WO2004055797A1

WO2004055797A1 - Storage medium for the optical storage and retrieval of information

Info

Publication number: WO2004055797A1
Application number: PCT/IB2003/005264
Authority: WO
Inventors: Christopher Busch; Johannes T. A. Wilderbeek
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2002-12-18
Filing date: 2003-11-18
Publication date: 2004-07-01
Anticipated expiration: 2005-06-18
Also published as: AU2003280089A1; TW200423119A

Abstract

The invention relates to a storage medium for the storage and retrieval of information, comprising: a substrate (1), a pre-determined pattern (4) of bit positions (14, 14', ...) provided with an active layer (2, 2', ...) comprising a recording medium for retention of data, a plurality of micro-optical elements (24, 24', ...) for receiving illumination from an external source of illumination. The micro-optical elements are provided according to the pre-determined pattern of 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, a pre-determined pattern of bit positions provided with an active layer comprising a recording medium for retention of data, a plurality of micro-optical elements for receiving illumination from an external source of illumination, the micro-optical elements being provided according to the 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. hi 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.

According to the invention the active layer in the storage medium is patterned beforehand such that recording or storing (coding) information in the recording medium of the active layer is possible only at pre-determined positions and with a certain shape. Because the active layer is not homogeneously distributed across the storage medium but only present at the pre-determined bit positions, (thermal) cross talk between adjacent bit positions is significantly reduced. As a consequence, the density of the bit positions can be increased as compared to the known storage media. When retrieving information from the storage medium, the size of the bit positions can even be smaller than the spot size of the retrieval means. 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. By employing a patterned active layer comprising a recording medium, cross-talk between bit positions is significantly reduced.

In addition, 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 comprising the recording medium, cross-talk between bit positions is largely reduced.

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 so-called near-field optical recording as described in the known storage medium, the cylindrical micro-lenses stretch along the track direction.

Preferably, the substrate of the storage medium is provided with the predetermined pattern of bit positions. This has the additional advantage that the active layer is provided at the bit positions in the substrate. Patterning the substrate of the storage medium largely facilitates the manufacturing of the storage medium according to the invention.

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 substrate material. For micro-optical elements based on stigmatic lenses, an even higher gain is possible than with a stigmatic 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 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 substrate and the micro-optical elements, the higher the gain in optical resolution. Preferably, 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. Preferably, the the refractive index n_me 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 substrate.

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 bit positions. 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 pre-determined pattern comprising a two-dimensional strip of bit positions is employed, the preferred encoding concept is bit-position encoding. 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 bit positions 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 the invention, a method of manufacturing a storage medium for the optical storage and retrieval of information comprises the following steps. An optical layer is deposited into cavities of a pressing tool, the pressing tool being provided with a pre-determined pattern of cavities. As a next step, the pressing tool is positioned in an unprintable layer disposed on a substrate, the unprintable layer comprising a recording medium. Subsequently, the imprintable layer is fixated, providing the substrate with bits of recording medium, the bits being arranged according to the pre-determined pattern. As a next step, the optical layer in the cavities of the pressing tool is released and the pressing tool is removed. Eventually, the optical layer is fixated, forming a micro-optical element on each of the bits of recording medium.

The method of manufacturing a storage medium according to the invention provides a storage medium provided with a pre-determined pattern of bits of recording medium and a lens at the location of each of the bits of recording medium. Elaborate positioning aligning the location where the lenses are to be deposited with respect to the location of the bits of recording medium is avoided. The pressing tool, which already contains the optical layer for each of the bits, provides the imprintable layer comprising the recording material with the same pre-determined pattern of bit positions. The lens material is provided at the same locations as the recording material. In addition, the method according to the invention is efficient, low-cost and high-yield and is very well suited for manufacturing.

Preferably, the micro-optical elements comprise lenses. Preferably, the curvature of the lenses is hemispherical or stigmatic. A favorable embodiment of the method of manufacturing a storage medium according to the invention is characterized in that the optical layer is released from the cavities by melting the optical layer. The optical layer is retained in the cavities of the pressing tool during the providing of the substrate with the pre-determined pattern of bits with recording material. The optical layer is readily then released from the pressing tool by increasing the temperature of the pressing tool above the melting temperature of the optical layer.

Preferably, the optical layer has a transition point above the transition point of the imprintable layer. This enables the retention of the optical layer in the pressing tool while the substrate is provided with the pre-determined pattern of bits with recording material. Preferably, the optical layer has a transition point above the ambient temperature.

A preferred embodiment of the method of manufacturing a storage medium according to the invention is characterized in that the imprintable layer is a liquid. This facilitates the patterning of the substrate with the pre-determined pattern of bits with recording material. The temperature of the substrate is preferably such that the imprintable layer at that temperature, which may be above (or below) ambient temperature, is in a liquefied state whereas the melting temperature of the optical layer is above that temperature. A favorable manner of fixating the pre-determined pattern of bits of recording material is by means of solidification. 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 DND, BD, optical memory cards, and similar products.

Preferably, a record carrier having information written thereon, is coded in a recording medium provided by a method of manufacturing as described in this patent application. 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 1 A; Fig. 2 shows the optical spot and bit pattern geometry of the pattern of bit positions of Figure IB;

Fig. 3 A shows a side view of an embodiment of the storage medium according to the invention, Fig. 3B shows a side view of an alternative embodiment of the storage medium according to the invention,

Figs. 4A-4F show steps of the method of manufacturing a storage medium according to the invention, and

Fig. 5 shows an alternative embodiment of the pressing tool used in the method of manufacturing a storage medium according to the invention.

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 1 A a substrate 1 is provided by a strip or track in the form of a spiral of 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 bit positions 14, 14', ... is shown. So-called guard bands 3 are shown between the strips or tracks of bit positions 14, 14', ...; the direction in which information is stored and retrieved from a strip of bit positions 14, 14', ... is indicated by a bold arrow. In the example of Figure IB, the pattern 4 of bit positions 14, 14', ... is a quasi-hexagonal pattern for which the number of nearest neighbors is six. In an alternative embodiment, the pattern of bit positions 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 bit positions. 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. The micro-optical elements are shown in Figures 3 A, 3B, 4D, 4E and 4F. Figure 2 shows the optical spot and bit pattern geometry of the pattern of bit positions of Figure IB. Individual bit positions 14, 14', ... are indicated (by the dashed lines) in the pre-determined pattern 4 as well as an optical spot 5. According to the invention, an active layer 2, 2', ... comprising a recording medium for retention of data is provided with the pre-determined pattern 4 of bit positions 14, 14', .... The active layer 2, 2', ... is provided only at the location of the bit positions 14, 14', .... It becomes clear from the geometry of the optical spot 5 and the bit pattern that cross-talk between neighboring bits is an important issue. For retrieving information from the storage medium, cross-talk can be resolved by adequate coding and signal processing techniques. For storing information on the storage medium, for instance by employing a thermal tip writing method, cross-talk can, by way of example, be avoided by tuning (the intensity of) the optical spot 5 such that upon storing in the active layer at the central bit position the information in the active layers at the nearest neighbor bit positions is not substantially effected. An effective way to reduce the effect of cross-talk is achieved by effectively shielding the active layer 2 at a bit position 14 from the active layer 2' at an adjacent bit position 14' .

Preferably, the[0] active layer is a recoding 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 storage medium is provided in the substrate 1 beforehand such that storing information is possible only at the pre-determined position and with a pre- determined shape. In this manner, a storage medium with a relatively high data density is obtained. Preferably, a pressing tool is employed to generate the pre-determined pattern 4 of bit positions 14, 14', .... In this manner the possible bit positions are known exactly beforehand. The pressing tool imprints the pre-determined bit position structure as shown in Figure IB in the form of a spiral as shown in Figure 1 A in a single print step. The pattern of bit positions 4 is embossed in the pressing tool.

Preferably, the scaled distance d_c* between centers of the bit positions 14, 14', ... is less than 0.84, preferably less than 0.63. The scaled distance d_c is a dimensionless distance. The distance d_c (see Figure 2) is scaled to the effective optical resolution of the system, i.e. dc* = d_c / (λ/2NA).

The expression λ/2NA is the so-called MTF cut-off, λ being the wavelength of (laser) light in nm and NA being the numerical aperture of the system. In this manner, d_c* is independent from the nature of the readout system. If a system with a blue laser (λ=405nm) and a NA=0.85 is used, d_c is, preferably, less than 200 nm, preferably less than 150 nm.

Similarly, the scaled distance d_aι* between the active layer at a first bit position and the active layer at an adjacent bit position is less than 0.42, preferably less than 0.3. The scaled distance d_aι is a dimensionless distance. The distance d_aι (see Figure 2) is scaled to the effective optical resolution of the system, i.e.

dai* = d_a, / (λ/2NA).

If a system with a blue laser (λ=405nm) and a NA=0.85 is used, d_aι is, preferably, less than 100 nm, preferably less than 70 nm. From experiments, it was found that a very suitable values for d_c* » 0.59 and d_aι* « 0.17. For a system with a blue laser and a NA=0.85 the corresponding distances are d_c ∞ 140 nm and d_aι « 40 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 Blue-ray Disc standard, the physical bit density is increased roughly by a factor of two. By employing a recording medium with pre-determined pattern of bit positions provided with an active layer, writing cross-talk between bit positions is significantly reduced.

When a pre-determined pattern comprising a two-dimensional strip of bit positions as shown in Figure 1 A, IB and 2 is employed, the preferred encoding concept is 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 bit- rows. 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 active layer at the pre-determined bit positions is known to a high accuracy beforehand.

Figure 3 A shows a side view of an embodiment of the storage medium according to the invention. In the example of Figure 3 A, the micro-optical elements comprise hemispherical lenses.

Preferably, the storage medium is covered by a transparent cover layer 18 made of a material with a relatively low refractive index, and the optical layer 22, 22', ... has a relatively high refractive index. Preferably, the refractive index n_cι of the cover layer 18 is n_s < 1.5 and the refractive index n_om of the optical layer is n_om ≥ 1.5. Preferably, the refractive index n_om of the optical layer is n_om > 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 cover layer material. By way of example, if the refractive index of the cover layer material n_cι=1.5 (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 3B shows a side view of an embodiment of the storage medium according to the invention, h the example of Figure 3B, the micro-optical elements 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 n_cι=1.5 (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 4A-4F show steps of the method of manufacturing a storage medium according to the invention. Figure 4A shows diagrammatically a pressing tool 10 provided with a pre-determined pattern 4 of cavities 11, 11', .... In each of the cavities 11, 11', ... of the pressing tool 10 an optical layer 22, 22', ... is deposited. In Figure 4B the pressing tool 10 is lowered (direction of movement indicated by the arrow) in an imprintable layer 13 disposed on a substrate 1 (situation depicted in Figure 4C). The imprintable layer 13 comprises a recording medium. Suitable recording media are layers or patterned layers of organic nature (e.g. organic dyes) or of inorganic nature (e.g. phase change materials). Preferably, the imprintable layer is a liquid (solution). Preferably, the temperature of the substrate is such that the imprintable layer at that temperature is in a liquefied state. As a next step, the imprintable layer 13 is fixated. Preferably, the imprintable layer 13 is fixated by means of drying. During drying the pressing tool 10 is kept in position. The fixation results in the substrate 1 being provided with (discrete) bits 12, 12', ... of the recording medium. According to the invention, the bits 12, 12', ... are arranged according to the pre-determined pattern 4. The lateral positioning of the bits 12, 12', ... comprising the recording medium is determined by the position and the lateral size of the cavities 11 , 11 ' , ... in the pressing tool 10.

As a next step of the method of manufacturing a storage medium, the optical layer 22, 22', ... in the cavities 11, 11', ... ofthe pressing tool 10 is released and the pressing tool 10 is removed, as shown in Figure 4D. Preferably, the optical layer 22, 22', ... is released from the cavities 11, 11', ... by melting the optical layer 22, 22', .... Preferably, the optical layer 22, 22', ... has a transition point (melting point) above the transition point ofthe imprintable layer 13 and/or a transition point above the ambient temperature. The release of the optical layer 22, 22', ... results in the deposition of a liquid droplet 23, 23', ... on the bits 12, 12', ... comprising the recording medium. In this manner, the droplets 23, 23', ... and the bits 12, 12', ... are arranged according to the pre-determined pattern 4.

As a next step, the droplets 23, 23', ... of the optical layer 22, 22', ... are fixated, forming a micro-optical element 24, 24', ... on each ofthe bits 12, 12', ... ofthe recording medium. A favorable manner of fixating the droplets 23, 23', ... is by means of drying. Preferably, the micro-optical elements 24, 24', ... comprise lenses. Preferably, the lenses are hemispherical lenses (see Figure 3 A) or stigmatic lenses (see Figure 3B).

In Figure 4F, the structured layer forming the storage medium obtained by the above described method of manufacturing is optionally covered by a transparent cover layer 18. Preferably, the transparent cover layer 18 is made from a material with a relatively low refractive index.

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 ofthe designated recording areas both in size, depth and position. Preferably, the widths ofthe cavities 11, 11', ... are of the size of the active material d_c-d_aι (e.g. for a blue system with NA=0.85 a value of d_c - d_aι = 90 nm might be chosen) and the depths ofthe cavities 11, 11', ... are approximately 1.5 to five times that value (135 nm to 450 nm) which can be readily made using this replication process.

Preferably, a transparent high refractive index material is used for the micro- optical element (nano-lenses). For a hemispherical and in particular for a stigmatic lens layout substantial de- wetting, i.e. high contact angles, ofthe optical layer on the recording medium is preferred. In addition, as soon as the desired hemispherical or stigmatic lens layout is established, the layout is fixated.

To realize the exact positioning ofthe optical layer on the bit with recording medium in one process step, the optical layer material is pre-deposited in the pressing tool 10. Preferably, this is done by immersion, optionally in combination with sonification, degassing, vapor deposition, spin coating with or without the aid of applied pressure (e.g. centrifugal force), ofthe, preferably, apolar, hydrophobic stamp (PDMS) in the, preferably, polar, hydrophilic liquid optical layer or lens precursor material. Alternatively, the optical layer material is dissolved in a moderately apolar solvent which still dissolves the optical layer material and has sufficient interaction with the pressing tool to allow the filling of the cavities. Fixation ofthe optical layer material in the pressing tool is, preferably, realized by a phase change, e.g. solidification. This enables the release ofthe optical layer material at a later stage by heating the stamp to a temperature exceeding the transition point ofthe optical layer material, but still below the transition point ofthe recording medium, and below transitions of the material used for the pressing tool.

Suitable materials for high index polymers are pentabromophenyl acrylate (n_om«l -7), pentabromophenyl methacrylate (n_om ⁼l-71), 2-vinylnaphtalene (n_om=1.68), 2-naphtyl methacylate (n_0m ⁼l-64), N-vinylphtalimide (n₀m-l-62), and pentachlorophenyl acrylate (n_om«1.6). Organic based, high refractive index (n_om∞l -7) materials have a predominant apolar character, which is of disadvantage with respect to the required contact angle on the recording material, which has generally also a predominant a-polar character. However, the polarity ofthe optical layer material can be adjusted by employing suitable co-monomers, or by introduction of suitable polar substituents, without affecting the refractive index ofthe resulting copolymer to a considerable degree. In addition, resulting polymers from optical layer material (monomer) are, preferably, transparent at the wavelength used during reading and writing of information. Favorable materials are amorphous polymers. Alternatively semi- crystalline polymers are suitable also provided the size and distribution ofthe crystallites is small enough to exclude scattering effects. The filling ofthe optical layer material in the cavities occurs at a certain ratio with respect to the groove depth (see Figure 4A). The ratio after solidification depends on several parameters, such as the contact angle between the optical layer material (pure or in solution) and the pressing tool, the temperature during deposition and the cooling rate after deposition, the evaporation and diffusion rate of any optionally used solvent, the depth and width ofthe cavities, etc. A filling ratio less than 1 is preferred, thereby enabling a wet embossing process at a later stage.

Once the optical layer material is enclosed and solidified in the pressing tool, the pressing tool is lowered into the liquid imprintable layer on the substrate, the imprintable layer containing the recording medium either in its pure form or in solution. The recording medium can be of organic nature (e.g. organic dye), but also of inorganic nature (e.g. inorganic nanoparticles, optionally stabilized). By employing, by way of example, the technique of wet (or liquid) embossing, the recording medium is forced into the remaining cavities ofthe pressing tool (see Figure 4B). Optionally, this step can be performed in a vacuum, to eliminate air inside the cavities preventing the liquid containing the recording medium from entering the cavities ofthe pressing tool. If the recording medium is dissolved or dispersed, the solvent is, preferably, chosen such that it is a non-solvent for both the optical layer material and the used pressing tool. Polar solvents are best suited for the specific example described here, e.g. acetone, acetonitril, γ-butyrolactone, lower alcohols, etc. Once the liquid layer is dried and the recording medium is subsequently deposited at the designated positions (Figure 4C), the pressing tool is heated above the melting temperature ofthe optical layer material, whereby the optical layer material melts. Subsequent removal ofthe pressing tool results in the deposition ofthe optical layer material on the previously deposited recording material. De-wetting ofthe optical layer material on the recording material results in a hemispherical or stigmatic lens design, depending on the established contact angles ofthe materials (Figure 4D).

The optical layer, i.e. the lens precursor material, is fixated (Figure 4E) using for instance radiation (e.g. UN-light, visible light) or thermal energy (e.g. thermally initiated polymerization at a curing temperature between the melting temperature ofthe optical layer material and that of the recording medium).

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. The nano-lens morphology manufactured according to the method according to the invention is, optionally, covered with a transparent cover layer consisting of a low refractive index material (n∞1.3) (see Figure 4F). Preferably, (amorphous) fiuorinated polymers are employed. The above-described method is an example for recording mediums with an apolar character. Different combinations, such as polar, hydrophilic recording materials and apolar, hydrophobic optical layer materials or the use of high refractive index (stabilized) organic or inorganic particles (e.g. nano-particles, quantum dots, nano-rods, nano-tubes, nano-wires) or metal complexes are also feasible. In addition, different materials for the pressing tool can be used, such as polyurethane stamps, enabling a wider choice of appropriate recording media, optical layer materials and intermediate solvents.

The above-described embodiment ofthe method of manufacturing a storage medium, as exemplified in Figures 4A-4F, is very suitable for depositing a polar, hydrophilic optical layer material into the cavities of an apolar, hydrophobic pressing tool. Alternatively an apolar optical layer material with an apolar pressing tool and depositing the optical layer material on a polar recording medium is also feasible. In the latter case it may appear to be difficult to force the liquefied optical layer material out ofthe cavities ofthe pressing tool. To simplify this, the pressing tool can modified as shown in Figure 5. Figure 5 shows an alternative embodiment ofthe pressing tool used in the method of manufacturing a storage medium according to the invention. In the example of Figure 5, the cavities 11, 11 ', ... of the pressing tool 10 are provided with a channel 15, 15', ... for influencing the release ofthe optical layer 22, 22 ' , ... from the cavities 11, 11', .... One or more small channels 15, 15', ..., e.g. capillaries are provided in each ofthe cavities ofthe pressing tool. The design can be realized in a simple fashion by appropriate adjustment ofthe mold used for preparing the pressing tool. Alternatively, the capillaries can be injected in the finished pressing tool when still inside the mould or during the formation ofthe pressing tool (curing ofthe monomer used for the foπnation ofthe pressing tool) by inserting a second mould on top ofthe first mould.

The modification ofthe pressing tool as shown in Figure 5, offers the possibility to adjust the external pressure such that entering (depositing) ofthe liquefied optical layer material in the cavities (under reduced pressure) and releasing (expelling) the optical layer material from the cavities is largely facilitated (under excess pressure). The modified pressing tool also offers an additional means of control ofthe filling ratio ofthe optical layer material. Preferably, a two-dimensional strip of bit positions 14, 14', ... in the form of a spiral is provided on the substrate by the method of manufacturing a storage medium (see Figure 1A and IB).

The scope ofthe 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 ofthe claims. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. Use ofthe 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, a pre-determined pattern of bit positions provided with an active layer comprising a recording medium for retention of data, a plurality of micro-optical elements for receiving illumination from an external source of illumination, the micro-optical elements being provided according to the pre-determined pattern of bit positions.

2. A storage medium as claimed in claim 1, characterized in that the substrate is provided with the pre-determined pattern of 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 ofthe cover layer is n_cι < 1.5 and the refractive index n_me ofthe micro-optical elements is n_me > 1.5.

7. A storage medium as claimed in claim 1 or 2, characterized in that the pre- determined pattern comprises a two-dimensional strip of bit positions.

8. A storage medium as claimed in claim 1 or 2, characterized in that the predetermined 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: an optical layer is deposited into cavities of a pressing tool, the pressing tool being provided with a pre-determined pattern of cavities, the pressing tool is positioned in an imprintable layer disposed on a substrate, the imprintable layer comprising a recording medium, the imprintable layer is fixated, providing the substrate with bits ofthe recording medium, the bits being arranged according to the pre-determined pattern, the optical layer in the cavities ofthe pressing tool is released and the pressing tool is removed, the optical layer is fixated, forming a micro-optical element on each ofthe bits ofthe recording medium.

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

11. A method of manufacturing a storage medium as claimed in claim 10 characterized in that the curvature of the lenses is hemispherical or stigmatic.

12. A method of manufacturing a storage medium as claimed in claim 9 or 10, characterized in that the optical layer is released from the cavities by melting the optical layer.

13. A method of manufacturing a storage medium as claimed in claim 12, characterized in that the optical layer has a transition point above the transition point ofthe imprintable layer.

14. A method of manufacturing a storage medium as claimed in claim 10, characterized in that the optical layer has a transition point above the ambient temperature.

15. A method of manufacturing a storage medium as claimed in claim 9 or 10, characterized in that the imprintable layer is a liquid.

16. A method of manufacturing a storage medium as claimed in claim 9 or 10, characterized in that the cavities ofthe pressing tool are provided with a channel for influencing the release ofthe optical layer from the cavities.

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

18. 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 made of a material with a relatively low refractive index, and the optical layer has a relatively high refractive index.

19. A method of manufacturing a storage medium as claimed in claim 18, characterized in that the refractive index n-i ofthe cover layer is n_cι < 1.5 and the refractive index n_om ofthe optical layer is n_om > 1.5.

20. A record carrier having information written thereon, characterized in that the information is coded in a recording medium provided by a method of manufacturing as claimed in claim 9 or 10.

21. A record carrier as claimed in claim 20, characterized in that the record carrier is an optical disc.