KR20010009662A - Phase change optical disc - Google Patents

Abstract

PURPOSE: An optical disk of phase changing type is provided to improve durability in repeated recording and overlapped recording characteristic through adding Ag and N to Ge-Sb-Te recording layer. CONSTITUTION: A first dielectric layer(31), a recording layer(32), a second dielectric layer(33), a first reflecting layer(34), a second reflecting layer(35) and a protect layer(36) are successively deposited. Ag and N are induced to Ge-Sb-Te alloy layer through placing Ag on a dielectric, sputtering the alloy layer, and injecting N2 to Ar as sputter gas. Metal is used for the reflecting layer to improve heat absorption and optical efficiency. Herein, Al-Cr alloy and Ag-Mg alloy are formed by adding Cr, Ni, Ti, Si to Al, Ag, Au, Cu.

Description

Phase change type optical disc {PHASE CHANGE OPTICAL DISC}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase change type optical disc, and more particularly, to a phase change type optical disc capable of freely recording and erasing and overlapping recording.

A typical optical disk has a disk structure as shown mainly in Fig. 1, where (1) represents the information area of the disk. A pit, which is a mark for digital information, is formed on a track like a Morse code on a substrate of an optical disc, and a reflective layer and a protective layer for reflecting a laser beam are sequentially formed thereon.

The process of manufacturing the optical disk as described above is largely composed of the step of manufacturing a stamper (stamper), the step of replicating the transparent resin substrate in a large amount by the manufactured stamper and the step of forming a thin film such as a reflective layer thereon. The stamper is made by first producing a glass master having the same format as the optical disk, and then plating Ni thereon. The optical disc produced in this way is a read only memory (ROM) which can only read the recorded contents of the recording layer.

On the other hand, with the rapid spread of CD-ROMs and the like, multimedia-related software programs including moving pictures, still images, and animations are rapidly spreading. Therefore, there is an urgent need for recording and reproducible recording media for effectively using such programs. In response to this demand, a write once write-once optical disc (CD-R) has been developed. However, the write-once optical disc has a fatal disadvantage that it is not possible to edit the recorded data freely and record new information since only one write is possible.

Accordingly, a slow-circuit type optical disc capable of erasing and rewriting (e.g., CD-RW (Compact Disc Rewritable)) has been developed. As a recording layer material, an optical magnetic disk using a magneto-optical material and a phase change optical disk using a phase change material have. Among these, the phase change type optical disc has a reproduction principle similar to that of a conventional CD.

The phase change type optical disc uses the principle that information is recorded and erased by controlling the heating temperature and the cooling rate of the material forming the recording layer to induce a reversible change between the crystalline structure and the amorphous structure. 2. Description of the Related Art A conventional phase change type optical disc generally has a first dielectric layer 21, a recording layer 22, a second dielectric layer 23, a reflective layer 24, and a protective layer on a transparent substrate 20, as shown in FIG. It has a structure in which 25 is sequentially stacked. Here, a track in which a recording signal is made is formed in the recording layer. When the laser beam is irradiated, the recording layer is reversibly changed from crystalline to amorphous or from amorphous to crystalline to repeat recording and erasing.

This phase change type optical disk is easier to superimpose recording than other optical disks using other recording methods, shortens the wavelength of the laser light to increase the recording density of the optical disk, and also because the optical system of the drive drive is simple, DVD-RAM (Digital Video) Disk random access memory).

Ge-Sb-Te tertiary alloys have been mainly used as the recording layer in the phase change type optical disk. However, since the grain growth tendency of the Ge-Sb-Te tertiary alloys is relatively large, there is a problem of increasing the overlapping jitter value. have.

In order to solve this problem, Japanese Laid-Open Patent Publication No. 2-139283 discloses that B and C are added to a Ge-Sb-Te tertiary recording layer to improve repeat recording durability, and Japanese Laid-Open Patent Publication No. 4 -16383 discloses the addition of N to the Ge-Sb-Te ternary recording layer to improve repeat recording durability. In addition, Japanese Patent Laid-Open Nos. 6-127135 and 7-172060 disclose that Pd or S are added to a Ge-Sb-Te ternary recording layer to improve repeat recording durability.

However, the optical disc manufactured by this method still has a satisfactory repeat recording durability, and particularly has a problem in that the superposition recording characteristics are poor.

Accordingly, an object of the present invention is to provide a phase change type optical disk having improved repeatability and superimposed recording characteristics by adding Ag and N to a Ge-Sb-Te ternary recording layer to solve the above problems.

1 is a view schematically showing an information area of a general optical disc,

2 is a schematic cross-sectional view of a general phase change type optical disk,

3 is a schematic cross-sectional view of a phase change type optical disk according to an embodiment of the present invention,

4 is a diagram schematically showing a superimposed recording process of a phase change type optical disk,

5 is a view schematically showing a process in which the superposition recording characteristics of a phase change type optical disc deteriorate,

6 is a graph showing the shape of the multi-pulse of the laser beam used in the embodiment,

7 is a graph showing the change of the erasing rate according to the N ₂ / Ar injection ratio in the formation of the recording layer;

8 is a graph showing a change in signal amplitude according to an N ₂ / Ar injection ratio in forming a recording layer.

Fig. 9 is a graph showing the change of the superposition recording jitter value of the phase change type optical discs manufactured in Examples and Comparative Examples.

<Explanation of symbols for main parts of drawing>

1: information area 20 of an optical disc

21: first dielectric layer 22: recording layer

23: second dielectric layer 24: reflective layer 25: protective film

30: substrate 31: first dielectric layer 32: recording layer

33: second dielectric layer 34: first reflective layer 35: second reflective layer

36: protective layer

In accordance with the above object, in the present invention, in a phase change optical disk including a first dielectric layer, a Ge-Sb-Te based recording layer, a second dielectric layer, and a reflective layer sequentially formed on a transparent substrate, the recording layer is formed of Ag and N. It provides a phase change type optical disk comprising a.

It is a feature of the present invention that Ag and N are included in the Ge-Sb-Te-based recording layer to improve the superimposed recording characteristics and repeat recording durability of the disk.

As shown in FIG. 4, a phase change type optical disc according to one aspect of the present invention has a first dielectric layer 31, a recording layer 32, a second dielectric layer 33, and a first dielectric layer on a transparent substrate 30. The reflective layer 34, the second reflective layer 35, and the protective layer 36 are sequentially stacked.

The substrate used in the present invention can be produced by a method such as injection molding using a stamper using polycarbonate or the like, which is commonly used as a substrate material for an optical disk. Grooves formed on the substrate are designed for the width and depth of the servo required for recording and playback, especially when recording and playing both land and grooves for higher density. In order to prevent cross talk, the depth of the grooves should be set to? / 5n to? / 7n (λ: recording / reproducing light wavelength, n: refractive index of polycarbonate substrate).

Since the dielectric layer of the present invention must have light transmission characteristics and thermal durability, the material constituting the dielectric layer is a metal oxide, metal carbide, metal nitride, or a mixture thereof having an absorption coefficient close to zero and excellent in thermal stability. Materials commonly used for phase change type optical disc dielectric layers include ZnS-SiO ₂ (8: 2), AlN, GeN, and the like, which can be made into dielectric layers by conventional methods such as RF-sputtering. have. The thickness is preferably 300 to 3000 mV for the first dielectric layer and 50 to 500 mV for the second dielectric layer.

In general, a chalcogen compound is used for the recording layer that is easily amorphous and crystallized within a short time. In order to maintain the mark phenomenon uniformly after superimposing recording, the grain size of the recording layer should be evenly distributed. Crystallization of most phase change recording layer materials, including Ge-Sb-Te based alloys, is subject to nucleation and growth. Grain growth is caused by the change of the thermodynamic stable state by the atomic diffusion, which occurs when heat energy is injected from the outside.

In the phase change type optical disc of the present invention, the superposition recording process is schematically shown in FIG. A high power laser beam is irradiated onto the recording layer to locally melt the recording layer, and then quenched to form an recording mark by amorphizing the irradiated portion. The recording mark here corresponds to a pit formed along the track of the reproduction-only optical disc. When the thus formed recording mark is irradiated with a laser beam having an output of about 1/3 to 1/2 of the power at the time of recording, a crystalline structure which is an erase area is formed to erase the recorded information. In order to make the shape of the recording mark uniform, it is preferable to use a multi pulse consisting of a series of several short pulses as the recording pulse.

However, the Ge-Sb-Te ternary alloy, which is most widely used as a recording layer material of a phase change type optical disk, has a problem that the jitter value increases when superimposed with a large grain growth tendency. In the actual phase change type disc, the recording mark in the amorphous state is crystallized by the erase pulse. In Fig. 5, the amorphous recording mark formed at the time of initial recording is indicated by a black ellipse, and the other white portions indicate a crystalline non-recording portion. After the first superimposed recording, the original recording mark becomes a crystalline non-recording portion, and a second recording mark is formed in an adjacent portion, where a grain size difference occurs in the non-recording portion crystallized from the first recording mark ( Represented by dots). When two or more overlap recordings occur, not only a large number of sites having different grain shapes occur but also a recording mark is deformed. The nonuniformity of the shape of the recording mark and the nonuniformity of the grain size both contribute to the reduction of the erase rate and the increase of jitter in the overlapping recording.

Another required property is repeat recording durability. This means that the playback signal characteristics do not deteriorate even after repeated recording and erasing. It is known that deterioration due to repetitive recording in a phase change type optical disc is mainly caused by deformation or precipitation of the recording layer due to accumulation of thermal load. As a method for suppressing such deterioration, there is a method of using a dielectric layer having high mechanical strength, introducing an insertion layer, and adding an alloying element to the recording layer.

The recording layer of the present invention is made by adding Ag and N to a Ge-Sb-Te based alloy. The content of Ag and N may vary depending on the structure of the disk, dielectric material, reflective material and the like of each layer, disk format and method. For example, in the case of manufacturing a disc for DVD-RAM, the content of Ag in the final thin film is adjusted within the range of 0.01 to 3 at (atomic)%, and a gas atmosphere having a nitrogen content of 10 to 40 vol (volume)%. Under the recording layer, a recording layer is formed to incorporate nitrogen into the recording layer. The thickness of the recording layer is preferably 100 to 1000 mm.

In one aspect of the present invention, the recording layer can be manufactured as follows. On a substrate that is coated with the recording layer, that is, first put an appropriate amount of the Ag metal, first in the dielectric, the N ₂ in the over the while sputter gas forming the Ge-Sb-Te based alloy layer by sputtering Ar N Ag and N were introduced into the Ge-Sb-Te-based alloy layer by injecting ₂ / Ar in a range of 10 to 40 vol%. At this time, the Ag metal may be in the form of any metal chip such as a rectangular fragment. Ag can also be incorporated into the recording layer through any conventional method, such as including the Ge-Sb-Te-based alloy layer in the starting material during film formation.

In the reflective layer, a metal material is used to improve the heat absorption effect and the light efficiency of the recording layer in amorphous form, and mainly a single element such as Al, Ag, Au, Cu or Cr, for recording sensitivity and oxidation resistance, Alloys with small amounts of Ni, Ti, Si, etc. (e.g. Al-Ti (1.5 wt% Ti), Al alloys such as Al-Cr (Cr 2 at (atomic)%), Ag-Al, Ag-Mg Ag alloys, etc.) are used. The reflective layer may be a single layer or may be in the form of a double layer as shown in FIG. 3.

The thickness of each layer is preferably designed in combination so that the difference in reflectance when the recording layer is in an amorphous state and reflectance when in a crystalline state can be realized by 10% or more in order to exhibit sufficient modulation signal characteristics. In addition, it should be designed to have heating and cooling characteristics such that the recording layer can be kept above the crystallization temperature for a time required for crystallization during erasing while being sufficiently quenched to form a uniform amorphous mark as described above. At this time, the proper heating and cooling characteristics of the disk may be varied depending on the amorphous rate and the crystallization rate of the recording layer. Therefore, the appropriate thickness combination should be set in consideration of their correlation.

When recording simultaneously to land and groove, in order to suppress the phase difference caused by the difference in refractive index between the recorded amorphous mark and the crystal state matrix of the unrecorded portion and the decrease in signal amplitude caused by the interference caused by the phase difference, It is additionally necessary to design each layer thickness so that the reflectance is close to zero or the difference between the signal amplitude in the groove and the signal amplitude in the land caused by these interferences is minimized.

In addition, in order to shorten the production time of material costs in actual production, it is advantageous that the thickness of each layer is as thin as possible so that various properties are in a good range.

UV protective resin etc. are used normally as a protective layer.

In phase change optical disks, the cooling rate is mainly determined by the thermal conductivity and thickness of each layer. Therefore, a good combination of the thickness and the selection of each of these layer materials can be achieved to show good overlapping recording characteristics. As an indicator of the superposition recording characteristic, jitter which means time deviation of the reproduction signal is used, and the lower the jitter, the better the signal characteristic.

In general, in a phase change type optical disk having a Ge-Sb-Te recording layer, the increase in jitter that occurs in the range of 1 to 1,000 times of superimposed recording is caused by a defect in superimposed recording characteristics, and jitter occurs at 10,000 or more times. The increase of may be due to the poor repeat recording durability.

The present invention will be described in detail with reference to Examples, but the present invention is not limited only to the following Examples.

Example 1

By injection molding using a stamper, a 0.6 mm thick polycarbonate disk substrate was formed in which a spiral groove having a depth of 70 nm was formed. At this time, the spacing between the groove of the substrate and the adjacent groove was 1.48 mu m, and the spacing between the groove and the land was 0.74 mu m corresponding to 1/2 thereof.

ZnS-SiO ₂ (8: 2) was RF sputtered on the groove forming surface of the disk substrate to form a first dielectric layer having a thickness of 950 kPa.

Subsequently, an Ag metal chip (10 mm × 10 mm) was placed on the first dielectric layer, and then a Ge—Sb—Te alloy thin film was formed to a thickness of 200 μs by DC sputtering. At this time, the injection amount of the stylized N and Ar ₂ as a sputtering gas ratio N ₂ / Ar in the recording layer is mixed with nitrogen to ensure that the 35 vol%, was the content of Ag is 1.5 at%.

Subsequently, ZnS-SiO ₂ (8: 2) was again RF sputtered to form a second dielectric layer with a thickness of 120 mW, and then an AlCr first reflection layer was formed with a thickness of 500 mW. Thereafter, Cu was sputtered on top of the first reflective layer to form a second reflective layer with a thickness of 400 GPa. As a protective layer, after spin-coating UV curable resin (brand name: SD17: DIC), the board | substrate was mounted and it hardened | cured.

In order to initialize the phase change type optical disc, the recording layer was crystallized by irradiating a semiconductor laser beam (wavelength: 830 nm) while rotating the disc while rotating the disc.

Comparative Example 1

The same procedure as in Example 1 was conducted except that Ag was not added.

Comparative Example 2

The same procedure as in Example 1 was carried out except that the Ag content was 3.2 at%.

Comparative Example 3

The same procedure as in Example 1 was carried out except that N was not added.

The characteristics of the optical disc were evaluated as follows.

(1) recording characteristics

The optical disc was mounted on a DVD-RAM-specific dynamic evaluator (manufacturer: Nakamichi, model name: OMS-2000), and then rotated at 6.0 m / s and mounted on a track of 3T to 14T (T _w = 34.27㎱). After recording a signal of a random pattern, the jitter value of the reproduction signal with respect to a certain clock was evaluated. Jitter refers to the signal deviation of the reproduction signal, the lower the better the signal characteristic.

The method of setting the overlapping proxy write power P _p and the erase power P _b is as follows. That is, after superimposing and recording the optical disc 10 times while changing the power value, the power value representing the double jitter value of about 5.0 ns is multiplied by 1.2 as the provisional recording power (P _p 1), and the recording power is fixed again. After the jitter was measured by changing the erase power, the power range representing the jitter value of about 5.0 ns or less was determined, and the median value was selected as the erase power. Then, under the erase power, the final recording power was selected again through the same process as the provisional recording power described above. At this time, the jitter value was measured after overlapping recording for a specific number of times while examining the laser pulses of the selected recording and erasing power.

At this time, the signal for jitter evaluation is synchronized, and the shape of the laser pulse for recording is shown in FIG. 6. Synchronization conditions and the shape of the laser pulses conformed to the DVD-RAM Specification for Rewritable Disc version 1. The wavelength of the light source used for pickup of the dynamics evaluator was 680 nm, and the NA of the objective lens was 0.60.

(2) Evaluation of the erasure rate

After recording the 3T single signal at the same power as the jitter characteristic evaluation, the 11T signal is overwritten again to reduce the carrier level of the 3T signal ((carrier level of the 3T signal before the 11T signal is overwritten)-(the residual 3T signal after the 11T signal is overwritten). Carrier level) is measured with a spectral analyzer (in dB).

7 is a graph showing the change in the erase ratio according to the N ₂ / Ar injection ratio, Figure 8 is a graph showing a change in signal amplitude according to the N ₂ / Ar injection ratio.

As can be seen in Figure 7 can be seen that when the ratio of N ₂ / Ar injected during the deposition is approximately 10% or more, the erasure rate increase phenomenon is apparent. It was confirmed by transmission electron microscopy (TEM) observation that the improvement of the superposition recording characteristic is caused by the grain growth inhibition effect by the introduction of N ₂ into the recording layer. However, as shown in FIG. 8, the signal amplitude decreases as the N ₂ / Ar injection ratio increases. Therefore, it is desirable to limit the removal rate to within 40% of the N ₂ / Ar injection ratio, which is almost saturated.

According to the recording characteristic evaluation method, the change of superposition recording jitter values of the phase change type optical discs manufactured in Examples and Comparative Examples was measured, and the results are shown in Table 1 below.

Ag content (at%) N ₂ / Ar (% by volume) Fujitsu 10% overlapping record 100% overlapping record jitter growth rate (%) Judgment Example 1 1.5 35 8 16 ○ Comparative Example 1 0 35 2 - × Comparative Example 2 3.2 35 13 16 × Comparative Example 3 1.5 0 32 24 × Criteria: Jitter growth rate after 10 overlapping records (%) 〈10% (Nested property) Jitter growth rate after 100,000 overlapping records (%) 〈50% (Repeat recording endurance)

As can be seen from the results of Table 1, the phase change type optical disc of the present invention according to Example 1 had excellent superposition recording characteristics and repeat recording durability.

On the other hand, in the case of Comparative Example 1 without Ag, the repeat recording durability was poor at 50% or more, and in Comparative Example 3 without N ₂ , the overlapping recording property was significantly poor at about 32%. On the other hand, even when the Ag content was excessive as in Comparative Example 2, the superposition recording characteristics were poor. This can also be seen from FIG. 9, in the case of Example 1 (○), the jitter value was kept low, whereas in the case of Comparative Example 1 (×), the jitter value was increased rapidly. In this case, the initial jitter value is very large.

As can be seen from the above results, by adding Ag and N to the GE-Sb-Te alloy reflective layer at the same time, a phase change type optical disc having excellent repeat recording durability and superimposed recording characteristics can be obtained.

The phase change type optical disc of the present invention exhibits excellent superposition recording characteristics and repeatable recording durability, and thus is suitable as a slow ring type optical disc.

Claims (2)

A phase change optical disc comprising a first dielectric layer, a Ge-Sb-Te based recording layer, a second dielectric layer, and a reflective layer sequentially formed on a transparent substrate, wherein the recording layer comprises Ag and N Fluorescent disk. The method of claim 1, A phase change type optical disc, wherein the recording layer is formed in a gas atmosphere having an Ag content of 0.1 to 3 atomic% and a nitrogen content of 10 to 40% by volume.