Method of fabricating glass substrate for disk

Abstract: A method of fabricating a glass substrate for a disk including guide track portions and sector (or track) address portions of which grooves and pits have different depths, providing the steps of (i) forming a photoresist film over a glass substrate for an optical memory element, (ii) carrying out exposure to define a plurality of guide track portion formation areas and sector (or track) address portion formation areas interposed between the guide track portion formation areas, and carrying out development so as to cause the glass substrate in the address portion formation area to appear and to cause the photoresist film in the guide track portion formation area to remain as a first residual film, (iii) carrying out first etching to sink the glass substrate in the address portion formation area by a predetermined thickness such that a pit is formed and to remove the surface layer of the first residual film such that a second residual film is formed, (iv) carrying out first ashing to remove the second residual film so that the glass substrate in the guide track portion formation area appears, (v) carrying out second etching to form a groove in the guide track portion formation area and to further sink the pit in the address portion formation area so that a deeper pit is formed, and (vi) carrying out second ashing to remove a third residual photoresist film which remains between the deeper pit and the groove.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of fabricating a glass substrate for, or for use in the manufacture of, an optical recording medium. 2. Description of the Prior Art Recently, there has been needed an optical memory medium as a high density mass memory medium. The optical memory medium can be classified into a read-only memory, a write-once memory and a rewritable memory according to its working form. The optical memory medium to be used as the write-once memory and rewritable memory uses a substrate for a disk. The substrate for a disk preliminarily comprises on a glass substrate guide tracks for guiding optical beams for recording, reading and erasing information to predetermined positions on the optical memory element, and track addresses for identifying track numbers. The track is divided into a plurality of sectors. In the case where information should be managed, sector addresses and the like are often provided in advance. As shown in Fig. 8, the depth of a guide track portion 1b is not equal to that of a sector (or track) address portion 1a, and the address portion 1a is provided between the guide track portions 1b. The reason is as follows. In the case where the optical beams are tracked by an optical differential method, it is preferred that the depth of the guide track portion approximates λ / (8 x n) and the depth of the address portion approximates λ / (4 x n) (where λ is a wavelength of light to be used and n is a refractive index of a substrate). As shown in Figs. 9 to 13, there has been proposed a technique for forming the guide track, the sector (or track) address and the like on a glass substrate by a contact printing method and a dry etching method. Referring to the technique described above, a photomask is used for forming a glass substrate for a disk by the contact printing method and the dry etching method. The photomask has such a shape that a quantity of light radiated onto the guide track portion and sector (or track) address portion on a photoresist film is varied. This technique will be summarized with reference to Figs. 9 to 13. As shown in Fig. 9, a positive type photoresist film 2 is provided on a glass substrate 1, and prebaking is then carried out. As shown in Fig. 10, a photomask 3 having a thin film 4 for a mask is caused to contact the photoresist film 2 through the thin film 4. In this case, the thin film 4 is not provided in a formation area A1 of a sector (or track) address portion 1a. Consequently, a quantity of transmitted light is not reduced. The thin film 4 remains at a thickness of about 10 to 200 Å in a formation area B1 of a guide track portion 1b. Consequently, a quantity of transmitted light in the formation area B1 is smaller than in the formation area A1. Then, ultraviolet rays 5 are radiated to expose the photoresist film 2 provided below the photomask 3 (see Fig. 10). Thereafter, the photomasks 3 and 4 are removed. As shown in Fig. 11, the photoresist film 2 thus exposed is developed, and postbaking is then carried out. Exposure is fully carried out by strong light in a formation area A2 of the address portion 1a on the photoresist film 2. Consequently, the photoresist film 2 is completely developed so that the glass substrate 1 appears. In a formation area B2 of the guide track portion 1b on the photoresist film 2, the exposure is carried out by weaker light than in the formation area A2. Consequently, development is stopped halfway. As a result, the photoresist film 2 remains by a thickness according to a quantity of transmitted light in the formation area B1 on the thin film 4, so that the glass substrate 1 does not appear. The reference numeral 22a denotes a residual photoresist film. As shown in Fig. 12, the glass substrate 1 is subjected to dry etching using gas such as CF 4 or CHF 3 . The photoresist film 2 shown in Fig. 11 is also etched simultaneously. Consequently, when etching is started, the etching of the glass substrate 1 is advanced in the formation area A2 so that a deep pit 11a is formed on the glass substrate 1. The residual photoresist film 22a is etched in the formation area B2. When the glass substrate 1 appears, the etching of the glass substrate 1 is advanced. Consequently, there is formed a groove 11b which is shallower than the pit 11a in the formation area A2. The reference numeral 22b denotes a photoresist film which remains when the etching is completed. As shown in Fig. 13, the photoresist film 22b remaining on the glass substrate 1 is removed to form on the glass substrate 1 the address portion 1a as a deep pit 111 and the guide track portion 1b as a shallow groove 112. Thus, there is formed a glass substrate 11 for a disk on which the address portion 1a and guide track portion 1b have different depths. The depth of the groove 112 forming the guide track portion 1b is defined by the thickness of the photoresist film 22a remaining on the guide track portion 1b, and the etching speeds of the photoresist film 1 including the residual photoresist film 22a and the glass substrate 1 as shown in Fig. 11. The thickness of the photoresist film 22a is defined by a quantity of transmitted light and developing conditions in the guide track portion 1b. Accordingly, it is difficult to uniformly form pits and grooves with good reproducibility. EP-A-0 241 028 discloses a process of forming grooves of different depths in a glass substrate in the manufacture of an optical memory disc. The process corresponds to that described above with reference to Figs. 9 to 13. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of fabricating a glass substrate for, or for use in the manufacture of, an optical recording medium, wherein two types of patterns distinguishable at least by different thicknesses of the substrate can be uniformly formed. The present invention provides a method of fabricating a glass substrate, as defined by claim 1. The sub-claims 2 to 10 are directed to embodiments of the invention. A method of manufacturing an optical recording medium is defined by claim 11 or claim 12. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view showing a first step according to an embodiment of the present invention; Figure 2 is a view showing a second step according to the embodiment of the present invention; Figure 3 is a view showing a third step according to the embodiment of the present invention; Figure 4 is a view showing a fourth step according to the embodiment of the present invention; Figure 5 is a view showing a fifth step according to the embodiment of the present invention; Figure 6 is a view showing a sixth step according to the embodiment of the present invention; Figure 7 is a view showing a seventh step according to the embodiment of the present invention; Figure 8 is a view showing the structure of a main part of a glass substrate for a disk fabricated; Figure 9 is a view showing a first step according to the prior art; Figure 10 is a view showing a second step according to the prior art; Figure 11 is a view showing a third step according to the prior art; Figure 12 is a view showing a fourth step according to the prior art; and Figure 13 is a view showing a fifth step according to the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A glass substrate fabricated according to the method of the present invention is used as a substrate for an optical memory medium or as a substrate for a master plate for preparing a stamper to be used for injection molding. At first, a photoresist film 2 on a glass substrate 1 is exposed (see Fig. 2). Then, development is carried out so that the surface of the glass substrate 1 appears in an address portion formation area A2 and the photoresist film 2 remains as a first residual film 6 on the surface of the glass substrate 1 in a guide track portion formation area B2 as shown in Fig. 3. It is preferred that exposure is carried out by radiating ultraviolet rays 5 onto the photoresist film 2 with the use of photomasks 3 and 4, or by a laser cutting method using laser beams. The photomasks 3 and 4 can completely remove the photoresist film 2 in the address portion formation area and can stop the removal of the photoresist film 2 halfway in the guide track portion formation area (see Fig. 2) The photomasks 3 and 4 are preferably composed of a quartz substrate 3 and a metallic pattern 4 such as tantalum (Ta), chromium (Cr) or titanium (Ti) which is formed on the quartz substrate 3. Referring to the laser cutting method, Ar ion laser beams having a large quantity of light for exposing the address portion formation area and Ar ion laser beams having a small quantity of light for exposing the guide track portion formation area are radiated onto a photoresist plane, and scan the photoresist plane while rotating the glass substrate so that each formation area is exposed like spirals. The glass substrate 1 which includes the photoresist film 2 having the first residual film 6 is subjected to first etching. Consequently, the glass substrate 1 in an address portion formation area A3 is sunk by a predetermined thickness. In a guide track portion formation area B3, the first residual film 6 is removed to cause a second residual film 7 to remain (see Fig. 4). The first etching is a known method, for example, dry etching carried out at a power of 1500 W and a pressure of 12 mTorr in a plasma atmosphere using gas such as CF 4 or CHF 3 . Then, the second residual film 7 is removed by first ashing so as to cause the surface of the glass substrate 1 to appear in a guide track portion formation area B4 (see Fig. 5). The first ashing serves to remove an organic material such as a photoresist film. By the first ashing, the second residual film 7 can be removed without depending on the non-uniformity of thickness distribution of the first and second residual films 6 and 7. Consequently, the surface of the glass substrate 1 can completely appear in the guide track portion formation area B4. More specifically, the glass substrate 1 cannot be removed by the first ashing. Accordingly, pits in an address portion formation area A4 can be defined only by the first etching without depending on the selection ratio of the glass substrate to the photoresist film (for example, 1 : 1 or 1 : 2), and the thicknesses of the first and second residual films 6 and 7. The first ashing is a known method, for example, plasma etching carried out at a power of 200 W and a pressure of 10 to 50 mTorr in an oxygen plasma atmosphere. Then, second etching is carried out to sink by a predetermined thickness the glass substrate 1 which is caused to appear by the first ashing. Consequently, there is formed a shallow groove as a guide track portion 1b in the guide track portion formation area B4. At the same time, the glass substrate 1 is sunk by a predetermined thickness in the address portion formation area A3 so as to form a deep pit as an address portion 1a (see Fig. 6). The glass substrate 1 is sunk by the second etching. More specifically, the guide track portion 1b can be defined only by the second etching without depending on the selection ratio of the glass substrate to the photoresist film, and the thicknesses of the first and second residual films 6 and 7. At this time, the address portion 1a forms a pit having a desired depth by the first and second etching. Subsequently, a third residual film 8 (see Fig. 6) provided between the address portion formation area A4 and the guide track portion formation area B4 is removed by second ashing. In this case, the depths of the address portion 1a and guide track portion 1b are not affected by the second ashing. According to the present invention, the depths of the guide track portion and address portion can be defined only by the first and second etching without depending on the selection ratio of the glass substrate to the photoresist film, and the thicknesses of the first and second residual films. Consequently, the guide track portion and address portion can have grooves and pits uniformly formed on the glass substrate with good reproducibility. A method of fabricating a glass substrate for a disk according to the present invention will be described in detail with reference to the drawings. Figs. 1 to 7 show fabricating steps according to the present invention. As shown in Fig. 1, a positive type photoresist film 2 is provided on a glass substrate 1 as a glass substrate for a disk. Then, prebaking is carried out. As shown in Fig. 2, a photomask 3 is caused to contact the photoresist film 2 through a thin film 4 for a mask. Ultraviolet rays 5 are radiated onto the photomask 3 so as to expose the photoresist film 2. In this case, the thin film 4 is not provided in a formation area A1 of a sector (or track) address portion 1a. Consequently, a quantity of light transmitted to the photoresist film 2 is not reduced. The thin film 4 remains at a thickness of about 10 to 200 Å in a formation area B1 of a guide track portion 1b. Consequently, the quantity of light transmitted to the photoresist film 2 is reduced. After the photomasks 3 and 4 are removed, the exposed photoresist film 2 is developed as shown in Fig. 3. Then, postbaking is carried out. A formation area A2 of the address portion 1a on the photoresist film 2 is fully exposed by strong light. Consequently, the photoresist film 2 is completely developed so that the glass substrate 1 appears. A formation area B2 of the guide track portion 1b on the photoresist film 2 is exposed by weaker light than in the formation area A2 of the address portion 1a. Consequently, the development of the photoresist film 2 is stopped halfway. The photoresist film 2 in the formation area B2 of the guide track portion 1b remains as a first residual film 6. The first residual film 6 is formed at a thickness according to a quantity of transmitted light in the formation area B1. The above-mentioned steps are the same as those of the prior art. Subsequently, the glass substrate 1 including the photoresist film 2 is entirely etched as shown in Fig. 4. Consequently, the glass substrate 1 in a formation area A3 of the address portion 1a and the first residual film 6 in a formation area B3 of the guide track portion 1b are simultaneously etched by a thickness of 200 to 800 Å. In this case, the first residual film 6 in the formation area B3 is not completely removed so as to remain as a second residual film 7. Then, the second residual film 7 in the formation area B3 of the guide track portion 1b is removed by ashing (first ashing). Consequently, the glass substrate 1 in the formation area B3 of the guide track portion 1b appears (see Fig. 5). In this case, the glass substrate 1 in the formation area A3 which has appeared is not removed. In this state, the glass substrate 1 appears in formation areas A4 and B4. The glass substrate 1 in other areas is covered by the photoresist film 2. Thereafter, the glass substrate 1 including the photoresist film 2 is entirely etched (second etching). Consequently, a portion in which the glass substrate 1 appears, i.e., the glass substrate 1 in the formation areas A4 and B4 of the address portion 1a and guide track portion 1b is also etched by a thickness of 300 to 1000 Å (see Fig. 6). In this case, the photoresist film 2 is removed by a thickness of about 300 to 1000 Å so as to remain as a third residual film 8. Finally, only the third residual film 8 is removed by ashing (second ashing). Consequently, there is formed a glass substrate 5 for a disk having address portions 1a and guide track portions 1b of which pits and grooves have different depths. In this case, the pit of the address portion 1a has a depth of 1400 Å and the groove of the guide track portion 1b has a depth of 700 Å. These depths are set to λ / 8n in the guide track portion 1b and λ / 4n in the address portion 1a (where λ is a wavelength of a laser beam radiated onto the glass substrate 5 and n is a refractive index of a substrate). According to the present invention, a new etching process is incorporated. Consequently, the depths of the pit and groove of the address portion and guide track portion are defined only by etching. Thus, there can be fabricated a glass substrate for a disk on which address and guide track portions are uniformly formed with good reproducibility. The address and guide track portions include pits and grooves which have different depths, respectively.

Claims: A method of fabricating a glass substrate (1) for, or for use in the manufacture of, an optical recording medium, the substrate carrying two types of pattern (1a, 1b) distinguishable at least by different thicknesses of the substrate, the method comprising a first etching step in which initially the surface of the substrate is exposed in a first area where one pattern (1a) is to be formed and is covered by a resist layer (6) in a second area where the other pattern (1b) is to be formed, characterised: in that, in the first etching step, the substrate is etched in said first area only; by a step of removing the remnant resist layer (7) in said second area after the first etching step, thereby exposing the substrate in said second area; and by a second etching step in which the substrate is further etched in said first area to complete the formation of the one pattern (1a) and is etched in said second area to form the other pattern (1b). A method of fabricating a glass substrate according to claim 1, wherein said two types of pattern consist of guide track portions (1b) and sector or track address portions (1a) having different groove and pit depth respectively, the method comprising: i) forming a photoresist film (2) over the substrate (1); ii) carrying out exposure (3, 4, 5) to define a plurality of guide track portion formation areas (B1) and sector or track address portion formation areas (A1) interposed between adjacent ones of the guide track portion formation areas, and carrying out development, the exposure causing, after development, the glass substrate to appear in the address portion formation area (A2) and the photoresist film to remain as a first residual film (6) in the guide track portion formation area (B2); iii) carrying out said first etching step to etch the glass substrate in the address portion formation area (A3) by a predetermined thickness such that a pit (1a) is formed and to remove a surface layer portion of the first residual film (6) such that a second residual film (7) is formed; iv) carrying out first ashing to remove the second residual film (7) so that the glass substrate appears in the guide track portion formation area (B4); v) carrying out said second etching step to form a groove (1b) in the guide track portion formation area (B4) and to further etch said pit in the address portion formation area (A4) so that a deeper pit (1a) is formed; and vi) carrying out second ashing to remove a third residual photoresist film (8) which remains of the photoresist film between the deeper pit (1a) and the groove (1b) after the second etching step. A method of fabricating a glass substrate according to claim 2, wherein the first and second ashing is carried out by means of a parallel plate type etching apparatus which is used for the first and second etching steps. A method of fabricating a glass substrate according to claim 2, wherein the first and second ashing is carried out in an oxygen plasma atmosphere. A method of fabricating a glass substrate according to claim 2, wherein the first and second etching is carried out in a plasma atmosphere using gas such as CF 4 or CHF 3 . A method of fabricating a glass substrate according to any of claims 2 to 5, wherein the exposure is carried out using a photomask (3, 4) capable of completely removing the photoresist film (2) in the sector or track address portion formation area (A1) and stopping the removal of the photoresist film (2) half-way in the guide track portion formation area (B1). A method of fabricating a glass substrate according to claim 6, wherein the photomask comprises a quartz substrate (3) and a metallic pattern (4) such as tantalum, chromium or titanium which is formed on the quartz substrate. A method of fabricating a glass substrate according to any of claim 2 to 5, wherein the exposure is carried out by a laser cutting method using two Ar ion laser beams. A method of fabricating a glass substrate according to any of claims 1 to 8, wherein the glass substrate is used as a substrate for said optical recording medium. A method of fabricating a glass substrate according to any of claims 1 to 8, where the glass substrate is used as a substrate for a master plate for preparing a stamper to be used for injection molding. A method of manufacturing an optical recording medium which includes the method of fabricating a glass substrate as a substrate for said medium according to claim 9. A method of manufacturing an optical recording medium which includes the method of fabricating a glass substrate as a master plate according to claim 10.

Inventors: HIROKANE JUNJI, INUI TETSUYA, MIEDA MICHINOBU, OHTA KENJI, HIROKANE, JUNJI, INUI, TETSUYA, MIEDA, MICHINOBU, OHTA, KENJI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G11B7
IPC codes: H01L21, G11B7