JPH10320857A - Optical recording medium and its ultra resolving/ reproducing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the optical recording medium and its ultra-resolving method generating the change of transmittance of a ultra-resolving film in the region of a practical reproducing optical power, having the large quantity of change, capable of forming an optical aperture at a high speed for the short time of about the passing time of a reproducing optical spot and showing stability even for repeating reproduction. SOLUTION: In the optical recording medium providing with a recording layer 15 and the ultra-resolving film 13 provided in the incident side of the reproduced light with respect to the recording layer 15 and allowing ultra- resolving reproduction by forming the optical aperture by absorptive saturation to the ultra-resolving film 13, the ultra-resolving film 13 is composed of the fine grain dispersed film dispersed semiconductor fine grains into the matrix or the continuous film of the semiconductor, and the percentage content of the matrix material or contamination mixed into the semiconductor fine grains or the continuous film of the semiconductor is specified to be 20 at.%, or below.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical recording medium and a super-resolution reproducing method thereof.

[0002]

2. Description of the Related Art An optical disk memory for reproducing or recording / reproducing information by irradiating a light beam is a storage device having a large capacity, a high-speed access property, and a medium portability. It has been put into practical use, and its development is expected in the future. As a technique for increasing the density of an optical disk, shortening of the wavelength of a gas laser for cutting a master, shortening of the wavelength of a semiconductor laser as an operating light source, increasing the numerical aperture of an objective lens, and reducing the thickness of the optical disk are considered. Further, there are various approaches to recordable optical disks, such as mark length recording and land / groove recording.

Further, as a technique that has a great effect of increasing the density of an optical disc, a reproduction super-resolution technique using a medium film has been proposed. The playback super-resolution technique was originally proposed as a technique unique to a magneto-optical disk. In the reproduction super-resolution technology of magneto-optical recording, a magnetic film (super-resolution film) having a super-resolution function is provided on the recording layer on the side irradiated with the reproduction light, and a medium in which both are exchange-coupled or magnetostatically coupled is provided. Used. Then, the super-resolution film is heated by irradiating the reproduction light to change the exchange force or the magnetostatic force between the layers, thereby forming a partial optical mask or an optical aperture for the reproduction spot on the super-resolution film.

[0004] After that, it has been reported that even in the case of a ROM disk other than a magneto-optical disk, an attempt is made to provide a super-resolution film whose light transmittance is changed by irradiation of the reproduction light on the recording layer irradiation side with respect to the recording layer to perform super-resolution reproduction. Have been. As described above, the reproducing super-resolution technology is a magneto-optical disk, a CD-ROM, a CD-R, a WOR.
It has been clarified that the method can be applied to all optical disks such as M and phase change optical recording media.

[0005] The super-resolution film proposed in the conventional super-resolution reproduction technique is roughly classified into a heat mode system and a photon mode system. In the heat mode method, the super-resolution film is heated by reproducing light irradiation to cause phase transition etc. in the super-resolution film,
An optical aperture having a high transmittance is formed. The shape of the optical aperture becomes the same as the isotherm of the super-resolution film. Since the size of the optical aperture tends to fluctuate due to the influence of the environmental temperature, it is necessary to strictly control the heat in accordance with the linear velocity of the optical disk. Further, it is difficult for the heat mode type super-resolution film to obtain sufficient repetition stability due to thermal fatigue during reproduction and recording.

In the photon mode system, a photochromic material is used as a super-resolution film, and color development or decoloration by irradiation of reproduction light is used. In a photochromic material, electrons are excited by light irradiation from a ground level to an excited state having a short lifetime, and further, electrons are transitioned from the excited level to a metastable excited level having a very long lifetime, and are captured. Develop changes in light absorption characteristics. Therefore, to reproduce repeatedly, it is necessary to de-excit the electrons trapped by the metastable excitation level to the ground level and close the optical aperture once formed. As means for this purpose, auxiliary beam irradiation is used, so that two-beam operation is performed in principle, which is disadvantageous for high-speed response. Further, in the case of a photochromic material, a change in transmittance occurs through a complicated process involving the movement of atoms or a change in a bonding state, so that the repetition stability is limited to about 10,000 times.

Further, Japanese Patent Application Laid-Open No. 6-28713 discloses that
An optical disk provided with a shutter layer for reducing the diameter of a light beam is described. This shutter layer is obtained by dispersing semiconductor fine particles in a glass or resin matrix. CdS, CdSe, C
dSSe, GaAs, a-Si, CdTe, CdSe,
ZnO, ZnS, ZnSe, ZnTe, GaP, Ga
N, AlAs, AlP, AlSb, a-SiC and the like are described. The semiconductor fine particles have a content of 5 to 70 mo.
1% and a particle size of 0.1 to 50 nm are described as preferable. As a method for forming the shutter layer, ultra-quenching and heat treatment, impregnation, a sol-gel method, spin coating, sputtering, and vapor deposition are described.

Similarly, Japanese Patent Application Laid-Open No. 6-44609 contains fine particles of a semiconductor, a metal or a metal compound, and has a low transmittance for a low-intensity light beam and a low transmittance for a high-intensity light beam. There is described an optical recording medium provided with a light control film having a light transmission characteristic to increase the transmittance. This light control film is made of SiO ₂ , Si ₃ N ₄ , Y ₂ O ₃ , Al ₂
Semiconductor fine particles such as CdS and CdSe are dispersed in a transparent dielectric or transparent resin such as O ₃ , Li ₃ N, Ta ₂ O ₅ , and Nb ₂ O ₃ . It is described that the semiconductor fine particles preferably have a particle size of 1 to 20 nm. As a method of forming the light control film, sputtering, spin coating, and plasma CVD are described.

[0009] However, these publications do not describe what principle the shutter layer or the light control film functions as a super-resolution film, and there are conditions under which characteristics suitable for the super-resolution film can be obtained. Unknown.

As described above, in order to realize the super-resolution reproduction of the optical disk, the transmittance of the super-resolution film changes in a practical reproduction light power region, and the change amount is large. It is required that the optical aperture can be formed at a high speed in a short time, such as the passing time of the optical disk, and that it is stable against repeated reproduction. However, none of the conventional super-resolution films satisfy all of these requirements.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to cause a change in the transmittance of a super-resolution film in the range of a practical reproduction light power, and the change amount is large, and the change amount is large, and the transmission time of the reproduction light spot is about the same. An object of the present invention is to provide an optical recording medium capable of forming an optical aperture at high speed in a short time and exhibiting stability against repeated reproduction, and a super-resolution reproduction method thereof.

[0012]

The optical recording medium of the present invention comprises:
A recording layer, and a super-resolution film provided on the recording beam incident side with respect to the recording layer, wherein the super-resolution film is a fine particle dispersion film in which semiconductor fine particles are dispersed in a matrix or a continuous semiconductor. It is characterized in that the content of the matrix material or contamination mixed in the semiconductor fine particles or the continuous film of the semiconductor is 20 at% or less.

According to the super-resolution reproducing method of the present invention, the above-mentioned optical recording medium is irradiated with reproducing light and transmitted through the super-resolution film to a region where the number of photons irradiated with the reproducing light is larger than the surrounding area by absorption saturation. An optical aperture having a high efficiency is formed, and a recording mark in a recording layer is detected through the optical aperture.

The super-resolution film used in the optical recording medium of the present invention preferably has a structure in which semiconductor fine particles are dispersed in a matrix. The super-resolution film is maintained in an excited state at least while the reproduction light is being irradiated, and is de-excited to a ground state within a predetermined time after the irradiation of the reproduction light (for example, during one rotation of the optical disk). It is preferable to return. Specifically, the time constant of de-excitation of the super-resolution film from the excited state is preferably at least twice as long as the time when the full width at half maximum of the reproduction light passes on the optical recording medium surface.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. FIG. 1 shows a sectional view of an example of the optical recording medium according to the present invention. In the optical recording medium of FIG.
A super-resolution film 2, an intermediate layer 3, a recording layer 4, and a protective layer 5 are sequentially formed. Note that the intermediate layer 3 and the protective layer 5 are provided as needed. The reproduction light is irradiated from the side of the substrate 1 through the super-resolution film 2 and the intermediate layer 3 to the recording mark array formed in the recording layer 3. As described above, the super-resolution film 2 is formed on the side where the recording layer 4 is irradiated with the reproduction light.

In the present invention, as the super-resolution film, a film composed of a semiconductor fine particle dispersed film having a structure in which semiconductor fine particles are dispersed in a matrix or a continuous semiconductor film is used. However, a super-resolution film having a structure in which semiconductor fine particles are dispersed in a matrix is preferable. In such a structure, the excitation lifetime can be lengthened and the absorption wavelength can be controlled. In the fine particle dispersion film, semiconductor fine particles may be aggregated.

In the present invention, the super-resolution film is kept in an excited state at least while the reproduction beam is being irradiated, and is removed within a predetermined time after the irradiation of the reproduction beam (for example, during one rotation of the optical disk). It is preferable to return to the ground state by excitation. Specifically, the time constant of the de-excitation of the super-resolution film from the excited state is preferably at least twice the time when the full width at half maximum (FWHM) of the reproduction beam passes on the optical disk surface.

The super-resolution film used in the present invention forms an optical aperture by absorption saturation of a semiconductor. Absorption saturation refers to a phenomenon in which when a semiconductor is irradiated with light, electrons in a ground state transition to an excited state and are retained, and electrons in the ground state decrease, so that the semiconductor no longer absorbs light. . When the semiconductor is absorbed and saturated and no longer absorbs light, the transmittance increases. When a reproduction beam is irradiated to an optical disc having the super-resolution film of the present invention, absorption saturation occurs in the super-resolution film corresponding to the region having a large number of photons in the reproduction beam spot, that is, the central portion, and the reproduction beam spot has A small optical aperture is formed. By detecting the recording marks on the recording layer through the optical aperture, super-resolution reproduction can be performed.

The present inventors have found that in order for a semiconductor constituting a super-resolution film to cause absorption saturation effectively, it is necessary to minimize the amount of foreign substances mixed into the semiconductor. Foreign substances include matrix materials and / or contamination. In the present invention,
It is necessary that the content of the matrix material and / or contamination mixed in the semiconductor fine particles or the continuous film of the semiconductor is 20 at% or less.

In order to form such a super-resolution film, it is necessary to prevent the matrix material or contamination from being mixed into the semiconductor during the film formation. For this purpose, it is preferable to use the following method.

For example, it is preferable to use a combination of a semiconductor material and a matrix material having poor wettability with each other. Specifically, for a Si-based semiconductor, SiO
₂ , SiC, BN, etc. are suitable as matrix materials. On the other hand, C, B ₄ C, etc. are not suitable as matrix materials because of their good wettability to Si-based semiconductors. For Al-based semiconductors, C, Si ₃ N ₄ , Si
C, SiO ₂ , Y ₂ O ₃ , ZrO _{2 and the} like are suitable as matrix materials.

Further, for example, when forming a super-resolution film by sputtering, it is preferable to apply a bias of an appropriate magnitude to the substrate. When sputtering is performed by applying a bias, particularly an RF bias, to a substrate, ions having appropriate energy are incident on the substrate, and the surface movement of sputtered particles deposited on the substrate surface is promoted. In this case, when the semiconductor material and the matrix material have poor wettability with each other,
Since the constituent atoms of the semiconductor are bonded to each other, they grow into particles of an appropriate size, and the amount of the matrix material or contamination mixed is reduced. Further, in order to bond the atoms constituting the matrix material or the contamination to each other, they grow in a network between the semiconductor fine particles.

At the time of sputtering, a target of a matrix material and a target of a semiconductor material may be simultaneously sputtered, or a composite target of a matrix material and a semiconductor material may be sputtered. In addition to RF sputtering, ion beam sputtering, vapor deposition, CVD, or the like can also be used.

When a semiconductor fine particle dispersed film is used as the super-resolution film according to the present invention, the matrix material is not particularly limited. For example, SiO ₂ , Si—N, Al—O, Al—
A wide range of materials, such as N and BN, that are transparent to the wavelength of the reproducing beam to be used can be selected.

The super-resolution film according to the present invention utilizes the fact that electrons in a semiconductor transition from a ground level to an excited level and are absorbed and saturated. (Forbidden band width) substantially corresponds to the energy of the reproduction beam. The two levels involved in the excitation are the conduction level, the valence band, the impurity levels in the forbidden band,
Selected from exciton levels. For example, a wavelength of 650 nm
When a (1.91 eV) reproducing beam is used, the most suitable semiconductor candidates are AlSb, CdSe, GaAs, and In.
P, CdTe, InSe and the like. Note that the energy gap of a semiconductor can be adjusted by forming an impurity level (deeply doped level or lightly doped level) in a forbidden band. For example, Cu ₂ O, Al
P, AlAs, GaP, ZnO, ZnS, ZnSe, Z
Semiconductors such as nTe, CdS, and TiO ₂ have a large energy gap. However, if these semiconductors are doped with impurities to form impurity levels in the forbidden band, the energy gap can be reduced to approach the energy of the reproduction beam. Table 1 shows the forbidden band width (Eg) of a typical semiconductor material (continuous film) and the corresponding wavelength (λg).

[0026]

[Table 1]

First, the basic characteristics of the super-resolution film used in the present invention will be described. Here, the characteristics were evaluated by forming only a super-resolution film having a structure in which CdSe fine particles were dispersed in a SiO ₂ matrix on a glass substrate. Cd
Se has a forbidden band width of 1.84 eV at 0K. Therefore, the wavelength 650 nm assumed as the reproduction light
Irradiation of (1.91 eV) laser light causes direct excitation from the full band to the conduction band.

This super-resolution film was formed by mounting a CdSe target, a SiO ₂ target and a glass substrate on a magnetron sputtering apparatus, and simultaneously performing RF sputtering on both targets. At this time, the CdSe content in the film can be adjusted by the sputtering power applied to each target. Further, the particle diameter of the CdSe fine particles can be adjusted by the substrate bias power applied to the substrate. When a substrate bias is applied, the surface movement of sputtered particles on the film formation surface can be promoted. For this reason, when the bias power is low, the size of the fine particles is small, and when the bias power is high, the size of the fine particles is large due to the surface movement effect and the aggregation effect of the same material. Further, the matrix material and the contamination mixed in the semiconductor fine particles can be suppressed to 20 at% or less.

A semiconductor laser beam having a wavelength of 650 nm is applied to the super-resolution film while changing the power with a pulse width of 50 ns.
Irradiation was performed through an NA 0.6 objective lens, and the transmittance was examined using a photodetector. The spot size on the sample surface is e
^-2 width was 0.89 µm, and full width at half maximum was about 0.5 µm. The reason why the pulse width is set to 50 ns is that the pulse width is set to 50 ns, which is the passage time of the full width at half maximum of the spot when the optical disk is operated at a linear velocity of 10 m / s.

FIG. 2 shows the relationship between the number of irradiated photons (N _p ) and the transmittance (Tr). The number of irradiation photons on the horizontal axis is proportional to the irradiation power of the reproduction light. That is, when the irradiation power was set to P (W), the irradiation number of photons N _p is given by the following equation.

N _p = P × τ _p (1240 / λ × 1.6 × 10 ⁻¹⁹ ) (1) where τ _p is the light irradiation time [sec], and λ is the wavelength [n
m]. In the formula (1), the numerator is the energy of the irradiated light, and the denominator is the energy [J] of one photon.
(1240 means a wavelength [nm] corresponding to 1 eV, and 1.6 × 10 ^-19 is a conversion coefficient from eV to J). Substituting τ _p = 50 ns and λ = 650 nm into equation (1), N _p is 1.6 for a reproduction power of 1 mW.
It becomes 4 × 10 ⁸ photos / mW. When the number of irradiated photons N _p is divided by the size of the full width at half maximum, the photon number density 8
× 10 ¹⁶ photons / mW · cm ² is obtained.

In general, it is considered that about 5 × 10 ¹⁶ molecules or atoms need to be excited in order for the transmittance to change by about several tens% due to absorption saturation. Judging from the above photon number density, it can be estimated that a sufficient change in transmittance occurs at a quantum efficiency that can be realized about 0.5.

In FIG. 2, up to the photon number N _p corresponding to an irradiation power of 0.7 mW, the molecular density of the ground level is large and light is efficiently absorbed, so that the light transmittance shows a low value of about 30%. I have. When the irradiation power becomes 0.7 mW or more, the light transmittance gradually rises and reaches a saturation value of 70% at 1.3 mW. For reference, laser light is not pulsed but DC
When the light is radiated, the number of photons integrated over time becomes extremely large even when the power is low, so that the transmittance shows a high value. This indicates that the characteristics in FIG. 2 are due to the absorption saturation phenomenon.

FIG. 3 shows the relationship between the recording mark array, the reproduction spot, and the optical aperture when reproducing the optical disk of FIG. 1 having the super-resolution film having the characteristics of FIG. In FIG. 3, TR
Represents a recording track, TR _i is a track on which reproduction is being performed, and TR _i-1 and TR _{i + 1} are adjacent tracks.
S indicates the e- ² diameter of the reproduction light spot, and M indicates a recording mark formed on the recording layer. Here, when no super-resolution film is provided, the inter-symbol interference is large, and the recording marks are recorded at such a narrow pitch that the marks cannot be identified. That is, as shown in FIG. 3, two or more recording marks exist in the spot diameter of the reproduction beam.

When the super-resolution film of the present invention is provided, a region having a high transmittance is formed only at a position where the number of irradiated photons is large by selecting an appropriate reproducing power. The number of irradiated photons here is the number of photons obtained by time integration with the movement of the medium with respect to the spot during reproduction. in this case,
The transmittance increases in the region A in FIG. 3, and light does not pass through the super-resolution film outside the region A. What contributes to the reproduction signal is a common set portion of the reproduction light spot S and the area A. Therefore, according to the present invention, even a high-density recording mark that cannot be identified by a conventional optical disk having no super-resolution film can be easily identified. Further, in the conventional optical disk, crosstalk occurs with the recording marks Mi _-1 and Mi _{+ 1} on adjacent tracks. For this reason, the track pitch cannot be reduced so much in the conventional optical disk. On the other hand, in the present invention, the recording marks M _i-1 and M _{i + 1} on the adjacent track are
Therefore, the track pitch can be reduced. In the present invention, absorption saturation due to excitation to a relatively long-lived excitation level is used, but this excitation level is not metastable and is completely de-excited to a ground state after a lapse of several hundred μs at the latest. . Therefore, in the present invention, the optical aperture can be closed by one-beam operation, and there is no need to irradiate an auxiliary beam to close the optical aperture unlike a conventional photon mode super-resolution film using a photochromic material.

[0036]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 First, only a super-resolution film was formed on a substrate, and its characteristics were examined. Glass substrate, AlSb in magnetron sputtering device
A target and a SiO ₂ target were mounted, and a 100 nm-thick super-resolution film having a structure in which AlSb fine particles were dispersed in SiO ₂ was formed on the substrate by dual simultaneous sputtering. The AlSb content in the film was adjusted by changing the RF power ratio applied to the AlSb target and the SiO ₂ target during dual simultaneous sputtering, and the size of the AlSb fine particles in the film was changed by changing the substrate bias during film formation. Specifically, the AlSb volume content in the super-resolution film is set to 10 to 10
0 vol% (100 vol% means an AlSb continuous film), and the AlSb particle size is 0.3 n
m (monomolecular) to 25 nm.
The content of SiO ₂ mixed into the AlSb fine particles was 20 at% or less.

Here, when discussing the electronic excitation in the semiconductor fine particles, it is appropriate to compare the content of the semiconductor fine particles by vol% (for the theory, see, for example, Staufer, translation of Tsunetaka Oda, " Fundamentals ", Yoshioka Shoten, Physics Series 54, p. 25). Therefore, in this specification, discussion will be made based on the volume content of the semiconductor fine particles.

The wavelength of the obtained super-resolution film is 650 nm.
The semiconductor laser light was irradiated in a pulse shape, and the time response of the transmittance was examined using a time-resolved spectrum analyzer. AlSb volume content of 75 vol% or more (100 v
ol% of the continuous film), the excitation efficiency was poor irrespective of the particle size, and the characteristic that the transmittance changed at high speed was not obtained. This is because the forbidden band width of the super-resolution film approaches the forbidden band width of the continuous film because the particles are connected to form a network structure. Such a film having a high AlSb volume content is not suitable for super-resolution reproduction of an optical disk. AlS
When the volume content b was less than 20 vol%, the change over time of the transmittance showed the desired characteristics. However, when the AlSb volume content was less than 20 vol% and the particle size was small, it took several minutes or more for the transmittance increased after pulse irradiation to return to the original state. This is probably because the Stark effect occurs preferentially. When the volume content of AlSb is less than 5 vol%, the amount of change in transmittance due to the Stark effect becomes insufficient.

The above-mentioned Stark effect means that the ground level and the excited level are disturbed by the electric field of the irradiated light (more precisely, the wave function of the electrons existing in each level is distorted). This is an effect that the wavelength of transition to the excitation level shifts. In this case, irradiation with light does not deplete the electrons at the ground level, but decreases the absorption coefficient associated with the electron transition to a specific excitation level and the absorption coefficient associated with the electron transition to another excitation level. The coefficient increases. Since the Stark effect becomes more remarkable as the electric field becomes stronger, the light absorptance of a predetermined wavelength is remarkably reduced near the center of the light spot, and an optical aperture is formed. However, in a state where the continuous film and the fine particles are associated, the Stark effect cannot be expected because the excited electrons move to the adjacent fine particles. That is, the Stark effect occurs preferentially only when the fine particles are surely dispersed.

On the other hand, when the volume content of AlSb is 75
In the case where the particles are not so associated and have an appropriate particle size at less than vol%, absorption saturation occurs mainly, and a practical transmittance change amount and an appropriate time response can be obtained. In a region where a transmittance change amount of 10% or more and a long de-excitation time constant of several tens ns or more are obtained, the volume content is 75 vol%, the particle size is 10 nm, the volume content is 50 vol%, and the particle size is 20 n.
m, the volume content is 20 vol%, and the particle size is within the range of a line segment connecting 32 nm. The larger the particle size, the shorter the time constant of deexcitation, because the effect of maintaining the lifetime of the excited level by the effect of fine particles is reduced. Further, when the volume content exceeds 75 vol%, the excitation efficiency is deteriorated because the fine particles are connected to each other to form a network structure, and thus approach the forbidden band width of the continuous film. The lower limit of the particle size is 2 to 3 nm at which the Stark effect mainly occurs. Summarizing the above results, FIG. 4 shows the relationship between the volume content of fine particles and the particle size of the super-resolution film in which AlSb fine particles are dispersed in a SiO ₂ matrix. In this figure, a region where absorption saturation mainly occurs and a good time response is obtained is indicated by oblique lines. In addition, the region where particle association is remarkable (AG),
A region in which both the absorption saturation and the Stark effect can occur is indicated by (AS + SE), a region in which the Stark effect mainly occurs is indicated by (SE), and a region in which the Stark effect hardly changes in transmittance is indicated by (NG).

Next, based on the above results, A
1Sb volume content [vol%] and average particle size [nm]
Is 60 vol%, 8 nm (Example 1-1), 50 vol
1%, 8 nm (Example 1-2), 50 vol%, 5 nm
A super-resolution film as (Example 1-3) was formed. These super-resolution films were irradiated with semiconductor laser light having a wavelength of 650 nm, and the time-dependent change in transmittance was examined using a time-resolved spectrum analyzer. We examined how the transmittance increased due to absorption saturation attenuated with time. The result is shown in FIG. In FIG. 5, a time “0” means a time at which irradiation of a light pulse having a sufficient intensity to cause absorption saturation is started.

The rise time of the transmittance is ns which is almost the same as that of the light pulse. From this, it can be seen that the response of the light excitation occurs extremely fast. After the irradiation of the light pulse, the transmittance decreases with de-excitation, and eventually returns to the level before the pulse irradiation. The time constant of the de-excitation is 200 ns in Examples 1-1 and 1-2, and 500 ns in Example 1-3. If the time constant is such a value as at least, it is maintained in an excited state at least while the reproduction light is being irradiated, and shows absorption saturation.

On the other hand, the FWHM transit time of the reproduction spot is 50 ns under the conditions of a wavelength of 650 nm and an NA of the objective lens of 0.6. The rotation speed of the optical disk is 360 at present
It is about 0 rpm, and it will be doubled to 7200 rpm in the future.
Although the speed may be increased to the extent, the time required for one rotation is 8.3 ms even in this case. That is, even when the same track is reproduced, the time between the first reproduction and the second reproduction is 10 times or more the time constant of the de-excitation.
Accordingly, it can be determined that de-excitation occurs before the next reproduction is performed after passing through the reproduction light spot, and the transmittance also returns to the original.

As described above, the time constant of de-excitation can be widely controlled by the content and particle size of the semiconductor particles in the super-resolution film. In addition, the lower limit of the time constant of de-excitation depends on the spot size. If re-excitation occurs significantly during irradiation of the reproduction spot, absorption saturation becomes difficult. For this reason, it is preferable that the time constant of the de-excitation is at least twice the transit time of the full width at half maximum of the reproduction spot. The upper limit of the time constant of de-excitation is preferably less than half of the reproduction interval in order to perform repeated reproduction by single beam operation. Although this depends on operating conditions such as the number of rotations of the disk, it is specifically preferable that the time be less than 4 ms.

In FIG. 5, comparing Example 1-1 and Example 1-2 with the same particle size and different volume content, the larger the volume content, the greater the advantage of the change in transmittance. A comparison between Example 1-2 and Example 1-3 having the same volume content and different particle diameters shows that Example 1-3 having a small particle diameter is obtained.
Since the effect of fine particles is more remarkable, the deexcitation time is longer. Further, the amount of change in transmittance is large due to the long de-excitation time. In the present invention, the particle size and the volume content, that is, the transmittance change amount and the de-excitation time constant can be freely changed within the range shown in FIG. 4, so that the degree of freedom in designing according to the operating conditions is wide.

Next, as shown in FIG. 6, a phase change optical recording medium (DVD-RAM) having a super-resolution film was manufactured. In FIG. 6, a disc substrate 1 made of polycarbonate is shown.
1, a 100 nm-thick SiN interference layer 12,
0 nm super-resolution film 13 and 150 nm thick ZnS-Si
An O ₂ lower interference layer 14, a GeSbTe recording layer 15 with a thickness of 20 nm, a ZnS—SiO ₂ upper interference layer 16 with a thickness of 150 nm, and an Al—Mo reflection layer 17 with a thickness of 50 nm are formed. A counter substrate 19 is bonded on the Al-Mo reflection layer 17 with an adhesive 18.

The phase change optical recording medium shown in FIG. 6 can be manufactured, for example, by the following method. The polycarbonate disk substrate 11 provided with the tracking guide grooves is set in a multi-chamber magnetron sputtering apparatus and evacuated. In the first chamber, a B-doped Si target is subjected to reactive DC sputtering in an N ₂ -Ar mixed gas plasma to form a 100 nm-thick SiN interference layer 12. In the second chamber, the AlSb target and the SiO ₂ target are simultaneously and simultaneously RF-sputtered with Ar plasma, and
An RF bias is applied to the substrate to form a 50 nm-thick super-resolution film 13 by bias sputtering. At this time, by adjusting the sputtering conditions, the super-resolution films of Examples 1-1 to 1-3 formed in the above preliminary experiment can be formed. In the third chamber, ZnS—SiO ₂ is R
The ZnS—SiO ₂ lower interference layer 14 having a thickness of 150 nm is formed by F sputtering. In the fourth chamber, a Ge ₂ Sb ₂ Te ₅ target is DC-sputtered with Ar plasma to have a thickness of 20 n.
The m GeSbTe recording layer 15 is formed. Room 5 with Zn
RF sputtering of S-SiO ₂ with Ar plasma to a film thickness of 1
Forming a ZnS-SiO ₂ upper interference layer 16 of 50nm. In the sixth chamber, an Al-2 at% Mo target is DC-sputtered with Ar plasma to form an Al-Mo reflection layer 17 having a thickness of 50 nm. Thereafter, the disk is taken out into the atmosphere. Further, an adhesive (hot melt adhesive or UV resin) 18 is spin-coated on the Al-Mo reflection layer 17,
The opposing substrate 19 is placed, and the adhesive 18 is cured to produce an optical disc having a bonded structure.

The SiN interference layer 12 is not necessarily provided, but is preferably provided to increase the change in transmittance of the super-resolution film 13 due to the interference effect. Counter substrate 19
May be a flat plate on which no film is provided, or may be a plate on which grooves are provided in the same manner as the disk substrate 11 to form a functional multilayer film.

For comparison, an optical disk (comparative example) having the same configuration as that of FIG. 6 except that the SiN interference layer and the super-resolution film were not provided was manufactured. The pitch of the grooves provided on the disk substrate 11 is determined according to the wavelength of the laser used for disk operation, the NA of the objective lens, and the characteristics of the super-resolution film. In the following experiment, the wavelength is 650 nm,
The condition of NA 0.6 of the objective lens is adopted. Under these conditions, when no super-resolution film is provided, the groove pitch can be reduced to at most about 0.6 μm, but when the super-resolution film is provided, the groove pitch can be reduced to about 0.4 μm. Talk can be suppressed to a predetermined amount or less.
However, considering the cross erase during recording, the groove pitch is 0.5 μm, which is equivalent to the full width at half maximum of the laser spot.
m is preferable. The groove depth is set to 150 to reduce crosstalk during reproduction in the land-groove recording method and to reduce cross-erase during recording.
nm (so-called deep groove).

The recording and reproducing characteristics were evaluated using the optical disks of Examples 1-1 to 1-3 and Comparative Example. First, the phase change optical recording layer 15 was initially crystallized over the entire surface of the disk using an initialization apparatus. Next, the optical disk is set to a wavelength of 650 n
m, an optical disk drive equipped with an objective lens with NA of 0.6, and a linear velocity of the disk of 10 m
/ S, the recording power level was set to 12 mW, the erasing power level was set to 6 mW, and a recording mark having a mark length of 0.3 μm was recorded at a single frequency in the overwrite mode while changing the mark interval. At this time, in order to prevent the influence of thermal interference, recording compensation for dividing the recording pulse was applied.

The optical disk recorded as described above was reproduced. First, when the mark interval (MP) is 0.2
Reproduction was performed while changing the reproduction power for the μm mark row. FIG. 7 shows the relationship between the reproduction power and the reproduction signal noise ratio (CNR).

In the optical disk of the comparative example, it is impossible to separate and identify mark rows at intervals of 0.2 μm and reproduce them.
The reproduced signal intensity was at an extremely low level due to the influence of intersymbol interference. When the reproduction power was increased, the signal intensity was increased in accordance with the increase in the light intensity, but the noise level was also increased, so that the CNR remained at a low level.
On the other hand, in the optical disk of the embodiment 1-3, 0.7
In a low power region of less than about mW, no signal is obtained because the super-resolution film does not absorb and saturate and remains in a low transmittance state. When the reproduction power becomes 0.7 mW or more, the absorption gradually becomes saturated, the transmittance increases, and the CNR is improved. When the reproducing power is about 1.3 mW, absorption saturation occurs sufficiently, the transmittance becomes extremely high, and the CNR is sufficiently high.
And a high CNR is maintained up to about 2.2 mW. When the reproducing power is further increased, the optical aperture formed in the super-resolution film becomes excessively large, so that the mark cannot be identified, the CNR gradually decreases, and finally reaches a level equivalent to that of the comparative example. ing. In addition, mark interval (MP)
Is narrower, the power range in which the CNR shows a constant value becomes narrower.

FIG. 7 shows only the results of Example 1-3, but a similar tendency was observed in Examples 1-1 and 1-2. Specifically, Example 1-1 is the same as Example 1
-3 showed almost the same characteristics. Further, although the CNR of the example 1-2 was lower than that of the example 1-3, the power range in which the CNR shows a constant value was wider than that of the example 1-3, which was advantageous in terms of power margin. Such a result can be expected from FIG. That is, while the reproduction light is being irradiated, de-excitation hardly occurs in any of Examples 1-1 to 1-3, and the transmittance change amounts are almost equal in Examples 1-1 and 1-3. In Example 1-2, the amount of change in transmittance is small, but the size of the optical aperture is equal to those in Examples 1-1 and 1--1.
It can be explained from the fact that it is smaller than 3.

Next, in FIG. 7, when CNR was set to a reproducing power showing a constant value, the high-density recording characteristics of reproducing tracks having different mark intervals were evaluated. The result is shown in FIG. In the optical disc of the comparative example, when the mark interval is less than 0.3 μm, the influence of intersymbol interference is strong, and the CNR is reduced. Also, since the crosstalk from the adjacent track is large, even if the mark interval on the track is long,
The level of CNR is not very high. In contrast, in the disc of Example 1-3, the mark interval was 0.1
Reproduction can be performed with a high CNR even at 5 μm. Further, since there is no influence of crosstalk at all, the CNR when the mark interval is longer than 0.15 μm is higher than that of the comparative example. The optical disc of Example 1-2 has a small transmittance change amount, so the CNR when the mark interval is long is low, but the optical aperture size is small, so that a constant CNR level can be maintained even when the mark interval is further shortened.

From the above results, it can be said that it is preferable to increase the transmittance change in order to increase the CNR, and it is preferable to reduce the optical aperture from the viewpoint of higher density and power margin.

Further, the optical disk of the present invention has an effect that the number of times of repeated reproduction is large. That is, unlike the conventionally known heat-resolution or photon-mode super-resolution reproduction method, the present invention uses only electronic excitation in principle, and there is no deterioration due to thermal fatigue or atom transfer or change in the bonding state. The repetition stability is extremely good.

Example 2 As a super-resolution film, a Be-doped GaP fine particle dispersion film or T
An optical disk having the same configuration as that of FIG. 6 was manufactured except that the e-doped GaP continuous film was used.

The GaP (continuous film) has a bandgap of 2.35 eV at 0 K and a corresponding wavelength of 530 nm. Therefore, when no impurity is doped, electronic excitation does not occur at a wavelength of 650 nm. Also, when GaP is finely divided and dispersed in a matrix, the forbidden band width is widened depending on the size and volume content of the fine particles, so that direct excitation from the full band to the conduction band is further more than in the case of a continuous film. Less likely to happen.

A super-resolution film having a structure in which Be-doped GaP fine particles having an average particle diameter of 5 nm were dispersed in Si-N at a volume content of 50 vol% was used. B contained in this super-resolution film
In the e-doped GaP fine particles, some of the lattice points of Ga are replaced with Be, and an acceptor level is formed above the full band. As a result, upon irradiation with reproduction light having a wavelength of 650 nm, absorption saturation occurred due to electron transition between the full band and the acceptor level, and super-resolution reproduction became possible.

A super-resolution film made of a Te-doped GaP continuous film was used. In the Te-doped GaP continuous film constituting this super-resolution film, a part of the lattice points of P is replaced by Te, and a donor level is formed below the conduction band. As a result,
Irradiation of reproduction light with a wavelength of 650 nm results in a donor level-
Electron transition between the conduction bands caused absorption saturation, which enabled super-resolution reproduction. The present invention provides a magneto-optical disk,
It can be applied to CD-ROM, CD-R, WORM, etc.
As in the above embodiment, high density can be realized.

[0061]

As described above in detail, according to the present invention, the transmittance of the super-resolution film changes in a practical reproduction light power range, and the change amount is large, and the transit time of the reproduction light spot is large. An optical aperture can be formed at a high speed in a short period of time, and a super-resolution reproduction method for an optical recording medium that exhibits stability against repeated reproduction can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a basic configuration of an optical recording medium according to the present invention.

FIG. 2 is a diagram showing the relationship between the number of irradiated photons (N _p ) and the transmittance (Tr) for the super-resolution film according to the present invention.

FIG. 3 is a diagram showing a relationship between a recording mark array, a reproduction spot, and an optical aperture during reproduction of the optical recording medium of the present invention.

FIG. 4 is a diagram showing the relationship between the AlSb particle volume ratio and the AlSb particle size in the super-resolution film according to the present invention.

FIG. 5 is a diagram showing a change over time in transmittance of a super-resolution film according to the present invention during reproduction.

FIG. 6 is a sectional view of an optical recording medium manufactured in an example of the present invention.

FIG. 7 shows an optical recording medium according to an embodiment of the present invention.
FIG. 4 is a diagram illustrating a relationship between a reproduction power and a CNR.

FIG. 8 shows an optical recording medium according to an embodiment of the present invention.
The figure which shows the relationship between a mark interval and CNR.

[Explanation of symbols]

1 ... substrate 2 ... super-resolution film 3 ... intermediate layer 4 ... recording layer 5 ... protective film 11 ... disk substrate 12 ... SiN interference layer 13 ... super-resolution film 14 ... ZnS-SiO ₂ lower interference layer 15 ... GeSbTe recording layer 16 ... ZnS-SiO ₂ upper interference layer 17 ... Al-Mo reflective layer 18 ... adhesive 19 ... counter substrate

Claims (4)

[Claims] 1. A fine particle dispersion comprising: a recording layer; and a super-resolution film provided on a reproduction light incident side with respect to the recording layer, wherein the super-resolution film has semiconductor fine particles dispersed in a matrix. An optical recording medium comprising a film or a continuous film of a semiconductor, wherein the content of a matrix material or contamination mixed in the semiconductor fine particles or the continuous film of the semiconductor is 20 at% or less. 2. The optical recording medium according to claim 1, wherein the optical aperture of the super-resolution film is irradiated with the reproduction light, and an optical aperture having a transmittance higher than that of the surrounding area is formed by absorption saturation in a region where the number of photons irradiated with the reproduction light is large. A super-resolution reproduction method for an optical recording medium, comprising forming a recording mark in a recording layer through the optical aperture. 3. The super-resolution film has a structure in which semiconductor fine particles are dispersed in a matrix. The super-resolution film is kept in an excited state during irradiation with reproduction light, and returns to a ground state within a predetermined time after irradiation with reproduction light. 3. The method according to claim 2, wherein de-excitation of the compound is performed. 4. The time constant of deexcitation of the super-resolution film from the excited state to the ground state is at least twice the time when the full width at half maximum of the reproduction light passes through the surface of the optical recording medium. The method according to claim 3, wherein