1. Field of the Invention
The invention relates to a recording method which is applied to manufacturing of a master for an optical disk, the master for the optical disk, and an optical recording medium.
2. Description of the Related Arts
In recent years, a “Blu-ray Disc (registered trademark of Sony Corporation)” format has been proposed. The Blu-ray Disc (hereinbelow, properly referred to as a BD) format is a format of a high-density optical disk having a recording capacity of about 25 Gbytes for a one-side single-layer or about 50 Gbytes for a one-side double-layers.
Further, according to the BD format, in order to reduce a spot diameter of a laser beam for recording and reproduction, a wavelength of a light source is set to 405 nm and a numerical aperture NA of an objective lens is set to a large value of 0.85. In the BD format, since the spot diameter can be decreased to 0.58 μm, it can be decreased to about ⅕ of that of a DVD (Digital Versatile Disc). Further, since the numerical aperture NA of the objective lens has been increased, an angular error (called a tilt margin) which is permitted to an inclination of an angle between a disk surface and an optical axis of the laser beam from 90° decreases. Therefore, a cover layer covering an information layer is thinned to 0.1 mm.
In the case of an optical disk of the rewritable BD format, an on-groove recording is used. According to the on-groove recording, the recording is executed only to grooves in convex portions having a rough shape by what are called guide grooves formed on a disc substrate along recording tracks. Grooves in a data recording area are wobbled grooves formed by a multiplexed signal of an MSK (Minimum Shift Keying) signal and an STW (Saw Tooth Wobble) signal. The MSK system is a modulating system of the wobbles and addresses have concentratedly been embedded in a specific position of the wobbles formed by a sine wave.
By concentratedly embedding information as mentioned above, if there is a defect in such a portion, the information is influenced. Therefore, the STW is multiplexed to the wobbles of the MSK. The STW system is a system in which the wobble shape is set to a form of a saw-tooth wave and “0” and “1” of the address information are discriminated on the basis of the direction of the tooth. According to the STW system, since the same information is continuously arranged in a wide range, there is such a feature that even if a partial defect occurs, a possibility that it can be restored is high. In the case of the rewritable BD format, by combining the MSK and the STW, the address information having high survivability for the error can be obtained.
Since the optical disc substrate has generally been formed by an injection molding of a resin material, the low price of the optical disk has been realized. In the injection molding of the optical disc substrate, in order to provide patterns of grooves, pits, and the like for the optical disk, a stamper as a master for manufacturing the optical recording media onto which those patterns are transferred is arranged in a cavity of an injection molding apparatus.
An outline of a manufacturing method of the stamper will now be described. First, a glass master is coated with a very thin photoresist (sensitive material) by a spin coating method or the like and the disc is exposed by a laser of a cutting apparatus while rotating the glass master. A latent image of the patterns corresponding to the grooves or pits is formed onto the photoresist film by the exposure.
After that, by dropping a developer onto the rotating glass master and executing a developing process, the concave/convex resist patterns corresponding to the grooves or pits of the optical disk are formed onto the glass master.
Subsequently, a metal such as nickel or the like is precipitated onto the glass master by a plating process, peeled off, and trimmed, so that the stamper is obtained. The stamper is arranged in the cavity of the injection molding apparatus. By injecting the resin into the cavity, the disc substrate is formed.
An exposure beam diameter d at the time of exposing the photoresist film is expressed by the following equation (1).d=1.22×λ/NA  (1)where,                λ: wavelength of the light source (laser) for exposure which is used        NA: numerical aperture of the objective lens for converging a light flux emitted from the light source onto the sensitive material        
Table 1 (refer to FIG. 5) shows the exposure wavelength of the laser, a track pitch, a wobble amount, and the exposure beam diameter in the BD format.
As shown in Table 1, in the BD of the high-density optical disk, a deep ultraviolet laser (wavelength: 266 nm) and a resist of a stable novolak system are used, thereby enabling a format of a track pitch smaller than a diameter of the recording beam to be formed. That is, even in the case of using a stable laser of 266 nm in which a track pitch of the data recording area is equal to 320 nm and a wavelength is short, the following relation is obtained.Exposure beam diameter (about 360 nm)>track pitch (320 nm)
From the above relation, the exposure beam leaks to the adjacent track. A technique in which even if the track pitch is narrowed, by uniforming an overlap of the exposure beams, fluctuations of a pit size and a pit shape are suppressed and a jitter and crosstalks of the high-density optical disk are reduced has been disclosed in Patent Document 1 (JP-A-2003-346390).
However, in the case of the optical disk of the rewritable BD format, there is a case where the wobbled grooves extremely approach each other in dependence on a phase relation between the adjacent wobbled grooves, and an amount of leakage of the exposure beam to the adjacent track changes. In addition, since an amount of leakage of the exposure beam to a land portion further changes by the wobbles, a height of land portion changes in the BD disc formed by the steps of the exposure, development, forming and molding of the stamper, and the like.
In the rewritable BD format, since a phase-change recording film is used, a depth of groove is so shallow to be about 23 nm and in the data recording portion, when a change amount of the height of land portion due to the wobbles is equal to or larger than 1.4 nm, an influence of the height change amount increases. Thus, a fluctuation amount of a push-pull signal is large, secondary distortions of the MSK and the STW wobbles are large, and it is difficult to satisfy the standard.
A technique which can solve the problem in the case of using the organic resist in the related art and enables the high-density optical disk to be manufactured has been disclosed in Patent Document 2 (JP-A-2003-315988). There has been disclosed such a technique that in an inorganic resist material made of an incomplete oxide of a transition metal disclosed in Patent Document 2, even by the exposure using a visible laser of about 405 nm, the exposure of patterns smaller than the spot diameter can be performed showing to the thermal recording characteristics. An attention has been paid to such a technique as a technique that is useful to a mastering technique of the optical disk corresponding to the BD format of a ROM (Read Only Memory) or the recording density higher than that in the BD format.
The incomplete oxide of the transition metal disclosed here denotes a compound in which an oxygen content is deviated in such a direction as to be smaller than that of a stoichiometric composition according to a valence number which the transition metal can have, that is, a compound in which the content of oxygen in the incomplete oxide of the transition metal is smaller than the oxygen content of the stoichiometric composition according to the valence number which the transition metal can have. In the incomplete oxide of the transition metal, since a latent image forming portion by the exposure has been oxide-altered, it is soluble to an alkali developer and microfabrication of the master for the optical disk can be realized.
The exposure beam diameter d in the case of the laser wavelength of 406 nm is obtained byd=1.22×406 [nm]/0.85=about 583 [nm]
The following relation is obtained.Exposure beam diameter (583 nm)>>track pitch (320 nm) in the BD format of the ROM
As mentioned above, by using the inorganic resist, the optical disk format of the track pitch which is equal to about the half of the exposure beam diameter can be formed.
A technique in which by using a phase-change (inorganic) resist and a laser of the wavelength of 480 nm, super-high density concave/convex marks each having a diameter of about 40 nm as shown in an SEM (Scanning Electron Microscope) photograph in FIG. 4 can be formed has been disclosed in Non-Patent Document 1 (Nano-pattern machining technique of a phase-change recording film for a disk, “HAITAKKU”, Hitachi, Ltd., pages 9-10, April, 2004).