1. Field of the Invention
The present invention relates to an optical information recording medium capable of recording and reproducing large-capacity information using a light beam.
2. Description of the Related Art
Recently, in the field of information recording, research into optical information recording media and optical information recording methods are progressing at various laboratories. Optical information recording media can record/reproduce information in a noncontact state. Optical information recording media are classified into read-only-type, write-once, read-many-type, and rewritable media and can cope with various memory forms. Such optical information recording media can inexpensively store large-capacity files and are expected to be widely used as industrial and consumer devices.
CDs, LDs, and DVDs (Digital Versatile Discs) corresponding to a read-only memory form have already been widely proliferated. These optical disks have a transparent substrate on which a three-dimensional pattern such as pits and grooves that indicate an information signal is formed. A reflecting film formed from a metal thin film of, e.g., aluminum is formed on the transparent substrate. A protective film for protecting the reflecting film from oxidation is formed on the reflecting film. A light beam incident on the optical disk is reflected by the reflecting film. The three-dimensional pattern such as pits and grooves that indicate an information signal is reflected on reflected light reflected by the reflecting film. Hence, when a change in reflected light is detected, the information signal can be reproduced.
Phase change optical disks corresponding to a rewritable memory form are already forming a market of PDs, DVD-RAMs, and DVD-RWs. The disk structure will be described below. A transparent dielectric film is formed on a transparent substrate. A phase change recording layer essentially consisting of Ge, Sb, Te, In, or Ag is formed on the transparent dielectric film. Another transparent dielectric film is formed on the phase change recording layer. A reflecting film made of, e.g., aluminum is formed on the transparent dielectric film. In addition, a protective film made of, e.g., a UV curing resin is formed on the reflecting film. Upon receiving a light beam from a semiconductor laser, the phase change recording layer on the transparent substrate reversibly transits between an amorphous state and a crystal state. In an information reproduction mode, an information signal is reproduced by detecting a change in reflected light from a recording portion of the phase change recording layer. In an information recording mode, a recording portion of the phase change recording layer is irradiated with a short-pulse light beam having a relatively high power to heat the recording portion to a temperature equal to or more than the melting point. Then, the recording portion is quickly cooled to form an amorphous recording mark at the recording portion. In an information erase mode, the recording portion of the phase change recording layer is irradiated with a long-pulse light beam having a lower power than in the recording mode to hold the recording portion at a temperature between the crystallization temperature (inclusive) and the melting point (exclusive) or cool the recording portion from a temperature equal to or more than the melting point, thereby crystallizing the recording portion. As described above, in the phase change optical recording, information is recorded using a change in reflectance between the amorphous state and the crystal state. For this reason, an apparatus can have an optical system with a simple structure. Phase change optical recording requires no magnetic field, unlike magnetooptical recording. Additionally, in phase change optical recording, an overwrite by light intensity modulation is easy, and the data transfer rate is high. Furthermore, phase change optical recording has good compatibility with a read-only disk represented by a DVD-ROM and CD-ROM.
As a method of increasing the capacity of such an optical disk, the NA (Numerical Aperture) of the objective lens of an optical pickup is increased to reduce the spot diameter of reproduction light, thereby attaining a high recording density. In a shift from, e.g., a CD to a DVD, the substrate thickness is decreased from 1.2 mm to 0.6 mm to cope with an optical system with a high NA. To increase the NA, the transparent substrate through which reproduction light passes must be made thinner. This is because when the NA is increased, the allowable amount of aberration generated by the angle of shift of the disk surface from a plane perpendicular to the optical axis of the optical pickup, i.e., the tilt angle becomes small. For this reason, as the NA increases, the transparent substrate must be made thin, and the substrate thickness distribution in the disk must fall within a predetermined range.
For a recording/reproduction optical disk such as a DVD, a light beam becomes incident from the substrate side. That is, a light beam irradiation surface in the reflecting layer is formed on the protective layer. An interface is formed between the reflecting layer and the protective layer. Since the surface of the protective layer is reflected on the light beam irradiation surface in the reflecting layer, an equilibrium is maintained. On the other hand, in a high-NA-compatible optical disk which is applied to an apparatus having an optical pickup with a high-NA lens, layers are formed in an order reverse to that of the above-described conventional optical disk to ensure a tilt margin. In such a high-NA-compatible optical disk, a reflecting layer, second protective layer, phase change recording layer, and first protective layer are formed in this order. For this reason, the surface state of the reflecting layer is reflected on the second protective layer formed on the reflecting layer, the recording layer formed on the second protective layer, and the first protective layer formed on the recording layer. Generally, crystal grains on the surface of the reflecting layer formed from an AL alloy tend to have a large side due to columnar growth unique to a metal thin film. The surface roughness of the reflecting layer also roughens the surface of the recording layer through the second protective layer. A mark recorded on the high-NA-compatible optical disk which aims at increasing density by increasing the NA is finer than a mark recorded on the conventional optical disk. That is, the above-described surface roughness of the recording layer greatly influences the recording/reproduction characteristic of the high-NA-compatible optical disk. More specifically, the surface roughness of the recording layer produces noise in the reproduction mode or causes strain at a mark edge in forming a recording mark. Hence, in a high-NA-compatible optical disk, such surface roughness (three-dimensional pattern) of the reflecting layer is preferably suppressed.
In the conventional optical disk in which a light beam is incident from the substrate surface side, a reflecting layer is divisionally formed for efficient mass production. With the divisional reflecting layer formation, the columnar growth of the reflecting layer is slightly suppressed. However, the surface roughness of the reflecting layer generates noise in the reproduction mode or fluctuates a mark edge at a 1/10 wavelength or more. For this reason, even the above-described columnar growth suppression by divisional film formation does not suffice in obtaining a satisfactory recording/reproduction characteristic. In the conventional optical disk, an Al-based material is used for the reflecting layer. However, this material readily forms large-size crystal grains and is therefore unsuitable for a high-NA-compatible optical disk in which layers are formed in a reverse order. As a method of reducing the surface roughness of the recording layer of a high-NA-compatible optical disk, a method of inserting a metal undercoat between the reflecting layer and the substrate is proposed in Jpn. Pat. Appln. KOKAI Publication No. 11-327890. However, this method has another problem that the disk manufacturing cost increases because an additional layer is formed.