KR19990069853A - Phase change optical disk - Google Patents

Abstract

The present invention relates to a phase change optical disk in which a first dielectric layer, a recording layer, a second dielectric layer, and a reflective layer are sequentially formed on a transparent substrate, wherein the thickness of the first dielectric layer is 180 to 220 nm and the thickness of the recording layer. Is 10 to 25 nm, the thickness of the second dielectric layer is 5 to 25 nm, and the thickness of the reflective layer is 50 to 150 nm. The phase change optical disc of the present invention has excellent carrier level, noise level, modulation amplitude, and recording mark shape symmetry characteristics by controlling the thickness of each layer within a predetermined range, thereby obtaining stable signal quality.

Description

Phase change optical disk

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

A general optical disc has a structure as shown in FIG. Here, when the information area of the optical disc is roughly shown, a denotes a recording start surface, b denotes a program surface and c denotes a recording end surface.

2 is an enlarged cross-sectional view of the line AA ′ of FIG. 1A. Referring to this, a pit 22, which is a mark for digital information, is formed on the track 21 as if it is a Morse code, and a reflective layer 23 and a protective layer 24 for reflecting a laser beam are formed thereon. It is.

The process of manufacturing the optical disk as described above is largely composed of the step of manufacturing a stamper (stamper) and the step of replicating the optical disk in large quantities by the manufactured stamper. The stamper is obtained by first producing a glass master having the same format as the optical disc, and then manufacturing a mold for the obtained glass master.

Optical discs can be classified into a read-only type that can only read and write once, and a recordable write once type, and a slow ring type that can be erased and rewritten after recording, depending on whether information can be recorded and erased. Here, the slow-circulating optical disk includes an optical magnetic disk using a magneto-optical material as a recording layer material, a phase change optical disk using a phase change material, and the like.

A phase change optical disc uses the principle that information is recorded and erased by controlling a heating temperature and a cooling rate of a material forming a recording layer to induce a reversible change between a crystalline structure and an amorphous structure. Herein, the formation process of the crystalline structure and the amorphous structure will be described in detail. In other words, when the recording layer is locally melted using a laser beam and then quenched, an amorphous structure which is a recording mark is formed, and if it is maintained for a proper time below the melting point above the crystallization temperature, a crystalline structure which is an erase region is formed.

A phase change optical disc generally has a structure in which a first dielectric layer, a recording layer, a second dielectric layer, a reflective layer, and a protective layer are sequentially stacked on a transparent substrate. Here, in the recording layer, a track on which a recording signal is made is formed. When the laser beam is irradiated, the recording layer is changed from crystalline to amorphous or from amorphous to crystalline so that recording and erasing are repeated.

Such a phase change optical disk is easier to superimpose recording than an optical disk using other materials. In addition, shortening of the wavelength of the laser light which increases the recording density of the optical disc is possible, and the optical system of the drive drive is attracting attention as a next generation high density optical disc which can be recorded simply.

In a phase change optical disc, recording and erasing of information is related to the intensity of the laser beam to be irradiated. For example, when a high power laser beam is irradiated to the recording layer, the recording layer is locally melted, and then quenched, the irradiated portion is amorphous to form a recording mark. 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, the recording mark portion is crystallized and the recorded information is erased.

The phase change optical disk has the advantage of being able to repeatedly record the information since the recording and erasing of information are performed by forming and erasing the recording mark as described above. When superimposing recording on a phase change optical disc, it is common for phases at positions where recording marks are formed to be different from each other.

The reproduction signal of the phase change optical disc recorded as described above is generated by the difference between the reflectance of the amorphous state formed by the laser beam and the reflectance of the crystalline state which is an unrecorded portion. Carrier-to-noise ratio (CNR) and modulated amplitude characteristics, which are the main characteristics related to the reproduction signal, are determined by reflectance in the amorphous state and the crystalline state.

However, the reflectance of the amorphous state and the crystalline state varies depending on the thickness of each layer constituting the optical disc. In addition, the left and right shape symmetry of the recording marks is also determined by the amount of heat accumulation by the combination of the thicknesses of the layers.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a phase change optical disc having improved signal quality by adjusting the thickness of each layer constituting the optical disc within a predetermined range to improve symmetry characteristics of CNR, modulation amplitude, and recording mark shape.

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

FIG. 2 is an enlarged cross-sectional view taken along line AA ′ of FIG. 1;

3 is a diagram showing a recording signal level,

4 is a view showing a change in reflectance according to the thickness of the first dielectric layer,

5 is a diagram showing a change in reflectance according to a recording layer thickness;

6 is a view showing a change in reflectance according to the thickness of the second dielectric layer,

7 is a view showing a change in reflectance according to the thickness of the reflective layer,

8 is a view showing a thickness combination range of a first dielectric layer and a second dielectric layer satisfying good reflectance characteristics;

9 is a view for explaining the change in the oscilloscope signal waveform according to the recording mark shape symmetry due to thermal accumulation,

10 is a view schematically showing an apparatus for evaluating dynamic characteristics of a phase change optical disk used in the present invention.

11 is a view showing a laser power pattern used in the present invention.

<Explanation of symbols for the main parts of the drawings>

101 ... Record Signal Generator

102. Write and erase power control circuit

103 ... laser diode driving circuit

104 ... laser diode

105 ... spindle motor

In order to achieve the above object, in the present invention, in a phase change optical disk in which a first dielectric layer, a recording layer, a second dielectric layer, and a reflective layer are sequentially formed on a transparent substrate,

The thickness of the first dielectric layer is 180 to 220 nm,

The recording layer has a thickness of 10 to 25 nm,

The second dielectric layer has a thickness of 5 to 25 nm,

It provides a phase change optical disk, characterized in that the thickness of the reflective layer is 50 to 150nm.

CNR, one of the signal characteristics of a phase change optical disk, is defined as a difference between a carrier level and a noise level. In order to be used as a product, a CNR is required to have a value of 50 dB or more. In addition, in order to be driven by the drive, the level of the reproduction signal must be maintained above an appropriate level. For this purpose, the modulation amplitude characteristic, i.e., l14 / l14H, must be good. The level values of l14 and l14H are as shown in FIG. 3.

The DVD-RAM specification, a representative product of high density phase change optical disks, requires that the modulation amplitude characteristic be maintained at 0.43 or more. Accordingly, the phase change optical disk according to the present invention has good signal quality by adjusting the thickness of each layer within a predetermined range to satisfy a carrier level of -15 dB or more, a noise level of -65 dB or more, and a modulation amplitude of 0.43 or more.

Determinants of the carrier level in the phase change optical disk will be described as follows.

When reproducing the recording of the optical disc, the optical signal intensity detected by the photodetector is expressed by the equation (1).

Signal strength = 10logP

However, P represents photodetector generated power, and when P is watt or milli watt, the signal strength is expressed in dBW or dBm.

When relative amounts of generated light of two kinds of signals are compared and expressed on a log scale of P ₁ / P ₂ , the unit of signal strength is expressed in dB.

Also, since the generated power P of the photodetector is V ² / R or RI ² (where V is voltage, R is resistance, and I is current), when the resistance is constant, the photodetector's generated power ratio is represented by Equation 2 And can be expressed as a current ratio or a voltage ratio as shown in FIG.

10log (P ₁ / P ₂ ) = 20log (I ₁ / I ₂ )

10log (P ₁ / P ₂ ) = 20log (V ₁ / V ₂ )

The CNR is expressed by the following expression 4 as the ratio of the signal strength at the carrier frequency f _o and the average noise level.

CNR = 10log (P _{s (fo)} / P _{N (av)} ) = 20log (I _{S (fo)} / I _{N (av)} ) = 20logI _{S (fo)} -20logI _{N (av)}

However, P _{s (fo)} (or I _{s (fo)} ) represents the power (or current) generated by the photodetector by the signal of frequency f _o , and P _{N (av)} (or I _{N (av)} ) Power (or current) generated by the photodetector by a signal of any frequency except the frequency f _o .

The current value I generated by the photodetector during optical disc reproduction may be represented by Equation 5 below.

I = η _s * P _o * Rf (η _s denotes the sensitivity coefficient of the photodetector, P _o denotes the reproduction power, and R f denotes the reflectance.

Therefore, the phase change optical disk can be developed as follows.

I _{S (fo)} = η _s P _o {Rf (c) -Rf (a)}

Carrier Level = 20log [η _s P _o {Rf (c) -Rf (a)}] = 20logη _s P _o + 20log {Rf (c) -Rf (a)}

However, Rf (c) represents the amount of reflected light when the laser spot passes through the unrecorded crystal portion, and Rf (a) represents the amount of reflected light when the laser spot passes through the amorphous recording mark portion.

Since η _s and P _o vary depending on the characteristics of the drive or the evaluation device, the carrier level can be developed by Equation 6 below.

Carrier level = K + 20log {Rf (c) -Rf (a)}, where K is a constant

In the above formula, Rf (c) and Rf (a) are affected by the size of the laser spot, the groove shape including the track pitch, the width and the length of the recording mark, etc., but {Rf (c) -Rf (a)} It mainly depends on the reflectance difference (Rc-Ra) of the crystalline state and the amorphous state, and has a linear increase or monotone increase relationship with each other.

On the other hand, the disk reproducing signal factors affecting the noise component include noise generated in the drive electronic circuit, short noise in the photodetector, medium noise, and the like. Among them, the noise generated from the drive electronics and the short noise from the photodetector are related to the drive itself, so the disk design does not need special consideration.

Although the factors affecting the medium noise have not been clearly identified, the main noise source of the phase change optical disk is known to be caused by the variation in the amount of reflected light. This variation in the amount of reflected light is greatly influenced by the reflectance variation of the unrecorded portion resulting from the irregularities in the shape of the recording mark and the irregularities in the crystal grain size, all of which depend on the reflectance size of the crystal state.

One of the main signal characteristics of an optical disc, the modulation amplitude is defined as the ratio of l14 to l14H (where l14 is defined as the difference between l14H and l14L). Characteristics are required. At this time, l14H is determined by the generated current value of the photodetector by the amount of reflected light in the unrecorded portion, and l14L is determined by the generated current value of the photodetector by the amount of reflected light in the recorded portion. Here, recalling that the unrecorded portion is in the crystalline state and the recording portion is in the amorphous state, it can be seen that each of them depends on the reflectance of the crystalline state and the reflectance of the amorphous state.

In order to have good CNR and modulation amplitude characteristics, the reflectance Rc of the crystalline state should be about 23% or less, the difference between the crystalline reflectance Rc and the amorphous state reflectance Ra of about 11% or more, and (Rc-Ra) / Ra of about 0.65 or more. do. In this case, reflectance characteristics of the crystalline state and the amorphous state vary depending on the thickness of each layer.

4-7 illustrate the change in reflectance according to the thickness of the first dielectric layer, the recording layer, the second dielectric layer, and the reflective layer. The wavelength of the laser beam used in the simulation is 680 nm, and the material and optical constant of each layer are as shown in Table 1 below.

division material Complex refractive index (680 nm wavelength) Reflective layer Al-Cr ^* 2.06 + 6.83i First dielectric layer ZnS-SiO ₂ (8: 2) 2.08 Second dielectric layer Recording layer Ge ₂ Sb ₂ Te ₅ (Amorphous) 4.31 + 1.69i Ge ₂ Sb ₂ Te ₅ (crystalline) 4.69 + 3.79i Board Polycarbonate 1.55

* Cr content: 2at.% Based on Al.

In the phase change optical disk having the substrate, the recording layer, and the first and second dielectric layers composed of the materials of Table 1, the preferred thickness range of each layer capable of satisfying the above-described reflectance characteristics was investigated through simulation. As a result, the thickness of the recording layer is 10 to 20 nm, the thickness of the reflective layer is 50 nm or more, the thickness of the first dielectric layer is 100 to 130 nm, the thickness of the second dielectric layer is 15 to 60 nm, and the thickness of the recording layer. Is 10 to 20 nm, the thickness of the reflective layer is 50 nm or more, the thickness of the first dielectric layer is 180 to 210 nm, and the thickness of the second dielectric layer is 5 to 35 nm.

FIG. 8 shows the thickness ranges of the first dielectric layer and the second dielectric satisfying the reflectance characteristics described above when the thickness of the reflective layer is 90 nm and the thickness of the recording layer is 15 nm.

On the other hand, the left and right symmetry of the recording mark shape formed at the time of recording depends on the amount of heat accumulation according to the thickness of each layer except for the first dielectric layer.

9 is a view for explaining the change in the symmetry of the shape of the recording mark due to thermal accumulation and the change in signal characteristics accordingly. Referring to this, (a) is a case where the heat accumulation is insufficient, (b) is a case where the heat accumulation is suitable, (c) is for a case where the heat accumulation is excessive.

Tables 2 and 3 show changes in the left and right symmetry characteristics of the recording mark shape according to the thickness change of the recording layer and the second dielectric layer and the thickness change of the reflective layer.

Record Mark Shape Symmetry Thickness of recording layer (nm) 15 20 25 30 35 40 Thickness of the second dielectric layer / nm 10 ○ ○ ○ × × × 15 ○ ○ △ × × × 20 ○ △ △ × × × 25 △ △ × × × × 30 × × × × × × 40 × × × × × ×

Reflective layer thickness (nm) 10 20 30 50 70 120 150 200 250 Record Mark Shape Symmetry × × × △ ○ ○ ○ △ ×

In the above Tables 2 and 3, " ○ " means that the recording mark shape symmetry is good, " △ " is relatively good, and " x "

Tables 2 and 3 show that recording mark symmetry is good when the thickness of the recording layer is 25 nm or less, the thickness of the second dielectric layer is 25 nm or less, and the thickness of the reflective layer is in the range of 50 to 150 nm.

In summary, in a phase change optical disk having a recording layer made of Ge-Sb-Te alloy, a dielectric layer made of ZnS-SiO ₂ , and a reflective layer made of aluminum, the thickness of each layer is as follows. It is desirable to set. That is, the thickness of the first dielectric layer is preferably 180 to 220 nm, the thickness of the recording layer is 10 to 25 nm, the thickness of the second dielectric layer is 5 to 25 nm, and the thickness of the reflective layer is 50 to 150 nm.

10 is a block diagram of an apparatus for evaluating recording characteristics of a phase change optical disk according to the present invention.

Referring to this, it consists of a write signal generator 101, a write and erase power control circuit 102, a laser diode drive circuit 103, a laser diode 104 and a spindle motor 105 for rotating the optical disk 107. It is.

11 shows the form of a write pulse used in the present invention. Referring to this, the write pulse of the present invention is composed of a write power level, an erase power level, and a bias level.

Hereinafter, 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>

An injection molding method using a stamper produced a polycarbonate circular substrate having a thickness of 0.6 mm, a track pitch of 0.74 µm, and a groove depth of 70 nm.

ZnS-SiO ₂ (8: 2) was RF sputtered on the polycarbonate circular substrate to form a first dielectric layer (thickness: 200 nm). Subsequently, Ge ₂ Sb ₂ Te ₅ was sputtered on the first dielectric layer to form a recording layer (thickness: 15 nm), and ZnS-SiO ₂ (8: 2) was RF sputtered to form a second dielectric layer (thickness: 15 nm). ) Was formed.

An Al—Cr alloy was DC sputtered on the second dielectric layer to form a reflective layer (thickness: 90 nm). After spin-coating a UV curable resin on the reflective layer, a phase-change optical disk (thickness: 1.2 mm) was completed by irradiating UV with a polycarbonate disc substrate (thickness: 0.6 mm) having no groove formed thereon. It was.

The optical disk was initialized by crystallizing the recording layer by irradiating a laser beam while rotating the completed phase change optical disk.

The initialized phase change optical disk was mounted in the recording characteristic evaluation apparatus of FIG. 10, and then rotated at 6 m / s. At this time, the laser pulse used for recording was the same as in Fig. 11, the wavelength of the light source was 680 nm, and the NA of the objective lens was 0.6.

Then, the signal generated from the recording mark was input to the spectrum analyzer and analyzed to measure the carrier level and the noise level. The left and right symmetry of the level of each signal and the shape of the recording mark were evaluated with an oscilloscope.

<Example 2>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 210 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 90 nm.

<Example 3>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 220 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 90 nm.

<Example 4>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 14 nm, the thickness of the second dielectric layer was 10 nm, and the thickness of the reflective layer was 90 nm.

Example 5

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 25 nm, and the thickness of the reflective layer was 90 nm.

<Example 6>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 70 nm.

<Example 7>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 120 nm.

<Example 8>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 150 nm.

Comparative Example 1

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 120 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 90 nm.

Comparative Example 2

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 140 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 90 nm.

Comparative Example 3

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 160 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 90 nm.

<Comparative Example 4>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 180 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 90 nm.

Comparative Example 5

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 240 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 90 nm.

Comparative Example 6

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 5 nm, and the thickness of the reflective layer was 90 nm.

Comparative Example 7

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 27 nm, the thickness of the second dielectric layer was 25 nm, and the thickness of the reflective layer was 90 nm.

<Comparative Example 8>

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 32 nm, the thickness of the second dielectric layer was 20 nm, and the thickness of the reflective layer was 90 nm.

Comparative Example 9

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 40 nm, the thickness of the second dielectric layer was 10 nm, and the thickness of the reflective layer was 90 nm.

Comparative Example 10

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 10 nm.

Comparative Example 11

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 20 nm.

Comparative Example 12

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 40 nm.

Comparative Example 13

The same procedure as in Example 1 was carried out except that the thickness of the first dielectric layer was 200 nm, the thickness of the recording layer was 15 nm, the thickness of the second dielectric layer was 15 nm, and the thickness of the reflective layer was 200 nm.

Carrier level (CL), noise level (NL), modulated amplitude (MA) and recording mark symmetry of phase change optical disks prepared according to Examples 1-8 and Comparative Examples 1-13. It was measured and shown in Table 4 below.

division CL (dB) NL (dB) MA Record Mark Symmetry Example 1 -11.3 -65.9 0.683 Good Example 2 -12.8 -66.4 0.663 Good Example 3 -15.4 -66.1 0.653 Good Example 4 -12.5 -65.7 0.545 Good Example 5 -12.2 -65.0 0.430 Good Example 6 -12.8 -67.4 0.641 Good Example 7 -12.5 -66.2 0.633 Good Example 8 -12.9 -66.2 0.609 Good Comparative Example 1 -11.6 -64.4 0.380 Good Comparative Example 2 -10.2 -63.2 0.376 Good Comparative Example 3 -9.5 -62.9 0.406 Good Comparative Example 4 -9.9 -64.0 0.582 Good Comparative Example 5 -17.4 -67.3 0.512 Good Comparative Example 6 -13.1 -64.7 0.415 Bad Comparative Example 7 -12.6 -63.0 0.421 Bad Comparative Example 8 -16.1 -60.7 0.432 Bad Comparative Example 9 -14.7 -57.0 0.327 Bad Comparative Example 10 -12.6 -67.6 0.603 Bad Comparative Example 11 -12.3 -66.6 0.736 Bad Comparative Example 12 -12.8 -66.5 0.659 Bad Comparative Example 13 -12.3 -66.9 0.696 Bad

From Table 4, it can be seen that the phase change optical disc manufactured according to Example 1-8 has a carrier level of -15 dB or more, a noise level of -65 dB or less, a modulation amplitude of 0.43 or more, and good recording mark shape symmetry. there was.

On the other hand, in the phase change optical disc manufactured according to Comparative Examples 1-13, it was found that the symmetry characteristics of the carrier level, noise level, modulation amplitude and recording mark shape could not simultaneously satisfy the above range.

The phase change optical disc of the present invention has excellent carrier level, noise level, modulation amplitude, and recording mark shape symmetry characteristics by adjusting the thickness of each layer within a preferable range, thereby obtaining stable signal quality.

Claims (1)

In a phase change optical disc in which a first dielectric layer, a recording layer, a second dielectric layer, and a reflective layer are sequentially formed on a transparent substrate, The thickness of the first dielectric layer is 180 to 220 nm, The recording layer has a thickness of 10 to 25 nm, The second dielectric layer has a thickness of 5 to 25 nm, The thickness of the reflective layer is a phase change optical disk, characterized in that 50 to 150nm.