JPH0695389B2 - Optical recording medium - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a structure of an optical recording medium in which the cooling rate of a medium layer after heating by irradiation with light is optimized.

[Prior Art] With the shift to an information-oriented society, an optical recording system is being put to practical use as a means for recording a huge amount of information. In particular, optical discs are 10 times better than the magnetic recording media that have been widely used in the past.
It is expected to be used as a high-density, large-capacity recording method because it has a recording density of up to 100 times and has a long life because the head and disk are not in contact with each other.

This optical recording is classified into three types, a read-only type, a write-once type, and a rewritable type, depending on the application. The read-only type can only read information and is already widely used as a compact disc. The write-once type can record and read information, but cannot rewrite the recorded information, and is being put to practical use for document files. The rewritable type can record, read, and erase information, and is expected to be used as a data file for computers.

For this rewritable type, two recording methods, "magneto-optical recording" and "phase change recording", are being developed.
Both systems leave room for further improvement in terms of recording materials and writing mechanism.

Of these, phase transformation recording generally controls the phase state (crystal-amorphous, phase transition, etc.) of the recording material by focusing and heating laser light on the recording surface and controlling the pulse output and duration. Information is recorded based on the difference in reflectance in each state.

An example of a concrete phase change type optical recording medium is shown in FIG. A silicon oxide (SiO ₂ ) film 2 is formed on the surface of a substrate 1 made of polycarbonate or the like by a method such as sputtering,
Further, the recording medium film 3 is formed thereon. A SiO ₂ film 4 and an organic protective layer 5 are further formed thereon. The structure in which the medium film 3 is sandwiched between the SiO ₂ films 2 and 4 has a high temperature due to light heating during signal writing and erasing, but the reaction with the substrate material and evaporation of the medium at that time occur. , To prevent scattering and to prevent alteration of the medium material. Laser light is generally incident from the substrate 1 side.

In order to actually write information, first, the medium is sufficiently crystallized by light irradiation of a flash lamp or the like to be initialized. Next, the medium is irradiated with high-power, short-pulse laser light to melt the medium and then quench it. This causes the medium to transform from crystalline to amorphous. In the case of erasing information, since the amorphous medium is made crystalline, the medium is heated to the crystallization temperature by a relatively low output laser beam and annealed. The irradiation time at this time is determined by the crystallization speed of the medium.

In order to increase the data transfer rate, it is necessary to shorten the time required for writing and erasing this data. The rate-determining factor for this is the crystallization time of the medium during erasing.

The crystallization rate of the medium generally has temperature dependence as shown in FIG. That is, it increases at a constant activation energy from around room temperature to a temperature slightly below the melting point of the medium. Crystal nuclei are not formed in the vicinity of the melting point, and the crystallization rate decreases. At the time of erasing the recording by the laser described above, it is advantageous to raise the temperature of the medium to a temperature slightly below the melting point (crystallization rate) at which the crystallization rate becomes maximum.

In order to shorten the crystallization time for erasing, in addition to raising the temperature of the medium film to the crystallization temperature as described above, it is important to select a material having a large crystallization rate at the crystallization temperature, For example, using GeTe compound, 3 × 10 ⁷ (1 / sec)
It is known that the above crystallization rate can be obtained. This is because the medium layer is 33 n se at the crystallization temperature of the GeTe compound.
This means crystallization at c or less, and is considered to be suitable as an optical recording medium for shortening the erasing time.

[Problems to be solved by the invention]

However, even if such a GeTe medium film capable of shortening the erasing time is used for an optical recording medium having a conventional structure as shown in FIG. 4, information cannot be recorded and it functions as an optical recording medium. I can't. Information is recorded by irradiating the medium film 3 with laser light to melt the medium film and quenching it to obtain an amorphous medium film 3 as described above, but the GeTe medium film is crystallized. Since the speed is high, the cooling rate for making the medium film amorphized must also be increased. However, in the structure shown in FIG. 4, since the medium film 3 is sandwiched by the silicon oxide films 2 and 4, it is thermally shielded and the cooling rate necessary for amorphizing the GeTe medium film is obtained. I can't. For example, in the conventional structure shown in FIG. 4, the cooling rate is about 10 ⁹ (° C /
This is because a cooling rate higher than the above value is required to amorphize the GeTe compound (the crystallization rate of the GeTe medium film is 3 × 10 ⁷ (1 / sec)). If the temperature range above is 30 (℃), 3n
Crystallizes in the presence of sec 30 (° C) / 33 (n sec) ≒
The GeTe medium film is crystallized at a cooling rate of 10 ⁹ (° C / sec). Therefore, 10 ⁹ (° C / sec) is required for amorphization.
It will be said that a cooling rate exceeding the above is required). Therefore, the GeTe medium film does not become amorphous even if it is melted by being irradiated with laser light and cooled, and information cannot be recorded.

As a countermeasure against this, for example, the material of the SiO ₂ layer 4 in FIG. 4 may be a high thermal conductivity material such as Al to increase the thermal diffusion rate, but such a metal material is alloyed with the medium material. Not only is there a danger, but the thermal diffusion is generally too great, requiring a large laser input to heat and melt the medium.

The present invention has been made in view of the above points, and an object thereof is to adjust a cooling characteristic of a medium film to an optimum range so that a required optical input energy density is small and information can be recorded and erased. Moreover, it is to provide an optical recording medium which can be performed at high speed.

[Means for solving problems]

To achieve the above object, the present invention provides a substrate, a protective layer on the substrate, a medium layer on the protective layer, a heat shield layer on the medium layer, and a cooling layer on the heat shield layer. In the optical recording medium with and, the thickness x of the heat shield layer is ks Tm / P <x <2 × ks t / ρs Cs) ^1/2 (where ks is the thermal conductivity of the heat shield layer). , Tm is the melting point of the medium layer, P is the energy density of the irradiated light, t is the irradiation time of the light, ρs is the density of the heat shield layer, and Cs is the specific heat of the heat shield layer. There is.

The substrate 31 is a structural material of an optical recording medium formed of a light transmissive material such as polymethylmethacrylate (PMMA), polycarbonate (PC) or glass.

The protective layer 32 includes silicon oxide (SiO ₂ ), zirconium oxide (ZO
₂ ), aluminum oxide (Al ₂ O ₃ ), silicon nitride (Si ₃ N
₄ ), aluminum nitride (AlN), sialon, etc., and prevents oxidation of the medium layer described later and prevents reaction with the substrate 31. The protective layer 32 is formed with a spatter or the like to a thickness of about 0.1 μm.

For the medium layer 33, a material having a high melting point and a high crystallization rate such as a GeTe compound or an InTe compound is used. The medium layer 33 is formed to a thickness of about 0.1 μm by vapor deposition, sputtering or the like. Information is recorded on and erased from the medium layer 33 by irradiation with laser light. Information recording is performed by irradiating the medium layer 33 with a thin beam of laser light, melting the spot of the medium layer irradiated with the beam, and then rapidly cooling it to amorphize the spot of the medium layer. . Information is erased by irradiating the above-described amorphized medium layer portion with a laser beam having a weaker input than during recording, and annealing the amorphous portion to crystallize.

Cooling of the medium layer 33 is performed by the cooling layer 35 described below through the heat shield layer 34.

The heat shield layer 34 is made of the same material as the protective layer and is formed by the same method. The heat shield layer 34 prevents the medium layer 33 from being oxidized, prevents the medium layer 33 from reacting with the cooling layer described below, and further adjusts the cooling rate of the medium layer 33. The heat energy of the medium layer 33 heated by the laser light irradiation as described above passes through the heat shield layer 34 and reaches the cooling layer described below.

The cooling layer 35 is made of aluminum (Al), gold (Au), silver (Ag),
It is formed using a material having a good thermal conductivity, such as copper (Cu). The cooling layer 35 is formed of such a material to a thickness of about 0.1 μm by a method such as vapor deposition. The cooling layer 35 cools the medium layer 33 heated by laser light irradiation.

[Action]

The thermal energy of the laser heating of the medium layer 33 is guided to the cooling layer via the heat shield layer 34, and the medium layer 33 is cooled.
The heat shield layer 34 adjusts the heat conduction to the cooling layer 35 by its thickness. This heat conduction is made large enough for the medium layer 33 to become amorphous, and its size is limited so that the medium layer can be melted by light irradiation. In this way the cooling properties of the medium layer are optimized.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic sectional view of an optical recording medium according to an embodiment of the present invention. In the optical recording medium having the structure shown in FIG. 1, 3 m thick polycarbonate is used as the substrate 31. The protective layer 32 and the heat shield layer 34 are formed by sputtering silicon oxide to a thickness of 0.1 μm.
It is formed thick. The medium layer 33 is formed to have a thickness of 0.1 μm by sputtering a GeTe compound. The cooling layer 35 is formed to a thickness of 0.1 μm by vapor deposition of metallic aluminum. The organic protective layer 36 is formed by applying a UV curable resin by spin coating to a thickness of 2 mm. The organic protective layer 36 mechanically protects the layer structures 32 to 35.

A laser beam (830 nm) is incident from the side of the substrate 31 and the medium layer 33
At, the light is focused on a spot with a diameter of 1.2 μm. Laser light input energy density is 15 mW and irradiation time is 0.1 μsec. The medium layer 33 is annealed in advance at 250 ° C. to be crystallized and initialized. Amorphous spots were formed in the medium heating portion by laser light irradiation, and a change in reflectance was observed. The temperature change of the medium at this time is as shown in FIG.
The temperature of the medium layer rises due to the laser light irradiation, and eventually reaches the melting point 720 (° C). Since the melting of the medium layer proceeds at this temperature, the temperature is maintained as it is for a while. The temperature of the molten medium layer starts to rise after the medium layer has completely melted, but when the laser beam irradiation time is 0.1 μsec and the laser beam irradiation ends, the temperature of the molten medium begins to drop and the melting point remains constant for a while. Hold the temperature. After the media layer has completely solidified, the temperature of the media layer begins to drop again.
Below the solidification temperature, when the heat shield layer is 0.1 μm thick, the temperature drops between 700 ° C. and 600 ° C. at a rate of 9 × 10 ⁹ (° C./sec), and the medium layer is rapidly cooled. This value is
Since the cooling rate for crystallization of the medium layer at a crystallization temperature of 700 to 600 (° C.) is sufficiently higher than 10 ⁹ (° C./sec), the medium layer becomes amorphous and information can be recorded. Information can be erased by irradiating with a laser light input energy density of 7 mW for 0.1 μsec because the crystallization speed of the GeTe medium layer is fast. At this time, the GeTe medium layer is maintained at the crystallization temperature for a predetermined time and undergoes a phase change from an amorphous state to a crystalline state at a high speed, so that the erasing time becomes short.

In the above example, the thickness of the protective layer 32 made of silicon oxide is set to 0.1 μm.
However, even if the thickness is 0.3 μm, there is almost no change in the cooling characteristics. The cooling rate is 9.5 × 10 ⁹ (℃ / sec). This is because the cooling of the medium layer 33 is mainly performed by the cooling layer 35.

Regarding the thickness of the cooling layer 35, 0.1 μm is used in the above example, but if this is 0.3 μm, the cooling rate is 9.2 × 10 ⁹
(° C / sec), which is almost the same as the case of 0.1 μm thickness.

Next, optimization of the thickness (x) of the heat shield layer 34 will be described.
When the medium layer is heated by laser irradiation to reach the melting point,
The heat flux J flowing from the medium layer 33 to the cooling layer 35 through the heat shield layer 34 has a thermal conductivity of the heat shield layer of ks (cal / cm · sec · ° C),
If the melting point of the medium layer is Tm (° C.) and the input energy density of the laser light is P (cal / cm ² · sec), then J = ks Tm / x (1) The condition that the heat flux J is smaller than the input energy density P of the laser light is the condition for the medium layer 33 to reach the melting point, so the following equation is obtained.

J = ks Tm / x <P (2) Therefore, ks Tm / P <x (3).

Next, the cooling effect of the cooling layer 35 becomes remarkable.
It is necessary that the thickness x of the laser light be less than or equal to the thermal diffusion distance D in the heat shield layer.
c), the density of the heat shield layer is ρs (g / cm ³ ) and the specific heat is Cs (cal /
Eq. (4) is obtained, where K is the coefficient in ° C.

x <K × D = K × (ks · t / ρs · Cs) ^1/2 (4) The coefficient K can be experimentally determined. When laser light irradiation is performed with the thickness of the heat shield layer 34 being 0.3 μm, a change in reflectance can be obtained, but the diameter thereof is about 0.3 μm as compared with 1.0 μm described above. The cooling rate is about 2 × 10 ⁹ (° C./sec) at the crystallization temperature. Therefore, assuming that the thickness of the shielding layer is 0.3 μm, which is the larger limit, K = 2 is obtained from this. Therefore, equation 4 is expressed by the following equation.

ks Tm / P <x <2 (ks · t / ρs · Cs) ^1/2 (5) In equation (5), ks · Tm / P is 0.054 μm at an input energy density of 15 mW of laser light. It can be seen that if the thickness x of the heat shield layer 34 is set to 0.05 μm and laser irradiation is performed, an amorphous spot cannot be obtained. This indicates that the thickness of the heat shield layer is too thin and the heat flux is large, and the medium layer does not reach the melting point due to the irradiation of the laser beam, so that it cannot be amorphized. When the input energy density of laser light is increased, it is possible to melt and amorphize the medium layer,
Laser input 15mW in medium layer with current laser technology
Obtaining the above is not only technically difficult but also economically disadvantageous. Further, it is considered that the necessity of obtaining the cooling rate higher than that shown in the embodiment is practically small, and when SiO ₂ is used as the material of the heat shield layer 34, the film thickness thereof is preferably 0.05 μm or more. To summarize the above, when silicon oxide is used as the heat shield layer 34, it is necessary that the thickness x be in the range of 0.05 μm <x <0.3 μm (6). In the above example, good characteristics are shown when the thickness of the heat shield layer 34 is 0.1 μm, which satisfies the equation (6).

When silicon oxide (SiO ₂ ) is used as the heat shield film 34 in this way, if the thickness x is set to the optimum range satisfying the expression (6), the medium layer 33 with a small laser light input energy density of 15 mW is formed. Can melt the medium layer
The cooling rate of 33 can also be made sufficiently fast, and as a result, recording of information and 0.1 μs by using the medium layer 33 having a fast crystallization rate.
It becomes possible to erase high-speed information called ec.

As described above, the material of the heat shield layer 34 is not limited to SiO ₂ , and the thickness x of the shield layer may be set so as to satisfy the expression (5) for the material used. Of.

Comparative Example An experiment similar to that of the example is performed on a magneto-optical recording medium having a structure in which the cooling layer 35 is removed from the structure shown in FIG. At this time, no amorphous spot is formed. The temperature change is shown in FIG. Cooling rate at crystallization temperature is 1 × 10 ⁹ (℃ / sec)
This is because crystallization occurs at this cooling rate.

〔The invention's effect〕

According to the present invention as described above, the substrate, the protective layer on the substrate, the medium layer on the protective layer, the heat shield layer on the medium layer, and the cooling layer on the heat shield layer are provided. In the provided optical recording medium, the thickness x of the heat shield layer is ks Tm / P <x <2 × ks t / ρs Cs) ^1/2 (where ks is the thermal conductivity of the heat shield layer, Tm Is the melting point of the medium layer, P is the energy density of the irradiated light, t is the irradiation time of the light, ρs is the density of the heat shield layer, and Cs is the specific heat of the heat shield layer. The cooling effect of the layers,
Due to the heat conduction adjusting effect of the heat shield layer, the cooling rate necessary for amorphization of the medium layer and the melting of the medium layer at a small light input energy density are simultaneously achieved, and as a result, the medium having a fast crystallization rate is obtained. The layer can be used for high speed recording and erasing of information under a low light input energy density.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an optical recording medium according to an embodiment of the present invention, FIG. 2 is a graph showing cooling characteristics of a medium layer of an optical recording medium according to the embodiment of the present invention, and FIG. 3 is a comparative example. FIG. 4 is a graph showing the cooling characteristics of the medium layer of such an optical recording medium, FIG. 4 is a schematic sectional view of a conventional optical recording medium, and FIG. 5 is a graph showing the crystallization rate of the medium layer. 31: substrate, 32: protective layer, 33: medium layer, 34: heat shielding layer, 35: cooling layer.

Claims (1)

[Claims] 1. An optical recording medium comprising a substrate, a protective layer on the substrate, a medium layer on the protective layer, a heat shield layer on the medium layer, and a cooling layer on the heat shield layer. Where the thickness x of the heat shield layer is ks Tm / P <x <2 × ks t / ρs Cs) ^1/2 (where ks is the thermal conductivity of the heat shield layer and Tm is the medium layer). Melting point, P is the energy density of the irradiated light, t is the irradiation time of the light, ρs is the density of the heat-shielding layer, and Cs is the specific heat of the heat-shielding layer).