Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2000-03-21
2002-07-09
Evans, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064500, C428S064600, C430S270130
Reexamination Certificate
active
06416837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optically recordable, reproducible, erasable and rewritable information recording medium, a method for manufacturing the medium and a method for recording/reproducing information thereon.
2. Description of the Prior Art
Conventionally, for a phase changeable information recording medium, a multilayered film including a recording layer where a reversible phase change is caused between a crystalline state and an amorphous state is formed on a transparent disk substrate by sputtering or the like in the film formation process. The structure of the recording layer is amorphous after the film formation, and then the recording layer is subjected to a process for changing the entire surface of the recording layer from the amorphous state to the crystalline state by optical or thermal means (hereinafter, referred to as an initialization process). Thus, a phase changeable information recording medium is manufactured. (In the specification of the present invention, the thus formed amorphous state in the film formation process is referred to as “as-depo amorphous” to be distinguished from the amorphous state formed by quenching after melting by power laser irradiation as described below.)
In the phase changeable information recording medium, signals can be recorded or rewritten by irradiating the recording layer with a single laser beam while changing the power between high and low. When the recording layer is irradiated with a high power laser beam to be molten and then quenched, the recording layer becomes amorphous (recorded state). When the recording layer is irradiated with a low power laser beam to be warmed and then cooled gradually, the recording layer becomes crystalline (erased state). Thus, a recording mark on the order of several tenths Am (several 100 nm) is formed on the track. The signals are reproduced by utilizing the difference &Dgr;R(%) (&Dgr;R=|Rc−Ra|) between the reflectance Rc (%) of the medium when the recording layer is in the crystalline phase and the reflectance Ra (%) of the medium when the recording layer is in the amorphous phase. In either the medium in which Rc>Ra or Ra>Rc, signals can be recorded/reproduced.
In the initialization process, the reflectance of the medium changes from Ra to Rc. In particular, in the medium optically designed to achieve Ra>Rc, the reflectance is reduced so that it is preferable that Rc is 10% or more.
The initialization process requires equipment provided with optical or thermal means. For example, in the case where a semiconductor laser is used as the optical means, operations for optimizing various conditions such as the shape of the laser beam, the power of laser irradiation, the cooling rate, the rotational speed of the medium and the period of time for irradiation with respect to each particular medium are required. In addition, other problems arise. For example, it is known that the volume of the recording layer is contracted by several % at the time of the phase change from the amorphous phase to the crystalline phase. Therefore, when the recording layer is crystallized after the multilayered film is formed, the volume contraction of the recording layer generates new internal stress, which was not present immediately after the film formation, at least in the layer in contact with the recording layer. If the recording layer is as thin as 10 nm or less, light absorption is small and heat is diffused readily, so that crystallization requires more power density so that a load is applied to grooves or address pits that previously have been transferred on the substrate. Thus, the initialization process poses a large number of problems.
If the initialization process is eliminated, the plant investment and the development cost can be reduced, leading to a significant reduction in the cost of the medium. Different systems to eliminate the initialization process can be conceived for (1) the medium of Rc>Ra and (2) the medium of Ra>Re. In order to obtain good servo characteristics, it is preferable to keep the reflectance high, and it is required that in the case of (1), the recording layer is in the crystalline phase (initial state Rc) after the film formation, and that in the case of (2), the recording layer is in the amorphous phase (initial state Ra) after the film formation. Herein, the initial state refers to the state of the medium before recording. In order to meet these requirements, a technique to crystallize the recording layer during the film formation and a technique to record signals in an amorphous recording layer are required.
A method for crystallizing a recording layer of a phase changeable optical information recording medium during the film-formation is disclosed in WO98/47142. In this method, a crystallization accelerating layer made of a material whose crystal structure is face-centered cubic lattice or rhombohedral lattice is provided, and then the recording layer is formed directly on the crystallization accelerating layer and the substrate temperature is changed from 45° C. to 110° C. during the formation of the recording layer. Furthermore, the examples show that the crystallization accelerating layer is formed of a material comprising at least one of Sb, Bi and Sb compounds and Bi compounds, and the recording layer of the phase changeable optical information recording medium manufactured by this method is formed in the crystalline state.
Furthermore, PCT International Publication No. WO98/38636 discloses methods for manufacturing a phase changeable optical information recording medium that is designed to attain Ra>Rc. In this disclosure, a method in which the substrate temperature is from 35° C. to 150° C. during formation of a recording layer, and a method in which the substrate temperature is from 35° C. to 95° C. immediately before formation of the recording layer are described. The thus produced phase changeable optical information recording medium can achieve high recording characteristics, even if recording is performed first on the as-depo amorphous recording layer without performing an initialization process.
However, in WO98/47142, Bi has a melting point as low as about 271° C., so that it is impossible to raise sputtering power. In WO98/38636, in order to form a film having an as-depo amorphous recording layer by heating the substrate, the entire surface of the substrate is heated uniformly and the temperature is required to be kept. For example, when heating the substrate holder itself, it is very difficult to heat the entire substrate uniformly without contacting the entire surface of the substrate with the substrate holder so as to conduct heat to the substrate. However, when the substrate is contacted with the holder on its entire surface, scratches or dirt are likely to be generated on the surface of the substrate. In addition, when high frequency induction or flash heating is performed, complicated film-forming equipment is required in order to heat the substrate uniformly in a contactless manner in a vacuum apparatus. Moreover, it is difficult to keep a constant temperature stable immediately before or during formation of the film. Furthermore, it is necessary to measure the temperature of the substrate in a contactless manner in the vacuum apparatus and to monitor the temperature outside the apparatus, so that the apparatus inevitably becomes complicated and largescale.
It is believed that the reason why it conventionally is difficult to perform recording on an as-depo amorphous phase is that the as-depo amorphous phase is different in nature from the amorphous phase formed by irradiating a crystalline phase with laser. In general, the amorphous phase has several metastable energy states. When a medium is stored for a long time or under high temperature conditions, the energy state can be changed to a different energy state after the storage. For this reason, since optimal conditions for recording/reproducing are different between before and after the sto
Kitaura Hideki
Kojima Rie
Yamada Noboru
Evans Elizabeth
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P,C,
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