SHADOW MASK

Abstract: A cast ingot of an invar alloy is forged, hot-­ and cold-rolled, annealed and subjected to a controlled rolling to provide a shadow mask plate. An X-ray dif­ fration pattern is taken in relation to an electron-­ beam hole-formation surface of the shadow mask plate, and a draft at a controlled rolling step is so control­ led tat a "g" value indicated by g = (I₁ + I₂)/I₃ is above 2, where I₁ = diffraction intensity at a {200} crystal face; I₂ = a diffraction intensity at a {lll} crystal face; and I₃ = a diffraction intensity at a {220} crystal face. The shadow mask plate is etched to provide shadow masks (3) each with electron-beam holes (5) formed therein, noting that one hole surface side which has a higher orientation is used as a larger-diameter hole surface side.

Description: This invention relates to a shadow mask for use in a color CRT (cathode ray tube). A shadow mask for use in a color CRT is arranged in proximity to a tri-color fluorescent screen. Electron beams emitted from electron guns pass through a corresponding electron-beam hole in a shadow mask and a land precisely on predetermined spots on the fluorescent screen. The shadow mask functions to permit the electron beam to be directed onto the fluorescent screen. For this reason, the relative position, diameter, and configuration of the electron-beam holes directly influence the resultant image quality, and a high manufacturing accuracy, therefore, is required when forming the electron-beam holes. Low manufacturing accuracy would lead to the degeneration of image quality, resulting from a "doming" phenomenon. In order to prevent the generating of scattering electrons, a special process is required which makes the beam-exit surface side larger than the beam-entry surface side of the electron-beam holes, and thus makes the electron-beam hole semispherical in configuration. Conventionally, high manufacturing accuracy has been achieved by etching a shadow mask plate with a predetermined etchant, to form electron-beam holes. Recently, due to widespread demand for "fineness of texture" on the television pictures, a high quality television system is being developed as part of a communication system. This requires a CRT having improved image resolution. In order to improve image resolution, it is necessary to form electron-beam holes of a shadow mask more finely, and achieve a high density thereof. In order to provide very fine, high-density electron-beam holes in the shadow mask, it is required that electron-beam holes to be formed with high accuracy than has been required in the conventional CRT. However, the electron beam also hits an area around the beam holes in the shadow mask, causing the shadow mask to be heated up, with the result that the electron-beam holes become somewhat displaced due to the thermal expansion of the shadow mask. In this case, a PD (purity drift) value is employed in proportion to the extent of such a displacement. An invar type alloy of a low thermal expansion coefficient has, therefore, been used so as to lower the PD value. It has, however, been difficult to form electron-beam holes by a conventional etching method, because the invar type alloy has high resistance to etching. Even if it is possible to form electron-beam holes, it is difficult to form them with the required high accuracy. In connection with this problem, the inventors showed, in US Patent No. 4,528,246, the concept of forming fine electron-beam holes with better accuracy by forming {l00} crystal faces, by a preliminary rolling and heat treatment, along the electron-beam hole formation surface with above 35% orientation. Here, an f value has been adopted as a parameter the extent of orientation for the crystal plane. However, it could be that the accuracy with which the electron-beam holes are made may be inadequate, thus making it difficult to evaluate such an "f" value, the parameter of the extent of orientation along the {l00} crystal plane. It is accordingly the object of this invention to provide a shadow mask having electron-beam holes formed more finely and with a higher density than in the conventional CRT. According to the present invention there is provided a shadow mask for a color cathode ray tube, comprising a plate-like body made of an invar type alloy of a face-centered cubic lattice structure, and having holes through which electron beams from electron guns pass, characterized in that said plate-like body has a "g" value of 2 or more at both the surface sides of said electron-beam holes, the "g" value being given by g = (I₁ + I₂)/I₃ where, using in each case the same measurement technique, I₁ = the X-ray diffraction intensity of a {200} crystal face; I₂ = the X-ray diffraction intensity of a {111} crystal face; I₃ = the diffraction intensity of a {220} crystal face. The inventors have found that improper configuration and inexact relative positioning of electron-beam holes in the shadow mask are related to nonuniformity in the orientation of crystal faces. That is, where electron-beam holes are formed by etching in the shadow mask plate made of an invar type alloy of a face-centered cubic lattice structure, they can be formed with high accuracy if more {100} and {111} crystal faces are oriented in an etching surface. If, however, more {110} crystal faces are oriented, then elliptical holes will be formed by etching, thus failing to form electron-beam holes with high accuracy. The extent of orientation can be shown using X-ray diffraction intensity. The higher the extent of orientation along the {100} and {111} crystal faces, the greater the diffraction intensities I₁ and I₂ and, conversely, the higher the extent of orientation along the {110} crystal faces, the higher the diffraction intensity I₃. Thus, the greater the "g" value indicated by (I₁ + I₂)/I₃ , the higher the accuracy with which electron-beam holes are formed. With the "g" value set to be above 2, the electron-beam holes can be made very finely and with a high density, thus offering better image quality. The inventors have found that the extent of orientation along the respective {100} crystal faces varies at the surface sides (obverse and reverse surfaces) of the electron-beam holes in a shadow mask plate. In order to prevent the generating of scattering electrons, the electron-beam holes are so formed that their diameters are greater at the beam-exit surface side than at the beam-entry surface side. For this reason, a greater amount of working is required at the beam-exit surface side than at the beam-entry surface side. If, therefore, one shadow-mask surface side higher in the extent of orientation along the {100} crystal plane is used as the beam-exit surface side of the shadow mask sheet, then high accuracy can be attained at that surface side of the shadow mask sheet where a greater amount of working is required. It is, therefore, possible to form the electron-beam holes with high accuracy. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a perspective view showing a color CRT using a shadow mask according to one embodiment of this invention; Fig. 2 is a cross-sectional view, partly enlarged, showing electron-beam holes in the shadow mask; Fig. 3 is a block diagram showing a manufacturing process of a shadow mask sheet; Figs. 4 to 6 each show a modeling diagram of a face-centered cubic lattice structure; Figs. 7 to 9 are each a modeling diagram showing an etching progress state at a crystal plane of a shadow mask sheet; Fig. 10 is a modeling diagram showing an etching progress direction when an electron-beam hole-formation surface of a shadow mask sheet is oriented along a {110} or {111} crystal plane; Fig. 11 is a modeling diagram showing an etching progress direction when an electron-beam hole-formation surface of a shadow mask sheet is oriented along a {110} crystal plane; Fig. 12 is a graph showing a relation of a PD value to a "g" value; Fig. 13 is a graph showing a relation of a "g" value to a controlled rolling; and Fig. 14 is a graph showing an X-ray diffraction pattern for an electron-beam hole-formation surface of a shadow mask sheet. A shadow mask according to one embodiment of this invention will now be described below with reference to the accompanying drawings. Fig. 1 is a perspective view showing a color TV CRT using a shadow mask according to the embodiment of this invention. The TV CRT has electron guns 2a, 2b, and 2c, shadow mask 3, and fluorescent screen 4. Shadow mask 3 is located between the electron guns and fluorescent screen 4. A plurality of electron-beam holes are formed in shadow mask 3, and extend from beam-entry surface side 3a to beam-exit surface side 3b. Red, blue, and green beams 6a, 6b, and 6c, respectively, are emitted from electron guns 2a, 2b, and 2c and are directed as beam spots onto red, green, and blue fluophors 7a, 7b, and 7c on fluorescent screen 4, through corresponding electron-beam holes. Fig. 2 is an enlarged, cross-sectional view showing the aforementioned electron-beam hole which is formed in shadow mask 3. Electron-beam hole 5 is comprises of a frusto-conical input section 5a formed on the electron beam-entry side and semispherical output section 5b formed at the beam-exit side, with a pinched portion defined between input section 5a and output section 5b. Electron-beam hole 5 is so formed that electron beam-exit side 3b of output section 5b has a greater diameter than electron beam-entry side 3a of input side 5a. By widening the electron-beam hole at output section 5b, in this case, the scattering of electron beams is prevented, thereby offering improved image quality. In the color CRT so arranged, respective electron beams 6a, 6b, and 6c are directed onto the corresponding fluophors 7a, 7b, and 7c, to allow the emission of the respective colored light. Shadow mask 3 allows the passage of electron beams of the respective colors, so that the respective electron beams are directed precisely onto predetermined locations on the fluorescent screen. It is therefore necessary that the electron-beam holes of shadow mask 3 be formed with high manufacturing accuracy. Shadow mask 3 is made of an invar type alloy of a face-centered cubic lattice structure. In the face-centered cubic lattice structure, the crystal faces are easy to orient and, in the formation of electron-beam holes 5, those crystal faces can be favorably utilized to provide electron-beam holes with adequate accuracy. Usable invar type alloys of a face-centered cubic lattice structure are, for example, an invar alloy (36Ni-Fe), super invar steel (32Ni-5Co-63Fe), invar stainless steel (54Co-9.3Cr-36.5Fe), and 43Pb-63F alloy. These alloys all have a low thermal expansion coefficient, as is set forth above, and even if the shadow mask is heated up, almost no color drift, due to the thermal expansion of shadow mask 3, occurs, nor does a resultant change in the position and configuration of electron-beam holes 5. It is preferable that shadow mask 3 be of an erinvar type alloy with a predetermined tension applied thereto. In this case, shadow mask 3 undergoes no substantial expansion due to a temperature rise. The process of manufacturing a shadow mask plate will now be explained below, with reference to Fig. 3. First of all, the aforementioned alloy is dissolved and cast, at step 1, to provide an ingot. The ingot is forged at step 2 and rolled, by a continuous hot-rolling process, into a 2 mm-thick sheet, at step 3. As a result of the hot-rolling process, {100} crystal faces are oriented along the surface of the rolled sheet. Then, the sheet is cold-rolled once, or a plurality of times, with a draft of above 80% (for example 90%), to obtain a sheet of, for example, 0.2 mm, at step 4. This cold-rolling step allows the crystal axis to rotate, so that {110} crystal faces of the sheet are oriented along the surface of the rolled plate. Then, the sheet so obtained is annealed at a temperature exceeding a recrystallization temperature, at step 5, so that {100} crystal faces are again oriented. Thereafter, the plate is cold-rolled by a controlled rolling step with a predetermined draft, and its shape, etc. is corrected, to thereby obtain a shadow mask sheet, at step 6. Shadow mask 3 is manufactured by forming electron-beam holes 5 in the surface of the rolled shadow mask plate. Electron-beam holes 5 are formed by etching both surfaces of the shadow mask plate. The accuracy with which the electron-beam holes are formed varies depending upon the crystal faces of the emerging rolled-surface of the plate. As is set forth above, the shadow mask is made of an alloy of a face-centered cubic lattice structure and has {100}, {110}, and {111} crystal faces, as is shown in Figs. 4, 5, and 6, respectively. Since the {100} crystal face emerges on the etched surface of the plate, isotropic etching progresses, as is shown in Figs. 7 and 9, with the {100} and {111} faces oriented. The etching direction is perpendicular to the rolled surface, as is indicated by arrows in Fig. 10. With these crystal faces oriented along the rolled surface of the plate, it is possible to form very fine holes with high accuracy. With the {110} faces oriented along the rolled surface, as is shown in Fig. 8, the longitudinal and lateral etchings are different from each other with respect to their etching rate, offering an elliptical hole configuration. As a result, the etching direction will be inclined with respect to the rolled surface, as is indicated by arrows 12 in Fig. 11. With the {110} crystal faces oriented, it is difficult to form very small holes with high accuracy. In the rolled surface of the shadow mask plate, the accuracy with which electron-beam holes 5 are made can be enhanced by means of the extent of orientation being higher in the {100} and {111} crystal faces and lower in the {110} crystal face. The extent of orientation at the {100} and {111} crystal faces can be evaluated by taking an X-ray diffraction pattern on the rolled surface of the shadow mask plate. In the actual evaluation of the X-ray diffraction, the diffraction intensity at the {200} crystal face corresponds to the extent of orientation at the {100} crystal face, and the diffraction intensity at the {220} crystal face corresponds to the extent of orientation at the {110} crystal face. With the diffraction intensities of the {200}, {111}, and {220} crystal faces indicated by I₁, I₂, and I₃, respectively, the extent of orientation, g, of the {100} and {111} crystal faces can be expressed by Equation (1) below. g = (I₁ + I₂)/I₃ (1) The "g" value varies, depending upon the draft of the controlled rolling. The greater the "g" value, the smaller the draft. The relation of the "g" value to image quality will be explained below. Fig. 12 is a graph showing a relation of the "g" value as the abscissa, to a PD value as the ordinate. Here, the PD value shows the extent of color drift, µm, arising from a relative positional relation of electron-beam hole 5 in shadow mask 3, to fluorescent screen 2. The smaller the PD value, the lesser the color drift. In Fig. 12, if the "g" value becomes smaller than 2, the PD value increases sharply and exceeds 100 µm. When the "g" value becomes smaller than 2, then, a drastic color drift occurs, indicating a lower manufacturing accuracy of electron-beam holes 5 in shadow mask 3, and thus prominently lowering image quality. When, on the other hand, the "g" value exceeds 2, color drift decreases at the shadow mask thus, indicating the high manufacturing accuracy with which the electron-beam holes are formed. As a result, better image quality is obtained. For the "g" value exceeding 2, it is possible to form very small electron-beam holes in the shadow mask, with high accuracy, and thus to obtain improved image quality. This embodiment will now be explained below, in connection with the following example: Example 1 An invar alloy (36Ni-Fe) was dissolved and cast to provide an ingot. The ingot, after being forged, was rolled, by a continuous hot-rolling process, into a 2 mm-thick plate. The sheet was further cold-rolled into a 0.2 mm-thick plate, with a 90% draft. Then the plate was annealed at 750°C, a temperature exceeding the recrystallization temperature. The annealed plate was subjected, as required, to a controlled rolling to obtain a shadow mask plate. After the respective steps, an X-ray diffraction pattern was taken on the surface of the rolled sheet, and the aforementioned "g" value was obtained. As a result, the "g" value was 13 after the sheet was hot-rolled. After a cold working, the "g" value was 0.5. After a subsequent step, the "g" value was 108. Fig. 13 is a graph showing a relation of a draft in a controlled rolling step, to the "g" value, with the draft plotted as the ordinate, and the "g" value as the abscissa. As can be seen from the graph of Fig. 13, in the controlled rolling step, there is a tendency wherein the higher the draft, the lower the "g" value. The "g" value was 87 for a 4.8% draft, 26 for 16.7% draft, 5 for a 19% draft, and 2.2 for a 27% draft. Then, an X-ray diffraction pattern was taken on the surface of the rolled shadow mask sheet. Fig. 14 shows an X-ray diffraction pattern of the shadow mask sheet when the draft at the controlled rolling step was 4.8%. As the X-ray, a kα ray from copper was used with an acceleration voltage of 50 kV and electric current of 30 mA, noting that as a definer of the diffraction intensity of the X-ray, use was made of a value corresponding to the area of each peak value in the graph of Fig. 14. A shadow mask sheet having a "g" value of below 2, that is, a draft of below 30% in the controlled rolling step, was thus manufactured, followed by placing an etchant onto both the surfaces (the beam-entry side and beam-exit side in Fig. 2), to provide electron-beam holes 5. As the etchant, an aqueous solution containing 43% ferric chloride, 6% ferrous chloride, and 0.1% hydrochloric acid was used, having a temperature of 65°C. By so doing, 520,000 electron-beam holes 5 were formed at a pitch of 0.3 mm in the shadow mask plate, to thereby provide shadow masks for a 14" type television CRT. It has been found that the electron-beam holes thus formed have excellent manufacturing accuracy. The shadow mask with these electron-beam holes assures excellent image quality with less color drift. Another embodiment of this invention will now be explained below. In the shadow mask plate thus manufactured by the steps of the first embodiment, the "g" value varies at both surfaces. As was set out above, an etchant was placed on both the surfaces (beam-entry and beam-exit surfaces) of shadow mask 3, to provide a beam input section 5a and a beam output section 5b at each electron-beam hole 5. In order to prevent the generating of scattering electrons, electron-beam hole 5 is so formed as to be larger at beam output section 5b than at beam input section 5a. For this reason, a greater amount of working is required at the side of the beam output section than at the beam input section of the shadow mask. As is set out above, the greater the "g" value, the higher the manufacturing accuracy. If the shadow mask side whose "g" value is greater than that of the other side of the shadow mask is used as that side of the shadow mask which requires a greater amount of working, then it is possible to enhance the manufacturing accuracy of the electron-beam hole 5. In the respective rolling and annealing steps of the manufacturing of the shadow mask plate, the extent of orientation varies principally at the {100} and {110} crystal faces. In place of the "g" value, therefore, the "a" value, as indicated by Equation (2) below, can be employed as the parameter of the manufacturing accuracy. a = I₁/I₃ (2) The "a" value can be employed in the same was as the "g" value. That is, if the shadow mask surface side whose "a" value is greater than that of the other side surface of the shadow mask is used as that side of the shadow mask which requires a greater amount of working, then it is possible to enhance the manufacturing accuracy of the electron-beam hole 5. This embodiment will now be explained below in more detail, with reference to the following example: Example 2 A shadow mask sheet was manufactured in the same way as in Example 1, by using an invar allow. The shadow mask sheet was wound around a take-up roll for storage. Table 1 below shows the "g" value of both sides (surfaces) of the shadow mask sheet. In Table 1, surface A was used as a surface whose "g" value is higher, and surface B as a surface whose "g" value is lower. Similarly, the "a" value was, for example, 30 at surface A and 20 the surface B. In the shadow mask plate, electron-beam hole 5 was formed, as in Example 1, such that the shadow mask plate surface side whose "g" or "a" value is greater than that of the other surface side of the shadow mask plate is used as the plate side which requires the greater amount of working. As a result, it was possible to obtain electron-beam holes 5 with high manufacturing accuracy. Electron-beam holes 5 were formed with the beam output side as surface A when the "a" value was, for example, 30 at surface A and 20 at surface B, in which case the PD value was 25 µm. When electron-beam hole 5 was formed with surfaces A and B interchanged, the PD value was then 50 µm. From this it will be appreciated that the advantage of this embodiment is clear, since the PD value was 25 µ. Although in the aforementioned embodiment the electron-beam holes have been explained as being circular, this invention is not restricted thereto. For example, the electron-beam holes may be so formed as to be of a wide or narrow slot type.

Claims: A shadow mask for a color cathode ray tube, comprising a plate-like body (3) made of an invar type alloy of a face-centered cubic lattice structure, and having holes (5) through which electron beams (6a, 6b, 6c) from electron guns (2a, 2b, 2c) pass, characterized in that said plate-like body (3) has a "g" value of 2 or more at both the surface sides of said electron-beam holes, the "g" value being given by g = (I₁ + I₂)/I₃ where, using in each case the same measurement technique, I₁ = the X-ray diffraction intensity of a {200} crystal face; I₂ = the X-ray diffraction intensity of a {111} crystal face; I₃ = the diffraction intensity of a {220} crystal face. The shadow mask according to claim 1, characterized in that said electron-beam holes (5) are so formed that each is smaller at one surface side than at the other surface side. The shadow mask according to claim 2, characterized in that said electron-beam holes (5) are so formed that one surface side whose "g" value is greater than that of the other surface side is used as a surface side (3b) in which the beam hole diameter is larger than that of the other surface side (3a). The shadow mask according to claim 2, characterized in that said electron-beam holes (5) are formed such that one surface (3b) wherein the diameter of each hole is greater than that of the other surface, has a higher orientation of the {100} and {111} crystal faces than the other surface (3a). The shadow mask according to claim 2, characterized in that electron beams which have been emitted from said electron guns (2a, 2b, 2c) pass through the shadow mask, from a smaller-diameter side of said electron-beam holes (5). The shadow mask according to claim 4, characterized in that the surface of said shadow mask having a greater "g" value is used as an electron beam-exit surface (6a, 6b, 6c). The shadow mask according to claim 3, in which the surface side whose "g" value is greater also has a greater ratio of I₁/I₃ than the other surface side. A colour cathode ray tube comprising electron guns (2a, 2b, 2c), a tri-colour fluorescent screen (4), and a shadow mask (3) therebetween in proximity to the screen (4) and having holes (5) through which respective electron beams (6a, 6b, 6c) from the electron guns pass to the screen (4), characterized in that the shadow mask (3) is in accordance with any preceding claim.

Inventors: HIGASHINAKAGAWA, EMIKO PATENT DIVISION, INABA, MICHIHIKO PATENT DIVISION, KANTO, MASAHARU PATENT DIVISION, OHTAKE, YASUHISA PATENT DIVISION, SUGAI, SHINZO PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01J29
IPC codes: H01J9, H01J29