Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-04-12
2003-01-21
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S621000, C148S651000, C420S094000
Reexamination Certificate
active
06508892
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a Fe—Ni alloy blank for use in making a shadow mask by fine etching, and more specifically to a Fe—Ni alloy shadow mask blank which, when perforated by fine etching to form apertures through which electron beams pass, can improve the unevenness of aperture diameters due to the presence of irregular apertures and can provide electron beam apertures of uniform diameter and also relates to a shadow mask blank which has been formed with apertures for the passage of electron beams having improved unevenness of aperture diameters due to the presence of irregular apertures. The invention further relates to a method for manufacturing a Fe—Ni alloy blank with such properties.
In the following description the concentrations of alloy components are given on the basis of mass proportions (%=mass percentage; ppm=mass proportion).
As material of shadow masks for color picture tubes, mild steel has been commonly used. The mild steel, however, presents a problem. Continuous use of a color picture tube increases the temperature of its shadow mask due to irradiation with electron beams. Consequent thermal expansion of the mask gradually brings the points of the screen that the electron beams strike through the mask out of register with the phosphor dots of the screen, causing color misregister or mismatching. The temperature rise of the shadow mask results from the fact that when a television is turned on, only less than one-third of the total amount of the electron beams passes the apertures of the shadow mask, the remainder of the electron beams striking the mask itself. More recently, therefore, a Fe—Ni alloy of low thermal expansion coefficient known as “36 (iron-36% nickel) alloy” has come into use in the art of shadow masks for color picture tubes because of its merit in preventing color mismatching.
For the manufacture of a Fe—Ni alloy blank for shadow mask, a Fe—Ni alloy of a desired composition is melt-refined, for example, by vacuum melting in a vacuum induction melting (VIM) furnace or by secondary refining in a ladle furnace (LF). The molten metal is cast into an ingot, which in turn is forged or rolled by a blooming mill to a slab. The slab is hot rolled, descaled to remove oxide from the surface, repeatedly cold rolled and annealed for recrystallization, and, after the last recrystallization annealing, the rolled slab is finished by final cold rolling to a sheet of desired thickness in the range of 0.05 to 0.3 mm. The finally cold rolled sheet is slitted into blanks of desired width as shadow mask blanks. The blanks are degreased, coated with photoresist on both sides for patterning, exposed to light and developed to form a pattern, perforated by etching, and then cut to individual flat mask blanks. The flat mask blanks are annealed in a non-oxidizing atmosphere to impart press workability. (In the preannealing process this annealing is done on the finally cold rolled stock prior to etching.) The blanks are spherically pressed to the form of masks. Lastly, the spherically shaped masks are degreased, annealed in water vapor or combustion gas atmosphere to form a black oxide film on the mask surface. In this way shadow masks are manufactured.
For the purposes of this invention, the blanks to be etched for perforation after the final cold rolling for the passage of electron beams are collectively called shadow mask blanks. The term also encompasses the blanks, including flat masks, that have been perforated for the passage of electron beams and are yet to be press formed, as shadow mask blanks that have been formed with apertures for the passage of electron beams.
These shadow mask blanks are usually formed with apertures for the passage of electron beams by the well-known etching technique using aqueous ferric chloride. For the etching, photolithography is applied, and resist masks are formed on both sides of a blank, e.g., the mask on one side having a number of round openings 80 &mgr;m in diameter and the corresponding points of the mask on the other side having round openings 180 &mgr;m in diameter, and then aqueous solution of ferric chloride is sprayed over the both sides.
The etching provides the shadow mask blank a multiplicity of tiny apertures in a close arrangement. However, localized variation of etching conditions and other factors can result in unevenness of aperture diameters. If the unevenness is excessive, the shadow mask incorporated into a color picture tube can cause color mismatching and make the product defective. This unevenness of aperture diameters has hitherto been an important cost-raising factor as it decreases the yield in etch-perforation of shadow mask blanks for the passage of electron beams.
Various attempts have heretofore been made to control the unevenness of aperture diameters. From the material viewpoint, for example, Japanese Patent Application Kokai Nos. 5-86441 and 10-111614 propose precluding the unevenness through the control of the texture. They intend to secure the uniformity of etching by the texture control.
Our intensive research has, however, revealed that there is a phenomenon of unevenness of aperture diameter that cannot be coped with by the techniques of the prior art.
FIG. 1
shows scanning electron micrographs (SEMs) of a “normal aperture” formed by etching for the passage of electron beam and an “abnormal aperture” newly found to be a cause of unevenness of aperture diameters. (The shapes of the apertures formed upon etching of only one side were comparatively observed.) The abnormal aperture is characterized by rough wall surface compared with the normal aperture. The profile of the aperture is fringed and blurred with unusual etching, the diameter tending to be larger than the target value. The characteristic configuration of the abnormal aperture varies in degree with etching and other conditions; sometimes the surrounding wall is not roughened or the fringe or blur is not clearly observed. The unevenness of the aperture diameters with the formation of abnormal apertures has not been precluded by the prior art.
OBJECT OF THE INVENTION
This invention is aimed at providing a shadow mask blank of Fe—Ni alloy which, in perforation by etching to form apertures for the passage of electron beams, will not have unevenness in the diameters of the apertures due to the formation of abnormal apertures, even if the etching conditions are locally varied, and is also aimed at providing a method of manufacturing the blank.
SUMMARY OF THE INVENTION
We have made intensive study on the problems of the prior art from an entirely new, unique viewpoint and have found that, with a shadow mask blank of Fe—Ni alloy which contains many minute inclusions, the perforation by etching scarcely causes the unevenness of aperture diameter due to the formation of abnormal apertures. Of the minute inclusions, particularly fine MnS has been found effective in controlling the unevenness of aperture diameter. In this case the MnS that proves effective in restricting the unevenness of the diameter of etched apertures for electron-beam passage is in the form of particles from 50 to 1,000 nm in diameter. The restricting effect was shown when the density (which means abundance, that is probability or frequency of existence) of MnS particles exceeded 1,500/mm
2
. For an elliptical, bar-like, or needle shape in the purposes of this invention, as shown in
FIG. 2
, the diameter of MnS particle is represented by the mean value of the shorter axis L
1
and the longer axis L
2
.
Although the detailed mechanism by which MnS controls the unevenness of the diameter of etched apertures for the passage of electron beams is not yet clarified, it is presumed to be as follows:
A rolled blank of Fe—Ni alloy according to this invention is usually etched to be a shadow mask, using an aqueous solution of ferric chloride. For that purpose a resist film is applied to the blank to cover the portions not to be perforated, so that only the portions to be perforated are exposed to the aqueous ferric chloride. If minute Mn
Hatano Takaaki
Kita Yoshihisa
Drinker Biddle & Reath LLP
Nippon Mining & Metals Co., Ltd.
Yee Deborah
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