the method comprising the following steps: a hydrophilic wetting material is deposited on a substrate provided with an active layer for retention of data, the hydrophilic wetting material being provided according to a pre-determined pattern, a hydrophilic optical layer material is deposited on the patterned hydrophilic wetting material, the hydrophilic optical layer material is fixated, forming a micro-optical element according to the pre-determined pattern.

' 10. A method of manufacturing a storage medium for the optical storage and retrieval of information, the method comprising the following steps: a hydrophobic wetting material is deposited on a substrate provided with an active layer for retention of data, the hydrophobic wetting material being provided according to a pre-determined pattern, a hydrophobic optical layer material is deposited on the patterned hydrophobic wetting material, the hydrophobic optical layer material is fixated, forming a micro-optical element according to the pre-determined pattern.

11. A method of manufacturing a storage medium as claimed in claim 9 of 10, characterized in that the wetting material is deposited on a pressing tool provided with protrusions according to the pre-determined pattern.

12. A method of manufacturing a storage medium as claimed in claim 9 or 10, characterized in that the micro-optical elements comprise lenses.

13. A method of manufacturing a storage medium as claimed in claim 12 characterized in that the curvature of the lenses is hemispherical or stigmatic.

14. A method of manufacturing a storage medium as claimed in claim 9 or 10, characterized in that the storage medium is covered by a transparent cover layer.

15. A method of manufacturing a storage medium as claimed in claim 14, characterized in that the cover layer is made of a material with a relatively low refractive index, and the micro-optical elements have a relatively high refractive index.

16. A method of manufacturing a storage medium as claimed in claim 14, characterized in that the refractive index n₀ι of the cover layer is n-i < 1.5 and the refractive index n_mo of the micro-optical elements is n_mo > 1.5.

17. A method of manufacturing a storage medium as claimed in claim 9 or 10, characterized in that a two-dimensional strip of micro-optical elements in the form of a spiral is provided on the substrate.

18. A record carrier having information written thereon, characterized in that the information is coded in an active layer and that micro-optical elements are embedded in the record carrier, said micro-optical elements being provided by a method of manufacturing as claimed in claim 9 or 10.

19. A record carrier as claimed in claim 18, characterized in that the record carrier is an optical disc.