AN OPTICAL MEMORY ELEMENT

Abstract: An optical memory element comprising a substrate with grooves functioning as grooves for, for example, controlling the position of a laser beam, has the substrate formed from a glass containing SiO₂ in the range of 51 to 71 % by weight, Na₂O in the range of 10 to 18 % by weight, K₂O in the range of 0 to 8 % by weight, Al₂O₃ in the range of 12 to 22 % by weight, and B₂O₃ in the range of 0 to 9% by weight.

Description: This invention relates to an optical memory element which optically records information therein. In recent years, an optical memory system has been developed, which optically stores information in high density and in a mass storage order. The optical memory system ensures high density and mass storage because the information recording unit (bit) is determined by the diameter of the beam which is used in the recording operation and the reading operation. Generally, the bit size is selected to be about 1 µm. However, in order to have the optical memory system ensure high density and mass storage, the optical beam position must be accurately controlled so that the information is recorded at a predetermined position and the information is read out from a preselected position. With the optical memory system of a read only type, in general, address information can be recorded on the optical memory element when data information is recorded on the optical memory element. Accordingly, the optical beam position is controllable while the data information is read out from the optical memory element. However, in the optical memory system of an add-on-memory type or an erasable memory type, it is difficult to record the address information on the optical memory element while the data information is recorded on the optical memory element. Therefore, in the add-on-optical memory or the erasable optical memory, guide signals or guide addresses are normally recorded on the optical memory element, which are used to control the optical beam position. In the accompanying drawing, Figure 3 schematically shows the conventional optical memory element of an add-on-memory type (e.g., a DRAW type disc using TeOx, etc., as a recording material) or an erasable memory type (e.g., an optically magnetic disc using an alloy including rare earth elements and transition metals as a recording material), in which striped grooves are formed on the substrate of the optical memory element. The information recording operation and the information playback operation are conducted along the striped grooves. Some methods, typically three kinds of methods, for the formation of the striped grooves on the substrate of an optical memory element have been proposed as follows: (1) A method for the formation of the above-mentioned grooves on the substrate, in which a mold having striped grooves is employed to transcribe the striped grooves onto a resin substrate made of acrylic resin or polycarbonate resin by an injection molding technique. (2) A method for the formation of the above-mentioned grooves on the substrate, in which a mold having striped grooves is employed to transcribe the striped grooves onto a resin substrate made of acrylic resin or polycarbonate resin by a casting technique. (3) A method for the formation of the above-mentioned striped grooves on the substrate, in which UV-setting resin is interposed between the stamper having striped grooves therein and the substrate made of acrylic resin, epoxy resin, glass, etc., and the interposed resin is then exposed to UV rays through the substrate so as to be cured, following by removing the stamper. This method is known as "the 2P method". Since all of the three methods use a resin layer, there is a possibility that oxygen or moisture will reach the recording material through the resin layer, which will cause deterioration of the recording material disposed on the substrate. That is, the conventional method does not ensure stable operation of the optical memory element. Especially, in optically magnetic memory elements in which a rare earth metal-transition metal alloy film made of GdTbFe, TbFeCo or the like is used as a recording material, it is undesirable to employ the above-mentioned resin layer for the substrate since the rare earth metals such as Gd, Tb, etc., are extremely active elements. In order to eliminate the above-mentioned problems, there has previously been proposed in our US Patent No. 4,544,443, a method for the formation of striped grooves on a glass substrate in which a photoresist material is coated on the glass substrate to form a photo-resist film; UV rays are applied to the photo-resist film through a photo-mask to form a guide groove pattern latent image on the photo-resist film; the said guide groove pattern latent image is developed; and then an etching operation is conducted through the said developed guide groove pattern so as to form the guide grooves in the glass substrate. However, we have found that the smoothness of the surface of the striped grooves on the glass substrate is significantly damaged by the etching operation, which results in an increase in the noise level of signals reproduced from the grooves. EP-A- 0 130 399 discloses different glass compositions, including aluminosilicate glass, suitable for optical disc supports. It is an object of the present invention to provide a novel glass substrate for an optical memory element which will allow the formation thereon by an etching operation of striped grooves which will have a sufficiently smooth surface to reduce the noise level of signals reproduced from the grooves. Thus in accordance with the invention, a substrate for an optical memory element on which substrate striped grooves for the recordal and reproduction of information are to be formed by an etching operation is formed from a glass composition containing Si0₂ in the range of from 51 to 71% by weight, Na₂0 in the range of from 10 to 18% by weight, K₂0 in the range of from 0 to 8% by weight, A1₂0₃ in the range of from 12 to 22% by weight, and B₂0₃ in the range of from 0 to 9% by weight. In a preferred embodiment, the grooves are formed by disposing a photo-resist film on said glass substrate; disposing a masking board, on which groove information is patterned, on said photo-resist film; applying light to said masking board to form a guide groove pattern latent image on said photo-resist film; developing said guide groove pattern latent image formed on said photo-resist film to form the grooves in the photo-resist film; subjecting said glass substrate covered with said photo-resist film containing the grooves to an etching treatment to form the grooves in said glass substrate; and removing said photo-resist film from said glass substrate. In preferred embodiment, the etching treatment is conducted by either a dry etching technique or a wet etching technique. Thus, the invention described herein makes possible the object of providing an optical memory element comprising a novel substrate made of a particular glass, the surface smoothness of which is maintained at the same level as prior to an etching treatment, even though the glass substrate has been subjected to the etching treatment so as to form striped grooves therein, so that noise arising when the information is read out from the optical memory element can be reduced. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1(a) to 1(e) are schematic diagrams showing a process for the production of an optical memory element of this invention. Figure 2(a) shows a graph illustrating the relationship between the glass substrates and the amount of noise arising when information is read out from the optical memory element using each of the said glass substrates. Figure 2(b) is a front view showing a part of the glass substrate, wherein B, C, and D are the positions to each of which a laser beam is applied. Figure 3 is a perspective view showing a part of the glass substrate of the optical memory element of this invention. Figures 1(a) to 1(e) show schematic diagrams illustrating a process for the production of the substrate of an optical memory element of this invention. Step(1): A glass substrate having the surface polished with high accuracy is washed as a glass substrate of the optical memory element, and then, as shown in Figure 1(a) , a photo-resist material is coated on the glass substrate 1 by a spinner to form a photo-resist film 2 . The glass substrate 1 is composed of a glass containing SiO₂ in the range of 51 to 71 % by weight, Na₂0 in the range of 10 to 18 % by weight, K₂0 in the range of 0 to 8 % by weight, Al₂O₃ in the range of 12 to 22 % by weight, and B₂O₃ in the range of 0 to 9 % by weight. Step(2): As shown in Figure 1(b) , a masking board 3 , on which guide groove information is patterned, is disposed on the glass substrate 1 covered by the photo-resist film 2 , and the UV rays A are applied to the masking board 3 to form the guide groove pattern latent image on the photo-resist film 2 . Step(3): The photo-resist film 2 , which has a guide groove pattern latent image thereon corresponding to the guide groove pattern of the masking board 3 , is subjected to a developing treatment to form the guide grooves in the photo-resist film 2 as shown in Figure 1(c) . Step(4): The glass substrate 1 coated with the photo-resist film 2 having the guide grooves is subjected to a dry etching treatment such as a sputtering treatment (a reactive etching treatment) or the like within an etching gas such as CF₄, CHF₃ or the like, resulting in striped grooves 4 in the glass substrate 1 as shown in Figure 1(d) . Step(5): The photo-resist film 2 is removed from the glass substrate 1 by use of a solvent such as acetone, etc., or by means of the sputtering method, etc., in an O₂ atmosphere. Thus, the striped grooves 4 which function as guide grooves or the like of a laser beam are formed in the glass substrate 1 as shown in Figure 1(e) . Figure 2(a) shows the relationship between the glass substrates and the amount of noise, wherein "GI" is the characteristics of the novel glass, which is used for the substrate of this invention, with a composition containing SiO₂ of 61 % by weight, Na₂O of 13 % by weight, K₂O of 3 % by weight, Al₂O₃ of 17 % by weight, B₂O₃ of 4 % by weight, and the others of 2 % by weight, and "G2" is the characteristics of a glass (i.e., a soda-lime glass), which is used for a conventional substrate, with a composition containing SiO₂ of 72 % by weight, Na₂O of 15% by weight, Al₂O₃ of 2% by weight, CaO of 7 % by weight, and MgO of 4 % by weight. The amount of noise shown in Figure 2(a) is determined as follows: As shown in Figure 2(b) , noise arising at the position D of the glass substrate, at which no groove is formed, is regenerated by an optical head, and the frequency of the noise is analyzed by a spectrum analyzer to obtain the amount of noise, S₀. In the same manner as the mentioned above, the amount of noise, S₁, at the position B of the glass substrate located between the grooves and the amount of noise, S₂, at the position C located in the groove of the glass substrate are obtained. Then, the difference N₁ in the amount of noise between the positions B and D and the difference N₂ in the amount of noise between the positions C and D are calculated using the values S₀, S₁ and S₂. The values S₀, N₁ and N₂ are plotted as shown in Figure 2(a) , which indicates that the amount of noise at the position C in the groove of the conventional glass substrate made of the ordinary G2 type glass is greater by - 70 dBm than that at the position D of the same glass substrate made of the ordinary G2 type glass, whereas the amount of noise at the position C in the groove of the glass substrate made of the novel G1 type glass of this invention is greater by - 80 dBm than that at the position D of the same glass substrate made of the novel G1 type glass of this invention. The amount of noise at each of the positions D and B of the glass substrate made of the novel G1 type glass of this invention is nearly equal to that at each of the positions D and B of the conventional glass substrate made of the ordinary G2 type glass, and accordingly it can be seen that the polished state of the G1 type glass of this invention prior to the etching treatment is the same as that of the conventional G2 type glass prior to the etching treatment. The difference in the amount of noise between the G1 type glass and the G2 type glass caused by the formation of grooves in the glass substrates by an etching technique, is due to the difference in the composition between the G1 type glass and the G2 type glass. That is, the glass substrate made of the G1 type glass of this invention contains a relatively large amount of Al₂O₃ so that even though it is subjected to an etching treatment, the surface smoothness of the glass substrate can be maintained at the same level as before etching. On the contrary, the conventional glass substrate made of the G2 type glass, which is used for a conventional optical memory element, contains a small amount of Al₂O₃, so that once it is subjected to an etching treatment, the surface smoothness of the glass substrate is significantly damaged, which causes an increase in noise. Although the above-mentioned example disclosed the formation of grooves by a dry etching technique such as a reactive ion etching technique, a wet etching technique can be, of course, applied to the glass of this invention to thereby produce a high quality glass substrate which is useful for an optical memory element.

Claims: The use, for the production of a substrate for an optical memory element on which substrate striped grooves for the recordal and reproduction of information are to be formed by an etching operation, of a glass composition containing from 51 to 71% by weight of SiO₂, from 10 to 18% by weight of Na₂O, from 0 to 8% by weight of K₂O, from 12 to 22% by weight of Al₂O₃, and from 0 to 9% by weight of B₂O₃, in order to reduce the noise level of signals reproduced from the grooves. A substrate for an optical memory element having formed thereon striped grooves functioning as grooves from which signals can be read, wherein said substrate is composed of a glass composition containing SiO₂ in the range of 51 to 71% by weight, Na₂O in the range of 10 to 18% by weight, K₂O in the range of 0 to 8% by weight, Al₂O₃ in the range of 12 to 22% by weight, and B₂O₃ in the range of 0 to 9% by weight. A method of producing an optical memory element comprising a substrate composed of a glass composition as defined in Claim 2, wherein striped grooves are formed on the substrate by disposing a photo-resist film on the glass substrate; disposing a masking board, on which groove information is Patterned, on said photo-resist film; applying light to said masking board to form a guide groove pattern latent image on said photo-resist film; developing said guide groove pattern latent image formed on said photo-resist film to form the grooves in the photo-resist film; subjecting said glass substrate covered with said photo-resist film containing the grooves to an etching treatment to form the grooves in said glass substrate; and removing said photo-resist film from said glass substrate. A method according to Claim 3, wherein said etching treatment is conducted by either a dry etching technique or a wet etching technique.

Inventors: HIROKANE, JUNJI, INUI, TETSUYA, KATAYAMA, HIROYUKI, MURAKAMI, YOSHITERU, OHTA, KENJI, TAKAHASHI, AKIRA
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1986
Main classification: G11B7
IPC codes: G11B11, C03C3, G11B7, C03C15, C03C4