JP3905581B2 - Optical recording medium and recording / reproducing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical recording medium and a recording / reproducing method. More specifically, the present invention relates to an optical information recording medium for recording, reproducing, and erasing information in both a groove portion and a groove of a substrate by irradiation with a laser beam, and an optical information recording medium The present invention relates to the recording / reproducing method used.
[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 the recording media and recording devices were imposed on handling vast amounts of image information and audio signals. Digital modulation technology and data compression technology Together with progress, the progress is just a step forward.
[0003]
As specific means for increasing the density, in an optical disc, the convergent beam diameter is reduced by shortening the wavelength of the light source and increasing the NA (Numerical Aperture) of the focusing lens, the recording mark length is shortened, and the rotational speed is the same. Development and use of MCAV (Modified Constant Angular Velocity), which increases the recording frequency toward the outer circumference under constant conditions, and makes the recording density at the inner and outer circumference constant, and mark edge recording that puts information on the start and rear ends of the mark 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 and the groove is abbreviated as L & G recording.
[0006]
As a proposal for L & G recording, there is JP-B-63-57859. However, 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 is irradiated up 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 the crosstalk, for example, there is a technique for reducing the crosstalk by providing a special optical system and a crosstalk cancel circuit in the optical disc reproducing apparatus. (SPIE Vol. 1316, Optical Data Storage (1990) pp. 35)
[0008]
However, this method has a demerit that the optical system and signal processing system of the apparatus become more complicated.
As a method of reducing crosstalk without providing a special optical system or signal processing circuit for reducing reproduction crosstalk, the groove and land widths are made equal, and the groove depth corresponds to the reproduction light wavelength. There is a proposal that it is effective to be within a certain range. (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.
However, although the effect of reducing crosstalk can be obtained by setting the groove depth to an optimum value, the CN ratio between the land and the groove is unbalanced.
[0010]
When L & G recording is performed, a difference between the land carrier level (signal quality) and the groove carrier level is caused, and as a result, one of the CN ratios is remarkably lowered.
[0011]
On the other hand, when packing the track pitch for high density, usually, the track pitch and the groove shape should be selected so that the amount of crosstalk is below a predetermined level. There is another issue to consider.
That is, when a track is repeatedly overwritten, the amorphous bit of the adjacent track disappears, that is, recrystallizes.
The reason for this is not necessarily clear, but when recording on an adjacent track, the temperature of the track is raised by a weak laser beam at the bottom of the focused light beam intensity distribution, and the temperature of the amorphous bit part is heated above the crystallization temperature. It is thought that it is to be done.
[0012]
Although the time is several hundred nanoseconds at a time, it is gradually recrystallized while being repeatedly heated.
For example, when the overwriting is repeated 10,000 times, the CN ratio of the adjacent track may decrease from the initial 55 dB to less than 50 dB.
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.
[0013]
In addition, if the density is increased by narrowing the track pitch while keeping the groove and land width at 1: 1, the land mark disappears after repeated overwriting and the jitter of the recording mark deteriorates. It became clear that the characteristic deterioration of was remarkable.
[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 and groove recording marks is eliminated, and the land and groove It is an object of the present invention to provide a high-density optical disk that can obtain the same high signal quality and durability against repeated overwriting regardless of which is recorded.
[0015]
[Means for Solving the Problems]
The present invention has been made as a result of repeated studies on the phase difference of reflected light from unrecorded areas and recorded marks, and the shape of lands and grooves. The gist of the present invention is that a lower dielectric is formed on a transparent substrate on which grooves are formed. A body protective layer, a phase change recording layer, an upper dielectric protective layer, and a metal reflective layer are sequentially stacked. Both the groove and the groove are used as a recording region, and a laser beam having a wavelength of 700 nm or less is irradiated. An optical recording medium for recording, erasing and reproducing information,
(1) Phase difference between reflected light from unrecorded area and reflected light from recorded area defined below2πα
2πα= (Phase of reflected light from unrecorded area)-(Phase of reflected light from recorded area)
Satisfies the following equation:
[0016]
[Equation 8]
(M−0.1) π ≦2πα  ≦ (m + 0.1) π (m is an integer)
[0017]
(2)The groove width and the width between the grooves are almost equal,The groove width GW, the width LW between the grooves, and the groove depth d satisfy the following formula:
[0018]
[Equation 9]
0.3 μm ≦ GW ≦ 0.8 μm
[0019]
[Expression 10]
0.3 μm ≦ LW ≦ 0.8 μm
[0020]
## EQU11 ##
      0.63× (λ / NA) ≦ LW ≦ 0.8 × (λ / NA)
[0021]
[Expression 12]
(GW + LW) / 2> 0.6 × (λ / NA)
[0022]
[Formula 13]
λ / 7n <d <λ / 5n
(Where λ is the wavelength of the irradiated light, n is the refractive index of the substrate,
NA: numerical aperture of focusing lens)
It exists in the optical recording medium characterized by this.
[0023]
In addition, using such an optical recording medium, both the grooves are defined 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. It exists in the recording / reproducing method.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
With the configuration described above, in the optical disc of the present invention, the carrier level of the recording mark is the same regardless of whether recording is performed on the land or the groove, and durability against repeated overwriting on the land is enhanced.
These are indispensable provisions in terms of guaranteeing the reliability of an L & G recording type optical disc using a laser beam having a wavelength of 700 nm or less as a light source in order to perform high density recording.
[0025]
Next, how the present invention acts in the reproduction process of the optical recording medium for land and groove recording and brings about an effect will be described in detail using a simple model.
FIG. 1 to FIG. 4 show schematic views when a reproduction light beam is irradiated on a land or a groove of an optical disc for L & G recording. In order to make the figure easy to see, layers other than the recording layer 2 are omitted.
The reproducing convergent beam 5 is condensed using an objective lens or the like, and is irradiated onto the recording layer 2 from the substrate 1 side.
It is assumed that the width of the land 3 is equal to the width of the groove 4 and is half the beam diameter, and the step (groove depth) between the land 3 and the groove 4 is d.
[0026]
Here, the recording layer when not recorded is in a crystalline state, and the recording layer when recording is in an amorphous state. The intensity of the convergent beam 5 is Gaussian according to the actual model and is 1 / e of the center intensity.²The beam diameter is in the range (e is the base of natural logarithm), that is, 0.82 (λ / NA) (λ is the wavelength of the convergent beam 5 and NA is the numerical aperture of the focusing lens).
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 5 exists on the recording mark 8.
In order to simplify the calculation, it is assumed that the recording mark 8 is sufficiently longer than the convergent beam 5. 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.
[0027]
Since the convergent beam 5 is irradiated from the substrate 1 side, it is incident and reflected from the other side of the page. Therefore, when viewed from the light source side, the land 3 is concave and the groove 4 is convex.
When the surface of the groove 4 is used as a phase reference, the phase of the reflected light from the land 3 is delayed by 2π · 2nd / λ (n is the refractive index of the substrate 1 and λ is the wavelength of the convergent beam 5).
[0028]
The change in phase is not caused solely by the groove depth, but generally the phase difference also changes depending on the change in 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.2παIt is defined that the phase is only delayed. Hereafter, the groove surface is used as a phase reference, and the amplitude reflectance of the convergent beam is changed as necessary.2παWe will formulate while using.
[0029]
The amount of reflected light from the central portion of the convergent beam 5 irradiated to the crystal region is R_c1, The amount of light reflected from both ends is R_c2, The amount of reflected light from the center of the convergent beam 5 irradiated to the amorphous region is R_a1, The amount of light reflected from both ends is R_a2And Since land width = groove width, this can be applied when the beam center is in either the land or the groove.
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: Note that i is an imaginary unit.
[0030]
[Expression 14]
φ₁= R_c1Exp [-2πi · 2nd / λ]
+ R_c2Exp [-2πi · 0] (a)
However, R_c1Is the amount of light reflected from the area 6 of the land irradiated with the convergent beam, R_c2Is the amount of reflected light from the groove region 7 irradiated with the convergent beam.
[0031]
As shown in FIG. 2, the amplitude reflectance φ when the convergent beam 5 is in the land where the amorphous recording mark 8 is located.₂Can be expressed as:
[0032]
[Expression 15]
φ₂= R_a1Exp [-2πi · (2nd / λ + α)]
+ R_c2Exp [-2πi · 0] (b)
However, R_a1Is the amount of light reflected from the area 6 of the land irradiated with the convergent beam, R_c2Is the amount of reflected light from the groove region 7 irradiated with the convergent beam.
[0033]
As shown in FIG. 3, the amplitude reflectance φ when the convergent beam 5 is in the groove 4 without the amorphous recording mark._ThreeCan be expressed as:
[0034]
[Expression 16]
φ_Three= R_c1Exp [-2πi · 0]
+ R_c2Exp [-2πi · 2nd / λ] (c)
However, R_c1Is the amount of light reflected from the region 7 of the groove irradiated with the convergent beam, R_c2Is the amount of reflected light from the land area 6 irradiated with the convergent beam.
[0035]
As shown in FIG. 4, the amplitude reflectance φ when the focused beam 5 is in the groove having the amorphous recording mark 8_FourCan be expressed as:
[0036]
[Expression 17]
φ_Four= R_a1Exp [-2πi · α]
+ R_c2Exp [-2πi · 2nd / λ] (d)
However, R_a1Is the amount of light reflected from the region 7 of the groove irradiated with the convergent beam, R_c2Is the amount of reflected light from the land area 6 irradiated with the convergent beam. here,
[0037]
[Expression 18]
R_c2= ΒR_c1                                      (E)
[0038]
[Equation 19]
R_a2= ΒR_a1                                      (F)
(Where 0 <β <1). R_c= R_c1+ R_c2, R_a= R_a1+ R_a2When formula (e) and formula (f) are rearranged,
[0039]
[Expression 20]
R_c1= R_c/ (1 + β) (g)
[0040]
[Expression 21]
R_c2= ΒR_c/ (1 + β) (h)
[0041]
[Expression 22]
R_a1= R_a/ (1 + β) (i)
[0042]
[Expression 23]
R_a2= ΒR_a/ (1 + β) (j)
It becomes.
Substituting the formulas (g) to (j) into the formulas (a) to (d) and rearranging them,
[0043]
[Expression 24]
φ₁= [R_c/ (1 + β)] [β + exp [−4πind / λ]] (k)
[0044]
[Expression 25]
φ₂= [1 / (1 + β)]
[ΒR_c+ R_aExp [-4πind / λ-2πiα]] (l)
[0045]
[Equation 26]
φ_Three= [R_c/ (1 + β)] ・
[1 + β · exp [−4πind / λ]] (m)
[0046]
[Expression 27]
φ_Four= [1 / (1 + β)] · [R_aExp [-2πiα]
+ ΒR_cExp [-4πind / λ]] (n)
Here, the reproduction carrier level CL ′ (L) when recorded on the land is:
[0047]
[Expression 28]
CL ′ (L) = | φ₁｜²− | Φ₂｜²                  (O)
Is proportional to Similarly, the reproduction carrier level CL ′ (G) when recorded in the groove is:
[0048]
[Expression 29]
CL ′ (G) = | φ_Three｜²− | Φ_Four｜²                  (P)
Is proportional to
[0049]
The fact that there is no difference between the land and groove carrier levels is that the difference between the equations (o) and (p) is zero. Substituting Equation (k) to Equation (n) into Equation (o) and Equation (p) to calculate the difference, and obtaining the necessary condition for the difference to be 0,
[0050]
[30]
2πα= Mπ (where m is an integer) (q)
It becomes. Phase difference2παDoes not necessarily need to be exactly mπ, and any disk reflectivity is effective if it is within a range of ± 0.1π.
[0051]
On the other hand, if the phase difference is less than (m−0.1) π, it is a bad point that the land carrier level becomes significantly smaller than the groove, and the phase difference is ( When m + 0.1) π is exceeded, it is a bad point that the carrier level of the groove becomes significantly smaller than that of the land.
[0052]
Thus, the phase difference2παBy setting the value within (m ± 0.1) π, it is possible to guarantee high signal quality regardless of whether recording is performed on either the land or groove track. The identification can be realized by appropriately selecting the optical constant and the film thickness of each layer.
[0053]
Next, the groove shape of the present invention will be described in detail.
First, a method for measuring the groove width and groove depth will be described. The measurement is performed by irradiating a He—Ne laser beam (wavelength 630 nm) from the non-grooved side of the substrate and diffracting the transmitted light by the groove of the substrate.₀Primary light intensity I₁Secondary light intensity I₂And by measuring the angle of the diffracted light.
P is the groove pitch (groove pitch), w is the groove width, d is the groove depth (groove depth), n is the refractive index of the substrate, λ is the laser wavelength, θ is the angle between the 0th order light and the 1st order light. When the groove is rectangular,
[0054]
[31]
P = λ / sinθ
It becomes. Also,
[0055]
[Expression 32]
ε = w / P, δ = 2 (n−1) πd / λ
After all,
[0056]
[Expression 33]
I₂/ I₁= Cos²(πε)
[0057]
[Expression 34]
I₁/ I₀= {2sin²(πε) (1-cosδ)}
/ [Π²{1-2ε (1-ε) (1-cosδ)}]
[0058]
Therefore, the groove width and groove depth can be calculated. Although the actual groove shape is not a perfect rectangle, in the present invention, the groove shape uses values in which the groove width and groove depth are uniquely determined by the measurement method described above. Therefore, the actual groove shape may deviate from the rectangle.
[0059]
The width of the land and groove needs to be in the range of 0.3 to 0.8 μm. If the land and groove widths are smaller than 0.3 μm, stable tracking cannot be obtained, and the current cutting technique makes it difficult to manufacture. On the other hand, when the width is larger than 0.8 μm, the purpose of high density recording cannot be achieved.
[0060]
Further, as described above, the groove depth of the substrate needs to be within this range since the crosstalk from the adjacent track is reduced when the groove depth is λ / 7n to λ / 5n.
[0061]
As described above, after good initial characteristics can be obtained in both the land and the groove, it is necessary to further improve the durability against repeated overwriting and the above-mentioned cross erase.
According to the study by the present inventors, the repeated overwriting durability of the groove is better when the groove width is narrower. Of course, it is necessary that the recording mark does not protrude from the groove.
For example, Japanese Patent Laid-Open No. 6-338064 has a description associated with a convergent beam shape. However, there is no mention that the repeated overwrite durability is affected by the groove width or land width.
[0062]
On the other hand, it has been found that the repeated overwriting durability of the land depends on the relative relationship between the land width and the beam diameter, and deteriorates rapidly when the land width becomes narrower than a specific value with respect to the beam diameter.
[0063]
  That is, the land width is0.63Within the range of × (λ / NA) to 0.8 × (λ / NA), there is no disappearance of the previous mark or jitter of the recording mark when overwriting repeatedly, and recording in the groove The same characteristics are maintained.
  If the land width is narrower than this, when the recording mark is repeatedly overwritten on the land, the disappearance of the previous mark becomes remarkable, and the jitter of the recording mark is remarkably deteriorated.
  If the land width is larger than this, there is no problem in the repetitive overwriting characteristics of the land, and good characteristics can be obtained, but from the viewpoint of high-density recording, it is meaningless to increase the land width and lower the recording density. It's not a good idea.
[0064]
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 only on the beam spot diameter.
[0065]
The groove pitch (GW + LW) of L & G recording is made larger than 1.2 (λ / NA), that is, the actual track pitch {(GW + LW) / 2} of L & G recording is made larger than 0.6 (λ / NA). If so, signal deterioration of adjacent tracks due to cross erase can be prevented.^FourThe decrease in the CN ratio after the overwriting can be suppressed to less than 3 dB, and there is no practical problem.
[0066]
This theoretical meaning is discussed below.
FIG. 5 is a schematic diagram showing the shape of the convergent beam.
The convergent beam 9 passing through the focusing lens 12 has an intensity distribution 10 having a main peak and a sub peak. The diameter of the central spot, which is the main peak, is approximately 1.2 (λ / NA). This is referred to as an airy disk 11.
The above value of 0.6 (λ / NA) theoretically corresponds to exactly half of the Airy disk 11. From this, it can be considered that the cross erase phenomenon has a physical meaning that, as a first approximation, the temperature of adjacent tracks is raised by weak laser light at the base of the Airy disk 11 of the convergent beam 9.
[0067]
By the way, in the phase change type recording layer containing GeatbTe, AgInSbTe, InSnTe, InSbTe, etc. which is currently known and containing at least 40 at% of any of IIIb, IVb, Vb, and VIb group elements or a mixture (alloy) thereof as a main component, The thermal conductivity is on the order of 2 to 3 orders of magnitude smaller than that of the magneto-optical recording layer. Moreover, it is substantially heat insulation in the order of 10 to 100 nanoseconds required for recording.
For this reason, the cross erase phenomenon is hardly influenced by the heat conduction of the phase change recording layer.
Therefore, the minimum track pitch is substantially determined only by the beam spot diameter, and therefore the light beam wavelength and NA.
[0068]
However, repeated overwriting 10^FourIn order to reduce the cross erase at the time of more than the number of times, there is an effect although there are some restrictions on the layer configuration of the recording medium and the physical properties of the recording layer.
Among the above-described alloy recording layers, many compositions having a melting point Tm of less than 700 ° C. and a crystallization temperature Tg of 150 ° C. or more are available in a composition that can reversibly change between crystal and amorphous and has little cross erase.
[0069]
When Tg is lower than 150 ° C., the amorphous state is not stable and is easily cross-erased. 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.
In fact, Ge₁Sb₂Te_FourOr Ge₂Sb₂Te_FiveIn the vicinity composition, Tm is 600 to 620 ° C. and Tg is 150 to 170 ° C. Also, Ag₁₁In₁₁Te_{twenty three}Sb₅₅Then, Tm is about 550 ° C. and Tg is about 230 ° C.
In general, when the recording layer thickness exceeds 30 nm, the recording sensitivity decreases, and heat easily escapes to an adjacent track during recording, so that cross erase is likely to occur.
[0070]
As an important parameter related to the melting of the phase change recording layer during the formation of the amorphous recording mark, there is a light absorption rate of the recording layer.
In terms of obtaining a high carrier level in both the land and groove tracks, the effect is further amplified by specifying the ratio of the optical absorptance of the recording layer before and after the phase change within a certain range.
[0071]
One feature of the phase change type optical disc is a one-beam overwrite as disclosed in Japanese Patent Publication No. 5-32811.
In PWM recording, information of 0 or 1 is assigned to the front end and rear end of the recording mark, so that it is particularly required that the shape of the mark front end and rear end is not distorted during recording.
However, in the case of 1-beam overwrite, the temperature rise and fall processes are non-uniform because the thermal conductivity differs depending on whether the recording layer before recording is in an amorphous state or a crystalline state, and the recording mark is distorted. Has been pointed out. Further, for example, as described in Japanese Patent Laid-Open No. 5-298747, a larger CN ratio, a higher erasure rate, and a wider power allowance (margin) are obtained when the light absorption rate in the crystalline state is larger than the light absorption rate in the amorphous state. There is a proposal that can be obtained.
[0072]
However, according to the study by the present inventors, it is not necessary to make the light absorption rate in the crystalline state significantly larger than the light absorption rate in the amorphous state._c, A light absorption rate in the amorphous state_aRatio A_c/ A_aBut,
[0073]
[Expression 35]
0.84 ≦ A_c/ A_a  <1.01
It was found that the CN ratio and the jitter of the recording mark are particularly excellent in the disk in which the layer structure of the disk is designed so as to be in the above range.
This is not limited to the case where the rotational speed of the disk is in a specific narrow range, and the effect is noticeable over a wide range from a linear speed of 1.4 m / s to 15 m / s.
[0074]
A_c/ A_aIf it is less than 0.84, the presence or absence of a recording mark pre-existing on the recording track causes an imbalance in the temperature rise / fall process during melting of the recording layer at the time of overwriting, which causes distortion of the mark shape. .
Further, in a disc in which the initial state (unrecorded state) of the disc has a high reflectivity and the recording state has a low reflectivity, the recording sensitivity tends to deteriorate if it is less than 0.84. 84 or more is desirable.
[0075]
The metal reflective layer is preferably made of Al and Ti or an alloy of Al and Ta in consideration of sensitivity and stability.
The content of Ti or Ta is desirably 0.5 at% to 3.5 at%. At this time, the loss of the reflectivity of the disk is small, and it can easily serve as an appropriate heat dissipation layer.
[0076]
As a method for forming these thin films, a known method can be used, and there is a method of forming a normal optical thin film such as magnetron DC sputtering or RF sputtering on a substrate disk such as resin or glass on which grooves have been formed in advance.
It is desirable to form a resin layer on the metal reflective layer by coating or spin coating in order to protect the film.
[0077]
Various combinations can be used as the dielectric used for the protective layer, and the dielectric is 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.
[0078]
Of these, ZnS and SiO₂Or Y₂O_ThreeWhen using a mixed film of at least one of₂Or Y₂O_ThreeA content of 5 to 40 mol% is preferable because the storage stability of the recorded disk is excellent.
[0079]
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. The phase difference can be measured with a laser interference microscope or the like.
In addition, said A_c/ A_aIt is more desirable to accurately grasp the above and to make it within a specific range in terms of improving the CN ratio and reducing the jitter of the recording mark.
A_c/ A_aIs an absorptance ratio of only the recording layer in the multilayer structure and cannot be directly measured. 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 thicknesses of each layer (the calculation method is “spectrum fundamentals and methods” (Keiei Kudo, Ohmsha, 1985), Chapter 3).
The calculated values of the phase difference and the absorption ratio in the present invention were calculated based on the method described in this document.
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.
[0080]
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, the rotating disk is irradiated with a pulsed laser beam to change the phase of the recording layer into two reversible states, a crystalline state and an amorphous state, and a recorded state or erased state (unrecorded state). To do.
At this time, the marks that existed before recording can be erased (overwritten) at the same time while recording.
[0081]
During reproduction, a rotating disk is irradiated with a laser beam having a power lower than that during recording and erasing, and a change in intensity of reflected light is detected by a photodetector to determine a recorded or unrecorded state. At this time, the phase state of the recording layer immediately before reproduction must not be changed.
[0082]
The wavelength of the laser beam used for recording / reproduction must be 700 nm or less in order to realize high-density recording. Further, considering the ultraviolet absorption of the disk substrate, 300 nm or more is preferable. As the laser beam, for example, a laser beam of 500 to 700 nm output from a III-V semiconductor laser such as AlGaAs, AlGaAsP, AlGaInP, or GaN, or a gas laser such as Ar, Kr, He—Cd, or He—Ne is output from 400 to 400 nm. 650 nm laser light, 400-500 nm laser light output from II-VI semiconductor lasers such as ZnCdSe, ZnSe, ZnCdS, ZnSeS, and III-V semiconductor laser output light are passed through SHG (second harmonic generation) elements. Examples thereof include laser light of 340 to 390 nm obtained, and laser light of 350 nm obtained by passing YAG laser output light by semiconductor laser excitation through a THG (third harmonic generation) element. Considering miniaturization of the optical disk system, a laser beam of a semiconductor laser is preferable.
[0083]
In order to reduce the leakage of the signal from the adjacent track in either the groove or the land, the groove width and the land width are preferably 1: 1.
However, for the purpose of securing the track cross signal or preventing the deterioration of the characteristics when repeated recording erasure is performed many times, a problem occurs in the crosstalk in consideration of the optimum shape of the land and groove. It may be slightly deliberately deviated from 1: 1 within a range.
[0084]
The disc of the present invention can be used as a single disc using only one side, and two discs can be bonded to each other with the opposite side of the substrate facing each other.
Even in the case of a double-sided disk, by adopting a drive having a structure in which optical pickups are arranged on both sides of the disk, recording, erasure and reproduction can be performed simultaneously on both sides without replacing the disk surface.
This is an important feature not found in magneto-optical disks that require a magnet on the side opposite to the laser beam irradiation side.
[0085]
The L & G recording optical disc of the present invention is a rewritable optical information recording medium, and can also be used as a write-once type that can be recorded only once. For example, a signal for prohibiting writing of information may be recorded on the disk so that erasure or rewriting cannot be performed.
[0086]
【Example】
Hereinafter, the present invention will be described in more detail with specific examples. However, the present invention is not limited to the following examples as long as the gist thereof is not exceeded.
The substrates used in the examples and comparative examples were all the same polycarbonate substrates obtained by injection molding, and the refractive index was 1.56 for laser light having a wavelength of 680 nm. Further, the noise level when recording on the land and the noise level when recording on the groove were almost the same under any of the recording conditions shown in the examples and comparative examples. Therefore, CN ratio comparison is synonymous with record carrier level comparison.
[0087]
Example 1
A substrate provided with a spiral groove was prepared. The groove width and land width were both 0.75 μm and the groove depth was about 70 nm. A lower dielectric protective layer, a recording layer, an upper dielectric protective layer, and a reflective layer were provided on the substrate by sputtering.
[0088]
The lower dielectric protective layer and the upper dielectric protective layer are (ZnS)₈₀(SiO₂)₂₀The thickness of the lower dielectric protective layer was 100 nm, and the thickness of the upper dielectric protective layer was 20 nm. The recording layer uses a material mainly composed of Ge, Sb, and Te that causes a reversible phase change between an amorphous phase and a crystalline phase by laser irradiation, and has a composition ratio of Ge: Sb: Te = 2: 2: 5 (atom Ratio). The film thickness of the recording layer was 25 nm. Reflective layer is Al_97.5Ta_2.5Was 100 nm. An ultraviolet curable resin was further provided as a protective coating on the reflective layer.
Since the recording layer immediately after the film formation was in an amorphous state, the entire surface was annealed with a laser beam to change the phase to a crystalline state, which was set as an initial (unrecorded) state.
[0089]
Recording was performed by irradiating the track with a focused beam of a high-power laser to change the recording layer to an amorphous state, and the recording mark was detected by the change in the amount of reflected light from the amorphous recording mark.
Next, the disk is rotated at a linear velocity of 10 m / s, and a semiconductor laser beam of 680 nm is condensed on the recording layer by an objective lens having a numerical aperture of 0.60, and signal recording and reproduction is performed while tracking control is performed by a push-pull method. Went.
[0090]
First, an arbitrary groove was selected, and a signal having a frequency of 7.47 MHz was recorded. The recording power was changed in increments of 1 mW between 10 and 12 mW, and one beam overwrite was performed with the erasing power and base power set to 6 mW. After that, when it was reproduced and measured with a spectrum analyzer at a resolution bandwidth of 30 kHz, the CN ratio was a good value of 54 to 55 dB.
Next, an arbitrary land was selected, and the same recording was performed to measure the CN ratio. As a result, a CN ratio of 54 to 55 dB that was exactly the same as that in the groove was obtained.
[0091]
The phase difference of the reflected light between the crystalline state and the amorphous state of the recording layer was calculated by calculating that the reflected light in the amorphous state was advanced by 0.01π. Also, the absorption ratio A of the recording layer_c/ A_aWas 0.84 by calculation.
[0092]
Example 2
The same disk as in Example 1 was rotated at a linear velocity of 15 m / s, an arbitrary groove was selected, and a signal having a frequency of 11 MHz was recorded using the same signal recording apparatus as in Example 1. One-beam overwriting was performed with a recording power of 12 mW, an erasing power, and a base power of 7 mW. After that, reproduction and measurement at a resolution bandwidth of 30 kHz gave a CN ratio of 52 dB.
After recording, when the recording track was irradiated with a DC laser beam having a power of 7 mW, the carrier level was reduced by 25 dB, and an erasing ratio of 25 dB was shown. Next, when an arbitrary land was selected and the same recording was performed and the CN ratio was measured, a CN ratio of 52 dB exactly the same as that of the groove was obtained, and an erasure ratio of 24 dB equivalent to the groove was obtained.
[0093]
Example 3
The composition of the recording layer is Ge_{twenty two}Sb_{twenty five}Te₅₃A disc was manufactured in exactly the same manner as in Example 1 except that.
The stoichiometric Ge: Sb: Te = 2: 2: 5 is suitable for recording / reproduction at a linear velocity of 10 m / s or higher, and a recording mark by recrystallization at a linear velocity of 10 m / s or lower. In order to prevent shape distortion, it is effective to slightly increase the amount of Sb.
[0094]
The disc was rotated at a linear velocity of 3 m / s, an arbitrary groove was selected, and a signal with a frequency of 2.24 MHz was recorded using the same signal recording apparatus as in Example 1. The recording power was changed in increments of 0.5 mW between 8.5 and 10.5 mW, and one beam overwrite was performed with the erasing power and base power set to 4.5 mW. After that, when it was reproduced and measured at a resolution bandwidth of 10 kHz, the CN ratio was a good value of 57 to 59 dB.
Next, an arbitrary land was selected and the same recording was performed to measure the CN ratio. As a result, a CN ratio of 57 to 59 dB, which was exactly the same as that of the groove, was obtained.
At this time, the jitter of the recording mark was 8 ns. Jitter was measured by detecting the zero-cross point of the second derivative of the signal waveform from the beginning to the rear end of the mark.
[0095]
The phase difference of the reflected light between the crystalline state and the amorphous state of the recording layer was calculated by calculating that the reflected light in the amorphous state was advanced by 0.01π. Also, the absorption ratio A of the recording layer_c/ A_aWas 0.84 by calculation.
FIG. 6 shows the relationship between the recording power and the CN ratio obtained in this example.
[0096]
Example 4
A disk was manufactured in exactly the same manner as in Example 3 except that the thickness of the lower dielectric protective layer was 150 nm.
The disc was rotated at a linear velocity of 3 m / s, an arbitrary groove was selected, and a signal with a frequency of 2.24 MHz was recorded using the same signal recording apparatus as in Example 1. The recording power was changed in increments of 0.5 mW between 10 and 12 mW, and one beam overwrite was performed with the erasing power and base power set to 4.5 mW. After that, when it was reproduced and measured at a resolution bandwidth of 10 kHz, the CN ratio was 54 to 55 dB and a good value.
[0097]
Next, an arbitrary land was selected, and the same recording was performed to measure the CN ratio. As a result, a CN ratio of 54 to 55 dB that was exactly the same as that in the groove was obtained.
Although the CN ratio between the land and the groove was equal and sufficient, the CN ratio was reduced by about 4 dB as compared with Example 3, and the optimum recording power was required by about 1.5 mW. Deterioration of recording sensitivity is directly linked to a decrease in the laser light life of the drive used.
[0098]
The phase difference of the reflected light between the crystalline state and the amorphous state of the recording layer was found that the reflected light in the amorphous state was delayed by 0.03π. Also, the absorption ratio A of the recording layer_c/ A_aWas calculated to be 0.75.
[0099]
Comparative Example 1
A disc was manufactured in exactly the same manner as in Example 3 except that the thickness of the recording layer was 20 nm.
The disc was rotated at a linear velocity of 3 m / s, an arbitrary groove was selected, and a signal with a frequency of 2.24 MHz was recorded using the same signal recording apparatus as in Example 1. The recording power was changed in increments of 1 mW between 5 and 10 mW, and one beam overwrite was performed with the erasing power and base power set to 4.5 mW. After that, when it was reproduced and measured at a resolution bandwidth of 10 kHz, the CN ratio was 56 dB, which was a good value.
[0100]
Next, when an arbitrary land was selected and the same recording was performed and the CN ratio was measured, it was 53 dB. As described above, the signal quality of the land and the groove is not equal, and a difference of 3 dB is generated in the CN ratio.
The phase difference of the reflected light between the crystalline state and the amorphous state of the recording layer was calculated by the calculation of the reflected light in the amorphous state by 0.20π. Also, the absorption ratio A of the recording layer_c/ A_aWas 0.85 by calculation.
[0101]
Comparative Example 2
A disc was manufactured in exactly the same manner as in Example 3, except that the thickness of the lower dielectric protective layer was 180 nm, the thickness of the recording layer was 20 nm, and the thickness of the upper dielectric protective layer was 80 nm.
The disc was rotated at a linear velocity of 3 m / s, an arbitrary land was selected, and a signal with a frequency of 2.24 MHz was recorded using the same signal recording apparatus as in Example 1. The recording power was changed in increments of 0.5 mW between 8 and 9 mW, and 1-beam overwrite was performed with the erasing power and base power set to 4.5 mW. After that, when it was reproduced and measured with a resolution bandwidth of 10 kHz, the CN ratio was 50 to 51 dB.
[0102]
Next, an arbitrary groove was selected, the same recording was performed, and the CN ratio was measured. As a result, only 39 to 40 dB was obtained. As described above, one of the signal quality of the land and the groove is remarkably deteriorated, and the difference of CN is as much as 11 dB.
[0103]
The phase difference of the reflected light between the crystalline state and the amorphous state of the recording layer was found to be delayed by 0.16π from the reflected light in the amorphous state.
Absorbance ratio A of recording layer_c/ A_aAlthough the calculated value was 1.19, the jitter of the recording mark measured at the land was 13 ns, which was inferior to that of Example 3.
FIG. 7 shows the relationship between the recording power and the CN ratio obtained by this comparative example.
[0104]
Comparative Example 3
A disk was fabricated in exactly the same manner as in Example 3, except that the thickness of the lower dielectric protective layer was 220 nm, the thickness of the recording layer was 20 nm, and the thickness of the upper dielectric protective layer was 80 nm.
The disc was rotated at a linear velocity of 3 m / s, an arbitrary land was selected, and a signal with a frequency of 2.24 MHz was recorded using the same signal recording apparatus as in Example 1. The recording power was changed in increments of 0.5 mW between 8 and 9 mW, and 1-beam overwrite was performed with the erasing power and base power set to 4.5 mW. After that, reproduction and measurement at a resolution bandwidth of 10 kHz showed a CN ratio of 51 to 52 dB.
Next, an arbitrary groove was selected, the same recording was performed, and the CN ratio was measured. As a result, only 44 to 45 dB was obtained. Thus, one of the signal quality of the land and the groove is remarkably deteriorated, and a difference of 7 dB is generated in the CN ratio.
[0105]
The phase difference of the reflected light between the crystalline state and the amorphous state of the recording layer was found to be delayed by 0.25π from the reflected light in the amorphous state.
Absorbance ratio A of recording layer_c/ A_aHowever, the jitter of the recording mark measured at the land was 10 ns, which was inferior to that of Example 3, although 1.21 was calculated.
[0106]
Example 5
A plurality of spiral grooves were prepared by changing the groove pitch from 1.1 to 1.6 μm in steps of 0.5 μm. In both cases, the groove width and land width were the same. The actual recording track pitch is 0.55 to 0.8 μm. The groove depth was about 70 nm.
A lower dielectric protective layer, a recording layer, an upper dielectric protective layer, and a reflective layer were provided on the substrate by sputtering. The composition of the recording layer is Ge_{twenty two}Sb_23.5Te_54.5Except for the above, the layer configuration was the same as in Example 1.
[0107]
This disk was repeatedly overwritten and evaluated for the degree of cross erase.
First, recording was performed on the groove or land, and then overwriting was repeatedly performed on both adjacent lands or grooves, and the decrease in the CN ratio of the signal first recorded on the groove or land was measured.
[0108]
The disc is rotated at a linear velocity of 10 m / s, and a 680 nm semiconductor laser beam is focused on the recording layer by an objective lens having a numerical aperture of 0.60, and signals are recorded and reproduced while tracking control is performed by a push-pull method. It was.
First, an arbitrary groove was selected, and a signal having a frequency of 2.24 MHz was recorded with a duty of 25%. One-beam overwriting was performed with a recording power of 8 to 9 mW, an erasing power and a base power of 4.5 mW.
[0109]
As a result, when the groove pitch is 1.45 μm (recording track pitch 0.725 μm) or more, the CN ratio between adjacent grooves or lands after 10,000 overwrites can be reduced to less than 3 dB, and there is no practical problem. there were.
The phase difference of the reflected light between the crystalline state and the amorphous state of the recording layer was calculated by calculating that the reflected light in the amorphous state was advanced by 0.01π. Also, the absorption ratio A of the recording layer_c/ A_aWas 0.85 by calculation.
[0110]
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 diffusion, and the most severe condition regarding cross erase. It will be examined in. Therefore, it can be considered that there is no problem with other layer configurations as long as the minimum track pitch is exceeded.
[0111]
Example 6
The same disk as in Example 5 was repeatedly overwritten on the land, and the mark length jitter of the signal on the land was measured. The recording / reproducing conditions were the same as in Example 5 except that the focusing lens with NA = 0.55 was used.
Only when the groove pitch is 1.55 μm and 1.6 μm (recording track pitch 0.775 μm and 0.8 μm), 10^ThreeThe increase in jitter with respect to the overwriting times was suppressed to about 20%.
On the other hand, when the groove pitch is 1.4 μm (recording track pitch 0.7 μm), the jitter is remarkably increased.^ThreeMore than twice the number of overwrites.
[0112]
【The invention's effect】
As described above, according to the optical recording medium and the recording / reproducing method of the present invention, it is possible to eliminate the undesirable difference between the carrier level of the land recording mark and the carrier level of the groove, and to both the land and the groove. Even when a signal is recorded, the influence of crosstalk from adjacent tracks can be reduced.
Therefore, a reproduction signal amplitude of the same level can be obtained regardless of whether recording is performed on the land or the groove, and a high quality and highly reliable land and groove recording disk can be provided.
[0113]
Furthermore, the ratio of the irradiation light absorbed in the recording layer between the case where the recording layer of the optical recording medium of the present invention is in the amorphous state and the case where the recording layer is in the crystalline state is specified, so that a high CN ratio and recording can be performed. An excellent disk with low mark jitter can be provided.
[Brief description of the drawings]
FIG. 1 is an enlarged perspective view for explaining the positional relationship between a groove shape of an optical disk and a convergent beam of irradiated laser light in the present invention.
FIG. 2 is an enlarged perspective view for explaining the positional relationship between the groove shape of an optical disk and the convergent beam of irradiated laser light in the present invention.
FIG. 3 is an enlarged perspective view for explaining the positional relationship between the groove shape of the optical disc and the convergent beam of the irradiation laser beam in the present invention.
FIG. 4 is an enlarged perspective view for explaining the positional relationship between the groove shape of the optical disc and the convergent beam of the irradiation laser beam in the present invention.
FIG. 5 is a schematic diagram showing the shape of a convergent beam.
6 is a graph showing the relationship between recording power and CN ratio in Example 3. FIG.
7 is a graph showing the relationship between recording power and CN ratio in Comparative Example 2. FIG.
[Explanation of symbols]
1 Substrate
2 Recording layer
3 rand
4 Groove
5 Focused beam
Area of convergent beam irradiated to 6 lands
7 Focused beam area irradiated to groove
8 Record mark
9 Focused beam
10 Intensity distribution of convergent beam
11 Airy Disc
12 Focusing lens

Claims (5)

It consists of a lower dielectric protective layer, a phase change recording layer, an upper dielectric protective layer, and a metal reflective layer sequentially laminated on a transparent substrate on which grooves are formed, and both the grooves and the grooves are used as recording areas. An optical recording medium for recording, erasing and reproducing information by irradiating a laser beam having a wavelength of 700 nm or less,
(1) Phase difference 2πα between reflected light from an unrecorded area and reflected light from a recorded area defined below
2πα = (phase of reflected light from unrecorded area) − (phase of reflected light from recorded area) satisfies the following equation:
[Expression 1]
(M−0.1) π ≦ 2πα ≦ (m + 0.1) π (m is an integer)
(2) The groove width is substantially equal to the width between the grooves, and the groove width GW, the groove width LW, and the groove depth d satisfy the following formula:
0.3 μm ≦ GW ≦ 0.8 μm
[Equation 3]
0.3 μm ≦ LW ≦ 0.8 μm
[Expression 4]
0.63 × (λ / NA) ≦ LW ≦ 0.8 × (λ / NA)
[Equation 5]
(GW + LW) / 2> 0.6 × (λ / NA)
[Formula 6]
λ / 7n <d <λ / 5n (where λ: wavelength of irradiated light, n: refractive index of substrate, NA: numerical aperture of focusing lens). Assuming that the absorption rate of the irradiation laser beam in the recording layer is Ac when the recording layer is in the crystalline state and Aa when the recording layer is in the amorphous state, the ratio of the absorption rate between the crystalline state and the amorphous state Ac / Aa Is [Equation 7]
0.84 ≦ Ac / Aa <1.01
The optical recording medium according to claim 1, wherein: 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 are a mixed film of ZnS and either SiO ₂ or Y ₂ O ₃ , and SiO ₂ or Y ₂ O ₃ The optical recording medium according to claim 1, wherein the content of the optical recording medium is 5 to 40 mol%. Using the optical recording medium according to any one of claims 1 to 4, recording and erasing by one-beam overwriting with a laser having a wavelength of 700 nm or less in both regions, using both the grooves and the grooves as recording regions. A recording / reproducing method characterized by causing reproduction.

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