JPH0792931B2 - Optical recording / reproducing method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for recording / reproducing information on / from an optical information recording member at high speed and high density by using light, heat or the like.

2. Description of the Related Art In recent years, an optical disk using a laser beam has been attracting attention as the amount of information increases, the speed of recording and reproducing increases, and the density increases. There are two types of optical discs, a write-once type that can be recorded only once and a rewritable type that can erase a recorded signal and can be used many times. Some write-once type optical discs are played back with a recording signal in a perforated state, and others are played back by generating irregularities. There are attempts to use chalcogenides as rewritable ones, and Te-Ge and other Te
It is well known that As, S, Si, Se, Sb, Bi, etc. are added.

On the other hand, a recording medium using a composition composed of Te, Ge, Se and Au as a recording layer has been proposed. This material has excellent heat resistance and moisture resistance, high sensitivity, and good repeatability for recording and erasing.

Problems to be Solved by the Invention The materials of the system consisting of Te, Ge, Se and Au have the above-mentioned advantages, but have the following problems.

That is, recorded state (amorphous state) and erased state (crystalline state)
That is, the difference in the optical characteristics of is small and the signal amplitude at the time of reproduction is small.

The optical characteristics of a thin film are determined by the refractive index n, extinction coefficient k, and film thickness of the material, but the change in refractive index n is small in the Te, Ge, Se, Au system, and changes when reading the optical state as reflectance. The amount is small.

Means for Solving Problems A first transparent thin layer is provided on a transparent substrate, a Te, Ge, Se, Au-based recording thin film is provided on this thin layer, and a second transparent thin film is further provided thereon. Information is recorded by changing the optical characteristics of the recording thin film using a recording medium with a transparent thin layer, and the light of wavelength λ (nm) emitted from the light source is irradiated onto this to detect the reflected light. To detect the signal and reproduce the signal.

Here, the thickness t ₂ of the recording thin film is in the range of 70t ₂ 140 nm, and the thickness t ₁ of the first transparent thin layer is n ₁ as the refractive index. The range is. However, t ₁₀ and i are 0 or a positive integer.

Effect According to the present invention, the reflectance of the recording medium with respect to the reproduction light can be relatively reduced and the reflectance difference between the recording state and the erasing state can be increased, and the SN ratio of the reproduction signal can be comprehensively increased.

Practical Example As shown in FIG. 1, a transparent thin layer is provided on a transparent substrate 1, and a Te, Ge, Se, Au-based recording thin film 3 is provided thereon. Further, a second transparent thin layer 4 is provided thereon, and the one covered with a protective substrate 5 is used as a recording medium. Light from the light source 6 is applied to this medium, and the reflected light from the medium is detected by the photodetector 8 via the heme splitter 7.

The Te, Ge, Se, Au-based materials used in the present invention will be described below. FIG. 2 shows the range in which a crystalline / amorphous phase transition is possible in this system and which can be used as an optical recording medium.
The figure is composed of Te, Ge and Se, but the Au concentration is 5 to 40 at% with respect to the composition of Te, Ge and Se shown in FIG. (Au
The concentration corresponds to m in the case of (Te _x Ge _y Se _z ) _m Au _m , where x + y + z = 100). In FIG. 2, each point has the following composition.

A ₁ point (Te ₉₀ Ge ₅ Se ₅ ) B ₁ point (Te ₆₀ Ge ₅ Se ₃₅ ) C ₁ point (Te ₄₀ Ge ₂₅ Se ₃₅ ) D ₁ point (Te ₄₀ Ge ₄₀ Se ₂₀ ) E ₁ point (Te ₅₅ Ge ₄₀ Se ₅₎ range surrounded by _{a 1 -B 1 -C 1 -D 1} -E 1 is a usable range. Further the transition temperature from amorphous to crystalline, the range a and Au concentration practical range is surrounded by _{A 2 -B 2 -C 2 -D 2} -E 2 from speed of crystallization of 10～35At% Area.

In this case _{A 2 Te 83 Ge 7 Se 10} B 2 Te 63 Ge 7 Se 30 C 2 Te 45 Ge 25 Se 30 D 2 Te 45 Ge 35 Se 20 E 2 Te 55 Ge 35 Se 10 further Figure 2, A ₃ -B ₃ -C ₃ -D ₃ area Au concentration of 10～25At% in range surrounded by -E ₃ is a high and stable material composition recording sensitivity.

Here, it is A ₃ Te ₇₅ Ge ₁₀ Se ₁₅ B ₃ Te ₆₅ Ge ₁₀ Se ₂₅ C ₃ Te ₅₀ Ge ₂₅ Se ₂₅ D ₃ Te ₅₀ Ge ₃₀ Se ₂₀ E ₃ Te ₅₅ Ge ₃₀ Se ₁₅ .

Above all, (Te _80-p Ge _p Se ₂₀ ) _100-m Au _m 10p25,15m
The 25at% region is the most sensitive and stable composition region.

Next, a method for producing the optical information recording member used in the present invention will be described.

FIG. 3 is a schematic view of a cross section of an optical disc formed by using the recording layer of the present invention. In the figure, 1 and 5 represent substrates, and the materials are polycarbonate, acrylic resin,
It is possible to use a transparent substrate such as glass or polyester. 2 and 4 are transparent thin layers, and various oxides, sulfides, and carbides can be used. The layers 2 and 4 prevent thermal deterioration of the base material due to repeated recording / erasing of the recording film 3, and further protect the recording film 3 from humidity. Therefore, the material of the transparent layers 2 and 4 is determined from the above viewpoint. The recording film 3 is formed by vapor deposition, sputtering or the like. When vapor deposition is used, it is preferable to use a 4-source vapor deposition machine capable of vapor depositing each composition independently, because a uniform film can be formed.

Next, the optical characteristics of the Te, Ge, Se, Au system will be described.

The optical constants of the Te, Ge, Se, and Au thin films were λ = 780 to 830 nm in the amorphous state with the refractive index n _a 4.2 extinction coefficient k _a 1.3 crystal 〃 〃 n _c 4.6 〃 k _c 2.2. This value was deposited on a glass substrate (Te ₆₀ Ge
₂₀ Se ₂₀ ) ₈₀ Au ₂₀ Derived from the transmittance and reflectance of a 100 nm thin film. The amorphous state is the measured value in the as depo state, and the crystalline state is the measured value in the state annealed in nitrogen gas at 300 ° C. for 5 minutes. When the same measurement was performed in other composition regions, in FIG. 2, A ₁ -B ₁ -C ₁ -D ₁ -E ₁ and an Au concentration of 5 to 5 were obtained.
At 40 at%, the range was n _a 4.0 to 4.5 k _a 1.1 to 1.6 n _c 4.5 to 5.0 k _c 2.0 to 2.8.

Normally, a phase-transition type optical recording medium reproduces a signal by irradiating a laser beam on the medium and detecting a change in its reflected light or a change in transmitted light, but detects a reflected light. The method is advantageous because the optical system becomes compact, and it is the mainstream method. For signal reproduction by reflectance, the difference in reflectance R _c of the unrecorded state or erased state and the reflectance R _a recorded state (amorphous state) (crystalline state), as R _c -R _a, is large signal The swing becomes large. Further, the noise of the medium is reproduced in proportion to the reflectances R _a and R _c . Therefore, the signal-to-noise ratio can be expressed by the signal contrast defined below.

The reflectance of the recording medium used in the present invention is treated as a problem of interference of the multilayer thin film, and the reflectance is determined by the thickness of each phase, the refractive index, and the extinction coefficient. The method of obtaining the total reflectance from the constants of each layer is described in detail in a general textbook of optics, for example, Hiroshi Kubota "Optics" Iwanami Shoten (P200-).

As shown in FIG. 3, the recording medium used in the present invention comprises five layers of a substrate, a first transparent thin layer, a recording film, a second transparent thin layer and a protective substrate.

The refractive index of the glass and resin used as the base material is 1.4 to 1.
6, absorption is 0.

The refractive index of the transparent thin layer is an intermediate value between the refractive index of the base material and the refractive index of the recording film in order to reduce the reflectance of the medium, R _a , R _c , and the difference between the two is large. Is desirable. Therefore, the refractive index of the transparent thin layer can be 1.7 to 4.0, and is preferably about 2 to 3.5. Examples of oxides such as GeO ₂ , carbides such as Sic and Tic, chalcogenides such as ZnS and ZnTe, and nitrides such as SiN and BN are those that satisfy this condition and have the above-mentioned heat resistance and moisture resistance. Can be used.

First transparent layer using optical constants of Te, Ge, Se and Au thin films,
The calculation results of the signal contrast SC when the thicknesses of the three layers of the recording film and the second transparent layer are changed are shown in FIGS. 4A to 4H. The matrix method was used for the calculation. In each figure, the recording film thickness is constant, and the vertical axis shows the thickness of the first transparent layer and the horizontal axis shows the thickness of the second transparent layer by the product of the refractive index n and the thickness t, that is, the optical length. Refractive index of substrate used for calculation 1.5, refractive index of transparent thin layer
2.4, the wavelength is 830 nm.

In the figure, the transparent thin layer shows the range of 0 to λ / 2, because the thin layer without absorption has the same effect in the period of λ / 2. Therefore, for the optical length shown here ± λ / 2
The same signal contrast is provided even at a thickness corresponding to an optical length of i (i is 0 or a positive integer).

Generally speaking, in light interference, light intensity and phase can be considered separately. The phase is determined by the optical length,
The intensity is determined by the reflectance of the interface, that is, the refractive index on both sides of the interface. Therefore, when a transparent thin layer having a refractive index different from that calculated here is considered, the effect of the phase is the same when considered in terms of the optical length, and the effect of the reflectance at the interface is different. However, the effect is small in the constant range used here, which is a range that can be discussed with the calculation result shown in FIG.

From FIG. 4, Table 1 shows the range where the signal contrast is 50% or more for each film thickness. From Table 1, the thickness t _{2 of the} recording film is 70 t ₂ 140 nm and the optical length n ₁ t _{1 of} the first transparent thin layer is It can be said that the signal contrast is large in the range. Further, Table 2 shows the range where the signal contrast is 80% or more. From this, the thickness t _{2 of the} recording film is 80 t ₂ 110 nm and the optical length n ₁ t _{1 of} the first transparent layer is The optical length n ₃ t ₃ of the second transparent layer is The range of is a region where the signal contrast is particularly large.

Example 1 PMMA resin (n = 1.49) as the base material and the protective base material, the first and second
ZnS deposited film (n = 2.4) on the transparent layer and (Te ₆₀ Ge ₂₀
Se ₂₀ ) ₈₀ Au ₂₀ is used to construct a disk with a diameter of 200 mm, and a semiconductor laser with a wavelength of 830 nm is used with an NA 0.5 objective lens to achieve a diameter of 0.9 μ.
Recording and reproduction were performed by focusing on m and converging on the recording film surface. Further, the signal was erased by irradiating a semiconductor laser having a wavelength of 780 nm with the same objective lens to narrow it down to an ellipse of 10 μm in the rotation direction of the disk.

Table 3 shows the results of the recording / reproducing experiment in which the recording film thickness and the transparent layer thickness were changed.

The CNR shown here is the CNR of a reproduction signal when recording is performed at a single frequency of 2 MHz in a state of being erased in advance at a position of φ120 on a disc rotating at 900 rpm. The recording and erasing power was adjusted so that the same energy was absorbed by previously calculating the absorption coefficient for the semiconductor laser light used by each disk. 7mW when recording, 16mW when erasing,
Absorbed on the membrane. The reproduction power is 0.9mW. CNR was measured with a bandwidth of 30 KHz.

Table 3 shows that CNR of 45 dB is obtained when the recording film thickness is 50 to 130 nm and the optical length of the first transparent layer is 9 / 64λ to λ / 2.

Effects of the Invention According to the optical recording / reproducing method of the present invention, signal recording / reproducing with a high SN ratio can be performed by using a material having a relatively small change in optical constant.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an optical recording / reproducing method in one embodiment of the present invention, FIG. 2 is a composition diagram showing a composition range of a recording film used in the present invention, and FIG. 3 is a recording medium used in the present invention. FIGS. 4 (a) to 4 (h) are schematic views showing the cross section, and graphs showing the relationship between the calculated values of the signal contrast in each recording film thickness and the optical tables of the first and second transparent layers. is there. 1 ... Substrate, 2,4 ... Transparent thin layer, 5 ... Protective substrate, 6 ...
... light source, 7 ... beam splitter, 8 ... photodetector.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G11B 7/24 A 7215-5D

Claims (3)

[Claims] _1. A thin layer having a _first transparent refractive index n ₁ is provided on a transparent substrate, and a layer of a recording thin film whose optical characteristics are changed by light irradiation is provided on the thin layer. Second on this
A method for recording information by changing the optical characteristics of the recording thin film on an optical recording medium provided with a thin layer having a transparent refractive index n ₃ and optically reproducing the information with light having a wavelength λ (nm). The recording thin film is formed of a material composed of Te, Ge, Se and Au, and the atomic ratio of Te, Ge and Se is A ₁ (Te ₉₀ Ge ₅ Se ₅ ), B ₁ ( Te ₆₀ Ge ₅ Se ₃₅ ) C ₁ (Te ₄₀ Ge ₂₅ Se ₃₅ ), D ₁ (Te ₄₀ Ge ₄₀ Se ₂₀ ) E ₁ (Te ₅₅ Ge ₄₀ Se ₅ ).
Of a thin film having a concentration (at%) of 5 ≦ m ≦ 40 at% in the entire composition (Te _x Ge _y Se _z ) _100-m Au _m and a thickness t ₂
Is in the range of 70 ≦ t ₂ ≦ 140 mn, and the thickness t _{1 of} the first transparent thin layer is relative to the wavelength λ of the light used for reproduction. The optical recording / reproducing method is characterized in that 2. The thickness t _{2 of the} recording thin film is in the range of 80 ≦ t ₂ ≦ 110 mn, and the thickness t _{1 of} the first transparent thin layer is The thickness t _{3 of} the second transparent lamina is The optical recording / reproducing method according to claim 1, wherein the optical recording / reproducing method is in the range. 3. The optical recording / reproducing method according to claim ₁ , wherein the refractive indices n ₁ and n _{3 of} the first and second transparent thin layers are 1.7 to 4.0.