KR100565569B1 - optical recording medium - Google Patents

Abstract

An optical recording medium capable of recording at low laser power and improving recording density above the diffraction limit of light, comprising: forming a first dielectric layer of Si ₃ N ₄ or the like on a substrate with a thickness of about 700 kHz or less; And a photoreaction window layer 13 for transmitting only the laser light of the central region having the highest temperature thereon to a thickness of about 100 to 450 mW, and then a second of Si ₃ N ₄ or the like to a thickness of about 100 to 450 mW. By sequentially stacking the dielectric layer 14, the recording layer 15 made of an azo group liquid crystal polymer, and the Al reflecting layer 16, recording and reproducing are possible even with a low power laser, and recording density can be dramatically improved. have.

Liquid crystal polymer, photoreaction window layer, azobenzene, photoisomerization

Description

Optical recording medium

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows photoisomerization of azobenzene polymer used in a recording layer of an optical recording medium according to the present invention.

2 shows an optical recording medium according to the present invention;

Explanation of symbols for main parts of the drawings

11 substrate 12 first dielectric layer

13 photoreaction window layer 14 second dielectric layer

15 recording layer 16 reflective layer

TECHNICAL FIELD The present invention relates to an optical recording medium, and more particularly, to an optical recording medium using an azo group liquid crystal polymer as a recording material.

Generally, there are several methods of recording a rewritable or recordable optical disc using a laser.

Typical rewritable recording methods include magneto-optical recording and phase change recording, and recordable recording methods are typically ablation by laser energy.

First, magneto-optical recording uses a phenomenon in which the angle of the polarization plane is rotated when the polarized light is a magnetic thin film having magnetic anisotropy in a direction perpendicular to the film plane when the polarized light is reflected from the magnetic film plane.

This magneto-optical effect is called a Kerr effect, and the angle of the rotating polarization plane is called a rotation angle.

The signal-to-noise ratio (SNR) of a magneto-optical disk is

Where η is the quantum efficiency of the photodiode, P is the read power, R is the medium reflectivity, θ _K is the rotation angle, e is the electron charge, and B is Band width.

In the above equation, the figure of merit when considering only the parameters R and θ _K representing the characteristics of the medium is Rθ _K ² Proportional to

Therefore, it is more efficient to increase θ _K than R.

Usually, the rotation angle of the magnetic thin film used for magneto-optical recording has a small value of about 0.3 degrees.

Therefore, in order to increase this value, a multilayer is used using a dielectric and a reflective layer.

The phase change recording is read out by reflectance of reflected light as in the case of a CD or a CD-ROM.

That is, a representative phase change recording material is Ge-Sb-Te, and the phase (crystalline, amorphous) change of this material is used.

In general, the crystalline case uses a phenomenon in which the reflectivity is greater than that of the amorphous form.

In order to induce a phase change from the crystalline phase to the amorphous phase, fast cooling is required along with a high laser power, and conversely, a slow cooling method using a lower laser power than the previous case is required to form the crystalline phase in the amorphous phase.

On the other hand, rewritable phase change recording basically uses the same principle as in the case of using a recordable polymer ablation.

That is, the recorded portion and the unrecorded portion are distinguished using the difference in reflectance.

In the case of phase change recording, the difference in reflectivity between amorphous and crystalline is used, and in the case of using recordable polymer ablation, the difference in reflectance between the unrecorded portion and the portion where the polymer is removed is recorded.

The SNR in this case has the following relationship with reflectivity.

Where R is the average reflectivity of the medium, and ΔR is the difference between the reflectances of the write bit and the unrecorded bit.

On the other hand, in the related art, a recordable disc having a simple structure using a die polymer as a recording layer has been developed, but this also has a disadvantage in that recording power is high when information is recorded in the recording layer.

That is, in the case of a magneto-optical disk, a phase change disk, and a disk using a die polymer, the recording power needs about 10 mW.

In order to obtain this value, a laser diode having an output of about 30 mW should be used, but there is a disadvantage in that it is difficult to produce such a high power laser diode in the blue wavelength band.

Even if high power laser diodes in the blue wavelength band are produced by technological innovation in the near future, they will not be very effective because they are expensive in terms of production cost.

Therefore, to produce competitive discs in the future, it is necessary to develop discs capable of recording with low laser power.

In the optical recording medium according to the prior art, there are the following problems.

First, there is a limit to increasing the recording density beyond the diffraction limit of light.                         

The reason is that the technique of recording information on the optical recording medium depends on the wavelength of the laser light source used for recording / reproducing.

That is, the minimum length of one bit of data mark in the optical recording technique is determined by the following equation.

Where? Is the wavelength of light and NA is the numerical aperture (or numerical aperture).

Therefore, since the minimum pitch between data tracks that can be determined is approximately the wavelength of light, it is necessary to overcome the diffraction limit of such light in order to increase the recording density.

Second, when recording information in the recording layer, the recording power is high.

The present invention has been made to solve such problems, and an object thereof is to provide an optical recording medium capable of recording with low laser power.

Another object of the present invention is to provide an optical recording medium capable of improving the recording density beyond the diffraction limit of light.

The characteristics of the optical recording medium according to the present invention include a substrate, a first dielectric layer formed on the substrate, a photoreaction window layer formed on the first dielectric layer and transmitting only light above a specific temperature, and formed on the photoreaction window layer. It consists of a 2nd dielectric layer, the recording layer formed on the 2nd dielectric layer and consisting of an azo group liquid crystal polymer, and the reflective layer formed on the recording layer.

Another feature of the present invention is that the photoreaction window layer is made of any one of Ge, As, Se, Sn, Sb, Te, Bi, Si, or an alloy thereof.

Referring to the accompanying drawings, an optical recording medium according to the present invention having the above characteristics is as follows.

First, the concept of the present invention is to use azo group liquid crystal polymer showing high absorption in the blue wavelength band as a recording material of the rewritable and recordable optical discs, so that even with low laser power. It enables recording and forms a photoreaction window layer between the recording layer and the substrate and transmits only the central region of the recording / reproducing laser with the highest temperature, thereby stably recording and reproducing information at a density above the diffraction limit of light. It is.

The azobenzene group of liquid crystal polymers used as the recording layer material of the present invention is sensitive to light, and thus, when irradiated with linearly polarized light or non-polarized light, information can be recorded or erased as well as rewritten the erased information. There is this.

This principle of information storage is recorded by the information being recorded or accompanied by the deformation caused by the difference in birefringence induced by the optical dichroism of the azobenzene group introduced into the polymer material.

That is, the azobenzene groups cause the isomerization reaction from the trans structure to the cis structure by the light as shown in FIG. 1, and thus affect the arrangement of the polymer chain including the azobenzen group. To accompany the deformation.

In addition to the photoisomerization reaction, the polymer molecules are aligned in a direction perpendicular to the polarization direction, and birefringence occurs.

As described above, deformation occurs in the polymer thin film together with the migration phenomenon of the polymer molecule. When the degree of movement of the polymer molecule is greater than the size of the free volume inside the thin film, the deformation phenomenon occurs.

This deformation phenomenon may be used as a method of recording other than recording due to birefringence change.

On the other hand, the state in which the polymer molecules before being exposed to light in the absorption region maintains isotropy is called a trans state, and the state in which anisotropy is exposed after being exposed to linearly polarized light is cis. ) State.

As such, the phenomenon of trans to cis by linearly polarized light is called photo isomerization, and this trans-cis-trans photoisomerization reaction is a record in which azobenzene groups that are not circulated are irradiated. It is arranged perpendicular to the plane of polarization of light.

Thus, birefringence is induced and accompanied by deformation as the azobenzene group changes from an isotropic state to an anisotropic state.

Therefore, by measuring the induced birefringence, the arrangement degree of the azobenzene group can be confirmed.

That is, the information is recorded by such a birefringence difference or the information is recorded by the accompanying deformation.

Then, the irradiated light or circularly polarized light or heat is applied to erase the recorded information.

At this time, the azobenzene group is arranged in a disordered manner through the aforementioned cycle of the trans-cis-trans photoisomerization reaction, so that the induced birefringence disappears or the modified portion is returned to its original state.

This erasing process is also possible by heating the polymer thin film used above the melting point.

On the other hand, reversible optical record carriers are widely used polymer materials in which azobenzene groups are introduced into the main chain or side chain of the polymer chain through chemical bonds, and when the azobenzene groups are introduced into the main chain, the movement of the polymer is slowed, making it difficult to arrange the azobenzene groups. .

Therefore, in the present invention, side chain type liquid crystal polymer materials in which azobenzene group is introduced into the side chain are used.

In addition, these side chain type liquid crystal polymers are suitable for application to the present invention because the rearrangement of molecules in the transition temperature range is easy and the stability of the arranged molecules is good.

In addition, in the present invention, the azobenzene group is not connected to the polymer by a chemical bond, but may be simply a brand-type material introduced into the polymer matrix.

The material of this brand type has less storage stability of recorded information than the side chain type liquid crystal polymer materials, but the polymer matrix of various structures can be selected according to the characteristics of the polymer matrix to be used, and it is easy to manufacture a polymer thin film. There is this.

Azo group liquid crystal polymers having such characteristics show a very sensitive response to recording laser light as compared to conventional phase change or magneto-optical recording films, and thus are well suited for the purpose of the present invention to increase the recording density.

The reason for this is that when the photoreaction window layer of the present invention is used for an existing optical recording medium, it is difficult to record information in the recording layer because the intensity of the recording laser light decreases drastically.

However, when the azo group liquid crystal polymer is used as the recording layer material as in the present invention, even if the recording laser light passes through the photoreaction window layer and the intensity of the recording laser light is reduced, the sensitivity of the polymer recording layer is high enough to make it difficult to record information. Not only that, but also the recording density can be increased beyond the diffraction limit of light.

FIG. 2 is a structural cross-sectional view showing an optical recording medium according to the present invention. As shown in FIG. 2, the present invention provides a first dielectric layer made of Si ₃ N ₄ or the like on a substrate 11 with a thickness of about 700 GPa or less. 12), and a photoreaction window layer 13 through which only the laser light in the central region having the highest temperature is transmitted is formed to a thickness of about 100 to 450 kPa.

The second dielectric layer 14 made of Si ₃ N ₄ or the like, the recording layer 15 made of an azo group liquid crystal polymer, and the Al reflecting layer 16 are successively laminated to a thickness of about 100 to 450 mW.

Here, the photoreaction window layer 13 is located between the first and second dielectric layers 12 and 14 made of Si ₃ N ₄ , and the recording and reproducing laser light is transferred from the substrate 11 to the recording layer 15 and the reflecting layer. Incident on (16), the second dielectric layer 14 is formed to a thickness within a limit where light diffraction does not occur.

In addition, the photoreaction window layer 13 has a characteristic of changing its physical properties as it is heated by laser light. The photoreaction window layer 13 of the photoreaction window layer 13 is located near the center of the region where the temperature of the laser light is highest during information recording and reproducing. The physical properties change and only the laser light in the area transmits.

Suitable materials used for the photoreaction window layer 13 having such characteristics are any one of Ge, As, Se, Sn, Sb, Te, Bi, Si or alloys thereof.

Looking at the recording, reproducing and erasing process of the present invention is as follows.

During recording, first, when a laser beam in the blue wavelength band (400 to 480 nm) is irradiated to the photoreaction window layer with a low laser power of about 5 mW or less, the physical properties of the photoreaction window layer change only at the center of the laser light.

The reason is that the intensity I of the laser light

This is because the center temperature of the laser light is the highest since it has a Gaussian shape such as.

In this way, the photoreaction window layer closer to the center region of the high-temperature laser light produces a small aperture as the physical properties change, and only a small diameter laser light passes therethrough to reach the recording layer.

Laser light at this time is light whose intensity of light is rapidly reduced, but polymer molecules of the azobenzene group in the recording layer are sufficiently sensitive to the light.

Subsequently, the polymer molecules of the azobenzene group in the recording layer are induced by birefringence while causing a photoisomerization reaction to absorb the laser light (information recorded) and the non-absorption (information not recorded) portion of the laser beam. Difference appears.

This difference in absorbance appears as a reflectance, so it is used as an information recording unit of "0" and "1".

In the process of reproducing the recorded information, first, when the laser beam for reproducing is irradiated to the photoreaction window layer with a low laser power of about 3 mW or less, the physical properties of the photoreaction window layer are changed only at the center of the laser light. (aperture) is generated, and only a small diameter laser light passes through to reach the recording layer.

When the laser light passes through the portion where the information is recorded and the portion where the information is not recorded on the surface of the recording layer, a difference in reflectance appears, which is used to reproduce "0" or "1".

Here, the bit recorded by the laser of the blue wavelength can be reproduced by the laser of the red or infrared wavelength as well as the laser of the blue wavelength.

Further, when erasing the recorded information, it is possible to simply erase the recorded information by re-irradiating unpolarized light or circular polarized light on the recording layer on which the information is recorded or by applying heat of a predetermined temperature to the recording layer.

As described above, the present invention can dramatically improve the recording density by 15 times or more while using the existing optical system as it is.

That is, the conventional optical disk has a recording density of about 4 Gb / in ² , but in the case of the present invention, about 60 to 70 Gb / in ² or more is possible.

The optical recording medium according to the present invention has the following effects.

First, the recording density can be dramatically improved by more than 15 times while using the existing optical system, thereby reducing the production cost.

Second, since it is possible to record with a low power laser of 5mW or less, it is possible to use a low-power laser diode without using a high-power laser diode in a difficult blue area.

Claims (4)

A substrate; A first dielectric layer formed on the substrate; A photoreaction window layer formed on the first dielectric layer and having a physical property changed by heat to transmit laser light; A second dielectric layer formed on the photoreaction window layer; A recording layer formed on said second dielectric layer and composed of an azo group liquid crystal polymer; And a reflective layer formed on said recording layer. The optical recording medium of claim 1, wherein the photoreaction window layer transmits only a central region of the recording and reproducing laser beam. The optical recording medium according to claim 1, wherein the photoreaction window layer is any one of Ge, As, Se, Sn, Sb, Te, Bi, Si, or an alloy thereof. The optical recording medium according to claim 1, wherein the thickness of the first dielectric layer is 700 GPa or less, and the thickness of the second dielectric layer and the photoreaction window layer is 100 to 450 GPa.

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