JP2005078738A - Optical disk substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a recording method by heat is proposed for manufacturing a ROM disk in an optical disk, but recording accuracy of a fine mark has a limit since a recording power margin is low. <P>SOLUTION: The fine mark is highly accurately recorded utilizing a recrystallization phenomenon by using a phase transition film as a recording film and performing melting recording. In this case, the shape of the shortest recording mark is different from conventional one and the mark longer in a disk radius direction than in a track direction is recorded. After recording, a phase transition mark is converted into a rugged pattern to manufacture an original plate by utilizing difference between etching rates of a crystal part and an amorphous part by reactive ion etching. The recording power margin is increased and jitter of data reproducing signals can be reduced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to optical disc technology, and more particularly to an optical disc substrate.

  Optical discs are roughly classified into read only memory (ROM), write once read memory (WORM), and rewritable memory (Random Access Memory; RAM). Among these, the ROM has a pit corresponding to the recording data formed on the substrate, and the data is read by irradiating the focused laser beam on the pit and detecting the amount of reflected light. In WORM and RAM, a groove (groove groove) corresponding to a track is formed on the substrate, and user data is recorded along the groove. In addition to the grooves, WORM and RAM have pits indicating the disc type information and address information, and the optical disc drive reads the pit information to recognize the disc type and characteristics. To do.

  The disk substrate having the pits and grooves as described above is roughly manufactured by the following process; 1. Apply a photosensitive resist to the glass substrate. 2. The substrate is rotated, the laser beam condensed by the objective lens is incident on the substrate, and the resist is exposed. 3. Develop the substrate so that the photosensitive pattern becomes a concavo-convex pattern. A metal such as Ni is plated, and the melted polycarbonate is poured using it as a master and solidified to produce a substrate. The exposure with laser light is called cutting, and the apparatus is called a cutting apparatus. A series of processes for producing a master disk is called mastering.

  In 2 above, the incident laser beam is DC light when a groove is formed, and pulsed light with appropriate conditions is used when a pit is formed. The conditions are optimized in consideration of the photosensitivity of the resist.

  In order to produce a high-density optical disk, it is necessary to produce small pits and narrow track grooves with high accuracy. For this purpose, it is necessary to reduce the spot size of incident light. The diameter of the focused light spot is proportional to λ / NA, where λ is the wavelength and NA is the numerical aperture of the objective lens. In the specifications currently proposed for the next generation optical disk, the track pitch is the shortest mark length of 0.15 to 0.2 μm, the track pitch is about 0.3 to 0.35 μm, and the disk with a diameter of 120 mm is 20 to 30 GB. With a capacity of In order to produce pits of this size, the wavelength of the cutting device is set to 250 to 270 nm and the NA is set to about 0.9.

  On the other hand, phase change recording is used as a rewritable medium. Phase change recording uses a phenomenon in which a part of the recording film is melted by the heat generated by absorbing the incident laser beam and then becomes amorphous in the process of rapid cooling after being absorbed into the crystalline film. This is a method of recording a mark. This method is used in, for example, CD-RW, DVD-RAM, DVD-RW, and DVD + RW, and recording is performed by a drive as in reproduction. In the next generation optical disk drive, it is considered to use a blue-violet laser having a wavelength of 400 to 410 nm and an objective lens having an NA of 0.65 to 0.85.

  In normal phase change recording, in the cooling process after heat is applied, the mark periphery cools more slowly than the mark center, so it crystallizes without becoming amorphous. This is called recrystallization. Due to this recrystallization, the mark becomes smaller than the melted region, and a minute mark can be recorded. The degree to which the recrystallization occurs is determined by the crystallization rate of the recording film and the heat cooling rate. That is, when the same amount of heat is applied to a medium having the same cooling speed at the same speed in a medium having the same cooling rate, recrystallization occurs in a larger area in a recording film having a high crystallization speed. The crystallization rate is determined by the chemical composition of the recording film. For example, a recording film often used for CD-RW and DVD-RW is AgInSbTe, which has the largest amount of Sb and has a relatively high crystallization speed. The recording film typically used for DVD-RAM is an alloy of GeTe and Sb2Te3, and the crystallization speed is relatively slow. In addition, even with the same GeSbTe, the crystallization speed increases with an increase in the amount of Sb. This is described in, for example, Technical Digest of Optical Data Storage Topical Meeting 2001 pp. 37-39.

  The spot size necessary for recording for the next generation optical disc is about 500 nm for phase change recording, and about 300 nm for cutting. That is, it can be seen that if the same size light spot is used, smaller marks can be recorded in the phase change recording, and the capacity and density of the recorded data can be increased. This difference can be attributed to the difference in the recording mechanism between the two.

  Cutting is a photon mode recording that records a pattern by inducing a photochemical reaction in the resist by exposing the photosensitive resist as described above, whereas phase change recording is a heat that changes the physical properties of the material by heat. Use mode recording.

  In photon mode recording, reactivity is determined by the total number of photons irradiated. That is, the integration of irradiated photons is important. Usually, the minimum mark size is smaller than the spot size to be recorded. Consider a case where such a minimum mark row is recorded. One of the marks is named Mark A. When a mark is recorded on the front, back, left and right of the mark A, a part of the light spot is irradiated on the area of the mark A. The size of will increase. The total exposure amount depends on the front, rear, left, and right mark patterns, and the pattern is random. Therefore, the mark size varies randomly. This causes an improvement in jitter during data reproduction, that is, an improvement in data error rate.

  Focusing on this, a ROM manufacturing technique using phase change recording is described in Technical Digest of Optical Data Storage Topical Meeting 2003 pp. 52-55. In this method, a transition metal oxide exhibiting a crystal-amorphous phase transition is used as a resist, a crystal mark is produced in the amorphous film by heat generated by incidence of laser light, and one of the crystal and the amorphous is melted. A pattern is produced by developing with a developing solution. By this method, a mark having a recording capacity of about 25 GB is successfully produced with high accuracy in a disk having a diameter of 120 mm by an optical system having a wavelength of 400 nm and NA of 0.85.

Technical Digest of Optical Data Storage Topical Meeting 2001 pp. 37-39

Technical Digest of Optical Data Storage Topical Meeting 2003 pp. 52-55

  As described above, the thermal mode recording can achieve a larger capacity of recording data than the photon mode recording, but the general thermal mode recording (simple thermal mode recording) as described above is a normal phase change. Compared to recording, it is difficult to increase the capacity. The reason is that recrystallization is present in normal phase change recording, and accordingly, the mark has a smaller size than the melting region, and fine marks can be recorded. Results of investigating the relationship between recording power and mark length for a phase change film with an extremely slow crystallization speed, a phase change film with a relatively slow crystallization speed, and a phase change film with a relatively fast crystallization speed Is shown in FIG. The horizontal axis represents the ratio between the recording power and the optimum recording power, and the vertical axis represents the difference between the optimum value of the mark length. From this result, it can be seen that in the film having a high crystallization speed, the mark length variation is the smallest with respect to the recording power variation, that is, the recording power margin is the largest. The reason is considered as follows. In the simple thermal mode recording, when the recording laser power changes, the reaction area changes accordingly, and the mark size changes proportionally. In phase change recording, the size of the melting region changes due to fluctuations in the laser power, but the recrystallization region also changes accordingly. Less. If the crystallization speed is high, the power fluctuation and the fluctuation of the recrystallization region are likely to be linked, so that it can be interpreted that the correction effect is larger.

  In the example of mastering using the phase change described in the prior art, the change from amorphous to crystal is used, and since it has not undergone melting, there is no recrystallization. In addition, since a special material is used, the crystallization rate cannot be controlled.

  In the present invention, in order to enable recording of a minute mark by recrystallization, a method of recording a mark by melting and amorphizing the crystal is used. As a result, the recording power margin is increased as shown in FIG. 2, and the jitter of the data reproduction signal can be reduced.

  FIG. 1 shows the crystallization speed dependence of the recorded mark shape. In the figure, (a), (b), and (c) respectively show a case where the crystallization speed is high, a case where the crystallization speed is low, and a case where there is no crystallization. The recording film as shown in (a) in the figure is a DVD-RW, and (b) is a recording film used in a DVD-RAM. (C) is a case of simple thermal mode recording or photon mode recording. When recording a minute mark with a circular light spot, a mark recorded by simple thermal mode recording or photon mode recording is circular or elongated in the spot scanning direction, and therefore xmin ≧ ymin. On the other hand, in a system where recrystallization exists, after a spot passes, recrystallization occurs behind the mark due to heat from the spot that has passed, and a mark as shown in FIGS. Therefore, xmin <ymin. This mechanism is described in detail in, for example, Japan Journal of Applied Physics, Vol. 41, pages 631-635. The mechanism for recording a mark satisfying xmin <ymin can be performed only by the method described in this document as long as the recording film and the resist are not anisotropic, and cannot be performed by photon mode recording or simple thermal mode recording. It is.

AgInSbTe and GeSbTe films are crystalline and amorphous and have different physicochemical properties. Therefore, for example, the etching amount (etching rate) per unit time for reactive ion etching (RIE) differs between crystal and amorphous. For example, when a Ge ₅ Sb ₇₀ Te ₂₅ thin film was etched at a power of 100 W using a gas in which Ar and CHF ₃ were mixed, the etching rate per unit time was 5 nm / min for amorphous and 10 nm / min for crystal. It was. Incidentally, it was 40 nm / min for SiO ₂ . As a result, the phase change mark recorded by the above method can be made into an uneven pit, and a ROM pit satisfying xmin <ymin is produced.

  It is necessary to control the depth of the ROM pit. For example, in a CD-ROM or DVD-ROM, if the wavelength is λ and the refractive index of the substrate is n, λ / 4n. That is, it is about 130 nm for CD-ROM and about 100 nm for DVD-ROM. When the recording film is thickened to that extent, the diffusion of heat generated in the medium is suppressed and recrystallization occurs excessively. In order to prevent this, a concavo-convex pattern having a desired depth can be produced by providing a material having a relatively large RIE etching rate under the recording film and using the amorphous portion remaining by RIE as a mask.

  The recording power margin is increased and the jitter of the data reproduction signal can be reduced.

Here, the case where Ge ₅ Sb ₇₀ Te ₂₅ is used as the recording film will be described. Ge ₅ Sb ₇₀ Te ₂₅ has a high crystallization speed, and the shape of the shortest mark is as shown in FIG.

The media structure is shown in FIG. The medium was produced by forming a film of Ag alloy 302 on a glass substrate 301 at 200 nm, SiO ₂ 303 at 60 nm, Ge ₅ Sb ₇₀ Te ₂₅ 304 at 20 nm, and SiO ₂ 305 at 20 nm by sputtering. Laser light for mark recording is incident from above. The wavelength of the light is 400 nm, and the numerical aperture NA of the objective lens is 0.85.

  1-7 modulation was used as a modulation code for data recording. In this case, if the window width is Tw, the shortest mark is 2 Tw and the longest mark is 8 Tw. FIG. 4 shows a pattern waveform of the output power of the laser used for recording. This waveform is composed of three power levels of recording power Pw, bottom power Pb, and erasing power Pe in units of 0.5 Tw as shown in the figure. An nTw mark is recorded with n-1 pulses, such that a 2Tw mark is one pulse and an 8Tw mark is seven pulses. This recording laser waveform is the same as that used for normal rewritable phase change recording. By using such a waveform, recrystallization is induced, and the shape as shown in FIG. Marks can be recorded.

  Here, the length of the 2Tw mark is 0.15 μm, and the track pitch is 0.32 μm. This makes it possible to record about 25 GB of data on a 120 mm diameter disk. The recording power was set so that Pw / Pe / Pb was 5 mW / 2.5 mW / 0.3 mW on the medium, respectively. The spot scanning speed was 5 m / s.

The process is shown in FIG. FIG. 4A shows the recording film of the medium shown in FIG. 4 crystallized with a phase change medium initialization machine. The medium was irradiated with laser light, and an amorphous mark 506 was recorded on the recording film 504 which is a crystal as shown in FIG. Thereafter, the sample of (b) was etched by RIE using a mixed gas of Ar and CHF ₃ for 219 seconds, whereby a pit with a depth of 50 nm could be produced as shown in (c). The reason why the thickness is 50 nm is as follows. As described in the solution, the RIE etching rate of this film was 5 nm / min for amorphous, 10 nm / min for crystal, and 40 nm / min for SiO ₂ . As a result, the entire SiO ₂ 505 is etched in the first 30 seconds. Next, the phase change film is etched, but the crystal is completely etched in 120 seconds, and at that time, the amorphous portion 506 remains by 10 nm. At this time, the remaining amorphous plays a role of a mask, and SiO ₂ is etched at a high etching rate. When etching is performed for 69 seconds, SiO ₂ is etched by about 46 nm, amorphous is etched by about 6 nm, and 4 nm remains. Therefore, the total pit depth is about 50 nm.
An Ag alloy was sputtered to a thickness of 50 nm on a ROM substrate manufactured using FIG. 5C as a master, and a polycarbonate sheet having a thickness of 0.1 mm was attached. When this disk was measured for optical jitter at a linear velocity of 4.92 m / s using an optical disk evaluation machine having a wavelength of 400 nm and an objective lens NA of 0.85, it was 4.5%.

  Using the same material and process as in Example 1, a further minute mark was produced. However, here, the recording laser wavelength was 266 nm, and the NA of the objective lens was 0.9. In addition, recording was performed with a 2Tw mark length of 0.11 μm and a track pitch of 0.24 μm. The recording laser waveform was the same as that shown in FIG. 5, but the recording power Pw / Pe / Pb was set to be 1 mW / 0.5 mW / 0.05 mW on the medium. The length of the produced ROM pit was measured with an atomic force microscope (AFM). When the length of 200 pits was measured and the standard deviation was calculated, the standard deviation was 5.2 nm. This corresponds to about 9.5% of the window width.

Here, an example in which Ag ₂ In ₈ Sb ₇₀ Te ₂₀ is used for the recording film of the medium described in the first embodiment will be described. This recording film has a high crystallization speed as in the first and second embodiments, and the mark shape is as shown in FIG. The film thickness of the medium and the recording conditions were the same as those in Example 1.

The etching rate of Ag ₂ In ₈ Sb ₇₀ Te ₂₀ by RIE was 12 nm / min for crystals and 5.5 nm / min for amorphous under the same conditions as RIE of Example 1. By etching the sample in the state of FIG. 5B for 201 seconds, a pit with a depth of 50 nm could be produced. The reason is as follows. All SiO ₂ 505 is etched in the first 30 seconds. All of the crystals of the phase change film are etched in 100 seconds, and at that point, amorphous remains about 9 nm. SiO ₂ crystals have been removed portion is etched, SiO ₂ is approximately 47nm etched in about 71 seconds. At that time, the amorphous portion of the recording film is etched by about 6 nm and remains 3 nm, so that a pit having a total depth of about 50 nm is formed in the SiO ₂ portion and the recording film portion.

  Using this sample as a master, a ROM disk was prepared, and jitter was measured with an optical disk evaluator. A jitter of 5.2% was obtained.

Here, an example in which Ge ₂ Sb ₂ Te ₅ is used for the recording film of the medium described in the first embodiment will be described. Ge ₂ Sb ₂ Te ₅ has a relatively low crystallization speed, and its mark shape is as shown in FIG.

The medium structure in this case is shown in FIG. Although this structure is similar to that of FIG. 4, an Al alloy having a thermal conductivity lower than that of the Ag alloy is used for the reflective film, and the film thickness is set to 100 nm. The reason for this is that the crystallization rate of Ge ₂ Sb ₂ Te ₅ is relatively slow as described above, so that the amount of heat diffusing from the recording film to the reflection film during recording is suppressed, and the temperature drop of the recording film is slowed down. This is because the purpose is to induce crystallization.

The process is similar to that of FIG. The etching rate of Ge ₂ Sb ₂ Te ₅ by RIE was 15 nm / min for crystals and 8 nm / min for amorphous under the same conditions as in Examples 1 to 3.

By etching the sample in the state of FIG. 5B by RIE for 168 seconds, a pit with a depth of 40 nm could be created. The reason why the depth is 60 nm is as follows. All SiO ₂ 605 is etched in the first 30 seconds. In the next 80 seconds, all of the crystal portion of the recording film is etched, and at that time, an amorphous portion remains about 9.3 nm. In the next 58 seconds, SiO ₂ 605 is etched by about 38.5 nm, at which point the amorphous part remains about 1.5 nm, resulting in a total pit depth of 40 nm.

  Using this sample as a master, a ROM disk was prepared in the same manner as in Example 1, and the jitter was about 6.2% when measured with an optical disk evaluator.

  The present application can be applied to a medium for recording and reproducing information by light irradiation.

The relationship between the shape of the recording film and the shortest mark. (A) In the case of a recording film having a high crystallization speed, (b) In the case of a recording film having a relatively low crystallization speed, (c) When there is no crystallization speed. Relationship between recording power and mark length. The recording power on the horizontal axis is indicated by the ratio of the mark length to the appropriate recording power, and the mark length on the vertical axis is indicated by deviation from the appropriate mark length. The medium structure used in the first to third embodiments. Recording laser waveform. ROM disk master production process. Embodiment Medium structure used in the fourth embodiment.

Explanation of symbols

301: Glass substrate, 302: Ag alloy, 303: SiO ₂ , 304: Recording film, 305: SiO ₂ ,
501: Glass substrate, 502: Ag alloy or Al alloy, 503: SiO ₂ , 504: Recording film in crystalline state, 505: SiO ₂ , 506: Amorphous marks in the recording film,
601: Glass substrate, 602: Al alloy, 603: SiO ₂ , 604: recording film, 605: SiO ₂ .

Claims (1)

In an optical disk that records information by using the unevenness produced on the substrate of the optical disk as recording data and reads the unevenness by irradiating light, the length of the uneven mark in the light spot scanning direction is x, and the disk is perpendicular to x It is characterized by having a mark satisfying xmin <ymin, where y is the length in the radial direction, and xmin and ymin are the minimum values of x and y among all mark lengths existing on the disc. An optical disk substrate.

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