JPS6129433A - Optical disk - Google Patents

Abstract

PURPOSE:To reduce the variation of track detecting sensitivity by performing processing in order to increase the absolute value of a reflectance by formation of a recording pit and also to obtain a prescribed amount of phase variation of the reflectance. CONSTITUTION:A foundation layer 11 having a higher dielectric constant than a substrate 2 is set at a proper level of thickness. A recording medium consists of the layer 11 and a recording layer 3'. The track detecting sensitivity is deteriorated by the variation of the phase difference owing to formation of a recording pit and also by the deterioration of the reflected light quantity itself with a recording medium having the deterioration of the reflectance at a pit position. Therefore it is possible to prevent the deterioration of the track detecting sensitivity due to the deterioration of the reflected light quantity with a recording medium having an increase of the reflection factor at the pit position. Thus it is possible to obtain a recording medium that produces virtually no change of the track detecting sensitivity by having an optimum form of a groove track as long as the phase difference of the reflectance owing to formation of a recording pit is set at ¦DELTAphi¦<90 deg. and the recording pit is equal to a reflectance increasing type pit.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical disc on which recording and reproduction are performed by light irradiation.

(Prior art and its problems) As a method for forming recording pits on defined tracks in an optical disc, a method is used in which concave or convex grooves are provided in advance on a substrate.

FIG. 1(a) is a diagram showing a track position detection method.

The f incident light passes through the substrate 2 by the condensing lens 1 and forms a spot on the recording medium 3. When the center of this spot coincides with the center of the track 7, the distribution of reflectance from the spot becomes symmetrical as shown by curve 4 in FIG. 1(b). On the other hand, if there is a misalignment between the spot center and the track center, the diffraction due to the grooves will become uneven.
As shown by curve 5 indicated by a broken line, the reflected light distribution becomes uneven. Therefore, if the reflected light is received by the two-split photodetector 6 shown in FIG. 1(c) and the difference between the left and right detection outputs is calculated,
You can know the deviation of the spot from the track.

The differential output is, for example, as shown in FIG. 1 (fd).

The slope of the differential output before and after the track center provides the detection sensitivity of the track error signal. This detection sensitivity varies depending on the depth of the groove. The depth of the groove gives a phase difference to the reflected light.If the groove depth is d, the substrate refractive index is n, and the wavelength is λ, then the phase difference between the reflected light 7 on the track and 8 between the tracks is ψ.
becomes ψ=2π·2dn/λ. As shown in FIG. 2, the track detection sensitivity with respect to ψ is a sine wave function about #1. The inversion of the sign indicates that the slope of the differential output is inverted.

As shown in FIG. 2, the track detection sensitivity is at its maximum when the phase difference ψ is 90', so a groove depth of this value is used for conventional optical discs.

Although the above is true in a recording medium without uniform recording pits, in reality, the phase difference ψ deviates from 90' by forming pits.

For example, as shown in FIG. 3, the substrate 2 is coated with a low melting point medium (Bs).

In the case of an optical disc in which the recording medium 3 is formed by using Ts, phthalocyanine, etc., there are pit positions 9 and outside pits 10.
The difference △ψ in the amount of phase change of light is the reflection on the surface where the high refractive index material changes to the low refractive index material at the pit position, and the change from the low refractive index material to the high refractive index material outside the pit. Since it is reflected by the surface, it occurs at nearly 180°. Therefore, as shown in Figure 2, if ψ = 90', ψ +
This results in an inversion of the shift detection sensitivity such that Δψ=270°. Therefore, stable tracking servo operation becomes difficult.

In reality, pits are formed partially on the track, and the track detection sensitivity is the average value when the beam moves on the track. However, the inventor's experiments have confirmed that the sensitivity may drop to 50% or less compared to before pit formation.

(Object of the Invention) An object of the present invention is to provide an optical disc that is capable of stable tracking operation even after recording pits are formed.

(Structure of the Invention) The optical disc of the present invention is a recording medium in which recording pits are formed in which the complex reflectance of the surface changes due to phase change or hole formation etc. by irradiation with energy beams such as light, and concave or convex tracks. The optical disc provided on the substrate is characterized by processing so that the absolute value of the reflectance increases due to the formation of recording pits, and the amount of phase change Δψ of the reflectance becomes 90'>IΔψ1. be.

(Detailed Description of Configuration) As already pointed out, the decrease in track detection sensitivity due to the formation of recording pits occurs because the amount of phase change Δψ of the reflectance at the recording pit portion is close to 180°.

Looking at Figure 2, even if the groove phase difference ψ is 90°, which is the optimum value before recording pit formation, if the amount of phase change 1△ψ1 in reflectance due to recording pit formation is 90' or less, the track detection sensitivity will decrease. It can be seen that no reversal occurs. The same is true whether the track is concave or convex.

As a method for reducing 1Δψ1 to 900 or less, for example, the fourth
As shown in the figure, this can be realized by providing an underlayer 11 having a higher dielectric constant than the substrate 2 to an appropriate thickness, and constructing a recording medium with the underlayer 11 and the recording layer 3'.

The decrease in track detection sensitivity is caused not only by a change in phase difference due to the formation of recording pits, but also by a decrease in the amount of reflected light itself in a recording medium where the reflectance at the pit position decreases. Therefore, in a recording medium in which the reflectance at pit positions increases, it is possible to prevent a decrease in track detection sensitivity due to a decrease in the amount of reflected light.

Furthermore, the phase difference in reflectance due to recording pit formation is 1ΔψI
If the recording pit is <90' and is of the increased reflectance type, by optimizing the groove track shape, it is possible to realize a recording medium in which almost no change in track detection sensitivity occurs due to the formation of the recording pit.

By the way, in consideration of multi-stable reproduction of information from recording pits, it is desirable that the amount of reflected light from the recording medium be large. FIG. 5 shows the change in the amount of reflected light with respect to the phase depth ψ of the track, and below 1800, the smaller ψ is, the larger the amount of reflected light is.

This is true for both convex and concave tracks.

For example, if the phase change amount △ψ is positive in a reflectance-increasing recording pit, if the phase depth ψ is set to be smaller than 900 with a convex track, the phase depth ψ will increase due to the recording pit formation.
It can be seen from FIG. 2 that when +Δψ, the decrease in track detection sensitivity is smaller than when ψ is set to 900. When a concave track is used, the phase depth becomes ψ-Δψ due to the formation of recording pits, and in order to reduce changes in track detection sensitivity, ψ needs to be set larger than 90'. In this case, as shown in FIG. 5, the reflectance before the recording pits are formed is reduced, which is not desirable in practice.

Conversely, when the amount of phase change Δψ is negative, the phase depth σ becomes ψ−△ψ for a convex shape and ψ10△ψ for a concave shape due to the formation of recording pits, so the initial setting value of ψ is changed from 900. It can be seen that a concave track is advantageous in order to make it smaller, contrary to the case where Δψ is positive.

In this way, by appropriately selecting a track (concave or convex) depending on the sign of Δψ, it is possible to increase the reflectance after recording pits are formed. Another method for increasing the reflectance may be a structure in which a reflective layer made of a material with a high reflectance, such as metal, is provided.

(Example 1) In the configuration shown in FIG. 4, vanadyl-7-thalocyanine (VoFc) [''n = 2.9.
), Show (n = 2.2) was used for the underlayer 11, P1'viMA (n = 1.5) was used for the substrate 2, the recording layer was 30 nm, and the underlayer thickness was 80 nm. The reflectance increases from 896 to 27%, and the amount of phase change becomes approximately +70'.

Therefore, with a convex track, the phase depth ψ is set to 50 to 60'.
Just keep it to a certain level.

(Example 2) In a structure having a surface incidence structure and a reflective layer 12 under an underlayer 11 as shown in FIG. 6, the recording layer has Te (n=
=4. ]c=2)-PMMA (n=1.
5) AI (H=2.k=-7) is used for the reflective layer, the recording layer thickness is IOnm, and the underlayer thickness is 40 nm. Recording pits are formed by removing the recording layer 3', and the reflectance increases from 10g6 to 84% due to pit formation, and the amount of phase change becomes approximately -30'. Therefore, it is sufficient to use a concave track and set the phase depth ψ to about 70 to 80'.

(Effects of the Invention) As described above, according to the present invention, an optical recording layer disc with little change in track detection sensitivity can be realized by forming pits in the recording layer.

[Brief explanation of the drawing]

Figures 1 (a) to fd) are diagrams showing the principle of track error detection, Figure 2 is a diagram showing changes in track detection sensitivity with respect to the phase difference ψ of the groove, and Figures 3, 4, and 6 are diagrams showing the principle of track error detection. FIG. 5, which is a diagram showing an example of the configuration of an optical disc, is a diagram showing changes in the amount of reflected light with respect to the phase difference ψ of the grooves. In the figure, 1... condenser lens, 2... substrate, 3...
Recording layer, 4.5-・Reflected light distribution, 6-2 split photodetector,
9...Pit position, 10--Recording medium surface outside the pit, 1
1... Base layer, 12... Reflective layer. 71-1 Mouth 71-2 Fig. O 3 Fig. 4 O 5 Fig. 76 Fig./L, 2

Claims (1)

[Claims] In an optical disk comprising a recording medium on a substrate having concave or convex tracks, in which recording pits are formed whose surface complex reflectance has changed due to phase change or hole formation due to irradiation with energy beams such as light, recording pits are formed on a substrate having concave or convex tracks. The absolute value of the reflectance increases due to the formation, and the phase change amount Δψ of the reflectance is 90
An optical disc characterized by being processed so that °＞｜△ψ|.