KR20050094987A - Super resolution information storage medium having a plurality of recording layers - Google Patents

Abstract

A super resolution information storage medium having a plurality of information recording layers is disclosed.

The disclosed information storage medium is an information storage medium capable of reproducing information recorded with a recording mark having a size equal to or less than the resolution of an incident light beam, the information storage medium comprising a plurality of recording units having a substrate and an information recording layer, and neighboring information. The interval between the recording layers is characterized by having a length at which the C / N value at least according to the defocus starts to become zero.

This configuration improves playback performance by minimizing the gap between neighboring recording layers.

Description

Super resolution information storage medium having a plurality of recording layers

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a super resolution information storage medium having a plurality of information recording layers, and more particularly, to an information storage medium having improved reproduction performance by minimizing a gap between neighboring information recording layers.

The optical recording medium is used as an information storage medium of an optical pickup apparatus for performing non-contact recording and reproducing of information, and it is required to increase the recording density of information stored with industrial development. To this end, information storage media have been developed in which information storage media having recording marks having a resolution smaller than the resolution of a laser beam can be reproduced using super resolution.

The information storage medium includes a read only memory (ROM) designed to reproduce recorded information only, a write once read many storage medium capable of recording only once, and a recording, erasing and There is a rewritable storage medium capable of rewriting.

Here, in the read-only storage medium, information is recorded on the substrate in the form of pits, and the information is reproduced by the reflectance difference of the reproduction beams. In other words, where there is a pit, the reflection amount is small, and where there is no pit, the information is reproduced using a relatively high reflection amount.

As technology advances, the performance required for such information storage media increases, and one of the most important is the capacity of the storage media. The increase in capacity of the storage medium depends on how small the mark can be recorded within a given area of the storage medium and how accurately the recorded mark can be reproduced.

In particular, the information reproduction performance depends on shortening the wavelength of the light source used for information reproduction or increasing the numerical aperture of the objective lens. However, current technology has a limitation in providing a laser having a short wavelength, and manufacturing cost is high in order to manufacture an objective lens having a large numerical aperture. In addition, as the numerical aperture of the objective lens increases, the working distance between the optical pickup and the storage medium becomes very short, so that the surface of the storage medium is damaged by the collision of the optical pickup and the storage medium, and thus the information recorded on the storage medium is lost. There is also a growing concern. For these reasons, it is not easy to solve the high capacity and high density of the storage medium.

On the other hand, there is a method of constituting a plurality of recording layers as a method for increasing the capacity. For example, a disc of BD (Blu-Disc) format having a capacity of about 25 GB including the first and second recording layers has been developed. 1 shows a disc including a first recording layer L0 and a second recording layer L1. By the way, when the reproduction beam is irradiated to the first recording layer to reproduce the data recorded on the first recording layer L1, the reproduction beam is partially transmitted to form the second recording layer L2 and defocused. The undesired data of the second recording layer L2 can be reproduced together with the data of the first recording layer by the defocused reproduction beam. In order to prevent such interference of the reproduction signal, a space layer SL needs to be provided between each recording layer. That is, a space layer is necessary to prevent the beams reflected from other recording layers adjacent to the recording layer to be reproduced from being reproduced together.

FIG. 2 shows the signal-to-noise ratio (hereinafter referred to as C / N) according to the defocus of the information storage medium having the structure shown in FIG. 1 in the case where the mark length is 2T (150 nm) and 8T (600 nm). According to this graph, in a conventional general information storage medium, C / N is less affected by defocus, and the defocus margin is approximately 600 nm.

Therefore, in order to avoid signal interference in the structure having the first recording layer and the second recording layer, the distance between the first and second recording layers should be at least greater than the defocus margin. In this way, a space layer is required to prevent signal interference between the plurality of recording layers. The thickness of the space layer is better in consideration of only the signal interference side.

However, since a disc having a plurality of information recording layers must also consider other reproduction characteristics in addition to signal interference, the thickness of the spacing layer cannot be determined solely by the defocus margin.

Since the pickup for reproducing data is optimally configured for a predetermined disc thickness, the reproducing performance is degraded for other thicknesses. Further, when reproducing each recording layer, spherical aberration occurs because the distance to each layer on which the reproduction beam is incident is different. Furthermore, the depth of focus is shortened due to the short wavelength of the reproduction laser and the high aperture of the objective lens for reproducing the high density disk, which causes spherical aberration. In particular, when accessing the recording layer located far from the incident surface, spherical aberration becomes larger, and when the spherical aberration becomes larger, the reproduction signal characteristic is inferior because the laser spot spreads without gathering at one point. Moreover, when the space layer is thick, it is difficult to minimize the thickness variation of each recording layer.

For these reasons, the spot size of the reproduction beam varies depending on the distance from the pickup to each recording layer. Accordingly, the amount of reflection in each recording layer is changed.

In consideration of the above reasons, the thickness of the space layer SL is defined as 50 ± 15 (μm) in the case of a DVD.

On the other hand, as another method of increasing the recording capacity, super resolution discs have been developed. Super-resolution discs have different playback characteristics than conventional discs such as DVDs and BDs, and accordingly, in a structure having a plurality of recording layers, the thickness of the space layer needs to be defined differently from the general discs.

A super resolution disc refers to a storage medium capable of reproducing a recording mark having a size smaller than resolution. When the wavelength of the light source for reproducing the information of the storage medium is λ and the numerical aperture of the objective lens is NA, λ / 4NA appears as a limitation of the reproduction resolution, even if it is possible to form a recording mark extremely small, reproduction is possible. It may be impossible. That is, it is common that information reproduction cannot be performed because the recording marks irradiated with light emitted from the light source having a size smaller than λ / 4NA cannot be distinguished.

By the way, a super resolution phenomenon occurs in which a recording mark having a size exceeding a resolution limit is reproduced, and a cause analysis and research and development for such a super resolution phenomenon are in progress. According to the super resolution phenomenon, since a recording mark having a size exceeding the resolution limit can be reproduced, the super resolution recording medium can meet the demands of high density and high capacity.

In order to further increase the recording capacity of such a super-resolution information storage medium, when the recording layer is composed of a plurality of layers, the playback characteristics are sensitively dependent on the thickness of each layer, thereby providing a structure that can guarantee optimized playback performance. It is necessary to do

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object thereof is to provide a super resolution information storage medium having a plurality of information recording layers and having a thickness for optimizing the reproduction performance of each recording layer.

In order to achieve the above object, a super resolution information storage medium according to the present invention is an information storage medium capable of reproducing information recorded with a recording mark having a size equal to or less than the resolution of an incident light beam,

And a plurality of recording sections each having a substrate and an information recording layer, and having a length at which the distance between neighboring information recording layers starts to become zero at least with respect to defocus.

Preferably, a space layer is provided between the recording units.

Preferably, the interval between the neighboring information recording layers has a length of at least 500 nm.

The information recording layer is preferably made of at least one material or polymer compound selected from metal oxides consisting of PtOx, AuOx, PdOx and AgOx.

The recording unit may include at least one super resolution auxiliary layer that absorbs heat of an incident light beam.

Hereinafter, a super resolution information storage medium according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The super-resolution information storage medium according to the present invention, with reference to FIG. 3, includes a plurality of information recording layers, and a space layer provided between each information recording layer.

A super-resolution auxiliary layer is provided in at least one of the upper and lower portions of the information recording layer, and a dielectric layer is provided between the layers.

Specifically, FIG. 3 shows a two-layer information storage medium including first and second information recording layers L1 and L2. The first recording unit 20 and the second recording unit 30 are stacked on the substrate 15. The first recording unit 20 includes a first super-resolution auxiliary layer 21 having a thickness of 15 nm, a first dielectric layer 22 having a thickness of 25 nm, a first recording layer L1 having a thickness of 3.5 nm, and a second dielectric layer 24 having a thickness of 25 nm. And a second super resolution auxiliary layer 25 having a thickness of 15 nm, and the second recording unit 30 includes a first super resolution auxiliary layer 31 having a thickness of 15 nm, a first dielectric layer 32 having a thickness of 25 nm, and 3.5 nm. A second recording layer L2 having a thickness, a second dielectric layer 34 having a thickness of 25 nm, and a second super resolution auxiliary layer 35 having a thickness of 15 nm.

The space layer SL is provided between the first recording unit 20 and the second recording unit 30. In addition, a 95 nm-thick dielectric layer 17 is further provided between the substrate 15 and the first super-resolution auxiliary layer 21 of the first recording unit 20, and is 95 nm thick on the second recording unit 30. The dielectric layer 37 and the cover layer 38 may be further provided. Here, a laser beam may be incident through the substrate 15 or may be incident through the cover layer 38.

The substrate 15 is formed of any one material selected from polycarbonate, polymethylmethacrylate (PMMA), amorphous polyolefin (APO), and glass materials.

Preferably, the first and second recording layers L1 and L2 are made of at least one material or polymer compound selected from PtOx, PdOx, AuOx, and AgOx. The super resolution recording layer is deformed by absorbing heat by the recording beam or the reproduction beam.

The dielectric layers 17, 22, 24, 32, 34 and 37 prevent the deformation of the first and second recording layers L1 and L2 from spreading due to repeated irradiation of the reproduction beam. It intended to, preferably made of a ZnS-SiO _2.

The super resolution auxiliary layers 21, 25, 31, and 35 may be made of a Ge-Sb-Te-based alloy or an Ag-In-Sb-Te-based alloy. The super-resolution auxiliary layer is deformed by the reproduction beam to serve to assist the deformation in the first and second recording layers L1 and L2.

The reproduction operation of the information storage medium configured as described above is as follows. Here, the case where the first and second recording layers L1 and L2 are made of platinum oxide (PtOx) will be described as an example.

When the recording layers L1 and L2 are irradiated with a laser beam of recording power, deformation occurs in a region where the laser beam is formed. When the laser beam is irradiated above a predetermined temperature, the platinum oxide (PtOx) is separated into platinum and oxygen, and the oxygen swells in a bubble form, causing volume expansion in a portion where the light spot is formed. Corresponding to this deformation, the super-resolution sublayers 22, 24, 32 and 34 are also deformed. Through this deformation, a recording mark is formed. The recording mark is kept unchanged by the reproduction beam of less power than the recording beam.

 On the other hand, the recording mark may be formed by a phase change in addition to the deformation into the bubble shape as described above. In this case, the super-resolution auxiliary layer is made of a phase change material such as a Ge-Sb-Te-based alloy or an Ag-In-Sb-Te-based alloy, and the portion to which the recording beam is irradiated is in an amorphous state to form a recording mark. Can be. When a recording mark is formed on the super resolution auxiliary layer, the recording medium is an information storage medium capable of recording and erasing.

FIG. 4 shows the results of experimenting with the signal-to-noise ratio (hereinafter referred to as C / N) according to the defocus of the super-resolution information storage medium configured as described above when the mark length is 2T (75 nm) and when 8T (300 nm). To show.

As described above, the resolution limit is defined as λ / 4NA, where λ is 405 nm, NA is 0.85, and the resolution limit is approximately 120 nm.

 2 and 4, in the case of a recording mark having a size greater than or equal to resolution, C / N does not change significantly according to defocus, while a super resolution information storage medium having a size less than resolution (2T of 75 nm length) is shown. Shows a significant decrease in C / N from about 0.3-0.4㎛ defocus. When defocused more than this, it was no longer possible to measure C / N characteristics.

According to the above results, it can be seen that an information storage medium recorded with a recording mark having a size less than or equal to resolution is very sensitive to defocus. Referring to Fig. 2, the defocus margin of a recording mark having a size greater than or equal to the resolution is approximately 600 nm, whereas the defocus margin is 360 nm for a super resolution information storage medium having a size less than or equal to the resolution. This small defocus margin means that even if the distance between the recording layers is small, the possibility of signal interference from other recording layers neighboring the recording layer to be reproduced is reduced. For example, when focusing a reproduction beam on the first recording layer to read information recorded in the first recording layer, the reproduction beam is defocused on the second recording layer, which is defocused by the second recording layer. Even if a signal is read from the C / N value, the C / N value is very small so that it does not act as a reproduction signal.

Therefore, the thickness of the space layer provided between each recording layer can be formed relatively small.

The above phenomenon shows that the reproduction of the conventional phase change disk is caused by a mechanism due to the difference in reflectance of light, whereas the super resolution is performed by a mechanism other than reflectance.

5 shows C / N according to the reproduction power for a 75 nm mark in an optical system having a wavelength of 405 nm and an NA of 0.85. Here, the super-resolution auxiliary layer is made of Ge-Sb-Te-based alloy (GST), the recording power (Pw) is recorded at 9.0mW, and made of Ag-In-Sb-Te-based alloy (AIST) The cases where the recording power Pw is made 10.0 mW are shown, respectively. According to this graph, when the reproduction power is 1.0 mW, C / N is almost zero, but from 1.2 mW, there is a threshold in which C / N rapidly increases. This shows that in the super-resolution information storage medium, unlike the conventional phase change optical disc, the signal is detected only when the reproduction power is above a predetermined threshold power, and the signal is hardly detected below the threshold power.

The reproducing power means the temperature of the reproducing beam, and it can be seen that C / N reacts very sensitively according to the reproducing beam temperature in the super resolution information storage medium. In addition, since the response is sensitive to the temperature of the reproduction beam, the C / N value is significantly different between the focused portion and the defocused portion. This in other words indicates that recording marks having a size below the resolution are sensitive to defocus.

As described above, in the super-resolution information storage medium having a plurality of information recording layers, since the defocus margin is small, it can be seen that signal interference between layers is less likely to occur even when the spacing layer between the recording layers is relatively small. Referring to FIG. 4, if the amount of defocus predicted to be almost zero by the extrapolation method is found, it can be seen that it is approximately ± 500 nm. Therefore, it is preferable that the interval d between neighboring information recording layers is at least 500 nm.

In this way, the minimum distance where signal interference from neighboring recording layers does not occur by defocusing is obtained, and the distance between neighboring recording layers is arranged at least this distance away, thereby minimizing the thickness of the space layer SL. Can be. Furthermore, even without the space layer, signal interference may not occur between neighboring recording layers.

It is possible to minimize the space layer in an information storage medium on which data is recorded with a recording macro having a length of resolution or less, thereby reducing spherical aberration.

As described above, the super-resolution information storage medium according to the present invention has a plurality of layers of information recording layers, and allows spaces between the information recording layers to reduce spherical aberration while preventing signal interference in each recording layer. Provide at least one structure. This further increases the recording capacity of the super-resolution information storage medium and improves the reproduction performance of the information storage medium of a plurality of layers.

1 shows a recording layer structure of a conventional information storage medium.

FIG. 2 shows C / N according to defocus in the information storage medium shown in FIG. 1 for each record mark length.

3 shows a recording layer structure of a super resolution information storage medium according to a preferred embodiment of the present invention.

FIG. 4 shows C / N according to defocus in the information storage medium shown in FIG. 2 for each record mark length.

FIG. 5 shows C / N according to reproduction power of each material of the super resolution auxiliary layer in the information storage medium according to the present invention.

Claims (7)

An information storage medium capable of reproducing information recorded with a recording mark having a size equal to or less than the resolution of an incident light beam, And a plurality of recording units having a substrate and an information recording layer, wherein the interval between neighboring information recording layers has a length at which the C / N value at least according to defocus starts to become zero. The method of claim 1, An information storage medium comprising a space layer between each recording unit. The method according to claim 1 or 2, An information storage medium having a length of at least 500 nm between the neighboring information recording layers. The method of claim 1 or 2, wherein the information recording layer, An information storage medium comprising at least one material or a polymer compound selected from metal oxides consisting of PtOx, AuOx, PdOx and AgOx. The method according to claim 1 or 2, And the recording unit includes at least one super resolution auxiliary layer that absorbs heat of an incident light beam. The method of claim 5, And each recording unit comprises a first super resolution auxiliary layer, a first dielectric layer, an information recording layer, a second dielectric layer, and a second super resolution auxiliary layer, which are sequentially stacked from the light incident side. . The method of claim 5, The super-resolution auxiliary layer is formed of a Ge-Sb-Te-based alloy or an Ag-In-Sb-Te-based alloy.