MANUFACTURING METHOD FOR AN OPTICAL MEMORY ELEMENT

Abstract: A method for manufacturing an optical memory element comprising the steps of providing a substrate, disposing a resist film on the substrate, providing a photo-mask carrying a guide groove pattern on the resist film so as to position the photo-mask over a predetermined position of the substrate, forming guide groove pattern latent image on the resist film, developing the guide groove pattern latent image formed on the resist film, conducting an etching operation through the developed guide groove pattern sa as to form guide grooves in the substrate, removing the resit film from the substrate, and disposing a recording medium layer on the substrate hav­ ing the guide grooves formed therein.

Description: Background of the Invention The present invention relates to an optical memory element capable for at least one of the recording, reroducing and erasing operations by means of optical beam irradiation, or more specifically to an optical memory element substrate and a photo-mask for use in transcribing guide track and track address pattern onto the optical memory element substrate. Of late, demand for optical memory elements is increasing year by year due to their high density and large capacity. Optical memory elements can be classified nto the three types in terms of their use: read-only memory, add-on memory and erasable memory. The optical memory elements ensure high density and large capacity because a bit (information recording unit), which is determined solely by the optical beam diameter, can be made as small as about 1µm in size. This fact, however, causes a number of restrictions on an optical memory system; optical beam positioning must be extremely accurate in order to record information precisely in a predetermined point or reproduce proper information recorded in a preselected point. In the case of read-only type optical memory, in general, address data can be recorded together with data information so that it is possible to properly position the optical beam while recorded data information is being reproduced. For add-on memory or erasable memory type, on the other hand, it is difficult to record address data together with data information in the memory. Therefore, in the add-on memory or the erasable memory, guide signals or guide addresses are normally recorded on the memory substrate. For instance, an optical memory element to be used as add-on or erasable memory normally has guide tracks in the substrate, to guide information-recording or -reproducing beam spot to a specified position on the optical memory element. In many cases, track address-indicating data is written in a part of each guide track to locate the guide track. Fig. 1 shows the essential part of the memory substrate of the add-on or erasable memory in perspective view. As shown, stripe-shaped grooves are formed in the substrate, and information is recorded or reproduced along the grooves. Though not shown, the grooves are intermittent in the circumferential direction so as to provide address bit information of each groove. In an optical memory element of disc shape as shown in Fig. 2, in particular, guide tracks 3 and track address portion 2 (together with sector address if the tracks are divided into many sectors) are formed concentrically or spirally in the substrate 1. For the purpose of simplicity, only one track 3 and only one address portion 2 are shown in Fig. 2. A number of methods have been practised for manufacturing a disc substrate provided with tracks and addresses as described above. A mask which carries guide tracks and addresses thereon may be airtightly placed on a resist film applied on a disc glass, to form the tracks and addresses in form of grooves or pits directly on the disk glass. In this method, it is essential to form the guide tracks in such a manner that the guide track centre coincides with the centre hole of the disc glass as precisely as possible. With poor concentricity between the guide tracks and the disc glass, when the disc glass is rotated for recording, reproduction or erasure with the centre hole fixed on a rotary shaft 4, as shown in Fig. 3, the guide tracks vibrate significantly with respect to a recording, reproducing or erasing laser beam 5, hampering track servo operation for controlling the lens position. For the above reason, it is necessary to join the guide track centre and the disc glass centre hole as precisely as possible (with eccentricity allowance preferably within 20µm), in the optical exposure for guide tracks. The above manufacturing method can cause another problem in the guide track and track address pattern transcription process. The resist film 6, when applied, tends to have a partial rise 8 at the peripheral portion of the disc glass as shown in Fig. 4. This rise 8 often impairs close contact between the resist film 6 and the photo-mask 7, although it is desirable that they are made in close contact with each other. The width of this deficient close contact is usually 1 ∼ 2mm. In Fig. 4, the guide track and address pattern on the photo mask 7 is omitted. The deficiency in close contact prevents the guide tracks and track addresses from being formed at proper positions. Various solutions to the problem of impaired close contact between the resist film and the photo-mask have been proposed. In Patent Abstracts of Japan, Vol. 7, No. 207 (M-242)(1352) and U.S. 3,936,301 (Schneider) a groove is provided in the photo-mask to accommodate the peripheral rise in the resist film. In U.S. Patent 3,824,014 (Abita) the problem is solved by providing the central portion of the mask with a raised mound. In U.S. Patent 4,111,698 (Sato) the periphery of the support for the mask is removed so that the surface of the coated layer of the periphery is not raised as compared to the surface of the coated layer at the centre. Co-pending European patent application EP-A2-0155000 by the present applicants discloses a method for manufacturing an optical memory element, comprising the steps of: providing a disc-shaped substrate; disposing a resist film on said substrate; providing a photo-mask carrying a guide groove pattern on the resist film; forming a guide groove pattern latent image on said resist film; developing said guide groove pattern latent image formed on said resist film; conducting an etching operation through said developed guide groove pattern so as to form guide grooves in said substrate; removing said resist film from said substrate; and disposing a recording medium layer on said substrate having said guide grooves formed therein. It would be desirable: to provide an optical memory element in which guide track and track addresses are accurately formed on the substrate of the optical memory element; to provide a method for manufacturing an optical memory element, which method ensures accurate formation of guide tracks and track addresses; to provide a mask which facilitates coincidence between the guide track centre and the disc centre hole when transcribing guide tracks and addresses on a resist film applied on the disc; to provide a photo-mask which prevents deficient close contact between a resist film and the photo-mask from being caused by a rise of the resist film on the peripheral portion of the disc glass; and to provide an improvement in the configuration of a glass substrate so as to achieve guide tracks and address-indicating grooves of more accurate shape. According to the present invention as defined in appended claim 1 there is provided a method as above-defined in relation to EP-A2-0155000, wherein said substrate has a hole in the centre thereof and said photo-mask has a marker of a shape corresponding to said centre hole in the substrate and a pattern for guide tracks to be transcribed onto the optical memory element, said marker and said pattern having been formed on said photo-mask simultaneously, thereby to facilitate centring of the resulting transcribed guide tracks on said optical memory element. A cut-away portion is preferably preliminary formed in said photo-mask at a position facing a rise of said resist film. Alternatively, the peripheral end portion of said substrate is ground to be inclined so that a rise of the resist film which is formed at the peripheral edge of the substrate when applied on the substrate is lower than the plane of the substrate portion where the guide grooves are formed. Other features of the present invention will become apparent from the detailed description given hereafter. It should be understood, however, that the detailed description as specific examples, while indicating preferred embodiments of the invention are given by way of illustration only, since various changes and modifications within the scope of the invention as defined in the appended claims will become apparent to those skilled in the art from this detailed description. The peripheral portion of the substrate may be ground to be inclined so that the plane of the substrate with the grooves formed therein is higher than the rise of the resist film which can occur on the peripheral portion of the substrate when the resist film is applied on the substrate. Brief Description of the Drawings The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a perspective view of the essential part of a glass substrate of an add-on memory or erasable memory after grooves have been formed thereon; Fig. 2 is a perspective view of a typical optical memory element (optical disc); Fig. 3 is a sectional view showing a part of the optical disc system; Fig. 4 is a sectional view showing the contact between the glass substrate and the photo-mask of the conventional optical memory element; Fig. 5 is a sectional view for explaining manufacturing steps of an optical memory element of the present invention; Fig. 6 is a plan view of an embodiment of an optical disc photo-mask of the present invention; Fig. 7 is a plan view showing a part of another embodiment of an optical disc photo-mask of the present invention; Fig. 8 is a partially enlarged sectional view of another embodiment of an optical disc photo-mask of the present invention; Fig. 9 is a sectional view showing the entire photo-mask of Fig. 8; Figs. 10 (a) and 10 (b) are partially enlarged sectional veiws showing other embodiments of an optical memory element photo-mask of the present invention; Fig. 11 is also a partially enlarged sectional view showing still another embodiment of an optical memory element photo-mask of the present invention; Figs. 12 (a) and 12 (b) are a plan view and a sectional view, respectively, of the glass substrate for an optical memory element; Figs. 13 (a), 13 (b) and 13 (c) are partially enlarged sectional views of glass substrates for an optical memory element of the present invention; and Figs. 14 (a), 14 (b) and 14 (c) are partially enlarged sectional views of the optical memory element glass substrates on which resist film is applied. Detailed Description of the Invention An embodiment of an optical memory element of the present invention will be described in detail below with reference to the accompanying drawings. Fig. 5 shows manufacturing steps of the substrate of the optical memory element of the present invention. Referring to Fig. 5, manufacturing method of the optical memory element substrate is described by steps. Step I An optical memory element glass substrate which is highly reliable with respect to oxygen and moisture penetration (or which does not admit oxygen and moisture penetrations) is cleaned. A photo resist film 6 is applied on the glass substrate 1 (See Fig. 5 (a)). The thickness of the photo resist film 6 is preferably about 100nm ∼ 500nm. Step II A mask plate 7 on which guide truck and address information have been patternized is placed airtightly over the photo resist film 6 on the glass substrate 1. Then, the mask plate 7 is irradiated with ultraviolet ray A to transcribe the mask pattern of the mask plate 7 onto the photo resist film 6 (See Fig. 5 (b)). Since the optical memory element is of disc shape, it is desirable that the mask plate 7 is round. Step III The photo resist film 6 with the mask pattern thereon is developed so as to form grooves in the resist film 6 (See Fig. 5 (c)). Step IV The glass substrate 1 covered with the photo resist film 6 having grooves formed therein is subjected to wet etching operation or dry etching operation such as sputtering (reactive ion etching) in an etching gas atmosphere such as CF₄ or CHF₃. Grooves 8' are then formed in the glass substrate 1 (See Fig. 5 (d)). Step V The developed resist film 6 is removed from the glass substrate 1 by means of sputtering in the O₂ atmosphere, or dissolving in a solvent such as acetone. In this way, the glass substrate 1 with the grooves 8' formed therein is produced (See Fig. 5 (e)). Step VI After the step V, a recording medium layer is formed on the glass substrate 1 having the grooves 8 formed therein. The glass substrate 1 having grooves for guide tracks and guide address information is completed through the above process. According to this process, the mask plate 7 with the pattern for guide tracks and guide address information thereon, prepared in advance, is placed airtightly over the glass substrate 1 coated with the photo resist film 6, so as to transcribe the mask pattern of the mask plate 7. Therefore, the time required for transcribing the guide pattern can be substantially reduced. Now, the optical disc mask 7 used in the above manufacturing process is described in the following. Fig. 6 is a plan view of an embodiment of an optical disc mask of the present invention. The mask 7 has been produced by forming Cr or CrO film over, say, a quartz glass disc plate and etching the film to make the pattern shown in Fig. 6. Referring to Fig. 6, the shaded part indicates the area where Cr or CrO remains unremoved. "a" is an information area in a part of which are formed tracks 3 and addresses 2 spirally or concentrically. Cr or CrO is removed in the area "b" of the mask 7. A positioning reference marker C is provided in the area "b". The concentricity with the tracks can be secured by recording the reference marker C and the guide tracks simultaneously by using the same recording device. For example, when the marker C has virtually the same diameter as the glass disc centre hole, the marker C can be easily adjusted to the disc centre hole when placing the mask on the glass disc for pattern transcription, so that it is easy to achieve the concentricity of the tracks with the disc. The marker need not be limited to the above embodiment. As shown in Fig. 7, two markers C may be recorded at an interval of about 10 ∼ 20µm and with such diameters that the glass disc centre hole is positioned between the two markers. Assuming the glass disc centre hole is about 15mm in diameter, for instance, the two markers may have the diameters of about 14.09mm and 15.01mm, respectively. With such markers, it is easy to position the mask with respect to the glass disc centre hole. Generally, the centre hole of a glass disc has a high degree of roundness but often differs in the diameter, for instance within the range of about 15 ± 0.5mm, depending upon the manufacturing process. To provide for various centre hole diameters of the disc, a plurality of markers C with different diameters may be formed concentrically on a mask so that the same mask can be used for discs with different centre hole diameters. Alternatively, instead of using the marker C of Fig. 6, it is possible to use the area "b" as a marker if the area "b" is made narrower. In the above embodiment, Cr or CrO is used in the shaded area of Fig. 6, although clearly another metal film such as Ni, Ti or Ta may be used in place of Cr or CrO. As understood from the above, by using the mask of the present invention, it is possible to improve the concentricity of the memory disc guide track with the memory disc center hole, thus minimizing vibration of the guide track with respect to the optical beam in recording, reproducing or erasing information on the disc. Consequently, servo operation of an objective lens for condensing a laser beam becomes easy. Another embodiment of a mask 7 used in the manufacturing method of the present invention is described now. Fig. 8 is a partially enlarged sectional veiw of another embodiment of an optical memory element photo mask of the present invention. As shown, a groove 9 is formed in the photo-mask 7 at the position facing the peripheral portion of the glass disc 1, to accommodate the rise 8 of the resist film 6, thus eliminating deficient close contact between the glass disc 1 and the photomask 7. For a glass disc of about 130 ± 0.1mm in diameter, for instance, the groove desirably has the width "Y" (about 4mm for example) with the inside diameter of a circle defined by the groove being "X" (127mm for example) as shown in Fig. 9. The depth "Z" of the groove should be about 0.2 ∼ 0.5mm. The section of the groove is not necessarily square as shown in Fig. 9. It may be triangular as shown in Fig. 10 (a) or semi-circular as shown in Fig. 10 (b). In short, the groove provided in the photo mask may be of any shape as long as it can absorb the rise 8 of the resist film 6 on the periphery of the glass disc 1. In this sense, instead of forming a groove, the peripheral portion 10 of the photo-mask 7 may be made thinner than the central portion as shown in Fig. 11, to absorb the rise 8 of the resist film 6. By using the photo-mask of the present invention, the guide track and track address pattern for an optical memory element can be properly transcribed onto the resist film applied on the optical memory element substrate. Now, the shape in section of the glass disc substrate is described with reference to the drawings. Fig. 12 (a) is a plan view of the glass disc substrate 1 and Fig. 12 (b) is a sectional view of the glass disc substrate 1. The outer edges 15 of the glass disc substrate 1 are chamfered, as shown. Fig. 13 is an enlarged sectional view of the chamfered edge 15. Fig. 13 (a) is a sectional view of the chamfered edge of the conventional glass disc substrate, and Figs. 13 (b) and 13 (c) are sectional views showing the chamfered edge configuration of the glass disc substrate of the present invention. In both of Figs. 13 (b) and 13 (c), the surface portion 17 without guide grooves is ground to be lower than the glass disc substrate surface plane 18. Figs. 14 (a), 14 (b) and 14 (c) are sectional views of the glass disc substrates having the edges shown in Figs. 13 (a), 13 (b) and 13 (c), respectively, on which resist film is applied by the spin coat method. At the edge of the glass disc substrate, the resist film has a rise 8 due to the surface tension. On the glass disc substrate of the conventional shape shown in Fig. 14 (a), the rise 8 of the resist film 6 is higher than the resist film surface plane 11. When placing the mask plate 7 on the glass substrate 1 in the manufacturing step II, under this condition, the mask plate 7 will be positioned on the plane 11', resulting in defective contact over a large area between the mask plate 7 and the resist film 6. On the glass disc substrate of the shape shown in Figs. 14 (b) or 14 (c), in contrast, the rise 8 of the resist film 6 is lower than the resist film surface plane 11 so that the mask plate 7 can be positioned on the resist film surface plane 11. Consequently, sufficient contact can be obtained between the mask plate 7 and the resist film 6 over the area 16 with the guide grooves formed thereon. As appreciated from the above the guide grooves formed in the optical memory element are entirely of proper shape, thus reducing noise in reproduced signals. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the present invention as claimed.

Claims: A method for manufacturing an optical memory element, comprising the steps of: providing a disc-shaped substrate (1); disposing a resist film (6) on said substrate; providing a photo-mask (7) carrying a guide groove pattern on the resist film; forming a guide groove pattern latent image on said resist film; developing said guide groove pattern latent image formed on said resist film; conducting an etching operation through said developed guide groove pattern so as to form guide grooves in said substrate; removing said resist film from said substrate; and disposing a recording medium layer on said substrate having said guide grooves formed therein; wherein said substrate has a hole in the centre thereof and said photo-mask has a marker (C) of a shape corresponding to said centre hole in the substrate and a pattern for guide grooves (3) to be transcribed onto the optical memory element, said marker and said pattern having been formed on said photo-mask simultaneously, thereby to facilitate centring of the resulting transcribed guide grooves (3) on said optical memory element. The method of claim 1, wherein a cut-away portion (9) is preliminarily formed in said photo-mask at a position facing a rise (8) of said resist film. The method of claim 1, wherein the peripheral end portion (10) of said substrate is ground to be inclined so that a rise (8) of the resist film which is formed at the peripheral edge of the substrate when applied on the substrate is lower than the plane of the substrate portion where the guide grooves are formed.

Inventors: DEGUCHI, TOSHIHISA DAIYA-HEIGHTS, HIROKANE, JUNJI, INUI, TETSUYA, KATOH, SHOHICHI, OHTA, KENJI, TAKAHASHI, AKIRA
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1986
Main classification: G11B7
IPC codes: G11B7