Information recording medium having pair of electrodes Number:7,426,174 from the United States Patent and Trademark Office (PTO) owispatent Disclosed herewith is a method for enabling fast and high density recording of information. A voltage is applied to a recording layer formed between a pair of electrodes. The distance between the pair of electrodes is set wider at one of land and groove areas of a subject optical disk and narrower at the other or the distance is set so that light absorption occurs only in either of the land and groove areas. The optical disk is also provided with a layer of which light absorption spectrum changes according to the application of an electric current, thereby absorbing the light. The new layer may be the recording layer itself or a layer adjacent to the recording layer. Because a heat generates only from a small area of the optical disk at the time of recording, the disk can be turned rapidly and permissively to the auto focusing and tracking offsets, thereby enabling fast and high density recording. The disk can thus be formed with easily selectable multiple layers.<H Information, electrodes, Information, rec, 7426174.html, Patent, pto, 7426174

Title: Information recording medium having pair of electrodes

Abstract: Disclosed herewith is a method for enabling fast and high density recording of information. A voltage is applied to a recording layer formed between a pair of electrodes. The distance between the pair of electrodes is set wider at one of land and groove areas of a subject optical disk and narrower at the other or the distance is set so that light absorption occurs only in either of the land and groove areas. The optical disk is also provided with a layer of which light absorption spectrum changes according to the application of an electric current, thereby absorbing the light. The new layer may be the recording layer itself or a layer adjacent to the recording layer. Because a heat generates only from a small area of the optical disk at the time of recording, the disk can be turned rapidly and permissively to the auto focusing and tracking offsets, thereby enabling fast and high density recording. The disk can thus be formed with easily selectable multiple layers.

Patent Number: 7,426,174 Issued on 09/16/2008 to Terao, et al.

Inventors: Terao; Motoyasu (Hinode, JP), Miyamoto; Harukazu (Higashimurayama, JP), Shintani; Toshimichi (Kodaira, JP), Kojima; Kyoko (Kunitachi, JP), Tsuchiya; Yuko (Tokorozawa, JP)
Assignee: Hitachi, Ltd. (Tokyo, JP)

Appl. No.: 10/988,548

Filed: November 16, 2004

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
10336717	Jan., 2003	6977883

Foreign Application Priority Data


May 27, 2002 [JP]			2002-151713

Current U.S. Class: 369/276 ; 369/126

Current International Class: G11B 5/84 (20060101)

Field of Search: 369/276,126

References Cited [Referenced By]

U.S. Patent Documents


3986771	October 1976	Tsukada
3989530	November 1976	Robillard
4075610	February 1978	Crandall et al.
4324622	April 1982	Deb
4500174	February 1985	Conner
4701880	October 1987	Ichihara
4773060	September 1988	Shimada et al.
4832456	May 1989	Yamazaki et al.
4842381	June 1989	Green
4945515	July 1990	Ooumi et al.
5279932	January 1994	Miyasaka et al.
5430705	July 1995	Takanashi et al.
5629920	May 1997	Sakano et al.
5717626	February 1998	Aoki et al.
5903296	May 1999	Shimizu et al.
6821596	November 2004	Terao et al.
7203152	April 2007	Van Der Mark et al.
7263053	August 2007	Terao et al.
2005/0047309	March 2005	Terao et al.
2005/0052983	March 2005	Vincent et al.
2006/0140100	June 2006	Wilderbeek et al.
2007/0086316	April 2007	Hirotsune et al.
2007/0247999	October 2007	Mukoh et al.

Foreign Patent Documents


60-117431	Jun., 1985	JP
63-122032	Nov., 1986	JP
04-319545	Apr., 1991	JP
10-260432	Mar., 1997	JP
11-185288	Dec., 1997	JP
2001-344807	Jun., 2000	JP

Other References

M Terao, H. Yamamoto, and E. Maruyama, "Highly Sensitive Amorphous Optical Memory", 1973, Supplement to the Journal of the Japan Society of Applied Physics, vol. 42, p. 233-238. cited by other.

Primary Examiner: Dinh; Tan X
Attorney, Agent or Firm: Reed Smith LLP Fisher, Esq.; Stanley P. Marquez, Esq.; Juan Carlos A.

Parent Case Text

This application is a Continuation application of nonprovisional U.S. Ser. No. 10/336,717 filed on Jan. 6, 2003 now U.S. Pat. No. 6,977,883. Priority is claimed based upon U.S. application Ser. No. 10/336,717 filed on Jan. 6, 2003, which claims the priority date of Japanese Patent Application 2002-151713 filed on May 27, 2002.

Claims

What is claimed is:

1. An information recording method for recording information to a medium by irradiating the medium with a laser beam and applying an electric current to the medium, said medium comprising a first recording layer including electro-chromic material and a pair of electrodes, said first recording layer disposed between the pair of electrodes, said medium having a land area and a groove area in a radial direction of said medium, the information recording method, comprising steps of: applying a controlled voltage to said electrodes so as to develop color of said land area of said first recording layer and suppress color development of said groove area of said first recording layer; and recording marks by irradiating said first recording layer with a laser beam having predetermined power so as to weaken electrochromic property of said first recording layer, wherein a thickness of the first recording layer in said land area is thinner than a thickness of the first recording layer in said groove area.

2. An information recording method for recording information to a medium by irradiating the medium with a laser beam and applying an electric current to the medium, said medium comprising plural recording layers including an electrochromic material and a pair of transparent electrodes, each of said plural recording layers disposed between the pair of transparent electrodes, said medium having a land area and a groove area in a radial direction of said medium, the information recording method, comprising steps of: applying a controlled voltage to said transparent electrodes of a target recording layer of said recording layers so as to develop color of said land area of said target recording layer and suppress color development of said groove area of said target recording layer; and recording marks by irradiating said target recording layer with a laser beam having predetermined laser power so as to weaken electrochromic property of said target recording layer, wherein a thickness of each of said plural recording layers in said land area is thinner than a thickness of each of said plural recording layers in said groove area.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to media, methods, and apparatuses for recording and reproducing information by means of light irradiation respectively.

2. Description of Related Art

The most significant features of optical disks are removability from their recording/reproducing apparatuses and availability with low prices. Optical disk drives that use those disks are therefore configured so as to enable fast and high density recording without losing such the features of the optical disks.

So far, various well-known principles have been employed for those optical disks; each records information in its recording layer by irradiating a light therein. One of such the principles makes good use of changes of atomic configuration caused by a thermal process, that is, phase-changes of a subject film material (the phase-change is also referred to as phase transition or phase transformation) to enable the information recording medium made of the material to be rewritten many times. For example, a phase-change optical disk disclosed by JP-A No. 344807/2001 is basically configured by a protective layer, a recording film made of a GeSbTe material, a protective layer, and a reflection layer, which are formed sequentially on a substrate.

On the other hand, there are also other well-known optical disks referred to as electric-field-effect type disks. In the case of this type disk, a laser beam is irradiated to its phase-change recording film while an electric field is applied thereto so as to record information therein. This type disk has an element structure in which a phase-change recording layer made of a GeSbTe material or the like is disposed between upper and lower electrodes. This electric-field-effect type optical disk is disclosed, for example, by JP-A No.122032/1988. This type optical disk receives an electric-field in its recording film, thereby the phase-change in the recording film is promoted (crystallization) more than other optical disks for each of which only a laser beam is irradiated to its recording film. FIG. 1 shows a configuration of such an electric-field-effect optical disk. A light 9 is condensed by a lens 8. The disk has electrodes and layers formed on a substrate 7 sequentially from the light incidence side in order of a transparent electrode 1, a UV-resin guide groove layer 2, a recording film 3, an insulator electrode 4, an AI electrode 5, and a protective layer 6. A voltage is applied to both of the transparent electrode and the AI electrode. The UV-resin layer 2 that has a guide groove on its surface is an insulator whose specific resistance is 10.sup.6 .OMEGA.cm. The UV-resin layer 2 functions as an electric-field-effect and to form the guide groove.

A paper written by the present inventor et al (M. Terao, H. Yamamoto and E. Maruyama) and entitled as "Highly Sensitive Amorphous Optical Memory: supplement to the J. of the Japan Society of Applied Physics Vol. 42, pp 233-238" reports an experimental result that light irradiation to both of a photo-conductor and a phase-change recording film disposed between transparent electrodes while a voltage is applied to them from those transparent electrodes makes it possible to record information in the recording layer only with a laser beam that is weaker by almost two digits than any case in which only a light is irradiated thereto. On the other hand, there are also still other type optical disks like CR-R and DVD-R, each of which uses an organic-material for its recording layer. In the case of those optical disks, a laser beam is irradiated to both of a recording layer and a substrate surface adjacent to the recording layer. The recording layer includes a dye to absorb the wavelength of the recording laser, thereby the quality of the substrate surface is changed to enable information to be recorded therein.

As described above, while optical disks are all characterized by removability from their recording units, as well as availability at low prices realized by using plastic substrates, rapid operation is one of the indispensable requirements for them. And, because of such the structure, optical disks have been confronted unavoidably with problems; each of those disks repeats pitching, resulting in tracking offsets sometimes when it rotates faster. And, this comes to generate a high frequency that makes it difficult for the disk to follow up with auto-focusing and tracking. This is why the recording media have long been required to be permissive to such the tracking offset, especially when in recording during which such the trouble is apt to occur. And, this conventional problem has to be solved to speed up the operation of the recording apparatus to enable such the following-up over the mechanical vibration limit of the apparatus.

According to the technique disclosed in the above-described paper "supplement to the J. of the Japan Society of Applied Physics Vol.42" written by the present inventor et al, as well as the electric-field-effect type recording medium disclosed by JP-A No. 122032/1988, it never occurs that only one of the land area and the groove area becomes easier to be recorded, since almost the same voltage is applied to both of the areas and almost the same light absorption occurs in them. Consequently, the media in the above cases are not permissive so much to the tracking offset and accordingly they cannot cope with fast recording satisfactorily. On the other hand, in the case of the CD-R and the DVD-R, light absorption makes no difference practically between the land area and the groove area, so that no electric current application can assist the recording.

Under such circumstances, it is an object of the present invention to solve the above-described conventional problems and enable mass of information to be recorded stably and rapidly.

SUMMARY OF THE INVENTION

Hereunder, the configuration of the present invention for solving the above conventional problems will be described.

Note that, however, a long recessed portion formed on the substrate will be referred to as a groove in the present invention. An area between such grooves will be referred to as a land. Upon a light incidence to a film through the substrate, such a groove looks like a convex from the light incident side. Therefore, even in the case of the method that applies a light from an opposite side of the substrate, such a portion that looks like a convex at a view from the light incident side might be referred to as a groove in some cases. This portion is recognized as a convex when only the substrate is watched, but it is actually a land between grooves. Strictly, such a portion is not referred to as a groove in the definition by the present invention. In the case of a method for recording information in either lands or grooves, that is, in the case of the so-called in-groove recording method, the recording characteristics are often better when in recording at convex portions at a view from the light incident side regardless of whether the light incidence is done from the substrate side or from an opposite side of the substrate. However, those two methods are basically similar to each other, so that recording may also be done at recessed portions at a view from the light incident side.

Concretely, the configuration of the present invention will be as follows. (1) A first electrode, an electro-chromic material layer, and a second electrode are disposed on a substrate of the information recording medium of the present invention, then a voltage is applied to between the first and second electrodes to flow an electric current in the electro-chromic material, which is thus colored. The information recording medium is preferably configured so that the electro-chromic material is colored in a first area while it is not colored in a second area. The first area is equivalent to a land area and the second area is equivalent to a groove area. And, because the light is absorbed only in the first area or in the second area, the easily recordable range can be identified. Consequently, information can be recorded in the target place stably regardless of slight changes of the light spot and the light condensing level, thereby the medium can record information fast and permissively to both AF and tracking offsets. The medium can also cope with high density recording.

FIG. 2 shows a structure of the information recording medium of the present invention. In order to make it easier to understand, the structure is illustrated so that a light is applied from the upper side. The medium is configured by layers and electrodes formed sequentially from the light incident side on a substrate 17 in order of a protective layer 11, a first electrode (transparent electrode) 12, an electro-chromic material layer 13, a second electrode 14, a UV-resin layer 14, and another protective layer 16. Other reference numerals are defined as follows; 18 denotes a groove area and 19 denotes a land area. Another layer, which is, for example, a thin insulator layer or layer referred to as a boundary layer may be formed between the recording layer (electro-chromic material layer) and the first or second electrode. The additional layer should preferably be 20 nm or under in thickness.

In another aspect, the information recording medium of the present invention is configured by a first electrode, an insulator film having a through opening to the substrate, a recording film formed so as to be extended from the opening onto the insulator film and enabled to record information therein, and a second electrode formed on the recording film. In this case, one of the land area and the groove area is formed as the opening of the insulator film and the insulator film is formed in the other. Consequently, only one of the groove and land areas becomes the opening of the insulator film, so that the upper and lower electrodes come closer to each other. An electric current thus flows in the area. On the other hand, an insulator film is formed in the other (the land area or the groove area), so that little electric current flows there. This is why almost no current flows between those electrodes. The current flowing range is thus limited, thereby the easily recordable range is limited.

And, due to the features described above, the medium assures stable recording regardless of slight changes of both light spot position and light condensing level, thereby enabling fast recording permissively to AF and tracking offsets. The medium can also cope with high density recording.

The present invention, therefore, intended to further improve the recording density by multi-layer recording. To form multiple layers to improve such the effective recording density (effective surface density) is also desirable for any of the conventional media. However, for a medium consisting of three or more layers, the transmission factor and the recording density comes into a trade-off relationship, and accordingly either of the reproduced signal quality or the recording sensitivity in each of the layers have been forced to be sacrificed. In order to solve such the problem, the well-known three-dimensional recording methods uses the thickness direction of each transparent organic material for recording information. One of the methods utilizes two-light-quantum absorption. The recording sensitivity of this method, however, is very low. And still another method that employs light polymerization has a problem that both storage stability and recording sensitivity are low.

In the present invention, two or more recording layers are formed and each recording layer is disposed between transparent electrodes to improve the transmission factor of each layer, thereby improving both recording sensitivity and reproduced signal quality.

For multi-layer recording when the subject medium consists of two or more recording layers, each of the recording layers other than the farthest one from the light incident side, when information is to be recorded therein or to be read therefrom, should preferably be disposed between electrodes and the recording/reading laser beam absorption factor should increase upon applying of a voltage between those electrodes. The farthest recording layer from the light incident side may also be processed similarly. Therefore, every layer can be disposed in the focal depth of the focusing lens, since the layer is not interfered by any other layers. The information recording medium can thus be provided with multiple layers and a large capacity than any of the conventional disks consisting of a plurality of layers respectively. To achieve this object, each recording layer or a layer adjacent to the recording layer may be stacked layer made of an organic or non-organic electro-chromic material or a mixed material layer of electro-luminescent material and a photo-chromic material. Consequently, the medium can be configured so that a light is absorbed in a target layer and almost not in other layers. It is also possible to dispose some of the layers out of the focal depth of the focusing lens and move the focal point to record/read information in/from those layers, of course. In this case, although a stacked layer consisting of many layers might cause pits and grooves for representing address information to be deformed, such the problem can be avoided by forming another layer in which those pits and grooves are copied so as to prepare for reading address information from at least one of the layers in the focal depth at the moved focal point. The above described electro-chromic material may be, for example, a polymer consisting of tungsten oxide or thiophene organic polymer molecules.

Various other electro-chromic materials such as those described in "Electro-chromic Display" issued by Sangyo Tosho (Co.) on Jun. 28, 1991 and those described in papers at present are also usable.

A phase-change recording layer (ex., Ge.sub.2Sb.sub.2Te.sub.5 layer) may be disposed between the electro-chromic material layer and the first or second electrode. The layer should preferably be formed at the other side of the light incidence from the optical point of view. In this connection, the electro-chromic layer should have a high recording threshold value and the phase-change recording layer preferably has a high light-transmissivity and low fusing point by choosing materials including sulfur compound such as Sb.sub.4Te.sub.3S.sub.2 so that information is recorded only in the phase-change recording layer. (2) As shown in FIG. 3, a photo-conductor layer 23 may be formed between the first or second electrode 22 and the recording film 24. The photo-conductor layer 23 should be formed closer to the light incident side electrode than the recording film. In this connection, photo-carriers generated in the photo-conductor layer 23 due to the light irradiation are moved, thereby the resistance of the photo-conductor layer 23 lowers and both voltage and electric current applied to the recording layer 24 comes to increase sharply. As a result, the temperature of the light irradiated portion of the recording layer rises. The recording layer is thus enabled for recording. The photo-carriers may also increases through the avalanche multiplication effect. A photo-conductor film formed such way has a high-density electric current flown in the recording film, thereby the required incidence light energy is reduced. The electro-chromic material layer may also be used as a photo-conductor layer.

The recording layer may also be used as a photo-conductor layer or the recording layer may be of a type in which the electric resistance may drop upon rising of the temperature of the layer. Such a chalcogenide material as Ge--Sb--Te or the like, such an organic conductor material as polythiophene or the like are equivalent to the layer of which electric resistance drops due to a temperature rise in the layer. In FIG. 3, reference numerals are defined as follows; 21 denotes a protective layer, 22 denotes a transparent electrode, 25 denotes an insulator layer, 26 denotes a second electrode, and 27 denotes a protective substrate. The UV-adhesive layer is omitted here. (3) A circular information recording medium may be used and provided with a third electrode that is long in the radial direction of the medium used to supply an electric current to the first and second electrodes. The third electrode can apply the same voltage up to the outer periphery of the medium. (4) The voltage application to between the first and second electrodes during light irradiation should preferably be continued after the light irradiation. When a light is irradiated to between the first and second electrodes concurrently with a voltage, the electric current increases around the light irradiated portion as shown in FIG. 8. And, if the voltage is kept applied thereto even after the end of the light irradiation, the resistance in the portion rises due to any of the reduction of excitation carriers, the fusion of the recording film itself, the disarray/resolution of the atom/molecule configuration, thereby the electric current flow in the layer decreases automatically. After the current returns to its initial value, the state of the recording layer changes. For a disk-like medium, therefore, the powering/heating time for recording becomes almost the same at both inner periphery and outer periphery. Thus, the method will also be adaptable easily for the CAV (Constant Angular Velocity) recording. And, the electric current that flows throughout the recording medium grows into a large current, thereby preventing occurrence of insufficient supply of the current, state changes of the recording film, and excessive expansion of damaged areas. In order to achieve this object, the recording apparatus is just provided with a control circuit for suppressing the voltage application under 80% of that for enabling recording. (5) One of the first and second electrodes should preferably be divided into a plurality of electrodes. If such an electrode is divided in the radial direction, the medium will also be suited for the CAV (Constant Angular Velocity) recording and the capacitance between electrodes can be reduced to improve the response speed. (6) When in recording, the recording laser power can be set at over 0.2 mW to 2 mW (excl.) even at a linear recording speed of 15 m/s or over. If the recording sensitivity is such improved, a higher transfer rate can be achieved without causing insufficient laser power application even for high linear speed recording when an array laser/surface emission laser is used for simultaneous light irradiation on a plurality of spots on the medium. In this connection, it is also recommended to make pulse-like light irradiation also in erasure areas and set a pulse width wider than that of the recording mark forming pulses. Consequently, rewriting is enabled satisfactorily while it is avoided to widen the erasing width excessively. A voltage may also be applied simultaneously to at least the plurality of electrode pairs on the recording medium in units of two pairs. This method is necessary when a low material maintenance voltage is applied to cause the material color to be changed.

The recording medium of the present invention may also be provided with a plurality of recording layers and a voltage or zero-voltage is applied to only between those pairs of electrodes while a different voltage is applied to between electrodes at both sides of a target layer for recording, erasing, or reading. (7) In order to achieve the above object, the recording apparatus of the present invention is provided with two means; one means for positioning a plurality of electrodes disposed at a portion where the rotary shaft of the disk motor or the disk receiving part attached to the rotary shaft comes in contact with the place close to the center hole of the disk so that each of the plurality of electrodes faces each of the predetermined electrodes disposed in the place close to center hole of the disk when the disk is loaded and the other means for making each electrode disposed at the rotary shaft side contact with each electrode disposed at the disk side. Consequently, a predetermined voltage comes to be applied to each electrode on the disk.

In another aspect, the information recording apparatus of the present invention is provided with a tapered projection in the vertical direction at least at one place in the circumferential direction of a side surface of the disk motor rotary axis or disk receiving part attached to the rotary shaft. The disk is provided with a plurality of divided electrodes at a portion having a height, where the disk is to be loaded. Consequently, the disk is positioned accurately in the turning direction, thereby a power is supplied accurately to each of the electrodes disposed in the multiple layers.

The present invention is thus effective for the recording density (track pitch, bit pitch) over the 2.6 GB DVD-RAM standard and more effective for the recording density over the 4.7 GB DVD-RAM standard. When the light source wavelength is not around 660 nm and when the numerical aperture (NA) of the focusing lens is not 0.6, the present invention is effective for the recording density over a value calculated on those conditions in terms of wavelength ratio or NA ratio in both of the radial direction and the circumferential direction.

In this specification, a term "phase-change" is used and the "phase-change" includes not only the phase-change between crystal and non-crystal, but also the phase-change to fusion (change to the liquid phase) and re-crystallization, as well as the phase-change between crystal states.

In this specification, the present invention premises that the electro-chromic material layer mentioned above means a layer made of a material that develops its color directly (due to a light absorption spectrum change) by an applied voltage defined ordinarily and its support layer(electrolyte layer), as well as a layer including an area that emits a light due to an applied voltage (a flown electric current) and an area that develops its color or distinguishes its color with a light received from the light emitting area.

Furthermore, "the electro-chromic material of the present invention is conductive" is defined as a constant flow of an electric current of 0.1 mA or over when 2 V is applied to between the first and second electrodes of a disk whose diameter is 80 mm or over.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structure of a conventional information recording medium;

FIG. 2 is a cross sectional view of an information recording medium in an embodiment of the present invention;

FIG. 3 is another cross sectional view of the information recording medium in another embodiment of the present invention;

FIG. 4 is a 1/4 view of the information recording medium in the embodiment of the present invention;

FIG. 5 is a bird's-eye view of part of the information recording medium in the embodiment of the present invention;

FIG. 6 is a cross sectional view of the recording medium to be subjected to an etching process for exposing an insulator layer thereof in the embodiment of the present invention;

FIG. 7 is an electrode disposed in part of a disk holder in which the information recording medium in the embodiment of the present invention is to be loaded;

FIG. 8 is a chart for denoting changes of a current with time, which flows in one recording spot of the information recording medium of the present invention;

FIG. 9 is a block diagram of the information recording medium of the present invention;

FIG. 10 is a chart for describing a relationship between a ratio of the distance between electrodes and a drop of a signal level due to a tracking offset;

FIG. 11 is a structure of a stacked layer of a multi-layer disk of the present invention;

FIG. 12 is a molecule structure in an organic electro-chromic material;

FIG. 13 is another molecule structure in an organic photo-chromic material;

FIG. 14 is a structure of the information recording medium and an optical system in the embodiment of the present invention;

FIG. 15 is a block diagram of an applied voltage reversing and controlling circuit in the embodiment of the present invention; and

FIG. 16 is part of an electrode in a disk holder in which the information recording medium in the embodiment of the present invention is to be loaded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

(Structure and Fabrication Method)

FIGS. 4 and 5 show structures of a disk-shaped information recording medium in the first embodiment of the present invention. FIG. 4 shows a 1/4 size view of the structure of the disk and FIG. 5 shows an expanded view of the 1/4 size portion of the disk. In FIG. 4, only two upper radial-patterned transparent electrodes 33,34 are shown while many more are disposed actually all over the disk surface. A light for recording/reading is irradiated from above through the substrate while the top substrate is not shown in FIG. 4. FIG. 5 shows an expanded view of part of the disk. Also in FIG. 5, the top substrate and the insulator layer are not shown so as to simplify the description. In FIG. 5, reference numerals are defined as follows; 45 denotes a reflection electrode, 43 denotes a recording layer, 44 denotes an insulator layer, 42 denotes a photo-conductor layer, 41 denotes a transparent electrode, 46 and 50 denote light spots, 47 denotes a groove area, and 48 denotes a land area. Usually, information is recorded/read in/from portions referred as to groove areas, which look like convex portions at a view from the light spot. In this embodiment, however, information is recorded in land areas. The notches shown beforehand in FIG. 5 are part of the A-A' cross section and the upper notches 49 shown in FIG. 5 correspond to gaps 38 between radial patterned electrodes shown in FIG. 4. The entire A-A' cross sectional view becomes as shown in FIG. 3.

This recording medium is fabricated as follows. At first, as shown in FIG. 6, a tracking groove (width: 0.25 micron) is formed on a polycarbonate substrate 58. The groove is used for in-groove recording (recording in the land area at a view from the light spot here) at 0.45-micron track pitches at a diameter of 12 cm and a thickness of 0.6 mm. Addresses are represented by the groove wobbling on the substrate. On the substrate are also formed transparent electrodes (ITO) 51 made of (In.sub.2O.sub.3).sub.90(SnO.sub.2).sub.10 at a film thickness of 30 nm. A groove pattern is then copied onto the surface of the substrate with use of a mother to which the pattern is copied once from a nickel master formed by plating the photo resist of the original disk. This is to correspond the exposed photo-resist of the groove to the land. FIG. 6 shows processes for forming the films on the substrate 57. Note that the structures shown in FIGS. 4 and 5 are upside down in FIG. 6. Then, an As.sub.3Se.sub.97 layer 52, which is a photo-conductor layer, is formed at a thickness of 50 nm. After this, a Ge--Sb--Te recording layer 53 is formed at an average film thickness of 10 nm. This transparent electrode layer is formed by means of spattering through a mask, thereby the electrode is isolated in the radial pattern area 20 corresponding to the recording sector. Then, a SiO.sub.2 layer, which is an insulator layer 54, is formed at a thickness of 120 nm. It is followed by forming an acrylic-resin layer 55 at an average thickness of 25 nm as shown at the top in FIG. 6. A ultra-violet light having a wavelength of 254 nm is applied to the layer 55 to etch until the relatively thin resin layer 56 above the land area 59 is removed. At this time, the relatively thick layer 57 above the groove area 60 becomes thin relatively, but remains as a mask. This is followed by performing reactive ion etching for the layer 56 to remove the SiO.sub.2 layer from the land area, thereby the recording layer 53 is exposed. In other words, a through-opening to the insulator layer is formed. Such the method in which the film thickness of a layer is changed between the land and the groove to improve the recording sensitivity in one of the land and the groove, thereby increasing the permissiveness to both tracking and auto-focusing offsets is also effective for recording media to which only a light is applied to record information (while no voltage is applied). After this, the acrylic resin is almost completely removed from the substrate surface by means of plasma asher. As a result, the information recording medium in this embodiment comes to be structured as shown in FIG. 3. In the case of the recording medium in this embodiment, the total film thickness at each portion between electrodes depends significantly on whether or not an insulator film exists there; the film thickness differs by twice or more between existence and non-existence of the insulator film. And, because the resistance of the SiO.sub.2 layer is larger than those of other layers, the resistance of every layer between electrodes takes a ratio of 1:2 or over between the through-opening and each of other portions. When the recording layer is a phase-change recording layer, any of such the Ge--Sb--Te recording materials as known Ge.sub.2Sb.sub.2Te.sub.2, Ge.sub.5Sb.sub.70Te.sub.25, etc, as well as an Ag--In--Sb--Te recording materials is usable. The recording layer may also be made of an electro-chromic material such as polythiophene to be described in detail in the second embodiment. Next, a W.sub.80Ti.sub.20 film is formed at a thickness of 50 nm. The film is used as both of a reflection layer and a second electrode layer 45. A magnetron spattering apparatus is used for forming the stacked film.

Voltage application to the outer periphery of the disk might often be disturbed by the transparent electrode sheet resistance and the thinner portions of the transparent electrodes formed at angles of convex and concave portions of the groove. And, to avoid this problem, one or two thin metallic (AI or Ag is recommended) electrodes 39/40 should preferably be formed on the substrate before the transparent electrodes are formed on the substrate. The electrode 39/40, which is narrower than the radial patterned transparent electrode, is a thin metallic electrode extended from the inner periphery to the outer periphery of the disk and about 100 microns in average width in the radial direction and 50 nm to 200 nm in film thickness. These metallic electrodes are formed on the medium by means of spattering through a mask. These electrodes are avoided when in recording/reading.

Unlike the above embodiment, when information is to be recorded in a portion that looks like a groove at a view from the light spot, it is just required that the transparent electrode layer and the reflection & electrode layer are replaced with each other and the light is irradiated from the protective substrate side. In this case, the nickel master is used to form the substrate, not the mother one. The protective substrate may be thinned to about 0.1 mm and the NA of the focusing lens may be increased to 0.85. As a result, the track pitch can be narrowed to about 0.33 micron, which is about 3/4 of that in the above embodiment.

Although a photo-conductor layer is employed in this example, if the recording film is also used as a photo-conductor layer, there is no need to form the above-described photo-conductor layer; for example, only an electro-chromic material layer or phase-change recording layer may be formed between electrodes. In such a case, the insulator layer can be omitted, so that the cross sectional view of the recording medium becomes as shown in FIG. 2. The electro-chromic material layer will be described in detail in the second embodiment. In this embodiment, a thiophene derivative material is used. Other materials to be described in the second embodiment are also usable, of course. The electro-chromic material layer, as to be described in detail in the second embodiment, consists of three layers, which are formed by means of coating, vacuum deposition, or electrical field polymerization. The polymer layer of the three layers is formed by means of coating, so that the film thickness is thin in the land area and thick in the groove area on the substrate as shown in FIG. 2. The electro-chromic material layer develops its color when a voltage is applied to between the transparent electrode layer and the electrode layer. However, because the distance between electrodes in the land area is shorter, the land area develops its color earlier. If the land area develops its color due to a voltage applied thereto as described above and the voltage application stops or it is controlled low while the groove area does not develop its color enough to suppress or keep the color development, then recording and reading are done accurately even upon occurrence of a tracking offset caused by the faster rotation of the disk during recording/reading. This is becasue the light absorption becomes higher only in the land area.

A very thin conductor layer (metallic layer or transparent electrode layer) should preferably be formed between a recording layer and a photo-conductor layer so as to suppress mutual dispersion/reaction. If the layer is formed, the reliability of the medium will increase more when in repetitive rewriting. In this case, however, the film thickness should be 1 nm or over and 10 nm or under so that the photo-carrier generated in the photo-conductor layer breaks through the layer. The film may be a band-like or reticulate discontinuous film. For example, if a 5 nm thick W.sub.80Ti.sub.20 electrode layer is formed between the recording layer and the photo-conductor layer, the surface potential can be set in uniform, mutual dispersion between the recording layer and the photo-conductor layer can be prevented during rewriting, thereby the number of rewriting times is improved by a single digit.

A plurality of radial pattern transparent electrodes may be replaced with one electrode formed all over the disk surface. However, when the plurality of radial pattern electrodes are used, the capacitance between electrodes becomes smaller, thereby the voltage supply can start and stop quickly. The capacitance between electrodes should preferably be 0.1 F or under, since both time and electric current required for coloring/decoloring are within a predetermined range. The recording medium should be structured so as to control the capacitance between electrodes to 0.1 F or under to keep the good characteristics of the elements. Instead of dividing the transparent electrode, the metallic electrode may be divided or both of the upper and lower electrodes may be divided. If an electrode is divided such way, whether or not the breaks of the divided upper and lower electrodes are aligned is no matter.

A leader electrode is formed at the innermost periphery of each of the above transparent electrodes and a layer as both the reflection layer & electrode. These leader electrodes reaches the innermost area of the disk and is connected to corresponding one of a plurality of electrodes 35 and 36 disposed at the end face of the disk's center hole so that it is connected to another electrode on the disk rotary shaft of the recording/reading apparatus as shown in FIG. 4. As shown in FIG. 7, six electrodes are bonded separately (only three 65 to 67 of the six electrodes are shown in FIG. 7) on the side of the rotary shaft 61 which extends through the disk sustainer 68 at a height, where the disk is to be loaded o. At one place around the rotary shaft is formed a tapered projection 70 or a concave in the vertical direction, which is fit by a concave or convex formed in the disk's center hole to positioning the disk, thereby predetermined electrodes come in contact with each other. Each electrode of the disk's rotary shaft receives a power supplied from the circuit board of the recording apparatus by a combination of a plurality of brushes and rings 62 to 54. The power supply method may be replaced with another. For example, instead of the brushes and rings (slip rings), a non-contact method may be employed. The non-contact method combines laser, or LED with a solar battery. The rotary-connector method uses a rotary connector in which a tip of a contact put in a mercury tank is rotated. The rotary connector is available on the market.

A UV-curing resin is coated on the film surface of the above-described disk member, then stacked on another same shape substrate to complete the disk-like information recording medium of the present invention.

The laser beam for recording/reading is irradiated on the medium from the substrate side. The transparent electrode layer formed on the medium last may be used as a transparent electrode layer so as to irradiate the laser beam on the medium from the protective substrate side. In this case, the thickness of the recording film is decided so that the reflection rate becomes about 10% to satisfy a required read contrast ratio.

[Initial Crystallization]

The phase-change recording layer of the disk fabricated as described above is crystallized in the initial stage as follows. The disk is turned and a laser beam having a power of 800 mW is applied to the recording layer 24 through the substrate 28. The laser beam spot of the semiconductor laser (wavelength: approx. 810 nm) is elliptic long in the radial direction of the medium. The laser beam spot is then moved step by step in units of 1/4 of the spot length in the radial direction of the medium. The medium is thus crystallized in the initial stage. This initial crystallization may be done only once. When this crystallization is done twice, the noise to be caused by crystallization is a little suppressed.

(Recording, Erasing, and Reading)

Information is recorded/read in/from the above-described recording medium as follows. Hereinafter, how the recording/reading is done will be described with reference to FIG. 9. The ZCAV (Zoned Constant Linear Velocity) method is employed for controlling the motor to change the rotation speed of the disk in each zone for recording/reading.

Information from external is received in units of 8 bits and transferred to an 8-16 modulator 8-8. When information is to be recorded on the information recording medium (hereinafter, to be referred to as the optical disk) 8-1, a so-called 8-16 modulation method is used to convert 8-bit information to 16-bit information. This modulation method records information having a 3T to 14T mark length corresponding to 8-bit information. The 8-16 modulator 8-8 in FIG. 9 performs such the modulation. The "T" mentioned above means a clock cycle at the time of recording. The optical disk is turned so that the relative speed with respect to each light spot becomes a linear speed of 15 m/s.

The 3T to 14T digital signals converted by the 8-16 modulator 8-8 are transferred to a recording waveform generator 8-6, so that a multi-pulse recording waveform is generated there.

At this time, the power level for forming recording marks is set at 5 mW, an intermediate power level for erasing the recording marks is set at 2 mW, and a reduction power level is set at 0.1 mW respectively. The laser power for forming recording marks can be lowered in response to the rising of the applied voltage. The recording is done satisfactorily within a range over 0.5 mW to 5 mW. No significant change appeared within the range even when the linear speed is changed to another from 15 m/s. Reading from the medium is done at 1 mW with no voltage application. Reading is practically possible within a range over 0.2 mW to 2 mW. When in reading at a power level over 2 mW for a long time, recorded data is degraded. In the above-described recording waveform generator, 3T to 14T signals are corresponded to "0" and "1" alternately in a time series. At this time, non-crystallization occurs in each area (mark area) in which high power level pulses are applied. The recording waveform generator 8-6 has a multi-pulse waveform table corresponding to a method (adaptive recording waveform control) for changing the pulse widths of both leading and trailing pulses of the multi-pulse waveform in correspondence with the length of each space before and after each mark area. The recording waveform generator 8-6 thus uses the table to generate multi-pulse recording waveforms free of the thermal interference that might occur between recording marks.

The recording waveform generated by the recording waveform generator 8-6 is transferred to a laser driver circuit 8-7 and the laser driver circuit 8-7 emits a light of the semiconductor laser disposed in the optical head 8-3 according to this recording waveform.

The optical head 8-3 installed in this recording apparatus uses a semiconductor laser having a light wavelength of 400 nm used for recording information. The objective lens having a lens NA of 0.65 focuses this laser beam on the recording layer of the optical disk 8-1 to apply the laser beam to the target area so as to record information therein.

In the phase-change recording layer, the reflection rate of the medium becomes higher in the crystal state than that in the non-crystal state which is set after information is recorded therein. While information is recorded by means of laser beam irradiation, a 5 V voltage is applied continuously to between upper and lower electrodes of the recording layer. And, photo-carriers (electrons, pairs of positive holes) are generated in the Se--As layer, which is a photo-conductor layer, due to the irradiated pulse laser beam, thereby the electric resistance lowers. As a result, the voltage applied to this portion in the recording layer rises, thereby an electric current path is formed in the recording layer. In addition, an area is formed in the recording layer. In the area, the fusing point is exceeded by the Joule heat of the electric current. After this fusing, the electric resistance in this area rises, thereby the electric current path disappears and the area cools down to become non-crystallized. Consequently, both deflection rate and extinction coefficient are changed, thereby signals can be read from this area optically. By repeating the irradiation of this pulse laser beam according to information signals, non-crystal recording mark strings are formed. When the recording is speeded up, the laser beam point is moved fast and the electric current keeps flowing until the recording layer is fused and the electric resistance in the area rises even after the laser beam irradiation as shown in FIG. 8 (electric current changes with time). The electric current thus flows in a plurality of places simultaneously, then the current stops sequentially in order of current application start.

Because the recording is done in such the mechanism, the current flowing time is almost fixed regardless of the radius of the disk, although the laser spot passing time depends on the radius of the recording track. It is therefore easy to record information at a fixed ration speed (CAV) that is difficult on ordinary phase-change optical disks. A voltage is applied sequentially to each of the plurality of divided transparent electrodes in each laser-beam-irradiated area.

Because of the high recording sensitivity, the above recording medium enables recording to be made in a plurality of laser spots simultaneously. In addition, because the light absorption is not required so much, both high reflectivity and high transmittance are obtained for recording and a high S/N ratio is assumed for reading. When voltage application is suppressed upon reading, a high laser power is assumed for reading, thereby a high S/N ratio is obtained.

The recording medium in this embodiment is structured so that upper and lower electrodes are disposed closely to each other only in the groove area. A high electric field is thus applied just in a narrow range in the recording film. Consequently, stable information recording is assured regardless of slight changes of the laser spot and/or the light condensing level, thereby the recording medium becomes permissive to the AF and tracking offsets, sensitive to a light, and suited for recording at a fast rotation of the medium.

The recording medium in this embodiment can also obtain a light reflection contrast ratio of about 2:1 between recording marks and other portions. When the contrast ratio lowers, the fluctuation of read signals to be caused by noise exceeds 9% of the upper limit, thereby practical read signal quality comes to exceed the limit. To avoid this problem, SiO.sub.2 is included in the transparent electrode layer to form a (SiO.sub.2).sub.40(In.sub.2O.sub.3).sub.55(SnO.sub.2) layer. As a result, the reflection rate lowers to improve the contrast ratio to 2.5:1 or over.

Because of the recording principles as described above, a plurality of laser beam spots are formed in the same recording track or different recording tracks with use of a single or a plurality of optical heads, thereby information is recorded easily in the track(s) simultaneously.

When in erasing, an applied voltage is lowered and the laser beam is applied continuously to the target non-crystal area to crystallize the area. A pulse laser beam may be used for this erasing and the laser beam pulse may be wider than any of the recording pulses.

The recording apparatus of the present invention can employ a method for recording information in the land area (a variation of the so-called in-groove recording method).

The above optical head(s) are also used for reading recorded information. Concretely, a laser beam is irradiated on each recorded mark and reflected beams from the mark and another portion are detected to obtain a read signal. The amplitude of this signal is amplified by a preamplifier circuit, then converted to 8-bit information by an 8-16 demodulator 8-10 in units of 16 bits. This completes the reading of the recorded marks.

When in recording with use of mark edges under the above conditions, the shortest mark 3T becomes about 0.20 .mu.m and the longest mark T14 becomes about 1.96 .mu.m in length respectively. Each record signal includes dummy data in both start and end parts respectively; in the dummy data, a 4T mark and a 4T space are alternated and the start part also includes a VFO.

(Mark Edge Recording)

The mark edge recording method is employed for high density recording in DVD-RAM and DVD-RW. This mark edge recording makes both edges of each recording mark formed on the subject recording film correspond to "1" of digital data, thereby the length of the shortest recording mark can be corresponded to 2 to 3 reference clock pulses so as to realize high density recording. The DVD-RAM employs the 8-16 modulation method and extends the length of the shortest recording mark to three reference clock pulses. When compared with the mark position recording method that makes the center of each circular recording mark to "1" of digital data, the mark edge recording method is more effective, since it realizes high density recording without reducing the recording marks so much in size. In spite of this, it is required that shape distortion of recording marks must be minimized for the recording media that employ this mark edge recording method.

(ZCLV Recording Method and CAV Recording Method)

Phase-change recording media, when the recording waveform remains the same, should preferably be enabled to record information at an optimal linear speed corresponding to the crystallization speed to obtain satisfactory recording/reading characteristics. When accessing a space between recording tracks that are different from each other in radius on a disk, it takes much time to change the disk rotation speed so as to equalize the linear speed between those tracks. In order to solve this problem, the DVD-RAM employs the ZCLV (Zoned Constant Linear Velocity) method, which divides the disk surface into 24 zones in the radial direction, fixes the disk rotation speed in each zone, and changes the disk rotation speed only when a zone to access must be changed to another. According to this method, the linear speed differs slightly between the innermost track and the outermost track in each zone, so that the recording density also comes to differ between those tracks. Nevertheless, the method makes it possible to record information almost at the maximum density all over the disk.

On the other hand, the CAV recording method that keeps a fixed disk rotation speed is suited for recording in which the disk rotation is kept as a fixed speed even upon an access to be made by skipping a far distance in the radius direction. This method is also suited for mobile devices that can suppress the power consumption required for changing the disk rotation speed. As described above, because the present invention also makes it possible to keep a fixed heating time regardless of the position in the radial direction of the disk, it makes the CAV recording easier.

The present invention also considers preventing of re-crystallization as important. This is because the temperature in adjacent tracks is apt to rise when re-crystallization occurs in peripheral areas of a recording film that is fused due to the recording therein, thereby remaining non-crystallized recording mark area is narrowed and a wider area must be fused to form recording marks in a predetermined size. The present invention can also prevent such the re-crystallization, since the heat conductivity of the transparent electrodes is low, so that the heat dispersion towards the inner periphery of the disk is not so much. The present invention also makes it possible to prevent a problem that the heat in the center of each recording mark is diffused in the transverse direction and the peripheral area of the fused area cools down slowly, thereby the re-crystallization that is apt to occur around there is suppressed.

(Tracking Margin)

In this embodiment, the upper electrode is in direct contact with the recording film in the land area and an SiO.sub.2 layer that is an insulator layer is disposed b