KR20050020991A - Optical data storage medium and use of such medium - Google Patents

Abstract

An optical data storage medium 10 for reading using a focused radiation beam 19 having a wavelength λ and a numerical aperture NA is described. The medium comprises a substrate 10, a first stack 12 of a plurality of layers represented by L0 comprising a first information layer, and optionally a plurality of layers represented by Ln comprising an additional information layer. It has at least one additional laminate 13 composed of. Between each of L0 and Ln is a spacer layer 14 transparent to the radiation beam 19. The transmissive laminate, denoted _TS0 with a thickness d _TS0 , comprises all the layers between the LO 12 and the incident surface 16 of the medium 10. The transmission laminate represented by TSn having a thickness d _TSn includes all the layers between the Ln 13 and the incident surface 16. When measured over the information area of the medium 10, the maximum deviation of d _TS0 and, if applicable, d _TSn does not exceed the predetermined value DEVd _TS0 or DEVd _TS1 , and this value is set depending on λ and NA. . According to this, reliable reading of the information layer (s) is achieved without requiring dynamic spherical aberration correction.

Description

Optical data storage media and their uses {OPTICAL DATA STORAGE MEDIUM AND USE OF SUCH MEDIUM}

The present invention provides a data storage medium having at least a reading using a focused radiation beam having a wavelength λ and a numerical aperture NA and incident through an incidence plane of a medium during reading.

A substrate present on one side of the storage medium,

A first stack of a plurality of layers represented by L0 comprising a first information layer,

A cover layer transparent to the radiation beam adjacent to the incident surface,

An optical data storage medium having a transmissive laminate represented by TS0 having a thickness d _TS0 and including all layers between L0 and the plane of incidence.

The present invention also relates to the use of such a medium.

One embodiment of such an optical recording medium is described in the article "New Replication Process Using Function-assigned Resins for Dual-layered Disc with 0.1 mm thick Cover layer," by K. Hayashi, K. Hisada and E. Ohno, Technical Digest ISOM. Known in 2001, Taipei, Taiwan.

There is an ongoing need to obtain an optical storage medium having a storage capacity of 8 gigabytes (GB) or more and suitable for recording and reproduction. This need is met by a format (DVD), also called some digital video discs or sometimes digital multifunction discs. DVD formats can be classified into DVD-ROM that is dedicated to playback, DVD-RAM, DVD-RW and DVD + RW that may be used for storing rewritable data, and DVD-R, which is a write once type. Currently, DVD formats include discs with capacities of 4.7 GB, 8.5 GB, 9.4 GB and 17 GB.

The 8.5GB, and especially the 9.4GB (DVD-9) and 17GB (DVD-18) formats represent a more complex structure and usually include multiple information storage layers. The 4.7 GB single layer rewritable DVD format is easy to handle, for example, similar to a conventional compact disc (CD), but provides insufficient storage capacity for video recording purposes.

A recently proposed high storage format is digital video recording (DVR). Currently, two formats, DVR-red and DVR-blue, are being developed, the latter being called Blu-ray Discs (BD), where red and blue refer to the laser beam wavelength used for recording and reading. Such discs solve capacity problems, the simplest form of which has a capacity of over 22 GB in DVR-Blue format and has a single storage layer format suitable for high density digital video recording and storage.

DVR disks generally include a disk-shaped substrate having an information storage layer on one or both sides of the information layer. The DVR disk further includes one or more radiation beam transmissive layers. These layers are transparent to the radiation beam used to read or write to the disc. For example, a transmissive cover layer is applied over the information storage layer. In general, for high density discs, lenses with a high numerical aperture (NA), for example greater than 0.60, are used to focus such a radiation beam with a relatively small wavelength. For systems with NA greater than 0.60, it is even more difficult to apply substrate incidence recording using substrate thickness of 0.6-1.2 mm, for example due to reduced tolerances for thickness variations and disc tilt, and the like. For this reason, when using discs recorded and read using high NA, focusing of the first recording stack onto the recording layer is performed on the opposite side of the substrate. Since the first recording layer must be protected from the external environment, for example, at least one relatively thin radiation beam transmissive cover layer thinner than 0.5 mm is used, through which the radiation beam is focused. Obviously, since the need for the substrate to be transparent to the radiation beam no longer exists, other substrate materials, for example metals or alloys, may be used.

The dual layer optical storage medium has two reflective information layers that are read on the same side of the medium. In the case of such a double layer medium in which the second recording stack is present, a radiation beam transmissive spacer layer is required between the recording stacks. In order to enable reading from the first information layer of the first recording stack, this additional recording stack must be at least partially transparent to the radiation beam wavelength. The thickness of these spacer layers is chosen such that the information layers are separated from each other. In such a case, the laser beam can be individually focused on each information layer, and no signal interference with other storage layers occurs. The radiation beam transmissive layer or transmissive layers existing between the radiation beam light source and the recording stack located furthest from the substrate are commonly called cover layers. If prefabricated sheets are used as the transmissive layers, additional transmissive adhesive layers are needed to adhere the cover layers to each other.

In a DVR disk, in order to minimize the variation in the optical path length with respect to the incident radiation beam, it is necessary to carefully adjust the variation or irregularities in the thickness of the radiation beam transmitting layer over the radially extending portion of the disk. In particular, the optical quality of the radiation beam at the focal point of a BD, ie a DVR-blue disk, using a radiation beam with a wavelength such as approximately 405 nm and an NA of approximately 0.85, is relatively sensitive to variations in the thickness of the transmissive layers. The total layer thickness has an optimum value, for example, to obtain the minimum optical spherical aberration of the focused radiation beam on the first information recording layer. From this optical thickness a deviation of, for example, +/- 5 占 퐉 is already a significant amount and introduces an unacceptable amount of this kind of aberration. Depending on the system, the spacer layer thickness can range from several μm to about 100 μm. Moreover, the disk may have a cover layer. Such layers are usually manufactured using plastic foils or by spin coating of resin layers. Other manufacturing processes may be employed. These technical processes cause thickness variation of the spacer / cover layers. This fits very well against variations in the radial direction of the disc, especially when spin coating processes are applied. As a result, the depth position of the information layers with respect to the incidence plane of the disc and the distance between the information layers can vary considerably within the disc. Such fluctuations cause additional spherical aberration inside the system, resulting in poor signal quality, delays in jumping between information layers, and ultimately system failure. For successful reading and writing, a clear focus must be available for each of the information layers on the disc. Therefore, additional spherical aberrations due to spacer / cover layer variations have to be corrected by the optical drive. If these variations are small enough to fall within the actuator's focusing range, these variations can be corrected by the actuator itself. If these fluctuations are greater than the operating range of the actuator, dynamic spherical aberration correction must be employed or other servo methods must be introduced into the system.

As mentioned above, mass-produced single layer and multilayer optical data storage media have a thickness variation of the spacer layer / cover layer. Due to these variations, the depth position of the information layer deviates from a predetermined value. If these fluctuations are so large that the focusing system cannot handle them well, deterioration of the servo and data signals will result, resulting in poor system performance. A solution for compensating for large depth positional deviations within the disc is a spherical aberration (SA) correction in the optical pickup unit (OPU) inside the optical drive. The problem with this method is that it is necessary to continuously generate the SA error signal.

Finally, it is an object of the present invention to provide a storage medium of the kind described at the outset for reliably reading data from the information layer (s).

The above object is, according to the present invention, when measured over the entire area of the medium, the maximum deviation from each of the average values of d _TS0 of the predetermined area of the medium does not exceed the predetermined value DEVd _TS0 , and DEVd _TS0 is equal to λ. It is achieved by the optical data storage medium, characterized in that it is set depending on the NA.

This results in a placement of media that does not require dynamic spherical aberration (SA) correction for calibration of focus and reliable reading of data. However, according to the present invention, when the first information layer of the medium is scanned by the optical medium drive, no substantial correction of spherical aberration is necessary. During scanning, the OPU moves radially inward or radially outward while the medium is rotating. In the case where the thickness variation of TS0 falls within the above limit value, spherical aberration is also maintained in an acceptable range throughout the information area of the medium. The thickness variation of the transmission laminate mainly causes spherical aberration A40. Spherical aberration causes wavefront errors. For accurate focusing of the radiation beam, the wavefront rms error should not exceed 0.333λ. Calculations show that the wavefront error is highly dependent on the NA, thickness d of the radiation beam, the refractive index n of the transmissive laminate passing as the focused radiation beam moves, and the error is expressed in units of λ. . The general formula for A40 is:

Usually, in the optical disk system, the aberration at the predetermined thickness d is canceled by the objective lens designed to introduce the same amount of spherical aberration having the opposite sign. Therefore, in practice, a problem arises only when the thickness d deviates by a magnitude of Δd from its predetermined value, so the above equation must also be interpreted accordingly. The maximum allowable Δd may be converted to DEVd _TS0 .

In one embodiment, DEVd _TS0 = ± 3 μm. In particular, for media using short radiation beam wavelengths λ smaller than 500 nm and high NAs, for example higher than 0.75, this value should be kept within the above limits.

In a preferred embodiment, the medium, at least,

One additional stack consisting of a plurality of layers, denoted by Ln, where n is an integer ≧ 1 and comprises an additional information layer and is located at a position closer to the entrance plane than L0,

A spacer layer transparent to the radiation beam present between each of L0 and Ln,

A transmissive laminate having a thickness d _TSn and represented by TSn including all layers between the Ln and the incident surface, and when measured over the entire area of the medium, the maximum deviation of d _TSn does not exceed the predetermined value DEVd _TSn Instead, DEVd _TSn is set depending on λ and NA. In order to enable reading and writing to the L0 stack, the stack Ln must be at least transparent to the radiation beam. Likewise, especially in the case of a medium using a short radiation beam wavelength [lambda] smaller than 500 nm and a high NA, for example greater than 0.75, this value DEVd _TSn should be kept within ± 3 μm. When such multilayer media are used, the only case where spherical aberration correction is needed is when the OPU is switched between focusing from one information layer to another. This calibration is done at the OPU with special means and need not be dynamic.

Preferably, DEVd _TS0 = ± 2 μm. Considering possible media deformations due to manufacturing and operating conditions (temperature, humidity, etc.), the depth position variation of the storage layers should be slightly less than +/- 2 μm.

In a particular embodiment, there is only one additional stack consisting of a plurality of layers represented by L1 with one additional information layer, DEVd _TS1 = ± 2 μm, and λ has a range of 400 nm to 410 nm, NA Has a range of 0.84 to 0.86. These values apply to the Blu-ray Discs (BDs) described previously, in which case reliable readings can be made without using dynamic SA calibration. The BD comprises two transmission stacks TS0 and TS1 having effective refractive indices n _TS0 and n _TS1 and thicknesses d _TS0 and d _TS1 . In BD, n _TS0 and n _TS1 both have values 1.6 and close to 1.6, and the following conditions are satisfied: 95 μm ≦ d _TS0 ≦ 105 μm and 70 μm ≦ d _TS1 ≦ 80 μm. Most plastic materials used as transparent layers have a refractive index of 1.6 or nearly close to this value. The layer between L0 and L1 is called a spacer layer, and the layer between L1 and the incident surface is called a cover layer.

In yet another embodiment, the spacer layer thickness is nearly close to 20 μm or 20 μm and the cover layer thickness is close to 80 μm or 80 μm. From a manufacturing standpoint, it is advantageous to use spacer layers and cover layer thicknesses of substantially fixed values. For example, one manufacturing method applies a sheet comprising a pressure sensitive adhesive (PSA), which is UV cured after contact with other layers of the medium. Such materials are usually provided as sheets of foil having PSA on one or both sides, and these sheets are made to a predetermined thickness. In BD, the spacer layer thickness is 20 µm to 30 µm, and the cover layer thickness should be adjusted accordingly, for example, 80 µm to 70 µm.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings in which:

1 schematically illustrates the structure of an optical data storage medium according to the present invention having two information layers (Ln = L1).

Fig. 2 shows the calculated wavefront error rms as a function of the depth position deviation DEVd _TSn of the information layers.

1, an embodiment of a dual layer optical data storage medium 10 according to the present invention is shown. A focused laser beam 19 having a wavelength λ of 405 nm and a numerical aperture NA of 0.85 is incident through the entrance face 16 of the medium 10 during reading. The substrate 11 made of polycarbonate is represented by a first stack 12 of a plurality of layers represented by L0 including a first information layer on one surface thereof and an L1 including a second information layer. It has a second stack 13 of a plurality of layers. L1 exists at the position closest to the incident surface 16, and L0 is located farther from the incident surface 16 than L1. A transparent spacer layer 14 made of UV curable resin, for example SD694 made by DIC, is located between LO and L1. The transparent cover layer 15 is present between the entrance face 16 and L1 and may be made of a sheet of PC or PMMA with the same material or pressure sensitive adhesive (PSA). The spacer layer may be a sheet bonded to the PSA. The transmission laminate represented by TS0 has a thickness d _TS0 of 100 μm and an effective refractive index n _TS0 = 1.6 and includes all layers between L0 and the incident surface 16. The L1 laminate 13 has a relatively small thickness of up to several hundred nm, negligible of its influence. Of course, L1 affects the light transmittance, but this aspect is not dealt with in this specification. The transmission laminate represented by TS1 has a thickness d _TS1 of 80 μm and an effective refractive index n _TS1 of 1.6 and includes all layers between L1 and the incident surface 16. The laminate TS1 corresponds to the cover layer 15. The spacer layer 14 has a thickness of 20 μm. When measured over the information area of the medium 10, the maximum deviation of d _TS0 does not exceed the predetermined value DEVd _TS0 of ± 2 μm. When measured over the entire area of the medium 10, the maximum deviation of d _TS1 does not exceed the predetermined value DEVd _TS1 of ± 2 μm. Therefore, the wavefront error A40 does not exceed 0.033 lambda rms.

2 shows the calculation of the wavefront error caused by the variation in the thickness of the cover layer. Graph 21 of this figure corresponds to the calculation result for DEVd _TS1 when focused on an additional stack of L1 disposed closer to the incident surface 16 (FIG. 1). Graph 22 corresponds to the calculation result for DEVd _TS0 when focused on the first information layer of L0 disposed closer to the substrate 11 of the medium 10 (FIG. 1). As can be seen from this figure, in order to keep the wavefront error rms smaller than 0.033λ indicated by the dotted line 23, the deviation of the depth position of the information layer should not exceed ± 3 μm. Given the possible deformation of the medium due to manufacturing and operating conditions, this deviation should be slightly less than ± 2 μm.

At this time, it should be noted that the above-described embodiments are intended to illustrate rather than limit the present invention, and that various other embodiments can be envisioned to those skilled in the art without departing from the scope of the appended claims. I hope. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The term "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The term "a" or "an" in front of a component does not exclude the presence of such a plurality of components. The simple fact that a particular configuration is repeated in different subclaims does not suggest that a combination of these configurations may not be used advantageously.

According to the present invention, a double layer optical data storage medium for reading using a focused radiation beam having a wavelength lambda and a numerical aperture NA is described. The medium comprises at least one additional structure consisting of a substrate, a first stack of a plurality of layers represented by L0 comprising a first information layer, and optionally a plurality of layers represented by Ln comprising an additional information layer. It has a laminated body. Between each of L0 and Ln is a spacer layer transparent to the radiation beam. The transmissive laminate, denoted _TS0 with a thickness d _TS0 , comprises all the layers between LO and the incident surface of the medium. The transmission laminate represented by TSn with thickness d _TSn includes all layers between Ln and the incident surface. When measured over the information area of the medium, the maximum deviation of d _TS0 and, if applicable, d _TSn does not exceed the predetermined value DEVd _TS0 or DEVd _TS1 , and this value is set depending on λ and NA. According to this, reliable reading of the information layer (s) is achieved without requiring dynamic spherical aberration correction.

Claims (7)

A data storage medium having a wavelength λ and a numerical aperture NA and at least reading out using a focused radiation beam incident through an incidence plane of a medium during reading, at least: A substrate present on one side of the storage medium, A first laminate of a plurality of layers represented by L0 including a first information layer, A cover layer transparent to the radiation beam adjacent to the incident surface, An optical data storage medium having a thickness d _TS0 and having a transmissive laminate represented by TS0 comprising all layers between L0 and the incident surface, When measured over the entire area of the medium, the maximum deviation from each of the average values of d _TS0 of the predetermined area of the medium does not exceed the predetermined value DEVd _TS0 , and DEVd _TS0 is set depending on λ and NA. Optical data storage medium. The method of claim 1, DEVd _TS0 = ± 3㎛ optical data storage medium, characterized in that. The method of claim 1, At least, One additional stack consisting of a plurality of layers, denoted by Ln, where n is an integer ≧ 1 and includes an additional information layer and is located at a position closer to the entrance plane than L0; A spacer layer transparent to the radiation beam present between each of L0 and Ln, _{Having a} thickness d _TSn and having a transmissive laminate represented by TSn comprising all layers between the Ln and the incident surface, and when measured over the entire area of the medium, the maximum deviation of d _TSn does not exceed the predetermined value DEVd _TSn And DEVd _TSn is set depending on λ and NA. The method of claim 3, wherein DEVd _TSn is an optical data storage medium, characterized in that ± 3㎛. The method of claim 1, DEVd _TS0 = ± 2㎛ optical data storage medium, characterized in that. The method of claim 3, wherein There is only one additional stack consisting of a plurality of layers represented by L1, including an additional information layer, DEVd _TS0 = ± 2 μm and DEVd _TS1 = ± 2 μm, λ has a range of 400 nm to 410 nm, NA Optical data storage medium having a range of 0.84 to 0.86. Use of an optical data storage medium according to any one of the preceding claims for reliable data reading from at least one information layer.