Method of forming small through-holes in thin metal plate

Abstract: A method of forming small through-holes in a thin metal layer including the steps of forming resist films having openings formed in accordance with a predetermined pattern; etching one or both sides of the thin metal plate and stopping etching before through-holes are formed; removing the overhang of the resist film by spraying high pressure fluid onto the etched side of the thin metal plate; covering the surface by an etch-resisting layer; forming through-holes by etching the side not covered; and stripping the etch-resisting layer and the resist film.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a method of forming through-holes in a thin metal plate by means of etching, and more specifically, to a method of forming small through-holes in a thin metal plate by etching the thin metal plate from its both sides for use in manufacturing a shadow mask for a color CRT (Cathode Ray Tube), etc. Description of the Related Art Color CRT are now in wide use for computer display or monitoring units, etc. besides as a color television receiver tube for general television broadcasting. With such a variety of use more prevalent, extremely high definition picture qualities are in demand for color CRT. A color CRT includes, as shown in Fig. 1, an electron gun 30 for producing three electron beams B, phosphors 31 for receiving the electron beams B produced by the electron gun 30 and emitting light of three primary colors, and a shadow mask 32 disposed between the phosphors 31 and the electron gun 30 and provided with a number of through-holes (see Fig. 2) for passing selectively an electron beam B running in a desired direction among the electron beams B. The electron beams in undesired directions impinge upon the shadow mask 32 and do not reach the phosphors 31. As described above, high resolution is required especially for a color CRT used in a display unit or a monitor (hereinafter referred to as "high resolution TV tube"). A shadow mask to be used in a high resolution CRT must have through-holes small enough and of substantially the same shape. Naturally high resolution as intended can not be obtained unless through-holes are small enough, while the diversity in shapes of through-holes can be responsible for degradation in picture quality. A shadow mask is generally manufactured by a process including steps as shown in Fig. 3. More specifically, a low carbon aluminum-killed steel plate having a thickness in the range of about 0.1 to 0.3mm, or a piece of Invar alloy (ferric-nickel alloy containing 36% of nickel) is prepared for a shadow mask material. The surface of the shadow mask material is degreased and rinsed with water. Then, photosensitive agent is applied onto both sides of the shadow mask material and dried, and a photoresist film having a thickness of several µm is formed on each side of the shadow mask material. Masks are put into contact with the surfaces of the photoresist films on both sides of the shadow mask material. The mask for exposure is prepared by forming a desired image pattern on a surface of a transparent plate of, for example, glass. The masks on both sides of the shadow mask material are registered so that the positions of their picture images correspond to each other. The photoresist film covered by the mask for exposure is exposed to light and chemical reaction takes place in the exposed part. The photoresist film is developed after the exposure, and undergoes hardening treatment thereafter. Resist films having openings formed in accordance with the desired image pattern are formed on the top and bottom side surfaces. The shadow mask material is subject to spray-etching using ferric chloride aqueous solution. The metal part of the shadow mask material exposed and not covered by the resist films is removed, and a number of through-holes having a desired shape are formed in the shadow mask material. The resist films are stripped from the front and rear surface of the shadow mask material after the etching is completed. After the shadow mask plate material is rinsed with water and dried, the plate material is cut out into a prescribed shape. The desired shadow mask is thus obtained. Through-holes having a diameter smaller than the thickness of a shadow mask material are often produced for a shadow mask used in a high resolution CRT. A circular through-hole having a diameter of about 145µm is produced in a plate material having a thickness of about 0.2mm in some cases. In other cases, for example, in the case of an aperture grill used in a CRT of TRINITRON R ○ (a registered trademark by SONY CORPORATION) type, a long hole having a width of about 75µm is formed in a plate material having a thickness of about 0.1mm. Consider that a shadow mask sized as such is fabricated by forming through-holes through only a single etching process. In this case, it is empirically well known that it is extremely difficult to form through-holes having a desired shape and only a shadow mask which can not be used in practice is obtained. A solution to this problem is proposed, for example, in U.S. Patent No. 3,679,500 or Japanese Patent Publication No. 57-26345. Suggested in these documents are essentially forming through-holes by means of first and second etching processes as shown in Fig. 4. Now, referring to Fig. 4, after the first etching process, the surfaces of a shadow mask material are covered with etch-resisting layers. The covered part is protected against a subsequent second etching process. The first approach of these suggestions will be described in conjunction with Figs. 4A to 4G. As shown in Fig. 4A, a thin metal plate (a shadow mask) 1 having its front and rear surfaces covered respectively with resist films 2, 2' with desired image pattern is prepared by the steps as far as "hardening treatment" in Fig. 3. Referring to Fig. 4E, etching agent (ferric chloride aqueous solution) 4 is sprayed from a spray nozzle 3 to one side of the thin metal plate 1. The exposed part of the one side of the thin metal plate is etched, and a recess 5 having a shape as shown in Fig. 4C develops. This is "the first etching treatment." Referring to Fig. 4C, etching is stopped before the recess 5 reaches the other side of the thin metal plate 1. The thin metal plate 1 is rinsed with water and dried. Referring to Fig. 4D, an etch-resisting layer 6 formed of, for example, paraffin, asphalt, lacquer, UV light hardening type resin, etc. is formed entirely over the one side of thin metal plate 1. The layer 6 is formed using a spray or a roll coater. Now referring to Fig. 4E, the etching agent 4 is sprayed from a spray nozzle 3' onto the other side of thin metal plate 1. A recess 7 shown in Fig. 4F develops from the other side of thin metal plate 1 toward the side of the front surface by this treatment, and the recesses 5 and 7 come into communication. This step corresponds to "the second etching treatment." Referring to Fig. 4G, after the etching is completed, the resist films 2, 2' and the etch-resisting layer 6 are stripped off. A number of through-holes 8 are left in the thin metal plate 1. The through-holes are formed by etching the same metal plate 1 twice from the front and the rear. As compared to the case in which the through-hole 8 is formed by one etching process, the etchant does not pass the through-holes in accordance with this method, thereby preventing the diameter of the through-hole 8 from widening inappropriately. Figs. 5A to 5G show a variation of the method shown in Figs. 4a to 4g. The method shown in Figs. 5A to 5G is substantially identical to the one shown in Figs. 4A to 4G except that the first etching treatment is conducted from both sides of the thin metal plate 1 as shown in Fig. 5C. The etching treatment is completed before the recesses 5, 5' formed by the process communicate with each other. Referring to Fig. 5D, the entire surface of one side is covered by the etch-resisting layer 6. As shown in the figures after Fig. 5E, the treatment identical to the one after Fig. 4E is conducted, and a number of through-holes 8 shown in Fig. 5G are formed in the thin metal plate 1. In Figs. 4A to 4G, and Figs. 5A to 5G, the same reference characters and reference numerals denote the same or corresponding portions. Therefore, detailed description in conjunction with Figs. 5A to 5G will not be repeated here. The above-described conventional method, however, does have the following shortcomings. As shown in Figs. 4C and 5C, the recess 5 develops not only in the direction of the thickness of the thin metal plate 1 but also in the direction parallel to its surface. Consequently, the recess 5 expands as far as the portion below the circumferences of the opening of the resist film 2. In other words, the resist film 2 slightly protrudes from the circumferential edge of the recess 5 toward the center of the recess 5. This protruding part of the resist film 2 is hereinafter called "overhang." As shown in Figs. 4D or 5D, an etch-resisting layer 6 is formed on the resist film 2. However, in this process, a void 10 is liable to be formed beneath the overhang 9 as shown in Fig. 6. The surface of the thin metal plate 1 in the boundary with this void 10 can not be covered by the etch-resisting layer 6. At the time of the second etching, the etching agent enters into the void 10 when the recess 7 developing from the bottom and the void 10 come into communication. As a result, the thin metal plate 1 is etched in the boundary with the void 10, thereby expanding the recess 5 in that part. It is therefore not possible to form small through-holes shaped as desired, rendering a resultant shadow mask defective. A method for solving such a problem is disclosed in Japanese patent Laying Open No. 59-73834, which includes the steps of removing a resist film on the side to be provided with an etch-resisting layer from the surface of a thin metal plate after the first etching process, and forming the etch-resisting layer on the surface with that resist film removed. However in accordance with this method, at the time of the removal of the resist film after the first etching process, the resist film on the opposite side must be protected by a sheet, etc. so that the resist film on the opposite side will not be removed. A protective sheet must therefore be prepared. Devices for attaching or removing the sheet will also be necessary. Furthermore, a cleaning device will be necessary in addition to a device for removing a resist film, when the resist film is removed by spray of, for example, alkaline liquid. The shadow mask manufacturing device as a whole will be over-sized accordingly, and a number of devices will be necessary, complicating the operation of the device. A method is disclosed in Japanese Patent Laying Open No. 61-130492, which includes the steps of removing only the overhang of a resist film after the first etching process by bathing an entire thin metal plate in ultrasonic wave and then forming an etch-resisting layer. The void can be prevented in accordance with this method. However, this method bears problems as follows. That is, a small quantity of etching agent remains within a very small recess formed in a thin metal plate after the first etching process. It is almost impossible to remove completely the remaining etching agent by means of normal cleaning. The very small recess is corroded by the remaining etching agent and expands while the thin metal plate is bathed in ultra-sonic wave. Ultra-sonic wave capable of cutting off the overhang of a resist film, when applied to a thin metal plate can give rise to mal-adhesion between the thin metal plate and the resist film. The part other than desired to be etched will be etched in the second etching process due to this mal-adhesion state, and this will be another reason for rendering the shadow mask defective. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide, in a method of forming a number of through-holes in a thin metal plate through two etching processes, a method of removing the overhang of a resist layer produced by the first etching process while keeping the adhesion between the thin metal plate and the resist layer firmly. Another object of the present invention is to remove the overhang of a resist layer by simple steps while keeping the adhesion between a thin metal plate and the resist layer well. A further object of the present invention is to remove the overhang of a resist layer by a process other than ultrasonic wave bathing. Yet another object of the present invention is to remove the overhang of a resist layer by reduced steps. A still further object of the present invention is to provide a method of removing the overhang of a resist layer and cleaning the surface of a thin metal plate simultaneously, after the first etching process. An additional object of the present invention is to provide a method of removing the overhang of a resist layer without post-treatment. An additional object of the present invention is to remove the overhang of a resist layer without adversely affecting human bodies and environment. An additional object of the present invention is to provide a method of removing efficiently all the overhangs of a resist layer on the surface of a thin metal plate. A method in accordance with the present invention is directed to the formation of small through-holes in a thin metal plate having front and rear surfaces. The method includes a step of forming corrosion-resistant films having a plurality of openings formed in accordance with a set of patterns corresponding to each other on the front and rear surfaces of a thin metal plate, for exposing the front and rear surfaces; and a first etching step for etching the thin metal plate exposed by the openings in the corrosion-resistant film formed on the front surface to the extent that a recess formed by the etching does not reach the rear surface. Side etching in the first etching step sometimes causes the size of the recess on the front surface to be larger than the opening formed in the corrosion-resistant film. The method further includes the steps of removing at least that part of the corrosion-resistant film which protrudes over the recess by spraying fluid substance onto the front surface in which the recesses are formed at a spraying pressure larger than a predetermined pressure; covering the front surface and the overlying corrosion-resistant film by an etch-resisting layer; a second etching step for etching the rear surface exposed by the openings of the corrosion-resistance film until the etched rear surface comes into communication with the recess formed in the first etching step; and stripping the etch-resisting layers and the corrosion-resistant films formed on the front and rear surfaces. According to the method, subsequent to the first etching step, the fluid substance is sprayed onto the front surface of the thin metal plate etched by the first etching step at a spraying pressure more than a predetermined pressure. The thin metal plate beneath the circumferential edges of the openings of the corrosion resistant films has been etched in the first etching step. The overhangs of the corrosion-resistant films are formed in these portions, and at least these overhangs are cut off from the corrosion-resistant films by physical impact given by the fluid substance. Even if an etch-resisting layer is formed on the surface of the thin metal film and the corrosion-resistant film in a subsequent step after the removal of the overhangs, voids are unlikely to be formed anywhere. In the second etching step the thin metal plate will not attain an undesired shape in the part with the voids. Consequently, very small through-holes can be formed in the thin metal plate accurately. In accordance with another aspect of the present invention, fluid substance is shot from a nozzle, etc. and impinges upon a thin metal plate, and the relative position between the nozzle and the thin metal plate is changed. It is therefore possible to spray the fluid substance entirely over the thin metal plate, so that at least the overhangs of the corrosion resistant films can be removed from the entire surfaces of the thin metal plate. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view schematically showing a color picture tube; Fig. 2 is an enlarged view showing the surface of a shadow mask; Fig. 3 is a diagram showing a general manufacturing process of a shadow mask; Fig. 4 is a diagram showing a process of forming through-holes in a thin metal plate by two etching processes; Figs. 4A to 4G are sectional views showing a thin metal plate and films formed on both sides thereof in accordance with a process of forming through-holes by two etching processes; Figs. 5A to 5G are sectional views showing a thin metal plate and films formed on both sides thereof in accordance with a process of forming through-holes by two etching processes; Fig. 6 is a partially enlarged sectional view showing a thin metal plate for illustrating a void; Fig. 7 is a partially enlarged sectional view showing a thin metal plate for illustrating the formation of undesirably etched portions caused by the void; Fig. 8 is a diagram showing the essence of a method in accordance with the present invention; Figs. 8A to 8C are partially enlarged sectional views each showing a thin metal plate for illustrating the essence of a process of a method in accordance with the present invention; Fig. 9 is a view schematically showing the positioning and operation of a nozzle in an experiment on the present invention using one nozzle; Figs. 10 to 13 are enlarged electron microphotographs each showing the surface of a thin metal plate obtained by an experiment; Fig. 13A is a diagram showing the essence of a process in accordance with another embodiment of the present invention; Fig. 13B is a diagram showing the essence of a process in accordance with another embodiment of the present invention; Fig. 14 is a view illustrating the positioning and operation of three nozzles in accordance with another embodiment of the present invention; Fig. 15 is a view illustrating the placement of four nozzles in accordance with another embodiment of the present invention; Fig. 16 is a view illustrating the placement relation between a roller for supporting a thin metal plate and nozzles in accordance with another embodiment of the present invention; Fig. 17 is a sectional view schematically showing a picture tube with an aperture grill incorporated therein; Fig. 18 is a partially enlarged view showing the surface of an aperture grill; Figs. 19 to 22 are partially enlarged photographs showing the surface of a thin metal plate in the steps of manufacturing an aperture grill in an experiment conducted in accordance with the present invention ; Fig. 23A and 23B are schematic views illustrating the placement of plural spray tubes in accordance with another embodiment of the invention; and Fig. 24 is a schematic view showing an operation of rinsing the shadow mask material in accordance with another embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS When a shadow mask is manufactured in accordance with the present invention, for example, the steps of manufacture is identical to the steps shown in Fig. 3 except for the etching steps. Among the steps shown in Fig. 3, the steps other than etching can be implemented in a totally identical manner as conventionally practiced. And only the etching steps are replaced with the process in accordance with the present invention. Therefore, detailed description will be provided exclusively on the etching process of a method in accordance with the present invention. Referring to Fig. 8A through 8C, an etching process in accordance with a method of the present invention includes first etching treatment, overhang removing treatment using fluid substance, covering the surface of a thin metal plate with an etch-resisting layer after the overhang is removed, and second etching treatment. As can be apparent from Figs. 4A through 4G and 8A through 8C in comparison, the method in according with the present invention is characterized by the step of removing the overhang of a resist layer by spraying fluid substance onto the surface of a metal plate after the first etching treatment and prior to the formation of an etch-resisting layer. The step of removing the overhang of the resist film, for example, can be put between the step shown in Fig. 4C and the step shown in Fig. 4D. Alternatively, the step of removing the overhang can be put as it is between the step shown in Fig. 5C and the step shown in Fig. 5D. The process shown in Figs. 4A to 4G and the process shown in Figs. 5A to 5G can be applied as they are to a method in accordance with the present invention with exception being that the step of removing an overhang is inserted and the overhang is removed thereafter. The same characters and reference numerals denote the same or corresponding elements to the elements shown in Figs. 4A to 4G, and Figs. 5A to 5G hereinafter, and detailed description will not be given on the steps other than the step of removing the overhang. Referring to Fig. 8A, a thin metal plate 1 having a resist film 2 formed on its surface is prepared. Formed on the surface of the thin metal plate 1 is a recess 5 produced by a preceding first etching step. A part of the resist film 2 protrudes onto the circumferential edge of the recess 5, forming an overhang 9. A high pressure spraying nozzle 11 is provided a prescribe distance apart from the surface of the thin metal plate. The nozzle 11 is provided in an posture in which its center axis intersects the surface of the thin metal plate 1 at a right angle, and is capable of moving relative to the thin metal plate 1 in the direction parallel to the surface of the thin metal plate 1. Referring to Fig. 8B, as the nozzle 11 moves, fluid substance blown from the nozzle 11 removes the overhang 9. Referring to Fig. 8C, after the passage of the nozzle 11, no overhang is appreciated, and a resist film 13 without the overhang remains. The recess 5 is entirely exposed. An etch-resisting layer is then formed entirely over the resist film 13 and the recess on the surface of the thin metal plate 1. As can be seem from Fig. 8C, there is no possibility of a void occurring in the recess 5. Therefore, it is most unlikely that through-holes are formed in an undesired manner in subsequent second etching treatment. As shown in Figs. 8A to 8C, a single or multiple high pressure spray nozzles 11 are kept a certain distance apart from the surface of the thin metal plate 1. Furthermore, the high pressure spray nozzles 11 move relatively to the thin metal plate 1 in the direction parallel to the surface of the thin metal plate 1. Thus, high pressure fluid substance is spayed entirely over the surface of the thin metal plate 1, and the overhang is entirely removed. Alternatively, an arrangement of a sufficient number of high pressure spray nozzles may be used in order to treat the entire surface of the thin metal plate 1 without the relative movement of the nozzles. Any fluid substance may be used, which can be shot from the high pressure spray nozzle and give physical impact to the resist film. For example, liquid such as water, acid, alkaline water, various chemical agents, and organic solution, powdery grain such as ice grain, sand, titanium oxide, and quartz, or a mixture of these liquid and powdery grain may be used. Now, description will be provided on experiments by the application of the present invention in practice and the results in conjunction with Figs. 9 to 13 as well as a table. A web of shadow mask material 14 (low carbon aluminum-killed steel) was used as a sample, having a circular recess having a diameter of about 100µm on its opposite surfaces, rinsed with water and dried thereafter. As shown in Fig. 9, the shadow mask material 14 was fed along the front surface of a base stage (not shown), while a spray system 18 for shooting high pressure fluid substance was moved along a guide 19 in the direction intersecting the direction in which the shadow mask material 14 was fed and in parallel to the front surface of the shadow mask material 14. Fluid substance was shot upon the surface of the shadow mask material 14 from the spray system 18. The spray system 18 thus scanned the surface of the shadow mask material 14, and the fluid was sprayed in an even manner entirely on the shadow mask material 14. An A7A type airless automatic gun manufactured by NORDSON K.K. (Japanese Corporation) is used for the spray system 18 for shooting high pressure fluid substrate. The fluid shot was water having an ordinary temperature of 23.2°C, and the diameter of the water shot was about 2mm. The speed of feeding the shadow mask material 14 was 1mm/sec, the speed of moving the spray system 18 was 400mm/sec and the width of the shadow mask material 14 was 600mm. Experiments were made, in which the spraying pressure of the spray system 18 was changed as shown in the table, and water of ordinary temperature was ejaculated from the height 2cm above from the surface of the shadow mask material 14 for each pressure. A FUJI FILM PRESCALE, super low two sheet type manufactured by FUJI PHOTO FILM CO., LTD. was used for the measurement of the spraying pressure. Then, after the step of water shooting, UV resin of about 100cps was applied as an etch-resisting layer by a roll coater as thick as about 10µm onto the surface of one side of the shadow mask material 14 having a plurality of recesses formed by the first etching process. Subsequently, as shown in Fig. 4E, etching agent was sprayed from the spray nozzle onto the surface not covered by the etch-resisting layer, thereby performing the second etching. As shown in Fig. 4F, very small recesses formed on the surface and very small recesses formed by the first etching process were communicated to each other. Then, the etch-resisting layer and the resist film were stripped off from the surface of the shadow mask material in the order shown in the flow chart in Fig. 3, and using with water and drying are conducted thereafter. Ferric chloride aqueous solution of 40°C and 45° baumé were used in the above-described first and second etching processes. The effect of the removal of the overhangs was evaluated by observing with an optical microscope the surface of the shadow mask material 14 after the shooting. Electron microphotographs each showing the effect are in Figs. 10 to 13. Fig. 10 shows the surface of the shadow mask material having a plurality of recesses by the first etching process on its surface. That surface of the shadow mask material 14 becomes as shown in Fig. 11 after the shooting process. It can be seen from Figs. 10 and 11, that the diameter of the opening of the resist film increases after the shooting, and all of the recesses formed in the shadow mask material 14 are exposed. Fig. 12 and 13 are photographs enlarged by the factor of 500 and 2500, respectively, each showing the surface of the shadow mask material 14 after the overhangs were removed. It can be appreciated from Figs. 12 and 13 that not only the overhangs formed by the first etching process but also the resist film surrounding these overhangs are removed slightly. As for the formation of voids, evaluation was made by determining the existence of a void, observing with an optical microscope the surface of the shadow mask material 14 supplied with the UV resin as an etch-resisting layer after the removal of the overhangs. The finish of the etching was evaluated by visually confirming whether apertures are formed in desired shapes in the shadow mask material 14. Table 1 represents evaluations on the removal of the overhangs, the presence or absence of a void, and the finish of etching as to different values for the spraying pressure from the nozzle under the above-described conditions. It can be appreciated from the above-described results of the experiments that very small through-holes having a desired shape can be formed by setting the spraying pressure of the spray system 18 for high pressure fluid substance spraying to be more than 10kgf/cm², for example, 15kgf/cm². This is because the overhangs of the resist film were entirely removed by the fluid sprayed at a sufficiently high pressure, and, therefore, voids can not be formed when the recesses are covered by the etch-resisting layer. Conventionally, it has been commonly recognized that the water pressure in the step of rinsing with water conducted subsequent to the etching must be low, for example, as low as 0.5 to 1.5kgf/cm². It has been believed that otherwise a resist film would partially comes off, mal-adhesion between a resist film and a thin metal plate takes place due to cracks formed in the resist film, or a thin metal plate itself would be broken. As in the foregoing, however, putting aside such a conventional prejudiced idea, the applicant conceived a possibility that overhangs are removed by positively applying high pressure fluid substance, and completed the present invention by confirming this conception by experiments. In these experiments, the removal of the overhang as described above was conducted immediately after the first etching treatment shown in Fig. 8 and immediately before the surface of the shadow mask material was covered by the etch-resisting layer. The present invention however is by no means limited thereto, but any appropriate method may be employed for the removal of the overhang between the first etching treatment and the drying step before the formation of the etch-resisting layer. For example, referring to Fig. 13A, the overhang may be removed in a separate step after the surface of the shadow mask material is rinsed with water subsequent to the first etching treatment. In this case, sprayed water, etc. can be recycled and used repeatedly for the removal of the overhang, because the surface of the thin metal plate is clean. Another possible method is to remove the overhangs in the step of rinsing with water after the first etching process is completed. In this case, water sprayed from the spray nozzle of a rinsing device is utilized for the removal of the overhang as well, and, therefore, no device dedicated to the purpose of the removing the overhangs will be necessary. As the case may be, the step of rinsing with water may be added subsequent to the removing step for the purpose of expelling cut away resist film. When performing such rinsing, preferably, water is sprayed from the spray nozzles not only to the front surface of the shadow mask material alone but also to the front and rear surfaces thereof alike as illustrated in Fig. 24, in order to prevent any scums of the overhangs cut on the front surface from adversely going round to the rear surface. In the experiment, no deformation of the web of shadow mask material due to the water pressure at the time of high pressure water spraying was observed. Now, to be more careful, high pressure water, etc. may be sprayed to the front surface of the thin metal plate by supporting the thin metal plate on a stage and moving the front surface, or high pressure water, etc. may be sprayed to the part supported by a roll by feeding the web of thin metal plate into the rotating roll. In the following, some of other embodiments in accordance with the present invention as described above will be illustrated. Referring to Fig. 14, the web of shadow mask material 14 after the first etching process is fed along the front surface of a base stage (not shown). Three spray nozzles 48 are provided a prescribed distance above from the surface of the shadow mask material 14. These spray nozzles 48 are moved along a guide 19 in the direction orthogonal to the direction of feeding the shadow mask material 14 and in parallel to the front surface of the shadow mask material 14. Fluid substance is sprayed in an even manner entirely on the surface of the shadow mask material 14 from the three spray nozzles 48 moving in the widthwise direction of the shadow mask material 14. A good result was also given by implementing the present invention in a manner as shown in Fig. 14. A spray nozzle used was a nozzle (model number 1/4MVEP11519) manufactured by H. IKEUCHI & Co., LTD. The spray nozzle 48 is placed in a position 7cm above from the surface of the shadow mask material 14 and sprays the flow of fluid substance fanwise in the range of about 8cm in the direction of feeding the shadow mask material 14. Water of ordinary temperature was used for the fluid substance. The speed of feeding in the shadow mask material 14 was 3m/min, and the moving speed of each of the spray nozzles 48 was more than 250mm/sec, for example, 300mm/sec. The spray nozzle 48 makes the reciprocating movement for 200mm in the widthwise direction of the shadow mask material 14 at this speed. The width of the shadow mask material 14 was 600mm. Fluid substance having a pressure of more than 10kgf/cm² at the time of spraying, e.g. a spraying pressure of 15kgf/cm² was sprayed onto the surface of the shadow mask material. Another possible embodiment is in Fig. 15. Referring to Fig. 15, the web shaped shadow mask material 14 after the first etching process is fed in a prescribed direction. A spray tube 15 is provided in the direction orthogonal to the direction of feeding of the shadow mask material 14. The spray tube 15 is provided with a plurality of fixed nozzles 16. The fixed nozzles 16 discharge fluid substance toward the shadow mask material 14, and the fluid substance has an even pressure across the entire width of the shadow mask material 14. Water of ordinary temperature is used for the fluid. The spraying pressure applied to the surface of the shadow mask material 14 is determined to be more than 10kgf/cm², for example, 15kgf/cm². The speed of feeding of the shadow mask material 14 is determined on demand, e.g. 0.5 to 4m/mic. A base stage (not shown) for supporting the rear surface of the shadow mask material 14 is provided in the feeding path. The shadow mask material 14 was fed along the front surface of the stage. Fig. 16 shows an example in which a rotating roller 17 is provided in the path instead of the stage, for supporting the rear surface of the shadow mask material 14. Water of ordinary temperature is shot from the nozzles 16 of the spray tubes 15 to the part supported by the rotating roller 17. Either a roller covered by rubber or a metal roller may be used for the rotating roller 17. The spraying pressure as well as the speed of feeding the shadow mask material 14 should be identical to the above-described conditions. Further, as shown in Figs. 23A and 23B, the two spray tubes 15 may be provided, being juxtaposed each other and extending orthogonally to the fending direction of the shadow mask material 14, so that two-staged fluid substance discharging can be conducted. Needless to say, three spray tubes 15 or more may be provided as well for three-staged discharging or more. Good results were obtained for all of the above-described cases. Although, in the above-described experiments as well as the embodiments of the step of spraying high pressure fluid substance the cases have been described in which a circular shaped recess is produced, the present invention is not limited to forming of through-holes of this shape. The present invention can be applied to the case in which such slot shaped through-holes as illustrated in Fig. 2 are produced in a shadow mask material. A CRT part called an aperture grill similar to a shadow mask can be manufactured by the application of the present invention. Referring to Fig. 17, the aperture grill is used in a TRINITRON (a trademark by Sony Corporation) type CRT manufactured by SONY CORPORATION. The trinitron type CRT includes an electron gun 30b provided in a glass tube 51, and an aperture grill 53 provided close to the phosphors 52 of the glass tube 51. The aperture grill 53 includes a thin metal plate in which a plurality of slots are formed as shown in Fig. 18. The slot is provided for passing only an electron beam in a desired direction. Electron beams in unnecessary directions impinge upon the thin metal place and will not reach the phosphors 52. The present invention can be applied for forming the plurality of small slots in the aperture grill. More specifically, it is only required to change the exposure patterns to be formed from the formation of round through-holes, and, therefore detailed description will not be given on the manufacturing process of the aperture grill. Figs. 19 to 22 each illustrates the result of an experiment of manufacturing an aperture grill in accordance with the present invention. As shown in Fig. 19, and more specifically in Fig. 20, a plurality of thin long grooves are formed on one side of a thin metal plate by the first etching process. The thickness of the thin metal plate was 0.1mm, and the width of etched groove was about 65µm. The surface of the thin metal plate shown in Figs. 19 and 20 after being subject to treatment in accordance with a method of the present invention becomes as shown in Fig. 21, and in more detail in Fig. 22. It will be apparent from comparison between Figs. 19 and 20, and Figs. 21 and 22 that the overhangs of the resist film left behind on both sides of the groove were removed by the treatment in accordance with a method of the present invention. When an aperture grill was fabricated in accordance with a method of the present invention, the one with even shaped slots and of high quality was obtained. Although in the above-described embodiments, the cases have been described in which the present invention is applied to the manufacturing of a shadow mask for a color picture tube, the present invention is not limited thereto, and can be applied to the step of etching perforation in manufacturing precision electronic parts such as a lead frame, etc. for, for example, connecting electrically a semiconductor element to external associated circuitry. As the method will be readily understood by those skilled in the art by the illustration of the above-described embodiments, therefore, detailed description thereon will not be provided herein. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: A method for forming small through-holes in a thin metal plate having first and second surfaces, comprising the steps of: forming a corrosion-resistant film having a plurality of openings formed in accordance with a set of predetermined patterns corresponding to each other for exposing said first and second surfaces; conducting a first etching to said first surface or said first surface and said second surface of said thin metal plate exposed by said openings of said corrosion-resistant film in order to form recesses on said first surface or recesses on said first and said second surfaces as far as the recesses formed by the etching do not form through-holes, the size of said recesses on said first surface or said first and second surfaces being liable to be larger than the size of the openings formed in said corrosion-resistant film on said first surface or said first and second surfaces in said first etching step; removing at least that part of said corrosion-resistant film which protrudes from a circumferantial edge of said recess by spraying predetermined fluid substance onto said first surface or said first and second surfaces on which said recess is formed, at a blowing pressure larger than a predetermined pressure; covering said recesses on said first surface with an etch-resisting layer; conducting a second etching to said second surface exposed by the opening of said corrosion-resistant film or to said recesses on said second surface until said recesses are formed in said first etching and said recesses formed on said second surface from through-holes; and stripping said etch-resisting layer and said corrosion-resistant film formed on said first and second surfaces. A method as recited in claim 1, further comprising conducting rinsing of the surface of said thin metal plate with water after said first etching step. A method as recited in claim 2, wherein said removing step is performed after said rinsing. A method as recited in claim 3, further comprising collecting said fluid substance used in said removing step and supplying said collected fluid substance once again for use in said removing step. A method as recited in claim 3, further comprising conducting rinsing on said first surface of said thin metal plate or said first and second surfaces thereof with water after said removing step. A method as recited in claim 1, further comprising conducting rinsing of said first surface of said thin metal plate or said first and second surfaces thereof with water after said removing step. A method as recited in claim 1, wherein said removing step is effected by said rinsing. A method as recited in claim 1, wherein said corrosion-resistant films formed on said first and second surfaces have a plurality of apertures arranged in a predetermined pattern. A method as recited in claim 1, wherein said corrosion-resistant films formed on said first and second surfaces have a plurality of aperture grills arranged in parallel to each other. A method as recited in claim 1, wherein said removing step includes a step of spraying said fluid substance from a nozzle onto said first surface and changing the relative position between said nozzle and said thin metal plate so that said fluid substance impinges upon substantially the entire surface. A method as recited in claim 10, wherein the distance between said nozzle and said first surface is kept constant. A method as recited in claim 1, wherein the posture of said nozzle is kept so that said fluid substance impinges upon said first surface at an approximately constant angle. A method as recited in claim 12, wherein said angle is approximately a right angle. A method as recited in claim 10, wherein said second surface is supported by a predetermined supporting means in said removing step. A method as recited in claim 14, wherein said supporting means includes a roller rotatable around a predetermined rotating axis. A method as recited in claim 10, wherein said blowing pressure is more than 10 kgf/cm². A method as recited in claim 16, wherein said blowing pressure is less than 150 kgf/cm². A method as recited in claim 17, wherein said blowing pressure is more than 15 kgf/cm². A method as recited in claim 10, wherein said nozzle moves relatively to said first surface so as to scan said first surface in a predetermined main scanning direction and in a subscanning direction intersecting said main scanning direction, in said spraying step. A method as recited in claim 19, wherein the distance between said nozzle and said first surface is kept constant. A method as recited in claim 20, wherein the speed of said nozzle relative to said first surface is kept constant. A method as recited in claim 10, wherein said fluid substance is blown from a plurality of said nozzles. A method as recited in claim 22, wherein said thin metal plate is a long web of the metal and one or a plurality of spray tubes having the nozzles are provided extending in parallel to the widthwise direction of said first surface, said plurality of spray tubes being juxtaposed each other. A method as recited in claim 23, wherein said plurality of nozzles move relatively to said first surface so as to scan said first surface in a predetermined scanning direction. A method as recited in claim 24, wherein each of distances between said nozzles and said first surface are kept constant. A method as recited in claim 25, wherein the speed of said nozzles relative to said first surface is constant. A method as recited in claim 25, wherein the distances between said nozzles and said first surface are equal to each other. A method as recited in claim 1, wherein said fluid substance is selected from the group consisting of liquid, powdery grain, and a combination of liquid and powdery grain. A method as recited in claim 28, wherein said liquid is selected from the group consisting of water, acid, alkaline water, and organic solution. A method as recited in claim 29, wherein said powdery grain is selected from the group consisting of ice grain, sand, titanium oxide, and quartz powder.

Inventors: INAGAKI AKIHIRO DAINIPPON SCRE, MIYAJI YASUYOSHI DAINIPPON SCR, OMOTO KOICHI DAINIPPON SCREEN, SATO MASANOBU DAINIPPON SCREEN, TOEI KEIJI DAINIPPON SCREEN MF, TONOGAI SEIJI DAINIPPON SCREEN, INAGAKI, AKIHIRO, DAINIPPON SCREEN MFG. CO., LTD, MIYAJI, YASUYOSHI, DAINIPPON SCREEN MFG. CO., LTD, OMOTO, KOICHI, DAINIPPON SCREEN MFG. CO., LTD, SATO, MASANOBU, DAINIPPON SCREEN MFG. CO., LTD, TOEI, KEIJI, DAINIPPON SCREEN MFG. CO., LTD., TONOGAI, SEIJI, DAINIPPON SCREEN MFG. CO., LTD
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1991
Main classification: C23F1
IPC codes: H01J9, H01L21, C23F1