JPH0677337B2 - Light disk - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical disc for recording / reproducing by light irradiation.

(Prior Art and its Problems) As a method of forming recording pits on a predetermined track of an optical disc, a method of previously providing a concave or convex groove on a substrate is used.

FIG. 1 (a) is a diagram showing a track position detecting method.
Incident light passes through the substrate 2 by the condenser lens 1 to form a spot on the recording medium 3. When the center of this spot coincides with the center of the track 7, the distribution of the reflectance from the spot becomes symmetrical as shown by the curve 4 in FIG. 1 (b). On the other hand, when a deviation occurs between the spot center and the track center, diffraction due to the groove becomes non-uniform, so that the reflected light distribution becomes biased as shown by a curve 5 indicated by a broken line. Therefore, if the reflected light is received by the two-divided photodetector 6 shown in FIG. 1 (c) and the difference between the left and right detection outputs is taken, the deviation of the spot from the track can be known. The difference output is as shown in FIG. 1 (d), for example.

The inclination of the differential output before and after the track center gives the detection sensitivity of the track error signal. This detection sensitivity depends on the depth of the groove. The groove depth 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, the phase difference between the reflected light on track 7 and between tracks 8 is = 2π · 2dn / λ. The track detection sensitivity with respect to is approximately a sine wave function as shown in FIG. The inversion of the sign indicates that the gradient of the difference output is inverted.

Since the track detection sensitivity becomes maximum when the phase difference is 90 ° from FIG. 2, the groove depth of this value is used in the conventional optical disc.

This is true for recording media that do not have uniform recording pits, but in reality, the phase difference can be reduced by forming pits.
It will deviate from 90 °.

For example, in an optical disc in which the recording medium 3 made of a low melting point medium (Bi, Te, phthalocyanine, etc.) is formed on the substrate 2 as shown in FIG. 3, the difference in the phase change amount of the reflected light between the pit position 9 and the outside of the pit 10 Δ indicates near 180 ° because reflection occurs on the changing surface from the high refractive index material to the low refractive index material at the pit position and reflection occurs on the changing surface from the low refractive index material to the high refractive index material outside the pit. . Therefore, from FIG. 2, if = 90 °, + Δ = 270 ° at the pit forming portion, and the track detection sensitivity is inverted. Therefore, stable tracking servo operation becomes difficult.

Actually, the pits are partially formed on the track, and the track detection sensitivity is an average value when the beam moves on the track. It has been confirmed by the inventor's experiment that the value may decrease to 50% or less compared to before the formation of pits.

(Object of the Invention) An object of the present invention is to provide an optical disk capable of a stable tracking operation even after formation of recording pits.

(Structure of the Invention) The optical disk of the present invention has a concave or convex track on a recording medium in which a recording pit is formed in which the complex reflectance of the surface is changed due to phase change or hole formation by irradiation of energy rays such as light. The optical disk provided on the substrate is characterized in that the absolute value of the reflectance increases due to the formation of recording pits, and that the phase change amount Δ of the reflectance is 90 °> | △ | There is.

(Detailed Description of Configuration) It has already been pointed out that the decrease in the track detection sensitivity due to the formation of the recording pit occurs because the phase change amount Δ of the reflectance at the recording pit portion is close to 180 °.

As shown in FIG. 2, even if the groove phase difference is 90 ° which is the optimum value before the formation of the recording pits, if the phase change amount of reflectance │ △ │ due to the formation of the recording pits is 90 ° or less, the track detection sensitivity is reversed. It turns out that does not occur. The same can be said whether the track is concave or convex.

As a method of setting | Δ | to 90 ° or less, for example, as shown in FIG. 4, an underlayer 11 having a higher dielectric constant than the substrate 2 is provided with an appropriate thickness, and the underlayer 11 and the recording layer 3'are combined. It can be realized by configuring a recording medium.

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

Furthermore, the phase difference of the reflectance due to the formation of the recording pit is | △ |
If the recording pit is of the type of <90 ° and the reflectance is increased, by optimizing the groove track shape, it is possible to realize a recording medium in which the track detection sensitivity hardly changes due to the formation of the recording pit.

By the way, in consideration of stably reproducing information from the recording pit, it is desirable that the amount of light reflected from the recording medium is large. FIG. 5 shows the change in the amount of reflected light with respect to the phase depth of the track. Below 180 °, the smaller the value, the greater the amount of reflected light. This is true for both convex and concave tracks.

For example, in the case of a reflectivity increasing type recording pit and the amount of phase change Δ is positive, if the phase depth is set to be smaller than 90 ° in the convex track, the phase depth will be + due to the formation of the recording pit.
It can be seen from FIG. 2 that the value becomes Δ, and the decrease in the track detection sensitivity is smaller than that when the setting is 90 °.
When a concave track is used, the phase depth becomes −Δ due to the formation of recording pits, and it is necessary to set the phase depth larger than 90 ° in order to reduce the change in track detection sensitivity. At this time, the reflectance before the formation of the recording pits is reduced as shown in FIG. 5, which is not desirable in practice.

On the contrary, when the amount of phase change Δ is negative, the recording pits form a phase depth of −Δ in the convex shape and + Δ in the concave shape. Therefore, in order to reduce the initial setting value to less than 90 °, It can be seen that the concave track is advantageous as opposed to the case where Δ is positive. As described above, the reflectance after forming the recording pits can be increased by properly selecting the track (concave or convex) depending on whether the Δ is positive or negative. As another method of increasing the reflectance, for example, a structure provided with a reflecting layer made of a material having a large reflectance such as metal may be used.

(Example 1) In the structure of FIG. 4, vanadyl phthalocyanine (VoFc) [n = 2.9, k = -0.8] was used as the recording layer 3 ', and Sn was used as the underlayer 11.
O ₂ [n = 2.2], PMMA [n = 1.5] is used for the substrate 2, the recording layer is 30 nm, and the underlayer thickness is 80 nm. Upward reflectance increased from 8% to 27% and the amount of phase change was about +70 when forming recording pits.
It becomes °. Therefore, with a convex track, the phase depth is 50
It should be about 60 °.

(Embodiment 2) As shown in FIG. 6, the reflection layer 12 is formed under the underlayer 11 in a front-illuminated structure.
In the structure having the following structure, Te [n = 4, k =
-2], PMMA [n = 1.5] for the underlayer 11, and Al [n for the reflective layer
= 2, k = −7] and the recording layer thickness is 10 nm and the underlayer thickness is 40 nm. The recording pits are formed by removing the recording layer 3 ', the reflectance increases from 10% to 84% due to the formation of the pits, and the amount of phase change becomes about -30 °. Therefore, it is sufficient to set the phase depth to about 70 to 80 ° in the concave track.

(Effects of the Invention) As described above, according to the present invention, it is possible to realize an optical recording disk with little change in track detection sensitivity due to the formation of pits in the recording layer.

[Brief description of drawings]

1 (a) to 1 (d) are views showing the principle of track error detection, FIG. 2 is a view showing changes in track detection sensitivity with respect to groove phase difference, and FIGS. 3, 4, and 6 are FIG. 5 is a diagram showing an example of the configuration of an optical disc, and FIG. 5 is a diagram showing changes in the amount of reflected light with respect to the phase difference of the groove. In the figure, 1 ... Focusing lens, 2 ... Substrate, 3 ... Recording layer, 4,5 ... Reflected light distribution, 6 ... 2-split photodetector, 9 ...
... pit position, 10 ... recording medium surface outside pit, 11 ... underlayer, 12 ... reflective layer.

Claims (1)

[Claims] 1. A recording medium on which a recording pit is formed on which a concave or convex track is formed, in which a recording pit having a complex reflectance on the surface is changed due to phase change or hole formation by irradiation of energy rays such as light. The optical disk is characterized in that the absolute value of the reflectance increases due to the formation of recording pits and the phase change amount Δ of the reflectance is 90 °> | Δ |.