WO2009040239A1

WO2009040239A1 - Recordable optical recording medium

Info

Publication number: WO2009040239A1
Application number: PCT/EP2008/061873
Authority: WO
Inventors: Larisa Von Riewel; Christophe Fery
Original assignee: Thomson Licensing
Priority date: 2007-09-24
Filing date: 2008-09-08
Publication date: 2009-04-02

Abstract

The recordable optical storage medium comprises a substrate layer (11), a recording layer (12, 13), a cover layer (15), a reflecting layer (14) arranged between the recording layer and the cover layer, a first dielectric layer (16) arranged between the substrate layer and the recording layer, and a second dielectric layer (17) arranged between the recording layer and the reflecting layer. The recording layer (12, 13) comprises a first film (12) containing essentially copper being arranged on the first dielectric layer (16) and a second film (13) containing essentially silicon arranged between the first film (12) and the second dielectric layer (17). The thicknesses of the first film (12) and of the second film (13) are in particular each in a range of 2 - 20 nm, wherein the thickness of the first film (12) is larger than the thickness of the second film (13), and the thickness (d1) of the first dielectric layer (16) is larger than the thickness (d2) of the second dielectric layer (17).

Description

RECORDABLE OPTICAL RECORDING MEDIUM

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a optical recording medium, which comprises a substrate layer, a recording layer, a reflecting layer and a cover layer, the recording layer consisting essentially of an inorganic material. The optical recording medium is in particular suitable for recording of data with a laser having a wavelength of about 405 nm and for reading of data with a laser having a wavelength of about 650 nm.

BACKGROUND OF THE INVENTION

Optical storage media are media in which data are stored in an optically readable manner, for example by means of a pickup comprising a laser for illuminating the optical storage medium and comprising a photo-detector for detecting the reflected light from the storage media. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB) . The formats include read-only formats such as Audio CD and Video DVD, write-once optical media like DVD+R and DVD-R as well as rewritable formats like CD-RW, DVD-RW, DVD+RW and DVD-RAM for example. Digital data are stored in these media along tracks in one or more layers of the media.

The storage medium with the highest data capacity is at present the Blu-Ray disc (BD) , which allows to store up to 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of about 405 nm is used. As a recording layer, a phase change material is used for a Blu-Ray disc. Further information about the Blu-Ray disc system is available for example from the Blu-Ray group via Internet: www.blu-raydisc.com. For reading and writing of data of a DVD, an optical pickup having a laser wavelength of 650 nm is used. For a recordable DVD, a recording layer with an organic dye is used. Additional information about the specification of read only and recordable DVDs can be obtained for example from the DVD forum.

Applications exist where it is wanted to record data on to an optical storage medium by using a laser diode having a wavelength in the range of 400-450 nm, for example for a fast recording of the storage medium with a high data rate, and for reading of the data of the optical storage medium by using a laser diode having a wavelength in a range of 650 nm. A storage medium of this kind allows it therefore for example to record a movie in a store from a server within a short time, on request of a user, which the user can replay later at home with his DVD player. For a recording with such a wavelength, an optical storage medium having a recording layer with an organic dye is less suitable .

Optical storage media having a recording layer consisting of an inorganic material are also known in the meanwhile, for example from US 7,018,695, which describes an optical recording medium having a recording layer with a first recording film containing an element selected from a group consisting of Si, Ge, C, Sn, Au, Zn and Cu as a primary component and a second recording film containing Cu as a primary component.

A further optical storage medium having a recording layer with an inorganic material comprising a first recording film with Si as a main component and a second recording film with Cu as a main component is known from US

7,141,289. A recordable storage medium Ml of this kind is shown in a cross section in figure 1 in a simplified manner. The recordable storage medium Ml comprises a substrate layer 1, on which a dielectric layer 6 is arranged. On the dielectric layer 6 a recording layer with a first film 2 having as a main component silicon, Si, and with a second film 3 having as a main component copper, Cu, is arranged. The first film 2 is deposited on top of the dielectric layer 6 and the second film 3 is deposited on top of the first film 2 for example by means of sputtering.

Above the recording layer 2, 3 then a second dielectric layer 7 is arranged. The two dielectric layers 6 and 7 contain essentially for example the material ZnS-SiO₂ and are arranged as a protection layer for the recording layer 2, 3. On top of the second dielectric layer 7 a reflective layer 4, for example an aluminum layer, is arranged, on which as a final layer a cover layer 5 is placed. For recording and reading of the data, a laser beam L is applied from below the optical storage medium Ml, penetrating first the substrate layer 1.

The storage medium Ml as shown in figure 1 provides a high reflectivity when a wavelength of 405 nm is used, but when using a wavelength of about 650 nm, the reflectivity is comparatively low and therefore, no sufficient signal-to- noise ratio can be obtained, when using a DVD pickup for reading of data from the storage medium Ml .

SUMMARY OF THE INVENTION

The recordable optical storage medium according to the invention comprises a substrate layer, a recording layer, a cover layer, a reflecting layer arranged between the recording layer and the cover layer, and a first and a second dielectric layer for a protection of the recording layer. The first dielectric layer is arranged on the substrate layer, on which a first film of the recording layer is arranged comprising essentially copper. The recording layer comprises further a second film containing essentially silicon, element Si, which is arranged on top of the first film of the recording layer. The first dielectric layer is arranged in particular between the first film of the recording layer and the substrate layer and the second dielectric layer is arranged between the second film of the recording layer and the reflective layer .

In a preferred embodiment, the recordable optical storage medium is a disk with outside dimensions, cover layer and substrate layer similar to a DVD. The thickness of the first dielectric layer is advantageously in a range of 40 - 100 nm, and the thickness of the second dielectric layer within a range of 5 - 30 nm. The thicknesses of the films with the copper layer and of the silicon layer are each within a range of 2 - 20 nm and can be manufactured on the first dielectric layer successively by means of a sputtering method. The thickness of the copper layer is preferably larger than the thickness of the silicon layer, the thickness of the copper layer being within a range of 8 - 12 nm and the thickness of the silicon layer being within a range of 4 - 8 nm.

In another aspect of the invention, the thickness of the first dielectric layer is advantageously in a range of 40 - 100 nm, and the thickness of the second dielectric layer within a range of 50 - 120 nm, to provide a high reflectivity of the recording layer.

The optical storage medium is designed in particular as a write-once-read-many (WORM) optical storage disc, on which data can be recorded with a laser wavelength of 405 nm and read with a red laser, e.g. wavelength of 650 nm, and the WORM disc has physical properties, according to which a consumer DVD player recognizes the WORM disc as a DVD ROM disc.

In a further aspect of the invention, the thicknesses of the two films of the recoding layer and of the two dielectric layers are optimized for providing a high push/pull signal when recording data with a Blu-laser beam on the recordable optical storage medium, and for providing a low push/pull signal for reading of the respective data with a red laser beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are explained in more detail below by way of example with reference to schematic drawings, which show:

Fig. 1 an optical storage medium with a recording layer consisting of an inorganic material according to prior art,

Fig. 2 a recordable optical storage medium comprising a recording layer in accordance with the invention, Fig. 3 reflectivity and modulation values of a recordable optical storage medium according to the invention in dependency of the thickness of the second dielectric layer at a wavelength of 650 nm, and Fig. 4 normalized push/pull signals in dependency of the thickness of the second dielectric layer of an optical storage medium according to the invention at a wavelength of 650 nm.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In figure 2 an optical storage medium M2 in accordance with the invention is shown in a simplified manner in a cross section. The optical storage medium M2 comprises a substrate layer 11, a recording layer with two films 12, 13 arranged between first and second dielectric layers 16, 17, the first dielectric layer 16 being placed on the substrate layer 10 and the second dielectric layer 17 being placed on the recording layer, and a reflective layer 14 being arranged between the second dielectric layer 17 and a cover layer 15, similar to the optical storage medium Ml as shown in figure 1. As an essential difference, the first film 12 with a copper layer Cu is arranged as a first recording layer on top of the first dielectric layer 16, and then, the second film 13 with a silicon layer Si is deposited on top of the copper layer 12. The first and second dielectric layers 16, 17 are arranged in particular as protective layers for the recording layer, for improving the optical performance of the storage medium M2.

By arranging first the copper layer Cu on top of the first dielectric layer 16, the copper layer has a more stable consistency. When a copper layer is arranged on top of a silicon layer, after the silicon layer is arranged on top of the first dielectric layer as shown in figure 1, the copper reacts readily with the silicon when the copper is deposited. This can be overcome by depositing first the copper layer 12, and then depositing the silicon layer 13 on top of the copper layer 12, as shown in figure 2.

The copper layer 12 and the silicon layer 13 can be arranged on the first dielectric layer 16 for example by means of sputtering, also the first and second dielectric layers 16, 17. Further, also the reflective layer 14 is preferably deposited by means of sputtering.

In a first preferred embodiment, the substrate layer 11 consists of a 0,6 mm polycarbonate substrate, the reflective layer 14 of a 100 nm silver or silver alloy, aluminum or aluminum alloy layer, and the cover layer 15 has a thickness of about 0,1 mm. The thickness of the copper layer Cu and the silicon layer Si have for example each a value in a range of 2 - 20 nm, with the thickness of the copper layer being larger that the thickness of the silicon layer. The recordable optical storage medium is in particular an optical disc with outside dimensions similar to a DVD. The thickness of the cover layer are in particular within a range of 0,5 - 0,15 mm, the thickness of the substrate layer in a range of 0,5 - 0,7 mm, and the thickness of the first dielectric layer 16 in a range of 40 - 70 nm. The thickness of the reflective layer 14 may vary within a range of < 200 nm.

In figure 3 reflectivity values R and modulation factors M of recordable optical storage media in accordance with figure 2 are shown for a wavelength of 650 nm and for various thicknesses d2 of the second dielectric layer 17. The measured reflectivity values R show, that in a range of 30 - 60 nm for the thickness d2, the reflectivity is below 40% and therefore too low for practical applications for optical storage media. For thickness values d2 below 20 nm, the reflectivity is above 45%, and therefore sufficiently high. For values d2 above 60 nm, the reflectivity is increasing sharply, reaching a reflectivity value of 0,5 at a thickness d2 of about 70 nm, and obtaining almost a reflectivity value of 1,0 at a thickness d2 of 120 nm.

For thicknesses d2 of 10 nm, 40 nm and 90 nm of the second dielectric layer 17, for which reflectivity values have been measured as shown in figure 3, normalized push/pull signals NPP have been determined for a wavelength of 650 nm, as shown in figure 4. As can be seen, not only the reflectivity and modulation values as shown in figure 3, but also the normalized push/pull signals are significantly influenced by the thickness d2 of the second dielectric layer 17. For the measurements as shown in figures 3 and 4 an optical disk with the following values have been used: Cu = 8 nm, Si = 8 nm, dl = 60 nm, and a 100 nm Ag alloy reflective layer.

As shown in figure 4, the value of the push/pull signal NPP is far above the specified value of 10% for a thickness d2 = 10 nm, but for thickness values d2 of 40 and 90 nm clearly below 10% and hence within the specification of a DVD. It appears therefore, that optical storage discs with a thickness d2 of the second dielectric layer 17 below 60 nm are not suitable for a DVD download format, but only for values d2 above 60 nm sufficient reflectivity values and normalized push/pull values can be obtained.

Nevertheless, a highly reflective layer arrangement does not allow a high modulation factor M of the data signal, which is required for providing a high signal-to-noise ratio, respectively a low bit error rate, because then the change in reflectivity between marks and spaces is too low to provide required modulation values M, as shown in figure 3. To obtain a modulation factor M of at least 60%, the thickness d2 has to be less than about 70 nm.

Simulations have been performed, in which the thicknesses for copper layer and silicon layer and for both dielectric layers where varied simultaneously in 1 nm steps around the values as described before with regard to figure 2 : Cu = 8 nm, Si = 8 nm, dl = 60 nm, and d2 = 10 nm. According to the simulations, the reflectivity values in the range d2 < 20 nm can be increased and the value of the push/pull signal can be reduced. It has been found that the main parameter that influences the modulation and the reflectivity is the Cu layer. But the other three parameters, dl, d2 and the thickness of the silicon layer, influence also the reflectivity value.

According to the simulations, the thickness of the silicon layer should be below the thickness of the copper layer, and good values have been obtained in particular with layers, in which dl is in a range of 40 - 100 nm, the thickness of the copper layer in a range of 8 - 12 nm and of the silicon layer in a range of 4 - 8 nm, and d2 in a range of 5 - 20 nm. Best results have been obtained with a following layer stack: Thickness of the substrate: 0,6 mm, dl = 54 nm, d2 = 12 nm, thickness of the copper layer: 9 nm, and of the silicon layer: 7 nm, by using a silver or silver alloy layer as the reflective layer. For recording data on this storage medium, a laser recording power of about 24 - 26 mW is required, to provide sufficiently low jitter values. For improving the optical storage medium with regard to the recording laser power, therefore simulations have been performed, in which the thickness of the reflectivity layer has been modified. According to the simulations, when using a silver layer with a thickness in a range of 50 - 90 nm, jitter values below 8% can be obtained with a laser write power below 20 mW.

An optical storage medium of this kind can be use therefore for recording of data with a Blu-Ray type or HD-DVD type laser, wavelength e.g. 405 nm, and which can be read with a red laser with a wavelength of 650 nm, having a push/pull signal below 18%, reflectivity value above 45%, modulation value above 50%, and a jitter below 8%.

Also other embodiments of the invention can be made by a person skilled in the art without departing from the spirit and scope of the invention. The invention resides therefore in the claims herein after appended.

Claims

1. Recordable optical storage medium comprising a substrate layer (11), a recording layer (12, 13), a cover layer (15), a reflecting layer (14) arranged between the recording layer and the cover layer, a first dielectric layer (16) arranged between the substrate layer and the recording layer, and a second dielectric layer (17) arranged between the recording layer and the reflecting layer, characterized in that the recording layer (12, 13) comprises a first film (12) containing copper being arranged on the first dielectric layer (16) and a second film (13) containing silicon arranged between the first film (12) and the second dielectric layer (17).

2. The recordable optical storage medium of claim 1, wherein the thickness of the first film (12) and of the second film (13) is each in a range of 2 - 20 nm.

3. The recordable optical storage medium of claim 1 or 2, wherein the thickness of the first film (12) is larger than the thickness of the second film (13) .

4. The recordable optical storage medium of claim 3, wherein the thickness of the first film (12) is in a range of 8 - 12 nm and of the second film (13) in a range of 4 - 8 nm.

5. The recordable optical storage medium of one of the preceding claims, wherein the first film (12) is arranged on top of the first dielectric layer (16) by means of sputtering, and wherein the second film (13) is arranged on top of the first film (12) by means of sputtering.

6. The recordable optical storage medium of one of the preceding claims, wherein the thickness (dl) of the first dielectric layer (16) is larger than the thickness (d2) of the second dielectric layer (17).

7. The recordable optical storage medium of claim 6, wherein the thickness (dl) of the first dielectric layer (16) is in a range of 40 - 100 nm, and the thickness (d2) of the second dielectric layer (17) in a range of 5 - 30 nm.

8. The recordable optical storage medium according to one of the preceding claims 1-5, wherein the thickness (dl) of the first dielectric layer (16) is in a range of 40 - 100 nm and the thickness (d2) of the second dielectric layer (17) in a range of 50 - 120 nm.

9. The recordable optical storage medium according to one of the preceding claims, wherein the first film (12) contains copper (Cu) with a content of > 90%, preferably 100%, and the second film (13) contains silicon (Si) with a content of > 90%, preferably 100%.

10. The recordable optical storage medium of one of the preceding claims, being designed for providing a high push-pull signal when recording data with a laser beam having a wavelength of 400 - 430 nm and for providing a low push-pull signal for a reading of the respective data with a laser beam having a wavelength of larger than 630 nm, both laser beams penetrating first the substrate layer (11).

11. The recordable optical storage medium of claim 10, wherein the thicknesses of the first and second films

(12, 13) and of the second dielectric layer (17) are optimized for providing a high push-pull signal when recording data with a laser beam having a wavelength of 400 - 430 nm and for providing a low push-pull signal for a reading of the respective data with a laser beam having a wavelength of larger than 630 nm.

12. The recordable optical storage medium of one of the preceding claims, wherein the cover layer (15) has a thickness of 0,05 - 0,15 mm and the substrate layer (11) has a thickness of 0,5 - 0,7 mm.

13. The recordable optical storage medium of one of the preceding claims, wherein the reflective layer (14) is a silver or silver alloy layer, an aluminum or aluminum alloy layer with a thickness of 50 - 100 nm, which is arranged as a sputtered layer onto the second dielectric layer (16).