FR2929747A1 - SUPER-RESOLUTION OPTICAL DISK WITH HIGH READING STABILITY - Google Patents

Abstract

The invention relates to a super-resolution optical disc with high read stability. The disc comprises at least one protective layer (102) on the surface, a substrate (110) on which is deposited a stack (120) comprising at least one active layer (108) with non-linear reversible properties, the active layer (108) being interposed between at least a first dielectric layer (104a) deposited under the protective layer and a second dielectric layer deposited on the substrate (110), the disc further comprising an interface layer (106) interposed between at least one the two dielectric layers (104a, 104b) and the active layer (108), the interface layer (106) being made of a material selected from the following materials: ZrO2; HfO2; TiO2; Cr2O3, Si3N4. The invention is particularly applicable to the production of ultra-high density super-resolution disks, in particular to the production of pre-recorded super-resolution disks.

Description

Super-resolution optical disk with high readability

The invention relates to a super-resolution optical disc with high read stability, that is to say a super-resolution disc that accepts a large number of read cycles. The invention applies in particular to the production of ultra-high density super-resolution disks, in particular to the production of prerecorded super-resolution disks.

Pre-recorded optical discs or ROMs, according to the acronym Read Orily Memory, comprise a structured substrate covered with a reflective layer, the structure of the substrate comprising a series of marks large enough to be detected by an optical reading head. For some discs, called super-resolution discs, the substrate structure includes marks theoretically too small to be detected by an optical pickup. Also, to make possible the detection of these small marks, the substrate is covered with a stack of thin layers, this stack sometimes being called active stack because at least one layer of the stack has optical nonlinear properties. By way of example, a super-resolution disk comprises small marks with a length of 80 nm and an active stack comprising a layer of InSb semiconductor material encapsulated between two layers of ZnS-SiO2 dielectric material. The passage of a high power laser on the active stack makes it possible to detect the small marks of the structured substrate. However, a problem is raised when marks of a disc are read a large number of times because the reading of the high power disc produces a rise in the temperature of the active stack under the laser spot. The repetition of this heating at each reading cycle is likely to cause irreversible structural changes in the materials constituting the disc, and may ultimately result in a degradation of the reading of the signal obtained. After a given number of read cycles, several degradation can occur, in particular a diffusion of material between the layers of the active stack, a gradual detachment of the thin layers constituting the stack, a modification of the crystalline microstructure of the material with no properties. linear lines, or deformation of the marks of the structured substrate. Thus, unlike ROMs that do not have super-resolution read capabilities, which can be read almost indefinitely, the super-resolution ROMs accept a limited number of read cycles. This phenomenon is particularly critical in the case of super-resolution disks.

An object of the invention is to improve the reading stability of a super-resolution optical disk, the quality of the signal delivered during its reading, and overall the number of possible read cycles during the lifetime of the disk. . For this purpose, the subject of the invention is a super-resolution optical reading disk comprising at least one surface protection layer, a substrate on which is deposited a stack comprising at least one active layer with reversible non-linear properties. the active layer being interposed between at least a first dielectric layer deposited under the protective layer and a second dielectric layer deposited on the substrate, the disc being characterized in that at least one interface layer is interposed between at least one the two dielectric layers and the active layer, the interface layer consisting of a material selected from the following materials: ZrO2; HfO2; TiO2; Cr2O3 Si3N4.

The presence of this interface layer between the active layer and the dielectric layer makes it possible in particular to limit the degradation of the signal when increasing the number of read cycles performed. Preferably, the material chosen for the interface layer is ZrO2. According to one embodiment, said interface layer is deposited between the first dielectric layer and the active layer. According to another embodiment, it is deposited between the second dielectric layer and the active layer. According to a third embodiment, a first interface layer is deposited between the first dielectric layer and the active layer, and a second interface layer is deposited between the second dielectric layer and the active layer. According to one embodiment, each dielectric layer has a thickness substantially equal to 50 nm, the active layer has a thickness substantially equal to 20 nm, and the interface layer (s) has (have) a thickness of between 3 nm and 20 nm.

According to one embodiment, the sum of the thicknesses of interface layers and dielectric layers above the active layer is substantially equal to the sum of the thicknesses of dielectric layer interface layers below the active layer. According to one embodiment, the dielectric layers comprise ZnS-SiO2.

Other characteristics will become apparent on reading the detailed description given by way of non-limiting example, which follows, with reference to appended drawings which represent: FIG. 1, a first embodiment seen in section of a disk structure according to the invention, Figure 2, a second embodiment seen in section of a disk structure according to the invention.

FIG. 1 shows a first embodiment of a structure used in an optical disk according to the invention.

The disc comprises a substrate 110 covered with a stack 120 of thin layers, a transparent protective layer 102 being deposited on the stack. By way of illustration, numerical values are given for the disk of FIG. 1. The disk is adapted to be read by an optical head with a numerical aperture equal to 0.85, whose resolution limit is 120 nm, the head producing a beam laser whose wavelength is equal to 405 nm. The information carried by the disk is formed by the structured substrate, the latter comprising coded sequences in the form of spaced marks, the marks having lengths between 2T and 9T, T being an equal elementary dimension, in the example , at 40 nm, against 160 nm for the classic Blu-Ray format. The small marks, whose length is equal to 2T or 3T, therefore have a length equal to 80 nm and 120 nm, respectively, less than or equal to the resolution limit of the optical head. Therefore, the super-resolution effect is necessary for the detection of these small marks.

The substrate 110 is formed, for example, of a polymeric material such as polycarbonate, or glass which bears hollow physical marks and bumps representing the stored information. The protective layer 102 is formed, in the example, of a polycarbonate, preferably of a thickness of the order of 100 micrometers.

The thin film stack 120 comprises a first dielectric layer 104a, an interface layer 106, an active layer 108, and a second dielectric layer 104b, in order, on the side of the protective layer 102 towards the substrate side 110. The stack 120 makes it possible to obtain the desired super-resolution effect by being modified under the effect of a high power laser. More precisely, it is the active layer 108 that makes it possible to obtain this super-resolution effect because it exhibits optical non-linear properties at the working wavelength. The active layer 108 of the example consists of the compound InSb; this layer 108 may also comprise one or more other doped or non-doped semiconductor materials, such as GaSb or ZnO compounds. According to another embodiment, the active layer 108 comprises phase-change materials such as the AgInSbTe compound or the GeSbTe compound. In the example, the active layer 108 has a thickness of 20 nm. As for the dielectric layers 104a, 104b flanking the active layer 108, they make it possible in particular to adjust the optical properties of the disk, but also to thermally isolate the active layer 108. In the example, the dielectric layers 104a, 104b are formed from the compound ZnS-SiO2. Other materials are conceivable for constituting these dielectric layers 104a, 104b, these materials being preferably dielectric materials such as compounds Si3N4, AlN, HfO2, TiO2 or SiO2. In the example, the first dielectric layer 104a has a thickness equal to 35 nm and the second dielectric layer 104b has a thickness substantially equal to 50 nm. Unlike conventional super-resolution disks, the disk 35 according to the invention comprises an interface layer 106 interposed preferably between the first dielectric layer 104a and the active layer 108, but the interface layer could also, although this is less favorable, be interposed between the second dielectric layer 104b and the active layer 108. It is the presence of this interface layer 106 which allows, on the one hand, to increase the quality of the read signal and, d on the other hand, to improve the reading stability of the disc and thus the number of possible playback cycles during the lifetime of the disc. The interface layer 106 has a low absorption, so as not to disturb the optical process for reading the super-resolution disk. In the example, this layer is in ZrO2. According to another embodiment, the interface layer 106 comprises one or more dielectric materials chosen from the oxides HfO2, TiO2, Cr203, or Si3N4. The thickness of the interface layer 106 is between 3 nm and 50 nm, preferably between 5 nm and 20 nm. In the example, the thickness of this interface layer 106 is equal to 15 nm, the thickness of the first dielectric layer 104a being adjusted according to the thickness of the interface layer 106, so that the total thickness of dielectric materials on each side of the active layer 108 is equal to 50 nm. According to another embodiment, a plurality of interface layers 20 are arranged between the first dielectric layer 104a and the active layer 108. For a disk made according to this configuration and read at a speed of 2.65 m / s, the BER BER bit error rate of the read signal obtained is equal to 4.103. By setting the maximum error rate TEBmax acceptable at 6.10-3, the reading stability of the disk is here understood as the number of read cycles allowed before the error rate TEB of the signal exceeds TEBmax. However, with a conventional super-resolution disk, ie with an active stack without an interface layer 106, the reading stability of the disk is of the order of 3000 read cycles. Stability increases with an interface layer located between the second dielectric layer and the active layer. It increases again and reaches more than 20000 cycles if the interface layer is located between the laser and the active layer and therefore between the first dielectric layer and the active layer. In addition, the quality of the read signal is improved during the first cycles. Furthermore, as illustrated in FIG. 2, showing a second embodiment seen in section of a disk structure according to the invention, the active layer 108 may be framed by two interface layers 106a, 106b placed respectively at above the active layer 108 and below it. Thus, with respect to the first embodiment shown in FIG. 1, a second interface layer 106b, placed on the side of the substrate 110 has been added. This second interface layer 106b makes it possible to improve the gain on the amplitude of the read signal and to further improve the number of possible readings.

The reading of the super-resolution discs generally involves the processing of signals of very low amplitude, signals that are sometimes difficult to exploit with conventional electronic circuits. An advantage of the disk structure according to the invention is that the quality of the read signal is improved, including during the first read cycles, which facilitates the processing of the signals. 20

Claims (7)

REVENDICATIONS1. Super-resolution optical storage disc comprising at least one protective layer (102) on the surface, a substrate (110) on which is deposited an elongation (120) comprising at least one active layer (108) with non-linear properties reversible, the active layer (108) being interposed between at least a first dielectric layer (104a) deposited under the protective layer and a second dielectric layer deposited on the substrate (110), the disc being characterized in that at least one interface layer (106) is interposed between at least one of the two dielectric layers (104a, 104b) and the active layer (108), the interface layer (106) being made of a material selected from the following materials: ZrO2; HfO2; TiO2; Cr2O3; Si3N4. An optical storage disk according to claim 1, characterized in that said interface layer (106a) is deposited between the first dielectric layer (104a) and the active layer (108). 3. Storage disk according to claim 1, characterized in that said interface layer (106b) is deposited between the second dielectric layer (104b) and the active layer (108). Storage disk according to claim 1, characterized in that a first interface layer (106a) is deposited between the first dielectric layer (104a) and the active layer, and a second interface layer (106b). is deposited between the second dielectric layer (104b) and the active layer. 5. Optical storage disk according to one of claims 1 to 4, characterized in that each dielectric layer (104a, 104b) has a thickness substantially equal to 50 nm, the active layer (108) has a thickness substantially equal to 20 nm, and the interface layer (106a, 106b) has a thickness of between 3 nm and 50 nm, preferably between 5 and 20 nm. An optical storage disk according to any one of the preceding claims, characterized in that the sum of the thicknesses of interface layers (106) and dielectric layers (104a) above the active layer (108) is substantially equal to the sum of the thicknesses of interface layers (106) and dielectric layers (104b) below the active layer (108). An optical storage disk according to any one of the preceding claims, characterized in that the dielectric layers (104a, 104b) comprise ZnS-SiO2.