JP3625541B2 - Optical recording medium and recording / reproducing method - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an optical recording medium and a recording / reproducing method, and more particularly to an optical information recording medium and a recording / reproducing method for recording, reproducing, and erasing information in and between a groove portion of a substrate by irradiation with a laser beam.
[0002]
[Prior art]
In recent years, as the amount of information increases, a recording medium capable of recording and reproducing a large amount of data at high density and at high speed is demanded. However, an optical disc is expected to meet such a use.
The demand for higher capacity and higher density of such recording media is inevitable in the era when it was imposed on recording media and recording devices to handle enormous amounts of image information and audio signals. Digital modulation technology and data compression technology Together with the progress of, the progress is just a step forward.
[0003]
As a specific means of increasing the density, in an optical disk, the convergent beam diameter of irradiated light is reduced by shortening the wavelength of the light source and increasing the NA (Numerical Aperture) of the lens, shortening the recording mark length, and maintaining a constant rotation speed. Originally, MCAV (Modified Constant Angular Velocity), which increases the recording frequency toward the outer periphery and keeps the recording density at the inner and outer periphery constant, mark edge recording that puts information on the start and rear ends of the mark, has been developed and used. Therefore, the present situation is that a method for further densification is being sought for the future.
[0004]
In a recordable optical disc, a guide groove is cut in advance on the disc to form a so-called track.
Usually, information signals are recorded, reproduced or erased by condensing laser light between guide grooves or in guide grooves.
In general optical disks currently on the market, information signals are usually recorded only between guide grooves or within guide grooves, and the other is a boundary for separating adjacent tracks to prevent signal leakage. It only plays a role.
[0005]
If information can be recorded in the boundary portion, for example, between guide grooves when recording between guide grooves, and between guide grooves when recording within guide grooves, the recording density is The recording capacity is doubled and a significant improvement in recording capacity can be expected.
Hereinafter, a method of recording information on the guide groove as a groove, between the guide grooves as a land, and on both the land portion and the groove portion is described as L & G recording.
[0006]
Japanese Patent Publication No. Sho 63-57859 is proposed as an L & G recording proposal. When such a technique is used, it is necessary to pay special attention to reducing crosstalk.
That is, in the above-mentioned L & G recording described in Japanese Patent Publication No. 63-57859, the interval between the recording mark row of a certain track and the recording mark row of the adjacent track is half of the convergent beam diameter. The convergent beam diameter overlaps to the adjacent recording mark row.
[0007]
For this reason, there is a problem that crosstalk during reproduction increases and reproduction S / N deteriorates.
In order to reduce this crosstalk, for example, SPIE Vol. 1316 Optical Data Storage (1990) pp. 1316. As shown in FIG. 35, there is a technique for reducing crosstalk by providing a special optical system and a crosstalk cancel circuit in the optical disk reproducing apparatus.
[0008]
However, this method has a disadvantage that the optical system and signal processing system of the apparatus become more complicated.
As a method of reducing crosstalk without providing any special optical system or signal processing circuit for reducing reproduction crosstalk, the width of the groove (guide groove) and land (between the guide grooves) are made equal. There is a proposal that it is effective to set the groove depth within a certain range corresponding to the reproduction light wavelength. (Jpn. J. Appl. Phys. Vol 32 (1993) pp. 5324-5328).
[0009]
According to this calculation, it is calculated that crosstalk is reduced when land width = groove width and groove depth is λ / 7n to λ / 5n (λ: reproduction light wavelength, n: refractive index of substrate). And shown as experimental facts.
This is also described in JP-A-5-282705.
According to the CN ratio (carrier / noise ratio) and the groove depth dependency of crosstalk described in this paper, the effect of reducing crosstalk can be seen by setting the groove depth to an optimum value. And the CN ratio in the groove part is unbalanced.
[0010]
When L & G recording is performed, a difference occurs between the carrier level of the land portion and the carrier level of the groove portion, and as a result, the CN ratio of one of them is remarkably lowered.
On the other hand, when packing the track pitch due to high density, it is usually sufficient to select the track pitch, groove shape, etc. so that the amount of crosstalk is below a predetermined level. There is another issue that must be considered.
[0011]
That is, when a track is repeatedly overwritten, the amorphous bit of the adjacent track disappears (recrystallizes).
The reason for this is not necessarily clear, but the adjacent track is heated by the weak laser beam at the bottom of the intensity distribution of the focused light beam at the time of recording the adjacent track, and the temperature of the amorphous bit part exceeds the crystallization temperature. This is thought to be due to heating.
[0012]
Although the time is several hundred nanoseconds at a time, it is gradually recrystallized while being repeatedly heated.
For example, when the overwrite is 10,000 times, the C / N ratio (carrier-to-noise ratio) of the adjacent track that initially was 55 dB may be reduced to less than 50 dB.
[0013]
This problem is hereinafter referred to as cross erase. In the phase change medium, attention must be paid to the minimum track pitch due to cross erase rather than the optical diffraction limit, but the limit is not necessarily clear.
Furthermore, as a result of our diligent investigations, when the groove width was narrowed and the density was increased by narrowing the track pitch while keeping the groove and land width at 1: 1, the previous mark after repeated overwriting was improved. It was found that the characteristic deterioration in the land portion was remarkable in terms of unerased residue and deterioration of jitter of the recording mark.
[0014]
[Problems to be solved by the invention]
The present invention solves such a problem. In particular, in an L & G recording type optical disc using a laser beam having a wavelength of 700 nm or less as a light source, the carrier level imbalance between the land portion and the groove portion is eliminated. It is an object of the present invention to provide a high-density optical disk that can obtain the same high signal quality regardless of whether it is recorded in the groove portion.
[0015]
[Means for Solving the Problems]
The present invention was made as a result of repeated studies on the definition of the groove depth and the phase difference between the reflected light from the unrecorded area and the recorded mark. The gist of the present invention is that the dielectric is formed on the transparent substrate on which the groove is formed. A body layer, a phase change recording layer, a dielectric layer, and a metal reflective layer are sequentially laminated. Information is obtained by irradiating a laser beam having a wavelength of 700 nm or less using both the groove and the land as a recording region. An optical recording medium for recording, erasing and reproducing
(1) The groove width is 0.3 μm or more and 0.8 μm or less, the land width is 0.3 μm or more and 0.8 μm or less,The groove width and land width are almost equal,And the groove depth d satisfies the following inequality,
[0016]
[Expression 7]
λ / 7n <d <λ / 5n
(Λ: wavelength of irradiated light, n: refractive index of substrate, d: depth of groove)
[0017]
(2) Of the reflected light from the unrecorded area and the reflected light from the recorded area defined below, the one with the higher reflectivity is R_high(%), The lower one is R_low(%) And the phase difference between the reflected light from the unrecorded area and the recorded area.2παThen, the following equation is satisfied,
[0018]
[Equation 8]
10 ≦ R_high≦ 40
[0019]
[Equation 9]
R_low/ R_high≦ 0.15
mπ ≠ 2πα    (M is an integer)
However,
2πα= (Phase of reflected light from unrecorded area)-(Phase of reflected light from recorded area)
[0020]
(3) The land width LW satisfies the following equation:
[0021]
[Expression 10]
0.62λ / NA ≦ LW ≦ 0.8λ / NA
λ is the irradiation light wavelength, NA is the numerical aperture of the focusing lens
[0022]
(4) The groove width GW and the land width LW satisfy the following formula:
[0023]
## EQU11 ##
(LW + GW) / 2> 0.6λ / NA
[0024]
This is an optical recording medium.
With the configuration described above, in the optical disc of the present invention, the signal quality (carrier level) of the recording mark is the same regardless of whether recording is performed on either the land portion or the groove portion.
Furthermore, the present invention relates to an optical information recording medium, wherein the recording track pitch is larger than 0.6λ / NA (λ: light beam wavelength, NA: numerical aperture of focusing lens).
[0025]
Further, in order to guarantee durability against repeated overwriting in the land portion, the land width (and therefore the groove width is substantially equal) is set to 0.62λ / NA or more and 0.8λ / NA or less. The present invention relates to an information recording medium.
These are indispensable provisions in terms of guaranteeing the reliability of an L & G recording type optical disk using a laser beam having a wavelength of 700 nm or less as a light source.
How the present invention works in the reproduction process of the optical recording medium for land and groove recording and brings about an effect will be described in detail below using a simple model.
[0026]
1 to 4 schematically show a case where a reproduction light beam is irradiated on a land or a groove of an L & G optical disk.
In order to make the figure easy to see, layers other than the recording layer 2 are omitted.
The reproduction light beam is condensed using an objective lens or the like and is applied to the disk from the substrate 1 side, and is hereinafter referred to as a convergent beam.
1 and 3 show the case where the convergent beam 5 exists in the unrecorded area, and FIGS. 2 and 4 show the case where the convergent beam 6 exists on the recording mark 8.
[0027]
It is assumed that the recording mark 8 is sufficiently longer than the convergent beam 5 in order to simplify the calculation.
As will be shown later in the embodiment, there is no problem even if the recording mark is actually shorter than the convergent beam diameter.
Here, the state of the recording layer when not recorded is defined as a crystalline state, and the state of the recording layer during recording is defined as an amorphous state.
[0028]
The intensity of the convergent beam is Gaussian according to the actual model, and the beam diameter is 1 / e of the center intensity.²It is defined as
Assume that the width of the land 3 (land width) is equal to the width of the groove 4 (groove width) and is half the beam diameter, and the step between the land 3 and the groove 4 is d.
Since the convergent beam is irradiated from the substrate side, it is incident and reflected from the other side of the paper.
[0029]
Therefore, when viewed from the light source side, the land portion 3 is concave and the groove portion 4 is convex.
When the groove surface is used as a phase reference, the phase of the reflected light from the land portion is delayed by 2π · 2nd / λ from the reflected light from the groove portion.
Where n is the refractive index of the substrate, d is the depth of the groove, and λ is the wavelength of the convergent beam.
[0030]
The phase change is not caused only by the groove depth, but the phase difference generally changes also by the change of the optical constant before and after the phase change of the recording layer.
Here, the reflected light from the amorphous region is more than the reflected light from the crystalline region.2παAssume that the phase is only delayed.
Below, the amplitude reflectivity of the convergent beam is set as required with the groove surface as the phase reference.ΑWe will formulate while using.
As shown in FIG. 1, the amplitude reflectivity φ when the convergent beam 5 is in the land 3 without the amorphous recording mark₁Can be expressed as:
[0031]
[Expression 12]
φ₁= R_c1Exp [-2πi · 2nd / λ] + R_c2Exp [-2πi · 0] (a)
[0032]
However, R_c1Is the amount of light reflected from the area 6 of the land portion irradiated with the convergent beam, R_c2Is the amount of light reflected from the region 7 of the groove portion irradiated with the convergent beam, n is the refractive index of the substrate, d is the depth of the groove, λ is the wavelength of the irradiated light, and i is the imaginary unit.
As shown in FIG. 2, the amplitude reflectance φ when the convergent beam 5 is in the land portion where the amorphous recording mark is present.₂Can be expressed as:
[0033]
[Formula 13]
φ₂= R_a1Exp [-2πi (2nd / λ + α)] + R_c2Exp [-2πi · 0] (b)
[0034]
However, R_a1Is the amount of light reflected from the area 6 of the land portion irradiated with the convergent beam, R_c2Indicates the amount of light reflected from the region 7 of the groove portion irradiated with the convergent beam.
No amorphous recording mark as shown in Fig. 3Groove partAmplitude reflectivity φ when there is a convergent beam 5₃Can be expressed as:
[0035]
[Expression 14]
φ₃= R_c1Exp [-2πi · 0] + R_c2Exp [-2πi (2nd / λ)] (c)
[0036]
However, R_c1Is the amount of reflected light from the region 7 of the groove part irradiated with the convergent beam, R_c2Indicates the amount of reflected light from the area 6 of the land portion irradiated with the convergent beam.
As shown in FIG. 4, the amplitude reflectance φ when the convergent beam 5 is in the groove portion where the amorphous recording mark is present.₄Can be expressed as:
[0037]
[Expression 15]
φ₄= R_a1Exp [-2πiα] + R_c2Exp [-2πi (2nd / λ)] (d)
[0038]
However, R_a1Is the amount of reflected light from the region 7 of the groove part irradiated with the convergent beam, R_c2Indicates the amount of reflected light from the area 6 of the land portion irradiated with the convergent beam.
Here, it is assumed that land width = groove width, and the width is half of the convergent beam diameter, so if 0 <β <1,
[0039]
[Expression 16]
R_c2= ΒR_c1                        (E)
[0040]
[Expression 17]
R _a2 = ΒR_a1                        (F)
[0041]
Call it.
R_c=R _c1 + R_c2, R_a= R_a1+R _a2 The formula (e) And formula (f)
[0042]
[Expression 18]
R_c1= R_c/ (1 + β) (g)
[0043]
[Equation 19]
R_c2= ΒR_c/ (1 + β) (h)
[0044]
[Expression 20]
R_a1= R_a/ (1 + β) (i)
[0045]
[Expression 21]
R_a2= ΒR_a/ (1 + β) (j)
[0046]
It becomes.
Substituting the formulas (g) to (j) into the formulas (a) to (d) and rearranging them,
[0047]
[Expression 22]
φ ₁ = [R_c/ (1 + β)] [β + exp [−4πind / λ]] (k)
[0048]
[Expression 23]
φ₂= [1 / (1 + β)]
[ΒR_c+ R_aExp [-4πind / λ-2πiα]] (l)
[0049]
[Expression 24]
φ₃= [R_c/ (1 + β)] [1 + β · exp [−4πind / λ]] (m)
[0050]
[Expression 25]
φ₄= [1 / (1 + β)] [R_a・ Exp[-2πiα] + βR_cExp [-4πind / λ]] (n)
[0051]
Here, when recorded on the land portion, the reproduction carrier level CL ′ (L) is
[0052]
[Equation 26]
CL ′ (L) = | φ₁｜²− | Φ₂｜²                        (O)
[0053]
Is proportional to
Similarly, when recorded in the groove part, the playback carrier level is
[0054]
[Expression 27]
CL ′ (G) = | φ₃｜²− | Φ₄｜²                        (P)
[0055]
Is proportional to
The fact that there is no difference in the carrier level between the land part and the groove part means that the difference between the expression (o) and the expression (p) becomes zero.
Formula (k)-Formula (n)Formula (o) and Formula (p)Substituting into, calculating the difference, and finding the necessary condition for the difference to be zero,2πα= Mπ (where m is an integer).
[0056]
This result is obtained when the phase difference between phase transitions is an integral multiple of π (including 0).=In the case of the groove width, the reproduction signal amplitudes of the land portion and the groove portion are equal.
On the other hand, we have intentionally made a layered disk with a phase difference between phase transitions, and have made extensive studies.
[0057]
As a result, a new condition has been found in which no difference occurs in the signal amplitude between the land portion and the groove portion regardless of any arbitrary value of the phase difference between the phase transitions.
This condition is the reflectivity R of the mirror surface of the disk when the recording layer is in a crystalline state._cAnd mirror surface reflectivity R in the amorphous state_aThe ratio is limited to a certain range.
[0058]
In the first place, when the land width is equal to the groove width, the difference in the reproduction signal amplitude between the land and the groove is related to the phase difference depending on the groove shape and the phase difference between the phase transitions. That is, the difference in the reproduction signal amplitude) is also largely dependent on the degree of interference effect depending on the reflectance ratio between the phase transitions.
That is,R _c And R_aR with higher reflectivity_high, R with the smaller reflectance_lowThen R_lowIs R_highIf the phase difference is sufficiently small, the difference in the reflected light amount between the land and the groove due to the interference is substantially small no matter how the phase difference occurs.
[0059]
For the purpose of actually investigating this, we will compare the phase difference between the phase transitions and R_high, R_lowA large number of discs with different sizes were manufactured, and the influence on the difference in the reproduction signal amplitude between the land and the groove was investigated. As a result, as indicated in claim 1 of the present invention, R_highIn a disc whose range is limited to a range of 10% to 40%,R _low / R _high It has been found that the land recording signal quality and the groove recording signal quality in L & G recording can be made equal even if the phase difference between phase transitions is 0.15 or less.
[0060]
Necessary for thisR _low / R _high The range can be specified by appropriately selecting the optical constant and film thickness of each layer.
For the groove depth of the substrate, see Jpn. J. et al. Appl. Phys. Vol 32 (1993) pp. As described in 5324-5328, crosstalk from adjacent tracks is reduced when the groove depth is λ / 7n to λ / 5n (λ: reproduction light wavelength, n: refractive index of the substrate). In this range, it is desirable.
[0061]
Here, a method for measuring the groove width and the groove depth will be described. Measurement, Les-0th-order light intensity I irradiating the laser beam (wavelength 630 nm) from the side of the substrate not having grooves and diffracting the transmitted light by the substrate grooves₀Primary light intensity I₁Secondary light intensity I₂And by measuring the angle of the diffracted light. When P is the groove pitch, w is the groove width, d is the groove depth, λ is the laser wavelength, θ is the angle between the 0th order light and the 1st order light,
[0062]
[Expression 28]
I₂/ I₁= Cos²(Πε)
[0063]
[Expression 29]
I ₁ / I ₀ = {2sin²(Πε) (1-cosδ)} / [π²{1-2ε (1-ε) (1-cosδ)}]
[0064]
[30]
ε = w / P, δ = 2 (n−1) πd / λ (n is the refractive index of the substrate)
[0065]
[31]
P = λ / sinθ
[0066]
Therefore, the groove width and the groove depth are calculated. The actual groove shape is not a perfect rectangle, but the groove shape in the present invention uses values in which the groove width and groove depth are uniquely determined by the measurement method described above.
Therefore, the groove shape in the present invention is applied even when it is deviated from a rectangle. High signal quality is assured regardless of whether it is recorded on a land or groove track.
[0067]
It is desirable that a resin layer is applied or spin-coated on the metal reflective layer according to claim 1 to protect the film.
Various combinations are possible for the dielectrics used for the dielectric layer in the present invention, and are determined in consideration of the refractive index, thermal conductivity, chemical stability, mechanical strength, adhesion, and the like.
Generally, Mg, Ca, Sr, Y, La, Ce, Ho, Er, Yb, Ti, Zr, Hf, V, Nb, Ta, Zn, Al, Si, Ge, which have high transparency and high melting point, Oxides such as Pb, sulfides, nitrides, and fluorides such as Ca, Mg, and Li can be used.
[0068]
Of these, ZnS and SiO₂Or Y₂O₃When using a mixed film of at least one of₂Or Y₂O₃When the content is 5 to 40 mol%, the storage stability of the recorded disc is excellent.
The disc can be used as a single plate specification using only one side, and the capacity can be doubled by bonding two discs with the opposite side facing the substrate.
[0069]
In addition, when a laminated disk is used, by adopting a drive having a structure in which optical pickups are set on both sides of the disk, recording, erasure and reproduction can be performed simultaneously on both sides without replacing the disk at all.
This is an important feature that cannot be performed with a magneto-optical disk that requires a magnet on the side opposite to the laser irradiation side.
In order to design the disc of the present invention, it is necessary to accurately grasp the phase difference of the reflected light before and after the phase change.
[0070]
In addition, hopefully said A_c/ A_aIt is more desirable to accurately grasp the above and to make it within a certain range in terms of CN ratio and recording mark jitter.
The phase difference can be measured with a laser interference microscope or the like.
A_c/ A_aIs an absorptance ratio of only the recording layer in the multilayer structure and cannot be directly measured.
[0071]
However, the phase difference of the reflected light before and after the phase change is also the absorptance ratio A._c/ A_aCan also be obtained by calculation using the optical constants and film thickness of each layer.
The calculation method is described in detail in Chapter 3 of “Spectros Fundamentals and Methods” (Keiei Kudo, Ohmsha, 1985).
The calculated values of the phase difference and the absorptance ratio in this example and the comparative example were calculated based on the method described in this document.
[0072]
The optical constant of each layer may be measured in advance by preparing a single layer film by a method such as sputtering and using an ellipsometer.
For recording / erasing / reproducing of the optical disk of the present invention, a one-beam laser beam condensed by an objective lens is used and irradiated from the substrate side of the rotating optical disk.
During recording and erasing, a rotating disk is irradiated with a pulsed laser beam to change the phase of the recording layer into two reversible states, a crystalline state or an amorphous state, and a recorded state or erased state (unrecorded state). To do.
[0073]
At this time, it is also possible to simultaneously erase marks existing before recording while recording by overwriting.
During reproduction, the rotating disk is irradiated with a laser beam having a power lower than that for recording and erasing.
At this time, the phase state of the recording layer immediately before reproduction must not be changed.
Reproduction is performed by detecting a change in the intensity of the reflected light with a photodetector and determining a recorded or unrecorded state.
[0074]
Now, as described above, it is desired to further improve the durability against repeated overwrite and the above-described cross erase after the good initial characteristics can be obtained in the land and the groove. According to the study by the present inventors, the durability of the groove portion against repeated overwriting is better when the groove width is narrower, and is consistent with the increase in the track pitch density. If the lower limit is provided, the recording mark does not protrude from the inside of the groove, and the condition is defined in association with the focused beam shape in, for example, Japanese Patent Laid-Open No. 6-338064.
[0075]
However, there is no mention that the durability against repeated overwrite is affected by the groove width or land width..According to the study by the present inventors, the repeated overwrite on the land depends on the relative relationship between the land width and the beam diameter, and can deteriorate rapidly when the land width becomes significantly narrower than the beam diameter. found.
[0076]
Therefore, in the present invention, the land width is preferably in the range of 0.62 × (λ / NA) to 0.80 × (λ / NA).
When the land width is narrower than this range, when the recording mark is repeatedly overwritten on the land, the remaining mark disappears remarkably, and the jitter of the recording mark is remarkably deteriorated.
[0077]
If the land width is within this range, there is no disappearance of the previous mark or remarkably deteriorated jitter of the recording mark when overwriting is repeated, and characteristics equivalent to those recorded in the groove are maintained.
If the land width is larger than this range, there is no problem with the repeated overwriting characteristics of the land, and good characteristics can be obtained, but from the viewpoint of high-density recording, the land width is meaninglessly expanded to lower the recording density. Is not a good idea.
[0078]
Further, it has been found that the cross erase phenomenon also depends on the relative relationship between the beam spot diameter and the recording track pitch. That is, there is a minimum track pitch that can reduce the cross erase to a level that can be ignored in practice, and it depends on the size of the beam spot.
Since the light beam spot diameter is proportional to λ / NA, the allowable minimum pitch can be regarded as proportional to λ / NA.
[0079]
The proportionality coefficient may be determined based on experiments.
In fact, the present inventors have made various studies, and as a result, the groove pitch for L & G recording is 10 (if greater than 1.2 (λ / NA)).⁴The C / N ratio (carrier-to-noise ratio) can be reduced to less than 3 dB after overwriting, and the level can be practically not problematic.
Since the actual recording track pitch of L & G recording is half of the groove pitch (groove pitch), if the minimum recording track pitch is made larger than 0.6 (λ / NA), the signal deterioration of the adjacent track due to cross erase will be caused. It was confirmed experimentally that this can be prevented.
[0080]
The value of 0.6 theoretically corresponds to exactly half the beam spot of the convergent light 10 that has passed through the lens 9.
That is, the convergent light beam 10 has a shape as shown in FIG. 5, and a sub-peak appears in the intensity distribution (the figure 11 in FIG. 5 shows the intensity distribution) due to the diffraction effect.
The diameter of the central spot is approximately 1.2 (λ / NA).
[0081]
This is called an airy disk 12.
Also, the light intensity distribution in this is not uniform and the intensity is 1 / e.²The diameter (e is the base of natural logarithm) is expressed as 0.82 (λ / NA).
Since the minimum pitch of the track 1 corresponds to the radius of the area disk, the cross erase phenomenon is a weak laser beam at the base of the Airy disk of the focused light beam spot as shown in FIG. 5 as a first approximation. As a result, the physical meaning that the temperature of the adjacent truck is heated is also clarified.
[0082]
The fact that the cross erase phenomenon is hardly influenced by the heat conduction of the recording layer is currently known, such as GeSbTe, AgInSbTe, InSnTe, InSbTe, etc. IIIb, IVb, Vb, VIb group elements, or a mixture (alloy) thereof 40 at. This is because the thermal conductivity of the recording layer containing at least% is on the order of 2 to 3 orders of magnitude smaller than that of the magneto-optical medium.
This is because the order of 10 to 100 nanoseconds required for recording is substantially adiabatic.
[0083]
Therefore, the minimum track pitch determined by the above 0.6λ / NA is substantially determined only by the beam spot diameter, and therefore the light beam wavelength and NA.
However, although the cross-erasing at 10,000 times or more of repeated overwriting is slight, further reduction can be achieved by limiting the layer structure of the recording medium and the physical properties of the recording layer.
[0084]
Although depending on the melting point and crystallization temperature of the recording layer, the composition currently known to allow reversible change between crystal / amorphous in the alloy recording layer has a melting point Tm of less than 700 ° C., Many have a crystallization temperature Tg of 150 ° C. or higher.
In fact, Ge₁Sb₂Te₄Or Ge₂Sb₂T_sIn the vicinity of the composition, the melting point is 600 to 620 ° C., and the crystallization temperature is 150 to 170 ° C.
Ag_0.11In_0.11Te_0.20Sb_0.55The melting point is about 550 ° C., and the crystallization temperature is about 230 ° C.
When Tg is lower than 150 ° C., the amorphous state is not stable and is easily cross-erased.
[0085]
Further, when Tm is 700 ° C. or higher, energy to be irradiated at the time of recording becomes high, and it is easy to cause cross erase in the adjacent track.
As for the layer structure, when the recording layer thickness exceeds 30 nm, the recording sensitivity is lowered, and cross-erasing is likely to occur because heat easily escapes to the adjacent track during recording.
Examples will be shown below, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded.
[0086]
Example 1
A polycarbonate resin substrate was obtained by injection molding as the substrate.
A plurality of substrates were prepared in which the groove pitch was changed from 1.3 μm to 1.6 μm in steps of approximately 0.05 μm.
Therefore, the substantial recording track pitch is 0.65 to 0.8 μm. (ZnS) as a lower protective layer on the obtained substrate₈₀(SiO₂)₂₀2100 mm, Ge as the recording layer₂₂Sb_23.5Te_54.5As a top protective layer (ZnS)₈₀(SiO₂)₂₀200 mm, Al as the reflective layer_97.5Ta_2.5Was formed by 1000-mm sputtering. An ultraviolet curable resin was further provided as a protective coating on the reflective layer.
[0087]
By the initialization, the entire crystallized state is changed to an unrecorded state, and the recording mark is amorphous. Unrecorded reflectance R_highIs 14.2%, reflectance ratio R_low/ R_high= 0.09, A_c/ A_a= 0.91 and the phase difference is -0.44π.
An optical head having a wavelength of 680 nm and NA = 0.55 was used. The linear velocity was 3 m / s, Pw = 8 to 9 mW, and Pe = 4.5 mW. The recording power was modulated with a single pattern having a frequency of 2.24 MHz and a duty of 25%.
When recording was performed on the groove, overwriting was repeatedly performed between the adjacent grooves, and a decrease in the C / N ratio of the signal first recorded in the groove was measured.
[0088]
The same measurement was performed when recording was performed on the land and repeated overwriting was performed on both adjacent grooves.
If the groove pitch is larger than 1.5 μm (recording track pitch 0.75 μm), the decrease in the C / N ratio between adjacent grooves after overwriting 10,000 times or between the grooves can be less than 3 dB, and there is no practical problem. It was a level.
[0089]
(680 / 0.55) × 0.6 = 741 nm = 0.441 μm, which can be considered to be larger than the requirement 0.6λ / NA regarding the minimum track pitch.
On the other hand, when a similar experiment was performed using a head of 680 nm and NA = 0.6, there was no problem up to a groove pitch of 1.4 μm (recording track pitch of 0.7 μm).
This satisfies the minimum recording track pitch condition of (680 / 0.6) × 0.6 = 0.680 μm.
[0090]
On the other hand, when recording was performed on the land, overwriting was repeated in both adjacent grooves, and deterioration due to cross erase of the signal on the land was measured, the same result was obtained except for a difference of 1 to 2 dB. .
Furthermore, overwriting was repeatedly performed on the land, and the mark length jitter of the signal between the grooves was measured. In this case, measurement was performed only with a head of λ = 680 nm and NA = 0.55. Only when the groove pitch is 1.6 μm (land width≈0.8 μm).³There was almost no increase in jitter for each overwrite (only an increase of about 20%). In this case, the land width is wider than 0.62λ / NA≈0.77 μm, which satisfies the requirements of the present invention. On the other hand, when the groove pitch is 1.4 μm (land width 0.7 μm), the jitter is remarkably increased.³The number of overwriting times has more than doubled.
[0091]
According to the analysis results obtained by solving the thermal diffusion equation of the present inventors by numerical calculation, the layer configuration used in this example is one of the most lateral thermal diffusions, and the most in terms of cross erase. It is considered under severe conditions. Therefore, it can be considered that the definition regarding the minimum track pitch is established regardless of the layer structure.
[0092]
【The invention's effect】
As described above in detail, according to the optical recording medium and the recording / reproducing method of the present invention, even if a signal is recorded on both the land and the groove, the groove depth is limited. Can be reduced.
In addition, since the ratio of the reflectance of the reflected light from the unrecorded area to the coherent light having the same wavelength as the wavelength of the reproduction light and the reflectance of the reflected light from the recording area are defined, Undesirable differences between the carrier level and the groove carrier level can be eliminated.
[0093]
Accordingly, a reproduction signal amplitude of the same level can be obtained regardless of whether recording is performed on either the land portion or the groove portion, and a high-quality and highly reliable land / groove recording disc can be provided.
Further, the ratio of the irradiation light absorbed in the recording layer when the recording layer of the optical recording medium of the present invention is in an amorphous state, and the irradiation light absorbed in the recording layer when the recording layer is in a crystalline state. Ratio, i.e., when the recording layer is in an amorphous phase._aA when the recording layer is in a crystalline state_cThe ratio of the absorption ratio between the crystalline state and the amorphous stateA _c / A_aThe
[0094]
[Expression 32]
0.84 ≦A _c / A_a<1.01
[0095]
By prescribing within this range, it is possible to guarantee excellent characteristics with a high CN ratio and low recording mark jitter, and an excellent disk can be provided.
In addition, by using the optical recording medium of the present invention, both on and between the grooves are used as recording areas, and recording, erasure and reproduction can be performed by one-beam overwriting with a laser having a wavelength of 700 nm or less in both areas. A characteristic recording / reproducing method can be provided.
[Brief description of the drawings]
FIG. 1 is an enlarged perspective view for explaining a positional relationship between a groove shape of an optical disk and a convergent beam of irradiation laser light in an embodiment.
FIG. 2 is an enlarged perspective view for explaining a positional relationship between a groove shape of an optical disk and a convergent beam of irradiation laser light in the embodiment.
FIG. 3 is an enlarged perspective view for explaining a positional relationship between a groove shape of an optical disk and a convergent beam of irradiation laser light in the embodiment.
FIG. 4 is an enlarged perspective view for explaining a positional relationship between a groove shape of an optical disc and a convergent beam of irradiation laser light in the embodiment.
FIG. 5 is an explanatory diagram of intensity distribution of a convergent light beam.
[Explanation of symbols]
1 Substrate
2 Recording layer
3 Land
4 Groove
5 Focused beam
Area of convergent beam irradiated to 6 lands
7 Focused beam area irradiated to groove
8 Record mark
9 Lens
10 Convergent light
11 Intensity distribution
12 Airy Disc

Claims (6)

It consists of a structure in which a dielectric layer, a phase change recording layer, a dielectric layer, and a metal reflective layer are sequentially laminated on a transparent substrate on which grooves are formed. Both the groove and the land are used as a recording area, and 700 nm or less. An optical recording medium for recording, erasing and reproducing information by irradiating a laser beam having a wavelength of
(1) The groove width is 0.3 μm or more and 0.8 μm or less, the land width is 0.3 μm or more and 0.8 μm or less, the groove width is substantially equal to the land width, and the groove depth d satisfies the following inequality,
[Expression 1]
λ / 7n <d <λ / 5n
(Λ: wavelength of irradiated light, n: refractive index of substrate, d: depth of groove)
(2) Of the reflected light from the unrecorded area and the reflected light from the recorded area defined below, the higher reflectance is R _high (%) and the lower reflectance is R _low (%). And the phase difference of the reflected light from the recording area is 2πα, the following equation is satisfied:
[Expression 2]
10 ≦ R _high ≦ 40
[Equation 3]
R _low / R _high ≦ 0.15
mπ ≠ 2πα (m is an integer)
However,
2πα = (phase of reflected light from unrecorded area) − (phase of reflected light from recorded area)
(3) The land width LW satisfies the following equation:
[Expression 4]
0.62λ / NA ≦ LW ≦ 0.8λ / NA
λ is the irradiation light wavelength, NA is the numerical aperture of the focusing lens (4) The groove width GW and land width LW satisfy the following formula
(LW + GW) / 2> 0.6λ / NA
An optical recording medium characterized by the above. A ratio of the absorption laser light of wavelength λ absorbed by the recording layer is expressed as A _a when the recording layer is in an amorphous phase. When the case where the recording layer is in the crystalline state was A _c, the ratio _{A c /} A _a in the absorption of the crystalline and amorphous states is [6]
0.84 ≦ A _c / A _a <1.01
The optical recording medium according to claim 1, wherein The optical recording medium according to claim 1, wherein the recording layer is made of an alloy mainly composed of Ge, Sb, and Te and has a thickness of 20 ± 5 nm. The optical recording medium according to claim 1, wherein the reflective layer is an alloy of Al and Ti or Ta, and the content of Ti or Ta is 0.5 to 3.5 at%. One or both of the lower dielectric protective layer and the upper dielectric protective layer is a mixed film of ZnS and either SiO ₂ or Y ₂ O ₃ , and is made of SiO ₂ or Y ₂ O ₃ . Content is 5-40 mol%, The optical recording medium in any one of Claims 1-4 characterized by the above-mentioned. The optical recording medium according to claim 1 is used, both on the groove and between the grooves are used as recording areas, and recording, erasure and reproduction are performed by one-beam overwriting with a laser having a wavelength of 700 nm or less in both areas. Recording and playback method.

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