Method of manufacturing a photomask for an optical memory

Abstract: A method of manufacturing a photomask for an optical memory, the photomask having two types of pattern where guide tracks (1b) and formatting pits (1a) are different in amount of optical transmission, including the steps of (a) forming on a transparent substrate (1) a thin film (2) of which light transmission amount depends upon its thickness; (b) forming a photoresist film (3) on the thin film; (c) exposing (4, 5) the photoresist film to light with different light intensities depending selectively on the guide tracks or formatting pits; (d) eliminating the photoresist film exposed to the more intense light (4) by the development until the thin film is revealed; (e) etching the exposed thin film; (f) eliminating the photoresist film until the thin film in the areas which were exposed to the less intense light (5) is revealed; (g) etching away the thin film until the transparent substrate is revealed in the areas which were exposed to the more intense light (4); and (h) eliminating the remnant photoresist.

Description: Field of the Invention The present invention relates to a method of manufacturing a photomask for preparation of an optical memory device. Description of the Related Art In recent years, an optical memory device as high-density, large-capacity memory means has been increasingly needed. Such optical memories can be classified into three types in accordance with their applications; memories of read-out-only, write-once-read-many memories, and rewritable memories. Of them, optical memories employed as the write-once-read-many memory and rewritable memory usually have guide tracks on their substrates to guide optical beams for recording, reproducing and erasing information to specified locations in the optical memories, and also have formatting pits for identifying the order of the tracks. If a single track is sectioned into a plurality of sectors in use, the formatting pits are often provided in advance with information about sector addresses and the like. Specifically, as shown in Fig. 4, guide tracks 11b of an optical memory are different from formatting pits 11a in depth, and each of the formatting pits 11a is positioned between the adjacent guide tracks 11b. It has been known that, in the reading, when optical beams track in accordance with an optical differential system, preferably, the depth of the guide tracks 11b is fixed in approximately λ/8n while the depth of the formatting pits 1a is fixed in approximately λ/4n, where the wavelength of the employed light is λ and the refractive index of the substrate is n. There are known several methods of forming such guide tracks and formatting pits on a glass substrate by the process including the contact printing and dry etching. An example of such methods is the art including the steps described below. Now the scheme of the art will be described with reference to Fig. 3. (a) First Step (Fig. 3(a)): A thin film 2 of Cr (chrome), Ti (titanium), Ta (tantalum), or the like, is formed on a photomask substrate 1 so that light transmission amount depends upon a film thickness. (b) Second Step (Fig. 3(b)): A positive type photoresist film 3 is formed on the thin film 2 , and then it is pre-baked. (c) Third Step (Fig. 3(c)): Two beams of laser ray, argon laser and the like, are focused, respectively, and the laser beams are moved with the rotation of the photomask substrate 1, so that a photoresist film 3 can be exposed to the beam in a spiral. One of the beams, laser ray 4, is for exposing the formatting pits 1a by intermittently irradiating them with sufficient intensity, while the other, laser ray 5, is for exposing the guide tracks 1b by continuously irradiating them with less intensity than the laser ray 4. (d) Fourth Step (Fig. 3(d)): After the exposure, developing and post-baking processes are performed in this order. At this time, the formatting pits 1a are subjected to a great exposure, and the photoresist film 3 in the corresponding region is removed to expose the thin film 2. On the other hand, the photoresist film 3 in the guide tracks 1b exposed to the less intense ray is also removed, but not so deep as to expose the thin film 2. (e) Fifth Step (Fig. 3(e)): The thin film 2 is etched by the dry etching. In this step, the photoresist 3 is also etched together. In the formatting pits 1a where the thin film 2 is exposed after the development, the etching of the thin film 2 starts in accord with the start of the etching, and it ends when all the thin film 2 in this region is completely removed to expose the substrate 1. In the guide tracks 1b where the photoresist 3 still remains, however, the photoresist 3 is first etched and then the thin film 2 is done. Thus, when the etching ends, the guide tracks 1b have the remnant thin film 2 of a specified thickness. (f) Sixth Step (Fig. 3(f)): Eventually, eliminating the still remaining photoresist 3, the formatting pits 1a are provided with a photomask transmitting a great amount of ray or light, while the guide tracks 1b are provided with a photomask transmitting less ray or light. With use of the photomask manufactured in this way, a photoresist pattern is formed in a photoresist coated on a glass substrate by the contact printing so that different depths between the formatting pits and guide tracks result on the pattern, and the dry etching is performed to form grooves on the glass substrate so as to have different depths between the formatting pits and guide tracks. Therein, importance lies in controlling to provide the thin film 2 remaining in the guide tracks 1b in the photomask with an even thickness. This is because, assuming that a thin film of Ta is employed, for example, an amount of the transmitted light is 1.00 with no Ta film remaining, 0.65 with the Ta film remaining by 50 Å, and 0.52 with the Ta film remaining by 75 Å. In other words, the 25 Å difference in thickness of the thin films causes the amount of the transmitted light to vary by 13 %, and the similar variation is transferred to the photoresist pattern formed by the contact printing. With a photomask manufactured in accordance with such a conventional method, the thickness of the thin film 2 in the guide tracks 1b is effected depending upon three factors: a laser power in the exposure, a thickness of the photoresist 3 unremoved in the developing step (this is determined by requirements of the development and the like), and a difference in speed between etching the photoresist 3 and thin film 2 in the etching step. Therefore, there is the disadvantage that preparations of the thin film 2 in the guide tracks with even thickness and with good reproducibility is difficult. SUMMARY OF THE INVENTION In one aspect, the present invention, as defined by claim 1, provides a method of manufacturing a photomask for an optical memory device, said photomask comprising a substrate and a thin film formed on said substrate, the photomask carrying two types of pattern distinguishable at least by different thicknesses of the mask, the method being characterised by the steps of two separate etching operations on said thin film, one pattern being formed as a result of both etching operations and the other pattern being formed as a result of one only of the two etching operations. In another aspect, the present invention provides the method defined by claim 2 of manufacturing a photomask for an optical memory device. Claims 3 to 11 are directed to preferred embodiments of the method of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing the steps of manufacturing a photomask according to the present invention; Fig. 2 is a partial enlarged perspective view showing the photomask; Fig. 3 is a diagram for explaining the steps of manufacturing a conventional photomask; and Fig. 4 is an enlarged partial perspective view showing a substrate of the optical memory. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT While the present invention will be described in detail in conjunction with preferred embodiments shown in the accompanying drawings, it is not intended that the present invention be limited to the precise form disclosed in the description. Fig. 1 shows a method of manufacturing a photomask according to the present invention, while Fig. 2 perspectively shows an enlarged part of the photomask. (a) First Step (Fig. 1(a)): A thin film 2 of Cr (chrome), Ti (titanium), Ta (tantalum), or the like, of which light transmission amount depends upon a film thickness, is formed on a photomask substrate 1, the thin film 2 having a thickness of 100 Å to 1000 Å. (b) Second Step (Fig. 1(b)): A positive type photoresist film 3 is formed with thickness of 1000 Å to 4000 Å on the thin film 2 , and then it is pre-baked at about 95 °C. (c) Third Step (Fig. 1(c)): Two argon laser beams are focused, and the laser beams are moved with the rotation of the photomask substrate 1, so that a photoresist film 3 can be exposed to the beam in a spiral. One of the laser beams, laser light 4, is for exposing the formatting pits 1a by intermittently irradiating them with sufficient intensity (e.g., 1.2 mW in an inner edge of the data area of the optical memory; 2.4 mW in its outer edge), while the other, laser ray 5, is for exposing the guide tracks 1b by continuously irradiating them with less intensity than the laser ray 4 (e.g., 0.9 mW in the inner edge of the data area of the optical memory; 1.8 mW in its outer edge). (d) Fourth Step (Fig. 1(d)): After the exposure, developing and post-baking processes are performed in this order. At this time, the formatting pits 1a are subjected to a great exposure, and the photoresist film 3 in the corresponding region is removed to expose the thin film 2. On the other hand, the photoresist film 3 in the guide tracks 1b exposed to the less intense ray is also removed, but not so deep as to expose the thin film 2. (e) Fifth Step (Fig. 1(e)): The thin film 2 is etched by 10 Å to 200 Å by the dry etching. (f) Sixth Step (Fig. 1(f)): The photoresist film 3 is eliminated by the ashing till the thin film 2 in the guide tracks 1b is exposed. (g) Seventh Step (Fig. 1(g)): The dry etching is performed till the thin film 2 in the formatting pits 1a is completely eliminated and the photomask substrate 1 is exposed. (h) Eighth Step (Fig. 1(h)): Eventually, the still remaining photoresist is eliminated by the ashing. According to this method, the resultant photomask has the thin film 2 in the guide tracks 1b, of which thickness is enabled to be determined by the etching of the thin film 2 in the above-mentioned fifth and seventh steps. In accordance with the present invention, with an additional one etching step and one photoresist eliminating step introduced to the conventional manufacturing method, a thickness of a thin film in the guide tracks can be determined only by the etching step of the thin film. Thus, the causes of error in manufacturing are removed, and so, a photomask for an optical memory is permitted to be manufactured to have an even thickness and a good reproducibility. Whereas the described embodiment is particularly concerned with the formation of guide tracks and formatting pits on an optical memory disc, it will be appreciated that the invention is not so limited and that other patterns may be formed in the thin film to suit different requirements. It will also be appreciated from the foregoing description that the invention is characterised at least in part by the use of two separate etching operations on the thin film to form said patterns, compared with the single etching operation in the prior art method (Fig. 3(e)). Whilst the two etching operations necessitate two additional process steps, this is more than compensated by the significant benefit of the controllable, uniform and repeatable thickness of the etched thin film pattern which is obtained. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention as claimed.

Claims: A method of manufacturing a photomask for an optical memory device, said photomask comprising a substrate (1) and a thin film (2) formed on said substrate, the photomask carrying two types of pattern (la,lb) distinguishable at least by different thicknesses of the mask, the method being characterised by the steps of two separate etching operations on said thin film (2), one pattern (la) being formed as a result of both etching operations and the other pattern (1b) being formed as a result of one only of the two etching operations. A method of manufacturing a photomask for an optical memory device, said photomask carrying two types of pattern (1a,1b) distinguishable at least by different thicknesses of the mask, the method comprising the steps of: (a) forming a thin film (2) on a substrate (1); (b) forming a photoresist film (3) on the thin film; (c) exposing areas of the photoresist film to radiation (4,5), the degree of exposure being dependent upon the required type of pattern; (d) removing the photoresist film (3) until the thin film (2) is revealed in the areas (1a) which received the greater exposure (4) to said radiation; (e) etching the thin film (2) in the areas revealed in step (d); (f) removing the photoresist film (3) until the thin film is revealed in the areas (1b) which received the lesser exposure (5) to said radiation; (g) etching away the thin film (2) until said substrate is revealed in the areas (1a) which received the greater exposure (4) to said radiation; and (h) removing the remnant photoresist film (3). A method according to claim 2, wherein step (c) comprises exposing areas of the photoresist film to light (4,5) of different light intensities in dependence upon the required type of pattern. A method according to claim 2 or claim 3, wherein the substrate (1) is transparent and the thin film (2) has a light transmission property which depends upon its thickness. A method according any of claims 2 to 4, wherein the thin film (2) formed in step (a) has a thickness of 100 Å to 1000 Å. A method according to any of claims 2 to 5, wherein the photoresist film (3) formed in step (b) has a thickness of 1000 Å to 4000 Å. A method according to any of claims 2 to 6, wherein the thin film (2) subjected to the etching in step (e) is etched until its thickness becomes 10 Å to 200 Å. A method according to any of claims 2 to 7, wherein the etching in steps (e) to (g) is a dry etching. A method according to any of claims 2 to 8, wherein the photoresist film (3) formed in step (b) is a positive-type resist, and the method of eliminating the resist in steps (f) and (h) is an ashing method. A method according to any preceding claim, wherein the thin film (2) is made of Cr, Ti or Ta. A method according to any preceding claim, wherein the two types of pattern of the photomask respectively correspond to formatting pits (1a) and guide tracks (1b) of the optical memory.

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: 1991
Main classification: G03F1
IPC codes: G03F1, G11B7, H01L21