JP3165354B2 - Optical information recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium. More specifically, the present invention relates to a phase change type recording medium having a high density and having little deterioration with respect to a large number of repeated recordings.

[0002]

2. Description of the Related Art In recent years, as the amount of information has increased, there has been a demand for a recording medium capable of recording and reproducing a large amount of data at a high density and at a high speed. However, an optical disk is expected to exactly meet such a use. I have. Optical discs include a write-once type, which allows recording only once, and a rewritable type, which allows recording / erasing any number of times.

[0003] Examples of the rewritable optical disk include a magneto-optical recording medium utilizing a magneto-optical effect and a phase change medium utilizing a reflectance change accompanying a reversible change in crystal state. The phase change medium does not require an external magnetic field, and has the advantage that recording and erasing can be performed only by modulating the power of laser light, and the recording and reproducing apparatus can be downsized. Further, there is an advantage that a medium capable of recording and erasing at a wavelength of about 800 nm, which is currently mainstream, can be converted into a high-density recording medium with a short wavelength without changing the material.

As such a recording layer material of a phase change medium, a chalcogen-based alloy thin film is often used. For example, GeTeSb, InSbTe, GeSnTe
And the like. Generally, in a rewritable phase change recording medium, an unrecorded / erased state is changed to a crystalline state, and an amorphous bit is formed.

[0005] Amorphous bits are formed by heating the recording layer to a temperature higher than its melting point and rapidly cooling it. In this case, the dielectric layer functions as a heat radiation layer for obtaining a sufficient supercooled state. On the other hand, erasing (crystallization) is performed by heating the recording layer to a temperature higher than the crystallization temperature of the recording layer and lower than the melting point.

In this case, the dielectric layer functions as a heat storage layer for keeping the temperature of the recording layer high until crystallization is completed.
In a so-called one-beam overwrite-capable phase change medium, the above-described erasing and re-recording processes can be performed only by intensity modulation of one focused light beam (Jp).
n. J. Appl. Phys. , 26 (1987) s
uppl. 26-4, p. 61-66).

The phase change recording medium capable of one-beam overwriting has the advantages that the time required for writing information is reduced, the magnetic field is unnecessary, the drive configuration is simple, and the cost is low. Further, the phase change type medium has the same track pitch, (linear recording density) as compared with the magneto-optical medium.
And when recording with the shortest pit length (vertical recording density),
Since there is little deterioration due to a decrease in optical resolution and a large signal amplitude can be obtained, there is an advantage that high density can be easily achieved.

In order to improve the recording density, especially the linear density, the track pitch must be reduced. In the conventional method in which tracks are formed by grooves and unevenness between grooves, it was difficult to make the track pitch smaller than 1.0 μm due to the limit of fine processing using a photoresist.

In recent years, as a means for dramatically reducing the track pitch for the purpose of improving the recording density, a method of performing recording both in and between guide grooves provided on a substrate (land). And groove recording, L & G recording)
In addition, a sample servo method has been proposed in which a light beam is guided by an arrangement of concave pits without providing a guide groove.

When these methods are used, a linear density of 1.0 μm
Track pitches of less than m are easily achieved. In the L & G recording method using the phase change medium, 68
By combining with an optical head having 0 nm and an NA (numerical aperture of a focusing lens) of 0.6, the CD size (diameter of 120
mm), it is reported that 3 to 4 times higher densification can be achieved (Jpn. J. Appl. Phys., 32 (1)
993), pp5324-5328). This allows
It is said that a high-quality moving image of one hour or more can be recorded by using the image compression technique together.

[0011]

However, when recording is repeatedly performed on a specific track with a track pitch of less than 1.0 μm in a phase change medium, a track or land adjacent to the specific track is also raised. Since the temperature rises, there is a problem that amorphous pits are gradually crystallized and information is lost.

[0012] We call this cross-erase. When performing L & G recording or sample servo recording,
This is because, unlike conventional recording on one of the lands or grooves, there is no unevenness between the adjacent tracks that has the effect of blocking heat conduction, so that the temperature of the adjacent track is easily increased by thermal diffusion.

Therefore, the limit of the substantial linear density is limited by the limit of thermal separation (cross erase) rather than the optical resolution, that is, signal leakage (cross talk) of adjacent tracks. According to the study by the present inventors, a semiconductor laser having a wavelength of 680 nm and an optical head having an NA of 0.55 each have a groove and a land of 0.1 mm each.
When L & G recording is performed on a medium having a width of 7 μm at a linear velocity of 3 m / s, the carrier level of a signal recorded on an adjacent land or groove by overwriting 1000 times is 3 to 5
dB decreased.

However, in a normal recording medium, 10
In many cases, recording is repeatedly performed 0 or more times only when the file management information on the recording medium is rewritten. That is, only a very limited area, which is provided on the inner or outer circumference of the disk, called FAT in the DOS format or TOC in the CD format, is frequently rewritten.

This frequently rewritten area is usually
Less than 1% of the entire recordable area. Like UNIX,
The file management information may be physically distributed, but in that case, the average number of rewrites may be considered, and there is almost no possibility that a specific area is rewritten 10,000 times or more. In general, with regard to future formats as well, it can be considered that the situation is such that rewriting is concentrated only in a very narrow area that is physically distinguished by performing recording while distinguishing between the file management area and the file content itself. That is, the fact is that the recording density of the entire medium is limited by the frequently rewritable area of less than 1%.

[0016]

In view of these circumstances, the present inventors have found a method for achieving a high linear density without substantially impairing the durability against rewriting many times.
The present invention has been reached. That is, the gist of the present invention is that file management information is recorded on an optical information recording medium that reversibly records, erases, and reproduces information by utilizing an optically identifiable crystal or amorphous state. The track pitch of the area where the file management information is recorded is set to 0.6λ / NA, and the track pitch of the area where the file management information is recorded is set to 0.6λ / NA.
(Λ: wavelength of used laser beam, NA: numerical aperture of lens for converging laser beam).

The track pitch of a general user recording area is determined by the optical diffraction limit. In practice, the track pitch is selected so that the amount of signal leakage, that is, crosstalk, from adjacent two adjacent tracks is -20 dB or less. on the other hand,
The track pitch in the file management area is set to be 1.05 to 1.5 times wider than the recording area. Substantially less than 100 such tracks are sufficient. 1.05
If it is less than twice, it is not preferable because the recording track pitch in the user area can be improved only by about 5%.

If it is 1.5 times or more, the user area and
The track pitch difference from the file management area is too large,
The track servo circuit must be separate,
Improper. Further, the track pitch of the file management area is 0.6λ / NA (λ: used laser beam wavelength, N
A: If not larger than (the numerical aperture of the laser beam converging lens), the cross erase becomes remarkable.

The coefficient of 0.6 is explained as follows. That is, the cross erase first depends on the size of the convergent light beam spot diameter. Since the light beam spot diameter is proportional to λ / NA, the minimum allowable pitch is λ / N
It can be considered proportional to A. The proportionality coefficient may be accurately determined based on experiments. In fact, the present inventors have conducted various studies and found that the C / N ratio (carrier to carrier ratio) after 10 ⁴ overwrites was found to be larger than 1.2 (λ / NA) for the groove pitch or the space between grooves for L & G recording. Noise ratio) can be reduced to less than 3 dB, which can be a practically acceptable level.

Since the substantial recording track pitch of L & G recording is half of the groove pitch, if the minimum recording track pitch is larger than 0.6 (λ / NA), signal deterioration of adjacent tracks due to cross erase can be prevented. It was confirmed experimentally that it was possible. The value 0.6 above theoretically corresponds to exactly half of the beam spot of the convergent light passing through the lens. That is, the convergent light beam has a sub-peak in the intensity distribution due to the diffraction effect. The diameter of the central spot is
It is approximately represented by 1.2 (λ / NA). This is called an airy disk. Further, the light intensity distribution therein is not uniform, and the diameter at which the intensity is 1 / e ² (e is the base of natural logarithm) is expressed as 0.82 (λ / NA).

Since the minimum pitch of the track corresponds to the radius of the area disk, the cross-erase phenomenon is, as a first approximation, caused by the weak laser light at the foot of the Airy disk of the focused light beam spot. The physical meaning of the temperature rise was also clarified.

As a more specific means for realizing the present invention, the file management area having a wide track pitch is provided at the innermost or outermost circumference of the recording area of the disk. Further, by providing an unrecorded area for at least one track at the boundary between the wide area and the narrow area of the track pitch, the boundary at which the track pitch changes is clarified.

The recording medium of the present invention utilizes, as a recording layer, a reversible change between an optically identifiable crystalline and amorphous state or two metastable and identifiable crystalline states. More specifically, GeSbTe, GeSb
Alloys containing a chalcogen element as a main component, such as Sn, InSbTe, and AgInSbTe, are known.

The fact that the cross-erase phenomenon is hardly affected by the heat conduction of the recording layer is known from GeS.
bTe, AgInSbTe, InSnTe, InSbT
e or any of the group IIIb, IVb, Vb and VIb elements or a mixture (alloy) thereof as a main component at 40 at. This is because the thermal conductivity of the recording layer containing at least% is smaller by two to three orders of magnitude than that of a magneto-optical medium or the like. And, it is substantially adiabatic in the order of 10 to 100 nanoseconds required for recording.

Therefore, the minimum track pitch determined by the above 0.6λ / NA is substantially the beam spot diameter, and
It is determined only by the light beam wavelength and NA. However, the cross-erase after repeated overwriting of 10,000 times or more is slight, but can be further reduced by limiting the layer structure of the recording medium and the physical properties of the recording layer.

Although it depends on the melting point and the crystallization temperature of the recording layer, the melting point (Tm) of the alloy recording layer is known to be capable of reversible change between crystal and amorphous. Less than 700 ° C, crystallization temperature (Tg) 150
Many are at or above ° C. Actually, in the vicinity of Ge ₁ Sb ₂ Te ₄ or Ge ₂ Sb ₂ Te ₅ composition, the melting point is 600
６620 ° C., crystallization temperature 150１５０170 ° C.

Ag _0.11 In _0.11 Te _0.23 Sb _0.55
Has a melting point of about 550 ° C. and a crystallization temperature of about 230 ° C. If the Tg is lower than 150 ° C., the stability of the amorphous state is poor and cross-erasing is likely to occur. Also, Tm is 700 ° C.
In this case, the energy to be irradiated at the time of recording increases, and cross-erasing is likely to occur in the adjacent track. Regarding the layer configuration, if the thickness of the recording layer exceeds 30 nm, the recording sensitivity is reduced, and heat is easily released to an adjacent track during recording, so that cross-erasing is likely to occur.

The minimum track pitch is determined by the recording track pitch and the radius of the convergent light beam. Therefore, not only in the case of L & G recording, but also sample servo recording provided with pre-pits and conventional grooves. The same applies to the inter-groove (land) recording provided with. However, in the case of L & G recording, it is known that it is desirable to set the groove depth to approximately λ / 6n (n is the refractive index of the substrate) in order to reduce crosstalk.
It is desirable to apply it in conjunction with the present invention.

Further, it is desirable to make the widths of the groove and the land substantially equal, and to make the signal amplitudes of the two substantially equal. It is known that when recording is performed on only one of a groove and a land at a narrow pitch, it is desirable to set the groove depth to approximately λ / 8n in order to obtain a servo signal by the push-pull method. Further, in sample servo recording, it is known that it is desirable to set the depth of the servo pit to approximately λ / 4n, so it is desirable to apply the present invention together with the present invention.

Usually, these recording layers are sandwiched between protective layers made of a dielectric thin film having a melting point much higher than the melting point of the recording layer. Further, in order to utilize the interference effect, a reflective layer may be further provided. These multilayer structures are formed on a resin or glass substrate transparent to light used for recording and reproduction.

Usually, a resin substrate made of polycarbonate, polyolefin or the like, which is inexpensive and can easily form a fine uneven structure (preformat information) by injection molding, is often used. Overwriting on the phase change medium is
As shown in the figure, this is done by modulating the focused light beam. For example, in a GeSbTe phase change medium, the initial and erased states are crystalline, and the recording pits are amorphous.

An amorphous pit is formed by melting the recording layer at least at the melting point or higher with the recording power Pw.
With the bias power Pe, the recording layer is heated to a temperature higher than its glass transition point and lower than its melting point to be recrystallized to be in an erased state. In the InSbTe system, the erasing power Pe is set to P
Melt erasure, in which the temperature is raised to a value lower than w and higher than the melting point of the recording layer, is also performed. The present invention is not limited to these recording layer materials, layer configurations, and recording / erasing methods.

[0033]

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples. Example 1 A polycarbonate resin substrate having a spiral groove is formed by injection molding.

As a recording layer, a GeSbTe phase change medium was formed. The widths between the grooves are almost equal in order to provide for L & G recording. As the groove pitch (distance from groove to groove),
A 1.6 μm portion and a 1.4 μm portion were formed. In each case, the width of the grooves or the space between the grooves is approximately 0.8 μm and 0.7 μm, respectively.

Since recording is performed in both the groove and the space between the grooves, the substantial track pitch is 0.8 μm and 0.7 μm.
Becomes The 0.8 μm track pitch is 1.14 times the 0.7 μm pitch. The groove depth is about 700 ° in each case. An optical head having a wavelength of 680 nm and an NA of 0.55 was used. Linear velocity is 3m / s, PW = 8-9m
W, Pe = 4.5 mW.

The recording power is a frequency of 2.24 MH.
The modulation was performed in a single pattern of z and 25% duty. Even if the track pitch is different, if it is within this range,
There is no problem in tracking. FIG. 2 shows a decrease in carrier level of a signal recorded on an adjacent land when overwriting is repeatedly performed in a groove.

[0037] In the groove pitch 1.4 mu m (substantially track pitch 0.7 [mu] m) (land width 0.7 [mu] m sections), although the carrier level with the repetitive overwriting was reduced about 3dB at 1000 times, usually There is no problem if it is used as a data recording area. On the other hand, groove pitch 1.6 μm (effective track pitch 0.8 μm)
Since almost no deterioration is observed in the portion after overwriting 10,000 times, it can be understood that the portion can be used as the file management information area. Since 0.6λ / NA = 0.74, the 0.8 μm pitch is larger than 0.6 / NA.

Since the file management area is less than 1%, the actual recording capacity is determined by the pitch of 1.4 μm . Of course, it is possible to further reduce the track pitch by shortening the laser light wavelength, increasing the glass transition point of the recording layer to improve heat resistance, or increasing the NA. In any case, by setting the track pitch of the file management area to be 1.05 to 1.5 times the track pitch of the data area as in the present invention, it is possible to prevent repeated recording as compared to the case where all track pitches have the same width. An optical information recording medium with little deterioration can be obtained.

Making the track pitch different between the file management area and the data area as in the present invention is always effective in a format in which the file management area is physically concentrated in a specific area. is there. Currently,
As described above, the FAT area in the DOS format and the TOC area in the CD format can be applied.

However, the present invention is not limited to storage media using these formats. Even if the format does not exist at present, as long as the file management area is physically concentrated. Needless to say, it is effective. The present invention is more effective as the ratio of the capacity of the file management area to the total capacity of the medium is smaller.

The ratio is not necessarily limited. Further, in the above specific example, so-called L & G recording has been described. However, the present invention is effective for all phase change media having a small track pitch and causing a problem of cross erase. Therefore, for example, a sample servo system with a track pitch of less than 1.0 μm is also effective.

Also, even with the method of recording data on one of the current group and land, if the track pitch becomes smaller than 0.8 μm with the progress of fine processing technology in the future, it is still effective. It becomes a means of densification. The one-beam overwriting method also does not depend on a recording method such as a method of dividing a long-bit recording pulse for recording, and it goes without saying that the present invention is effective.

[0043]

According to the optical information recording medium of the present invention, it is possible to obtain a phase-change type recording medium having a high density and a small deterioration with respect to a large number of repeated recordings.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a method of overwriting a phase change medium.

FIG. 2 is a graph showing a change in a carrier level of a signal according to the first embodiment.

[Explanation of symbols]

 Pw Recording power Pe Bias power

Continued on the front page (72) Inventor Kenichi Takada 1000 Kamoshita-cho, Aoba-ku, Yokohama-shi, Kanagawa Prefecture Mitsubishi Chemical Corporation Yokohama Research Laboratory (72) Inventor Hironori Mizuno 1000 Kamoshida-cho, Aoba-ku, Yokohama-shi Kanagawa Prefecture Inside Yokohama Research Institute (56) References JP-A-7-134826 (JP, A)

Claims (5)

(57) [Claims] 1. An area in which file management information is recorded in an optical information recording medium for reversibly recording, erasing and reproducing information by utilizing an optically identifiable crystal or amorphous state. Characterized in that the track pitch of the optical recording medium is 1.05 to 1.5 times wider than the track pitch of the other data recording area. 2. The method according to claim 1, wherein a track pitch of an area in which the file management information is recorded is larger than 0.6λ / NA (λ: wavelength of laser light used, NA: numerical aperture of a laser light focusing lens). The optical information recording medium according to claim 1. 3. The optical information recording medium according to claim 1, wherein the file management area having a wide track pitch is provided on an inner circumference or an outer circumference of a recording area of the disk. 4. An unrecorded area for at least one track is provided at a boundary between a wide area and a narrow area of the track pitch.
The optical information recording medium according to any one of the above. 5. The optical information recording medium according to claim 1, wherein the melting point of the recording layer is less than 700 ° C., and the crystallization temperature is 150 ° C. or more. .