JP2002517058A - Optical recording medium - Google Patents

Abstract

(57) Abstract: An optical recording medium having a grooved recording layer is disclosed. The structure of the unrecorded tracks must allow the scanning device to obtain a radial tracking error signal by the push-pull method. The structure of the recorded track must allow the scanning device to obtain a radial tracking error signal by a high frequency phase detection method. To this end, the width of the groove is in the range of 0.3 to 0.6 times the wavelength of the radiation beam used to scan the recording medium divided by the numerical aperture, and the depth of the groove is The wavelength is in the range of 1/24 to 1/7 times the value obtained by dividing the wavelength by the refractive index. The phase difference between the radiation beam reflected from the area of the track between the recording marks and the radiation beam reflected from the marks is in the range of 0.4 to 2.0 radians.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention has a recording layer which is irradiated by a radiation beam and changes from a first state to a second state, and the recorded information is recorded in the first state area in the second state. Represented by a marked mark, said mark being arranged in the form of a track having a guide groove including a width and a depth, the reflection from an area on the track in the first state and the track in the second state The first optical phase difference that exists between the reflection from the upper area and the area between the tracks in the first state and the area on the track in the first state is the optical phase difference. The present invention relates to an optical recording medium for recording and reading information using a radiation beam having a predetermined wavelength and a predetermined numerical aperture.

[0002] Information can be stored on such recording media by a scanning device having an optical head. The head focuses the radiation beam on the information layer of the medium and follows the unrecorded track using tracking information obtained from the track grooves. When the medium is disk-shaped, the grooves are annular or spiral, and the tracking information is in the form of a radial tracking error signal. When a relatively high power radiation beam is modulated by a signal representing the information to be recorded, the information is recorded on the track in the form of photodetectable marks. During reading, the radiation beam modulated by the mark upon reflection from the information layer has a relatively low power. During reading, tracking information may be obtained from the grooves or the recorded information.

An optical recording medium according to the preamble is known from Japanese Patent Application No. 5174380. This medium has a stack of optical thin layers with embedded recording layers. The thickness of the transparent layer of the laminate adjacent to the recording layer is the same as that of the area between the tracks where the first optical phase difference between the unrecorded area of the track and the recording area is in the first state. An adjustment is made to increase the second optical phase difference between the region on the track in the state. The relationship between these phase differences increases the information signal obtained from the scanned mark.
A disadvantage of this conventional recording medium is that a track cannot be correctly traced by a scanning device using a method called a push-pull method for obtaining a tracking signal from a mark recorded on an information layer and a method called a phase detection method. . The push-pull method and the phase detection method for obtaining tracking information are described in U.S. Pat.
No. 785,441.

[0004] It is an object of the present invention to provide a mark recorded by the push-pull method, and
An object of the present invention is to provide an optical recording medium from which tracking information can be obtained from recorded information.

The object of the present invention is to provide a recording medium according to the preamble, wherein the width of the guide groove is in the range of 0.3 to 0.6 times the value obtained by dividing the wavelength by the numerical aperture, and the depth of the guide groove is determined by the radiation beam. This is achieved by a recording medium characterized in that the first optical phase difference is 1/24 to 1/7 times the value obtained by dividing by the obtained refractive index, and the first optical phase difference is in the range of 0.4 to 2.0 radians. . The combination of the groove width and the specific value of the phase difference provides both a high quality push-pull signal and a phase detection signal and a high quality information signal from the recorded mark. The phase difference enhances both the information signal and the push-pull signal. Minimizing the depth of the groove allows the scanning device to obtain a high quality push-pull signal from the groove. Grooves deeper than the maximum depth of the groove degrade the quality of the phase detection signal. Further, the phase detection signal from the deeper groove becomes more dependent on the defocus of the radiation beam. The depth of the groove is provided as a mechanical depth. The refractive index experienced by the radiation beam is the refractive index of the material between the grooves. When the radiation beam enters the recording layer via the substrate or the protective layer, the material is the material of the transparent substrate or the protective layer, respectively. It is desirable that the width of the groove is larger than the width of the mark designed to be recorded on the recording layer.

[0006] The tracking signal generated by the phase detection method is usually influenced by the axial position of the focus of the radiation beam with respect to the position of the recording layer. When the radiation beam is not focused on the recording layer, the amplitude of the tracking signal can be greatly reduced and change sign. This effect is already mitigated when the first phase difference is in the range of 0.4 to 2.0 radians. Further mitigation of the effect is achieved when the width and depth of the guide groove satisfy 8.33 NA D / n +121 NA / λ −400 NA Φ / λ <W, where NA is the numerical aperture and λ is Wavelength in nanometers, Φ
Is the first optical phase difference in radians, n is the refractive index, D is the depth in λ / n,
W is the width in λ / NA units.

Preferably, the first optical phase difference is in a range of 0.4 to 1.1 radians.
Media having a phase difference less than this value will exhibit an irregular tracking signal as obtained by the phase detection method. As the radiation beam focuses below and above the recording layer, this asymmetry manifests itself as different amplitudes of the tracking signal. The possible measurement of the asymmetry is (xy) / (2z), where x is the highest value of the phase detection tracking error signal when the radiation beam is focused 1 μm above the information plane;
y is the maximum value of the phase detection tracking error signal when the radiation beam is focused 1 μm below the information surface, and z is the maximum value of the phase detection tracking error signal when the radiation beam is focused on the information surface.

[0008] A further improvement of the phase detection tracking signal is that the ratio of the intensity reflection of the area on the track in the second state to the intensity reflection of the area on the track in the first state is 0.1.
It can be achieved when it is greater than 5. Such a medium is called a dark / write medium. The medium referred to as the white / write medium preferably has a ratio of the intensity reflection of the area on the track in the first state to the intensity reflection of the area on the track in the second state of more than 0.15. . The averaging of the tracking signal as obtained from the phase detection method is improved when the ratio is in the range of 0.3 to 0.5 for both dark and white recording media.

The intensity reflection of the region on the track in the first state is preferably larger than 0.15 in a dark / write medium in order to obtain a good information signal from the radiation reflected from the information layer. In the white / write medium, it is preferable that the intensity reflection of the area on the track in the second state is larger than 0.15.

[0010] The grooves in the information layer may be used to store information, such as addresses used in accessing the information. Such information can be stored in the form of groove depth or position fluctuations. The depth of the groove is preferably in the range of 1/12 to 1/7 times the wavelength of the radiation beam divided by the refractive index.

The optical retardation of a medium can be realized by embedding a recording layer in a stack of optical thin layers and adjusting the thickness of the layer. When the imaginary part of the refractive index of the material of the recording layer in the first state is larger than 3.4, the design of the lamination becomes easy.

The material of the recording layer is preferably of a phase change type. When an amorphous mark is recorded on a crystalline layer, the recording speed can be relatively high. In this case, the first state is a crystalline state and the second state is an amorphous state.

In view of the unpublished International Patent Application No. IB97 / 01470, the combination of groove depth, groove width, and track period is 40 nm, 500 nm and 900 nm,
Recording media having 55 nm, 400 nm and 900 nm, 51 nm, 500 nm and 870 nm, all having a refractive index n of 1.58 and a design wavelength of 670 nm have been discarded.

[0014] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

FIG. 1 shows an information recording medium 1 according to the invention designed for recording and reading information using a focused radiation beam having a design wavelength and a numerical aperture. The medium 1 has a transparent substrate 2 and a recording layer 3. The recording layer can be scanned by a radiation beam incident on the recording layer via the substrate 2. The recording layer 3 is embedded in a stack 4 of optical thin layers arranged on a substrate 2. This lamination is performed from the substrate side with the transparent interference layer 5, recording layer 3
, An additional interference layer 6 and a reflection layer 7. The laminate 4 is shielded from environmental influences by a protective layer 8.

The substrate 2 has a groove pattern on the side where the stack 4 is arranged. In a disk-shaped medium, the groove pattern is an annular or spiral groove. The portion of the groove pattern in the figure that forms a depression in the substrate when viewed from the lamination side is referred to as a groove 9. The portion of the pattern in the figure that forms the raised portion when viewed from the same side is called a land 10. The width of the groove is determined at the upper end of the groove, that is, at the land level. Since the thickness of the layer of the laminate 4 is very thin, the pattern of the substrate 2 appears on the recording layer 3. The following example is designed for recording in the groove 9 of the recording medium. Conversely, when the recording medium is designed to record on the part 10, the optimum parameter values of the depth and width of the groove according to the invention are likewise adapted, but the pattern part 9 in the figure is a land or a pattern part. 10 is referred to as a groove. Example I The substrate of the recording medium is made of polycarbonate (PC) having a design wavelength of 670 nm, a numerical aperture of 0.60 and a refractive index of 1.58. The interference layer 5 is a 90 nm thick layer having a refractive index of 2.13 and made of ZnS 80% and SiO ₂ 20%. The recording layer 3 has a refractive index of 4.26-i1.69 in an amorphous state and 4.26-i1.69 in a crystalline state.
Phase change material GeSb ₂ Te ₄ is a 44-i1.69 a layer thickness of 30 nm. The interference layer 6 is a layer of the same material as the interference layer 5 having a thickness of 30 nm. The reflective layer 7 is a layer of aluminum alloy having a refractive index of 1.69-i7.81 and a thickness of 100 nm. The mechanical depth d of the groove 9 is 55 nm, the width w of the groove is 450 nm, and the pitch of the groove, which is the track pitch, is 740 nm. The dimensionless groove depth D is dn
/Λ=0.130, where n is the refractive index of the substrate and the dimensionless groove width W is equal to wNA / λ = 0.403.

FIG. 2 is a diagram showing a part of the recording layer 3 having the grooves 9 and the lands 10. Information is recorded in the grooves. The recording layer 3 is initially in a crystalline state. During recording, an amorphous area 11 called a mark is formed in the recording layer. The length and position of the mark represent information recorded on the medium. The intensity reflectance of the laminate 4 in the region of the recording layer in the amorphous state is equal to 0.07. The intensity reflectance of the laminate 4 in the region in the crystalline state is 0.1
Equal to 8. The value of the ratio of the amorphous reflectance divided by the crystal reflectance is therefore 0.39
It is. Both intensity reflectances are measured using a focused radiation beam in the groove-free area.

The radiation reflected from the region of the track that is in the crystalline state and indicated by “a” in FIG. 2 has a phase that is similar to that of the radiation reflected from the region “b” between the tracks that is also in the crystalline state. Advance by 1.64 radians. The radiation reflected from the region of the track indicated by "c" in FIG. 2 in the amorphous state advances the phase by 0.6 radians compared to the radiation reflected from the region between the tracks in the crystalline state. . As a result, the phase difference between the land and the groove is increased by the phase difference between the mark and the area between the marks.
In other words, the effective depth of the groove is increased by the position of the mark.

The push-pull tracking error signal has a measured maximum that is greater than 95% of the value obtained for a medium having a groove depth optimized for the maximum push-pull signal. The phase detection tracking error signal of the method referred to as DTD-2 type has a maximum of clock period 0.69 with 0.1 μm radial tracking deviation of the focal point as measured by the scanning device described below. The value of the tracking error signal is a time difference normalized to a channel clock cycle used to record information on a recording medium. When the focus shifts by the depth of focus, the fluctuation of the tracking error signal is less than 10%. Example II The substrate of the recording medium is made of polycarbonate (PC) having a design wavelength of 670 nm and a refractive index of 1.58. This stack has the same layer order as the stack shown in FIG. The interference layer 5 is a layer having a refractive index of 2.13, 80% made of ZnS and 20% made of SiO ₂ and having a thickness of 95 nm. The recording layer 3 has a refractive index of 4.26 in an amorphous state.
Phase change material Ge which is -i1.69 and 4.44-i3.08 in the crystalline state
This is a layer of Sb ₂ Te _{4 having} a thickness of 25 nm. The interference layer 6 is a layer made of the same material as the interference layer 5 and having a thickness of 35 nm. The reflective layer 7 is a layer of an aluminum alloy having a refractive index of 1.98-i7.81 and a thickness of 100 nm. The depth of the groove 9 is 35 nm, and the groove width w
Is 550 nm, and the pitch of the grooves is 740 nm.

Information is recorded in the groove in the form of an amorphous mark near the crystal. The intensity reflectance of the laminate 4 in the region of the recording layer in the amorphous state is equal to 0.05. The intensity reflectance of the laminate 4 in the region in the crystalline state is equal to 0.16. The value of the ratio of the amorphous reflectance divided by the crystal reflectance is therefore 0.31. Both regions have no grooves.

The radiation reflected from the region of the track indicated as “a” in FIG. 2 in the crystalline state has a phase which is similar to that of the radiation reflected from the region “b” between the tracks in the crystalline state. Advance by 1.04 radians. The radiation reflected from the region of the track, which is in the amorphous state and indicated by "c" in FIG. 2, is compared to the radiation reflected from the region between the tracks in the crystalline state indicated by "a" in FIG. The phase advances by 0.7 radians.

The push-pull tracking error signal has a measured maximum value that is greater than 85% of the value obtained for a medium having a groove depth optimized for a maximum push-pull signal. The phase detection tracking error signal has a maximum of clock period 0.72 with a radial tracking deviation of 0.1 μm. The variation of the tracking error signal is less than 25% when the focus shifts by the depth of focus.

Although the above example of the recording medium according to the present invention relates to a medium in which an amorphous mark is recorded near a crystal, the present invention can be similarly applied to a medium in which a crystalline mark is recorded near an amorphous. The present invention is not limited to a medium on which information is recorded in a groove, and the present invention is also applicable to a recording medium on which information is recorded on lands between grooves. The stack 4 can have various forms. As shown in FIG. 1, a further reflective layer can be arranged between the substrate 2 and the interference layer 5 of the stack 4. On the other hand, further interference and reflection layers can be sandwiched between the stack and the substrate. The stack may also have only layers 3, 4 and 5, which are very suitable for write-once media. The material of the recording layer may be a phase change material, a dye or other material suitable for optically recording information. (Scanning Apparatus) FIG. 3 shows an optical scanning apparatus suitable for recording and reading information from a medium according to the present invention. FIG. 1 shows a part of a recording medium 1 having information expressed in the form of depressions and protrusions. The information layer is scanned through the substrate 2. The recording medium may have two or more information layers arranged one on top of the other.

The device has a radiation source 12 for emitting a radiation beam 13, for example a semiconductor laser. Although shown with a single lens for simplicity in the figure, the radiation beam is focused on the information layer 3 by the objective system 14. The radiation reflected by the information layer is directed to a detection system 15 via a beam splitter. The beam splitter may be a translucent plate, a diffraction grating, or may be polarization dependent. Detection system, the incident radiation is converted into one or more electrical signals and control signals, electrical signals are sent to the electronic circuit 16 in order to obtain the information signal S _i representative of the read out from the recording medium information . One of the control signals is a radial tracking error signal S _r representing the distance between the center line of the track to be scanned with the center of the spot of the information surface formed by the radiation beam. Another control signal is a focus error signal _Sr , which represents the distance between the focus of the radiation beam and the information plane. These two error signals are sent to a servo circuit 17 for controlling the position of the focal point of the radiation beam. In the figure, focusing control is realized by moving the objective system 14 in the direction of the optical axis according to the focus error signal, and radial tracking is realized by moving the objective system in the lateral direction of the track according to the radial tracking error. Is done. During recording, the intensity of the radiation source is modulated by the information to be recorded.

FIG. 4 shows the arrangement of a part of the detection system 15 and associated electronics 16 for obtaining a radial tracking error signal from the detection signal. FIG. 4A shows a circuit for obtaining a radial tracking error signal by the push-pull method. The detection system 15 has a quadrant detector having four radiation-sensitive detection elements A, B, C and D. The detection signals from the detection elements A and B are applied to the amplifier 18 and amplified. Similarly, the detection signals from the detection elements C and D are added to the amplifier 19 and amplified. The outputs of amplifiers 18 and 19 are connected to a differential amplifier 20, which forms the difference between the two input signals. The output signal of the differential amplifier 20 is a push-pull radial tracking error signal S _r (PP). This error signal is very suitable for controlling a radial tracking servo in a portion of a recording medium having a track without a recording mark.

FIG. 4B shows a circuit for obtaining a radial error signal by a high-frequency phase detection method. Detection signals from elements A and C of detection system 15 are applied to amplifier 21 and amplified. The output of the amplifier 2 is sent to the slicer 22. The slicer detects a level crossing with the detection level of the input signal and digitizes the input signal. The detection signals of elements B and D of detection system 15 are applied to and amplified by amplifier 23, the output of which is connected to the input of slicer 24. The output signals of the amplifiers 21 and 23 may be shaped by an equalizer before being sent to the slicers 22 and 24, respectively, to compensate for the effect of the response of the optical system of the scanning device to the detection signal. Slicers 22 and 2
The four digital output signals are sent to a phase comparator 25, which produces an output signal that depends on the phase between the two input pulses. The output signal of the comparator 25 is low-pass filtered by the filter 26. Output signal S _r filter 26 is a radial tracking error signal obtained by the specific embodiments in which the diagonal time difference (DTD) method of the phase detection method. This error signal is very suitable for controlling the radial tracking servo of the part of the recording medium having the recording marks.

A recording mark of a recording medium according to the present invention is an analog version of the method shown in FIG. 4B,
Alternatively, tracking can be performed using radial tracking error signals obtained by other high frequency phase detection methods, such as analog or digital versions of the phase detection method known from U.S. Pat. No. 4,785,441, among others.

[Brief description of the drawings]

FIG. 1 is a sectional view of a recording medium according to the present invention.

FIG. 2 is a plan view of a recording layer of a medium.

FIG. 3 shows a scanning device for scanning a medium according to the present invention.

FIG. 4A is a circuit diagram of a device for generating a push-pull radial tracking error signal, and FIG. 4B is a circuit diagram of a device for generating a DTD radial tracking error signal.

──────────────────────────────────────────────────の Continued on the front page (71) Applicant Groenewoodseweg 1, 5621 BA Eindhoven, The Netherlands F term (reference) 5D029 JA01 JC02 JC06 WB14 WB17 WC04 WC05 WC06 5D090 AA01 BB03 CC01

Claims (10)

[Claims] 1. A recording layer which is irradiated by a radiation beam and changes from a first state to a second state, and recorded information is recorded in an area of the first state in the second state. The mark is arranged in a track having a guide groove including a width and a depth, and is reflected from an area on the track in the first state and on a track in the second state. Between the reflection from the region of
Increases the optical phase difference between the area between the tracks in the first state and the area on the track in the first state, the radiation having a predetermined wavelength and a predetermined numerical aperture. An optical recording medium for recording and reading information using a beam, wherein a width of the guide groove is in a range of 0.3 to 0.6 times a value obtained by dividing a wavelength by a numerical aperture, and a depth of the guide groove. Is in the range of 1/24 to 1/7 times the value obtained by dividing the wavelength by the refractive index obtained from the radiation beam, and the first optical phase difference is 0.4 to 2.0.
An optical recording medium characterized by being radians. 2. The width and depth of the guide groove satisfy 8.33 NA D / n + 121 NA / λ−400 NA Φ / λ <W, where NA is a numerical aperture, and λ is a nanometer unit. The wavelength of the radiation beam, Φ is the first optical phase difference in radians, n is the refractive index, D is the depth in λ / n, and W is the width in λ / NA. 2. The optical recording medium according to 1. 3. The optical recording medium according to claim 1, wherein the ratio of the intensity reflection of the area on the track in the second state to the intensity reflection of the area on the track in the first state is greater than 0.15. 4. The optical recording medium according to claim 1, wherein the ratio of the intensity reflection of the area on the track in the first state to the intensity reflection of the area on the track in the second state is greater than 0.15. 5. The intensity reflection of an area on the track in the first state is 0.1.
5. The optical recording medium according to claim 4, wherein the value is larger than 5. 6. The intensity reflection of the area on the track in the second state is 0.1.
The optical recording medium according to claim 5, which is larger than 5. 7. The optical recording medium according to claim 1, wherein the depth of the guide groove is in a range of 1/12 to 1/7 times a value obtained by dividing a wavelength by a refractive index. 8. The optical recording medium according to claim 1, wherein the recording layer is made of a material having an imaginary part of a refractive index larger than 3.4 in the first state. 9. The optical recording medium according to claim 1, wherein the recording layer has a phase change material. 10. The optical recording medium according to claim 9, wherein said second state is amorphous.