Optical memory element and manufacturing method thereof

Abstract: A method for manufacturing an optical memory element comprising the steps of providing a substrate (1), disposing a resist film (6) on the substrate, providing a photo-mask (7) carrying an address pattern on the resist film, wherein either the substrate or the photo-mask is shaped so as to accommodate a peripheral rise (8) in the photo-resist film, thereby improving alignment between the substrate (1) and the photo-mask (7).

Description: Background of the Invention The present invention relates to an optical memory element capable for at least one of the recording, reproducing 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 into 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, strip-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 disc 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 centre hole as precisely as possible (with eccentricity allowance preferably within 20 µm), 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 track 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. 8, No. 219 (P-306)(1656) 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. European patent application EP-A2-0,155,000 by the present applicant 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 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 a first aspect of the present invention there is provided a method for manufacturing a disc-shaped optical memory element having guide tracks and address tracks for storing information, which method comprises the steps of: applying a photo-resist film onto a disc-shaped substrate; placing a disc-shaped photo-mask plate having a circular guide track mask pattern on said photo-resist film, wherein said mask plate has a diameter larger than said substrate; transcribing said mask pattern from said mask plate onto said substrate so as to form guide tracks and address tracks thereon; and disposing a recording medium on said substrate so as to form said optical memory element; characterised in that: said mask plate includes a peripheral annular mask portion which extends beyond the periphery of said photo-resist film and is shaped so as to receive any risen portion of improper thickness of said photo-resist film; said mask plate includes at least one circular marker (C) formed simultaneously with said guide track mask pattern and concentric with said pattern; and by the step of: aligning said at least one circular marker (C) with a centre hole of said disc-shaped substrate. A disc-shaped mask plate for use in the above method is defined in appended claim 11. According to a second aspect of the present invention there is provided a method for manufacturing a disc-shaped optical memory element having guide tracks and address tracks for storing information, which method comprises the steps of: applying a photo-resist film onto a disc-shaped substrate; placing a disc-shaped photo-mask plate having a circular guide track mask pattern on said photo-resist film; transcribing said mask pattern from said mask plate onto said substrate so as to form guide tracks and address tracks thereon; and disposing a recording medium on said substrate so as to form said optical memory element; characterised in that: said substrate has a peripheral portion which is removed so as to receive any risen portion of improper thickness of said photo-resist film; said mask plate includes at least one circular marker (C) formed simultaneously with said guide mask pattern and concentric with said pattern; and by the step of: aligning said at least one circular marker (C) with a centre hole of said disc-shaped substrate. According to a third aspect of the present invention there is provided a substrate used to form a disc-shaped optical memory element, said memory element having guide tracks and address tracks for storing information, said substrate comprising a disc-shaped plate having a first substrate portion on which, in the manufacture of said memory element, grooves are formed for said guide tracks and address tracks, and a second substrate portion on which said grooves are not formed, characterised in that, in order to accommodate any rise in the peripheral portion of a resist film formed on the plate in the manufacture of said memory element, said second substrate portion has an outer chamfer surface, which intersects the edge of said plate, and an inner surface which tapers from the surface plane of said plate to said outer chamfer surface. Other features of the present invention will become apparent from the detailed description given hereafter, with reference to the accompanying drawings. It should be understood, however, that the detailed description of specific examples, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. Brief Description of the Drawings 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 an 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 section 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 views showing other embodiments of an optical memory element photo-mask of the present invention; Fig. 11 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 100 nm ∼ 500 nm. Step II A mask plate 7 on which guide track 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 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 ∼ 20 µm and with such diameters that the glass disc centre hole is positioned between the two markers. Assuming the glass disc centre hole is about 15 mm 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 ± 0.5 mm, 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 centre 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 feature of the mask 7 used in the manufacturing method of the present embodiment is described now. Fig. 8 is a partially enlarged sectional view 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 photo-mask 7. For a glass disc of about 130 ± 0.1 mm in diameter, for instance, the groove desirably has the width "Y" (about 4 mm for example) with the inside diameter of a circle defined by the groove being "X" (127 mm for example) as shown in Fig. 9. The depth "Z" of the groove should be about 0.2 ∼ 0.5 mm. 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.

Claims: A method for manufacturing a disc-shaped optical memory element having guide tracks (3) and address tracks (2) for storing information, which method comprises the steps of: applying a photo-resist film (6) onto a disc-shaped substrate (1); placing a disc-shaped photo-mask plate (7) having a circular guide track mask pattern on said photo-resist film, wherein said mask plate has a diameter larger than said substrate; transcribing said mask pattern from said mask plate onto said substrate so as to form guide tracks and address tracks (8') thereon; and disposing a recording medium on said substrate so as to form said optical memory element; characterised in that: said mask plate (7) includes a peripheral annular mask portion which extends beyond the periphery of said photo-resist film and is shaped (9) so as to receive any risen portion (8) of improper thickness of said photo-resist film (6); said mask plate (7) includes at least one circular marker (C) formed simultaneously with said guide track mask pattern and concentric with said pattern; and by the step of: aligning said at least one circular marker (C) with a centre hole of said disc-shaped substrate (1). The method of claim 1, wherein said peripheral mask portion includes a groove (9) which receives said any risen portion of improper thickness of said photo-resist film. The method of claim 2, wherein said groove (9) is square-shaped, tapered, or curved. The method of claim 2 or claim 3, wherein said groove is formed with a depth (Z) of from about 0.2 to 0.5 mm. The method of claim 1, wherein said peripheral mask portion (10) is thinner than the central portion of said photo mask plate (7) so as to receive any risen portion (8) of improper thickness of said photo-resist film (6). A method for manufacturing a disc-shaped optical memory element having guide tracks (3) and address tracks (2) for storing information, which method comprises the steps of: applying a photo-resist film (6) onto a disc-shaped substrate (1); placing a disc-shaped photo-mask plate (7) having a circular guide track mask pattern on said photo-resist film; transcribing said mask pattern from said mask plate onto said substrate so as to form guide tracks and address tracks (8') thereon; and disposing a recording medium on said substrate so as to form said optical memory element; characterised in that: said substrate (1) has a peripheral portion (17) which is removed so as to receive any risen portion (8) of improper thickness of said photo-resist film (6); said mask plate (7) includes at least one circular marker (C) formed simultaneously with said guide mask pattern and concentric with said pattern; and by the step of: aligning said at least one circular marker (C) with a centre hole of said disc-shaped substrate (1). The method of claim 6, wherein said peripheral portion (17) is removed to form an outer inclined substrate edge. The method of claim 7, wherein said outer edge of said substrate comprises a chamfered edge. The method of any one of claims 1 to 8, wherein said mask plate (7) includes two small circular markers (C) having diameters respectively smaller and greater than the diameter of said centre hole, said step of aligning comprising positioning said centre hole between the two markers. The method of any one of claims 1 to 9, wherein said mask plate (7) includes a plurality of circular markers (C) of different diameter so as to accommodate tolerance in the diameter of said centre hole. A disc-shaped mask plate (7) for use in the manufacture of a disc-shaped optical memory element, said plate comprising: a circular mask pattern formed thereon, said mask pattern comprising guide track and address track patterns; characterised in that : said mask plate includes at least one circular marker (C) formed simultaneously with said mask pattern and concentric with said pattern, for positioning said mask plate in proper alignment with a centre-hole of a disc-shaped substrate (1) of said optical memory element; and said mask plate includes a circular peripheral removed section (9) which corresponds in position to a peripheral portion of said substrate (1) for accommodating any rise (8) in the peripheral portion of a photo-resist film (6) applied to said substrate in the manufacture of the optical memory element. The mask plate as defined in claim 11, wherein said peripheral removed section is a groove (9) which is square-shaped. The mask plate as defined in claim 11, wherein said peripheral removed section is a groove (9) which is triangular-shaped. The mask plate as defined in claim 11, wherein said peripheral removed section is a groove (9) which is semi-circular-shaped. The mask plate as defined in claim 11, wherein said peripheral removed section extends from a central portion to the peripheral edge of said mask plate. The mask plate as defined in any one of claims 11 to 15, including two said circular markers (C) having diameters respectively smaller and greater than the diameter of said centre hole of the substrate. The mask plate as defined in any one of claims 11 to 16, including a plurality of circular markers (C) of different diameter so as to accommodate tolerance in the diameter of said centre hole. A substrate used to form a disc-shaped optical memory element, said memory element having guide tracks (3) and address tracks (2) for storing information, said substrate comprising a disc-shaped plate (1) having a first substrate portion (16) on which, in the manufacture of said memory element, grooves are formed for said guide tracks and address tracks, and a second substrate portion (17) on which said grooves are not formed, characterised in that, in order to accommodate any rise (8) in the peripheral portion of a resist film (6) formed on the plate in the manufacture of said memory element, said second substrate portion has an outer chamfer surface (15), which intersects the edge of said plate, and an inner surface (19) which tapers from the surface plane (18) of said plate to said outer chamfer surface (15). The substrate as defined in claim 18, wherein said inner surface (19) provides a curved taper.

Inventors: AKIRA TAKAHASHI, JUNJI HIROKANE, KENJI OHTA, SHOHICHI KATOH, TETSUYA INUI, TOSHIHISA DEGUCHI DAIYA-HEIGHT, AKIRA, TAKAHASHI, JUNJI, HIROKANE, KENJI, OHTA, SHOHICHI, KATOH, TETSUYA, INUI, TOSHIHISA, DEGUCHI, DAIYA-HEIGHTS
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G11B7
IPC codes: G11B7