Abstract:
The present invention relates to a method of producing high contrast optical storage discs. The method comprises usage of so-called liquid embossing technology process steps that turn out to be beneficial for mass-producing fluorescent optical data storage discs. The present invention also relates to a high contrast optical storage disc comprising an information layer that includes a fluorescent dye on a substrate. The information layer comprises a structure of lands and pits and wherein the lands have a thickness of substantially zero; and the pits have a finite thickness. The optical storage disc may well be multi-layered. The present invention also relates to an apparatus suitable for producing high contrast optical fluorescent storage discs.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method of manufacturing a fluorescent optical information carrier.  
         [0002]     The present invention also relates to a fluorescent optical information carrier.  
         [0003]     Moreover the present invention relates to an apparatus for manufacturing a fluorescent optical information carrier.  
         [0004]     The present invention is particularly relevant for optical data storage and the production of an optical data storage disc, especially for a high contrast multilayer fluorescent optical disc that can be used as data storage medium.  
       BACKGROUND OF THE INVENTION  
       [0005]     In the field of optical recording, increasing the capacity of the information carrier is the trend. An already investigated way for increasing the data capacity consists in using a plurality of information layers in the information carrier. For example, a DVD (Digital Video Disc) can comprise two information layers. Information is recorded on or read from an information layer by means of an optical beam, using local refractive index variations or the presence of surface relief structures.  
         [0006]     In order to increase the number of layers of an information carrier, a fluorescent multi-layer information carrier has been proposed. Such a fluorescent multi-layer information carrier, as well as an optical disc apparatus for reading from this carrier, is described in patent U.S. Pat. No. 6,009,065, granted on Dec. 28, 1999.  
         [0007]     In each information layer, the information is deposited or recorded as a sequence of fluorescent and non-fluorescent cells, the fluorescent cells being made of a fluorescent material capable of generating a fluorescent radiation when interacting with an optical beam. The layers of the carrier are separated by spacer layers, which are transparent for the wavelengths of the optical beam and the fluorescent radiation.  
         [0008]     The optical beam is focused with an objective lens on a layer of the carrier. When a fluorescent cell of the addressed layer absorbs the energy of the optical beam, a fluorescence signal is generated. This fluorescence signal has a wavelength, which is different from the wavelength of the exciting beam, due to the so-called Stokes-shift. Hence, the interactions between the fluorescence signal and the non-addressed layer are relatively small, because the absorption of the non-addressed layers at the wavelength of the fluorescence signal is relatively small.  
         [0009]     A detector unit then detects the fluorescence signal. The detector unit comprises means for separating the fluorescence signal coming from the addressed layer from the fluorescence signals coming from the non-addressed layers. For example, a co-focal pinhole is inserted in front of a photodiode in order to spatially block the fluorescence signal coming from the non-addressed layers.  
         [0010]     Fluorescent data storage is interesting for application in multi-layer media systems because of the photo-induced emission of light, which is incoherent and of a different wavelength as the excitation beam. Therefore no adverse interference effects occur between photons coming from different layers. In contrast to phase grating systems, however, the contrast of the emission between a ‘one’ and a ‘zero’ is not achieved by interference of the refracted or reflected beam. It is achieved solely by the difference in the intensity of the emitted light. The two possibilities for modulation of emission are spatial modulation of absorbance and or emittance. Both possibilities can be achieved via an effective local concentration of dye per unit area of beam diameter. This effective local concentration can be modulated either as a concentration in the chemical sense (molecules per unit of volume) or the physical sense (absorbance per molecule) or simply as the variation in layer thickness. The latter is the most obvious although variation of the absorbance by variation of the molecular orientation (transition moment with respect to the incoming polarized beam) has been proposed.  
         [0011]     Variation in layer thickness to create a data-layer can be achieved by (i) structuring a substrate on which the fluorescent layer is applied by spin coating or (ii) by structuring the fluorescent layer after application on a flat substrate by embossing. The former approach (i) is sketched in  FIG. 1 . The substrate can be structured with the same technology as used for conventional optical recording media (ROM), like injection molding. A problem with this approach is that with spin coating a continuous layer is formed, which after drying will have a modulation in layer thickness but the layer thickness will not be zero. Increasing the depth of the pits can increase the modulation but the replication process of such structures limits this. The second approach (ii), as sketched in  FIG. 2 , suffers from a similar problem. The ratio of layer thickness in the pit vs. on the land is limited by the fact that it is not possible to reduce the layer thickness in the lands to zero and the aspect ratio of the pit structures is limited by the replication process. Prior art process steps are shown in  FIG. 1  for producing a fluorescent layer on a structured substrate for an information carrier disc. In step  100  a structured substrate  104  is taken on which a fluorescent layer  102  is applied. Substrate  104  can be structured using similar technology as used for conventional optical recording media (such as ROM), like injection molding. After the step  110  of spin coating that forms a continuous layer and after drying layer  108  is formed. Lands will however have a thickness unequal to zero.  
         [0012]     Prior art process steps are shown in  FIG. 2  for producing a structured fluorescent layer on an un-structured substrate for an information carrier disc. In step  200  a hard stamp  202  is used with an unstructured substrate  206  on which a fluorescent layer  204  is applied. In step  210  hard stamp  202  is applied and fluorescent layer  204  will be deformed to a structured fluorescent layer  208 . In step  220  the hard stamp  202  will be removed and a final structured fluorescent layer  210  is formed, possibly after a step of hardening. As stamp  202  is hard it will not be possible using this method to achieve a structured surface wherein the lands have a thickness of zero (or even not close to zero).  
         [0013]     Producing a disc with a fluorescent data layer with a highest possible contrast between pits and lands turns out not to be feasible until now. An essential step in improving the modulation would be the reduction of emission in the land (or pit) to virtually zero. On top of this, for an optical ROM medium, a process is preferred that structures a whole layer in a single step in order to speed-up the process and to make it at cheap as possible.  
       SUMMARY OF THE INVENTION  
       [0014]     It is, accordingly, an object of the present invention to provide an easily practiced, low cost process for producing an optical information layer on a substrate and an apparatus enabled to perform the process. The process is especially suited for optical storage discs for which it must be possible to be mass-produced such as read only optical discs (or hybrid discs thereof). The method is targeted for fluorescent optical storage discs that and these discs may be multi layered.  
         [0015]     It is another object of the invention to provide an optical storage disc comprising an information layer that includes a fluorescent dye on a substrate. The information layer comprises a structure of lands and pits and wherein the lands have a thickness of substantially zero; and the pits have a finite thickness.  
         [0016]     In order to maximize modulation of the information layer the inventor proposes to structure a fluorescent medium in a way that the layer thickness becomes virtually zero in the land (or pit) areas, whereas it has the required thickness for a strong signal in the remaining areas. For a multi-layer medium the inventor found a strong preference to have the continuous (land) area with zero thickness and the pits with maximum thickness in order to minimize background radiation from different layers.  
         [0017]     In one embodiment, e.g., starting from a medium as obtained by the processes (i) and (ii) (as described above and if  FIG. 1  and  FIG. 2  respectively) the inventor achieved the goal by etching the structured fluorescent layer, for instance in a reactive-ion etcher (RIE). In such a process material is removed from the surface by ion bombardment, as sketched in  FIG. 3 . It will be removed preferentially in the direction perpendicular to the surface. In this way the lateral resolution of the pattern is not affected. [The etching plasma composition can be chosen such that there is a strong difference in etching rate between the fluorescent layer and the substrate (or a coating applied between substrate and fluorescent layer). In this way the etching will virtually stop at the interface.] The potential disadvantages of this approach are the extra etching steps, which add cost to the medium and the potential damage to the fluorescent dyes during etching.  
         [0018]     In another embodiment, a preferred technique to achieve a virtually zero thickness involves the so-called liquid embossing process. In this process the structuring is carried out in a liquid layer with the aid of a soft stamp. Liquid embossing techniques have until now only been envisioned for use in the semiconductor technology and alike (see also WO0120402-A1 “Fabrication of finely featured devices by liquid embossing”). The inventor shows how to apply liquid embossing technology for producing a high contrast fluorescent data storage medium. This technology is especially interesting for optical storage media that comprise Read Only Memory (ROM) since they typically need to be produced in mass.  
         [0019]     These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein:  
         [0021]      FIG. 1  shows prior art steps of producing a fluorescent layer on a structured substrate;  
         [0022]      FIG. 2  shows prior art steps of producing a structured fluorescent layer on a flat substrate by embossing;  
         [0023]      FIG. 3  shows steps of producing a structured fluorescent layer on a flat substrate by embossing followed by etching in accordance with the invention;  
         [0024]      FIG. 4   a  shows steps of producing a fluorescent information carrier disc in accordance with the invention;  
         [0025]      FIG. 4   b  shows alternative steps of producing a fluorescent information carrier disc in accordance with the invention;  
         [0026]      FIG. 5  shows an apparatus for producing a fluorescent information carrier disc in accordance with the invention that also shows stages or steps of producing the disc;  
         [0027]      FIG. 6  shows an alternative apparatus for producing a fluorescent information carrier disc in accordance with the invention that also shows stages or steps of producing the disc; and  
         [0028]      FIG. 7  shows yet another apparatus for producing a fluorescent information carrier disc in accordance with the invention that also shows stages or steps of producing the disc. 
     
    
       [0000]     Throughout the drawings, the same reference numeral refers to the same element, or an element that performs substantially the same function.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0029]      FIG. 3  shows steps of producing a structured fluorescent layer on a flat substrate by embossing followed by etching in accordance with the invention.  
         [0030]     Step  300  of  FIG. 3  starts from a medium as obtained by the processes (i) and (ii) (as described  FIG. 1  and  FIG. 2  respectively). In step  300 , structured fluorescent layer  304  is etched, e.g., in a reactive-ion etcher (RIE). In such a process material from layer  304 , which resides on carrier  306 , is removed from the surface by ion bombardment. It will be removed preferentially in the direction perpendicular to the surface. In this way the lateral resolution of the pattern is not affected. The etching continues until fluorescent  304  is removed at the pits. The etching plasma composition can be chosen such that there is a strong difference in etching rate between the fluorescent layer and the substrate (or a coating applied between substrate and fluorescent layer). In this way the etching will virtually stop at the interface, carrier  306 .  
         [0031]     After etching a fully structured fluorescent layer  308  is created as shown in step  310 . As shown in  310  lands of layer  308  have a sufficient thickness and pits have a zero thickness.  
         [0032]     Process steps of producing a fluorescent information carrier disc in accordance with an embodiment of the invention are shown in  FIG. 4   a.    
         [0033]     In step  410  a soft stamp  400  is cast in, e.g., PDMS (polydimethoxysiloxane) from a mold  402 , which typically contains a required microstructure. The mold  402  can be a Ni shim, which is produced with existing stamper technology as used for injection molding of DVD substrates, except for a higher structure depth.  
         [0034]     In step  420  the stamp  410  is transferred to a solid substrate  403  to facilitate handling.  
         [0035]     In step  425  a substrate  406  (typically an optical substrate), is coated with a solution  404  of a fluorescent dye, like Coumarin-30 and a polymer, like polyvinylbutyral (PVB) or polyvinylalcohol (PVA), in a common solvent, like ethyl-lactate or ethanol. The concentration of the polymer in the solution  404  has been adjusted for the optimum solution viscosity for subsequent spinning and embossing process steps. The concentration of the dye in the solution  404  has been adjusted to the polymer to have maximum efficiency (avoid quenching).  
         [0036]     Step  430  comprises spinning solution  404  to a layer  407  with a required thickness (typically in the order of magnitude of less than half the structure depth; see below why).  
         [0037]     In step  440  stamp  400  is applied to layer  407  (typically a wet layer). Stamp  400  at least resides on the substrate  406  until a liquid film of solution  404  underneath stamp  400  (the stamp typically resembles a rubbery material) is squeezed out to form a structured layer  408  of solution  404 . Interfacial forces typically perform the squeezing out. It is preferred that liquid film material that is being squeezed out moves in cavities  409  that are present in stamp  450 . After the movement of the liquid film material thickness d 1  of a cavity  409  should be larger than thickness d 2  of the structured layer  408 . Otherwise layer  407  was too thick. On the other hand if layer  407  is not thick enough, the structured layer  408  will not be thick enough. So in an optimum situation thickness  407  should be such that thickness d 2  is almost as large as thickness d 1 . In other words: the surface of the pits (e.g., the upper surface of the squares of layer  416 ) versus the total surface of the stamp at the side where it contacts substrate  406  determines the maximum allowed thickness of layer  407 .  
         [0038]     In step  450  stamp  400  is released carefully.  
         [0039]     Structured layer  408  is dried in step  460  at slightly elevated temperature to form dried structured layer  412 . The process step described so far are cost effective and are compatible with processing steps on thin substrates. There is no thermal load on the dye. Stamp  400  can be reused. However a limitation comes from a required low viscosity of solution  404 . The low viscosity is required in order to achieve a reasonable rate of material displacement underneath stamp  400 . This leads to a reduced thickness of dried structured layer  412  after drying structured layer  408  in the case of a solvent, which is evaporated.  
         [0040]     Alternative process steps of producing a fluorescent information carrier disc in accordance with an embodiment of the invention are shown in  FIG. 4   b . Steps  410 ,  420 ,  425 ,  430  and  440  are substantially similar as described for  FIG. 4   a.    
         [0041]     In a preferred embodiment a solvent in layer  414  is used that is cured in step  470  to a polymer network after embossing (e.g., by UV irradiation that can start or speedup a polymerization reaction). It should be noted that step  470  of curing could be substantially simultaneously with step  440  of applying the stamp  400  forming structured layer  408 . In case of curing, the polymer is not necessary, as a special (active) solvent is used that will cure into a polymer (the active solvent typically forms radicals under exposure by UV-light that will in turn react to form the polymer). A curing process can be performed within a second. Typically the curing process executed in an oxygen poor environment such as a nitrogen gas environment.  
         [0042]     A drying process may involve a diffusion process of the solvent that is residing in layer  414  into stamp  400 . Stamp  400  can be used multiple times but care should be taken that it does not get too saturated with the solvent or else the diffusion process would slow down. The drying process can be executed quite fast as the amount of solvent is limited, e.g., due to the limited thickness of layer  414  (e.g., typically in the order of magnitude of less than 1 micrometer). As an alternative or in combination, drying may also be performed after layer  416  has been formed. A drying process may be sped-up by, e.g., elevating the ambient temperature.  
         [0043]     Alternatively in another preferred embodiment of step  470 , a chemical reaction of certain components of layer  414  solidifies layer  414 .  
         [0044]     In step  480  a full structured layer  416  is formed and retained on substrate  406  after removing stamp  400 .  
         [0045]     It should be noted that  FIGS. 4   a  and  4   b ,  5 ,  6 , and  7  show structures not drawn to scale. Also structures are typically drawn only partial in order to improve their function more clear. Moreover it is possible that a shown part of the stamp, e.g., stamp  400 , is actually part of a bigger stamp with, e.g., a curved shape. For instance, part of a curved stamp is above layer  404  and  407  and another part of the curved stamp is forming structured layer  408  or  414  as yet another part of the curved stamp is above  412  or  416 .  FIGS. 5, 6  and  7  will clarify this in more details.  
         [0046]      FIG. 5  shows one embodiment of an apparatus for producing a fluorescent information carrier disc in accordance with the invention that also shows stages or steps of producing the disc.  
         [0047]     The apparatus shown in  FIG. 5  comprises rotating drum  520 , soft stamp  500  located at the outer surface of drum  520 , reticle  530  with a hole  550  in it and UV-source  540 .  FIG. 5  also shows an information carrier comprising substrate  506 , newly formed structured layer  512 , solution  504  that has been applied on substrate  506 , and new structures  508  being formed. The information carrier is moving relative with respect to the apparatus as the drum  520  and stamp  500  are rotating whereby the speed of the outer surface of the stamp  500  is substantially the same as that of the information carrier at the point where new structures  508  are being formed. The speed of rotating drum  520  determines the time that stamp  600  is in contact with structures  608 . UV-source  540  irradiates new structures  508  though hole  550  and through substrate  506  with UV-light. The UV-light will start a polymerization reaction in structures  508  to eventually produce a newly formed structured layer  512 . Layer  512  typically comprises pits (the squares of  512 ) and lands (the empty spaces between the squares). The UV-light activates a photo-initiator that initiates a polymerization reaction of a solvent in solution  504 . The reaction is in progress in new structures  508 . The photo-initiator can, when being exposed to UV-light, e.g., dissociate into radicals that in turn can start a reaction with a reactive solvent to produce a polymer. Solution  504  typically comprises a reactive solvent and a fluorescent dye. In an alternative embodiment of the apparatus in  FIG. 5 , hole  550  can be located, at least partly under layer  512  since it is also possible to start the reaction until after stamp  500  releases from substrate  506  forming layer  512 .  
         [0048]      FIG. 6  shows another embodiment of an apparatus for producing a fluorescent information carrier disc in accordance with the invention that also shows stages or steps of producing the disc.  
         [0049]     The apparatus shown in  FIG. 6  comprises rotating drum  620 , soft stamp  600  located at the outer surface of drum  620 .  FIG. 6  also shows an information carrier comprising substrate  606 , newly formed structured layer  612 , solution  604  that has been applied on substrate  606 , and new structures  608  being formed. The information carrier is moving relative with respect to the apparatus as the drum  620  and stamp  600  are rotating whereby the speed of the outer surface of the stamp  600  is substantially the same as that of the information carrier at the point where new structures  608  are being formed. Solution  604  typically comprises a solvent, a fluorescent dye and a polymer. When stamp  600  is touching or in substantially close enough proximity of structures  606  the solvent will substantially diffuse into soft stamp  600  as the soft stamp  600  moves over the information carrier. A result is shown in  FIG. 6  as diffused solvent  660 . Structures  608  eventually produce newly formed structured layer  612 . Layer  612  typically comprises pits (the squares of  612 ) and lands (the empty spaces between the squares). In an alternative embodiment of the apparatus of  FIG. 6 , the solvent can be removed from solution  604  after newly formed structured layer  612  has been formed by a drying process. It is possible to achieve enough solvent removal any solvent diffusion into soft stamp  600  but a combination is also possible.  
         [0050]     The apparatus shown in  FIG. 7 , a so-called wave printing apparatus, comprises a pressure application substrate  770  and soft stamp  700 .  FIG. 7  also shows an information carrier comprising substrate  706 , newly formed structured layer  712 , solution  704  that has been applied on substrate  706 , and new structures  708  being formed. A traveling wave  780  is moving relative with respect to the apparatus and the information carrier. Substrate  770  is adapted to induce a traveling wave  780  in stamp  700 . Wave  780  moves from one side of stamp  700  to the other side. In the process stamp  700  will make contact with solution  704  and substrate  708  thereby forming layer  712 . The speed of wave  780  needs to be well controlled. Solution  704  typically comprises a solvent, a fluorescent dye and a polymer. In one embodiment, when stamp  700  is touching or in substantially close enough proximity of structures  706  the solvent will substantially diffuse into soft stamp  700  as the soft stamp  700  moves over the information carrier. Structures  708  eventually produce newly formed structured layer  712 . Layer  712  typically comprises pits (the squares of  712 ) and lands (the empty spaces between the squares). In an alternative embodiment of the apparatus of  FIG. 7 , the solvent can be removed from solution  704  after newly formed structured layer  712  has been formed by a drying process. It is possible to achieve enough solvent removal any solvent diffusion into soft stamp  700  but a combination is also possible.  
         [0051]     One of ordinary skill in the art will recognize that alternative schemes can be devised to create a fluorescent layer by making tweaks in the steps described.  
         [0052]     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.