Method for preparing a shadow mask for a colour picture tube

Abstract: A method for preparing a shadow mask 6 with a plurality of apertures 10 for a colour picture tube from a metal shadow mask sheet 15 includes the step of exposing the entire surface of the photosensitive layers 14 provided on the both surfaces of the metal sheet 15 to an unpattered light before or after the step of exposing the photosensitive layers to patterned light in order to form resist films 18a and 18b with pattern of recesses.

Description: This invention relates to a method of manufacturing a shadow mask for a colour cathode ray picture tube. A colour cathode ray picture tube, as shown in Figure 1, generally comprises a glass envelope 1, in-line electron guns 3 emitting three electron beams 11R, 11G and 11B, and a phosphor screen 5 containing red, green and blue phosphor dots (not shown) which emit visible light when excited by the electron beams 11R, 11G and 11B. Electron guns 3 are located in a neck portion 2 of the envelope 1, while the phosphors, arranged in dotted shapes of cyclically repeating colours, are coated on the inner surface of the panel portion 4 of the envelope 1. Connecting neck portion 2 with the panel portion 4 is a funnel portion 12. The electron beams 11R, 11G and 11B are deflected by magnetic fields produced by a deflection yoke (not shown) surrounding a portion of the neck portion 2. Near the screen 5 is a shadow mask 6 having a plurality of circular apertures, shown in Figure 2, corresponding to the position of the phosphor dots. The shadow mask 6 is attached to a mask frame 7 supported within the envelope 1 by frame holders 8 which are releasably mounted on panel pins 9 embedded in side walls of the panel portion 4. The shadow mask 6 has the function of causing the electron beams to accurately bombard the phosphor dots. Therefore, the sectional shape of the apertures 10 is carefully designed. Namely, as shown in Figure 2, to form the apertures 10, the shadow mask 6 has a front opening 13a facing toward the phosphor screen, a rear opening 13b facing toward the electron guns and an inner wall 13c connecting the front opening to the rear opening. The inner wall 13c has a most constricted portion 13d between the openings 13a and 13b to determine the spot size of the electron beam 11B projected on the phosphor screen. Further, to avoid undesirable bombardment of the electron beam 11B on the inner wall 13c, the front opening 13a is larger than the rear opening 13b, while the inner wall 13c is inclined. In the shadow mask, if the position between the front and rear openings 13a and 13b and the size of the most constricted portion 13d are changed from designed values, the amount of electron beam passing through aperture 10 will change and thus the picture quality will deteriorate. Also, if the electron beams 11B, 11G and 11R bombard the inner wall 13c, the colour purity of the picture will deteriorate due to reflected electron beams. This type of shadow mask has been manufactured by photolithography. A typical process for manufacturing the shadow mask is disclosed in US-A-3,973,965. Namely, as shown in Figure 3A, a pair of photosensitive layers 14 are formed on respective opposite surfaces of a metal sheet 15 by coating photosensitive resin liquid on both surfaces, after the surfaces have been washed and dried. A pair of negative films 16a and 16b, having patterns of different sizes formed thereon, are put on the photosensitive layers 14. Then, the photosensitive layers 14 are exposed to light emitted from light sources 17 through the negative films 16a and 16b, respectively (Figure 3B). Next, the photosensitive layers 14 are developed. Then, the unexposed portions of the layers 14 are removed. Thus, a pair of resist films 18a and 18b, which are composed of patterns corresponding to the patterns of the negative films 16a and 16b are formed on the sheet 15 (Figure 3C). The resist films 18a and 18b are dried and baked to increase the degree of corrosion resistance. After this, an etching solution is sprayed on both surfaces of the sheet 15 to form the apertures 10. As a result of etching, large holes 19a growing from large openings 20a in the resist film 18a and small holes 19b growing from small openings 20b in the resist film 18b are linked to each other (Figure 3D). The large openings 20a are larger than the small openings 20b. Finally, the resist films 18a and 18b are peeled off and removed from the surfaces (Figure 3E). During the etching process memtioned above, lateral extensions 21a and 21b of the resist films 18a and 18b are inevitably formed because of the lateral etching beneath the resist films which accompanies the growth of the holes 19a and 19b, as shown in Figure 4. In Figure 4, the lateral extension 21a of width d results from the isotropic etching. The extension 21a is destroyed and peeled off due to the pressure of spraying of the etching solution. Consequently, the configuration of the apertures is altered from the design configuration. The lateral etching mentioned above is liable to occur in thick shadow masks, which are used when the shadow mask is required to have a high mechanical strength, for example, with flattened shadow masks. For instance, when the sheet thickness is increased to 0.3 mm from a thickness of 0.15 mm used for a normal shadow mask, the etching time increases approximately 3 times. Thus, the lateral etching progresses in proportion to the progress of etching. Consequently, the lateral extension of the resist film increases and, finally, it is peeled off. Also, the lateral etching is liable to occur in a high-resolution colour picture tube shadow mask having small-sized apertures and a smaller array pitch compared with ordinary shadow masks. Namely, since it is hard to circulate the etching solution in the aperture due to the small openings, the etching time increases compared with that of an ordinary shadow mask. It is known from Patent Abstracts of Japan Vol.6 No.132 of 17th July, 1982 for a shadow mask to be manufactured by a method which includes providing layers of photosensitive material on respective opposite faces of a metal sheet. The layers are exposed to light through negative films so that some regions are exposed to light and other are not exposed. The non-exposed areas are removed and a temporary etching proof film is formed on the metal sheet. An etching process with reduced lateral etching is then carried out to form openings through the plate. An object of the invention is to provide a method for preparing a shadow mask with a high reliability. Therefore, the invention may provide a method of manufacturing a shadow mask for a colour cathode ray tube in which first and second photosensitive layers are provided on respective opposite surfaces of a metal shadow mask sheet; the first and second photosensitive layers are exposed to respective patterns of light so that each layer has regions which are exposed to the light and regions which are unexposed, the exposure being continued until the accumulated exposure amount of each of the exposed regions is at a first predetermined level; all of at least one of the first and second photosensitive layers is exposed to light, the exposure being continued until the accumulated exposure amount of the layer is at a second predetermined level, the second level being less than the first level; all of the parts of the first and second photosensitive layers which have been exposed to light are converted to resist material with those regions of the layers which received only the accumulated exposure at the second level being converted to a thinner resist portion than those regions which received at least the accumulated exposure at the first predetermined level; the sheet is etched through any portions of the sheet which are unprotected by resist and through said thinner portions of resist to form apertures extending through the sheet; and the resist material is removed from the sheet. It has been found that, when the photosensitive layer is exposed to light before or after the exposure to the patterned light in order to form the resist film, the lateral etching can be greatly decreased. The ratio of the accumulated exposure amount of the exposure to the unpatterned light to the accumulated exposure amount of the exposure to the patterned light is preferably from 10% to about 45%. When it becomes less than about 10%, the lateral etching is not effectively decreased. When exposure exceeds about 45%, the etching is not successfully completed, since the residual photosensitive film in the unexposed areas to the patterned light becomes too thick. The accumulated exposure amount means an accumulated total energy per unit area of the photosensitive layer during duration of exposure time. Namely, the accumulated exposure amount is represented by the following equation: ( Accumulated Exposure Amount ) [mJ/cm²] = ( Illuminance ) [mW] x ( Exposure Time ) [second] The accumulated exposure amount can be adjusted to an appropriate value in accordance with the thicknes and kind of material of the photosensitive layer, and transmissivity of the negative film. When the photosensitive film was exposed to the unpatterned light in addition to the exposure to the patterned light, the unexposed areas were covered with a residual photosensitive film as residual portions of a resist film after the unexposed portions of the photosensitive film were removed for forming the resist film on the shadow mask sheet. As a result, the resistive film composed of pattern of recesses was obtained. The periphery of the unexposed area was covered with the resist film of sufficient thickness to protect against etching. The residual portions of the resist film covering the unexposed areas was peeled off so as to etch those areas. Thickness of the residual portions is less than about 20% of that of the resist films. Preferably, the thickness is in the range from about 0.1 µm to 1 µm. To adjust the ratio of the accumulated exposure amount of the exposure to the unpatterned light to the accumulated exposure amount of the exposure to the patterned light, exposure duration of time and illuminance of the lights can be changed, respectively. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a sectional view of a colour picture tube which may incorporate the present invention; Figure 2 is a sectional view of a shadow mask of the colour picture tube shown in Figure 1; Figures 3A to 3E are sectional views explaining the steps of preparing a shadow mask sheet with a plurality of apertures according to the conventional method; Figure 4 is a sectional view of the shadow mask shown in Figure 3D; Figures 5A to 5F are sectional views explaining the steps of preparing a shadow mask sheet with a plurality of apertures according to the invention; Figure 6A is an enlarged sectional view of a portion of the photosensitive film formed on a metal sheet according to the invention and Figure 6B is a similar sectional view of a photosensitive film formed on the metal sheet according to the conventional method; and Figure 7 is a graph showing the relationship between lateral etching amount and exposure rate. At first, metal sheet composed of, for example, aluminium-killed low carbon steel was washed on both surfaces. As shown in Figure 5A, photosensitive layers 32 of thickness of approximately 6 µm were formed by coating and drying a photosensitive resin liquid on both surfaces of the metal sheet 31. As the photosensitive resin liquid, for example, an alkali milk caseinate including approximately 1 wt % of ammonium dichromate as a sensitizer was used. Next, as shown in Figure 5B, the photosensitive layers 32 were simultaneously exposed to light, such as ultraviolet, from light sources 33 after a pair of negative films 34a and 34b were applied to the respective photosensitive layers 32 to print the patterns of the negative films 34a and 34b on the photosensitive layers 32. The negative films 34a and 34b have different circular negative patterns. The first film 34a was formed with circular patterns so as to make large openings in one photosensitive layer 32. The second film 34b also was formed with circular patterns so as to make small openings in the other photosensitive layer 32. The light sources 33 were composed of 5 kW superhigh pressure mercury discharge lamps and they were positioned approximately 1 m from the surface of the metal sheet 31. The exposure was carried out for approximately 1 minute. For adjusting the accumulated exposure amount, for example, an integrating exposure meter detecting the accumulated exposure amount was used. It, however, was difficult to detect the actual accumulated exposure amount at the surface of the photosentitive layer. The accumulated exposure amount at the surface of the negative film could be used for adjusting exposure of the photosensitive layer instead of the accumulated exposure amount at the surface of the photosensitive layer. The exposure to the patterned light was continued until the exposed portions of the photosensitive layer were solidified. In other words, the exposure was executed until the accumulated exposure amount was at a predetermined level. Further, as shown in Figure 5C, the photosensitive layers 32 were additionally exposed to light from the light sources 33, after the negative films were removed, for about 10 seconds. Namely, the photosensitive layers 32 were exposed to unpatterned light in addition to the exposure by the patterned light. After the exposure, spraying by using warm water at about 40° C and a pressure of about 1 kg/cm² was carried out to develop and remove portions of the photosensitive layers 32. Then, the photoresistive layers 32 were dried in an atmosphere at a temperature of about 150° C and burned in an atmosphere at a temperature of about 200° C. As the result of the steps mentioned above, as shown in Figure 5D, a pair of resist films 35a and 35b having patterns of regions with different sizes corresponding to the negative patterns of the negative films were obtained on the surfaces of the metal sheet 31. It should be noted that the lightly exposed areas 36a and 36b, which were not exposed to the patterned light but were exposed to the unpatterned light, were covered with thin film portions 37. In other words, the resist films 35a and 35b have different patterns of recesses having the residual portions 37 of the resist films 35a and 35b. Next, the metal sheet covered with the resist films 35a and 35b was sprayed with ferric chloride solution at a temperature of about 67° C for etching. The specific gravity of the solution was about 1.467. As shown in Figure 5E, specified apertures were formed by linking large holes 39a to small holes 39b. The large and small holes 39a and 39b grew from the large openings 40a and the small openings 40b in the resist films 35a and 35b, respectively, during etching. Finally, resist films 35a and 35b were removed by spraying a 15% solution of caustic soda at a temperature of about 90° C and a pressure of about 1 kg/cm² after washing by water, and then the resultant shadow mask sheet with a plurality of apertures 38, shown in Figure 5F, was obtained by washing with water and drying. According to the embodiment, since the photosensitive layer was exposed to the unpatterned light for a short duration of time, a thin photoresist layer still remained on the areas of the photosensitive layer which were not exposed to the patterned light. Consequently, as shown in Figure 6A, the resist film 35a has the thin film portions 37 covering the areas unexposed by the patterned light. The residual film portions 37 of the resist film 35a were so thin that the etching solution could penetrate through, and could be removed, during etching. Consequently, the lateral etching underneath the resist film was decreased. On the contrary, in the case of the conventional method without exposure to the unpatterned light, the periphery of the resist film 18a surrounding the unexposed area by the patterned light was removed, as shown in Figure 6B. Due to the removal, an edge portion of the resist film 18a formed an obtuse angle as shown by ϑ₁. Consequently, since the etching solution penetrated to the edge portion, the lateral etching was promoted undesirably. In the case of the embodiment of the invention, since the edge portion of the resist films 35a could be kept an acute angle ϑ₂, even when the residual film portions 37 of the resist film were removed, the lateral etching could be decreased. Figure 7 shows the relationship between the exposure amount of unpatterned light and the amount of the lateral etching. In this graph, the horizontal axis indicates the ratio (%) of the accumulated amount of exposure for the accumulated amount of exposure by the unpatterned light to the amount of exposure required for patterning by using negative films in close contact to the metal sheet. In other words, the ratio (%) is obtained by dividing the accumulated amount of exposure of the unpatterned light by the accumulated amount of exposure of the patterned light which is required for patterning. The vertical axis indicates the ratio (%) of the amount of the lateral etching when exposed to the unpatterned light, taking as 100% the amount of the lateral etching when exposed to the patterned light alone. As seen from the graph, the ratio of the exposure amount of the unpatterned light to the exposure amount of the patterned light preferably ranges from about 10% to about 45%. The length of the lateral extension of the photosensitive film was decreased to about 25% to about 45% of the thickness of the metal sheet due to the exposure to unpatterned light, while the length of the lateral extension of the photosensitive film was 45% to 65% of the thickness of the metal sheet in the case of the conventional method, shown in Figures 3A to 3E. Further, the invention can prevent the metal sheet from rusting due to the residual photosensitive layer on the unexposed areas. In the method of this invention, the exposure of the unpatterned light may be executed before or after the exposure of the patterned light. In addition, the exposure of the unpatterned light can be executed for one of the photosensitive layers, especially for the photosensitive layer forming the larger opening pattern or both photosensitive layers. In the method of the invention, thickness of the first and second photosensitive layers can be made different from each other for preparing the shadow mask of a high resolution colour picture tube. Namely, it is preferable to increase the thickness of the first photosensitive layer formed on the first surface of the metal sheet, in which large apertures are formed, compared with the second photosensitive layer. In this case, an accumulated exposure amount of the exposure to the first patterned light exposing the first photosensitive layer is adjusted to be larger than that of the exposure to the second patterned light. As previously mentioned, illuminance of the exposure light and/or duration of exposure time are increased for increasing the accumulated exposure amount. The invention is applicable for the method shown in European Patent Application No. 137,366 (U. S. Patent No. 4,689,114) which is effective for forming small apertures to reduce the lateral etching.

Claims: A method of manufacturing a shadow mask for a colour cathode ray tube in which first and second photosensitive layers (32) are provided on respective opposite surfaces of a metal shadow mask sheet (31); the first and second photosensitive layers (32) are exposed to respective patterns of light so that each layer has regions which are exposed to the light and regions which are unexposed, the exposure being continued until the accumulated exposure amount of each of the exposed regions is at a first predetermined level; all of at least one of the first and second photosensitive layers (32) is exposed to light, the exposure being continued until the accumulated exposure amount of the layer is at a second predetermined level, the second level being less than the first level; all of the parts of the first and second photosensitive layers which have been exposed to light are converted to resist material (35a, 35b) with those regions of the layers which received only the accumulated exposure at the second level being converted to a thinner resist portion (37) than those regions which received at least the accumulated exposure at the first predetermined level; the sheet is etched through any portions of the sheet which are unprotected by resist and through said thinner portions of resist to form apertures (38) extending through the sheet (31); and the resist material (35a, 35b) is removed from the sheet. A method as claimed in claim 1, in which the exposure of all of at least one of the photosensitive layers to light is prior to the exposure of the photosensitive layers to patterns of light. A method as claimed in claim 1 or 2, in which the second predetermined level of accumulated exposure is between 10% and 45% of the first predetermined level of accumulated exposure. A method as claimed in any preceding claim, in which the layers of resist on both of the faces have a predetermined pattern of regions of reduced thickness, said regions on opposite faces being mutually aligned.

Inventors: SAGOU SEIJI C O PATENT DIVISIO, SAGOU, SEIJI C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01J9
IPC codes: H01J9